Selected Characterization Techniques

Transcription

1 Selected Characterization Techniques Tip-enhanced Raman spectroscopy Scanning helium ion microscope Magnetic resonance sub-nanometer imaging Terence Kuzma

2 Outline Tip-enhanced Raman spectroscopy (Lecture 1) Scanning helium ion microscope (Lecture 2) Magnetic resonance sub-nanometer imaging

3 Introduction This presentation will cover some unique characterization methods that could be employed to interrogate bottom up materials. In some cases we will review a related conventional characterization method, then detail how the method is enhanced for special applications. The first method under review uses light as an interrogating force, the second method uses ions, and the third method measures magnetic displacement. To fully understand these methods, we will contrast other methods, conceptually build a system model, then review the detailed physics to fully understand the method and limitations.

4 Tip-enhanced Raman spectroscopy Tip-enhanced Raman Spectroscopy (TERS) is a refined method of Surface Enhanced Raman Spectroscopy (SERS) TERS is desirable for detailed molecular information because it offers a stronger signal than SERS So the structure of this discussion will be: Overview SERS Understand the physics of the light to material interaction Discuss how TERS is an enhancement of the SERS technique

5 Tip-enhanced Raman spectroscopy Let s first look at the parent technique, then the enhanced technique. SERS is a vibrational technique similar to the acoustic interaction of a wine glass from a tuning fork. Using this analogy for Raman, the tuning fork is indirect coupling of high frequency radiation such as light, and the chemical bonds act as the wine glass to absorb and shift the radiate energy. The radiant energy shift is then correlated to the chemical makeup.

6 Tip-enhanced Raman spectroscopy It is important to state that Raman differs from IR absorbance techniques such as FTIR, SERS is a reflection measurement SERS is very sensitive to chemical bond length, strength, stress, strain, and arrangement in a material. SERS is often used to derive structural characterization information, more than chemical analysis Often used in pharmaceutical quality control, and and materials such as nanotubes.

7 Raman spectroscopy The Raman effect is relatively weak, so high power monochromatic continuous gas lasers are used to excite the bonds. This inherent weakness is a reason to desire TERS. Laser (photon) energy excites the chemical bond to a higher energy state Rayleigh scattering can occur from this interaction. Rayleigh scattering is the radiated wave is the same frequency of the input wave Electromagnetic radiation from the illuminated spot is collected and provides both elastic and inelastic information

8 Raman spectroscopy There are 3 major light peaks that are reflected from the laser/sample interface providing chemical bond interaction. The strongest radiation is the Rayleigh line. This radiation is the elastic interaction of the laser and the bond. This line is often filtered from the signal to better analyze other interaction. A second lower energy interaction denotes residual vibrational energy is known as Stokes scattering. This re-radiation has a lower frequency than the input signal. It is a reflected wave that is derived from the adsorption of the laser to vibrational modes in the sample

9 Raman spectroscopy Systems can be large and complex, and also smaller instruments can be used for macro drug and explosive detection by law enforcement. Even this handheld unit features sophisticated algorithms automatically determine presence of mixed and contaminated chemicals, and an extensive substance library

10 Raman spectroscopy Vibrational energy of the chemical bond is deduced from measuring the difference between the frequency of the Raleigh line and the Stokes line. This identifies the state of the bond This state shows chemical bonds and the stress and strain on the sample. This information is collected as wavenumbers, and this provides a fingerprint of the material under test. Another spectral line is formed from thermal excitation of the chemical bonds. This spectrum is known as the Anti- Stokes line. It mirrors the Stokes line and has higher frequencies with reference to the Raleigh line.

11 Raman spectroscopy The Anti-Stokes line has the same distance from the reference Rayleigh line as the Stokes line because it reports on the same bonds. The difference is that the Anti-Stokes incorporates ambient thermal energy. Therefore the Anti-Stokes line mirrors the Stokes line and has higher frequencies with reference to the Raleigh line. In a typical powdered substance, the intensity Raman scattering is roughly 10 million times less than the intensity of Rayleigh scattered light, and therefore very sensitive instrumentation is needed for Raman spectroscopy. This gives rise of the need of TERS discussed later.

