Supramolecular Structures of Coronene and Alkane Acids at the Au(111)-Solution Interface: a Scanning Tunneling Microscopy Study

Langmuir 2005, 21, 919-923 919

Supramolecular Structures of Coronene and Alkane Acids at the Au(111)-Solution Interface: A Scanning Tunneling Microscopy Study

Brett J. Gyarfas, Bryan Wiggins, Monica Zosel, and K. W. Hipps*

Department of Chemistry and Materials Science Program, Washington State University, Pullman, Washington 99164-4630

Received September 11, 2004. In Final Form: October 29, 2004

Scanning tunneling microscopy (STM) is utilized to study the solution-solid interface formed between Au(111) and solutions of coronene in hexanoic, heptanoic, and octanoic acids. In all three cases adsorbed coronene is observed and lays flat on the metal surface. Heptanoic and hexanoic acid solutions produce a hexagonal symmetry monolayer. For the heptanoic and hexanoic cases, dipole-image dipole interactions and H bonding stabilize a surface structure in which 12 acid molecules surround each coronene and produce a coronene spacing of 1.45 nm. In the case of octanoic acid as solvent, the incorporation of the solvent into the monolayer is not as strongly favored. The coronene spacing can range from close-packed (1.2 nm) with no solvent presumed present in the monolayer, to 1.50 nm with up to 12 solvent molecules surrounding each coronene. The close-packed regions have hexagonal symmetry, as do those with the largest (1.5 nm) spacing. Heptanoic acid solutions give the clearest STM images and are associated with the most stable two-component monolayer. The present paper demonstrates that non-covalent interactions at the solution-metal interface can lead to complex multicomponent monolayer structures.

Introduction Most recently, the desire to understand the solution- solid interface,17-21 and the design of nanostructures by Supramolecular chemistry is chemistry that uses 22-26 molecules rather than atoms as building blocks. Weak bottom-up methods, has driven the field of supramo- intermolecular forces, not covalent bonds, are used to lecular chemistry from the three-dimensional (3D) realm assemble by design large structures from tailored mol- to the two-dimensional (2D). The discovery and application ecules.1,2 Since the pioneering work of Lehn, Cram, and of the scanning tunneling microscope has made this Pedersen,3 there has been a steadily increasing interest evolution to 2D supramolecular studies possible. The in the development and application of supramolecular design strategies discovered for 3D supramolecular chem- chemistry. In its beginning, focus was on molecular istry can be applied to the adsorbed state. Moreover, there recognition, which is the selective binding of a guest by is a particular relevance of the surface state to supramo- a host using non-covalent interactions.4 As the field grew, lecular synthesis with physisorbed molecules, because the rational design of molecular crystals using supramo- many of the weaker interactions used in generating lecular interactions became an area of interest.5-7 Hy- synthons are probably distorted or destroyed in fluid drogen bonding is the most common supramolecular solution and in chemisorption. The weak lateral forces interaction; however, other interactions including halogen- exerted by the surface upon physisorbed molecules, and halogen,8 halogen-nitrogen,9,10 halogen-oxygen,7 elec- the image charges that occur in metal substrates, allow trostatic interactions, 11-15 and weak electron donor- these weaker intermolecular forces to play a significant acceptor complexation16 have been used to organize role in the formation of long-range order in the adsorbed molecules within a crystal. phase. While some of the supramolecular synthons developed for crystal synthesis may be inappropriate for * To whom correspondence should be addressed. generating surface structures because of the planar E-mail: [email protected]. template effect of the substrate, others may have their (1) Lehn, J. M. Supramolecular Chemistry: Concept and Perspectives; stability enhanced by the reduction in entropy, the steric VCH: Weinheim, Germany, 1995. constraints imposed by the surface, and the electrostatic (2) Lehn, J. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4763. (3) 1987 Noble Prize in Chemistry. (4) Gale, P. A. Philos. Trans. R. Soc. London, Ser. A 2000, 358, 431. (16) Bosch, E.; Radford, R.; Barnes, C. Org. Lett. 2001, 3, 881. (5) Reddy, D. S.; Craig, D. C.; Desiraju, G. M. J. Am. Chem. Soc. (17) De Feyter, S.; De Schryver, F. C. Chem. Soc. Rev. 2003, 32, 139. 1996, 118, 8, 4090. (18) Yablon, D. G.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B (6) Gillard, R.; Stoddard, J.; White, A.; Williams, B.; Williams, D. J. 2000, 104, 7627. Org. Chem. 1996, 61, 4504. (19) Yablon, D.; Guo, J.; Knapp, D.; Fang, H.; Flynn, G. W. J. Phys. (7) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311. Chem. B 2001, 105, 4313. (8) Scmhidt, G. M. Pure Appl. Chem. 1971, 27, 647. (20) Cai, Y.; Bernasek, S. L. J. Am. Chem. Soc. 2003, 125, 1655. (9) Reddy, D. S.; Ovchinnikov, Y.; Shishkin, Y.; Struchkov, Y.; (21) Yoshimoto, S.; Suto, K.; Itaya, K.; Kobayashi, N. Chem. Commun. Desiraju, G. J. Am. Chem. Soc. 1996, 118, 4085, and references therein. 2003, 2174. (10) Xu, K.; Ho, D.; Pascal, R. J. Org. Chem. 1995, 60, 7186. (22) Whitesides, G. M.; Boncheva, M. Proc. Natl. Aacad. Sci. U.S.A. (11) Coates, G.; Dunn, A.; Henling, A.; Dougherty, D.; Grubbs, R. H. 2002, 99, 4769. Angew. Chem., Int. Ed. Engl. 1997, 36, 248. (23) Griessl, S. J. H.; Lackinger, M.; Jamitzky, F.; Markert, T.; (12) Coates, G.; Dunn, A.; Henling, L.; Ziller, J.; Lobkovsky, E.; Hietschol, M.; Heckl, W. M. J. Phys. Chem. B 2004, 108, 11556. Grubbs, R. H. J. Am. Chem. Soc. 1998, 120, 3641. (24) Hipps, K. W.; Scudiero, L.; Barlow, D. E.; Cooke, M. P. J. Am. (13) Gillard, R.; Stoddard, J.; White, A.; Williams, B.; Williams, D. Chem. Soc. 2002, 124, 2126. J. Org. Chem. 1996, 61, 4504. (25) Scudiero, L.; Hipps, K. W.; Barlow, D. E. J. Phys. Chem. B 2003, (14) Williams, J. H. Acc. Chem. Res. 1993, 26, 593. 107, 2903. (15) Hunter, C.; Lu, X.; Kapteijn, G.; Koten, G. J. Chem. Soc., Faraday (26) Lei, S.; Wang, C.; Wan, L.; Bai, C. J. Phys. Chem. B 2004, 108, Trans. 1995, 91, 2009. 1173.

