Mapping Chemical Landscapes: smart polymer surfaces on nanometer scale

Transcription

1 Mapping Chemical Landscapes: smart polymer surfaces on nanometer scale Dissertation submitted for the degree of Dr. rer. nat. (Doctor rerum Naturalium) in the faculty of chemistry and biochemistry at the Ruhr-University Bochum Germany M.Sc. Marlena Filimon Department of Physical Chemistry II Bochum, 2010

2 Mapping Chemical Landscapes: smart polymer surfaces on nanometer scale 1 st Examiner: Prof. Dr. M. Havenith-Newen 2 nd Examiner: Prof. Dr. W. Schuhmann Thesis Committee Head: Prof. Dr. R. Heumann

3 This work was carried out between October 2006 and March 2010 at the Department of Physical Chemistry II, Ruhr University of Bochum, under the supervision of Prof. Dr. M. Havenith-Newen.

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5 Dedicated to my daughter Karina

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7 If If you can keep your head when all about you Are losing theirs and blaming it on you; If you can trust yourself when all men doubt you, But make allowance for their doubting too; If you can wait and not be tired by waiting, Or, being lied about, don't deal in lies, Or, being hated, don't give way to hating, And yet don't look too good, nor talk too wise; If you can dream and not make dreams your master; If you can think and not make thoughts your aim; If you can meet with triumph and disaster And treat those two imposters just the same; If you can bear to hear the truth you've spoken Twisted by knaves to make a trap for fools, Or watch the things you gave your life to broken, And stoop and build 'em up with wornout tools; If you can make one heap of all your winnings And risk it on one turn of pitch-and-toss, And lose, and start again at your beginnings And never breathe a word about your loss; If you can force your heart and nerve and sinew To serve your turn long after they are gone, And so hold on when there is nothing in you Except the Will which says to them: "Hold on"; If you can talk with crowds and keep your virtue, Or walk with kings nor lose the common touch; If neither foes nor loving friends can hurt you; If all men count with you, but none too much; If you can fill the unforgiving minute With sixty seconds' worth of distance run - Yours is the Earth and everything that's in it, And which is more you'll be a Man my son! Rudyard Kippling

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9 Contents 1. Introduction Chemical mapping - from micro to nanometer resolution Classical microscopy and resolution Infrared (micro-)spectroscopy Infrared spectroscopy Fourier transform infrared (FTIR) microspectroscopy Raman microspectroscopy Theory of scanning near-field optical microscopy (SNOM) Theory of scanning probe microscopy (SPM) Beyond the diffraction limit Apertureless or scattering SNOM (s-snom) Theory of s-snom Higher harmonic detection and background suppression Applications of SNOM...45

10 4. Smart surfaces- From concept to reality Materials Self-Assembly Self-assembled monolayers (SAMs) Microcontact printing (µcp) Block copolymer self-assembly Polymer brushes- an induced self assembly system Synthesis of polymer brushes Smart surfaces: switchable surface energy Theory of contact angle Experimental set-up Scattering scanning near-field infrared microscope (s-snim) Set-up AFM microscope Tunable carbon monoxide laser (CO laser) Coupling the laser to the AFM Raman microscope ph sensitive self-assembled monolayers as smart surfaces Introduction Materials and Methods Materials...77

11 Preparation of micro-structured SAMs Contact angle measurements Fourier Transform Infrared Spectroscopy (FTIR) s-snim Results and Discussions Conclusion Chemical image for microphase-separated block copolymer film Materials and methods Materials Methods AFM Raman microspectroscopy Results and discussions Imaging of microphase separation of block copolymers using AFM Chemical imaging of microphase separation of block copolymers using Raman microscopy Probing local chemical composition of nanophase-separated polymer brushes using SNIM Materials and methods Tailoring surfaces-mixed polymer brushes Atomic force microscopy (AFM)...107

12 Wetting properties - Contact angle Fourier Transform infrared spectroscopy (FTIR) Scanning near-field infrared microscopy (s-snim) Results and discussion Experimental results of microphase - segregation in polymer brushes Wettability switching probed by contact angle Study of chemical functional groups using FTIR spectroscopy Probing local chemical composition using s-snim Conclusions Appendices A. Typical IR and Raman vibration bands specific for polymers B. CO-laser emission lines Bibliography..129 Curriculum Vitae and Publications 143

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14 1. Introduction At the end of 1959, Richard P. Feynman unlocked a new door for scientific world: There s Plenty of Room at the Bottom [1]. Twenty years had passed until the door was open widely with the emergence of the techniques capable to visualize the bottom The discovery of processes, phenomena and new properties of materials at nanoscale, as well as the development of new experimental techniques provides new opportunities for the development of innovative nanosystems and nanostructured materials. The conventional optical microscopes cannot observe directly these objects due to the limited resolution imposed by the wave-like nature of light. The human curiosity push the limit of technology to find where, what and how much. Infrared spectroscopy is a powerful tool for characterizing the chemical composition and structure of the species under the sample, resolving the questions what and how much. An overview of the most widespread chemical imaging methods is presented in the Chapter 2. The biggest challenge in microscopy field remains the question where. Even the best optical microscopes cannot resolve structures smaller then 200nm due to the diffraction of the light waves. To understand totally the mechanism of the nanosystems or to visualize nanostructures materials it is necessary to break this limit. A resolving solution is aperturless or scattering-type scanning near-field optical microscopy (s-snom). This is a noninvasive and nondestructive optical imaging technique that can provide high spatial resolution with chemical sensitivity. Near-field microscopy breaks the diffraction limit using the aid of focused laser beam and the sharp probing tip placed in the immediate proximity of the sample surface. The combination of the s-snom and IR spectroscopy provides chemical sensitive nanometer scale mapping. This nanoscope is desirable in many research fields (e.g. polymer research, surface chemistry, microelectronic, life science) to create chemical mapping with nanometer resolution. 1

15 1. Introduction Functional coating allows the coverage of the surface with chemical groups that interact with other molecules in their environmental. These coatings are compositionally complex; provide excellent properties (e.g. controlling the adhesion, wetting, adsorption of molecules from surrounding environment). New tailor-made polymers that are designed with smart response to external stimuli are an important goal of modern material science. The polymer brushes are defined as polymers that undergo reversible physical and chemical change in response to external modifications in environmental conditions such as selective solvents, ph or temperature. Polymer brushes are assemblies of polymer chains in which one of the chains is tethered to a surface. Growing of these polymer-films with thickness on the molecular scale from solid surface, allows to tailor the surface properties of materials. The analysis of these polymer films is a challenge due to the reduced dimensionality, from 15 nm to 100 nm, and unique properties. In contrast to conventional brushes, which consist of a single homopolymer, mixed polymer brushes can amplify the response to external stimuli by combining conformational changes and nanophase separation. Although the assumptions based on analogous bulk polymer mixed system are made, there are still some lacks to understand their fundamental properties. To improve the design of these smart surfaces, it is essential to understand the nanophase separation and surface chemistry after expose to the appropriate chemical and physical stimulus. The goal of this work is to investigate smart nanostructured surfaces: their physical and chemical properties and their sensitivity and response at different stimuli. The employed experimental technique is s-snim that accomplishes this task by chemically mapping the investigated nanostructures. Mixed polymer brushes have proved to be well suited for this goal, due to the strong response at various stimuli. Until now, the study of these smart nanostructured surfaces encountered difficulties in nanoidentification of each component of the polymer brush. This nanoidentification is the key of understanding the properties and the ordering of the structure. In this thesis, s-snim provides the only route to create nanometer-resolved chemical landscape maps of smart surfaces without dyes or sample chemical modifications. Trough s-snim, the nanoscale chemical surface analysis of mixed polymer brushes modified by different selective solvents is presented. The polymeric system chosen for this study is poly (styrene-methyl methycrylate): PS-PMMA. The dependences of the thickness and the shape of mixed polymer brushes film are exploited as well. The ability of s-snim to provide infrared spectroscopic information with nanometer resolution on smart surfaces was demonstrated. At a wavelength of 5.75 µm, a lateral resolution of ~ 80 nm was achieved, comparable with a diffraction limit resolution of λ/70. 2

16 2. Chemical mapping - from micro to nanometer resolution 2.1. Classical microscopy and resolution Infrared (micro-)spectroscopy Infrared spectroscopy Fourier transform infrared (FTIR) microspectroscopy Raman microspectroscopy

17 2. Chemical mapping From micro to nano Chemical imaging spectroscopy is a new analytical technique that provides answer to commonly asked questions such as: what chemical species are in the samples, how much of each is present and where are they located. To understand chemical and biological workflows in material science and life science, it is crucial to resolve the design of the involved macromolecules and polymers. Different in-situ spectroscopic methods are used to obtain structural information on the participating molecules, such as mass spectroscopy, fluorescence spectroscopy, NMR, IR and Raman spectroscopy. In polymers and biological studies, one requires not just a suitable spectroscopic method but also, that such a method to possess a high spatial resolution as well. For this reason, microscopy plays an essential role in science, which aims to provide images of the small structure. Chemical imaging enables the researcher to obtain spatial and spectral information characterizing samples with unprecedented ease, speed, spatial and spectral resolution. In this chapter, two different microscopic techniques, FT-IR and Raman microspectroscopy will be outlined. In the first part, a brief introduction in classical microscopy will be presented. The second part introduces infrared microscopy as the key for chemical mapping with the main accent on infrared spectroscopy. In the last part, confocal Raman scanning microscopy will be described Classical microscopy and resolution Spatial resolution represents the minimum distance between two points at which an image can still resolve them, in other words, when they can still be distinguished as two distinctive points. It is worthy to remember that the imaging process is basically a measurement of the interaction between the object (to be imaged) and the radiation (usually electromagnetic) that probes that object. Since the photons (for light) are characterized by their wavelength λ, there is an intrinsic limit of spatial resolution: features with sizes smaller than λ/2 cannot be resolved. Another issue is that the resolution of a real optical imaging system is also limited by factors such as imperfections in the lenses, optical aberrations or misalignment. However, even with the perfect materials, the intrinsic resolution can still not be reached due to the diffraction phenomena; and this is the fundamental limit of spatial resolution of any real optical system [2]. Diffraction appears when the light encounters objects with similar sizes to its wavelength: they could be either objects (such as a hair for red light) or slits (aperture closed to the wavelength of light passing through it). Due to the fact that the interaction of matter with electromagnetic radiation differs as a function of radiation wavelength (e.g. the refraction index, diffraction limit etc), monochromatic light, where all photons had the same λ, is always preferable to the 4

18 2. Chemical mapping From micro to nano polychromatic light case in which the light beam is composed from photons with different wavelengths. Spatial resolution refers to both the lateral resolution as minimum distance between two resolved distinct points in plane (x,y) as well as the depth resolution If monochromatic radiation from a point source is passed into the microscope, then a series of concentric rings of decreasing intensity is seen at the beam focus. In practice, this is made by allowing the light to pass through a small aperture. In optics, the Airy disk and Airy pattern are descriptions of the best-focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light. The Airy disk is the central bright circular region of the pattern produced by the light diffracted when passing through an aperture. The extent of and the magnitude of the diffraction patterns are affected both by the wavelength of light (λ), the refractive materials used to manufacture the objective lens and the numerical aperture (NA) of the objective lens. There is therefore a finite limit, beyond which is impossible to resolve separate points in the objective field. In 1870, Ernst Abbe demonstrates that lateral resolution in optical systems is limited by the diffraction to: (2.1) Here, NA is numerical aperture of the lens defined as: NA = n sinθ (2.2) where n is the refractive index of the medium and θ is the half-angle of the maximum cone of light that enters the objective. Following Abbe s explanation, every object behaves as a superposition of diffraction gratings. In 1879, Lord Rayleigh found out that two objects are completely resolved if they are separated by 2Δx and barely resolved if they are separated by Δx. This later condition is known as the Rayleigh criterion of resolution. Usually, λ of 550 nm is assumed, corresponding to green light. With air as medium, the highest practical NA is 0.95, and with oil, up to 1.5. In practice, the lowest value of Δx obtainable is about 200 nm. Because of this limitation, the easiest way to improve spatial resolution is to reduce the wavelength of the light source. However, at smaller wavelength, the photon energy increases and this cause sample damages by ionization effects. Electron microscopes easily achieve 10 nm spatial resolution and beyond, but they are relatively poor performers with respect to spectral and dynamic properties. A new path was open when the confocal microscope was invented. With this kind of microscope, the resolution improved and a 3D map was possible. Still, the barrier for chemical information was closed. A powerful imaging technique making use of the confocality is fluorescence microscopy. 5

19 2. Chemical mapping From micro to nano Although the Abbe resolution limit of classical microscopes continues to be pushed, especially for visible wavelength by using special geometries of illumination, nonlinear optical effects, 4-Pi-microscopy [3] or stimulated-emission-depletion microscopy (STED) [4]. The main problems of these microscopes remain the fluorophores. By attaching fluorescent markers, these may cause significant changes in the natural environment. Another problem of these mentioned techniques is that not all samples are suitable to have fluorophores attached. For polymers that have been studied in this thesis (polystyrene and polymethylmethacrylate), it is impossible to attach such marker. Table 2.1 point out the resolution and some advantage and disadvantage of some most known microscopy techniques. Technique Typical wavelength λ (nm) Lateral resolution (nm) Observations Optical microscopy (VIS) ~250 + easy to handle + imaging in liquids - no chemical selectivity FT-IR microscopy (MID IR domain) ~1000 and 3-5 µm using synchrotron radiation + easy to handle, very fast + chemical selectivity - very low resolution Confocal Raman microscopy (VIS and NIR and MID IR) Δx ~ 300 Δz ~ high chemical selectivity + imaging in liquids - special sample preparation 4Pi (VIS) Δx ~ 100 Δz ~ fluorescence dye SPM - Δx ~ 0,5 Δz ~ 1 + spatial image (high resolution, 3D image) + phase contrast - no chemical selectivity - small sample Table 2.1: some of the well established microscopy techniques with some advantages + and disadvantages -; FT-IR microscopy = Fourier Transform infrared microscopy, NIR = near infrared, MID IR = middle infrared, SPM = scanning probe microscopy 6

20 2. Chemical mapping From micro to nano Vibrational spectroscopy, such as: infrared (IR), near IR, Raman, inelastic neutron scattering, electron energy loss spectroscopy are chemically specific techniques The positions and intensities of vibrational absorption bands can be used to confirm or identify the presence of a particular group, whereas spectral correlations can be used to access structural and environmental information on selected groups. Chemical imaging shares the fundamentals of vibrational spectroscopic techniques, but provides additional information by simultaneous acquisition of spatially resolved spectra. It requires an image of the sample to be focused onto the detector, where the intensity of the radiation passing through each region of the sample is measured at each pixel. In the chemical imaging experiment, the sample is moved in both directions (first in x and then in y) and for each pixel, a spectrum is acquired. It combines the advantages of digital imaging with the attributes of spectroscopic measurements. As shown in the Figure. 2.1, a chemical imaging data set is represented by a three-dimensional cube where two axes describe horizontal and vertical spatial dimensions, determined by the size of the pixel M and M, while the third axis represent the spectral wavelength dimension. Figure 2.1: Chemical mapping 3D image: x,y spatial resolution, z intensity as a function of wavelength. 7

21 2. Chemical mapping From micro to nano Reconstruction of a chemical map is made using a specific algorithm that combines the spatial resolution with spectroscopic information. The single pixel spectrum shown corresponds to intensity as function of wavelength at a fixed pixel and provides the spectral signature of chemical component present in that part of the sample. Some approaches for exploring a 3D global intensity plot are carried out. Multivariate Image Analysis (MIA) and Principal Component Analysis (PCA) are the two common methods to analyze a chemical image [5-6]. IR and Raman spectroscopy are noninvasive techniques that require a small amount of sample and that can be easily coupled with microscopy to provide spatial information. Infrared and Raman microscopic techniques have the experimental flexibility to characterize chemically a variety of samples using transmission and/or reflectance modes. More details about these techniques are given in Chapter 2, Sections 2.2 and 2.3. Even if the chemical selectivity is very high for these techniques, the spatial resolution is low, especially for FT-IR microscopy due to the Abbe limit. In the last two decades, a different approach has been developed: Scanning Probe Microscopy (SPM), which pushes the spatial resolution beyond the diffraction limit of conventional microscopy. This technique is a branch of microscopy that forms images of surfaces by using a physical probe that scans across specimen sample surface. In comparison with classical microscopy, SPM does not require a light source anymore and, intrinsically, there is not the case of diffraction limit. An image of the surface is obtained by mechanically moving the probe (a small tip) in a raster scan over the sample serially, and recording the probe-surface interaction as a function of position. The resolution of the microscopes is limited, by the size of the tipsample interaction volume, which can be as small as a few picometers, a good approximation of a single atom. However, the lack of chemical information is still present and just indirect molecular structure and composition of the sample can be obtained. One of the most conceptual ways to break the diffraction barrier is to use both a light source and a detector that are at nanometer scale. In the last two decades it was proven that near-field optical microscopy is able to extend the range of optical measurements beyond the diffraction limit [7]. Although the attainable spatial resolution does not compare to the related SPMs, the combination of resolution and chemical information makes near-field optical microscopy (SNOM) attractive. When the tip is brought few nanometers away from a molecule, the resolution is not limited by diffraction but by the size of the tip aperture; only that one molecule will see the light coming out of the tip. Scanning near-field optical microscopy is a concept that detects simultaneously optical and topographical information. The near-field information of the surface can be collected using a combination of a nanodetector and a nanofinger. In Chapter 3, this technique is described in details. Reviews about high-resolution microscopy techniques can be found in references [8 9]. 8

22 2. Chemical mapping From micro to nano 2.2. Infrared (micro -) spectroscopy Molecular recognition is a complex phenomenon. Infrared spectroscopy is a wellestablished noninvasive and efficient analytical method for material characterization. Most commonly, the spectrum is obtained by measuring the absorption of IR radiation passing through the sample, the so called transmission mode, but also opaque samples can be measured in reflection mode. Infrared spectrometry finds its widest applicability in the analysis of organic materials, but it is also useful for polyatomic inorganic molecules and organo-metallic compounds. The IR spectrum represents a molecular fingerprint of the studied molecule and is thus highly useful in compound identification. IR spectroscopy can be easily coupled with an optical microscope to access structural and environmental information simultaneously with spatial information in order to obtain a chemical map of the samples Infrared spectroscopy Infrared spectroscopy deals with interaction of infrared radiation with matter. The IR spectrum of a compound can provide important information about its chemical nature and molecular structure. The Infrared region refers to the region of electromagnetic wave from 770 nm to 1000 µm between the visible and microwave ranges. The corresponding wavenumber range is from to 10 cm -1. The IR region is divided in three subregions: Near-IR (NIR) 770 nm < λ < 2.5 µm < ν < cm -1 Middle-IR (MID) 2.5 µm < λ < 50 µm < ν < 200 cm -1 Far-IR (FIR) 50 < λ < 1000 µm 200 < ν < 10 cm -1 The IR spectrum of polyatomic molecules can be complex because of the many possible vibrational transitions and the existence of overtones and overlaying bands. With a few exceptions, nearly all molecules absorb infrared radiation. Their vibrational energy corresponds to the energy of electromagnetic waves in the MIR region. Exceptions are symmetrical diatoms such as O 2, N 2 and H 2. 9

23 2. Chemical mapping From micro to nano However, for certain functional or structural groups, it is known that their vibrational frequencies are nearly independent of the rest of the molecule. One example is the stretching vibration of the carbonyl group, in various aldehydes and ketones, which is always observed, in the range of 1650 cm -1 to 1740 cm -1. Such frequencies are characteristic for the functional or structural group involved and are known as group frequency. The presence of various group vibrations in the infrared spectrum is a good assistance in identifying the absorbing molecules. This fact is used in this thesis when.l- Cysteine is indentified due to its characteristic COOH group after hydrolization of L- Cysteine ethyl ester at ph 10 (more details are presented in Chapter 6). In practice, the region from 4000 to 1300 cm -1 is called the group frequency region. In the region from 1300 to 400 cm -1, vibrational frequencies are affected by the entire molecule as the broader ranges for group absorptions shown in Table 2.2 [10]. Table 2.2: Frequencies of various group vibrations This region is known as fingerprint region. Since the absorption in this region is characteristic of the molecule as a whole, this region finds widespread use for identification purposes by comparison with library spectra. In the infrared spectroscopy, it is common to use wavenumbers (ν), cm -1 units, rather than wavelength (λ) since the wavenumber is directly proportional with energy. Excitation of molecular vibrations and the formation of an infrared spectrum can be described by the simple classical mechanical model. Molecular vibration of diatomic molecules can be described by the harmonic oscillator model [11]. In the model, two rigid balls, with mass m 1 and m 2, connected together by a spring, which has no mass, is considered. The two atoms correspond to the two balls and the chemical bond to the spring. 10

24 2. Chemical mapping From micro to nano The displacements of the two atoms during the vibration are illustrated in Figure 2.2 where r 0 is the equilibrium distance between the two atoms; and r is the instantaneous distance between the two atoms during the vibration (expended or compressed). Figure 2.2: Atomic displacement during a diatomic vibration The mass is under the action of a resorting force proportional to the displacement of the particle from the equilibrium position and the force constant k of the spring. From Hooke s law given by equation 2.1: (2.1) The potential energy V is proportional with to the square of the displacement. The frequency of the vibration is given by (2.2): (2.2) where ν 0 is the vibrational frequency, k is the force constant of spring, and μ = is the reduced mass. Based on classical mechanics, if the diatomic molecule possesses a dipolar moment, the vibration of the dipole produces an electromagnetic wave, which can act on the incident electromagnetic wave, leading to an absorption band at the vibrational frequency. According to the Schrödinger s equations 2.3: (2.3) 11

25 2. Chemical mapping From micro to nano The vibrational energy of the system at energy level ν is described by equation 2.4: (2.4) where v is the vibrational quantum number (v = 0,1,2,3 ) The lowest value of energy is known as the ground-state vibrational energy. When ν=0, the energy is:. The selection rule for transitions is Δν = ±1. For the transition from ν = 0 (ground state) to ν =1 (first excited state), the energy difference ΔE is hν. On the other hand, the absorption frequency can be calculated with equation 2.5: (2.5) This frequency is known as the fundamental frequency. The potential energy of vibrations fits a parabolic function well only near the equilibrium internuclear range but this model is not able to explain some phenomena such as molecular dissociation. A better description is the model of an anharmonic oscillator: the Morse potential function. Figure 2.3 shows a qualitative description of both models. If the Morse potential function is used in the Schrödinger equation, the solution is more complex. The vibrational states are not equidistant anymore. According to this model, large vibrational amplitude can result in a dissociation of the molecule and moreover, the potential curve is much steeper during bonding compression due to the Pauli principle. Figure 2.3: The harmonic (Hook s law) and anharmonic (Morse type) potentials for a diatomic molecule. 12

