Langmuir 2003, 19, 6135-6139
6135
Friction and Molecular Order of Alkanethiol Self-Assembled Monolayers on Au(111) at Elevated Temperatures Measured by Atomic Force Microscopy Xinju Yang† and Scott S. Perry* Department of Chemistry, University of Houston, Houston, Texas 77204-5003 Received February 27, 2003. In Final Form: April 28, 2003 The influence of temperature on the frictional properties and molecular structure of hexadecanethiol self-assembled monolayers (SAMs) adsorbed on gold has been measured by atomic force microscopy (AFM) in a vacuum environment. The frictional response of hexadecane thiol films decreases significantly when the as-deposited SAM film is heated from room temperature to 330 K, with a corresponding increase in surface order. The changes observed during the first heat treatment are irreversible, with the roomtemperature frictional response lowered by approximately a factor of 4. However, subsequent heating cycles produce a reversible change in interfacial friction for temperatures up to 350 K, with interfacial friction increasing with increasing temperature. Further heating the SAM film above 370 K produces a significant and irreversible increase in friction. At 400 K, the lattice-resolved structure of the Au(111) surface is observed, indicating the instability and initial stages of desorption of the alkanethiol film at this temperature. Following surface anneals to 500 K, only small three-dimensional islands of residual thiol are observed in large-scale topographic images and the frictional properties largely reflect those of bare gold. The reversible increase in the frictional properties of the hexadecanethiol film with increasing temperature is ascribed to a decrease in the molecular order and the effective density of the film. Above the temperature threshold for film damage, the irreversible increase in frictional response is ascribed to energy being dissipated through ploughing and displacement of the film.
Introduction The structure and stability of self-assembled monolayers (SAMs) have been extensively studied in recent years due to their model character and the potential use in a wide number of applications.1,2 One of the most extensively studied systems entails alkanethiols adsorbed on Au(111). For some applications, the thermal stability of the film structure will influence the function of the film or act as a limiting factor in the performance of a given application. To date, there have been several reports of the temperature-dependent properties of SAM films. Specifically, scanning tunneling microscopy (STM), infrared spectroscopy, and X-ray diffraction have been used to assess the thermally induced surface ordering or disordering of alkanethiols on gold.1,3-9 STM studies4,5 have shown that alkanethiols on gold can be annealed to 325 K with no major structural changes, except for a coarsening of the domain boundary network and a ripening of the vacancy island distribution. In these investigation of CH3(CH2)xSH monolayers on Au (where x ) 7, 9, and 11), films annealed above 350 K exhibited significant structural changes accompanied by a disappearance of vacancy islands and molecular desorption of a fraction of surface * Corresponding author: e-mail
[email protected]. † Present address: Surface Physics Laboratory, Fudan University, Shanghai 200433, China. (1) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (2) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (3) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. Langmuir 1998, 14, 2361. (4) Camillone, N. J. Chem. Phys. 1994, 101, 11031. (5) Xiao, X.; Wang, B.; Zhang, C.; Yang, Z. Surf. Sci. 2001, 472, 41. (6) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767. (7) Fenter, P.; Eisenberger, P.; and Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (8) Bucher, J. P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979. (9) Delamarche, E.; Michel, B.; Gerber, C.; Anselmetti, D.; Guntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869.
