J. Phys. Chem. 1995, 99, 10966-10970
10966
Molecular Ordering and Gold Migration Observed in Butanethiol Self-Assembled Monolayers Using Scanning Tunneling Microscopy G. E. Poirier” and M. J. Tarlov National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received: March 1, 1995; In Final Form: April 27, 1995@
Functionalized alkanethiols offer tremendous flexibility in customizing the physicochemical properties of Au surfaces. For this reason they are being investigated for applications in sensing, tribology, and corrosion inhibition. We present a scanning tunneling microscopy characterization of molecular ordering in an alkanethiol monolayer system. When dense monolayers of butanethiol molecules are assembled onto Au( 11l), randomly distributed single-layer-deep pit defects are generated in the Au substrate. The monolayer spontaneously crystallizes via homogeneous nucleation and grows into a complex domain network. Simultaneously, the pit number density and size distribution are observed to evolve in a manner consistent with Ostwald ripening. The number density of pit features follows a t-”2 power law. When the monolayer has completely crystallized, the redistribution of Au atoms ceases. Thus, this is a special case of order-disorder-mediated Ostwald ripening.
The recent interest in alkanethiol self-assembled monolayers (SAMs) is attributable to their ease of preparation and highly reproducible, well-defined structure. I s 2 By virtue of the relatively strong binding of the sulfur head group to Au, the surface properties of SAMs can be precisely tuned by varying the terminal functionality of the alkanethiol molecules. The tremendous flexibility in customizing SAM surfaces has motivated researchers to investigate these systems for sensing, tribological, and microelectronic applications. Because many of the projected technological uses of SAMs will hinge on the ability to manipulate the surface molecular architecture, it is imperative that the structure of SAMs be understood on a molecular level. It has been established unequivocally in several studies3-I3 that n-alkanethiol SAMs formed on single crystal Au( 111) surfaces consist of ordered, crystalline domains exhibiting different chain-length- and coverage-dependent periodicities. Although much is known about the static, equilibrium structure of SAMs, far less is known about the early stages of SAM formation and ordering. Scanning tunneling microscopy has successfully elucidated previously unknown or controversial surface structure^.'^^^^ The data, an atomic resolution surface relief of the electron density near the Fermi level, reveal information about crystalline packing and surface defects. The technique’s reliance on a tunneling current limits its application to conducting and semiconducting surfaces. Although alkanethiols do not fall into this category, we achieved success by using sensitive, low-noise electronics and relatively short-chain alkanethiols. In previous articles, we elucidated the structure of long- and short-chain alkanethiol SAMs on a Au(ll1) ~ u r f a c e . ~In. ~this report we have used STM to obtain unprecedented glimpses of the selfassembly process and the surface dynamics of monolayer ordering.
Experimental Section Samples were characterized in a multichamber ultrahigh vacuum surface analysis system with a base pressure of 3 x Pa ( 3 x Torr) and a rapid-entry load-lock. A single crystal of Au( 111) was prepared by chemical etching, electro-
* Author to whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, June 1, 1995
chemical etching, and flame annealing. Prior to SAM deposition, a clean Au(ll1) surface was prepared by sputtering and annealing (500-600 “C) for 10 min. After this preparation, X-ray photoelectron spectroscopy revealed a contamination-free surface and STM topographs showed the herringbone reconstruction characteristic of clean Au( 11l).I53l6 These topographs were used to establish the sample’s crystallographic orientation with respect to the STM scan direction. For SAM preparation, the clean crystal was removed from the vacuum chamber, transported through air, and incubated 14-16 h in (1-2) x M butanethiol-ethanol solution. Following incubation, the crystal was rinsed with copious quantities of ethanol and transferred to the vacuum chamber. 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 3003000 ks scan rate. All monolayer preparation and imaging was done at room temperature.
Results and Discussion The time sequence images of butanethiol on Au(ll1) displayed in Figure 1 reveal the manifold complex processes that occur during SAM ordering. The sequence of topographs were acquired after withdrawal of the sample from the adsorbate solution over a period of several days. All topographs were acquired from the same surface region by aligning the imaging frame with certain landmark features and thereby counteracting instrumental drift. The self-assembly of alkanethiols on Au(111) results in simultaneous formation of vacancy-island or “pit” defects in the top layer of A u . ~ , ” - ’ These ~ defects might arise from adventitious etching in the thiol ~ o l u t i o nor ’ ~may be associated with a lifting of the Au herringbone reconstruction. For SAMs comprised of butanethiol we observe that these defects evolve via facile migration of surface Au atoms. In Figure 1 the uppermost terrace and the vacancy islands are mapped to shades of blue and orange, respectively. We point out two remarkable features of Figure 1: (A) the pit size distribution evolves from a large number of small pits to a small number of large pits and (B) the surface evolves from a disordered to an ordered state. The decreasing pit density or “pit coarsening” indicates the presence of room-temperature Au
This article not subject to U.S. Copyright. Published 1995 by the American Chemical Society
Molecular Ordering and Gold Migration
J. Phys. Chem., VoE. 99, No. 27, 1995 10967
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Figure 1. Temporal evolution of a butanethiol S A M on Au(ll1). Topographs a-p were acquired 5, 9, 12, 13, 20, 21, 30, 31, 47, 56, 61, 72, 83, 100, 126, and 127 h after solution-phase deposition of the monolayer. The image area is 100 nm x 100 nm. The orange features are depressions in the top layer Au that are created by the deposition process. The S A M has reached 95% solidification by frame j. In the later stages of sintering, anisotropic domain-wall energy polygonalizes the domains as seen in the central domain in frame p.
