Langmuir 1995,11, 2609-2614
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Structure of Self-AssembledDecanethiol on Ag(111): A Molecular Resolution Scanning Tunneling Microscopy Study A. Dhirani, M. A. Hines, A. J. Fisher, 0. Ismail, and P. Guyot-Sionnest" James Franck Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637 Received March 16, 1995@ Self-assembly of decanethiol on Ag(111)is studied using ultrahigh impedance scanning tunneling microscopy. The surface frequently shows monaiomic silver islands covering about 20%of the surface. Molecular domains with an average size of -70 A as well as the molecules themselves are also imaged. From these images, three important quantities regarding the molecular structure are determined: (1)the lattice constant is 4.61 dc 0.15 A; (2) two sets of 6-fold lattice orientations separated by 20.7 i 2.3: are observed; (3) far from domain boundaries, height variations of nearest neighbors are less than 0.1 A. A model is proposed to explain how such a structure could have arisen.
Introduction Self-assembled monolayers (SAMs)have many potential applications including the modification of surface wetting properties,' the adsorption of chemicals and biomolecules to surfaces,2controlled electron transfer across interface^,^ and the protection and patterning of surfaces.4-6 SAMs, like all other adsorbates on surfaces, experience three types of interactions: those with the surface, with each other, and with the environment above the surface. In the case of SAMs,the adsorbate-adsorbate interaction due to van der Waals (VDW) forces plays a significant role and is sufficiently strong that the molecules spontaneously form well-ordered layers. An example of a S A M that has been the subject of some attention is long chain alkanethiols (CH3(CH2)n-1SH)on Ag(l11). X-ray photoemission spectroscopy, reflection infrared spectroscopy, and wetting measurements show that an ordered layer is in fact formed.' Furthermore, X-ray and He diffraction studies indicate a lattice with nearest neighbor distances of 4.77 f 0.03 8, and 4.67 f 0.23 A, respectively.6 The diffraction study also indicates the presence of two domains rotated by f12" with respect to the Ag(ll1) lattice or 24" with respect to each other. The relationship between this lattice and the underlying silver surface, however, seems unclear. Fenter et al. interpret their results as evidence for an incommensurate structure, while Sellers et al.9favor the ( 4 7 x d7)R10.9" structure. It is worth noting closely related systems in which the situation is clearer. One is the limit of short (zero or one carbon atom) chain thiols, that is, Ag(ll1) dosed with S2, H2S,and CHsSH. These systems have been studied by methods including low-energy electron diffraction (LEED), sum frequency generation, and Auger Abstract published in Advance ACS Abstracts, July 1, 1995. (1)Abbott, N. L.;Folkers, J. P.; Whitesides, G. M. Science 1992,257, @
1380. -__.
(2)Prime, K.L.;Whitesides, G. M. Science 1991,252,1164. (3)Chidsey, C. E. D. Science 1991,251,919. (4)Swalen, J. D.; et al. Langmuir 1987,3,932. ( 5 ) Ulman, A.A n Introduction to Ultrathin Organic Films;Academic Press: Boston, MA, 1991. (6)Ross, C. B.;Sun, L.; Crooks, R. M. Langmuir 1993,9,632.Sun, L.; Crooks, R. M. J . Electrochem. SOC.1991,138,L23. (7)Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.; Parikh, A. N.; Nuzzo, R. G. J . Am. Chem. SOC.1991,113,7152. (8) Fenter, P.; Eisenberger, P.; Li, J.; Camillone, N., 111; Bernasek, S.; Scoles, G.; Ramanarayanan, T. A.; Liang, K. S. Langmuir 1991,7, e , . n ZlJlJ.
