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Two-Dimensional Liquid Phase and the p x 43 Phase of Alkanethiol Self-Assembled Monolayers on Au(111) G. E. Poirier,*M. J. Tarlov, and H. E. Rushmeier National Institute of Standards and Technology, Process Measurements Division, Gaithersburg, Maryland 20899 Received June 17, 1994. I n Final Form: August 15, 1994@ Alkanethiols CH3(CH2),-1SH (C,, n = 4, 6 , 8, 10) were self-assembled from ethanolic solutions onto a single-crystalAu(111)surface and characterizedusing an ultrahigh vacuum scanningtunneling microscope (STM). Short-chain homologues (C4 and c6) exhibited a two-dimensional (2-D) liquid phase at room temperature. Facile mass transport of surface gold atoms was observed in the presence of the liquid phase. The short-chain homologues exhibited slow desorption of surface thiolate which led t o the nucleation and growth of ordered domains having a unit cell o f p x d 3 (8 5 p 5 10). No 2-D liquid phase was observed for longer chain homologues (CS and CIO). Monomolecularfilms of organic amphiphiles are subject to a number of forces that influence their ordering dynamics and equilibrium structures. These forces include interactions of the amphiphile’s “head group” with the corrugated surface adlattice, van der Waals interactions between neighboring alkyl chains, and interactions between the amphiphile “tail groups’). A monolayer amphiphile system that has received considerable attention recently is alkanethiols: CH3(CH2),-1SH, hereafter referred to as C,, self-assembled on Au(ll1). These selfassembled monolayers (SAMs)’are simple to prepare and have potential uses in chemically specific electrodes, biosensors, and pharmacological diagnostic^.^,^ Neat alkanethiols are liquids a t room temperature for n < 18. Conversely, interaction with the Au adlattice induces crystalline order in C, SAMs of intermediate and large n. One might expect, however, that C , SAMs would liquefy a t room temperature in the limit of small n. Fenter et al. have recently shown that (212 SAMs melt when the surface temperature is raised to 50 “C. Moreover, their X-ray diffraction data suggest coexistence of solid and liquid phases between 50 and 65 0C.4 The data described here suggest that C, SAMs, prepared by liquid-phase dosing, are room-temperature, 2-D liquids for n < 6. Due to the interesting structural variations expected, chain-length-dependent studies have been conducted by a number of researchers. A study that used ellipsometry, IR, and electrochemical measurements on most of the even-chain homologues from Cz to Czz found that there was a loss of order and packing density in SAMs composed of the homologues below Cse5 Similar conclusions were drawn from an FTIR study comparing c6, C ~ Zand , cl6 SAMS.~Another group used low energy electron diffraction and IR absorption to study SAMs prepared by vaporand liquid-phase dosing of most of the homologues between n = 4 and n = 23. They report less densely packed shortchain SAMs with unit cells of (mJ3 x 1/31 where m = 1-6 Abstract published in Advance ACS Abstracts, September 15,
1994. (1)Nuzzo, R.G.; Allara, D. L. J . Am. Chem. Soc. 1983,105,44814483. (2) Ulman, A. Introduction to Ultrathin Organic Films from Langmuzr-Blodgettto Self-Assembly;Academic Press: San Diego, CA, 1991. Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992,43, (3)Dubois, L. H.; 437-463. (4)Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993,70, 2447-2450. ~~. (5)Porter, M.D.;Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. Chem. SOC.1987,109, 3559-3568. (6)Chailapakul, 0.;Sun, L.; Xu, C.; Crooks, R. M. J . Am. Chem. SOC. 1993,115,12459-12467. ~~~
depending on chain length.’ Conversely, prior STM and AF’M studies concluded that alkanethiol SAMs composed of CZ,and c6-clS form the densely packed (1/3x J3)R30 l a t t i ~ e . The ~ , ~observed lack of order in short-chain SAMs has been attributed to a greater concentration of gauche defects and to weak interchain intera~tions.~JO In this Letter, we describe an STM-based structural characterization of alkanethiol SAMs composed of C4. Our results suggest a different interpretation of the disorder seen in short chain SAMs, namely, that the short-chain SAMs are in a 2-D liquid state when removed from saturation solution-phase dosing a t room temperature. Moreover, our results suggest that, for the short-chain SAMs, the surface thiolate density is depleted by desorption. We present a model in which crystalline domains nucleate from density fluctuations in the subsaturation coverage SAM. The crystalline domains have a primitive unit mesh ofpx1/3 where 8 5 p 5 10. The behavior of C4 SAMs sharply contrasts the behavior ofC8 and Cl0 SAMs which form dense-packed, stable monolayers with a c(4 x 2) superlattice of a (43x J31R30 l a t t i ~ e . ~ J ’ ,The ’ ~ c6 alkanethiol SAM alternately displayed characteristics of the long-chain and short-chain SAMs. Presumably, the melting point of saturation coverage c6 SAM lies close to room temperature. Our samples were characterized in a multichamber UHV surface analysis