diffusion and could be due to one of two phenomena: a coalescence mechanism involving diffusion of pits on the surface or an Ostwald-ripening mechanism involving diffusion of single Au atoms or Au vacancies. The signature of Ostwald ripening is the growth of large pits at the expense of small pits.20321Close examination of the data in Figure 1 reveals the ripening mechanism to be that of Ostwald ripening. Pits that are large on a local scale, such as pits 1 and 3 in frame “a”, grow larger with time (compare “p” and intervening frames) whereas smaller
pits (i.e., 2 and 4)shrink and eventually disappear. Moreover, the relative position of the pits remains fixed. In this study we observe stationary pits coarsening via Ostwald ripening whereas in two previous studies, mobile pits were observed to coarsen via c o a l e ~ c e n c e . ~The ~ . ~pit~ motion in the previous studies was aligned with the STM fast-scan direction, and moreover, the step edges were observed to crenulate under repeated scanning. These effects are consistent with an invasive tunneling mechanism whereby the tip is in
10968 J. Phys. Chem., Vol. 99, No. 27, 1995
Poirier and Tarlov
close contact with, or is submersed in, the film. This was pointed out by the previous authors.22 In the present study we used a higher tunneling impedance and shorter alkyl chain length which likely resulted in a noninvasive tunneling mechanism. Consequently, we did not observe migrating or coalescing pits. Another striking feature of Figure 1 is the time evolution of ordered domains. At short times, the SAM exists primarily in a disordered, liquid-like state (Figure la)! Ordered regions with a rectangular unit mesh4 nucleate homogeneously on the terraces, grow anisotropically, and eventually coalesce. This is seen more clearly in Figure 2. On the basis of the extended ordering time (several days) and the subsaturation thiolate density of the striped crystalline we speculate that the ordering kinetics are controlled by a desorption process rather than by the intrinsic order-disorder kinetics. The desorbing species that initiate the crystallization may be desorbing thiolates or desorbing SAM impurities. The former case would imply that the low-density phase has a higher melting point than the high-density phase and that the ordered domains nucleate from low-density fluctuations in the SAM. The latter case would imply that the SAM melting point is depressed by impurities such as water or ethanol and that the ordered domains nucleate from high-purity regions. As the surface approaches the completely ordered state, the liquidus regions form a percolation network connecting the pit features as can be seen clearly in frame f and in Figure 2A. It is noteworthy that the upper terrace is more than 95% ordered before we observe any order in the pit-level-terrace(see bottom center pit of frame i). In frames j through 1, we see pit 3 begin to order with one orientation, subsequently disorder, and then reorder with a different orientation. At present, we do not have an explanation for the delayed ordering observed in the pits. The relationshipbetween pit ripening and molecular ordering is addressed in Figure 3. We plot the time evolution of the pit number density (Figure 3A) and the fractional coverage of liquid-phase SAM (Figure 3B) as calculated from a series of (200 x 200) nm topographs (not shown). The pit number density falls off with a power-law time-dependence as expected for a random-walk-mediated process and then saturates at time ts % 50 h. During the coarsening phase, the data are fit well by a phenomenological equation:
N ( t ) = No/(1
+ Bt0.50)
M
0
(1)
where NOis the initial pit number density and B is an adjustable parameter. The time at which the number density saturates corresponds exactly with completion of ordering. These curves therefore establish a correlation between facile Au migration and the presence of liquid-phase SAM. To confirm that the Au redistribution is confined to the uppermost terrace, we calculate the fractional pit area by integrating height histograms of the time series. The fractional pit area is conserved at 5% of a monolayer (see Figure 3C) indicating that there was no net flux of vacancies to the bulk. Thus Figures 1-3 suggest that pits coarsen via migration of Au adatoms or Au-adatomvacancies and that this migration is confined to the liquidus surface phase. Of course, it would be interesting to know whether the ripening is mediated by Au atom migration or Au vacancy migration. Both mechanisms require a concerted motion of Au atoms and thiolate moieties. At present, we do not have sufficient data to differentiate between the two. For particles undergoing classical Ostwald ripening the size distribution will continue to coarsen monotonically. The system of the present study, however, is a special case that we call “arrested Ostwald ripening” whereby the pit-ripening dynamics are altered by simultaneous crystallization of the monolayer.