26 2. Chemical mapping From micro to nano The vibrational energy of the system at energy level ν is then corrected for anharmonicity, which leads to equation 2.6: where χ is the anharmonicity coefficient. (2.6) Because of the vibrational anharmonicity, the transitions are not strictly confined to the condition Δν = ±1; for example, the transition of Δν = ±2 can also occur, though with a lower probability. These weaker transitions called vibrational overtones are sometimes observed and correspond to Δν =2 or 3; their frequencies are less than two or three times the fundamental frequency due to anharmonicity. In addition, a change in dipole moment must occur so that diatomic molecules exhibit no vibrational transitions in IR absorption spectroscopy. A molecule with a small permanent dipole moment may have a large dipole moment derivative and vice versa. Carbon monoxide, for example, has a very small dipole moment but is a good infrared absorber because of a large dipole moment derivative. Vibrational modes that do not involve a change in dipole moment are said to be infrared-inactive. Polyatomic molecules give rise to much more complex vibrational motions (e.g. stretching, bending) than diatomic molecules. For linear polyatomic with N atoms, there are 3N-5 normal modes of vibrations while nonlinear polyatomics have 3N-6 normal modes. Each mode can be considered as an independent harmonic oscillator (neglecting anharmonicity). Often, the predicted 3N-6 (or 3N-5) bands are not observed in absorption vibrational spectra because some transitions are forbidden or two or more modes may be degenerate and thus, they have the same vibrational frequency. Additional vibrational bands can occur due to overtones. As it was described above, molecular vibrations can be divided in two groups: Stretching (ν): displacement oriented parallel to the bonding axis; the bond distance changes periodically. Symmetric stretching two atomic groups are symmetrically equivalent and their vibrations are coupled. Antisymmetric stretching vibrations depended if the atoms vibrate into the same direction. Deformation (δ): the bonding angle changes periodically and the bond distance remains almost constant (e.g. bending, rocking, wagging, and twisting). The symmetric and antisymmetric vibrations are present. A summary of typical vibrational bands of biomolecules is presented in appendix A. The IR absorption spectrum can be obtained with gas-phase or with condensed-phase molecules. For most routine analytical application of infrared spectroscopy, spectra are 13

27 2. Chemical mapping From micro to nano obtained with condensed-phase samples. The vibrational transitions can be observed with molecules present as pure liquids, solutions and solid state Fourier transform infrared (FTIR) micro-spectroscopy In contrast to the dispersive spectrometer, where the radiation from an infrared source is made monochromatic, the Fourier transform spectrometer uses polychromatic radiation. The collimated radiation from the infrared light source is partially reflected and partially transmitted by a beam splitter (BM) (see Figure 2.3). The beam originally transmitted through the beam splitter is then reflected by a fixed mirror (FM) while the beam originally reflected from the beam splitter is reflected from a movable mirror (MM). Both mirrorreflected beams are recombined and split again at the beam splitter, where one of the newly split beams goes back to the source while the other one passes the sample and reaches the detector. The recombination of the mirror-reflected beams leads to a complex interference pattern with the highest intensity in the center burst and decreasing intensity to the sides of the center with increasing or decreasing distance (L+Δx) between the movable mirror and the beam splitter. The mirrors (fixed and movable) and the beam splitter form a Michelson interferometer [12]. The detector measures the intensity I(x) of the resulting radiation as a function of the distance (Δx). Practically, instead of measuring each wavelength at a time as in basic monochromatic spectroscopic methods, all wavelengths are measured at the same time in an interferogram. Figure 2.4 shows the FTIR microscope system Spectrum Spotlight 200 -Perker-Elmer that combines a spectrometer with a microscope. This system was used for some general chemical maps of copolymer mixed films (PS-PMMA) in this thesis. A calibrating beam of a helium-neon laser goes the same path as the infrared radiation [8]. Because of the monochromatic He-Ne laser light, its interferogram is a sine function. The distance between two zero crossing points in simply half the wavelength, λ, of the laser light which is used as an internal frequency calibration. The relation between the intensity I(x) at the mirror position x and the intensity of a monochromatic spectral line S(x) at the wavenumbers ν (= 1/λ) is given by eq 2.7. s (2.7) The result of the data acquisition is a discrete interferogram I(nΔx), where n is the counting number of the distance difference Δx between two zero crossing points of the He-Ne laser light. 14

28 2. Chemical mapping From micro to nano Figure 2.4: FTIR micro-spectrometer set-up. After Michelson interferometer, the system has a movable mirror to allow the usage of the spectrometer or the microscope. This interferogram is Fourier transformed into the spectrum S(kΔν): equation 2.8a with k used as a counting number of the frequency increment Δν. S n (2.8a) where N is the total number of digitized points. S(kΔν) represents the so-called single channel spectrum. The ratio between two single channel spectra (S sample /S reference ) gives the transmission spectrum, which is transformed to the absorption spectrum by taking the negative logarithm (2.8b): FTIR spectroscopy has various advantages compared to the dispersive method: (2.8b) (a) the measurement time is very fast (b) the random noise is reduced by signal averaging (random noise is reduced by the square root of the number of the spectra averaged together) 15

29 2. Chemical mapping From micro to nano In order to avoid instrumental characteristics, such as influences from the emission spectrum, the radiation source from the components guiding the beam, called reference or background spectrum, has to be recorded. The increased signal-to noise ratio (SNR) of FT instruments allows infrared spectroscopy to probe very small amount of samples such as organic thin films. Depending on the sample properties, transmission and reflection mode can be used. A configuration commonly used to analyze IR spectra of thin organic films on gold (or any reflective) substrate is grazing angle reflection geometry. A piece of metal with a thin film on the top is positioned so that the incoming light is reflected under a large angle of incidence. As the light passes through the thin film, a small amount is absorbed by the film via vibrational excitation. This technique is known as FT- infrared reflection absorption spectroscopy (IRRAS). When the studied molecules (as powder) are mixed in KBr pellets, transmission spectra can be recorded (KBr is transparent in IR). Infrared microscopy has developed into a powerful analytical tool. Typically, a conventional microscope working at IR wavelengths is combined with a FT-IR spectrometer to acquire a spectrum at each pixel of the image. FT-IR microscopy is considered as an indispensable tool in most major vibrational spectroscopy laboratories, It is a practical technique for chemical analysis, often having a good chemical sensitivity. It provides spatially resolved chemical contrast for a vast range of samples such as: polymeric materials, biomedical tissues, pharmaceutical drugs, etc. It is non-intrusive technique and requires just a small amount of the sample. Practically, chemical images from infrared microspectroscopy reveal what, how and where various chemical species occur in nature or in manufactured materials. The optical aberrations are minimized by utilizing aspherical surfaces in a Cassegrain-type configuration [13]. The typical sample to be studied by FTIR microspectrometer will have a cross-sectional area of µm 2 [10]. Therefore, the area of the focal plane should be approximately 100 µm 2, at maximum. The detector, usually a mercury-cadmium telluride (MCT) will then respond to all of the transmitted radiation through this area. The Cassegrain condenser is used to refer to the lens prior to the sample. Since FTIR is a single-beam spectrometric method, always a background spectrum must be obtained. The spectrum of the sample is then divided by the background. Therefore, it is important that the aperture size to be the same for both: background and sample. The use of two apertures serves to mask the sample so that the most radiation reaching the detector has passed through the sample. The size of the aperture influences the spatial information of the chemical map that can be obtained. Another important parameter is signal-to noise ratio (SNR). This is dependent on the aperture and a good compromise between signal and spatial information must be made. A simplified diagram of an FTIR microscope in transmission mode is displays in Figure

30 2. Chemical mapping From micro to nano Figure 2.5: Optical diagram of infrared microscope used for infrared microspectroscopy. The optical component focuses the beam on the detector in transmission mode In order to display the spatial resolution achieved in practical measurement using FTIR microscopy and also the sensitivity of IR spectroscopy, a test sample made of mixed polystyrene (PS) polymethyl-methacrylate (PMMA) was investigated in transmission mode. The mixed polymer solution was a 50:50 wt % ratio using chloroform as solvent for both polymers. The mixed polymer film was spin coated on a cleaned Si wafer at very low speed (~ 100 rpm) for 30 seconds to create an uneven thick layer for a good signal-to noise ratio. After this, it was heated to 90 C in a drying oven for 30 min to remove any solvent residue. The sample was then imaged in transmission mode at scan size µm 2. The aperture of the objective was set at 15 µm. With these acquisition parameters, the spectral resolution is 4 cm -1. The pixel size is 15µm 2 (32 32 pixels). Figure 2.6 displays the FTIR transmission spectra for PS and PMMA (A) and an optical image of a mixture of PS-PMMA (B). The optical image in Figure 2.6 (B) shows a sharp separation between PS and PMMA polymer. 17

31 2. Chemical mapping From micro to nano Figure 2.6: (A) Single point FTIR spectra of PS (blue) and PMMA (red) (specified in visible image) in transmission mode, (B) visible image The FTIR image of the mixed polymer PS-PMMA based on the PS and PMMA specific absorption bands can be obtained. Single spectra of PS and PMMA were taken at spots marked (blue and red) in the visible image (2.6 B). The peaks of each spectrum are listed in Table 2.3. sample Band assignment (cm -1 ) Integration range (cm -1 ) PS-PMMA 1450 antisymetric bend (specific for both polymer) Mixed polymer 1600 ν(c=c)-aromatic ring (specific PS) ν(c=o) stretch (specific PMMA) Table 2.3: Integration range for the image evaluation of PS-PMMA polymer sample 18

32 2. Chemical mapping From micro to nano Two specific peaks for each polymer can be observed: for PMMA ν(c=o) due to the carbonyl group and ν(c=c) for PS due to the aromatic ring. A comparison between IR spectra of PS, PMMA and mixed PS-PMMA is displayed in Figure 2.7.(A). Most of the peaks can be assigned to either PMMA or PS vibrational modes. The IR chemical images of the test sample area were calculated by integration over the specific absorption band (see Table 2.2). For PMMA, the ν(c=o) vibration mode from COOCH 3 as specific peak and for PS ν(c=c) vibration mode from cis(ch=ch)-aromatic ring were used. Figure 2.7: FTIR spectra of PS, PMMA and mixed PS-PMMA polymer film with their specific absorption bands (A) and FTIR images of PS-specific absorption band (1600 cm -1 ) and PMMA-specific absorption band (1727 cm -1 ) (B). The white line is for eye-guide. 19

33 2. Chemical mapping From micro to nano The integral absorbance was converted into color code. The software used was Spectrum Spotlight. The white line is a guide for eye for following the profile of the polymers. In the visible image of mixture, PS-PMMA does not show a clear separation between PS- PMMA polymer films. However, this separation is clear distinguished in the IR images: Figure 2.7. (B) displays the evaluations of PMMA and PS at different specific absorption band. An IR chemical image (see Figure 2.7. (B) left) was calculated by integration of one common specific absorption band for both polymers: at δ(ch 2 ) from (CH 2 ) n functional group (1450 cm -1 ). In this image, the polymeric components of the mixed polymer film are impossible to be distinguished. For chemical identification, IR chemical images were calculated by integration of the PMMA, respectively PS, specific absorbtion bands ( cm -1 for PMMA and cm -1 for PS). A single polymer is clear distinguished in Figure 2.7 (B): PS after integration of the region cm -1 (ν(c=c)-aromatic ring) and PMMA after integration of the region cm -1 (ν(c=o) stretch. In conclusion, a clear distinction between PS and PMMA is observed. However, the spatial resolution for these measurements is limited at 20 µm. A better spatial resolution in FTIR microscopy can be obtain by using attenuated total reflection (ATR)-FTIR microscopy. More specific peaks for PS and PMMA are discussed in Chapter 7 and 8. Fourier Transform infrared microspectroscopy has demonstrated potential for rapid, high resolution, non-destructive, high chemical selectivity method in the identification and localization of mixed polymers. However, the spatial resolution is limited to a few µm (20 µm) due to the diffraction limit and the instrument limitation. Thus, chemical mapping at nanometer scale of micro-phase separation of copolymer blocks is not achievable. Raman micro-spectroscopy is an attractive tool which can overcome the resolution limitation of IR microspectroscopy because the practical diffraction limit is on the order of the excitation wavelength, which is about 10-fold smaller for Raman spectroscopy in visible light than for FTIR (in mid-ir range) spectroscopy Raman microspectroscopy Raman spectroscopy is based on inelastically scattered light, which occurs from optical modes of molecules. In 1922, Brillouin predicted the scattering of light by long wavelength elastic sound waves [14] and one year later, Smekal developed the theory of light scattering by a system with two quantized energy levels. In 1928, Raman observed experimentally the effect by using sunlight, a narrow band photographic filter to create monochromatic light and a "crossed" filter to block this monochromatic light. He found that light of changed frequency passed through the "crossed" filter [15]. The invention of 20

34 2. Chemical mapping From micro to nano the laser in 1960 had opened the way for studies of small scattering cross section reactions. Raman spectroscopy is today a well-established technique commonly used in chemistry, since vibrational information is specific to the chemical bonds and symmetry of molecules. It therefore provides a fingerprint by which the molecule can be identified. Raman microspectroscopy offers a number of potential advantages for chemical imaging of polymer science and pharmaceutical products, especially for biomedical application [16-17]. Raman scattering originates from a change in the polarizability of molecules or the susceptibility of crystals by the excited molecules. The optical phonons are the most often investigated species. In contrast to absorption spectroscopy, it is the modulation of the response by the vibrations, which is important rather than the contribution of the vibronic oscillators themselves. Light is treated as an electromagnetic wave and the molecules are modeled as small spheres connected by a spring (see Figure 2.2) The incident light can be described by the equation 2.9: s (2.9.) For an applied field E (x,t), the polarizability α of the orbitals shown leads to a induced dipole moment 2.10: s (2.10) If the molecule is vibrating with frequency ω, the distance between two atoms changes periodically and the polarizability will be modulated and depends on the conformation of the molecule; it changes as the molecule vibrates : α = α(q), where Q is the vibrational coordinate, Q = Q 0 cos(ω M t). The polarizability is a tensor and can be Taylor expanded: (2.11) By substituting the polarizability into equation 2.10, the induced dipole can be calculated as 2.12: s s s (2.12) From equation 2.12, it can be seen that, the three terms represent three different frequencies of the emitted light. The first term shows the frequency of the emission light is the same with incident light, named Rayleigh scattering. For the second and the third term, there is a shift of frequency of the emitted light from the incident light, which is named 21

35 2. Chemical mapping From micro to nano Stokes scattering, and anti Stokes scattering respectively (see Figure 2.8). When the energy of the incident light is not large enough to excite the molecules from the ground state to the lowest electronic state, the molecule will be excited to a virtual state between the two states. The electrons cannot stay for long into the virtual state and they will immediately go back to the ground state. If the electron goes to where it is originated from, the wavelength of the scattered light is the same as the light source, which is Rayleigh scattering. Depending on whether the molecule is absorbed or emitted, the energy of the scattered light is lower or higher than the energy of the incident light, which is called Stokes and anti-stokes scattering. Figure 2.8: Energy level diagram showing the states involved in Raman signal. A phonon can only contribute to a Raman process if it induces a change in the polarizability. This is not necessarily the case for any vibration but rather depends on the mechanical deformation induced in the molecule. In the IR absorption spectroscopy, the photons are absorbed and not just scattered like in Raman spectroscopy. The transition between different vibrations levels are IR active when the dipole moment of the molecule is modified:. (2.13) A comparison between Raman scattering and IR absorption for symmetric and asymmetric diatomic and symmetric triatomic molecules to a change in polarizability is displayed in 22

36 2. Chemical mapping From micro to nano Table 2.4. The geometric deformations that lead to a change of the dipole moment PD of the molecule are indicated. molecule vibration Raman active yes yes yes no no Infrared active no yes no yes yes Table 2.4: Selection rules for Raman and IR activity of vibrations. Owing to the different selection rules for Raman and IR spectroscopy, some functional groups are more sensitive to IR (e.g. high content of polar C=O, C-O, O-H and C-H groups such as in lipids, carbohydrates and proteins) while other groups such as carbon-carbon double and triple bonds in aromatic ring are more sensitive in Raman scattering. The ν(c=o) vibration band is more sensitive in IR absorption and less sensitive in Raman (more peak assignment for IR and Raman are presented in Table A1). Also Raman scattering is more sensitive than fluorescence and is often called the fingerprint of the molecule. IR absorption and Raman scattering are complementary techniques; IR requires dipole moment (P D ) changes as the molecule vibrates, while Raman requires the change of polarizability (α). In the past, studies have tended to use either Raman or FTIR spectroscopy alone to probe a sample. Recent studies [18-19] have exploited the complementary nature of these techniques, especially for polymers and biological sample, and integrated confocal laser Raman and FTIR microscope systems are now commercially available [20]. A nice review about IR and Raman spectroscopic imaging can be found in [21]. In this thesis, IR absorption and Raman scattering are used as a complementary spectroscopy techniques; some difficulties regarding the limited spatial resolution required for nano-phase separation of copolymer blocks and copolymer brushes will be discussed in Chapter 7, (see Section 7.3.2). Another important factor is that the cross section for Raman spectroscopy is much smaller then IR (~ ) or fluorescence (~10-16 ), only ~10-26 cm 2 per molecule. This is the major disadvantage of Raman and it is also the reason why Raman was not used on a very 23

37 2. Chemical mapping From micro to nano large scale until the surface enhanced Raman (SERS) effect was discovered [19]. Further developments of evolving technologies such as coherent anti-stokes Raman scattering (CARS) microscopy, imaging hyper-raman and tip-enhanced Raman spectroscopy (TERS) will have a strong impact on chemical mapping of polymers, biological tissues, proteins and DNA in the future. TERS combines the high spatial resolution of an atomic force microscope with the chemical information provided by Raman spectroscopy and its scanning optical near-field microscopy technique enables significantly enhanced optical sensitivity and spatial resolution (~10 nm) [23]. In the next chapter, a detailed description of theory of scanning optical near-field microscopy techniques is presented. 24

38 3. Theory of scanning near-field optical microscopy (SNOM) 3.1. Theory of scanning probe microscopy (SPM) Atomic Force Microscopy (AFM) Beyond the diffraction limit Apertureless or scattering SNOM (s-snom) Theory of s-snom Higher harmonic detection and background suppression Applications of SNOM

39 3. Theory of SNOM Methods for performing in-situ and nondestructive chemical analysis with nanometer spatial resolution are in a great request due to rapid developments in nanoscience and nanotechnology. These techniques are required to answer out-standing questions in surface chemistry, material science and biology. The requirements for methods that provide full spectroscopic information for each nanoscale pixel of the sample are clearly necessitating simultaneously a good spatial resolution, label free chemical identification and good sensitivity. These effectively rule out the established chemical analysis such as IR microspectroscopy, Raman microscopy as it was described in Chapter 2. In this chapter, scanning near-field optical microscopy will be outlined as technique to provide a solution toproblems in material science and biology regarding spatial resolution at nanometer scale and chemical information The theory of scanning probe microscopy (SPM) In the early 1980's, scanning probe microscopy dazzled the world with the first real space images of the surface of silicon at atomic resolution [24]. The history of scanning probe microscopy started with the scanning tunneling microscope (STM) invented by Gerd Binnig and Heinrich Rohrer, who shared the Nobel Prize for their invention in In the same year, a new microscope was developed: the atomic force microscope (AFM) which was created in Quate s laboratory at Stanford thanks to the work of Binning, Quate, and Gerber [25]. The two above microscopes are considered the archetypes of a large variety of scanning probe microscopes (SPMs). In fact, SPMs are used now in many different disciplines. They are especially crucial for nanotechnology: surface science, routine surface roughness analysis, and true three-dimensional imaging from surface atoms to micron-sized features on the surface of a living cell, measurement of magnetic, electric, and chemical forces, nanostructured design-dip pen lithography Atomic Force Microscopy (AFM) Scanning probe microscopy (SPMs) studies surface properties via the physical interaction between the sample surface and the probe tip. The main difference between these families of microscopy techniques resides in the probe. For instance, in STM a conductive subnanometric tip is maintained at a constant voltage in respect to the sample surface that, as said before, must be conducting or semiconducting. When the tip is kept at a small distance (often less than one nanometer) from the surface, electrons can flow due to the tunnel effect. This flow is measured point by point during the scanning so that electronic surface properties can be reconstructed and information about the morphology can be gathered. For second type of SPMs, AFM instruments, on the contrary, the probe is used in 26

40 3. Theory of SNOM contact or quasi contact with the sample surface. In this way, it is possible to collect information on the repulsive and attractive forces between the specimen surface and the atoms of the probe. More generally, the tip-sample interaction allows for rebuilding the distance between the two elements and the topography or a measure of the height of every single point analyzed during scanning can be acquired. There are three main regions of forces resultant depending on tip-sample interaction (see Figure 3.1): contact region, semi contact region and noncontact region that can give three types of AFM operation modes: contact mode, intermittent mode (tapping mode) and noncontact mode. Figure 3.1: Force as function of tip sample interaction. Qualitative shape of the Lennard Jones potential. Field fluctuations are universal, which makes Van der Waals forces ever-present, independent of the chemical composition of the surface or the medium. Forces that are measured in AFM include mechanical contact forces (e.g. van der Waals forces, capillary forces, chemical bonding, and electrostatic forces). However, different intermolecular surface and macroscopic effects give rise to interactions with distinctive distance dependence. In the absence of external fields, the dominant forces are Van der Waals interactions, short-range repulsive interactions, and adhesion and capillary forces. 27

41 3. Theory of SNOM An AFM instrument is essentially composed of three main parts (see Figure 3.2): A piezoelectric scanner capable of positioning the sample with nanometer precision in x, y and z directions. A probe which consists of very sharp tip (that could end with only few atoms) An electronic system consisting of a computer that records signals, sets up the parameters for the experiment and a controller that is able to detect signals from the microscope, allows for the movement of the sample and realizes the feedback. Traditionally, the sample is mounted on a piezoelectric scanner that can move the object under examination in the z direction for maintaining a constant force, and in the x and y directions for scanning the sample. Surface images are obtained by mechanically moving the probe in a raster scan over the sample (or analogous, by moving the sample in a raster scan under the tip), line by line, and recording the probe surface interaction as a function of position. Figure 3.2: Schematic AFM setup. The tip of AFM instruments is generally micro integrated on an elastic lever called cantilever. The cantilever can have different shapes, sizes and elastic constants depending on the operated mode, the sample or the purposes of measurements. A cantilever is typically made of silicon or silicon nitride with a tip radius of curvature on the 28

