Application of atomic force spectroscopy (AFS) to studies of adhesion phenomena: a review

Transcription

1 J. Adhesion Sci. Technol., Vol. 19, No. 3 5, pp (2005) VSP Also available online - Application of atomic force spectroscopy (AFS) to studies of adhesion phenomena: a review F. L. LEITE 1,2 andp.s.p.herrmann 1, 1 Embrapa Agricultural Instrumentation, Rua XV de Novembro 1452, CEP , São Carlos, São Paulo, Brazil 2 Institute of Physics of São Carlos, University of São Paulo (USP), CEP , São Carlos, São Paulo, Brazil Received in final form 22 February 2005 Abstract This review article describes the fundamental principles of atomic force spectroscopy (AFS) and how this technique became a useful tool to investigate adhesion forces. AFS is a technique derived from atomic force microscopy (AFM) and can determine, at every location of the sample surface, the dependence of the interaction on the probe sample distance. AFS provides valuable information, at the nano-scale, such as, for example: (i) how the magnitude of the adhesion force depends on long- and short-range interactions and (ii) the tip sample contact area. An overview about the theory and experiments with local force spectroscopy, force imaging spectroscopy, chemical force microscopy and colloidal probe technique is presented. The many applications of the AFS technique for probing surface interactions open up new possibilities to evaluate adhesion, an important characteristic of materials. Keywords: Atomic force spectroscopy; adhesion phenomena; surface properties; atomic force microscopy; interfacial phenomena. 1. INTRODUCTION In Binnig and co-workers at the IBM Zürich Research Laboratory developed a new type of microscope which they called the scanning tunneling microscope (STM) [1], being the first one in the scanning probe microscopy (SPM) family, that allowed visualization of surfaces on an atomic scale. Although the STM technique is limited to electrically conducting samples, it led to the development of numerous devices that utilize a range of physico-chemical interactions between a tip and sample surface. Equally important, this family of techniques includes one To whom correspondence should be addressed. Tel.: (55-16) Fax: (55-16) herrmann@cnpdia.embrapa.br

2 366 F. L. Leite and P. S. P. Herrmann of the most commonly used SPM systems, the atomic force microscope (AFM) [2], which can image surface topography of both insulating and conductive samples. In general, the AFM studies can be divided into topographical applications (imaging mode) and force spectroscopy or so-called atomic force spectroscopy (AFS), i.e., measuring forces as a function of distance [3 6]. The former group generates an image of the sample surface to observe its structural or dynamic features and it has been employed very effectively on a wide variety of surfaces, including semiconductors [7], biological systems [8 11] and polymers [12 15], with resolution in the micrometer to subnanometer range, thus facilitating imaging at the submolecular level. The second group (AFS) is one of the most promising and interesting research areas related to SPM [16], allowing the study of interand intra-molecular forces. AFS has already been successfully applied to studies of biological systems [17 19], polymers (Refs [20 23] and data not shown) and interfacial phenomena [3, 24 28]. The aim of this review is to provide a glimpse of the potential and limitations of the application of AFS to studies of adhesion phenomena. 2. ATOMIC FORCE MICROSCOPY 2.1. Principle of operation This section briefly introduces the basic elements of AFM and its principle of operation. The microscope scans over the sample surface with a sharp probe, or tip, situated at the apex of a flexible cantilever that is often diving board-shaped or V-shaped and normally made of silicon. AFM utilizes a piezoelectric scanner that moves the sample with a sub-nanometer displacement when a voltage is applied. This piezoelectric system is employed to move the sample in three dimensions relative to the tip (Fig. 1). To form an image, the tip is brought into contact with or close to the sample and raster-scanned over the surface, causing the cantilever to deflect because of a change in surface topography or in probe sample forces. A line-by-line image of the sample is formed as a result of this deflection, which is detected using laser light reflected off the back surface of the cantilever onto a position-sensitive photodiode detector [29, 30]. Forces acting between the sharp probe (tip) placed in close contact with the sample result in a measurable deformation of the cantilever (console) to which the probe is attached. The cantilever bends vertically upwards or downwards because of a repulsive or attractive interaction. The forces acting on the tip vary, depending on the operating mode and the conditions used for imaging. A number of AFM imaging modes are available. The most widely used is the contact mode (C AFM) [2, 31]; in this regime, the AFM tip is in intimate repulsive contact with a surface. Scanning can be done in two different ways: (1) in the constant-force mode the cantilever deflection is kept constant by the extending and retracting piezoelectric scanner; in this method, a feedback loop adjusts the height of the sample (to

3 Studies of adhesion phenomena by AFS: a review 367 Figure 1. A schematic drawing of an atomic force microscope. A detector consisting of four photodiodes is shown. Scanning perpendicularly to the long cantilever axis, the (A + B) (C + D) signal gives topographical data, while the (A + C) (B + D) signal responds to friction due to torsion of the cantilever, providing lateral force information. maintain constant deflection) by varying the voltage applied to the z portion of the xyz piezoelectric scanner. (2) In the variable-deflection mode or constant height mode the piezotube extension is constant and the cantilever deflection is recorded; in this method, the feedback loop is open, so that the cantilever undergoes a deflection proportional to the change in the tip sample interaction. Friction force microscopy (FFM) [32] is a variant of the contact mode, in which the laser beam detector is arranged so as to allow monitoring not only of the vertical component of the tip deflection (topography), but also the torsion deformation exerted by the lateral forces acting on the tip end. Yuan and Lenhoff [33] demonstrated clearly the versatility of the FFM technique. The authors measured surface mobility of colloidal latex particles adsorbed onto mica by moving the particles with an AFM tip in the lateral force microscopy mode. Their data showed that the mean lateral force was proportional to the particle diameter, while the effect of electrostatic interactions on the mobility of adsorbed particles was seen to be weak. The results were consistent with classical theories of friction in macroscopic systems. Recently Zamora et al. [34] showed that a water layer, adsorbed on the sample surface, affected both the normal force at the nanoasperity contacts by the effect of a meniscus loading force and the friction force. The influence of the water condensed at the tip surface contact on the friction force was studied for hydrophilic, partially hydrophilic and hydrophobic surfaces. The results showed that surface wettability affected significantly the dependence of friction on the normal force and scan velocity.