(9)Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J . Am. Chem. SOC.1993,115,9389.
and mass desorption spectroscopy.10-12 Molecular dynamic simulations of SH and SCH3 have also been p e r f ~ r m e d .The ~ consensus among these studies is that a ( 4 7 x 47)R10.9" structure is formed on the Ag(ll1) surface. The other related system is long chain alkanethiols on Au(ll1) where it is known that a (2/3 x d3)R3Oo structure is formed with the hollow site being occupied.13-15 Recent advances in the generalizations of scanning tunneling microscopy (STM), in particular the accommodation of ultrahigh impedances, have led to the ability to resolve the structure of SAMs down to the molecular level and may be able to contribute to a better understanding of these systems. The ultralarge impedance, typically hundreds of gigaohms, permits the tip to stay far enough from the surface so as not to penetrate the S A M and allows the layer to be imaged with molecular resolution. This technique has already been used on gold to unambiguously identify depressions as pits in the surface,16J7to observe C(4 x 2) superlattices,18J9and to study changes in domain structure as afunction ofthermal annealinge20 It is the purpose of the present study to apply ultrahigh impedance STM techniques to S A M s on Ag(ll1) in order to better understand this system. We present here results describing the effects of the monolayer on the topography of the underlying silver substrate as well as the nearest neighbor molecular distances, lattice orientations, and height variations among molecules. Finally, our results and those in the literature are used to propose a model explaining the structure of the S A M s observed. We note that although an earlier STM study of alkanethiols on ~~
(10)Schwaha,K.; Spencer, N. D.; Lambert, R. M. Surf. Sci. 1979,81, 273. (11)Rovida, G.; Pratesi, F. Surf, Sci. 1981,104,609. (12)Harris, A. L.; Rothberg, L.; Dubois, L. H.; Levinos, N. J.; Dhar, L. Phys. Reu. Lett. 1990,64, 2086. (13)Strong, L.; Whitesides, G. M. Langmuir 1988,4,546. (14)Chidsey, C. E. D.; Liu, G.; Rowntree, P.; Scoles, G . J . Chem. Phys. 1989,91,4421. (15)Alves, C. A,;Smith, E. L.;Porter, M. D. J.Am. Chem. SOC.1992, 114,1222. (16)Schonenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994,10,611. (17)Sondag-Huethorst,J.A. M.; Schonenberger, C.; Fokkink, L. G. J. J . Phys. Chem. 1994,98,6826. (18)Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Guntherodt, H. J.; Wolf, H.; Ringsdorf, H. Langmuir 1994,10,2869. (19)Poirier, G. E.; Tarlov, M. J. Langmuir 1994,10,2853. (20)Delamarche, E.; Michel, B.; Kang, H.; Gerber, Ch. Langmuir 1994,10,4103.
0743-746319512411-2609$09.00/0 0 1995 American Chemical Society
2610 Langmuir, Vol. 11, No. 7, 1995
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silver has been performed, it was not with molecular resolution.21
Experimental Section Preparation of Substrates. Silver was evaporated by resistive heating in vacuum onto freshly cleved mica sheets (muscovite, Mica New York Corp.) heated to 300 "C. During the evaporation, the background pressure was typically 5 x Torr. Films of 1600 A thickness were deposited at a rate of -5 A/s as measured by a quartz thickness monitor. After allowing the samples to cool to room temperature, the evaporation chamber was backfilled with nitrogen, and the samples were removed. The silver samples were used within 5 min, although exposure of the silver substrates to ambient conditions for -1 h did not seem to affect the quality of the results, in agreement with the observations of other groups.' Gold substrates were also used and were prepared in an identical way. Preparationof Monolayers. Silver substrates were placed in 5 m M ethanolic solution of decanethiol (Aldrich) for -5 h, rinsed thoroughly with ethanol, and blown dry with argon. Lower concentrations and exposure times were also tried, the lowest being a n exposure to 3 pM solution for 10 s. Comparisons are discussed in the Results section. Samples were scanned immediately after preparation and were found to be stable for several hours. However, samples stored for -48 h were found to be significantly degraded a s has also been observed by other^.^ Monolayers on gold substrates were prepared similarly, but showed little degradation even after 48 h. ScanningTunnelingMicroscopy. A homemade STM and current-voltage converter were used. The STM is a hybrid of two and can operate down to currents of -1 PA. The rest of the electronics and software are commercial (Topometrix, TMX 2000). The system is operated in ambient conditions and can routinely achieve atomic resolution on gold in both constant height and current modes. The monolayer samples were scanned using mechanically cut Pt-Ir wire (Goodfellow) for tips. Bias values used ranged from 100 to 1400 mV (sample positive), and current values used ranged from 10 to 400 PA. All images shown were taken in the constant current mode. The STM was calibrated in thex-y direction using40-A scans on graphite, while calibrations for larger scans were obtained by extrapolation. Due to effects such a s thermal drift, piezo hysteresis, nonlinearity, creep, etc., the larger scale calibrations can only be considered to be nominally accurate (approximately 10%). Therefore, where lateral distance measurements were important, for example, in the determination of nearest neighbor distances, only relatively fast, 40-A scans were used. Vertical calibration was performed by measuring step heights on gold.
