Overlayers of Long-Chain Organic Molecules Physisorbed on the

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Langmuir 2003, 19, 4969-4976

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Overlayers of Long-Chain Organic Molecules Physisorbed on the Surface of Self-Assembled Monolayers of Alkylthiols on Au(111) P. V. Schwartz,*,† D. J. Lavrich,‡ and G. Scoles Princeton Chemistry Department and Princeton Materials Institute, Princeton University, Princeton, New Jersey 08544 Received January 29, 2003. In Final Form: March 20, 2003 We have been able to grow well-ordered crystalline overlayers of either alkylthiols or dodecane on the surface of self-assembled monolayers (SAMs) of alkylthiols on Au(111) by exposing the surface of a preexisting SAM to the appropriate amount of flux at carefully controlled temperatures. Low-energy helium diffraction shows that, with one notable exception, all overlayers proved considerably more ordered than the underlying SAM. Debye-Waller measurements also reveal that the overlayer surfaces are stiffer than the underlying SAM surfaces that support them. Dodecane overlayers have the same structure whether grown atop SAMs made of alkylthiols containing either 10 or 11 carbon atoms. However, the overlayers of C10SH on a C10SH SAM surface were distinctly different from and of higher quality than the overlayers of C11SH on a C11SH SAM surface. To the best of our knowledge, this is the first structural characterization of an organic monolayer grown on an organic substrate of a different structure.

1. Introduction Self-assembled monolayers (SAMs) are intensely studied to reach a better understanding of the physics and chemistry of soft matter in two dimensions (including complex biological systems such as membranes) and the nature of organic/inorganic interfaces.1-3 They are also receiving considerable attention because of potential technological applications such as corrosion protection and the nanofabrication of electronic components. In previous work, our laboratory investigated the in-vacuum, vapordeposition growth of the standing-up (3 × 2x3) phase of decanethiol (hereafter referred to as C10SH) on Au(111). This high-density phase is preceded by the lower-coverage “striped” phases in which the alkylthiol molecules lay at least partially flat, with their hydrocarbon backbone parallel to the Au(111) substrate.4,5 During the course of this work, we have observed that, after growing a full coverage SAM, by lowering the substrate temperature we could grow an overlayer of long-chain molecules, which was considerably more ordered than the supporting SAM. In this paper, we will compare four different overlayer/ SAM/Au(111) systems. The molecules used are dodecane, C10SH, and undecanethiol (hereafter referred to as C11SH). Overlayers of C10SH and C11SH were grown atop the (3 × 2x3) phase of C10SH and C11SH SAMs, respectively. Overlayers of dodecane (C12H26) were grown atop the (3 × 2x3) phase of the C10SH and C11SH SAMs. In the overlayer/SAM/Au(111) notation, we will drop the Au* Author to whom correspondence should be addressed. E-mail: [email protected]. † Present address: Physics Department, California Polytechnic State University, San Luis Obispo, CA 93407. ‡ Present address: Restek Corporation, Bellefonte, PA 16823. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (2) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (3) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (4) 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: Condens. Matter 1998, 57 (19), 12476. (5) Schwartz, P.; Schreiber, F.; Eisenberger, P.; Scoles, G. Surf. Sci. 1999, 423, 208-224.

(111) because all data have been collected on a Au(111) substrate. Thereby, the four overlayer systems studied above will be referred to as C10SH/C10SH, C11SH/C11SH, dodecane/C10SH, and dodecane/C11SH. The behavior of these four systems is investigated as a function of the substrate temperature. This includes the study of the formation, ordering, Debye-Waller response, transition to disorder, and desorption of the overlayer. A structural comparison is also made with a monolayer of dodecane molecules physisorbed on a bare gold surface. The structure-sensitive probe used to carry out these studies is low-energy atomic-beam scattering that, being nonperturbative and totally unpenetrating, is proven here to be an excellent tool for structure-sensitive studies of the adsorption of organic molecules on organic surfaces. The adsorption energy of organic molecules on Au(111) has been reported to be substantially higher than its bulk heat of vaporization.6,7 For instance, C10SH has a bulk enthalpy of vaporization of 65 kJ mol-1 but a measured physisorption energy on Au(111) of 104 kJ mol-1.6 One would expect that the energy required to desorb from the surface of an organic monolayer (ML) would approach the bulk value of that molecule as the ML thickness is increased. The additional attractive potential of the underlying gold substrate should no longer be felt by the overlayer at sufficiently large distances. Contact-angle measurements2 have shown that the length of a butyl chain (≈4.5 Å) is sufficient to decouple the overlayer from the substrate. However, experiments with liquid crystals atop alkanethiols/Au(111)8 and theoretical studies9 report that the presence of the gold substrate can be felt by an overlayer across a ML of C17SH (thickness ≈ 22.5 Å). By measuring the desorption temperature of the overlayers from organic layers of different thicknesses on gold, we (6) Lavrich, D. J.; Wetterer, S. M.; Bernasek, S. L.; Scoles, G. J. Phys. Chem. 1998, 102, 3456. (7) Wetterer, S.; Lavrich, D.; Cummings, T.; Scoles, G.; Bernasek, S. J. Phys. Chem. B 1998, 102, 9266. (8) Miller, W. J.; Abbott, N. L.; Paul, J. D.; Prentiss, M. G. Appl. Phys. Lett. 1996, 69, 1852. (9) Miller, W. J.; Abbott, N. L. Langmuir 1997, 13, 7106.

