Preparation and Structure of a Low-Density, Flat-Lying Decanethiol

This study investigates the formation of low-density, flat-lying decanethiol chemisorbed on Au prepared by heating the surface covered with a densely ...
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Langmuir 2006, 22, 174-180

Preparation and Structure of a Low-Density, Flat-Lying Decanethiol Monolayer from the Densely Packed, Upright Monolayer on Gold Laura B. Picraux, Christopher D. Zangmeister,* and James D. Batteas† Chemical Sciences and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, 20899 ReceiVed August 16, 2005. In Final Form: October 19, 2005 This study investigates the formation of low-density, flat-lying decanethiol chemisorbed on Au prepared by heating the surface covered with a densely packed, upright monolayer to a surface temperature above that of the onset of desorption. We determined conditions for preparing the low-density phase by observing the evolution of the photoemission spectrum as a function of the surface temperature using polarized ultraviolet light and by utilizing scanning tunneling microscopy. The preparation conditions were similar for single- and polycrystalline gold surfaces. Once the lowdensity decanethiol phase was formed, reflection absorption infrared spectroscopy was employed to determine the orientation of the carbon chain backbone with respect to the Au surface. The nature of the valance electronic structure for flat-lying decanethiol is described.

Introduction Tuning interfacial chemical properties is essential in areas such as wettability, surface passivation, molecular and organic electronics, and nanomanufacturing.1,2 One available surface modification technique in each of these areas is self-assembled monolayers (SAMs), which have been shown to form robust, reproducible films on a wide variety of substrates. This relative ease of fabrication has led to great interest in SAMs for use in biological applications and for the production of nanostructures.3,4 A challenge in utilizing SAMs for nanometer-scale structures is the requirement of spatial control of a material at the molecular level. An example of utilizing molecular-scale modification to vary the interfacial surface structure is exhibited in the growth of C60 fullerenes (C60) on Au(111). Creating highly ordered C60 surface structures in either one or two dimensions is a difficult process because of the molecular mobility and heterogeneity of C60 domain sizes at low coverage.5,6 Zeng and co-workers used a low-density, flat-lying phase of decanethiol (CH3(CH2)9SH or C10) to modify the Au(111) surface.7 This creates an ∼0.3 nm thick template onto which evaporated C60 forms a highly ordered, one-dimensional C60 nanostructure. Others have demonstrated that a compact C10 SAM on Au(111) can vary the structure of epitaxially grown 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) films when compared to an unmodified surface.8 These investigations illustrate that different surface structures of the same monolayer (flat-lying versus upright C10 on Au) can modify the growth of thin films at the molecular level. These studies rely * To whom correspondence should be addressed. E-mail: [email protected]. † Present address: Department of Chemistry, Texas A&M University, College Station, TX 77842. (1) Nuzzo, R. G.; Dubois, L.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (2) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (3) Senaratne, W.; Andruzzi, L.; Ober, C. K. Biomacromolecules 2005, 6, 2427. (4) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (5) Yuan, L.; Yang, J.; Wang, H.; Zeng, C.; Li, Q.; Wang, B.; Hou, J. G.; Zhu, Q.; Chen, D. M. J. Am. Chem. Soc. 2003, 125, 169. (6) Guo, S.; Fogarty, D. P.; Nagel, P. M.; Kandel, S. A. J. Phys. Chem B 2004, 108, 14074. (7) Zeng, C.; Wang, B.; Li, B.; Wang, H.; Hou, J. G. Appl. Phys. Lett. 2001, 79, 1685. (8) Scheiber, F.; Gerstenberg, M. C.; Dosch, H.; Scoles, G. Langmuir 2003, 19, 10004.