Results Silver Substrates. Figure 1shows a typical example of the quality of the silver substrates pbtained. The surface consists of large grains, about 5000 Ain diameter. Grains on our silver substrates are about 5 times larger than those on gold prepared using the identical procedure indicating the greater mobility of the silver atoms. The silver films had a foggy appearance to the eye which was likely due to the large grain size. In the STM images, the top of the grains show atomically flat terraces which correspond to the (111) plane24 with 60" edges and monatomic steps. Note that the steps are smooth and are stable to repeated scanning as is the whole surface even at the low tunneling resistance of 250 MR used. Silver with Decanethiol. Island Formation. Figure 2 is a scan of silver treated with decanethiol. One of the most striking features about the scan is the presence of a large number of elevated island structures on the terraces which do not appear on the untreated samples. Several characteristics can be noted about these islands. (21)Bucher, J. P.;Santesson, L.; Kern, K. Langmuir 1994,10,979. (22)Besocke, K. Surf. Sei. 1987, 181, 145. (23) Curtis, R.;Pearson, C.; Gaard, P.; Ganz, E. Reu. Sei. Znstrum. 1993, 64, 2687. (24) Reichelt, K.; Lutz, H. 0. J. Cryst. Growth 1971, 10, 103.
Figure 1. An image of the untreated silver surface, R = 250 MQ. The image is displayed a s if illuminated from the left and is 400 nm x 400 nm.
Figure 2. A 170 n m x 170 nm image of silver treated with decanethiol, R = 110 GQ. A cross section of the image through the line jndicated is shown below. The vertical scale is from 0 to 16 A.
Although varied in shape, they do sometimes show edge angles characteristic of a (111) surface just like the terraces. Secondly, they sometimes appear oriented and apparently parrallel to step edges. Also, as Figure 2 indicates, the regions of the higher terrace near steps often 9ppear devoid of islands. Their average size is akout 70 A although they range in size from 20 A up to 180 A. (The upper bound is somewhat arbitrary since the distinction between a large island and a terrace is quite arbitrary.) On average, about 20% of the surface is covered with
Decanethiol SAM on Ag(111) islands. Finally, high-resolution images show that there is an ordered layer on the islands as well as off. There are two possibilities as to the nature of these elevated regions: either there is a difference in the thiol structure on and off the regions or there is a difference in the silver substrate. Similar issues have arisen for depressions observed on Au( 111)treated with alkanethi01s. It has now been determined that these differences in height are due to the s u b ~ t r a t e . ~ ~ In J ~ order * ~ ~ yto~ ~ investigate the nature of the islands on silver, some tests were performed. Firstly, Figure 2 shows a cross section of the surface including both a step and some islands. The heights of both are the same indicating that the islands are essentially small silver terraces. Secondly, in order to eliminate the possibility that the height differences are due to the alkanethiols themselves, scans were taken at lower tunneling impedances. Parts a and b of Figure 3 show scans of the same region at impedances of about 500 and 200 MR, respectively. One can see that islands can still appear at low impedances demonstrating that they are not due to differences in heights of thiols. Whereas at high impedances the surface is stable to repeated scanning, it is not at low impedances. for example, one can see in Figure 3b, which was taken a t low impedance, that a terrace has broken up leaving islands in its place. Evidently, under these conditions, the tip-alkanethiol interaction becomes strong enough that it exceeds the silver-silver interaction. One can also see that a large pit has been created in the middle of a terrace by the scanning process. Cross sections of the image show that although the floor of the pit has steps, some of which are monatomic, its depth can be as much as 15 A. It is interesting to note that upon returning to a large tunneling impedance, small scale scans of regions that have been altered in this way are covered once again by the alkanethiols, sometimes even appearing ordered, thus indicating the remarkable mobility of the molecules and/ or the molecule-silver complex. Finally, in Figure 3c, one can see islands that appear as though they are about to or already have broken