4 368 F. L. Leite and P. S. P. Herrmann The contact mode allows tracking of surface topography with a high precision and also provides a high lateral resolution of nm (down to true atomic resolution under appropriate conditions [35]), but imposes a high local pressure as well as shear stresses on the surface. In contact-mode imaging, the deflection of the tip is mainly caused by the repulsive forces between the overlapping electron orbitals between the tip atoms and the sample atoms. The dominant attractive force is a van der Waals force arising primarily from the induced dipole interactions among atoms of the tip and specimen [36]. When the image is obtained in air, layers of water are adsorbed, producing an additional strong attractive force due to the liquid air interfacial tension. While in liquids, contributions from electrostatic Coulomb interactions between charges on the specimen and tip (either occurring naturally or induced because of polarization), structural forces, such as hydration and solvation forces, and adhesion forces should be considered. However, in a fluid environment, the surface tension forces are abolished and van der Waals forces are typically also reduced due to screening of these forces by the intervening dielectric, resulting in a reduced imaging force. Another way of avoiding the problems caused by the capillary layer is to use the longer-range attractive forces to monitor the tip sample interaction. These attractive forces are weaker than the repulsive force detected in contact mode and, consequently, different techniques are required to utilize them. There are two main types of dynamic mode: the first is often known as the tapping or intermittent contact mode (IC AFM) [37 39], whilst the second is usually called the noncontact mode (NC AFM) [40 42]; the new techniques developed for the use of noncontact mode are achieving high lateral resolution (atomic resolution), and are showing new opportunities in sample analysis [43 45]. In the tapping mode, the cantilever is deliberately excited by an electrical oscillator to amplitudes of up to approximately 100 nm, so that it effectively bounces up and down (or taps) as it travels over the sample. The oscillation amplitude is measured as an RMS value of the deflection detector signal. The feedback system is set to detect the perturbation on the oscillation amplitude caused by intermittent contact with the surface [46, 47]. When the tapping mode is carried out in liquids, the tip of the cantilever taps the sample gently during part of the force curve; this mode is similar to the tapping mode operating in air, except that the sample is tapped against the tip instead of the cantilever being driven at resonance to tap the sample [48]. In the NC AFM, the oscillating cantilever never actually touches the surface of the sample, the spacing between the tip and the sample for NC AFM is on the order of tens to hundreds of Ångstroms, with an oscillation amplitude of only about 5 nm. Non-contact mode usually involves a sinusoidal excitation of the cantilever with a frequency close to its main resonant frequency. In order to excite the vibration of the probe, in some applications, it is convenient to externally modulate the long-range probe sample interactions. Therefore, the relatively long-range attractive forces induce changes in the amplitude, frequency and phase of the cantilever and maintain a constant distance during scanning [49]. These changes in amplitude or in phase

5 Studies of adhesion phenomena by AFS: a review 369 can be detected and used by the feedback loop to produce topographic data. Other forms will be to attach a bimorph piezoelectric to the cantilever, or if the sample can be excited by a suitable piezoelectric actuator. The force modulation mode [50, 51] is an extension of the dynamic mode that uses very large vertical oscillations in which the AFM tip is actually pressed against the surface and the z feedback loop maintains a constant cantilever deflection (as for constant-force mode AFM). The tip moves laterally, point-by-point, over the surface and a complete distribution of the surface elastic properties (amplitude signal) and/or energy dissipation characteristics (phase signal) is collected concurrently with the topographical image [52]. The amplitude damping is determined by the elastic surface deformation against a hard tip. Usually, the elastic constant of the cantilever should be large to achieve reasonable contrast in the force modulation mode. In this mode experiments are typically conducted at the resonant frequency of the driving bimorph element (8 10 khz) and oscillation amplitudes of 1 to 5 nm [53]. Figure 2 represents the tip sample interaction force (F (D)) with different AFM operation modes. At short distances, the cantilever mainly senses interatomic forces: the very short range ( 0.1 nm) Born repulsive forces and the longer-range (up to 10 nm) van der Waals forces. At very small tip sample distances, a strong repulsive force appears between the atoms of the tip and those of the sample. This repulsive force occurs between any two atoms or molecules that approach so closely Figure 2. Empirical force vs distance curve that reflects the type of interaction between the scanning tip and sample during AFM measurements using specific imaging modes (adapted from Ref. [55]).

6 370 F. L. Leite and P. S. P. Herrmann that their electron orbitals begin to overlap. It is thus a result of the so-called Pauli Exclusion Principle [54]. When this repulsive force is predominant in an AFM set-up, tip and sample are considered to be in contact (regime of contact mode). The total intermolecular pair potential is obtained by assuming an attractive potential, ( C 1 /z 6 ) and a repulsive potential, (C 2 /z 12 ). Superimposing the two gives an expression for the well-known Lennard Jones potential: U = C 2 /z 12 C 1 /z 6,whereC 1 and C 2 are the corresponding coefficients for the attractive and repulsive interactions, respectively, and z is the distance between the sample surface and rest position of the cantilever. To describe the AFM tip and sample interactions, one needs to sum the attractive and repulsive potential pairs over all interacting atoms. A simple summation for all the atoms of the tip and sample is a good approximation for repulsive force (the first term of equation above). However, the van der Waals interaction (second term) is non-additive, i.e., the interaction of two bodies is affected by the presence of other bodies nearby, and a simple sum of the pair-wise interactions is usually greater than the actual force between the macro bodies of interest [55, 56]. To take into account non-additivity of the van der Waals part of the interaction, some methods can be used [57, 58]. Nevertheless, an additive approximation is used in many practical applications, including atomistic simulation of AFM [59]. In particular, the van der Waals interaction between the atoms at the end of the tip and in the surface is taken into account explicitly by summing the interactions of all pairs of atoms. However, a full tip contains billions of atoms and it is impossible to sum all the interactions; therefore, an approximation must be made based on the local geometry, material properties and structure of the tip [60, 61]. Hamaker [62] performed the integration of the interaction potential to calculate the total interaction between two macroscopic bodies using the following approximations: (1) the total interaction is obtained by the pair-wise summation of the individual contributions (additivity); (2) the summation can be replaced by an integration over the volumes of the interacting bodies assuming that each atom occupied a specific volume, with a density ρ (continuous medium); (3) ρ and C (interaction constant defined by London [63] and is specific to the identity of the interacting atoms) should be uniform over the volume of the bodies (uniform material properties). However, for van der Waals interaction derived from second-order quantum perturbation theory [64] is only an approximation to reality, since the internal states of molecules, i and j will be modified by the presence of all other molecules of the system, which means that the assumption of pair-wise additivity is not completely correct, especially in condensed phases, where the mean distance between atoms is small and many-body effects cannot be ignored. This problem can be solved by a different approach, proposed by Lifshitz in 1956 [65]. Basically, the Lifshitz or macroscopic approach considers the interactions between electromagnetic waves emanating from macroscopic bodies. The detailed original treatment is very complicated [66] and requires sophisticated mathematics, but several more accessible accounts have subsequently been published [67, 68]. The Lifshitz approach has