42 3. Theory of SNOM order of a few nanometers. Special measurement techniques requests special type of cantilevers: coated with different metal coating (Pt, Au), different shapes (triangular or rectangular) or special nanoantennas. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a relatively small deflection of the cantilever ruled by Hooke's law. In static contact mode (or simply contact mode) in the repulsive regime, the tip at the end of the cantilever is brought very close to the surface and maintained over the sample that has to be analyzed. Practically, in contact mode, the tip is continuously in close contact with sample and feels the surface topography of the sample. Prevailing forces in this mode are the core repulsive forces among nearest atoms of the tip sample heterogeneous system, and the quantum repulsive forces due to the Pauli Exclusion Principle. In contact mode, the cantilever deflection is revealed and compared to a value preset by the operator. The cantilever with a sharp tip is raster scanned across the sample to monitor the surface topography. If the measured deflection differs from the deflection set point, the feedback provides a correction for the voltage applied to the piezo scanner in order to constraint the cantilever at the desired deflection and the corrections are used to get topographical information. A laser is focused on the back of the cantilever and deflected onto a foursegment photodiode. The top-down signal difference gives information about the normal force and left-right deflection provides information about lateral forces. The force on the tip due to its interaction with the sample is sensed by detecting the deflection of the compliant lever with a known spring constant. Other methods for detecting cantilever deflection can be: a tunneling current similar to that used in the STM in the pioneering work of Binnig et al [24] by capacitance [26], piezoresistive and piezoelectric detection and by optical techniques, namely by optical interferometry [27]. As the surface is scanned, the tip follows the surface topography and moves according to the surface profile. For this reason, this mode can damage and/or deform soft samples (e.g. polymers, biological sample). For hard samples, this is a good mode that can provide a scan of the surface at atomic level. In the last decade, AFM experienced a significant evolution when a vibrating probe was used to explore the surface topography. Since then, dynamic AFM methods emerged as powerful and versatile techniques for atomic and nanoscale characterization and manipulation of a wide variety of surfaces. High-resolution images of polymers, proteins and DNA have been obtained in air and liquids [28]. The interest and the sophistication of dynamic modes starts from different factors: i) the possibility of imaging soft samples while avoiding their damage; ii) the existence of several parameters sensitive to the tipsample interaction (amplitude, frequency, phase shift, and cantilever deflection); iii) the potential to develop quantitative methods to characterize material properties at nanometric scale. The amplitude, the resonant frequency, and the phase shift of the oscillation links the vibrating tip dynamics to the tip-sample interaction. Any of them could be used as a 29

43 3. Theory of SNOM feedback parameter to track the topography of a surface. Depending on the regime of forces involved in the interaction between the probe and the surface, dynamic modes can be divided in two major classes: Tapping mode (intermittent mode or dynamic mode) Non-contact mode The tapping mode keeps the cantilever at large oscillation amplitude that can feels both attractive and repulsive forces (see Figure 3.1). For instance, if the cantilever is kept oscillating, according to a sine equation of the type A = A 0 cosωt, the size of the oscillation amplitude is A 0» r 0 (where r 0 is the distance at the minimum energy in the Lennard Jones potential). In dynamic non-contact mode, the condition to be satisfied is that A 0 «r 0. Acoustic and magnetic excitation modes have been developed to excite the cantilever-tip ensemble. In the acoustic excitation mode, a piezoelectric actuator holding the cantilevertip ensemble is attached below the substrate [29]. Applying of an oscillating voltage to the actuator produces its vibration and this in turn gives rise to the oscillation of the cantilever. This excitation mode is widely used in air and in liquids. Other researchers have directly excited the tip by applying an oscillating magnetic field to a magnetized cantilever [30]. A stiff cantilever is oscillating at resonance frequency in attractive force regime. When the tip is close to the surface, the free oscillation is damped. By maintaining constant oscillation amplitude, a constant tip-sample interaction is achieved. Since the tip contacts the surface only shortly during the scanning frictional forces exert lateral forces onto the sample are eliminated. Compared with constant mode, tapping mode is much slower but allows high resolution images of the surface topography of samples that can be easily damaged, like soft matter samples (e.g. polymers, biological samples). Phase imaging is a powerful extension of atomic force microscopy in tapping mode. It is a special technique capable of revealing particular structures often not revealed by topographic images. This method of analysis can be used only in dynamic mode. By mapping the phase of the cantilever oscillation (to the driving oscillation) during tapping mode scans, phase imaging allows to overcome simple morphological mapping and hence detect variations in composition, adhesion and viscoelasticity. Applications include identification of contaminants, mapping of different components in composite materials, and differentiating regions of high and low surface adhesion or hardness. Phase imaging is a relatively new technique; it has the advantage of being performed at the same time of topographic imaging in tapping mode (both topographic and phase images can be obtained in a single scan). By taking into account that the interactions between the tip and the surface depend not only on the sample topography but also on other surface properties, the 30

44 3. Theory of SNOM response of the tip cantilever system to the oscillating driving force depends on these properties. In phase imaging, the phase of the sinusoidal oscillation of the cantilever is measured relative to the driving signal applied to the cantilever (see Figure 3.3) Figure 3.3: Schematic representation of phase imaging operation mode: the electronics measure the phase lag between the cantilever oscillation (solid line) and the driver piezo (dashed line). Practically, phase imaging can detect different polymer components related to their stiffness or areas of different hydrophobicity/hydrophilicity properties of the sample. Before the invention of tapping mode AFM, high-resolution imaging of polymers by scanning probe microscopy was restricted to a few polymer surfaces. Normal and lateral tip surface forces often produced dramatic modifications of the polymer surfaces. The dynamic mode did not just open the potential of scanning probe methods to investigate the nanostructure morphology of polymers, it also provided the phase imaging mode, especially suited to study polymer surfaces. Figure 3.4 displays the topography and phase imaging of mixed copolymer brushes (PS-PMMA) on silicon substrate. 31

45 3. Theory of SNOM Figure 3.4: Topography (left) and phase (right) images of mixed copolymer brushes (PS- PMMA). Phase separation is especially clear in the phase map: brighter and darker areas represent different phases of the polymer. The topography image (in the left panel) provides small details of the heterogeneous nature of the sample, whereas the right image clearly maps the two phases of the material resolving features as small as 50 nm. Therefore, this method of investigation can be used to collect information on the separation of the components constituting the surface of block copolymers or copolymer brushes. Phase imaging mode will be exploited in this thesis in the mapping of micro-phase separation of self-assembly block copolymer (see Chapter 7) and switched micro-phase separation domains for different mixed polymer brushes ( see Chapter 8). The success of STM and AFM applications led to the development of a number of related SPM techniques, which are used for mapping surface magnetic, electric, thermal, and optical properties. These methods are also known as magnetic force microscopy (MFM), electric force microscopy (EFM), scanning capacitance microscopy (SCM), scanning thermal microscopy (SThM), and scanning near-field optical microscopy (SNOM). Although AFM allows to characterize nano-mechanical properties of the sample with resolution down to the atomic scale, a disadvantage still remains: the lack of chemical specificity, essential for chemical mapping and for good understanding of the smart properties of the samples. Although Lieber et al. in 1994 developed the chemical force microscope (CFM) an AFM technique based on chemically functionalized tips, a real chemical map at nanometer resolution was not possible due to the specificity of this technique [31]. In the following, the scanning near-field infrared microscopy (s-snim) is an efficient analytical method that provides spectroscopic characterization of surfaces with nanoscale resolution and can create a real chemical map at nanometer scale. 32

46 3. Theory of SNOM 3.2. Beyond the diffraction limit In order to break the optical diffraction limit (see chapter 2 for more details), the most straightforward idea is to create a point light source which is scanned over the sample surface in the optical near-field (close proximity) to perform optical mapping. The first concept to break diffraction limit resolution was proposed in 1928 by Synge [32]. In the Synge s original concept, the local probe consisted of a tiny aperture in a perfectly reflecting metal screen in the range of the optical near-field. Near-field optical microscopy relies on a confined photon flux between a local probe and the sample surface [33]. The probe is raster-scanned over the sample and a remote detector acquires the optical response. In this way an optical contrast image is recorded. Figure 3.5 displays a comparison of diffraction-limited optical microscopy and near-field optical microscopy. Figure 3.5: Schematic representation of: (A) the Abbe diffraction limit minimum detectable separation of two light scattered and (B) the aperture scanning near-field microscopy. In near-field microscopy, the resolution Δx does not depend on λ but on a characteristic length d (e.g., aperture diameter) of a local probe. To first approximation, resolution is defined by the aperture size and not by the wavelength of the exciting radiation. The spatial resolution is in this case not restricted by the diffraction limit but is determined by the size of the light source. Immediately behind the irradiated screen, the light field is spatially confined to the size of the aperture (d). Due to the difficulties in fabrication, as well as the precise positioning and scanning with such a light source, the technique of scanning nearfield optical microscopy (SNOM) was realized in 1980 s, following the invention of scanning tunneling microscopy and scanning probe microscopy [34]. A metal-coated fiber tip was used to create a nanoscale optical aperture. The first evidence of a near-field signal was given by Ash and Nicholls in the microwave range (λ=3cm) [35]. The aperture size 33

47 3. Theory of SNOM and hence the light source may be several times smaller than the diffraction limit using visible light [14]. This type of SNOM is referred as aperture-snom (a-snom). In the last two decades, a-snom achieved success by imaging the fluorescence of single molecules at room temperature [36] and mapping of luminescence from a quantum well with unpredicted spatial resolution [37]. However, a-snom has two fundamental difficulties: the power emitted from the aperture is usually on the order of nanowatts- too weak for collecting a spectrum with short acquisition time the spatial resolution is limited to 50 nm in practice (in visible light range) but no imaging in the infrared range is difficult because of the absence of suitable IR light fibers (chemical information). In order to overcome these problems, different alternatives have been developed in the last decade namely tip-enhanced Raman spectroscopy (TERS) and scattering SNOM (known as apertureless SNOM) which is mainly used in the IR spectral range. Laser-irradiated pointed probes were introduced to overcome the limitation of aperture probes and to improve the resolution in near-field optical microscopy [39-40]. Figure 3.6 displays different SNOM methods: aperture SNOM, tip-enhanced Raman spectroscopy and apertureless or scanning-snom. Figure 3.6: Different SNOM methods: (A) aperture SNOM (a-snom), (B) tip-enhanced Raman spectroscopy (TERS) and (C) scanning SNOM (s-snom). Raman spectroscopy, which is a complementary method to IR absorption spectroscopy, is known as a weak process. In order to collect a spectrum with a reasonable signal-to-noise ratio and speed, signal enhancement techniques are needed, and this is possible by using TERS. The key of the TERS technique is a hot tip that can highly confine and enhance the electric field at the tip apex. There are two types of field enhancement: the localscattering approach [37] and the local-excitation approach [38]. In the local scattering approach, the tip, a pointed probe, is used to locally perturb the fields at the sample; the 34

48 3. Theory of SNOM response to this perturbation is detected in the far-field at the same frequency of the incident light. The field near any laser-irradiation object results in waves that propagate away from the sample (propagating waves) and also of waves that remain attached to the sample s surface (evanescent waves). The inability of evanescent waves to propagate constitutes the diffraction limit. Instead of using the pointed probe as a local scatterer, one can use it as a local light source. This is accomplished under the proper polarization and excitation conditions through the use of suitable probe materials and probe geometry and it was demonstrated by the Novotny s group [41]. As shown in Figure 3.7, the tip is held a few nanometers above the sample surface so that a highly localized interaction between the enhanced field and the sample is achieved. Figure 3.7: Principle of tip-enhanced near-field optical microscopy: (A) schematic representation and (B) the lightning rod effect. When the polarization of the incident radiation is parallel to the axis, the electric field will be highly enhanced at the tip apex. Simulations and subsequent experimental results demonstrated that the electric field could be laterally confined into a small area while the light intensity could be enhanced by several orders of magnitude [42-43]. The TERS strategy is not suitable for IR absorption spectroscopy (IRAS). This is because a broadband light source is used in IRAS and it is difficult to create a constant enhancement over the entire spectral range. For this reason, TERS is known as a scattering SNOM technique in the optical regime and s-ir SNOM (scattering infrared SNOM or SNIM) is commonly used for near-field IRAS (see Figure 3.6.). 35

49 3. Theory of SNOM 3.3. Apertureless or scattering SNOM (s-snom) The scattering SNOM has some certain advantages in comparison to the aperture SNOM. The attainable optical resolution exceeds that of the aperture-based microscopes for a suitable incident field polarization, field enhancement occurs in a localized area of the size of the tip radius [42]. Second advantage is that the problem of the strong attenuation in the metal coated taper fiber is alleviated in the apertureless SNOM since the light is focused from the far field directly on the tip without any loss. This makes s-snom a powerful technique in micro-spectroscopy with applications in the mid-ir and far-ir ranges, where the losses within the fiber are much larger. The third advantage is the tip fabrication step, relying on the use of commercial AFM tips. The least but not last advantage is that the range of usable wavelengths is not limited (like in aperture SNOM). In Figure 3.8, the principle of s-snom is illustrated. The main disadvantage of s-snom comes from strong scattered light in areas of the tip shaft, far from the sample, which usually creates a strong background for the detector. Furthermore, the background usually varies during scanning because of the z-motion of the tip in constant force scanning mode, which might yield topographic artifacts. However, several strategies have been found to eliminate this problem from s-snom images. Some of these will be presented in the next section (3.3.2.) and more details about the strategy used in this thesis will be described. Figure 3.8: Principle of s-snom 36

50 3. Theory of SNOM The tip-sample system is illuminated from the side [50-51], the near-field signal (i.e. the interaction between the tip and the sample) is modulated by the oscillation of the tip and the harmonic signal components in the scattered light that correspond to the near-field information are recorded to form a chemical image. The utilization of the AFM tip as an apertureless probe was proposed by Wessel [44], Boccara[45] and Wickramasinghe [46]. The experimental proof of the s-snom was realized by Zenhausern at al [47] in 1995, with a resolution of 1 nm. Usually, most abundant probe materials are platinum (Pt) and gold (Au) due to their high polarizability [44, 48-49]. The dielectric probes (Si 3 N 4 or Si) can provide a sufficient scattering signal [45, 46]. The tip should be close to the surface sample. Therefore, the AFM in tapping mode is the optimal configuration for s-snom Theory of s-snom In near-field optical microscopy, pointed metal probes are used in localizing and enhancing optical radiation. In principle, these probes fulfill the role of standard optical lenses used in imaging. However, in this context they do not work as linear elements and are no longer limited by the laws of diffraction. The theory of scattering SNOM can start from a classical description of the total dipole moment for a molecule [52-54]: (3.1a) (3.1b) where p is the dipole moment, p p is the permanent dipole moment (if exists), α is the tensor of polarizability with components α ij, E is the electric field with the amplitude E 0 of the incident light at frequency ν which induces this dipole, ω is the angular frequency and t is the time. The near and far-field terms describe regions with different physical properties around any electromagnetic radiation source. For large distances (r c/ω = λ/2π ) the 1/r term (where r is the distance with respect to the dipole center, from electric field equation) dominates the electric field and the radial component is canceled out [41]. Those waves propagate only perpendicular to the dipole moment because their intensity is higher when θ = π/2 and canceled out when θ = 0, π. This zone, where the detectable intensity decreases with 1/r 2 is called far-field. For small distances (r < c/ω = λ/2π) the 1/r 3 term becomes dominant and the intensity drops rapidly with 1/r 6. Because the electric field components do not reach the far-field, they are refereed as non-propagating waves. The zone around the dipole in which 37

51 3. Theory of SNOM these waves decay is called near-field. Within these zones, even components parallel to the dipole moment exist so that the near-field decays rapidly around the dipole. In contrast to the far-field, the near-field contains information about the spatial charge distribution of the dipole. If the oscillating electric point dipoles (atoms, molecule, etc) are in near-field contact, the interaction energy is proportional to 1/r 6. If the dipoles are located in far-field zone, the interaction energy is proportional with 1/r 2. In scattering near-field microscopy, only scattered light from the very apex tip contains near-field information. Pointed metal probes used in near-field optics are referred to as optical antennas. In a simplified model, the tip apex can be approximated as a polarizable sphere. The concept of near-field microscopy was adapted by the Keilmann et al.[50]. The contrast between two materials in the dipole theory is generated by a difference in the polarizability of two materials. A surface can generate a mirror dipole of the tip. In Figure 3.9, dipole-mirror dipole interaction is displayed. In order to describe better the optical interaction of a probe with a planar sample like polymer films, the dipole-mirror dipole theory can be employed (see equation 3.5). In this thesis, the dipole -mirror dipole theory is applicable for the investigated polymer samples and it will be explained in detail in the following sections. Figure 3.9: Dipole-mirror dipole interaction. The tip apex is approximated as polarizable sphere (p). When exposed to an electric field (E 0 ) a mirror dipole (p ) in a close surface is induced by the plane-sample. a- tip curvature radius, z- the distance tip-sample and r = a+z distance from dipole center to sample surface 38

52 3. Theory of SNOM When exposed to an electric field (E 0 ) a mirror dipole (p ) in a close surface is induced by the sample plane. As a consequence of the near-field interaction between the scattered light from the apex and the surface, the signal is modified and contains information on the local dielectric properties of the sample. When applying an electric field E 0, the sphere becomes polarized with the dipole moment p given by equation 3.1.a. The near-field signal is the emitted electric field E of the tip - mirror dipole system under the exciting electric field of the laser (E o ). The mirror dipole theory is able to explain the near-field contrast of a plane surface, with its complex index of refraction or dielectric constant. The complex refractive index is defined by equation 3.2: n n (3.2) with the real part, the refractive index n and with the imaginary part, the absorption. The dielectric constant is defined by equation 3.3: (3.3) According to the Maxwell equations [52], the refractive index and the dielectric constant are related to each other by equation 3.4: n (3.4a) where μ is the permeability, for μ = 1, (3.4b) n n n n The refractive index can be measured by IR ellipsometry or by dispersive Fouriertransform spectroscopy [55] but more often, the derivation of the refractive index by the Kramers-Kronig relations is used as a good approximation [56-57]. In the practical experiment, the polymers vary substantially in the vibrational infrared region as a result of absorption resonance, giving rise to dispersive features in the refractive index via Kramer- Kronig relations. This behavior was observed by Taubner et al.[58] and was observed in all near-filed measurements presented in this thesis. 39

53 3. Theory of SNOM In order to calculate the scattering and absorption of the incident electric field (E 0 ), the tipsample system is described by the effective polarizability. Considering the tip as a point dipole with a curvature radius (from the simplest approximation), the polarizability is given by equation 3.5: (3.5) Currently, in practice, all s-ir SNOM measurements are performed in air (including the measurements presented here) and value is 1 for air. For samples studied in liquids, the dielectric constant of water or other solvents has to be taken into account. For highly conductive materials like Au or Pt (usually materials used probe in near-field microscopy), the polarizability approaches. The dielectric constant of Au (coated tip used in this thesis) is: = i and the refractive index is: n = i [57], hence the polarizability of the gold coated tip is: α tip = i for a = 30 nm, curvature of the tip. If the dielectric constant of the tip approaches - (in air, the real and the imaginary part of approach -2 and 0, respectively), the so-called Fröhlich resonance occurs demonstrated experimentally by Hillenbrand et al.[57] for a gold nanoparticle on a SiC substrate. The polarizability α eff of the two-dipole system is the critical variable of the theory. It relates the scattered electric field to the incident field by equation 3.6: (3.6) If the electric field ( ) has a longitudinal polarization in reference to the dipoles, the expression of the induced electric field on the particle (1),, from the dipole moment of the particle (2) is given by equation 3.7a: (3.7a) The scattered electric field (E sca ) is given by equation 3.7b: s (3.7b) where σ is the scattering cross-section, E 0 is the incident electric field, s n is the near-field amplitude, ϕ n is the near-field phase, r is the Fresnel coefficient and the n th component of polarizability α. The scattering cross section is the sum of the total scattering crosssection and the absorption cross-section. The mirror-dipole model is employed for plane-samples (or surfaces) using the effective polarizability (see Figure 3.10). When the incident electric field is polarized parallel to the 40

54 3. Theory of SNOM surface, the electric field of the probe dipole induces an image dipole oscillating 180º out of phase with the tip dipole. Due to the opposite orientations of probe and mirror dipole, the resulting total dipole moment cancels out (see Figure 3.10.B). Figure 3.10: (A) The dipole-mirror dipole theory; the surface creates a dipole mirror which interacts with the tip dipole and (B) orientation of external electric field perpendicular (left B) to the sample (parallel to the tip) and parallel (right B) to the sample (perpendicular to the tip). Here, a is the tip curvature radius, z is the distance tip-sample and r = a+z is the distance from dipole center to sample surface When the incident electric field is polarized perpendicular to the surface, the electric field of the probe dipole generates a mirror dipole with the polarization (see Figure 3.10). In this thesis, the perpendicular polarization was used during s-snim measurements. Therefore, for simplicity, in the description of the near-field theory, only the perpendicular polarization will be considered. The mirror dipole is given by equation 3.8: (3.8) 41

55 3. Theory of SNOM where β is the surface response function containing the sample s complex dielectric function ( ) and it is constant. This constant can relate the polarizability of the sample to the polarizability of the tip. The probe dipole ( ) and the mirror dipole ( ) are pointing in the same direction resulting in an enhanced total dipole moment (see Figure 3.10.A). The total electric field acting at the apex of the tip is composed of the coherent sum of E 0 and the contribution of the dipoles, r distance from dipole center to sample surface (r = a+z). Therefore: (3.9) The effective polarizability of the coupled probe and mirror dipole system is described by the equation 3.10: (3.10) In near-field theory, the electric field of the light scattered from the apex tip is proportional to the effective polarizability ( ) and the detected intensity of the scattered light is proportional to the square of its electric field. In applications of this scattering theory to near-field microscopy,, the change scattered light due to the spatial variation of the optical properties of the sample thus provides the imaging contrast. The scattering and absorption cross-section σ is given by equations 3.11: n (3.11) 42

56 3. Theory of SNOM Higher harmonic detection and background suppression The area illuminated by the focused laser beam is much larger than the tip s apex (see chapter 5). Consequently, the light scattered from other elements (sample, tip shaft, and cantilever) sums up to a large background signal. As it was shown in the previous section, only the light scattered from the apex of the tip is influenced by near-field interactions with the sample surface. The tip of a AFM cantilever in tapping mode (mode used for near-field measurements) is in oscillation around the tip-distance (z0): z = z0 +Δzcos(ωt+υ). The local near-field scattering is discriminated from background scattering using lock-in detection at the tapping mode frequency and each non-modulated signal is suppressed since scattered signal is generated from the same driving field. This method was already demonstrated in the realization of near-field optical microscopy and it is based on a modulation of the tip-sample distance [59]. The signal detected is a sum of background (E b ) and near-field (E n ) components and an interface product: s where E* is the complex field conjugated to E and is the phase - is the pure near-field signal - is the background off-set s - the mix term Since the distance dependence of the near-field signal is strongly non-linear, a sinusoidal modulation at frequency ω r of the tip-sample distance generates higher harmonic component of the scattering signal. The lock-in technique used in the experimental set-up permits to measure, where (σ1, σ2 ) are measured values. These values are the second and the third term of the Fourier transform of σ. s n s s s s s s s (3.12) 43