off from a terrace. The presence of islands on the silver surface, as compared to the presence of just pits on gold, suggests that a larger amount of material has been displaced on silver than on gold. A test was performed to see whether this is reflected in the relative amounts of silver and gold in solution. In order to avoid the problem of macroscopic flakes of metal peeling off the mica and invalidating the results and to increase the surface area of the treated metals, thin sheets of silver and gold were used for this test. SinceX-ray diffraction scans showed a large fraction of the sheets were not (111)and since different lattice planes have different reactivities in general, this provides a very rough result. Nevertheless, the test shows 7 times more silver in solution than gold. It is interesting to note that similar measurements done by others on gold show that while pits cover 5%-15% of the terraces, only 2% of a monolayer goes into s01ution.l~This would imply 14% of a monolayer in solution for silver, in contrast to the 80% suggested by island coverage. Finally, the concentration of the alkanethiol solution was reduced to determine whether there was a threshold concentration required to form the islands. It was found that even a sample exposed to a 3pM solution for 10s showed islands although the tunneling was quite unstable, presumably due to an incomplete layer. Decanethiol Structure. At higher resolutions (see Figures 4-61, one can begin to see the decanethiol molecules themselves. There are several important features to note in these figures. Firstly, one can see that the entire surface is covered with an ordered layer, both \
Langmuir, Vol. 11, No. 7,1995 2611
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Figure 3. (a-c, top to bottom) Scans of silver treated with decanethiol a t R = 500 MQ, 200 MQ, and 110 GQ, respectively. Parts a and b are of the same region and are 120 nm x 120 nm; part c is of a different region and is 90 nm x 90 nm. In part c, arrows indicate islands t h a t are about to or already have broken from terraces.
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Figure 4. A 120 nm
x 120 nm scan
of silver with decanethiol,
R = 110 GQ. The power spectrum of the image is shown in the corner.
Figure 6. A 10 n m x 10 nm image ofthe decanethiol molecules on silver, R = 110 GQ. Regions A and B show the two allowed lattice orientations. A cross section through the line indicated is shown below. The vertical scale ranges from 0 to 1.7 A, and the amplitude of the corrugation of the molecules is about 0.7 A.
Figure 5. A magnified image, 10 nm
x 10 nm, of one of the islands in Figure 4. Note the point defect near the center.
on the islands and off. Secondly, the domaip structure is clearly apparent. The average sige is 70 A with values ranging from about 30 to 110 A. Within a domain, molecules show a 6-fold pattern, and from domain to domain, this pattern may be rotated although not arbitrarily. Indeed, the 2D Fourier transform of this image, shown in Figure 4, has only 12 peaks corresponding to the two orientations of the 6-fold pattern present. (The two orientations can be seen clearly in Figure 6.) From the Fourier transform, one can determine that the angle between the two orientations is 20.7 f 2.3" and that the two orientations appear roughly equally. This is consistent with results found by Fenter et al. using He and X-ray diffraction. Another feature that can be seen in the images is that between domains there are depressions and that there seem to be two kinds (see FiguTe 6). In some places, they are shallower than 0.5 A, and a continuous row of molecules can be seen. In other places,
the depressions appear deeper. Due to the finite S"l$ depth resolution, only for depressions wider than 15 A can we unambiguously say that those of the latter kind are due to missing silver. An important observation that Figure 6 and all other images that we have taken indicate is that far from domain boundaries, all nearest neighbor molec@esappear to have the same height to within less than 0.1 A. This is different from the case of gold where superlattices ocyur and give rise to differences in heights of about 0.5 A. This has been seen by other g r o u ~ s and ~ ~ has J ~ been confirmed by us for decanethiol. Finally, by averaging several scans, y e have found a nearest neighbor distance of 4.61 f0.15 A. Again, this is consistent with the results of Fenter et al.