12 Raman spectroscopy Elastic and inelastic scattering

13 Raman spectroscopy Schematic illustration of the Stokes (down-conversion) and anti-stokes (upconversion) shifts. Solid and dotted lines represent absorption and emission spectra, respectively.

14 Raman Spectroscopy Energy Modes

15 Raman spectroscopy Jablonski diagram

16 Raman spectroscopy Jablonski diagram A Jablonski diagram illustrates the electronic states of a molecule and the transitions between them Aleksander Jablonski s diagrams shows a portion of the possible consequences of applying photons from the visible spectrum of light to a particular molecule opy/electronic_spectroscopy/jablonski_diagram

17 Raman spectroscopy Jablonski diagram Within each column, horizontal lines represent eigenstates for that particular molecule. Bold horizontal lines are representations of the limits of electronic energy states

18 Raman spectroscopy Jablonski diagram The transition shown is the absorbance of a photon of a particular energy by the molecule of interest. Electrons then transitions to a different eigenstate corresponding to the amount of energy transferred. This transition will usually occur from the lowest (ground) electronic state due to the statistical mechanical issue of most electrons occupying a low lying state at reasonable temperatures.

19 Raman spectroscopy Jablonski diagram Once an electron is excited, there are a multiple paths that energy may be dissipated. Vibrational relaxation, a non-radiative process. This is indicated on the Jablonski diagram as a curved arrow between vibrational levels. Vibrational relaxation is photon energy given to other vibrational modes as kinetic energy. Internal conversion is identical to vibrational relaxation, where an excited electron can transition from a vibration level in one electronic state to another vibration level in a lower electronic state.

20 Raman spectroscopy Jablonski diagram This figure shows the kinetic energy transfer in the electron states with in the material

21 Raman spectroscopy Jablonski diagram Fluorescence is a pathway for molecules to deal with photon energy. Fluorescence is most often observed between the first excited electron state and the ground state, because higher energies are more likely to be dissipated through internal conversion

22 Raman spectroscopy Jablonski diagram Due to the large number of vibrational levels between electronic states, measured emission is usually distributed over a range of wavelengths

23 Raman spectroscopy Jablonski diagram Intersystem crossing is another path a molecule may take in the dissipation of energy. This where the electron changes spin multiplicity from an excited singlet state to an excited triplet state.

24 Raman spectroscopy Jablonski diagram Intersystem crossing is a forbidden transition, that is, a transition that based strictly on electronic selection rules should not happen. Intersystem crossing leads to several interesting routes back to the ground electronic state including phosphorescence

25 Raman spectroscopy Stokes absorption is quantified as wavenumbers. The family of absorption represented by wavenumbers gives a fingerprint of the material under test.

26 Raman spectroscopy A typical system from Thermo Scientific Modular, able to change the pre-aligned laser, filter, and gratings without any tools This allows rapid analysis of unique materials without recalibrating the system

27 Raman spectroscopy Pre-aligned laser Blue or green lasers can be good for inorganic materials and resonance Raman experiments (e.g., for carbon nanotubes and other carbon materials) and surface enhanced Raman scattering Red or near infra-red ( nm) are good for fluorescence suppression Ultra-violet lasers for resonance Raman on biomolecules (such as proteins, DNA, and RNA), and fluorescence suppression Uses corresponding filter, and gratings This technique of customizing a characterization chamber is quick, but also add integrity to the test. The same hardware is used every time for similar materials under test.