57 3. Theory of SNOM To limit the noise, σ 2 was preferred. The background is suppressed by signal demodulation at nω, where ω is the angular resonance frequency of the cantilever. Figure 3.11 shows the typical behavior of the near-field amplitude for non-contact mode of the cantilever used in this thesis. The tip is approaching the surface of the sample and the amplitudes are measured on the fundamental and first harmonic of the cantilever vibration frequency. Figure 3.11: near-field amplitude on the fundamental (A) and on first harmonic (B). The red line represents the exponential fit. The tip is approaching the surface of the sample and the amplitudes are measured on the fundamental and first harmonic of the cantilever vibration frequency. The near-field intensity on the fundamental frequency (fig A) is dominated up to 50 nm by an exponential behavior, which is typical for evanescent wave. For z 70 nm, the intensity is dominated by a pseudo-sinusoidal signal coming from the propagating waves. On the first harmonic (see figure 3.11.B), the near-field exponential part of the signal is observed and the fluctuation level is much lower. The detector signal fluctuation dominates the detector signal for z 50 nm. For this reason, the first harmonic is useful to lower the background. In the same time, value of the scattered signal on the first harmonic is much lower than on the fundament due to non-linearity in the tip-sample interaction. The non-linearity is the main reason for contrast. The main point is that the non-linearity of the background signal is weaker than that of the near-field signal, so that at higher harmonic demodulation the background is much lower than the near-field contribution at this harmonic. Therefore, the laser beam has to be well focused on the tip to diminish the losses. Finally, it has to be noted that the dipole-mirror dipole theory is not to able to describe properly the near-field 44

58 3. Theory of SNOM interaction on multilayer samples. Depending on the thickness of the upper layer of a multilayer sample (e.g mixed polymer films with a clear micro-phase separation), the penetration depth of the tip s near-field is probing not only the first layer but also the underlying one. A multilayer theory for different thickness of a polymer film (using different references) was proposed by Aizpurua et. al.[60] Applications of SNOM Spectroscopic imaging with nanometer resolution is a very attractive method to discover chemical information of the studied samples. In biological applications, aperture-snom is used combined with fluorescence spectroscopy [61]. Under physiological conditions, aperture-snom is difficult since the probe-sample distance control is not well suited for operation in liquids as under ambient condition. However, individual florescent molecules on the membrane of the cell in solution were investigated [62]. An approach of a-snom was to characterize polymer thin films, to correlate their optical properties with the sample morphology and to study the micro-phase separation of polymer blends [63]. The set-up used by Dragnea et al is based on an illumination-mode SNOM with a tunable IR laser source and a collection of the light transmitted through the sample by an IR objective [64]. Samples used in the study were prepared by mask-assisted deep-uv pattering of poly(terbutylmethacrylate) thin films, in exposed and unexposed regions. Imaging was performed at 2.80 µm and 2.94 µm and the resulting chemical contrast due to O-H stretching was recorded. The lateral resolution was ~290 nm, which is below the diffraction limit. Similar studies were extended to measurements of water vapor [64]. However, as it was shown in this chapter, a-snom is not a suitable tool for vibrational spectroscopic imaging because of its transmission at infrared frequencies for fiber-based apertures, while apertureless (scattering) SNOM provides a good spatial resolution and sensitivity. Having full spectroscopic information available at every pixel is also crucial for obtaining simultaneous chemical information. Tip-enhanced Raman spectroscopy (TERS) can provide vibrational information in a full spectrum at nanometer scale. In the last years, plenty of different TERS experiments were reported, especially regarding biological research [66-67]. The imaging of single-walled carbon nanotubes (SWNTs) on glass was demonstrated by Novotny s group for the first time [65-66]. Imaging with material-specific spectral contrast using excitation at fixed IR in an s-snom (or s-snim) configuration can be achieved with excellent spatial resolution. Keilmann s group applied IR s-snom (s- SNIM) at fixed wavelengths to image polymer mixtures with excellent spatial resolution [58, 69-70]. A high contrast can be achieved between metal and high-refractive index dielectric composition [71] and a spatial resolution of ~ 10 nm has been reported even at IR wavelengths of λ = 10 µm, corresponding to a resolution of λ/1000. Different types of heterogeneous systems have been characterized by s-snim such as: ion-implanted 45

59 3. Theory of SNOM semiconductor surfaces [49, 56], polymers blends [58, 69-71], SAMs [73], lipids [74], single virus [75], DNA [76] and carbon-like nanoparticles [77], all with a resolution well below the diffraction limit. Moreover, a combination of s-snom with synchrotron radiation is a promising concept that can eliminate one of the disadvantages of this technique chemical imaging for a single wavelength. Recently, Hillenbrand group s demonstrated the first evidence of nanoscale resolved THz near-field microscopy by scanning a single transistor with a spatial resolution of 40 nm (λ/3000) at 2.54 THz (λ = 118 µm) [78]. 46

60 4. Smart surfaces From concept to reality 4.1. Materials Self-Assembly Self-assembled monolayers (SAMs) Microcontact printing (µcp) Block copolymer self-assembly Polymer brushes- an induced self assembly system Synthesis of polymer brushes Smart surfaces: switchable surface energy Theory of contact angle

61 4. Smart surfaces From concept to reality In the field of nanomaterials it is a challenge to discover new structures or to rebuild known materials into desired nanoscale building blocks. Tailoring materials with dynamic switchable responses to external fields is a major goal of modern materials science. Micropatterned surfaces are important for microelectronics, molecular printing technologies, microfluidic and microanalytical devices, information storage or biosensors. Topographical relief and physicochemical properties such as wetting, charge etc can be generated by some techniques including microcontact printing [79-81, 83] and photolithography [82]. However, once a pattern is created, it cannot be easily changed, except if the molecules that are used are stimuli sensitive. This can limit the usability of a patterned surface to a single specific application. Therefore, the scientists have been trying to create so called smart surfaces. These are systems responding to external stimuli and adapting themselves to the changing conditions. Smart surfaces adapt their physical or chemical behavior in response to small external changes in the environmental conditions, such as solvents [83-86], ph [87], temperature [88], magnetic and electric field [89], biological molecules [90] etc. One approach to create such structures is based on the selective deposition of stimuli-response materials forming self-assembled monolayers (SAMs) or polymer brushes. In this thesis, some of these surfaces will be reported. The following chapter presents an outline material of self-assembly and gives a brief introduction about self-assembly monolayers and polymer brushes Materials Self-Assembly Self-organization is a powerful route to the bottom-up fabrication of nanostructures [91]. The ability of soft materials to form a variety of nanoscale periodic patterns offers the potential to fabricate high-density arrays for use in data storage, electronics, molecular separation etc Self-assembly monolayers (SAMs) Self-assembled monolayers (SAMs) are monomolecular films of biological or chemical moieties attached to a surface. Due to their great variety of different substrate-head group combinations, the wide choice of terminal functional groups, molecular order and simplicity SAMs have gained increasing interest [92]. A self-assembled monolayer is formed when molecules from solution or the vapor phase adsorb onto a surface and spontaneously organize into a single molecular layer. SAMs can be formed by adsorption of a variety of functional organic molecules onto suitable solid substrates. The first selfassembled film with a thickness of only one molecule was detected by Langmuir in 1917: 48

62 4. Smart surfaces From concept to reality amphililes on water [93]. In 1935 Blodgett [94] was the first who transferred monomolecular films from air-water interface to a solid support. Despite the early work of Bigelow [95], who published the first preparation of a monomolecular layer in 1946, it was not exploited until 1983 when the interest in SAMs was rapidly expanded. Nuzzo and Allara showed that SAMs of alkanethiolates on gold could be prepared from dilute solutions of alkyldisulfides [96-97]. Since then, a variety of adsorbate-substrate combinations forming SAMs has been found and investigated. Many systems undergo self-assembly, including long-chain carboxylic acid on metal oxides, organosilane species on hydroxylated glass, silicon oxides (SiO x ) and aluminum oxides (Al x O y ) and organosulfur-based species on noble metal surfaces. An excellent review about self-assembly monolayers can be found at [92]. SAMs offer unique opportunities for controlling surface properties such as wettability, tribology, adhesion, and corrosion, chemical and electrochemical reactivity. SAM surfaces have opened up a new area to increase fundamental understanding of self-organization, structure-property relationships, and interfacial phenomena. Self-assembled monolayers are formed by simply immersing a substrate into a solution of the surface-active material. The driving force for the spontaneous formation of the 2D assembly includes chemical bond formation of molecules with the surface and intermolecular interactions. Generally, molecules forming closely packed ordered SAMs consist of three different parts: head or anchor group which interacts with a substrate (-SH on Au; -SiCl 3 on SiO 2 ) linker or spacer (alkyls groups in generally) terminal functional group or tail (-CH 3, -OH, -COOH, -NH 2 ). The simple formation processes makes SAMs inherently manufacturable and thus technologically attractive for building superlattices and for surface engineering. Figure 4.1 displays the schematics of SAMs formation. Organosulfur-based molecules, e.g. alkanethiols, dialkyl disulfides are known to coordinate very strongly to noble metals (Au, Ag, Cu, Pt). Nevertheless, most SAMs are carried out on gold substrates. In the case of alkanethiols, the mechanism of binding is considered as an oxidative addition of the S-H bond followed by reductive elimination of the hydrogen, thus resulting in the formation of a thiolate species. This leads to an Au-S covalent bond and a small amount of released hydrogen as can be observed in the following chemical equation: RSH + Au n 0 RS-Au + Au n /2H 2 49

63 4. Smart surfaces From concept to reality Figure 4.1: Schematic procedure of ordered molecular assemblies formed by the adsorption of an active surfactant on different solid substrates: gold and silicon oxide. Alkylsiloxanes, the other type of SAMs using silicon oxide or glass as substrate, are obtained by reaction of the hydroxylated surface with a solution of alkyltrichlorosilane (or alkyltriethoxysilane). The reactive silane groups first undergo a fast hydrolysis to form silanols, followed by slow condensation to oligomers which then hydrogen bond to the surface hydroxyl groups and lead to the formation of covalent bonds during the final curing process. The bonding arrangement is not well-defined but it is sufficient just one bond from each of the silicon atoms of the silane compound to react with the substrate surface. The two remaining silanols of the compound are present in either the free or condensed form (Fig. 4.2.) [102]. Alkylsiloxane SAMs are significantly more thermally stable than alkanethiolates on gold and do not require any other vapor deposition of a metal layer or other specific methods for annealing (template stripped gold [53]) to create a very smooth surface (the roughness can induce defected in the SAMs). In this thesis, both types of SAMs, alkanethiols and alkylsiloxanes, will be used. An essential requirement for high quality SAMs is a clean substrate. Gold and silicon oxide are chemically inert but organic compounds can cover the substrate under ambient condition. The most common cleaning procedure is using piranha solution, which is a strong oxidant and removes most organic compounds. Piranha solution typically consists of a 3:1 (v/v) mixture of H 2 SO 4 (96%) and H 2 O 2 (30%) and should be used immediately 50

64 4. Smart surfaces From concept to reality after preparation. Depending on the type of substrates, the cleaning time/incubation time differs: gold substrates not longer then 5-10 min, since the roughness of the gold increases upon prolonged exposure [100] and silicon or glass substrates at least15-30 min at 90 C for hydroxylation [101]. The hydrophilicity is a good prove that the substrate is clean, even if the gold is hydrophobic in general. Many diffractive microscopy and spectroscopy techniques (XRD, XPS, STM, SNIM, FTIR) provide detailed information about the properties of SAMs such as surface bond, loss of the surface hydrogen, and alkane chains surface orientations [ ] Microcontact printing (µcp) Molecular patterning of surfaces plays an important role in materials science. In 1993, Whitesides developed a new soft lithography technique called microcontact printing (µcp) [107]. Microcontact printing is a method for pattering SAMs on the surfaces. This technique is like normal stamping a paper but makes use of a relief containing elastomer as stamp and a gold substrate (usually) as paper. Using µcp high-quality patterns and structures that cover areas of cm 2 can be created with lateral dimensions from 5 to 1000 µm. A nice review on self- assembled monolayers and microcontact printing with application in nanotechnology can be found in references [105]. The central tool in pattering surfaces through chemistry is poly(dimethylsiloxane) (PDMS). This polymer can be fashioned in the form of a soft elastomeric stamp utilized for printing micron and submicron-scale designs of alkanethiols chemical inks on planar and curved surfaces. A schematic illustration of the procedure for casting PDMS replicas from a master having a relief structures on its surface is shown in Figure 4.2. The master can be individually fabricated by some form of lithography (photolithography, anisotropic chemical etching etc) or just a rigid substrate with a desired relief (AFM calibration standards, TEM grids, etc) can be used. Representative ranges of the critical dimensions are h, d, and l with numerical values: , and µm, respectively, generally. Each master can be used to fabricate more than 50 PDMS replicas. Whereas each PDMS stamp, in turn, can be used to print its inscribed pattern hundreds of time. For this reason, µcp is a good alternative, low-cost, non-photolithographic microfabrication method of patterned molecular structures. The PDMS stamp can be used to print SAM patterns on substrate in different ways and for different purposes. One of the methods of microcontact printing of alkanethiols on goldcoated silicon substrate is displayed in Figure

65 4. Smart surfaces From concept to reality Figure 4.2: PDMS stamp manufacture. A patterned master with defined dimensions (d, l, h) is used. A PDMS prepolymer is poured onto the master, cured and removed, resulting in a PDMS stamp with an inverse master relief. Figure 4.3: Principles of microcontact printing with alkanethiols. A printed long chain thiol (SAM 1), forms an ordered surface layer in regions of contact; the unmodified bare gold regions can be subsequently modified with a different thiol (SAM 2) 52

66 4. Smart surfaces From concept to reality The most common procedure in SAMs manufacturing is to immerse the stamp in a solution of the ink molecules (e.g. an ethanolic solution of alkylthiols). Since PDMS is hydrophobic, it is supposed that polar inks remain at the surface of the stamp whereas unpolar molecules diffuse into the bulk. After some time (depending on the ink molecules) the stamp is dried and gently pressed onto the clean substrate surface typically for second (depending on the ink). Thereby the ink molecules are transferred to the surface only at those regions where the relief surface of the stamp contacts it. The un-inked regions of the gold can be subsequently functionalized by dipping the SAM 1 patterned substrate into a different alkanethiols, SAM 2. In the end, a structure with a defined molecular pattern is fabricated Microcontact printed SAMs have been utilized for patterning surface charge, acidity and basicity, hydrophobicity and hydrophilicity, hydrogen bonding and metal-ligand coordination. One of these applications studied in this thesis is presented in more details in Chapter Block copolymers self-assembly A block copolymer molecule contains two or more polymeric chains attached on their ends. If two polymers are blended, they will form phase separation on the macroscale due to the entropy of mixing two long chain macromolecules. A block copolymer can selfassembly to form a nanoscale structure with a domain spacing that depends on the molecular weight, size and strength interaction between blocks represented by the Flory- Huggins interaction parameter, χ [106]. Where χ = (δ 1 δ 2 ) 2 xv 1 /RT, δ 1 and δ 2 are solubility parameters, V 1 molar volume of solvent, T-temperature and R-molar constant. A typical periodicity is in the range of nm. For the simplest class of block copolymers, AB diblocks, the following structures are known to be stable, confirmed by theory and experiments [91, ]: lamellae, cylinders, spheres, bicontinuous structures, etc. In Figure 4.4, some of microphase separations for AB block copolymers are presented. The length scale of the separated domains and the architecture depend on the molecular weight, composition, interactions and architecture of the segment and on the nature of any co-assembled additives, which may swell or crosslink the system. In this context, the selfassembly and structural organization of a microphase separated block copolymers can also be used to control surface properties such as wetting, chemical attachment to the surface etc. One problem in application of phase-separated copolymer block is the difficulty to integrate its in devices. Further processing is requested to form the correct shape and architecture and many samples can be destroyed due to polycrystalline domain orientation. Good approaches to remove this impediment are thin film block copolymers. 53

67 4. Smart surfaces From concept to reality Figure 4.4: Typical self-assembly behavior of bicomponent block copolymers In thin films, in addition to composition and molecular weight, the structure is dependent on the surface energies of the blocks and on geometrical constraints induced by confinement in a thin film. Since block polymers assembly is highly dependent on surface energies these boundaries can effectively direct the orientation of nanoscopic polymer domains. An overview of thin films copolymers blocks can be found in references [110]. In thin films, the lamellae formed by symmetric block polymers can orient either parallel or perpendicular to the surface due to the properties of the substrate. A various arrangements of block copolymers are possible, depending on the mixtures of the polymers, surfaces energies and the wetting properties of the substrate. More experimental details regarding the micro-phase separation for thin films copolymer block are presented in Chapter 7. 54

68 4. Smart surfaces From concept to reality 4.2. Polymer brushes- an induced self assembly system Polymer brushes are assemblies of macromolecules chemically tethered at one end to a substrate. They provide an alternative to self-assembled monolayer because of their intrinsically large size of the building blocks and the ensuing entropic contribution to the film morphology. Generally, a polymer brush is build from an ensemble of polymer chains that are attached either chemically or physically with one end to a surface. In Figure 4.5, an example for attached polymer brush is shown. The thickness of the film (h) and the distance between two neighbor brushes can be controlled during the fabrication process. Figure 4.5: Schematic illustration of attached polymer brushes: h is the thickness of the polymer film and d is the distance between two neighbor chains for controlling the density of the film. In the recent years, a number of limitations of SAMs become problematic in applications. First of all, due to the self-assembling nature of their formation, it is practically impossible to obtain large area defect-free monolayer (~mm). Secondly, since monolayers are only several nm thick, they are mechanically and chemically fragile. Thirdly, chemical groups can only be introduced at the surface, whereas in polymer brushes these groups can be carried out along the polymer backbone and placed in a different pseudo-3d spatial arrangement. Finally and the most important for smart surfaces, SAMs can be used to introduce functional groups to the surface but it is very difficult to introduce reversiblychanging chemical functionalities without reverting to sequential chemical transformation. In Chapter 6 SAMs which can be chemical modified (L-Cysteine ethyl ester) by the ph are presented. However, polymer brushes provide a conceptually simple robust, functional and 55

69 4. Smart surfaces From concept to reality switchable surface [ ]. The crucial difference between SAMs and polymer brushes is the size of the components: SAMs are assemblies of small molecules while polymer brushes are monolayers of macromolecules. Table 4.1 displays a comparison regarding advantages and disadvantages of using SAMs and polymer brushes to control surface properties. Self-assembled monolayers (SAMs) Polymer brushes Advantages Disadvantages Simple formation (especially alkathiols on gold and chlorosilanes on silicon oxides) Molecularly well-defined layers End groups to tailor surface properties (modification with biological ligands) Thin: one monolayer Limited long term stability Presence of defects on large scale Long-term stability Options for preparation ( grafting to and grafting from ) Variety of polymerization methods Control over thickness, shape structure (thickness from few up to hundred nm) (homo or mixed polymers) (free, Y-shape, H-shape) Thick film might provide selfhealing of defects More complex preparation More complex structure Table: 4.1: Comparison between SAMs and polymer brushes used to control surface properties Synthesis of polymer brushes The two main approaches employed for synthesizing polymer brushes are: grafting to and grafting from. The first method ( grafting to ) is based on subsequent grafting of end-functionalized polymers, which is chemisorbed to a solid substrate carrying suitable anchor group (see figure 4.6.A) [ ]. 56

70 4. Smart surfaces From concept to reality Before grafting, a surface treatment is usually applied where a SAM is formed to introduce appropriate functional groups onto the substrate surface. Due to the simplicity of this approach, it can also be used for the generation of polymer brushes on spherical surfaces [ ]. The advantage of the grafting to approach is that molecular weight distribution of the attached polymer can be controlled and polymers can be prepared separated. The disadvantage is that the polymer chains have to diffuse toward the modified surface. Therefore, the thickness of the polymer brushes is limited typically around 3-5 nm and the grafting density is low and decreasing with increasing molecular weight of the adsorbed chains [ ]. To avoid all these inconvenient, the grafting from synthesis can be applied (fig. 4.6.B). Figure 4.6: Scheme of chemically attached polymer brushes (A) grafting to and (B) grafting from approaches. In the grafting to case, end-functionalized polymers interact with functional groups on the surface while in the grafting from technique the polymer chains grow from initiator immobilized on the surface. The red spheres represent the anchoring group. Here, the polymer chain is directly grown from the surface using immobilized initiators. During this procedure, a SAM is add onto a substrate and then the polymer chains are grown in-situ from immobilized initiator molecules utilizing free radical polymerization [125] and atom transfer radical polymerization [126]. There are some advantages of the grafting from approach compared with grafting to. First of all, polymer brushes with higher graft density and higher molecular weight can be generated. Secondly, the thickness of the polymer brushes can be controlled during the polymerization reaction by adjusting either grafting density or average molecular weight of the surface bound polymer [127]. Finally, and most important for the studies of this thesis, using the grafting from approach, not only homopolymer brushes but also mixed brushes can be synthesized. For the aim of this thesis only mixed polymer brushes, with different shapes and thickness were used. 57

71 4. Smart surfaces From concept to reality A complete description of these two approaches and other combinations for synthesis of polymer brushes can be found in a review by Rühe [128]. In a good solvent, the thickness of the anchored polymer (h) in the low grafting density mushroom regime scales as h Nσ 0, where N is the degree of polymerization of the polymer and σ the grafting density. At the higher grafting densities, the polymers interact with each other and there will be a degree of distortion from the random coil. At sufficient high grafting density, the so called brush regime will be reached [129]. In this regime, the brush height scales as h Nσ 1/3. Dense brushes are most easily prepared following grafting from approach and making use of the surface tethered initiators. The amount of overlap between the chains, and hence the degree of stretching, is firmly dependent on the grafting density and chain length and for any given polymer it is also dependent on the solvent quality. In good solvent, the chains will swell, thereby, forcing them to stretch away from the surface. An ensemble of polymer chains grafted at one end to a surface at high grafting density, e.g. so that the distance between neighboring grafting points is much smaller than the unperturbed radius of gyration, is termed a polymer brush [130]. In the grafted polymer systems, interchain interactions lead to strong stretching of the polymers from a surface to a much larger length than they would be in a free melt. Depending on the degree of polymerization, N, and the grafting density, σ, and the quality of the solvent, several regimes with different static and dynamic properties occur. Especially interesting systems are brushes consisting of two or three different components, in which phase separation can occur. Such systems may be classified into two categories: (i) brushes consisting of di- or tri-block copolymer chains with one end covalently attached to a solid substrate (see Figure 4.7);(ii) mixed brushes, composed of a mixture of two homopolymers A and B, covalently attached to a surface [130]. When two polymer brushes are used, the conformation of mixture will be affected both by the solvent and by the incompatibility of both polymers. The shape of the mixed polymer brushes can be controlled by self-assembled monolayers. Using SAMs with one or two functional tailors, a free mixed or an Y-shape mixed polymer brushes can be manufactured (see Figure 4.7.C and D). Both types of polymer films consisting of a mixture of poly(styrene)- poly(methyl methacrylate) PS-PMMA) will be used in this thesis. More details about mixed polymer brushes (synthesis, nanophase-separation, switching properties) will be described in Chapter 6. It is well known, that diblock-copolymer chains show microphase separation if the two blocks are incompatible (described in details for block copolymers self-assembly). If diblock-copolymer brushes are covalently attached to a surface, even more complex phase separation behavior is induced. These brushes show specific topographical structures that result from the interplay of the tendency of block copolymers to separate into microphases and the restricted motion induced by tethering of the polymer chains to the substrate [ ]. 58