Discussion The most obvious feature seen among the low-resolution
STM images is the large number of elevated structures. As discussed earlier these islands are due to silver rather than the alkanethiols themselves. The important question as to how the islands may have formed still remains and is addressed later. The other obvious feature that can be seen in the images are the alkanethiols themselves. Any explanation for the structure observed should reconcile three important properties of the molecules that have been determined from the images: (1) the averaged onearest neighbor distance within domains is 4.61 f .15 A, (2)there are two domain types correspondingto two orientations of a 6-fold symmetric lattice separated by 20.7 f2.3"; (3)fluctuations in height of nearest nejghbors far from domain boundaries are less than 0.1 A. This is a new result that adds
Decanethiol SAM on Ag(ll1)
Langmuir, Vol. 11, No. 7, 1995 2613
boundaries may then arise from the kinetics of the monolayer growth and/or may be related to the need to release the stress built up in the domains. The picture also suggests that the similarity in the size of the thiql domains and silver islands (both on average about 70 A in size)is not a coincidence. If the VDW interactions really are that significant, the molecules on the length scale of a domain may act as a sort of glue holding the islands together. Finally, we note that the possibility of the restructuring of the silver has also been suggested by other authors and seems plausible in view of the evidence presented here.7p9 An important issue of how the islands are actually formed still remains. One possible mechanism is the etching of silver a t domain boundaries as suggested by several observations. Firstly, these regions are susceptible Fipure 7. A diagram illustrating one of the two 2/7 x J7 to etching since they are likely to contain many defects stGctures. The open circles represent the silver atoms, while the shaded and striped circles represent the top and hollow such as missing silver-thiol complexes and shallower sites, respectively, of the overlaying lattice. depressions that do contain molecules but are perhaps indicative of lattice mismatch induced stress in the SAM. a stringent constraint on any potential structural model In addition, the observation of islands separated from for this system. matching notches in terraces by trenches similar in width We can begin constructing a picture by reviewing the to domain boundaries suggests that some islands are situation in the limit of short chains where things are formed by such etching. Finally, this mechanism would clearer. As mentioned before, dosing with SZ,HzS, and in fact give rise to the similarity in the sizes of domains CH3SH results in a ( 4 7 x 2/7)R10.9"structure at higher and islands that is observed. However, this picture also coverages as observed by LEED (see Figure 7). Th? has some difficulties. It predicts that the distance between resulting spacing between the sulfur atoms of 4.41 A islands should be the width of boundaries, and so predicts requires only less than 1%change iq the lattice spacing too many islands. In addition, it predicts more silver in from the bulk value of y-Ag2S of 4.43 A.10925Such a lattice solution than is observed. Therefore, although this process predicts two possible domain orientations separated by 2 likely contributes to island formation, it cannot represent x 10.9" or 21.8". This agrees well with observations for the complete picture. long-chain alkanethiols. However, the nearest neighbor There are also observations that indicate another distance is too small to explain the observed value. Also, mechanism for island formation, one that results in fewer molecular dynamic simulations of SH and SCH3 on Agislands from the start. As mentioned before, the intrinsi(111)predict differences in +eights of molecules on top cally greater mobility of silver than of gold is indicated by and hollow sites of about 0.5 A which we do not ~ b s e r v e . ~ the larger grain size. Further, the weakening of the Therefore, it appears that for the longer chain alsilver- silver bonds enhances this mobility as unambigukanethiols, the structure is different from the ( 4 7 x 47)ously demonstrated by the layer's ability to adapt after R10.9" structure observed for the shorter chains. This being damaged by a low impedance scan. Such a mobility probably arises from the larger VDW interactions. The would suggest the facility of transferring large amounts VDW interactions are indeed important, for example, on of material on the surface. This would be consistent both Au(ll1) about -26.6 kcal/mol for CH3(CH&S vs -44 with the small fraction of silver actually observed in kcal/mol for the chemisorption energy.g (This is in solution and with the unstable tunneling observed on the comparison with -2.6 kcal/mol for silver's bulk latent heat sample with an incomplete SAM. Such a redistribution of fusion.) Furthermore, the strong ionic character of the may in fact be driven by the silver to lower its surface sulfur-silver bond must significantly weaken the unenergy by moving material down steps and may be quite derlying