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Friction and Adhesion on Different Phases of a Biphenyl-Alkanethiol Self-Assembled Monolayer on Gold Studied with Scanning Force Microscopy Francis J. McCarthy, Manfred Buck, and Georg Ha¨hner* EaStCHEM School of Chemistry, UniVersity of St. Andrews, North Haugh, St. Andrews KY16 9ST, U.K. ReceiVed: August 21, 2008; ReVised Manuscript ReceiVed: October 14, 2008
We have investigated the friction and adhesion properties of two structurally different phases of ω-(4′methylbiphenyl-4-yl) butanethiol CH3(C6H4)2(CH2)4SH (BP4) self-assembled monolayers (SAMs) on gold under water with scanning force microscopy. While the identical chemistry of the two phases is reflected by very similar water contact angle values, lateral force measurements and force distance curves reveal the strong influence of the structure, that is, of the molecular and defect density on the mechanical properties of the SAM. A surprisingly high difference in the resistance of the films to shearing but a similar friction coefficient is found for the two phases indicating a crucial influence of the film structure on the energy dissipation in SAMs. The results highlight the importance of structural effects in the interpretation of surface properties. Introduction Organosulfur based self-assembled monolayers (SAMs) have seen an ever increasing level of attention since they were first introduced more than two decades ago. The majority of work published in the literature has been performed with aliphatic moieties on gold. More recently it has been recognized that SAMs of thiols containing aromatic groups are an interesting alternative to the alkane-based monolayers, e.g., refs 1-19. The films established from surfactants containing aromatic groups show a higher rigidity due to stronger intermolecular interactions compared to their alkanethiol counterparts.20-22 As a consequence, they allow a better control over the structure of the resulting monolayer films.21,22 In addition, they display a number of interesting properties in view of potential applications, such as their behavior toward electron radiation23 and charge transfer processes,24-26 which make them attractive for molecular electronics and electrode modification.27-31 Because of their generally high packing densities and their low adhesion and surface energy values, SAMs are also of significant interest as boundary lubricants in microelectromechanical systems as well as in high density storage systems.32 In these systems submicrometer scales are important, and the surface molecular structure has a significant influence on the resulting frictional properties. It has been reported that in addition to the macroscopic surface chemistry the molecular packing order and packing density can have a huge influence on the resulting friction properties and hence mechanical stability of SAMs.33 It is not a priori clear however to what extent knowledge derived from alkanethiols can be applied to aromatic SAMs, which differ significantly from aliphatic ones in geometry, conformational degrees of freedom, and intermolecular interactions. Knowledge of the mechanical stability of these films and how the order influences the dissipation of energy during friction on the submicrometer scale is therefore important for a possible use of such films as lubricants and for applications, for example, as antisticking or adhesion coatings, or where * To whom correspondence should be addressed. E-mail: gh23@ st-andrews.ac.uk. Fax: +44 1334 463808.
mechanical contact to the surface is involved, such as the recently introduced scheme for the deposition of metal electrodes.34 SAMs of ω-(4′-methylbiphenyl-4-yl) butanethiol (BP4) represent a model system which allows studies of the influence of structure on tribological properties without change in chemical properties. BP4 SAMs have been reported to establish different phases depending on the temperature during preparation.21,22 The phases differ in their packing density, the lateral extension of domains, and the number of defects. The R-phase obtained at lower preparation temperatures and the high temperature β-phase are described by a rectangular 53 × 3 and oblique 63 × 23 unit cell, respectively.21,22 In the present paper we report adhesion and lateral force measurements under water on the different phases of BP4 SAMs on gold as well as on a methylterminated alkane thiol film for comparison. The observed differences in friction and the underlying mechanisms are discussed. A basic characterization of the films under water will help to gain a better understanding of their properties and is important for future applications, for example, in tribology and in electrochemistry. Experimental Section SAM Preparation. Self-assembled monolayers of 1-octadecanethiol (ODT) (Fluka, 95%) and ω-(4′-methyl-biphenyl-4yl)-butane-1-thiol12 were prepared on epitaxial (111)-gold on mica substrates (Georg Albert PVD, Heidelberg, Germany). The gold substrates were flame annealed for a few minutes prior to the SAM preparation in order to increase the size of the flat terraces. The ODT monolayer was prepared at room temperature by immersing the substrate in an approximately 10 mM solution in ethanol overnight. The low-temperature R-phase BP4 monolayers were prepared by immersing the substrates in a BP4 solution at 72 °C overnight in order to form a stable and uniform low-temperature phase film.21 The high-temperature β-phase BP4 samples were prepared by annealing low-temperature phase BP4 samples in nitrogen at 140 °C for about 18 h.21 Samples where both phases coexist were obtained by annealing an R-phase sample at the same temperature but for shorter times, typically about 4 to 10 h. The concentration of the BP4 solution
10.1021/jp807491t CCC: $40.75 2008 American Chemical Society Published on Web 11/12/2008
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Figure 1. Lateral force image of a BP4 SAM annealed at 145 °C for 4 h and showing both R- (dark areas) and β-phase (bright areas).