28 Tip-enhanced Raman spectroscopy Tip enhanced Raman scattering (TERS) combines surface enhanced Raman spectroscopy (SERS) with Raman-AFM analysis. TERS narrows the Raman scattering near the point of a near atomically sharp probe that is typically coated with gold. TERS uses a metallic (usually silver-/gold-coated AFM or STM) tip to enhance the Raman signals of molecules situated in its vicinity. Spatial resolution is approximately the size of the tip apex (10 30 nm). Detection of Carbon Nanotubes using Tip-Enhanced Raman Spectroscopy By Jia Wang, Xiaobin Wu, Rui Wang and Mingqian Zhang

29 Tip-enhanced Raman spectroscopy TERS is intended to solve some intrinsic issues with SERS. The spot size or interrogation size of the area of interest with SERS is certainly greater than a few molecules. So TERS refines this method by making the interrogation area equal to the sharpness of an AFM tip. Typical tip radii could be 10nm, providing an equivalent interrogation area SERS has limitations because the Stokes emission light energy is very small. This is reduced even further by the desire to interrogate small areas.

30 Tip-enhanced Raman spectroscopy Given these issues and constraints, TERS offers an eloquent and difficult to physically achieve solution. An AFM is used to bring a sharpened probe tip in very close proximity to the sample. This is important because electric field usually decease as a function of 1/r 2. So the sensor needs to be in close proximity of the sample for a strong signal. This is still a very small signal, and fortunately the tip radius can become essentially a place to confine and integrate plasmon resonance. This plasmon resonance enhances the strength of the laser material interaction

31 Tip-enhanced Raman spectroscopy As shown, TERS combines surface enhanced Raman spectroscopy (SERS) with AFM/plasmon reduce interrogation area with plasmon aided signal strength enhancement. analysis. TERS narrows the Raman scattering near the point of a near atomically sharp probe that is typically coated with gold. RS_Tip_enhanced_Raman_Scattering_APP_Note_RevA1.pdf

32 Tip-enhanced Raman spectroscopy Tip-enhanced resonance Raman and resonance Raman spectra of Brilliant Cresyl Blue (BCB) at surfaces a) TERS and RRS of BCB at Au(111) b) TERS and RRS of BCB at Pt(111) Note the enhanced signal when the probe in in close proximity of the sample

33 Tip-enhanced Raman spectroscopy One monolayer of a DNA base (adenine, thymine, guanine or cytosine) is adsorbed on a Au(111) subtrate. When the gold tip is brought into tunneling contact, a strong enhancement of the Raman intensity is observed; if the tip is retracted, no Raman signal is detectable

34 Outline Tip-enhanced Raman spectroscopy Scanning helium ion microscope Magnetic resonance sub-nanometer imaging

35 Scanning helium ion microscope Scanning helium ion microscope, (SHIM), is a microscope similar to a conventional SEM SHIM offers a sharp images with a large depth of field on a wide range of materials SHIMs uses the short De Broglie wavelength of the helium ions, which is inversely proportional to their momentum = wavelength, h = Planks constant p = momentum

36 Scanning helium ion microscope SHIM has a very small wavelength due to the very low momentum of the helium ion SHIM shares the ability of ion systems that allow sample milling and adding observation levels at sub-nanometer resolution Compared to an SEM, the secondary electron yield is high, allowing for imaging with currents as low as 1 femtoamp Recall that secondary electrons provide topographic information The this will result in an image that shows all high and low points of the sample in great detail and focus.

37 Scanning helium ion microscope Sample damage is very, very low due to relatively light mass of the helium ion Detectors provide information-rich images which offer topographic, material, crystallographic, and electrical properties of the sample

38 Scanning helium ion microscope Electron beams have a large scattering effect and interact with a broad area. This causes the emission of secondary electrons from an area that is larger than the beam size The smaller the area of surface interaction, the higher will be the ultimate image resolution.