7 Studies of adhesion phenomena by AFS: a review 371 the great advantage of automatically incorporating many-body effects and of being readily applicable to interactions in a third medium [69 71] Atomic force spectroscopy Atomic force microscopy (AFM) can be used to determine the dependence of the interaction on the probe sample distance at every location [72]. To determine the spatial variation of the tip sample interaction, force curves can be recorded at a large number of sample surface locations, using the technique of atomic force spectroscopy (AFS). With AFS it is possible to obtain the following information: (i) the magnitude of the force which depends on long-range attractive and adhesion forces, (ii) estimation of the point of tip sample contact, (iii) the tip sample contact area and (iv) the elastic modulus and plasticity of thin and thick films [73, 74]. The point of contact is defined as the intersection of the contact region of the force curve and the non-contact region of the force curve, i.e. the point of contact is that height where the tip would have touched the sample, if there was no attractive force resulting in a mechanical instability so that the tip jumps to the sample [75]. The contact area can be described and expressed by several continuum contact mechanics theories [74], besides modern molecular dynamics calculations that have been the source of many important insights into nano-scale mechanics [76]. The choice of the appropriate theory depends on the relative magnitudes of the materials properties and surface forces. Mechanical properties such as elastic modulus and hardness can be obtained from the corrected slope of the force curve after contact [77]; for more details, see Refs [78, 79]. One must choose the proper mechanical relationships with which to evaluate the data in order to determine the materials properties of the sample as well as the tip sample contact area [80] Local force spectroscopy. With commercially available cantilevers, AFM may be used to measure forces accurately down to approx. 10 pn [81]. It is possible to investigate the complex inter- and intra-molecular interactions, the ranges, magnitudes and time-dependence of rupture forces, the mechanical properties of molecules and the strength of individual bonds [82, 83]. There are several features of AFM that make it ideal for force sensing, such as the sensitivity of the displacement (around 0.01 nm), a small tip sample contact area (about 10 nm 2 )and the ability to operate under physiological conditions [84]. In order to evaluate how the force mapping experiments are conducted, it is necessary to understand how single-point force distance curves are obtained and what information they provide about tip sample interaction. In local force spectroscopy (LFS) (Fig. 3a), the force curve is determined at a particular location on the sample surface. At the start of the cycle, a large distance separates the tip and sample, there is no interaction between the tip and sample and the cantilever remains in a non-interacting equilibrium state (point (a)). As separation decreases, the tip is brought into contact with the sample at a constant velocity until it reaches a point close to the sample surface. As the sample moves

8 372 F. L. Leite and P. S. P. Herrmann Figure 3. When performing force measurements, the AFM tip is brought into and out of contact with the sample at a fixed point. The effect that the sample has upon the deflection of the tip is plotted against the displacement of the sample in the z-direction. (a) Local force spectroscopy and (b) force imaging spectroscopy. towards the tip various attractive forces pull on the tip (long- and short-range forces). Once the total force gradient acting on the tip exceeds the stiffness of the cantilever, the tip jumps into contact with the sample surface (jump-to-contact) ((b) (c)). At point (d), the tip and sample are in contact and deflections are dominated by mutual electronic repulsions between overlapping molecular orbitals of the tip and sample atoms ((a) (d)) is the approach curve. The shape of segment (c) (d) indicates whether the sample is deforming in response to the force from the cantilever. The slope of the curve in contact region is a function of the elastic modulus and geometries of the tip and sample and will only approach unity for rigid systems [85 87]. This slope can be used to derive information about the hardness of the sample or to indicate differing sample responses at different loadings. The segment (d) (e) is showing the opposite direction of the segment (c) (d). The piezoscanner is travelling in the backward direction. If both segments are straight and parallel to each other, there is no additional information content. If they are not parallel, the hysteresis gives information on plastic deformation of the sample [88, 89]. During withdrawal curve (d) (h), as the tip sample surface distance decreases ((e) (f)), adhesion or bonds formed during contact with the surface cause the tip to adhere to the sample up to some distance beyond the initial contact point on the approach curve. As the piezotube continues retracting, the spring force of the