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on using SAMs as templates. While the upright, compact monolayers of C10 are well understood in terms of surface orientation and valence electronic structure, information on the electronic structure for the flat-lying C10 surface phases on Au are still absent. The details of the valence molecular orbitals of flat-lying C10 monolayers must be understood in order to predict the impact of such films on the electrical behavior, such as charge injection, of systems that may be templated on them. In addition, controllably accessing a discrete phase of a monolayer will aid future investigations for growing films on modified surfaces. The C10 monolayer on Au(111) has been described as the prototypical system for the growth and characterization of SAMs.9-14 Unlike longer alkanethiols, the relatively high C10 vapor pressure enables monolayer growth to be followed as a function of surface coverage in real time in vacuum.9-14 In one example of this, Poirier systematically followed the formation of the C10 monolayer on Au(111) using scanning tunneling microscopy (STM) to construct a phase diagram.11 Subsequent studies showed that C10 forms at least six surface structures at low coverage prior to adopting the densely packed (x3 × x3)R30° overlayer structure (φ phase) observed in both fully formed gas-phase and solution-grown alkanethiol monolayers on Au.1,2,9,13 More recently, several other low-coverage phases of C10 on Au(111) were observed by Lui and co-workers and Toerker and co-workers that have 26% to 62% of the saturation packing density of the fully formed φ phase.13,16 Beginning with the fully formed (x3 x x3)R30° φ-phase overlayer structure, the surface is heated above the desorption temperature to form several low-density phases (32% to 62% of the saturation packing density).13 These low-density phases have varying degrees of interchain interactions which may suggest the mechanism of film growth. Finally, the lowest surface coverage phase is obtained, termed the β phase, (9) Poirier, G. E. Chem. ReV. 1997, 97, 1117. (10) Poirier, G. E. Langmuir 1999, 15, 1167. (11) Poirier, G. E.; Fitts, W. P.; White, J. M. Langmuir 2001, 17, 1176. (12) Fitts, W. P.; White, J. M.; Poirier, G. E. Langmuir 2002, 18, 1561. (13) Qian, Y.; Yang, G.; Yu, J.; Jung, T. A.; Liu, G. Y. Langmuir 2003, 19, 6056. (14) Vericat, C.; Vela, M. E.; Salvarezza, R. C. Phys. Chem. Chem. Phys. 2005, 7, 3258. (15) Staub, R.; Toerker, M.; Fritz, T.; Schmitz-Hubsh, T.; Seelam, F.; Leo, K. Langmuir 1998, 14, 6693. (16) Toerker, M.; Staub, R.; Frita, T.; Schmitz-Hubsch, T.; Setlam, F.; Leo, K. Surf. Sci. 2000, 445, 100.

This article not subject to U.S. Copyright. Published 2006 by the American Chemical Society Published on Web 11/23/2005

Decanethiol on Au

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lay the groundwork for comparative changes in the electronic properties of molecules templated on the top of such monolayers. Experimental Section

Figure 1. Schematic illustrating the preparation of a flat-lying structure from a fully formed monolayer by sample heating.