system with a base pressure of 3 x Pa (3 x Torr). The A u ( l l l 1 single-crystal was prepared by sputter cleaning with 0.5-keV AI-+and annealing to 500-600 “C for 10 minutes. Following this preparation, X-ray photoelectron spectroscopy revealed a contamination-free surface and STM images showed the herringbone reconstruction.13J4 These images were used to establish the sample’s crystallographic orientation with respect to the STM scan direction. The clean crystal was removed from the chamber via a separate load-lock chamber and incubated 14-16 h in (1-2) x M ethanolic solutions of C, ( n = 4, 6, 8, or 10). Following (7)Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J . Chem.Phys. 1993, 98,678-688. ( 8 ) Widrig, C. A.; Alves, C. A,; Porter, M. D. J.Am. Chem. SOC. 1991, 113,2805-2810. (9)Alves, C. A.;Smith, E. L.; Porter, M. D. J . Am. Chem. SOC.1992, 114, 1222-1227. (10)Camillone, N.; Chidsey, C. E. D.; Liu, G-y.; Putvinski, T. M.; Scoles. G. J . Chem. Phvs. 1991.94.8493-8502. (11)Camillone, N.; dhidsey, C. E’. D.; Liu, G.-y.; Scoles, G. J. Chem. Phys. 1993,98,3503-3511. (12)Poirier, G. E.; Tarlov, M. J. Langmuir 1994,10,2859-2862. (13)Chambliss, D. D.; Wilson, R. J. J . Vac. Sci. Technol.B 1991,9, 928-932. (14)Woll, C.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Rev. B 1989,39,7988-7991.
This article not subject to US.Copyright. Published 1994 by the American Chemical Society
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x 95 nm region of a A u ( l l 1 ) facet covered by a butanethiol SAM. Presence of ordered and disordered domains is interpreted a s solid-liquid coexistence. This image was acquired 5 h after completion of self-assembly. (B) High-resolution scan acquired from the region indicated in (A) showing the liquid-solid coexistence more clearly. Each atomic terrace was mapped to a separate gray scale to enhance contrast. Domains of varying size are seen (Dml Dm3) and interstitial thiolate molecules occupy the ordered domains. (C) Same surface region a s (A) but this image was acquired 31 h after self-assembly. The ordered domains have grown and the liquid phase is now concentrated at finite-width domain boundaries. An antiphase boundary (AB)is formed from the confluence of two domains with like-orientation but differing substrate registry. (D) High-resolution scan acquired from the region indicated in (C).
Figure 1. (A) 95
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incubation, the crystal was rinsed with copious quantities of ethanol and transferred to the vacuum chamber via the load-lock. The samples were exposed to air for less than 15 min during transfers. STM tips were prepared from polycrystalline tungsten wire using a dc etch. STM imaging was done in constant current mode with 100300 mV sample bias, 10-100 PA set-point-current, and 300-3000 &s scan rate. Cq monolayers imaged immediately upon introduction of the crystal into the vacuum chamber revealed atomic terraces separated by single atom steps. No molecular features were visible on the Au terraces. Images acquired at later times revealed ordered patches growing amidst the featureless terraces. Parts A and B of Figure 1show a Au(ll1) facet 5 h after removing the crystal from the incubation solution. We draw your attention to three types of features in these figures: disordered regions, ordered regions, and depressions. We attribute the disordered
regions to areas of liquid-phase SAM. These regions have a mottled appearance because the molecules are sitehopping quickly on the time-scale of the STM scan. One might argue that these areas are actually due to molecules that are locked to the surface via thiolate linkage but disordered and liquid-like in the hydrocarbon region, however, our data are inconsistent with this hypotheses as explained below. Figure 1C shows the same surface region as Figure 1A after 26 h. Any sample drift was corrected by adjustment of the x- and y-scan offsets. During this time the ordered domains have grown, the disordered regions have collapsed to finite-width domain boundaries, and the number-density of depressions has decreased. Six images were acquired at intermediate times and show a smooth transition from Figure 1A to 1C.15 Figure 2Ais from a different, though representative region of the surface. This image was acquired 127 h after removing the crystal from the incubation solution
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Figure 2. (A) 120 x 70 nm region showing a Au(ll1) terrace and three single-atom steps. This image was acquired 127 h after self-assembly of butanethiol. The ordered S A M lattice uniformly covers the terraces in a mosaic of three rotationally distinct domains. (B)High-resolution scan of an individual pit feature showing the ordered SAM lattice at the base of the pit. (C) Crosssectional slice along the line segment in (A) showing that the pits are the same depth as the A u ( l l 1 ) single-atom step height. (D) Model of the pit features that is consistent with the STM data: single-layerdeep voids in the Au surface that are filled with thiolate.
