J. Phys. Chem. 1994, 98, 11136-11142
11136
A New Mechanism for Surface Diffusion: Motion of a Substrate-Adsorbate Complex S. J. Stranick,? A. N. Parikh,* D. L. AllaraJ7$ and P. S. Weiss*J Departments of Chemistry and Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received: May 2, 1994; In Final Form: July 20, 1994@
We propose a new mechanism for diffusion of surface adsorbates in which motion of the substrate atoms to which the adsorbates are attached results in the motion of the substrate-adsorbate complex. We show an experimental example-the motion of self-assembled monolayers of CH302C(CH2)15SH on gold which we have observed by time-lapse imaging using scanning tunneling microscopy. This mechanism is also used to explain previous data for motion of Cu-0 complexes on the Cu{ 1lo} surface.
I. Introduction Diffusion on surfaces is important in surface processes where adsorbates must reach special sites to undergo reaction, in film growth to allow epitaxial or other special growth modes, and in other surface phenomena such as etching, corrosion, and Diffusion involving complex molecules, such as those in self-assembled monolayers (SAMs), has important implications because molecular films are being used in attempts to create complex, multifunctional surface architectures such as artificial receptor sites4 and controlled-size transport chann e l ~ .The ~ dynamics of formation and the stability of these structures will depend upon the diffusion of the adsorbates. In this regard, recent evidence indicates that mixed composition alkanethiolate SAMs on Au{ 11l} surfaces can undergo phase segregation into domains which show time-dependent shapes and sizes.6 Using time-lapse scanning tunneling microscopy (STM), we have discovered evidence for motion of adsorbatesurface complexes of C H ~ O ~ C ( C H ~ ) I ~ Sacross - A U the SAMcovered Au{ 11l} surface and have measured the rates of this motion. The rates are significantly slower than previously observed rates of motion of Au atoms on the Au{ 111) surface. We also discuss how the previously recorded motions of chains of Cu-O complexes on the Cu{IIO) surface7 are consistent with this diffusion mechanism. These results indicate how dramatically adsorbates influence the motion of surface atoms.
11. Experimental Section Substrates were prepared by evaporating (pressure = (1-6) Torr) gold onto hot (340 "C) mica. Such surfaces are known to consist of large Au{lll) terraces.* SAMs of CH302C(CH2)15SH were prepared by soaking the substrate in M) for 4 days.9 a solution of the thiol in ethanol (1 x Companion samples on larger substrates were prepared in parallel and characterized by infrared reflection and X-ray photoelectron spectroscopies and by ellipsometry in order to verify the monolayer quality. Experiments were conducted in air using a microwave-frequency-compatible,beetle-style STM with a low drift rate.l0 The samples were sufficiently conductive (through the 22 8, monolayer thickness) to allow use of the dc tunneling current to control the tip-sample separation. The passive drift rate of this microscope is < 1 h m i n , which allows recording a series of time-lapse images of the same area for x
* To whom correspondence should be addressed. Department of Chemistry.
* Department of Materials Science and Engineering. +
@
Abstract published in Advance ACS Abstracts, September 1, 1994.
hours without drift correction. Images were recorded in constant tunneling current mode. All images are shown unfiltered.