10.1021/la047726j CCC: $30.25 © 2005 American Chemical Society Published on Web 12/30/2004 920 Langmuir, Vol. 21, No. 3, 2005 Gyarfas et al. environment provided in the case of a metallic substrate. We restrict our consideration to physisorbed molecules to ensure that the lateral intermolecular interactions can play a significant role in coadsorbate ordering. Scanning tunneling microscopy (STM) is the only technique that can provide detailed sub-nanometer structural analysis at the solid-solution interface in real time. Through its application, a view is provided of the elegant architectures that occur in what one might think was the relativelydisorderedinterfacebetweensolidandsolution.17-21,23,26 STM has also been key to characterizing 2D supramolecular structures resulting from vapor deposition on solid surfaces.24,25,27,28 In the present study we will use STM to Figure 1. Constant-current STM images of two different probe the interfacial layer that results when Au(111) comes samples of coronene in octanoic acid on Au(111). (A) Bias voltage, into contact with a solution of coronene in various alkane -0.40 V; set point, 500 pA; coronene spacing, 1.47 nm. (B) Bias acids. Our interest in this system was generated by the voltage, -0.70 V; set point, 300 pA; coronene spacing, 1.24 nm. rich spectrum of potential weak interactions. The alkane The gray scale extends over 0.12 nm. acid might interact with the gold through dipole-induced dipole interactions, leading to a standing configuration, The STM head used was produced by Digital Instruments or dispersion type interactions might produce a surface (now Veeco Metrology). A Digital Instruments Nanoscope E covered by alkane acids in the striped array (lamellae) controller was used to acquire the reported data. STM image analysis was performed with the SPIP38 commercial software normally seen on graphite. Coronene is known to physisorb package. Constant-current images are reported, and any filtering on graphite from the vapor in a close-packed hexagonal is indicated in the appropriate figure caption. Both etched and structure with the coronene lying flat on the surface and cut Pt0.8Ir0.2 tips were used. In-plane spacing measured by STM having a lattice spacing of about 1.1 nm.29 STM studies was calibrated using the graphite lattice. In-plane measurements of coronene adsorbed Au and Ag reported a coronene have a precision of less than (0.04 nm and an average absolute spacing of about 1.2 nm.30-32 Another possibility is that error of <0.04 nm. All measurements were made at 21 ( 2 °C. coronene may adsorb onto lamellae of alkane acids (similar Results to phthalocyanine and tridedycelamine (TDA) on graph- 33 STM images from these systems varied widely in ite ). Finally, the potential for hydrogen bonding between quality. Good clear images would be replaced by unstruc- coronene and the carboxyl must be considered. What we tured noise for no apparent reason. At other times, changes actually find is a competition between several of these in bias voltage would lead to significant changes in the factors. observed image. We attribute these changes as principally due to the fact that the system is in dynamic equilibrium Experimental Section between the adsorbed layer and solution. The intrinsic Hexanoic acid (C O H ), heptanoic acid (C O H ), and exchange processes between solution and adsorbed phase 6 2 12 7 2 14 molecules, the structural changes induced by the STM tip octanoic acid (C8O2H16), all g99%, were used as supplied by Sigma-Aldrich. Coronene was also provided by Aldrich and was during scanning, and the bias voltage as