was typically between 0.1 and 1 mM and is not critical for the film preparation.21 Contact Angle Measurements. Static water contact angles (deionized water) were measured with a G10 goniometer ¨ SS GmbH, Hamburg, Germany) under ambimicroscope (KRU ent conditions. Droplets of 3 µL were dispensed from a microburette. The reported values are the average of at least three measurements taken at different places on the surface. AFM Measurements. All AFM measurements were performed with a PicoPlus SPM system (Molecular Imaging, CA, USA). Friction and adhesion forces in ultrapure water were recorded using silicon nitride probes with a nominal spring constant of 0.06 N/m (DNP-S, Veeco Probes, CA, USA). The experiments were carried out by scanning 3 µm wide lines at a speed of 3 lines per second. A total of 50 lines were recorded for each load, with 512 data points per line. The load was initially set to 0 nN and increased in steps of about 3 nN up to 35 nN. Care was taken to ensure that the lateral deflection signal on the photodiode at zero load was 0 V to minimize alignment errors. The average of the friction force loop widths for each load was then determined and reported as the friction signal. Note that the resulting signal is proportional to the true lateral signal even if there is a small misalignment and cross talk present. In addition to the lateral force measurements, pull-off measurements were performed on the samples under water. About 500 measurements per sample on different locations on the surface were recorded with a maximum load of approximately 20 nN according to the nominal spring constant provided by the manufacturer. All measurements were performed at a frequency of 1 Hz per force distance cycle corresponding to an approach and retract rate of around 5 nm/ ms. The tips were characterized before and after the experiments using a calibration grating (grating no. TGT01, NT-MDT, Moscow, Russia) to ensure that wearing of the tip remained negligible. All measurements shown were performed with the same tip. Results Topographic images of the SAMs on a large scale (10 µm) were collected by recording the normal deflection of the cantilever during scanning. No features could be discerned on this scale except for terraces and steps from the underlying gold substrate which is consistent with well-formed monolayers. Figure 1 shows a lateral force image of a BP4 film that was annealed for 4 h at 145 °C and displays both the low temperature R-phase and the high-temperature β-phase.21 The two phases can be distinguished based on results obtained with STM and with lateral force microscopy on samples that consist entirely of one phase,21 indicating that the brighter areas correspond to the β-phase. Samples that consisted entirely of the low- or the hightemperature phase22 were employed to study the adhesion
Figure 2. Histograms of the pull-off forces measured on an ODT SAM and on the two phases of BP4 SAMs on gold under water. The solid lines represent the best fit Gaussians. There is a clear difference in the positions of the adhesion maxima.