72 4. Smart surfaces From concept to reality Figure 4.7: Scheme of mixed polymer brushes (A) general scheme for a mixed brush, (B) nanophase separation into structured topography for mixed brush, (C) free mixed polymer brushes and (D) Y-shape mixed polymer brushes The polymer-polymer interactions and the polymer-solvent interactions show an abrupt readjustment in small ranges (nanometer order) of the mixed polymer brushes films. 59

73 4. Smart surfaces From concept to reality Smart surfaces: switchable surface energy In contrast to conventional brushes, which consist of a single type of homopolymers, mixed polymer brushes can amplify the response to external stimuli such as solvents [83, 86], ph [87], temperature [88] by a combination of conformational changes and nanophase separation. The ability of mixed polymer brushes to switch properties such as surface energy and/or surface topography in response to changes in their environment can be used for tailoring of surface properties and is an important ingredient for the switching mechanism of mixed brushes. It was observed that contact with different solvents can influence the surface topography significantly. Exposure to selective and nonselective solvents (e.g. toluene, acetone and chloroform) allows reversible switching of the topography on a nanoscale. Phase segregation in mixed brushes upon exposure to solvent of different qualities cause a strong alternation of the surface chemical composition of the brushes. Figure 4.8 displays an example of topography surface switching using different solvents that are selective or non-selective for one of polymer brushes. Figure 4.8: Schematic illustration of possible morphologies of mixed brushes exposed to solvents with different selectivities (solvent1: selective for polymer red, solvent 2 selective for polymer blue, solvent 3 non-selective). Surface wettability is an important parameter and it is the first that can be controlled quickly in almost all surface modification strategies. It is closely related to a series of other surface-relevant phenomena like absorption, nucleation, reactivity and even mechanical properties. Wettability mainly depends on the surface chemical functionality, although surface roughness also plays an important role. SAMs provide a simple and convenient way to change surface wettability by altering the terminal groups [131]. However, SAMs have limitations in making surfaces with switchable wettability. As an example for switchable SMAs L-Cysteine and L-Cysteine ethyl ester (as SAMs on gold substrate) will be presented in this thesis. A rare exception are electro-switchable SAMs based on the 60

74 4. Smart surfaces From concept to reality terminal carboxyl groups, which can bend up and down when applying a potential to produce a wettability change. Mixtures with different polymer brushes that have different affinity to different solvents represent the simplest way to change surface wettability. Brushes incorporating neutral hydrophilic/hydrophobic polymers, charged polyelectrolytes and even extremely hydrophobic perfluorinated polymers have been exploited [ ]. In this thesis, stimuli-response of mixed PS-PMMA polymer brushes is studied by chemical mapping. The switchable topographic surface of the film is very difficult to chemically characterize with conventional microscopic techniques (even impossible in some cases). Experimental results using successfully SNIM as a new technique for nanophase-separated mixed polymer brushes are presented in details in Chapter 8. Since contact angle measurements are very sensitive to the wetting properties of a thin film [134] this method was used to get additional information on the switching mechanism in mixed polymer brushes Theory of contact angle The formation of a stable and ordered organic layer is crucial for the attaining well defined nanostructures. The quality of stable monolayers can be estimated from wetting measurements. While surface tension is a quantitative value that is defined at the interfaces formed by two different phases, the contact angle (θ) is an angle at the junction of three different phases. It is measured in the liquid, typically at the meeting point of gas-liquid-solid. Indeed, the shape of a liquid droplet on a plane homogeneous surface depends on the free energy of both the droplet and the surface (fig. 4.9.) The contact angle is related to the surface free energy by the Young s equation: γlv sθ γsv - γsl (4.1) where γlv γsv γsl are surface tension at the liquid-vapor, solid-vapor and solid-liquid interface, respectively [135]. 61

75 4. Smart surfaces From concept to reality complete wetting wetting complete nonwetting θ = 0 0 <θ < 180 θ = 180 hydrophilic hydrophobic Figure 4.7: Schematic representation of the forces acting on a water droplet placed on top of a plane surface, the principle of contact angle measurements and wetting properties of different surfaces. The angle of a droplet on a solid surface is the result of the balance between the cohesive forces in the liquid and the adhesive forces between the solid and the liquid. In the case of liquid on the surface, complete wetting means its contact angle on that surface is 0. If there is no interaction between the solid and the liquid,θ= 180, the surface is complete nonwetting. Ideally, complete wetting means the formation of liquid film on the surface while complete nonwetting means the formation of liquid drops with perfect sphere shapes. In the case of water, when the surface shows a tendency toward complete wetting, it is generally called a hydrophilic surface whereas a surface with a tendency toward complete nonwetting is generally called hydrophobic. In the case where the water contact angle is above 150, the surface is used as superhydrophobic surface. These wetting properties of surfaces can be easily observed with polymer brushes. 62

76 5. Experimental set-up 5.1. Scattering scanning near-field infrared microscope (s-snim) Set-up AFM microscope Tunable carbon monoxide laser (CO laser) Coupling the laser to the AFM Raman microscope

77 5. Experimental Set-up 5.1. Scattering scanning near-field infrared microscope (s-snim) Set-up As shown in Chapter 3, near-field infrared microscopy is a unique technique that combines two different instruments: AFM which determines the spatial resolution and a laser which provides the working wavelength for spectroscopic information of the sample. Figure 5.1 displays the schematic set-up of the s-snim. The external laser beam is guided by external mirrors to an off-axis parabolic mirror that focuses the laser beam onto the AFM tip that is used like a nanofinger to probe the sample surface (for details about SNIM theory see Chapter 3). Figure 5.1: Schematic set-up of the s-snim Our scattering scanning near-field infrared microscope (s-snim) is an AFM that was modified to gain additional space around the tip for irradiation and detection of light [1]. We have integrated two different radiation sources into the SNIM set-up: a tunable highpower opto-parametric oscillator (OPO) which covers the spectral range from 3400 cm -1 to 2600 cm -1 (λ = µm) and a tunable liquid nitrogen cooled sealed-off carbon monoxide laser (CO laser) that covers the IR region from 2100 cm -1 to 1600 cm -1 ( λ = µm). In this thesis all near-field measurements are performed using the tunable CO laser. A detailed description of the OPO system can be found in [9, 49, 53-54]. Both laser beams are p-polarized (polarization perpendicular to the sample) because polarization has a large influence on scattering scanning near-field infrared microscopy. The scattered light 64

78 5. Experimental Set-up from the tip, which contains the near-field information of the sample, is focused by a calcium fluoride lens onto a nitrogen cooled mercury-cadmium-telluride detector (MCT). Phase sensitive lock-in measurement is performed at each pixel and a near-field infrared image of the sample is collected. More details about coupling of a laser to the AFM will be described in Section AFM Microscope The AFM microscope is a commercially available open-access system available from Nanotec Electronica. The software used for data collection is WSxM [137]. Small changes were necessary to meet out experimental requirement: in the modified AFM, the sample is scanned under a fix cantilever. The macroscopic approach of the sample to the tip is realized by three steppermotors while the piezo-tube controls the height with nanometer accuracy as soon as the feedback loop range is reached. The feedback loop is based on the detection of the beam-deflection laser by a 4-quadrant detector. The beam deflection laser is focused on the tip of the cantilever as described in Chapter 2, Section 2.3. The piezotube controls the x-y-z directions: x-y transversal and the height in z direction. There are four channels to check the quality of the measurements, 3 channels are standard (topography, phase, and amplitude) and the fourth is adapted for near-field measurements. In Figure 5.2, the topography and amplitude for a calibration grating are shown. Calibration gratings from the TGZ series (TGZ02, MikroMasch) are 2-D arrays of rectangular SiO 2 steps on a Si wafer. The structure is coated by Si 3 N 4 to protect the Si from oxidation. The pitch value is 3 µm and height ~ 50 nm. Figure 5.2: Normal AFM images of calibration grid using Nanotec instrument. A - Topography, B - Amplitude and C - 3D image of topography 65

79 5. Experimental Set-up Commercially available silicon rectangular cantilevers coated with gold (NSC16/Cr-Au, MikroMasch) are used for SNIM probes. The gold coating ensures a high reflectivity across the entire infrared range, but increases the radius of the tip. For this reason, the radius of the tip for near-field measurements is typically < 50 nm. In addition, the gold coating provides a good polarizability, especially for 1740 cm -1 [57]. The microscope usually operates in tapping mode, which is suitable for near-field measurements. The resonance frequency ω r is approximately 170 khz ± 20, which is a typical value for tapping mode cantilevers (see Fig. 5.3). The oscillation frequency of the tapping mode is important for the phase sensitive lock-in detection. Figure 5.3: Resonance frequency of the standard AFM cantilevers used for near-field measurements (right a zoom of the resonance frequency) The incident infrared radiation is focused on the tip of the cantilever by an off-axis parabolic mirror (focal length = 101 mm). For each pixel, the scattered intensity of the infrared radiation at resonance frequency ω r (fundamental frequency) or 2ω r (first harmonic) is measured. The oscillation amplitude, the distance between tip-sample, and all characteristics of the feedback loop influence the quality of the images. As was shown in Chapter 2 (Section 2.3.), the resonance amplitude depends on the geometry of the cantilever and its resulting spring constant. Typical amplitude values between 0.5 V and 3.5 V can be reached. Other parameters that can influence the resonance amplitude are: quality of mounting the cantilever on the AFM head, focusing of the laser-diode on the cantilever and so on. The oscillation amplitude of the cantilever can be controlled by the voltage applied on the excitation piezo crystal. Because of the limitation of the noise in the near-field signal, small cantilever oscillation amplitude can be used for a good topography image. 66

80 5. Experimental Set-up Typical values that are used in this thesis are 1.2 V (between 1 and 1.5V). The set-point parameter is the value from the 4-quadrant detector when the tip is in contact with sample. The detector signal decreases when the tip is in contact, i.e. it is getting closer to the surface until the set-point value is reached. The standard value of the set-point is half of the resonance amplitude. In order to obtain good topography and to optimize the near-field contrast, the set-point is reduced. A typical value for near-field measurements of polymeric samples at amplitude of 1.2V is -0.6 V. The near-field signal is more sensitive to the setpoint than the signal for normal topographic measurements. For this reason, keeping the set-point fixed is necessary during scanning; since the topography and near-field images are optimized, the set-point can sometimes be changed for optimizing the near-field contrast images. The scan frequency is the parameter which determines how long the tip remains on one position on the sample. As a consequence, the feedback has to find the height defined by the set-point. A typical value for the scan frequency is f scan = 0.5 Hz which results in 1 line with 512 pixels in 2s. For a 512 x 512 image, the total time for image formation is approximately 17 min. The integration time of the lock-in amplifier (τ c ) has to match the scan frequency. If the f scan is 0.5 Hz and 512 pixels, the tip stays ~ 3 ms on a pixel. Therefore, the averaging time is 3 ms per pixel and integration time is 1 ms because the lock-in measurement is completed after 3τ c. The scan area can be measured from 0 to 100 µm but for scans larger than 80 µm, the system in not stable. For a good image with nanometer resolution, a scan image smaller than 75 µm is preferred. The measurement procedure is to select a large area as an overview and then move to the interested region with small areas. All SNIM measurements used in this thesis have a scan area of 5 µm which means that the pixel size is ~ 10 nm. At the same time, the variation of the near-field amplitude as function of the distance to the surface can be read-out. This function can be used to optimize the near-field signal [5] The tunable carbon monoxide laser (CO laser) In this thesis, all near-field measurements are performed using a nitrogen cooled sealed-off CO laser. The CO-laser is a gas laser which was developed at Bonn University and Ruhr University Bochum [ ]. A review of the CO-laser with some spectroscopic applications can be found in [140]. Figure 5.4 shows a scheme of this home-built CO laser. The laser consist a glass tube (diameter ~ 18 mm) immersed in a sand which is surrounded by the nitrogen cooling dewar and a vacuum chamber. The gas mixture is filled into the laser tube. Table 5.1 lists the composition of our standard laser gas mixture. 67

81 5. Experimental Set-up Component Debit (volume %) Xenon Carbon monoxide 1.8 (CO) Nitrogen (N 2 ) Helium (He) 6 10 ~0.1 Table 5.1: Standard CO-laser gas composition Figure 5.4: Schematic set-up of the tunable CO laser used for s-snim. 68

82 5. Experimental Set-up The plasma is created using a high voltage of 10 kv and a current of 10 ma. The system is cooled down to a temperature of -140 C, and the plasma is maintained at constant temperature with two temperature feed-back loops. The liquid nitrogen and heating wire, in combination with a temperature sensor on the outside of discharge tube, is used for controlling the temperature. The homogeneity of the temperature is ensured by a sand-vacuum chamber between the nitrogen reservoir and the laser glass tube (see Fig. 5.4.). Next to the cathode a gas intake valve builds the connection to the gas mixture and a turbo pump which permits a pressure of 2x10-5 mbar. An additional valve was attached to protect the system against leakages. Both ends of the laser tube are sealed with two CaF 2 windows mounted at the Brewster angle. The windows serve as polarizing filters. The laser resonator is made from a gold coated concave mirror and a coated reflection grating, in total having a length of 1.5 m. The angle between the resonator axis and normal of the grating determines the emitting CO vibration lines and the output wavelength of the CO laser. The wavelength range of the CO laser is µm with approximately one emitting line every 3 cm -1. (see Appendix B). The output power depends on the Einstein coefficient of the transition and is different from one line to another. A power between 0.2 and 1.5 W is possible with this laser, and for s- SNIM measurements, a low power of 0.3 W was used to avoid damaging the polymeric samples (see Table B.1.). Before any measurements, the laser must be cleaned. This is realized with a gas mixture of 3:1 He 2 and O 2. This is burned out in the laser tube at room temperature several times (3-4 times) to wash the organic deposited films out of the laser tube. The laser can run continuity if nitrogen cooling is maintained. For removing the special impurities, the laser has to be opened at anode. Anode cleaning is performed in mixture 1:1 ethanol and water in ultrasonic bath for 15 minutes. Also, citric acid can be used for some minutes. Further details about this tunable CO laser are given in [140] Coupling the laser to the AFM As was describes in the previous chapter for scattering scanning near-field microscopy two different light sources can be used: a tunable CO laser and an OPO system. Figure 5.5 shows a schematic of the optical path for the lasers focused on the AFM tip. The optical path is defined by two diaphragms (D 1 and D 2 ) to insure a perfect overlap between the measurement laser (CO-laser or OPO) and the helium-neon laser (He-Ne). The pre-adjustment is realized using a He-Ne laser. This pre-adjustment is used to simplify the alignment of the infrared laser on the nanometer AFM tip. A kinematic mirror (M 1 ) is used to make the alignment accessible for both lasers. 69

83 5. Experimental Set-up Figure 5.5: General set-up of the s-snim optical path for laser beam: M 1 kinematic mirror, M 2 gold mirror (beam lift), D 1,2,3 diaphragms, PM 1,2 concave mirrors, PM 3 parabolic mirror (all others are plane mirrors), MCT mercury cadmium telluride detector Ideally, the focal spot size is limited by the diffraction limit and can be calculated using equation 5.1: (5.1.) where w is the beam radius, f is the focal legth, λ is the wavelength, d is the diameter of the focusing element 70

84 5. Experimental Set-up According with this equation, the diameter of the focal spot changes from ~3 µm at λ = 633 nm (He-Ne laser) to ~28.5 µm at λ = 5.7 µm. Due to the different polarizations of the OPO and the CO-laser, a beam lift is used which rotates the polarization of the OPO laser light but does not make changes for CO-laser light (M 2 ). The infrared laser beam is expanded using a telescope with a magnification factor of The beam expander consists of two concave mirrors (PM 1 ; f = 100 mm and PM 2 ; f = 457 mm ). After the beam expander, it is directed to the AFM by plane mirrors to a 90 off axis parabolic mirror (PM3; f = mm). This mirror focuses the laser beam onto the cantilever with an ~85 angle of incidence. The signal scattered from the coupled tipdipole mirror system is collected by a calcium fluoride lens (CaF 2 lens) with a focal length of 40 mm, and focused onto a high sensitivity 1x1 mm 2 nitrogen cooled MCT detector (Judson Technology). Focusing onto the tip can be controlled by carefully observing the shadow of the AFM tip reflected by the He-Ne laser. When the tip shadow and its mirror image approach each other and finally meet, the tip is considered to be in the focus. The position of the lens, the detector, and the tip build a configuration where the image has the same size as the object (2f-2f configuration). The scattered light which is collected with this system can be calculated using equations 5.2 For our system, it is ~ 30%. S R 2 (5.2) The collection area of the lens is ΔS ( which is collected is Φ cm -2 ), hence the fraction of scattered light For good control of the alignment, the CaF 2 lens and MCT detector can be fine adjusted by using separate x-y-z translation stages. Afterward, the fine alignment is performed using the infrared radiation after approaching the AFM tip to a sample. The measured signal is electronically amplified by a factor of 1000 (a preamplifier Judson Technologies) after detection and sent to a lock-in amplifier (SR844 Stanford Research Instruments). The lock-in amplifier is sensitive to the signals which have the same frequency as the lock-in frequency. It can lock-in between 25kHz up to 200MHz with a very high sensitivity between µv up to 1V. Figure 5.6 displays the AFM with detection unit for s-snim. 71

85 5. Experimental Set-up Figure 5.6: The set-up of s-snm, the AFM, and the detection unit As shown previously, the s-snim measurements are performed in tapping mode, which means that the tip oscillates vertically with resonance frequency f. Due to the height oscillation of the cantilever, the scattered radiation from the tip is amplitude-modulated. In a typical near-field microscopy set-up, the lock-in frequency is the oscillation frequency of the cantilever. To suppress possible interferences between the signal and IR reflections which are not resulting from the tip, ω lock = ω r 170 khz. Detection with a higher-order harmonic of the resonance frequency increases the enhancement of near-field signal over the background signal and improves the signal-to-noise ratio. Locking on the first harmonic 2ω r (340 khz), reduced other near-field fluctuations. All s-snim measurements used in this thesis are made using 2ω r. A large integration time limits the noise in the near-field contrast images. This is limited by the scan speed. Typical values for s-snim data used in this thesis are: f scan = 0.5 Hz, τ c = 1 ms, 512 pixels, 512 lines, 5 5 µm 2 scan size. 72

86 5. Experimental Set-up 5.2. Raman microscope As shown in Chapter 2, Raman microspectroscopy is an attractive technique because the practical diffraction limit is on the order of half the excitation wavelength, which is smaller for Raman with visible light than for MIR microspectroscopy. Raman microspectroscopy has a high chemical selectivity and high sensitivity. The Raman microscope is a commercially available instrument from WITec (WITec alpha 300). It is a complex microscope that can perform confocal Raman microscopy, AFM, scanning near-field optical microscopy (transmission and reflection units are available), and confocal optical microscopy. The system can also be upgraded to perform tip-enhanced Raman microspectroscopy. For this thesis, confocal Raman microspectroscopy and AFM are performed. Figure 5.7 displays the set-up used for Raman measurements. More details about this setup can be found in [141]. Figure 5.7: Set-up of the Raman microscope. Excitation comes from an frequency doubled Nd:YAG laser. The Raman scattered light is detected by a CCD detector. AFM unit is attached. A 45 mw frequency doubled Nd:YAG laser (λ incident = 532 nm) is coupled in a Zeiss microscope using a wavelength-specific single mode polarization maintaining optical fiber. The laser beam is then collimated via an achromatic lens and passes a holographic band 73

87 5. Experimental Set-up pass filter. After that, the beam is focused onto the sample through a Nikon Fluor (20x dry/0.4 NA or 100x dry/0.9 NA) microscope objective. The scattered light is collected with the same microscope objective. The sample is located on a piezo driven microscope scanning stage, which has an x,y resolution of 3 nm and a z-resolution of 0,3 nm. Reflected laser and Rayleigh scattered light are blocked by a holographic edge filter while Raman scattering is focused into a multimode optical fiber, which serves as the entrance slit for the spectrometer and focused onto a charge-coupled device (CCD) camera. The camera is cooled with Peltier element to -60 C. The frequency resolution is <3cm -1. A full Raman spectra from 0 cm -1 to >3700 cm -1 can be recorded using the 600 l/mm grating of the spectrometer. The chemical contrast is visualized over a given spectral range or specific wavelength. Before and after each measurement, a light microscopic image was taken with the Rayleigh filter to check the sample for any damage. Data acquisition was driven by the WITec Control software. At every pixel a complete Raman spectrum was recorded covering the spectral region from -173 cm -1 up to 3750 cm -1. Details about Raman measurements, such as integration time, scan size, etc. will be presented in the next Chapter. Data analysis was performed using WITec Project

88 6. ph sensitive self-assembled monolayers as smart surfaces 6.1. Introduction Materials and Methods Materials Preparation of micro-structured SAMs Contact angle measurements Fourier Transform Infrared Spectroscopy (FTIR) s-snim Results and Discussions Conclusion