it brings the labeled as sublimed and 99% purity. Epitaxial Au(111) films tunneling electrons near resonance can all contribute to with well-defined terraces and single atomic steps were prepared image changes. Adsorption and desorption of molecules on mica by previously described methods.34-37 These films were from the STM tip must also play a role. The results 0.1-0.2 µm thick and had a mean single grain diameter of about presented here are a fair representation of all the types 0.3 µm. The gold films were hydrogen flame annealed just prior of results (other than the clearly structureless ones) to use. Solutions were prepared by adding 0.30 mg of coronene observed. Of the three acids studied, the eight-carbon to 10 mL of the alkane acid in a 10 mL volumetric flask. This s s concentration gave coronene adsorption but was not so high as chain octanoic acid produced the most variability in to cause crystallization problems. Future work should explore STM images. the effects of concentration on the structures observed. A small Pure Acid Solutions. These acids had very low magnetic stirring bar was placed in the flask and the solution conductivity and the nontunneling current (measured with stirred under low heat for 2-3 h until all the coronene had the tip ∼ 10 µm from the surface) was always less than completely dissolved. A new piece of gold was used for each day’s 50 pA. In no case did we observe a structured interfacial experiments and for each type of solution. A total of 5-10 samples layer for the pure alkane acid on Au(111). of each acid type was studied. Octanoic Acid Solutions. Coronene dissolved in octanoic acid can produce a hexagonal symmetry mono- (27) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, layer, as shown in Figure 1. Depending on the sample and W. M. Single Mol. 2002, 3, 25. the physical position on a sample, the structure observed (28) Barth, J. V.; Weckesser, J.; Lin, N.; Dmitriev, A.; Kern, K. Appl. Phys. A 2003, 76, 645. varied in lattice spacing from a low of about 1.2 nm to a (29) (a) Lackinger, M.; Griessl, S.; Heckl, W. M.; Hietschold, M. Anal. high of about 1.50 nm. Two representative examples are Bioanal. Chem. 2002, 374, 685. (b) Walzer, K.; Sternberg, M.; Hietschold, shown in Figure 1. These differences were real, reproduced M. Surf. Sci. 1998, 415, 376. (30) Uemura, S.; Sakata, M.; Taniguchi, I.; Hirayama, C.; Kunitake, on several occasions, and are not due to instrumental M. Thin Solid Films 2002, 409, 206. artifacts. The apparent height of each coronene in (31) Yoshimoto, S.; Narita, R.; Itaya, K. Chem. Lett. 2002, 356. constant-current mode was of the order of 0.05 nm. In no (32) Lackinger, M.; Griessl, S.; Heckl, W. M.; Hietschold, M. J. Phys. case did we observe the internal structure of coronene, Chem. B 2002, 106, 4482. (33) Lei, S.; Wang, C.; Wan, L.; Bai, C. J. Phys. Chem. B 2004, 108, and all molecular images were simply the “blobs” typified 1173. in Figure 1. Note however that the contrast (apparent (34) Lu, X.; Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391. height) seen in the densely packed layers (coronene spacing (35) Lu, X.; Hipps, K. W.; Wang, X.; Mazur, U. J. Am. Chem. Soc. ∼ 1996, 118, 7197. 1.24 nm) was generally higher than that observed for (36) Barlow, D.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 2444. (37) Scudiero, L.; Barlow, D.; Hipps, K. W. J. Phys. Chem. B 2000, (38) SPIP Software, Version 3.2.9; Image Metrology A/S, Diplomvej 104, 11899. 373, DK-2800 Lyngby, Denmark, Version 3.2.9, 2004. Coronene in Acid at the Au(111)-Solution Interface Langmuir, Vol. 21, No. 3, 2005 921