at 26% of the φ-phase packing density, in which the long molecular axis of C10 is lying flat on the surface in highly periodic, ordered striped pairs. Of the observed C10 phases on Au, the highest coverage φ phase and the lowest coverage flat-lying β phase have been isolated.9,15,16 The β phase can be isolated from the φ phase by heating the surface to desorb a fraction (∼74% of saturation coverage) of the monolayer until the lowest coverage phase is reached. Staub et al.15 showed that heating a surface coated with φ-phase C10 on a Au surface in a vacuum to ∼475 K can result in isolation of the β phase. The onset of C10 desorption on Au(111) is 425 K.17-19 Thus, forming the β phase of C10 requires careful control of the heating and cooling parameters to isolate the phase prior to full desorption of the overlayer. Flat-lying, striped surface structures similar to those of the β phase of C10 have been observed on both Au and Cu surfaces by vacuum deposition of polyethylene (PE) oligomer thin films.20-26 The studies of oriented PE monolayers have shown that the surface selection rules governing the electronic structure allow for facile determination between upright and flat-lying films.20-25 Similar to physisorbed PE thin films, C10 monolayers can adopt various structures and orientations on surfaces that allow for good comparison between low-density phases of C10 and PE on Au. In this study, a robust protocol was developed for forming flat-lying β phase of C10 from the fully formed φ phase of C10 (schematically drawn in Figure 1) on both single-crystal and polycrystalline Au surfaces. The formation process and nature of the molecular structure of the films formed was followed using a combination of polarized infrared spectroscopy, STM, and polarized ultraviolet photoelectron spectroscopy (UPS), noting the effect the surface crystal structure has on the formation of the flat-lying phase. Notably, from the UPS measurements, the details of the valence electronic structure for flat-lying phases of C10 on the single-crystal and polycrystalline Au surfaces could be unambiguously measured and followed as the monolayer transitioned from the fully formed φ phase. These measurements (17) Wetterer, S. M.; Lavrich, D. J.; Cummings, T.; Bernasek, S. L.; Scoles, G. J. Phys. Chem. B 1998, 102, 9266. (18) Lavrich, D. J.; Wetterer, S. M.; Bernasek, S. L.; Scoles, G. J. Phys. Chem. B 1998, 102, 3456. (19) Schreiber, F.; Eberhardt, A.; Leung, T. Y. B.; Schwartz, P.; Wetterer S. M.; Lavrich, D. J.; Berman, L.; Fenter, P.; Eisenberger, P.; Scoles, G. Phys. ReV. B 1998, 57, 12476. (20) Dudde, R.; Reihl, B. Chem. Phys. Lett. 1992, 196, 91. (21) Ishii, H.; Morikawa, E.; Tang, S. J.; Yoshimura, D.; Ito, E.; Okudaira, K.; Miyame, T.; Hasegawa, S.; Sprunger, P. T.; Ueno, N.; Seki, K.; Saile, V. J. Electron Spectrosc. Relat. Phenom. 1999, 101, 559. (22) Seki, K.; Ueno, N.; Karlsson, U. O.; Engelhardt, R.; Koch, E. Chem. Phys. 1986, 105, 247. (23) Seki, K.; Sato, N.; Inokuchi, H. Chem. Phys. 1993, 178, 207. (24) Fujimoto, H.; Mori, T.; Inokuchi, H.; Ueno, N.; Sugita, K.; Seki, K. Chem. Phys. Lett. 1987, 141, 485. (25) Seki, K.; Ueno, N.; Inokuchi, H. Chem. Phys. 1994, 182, 353. (26) Karpen, A. J. Chem. Phys. 1981, 75, 238.

Monolayer Growth Protocol. C10 monolayers were grown in an Ar-purged glovebox by immersion of Au substrates in 1 mM CH3(CH2)9SH in absolute ethanol for 12-48 h. Au surfaces were cleaned in a UV/O3 cleaner for 15-20 min, rinsed with 18.2 MΩ‚ cm H2O and dried under a stream of N2 prior to introduction into the glovebox. After the formation of the monolayers, the surfaces were rinsed with ethanol and removed from the glovebox. Samples to be used in the vacuum chamber were removed from the glovebox and quickly (400 K.

Figure 4. (a) UP spectra of a solution-grown, compact C10 monolayer on polycrystalline Au. (b) s + p-Polarization spectrum of surface after annealing to 375 K for 4 h and heating to 450 K at 10 K/min. (c) s-Polarization spectrum of surface after heating to 450 K. (d) s + p-Polarization spectrum obtained after heating sample to 473 K.

C10 on Polycrystalline Au. To our knowledge, all previous reports of low-density C10-thiol phases on Au have been on singlecrystal surfaces. To determine if flat-lying C10 is able to be prepared and isolated on polycrystalline Au, we used polycrystalline Au that is primarily composed of (111) domains with some (100) texture. The spectrum of a sample annealed for 1-4 h at 375 K (Figure 4a) is very similar to that obtained on Au(111)/mica and is characterized by broad emission intensity from 2 to 10.5 eV with a maximum at 6.1 eV from the 1b1gderived orbital. Heating the surface to 450 K and cooling to 298 K resulted in the spectrum shown in Figure 4b. Upon heating, the wide peak manifold associated with densely packed C10 is no longer observed, and there is a concomitant increase in the emission in