111. Results and Discussion Figure 1 shows a series of 20 images of a 500 x 500 .k2 area of the surface. After each image was recorded, the STM tip was repositioned at the center of the imaged area for 15 min before the next image was recorded. During both the delay and imaging periods the STM tip was biased at -2 V with a tunneling current of 2 nA." No special rearrangement of the surface exclusively at the center of the images was observed, and the rate of motion was independent of the delay periods and scan rates, showing that the STM tip did not drive the surface motion. It is important to note that the features (including steps) in the Au substrate are imaged through the thiolate monolayer.'' Thus, the difference in topographic height when the step position has changed is due to Au atoms moving, and yet the Au atoms are still covered by the thiolate layer. We believe that the simplest explanation for the observed motion is that the Au substrate atoms and the thiolate layer move in a concerted manner as developed below. McCarley et al. have observed motion of the alkanethiolate SAMs driven by the STM tip at low tunneling gap impedance, Le., small tip-sample separation^.'^ The resulting surface after imaging showed steps in the underlying Au substrate with extending "fingers". No such convoluted step structures are seen in any of our results. The images in Figure 1 are readily interpreted as a series of terraces separated by monatomic (single Au atom) height steps. The four to six steps in each frame appear as color changes. As a guide to the eye, Figure 2 schematically shows the positions of the step edges in selected frames of Figure 1. Note the changes in the shapes of the step edges from frame to frame. The steps change shape and position between frames at an average rate of approximately 10 atoms/site/h along the step edges. We estimate this rate by measuring the image to image fluctuations in the terrace widths as described by Pimpinelli et aZ.'5,16This rate is substantially lower than that found for bare Au{ 111) in air by Mamin and co-workers and by Cooper and c o - w ~ r k e r s , l ~and - ~ ~shows that the presence of the strongly bound thiolate monolayer20,21greatly increases the barrier to lateral motion. This is in sharp contrast to the observations of others that certain strongly bound adsorbates appear to lower diffusion barriers. Trevor and Chidsey showed that strongly bound C1 adatoms accelerate the diffusion of surface Au atoms in aqueous s o l ~ t i o n . ~Similarly, ~ , ~ ~ Cooper and co-workers
0022-3654/94/2098-11136$04.50/0 0 1994 American Chemical Society
Motion of a Substrate-Adsorbate Cam-'--
m
hys. Chem, Vol. 5
~
,
43, 1994 11137
-
images Figure 1. A sen I & C C I ~ H ~onHAi 0 scanning tunneling microscope images showing a 500 x 500 Aiarea of a SAM were recorded in air in constant current mode at a tunneling current of 2 nA and a tip bias of -2 V. Between each image the tip was positioned at the center of the area shown for 15 min. The start time is impinted in each image as houn:minutes. A number of terraces each separated bv a monatomic height step (2.5 A) are shown. The color scale from top to bottom is red, yellow, blue. The images are unfiltered. large compared to the adsorbate-adsorbate attractions in the case of atomic adsorbates (e&, the Cl-CI interactions). If the motion were to occu in a concerted fashion via an integral Authiolate complex, the chain-chain interactions would substant i d l p d u c e the motion of the "tethered" Au atoms relative to the rates for atomic adsorbates. An example of such motion is shown schematically in Figure 3. Note that as discussed helow Figure 2. Schematics of selected frames from Figure I as indicated the actual stoichiometry of the diffusing complex is not known. showing the motion of SAM-covered An step edges over the sequence of images. Importantly, the above mechanism results in adsorbate diffusion which is fastest at sites with low coordination numbers found that (unidentified) adsorbates accelerated the motion of for the Au surface atoms. This is consistent with observations Au atoms on the Au{ 111) surface in vacuum.ls of atomic adsorption on bare surfaces where the fastest rates of In order to correlate these various observations, it is important motion are at low coordination sites such as step edges and to consider the adsorbate-substrate energetics. It has been kinks.'8 This aspect of the diffusion mechanism should result shown that both chemisorbed S and CI adatoms hind strongly in a SAM fdm with missing adsorbate (point) defects approachto the Au surface atoms, thereby weakening the bonds between ing an equilibrium structure faster at and near step edges than the attached Au atoms and their n e i g h h ~ r s . ~ *This . ~ ~ would on large Au{ 111) terraces. In accord with this we observe that, lower the harrier to diffusion and lead to more mobile surface on narrow terraces of the Au (SIC€-200 8, wide as in Figure Au atoms. On this basis, the thiolate S atoms should similarly 1). few point defects in the film are found as compared to wider allow fast surface diffusion of Au atoms; however. the opposite terraces. effect is observed. The simplest underlying factor which For use helow, here we categorize defects in S A M s on a controls the relative rates of surface motion between the thiolate and the chloride cases is the adsorbate-adsorbate i n t e r a ~ t i o n . ~ ~ phenomenological hasis and briefly discuss the mobilities and This conclusion is strongly supported by the fact that attractive tentative assignments that have been made for these surface interactions between the alkyl chains in the alkanethiolates are features found in STM and other scanned probe microscopies~
11138 J. Phys. Chem.. Vol. 98, No. 43, 1994
Stranick et al.
1
3 Figure 3. Schematic showing the motion of a Au-thiolate complex at a Au( I I I ) step edge. For clarity, the alkyl chains are represented as tilted ban on the upper terrace and are not shown on the lower terrace. The moving Au atom in the complex is shown in black. The outermost Au atoms of the step edge are shaded in gray. In each frame. the final position of the moving thiolate S is circled.