Figure 2. Constant-current STM images of the interface between Au(111) and a coronene in hexanoic acid solution. The figures have been low-pass-filtered and were collected with a set-point current of 275 pA. Panel A was obtained with sample bias set to -550 mV, while B was obtained at -50 (bottom) and -100 mV (top). The coronene spacing is 1.43 ( 0.04 nm in A ( Figure 3. Constant-current STM images of the interface and 1.43 0.06 in B. The color scale spans 0.11 nm. between Au(111) and a coronene in hexanoic acid solution. Panel A has been plane-fit and low-pass-filtered and was collected the layers having widely spaced coronene molecules. On with a set point of 500 pA and -100 mV sample bias. The the basis of the molecular spacing and previous single coronene spacing is 1.41 nm, and the color scale spans 0.11 nm. component adsorption studies,30-32 the coronene is laying Panels B and C are correlation averages, with the circles added flat on the gold surface. While the structures seen with in C to assist in recognizing the weak features. the smallest (1.2 nm) and largest (1.5 nm) spacing were hexagonal in symmetry, those with intermediate spacing were only approximately so. Hexanoic Acid Solutions. The six-carbon acid solutions always gave surface structures with a coronene spacing near 1.45 nm. In some scans the coronene appeared to be low relative to some small bright features surrounding it (Figure 2A), while in others this pattern was reversed (Figure 2). Which of these patterns was observed could, at times, be controlled by changing the bias voltage (Figure 2B), while at other times bias voltage changes only produced sharpening or loss of a particular type of image. We note that images such as Figure 2A were more often obtained at the low current and high bias voltage associated with a larger tip-surface separation. Unfortunately, these settings also could produce images such as Figure 2B. The coronene spacing did not change as the image changed. While the coronene images always appeared rounded, hints of the internal hexagonal structure were sometimes observed. To better define the small round structures seen between coronene molecules, a correlation average38 was performed on a particularly clear constant-current image (Figure 3A), and the result is presented in Figure 3B. What is apparent with careful inspection of Figure 3B, and exaggerated by the overdrawn circles in Figure 3C, is the Figure 4. Constant-current STM image of the interface presence of 12 features distributed roughly symmetrically between Au(111) and a coronene in heptanoic acid solution. surrounding each coronene molecule. Note that these 12 The image has been plane-fit and low-pass-filtered and was features appear with different brightness (apparent collected with a set point of 700 pA and -600 mV sample bias. height) and area in the correlation average, suggesting The coronene spacing is 1.45 nm, and the color scale spans 0.12 that the molecules producing these features are in fluxs nm. The inset is a 3D representation of a correlation average structure. they may be exchanging in and out of the monolayer and/ or changing configuration as the tip scans over them (vide infra). smaller features of equal height. Note again that in the Heptanoic Acid Solutions. The Au(111) interface with actual image there are variations in the apparent height coronene in heptanoic acid gave the most reproducible of these surrounding features similar to, but not as and easily defined results. The coronene adsorbed flat pronounced as, those seen in the hexanoic acid case. onto the gold surface, was clearly surrounded by smaller Inverted contrast images were also seen with heptanoic molecules, and consistently gave a coronene-coronene acid as solvent. In these images the small rounded features spacing of 1.45 ( 0.04 nm. An example of the quality of appear very bright (high) relative to the coronene. These data obtained is presented in Figure 4. Note that for bright, small-diameter features appear brightest in the heptanoic acid the presence of 12 small spots surrounding regions between three adjacent coronene molecules. each coronene is much clearer in the basic STM image For completeness, we include one additional image that than was the case for hexanoic acid solutions. Correlation is representative of several obtained during one afternoon, averaging produces a clear sharp image of a hexagonal Figure 5. In this image, the coronene molecules are symmetry coronene surrounded symmetrically by 12 somewhat isolated and there are large areas that appear 922 Langmuir, Vol. 21, No. 3, 2005 Gyarfas et al.