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Figure 5. Plot of heating and cooling cycle normalized peak ratios as a function of surface temperature. Normalized peak ratio for Au 5d at 4.3 eV (solid circle), for C10 1b2g intensity at 8.0 eV (open square), and for C10 1b2u intensity at 9.8 eV (crosshatched circle).

the Au 5d bands. A polarization-dependent emission (cf. Figures 4b and 2b; p + s-polarization) from the C10-based 1b2u orbital is observed at 9.8 eV and allows us to conclude that flat-lying C10 is able to be prepared on polycrystalline Au surfaces. Heating the surface to 473 K results in the spectrum shown in Figure 4d and is consistent with clean polycrystalline Au. It should be noted that the desorption temperature for polycrystalline Au is lower than that of the Au/mica sample due to differences in thermal conductivity. Thermal programmed desorption (TPD) studies of fullcoverage C10 monolayers on Au show a desorption onset at ∼435 K and full desorption of the monolayer by 675 K using a 2 K/s heating rate.17-19 Staub et al. prepared β-phase C10 by heating the surface at 10 K/min from room temperature to 475 K.15 Assuming the desorption onset is 435 K, the surface is heated above the desorption temperature for 3-5 min plus the additional time for cooling below the onset temperature. To produce flatlying C10, we replicated the 10 K/min heating rate and followed the evolution of the valence electronic structure of the interfacial region as a function of the surface temperature. Shown in Figure 5 is a plot of the photoemission intensity temperature dependence of the Au 5d at 4.3 eV, the 1b2g orbital at 8.0 eV, and the 1b2u at 9.8 eV of C10 normalized to the photoemission intensity at 11.0 eV, where there is neither emission from the monolayer nor the Au surface. The 1b2g and 1b2u peaks were chosen because the 1b2g of C10 at 8.0 eV has no intensity from the Au surface (see Figure 4d), and the 1b2u spectral intensity captures the flatlying C10 phase. Each point in Figure 5 is the average temperature of an 8-10 K temperature window/spectrum (∼1 min/spectrum). From these data, there is a loss in the 1b2g spectral intensity and an increase in the Au 5d emission between 410 and 430 K, similar to that observed in thermal desorption studies on Au(111). The loss of the 1b2g orbital and simultaneous gain of Au 5d intensity continues to 445 K when the heating was suspended. Over this same temperature window, there is no detectable change in the intensity of the 1b2u of C10. Photoemission spectral data acquired during the cooling of the sample to 298