Point defects (type I) appear as 18-25 8, diameter, I A deep depressions in STM images. They are not mobile while the surface is scanned at large tip-sample separation. These have been proposed as due to missing chain sites (molecular voids) with conformational relaxation of the surrounding matrix of alkyl chains? Larger area (z30 8, diameter) defects (type 11) do move under the influence of the tip andor on their own. These generally exist in films with shorter deposition periods than those used If larger defects correspond to regions of multiple missing adsorbate chains, these may be expected to show some mobility with respect to the influence of the STM tip since bare substrate areas provide open sites for adsorbate translation as well as space for chain segment motion via conformational isomerization. There are other large area defects (type 111) which exhibit unmistakable signs of being due to monatomic pits in the Au substrate-the defect topographic height is depressed from the surrounding surface by the substrate atomic layer spacing. These defects are mobile, and this motion can be driven by scanning if the tip-sample separation is small enough. Type I11 defects are observed to annihilate at substrate step edges.’4,26-28There is not general agreement that type II and type 111 defects on Au( 11 I } terraces are distinct, and some type II defects have also been attributed to regions in which underlying Au atoms have been removed.26 Yet another defect has been identified by Michel and co-workers. who have recently recorded molecularly resolved STM images of SAMs showing stable nanometer scale domains which they assigned as nanometer scale regions of thiolate molecules having different stable molecular conformationsfrom the majority alkanethiolate species (type IV).” The topographic height differences of these domains were (1 A, and the domains were stable with time under ambient conditions but were larger upon annealing in solution. (The average domain size changed from 65 to 120 A final possibility is for having one or more impurity molecules within the SAM of another alkanethiolate (type V).6 In the case of intentionally mixed composition monolayers. phase-segregated domains as small as one molecule showed topographic height differences dependent upon the identity of the molecule. The smallest of these domains were 5-1 8, in diameter, consistent with the spacings of the alkanethiolates in
the SAMs, and smaller than any features observed in sioglecomponent films6 The “impurity domains” of similar length alkanethiolatesexhibited slow motion on the surface apparently via molecular exchange within the densely packed layers? It is critical to note that in our measurements that (type I) point defects on large terraces are never observed to move at room temperature. This finding argues against the ability of thiolates on terraces to move independently to step edges and supports defect annihilation by a fluctuating flow of step edges via motion of integral adsorbate-Au complexes. When the Au step edge reaches a point defect (type I) site. the defect is swept off the terrace. In this way regions of unligated Au atoms (missing thiolate) can accumulate at step edges. essentially forming type I1 defects there. Supporting observations for the above point defect removal mechanism are given in Figure 1, which shows regions beneath the step risers that appear lower than the lower terrace heights. Figure 4a shows the third frame of Figure I . One of these regions is visible as a blue (low) section in the image. Figure 4h shows a cross section along the black line indicated in Figure 4a. This cut shows two monatomic steps along with the associated depression below the step riser (indicated by arrows in Figure 4h). Such depressions are not observed on the bare Au( 11 I } surface. Going back to the frames shown in Figure I , these regions can be seen to change size and shape along the step edges with time. We assign the observed depressions at the step edges to two effects. First. the step edge breaks the translational order of the film at the boundary between the domains defined by the upper and lower terraces. This results in reduced density and conformational relaxation at this boundary as in type I1 defects. Second, these boundaries also function as regions of the film in which unligated Au atoms (missing chains) are accommodated. The observed depressions have various depths and widths as for type II defects. because the density varies according to the number of molecular voids accommodated ar any given region of the step edge. This assignment is discussed further below. Also. referring to Figure I. the motion of the steps is not smooth. Rather. the observed step motion is fastest at the sites of the largest depressions. This
Motion of a Substrate-Adsorbate Complex
0
50
J. Phys. Chem, Vol. 98, No. 43, 1994 11139
100
150
200
250
Lateral Distance (in A) Figure 4. (a, top) Frame 3 (035) from the series of 500 x 500 A* images in Figure 1, showing a line where the cross-sectional view in (b) is taken. The color bar shows a 12 8, range. @, bottom) A cross section along the line shown in (a). Two monatomic height steps in the Au{lll) surface are shown. The “defect pols” are labeled with mows.