9 Studies of adhesion phenomena by AFS: a review 373 bent cantilever overcomes the adhesion forces and the cantilever pulls off sharply, springing upwards to its undeflected or noncontact position ((f) (g)). Finally, the tip sample surface distance continues to decrease and the tip completely loses contact with the surface and returns to its starting equilibrium position ((g) (h)). Figure 3b shows a force volume data set, that contains an array of force curves and a so-called height image. Force volume imaging is based on collecting arrays of force curves. Individual curves are transformed into force distance curves and all the curves are assembled into a three-dimensional force volume [3] (for more details, see Section 2.2.2). Approach and withdrawal curves can be divided roughly into three regions: the contact line, the non-contact region and the zero line (Fig. 4). The zero line is obtained when the tip is far from the sample and the cantilever deflection is close to zero (when working in liquid, this line gives information on the viscosity of the liquid [74]). When the sample is pressed against the tip, the corresponding cantilever deflection plot is called the contact line and this line can provide information on sample stiffness. The most interesting regions of the force curve are two non-contact regions, containing the jump-to-contact and the jump-off-contact. The non-contact region in the approach curve gives information about attractive (van der Waals or Coulomb force) or repulsive forces (van der Waals in some liquids, double-layer, hydration and steric force) before contact; this discontinuity occurs when the gradient of the tip sample force exceeds the spring constant of the cantilever (pull-on force). The non-contact region in the withdrawal curve contains the jump-off-contact, a discontinuity that occurs when the cantilever s spring constant is greater than the gradient of the tip sample adhesion forces (pulloff force). A convenient way to measure forces with precision is to convert them into deflections of a spring, according to Hooke s law: F = k c δ c, (1) where the cantilever deflection δ c is determined by the acting force F and the spring constant of the cantilever, k c. Although the manufacturer describes spring constants for the cantilevers, the actual spring constant may deviate from this value by an order of magnitude. It is, therefore, necessary to determine the spring constant experimentally. This may involve determining: (i) the resonant frequency of the cantilever before and after adding a small mass to the tip [90], (ii) ascertaining the unloaded resonant frequency with knowledge of the cantilever s density and dimensions [91], or (iii) thermal fluctuation of the cantilever [92, 93]. In equation (1), the acting force leads to a total bending z of the cantilever due interaction with the surface. The real probesample distance is then given by: D = z z, (2) where z is the distance between the sample surface and rest position of the cantilever and z is the sum of the cantilever deflection, δ c, and sample deformation,

10 374 F. L. Leite and P. S. P. Herrmann (a) (b) Figure 4. (a) Force curve on sisal fibers illustrating the points where jump-to-contact (approach) and jump-off-contact (withdrawal) occur and the maximum values of the attractive force (pull-on force and pull-off force); (b) contact mode topography image of sisal fiber. δ s [74]. Since we do not know in advance the cantilever deflection and the sample deformation, the distance that can be controlled is the displacement of the piezotube. Therefore, the raw curve obtained by AFM should be called deflection

11 Studies of adhesion phenomena by AFS: a review 375 displacement curve rather than force distance curve [74]. This latter term should be employed only for curves in which the force is plotted vs. the true tip sample distance (Fig. 2). A complete force curve consists of two portions amounting to the movement of the probe towards the sample (approach) and its retraction back to its starting position (withdrawal). Figure 4a illustrates these two portions of the force curve for the case of measurements performed on a sisal fiber surface. The authors studied the surface chemistry by force spectroscopy and investigated the morphological changes caused by chemical treatments of sisal fibers. By AFM, it was possible to observe that the untreated sisal fiber (Fig. 4b) consisted of lengthwise macrofibrils oriented in the same direction. The adhesion force between the AFM tip and the surface of the fibers was found to increase after benzylation of the fibers, indicating a rise in their surface energy. The distribution of the measured adhesion force over an area of 1 µm 2 was very nonuniform in all samples, but the low adhesion sites disappeared after benzylation. These results illustrate how the AFM can be used to detect heterogeneity in the wettability of fibers, such as sisal, with nanometer resolution and can be applied in the study of fiber-matrix adhesion in polymer composites. The hysteresis apparent in Fig. 4a is due to the adhesion force between the probe and sample. For clean surfaces of probe and sample, adhesion can result from van der Waals interactions [94] or from covalent or metallic bonding between the probe and sample [95]. However, since the experiment was realized in ambient conditions, the pronounced hysteresis is due also to capillary forces [4, 96], as we will see in more detail in Section Force imaging spectroscopy. In the mid-1990s, the idea of collecting data from force distance curves obtained from many points on a sample was introduced, effectively to produce a map of the tip surface interactions [97, 98]. Layered imaging is an SPM technique in which several measurements of cantilever deflection are made at each image pixel. Each measurement of a deflection at a given displacement is recorded. When all measurements for the current pixel are completed, the process is repeated at the next pixel and so on through the scan area. The resulting spatial maps represent the lateral variation of adhesion force due to material inhomogeneities and the surface topography [22]. The resulting three-dimensional data set can be thought of as a stack of layers of images (see Fig. 5b). Each horizontal layer is an image which represents measurements taken throughout the scan area at a specified height z. Since several measurements are made at each pixel, the data set can also be processed vertically to yield the force distance curve at each pixel. This force imaging spectroscopy (FIS) mode of AFM can thus be used to measure adhesion [99], hardness, or deformability of samples. Many probe sample interaction mechanisms can be studied. For example, the spatial adhesion map for a 5 5 µm 2 mica surface contaminated by organic compounds is shown in Fig. 5a. The outward movement (withdrawal) of the cantilever (sections d e, e f and f g of the force distance curve shown in

12 376 F. L. Leite and P. S. P. Herrmann Fig. 3a) was monitored and plotted. The pull-off force contrast in adhesion map images was adjusted to range between 0 nn (white pixel in the upper left corner of each image) and 20 nn (black pixel in the upper left corner of each image). For the cleaved mica surface, a mean pull-off force of 19 nn and a variance (i.e., squared standard deviation) of 3 nn 2 were calculated from the best fit of a normal distribution to the pull-off force histogram. Adhesion maps can be constructed by measuring the vertical displacement of the sample, driven by the piezoscanner, with respect to its displacement when the cantilever is at rest position. Force curves can be digitally acquired at 100 or more points equally spaced from each other over the scanned area of the surface. Each force curve is comprised of a row of a maximum of 250 data points acquired during the vertical movements of approach and withdrawal of the cantilever; software is used to create the adhesion maps (Fig. 5b). (a) (b) Figure 5. (a) A 5 5 µm map of the pull-off force recorded with a Si 3 N 4 AFM tip on a contaminated mica surface; (b) adhesion map plot illustrating the variability of the adhesion force on mica in air.