89 6. ph sensitive SAMs as smart surfaces Materials and Methods 6.1. Introduction Interfacial phenomena where organic and biological macromolecules are involved are normally very complicated and occur on different levels of molecular complexity. This can, for example, include van der Waals type interactions between hydrophobic regions in the interior of a protein molecule and a hydrophobic surface [142]. Interactions of that type will result in a quite substantial structural change of the interacting protein molecule and thereby to changes in its function and activity. The behaviour of the adsorbed protein may also depend on: 1) specific interactions between active sites and functional groups on the solid surface and 2) amino acid side-chain groups at the outermost surface of the protein molecule. As amino acids have both the active groups of an amine and a carboxylic acid they can be considered as acid and base in the same time (though their natural ph is usually influenced by the R group) 1421]. At a certain ph known as the isoelectric point, the amine group gains a positive charge (protonated state) and the acid group is negatively charged (deprotonated state). The exact value is specific to each different amino acid. A zwitterion can be extracted from the solution as a white crystalline structure with a very high melting point, due to its dipolar nature [143]. Near-neutral physiological ph allows most free amino acids to exist as zwitterions. When a monolayer film of amino acids is formed on a solid substrate, however, their intermolecular interactions are different from those in the gas phase, solid, or aqueous solution, due to the surface-adsorbate interactions and somehow restricted intermolecular interactions between adsorbents in two dimensions. From previous studies it is know that at ph 1.5 and 5.7, L-Cysteine binds to gold by the thiol group (see Figure 6.1) and exists in a protonated form containing -COOH and -NH 3 + moieties [143]. At ph 10.5 and above, however, it binds to gold by both sulphur and amino nitrogen and exists in a deprotonated form containing -COO - and -NH 2 moieties. Thus, the chemical state of L-Cysteine adsorbed on gold from solution is correlated to the ph of the solution during adsorption [144]. Figure 6.1: The schematic hydrolysis of L-Cysteine ethyl ester with ph adjusted at ph

90 6. ph sensitive SAMs as smart surfaces Materials and Methods Of the 20 naturally occurring amino acids, L-Cysteine is such a modified alkanethiol containing not only a thiol group that binds to gold with high affinity but also carboxyl and amino functional groups that can be conjugated with biomolecules. One of the derivate of L-Cysteine, is L-Cysteine-ethyl ester. Adsorbed to a surface the system L-Cysteine ethyl ester L-Cysteine represents a smart surface that is sensitive to ph (see Figure 6.1) Materials and Methods Materials L-Cysteine (Aldrich, 98%) and L-Cysteine ethyl ester-hydrochlorite (Aldrich, 98%) were purchased from Sigma. A molecular structure of the molecules is displayed in Figure 6.2. The octadecanethiol (ODT) used for µcp (micro Contact Printing), (Aldrich, 98%) was purchased from Sigma. Aqueous solutions were prepared using HPLC water (J.T.Baker). The ph was adjusted using NaOH, 1N. The ph was checked using a ph-meter from Hanna. Figure 6.2: Molecular structures (2D and 3D) of L-Cysteine and L-Cysteine ethyl ester: S- yellow, N- blue and O red. 77

91 6. ph sensitive SAMs as smart surfaces Materials and Methods Preparation of micro-structured SAMs A gold coated n(100) silicon wafer (Anfatec Instruments AG) which was cleaned with hot piranha solution (3:1 mixture of H 2 SO 4 and H 2 O 2 ) for approximately 5 min, was used as substrate. Self-assembled monolayers of L-Cysteine and L-Cysteine ethyl ester were prepared by immersing the substrate in a 1 mm L-Cysteine or L-Cysteheine ethyl ester solution respectivly (see Figure 6.3). The samples were carefully washed with water at the same ph (for sample at ph 10, the ph for water was modified) and dried in stream nitrogen for 1 minute. In the end, they were kept in desiccators for 1 h before use to avoid excessive contamination. In order to locally hydrolyze L-Cysteine ethyl ester SAM a 1 ml drop of water with ph 10.5 was put onto the surface (Fig ). Since the water drop is difficult to control, the drop covered surface is estimated to an area of about ~ 10 mm 2 (estimation was done using contact angle measurements). Similar experiments were carried out by completely immersing the L-Cysteine ethyl ester SAM in water with ph ~ 10.5 for 1h. Afterwards the sample was rinsed with water of ph 10.5, dried in a stream of nitrogen for 1-2 minutes and measured immediately. Figure 6.3: The hydrolysis of L-Cysteine ethyl ester using ph ~ 10.5 (scheme 3: locally modified SAM) 78

92 6. ph sensitive SAMs as smart surfaces Materials and Methods Micropatterns of ODT and L-Cysteine were prepared by microcontact printing (µcp). PDMS stamps used in these experiments were fabricated following the protocol described in Chapter 4, Section The microcontact printing is schematically described in Figure 8.3. First the dry stamp was put in the Eppendorf tube filled with approx. 1 ml of 5 mm ethanolic solution of ODT for 15 minutes (Figure 6.4, 1-2). In order to avoid precipitation of ODT during stamp loading the tube with the stamp was put in a water bath at 36 C. Subsequently the loaded stamp was shortly rinsed with ethanol and dried in a stream of nitrogen for 1 minute. As substrates a gold coated n(100) silicon wafer (Anfatec Instruments AG) which was cleaned with hot piranha solution (3:1 mixture of H 2 SO 4 and H 2 O 2 ) for approximately 5 min, was utilized. Printing was done by gently pressing the stamp to the gold surface by hand (3). After a contact time of 120 s, the stamp was carefully peeled off from the substrate and temporarily placed in an Eppendorf tube because further treatment has to be given to the substrate as fast as possible to avoid contamination of the free gold areas (4-5). Afterwards, the Au substrate was immersed in a 1mM L-Cysteine solution for 1 h (6). In the end, the substrate was washed carefully with water and dried in a stream of nitrogen (7). Before usage, all samples were kept in a desiccator for 1 h. The microstructure selfassembled monolayers were checked using AFM. In the Figure 6.4 (8) a topographic image of a micropatterned ODT/L-Cysteine surface is displayed. The height difference shown in the topography line profile corresponds to ~ 2 nm. Nevertheless, the AFM measurements do not provide information of the chemical structure of sample surface. 79

93 6. ph sensitive SAMs as smart surfaces Materials and Methods Figure 6.4.: Preparation of a structured SAM (ODT/L-Cysteine) by microcontact printing and AFM image and line profile of ODT/L-Cysteine micropatterned surface. 80

94 6. ph sensitive SAMs as smart surfaces Results and discussion Contact angle measurements Contact angle measurements were performed using a home-made system (an optical microscope Leika 200 and a digital camera Nikon). The system, able to reach a maximum magnification of 120x, was used to acquire images of water droplets (100 µl) on the investigated surfaces. For all measurements, ultra pure water was used. The digital images were then processed with ImageJ, a public domain, Java image processing program developed at the National Institutes of Health (USA) Fourier-Transform Infrared Spectroscopy (FTIR) Fourier-Transform Infrared spectra were recorded on a Vertex V80 (Bruker) spectrometer equipped with a MIR source, a KBr beam splitter and a liquid nitrogen cooled MCT detector. Spectra were recorded either in transmission mode or in grazing incident reflection mode. IRRAS (infrared reflection absorption reflection spectroscopy). For IRRAS a BaF 2 polarizer (LOT) and a grazing incident reflection unit (Bruker, ~80 ) were utilized. For IRRAS spectra a cleaned bare gold substrate was used as reference. FTIR spectra on KBr pellets were recorded in transmission configuration using a MCT detector. For preparation of L-Cysteine and L-Cysteine ethyl ester-kbr pellets, 200 mg of dry KBr were carefully pulverized and thoroughly mixed with 2 mg of L-Cysteine and L-Cysteine ethyl ester, respectively. Afterwards the powder was filled into a hydraulic press to form a pellet. After mounting the pellet (a pure KBr pellet as reference or a sample pellet) the sample chamber was evacuated for 10 min. For FTIR volume spectra, spectral resolution was set to 4 cm 1 and a 256 spectra were coadded. For IRRAS measurmenst, Spectra were recorded at room temperature with a resolution of 4 cm 1 and a coadding of 512 spectra. Spectra were evaluated using OPUS-Software 6.5 (Bruker)using the region from 1000 to 3300 cm s-snim s-snim measurements were carried out as described in Chapter 5. Each image was recorded at a scan rate of 0.5 Hz and a time constant of 1 ms at the lock-in amplifier. Image processing was performed using WSxM imaging software 4.0 Develop 11.2 (Nanotec Electronica) [137]. Raw images were corrected using a linear flatten function and for presentation the contrast was enhanced. 81

95 6. ph sensitive SAMs as smart surfaces Results and discussion 6.3. Results and Discussions Figure 6.5. displays FTIR volume spectra of a KBr pellet containing L-Cysteine (purple) and L-Cysteine ethyl ester (green). Figure 6.5: FTIR volume spectra of L-Cysteine (purple) and L-Cysteine ethyl ester. Peak assignment is given in Table 8.1 Thiol group instead of thiol bond. The spectra give an overview over the spectral range from 1100 to 3300 cm 1 including the ν(c=o)(red), ν(s-h)(-blue) and C-H (gray) stretching vibration regions. A detailed assignment of the vibrational modes is done in Table

96 6. ph sensitive SAMs as smart surfaces Results and Discussion peak center position (cm -1 ) vibrational mode 1267 δ w (CH 2 ) 1320 δ s (CH 3 ) 1470 δ(ch 2 ) 1718 ν(c=o) -COOH functional group 1745 ν(c=o) -COOR functional group NH + 3 sym def. - NH 2 ν(s-h) ν(s-s) 2920 ν s (CH 2 ) 2950 ν as (CH 2 ) 3010 ν as (CH 3 ) Table 6.1: Peak assignment for the FTIR volume I spectra of L-Cysteine and L-Cysteine ethyl ester in Figure 6.3 in the spectral region from 1100 to 3000 wavenumbers. The long frequency range of the spectra, from 2700 to 3100 cm -1, contains the C-H stretching vibrations for both molecules which appear more pronounced for the L-Cysteine ethyl ester in comparison to L-Cysteine. This is due to the presence of the ethyl group (- CH 2 -CH 3 ) in ester. The band correspond to the S-H stretching vibration appears in the region cm -1 is prominent for both molecules. L-Cysteine tends to form dimmers called Cystine. Cystine forms by oxidation of two L-Cysteine residues which covalently link to make a disulfide bond S-S bond. This bond is shifted to higher wavenumbers ( cm -1 ) compared to single thiol groups. With respect to biomolecules, especially polypeptides, the spectral range from 1500 to 1800 cm -1 is well known that the amide II ((N-C(=NH)-N ) region and carboxyl vibrational region. L-Cysteine and L-Cysteine ethyl ester show two prominent peaks in this region. One peak at 1550 cm-1 (for L-Cysteine) and 1580 cm 1 (for L-Cysteine ethyl ester) is assigned NH 2 /NH + 3 symmetric deformation mode. The weaker shoulder at 1630 cm 1 is an to amide II vibration (N-C(=NH)-N 83

97 6. ph sensitive SAMs as smart surfaces Results and discussion consisting of N-H in-plane bending vibrations in the chain of the L-Cysteine molecule as self- assembled monolayers. For L-Cysteine ethyl ester, the zwitterionic form is not present and for this reason, the presence of the NH 3 + symmetric deformation mode which appears for L-Cysteine adsorbed by the Au substrate. Another prominent peak is centered at 1720cm 1 and caused by the C=O stretching vibration of the carboxy group (-COOH). The shift that appears in the L- Cysteine ethyl ester spectra, at 1754 cm -1, is due to the influence of the CH 2 -CH 3. This can be used as fingerprint peak to distinguish carboxy group (-COOH) from the carboxyl ester group (-COOR). In this thesis, these peaks will be used to prove the hydrolization of the ester group. Figure 6.6. presents IRRAS spectra of L-Cysteine and L-Cysteine ethyl ester on gold adsorbed from aqueous solution. A detailed assignment of the vibrational modes is done in Table 6.1. Figure 6.6: IRRAS spectra of L-Cysteine and L-Cysteine ethyl ester adsorbed on gold from aqueous solution. A zoom spectra in the low frequency region ( cm 1 ) containing the characteristic amino NH 2 /NH 3 + symmetric deformation region (1500 to 1700 cm 1 ) and carboxyl region ( cm -1 ) is displayed. Peak assignment is given in Table

98 6. ph sensitive SAMs as smart surfaces Results and Discussion In IRRAS spectra similar peaks as in FTIR volume spectra for L_Cysteine and L-Cysteine ethyl ester are observed. The peak shift for carbonyl group is more clear (red region from zoom area in Fig 6.6.) and a good evidence of amino group, NH 2 /NH 3 + for L-Cysteine and amide II for ester (green) can be observed. A clear evidence for monolayer formation of L-Cysteine and L-Cystein ethyl ester is given by the absence of the thiol (S-H) and disulfide peak in the frequency region cm -1. The hydrolization of L-Cysteine ethyl ester was observed using IRRAS. Figure 6.7 displays the peak shift of the C=O vibration from the ester group due to hydrolization. Figure 6.7: IRRAS spectra of hydrolized L-Cysteine ethyl ester (pink) at ph 10.2 in comparison to L-Cysteine ethyl ester (green) and L-Cysteine (purple). The inset shows a zoom in the low frequency region ( cm 1 ). Peak assignment is given in Table 6.1. Hydrolization of L-Cysteine ethyl ester can be clearly observed in the low frequency region ( cm 1 ). The C=O stretching vibration from hydrolyzed ester is shifted to higher/lower frequencies (1730 cm -1 ) because of the presence of CH 2 -CH 3. The NH 2 /NH 3 + symmetric deformation modes are shifted due to the presence of the COOH after hydrolization. In the zoom spectra, gray region correspond to amino group, Can be 85

99 6. ph sensitive SAMs as smart surfaces Results and discussion observed an increased shoulder specific for NH 3 + vibrational mode. The spectrum of L- Cysteine ethyl ester hydrolyzed still shows characteristic peaks for L-Cysteine ester indicating that the hydrolization efficiency is not 100%. However, it has been proved that L-Cysteine ethyl ester and L-Cysteine ethyl ester after hydrolization with water of ph 10.2 are clearly distinguishable due to their vibrational modes in the infrared region.. Since hydrolization changes the wetting properties of the surface also contact angle measurements can be used for studying hydrophobicity/hydrophylicity of the modified surfaces. The data of the contact angle experiments are presented in Figure 6.8. As reference a clean gold substrate was used showing hydrophobic behavior with a contact angle of All chemically modified Au substrate shows a higher hydrophilicity than bare gold except the substrate with the L-Cysteine ethyl ester. This hydrophilicity is due to + the functionalization of the Au surface with COOH and NH 2 / NH 3 which are hydrophilic functional groups. Figure 6.8: Contact angle for the drop of water placed on the chemically modified cleaned Au surface. The dotted white line marks the locus of the contact lines. 86

100 6. ph sensitive SAMs as smart surfaces Results and Discussion After treatment of L-Cysteine with water of ph 10.5 a very pronounced change of wetting behavior is observed. The L-Cysteine after treatment with water at ph 10.5 is more hydrophilic then Au surface modified with l-cysteine at normal ph (~ ph 5.5). When L- Cysteine is treatead with high ph (> ph 10) the carboxyl group is ionized [144]. The big contrast is shown for Au substrate after hydrolization of the L-Cysteine ethyl ester. In Figure 6.9, a topographic AFM image of a laterally structured SAM composed of ODT and L-Cysteine is presented. Stripes appearing bright are assigned to ODT covered regions while the darker areas belong to L-Cysteine covered regions. Figure 6.9: AFM measurements, the topography and line profile of the microstructured ODT/L-Cysteine self assembled monolayer. The corresponding height profile shows periodic changes in the height of the adsorbed molecules. For well-ordered ODT molecules stamped onto clean gold a step height of about 2 nm. The laterally structured ODT-L-Cysteine SAM surface was found suitable for characterization using s-snim measurements. Two distinct frequencies were measured, within to the characteristic absorption regions for L-Cysteine (amino group and carboxyl regions). The one at 1650 wavenumbers is specific for the NH 3 + symmetry deformation mode and the one at 1718 cm -1, is specific to C=O from carboxyl (-COOH) as functional group. As can be observed in the near-field image (see figure 6.10), a strong absorption appears at 1718 cm

101 6. ph sensitive SAMs as smart surfaces Results and discussion Figure 6.10: IRRAS spectrum of L-Cysteine-SAM on gold substrate with corresponding near-field images at two different wavenumbers, 1650 cm -1 and 1718 cm -1 (colored images) 6.4. Conclusion IRRAS measurements enables to differentiate between hydrolyzed and non-hydrolized L- Cysteine ethyl ester. Nevertheless the spatial resolution does not allow for detection of local chemical modifications. However, SNIM enables chemical mapping of micro-patterned surfaces and it is able to resolve periodic contrasts as a function of the wavenumber. It can be considered a promising new tool for surface chemistry, especially for chemically modified surface as a reader tool. Regarding to the sample preparation, future studies should aim at improving the spatial resolution for local chemical modification to be suitable for s-snim measurements. 88

102 7. Chemical image for microphaseseparated block copolymer film 7.1. Materials and methods Materials Substrate cleaning Methods AFM Raman microspectroscopy Results and discussions Imaging of microphase separation of block copolymers using AFM Chemical imaging of microphase separation of block copolymers using Raman microscopy

103 7. Chemical imaging Materials and Methods A brief introduction on microphase-separation of block polymers was presented in Chapter 4, Section 4.1.3). Microphase separation fabrication has the ability to create nanometer sized structures through a self- assembling bottom-up process that can be used to coat surfaces. Pattering of semiconductor surfaces (Si substrate) using block copolymer films is presented in this chapter. The effect of varying the concentration of polymers, changing the surface condition or effects of solvent selectivity are demonstrated as possible methods to change the patterns of the surfaces. Microphase separation is studied using confocal Raman microspectroscopy and AFM. Confocal Raman microspectroscopy is an evolving technique for the rapid, non-invasive chemical imaging of composite microscopic structure in material science, in such a way that it enables the diffraction-limited investigation of both the spatial distribution of the micro-domains components and their chemical nature Materials and methods Materials In this thesis, block copolymer polystyrene-polymethyl-methacrylate, (PS-PMMA), two incompatible polymers are used. Polystyrene ( mw) and polymethyl-methacrylate ( and mw) were purchased from Sigma Aldrich. Molecular structures of both polymers are presented in Figure 7.1 Toluene (98%), acetone (98%) and chloroform (98%) from Fluka are used as selective solvents. Figure 7.1: Molecular structure of PS and PMMA (A) and copolymer block PS-PMMA (B) 90

104 7. Chemical Imaging Materials and Methods Commercial silicon wafers (111) were used as substrates. The silanization process was performed by using dimethyl dichloro silane solution from Sigma Aldrich Substrate cleaning Sample preparation involved two steps: first the silicon substrate was prepared with multiple chemical treatments (degreasing) and activated in boiling water for 4h. The first step is to hydrophil the Si surface for a good adhesion of the polymeric film. The hydrophilized surface was treated with piranha solution (mixture solution: H 2 SO 4 :H 2 O 2, 3:1) to remove organic residues. The degreasing process involved also successive immersion in: a boiling solution of dichloromethane; methanol and acetone, each treatment 10 minute. In between treatments the substrates are dried in a steam of nitrogen. After all these treatments, the substrates are immersed in boiling ultra pure water for 2 h and can be kept in ultra pure water at room temperature for another 2 h to create a high hydroxyl group density on Si surfaces. The Si substrates were stored into water until they were used. The Si surface can be functionalized with dimethyl diclorosilane solution for controlled wetting conditions [101] Methods Atomic force microscopy (AFM) The AFM measurements were acquired using a WITec (Ulm, Germany) Model alpha 300. It is a complex microscope that can perform confocal Raman microscopy, AFM (in contact and tapping mode) and confocal optical microscopy. More details about the set-up can be found in chapter 5. The AFM was operating in tapping mode for all polymer sample measurements. The phase contrast imaging is used to identify more clearly the microphase separation of PS-PMMA copolymer block films. Details about tapping mode (phase contrast) can be found in chapter 3 (section 3.1). Commercial silicon nitrate cantilevers (Nanoworld NCR-10 detector side Al-coating) were used with a tip curvature radius ~ 10 nm. 91

105 7. Chemical imaging Materials and Methods Raman microspectroscopy Raman spectra were acquired using the same WITec (Ulm, Germany) set-up. An achromatic Nikon (Tokyo, Japan) E Plan (100x0.9) objective with a working distance of 0.23 mm and an excitation wavelength of 532 nm was used for all Raman measurements presented in this thesis. The laser power at the sample was estimated to be ~12 mw. The sample is placed on a piezoelectrically driven microscope scan Table x, y resolution of ~3 nm. The sample was scanned while moving stepwise through the laser focus with a constant speed of 0.3 µm per second. The Raman signal from the confocal microscope is directed to the spectrometer using a multimode optical fiber with a core diameter of 50 µm which acts as entrance slit of the spectrometer and was finally detected by a CCD camera, 1028 x 128 pixels. The spectra were recorded with a diffraction-limited spatial resolution of ~32 nm laterally. The scan size of the measurements was: 8x8 µm 2, 150 pixels pert line, 150 lines per image. The spectral resolution was about 4 cm -1. The integration time for single spectra was 1 s and for scanning 0.5 s. Images processing were performed in WITec Project version First, cosmic ray were removed followed by baseline correction using a 5 th order polynomial function. In addition, a Savitzky-Golay linear filter was used for Raman spectra [144]. Thereby integration over selected Raman bands was performed which leads to images and show the distribution of the intensity of single Raman vibrational bands. Basis analysis (from WITec software) was used to create a chemical map of microphase separation of copolymer block PS-PMMA Fabrication of nanopatterns using microphase-separation of block copolymer The polymeric solutions (PS and PMMA) are made using different solvents: acetone, chloroform or toluene. During the process, the concentration of the polymer can be changed. In this thesis concentrations from 0.5 mg/ml to 3mg/ml are used. Different mixtures of both polymers (see Results and Discussion section) are boiled at 90ºC for 2h for creating copolymer blocks. Also, in PMMA case, two different molecular weights are used ( and mw). Block copolymer films are prepared by the spin coating technique (see Figure 7.2), where droplets of a solution of mixed polymer in a volatile organic solvent are deposited on a 92

106 7. Chemical Imaging Materials and Methods spinning hydrolyzed Si substrate. The polymer film spreads by centrifugal forces, and the volatile solvent is rapidly driven off. Figure 7.2: Spin coating technique; thin films of copolymers can give different domain orientations depending on the wetting characteristics of the substrate or on the solvents used. The spin coater used is from Laurell Research Technologies (model WS-400BX- 6NPP/LITE). Cleaned Si substrates are taken from ultra pure water, dried in a steam of nitrogen and flooded with coating solution prior to spin-up. The samples are spun in open air for 60s at speed varying from 500 to 1500 rpm. As spin coating progressed, and the liquid layers thickness became about 1 µm or less, a progression of time-varying interference colors was seen. Film thickness is controlled, for PS-PMMA copolymer block film and different solvent, by the initial concentration of polymers, the initial solution viscosity and the spinning speed. The average film thickness was fitted using the following equation [2]: (7.1) where d is the thickness of the polymer film, A constant - parameter specific to the solvent (0.92 for toluene, 4.3 chloroform and 5.1 for acetone [145]). n and m specific parameter for polymer; c is the concentration of the initial polymer and ω is the speed. 93