11140 J. Phys. Chem., Vol. 98, No. 43, 1994 is consistent with the most mobile adsorbate-gold moieties having fewer neighbor alkanethiolate chains (and less conformational order, see below), thereby reducing the attractive interactions between chains. It is important to consider other possible origins of the features assigned as defect pools. The three important possibilities are (i) monatomic Au substrate pits, (ii) alkanethiolate molecules in domains of different molecular conformations, and (iii) segregated impurities. Monatomic (or deeper) pits in the Au substrate on the lower terrace (i) as in type I11 defects would appear as topographic features depressed one (or an integral number) of Au atoms deep with flat centers for the largest such features as on other Au terraces observed. Such doubly depressed features are not observed. With regard to different alkanethiolate conformations (ii), as in type IV defects, such densely packed structures are also unlikely to lead to greatly enhanced diffusion. Finally, segregated domains of impurities as in type V defects at step edges (iii) are ruled out on the basis of the motion, appearance, and disappearance of the defect pool regions. The observed fluidity of these regions would require the segregated impurities to have extremely high mobility through the SAM film. On the basis of these arguments, we conclude that the most reasonable interpretation of the defect pools is that they consist of surface regions of low thiolate density (as in type II defects) which have segregated to substrate step edges. An important aspect of the observed phenomena is the driving force for the relaxation of the SAM toward the de facto segregation of the unligated Au atoms to the (vicinity of the) Au step edges. This is understandable on the basis that the film structure on terraces is optimized so as to maximize the van der Waals attractions between chains. Since molecules attached to different terraces separated by step edges would not be expected to interact as favorably as those adjacent on the same terrace, it is energetically favorable to reduce the film density at the step edges and complete the film structure on terraces away from step edges to the extent possible. As noted above, this has the effect of accelerating the motion in these regions as observed e~perimentally.~~
IV. Energetics Some data are available on the energetics of kink motion on step edges, and it is critical that the proposed thermally activated diffusion mechanism be consistent with these energetics. Kinkdriven step edge motion is well-known on fcc metal surf a c e ~ . The ~ ~ activation ~ ~ ~ . energy ~ ~ for kink motion along the step edge of a large Ir cluster on clean Ir{lll} has been determined by field ion microscopy measurements31 to be -90 kJ/mol as compared to the cohesive energy of Ir, 670 kJ/m01.~~ Scaling to the cohesive energy of Au, 368 kJ/mol, we can estimate this activation energy for bare gold to be 50 kJ/mol, identical to the measured upper limit of 50 kJ/mol reported by Trevor and Chidsey.22 Bonding S or C1 atoms to the (001) faces of Ni and Cu reduces the bond strength of the bound metal atoms to their neighbors as evidenced by interlayer and intralayer relaxations and increased Debye-Waller factors in SEXAFS measurem e n t ~ .Bonding ~~ of S or C1 to the Au{ 11l} surface is expected to have similar consequences on the Au-Au bonding, leading to a small reduction of the cohesive energy of the bound Au atoms, which we estimate to be 10-15%.34 We might further hypothesize that the transition state energy, and thus the activation barrier, have likewise been reduced by the same fraction. Using a simple Arrhenius activation energy dependence, and 50 kJ/mol for bare Au from above, the diffusion
Stranick et al. rate at room temperature would thereby increase by a factor of 7-20 upon chemisorption of C1 or S adatoms. (The additional bonding of an alkyl chain to the S should reduce these effects somewhat.) Contrary to the accelerating effects of the S-Au bond on diffusion, the attraction between the alkyl chains should slow diffusion. The packing energy of a quasi-two-dimensional C16 alkanethiolate domain is -74 kJ/m01.