thiols. By infrared spectroscopy, Bensebaa et al.3 found the change of the SAM was reversible below 350 K; however, above 350 K irreversible disordering at elevated temperatures was observed. Dubois and co-workers6 concluded that the degree of disorder increases with increasing temperature in ultrahigh vacuum (UHV) by following the peak position and intensity of the asymmetric methylene stretch in the CH fingerprint region. Other studies have revealed that annealing as-deposited monolayers in a vacuum improves the surface order.7-9 It was thought that, at moderate temperatures (320 K), annealing monolayer films acts to heal defects within the film and increase the domain size. Although the thermal stability of SAMs has been investigated from several approaches, little effort has been made to understand the influence of temperature on the frictional properties of the monolayer. The friction and wear (tribological) properties of SAMs are relevant to applications involving systems where monolayer films serve as lubricant coatings or where films are subject to intermittent contact with an opposing surface. As applications involving SAM films as tribological coatings will operate over a range of temperatures, a fundamental understanding of the influence of temperature on interfacial friction is clearly required. Two previous investigations employing molecular dynamics simulations have addressed the influence of temperature on the frictional properties of ordered organic monolayers; however, these have been conducted only over a low-temperature range (5-300 K).10,11 This work focused on changes in interfacial friction at temperatures surrounding the rotator transition temperature of the films, the temperature at which thermal activation of molecular rotation within the film occurs. Some experimental work has been carried out on the temperature-dependent frictional properties of Lang(10) Glosli, J. N.; McClelland, G. M. Phys. Rev. Lett. 1993, 70, 1960. (11) Ohzono, T.; Fujihira, M. Phys. Rev. B 2000, 62, 17055.
10.1021/la034354q CCC: $25.00 © 2003 American Chemical Society Published on Web 06/26/2003
6136
Langmuir, Vol. 19, No. 15, 2003
muir Blodgett films and silane-based SAMs on Si; however, the nature of attachment to a surface has been shown to strongly influence numerous properties of SAMs.12-14 In addition, several studies have investigated the correlation between molecular order and friction, demonstrating that densely packed and well-ordered SAMs exhibited lower friction than loosely packed disordered SAMs.15,16 However, to our knowledge, no atomic-scale experiments have investigated the influence of temperature on frictional properties and its correlation to the surface structure of SAMs. In the present investigation, we have employed variabletemperature atomic force microscopy (VT-AFM) to explore the temperature-dependent changes in the surface order and interfacial friction between alkanethiol SAMs and a silicon nitride probe tip. AFM has become an important tool in nanotribological studies due to the access provided to atomic-scale interfacial forces for contacts of controlled geometry. VT-AFM offers the possibility of in situ measurements of friction and surface order over a range of surface temperatures. In this study, the structural and frictional properties of hexadecanethiol SAMs adsorbed on Au(111) have been investigated from room temperature to 500 K. An irreversible change in both film quality and frictional response is observed near 350 K. Experimental Section Gold substrates were prepared by thermal evaporation of gold onto mica. Following metal deposition, the gold substrates were rinsed with ethanol, dried under a stream of N2 gas, and immersed at room temperature for more than 24 h in 0.1 M solutions of CH3(CH2)15SH (henceforth denoted as C16) with ethanol as the solvent. After removal from the solution, the SAMs were again thoroughly rinsed with ethanol and dried under a stream of N2. Following this procedure, SAMs were introduced to the UHV chamber through a vacuum load lock, thus obviating the need for potentially damaging bakeout procedures. All AFM measurements were performed in an ultrahigh vacuum chamber with a base pressure of 4 × 10-10 Torr. Topographic images and interfacial force measurements were performed with an Omicron variable-temperature AFM, in turn controlled by Omicron Scala Pro 4.1 software. In this approach, samples were mounted on a pyrolytic boron nitride (PBN) heater, which traveled with the sample holder, while the tip was mounted on the piezoelectric scan tube. Temperature was recorded through a silicon diode mounted on the sample holder and calibrated through prior measurements where a thermocouple was also mounted on the sample surface. Temperatures are reported with an error of (5 K for the entire measurements set; however, the relative error between measurements of the same set are less than 1 K. The topographic and frictional data were recorded with Si3N4 tips (Digital Instruments, Santa Barbara, CA) with a nominal radius of ∼50 nm. Normal forces were recorded in accord with the manufacturer’s reported spring constant (0.58 N/m), while frictional forces, ascribed to the lateral torsion of the cantilever during scanning, were recorded as the raw voltage output of the photodetector (V). For the purposes of discussion, a frictional response is defined as the slope of the plot of friction versus load. For all measurements of interfacial friction, data reported in the same plot were recorded with the identical cantilever tip assembly in order to avoid errors introduced (12) Lio, A.; Charych, D. H.; Salmeron, M. B. Langmuir 1996, 12, 235. (13) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Lu¨thi, R.; Howald, L.; Gu¨ntherodt, H.-J.; Fujihara, M.; Takano, H. Nature 1992, 359, 133. (14) Briscoe, B. J.; Evans, D. C. B. Proc. R. Soc. London, Ser. A 1982, 380, 389. (15) Yoshizawa, H.; Chen, Y. H.-L.; Israelachvili, J. J. Phys. Chem. 1993, 97, 4128. (16) Hayashi, K.; Sugimura, H.; Takai, O, Japn. J. Appl. Phys. Part 1 2001, 40, 4344.