properties and the load dependence of the friction force on the two phases separately. An octadecane thiol film was prepared for comparison. The characterization of the films by water contact angle measurements showed values of 93° and 90° for the R- and β-phase of BP4, respectively, as has been reported earlier.21,22 The alkanethiol film displayed a contact angle value of 109°.35 Adhesion Force Measurements. Figure 2 shows histograms of the pull-off forces (adhesion force measurements) measured under water for the different films. The adhesion forces show relatively broad distributions of varying widths and with the most probable adhesion force for the R- and β-phase being different. The adhesion force with the highest probability can be clearly distinguished on the two phases, with a ratio of roughly 2.6 ((0.2) for the BP4 β relative to the R phase. The adhesion force measured on the ODT film was around 1.5 ((0.2) times lower than the one measured on BP4 R. The error is based on the sensitivity with which the forces can be measured, which is around 0.1 units in our experiment. According to the model by Johnson, Kendall, and Roberts (JKR) and by Derjaguin, Muller, and Toporov (DMT), which have both been used before to describe the contact between AFM tips and SAMs in the literature,36 the critical load Lc at which the surfaces separate (pull-off force) is proportional to the radius of the probe, R, and the surface energy, γ (which is related to the adhesion energy):
Lc ∝ Rγ
(1)
Equation 1 allows for the determination of the ratio of the surface energies from the adhesion force measurements, giving the same values as those obtained for the pull-off forces: 2.6 for the β-phase relative to the R-phase and around 1.5 for the R-phase relative to the ODT. Using a value of ∼7 mJ/m2 reported in the literature for the surface energy of the ODT film,37 values of 10.5 ( 1.4 mJ/m2 and 27.3 ( 1.4 mJ/m2 are obtained for the BP4 R- and the BP4 β-SAM, respectively. The error bars are based on the error in the ratios of the adhesion forces. Larger systematic errors are possible due to the fact that we did not independently calibrate the used cantilever. Friction Force Measurements. Friction forces were measured as a function of the applied load on the different SAM surfaces. Measurements were performed under water to avoid capillary forces and to gather information on the properties of
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Figure 3. Friction forces as a function of the applied load for the different SAMs. Error bars are based on the standard deviation of the widths of the friction loops. Solid lines represent the best linear fit to the experimental data.
Figure 4. Typical force distance curves for the different SAMs recorded under water. The adhesion hysteresis is given by the area enclosed between the approach and retraction curve and is clearly different for the three films investigated.
the films in a liquid environment. Figure 3 displays typical results obtained for the ODT and the two BP4 self-assembled monolayers. We note that the twofold symmetry of the phases on the threefold substrate22 should give rise to an anisotropy in the friction signal. The results reported here were obtained by scanning several micrometers and therefore present an average of the friction for a single domain. The friction forces show a linear increase with load within the error bars on all samples and in the range of normal forces that was studied. The highest friction force was observed on the BP4 β-phase. Relative friction coefficients µ were determined by fitting straight lines to the data according to
Discussion
Flat ) F0 + µL
(2)
where L is the applied load and µ the friction coefficient. F0 is the observed friction force when no load is applied. This analysis reveals slightly different coefficients of friction for the β- and R-phase with a ratio of 0.6 ((0.2) and a much larger deviation in the friction forces at zero load with a ratio of 5.4 ((0.7). The corresponding ratios for the BP4 R in comparison to the ODT film are 2.2 ((0.8) and 2.6 ((0.4), respectively. The errors were determined from the deviation of the data points from the best fit straight lines. A modeling of the friction data by a JKR type behavior based on the assumption that the lateral forces are proportional to the true contact area but different shear strengths for the two phases did not give satisfactorily results. The reason is the rather similar slope of the friction curves while at the same time showing a relatively large difference in the absolute friction forces. It has been suggested that the friction force is correlated to the adhesion hysteresis,38 i.e., the energy that is dissipated during an approach-retraction cycle due to the making and breaking of the contact during shearing. Figure 4 shows typical force distance curves recorded on the different SAMs displaying the hysteresis during an approach-retraction cycle. As mentioned earlier, the ratio of the friction forces of the β- and the R-phase at zero load is 5.4. The work of adhesion measured from the mechanical hysteresis loop is roughly proportional to the square of the adhesion forces. Therefore the ratio of the friction forces at zero load based on this model should be on the order of 2.6 × 2.6 ) 6.8. This supports that the friction forces are closer related to the adhesion hysteresis on the BP4 films, i.e., to the energy dissipated during an adhesion cycle, than to the adhesion force itself. The same model applied to the ODT film predicts a 2.3 times smaller friction force on this SAM at zero load compared to the BP4 R-phase, which correlates quite well with the observed ratio of 2.5.