~~ This is approximately 1.3 times the attraction per methylene unit found for alkane liquids (from heats of fusion). To estimate this contribution to the activation energy due to the alkyl chain tether, we take the difference in the attraction between chains in the SAM (-74 kJ/mol) and in the alkane liquid (-57 kJ/mol), resulting in a value of -17 k . T / m ~ l .The ~ ~ total activation energy for moving the gold-thiolate complex we thus estimate to be -59-62 kJ/ mol.37 At room temperature this would reduce the rate of motion of the attached gold roughly by a factor of 1000 relative to bound C1 (or S) and by a factor of 100 relative to bare gold.38 The stoichiometry of the diffusing complex remains unclear. Sellers et al. have calculated that the 3-fold hollow site, sp3, thiolate species has almost the same energy as the atop, sp, species.39 It may then be that the mobile thiolate-Au complex consists of the thiolate species attached to a single Au atom which moves along the surface as a step adatom. Further experiments are planned at reduced temperature to capture images of isolated hops in order to identify the diffusing adsorbate-substrate complex. Recent X-ray diffraction experiments bring into question the bonding arrangement of the alkanethiol to the surface, suggesting that a disulfide species is bound to the surface.40 Measurements of individual hops of the diffusing species may also cast light on this issue. We point out that this adsorbate-substrate diffusion mechanism is consistent with other observed properties of alkanethiolate SAMs on Au. In particular, we have shown that mixed composition SAMs can phase segregate6 and that motion of the adsorbed molecules is required to form the domains. The evidence presented here supports one contributing mechanism of such motion involving an integral gold-thiolate complex.41 This mechanism is also consistent with the proposal of Camillone et al. that the presence of mobile defects are responsible for thermal annealing of monolayer structures in vacuum as determined by He atom and X-ray diffraction?2 Finally, we note that other more complicated mechanisms may display similar kinetics and discuss some of these alternatives here. One possibility is that Au atoms move underneath the thiolate layer and are slowed by their interactions with the thiolate S atoms. As observed, motion would be more rapid where the thiolate density is lowest. This mechanism would then require independent motion of the thiolate species. However, such mobility is not observed for isolated point defects which implies that isolated regions of missing chains on terraces are generally immobile on the time scale of the observed step motions. Perhaps such thiolate motion would be possible at substrate step edges with reduced coordination and reduced adsorbate-adsorbate interactions. A good test of our proposed mechanism in this regard will be to measure the diffusion rates vs alkyl chain length. Longer alkyl chains should stabilize the SAM surface structures due to the increased chain-chain attractions. Shorter chains should allow faster surface motion consistent with the energetic considerations above. A series of experiments are being carried out to provide quantitative tests of our predictions. Another important consideration not addressed in this study is the role of small molecules such as water and ethanol adsorbed on or included in these films. Michel and co-workers have
Motion of a Substrate-Adsorbate Complex recently concluded that for hydrophilic S A M s water and ethanol can be organized on the exposed surface by hydrogen bonding with the exposed alcohol and amine terminal functional gr0ups.4~ We have previously published microwave frequency ac tunneling spectra which may be consistent with the presence of water in or on hydrophobic S A M S . ~The effects that adsorbates have on surface motion remains unexplored but will also be addressed in reduced temperature ultrahigh-vacuum experiments.
V. Other Adsorbate-Substrate Complex Motion An additional experimental example of a diffusion mechanism involving an integral adsorbate-substrate atom complex appears to be 0 adsorbed on Cu( 110). On this surface, STM images have shown chains of 0-Cu complexes moving cooperatively across the surface so as to coalesce into stripes of the O/Cu(110) 2 x 1 ~tructure.~Unlike the motion along step edges described above, the chains of the 0-Cu complex move on top of the Cu(ll0) substrate terraces. When the 0-Cu complex chains join larger 2 x 1 structures of the surface, the atoms are more highly coordinated and the mobility of the complex is r e d ~ c e d .We ~ interpret the motion of the chains of 0-Cu complexes as being due to the electronegative oxygen atoms weakening the bonds of the Cu atoms in the complexes to the substrate Cu, thereby enhancing the mohility of complex.