39 Scanning helium ion microscope Helium ions are about 7,000 times heavier than electrons and neon ions are 40,000 times heavier than electrons. Because of this, helium or neon ion beam exhibits very little diffraction when passed through an aperture or across an edge

40 Scanning helium ion microscope Non-conducting samples can be probed without metallic coatings, (as compared to the SEM) Since the impinging ion is positive, it is possible for positive charge to build up. This is overcome with a effective electron flood gun. This means that one it is not necessary to coat the sample with conductive material that may obscure the image Helium microscopes offer a large depth of field This depth of field can be as much as 5-10 times as compared to a typical high end FE-SEM

41 Scanning helium ion microscope The Zeiss Orion 30 is a commercially available tool capable of imaging resolution of 0.5 nm In addition to the imaging capabilities, the system can perform etching, deposition and lithography

42 Scanning helium ion microscope The Zeiss system features 3 beam sources for material processing. This allows adjustment of beam current, beam energy, and importantly ion mass. Uses include material removal using sputtering, gas induced etching or deposition or lithography The gallium FIB is used to remove relatively large amounts of material, this beam may contaminate the material

43 Scanning helium ion microscope Neon beam (larger mass than He) for precision nanomachining with speed, and the helium beam can fabricate delicate sub-10 nm structures Having more kinetic energy, neon can expose resist faster than helium Gallium having even more mass can be used for rapid prototyping, and larger feature sizes Naturally He and Ne will provide less contamination than Ga interaction Speed is traded for resolution

44 Scanning helium ion microscope The Orion NanoFab features the NanoPatterning and Visualization Engine - an integrated hardware and software control system Software is capable of creating a range of fully editable shapes including rectangles, trapezoids, polygons, lines, polylines, ellipses and spots. Vector fill these shapes while maintaining full control over dose variation and patterning parameters.

45 Outline Tip-enhanced Raman spectroscopy Scanning helium ion microscope Magnetic resonance sub-nanometer imaging

46 Magnetic resonance sub-nanometer imaging Conventional magnetic resonance imaging is limited to spatial resolution in the micrometer range Magnetic resonance sub-nanometer (MRFM) imaging is capable of detecting the weak magnetic forces from nuclear or electronic spins This method can provide information on the nanometer and possibly the atomic scale, ultimate resolution is single electron-spin sensitivity MRFM combines ultrasensitive force microscopy with magnetic resonance imaging techniques.

47 MRFM Principle: A sample containing nuclear spins is attached to an ultrasensitive cantilever and brought into close proximity ( nm) of a nanoscale ferromagnet. The resultant magnetic force slightly deflects the cantilever, measured by a laser interferometer. Radio-frequency pulses are used to modulate the nuclear spin magnetization and to create an MRI signal. Scanning can produce 3D images of the sample. So basically this technique works by measuring the response of the nuclear magnetic moments or spins of a sample to external magnetic fields and electromagnetic radiation

48 MRFM The nitrogen-vacancy center is a point defect in the diamond lattice. It consists of a nearestneighbor pair of a nitrogen atom, which substitutes for a carbon atom, and a lattice vacancy.

49 MRFM Diamond defects carry a spin that is very sensitive to magnetic fields the optical rate of emission depends on the defect s spin state, we can measure small fields simply by looking at the fluorescence intensity

50 MRFM N-V centers emit bright red light which can be conveniently excited by visible light sources, such as argon, krypton, He-Ne, and other lasers. At room temperature, no sharp peaks are observed because of the thermal broadening. However, cooling the N-V centers with liquid nitrogen or liquid helium dramatically narrows the lines down to few megahertz width. Transitions between the triplet ground state and each of the excited states can be individually addressed via resonant optical excitation

51 MRFM

52 MRFM

53 MRFM

54 Image height is about 5 cm MRFM

55 MRFM These achievements include the detection of a single electron spin in 2004 [1] and the demonstration of nuclear magnetic resonance imaging of a nanoscale biological sample in

56 MRFM Advantages include, possibility of site-specific image contrast, absence of radiation damage, single copy of an object is required, well-suited to provide structural information of large biomolecular complexes that are known to overwhelm nuclear magnetic resonance Disadvantages include identifying a specific area. Issues like gas in proximity of the probe could be mistakenly measured. Naturally this is a detailed test to look at small areas, so it is time consuming to interrogate large features.

57 Outline Tip-enhanced Raman spectroscopy (Lecture 1) Scanning helium ion microscope (Lecture 2) Magnetic resonance sub-nanometer imaging