Yang and Perry through differences in cantilever force constants. Furthermore, calibration and blank experiments determined that the coupling between the normal and lateral motions of the cantilever was less than 10% and was independent of temperature. These measurements (data not shown) entailed simultaneously monitoring the normal and lateral force channels during forcedistance measurements carried out as a function of temperature. Lateral force images were collected by plotting the lateral torsion of the cantilever as a function of the location across the sample surface. Frictional forces were measured as a function of normal load by rastering the sample in a line scan mode while first increasing and then decreasing the applied load. During this procedure, frictional and normal forces were measured simultaneously with a scan speed of 400 nm/s over a distance of 50-100 nm. The reported friction data represent the average of a number of friction results obtained at different locations across the SAM surface. Additional details of this procedure can be found elsewhere.15,16 The thermal effects on surface structure and friction were determined by performing these procedures at evaluated temperatures in the VT-AFM. Plots of friction versus load are displayed for data collected as a function of increasing load. Similar data were obtained for friction measured as a function of decreasing load with the exception of adhesion hysteresis apparent in the negative load regime.
Results The film character of SAMs generated by the adsorption of C16 onto Au(111) was evaluated in a vacuum by AFM. Multiple investigations were conducted with a number of C16 films and produced the consistent results presented below. A topographic image of the surface (500 nm × 500 nm) revealed no features other than the terraces and steps arising from the gold substrate. Steps within the image were approximately 0.25 nm in height and consistent with single steps of a Au(111) surface. Terrace widths were between 100 and 150 nm. All the friction measurements described below were performed within single terraces of the surface. As a result, step edges are not thought to have contributed significantly to the measured frictional properties. Molecular-level ordering within the film structure was revealed through the stick-slip motion of the cantilever evident in 10 nm × 10 nm lateral force images. The lateral force image of the as-introduced film at 294 K is shown in Figure 1 panel A, together with an FFT analysis of the image in panel B. While regions of local order are evident through the hexagonal structure of the (x3 × x3) known adsorption geometry of long-chain alkanethiols formed from solution on Au,17,18 long-range order throughout the imaged region is not apparent. The lack of long-range order is consistent with the FFT image of the region that includes substantial intensity at irregular spacings. (The presence of only four spots in FFT images has been observed by others in previous studies and arises from an asymmetry in apparent resolution with respect to fast and slow scanning directions.17) Nonetheless, the intensity at intermediate spacings in the FFT image clearly indicates a lack of molecular order or film stability under scanning. In contrast to these results, the lateral force and corresponding FFT image measured at 294 K, after the sample has been annealed to 350 K, are shown in Figure 1, panels C and D. Here, a greater degree of order within the lateral force image is observed and the FFT image exhibits the characteristic six spots with hexagonal symmetry. The spacing of the molecular-level stick-slip events is ∼0.5 nm, again consistent with the spacing for (17) Lee, S.; Shon, Y.-S.; Colorado, R., Jr.; Guenard, R. L.; Lee, T. R.; Perry, S. S. Langmuir 2000, 16, 2220. (18) Kim, H. I.; Koini, T.; Lee, T. R.; Perry, S. S. Langmuir 1997, 13, 7192.