VI. Conclusions and Prospects In summary, we have shown that an adsorbate can be transported across a surface via an integral substrate atomadsorbate complex. It is likely that this mechanism is a general feature of surface diffusion of adsorbates which bind strongly to soft substrates such as the coinage metals. We are currently quantifying the rates of motion due to this diffusion mechanism and determining how this mechanism influences the motion of defects in the SAMs. Further, we are working to compare the rates of motion for these c16 chains to those for shorter chains. By varying the chain lengths, we anticipate being able to control the rates of motion by changing the attractive interactions between the alkyl chains.35
Acknowledgment. The authors thank Urs Dung, Gert Ehrlich, Paul Fenter, Michael Natan, Giacinto Scoles, Mark Stiles, John Tully, and George Whitesides for helpful discussions and Kyle Krom for help in the preparation of the figures. The support of BRDC, NSF Chemistry and PYI Programs, and ONR (the above for S.J.S.and P.S.W.) and NSF DMR (Grant DMR9001270 for A.N.P. and D.L.A.) is gratefully acknowledged. References and Notes (1) Somorjai, G. A. Chemistry in Two Dimensions: Surfaces; Come11 University Press: Ithaca, NY, 1981. (2) For example: Allara, D. L.; Hebard, A. F.; Padden, F. J.; Nuzzo, R. G.; Falcone, D. R. J . Vac. Sci. Technol. A 1983, 1 , 376-382. (3) Lombardo, S. J.; Bell, A. T. Surf. Sei. Rep. 1991, 13, 1-72. (4) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988,332,426-429. Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991,254, 1312-1319. Haussling, L.; Ringsdorf, H.; Schmitt, F.-J.; Knoll, W. Langmuir 1991, 7, 1837-1840. ( 5 ) Takehara, K.; Takemura, H.; Ide, Y. J . ColloidZnteface Sci. 1993, 156, 274-278. (6) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J . Phys. Chem. 1994, 98, 7636-7646. (7) Jensen, F.; Besenbacher, F.; Lsesgaard, E.; Stensgaard, I. Phys. Rev. B 1990, 41, 10233-10236. Wintterlin, J.; Schuster, R.; Coulman, D. J.; Ertl, G.; Behm, R. J. J . Vac. Sci. Technol. B 1991, 9, 902-908. Kuk, Y.; Chua, F. M.; Silverman, P. J.; Meyer, J. A. Phys. Rev. B 1990,41, 1239312402. (8) Chidsey, C. E. D.; Loiacono, D. N.; Skater, T.; Nakahara, S. Surf. Sci. 1988, 20, 45-66. Hallmark, V. M.; Chiang, S.; Rabolt, J. F.; Swalen, J. D.; Wilson, R. J. Phys. Rev. Lett. 1987, 59, 2879-2882.
J. Phys. Chem., Vol. 98, No. 43, 1994 11141 (9) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J . Am. Chem. SOC.1990, 112, 558-569. (10) Stranick, S. J.; Weiss, P. S. Rev. Sci. Instrum. 1994,65, 918-921. (1 1) By measuring the tunneling current vs tip-sample separation into point contact with the Au substrate,I2 we have shown that the tunneling conditions used correspond to the (nominally Pt-Ir) STM tip being farther from the Au substrate than the outermost portion of the alkanethiolate layer. Durig et al. found for their combination STIWAFM that similar tunneling conditions in ultrahigh vacuum led to their (nominally W) STM tip applying pressure to and thus compressing self-assembled films of the equivalent alcohol-terminated thi01.I~ We have shown in studies of mixed compositionself-assembled monolayers that we are sensitive to the identity of the outermost functional group of the thiolate.6 We conclude that the STM tip is not in the thiolate layer but that, under the mild tunneling conditions used, it is unclear if and to what extent the STM tip is compressing the thiolate films. (12) Gimzewski, J. K.; Moller, R. Phys. Rev. B 1987, 36, 1284-1287. (13) Diirig, U.;Ziiger, 0.;Michel, B.; Haussling, L.; Ringsdorf, H. Phys. Rev. B 1993, 48, 1711-1717. (14) McCarley, R. L.; Dunaway, D. J.; Willicut, R. J. Langmuir 1993, 9, 2775-2777. (15) Pimpinelli, A.; Villain, J.; Wolf, D. E.; Mttois, J. J.; Heyraud, J. C.; Elkinani, I.; Uimin, G. Surf. Sci. 1993, 295, 143-153. (16) Bartelt et al. have described a more quantitative way of determining atomic motion along step edges in: Bartelt, N. C.; Goldberg, J. L.; Einstein, T. L.; Williams, E. D.; Heyraud, J. C.; Mttois, J. J. Phys. Rev. B 1993, 48, 15453- 15456. (17) Mamin, H. J.; Wimmer, P.; Rugar, D.; Birk, H. J. AppE. Phys. 1994, 75, 161-168. (18) Peale, D. R.; Cooper, B. H. J . Vac. Sci. Technol. A 1992, IO, 22102215. Cooper, B. H.; Peale, D. R.; McLean, J. G.; Phillips, R.; Chason, E. In Evolution of Surface and Thin Film Microstructures; Atwater, H. A,, Chason, E., Grabow, M. H., Lagally, M., Eds.; MRS Symposia Proceedings 280; Materials Research Society: Pittsburgh, 1993; pp 37-46. (19) Surface features of clean Au(ll1) apparently do not change in ultrahigh vacuum at room temperature.'