13 Studies of adhesion phenomena by AFS: a review 377 Tapping mode AFM (IC AFM) has also been used to map tip surface interactions [100, 101]. In this mode, the cantilever oscillates at its resonant frequency at a position just above the surface, so that the tip is in contact with the surface for only a very short time. A feedback loop ensures that the amplitude of the cantilever oscillation remains almost constant. It is possible to measure the phase difference between the driving oscillation and the detected cantilever oscillation, generating a phase difference map. An increase in the phase difference arises from a stronger tip sample interaction, creating contrast in the phase map [102]. However, there are still problems associated with many of the alternative methods for determining tip sample interactions. Although, the image contrast is very much under discussion [103]. Another possibility is to use the so-called dynamic mode AFM operated in the frequency modulation mode (FM AFM). Schirmeisen et al. [104] measured metal polymer adhesion properties by dynamic force spectroscopy with functionalisation of the tip by a thin layer of aluminum, while the polymer was plasma-etched. They found that plasma etching of the polymer resulted in strongly enhanced interactions, indicating a chemical activation of the polymer surface. Sokolov et al. [105] analyzed the possibility of using noncontact atomic force microscopy to detect variations in surface composition, i.e., to obtain a spectroscopic image of the sample. The authors concluded that long-range forces acting between the AFM tip and the sample depended on the composition of both tip and sample. They showed theoretically how van der Waals forces could be utilized for force spectroscopy. Various results have been achieved in detecting the van der Waals interactions by use of molecular dynamics (MD) simulations and AFM measurements [ ] Chemical force microscopy Adhesion is governed by short-range intermolecular forces which in many cases can be controlled by appropriate surface modification. This provides a specific chemical functionality on the probe surface. This technique is known as chemical force microscopy (CFM) [ ] and it can be used to evaluate the strengths of specific forces of attraction directly and adds chemical interaction to a mechanical surface probe [114]. The AFM tip is first covered with an ordered monolayer of organic molecules (a self-assembled monolayer) to give it a specific chemical functionality. The force of interaction can be estimated from the excess force required to pull the tip free from the surface. The functionalization of the cantilever surface is a methodology that has been applied to biosensors [115]. Development of CFM has enabled investigation of the adhesion [ ] and friction [ ] between surfaces in close, molecular contact and the measurement of nanometer-scale tribological phenomena [122]. Starting around 1993, several papers have been published on the topic of CFM. The pull-off force, friction force measurements [ ] and also simulations using molecular dynamics (MD) have been used to investigate the indentation and friction properties of SAMs and the rupture of films bonded to solid substrates [ ]. CFM

14 378 F. L. Leite and P. S. P. Herrmann is a newly emerging method introduced recently for probing surface chemical composition with high resolution [110, 129]. One of earliest examples used tungsten tips to perform force measurements on two chemical monolayers [85], demonstrating that it was possible to distinguish between two self-assembled monolayers (SAMs), one terminating in CH 3 groups and the other in CF 3, simply by comparing the force distance curves obtained from each surface. The investigation of force sensing has made rapid progress with the incorporation of surface chemistry techniques to bind specific chemical groups to the AFM tip [130]. An approach often employed is to produce gold-coated tips, which are modified with SAMs of thiol compounds terminated in a chosen functional group (Fig. 6). There is an extensive literature on the subject which should be referred for more detailed information on the formation and properties of self-assembled monolayers [ ]. The functionalized tips can then be used in force distance curve measurements. The general idea, in this case, is to probe the adhesion forces between the tip and the surface, both with well-defined chemical composition. This type of chemical functionalization is used in some research laboratories because of the well-defined surface properties of monolayers studied [ ]. The most consistent pulloff force studies involving CH 3 -terminated monolayers have been done in liquid environments [ ]. It is important to understand and characterize the fundamental interactions between different tips and sample surfaces under different environmental conditions. Eastman and Zhu [145] measured the adhesion force between modified AFM tips and a mica substrate by atomic force spectroscopy. The results show that the ad- Figure 6. Scheme for chemical modification of tips and sample substrates. Tips and substrates are first coated with a thin layer of Au ( nm) and then, upon immersion in a solution of organic thiol, a dense SAM is formed on the Au surface. Similarly, cleaned Si or Si 3 N 4 tips can be derivatized with reactive silanes. The functional groups comprise the outermost surface of the crystalline SAM, and the tip sample interaction can be fine-tuned by varying the chemistry at the free SAM surfaces. The R in RSH and RSiCl 3 represents an organic alkyl chain that ends with a functional group X (X = CH 3, COOH, CH 2 OH, NH 2, etc.) (reprinted with permission from Ref. [132]).

15 Studies of adhesion phenomena by AFS: a review 379 hesion force is sensitive to the surface energies of the materials coated on the tips, e.g., the adhesion force between a gold-coated tip and a mica surface is much larger than that between a paraffin-coated tip and a mica surface. The authors also show that both the van der Waals and capillary forces between the AFM tip and the substrate can account for this behavior of the adhesion forces. There have been only a limited number of attempts to correlate the measured adhesion forces and energies predicted by interfacial energy theories [123, 146, 147]. This is due to the difficulty in calculating the interfacial energy from the directly-measurable adhesion force, mainly because of the continuing uncertainty whether the Johnson Kendall Roberts (JKR) [148] or the Derjaguin Muller Toporov (DMT) [149] theory of the adhesion contact should be applied to analyze the adhesion forces between the tip and the substrate. Beach et al. [150] measured pull-off forces between hexadecanethiol monolayers, self-assembled on gold-coated silicon nitride cantilever tip and silicon wafer, using AFM. The authors concluded that the AFM technique appeared to be a very useful tool in the examination of surface free energy of engineered materials. The surface energy of the self-assembled monolayer of hexadecanethiol was calculated to be in the range ± 6.61 to ± 9.57 mj/m 2 using the measured pull-off force values. These values are between the values reported in the literature from contact angle and force curve measurements. Duwez and Nysten [136] used tips modified with methyl- and hydroxyl-terminated alkanethiols and showed that AFM tips functionalized with alkanethiol SAMs could be utilized to map the distribution of adhesion forces on polypropylene (PP) surfaces (Fig. 7). The image in Fig. 7 shows the lateral distribution of pull-off forces. The authors also found evidence for additives migrating toward the surface and modification of additive distribution on the surface due to material aging, utilizing laterally resolved adhesion force maps [151]. Recently, a study of the effect of topography on chemical force microscopy was carried out using adhesion force mapping [152]. The authors determined the distribution of adhesion forces measured in water by pulsed-force-mode atomic force microscopy (PFM AFM). The peaks with the higher adhesion forces were attributed to the hydrophobic interactions between the CH 3 -terminated surfaces of the tip and the patterned sample in water. The results showed that variation in the grain sizes and in the multiplicity of contacts between the tip and convexities of the grains resulted in differences in the width of the distribution of the observed adhesion forces AFM colloidal probe technique A fundamental understanding of the factors controlling adhesion and the possible development of adhesion-free surfaces can potentially benefit greatly from direct measurements of the strength of adhesion interactions. A number of studies have been carried out using the surface force apparatus technique (SFA) [153]. However, SFA requires molecularly smooth crossed cylindrical samples with a radius of