and shows that the S A M is completely ordered and the number density of depressions has decreased further. As discussed below, the depressions are single-layer-deep voids in the Au surface. The change in size distribution of these pit features requires mass transport of Au atoms and hence concerted motion of the Au and the thiolate species. The redistribution of pit features essentially ceases when the ordering transition to thep x 4 3 structure is complete. Thus, the facile mass-transport only occurs when disordered regions are present. Based on both the lack of long-range order and the facile mass-transport of Au, we assign the disordered regions to domains of liquidphase SAM. The data in Figure 1 are likely a rare observation of a 2-D liquid.I6 The phase transition from 2-D liquid top x 4 3 domains was consistently observed with four separate sample preparations of C4 SAMs. To confirm that the ordering was not induced by the STM scanning, a sample exhibiting the liquid phase was imaged until the ordering transition to p x d 3 was complete and then a new area, several millimeters away from the first, was imaged. The new region also showed a completely ordered surface. In contrast to the C4 SAMs, the liquid phase was never observed for CRand Clo SAMs. With five separate sample preparations of Clo SAMs and one sample preparation of CI, SAM, we consistently observed stable, c(4 x 2) domains and no evidence of Au mass transport as was the case for C4 SAMs. The structure of CSSAMs was less reproducible. With four separate sample preparations, the p x 4 3 structure was observed twice and the c(4 x 2) was also observed twice. The depressions seen in Figures 1and 2 have been seen in many other STM studies of SAMs.8*gJ7-19 Though they are not the central focus of this paper, we discuss them (15) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. To be submitted for publication. (16)Feenstra, R. M.; Slavin, A. J.; Held, G. A.; Lutz, M. A. Ultramicroscopy 1992,42-44,33-40.
briefly. These depressions are only observed after deposition of the SAM; they are not observed on the bare Au terraces. The depth of these depressions is identical to the Au(ll1) single-atom step height as shown in Figure 2C. In addition, molecular features due to the SAM lattice are seen in the base of these pit features (Figure 2B). Our data are therefore consistent with a previously proposed model in which the depressions are a collection of atomic vacancies in the top layer A u . ~ These ~ ~ vacancies ~ ~ . ~ are ~ created during the self-assembly process and they are uniformly covered with SAM (see model in Figure 2D). To address the mechanism of the decreasing numberdensity of pit features, STM images were assembled into time-lapse movies. These movies establish that, in the presence of liquid phase SAM, the pits coarsen by an Ostwald-ripening mechanism, i.e. large pits grow a t the expense of small pits by diffusion of single-atom vacancies. The pits were not observed to coarsen by a coalescence mechanism involving diffusion of whole pits. A detailed discussion is beyond the scope of this paper but will be addressed in a future article.15 The structure of the ordered phase is addressed in Figure 3. The structure shown in Figure 3A is not consistent with either the c(4 x 2)4J1.12 or the ( J ~ X J ~ ) R ~ O ~ . ~ J structures previously reported. Cross-sections B and C of Figure 3, taken perpendicular and parallel to the "N (next-nearest-neighbor) Au direction, respectively, show that the primitive unit cell is 9 x 43. We invariably observe a unit-cell-short-axis of d 3 , however, an as yet uncorrelated variation is observed in the long-axis depending on the chain length and history of the SAM. Because of this variability, we term this structure the p x d 3 where p (17) Haussling,L.; Michel, B.; Ringsdorf,H.; Rohrer, H.Angew. Chem. Znt. Ed. Engt. 1991,30,569-572. (18) Edinger, K.; Golzhauser,A.; Demota, K.; Woll, C.; Grunze, M. Langmuir 1993,9, 4-8. (19) Schonenberger, C.; Sondag-Huethorst,J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994,lO(3),611-614.