* (20) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463 and references therein. (21) The measured Au-thiolate bond strength is -185 kJ/mol.20 If the alkanethiolate layer is removed mechanically, a layer of Au remains attached to the monolayer (Whitesides, G. M., private communication). (22) Trevor, D. J.; Chidsey, C. E. D.; Loiacono, D. N. Phys. Rev. Lett. 1989, 62, 929-932. (23) Trevor, D. J.; Chidsey, C. E. D. J . Vac. Sci. Technol. B 1991, 9, 964-968. (24) In solution it is possible that adsorptioddesorption processes could accelerate diffusion. In vacuum experiments,'* and here, adsorption/ desorption processes are not possible. Other more complex possibilities for explaining the relative rates of motion for chloride and thiolate adsorption are discussed later in the text. (25) We note that when Diirig et al. used shorter incubation periods (20 h) and more dilute solutions (3.1 x M) than those used here, they observed larger film defects which moved under the influence of the STM tip.I3 McCarley et al. did not report deposition conditions but also observed larger defects compared to those reported here, which were mobile while scanning with the STM tip at low tunneling impedance^.'^ (26) Edinger, K.; Golzhauser, A,; Demota, K.; Wo11, Ch.; Grunze, M. Langmuir 1993,90,4-8. Kim, Y.-T.; Bard, A. J. Langmuir 1992,8,10961102. Also, see ref 13. (27) On a small number of occasions we have seen these regions gain sufficient mobility to cross the upper terrace as a mobile one atom deep depression in the Au substrate apparently covered by the thiolate film at reduced density?* These events occurred too infrequently to d e out motion driven by the tip using the strategies outlined above. (28) Cygan, M. T.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. Unpublished data. (29) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Giintherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869-2871. (30) Frohn, J.; Giesen, M.; Poensgen, M.; Wolf, J. F.; Ibach, H. Phys. Rev. Lett. 1991, 67, 3543-3546. (31) Wang, S. C.; Ehrlich, G. Surf. Sci. 1990, 239, 301-332. (32) Kittel, C. Introduction to Solid State Physics, 5th ed.; J. Wiley: New York, 1976; p 74. (33) Sette, F.; Hashizume, T.; Comin, F.; MacDowell, A. A.; Citrin, P. H. Phys. Rev. Lett. 1988, 61, 1384-1387. (34) To our knowledge, there are no available data from which to derive a value for the bond weakening. The value taken is bracketed by limits given by the bulk compressibility, >5%, and by correlating increased bond length vs reduced bond order for metal-metal bonds, '20%. (35) The attractive interactions between chains scale with increasing chain length. An interaction energy of -4.6 kJ/mol per methylene unit2' gives -74 kJ/mol for a -(CH2)&H3 chain.
11142 J. Phys. Chem., Vol. 98, No. 43, 1994 (36) At step edges, the stability of an alkanethiolate species may be reduced by having a smaller number of neighbor chains and by imperfect intermolecular alignment. This may result in a further increase in the expected rate of motion at step edges. (37) The contribution to the activation barrier from the gold bound by an electronegative S is estimated at -42-45 M h o 1 and from the alkyl chain is -17 kJ/mol, giving the total indicated. (38) The effect of the activation entropy has not been included in the estimate of these rates. Chain disorder in the transition state may increase the relative rate for the alkanethiolate-gold complex, but we have no information on the details of such motion with which to make an estimate. (39) Sellers, H.; Ulman, A,; Shnidman, Y.; Eilers, J. E. J. Am. Chem. SOC. 1993, 115, 9389-9401.
Stranick et al. (40) Fenter, P.; Eberhardt, A.; Eisenberger, P. To be published. (41) As described in ref 6, we have observed exchange of individual molecules within the S A M s leading to coalescence of the phase-segregated domains, but this motion is much slower than the step kink motion described here. Solution interchange, although also slow, and physisorbed species in the freshly deposited films may also contribute to the formation of these phase-segregated domains. (42) Camillone III, N.; Chidsey, C. E. D.; Eisenberger, P.; Fenter, P.; Li, J.; Liang, K. S.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 99, 744747. (43) Sprik, M.; Delamarche, E.; Michel, B.; Rothlisberger, U.; Klein, M.; Wolf, H.; Ringsdorf, H. Langmuir, in press. (44)Stranick, S. J.; Weiss, P. S.; Parikh, A. N.; Allara, D. L. J. Vue. Sci. Technol. A 1993, 739-141.