16 380 F. L. Leite and P. S. P. Herrmann (a) Figure 7. Typical adhesion map obtained on a few regions of the polypropylene surface with a CH 3 - terminated tip in water (a); histogram of adhesion force distribution corresponding to the adhesion map (b) (reprinted with permission from Ref. [151]; copyright 2001 American Chemical Society). (b) the order of 1 cm. Thus, the development of the AFM has provided another experimental option for the measurement of surface forces which does not require a large smooth cross-section. Of special note is the use of colloidal probes, formed by attaching a single particle in the size range 1 20 µm to the cantilever [ ]. Examples of the cantilever with attached particle are shown in Fig. 8. Bowen et al. [156] used AFM to quantify the adhesion interaction between a silica sphere and a planar silica surface. The authors found that the experimentally measured adhesion forces depended on sample preparation and solution ph and that the adhesion of such surfaces was a complex phenomenon in which non-dlvo (Derjaguin Landau Verwey Overbeek) interactions probably played a substantial overall role. AFM tips with a well-defined silica colloidal particle have also been used to measure the adhesion of lactose carriers [159]. With this method, maps of adhesion between an individual lactose particle and gelatin capsules have been obtained [160]. Cho and Sigmund [161] suggested using a multi-walled carbon nanotube (MWNT) as a micrometer-length spacer and as a nanosized probe. This small-size probe is generally used for high-resolution imaging of topography of the sample. They proposed a systematic approach for data collection with a nanosize colloidal probe and an example of a directly measured surface force curve obtained with the MWNT probe was presented. Finally, the use of MWNT in the conventional liquid mode

17 Studies of adhesion phenomena by AFS: a review 381 Figure 8. (a) Scanning electron micrograph of an 18 µm polyethylene (PE) particle at the end of an AFM cantilever (2000 ). (Reprinted with permission from Ref. [165]; Copyright 2003 American Chemical Society.) The particle was glued to the AFM cantilever with a small amount of epoxy resin using a procedure described in Ref. [158]. (b) Epoxy-based modification of cantilevers. Using commercially-available AFM cantilevers with integral tips, the free terminus of cantilever was coated with an epoxy resin. This epoxy-laden cantilever was then placed in direct contact with the sample. When the epoxy hardened, a portion of the sample was mechanically torn from the substrate to produce a cantilever-supported sample (reprinted with permission from Ref. [166]; copyright Elsevier). of AFM opens the possibility of directly measuring the interaction force. Other authors have used a carbon nanotube as an STM or AFM probe [ ]. 3. APPLICATION OF ATOMIC FORCE SPECTROSCOPY TO THE STUDY OF ADHESION FORCES 3.1. Adhesion mechanics In general, the total adhesion force (pull-off force) between an AFM tip and a sample surface should include the capillary force (F cap ), as well as the solid solid interactions, which consist of van der Waals forces (F vdw ), electrostatic forces (F e ) and the chemical bonding forces (F chem ). If the measurement of the pull-off force is made in the presence of a dry atmosphere, like nitrogen or vacuum, the adhesion force, F adh, is due mainly to dispersion forces. Much of the present understanding of elastic adhesion mechanics (adhesion and deformation) of spheres on planar substrates is based on the theoretical work of Johnson, Kendall and Roberts (JKR) [148] and of Derjaguin, Muller and Toporov (DMT) [149]. Thus, studies of adhesion require application of either the JKR or the DMT theory. For a dissimilar sphere/flat system, in the

18 382 F. L. Leite and P. S. P. Herrmann Derjaguin approximation, one can write: F DMT adh = 2πR t ϖ ikj, (3) where ϖ ikj is the work of adhesion between two surfaces i and j in a medium k and R t is the tip radius. In the JKR theory, separation will occur when the contact area between the surfaces is a adh = 0.63a 0,wherea 0 is the contact area at zero applied load. This separation will occur when the pull-off force is: F JKR adh = 3 2 πr tϖ ikj. (4) When plastic or elasto-plastic deformation occurs, both the DMT and JKR analyses do not hold. Instead, the Maugis and Pollock (MP) analysis [167] can be used, at least when full plasticity occurs. The MP analysis gives the pull-off force as [168]: Fadh MP = 3πϖ ikjk 2(πH ) P 1/2, (5) 3/2 where H is the hardness of the yielding material, K is reduced Young s modulus and P is applied load. Generally, for ideally smooth surfaces the theoretically predicted Fadh DMT and Fadh JKR represent the lower and the upper limits of the experimentally measured F adh, respectively. Hence, one can write [169]: F adh = α a R t ϖ ikj, (6) where α a is a constant with values between (3/2)π (for soft materials) and 2π (for hard surfaces). The JKR model should appropriately describe the adhesion for large spheres with high surface energies and low Young s moduli, while the DMT model should be appropriate for describing adhesion of small spheres of low surface energies and high Young s moduli. To decide on which model to use, the parameter µ is used, as suggested by Tabor [170]: ( ϖ 2 ) ikj R 1/3 t µ = 2.92 K 2 z0 3, where z 0 is the equilibrium size of the atoms at contact. Tabor suggested that when µ exceeds unity, the JKR theory was applicable (µ > 1),otherwise the DMTmodel should be used (µ <1). Descriptions of the transition between these limits (µ 1) are provided by Müller et al. [171], Maugis [172] and Johnson and Greenwood [173]. Contact area vs. load curves for each of the cases are shown in Fig. 9a [174]. The Maugis Dugdale (M-D) theory can be expressed mainly in terms of a single non-dimensional parameter, the