SAM Friction, Molecular Order at High Temperatures
Figure 1. (A) Lateral force image collected at 294 K of the as-introduced C16 SAM. (B) FFT image of the lateral force image in panel A indicating the presence of a degree of crystallographic order. (C) Lateral force image collected at 294 K of the C16 SAM following an anneal to 350 K. (D) FFT image of the lateral force image in panel C indicating an increase in surface order. The image size is 10 nm × 10 nm for all images.
the well-known hexagonal structure. These results are consistent with changes in the molecular order of a C12 monolayer observed by X-ray diffraction following an anneal to 363 K.7 The relative line widths of those studies suggested that the domain size of an as-deposited film grows from ∼9 nm to greater than 100 nm following the thermal treatment. While not quantitative, the lateral force images presented here are also consistent with an increase in the monolayer domain size. The effect of the thermally induced ordering on the interfacial friction between the C16 film and a silicon nitride probe tip is seen in the plots of friction versus load. The results obtained for increasing loads are shown in Figure 2A for both the as-introduced film and the film following an anneal to 350 K. These data indicate that an approximately 4-fold reduction in friction accompanies the increase in film order seen in the lateral force images. Further insight into the phenomenon is provided by the plot of the frictional response of the film as a function of temperature shown in Figure 2B. To create this plot, friction-load data were collected as a function of first increasing and then decreasing temperature over the range 294-350 K. From plots similar to those of Figure 2A, the frictional response was measured as the slope of the friction-load plot through a best-fit procedure and plotted as a function of film temperature. While this approach neglects the curvature of the plots at lower loads that arises from the elastic deformation of the tip-film contact, it is only used as a means of summarizing the changes occurring as a function of temperature. Error bars in this plot represent the statistical variance of friction measurements carried out in different locations on the film surface. The influence of vacuum annealing the C16 film is seen in the gradual decrease in the frictional response with increasing temperature, followed by a second decrease in the frictional response upon cooling
Langmuir, Vol. 19, No. 15, 2003 6137
Figure 2. (A) Friction is plotted versus increasing load for the as-introduced C16 SAM (b) and the same film following an anneal to 350 K (2). (B) Plot of the frictional response of the C16 SAM as a function of increasing and decreasing temperature over the range of 294-350 K depicts an irreversible change in frictional response occurring during the initial thermal treatment.
Figure 3. Frictional responses of the C16 SAM measured over the 290-350 K temperature range indicate a reversible change in friction following the initial thermal treatment.
back to ambient temperatures. The data points in the far upper left and far lower left correspond to the friction load plots of Figure 2A. These data were obtained in sequence with the sample residing at the different temperatures for approximately 30 min; no attempt was made to evaluate the kinetics of the process. The change portrayed by the data in Figure 2A represents an irreversible lowering of the interfacial friction upon vacuum annealing and again agrees well the lateral force images (Figure 1A) and with X-ray diffraction studies of similar films indicating an increase in film order.7 Prior studies in our laboratory of alkanethiol SAMs of similar chain length but different film order have demonstrated a close correlation between the frictional properties of the films and the degree of disorder as measured with infrared spectroscopy, with friction increasing with increasing disorder. Once annealed, the frictional properties of the C16 SAM exhibit a systematic and reversible temperature dependence for temperatures up to ∼350 K. Figure 3 displays the frictional response of the annealed film as a function of film temperature. An approximate 2-fold increase in friction is observed with increasing temperature. Upon cooling to ambient temperatures, the frictional response returns to its original value. The temperature dependence of the frictional response (and the corresponding frictional
6138
Langmuir, Vol. 19, No. 15, 2003
Yang and Perry
Figure 5. (A) Topographic image of sample surface after heating to 500 K. Cross-sectional plots of the surface height are shown for (B) line 1 and (C) line 2. Figure 4. (A) Plot of the frictional response of the C16 SAM as a function of increasing and decreasing temperature over the range of 294-500 K depicts an irreversible change in frictional response with a threshold of approximately 350 K. (B) Lateral force images collected with increasing film temperature portray increasing film disorder. (C) By 400 K, the underlying gold substrate was evident in the lateral force image due to the facile displacement of the monolayer at this temperature. The FFT image of the lateral force image at 400 K reveals the a 0.3 nm spacing, consistent with the dimensions of the Au(111) surface. The image size is 5 nm × 5 nm for all images.