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K shows that the 1b2g (8.0 eV) and Au spectral intensity trend of monolayer desorption continues until the surfaces cools below 420 K. Over this heating and cooling cycle, there is an ∼10% loss in the total signal intensity of the C10 1b2u peak at 9.8 eV, while the intensity at 8.0 eV (C10 1b2g) returns to that of clean Au. From these data, we conclude that we have prepared a lowdensity, flat-lying C10 phase during this heating/cooling cycle. These data illustrate that utilizing this heating/cooling rate (10 K/min and ∼1 K/min, respectively), flat-lying C10 surfaces can be readily prepared. It should be noted that utilizing this established protocol results in a time/temperature window for preparing flat-lying, low-density C10 phases on Au that is on the order of 10-20 K (or 1-2 min) prior to the full desorption of the monolayer. Our heating rate procedure closely follows that reported by Staub et al.15 for the preparation of β-phase C10 on Au(111); however, our maximum temperature of 440 K is ∼30 K lower than that report. It was noted that the temperature of the surface in the Staub study was not able to be monitored directly, and there was some uncertainty in the temperature of the surface during the heating cycle, which may explain the observed differences in the temperature variance between the two experiments.15 The similar desorption onset between the singleand polycrystalline surfaces indicates that the crystal face does not affect the temperature dependence of flat-lying, low-density C10 phases on Au. The temperature-dependent spectral data shown in Figure 5 provides certain insights into the mechanism of C10 desorption on Au. Initially, the C10 monolayer is a densely packed, upright monolayer. From Table 1, emission from the 1b2g is allowed in the upright phase and is forbidden in the flat-lying phases of C10, whereas the 1b2u orbital is relatively enhanced in intensity in the flat-lying configuration, suggesting that the 1b2g and the 1b2u orbitals are good indicators for upright and flat-lying phases. As shown in Figure 5, the intensity of the 1b2u peak remains nearly constant through the heating and cooling cycle: the intensity is first from the upright phase and later from the flat-lying phase. In contrast, the 1b2g band decreases as the surface temperature rises above 420 K. Using the 1b2g as a monitor of the upright component, we can deduce that, as C10 desorbs, the percentage of the upright component is reduced as the surface temperature is increased until a temperature is reached where the upright component is sufficiently low (surface coverage of 0.26-0.31 of the saturated C10 monolayer density) that the surface is solely composed of the flat-lying C10. This desorption picture is consistent with STM investigations observing multiple C10 intermediate surface coverage phases prior to the formation of flat-lying β-phase C10 on Au(111).13 In such a scenario, the C10 surface density decreases nearly uniformly as opposed to pockets of C10 desorption exposing areas of bare Au. This should be contrasted to benzenethiol on Au(111), where it was observed that a Schottky surface state persists at benzenethiol surface coverages of up to 60% of the saturation coverage.36 They postulated that benzenethiol forms islands on the surface leaving areas of clean, bare Au. In contrast, we observe a surface state associated with clean, ordered Au after the desorption of the full monolayer on Au(111), but not during the C10 desorption process or after the formation of a flat-lying phase (for reference, cf. Figure 2b-d). As the surface density of C10 is reduced, the extent of interchain methylene interactions diminishes until the surface coverage is such that it is replaced by the methylene/Au surface interaction. The methylene/Au surface interaction stabilizes the (36) Whelan, C. M.; Barnes, C. J.; Walker, C. G. H.; Brown, N. M. D. Surf. Sci. 1999, 425, 195.