Figure 5. Typical constant-current STM image of the interface between Au(111) and a coronene in heptanoic acid solution. The image has been plane-fit and low-pass-filtered and was collected with a set point of 350 pA and -600 mV sample bias. The color scale spans 0.20 nm. to be densely covered by the small molecules. The of any kind. Even with this very crude model using the hexagonal symmetry of coronene is well-developed in CPK radii from Accelrys ViewerLite48 and hand packing, Figure 5sbetter developed than in any of our other images. one predicts a coronene-coronene distance of 1.51 nms Note also that the smaller diameter features now appear very close to our observed value of 1.45 nm. With this much taller in the regions where they are densely packed. idealized model in mind, we can discuss the results for all three acids. Discussion For the C6 and C7 acids, the surface dipole interaction The most critical issue generated by the above data is and H bonding with coronene are such that having one the identity of the small-diameter features seen in the oxygen per hydrogen is more stable than completely heptanoic and hexanoic acid cases. The observed features covering the surface with coronene. The longer C7 chain and symmetry are completely inconsistent with the flat- has greater van der Waals stabilization in the vertical lying lamellae structure often observed for terminally direction than does the C6 chain, making it less likely to substituted alkanes on graphite.17-19 While the size of the distort from the idealized geometry. Also, the shorter chain features (about 0.35 nm in diameter) in very much less species is expected to be more mobile in terms of exchange than the length of even the shortest alkane acid, it is between adsorbed and solution species. Thus, the hep- roughly correct for the area of either a methyl or tanoic acid/coronene surface species provides a clearer carboxylate group. It has been known for some time that more stable STM picture of the surface. alkane acids adsorb on metals such as gold and platinum The variability in coronene spacing in the case of the with the terminal CH3 group out, producing an autophobic C8 chain indicates that the solvent adsorption is either - surface.39 42 Moreover, when we began this study it was kinetically or thermodynamically less favorable than those with the expectation that the polar end of the alkane acid for hexanoic and heptanoic acid. The variation of coro- would interact through dipolar forces with the metal nene-coronene spacing in octanoic acid solutions from substrate and might even be involved in C-H‚‚‚O close packing (1.2 nm) to the spacing typical of heptanoic 43-45 interactions. It is also possible that the acid is adsorbed acid solutions (1.45 nm) suggests that from 0 to 12 octanoic 46,47 dissociatively as carboxylate, thereby increasing the acids may be incorporated in the monolayer. The lack of - ‚‚‚ strength of the C H O interaction. Thus, we propose well-resolved structure, even for the 1.45 nm spacing, that the structure observed for coronene and hexanoic or suggests that the octanoic acid is either poorly ordered or heptanoic acid is one in which the acids are standing up rapidly (on the STM time scale) exchanging with solution. on the surface and surrounding each coronene. A crude The free energy decrease that occurs when the long alkane - - Corey Pauling Koltun (CPK) model is shown in Figure tail is surrounded in the disordered solution may be 6. sufficient to compete with that resulting from the car- Note that the structures shown in Figure 6 are idealized boxylate associating with the surface. That is, if the alkane cartoons of the true structure. For example, it is likely chain increases from six to seven carbons, the increase in that the alkane chains are not so well ordered and the mass and lateral van der Waals interaction may make surface species may be partially or completely deproto- the surface solvated species slightly more stable. At eight nated to form carboxylate ions at the surface. Species were carbons, the van der Waals attraction along the alkane positioned by hand, and there has been no optimization chains (in solution) may begin to compete with the van der Waals interaction and carboxylate attraction for the (39) Hare, E. F.; Zisman, W. A. J. Phys. Chem. 1955, 59, 335. (40) Timmons, C. O.; Zisman, W. A. J. Phys. Chem. 1964, 68, 1336. gold of the adsorbed species. In this picture, the free energy (41) Fox, H. W.; Hare, E. F.; Zisman, W. A. J. Phys. Chem. 1955, 59, of adsorption for octanoic acid becomes more positive than 1097. for heptanoic acid (but still negative), allowing the (42) Bewig, K. W.; Zisman, W. A. J. Phys. Chem. 1964, 68, 1804. coronene-gold interaction to dominate the equilibrium. (43) Jeffrey, G. A. J. Mol. Struct. 1999, 485-486, 293. (44) Bodige, S. G.; Zottola, M. A.; McKay, S. E.; Blackstock, S. C. Acidity does not seem to play a relative role since all three 49 Cryst. Eng. 1998, 1, 243. acids have a Ka ) 4.86 ( 0.03. (45) Corey, E. J.; Lee, Thomas W. Chem. Commun. 2001, 1321. (46) Han, S. W.; Joo, S. W.; Ha, T. H.; Kim, Y.; Kim, Kwan. J. Phys. Chem. B 2000, 104, 11987. (48) ViewerLite 5.0; Accelrys Inc.: San Diego, CA, Version 5.0, 2002. (47) Castro, J. L.; Montanez, M. A.; Otero, J. C.; Marcos, J. I. Spectrosc. (49) Handbook of Chemistry and Physics, 84th ed.; Lide, D. R., Ed.; Lett. 1993, 26, 237. CRC Press: Boca Raton, FL, 2003. Coronene in Acid at the Au(111)-Solution Interface Langmuir, Vol. 21, No. 3, 2005 923