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Figure 3. (A) 6 x 6 nm STM scan of the p x 4 3 lattice. The p x J 3 is built from double-bright row structures we call “pinstripes”.(B)Cross-sectional slice from line segment B in (A) showing repeat distance of nine Au atoms (2.6 nm). Left triangle points to a region within a pinstripe; right triangle points to a region between pinstripes. (C) Cross-sectional slice from line segment C in (A) showing repeat distance of 4 3 Au atoms (0.50 nm) parallel to the Au(ll1) N” direction.
typically falls in the range of 8-10. In some cases we observe noninteger p in which case the unit cell is monoclinic rather than orthorombic. The basic building block of the p x 4 3 lattice appears to be a paired bright row structure which we term a ”pinstripe” as labeled in Figure 3A. Pinstripe domains oriented in the three symmetry-equivalent directions are seen in Figures 1and 2. The bright, corrugated rows with 5.0 spacing may be assigned to rows of close-packed thiolate. The gray region confined by these rows within a pinstripe may also contain thiolate molecules. The darker regions between pinstripes may be devoid of molecules. Without being able to count individual thiolate features within the gray
region of a pinstripe, we cannot assign an absolute coverage. The lack of molecular features between pinstripes, however, suggests that the p x 4 3 structure has a lower packing density than the c(4 x 2) structure. The contention of lower packing density for thep x J3 structure is further supported by annealing experiments involving the longer-chain alkanethiolates. When annealed to around 100 “C, CSand Clo SAMs converted from the c(4 x 2) lattice to the p x J3. Annealing to 300 “C resulted in complete desorption of the monolayer as evidenced by the return of the Au herringbone reconstruction. The thermal conversion of c(4 x 2) t o p x 43 was also observed in recent helium diffraction experiments on C12 SAMS.~O;~~ Thus t h e p x 43 lattice is not unique to short-chain SAMs but rather appears to be the energetically favored structure at lower surface thiolate density. We address the ordering transition observed in Figures 1and 2 in light of the low-packing density model of the p x J3 lattice. Note in Figure 2A that the ordered domains exist in a range of sizes from large to small (Dml- Dm3) and that they do not seem to nucleate from any apparent defect site such as a pit edge. We also draw your attention to molecular features occupying the regions between pinstripes in Figure 2B. These molecular features disappear at later times (compare “clean” pinstripes in Figure 2D).We assign these molecular features to excess thiolate molecules, interstitials in the p x J 3 structure, that subsequently desorb. We speculate that room-temperature, solution adsorption of Cq onto Au(ll1) results in a dense liquid-phase monolayer, perhaps close to the saturation coverage of 1 thiolate per 3 Au atoms. This liquid-phase C4S A M has a finite vapor pressure at room temperature giving rise to slow desorption of thiolate species. Commensurate crystalline domains ofp x J 3 then nucleate from local density fluctuations in the subsaturation-coverage SAM. As a n alternate explanation of the liquid phase, we speculate that contaminants from the incubation solution are retained in the S A M and thereby lower its melting point. As these contaminants desorb, the S A M relaxes into the solid-state p x J3 lattice. In summary, we report that alkanethiol SAMs prepared by saturationliquid-phase dosing are a room-temperature, 2-D liquid for n < 6. Slow desorption of surface thiolate or entrained solvent molecules induces a liquid-solid phase transition at room temperature. Patches of ordered thiolate with a p x J 3 unit cell (8 Ip 5 10) nucleate homogeneously on Au terraces and grow laterally. These ordered structures have a lower packing density than the c(4 x 2) structure observed for longer-chain SAMs. We close by noting that, in addition to their broad spectrum of technological applications, SAMs are also useful for studying the physics of Ostwald ripening and crystallization in two dimensions. (20) Camillone, N.; Leung, T. Y. €3.; Scoles, G. Proceedings of the SPIE Symposium on Lasers and Surfaces, Los Angeles, 1994, in press. (21)Camillone, N.; et al. To be submitted.