forces at given loads) is interpreted in terms of the phononic model of friction and is discussed in greater detail below. Experiments conducted at film temperatures above 350 K result in an irreversible change in the properties of the film. Figure 4A displays the frictional response measured as a function of increasing and decreasing temperature in the range of 294-500 K. These data depict a significant threshold in the frictional response of the interface at ∼350 K, with a 4-fold increase in frictional response observed between 350 and 370 K. Above 370 K, the frictional response remains relatively constant up to 500 K; however, it is observed to increase slightly upon cooling to ambient temperatures. A lateral force image of the film at 370 K contained little indication of surface order, although the film was still clearly present. However, at 400 K, lateral force images of the surface indicated the onset film desorption (or the mobility of the film under the light load of the tip)
through the appearance of the lattice resolved image of the crystalline gold surface. Figure 4 displays the lateral force and corresponding FFT images (B) before annealing and (C) at 400 K. From the FFT image of the 400 K image, a 0.3 nm spacing of the stick-slip events was measured, corresponding to the nearest-neighbor distance of the Au(111) surface. Topographic images of the surface collected after annealing to 500 K also indicated significant film decomposition. Figure 5A displays a representative topographic image revealing the step and terrace structure of the gold substrate decorated by a number of islandlike structures. The stepped structure of the surface and the dimensions of the islands are depicted in the crosssectional plots of surface height shown in Figure 5, panels B and C. A rough estimation of coverage from their volume and distribution density indicates that their composition is consistent with the presence of residual thiol (∼8% of the monolayer). The exact mechanism by which these islandlike structures are formed is not revealed through these measurements. However, additional experiments performed with a different film sample ramped to 500 K and then rapidly quenched to room temperature resulted in a frictional response intermediate between the two ambient temperature frictional responses of Figure 4, indicating a relatively greater degree of film retention following this procedure. This result suggests that the islandlike structures arise from the coalescence of the alkanethiol film as one step in the desorption pathway.
SAM Friction, Molecular Order at High Temperatures
Discussion Within this study, three distinct influences of temperature on the friction and molecular order of hexadecanethiol films adsorbed on Au have been observed. In each case, there exists a close relationship between the molecular order and stability of the film and the recorded frictional response. First, annealing the as-deposited film under vacuum to 350 K results in a significant reduction in the room-temperature frictional response of the film, as well as an increase in the molecular order within the film. While the lateral force images and corresponding FFT analysis represent only a qualitative measure of order, the results agree well with the increase in domain size of adsorbed alkanethiol films observed by grazingangle X-ray diffraction following anneals to similar temperatures under vacuum.7 STM measurements of hexadecanethiol monolayers performed as a function of surface annealing have indicated that the healing of substrate vacancy defects likely plays a role in this process.8 With the understanding of an increase in film order, the decrease in interfacial friction is ascribed to the reduction of lower density domain boundaries within the film. Prior AFM studies of alkanethiols of systematically varying film density have shown that greater energy dissipation occurs at the surface of films of lower density through channels of molecular deformation.15 It is interesting to note, although not understood at this time, that previous attempts in our laboratory employing anneals to ∼350 K under ambient pressures did not result in a similar reduction in friction. The second influence of temperature observed in this study can be characterized as a reversible increase in friction with increasing temperature over the range 290350 K. Increasing friction again is ascribed to a decrease in the local effective density of the film and thermally induced molecular disorder. Reflection absorption infrared spectroscopy (RAIRS) of alkanethiols with chain lengths ranging from 15 to 22 indicates that the chains undergo a decrease in tilt angle (leading to a lower effective density) and an increase in the density of gauche defects within the film over the temperature range of 50-350 K.3 Identical effects have been