Figure 6. Stick and CPK models of proposed film structure. These are just manually placed scale structures of heptanoic acid and coronene and are in no way optimized. Note that some or all of the acid molecules may actually be present as carboxylate on the surface and that the alkane chains are likely to be more disordered than that shown in the figure.

Another issue raised by this study is the fact that the alkane acids appear taller than coronene in some images apparent height of the two different molecular species is and shorter in others (e.g., Figure 2). This type of contrast neither that expected from their crystal structures or reversal is sometimes seen and can occur either as a relatively consistent from image to image. Sometimes the function of bias voltage or some random change normally alkane acid appears taller than coronene; other times, it’s associated with adsorption of molecules on the STM shorter. None of the heights ever correspond to the tip.56-58 For the present study, the contrast switching does expected length of the alkane chain. To understand this, not affect the conclusions drawn about the structure of one must realize that while molecular spacings derived the film. In fact, the presence of some images where the from STM data are consistent with expectations generated alkane chains are clearly higher than the coronene by crystallography, the height values are not.50-54 In molecules makes the identification of the alkane acids constant-current STM imaging, a feedback loop is used to more convincing. ensure that the tip is following a contour of constant Our long-term goal is to be able to use non-covalent current. Thus, unless the conductivity of all species is interactions to design interfacial structures with nano- exactly the same (as in a clean metal surface), the apparent meter-scale variations in chemical and physical properties. heights are not physical heights. Coronene sits flat on the An important part of this goal is the use of noble metals gold surface, has both HOMO and LUMO within a few eV as substrates. The present paper demonstrates that non- - of the Fermi energy, and has a veritable forest of pz orbitals covalent interactions at the solution metal interface can to assist in transporting electrons through the gap between lead to complex multicomponent monolayer structures the gold and the tip. Alkanes, on the other hand, are poor and that we have the tools to both observe and to change conductors.55 Thus, we would expect the apparent height them. This work is an essential first step to the rational of the alkane acids to be significantly less than their true design of such structures. physical height. This qualitatively explains why none of the alkane acids ever appear as tall as their structures Acknowledgment. We thank the National Science indicate. There remains, however, the issue of why the Foundation for support in the form of Grants CHE 0138409 and CHE 0234726 and for support from the Materials (50) Cyr, D. M.; Venkataraman, Flynn, G. W.; Black, A.; Whitesides, Research REU Program 0139125. Acknowledgment is G. W. J. Phys. Chem. 1996, 100, 13747. made to the donors of the Petroleum Research Fund, (51) Hamers, Robert Annu. Rev. Phys. Chem. 1989, 40, 531. administered by the American Chemical Society. (52) Lu, X.; Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391. (53) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. LA047726J 1996, 118, 7197. (54) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 11899. (56) Nieminen, J.; Lahti, S.; Paavilainen, S.; Morgenstern, K. Phys. (55) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, Rev. B: Condens. Matter Mater. Phys. 2002, 66, 165421/1. C.; Kagan, S.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; (57) Inaba, H.; Yagi, Y.; Okuda, M. Jpn. J. Appl. Phys. 1995, 34, Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; 5779. Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. J. Phys. (58) Walzer, K.; Sternberg, M.; Hietschold, M. Surf. Sci. 1998, 415, Chem. B 2003, 107, 6668. 376.