silver-silver bonds. We, therefore, suggest a significant during the early stages of the monolayer simple model as to how the long chain SAM structure formation. However, as nucleation and crystallization of forms. It seems reasonable that initially both long and the monolayer begin, the mobility of the molecule-silver short chain alkanethiols chemisorb in the ( 4 7 x 47)complex in the domains as compared to the noncrystallized R10.9" structure since early in the formation of the SAM, surroundings must decrease,thus leading to the formation the silver-molecule interaction must dominate the molof islands. This would suggest that island formation occurs ecule-molecule interaction even for long chains. Howa t the very early stages of the monolayer formation as is ever, as the monolayer reaches saturated coverage, confirmed by the presence of islands on the sample with weakening of the underlying silver and strengthening of an incomplete layer. The model is also supported by the the VDW interaction drive the long chains to fall more similarity in sizes of the islands and domains. Although into registry with each other, thus distorting the initial both processes for island formation, namely, the etching ( 4 7 x 1/7)R10.9" structure to the one observed. of molecular domains at boundaries and differences in This picture would explain the 20.7 f 2.3" bond angle mobility of the molecule-silver complex as the SAM observed as a vestige of the ( 4 7 x 47)R10.9' precursor develops, may occur, the evidence presented here can only and would explain the $qual molecular heights. The finite be considered as suggestive. domain size of -70 A can be 9xplained too since the In general, the data reflect a striking difference in the observed lattice constant of 4.77 A (using the more precise behavior of Ag( 111)vs Au( 111)even though the neare5t X-ray measurement of Fenter e$ al.) and the ( 4 7 x 47)neighbor distances are so similar (2.89 vs 2.88 A, R10.9" lattice cons!ant of 4.41 A give rise to a common respectively). This is due to the more reactive nature of periodicity of -70 A. The two types of depressions, that silver as compared to gold. For example, silver is more is, continuous vs silver layer deep, observed a t the domain easily oxidized, the ionization energy is 1.7 eV lower, and (25) Djurle, S. Acta Chem. Scand. 1958, 12, 1427. the work function is 0.6 eVlower. As a result, even though
2614 Langmuir, Vol. 11, No. 7, 1995 alkanethiols chemisorb on A u ( l l l ) , sulfur corrodes Ag(111)at sufficient doses.1° However, by goingto an element such as iodine on silver, one once again sees the familiar (d3 x d3)R30° structure with the iodine occupying the hollow sites.26 When viewed in this context, the behavior of long chain alkanethiols on silver appears as just one piece in a reasonable spectrum of possible behaviors.
Conclusion The use of ultrahigh impedance STM is a useful method in the study of SAM. In the case studied here, decanethiol on Ag(lll), the method permitted the confirmation of results given in the literature, namely, the determination of the lattice constant and orientations. It also permitted new observations, that is topographical changes induced on the surface and the measurement of molecular height variations. Altogether, these results provide insight into how such a S A M may have formed. We suggest that substrate -molecule and intermolecular interactions play dominant roles a t different stages of the formation of the monolayer, resulting in a S A M that shows signs of both stages. Also, we suggest that differences in the S A M between crystalline and noncrystalline phases during early stages of self-assembly or even differences between domains and defects after self-assembly may have profound effects such as the formation of islands. (26) Van Hove, S. Y.; Tong, S. Y. Surface Crystallography by LEED; Springer-Verlag: New York, New York, 1979;pp 254-255.
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Note Added in Proof. Recently, we have become aware of an independent STM study of thiolates on Ag( 1ll)?' Although the formation of islands and the direct observation of two 6-fold symmetric lattices for the longer alkanethiols were not reported, the results of the study are consistent with ours. Most notably, they report that for the shorter chains (FZ= 0,1), differences between top and hollow sites and a pattern with a period of about 70 A were observed, while for the longer chains ( n > 11, these differences disappeared. Acknowledgment. We thank J. Rosner, D. Smith, and J. Crocker for helpful discussions on the construction of the current-voltage converter, as well as D. Grier and J. Crocker for permitting us the use of their image analysis software. We also acknowledge the assistance of A. E. Essling and J. Kiely of the Chemical Technology Division at Argonne National Laboratory in the determination of the silver and gold solution concentrations. This work was supported primarily by the MRSEC program of NSF under DMR-9400379. We also gratefully acknowledge support from the David and Lucille Packard Foundation and NSF CHE-9508404. LA950206R (27)Heinz, R.;&be, J. P.Langmuir 1995, 11, 506.