observed in molecular dynamics simulations of hexadecanethiol monolayers adsorbed on gold.19 The observed reversible nature of the friction is consistent with such a reversible change in film structure. As before, the increase in friction is ascribed to additional channels of energy dissipation through the excitation of molecular motion that exist in the less ordered film (the exact molecular pathway of this process is unfortunately not accessible through these AFM experiments). Third, an irreversible increase in interfacial friction is observed when the film temperature exceeds a threshold of ∼350 K. Several experimental results suggest that this represents the point of significant molecular mobility within the self-assembled film. Within the present work, the observation of the lattice-resolved image of the gold substrate under low loads at 400 K indicates that the monolayer is easily displaced by the AFM tip. At room temperature, significantly higher loads (and often very sharp tips) are required to produce similar results.17 In the previously referenced RAIRS measurements, substantial disordering was observed above 350 K and described in analogy to crystalline melting.3 By 450 K, spectroscopic changes indicated further disordering in the (19) Liu, G. Y.; Salmeron, M. B. Langmuir 1994, 10, 367. (20) Motomatsu, M.; Mizutani, W.; Nie H.-Y.; Tokumoto, H. Thin Solid Films 1996, 281-282, 548. (21) Hautman, J.; Klein, J. L. J. Phys. Chem. 1990, 93 (10), 7483.
Langmuir, Vol. 19, No. 15, 2003 6139
form of film desorption and/or decomposition.3 We thus conclude that the corresponding increase in friction observed over this temperature arises from energy dissipation in the form of plowing, thiol displacement, and film decomposition (tip-induced desorption). Finally, it is important to note that the influences of temperature on the friction and molecular order reported and described above are specific to the explicit temperature range explored and to alkanethiol monolayers. Theoretical studies have previously considered the influence of temperature on friction and molecular order over the range of 50-200 K in the neighborhood of the rotator transition temperature of the monolayer and reported complex (both increasing and decreasing friction) over this range. In the present studies, all films exist well above this transition and the described influences arise from separate effects. With respect to film composition or type, several other studies have reported different temperature-dependent frictional properties of Langmuir Blodgett films and organosilane SAMs.12-14 Variations in temperature-dependent behavior primarily can be linked to the chemical/ physical nature of film adsorption. For example, the decrease in friction with increasing temperature reported for organosilane SAMs over the temperature range ∼300600 K was encountered for chemically anchored films and for loads ∼2-10 times higher than the loads employed here, where substantial film deformation is occurring.14 The difference in results for the different film systems further highlights the role of molecular structure and adsorption properties in determining frictional properties and confirms the importance of considering multiple properties of a film in describing the frictional properties of the system. Conclusions The temperature-dependent surface structure and friction of hexadecanethiol SAMs have been examined by variable-temperature AFM. A number of different changes in the molecular order and structure of the film are observed over the 294-500 K temperature range and lead to correlated changes in the frictional properties of the film system. Initial annealing of the as-deposited film to 350 K produces an irreversible 4-fold decrease in the roomtemperature frictional response of the film due to an increase in the domain size of the film structure. Following this treatment, a reversible change in friction is observed between 294 and 350 K, with friction increasing with increasing temperature as a result of decreased effective film density. Above 350 K, irreversible and large increases in friction are observed with the corresponding “melting” and desorption of the alkanethiol film. The range of influences of temperature on the frictional properties of these alkanethiol films illustrates the complex relationship between friction and film structure on the molecular level. Similar temperature-dependent frictional effects are anticipated in other SAM systems as well as under ambient conditions, although the exact details of the temperature dependence will depend on the chemical and structural details of the system. Acknowledgment. This work has been supported by the National Science Foundation (CAREER Award to S.S.P., CMS-9876042) and the Air Force Office of Scientific Research under Contract 49620-01-1-0424. LA034354Q