Figure 2. (A) STM topograph acquired 20 h postincubation. Pit features (dark) are connected by a finite-width percolation network defined by ordered domain boundaries. (B) STM topograph acquired 130 h postincubation. Percolation network is reduced to molecular width as the ordered domains coalesce. Solid lines demarcate a faceted domain boundary. (C) Difference image (Figure 2A - Figure 2B). Light regions represent added pit area, dark regions represent lost pit area, and gray regions are unchanged. The largest pits in part A grow at the expense of smaller pits, a signature of Ostwald ripening.
Molecular Ordering and Gold Migration
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40 60 80 120 time (hours)
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Figure 3. (A) Pit number density vs time. The number density shows a power-law decay that saturates at t = 50 h. (B) Fractional liquidphase monolayer vs time. Saturation of the ordered phase corresponds exactly with the rollover in the etch pit number density suggesting that Ostwald ripening is confined to the disordered liquid phase. (C) Total area of exposed second layer molecules expressed as a fraction of total image area. This quantity is calculated by integrating height histograms of the topographs. The quantity remains constant suggesting that the redistribution of pit features does not involve an exchange of Au atoms with the bulk.
Specifically, at short times the particle diffusion occurs on a 2-D plane (frame a), at later times on a finite-width percolation network (frame f and Figure 2A), and ultimately is pinched off as the percolation network reduces to molecular width (frames j-p and Figure 2B). Thus, the dimensionality of the problem crosses over from 2-D to 1-D during the course of late-stage ripening. Random walks on percolation networks are expected to exhibit unique dynamics and have been studied theoretically by Ben-Avraham and Ha~lin.2~ The present study suggests that butanethiol SAMs might provide a good system for empirical study of this phenomenon. It may be possible to inhibit crystallization of the monolayer by conducting the experiment in an overpressure of butanethiol and thereby directly comparing 2-D and pseudo 1-D ripening kinetics. In the later stages of domain coarsening, the domain walls polygonalize due to anisotropic domain wall energy as can be seen in Figure 2B. The domain geometry at any given time is determined by a competition between the Wulff c o n ~ t r u c t i o n ~ ~ * ~ ~ (thermodynamics) and curvature-driven domain wall motion2* (kinetics). At intermediate times in Figure 11 we observe straight domain walls connecting etch pits while at later times Figure l p and Figure 2B show that the domain walls facet. This faceting is driven by minimization of the total edge energy (r) where
I
Figure 4. (A) 200 nm x 200 nm STM topograph of butanethiol SAM
P is the perimeter of the domain, y e ) is the edge energy of a domain wall oriented perpendicular to 5,and ds is an infinitesimal edge arc. This domain-wall faceting may be due to a highly anisotropic molecular packing in the p x d 3 unit mesh."
on Au( 111) showing pits, ordered S A M domains, and single-atom Au steps. (B) High-resolution scan acquired from region indicated in (A). Step edges facet in (112), the direction of dense molecular packing. (C) High-resolution topograph acquired from region indicated in (B). Faceted step edges are terminated by double-thiolate rows.
10970 J. Phys. Chem., Vol. 99, No. 27, 1995 The highly anisotropic molecular packing may also lead to the step-edge faceting observed in Figure 4. The topographs in Figure 4 were acquired 8 days after incubation. Straight step edges running parallel to (112) have developed from the curvilinear step edges normally seen on bare Au. It is interesting to note in parts B and C of Figure 4 that all faceted step edges terminate with a double row of thiolates. This step-edge faceting is not limited to butanethiol S A M s but was also observed on octanethiol SAMs annealed in vacuum. The annealing procedure desorbs a fraction of the monolayer and results in nucleation of subsaturation coverage crystalline phases.24
Conclusions We have observed the molecular ordering of butanethiol
S A M s in real time with molecular resolution. The ordered domains nucleate homogeneously, grow, coalesce, and then slowly coarsen. The coarsening is associated with faceting of domain walls and step edges. The Au atoms undergo facile surface migration in the presence of liquid-phase S A M as evidenced both by Ostwald ripening of pit defects and faceting of step edges. We observed a new type of order-disordermediated Ostwald ripening whereby the atomic species migrate on a two-dimensional plane at early times and a finite-width percolation network at later times. In future studies we hope to explore how the change in dimensionality affects the kinetics of ripening. The results presented here have implications on the structure and stability of sensors based on SAM-bound proteins. Moreover, they show that S A M s are useful for studying basic physical phenomena occurring in pseudo twodimensional systems.
Acknowledgment, G.E.P.gratefully acknowledges stimulating discussions with A. Zangwill, J. A. Stroscio, R. E. Cavicchi, S . Semancik, M. Sobolewski, and R. M. Crooks in the course of this work. References and Notes (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. SOC. 1983, 105,44814483. (2) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463.
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