Decanethiol on Au

flat-lying phase up to 6.1 kJ/mol per CH2 unit (61 kJ/mol for C10), which is a substantial increase in stabilization when added to the 125 kJ/mol bond strength for the Au/S bond.17,18 STM investigations have shown that the low- and intermediatecoverage phases of C10 on Au have a high degree of surface mobility. Thermal desorption measurements on alkanethiols by formation of the monolayer through gas-phase exposure have revealed that, initially, an alkanethiol monolayer is physisorbed on the gold surface, and it may take more than 103 s to form a covalent Au-S bond.17,18 From these observations, the potential exists that flat-lying C10 is physisorbed on Au. In our experiments, the perturbation of the Au valence structure suggests that there is a strong interaction between C10 and the Au 5d (5,6) states (see Figure 2c,d). Similar perturbations of the Au valence structure have been observed in the case of more ordered short (methane and ethane) thiols adsorbed on Au.27 Previous studies have compared the adsorption enthalpies of chemi- and physisorbed C10 and adsorbed alkanes on Au(111).17,18 The adsorption enthalpy of chemisorbed C10 is 128 kJ/mol, compared to 104 kJ/mol for the physisorbed monolayer. Using a 104 kJ/mol desorption enthalpy, one can calculate a desorption onset of 340 K from Au(111) for physisorbed C10. Here, we heated the surface to ∼440 K at 10 K/min and cooled the surface at a rate of 1-1.5 K/min. Under these conditions, the monolayer is aboVe the physisorbed desorption temperature for over 1 h. The amount of time required for complete desorption of a chemisorbed monolayer on Au(111) at 440 K is 1 min (2 orders of magnitude faster than our heating and cooling cycle). In addition, we collected X-ray photoemission spectra of the compact, upright monolayer and an annealed monolayer heated to 475 K; each showed a S 2p3/2 and 2p1/2 emission doublet at 163.5 and 162.4 eV, respectively (data not shown). These binding energies are consistent with a bound thiolate.37-39 Orientation of C10 Methylene Groups on Au. To explore the orientation of C10 on Au, RAIR spectra were obtained from solution-grown compact φ-phase C10 films and flat-lying monolayers prepared by the annealing and heating/cooling process described above. Spectra obtained from compact, upright C10 films (Figure 6a) are consistent in peak position and peak intensity to those previously published.1,2,40 The absorptions at 2919 and 2850 cm-1 indicate that the film formed is a well-ordered crystalline film with few defects. This film is referred to as the compact phase (φ phase) in Table 2. The spectra of flat-lying C10 and a desorbed monolayer obtained by heating to 575 K are shown in Figure 6, panels b and c, respectively. RAIR spectra obtained from a surface after which the film was desorbed has no detectable spectral intensity arising from the monolayer, which is consistent with the previous conclusion that C10 completely desorbs as an intact molecule (i.e., it has not reacted with the surface to leave an IR active species). The spectrum of C10 in the flat-lying phase (Figure 6b) has changed relative to that of the upright phase. First, the absorption at 2965 cm-1 in the upright phase is no longer present. Next, the absorptions at 2878 and 2850 cm-1 in the flat-lying phase are significantly less intense than those in the compact phase. The absorption at 2919 cm-1 has been shifted by 12 cm-1 to 2907 cm-1, and a new, low intensity absorption at 2816 cm-1 is present. These spectral changes may be compared to that of TTC adsorbed on Au, which forms a monolayer with its long molecular axis parallel to the surface: an identical geometric orientation (37) Yang, Y. W.; Fan, L. J. Langmuir 2002, 18, 1157. (38) Castner, D. G.; Hind, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (39) Rieley, H.; Kendall, G. K.; Zemicael, F., W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147. (40) Anderson, M. R.; Evaniak, M. N.; Zhang, M. Langmuir 1996, 12, 2327.

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Figure 6. p-Polarized RAIR spectra of (a) solution-grown compact C10 monolayer on polycrystalline Au, (b) flat-lying C10, and (c) surface after heating to 575 K. Table 2. RAIRS Peak Assignments for Compact and Flat-Lying C10 Phases on Au assignmenta

compact Phase (φ)

νas(CH3) νsym(CH3)+Fermi resonance νas(CH2) νsym(CH3) νsym(CH2) ν(CH2) νas(CH2)/νsym(CH2)

2965 cm-1 2938 cm-1 2919 cm-1 2878 cm-1 2850 cm-1 n/a

flat-lying 2935 cm-1 2907 cm-1 b 2880 cm-1 2852 cm-1 b 2816 cm-1 c 9.5

a

b

Abbreviations: asymmetric stretch (as), symmetric stretch (sym). Distal mode. c Proximal mode.

Figure 7. Schematic of flat-lying C10 with carbon backbone in (a) parallel orientation showing proximal and distal hydrogens, and (b) perpendicular orientation from above the surface plane.

to flat-lying C10.41-45 In the case of alkanes adsorbed on Au, the hydrogens can be in two chemically inequivalent environments: hydrogens that are proximal to the surface, and hydrogens that are distal from the surface, as shown in Figure 7a. Each environment impacts the position of the νas(CH2) differently. In the case of TTC on Au, the hydrogens that are distal from the surface display a shift in the νas(CH2) mode from 2924 cm-1 in the upright monolayer to 2907 cm-1 in the flat-lying phase.41 (41) Yamamoto, M.; Sakurai, Y.; Hosoi, Y.; Ishii, H.; Kajikawa, K.; Ouchi, Y.; Seki, K. J. Phys. Chem. B 2000, 104, 7363. (42) Yamamoto, M.; Sakurai, Y.; Hosoi, Y.; Ishii, H.; Ito, E.; Kajikawa, K.; Ouchi, Y.; Seki, K. Surf. Sci. 1999, 427, 388. (43) Yamamoto, M.; Sakurai, Y.; Hosoi, Y.; Ishii, H.; Kajikawa, K.; Ouchi, Y.; Seki., K. J. Phys. Chem. B 2000, 104, 7370. (44) Hoestetler, M. J.; Manner, W. L.; Nuzzo, R. G.; Girolami, G. S. J. Phys. Chem. 1995, 99, 15269. (45) Luongo, J. P.; Schonhorn, H. J. Polym. Sci., Part A 1968, 6, 1649.