107 7. Chemical imaging Materials and Methods Since the color changes stopped within approximately 20s of the spin-up for all concentrations, it can be inferred that the spinning time of 60s was adequate for providing the minimum final film thickness. This time was used for all samples. In the end, the polymeric films were dried in a nitrogen flux and then placed in an oven at 90º C for 30 min, for removing residual organic solvents (toluene, acetone or chloroform). The film thickness can be controlled through the spin speed, the concentration of the block copolymer solution or the volatility of the solvent, which also influence the surface roughness. In this thesis, the speed is constant for all samples and a dependence on the concentration and the solvents is shown Results and discussions Imaging of microphase separation of Block Copolymers using AFM In studies of the topography of thin heterogeneous polymer films, one should be aware of the possible drawbacks of AFM imaging when the topographic differences are a few nanometers in height. Application of the contact mode for these samples could induce sample deformation, which will differ for different sample components due to their different elastic properties. Therefore, the height image will not reflect the surface topography correctly. To this end in all AFM measurements for polymeric thin films, tapping mode is used. A good qualitative impression about sample can be gained using topography and phase contrast of PS-PMMA mixed polymeric film. Also, chemical information about sample can be obtained from phase contrast images. A microphase-separated mixed polymer thin film of PS-b-PMMA block copolymer is displayed in Figure 7.3. The mixed polymer solution were prepared using PS ( mw) with toluene as solvent and PMMA ( mw) with acetone as solvent. Concentration for both polymers is 1 mg/ml. The mixed polymer solution was obtain with a ratio 4:1 (PS- PMMA) for (A) and 2:1 (PS-PMMA) for (B). All other conditions described above were identical for both samples. Scan size for both measurements is 15x15 µm 2. Pixel dimensions are 29 x 29 nm 2. The color scale is not equal for a good visualization of pictures. The line profile is on the same y-scale for a good visualization of the changing topography of the surface. 94

108 7. Chemical imaging Results and Discussion Figure 7.3: Effect of varying the concentration of polymers. AFM images of 4:1 (A) and 2:1 (B) PS-PMMA mixed polymer films; topography, phase contrast and line profile. In Figure 7.3 is displayed a clear micro-phase separation of copolymer block matrix. One polymer creates some islands-ring with a diameter of about 1 µm in the other polymeric matrix as can be observed in Fig.7.3. (A). This behavior can be explained due to the incompatibility of PS and PMMA and solvent selectivity. A significant role in micro-phase separation of copolymer block is the selective solvent: toluene is a selective solvent for PS and acetone is a selective solvent for PMMA. Another important factor is the concentration of the polymer. In Fig 7.3 A, a small amount of PMMA was used. However, if the amount of PMMA increases, the microphase-separation pattern changes (Fig (B)). The polymeric rings are still visible but the surface is much smoother and the presence of the wires which penetrate the polymeric matrix is observed. For these samples, just the amount of PMMA was changed and a clear micro-phase separation of PS-PMMA copolymer block was observed with a good correlation. Therefore, as a first conclusion from these measurements, the PMMA polymer creates island-rings in a PS polymer matrix. Increasing the amount of the PMMA polymer, the density of the rings is higher and some PMMA wires appears in PS polymer matrix. 95

109 7. Chemical imaging Results and Discussion It was shown in Chapter 4 (Section ) that the block copolymer assembly is highly dependent on the wetting characteristics of the substrate. These can be easily controlled by coating the substrate with a self-assembled monolayer. The clean Si substrate is coated with a dimethyl dichloro silane solution resulting in a hydrophobic surface. A mixture of 1:1 PS-PMMA polymer, with chloroform as selective solvent for both polymers is used. In Figure 7.4, the effect of changing the substrate surface properties on the micro-phase separation is shown. Scan size for both measurements is 2.5x2.5 µm 2. Pixel dimensions are 5 x 5 nm 2. The color scale is not equal for a good visualization of pictures. The line profile is on the same y-scale for a good visualization of the changing topography of the surface. Figure 7.4: Effect of changing the surface wettability. AFM images (topography, phase contrast and line profile) of PS-PMMA (1:1 from chloroform) on a (A) hydrophobic and (B) hydrophilic surface. The wettability of the surface plays an important role in microphase-separated mixed polymer film as could be observed in Fig PS polymer tends to create small (~ 100 nm) islands in a PMMA matrix on a hydrophilic surface (Fig. 7.4 B). This behavior was shown in previous paper [ ]. Even if the PS islands are clearly distinguishable, especially in phase contrast, the roughness of the surface is small (line profile for B). 96

110 7. Chemical Imaging Results and Discussion This behavior will be clearly chemically indentify using Raman micro-spectroscopy. On a hydrophobic surface, the tendency of PS polymer to cover the surface with large areas is decreased (fig. 7.4 A). The tendency to create nanophase domains is obviously clear in the line profile plot. Another parameter that can influence the mixed polymeric film is solvent selectivity (see Chapter 4). Also, this parameter will be very exploited in the study of mixed polymer brushes (see Chapter 8). In the mixed PS-PMMA polymer film, chloroform is a good solvent for both polymers and acetone is a selective solvent just for PMMA. This solvent selectivity can induce nanophase-separation in mixed PS-PMMA polymeric film. In Figure 7.5, the effect of solvent selectivity for mixed PS-PMMA polymeric film is shown. Figure 7.5: Effect of changing the solvent. AFM images (topography, phase contrast and line profile) of PS-PMMA (1:1): PS/chloroform and PMMA/acetone on (A) and PS/chloroform and PMMA/chloroform on (B) on the same hydrophilic surface. For both samples, clean Si substrates (hydrophilic surface) were used. Chloroform is known as a selective solvent for PS and PMMA creating flat mixed polymer film (Fig. 7.5 B). For the other film, acetone was used as selective solvent for PMMA. 97

111 7. Chemical imaging Results and Discussion In this case, nanometer sized PMMA islands are formed in the PS matrix. Scan size for both measurements is 1.5x1.5 µm 2. Pixel dimensions are 3 x 3 nm 2. The line profile is on the same y-scale for a good visualization of the changing topography of the surface. A creative strategy has been employed to organize micro-phase separated features as it was shown. By using the properties of individual polymers (PMMA and PS) inherent in diblock copolymer chains it is possible to create patterned surfaces with nanometer features. The micro-phase separation which appears in this diblock copolymer (PS-PMMA) defines a different response of the PS (hydrophobic) and PMMA (hydrophilic) polymers. Concentration of polymers has an effect on the size of the micropater as could be demonstrated in Fig The size of the micro-phase domains and the thickness of the copolymer film are affected by the concentration. Also, the selectivity of the solvent used, can influence these micropatterns. Surface substrate condition can affect micro-phase separated domains. Changing the surface energy, by changing the wetting properties (hydrophilic or hydrophobic) may allows to reorganize other features of microdomains. As could be observed, the self-assembly and structural organization of microphase-separated block copolymer can be directed by shearing forces, surface control of wettability, chemical attachment to surfaces. The ability of the PS-PMMA block copolymer to selfassemble into a wide range of nanoscale architectures has provided lot of opportunities for nanotechnology Chemical imaging of microphase separation of block copolymers using Raman microscopy Confocal Raman microspectroscopy has the potential to combine the sensitivity and selectivity of vibrational spectroscopy with the spatial resolution of a confocal microscope while requiring only minimal sample preparation and small sample volumes. Raman spectra of PS-PMMA copolymer block film explain the microphase separation induced by the polymer (observed by the AFM also) and their chemical composition at the nanometer level. Using diagnostic Raman marker bands, the received Raman images describe the spatial distribution of the structural component of polymers. Several bands in polymer Raman spectra are already assigned to certain polymers (see Table A1). In this thesis, a test sample of a PS-PMMA copolymer film on a glass slide substrate is chosen. The Si wafer has a strong influence in Raman spectra if is used as substrate. Glass slides had similar properties as Si wafer and the Raman signal from glass background does not influence the polymer Raman spectra. 98

112 7. Chemical Imaging Results and Discussion A mixture 2:1 PS-PMMA polymer, with chloroform as selective solvent for both polymers is used, similar with sample that form micro-phase separared domeins like Fig. 7.4 (B). A cleaned glass slide is spin coated with a speed 500 rpm for a thickness film of ~100 nm. The glass slide is used as substrate in order to decrease the Raman background signal which is much stronger for Si wafer used as substrate. The sample is kept in the oven for 30 min at 90 C. Raman microspectroscopy is potentially suitable to detect PS-PMMA microphase separation. Figure 6.6A shows the bright field microscopic image of the sample, taken through a 100x objective. The Raman scan area (8 x 8 µm 2 ) is marked with a white-line rectangle in the optical image (see Figure 7.6.A) Figure 7.6: Optical image of the sample using a 100x objective. (B) Raman average spectra of PS (blue) and PMMA (red). (C) Raman images using integrated Raman intensity of the ν(c=o) stretching vibration in the range from 1720 to 1740) cm -1 (red - PMMA) and υ(=(c-h)) stretching vibration in the range from3040 to3065) cm -1 (blue PS) of the area shown in A, collected with an integration time of 0.5 s/point and a point spacing of ~53 nm (8 x8 µm 2 scan area). Peak assignment is given in Table 6.1 The Raman average spectra for PS and PMMA are shown in Fig. 7.6 B. The red spectrum exhibits a distinct carbonyl C=O stretch vibration at 1730 cm -1 that is specific for PMMA. The blue spectrum exhibits a distinct υ(=(c-h)) stretch vibration at 3056 cm -1 specific to the aromatic ring for PS. Briefly, other prominent bands are at 1452 cm -1 (δ(-ch 2 ) and δ(- CH 3 ) asymmetric), δ(-ch 3 ) at 1380 cm-, υ(-cc) aromatic ring chain vibrations at 1606 cm -1. The last peak, 1606 cm -1 can be considered as specific peak for PS. Distinctive specific peaks in Raman spectra can be observed in the range of C-H bands ( cm -1 ) where the signal in much stronger in Raman than IR. The functional group of aromatic ring (-CH=CH-) can be used as fingerprint peak to indentify PS in PS-PMMA copolymer matrix. 99

113 7. Chemical imaging Results and Discussion Peak center position (cm -1 ) , 2952, 2883 ( ) Vibrational mode υ(=(c-h)) from aromatic ring (PS) υ(c Η) stretch (PS and PMMA) 1730 υ(c=o) stretch (PMMA) υ(-c=c) - aromatic ring chain vibrations (PS) δ(-ch 2 ) and δ(-ch 3 ) asymmetric (PS and PMMA) 1335 δ(-ch 3 ) Table 7.1: Peak assignment for the Raman spectra of PS and PMMA in Figure 6.6. B in the spectral region from 1000 to 3500 wavenumbers. The area mapped by confocal Raman microscopy is displayed in panel C (see Fig- 7.6.). The chemical maps are obtained by using integrated Raman intensities of the ν(c=o) stretch at 1730 cm-1 for PMMA and the ν(=(c-h)) stretch vibration for PS. The chemical maps shown in panel C confirm a variety of PS islands (small red circles in the second image) in a PMMA matrix, observed also in phase contrast from AFM measurements. Basis spectra analysis fit procedure can optimize the signal-to-noise-ratio of Raman images by first measuring Raman spectra of small identified regions of interest and then using this spectrum to generate a basis spectrum by averaging all spectra in this region. These new spectra can generated a new image of the copolymer film surface (sentence structure). The Raman spectra for single polymer PS and PMMA are recorded for comparison with PS and PMMA spectra from matrix. Panel C from Figure 6.7 shows the general microphaseseparation of the PS-PMMA copolymer block film and especially the PS islands in a PMMA polymeric matrix are visible. Using the basis analysis, the spectrum of each pixel of the Raman image is compared to the two average spectra and a color is attributed to each pixel in consequence. The red color in the Panel C reflects the total integrated Raman intensity of the carbonyl vibration band (ν(c=o)) and practically the contribution of PMMA and the blue color reflect the total integrated Raman intensity of υ(=(c-h)) from aromatic ring, contribution for PS, respectively. 100

114 7. Chemical Imaging Results and Discussion Figure 7.7.: PS-PMMA copolymer block film (A), Raman spectra representing PS (blue) and PMMA (red) and PS-PMMA overlapped (green). Panel C shows the color-coded image of PS-PMMA copolymer film (PS blue and PMMA-red). In Figure 7.7. B, the Raman spectra from three different positions is displayed: 1, 2 and 3 respectively. It is clear shown that for Raman spectra 3 (green), an overlapping contribution from PS and PMMA can be observed (grey arrows). In Panel C, a color coded image of microphase separation PS-PMMA copolymer film is shown. It is clearly distinguish the PS micro-domains included in a PMMA matrix, an expected structure for this polymer film. The spatial resolution of these measurements is ~350 nm and it is nearby to the instrument limit (~320 nm). However, below this limit, it is impossible to create a chemical mapping with a spatial resolution below 300 nm. Therefore, Raman microscpectroscpy cannot resolve completely all polymeric structures, even for copolymer block films. The results presented here clearly show a substantial influence of concentration, solvents and surface on copolymer block film morphology. Changing the concentration, solvent or properties of the substrate surface, new designed microphase separated polymer structures with variable domains (from few 1-2 µm up to 100 nm) can be created. 101

115 7. Chemical imaging Results and Discussion Using AFM measurements a nice topographical image of the microdomains with a spatial resolution in the nanometer range can be achieved providing elastomechnanic properties of the sample. However, the chemical information obtained by AFM is poor. A successful differential and chemical identification of microphase separation PS-PMMA copolymer film was obtain using confocal Raman microscopy with a resolution of 350 nm. 102

116 8. Probing local chemical composition of nanophase-separated polymer brushes using SNIM 8.1. Materials and methods Tailoring surfaces-mixed polymer brushes Atomic force microscopy (AFM) Wetting properties - Contact angle Fourier Transform infrared spectroscopy (FTIR) Scanning near-field infrared microscopy (s-snim) Results and discussion Experimental results of microphase - segregation in polymer brushes Wettability switching probed by contact angle Study of chemical functional groups using FTIR spectroscopy Probing local chemical composition using s-snim Conclusions

117 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM Mixed polymer brushes represent responsive materials that allow switching behavior upon change of their environmental medium. As outlined in Chapter 4, section 4.2., polymer brushes are an assembly of polymer chains that are tethered by one end to a surface. The importance of using different analysis methods, spectroscopic, microscopic and other approaches is crucial. In this thesis, FTIR spectroscopy has been used to characterize the chemical composition, of the composing different polymers. Contact angle measurements allow the determination of surface energy, which is depending on the hydrophobicity, or hydrophylicity of surfaces. Additionally information on morphology and functional group distribution on the surface can be derived. AFM has been used to image microphase segregation and characterize morphological changes in mixed polymer brushes due to treatment with different selective solvents. As an ultimate challenge, s-snim was used for chemical identification of polymers in mixed polymer brushes by their chemical fingerprint. Using s-snim, the nanoscale surface analysis including a combined topographic and chemical mapping of the morphologies of mixed polymer brushes formed by different selective solvents in thin film was performed. As an example, polystyrene and poly methyl-methacrylate (PS-PMMA) polymer brushes attached to silicon oxide surface has been studied Materials and methods Tailoring surfaces using mixed polymer brushes The PS-PMMA mixed polymer brushes were synthesized in the Department of Microsystems Engineering, University of Freiburg*. The brushes were prepared by surface-initiated polymerization, grafting from [ ]. Conventionally, mixed brushes are fabricated by first growing one type of polymer via a surface-initiated reaction [ ]. The reaction is stopped before all initiator is consumed and the remaining initiator which is randomly distributed over the surface is utilized for the growth of the second type of polymer. Fluctuation in the local density of grafting of one component can be caused by local variation of the initiator density. These fluctuations can be eliminated by using so-called Y initiators that simultaneously anchor both types of polymer chains [151]. In this thesis, PS-PMMA mixed polymer brushes from conventional and Y initiators will be studied. Briefly, both procedures are described. * by Prof. Dr Svetlava Santer 104

118 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM For the conventional PS-PMMA mixed polymer brushes, the initiator was immobilized on the surface of a cleaned silicon wafer (111). The surface-initiated radical polymerization of styrene and methylmethacrylate (MMA) was carried out through thermal initiator at 60 C. In the first step the polystyrene (PS) homopolymer was grown from the initiator surface (see Figure 8.1). Figure 8.1: Scheme of synthesis of PS-PMMA mixed polymer brushes using conventional initiator with one functional group (cyan) After polymerization, the silicon wafer containing the PS brush was used for growing the second polymer, PMMA. The growth of second polymer was initiated from initiator which was not active during the first reaction. By varying the polymerization time at constant MMA/toluene concentration, mixed brushes with different height were synthesized. Surface-initiated polymerization allows to generate also thick polymer films and was used to synthesis homopolymer brushes of PS (~ 60 nm) and PMMA (~100 nm) as is required for FTIR spectroscopy due to the signal to noise ratio which is too low for thinner polymer films. The synthesis of the PS-PMMA mixed brushes via the conventional method is described in more detail in reference [113, 148]. The Y-shaped PS-PMMA mixed brushes were prepared from an asymmetric difunctional initiator-terminated (Y initiator) by combining two different living radical polymerization techniques - atom transfer radical polymerization and nitroxide-mediated radical 105

119 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM polymerization [149]. The Y-initiator was designed to ensure that the two initiators are well mixed on the molecular scale and thus results in good mixing of the both homopolymers (PS and PMMA). Figure 8.2 displays the scheme of synthesis of Y-shaped PS-PMMA mixed brushes using a Y-initiator. Figure 8.2: Scheme of synthesis of Y-shaped PS-PMMA mixed polymer brushes using Y- initiator with two functional groups (green and magenta) PMMA was grown first from the Y-initiator by atom transfer radical polymerization at 75 C, followed by the nitroxide-mediated radical polymerization of styrene at 115 C. The synthesis of the Y-shaped PS-PMMA mixed brushes using Y-initiator conventional method is described in more detail in reference [149]. Three different types of PS-PMMA polymer brushes will be studied in this thesis: two synthesised by conventional initiator with different thickness (~ 120 nm and ~15 nm) and one sample fabricated by Y-initiator (~ 30 nm). 106

120 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM Atomic force microscopy (AFM) Atomic force microscope (Nanotec Electronics) was used to characterize the morphology of the layers after each solvent treatment. Tapping mode images were acquired using commercial silicon nitrate cantilevers (Nanoworld NCR-10 backside Al-coating) with a tip curvature radius of ~ 10 nm. If I remember right they do not give a good signal in the Nanotec AFM). For image analysis WSxM 4.0 Develop 11.2 (Nanotec Electronica) was used [137]. The roughness of the surface in terms of the root-mean square deviation from mean plane (rms) was calculated taking into account different areas from 1µm 2 to 3 µm Wetting properties - Contact angle Contact angles measurements were performed using a home-made system (an optical microscope Leica and a digital camera Nikon). The system, able to reach a maximum magnification of 120x, was used to acquire images of water droplets (100 µl) on the investigated surfaces. For all measurements, ultra pure water was used. The digital images were then processed with ImageJ, a public domain, Java image processing program developed at the National Institutes of Health (USA) Fourier Transform infrared spectroscopy (FTIR) In order to select the optimum frequency range for near-field contrast imaging, we have first recorded Fourier Transform infrared (FTIR) spectra of pure homopolymer brushes on silicon substrates. FTIR spectra were recorded using a Vertex V80 (Bruker) FTIR spectrometer in transmission mode. Each spectrum was averaged over 256 scans with a frequency resolution amount of 4 cm -1. Spectra were evaluated using OPUS-Software 6.5 (Bruker) ScatteringScanning near-field infrared microscopy (s-snim) SNIM measurements were carried out as described in Chapter 5. Each image was recorded at a scan rate of 0.5 Hz and time constant of 1 ms at the lock-in amplifier. Scan size for all measurement is 5 x5 µm 2. Image processing was performed using WSxM imaging software 4.0 Develop 10.4 (Nanotec Electronica) [137]. Raw images were corrected using a linear flatten function. Raw near-field data were evaluated using Mathematica 5.2 (Wolfram Research). 107

121 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM 8.2. Results and discussion Microphase - segregation in mixed polymer brushes When two incompatible polymers are merged in a mixed brush segregation at the nanoscale occurs. Covalent grafting of the chains to a surface prevents macroscopic segregation but microphase segregation still occurs in order to avoid unfavorable interaction between the incompatible polymers. This behavior is a key to the tailoring of the surface properties and an important ingredient for the switching mechanism of mixed brushes. The segregation depends strongly on the chemical structure and molecular properties of the brushes (e.g. molecular weight, hydrophobicity) as well as on environmental conditions. In these experiments, three different mixed PS-PMMA polymer brushes are used (see Figure 8.3.): Sample 1 PS-PMMA mixed polymer brushes synthesis from a conventional initiator with a thickness of 120 nm* Sample 2 PS-PMMA mixed polymer brushes synthesized from conventional initiator with a thickness of 15 nm*. Si nano-particles with 50 nm diameter are placed on top of the polymer brushes as a reference for calculating near-field contrast. Sample 3 PS-PMMA mixed polymer brushes synthesized from Y initiator with a thickness of 30 nm*, so called Y-shaped PS-PMMA polymer brushes. Figure 8.3: Models of all types of samples used: (A) sample 1 - PS-PMMA mixed polymer brushes (h~ 120 nm); (B) sample 2 PS-PMMA mixed polymer brushes (h~ 15 nm) and (C) sample 3 Y-shaped PS-PMMA polymer brushes (h~ 30 nm) 30 nm height *The thickness of the mixed brushes was characterized using an ellipsometer by Prof. Svetlana Santer, Department of Microsystems engineering, University of Freiburg 108

122 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM The microscopic morphology of PS-PMMA mixed polymer brushes was visualized using AFM. After expose to a particular solvent, the polymer film is rapidly dried by a flow of nitrogen within a few seconds According to the reference [130], acetone is a selective solvent for PMMA, chloroform is good solvent for both polymers PS and PMMA (nonselective) and toluene is a selective solvent for PS. The time of solvent evaporation is much shorter than the characteristic time for transforming one morphology into a different one upon changing the solvent. The morphologies after treatment with a particular solvent are reproducible and the morphology reversibly switches upon exposure to another solvent. After drying, the morphology does not change and is stable for a long time. In the following, solvent stimulated morphological changes of at different samples are presented (sample 1, sample 2 and sample 3). Figure 8.4: AFM topography and phase micrographs of PS-PMMA mixed polymer brushes with conventional initiator (h ~ 120 nm) step by step exposed to different solvents: (A) acetone, (B) chloroform and (C) toluene. White horizontal lines mark origins of topography profiles. Scan size is 3 x 3 µm