10.1021/la034159b CCC: $25.00 © 2003 American Chemical Society Published on Web 05/06/2003

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Figure 1. Sketch of the inner part of the helium scattering. RS, radiation shield; XL, crystal; BD, bolometer detector; RD, rotatable doser shutter; MA, actuator magnet; DL, dose line; HB, incident helium beam. The inner part of the chamber is cryogenically shielded from the outer part and, therefore, also very effectively cryopumped.

obtain results in close agreement with the study based on contact-angle measurements.2 This study does not address all the aspects of the overlayer/SAM interaction. For instance, the exact overlayer structure is not determined, and the origin of the differences between the surfaces of the different overlayer/ SAM systems is not identified. Additional studies will be necessary to answer these important questions. 2. Experimental Section Scattering Apparatus. The general layout of the heliumbeam surface diffraction apparatus used in the present work has been described in detail in previous publications,10 as have been recent renovations.5,11 Briefly, the apparatus consists of two differentially pumped vacuum chambers. In the first, a helium beam is produced by supersonic expansion from a 70 K source providing helium atoms with an incident wave vector of 5.33 Å-1 (the de Broglie wavelength is about 1 Å). Via a skimmer, the beam passes into the second chamber (base pressure of 10-8 Torr), where it strikes the sample mounted on a manipulator (Figure 1). The in-plane diffracted beams are detected by a bolometer, which rotates about a vertical axis and is thermally anchored to a 1.6 K reduced-pressure liquid-helium cryostat. The detector/ crystal assembly is located inside two concentric, liquid-nitrogencooled, radiation shields, which keep thermal radiation from warming the bolometer (and its cryostat) and define an inner space that is very effectively cryopumped. Surface Preparation. During the course of this study, two routes of substrate preparation have been followed, which are explained elsewhere.5,11 Briefly, the single-crystal gold surfaces were prepared in a separate ultrahigh vacuum chamber via Ar+ sputtering and covered with a protective C10SH SAM. After the (10) Camillone, N., III; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737. (11) Schwartz, P. Molecular Beam Studies of the Growth and Kinetics of Self-Assembled Monolayers. Ph.D. Thesis, Princeton University, Princeton, NJ, 1998.

Schwartz et al. crystal was installed into the helium atom diffraction chamber, the C10SH ML was desorbed by heating the crystal to 650 K. In the second route to substrate preparation, followed after the installation of an Ar+ sputter gun in the diffraction chamber, clean Au(111) surfaces were obtained by means of sputter and annealing cycles within the 77 K enclosures of the diffraction chamber. The cleanliness and order of the gold surface in both methods of preparation were assessed by observation of the Au first-order diffraction peak and the surface reconstruction peaks using helium atom diffraction. C10SH (purity 96%) was obtained from Aldrich Chemical Co. and purified by partial evaporation under vacuum. The purification eliminated absorbed gases and the more volatile impurities. At elevated temperatures, thermal motion causes a decrease in the diffraction intensities (Debye-Waller effect, explained in section 4). For this reason, the structural diffraction scans were normally taken at 40 K. The crystal temperature was raised to deposit or anneal the organic overlayer or to measure DebyeWaller behavior. The crystal heating rate is about 1 K s-1, and the cooling rate is about 0.2 K s-1. A thermally isolated dosing nozzle (see Figure 1) allowed for the deposition of both the underlying (3 × 2x3) SAMs and the overlayers. The flux at the sample can be varied from less than 1014 (less than 1 langmuir s-1) to over 1016 molecules cm-2 s-1 (more than 100 langmuir s-1) and was calibrated to within a factor of 2.5 The exposure is controlled through a combination of the flux and exposure time. Because the shutter assembly is kept at 77 K along with the entire inner radiation shield, thiols that do not adhere to the crystal surface are rapidly removed, allowing the exposure times to be determined by the shutter speed. Diffraction Geometry. All diffraction measurements are made using an in-plane scattering configuration, that is, the primary helium beam, surface normal, and detector are all in the same plane. The diffraction patterns are taken by measuring the intensity of the scattered beam as a function of the polar angle, θ, as the detector is rotated in the scattering plane (for details, see refs 10-12). These data in θ space can be transformed into momentum space by the use of the equation

∆k| ) ki(sin θf - sin θi)

(1)

where ∆k| is the momentum transfer parallel to the surface, θf is the detector angle, and θi and ki are the incident angle and wavenumber, respectively. The surface periodicity, l, is given by

l ) 2πn/∆k|

or

∆k| ) 2πn/l

(2)

where n is the order of the diffraction peak.