19 Studies of adhesion phenomena by AFS: a review 383 (a) (b) Figure 9. JKR DMT transition. (a) The relationship between contact area and load for an elastic sphere contacting a plane depends upon the range of attractive surface forces. Area load curves for the JKR limit (short-range adhesion), DMT limit (long range adhesion), and an intermediate case are shown. All of these approach the Hertz curve in limit γ 0 (no adhesion). Load and area are plotted in nondimensional units as indicated (reprinted with permission from Ref. [174]; copyright 1997 American Chemical Society). (b) Map of the elastic behavior of bodies. When the adhesion is negligible, deformations fall in the Hertz limit (approximately F>10 3 πϖr); when the adhesion is small the behavior of materials is described by the DMT theory (approximately 10 2 <λ<10 1 ), whilst JKR theory predicts the behavior of bodies with high adhesion (approximately λ > 10 1 ). The Maugis theory suits the intermediate region (approximately 10 1 <λ<10 1 ) (adapted from Ref. [175]).

20 384 F. L. Leite and P. S. P. Herrmann so-called elasticity parameter, λ, related to µ and defined by: λ = 2.06 ( Rt ϖ 2 ) 1/3 ikj = 1.16 µ. (7) z 0 πk 2 Using this theory, Johnson and Greenwood [173, 175] constructed an adhesion map with co-ordinates µ and F (see Fig. 9b), where F is the reduced load and is given by: F = F adh. (8) πϖr t More recently, significant adhesion has been encountered in the area of nanotribology where the contact size is measured in nanometers. Most practical applications fall in the JKR zone of the map, but the small radius of an AFM tip, for example, leads to operating values of the parameter λ which are in the M- D transition zone. Such values for AFM systems were encountered by Carpick et al. (λ = 0.8) [176] and Lantz et al. (λ = ) [177]. Thus, by inserting appropriate estimates for ϖ,k and R t in (7), an appropriate choice between equations (3) and (4) can be made. The approximate values of the F can be determined by an empirical equation given by Carpick et al. [178]: F = ( 4.04λ λ ), (9) where λ 0(DMT)andλ (JKR). Substituting the values of F in equation (9) into equation (8), one can obtain the empirical values of the adhesion force. For values of λ encountered in the literature, the expression for the adhesion force is approximately: F M-D adh (1.9 a a 1.6)πRϖ ikj, (10) where the values of the work of adhesion ϖ ikj are calculated as described in Section 3.2. Recently, Shi and Zhao [179] made a comparative study of the three models, JKR, DMT and M-D, and the influence of the dimensionless load parameter. It was shown that both the dimensionless load parameter, F, and the transition parameter had significant influences on the contact area at the micro/nano-scale and, thus, should not be ignored in the nano-indentation tests. Finally, all the theories reviewed in this section, except the MP model, are continuum elastic theories and, hence, assume smooth surfaces with no plastic deformation and no viscoelastic behavior [74] Work of adhesion The work of adhesion, ϖ ikj, between surfaces of two equal solids (ii) can be expressed in terms of their surface tension (surface energy), γ ik, when interacting

21 through a medium, k. Studies of adhesion phenomena by AFS: a review 385 ϖ ikj = 2γ ik (same surfaces, i, in a medium k). Similarly, for two dissimilar surfaces (i and j), the work of adhesion is defined as: ϖ ikj = γ ik + γ jk γ ij. (11) A commonly used approach to treating solid surface energies is to express surface tension or surface energy (usually against air) as the sum of components due to dispersion forces (γ d ) and polar (e.g., hydrogen bonding) forces (γ p ) [180]. Thus, the interfacial tension between two phases α and β is expressed in terms of the two components for each phase (the cross-term is described by the geometric mean): γ αβ = γ α + γ β 2 γ dα γ dβ 2 γα p γ p β. (12) Four cases arise in describing the work of adhesion: (A): dissimilar surfaces i and j in contact with vapor (V) ( ) ϖ ivj = 2 γi dγ j d + γ p i γ p j. (13) (B): identical surfaces i and i in contact with vapor (V) ϖ ivi = 2 ( γiv d + γ iv) p. (14) (C): dissimilar surfaces i and j in contact with liquid (L) { ϖ ilj = 2 γ L + [ γ d [ γ d i γ d j + i γ d L + γ p i γ p L γ p i γ p j (D): identical surfaces i and i in contact with liquid ] [ ] γj dγ L d + γ p j γ p L ]}. (15) { [ ] } ϖ ili = 2 γ L 2 γi dγ L d + γ p i γ p L + γi d + γ p i. (16) In an attempt to relate components more clearly to the chemical nature of the phase, van Oss et al. [181] suggested that the polar component could be better described in terms of acid base interactions. Thus, surface energy can be expressed as γ αβ = γα LW + γβ AB. Unlike γ LW, the London van der Waals component, the acid base component γ AB comprises two non-additive parameters. These acid base interactions are complementary in nature and are the electron-acceptor surface tension parameter (γ + ) and the electron-donor surface tension parameter (γ ). The