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The hydrogens proximal to the surface exhibit a new absorption at 2814 cm-1, indicative of a metal-hydrogen agostic interaction.41,43 Similar shifts in the νas(CH2) are observed in the case of flat-lying C10 on Au (Table 2). The shift of the νas(CH2) band from 2919 to 2907 cm-1 and the appearance of the mode at 2816 cm-1 indicate that the long axis of the molecule is parallel to the surface. Ozaki and co-workers determined that comparing the νas(CH2) band intensity to that of the νsym(CH2), one can determine the orientation of the carbon chain with respect to the surface plane.46,47 Two possible flat-lying orientations with the carbonchain parallel to the surface are possible (shown in Figure 7). In Figure 7a, the carbon chain is parallel to the surface plane with one hydrogen on each methylene unit proximal to the Au surface and one distal. In the case where the carbon chain is perpendicular (Figure 7b) to the surface plane, there are two methylene environments that exist in equal number along the chain. In one methylene environment, the two hydrogens are oriented away from the surface, and in the other environment, both hydrogens are proximal to the surface. It has been shown that, in the case of oriented TTC on Au, the ratio of the peak intensity of νas(CH2) to the intensity of the νsym(CH2), Iνas(CH2)/Iνsym(CH2), may be used to determine the carbonchain orientation with respect to the surface plane.46,47 In cases where the Iνas(CH2)/Iνsym(CH2) is large, the carbon chain is flat-lying and parallel to the surface plane, whereas a Iνas(CH2)/Iνsym(CH2) ratio that is close to 1 is indicative of a flat-lying perpendicular orientation of the carbon chain. Using these ratios, we analyzed our RAIRS data and obtained a Iνas(CH2)/Iνsym(CH2) ratio of 9.5, (46) Nakagoshi, A.; Wang, Y.; Ozaki, Y.; Iriyama, K. Langmuir 1995, 11, 3610. (47) Wang, Q.; Zhao, B.; Zhang, X.; Shen, J.; Ozaki, Y. Langmuir 2002, 18, 9845 and the Langmuir-Blodgett films shown therein.

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indicating that the carbon chain is parallel to the surface. In this orientation, a single hydrogen is proximal to the Au surface on each methylene unit of the chain (as shown in Figure 7a). We were unable to obtain high-quality RAIR spectra on Au(111)/ mica. These substrates are optically thin, allowing sufficient IR beam transmission and reflection to hinder our RAIR spectral quality. Despite this hindrance, we detected a strong νas(CH2) absorption at 2907 cm-1 on Au(111)/mica. Thus, we assume an identical parallel orientation is adopted on Au(111) surfaces.

Conclusion We have employed a combination of surface heating and polarized UPS, RAIRS, and STM to follow the thermally induced desorption of the compact C10 phase on Au to form a flat-lying structure. RAIRS data suggest that the monolayer adopts an orientation in which each methylene unit has a distal hydrogen pointing away from the Au surface that can be used for interaction with epitaxially grown thin films to form oriented structures. Further studies will investigate whether monolayer templating affects the electronic structure compared to monolayers prepared on a clean substrate. These studies may have a significant impact on how the surface-molecule interaction affects the valence structure of an adsorbed species as well as conduction through templated structures. Acknowledgment. The authors gratefully acknowledge support from the NIST Competence Building Program in Molecular Electronics. The authors would like to thank Dr. J. M. Beebe for aid in the collection of C10 STM images. L.B.P. acknowledges support from the National Research Council for Postdoctoral Fellowships. LA052231V