123 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM It was found that the different polymer brushes segregate into very distinct morphologies on exposure to solvents of different selectivity. Figure 8.4 displays the morphologies of sample 1 step by step exposed to solvents with different selectivity. After treatment with acetone (A), which is a better solvent for PMMA than for PS, topography and phase show a characteristic nanophase separation into dimple like structures with both lateral and perpendicular segregation. The segregation of the two polymer components perpendicular to the surface results in an enhancement of the PMMA at the top of the brush. When the samples were exposed to the toluene (B), which has an increased selectivity for PS, the components of the mixed polymer brushes create a laterally segregated ripple phase comprising worm-like domains and almost flat topographies. Treatment with chloroform (C) which is a good solvent for both polymers, PS and PMMA, results in a ripple -like brush morphology but more flat that in case of the toluene. The surface flatness can be characterized by its root-mean-square roughness by: R (8.1) where N N is the number of pixels, h nm is the height value of the pixel nm and is the mean height of the pixel calculated from the NxN values and by its average roughness*. Corresponding root-mean-square surface roughness of the surfaces (R rms ) are: 5.5 nm (for acetone), 4.5 nm (for chloroform) and 5.12 nm (for toluene ) over an area of 3 µm 2. In order to study the influence of the film thickness on the solvent stimulated microphase segregation a thinner mixed polymer brush (sample 2, (h~ 15 nm)) was used. Figure 8.5 displays the morphologies of sample 2 after exposure to acetone and chloroform. The white spheres in the topographical images represent Si nano-particles of 50 nm diameter which are placed on top of the brushes as a reference for later s-snim measurements After exposure to acetone clear nanopattern formation was observed. Height differences of up to 16 nm (R rms of 4.2 nm over an area of 2.5 µm 2 ) indicate a rugged surface topography (A). After exposure to chloroform, which is a good solvent for both polymers, the surface relief becomes much less pronounced. A rather flat topography with a roughness of (R rms ) ~ 1.3 nm (over an area of 1.5 µm 2 without Si nanoparticles.) is obtained, and it becomes very difficult to distinguish the different polymers solely upon AFM measurements (B). The phase micrograph of the brushes after chloroform exposure show a pronounced distribution of two different phases due to the different elasticities of of PMMA and PS. The Young modulus ofpmma is higher than that of PS. * average roughness: R 110

124 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM Figure 8.5: AFM topography and phase micrographs of PS-PMMA mixed polymer brushes with conventional initiator (h ~ 15 nm) after exposureto different solvents: (A) acetone, (B) chloroform. White horizontal lines mark origins of topography profiles. Scan size is µm 2. As could be observed in both cases, treatment with acetone results in microphase segregation with a dimple like morphology. The domains are larger for the thick polymer brush sample (sample 1) than for the thinner film (sample 2). Also after chloroform exposure a similar surface morphology for both kind of samples was observable However, for the thin polymer brush, the topography became smoother and it became more difficult to distinguish both polymers. Similar treatments were applied to a Y-shaped PS-PMMA mixed polymer brush (sample 3). After treatment with acetone the roughness (R rms ) of the surface amounts to 3.1 nm (over an area of 3x3 µm 2. The height of the pattern increased up to 14 nm and the domain pattern can be described as dimple -like similar to conventional mixed polymer brushes (see Figure 8.6.A). 111

125 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM Figure 8.6: AFM topography and phase micrographs of Y-shaped PS-PMMA mixed polymer brushes with Y-initiator (h ~ 30 nm) step by step exposed to different solvents: (A) acetone, (B) chloroform and (C) toluene. White horizontal lines mark origins of topography profiles. Scan size is 3 x 3 µm 2. After treatment with chloroform, the topography becomes (use present) smoother, with a roughness (R rms ) of 2.2 nm (Figure 8.6.B). The height of the features for Y-shaped brushes is ~7 nm, while the depth of the holes [153] (black points that appear clear in topography and are different in phase) in brush is ~6 nm. A ripples -like features appear unclear compared to the conventional mixed PS-PMMA polymer brushes. After toluene exposure (selective solvent for PS) the nanophase separated domains appear more pronounced and the roughness of the surface is increased (R rms = 2.9 nm- over an area of 3 x3 µm 2 ). The phase image shows two components (C). 112

126 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM As a conclusion, it was observed that for all studied PS-PMMA polymer brushes, the solvent quality (selectivity) has a strong influence on the pattern formation. Furthermore the microphase segregation is influenced by the thickness of the polymer film. These observations are in good agreement with previous studies [129, 130, 148, 153] Wettability switching probed by contact angle measurements Changes upon exposure to different solvents are reversible implying that the surface properties of the mixed brush (e.g. wetting, adhesion, and adsorption) can be changed over many cycles. This can be demonstrated on the macroscopic scale for example by wetting experiments and quantified by contact angle measurements. Contact angle measurements are very sensitive to the very top composition of a thin film. This method was successfully used to investigate smart properties-switching mechanism in mixed polymer brushes [ ]. After exposure of the mixed brushes to different solvents, the polymer brushes are freezed in the morphology while the solvent is rapidly evaporated in a stream of nitrogen. Afterward the wetting behavior is probed by measuring the static contact angle of a water drop in contact with the brush. The experiments demonstrate the reversibility and reproducibility of the switching behavior as well as the high sensitivity of the mixed brushes to exposure to different solvents. In the case of PS-PMMA mixed brushes, the contact angle changes from 90 to 65º upon exposure to toluene and ethanol, respectively. The data of contact angle experiments are presented in Figure 8.7. Figure 8.7: Switching of the surface state of a mixed PS/PMMA brushes upon exposure to various solvents. The samples were exposed to a solvent as indicated in the graph. After drying the water contact angle was measured. 113

127 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM The different solvents induce changes in the average surface morphology and composition of the top layer. For example, exposure to toluene enriches PS in the top layer, while PMMA predominantly occupies the top layer when exposed to ethanol. Chloroform is a good solvent for both polymers and changes the film brush to a flat state with neither polymer enriching top most layers. As a reference for all visualized the hydrophobicityhydrophilicity properties, the visible images of PS-PMMA mixed polymer brushes after different exposure solvents (toluene-ethanol) are compared with cleaned Si wafer (a hydrophilic surface) Study of chemical functional groups using FTIR spectroscopy Figure 8.8 shows FTIR absorbance spectra of a PS (Fig. 8.8.A blue line) and PMMA polymer film (Figure 8.8.A - red line) with a thickness of about 100 nm for each as well as for PS (Figure 8.8.B - blue line) and PMMA (Fig 8.8.B - red line) homopolymer brushes with a thickness of 90 nm for PMMA homopolymer brushes and 50 nm for PS homopolymer brushes. Figure 8.8: FTIR absorbance spectra of PS (blue line) and PMMA (red lines). A: PS and PMMA as pure polymer films with a thickness of ~ 100 nm and B: PS (~ 50nm) and PMMA (~ 90nm) as homopolymer brushes. 114

128 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM Figure 8.9 displays a zoom in the low frequency region ( 1250 to 1760 cm -1 ) containing the characteristic region carbonyl and aromatic chain vibrations and the region of the C-H stretching (2750 to 3150 cm -1 ) enlarged. Figure 8.9: FTIR absorbance spectra of PS (blue) and PMMA (red) : the low frequency region (1250 to 1760 cm-1) containing the characteristic region carbonyl and aromatic chain vibrations and the region of the C-H stretching (2750 to 3100 cm -1 ) enlarged. A-pure polymer films and B homopolymer brushes. The blue bare shows the fingerprint region used for SNIM (due to the C=O vibration mode). Peak assignment is given in Table 8.1 There are obvious differences in the spectral response of both polymers The shadded blue region in Fig represents the spectral range used for later near-field measurements. The spectral range from 1700 to 1750 wavenumbers is characteristic for the (ν(c=o)) vibration mode. PMMA (pure polymer film and homopolymer brushes) shows a prominent peak in this region. This mode is centered at 1740 cm 1 and caused by the C=O stretching vibration of the ester group ( CH 2 COOCH 3 ). For PS homopolymer brushes, this vibration can be observed due to the influence of the initiator used in polymer brushes fabrication. The long frequency range of the spectra contains the signature of the C-H stretching vibrations for both molecules which appear sharper for the PS in comparison to PMMA as pure polymer film. 115

129 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM This is due to the influence of the aromatic ring and the influence of the methyl group. For PS homopolymer brushes, the polymer film is much thinner (PS ~ 50nm) then for PMMA (~90 nm) and therefore a good comparison between both is difficult. The anti-symmetric stretching vibration (ν(ch3) of the C CH 3 group occurs at 2956 cm -1 and anti-symmetric stretching vibration(ν(ch 2 )) of -CH = CH from the aromatic ring occurs at 3010 cm -1. In contrast to PS, PMMA brushes show an intense absorption at approximately 1740 cm-1 due to the C=O stretching vibration of the methyl methacrylate (MMA) group. Peak center position (cm -1 ) PMMA Vibrational mode Functional group Peak center position (cm -1 ) PS Vibrational mode Functional group δ w (CH 2 ) -(CH 2 ) n 1310 δ w (-CH 2 ) -(CH 2 ) n δ s (CH 3 ) - CH 3 δ s (CH 3 ) δ s (CH 3 ) - CH 3 δ(ch 2 ) -(CH 2 ) n 1470 δ(ch 2 ) -(CH 2 ) n ν as (O=C-O - ) RCOO - ν(c=o) ν s (CH 2 ) ν as (CH 2 ) -COO-CH3 carbonylester group -(CH 2 ) n -(CH 2 ) n ν as (CH 3 ) C-CH ν(c=c) ν s (CH 2 ) ν as (CH 2 ) cisch=ch aromatic ring -(CH 2 ) n -(CH 2 ) n ν as (CH 3 ) C-CH 3 ν as (CH) cis(ch=ch) aromatic ring Table 8.1: Peak assignment for the FTIR absorbance spectra of PMMA and PS in Figure 8.8 and 8.9 in the spectral region from 1200 to 3100 wavenumbers. The specific peaks for PMMA and PS are pointed out. 116

130 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM Probing local chemical composition of polymer brushes using s-snim In this thesis, s-snim is used as a new analytical tool to probe the chemical composition on the nanometer scale. Evanescent waves are generated at an interface and probed by a tip, which acts as a scatterer to convert the sample-bound near-field into propagating waves that are collected in the far field by a conventional lens system. Local variations in the chemical composition of the film cause a change in the local dielectric function which in turn influences strongly the amplitude of the scattered light. If the sample surface is scanned the resulting image represents the ac amplitude of the scattered IR radiation. In the former case, whereas the refractive index of polymers is essentially the same as at visible wavelengths, the polymers vary significantly in the vibrational infrared groups as a result of absorption resonances giving rise to dispersive features in the refractive index via Kramer-Kronig relation This is demonstrated in all near-field measurements presented in this thesis where PMMA brushes are detected/identified in PS-PMMA mixed polymer brushes films with different morphological features. All results are based on differences in the refractive index in a vibrational infrared region of PMMA compare with PS. As first example, the PS-PMMA mixed polymer brushes with a thickness of ~ 120 nm was used. In order to map the chemical distribution of the PS-PMMA mixed polymer brushes the CO-laser was tuned through the C=O absorption band (1675 cm -1 to 1835 cm -1 ). SNIM signal/scattering intensity is recorded for each pixel. In order to obtain chemical information it is necessary to measure the frequency dependent response resembling the characteristic absorption for each polymer. Therefore we have tuned the CO-laser to distinct frequencies (1679, 1708, 1711, 1720, 1738, 1742, 1754, 1765, 1780, 1800, 1812, 1835 cm -1 ) and recorded the scattered light for conventional PS-PMMA mixed polymer brushes with a thickness of ~ 120 nm (you wrote the thickness already above. Try to merge this two sentences). From AFM measurements it is known that conventional PS-PMMA mixed polymer brushes show a clear microphase separation in topography even after chloroform exposure. From preparation it is known that the height of PS polymer brush (~100 nm) > height of PMMA polymer brush (80 nm) [130] Fig shows exemplarily simultaneously recorded topographic and near-field infrared images of conventional PS-PMMA mixed brush after exposure to chloroform at four specific frequencies (1728 cm -1, 1738 cm -1, 1780 cm -1, 1835 cm -1 ). In all micrographs the same pattern is visible; however the contrast is changing due to the frequency dependent dielectric properties of the PMMA. 117

131 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM Near-field infrared contrast values, C, are calculated for each frequency according to: (7.1) where I sample and I reference are the scattering intensities of PMMA and PS, respectively. As reported recently, PS can be used as reference for extracting the spectral near-field contrast of PMMA. Its very weak absorption near 1750 cm -1 due to the C=O vibration from SAM-initiator does not significantly alter the spectral shape of the PMMA near-field contrast [36]. The marked area was used to calculate the near-field contrast of PMMA relative to the PS. From topography image, the black selected area correspond to the PMMA polymer brushes, and white selected area correspond to the PS polymer brushes, due to the difference in heights. The extracted near-field contrast of PMMA relative to PS was obtaining by averaging over the black - white regions indicated in the topography. Several other areas were chosen to calculate the near-field infrared contrast of PMMA to PS to have a good statistics. As can be observed, the near field images show that the nearfield contrast between PS and PMMA polymer brushes changes with frequency and even reverse. The diagram in Figure 8.10 shows the frequency dependent near-field contrast of PMMA with a resonance around 1740 cm -1, which is attributed to the C=O stretching vibration. For comparison, the FTIR absorbance spectrum of thin-film PMMA is displayed. The profile of the obtained near-field contrast for PMMA is in good agreement with theoretically calculated scattering amplitude [154] as well as with previous studies for bulk coblock polymer films [55, ]. PMMA has a characteristic resonance in both the absorption coefficient as well as the refractive index, both of which will contribute to the near-field contrast. As a consequence, the frequency dependence of the contrast C is a convolution of absorption and the refractive index. The PMMA could be easily identified because of the different contrasts in the 120 nm-thick polymeric matrix between PS and PMMA, near to 1740 cm -1 As it was proven previously, the formation of micro-phase separated domains depends also on the thickness of the film. For a thin film, the influence of the solvents is different than for a thick film. 118

132 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM Figure 8.10: Simultaneously recorded topographic and near-field images (different wavelengths) of 120 nm thick conventional PS-PMMA mixed brush after exposure to toluene. The diagram displays the FTIR absorbance spectrum of PMMA (gray) and the experimental near-field contrast for PMMA (red triangles). The black line severs as guide for the eyes. The extracted infrared near-field contrast of PMMA relative to PS originates from the marked area. Scan size: µm 2. For thin mixed polymer brushes, as it was shown from direct AFM measurements, after exposure to chloroform, which is a good solvent for both polymers, the surface relief becomes much less pronounced for a 15 nm thick PS-PMMA conventional mixed polymer brushes film (sample 2). Although the nanophase separation is still present, it is challenging to distinguish the different polymer brushes with AFM. Due to the nonselectivity of the chloroform for PS-PMMA mixed polymer brushes, AFM cannot determine surface chemical composition information and therefore cannot answer which 119

133 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM polymer enriches the surface. As mentioned previously, s-snim simultaneously provides topographic and chemical composition information, and is ideal for mapping the chemical landscape of very flat surfaces. It was shown previously that the local variations in the chemical composition of the film cause a change in the local dielectric function which in turn influences strongly the amplitude of the scattered light. If the sample surface is scanned the resulting image represents the ac amplitude of the scattered IR radiation. This will change considerably when the frequency coincides with a resonance in absorption. Figure 8.11 shows the results of the simultaneously recorded topographic and s-snim images for different wavenumbers, on resonance (1754 cm -1 ) and off-resonance (1660cm -1 ) after exposure to chloroform. Figure 8.11: Simultaneously recorded topographic and near-field images (1660 cm -1 and 1754 cm -1 ) of conventional PS-PMMA polymer brushes after exposure to chloroform. The bright, circular features correspond to 50 nm diameter Si nanoparticles, which are used as a reference (circled areas). The extracted infrared near-field contrast of PMMA and PS relative to Si nanoparticles originates from the marked area. Scale bar is 540nm and scan size is µm 2 As reference for contrast calculation of both polymer brushes (PMMA and PS), the Si nanoparticles highlighted by a outlines in topography and near-field images (black circle) are used. Chemical specificity of both polymers is obtained by tuning the CO laser through the C=O absorption band, e.g. from ca cm-1 to ca cm -1 (1680, 1730, 1754, 1780, 1804, 1820 cm -1 ) and calculating the wavelength dependence of the contrast. Contrast values, C, are calculated for each wavelength according to equation (7.1). Since the emitted laser 120

134 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM power changes with frequency, the frequency independent scattered light of Si nanoparticles is used as a reference. With regard to equation 7.1, I sample and I ref are the scattering intensities of PS and Si nanoparticles, respectively. As it was proved that PMMA shows a characteristic resonance in both the absorption coefficient as well as the refractive index, both of which will contribute to the near-field contrast. As a consequence, the frequency dependence of the contrast C is a convolution of absorption and the refractive index. Figure 8.12 shows the frequency dependent near-field contrast of chloroform-exposed PS-PMMA conventional mixed polymer brushes. The PS (blue dots) has a weak frequency dependence whereas the PMMA (red dots) has a strong frequency dependence showing a resonance around 1740 cm -1 (grey area) which is attributed to the C=O stretching vibration. For comparison, the theoretical 3 rd order scattering amplitude from Ref. [154] for a 10 nm thick uniform thin-film of PMMA, based on interacting dipoles of a layered system, is shown (green curve). Figure 8.12: Experimentally determined near-field infrared contrast C of mixed polymer brushes (PS-PMMA) compared to the Si-nanoparticles after exposure to chloroform. The black lines serve as a guide to the eye. Also shown is the theoretically predicted near-field contrast of PMMA compared to bulk SiO 2 [154]. For these predictions thickness of 10 nm was assumed. In the experiments it was recorded the contrast for polymer brushes of 15 nm thickness. In both cases, Si nanoparticles were used as reference. 121

135 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM The extracted near-field contrast of PS and PMMA relative to Si nanoparticle is obtained by averaging over white and dark region, observed in rectangular region in the topography and near-field images. As can be observed, there is a very good agreement between the experimentally determined frequency-dependent contrast for PMMA and the theoretical scattering amplitude. Also, the measured frequency dependent contrast shows an unambiguous profile for each compound with a clear resonance around 1740 cm -1 for PMMA. Based on the near-field contrast at 1754 cm -1, it is possible to obtain a chemical map of the solvent exposed nanophase separated brushes (see Figure 8.13). Figure 8.13: Topography and color-coded chemical images color at three different frequencies (1660, 1754 and 1820 cm -1 ) of 15 nm thick PS-PMMA conventional mixed polymer brushes after exposure to chloroform showing the PMMA and PS regions. The black objects are the Si-nanoparticles used as a reference. The resulting color-coded chemical image at different wavelengths shows the overall distribution of PMMA ( bright or light gray in topography) and PS ( dark in topography) regions after exposure to chloroform. The white (topography) or black spot (near-field images) is a Si-nanoparticle. Another important paramenter for micro-phase separated features on polymer brushes is the shape of the initiator. A different initiator used in this thesis is the Y-shaped initiator. Further experiments are performed with 30 nm thick Y-shaped PS-PMMA mixed polymer brushes (sample 3). Using only nanomechanical information, as obtained by AFM, it is difficult to clearly distinguish the two polymer components locally. As it was shown in Figure 8.6. after different solvent exposures, the morphology of Y-shape PS-PMMA polymer brushes was change significantly but micro-phase separation were similar. For solvents with specific selectivity, as toluene for PS and acetone for PMMA, the morphology composition is changed and the top layer of the polymer brush changes. Using only nanomechanical information, as obtained by AFM, it is challenging to clearly 122

136 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM distinguish the two polymer components locally since AFM cannot provide information on the local chemical composition. For s-snim measurements, since topography and phase are recorded simultaneously with infrared near-field contrast the chemical morphology can easily be correlated with those. Also, PS is used as reference for extracting the spectral near-field contrast of PMMA. This polymer shows a characteristic resonance in both the absorption coefficient as well as the refractive index, both of which will contribute to the near-field contrast. As a consequence, the frequency dependence of the contrast C is a convolution of absorption and the refractive index. Figure 8.14 displays simultaneously recorded topographic and s-snim images of acetone and toluene exposed Y-shaped PS-PMMA mixed polymer brushes on resonance (1754 cm - 1 ) and off-resonance (1797 cm -1 and 1820 cm -1 ). Figure 8.14: Simultaneously recorded topographic and infrared near-field images (different frequencies) of Y-shaped PS-PMMA mixed brushes after exposure to different solvents: acetone and toluene. The extracted infrared near-field contrast of PMMA relative to PS originates from the marked area. Scan size is: nm

137 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM Figure 8.15 predicts the experimental near-field contrast of PMMA relative to PS in Y- shaped PS-PMMA mixed brushes after treatment with acetone and toluene by using equation 7.1. Figure 8.15: FTIR transmission spectrum of thin-film PMMA (gray) and infrared nearfield contrast C of PMMA calculated from 30 nm thick Y-shaped PS-PMMA mixed brushes after exposure to acetone (red) and toluene (purple). Lines serve as a eye-guide. As can be seen in Fig. 8.16, it is essentially impossible to clearly distinguish between PS and PMMA regions based solely on topography (lower gray images). Based on the nearfield intensities at 1754 cm -1, it is possible to correlate the chemical morphology of the sample with its topography, as detected by AFM. Both polymers can be clearly distinguished according to their scattered near-field intensity (upper colored images). 124

138 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM Figure 8.16: - 3D topography (gray) and near-field (colored) images at 1754 cm -1 of 30 nm thick Y-shaped PS-PMMA mixed brushes after exposure to acetone (left) and toluene (right) the text in the images is too big If the topography is followed, similar features can be observed. If the topography is correlated with near-field images, a clear difference is visualized. Big features in morphology of Y-shaped PS-PMMA polymer brushes after acetone exposure can be attribute to PMMA. After toluene exposure, is it clear that the top-brush is PS due to the inverted absorption at the same wavelength. When comparing the line plots of topography and the near-field intensity signal (see Figure 8.17) it becomes obvious that the lateral resolution does agree with the topographic resolution and is ~ 80 nm (λ/70). By calculating the near-field contrast, as described above, we could prove that the raising features in the case of acetone exposure correspond to PMMA whereas after treatment with toluene PS is on top of the brush. These results are conforming to other polymeric studies. The combination of both chemical and topographic information can provide critical information on the chemical landscape of polymer surfaces and promises to be a valuable tool for characterizing and improving the design of smart materials on the nanometer scale. 125

139 8. Chemical composition of nanophase-separated polymer brushes probed by SNIM Figure 8.17: Topography and near-field line profile for Y-shaped PS-PMMA mixed brushes after exposure to acetone Conclusions In this chapter new tailoring materials with smart response were presented. It was proven that chemical mapping based on distinct dielectric properties of polymers with high spatial resolution is possible. At a wavelength of 5.75 µm, a lateral resolution of ~ 80 nm was achieved comparable with a diffraction limit resolution of λ/70. The present results are the first measurements of a new kind of diagnostic techniques of mapping chemical landscapes on the nanometer scale. 126