Contact Angles. The static contact angle values of 93° and 90° measured on the R- and β-phase, respectively, are in agreement with those reported earlier22 and indicate that the chemistry and structure of the surfaces on a macroscopic scale is similar. The measurements show that the BP4 films are less hydrophobic than the alkanethiol film with a value of 109°. It is known that the films have different packing densities. The highest density can be found in the ODT film with 21.4 Å2 per molecule.35 The molecular density in the BP4 films is 27 Å2 per molecule in the R-phase and 32.4 Å2 per molecule in the β-phase.21 As a consequence more of the surfactant backbone is exposed in the case of the BP4 films resulting in a lower water contact angle value. An additional contribution due to van der Waals forces can be attributed to the higher polarizability of the aromatic units compared to the alkane chains. The contact angle values indicate that the slight change in the packing density of the BP4 films does not have a significant effect on the overall wettability of the films. Adhesion Forces. The adhesion force measurements reveal that the surface energy and the adhesion on the β-phase are roughly two to three times stronger than on the R-phase and around four times higher than the surface energy of the alkanethiol film. Adhesion forces are dominated by electrostatic interactions. The domain size in these films and the number of defects and domain boundaries will influence the strength of the adhesion force to some extent. Domains of the R-phase are typically 10-30 nm in diameter21 and therefore similar to the contact area between the tip and the surface, while on the β-phase the domain size is much larger with diameters of up to 100-300 nm, as is known from STM measurements.21 Note that regions covered by the R or the β phase as shown in Figure 1 consist of several domains. One contribution to a higher adhesion on the β-phase can be related to its lower molecular sized density of defects such as pin holes and domain boundaries.21 Domain boundaries will have molecules with gauche conformations such that a smaller number of molecular contacts to the probe are established resulting in a lower adhesion on the R-phase. It has been reported before that differences in the lateral force can be due to different domain sizes if the latter are comparable to the contact area (i.e., the tip is sensing different densities of domain boundaries).39 However, this can hardly be the only reason in view of the magnitude of the difference between the two phases by a factor of 2.6 and requires another explanation. Similarly a change of the polarizability and thus of van der Waals forces due to the reorientation of the molecules which have an anisotropic polarizability is unlikely to account for the pro-
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nounced difference. One possibility is a different charging of the surfaces under water caused by the structural differences of the SAMs. As known from alkane thiols even nonpolar surfaces can acquire a significant negative charge in an aqueous environment,40,41 and such electrostatic forces would be expected to be significant on the BP4 films. Another influence could come from entropic, i.e., hydrophobic effects. The surface energies of 10.5 mJ/m2 and 27.3 mJ/m2 for the R- and the β-phase are higher than the value measured for the ODT film. This can be explained in terms of the forces acting between tip and surface in combination with the domain sizes of the monomolecular films. The different adhesion energies cause different nominal contact areas. This, however, cannot explain the observed deviation in the friction force, and the adhesion force is clearly not directly proportional to the kinetic friction observed at zero load based on the determined ratios of 2.6 and 5.4 for the adhesion forces and kinetic friction forces at zero load, respectively. Friction Forces. The results obtained by the adhesion force measurements in combination with the friction forces give a more detailed and comprehensive picture of the frictional properties of the films and the friction mechanism. On the basis of the difference in the surface energies the contact areas at zero load on both phases would differ roughly by a factor of 2 if the JKR model were applicable. The much larger discrepancy by a factor of 5.4 in the friction forces could be compensated by a 2.3 times higher shear strength on the β-phase. This assumption, however, would also lead to a much stronger load dependence of the lateral force on the β-phase compared to the R-phase, which is not observed (see Figure 3). In fact, the friction coefficient of the β phase is slightly lower than the value determined for the R phase. The fitting of the load dependent friction forces measured on both phases in combination with the known ratio of the surface energies of 2.6 reveals that the observed difference in friction cannot simply be explained in terms of the contact area and therefore the difference in the adhesion forces. The relationship between the dissipated energy and the kinetic friction force can be understood in a model that expresses the friction force in terms of one component that is proportional to the adhesion hysteresis, A, (energy dissipated), and a second that is proportional to the applied load L.38 In this model the lateral force is