22 386 F. L. Leite and P. S. P. Herrmann total interfacial energy between two phases is [182]: ( ) 2 ( γ γ αβ = γα LW γβ LW α γ α + γ + β γ β ) γ α + γ β γ α γ + β. (17) Several papers in the literature have provided different methodologies and theories for estimation of surface tension components from contact angle data; this subject still is under debate [ ] Capillary force If a liquid vapor is introduced, the surface energy of the solids is modified by adsorption. At a certain relative vapor pressure, capillary condensation will occur at the point of contact between the tip and sample. An annulus of capillary condensate will form around the tip and, consequently, a capillary force arises as a main contribution in the measured pull-off force. To study how this adsorbed water affects the AFS experiments under ambient conditions it is necessary to understand why this layer is present, and on which conditions and parameters it depends. When working in ambient conditions it is important to focus on the nanometer scale, where two main effects have to be considered in the adsorption process: the disjoining pressure,, experienced by thin films, and in the case of non-flat interfaces the Laplace pressure (L), which determines the curvature of the adsorbed layer. The disjoining pressure is the interaction force per unit area between gas and liquid interfaces, and is induced by long-range interactions. For films of micrometer thickness, the disjoining pressure is negligible, but for thin films of thickness in the range nm it has to be taken into account in the analysis of the free energy of the system. In general, several forces are responsible for the disjoining pressure. For some systems, the van der Waals interaction dominates and the disjoining pressure for a film of thickness, t, can then be written as: (t) = A slv 1 6π t. (18) 3 Depending on the sign of the Hamaker constant, A slv, i.e., on the dielectric properties of the three media (s, solid; l, liquid; v, vapor), the force responsible for the disjoining pressure can be attractive, repulsive or a mixture of both, as shown in Fig. 10. Curve A is typical of a stable film (wetting), curve C corresponds to an unstable film (non-wetting) and curve B corresponds to a metastable film [189, 190]. Another possible origin for the disjoining pressure is the so-called repulsive double layer force, which can be very important in the case of charged surfaces or ionic solutions [61]. For an electrolyte solution, the disjoining pressure can be described by: (t) = K s exp( 2χt), (19) where χ is the Debye screening length of ions in the solution and K s is a constant factor related to the surface charge. In the case of pure water, the ions come mainly

23 Studies of adhesion phenomena by AFS: a review 387 from the solid surface, their concentration being very low. The DLVO theory includes the effects of both long-range forces, namely, the van der Waals and the double layer, when calculating the disjoining pressure, so that the (t) plot can take complicated shapes, due to superposition of the two contributions (Fig. 10). One can then say that the disjoining pressure displaces the gas liquid interface away from or towards the solid liquid interface. This implies a change in the internal energy of the system and, as a consequence, a change in the chemical potential of the liquid, which will change from zero to µ liq = (t). In order to keep the equilibrium between vapor and liquid phases, both chemical potentials must be equal. From these considerations, it is possible to obtain the thickness of the film for a given temperature and vapor density. Considering only the van der Waals contribution to the disjoining pressure and a hydrophilic substrate, the thickness of the water film can be approximately described by: ( ) A slv v 1/3 m t =, (20) 6πkT ln(n v /n sat ) where n v is particle number density for vapor phase (n = N/V,whereN is the number of particles and V is the volume), n sat is a saturation density for which liquid vapor equilibrium is reached [190], k B is the Boltzmann constant and T is the temperature; the value n v /n sat is the relative humidity [61]. Figure 10. Dependence of disjoining pressure on film thickness and type of force involved. Curve (A) corresponds to a repulsive force and is a wetting case. Curve (C) is an attractive force and a non-wetting situation and curve (B) corresponds to a metastable film (adapted from Ref. [189]).

24 388 F. L. Leite and P. S. P. Herrmann As an AFM tip approaches the substrate, the capillary force on the tip is initially near zero until the tip contacts the surface of the water film. When contact is made, water wicks up around the tip to form a meniscus bridge between the tip and the substrate. The behavior of the force curve (pull-off force) depends directly on the height of the water film adsorbed on the substrate. The minimum required thickness of water film precursor for spreading [191, 192] is given by: ( ) 1/2 γsv s f = a m, (21) ς where a m is the molecular length given by a m = A/6πγ sv [193], ζ is the spreading coefficient given by ς = γ s γ sl γ sv and γ s is the solid vacuum interfacial energy. The formation of a capillary neck requires a certain minimum height of the water film. No capillary neck forms between two surfaces until the water film thickness reaches the minimum thickness, s f. Various techniques have been used extensively for the analysis of water films on surfaces, such as ellipsometry [194], surface force apparatus [195] and AFM [ ], among others. Miranda et al. [200] used a combination of vibrational sum frequency generation and scanning polarization force microscopy [201] and concluded that above the transition point (relative humidity where capillary condensation occurs) the AFM tip induces water nucleation and, therefore, formation of a capillary bridge. Forcada et al. [202] measured the thicknesses of solid-supported thin lubricant films using AFM, and the differences observed between the thicknesses measured with the force microscope and by ellipsometry were explained by appearance of instability in the liquid film. The theoretical description also predicts the dependence of these differences on the thicknesses of the film. In our group, measurements of water layer thickness have been realized on mica, quartz and silicon substrates. Figure 11a shows the thickness of the liquid film determined by AFM and the influence of the type of substrate used. Figure 11b shows a force curve enlarged in the attractive region (approach curve) to identify the jump-to-contact distance (D jtc ). The thickness of the liquid film is determined by D jtc values in the force curve (RH 70%), since in drier conditions (RH 36%) this distance drops to values equivalent to Djtc vdw, which is directly related to van der Waals forces (Djtc vdw = 2.1 nm). The theoretical values for mica surface, using equation (20), are 1.4 and 3.0 for dry and wet conditions, respectively, which agree with values from force curve (Fig. 11a). Luna et al. [203] used non-contact AFM to study water adsorption on graphite, gold and mica. Graphite surface is rather hydrophobic compared to gold and mica. They also showed that water adsorbed on graphite only under the influence of the scanning tip at 90% RH or more, while in the case of gold and mica, water adsorbed on the surface spontaneously at low RH values (30%). However, it is evident that for many processes in air, understanding the behavior of water on surfaces is fundamental to AFM studies. In fact, effects of water have been observed on adhesion by AFS [3, 204]. Ata et al. [205] studied the role of surface

25 Studies of adhesion phenomena by AFS: a review 389 (a) (b) Figure 11. (a) Histogram illustrating the values of jump-to-contact distance in air (RH 70%) for various sample surfaces (mica, quartz and silicon). (b) Typical force curve enlarged in the attractive region to show thickness of liquid film determined by AFS (k c 0.13 N/m) on muscovite mica. The experimental value of the jump-to-contact distance, D jtc, is about 3.4 nm.