Flat ) C1A + µL
(3)
While the second term corresponds to the µL term in Amontons’ law and depends on the topography, the first term varies in a more complex way with the applied load.38,42 Since our surfaces are very smooth the dominating contribution to the kinetic friction force comes from the adhesion component. The dependence of the lateral force on the applied load (Figure 3) for the two phases shows almost no change in the slope over the entire load range investigated. This suggests that the increase in the frictional force with load, L, comes principally from the second term in eq 3, which is linear in L and not from the first term. The observed adhesion hysteresis (Figure 4) contains a “mechanical” and a “chemical” contribution. The work of adhesion measured from the mechanical hysteresis loop is determined by the area enclosed between the approach and the retraction curve in a force-distance cycle. This area is approximated by a triangle with one side being the adhesion force. Since all resulting triangles are similar (because of the same spring constant) the area of the triangle (the adhesion hysteresis) approximately scales with the change in the size of the adhesion
force squared. Part of the mechanical contribution is due to the instabilities that occur while approaching and retracting the tip from the surface (jump to contact and jump-off). This mechanical instability hampers to distinguish unambiguously between the contributions from the cantilever and those from the system under investigation. Using a cantilever with a higher spring constant would enable us to follow a greater part of the true force distance curve, but at the same time leads to a loss in sensitivity. The interactions between the tip and the ODT film and the tip and the different phases of the BP4 are similar in terms of their origin (electrostatic, hydrophobic, van der Waals). Therefore it appears reasonable to assume that the overall shape of the chemical potential on the tip surface distance is also similar and does not change dramatically. In that case the linear relation between the hysteresis due to the film and the lateral force still holds, and this “true” (chemical) hysteresis scales with the square of the adhesion force. This also explains that the ODT film showed only a slightly lower surface energy value compared to the BP4 R-phase, but a much lower friction force under zero load. Adhesion between the tip and the sample will therefore be the most significant contribution to the coefficient of friction by causing the dissipation of energy in the shearing of intermolecular interactions as has been reported for SAM systems before.43,44 In terms of a molecular picture energy can be dissipated during friction through the creation of gauche defects in the monolayer films. For the β-phase there is more space per molecule available and therefore it is likely that a higher number of gauche defects can be simultaneously created per molecule during sliding on a single domain. It has been reported before that gauche defects can contribute significantly to the energy dissipation during friction on a SAM surface, e.g., ref 45. However, there are other possible energy dissipation channels. The size of the domains for example might contribute to how easily energy can be dissipated through the creation of phonons. This could also explain a higher friction on the β-phase compared to the R-phase. In addition, localized molecular vibrations can also play an important role. Conclusions The study demonstrates that the purely mechanical packing of molecules in combination with differences in the domain size can have a dramatic effect on the frictional properties on the submicrometer scale, even if the macroscopic surface chemistry is rather similar. We therefore conclude that the chemistry contributes via the number of molecular contacts that are established between probe and surface and the mechanical stability by determining the number of dissipating channels that are available. An increase of the friction due to a less densely packed structure on SAMs has been reported before and was assigned to an opening of additional dissipation channels.45 The same effect may play a crucial role here. The large difference in the friction force that is observed on the two phases of the BP4 SAM is remarkable in view of the similar water contact angle values and the relatively small change in the packing density of the molecules. Acknowledgment. Financial support from the EPSRC is gratefully acknowledged. We thank Christophe Silien for his help with the preparation of the samples. References and Notes (1) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529.
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