3. Structural Characterization C10SH/C10SH. The first observation that overlayers could be more ordered than the underlying SAM was made when approximately one ML of C10SH was deposited on a very disordered C10SH/Au(111) SAM at 220 K. Figure 2 is a diffraction scan of the overlayer along with a previous scan of the underlying surface. Although the underlying SAM surface is featureless, showing almost no specularity, the overlayer has a very large specularity [with 8 times the integrated intensity of the finest (3 × 2x3) phase we have ever recorded]. The additional presence of large, welldefined diffraction peaks indicates that not only is the overlayer surface smooth, but it is also well-ordered. The presence of higher-order diffraction peaks further indicates that the surface is highly corrugated.13 The diffraction patterns obtained at all the azimuthal angles were very similar, indicating that this overlayer is a two-dimensional powder. This is not surprising: the underlying surface showed no order and, therefore, could not have provided directionality. With an underlying (3 × 2x3) SAM of (12) Camillone, N. Ph.D. Thesis, Princeton University, Princeton, NJ, 1994. (13) Avrin, W. F.; Merrill, R. P. Surf. Sci. 1994, 311, 269. Poelsema, B.; Verheij, L. K.; Comsa, G. Phys. Rev. Lett. 1983, 51, 2410.

Overlayers of Long-Chain Organic Molecules

Figure 2. Diffraction scan of an ordered overlayer (above) along with that of the disordered underlying SAM.

Figure 3. C10SH/C10SH overlayer system. (a) The underlying C10SH (3 × 2x3), 〈11 h 0〉 direction; (b) overlayer, 〈11 h 0〉 direction; (c) overlayer, 〈112 h 〉 direction. The solid lines indicate multiples -1 of ∆k| ) -1.45 Å . The dotted lines indicate multiples of ∆k| ) -0.875 Å-1.

reasonable quality, the crystalline overlayer islands orientationally align themselves with the (3 × 2x3) structure. Figure 3 is a diffraction pattern of a C10SH (3 × 2x3) SAM of moderate quality (scan a) along with scans of a C10SH overlayer at two different azimuths (scan b is the 〈11 h 0〉 direction and scan c is the 〈112 h 〉 direction). The widths of both the specular and the hexagonal peaks of the (3 × 2x3) and overlayer are very close to the machinedetermined peak width, indicating ordered, crystalline domains of >100 Å. The distinct difference between the diffraction patterns in the 〈11 h 0〉 and 〈112 h 〉 directions indicates that the overlayer islands have a preferred orientation. From the (3 × 2x3) and overlayer scan in the 〈11 h 0〉 direction, one sees that the overlayer adopted the spacing of the (3 × 2x3) (indicated by solid lines) but that the peaks corresponding to the superlattice14 cannot be observed. Additional peaks at multiples of ∆k| ) -0.875 h 0〉 direction Å-1 exist in the overlayer scan in the 〈11 (dashed vertical lines in Figure 3). This corresponds to a periodicity of 8.3 Å. The only distance this relates to in the system that we identified is l/2 or l cos 60°, where l h〉 is the 17-Å length of the C10SH molecule. In the 〈112 direction, one sees smaller remnants of the same peaks as seen in the 〈11 h 0〉 direction. The overlayer may have only incompletely adopted the directionality of the underlying (3 × 2x3). Because the (3 × 2x3) is of only moderate quality, it is very likely that some “powderlike” domains exist, such as in the underlying SAM and (14) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503.

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Figure 4. Comparison of C10SH/C10SH (above) and C11SH/ C11SH (below). The darkened traces are the diffraction patterns for the overlayers. The thin traces are of the corresponding underlying (3 × 2x3) surfaces. The fast drop-off of the intensity of the C11SH scans after the specular peak is due to the larger incident angle used for C11SH (71.5° as opposed to 55.5° for C10SH).

overlayer shown in Figure 2. Also note that the specular beam produced by the overlayer surface is very wide at the base compared to that of the underlying (3 × 2x3). This could be the result of either an increased amount of inelastic scattering or a distribution of point defects.15 C11SH/C11SH. The surfaces of both the hexagonal and (3 × 2x3) phases of the SAMs are likely to be different for odd and even carbon-numbered thiols as a result of the different orientation of the final CH2-CH3 bond. For instance, in the hexagonal phase, the final CH2-CH3 bond of C10SH should be oriented more perpendicularly to the gold surface than that of the C11SH (at 27° and 55° with respect to the surface normal, respectively).16 In the (3 × 2x3) phase, this issue is complicated by the possible presence of sulfur atom dimerization,17,18 but the two surfaces are most likely still different. This difference could affect the structure of the overlayer. Figure 4 shows the diffraction scans of the overlayers of C11SH/C11SH resulting from the deposition of approximately two MLs on a C11SH (3 × 2x3) substrate at 200 K followed by annealing at 220 K. Also shown are the diffraction scans of the previously discussed C10SH/C10SH. The corresponding (3 × 2x3) scans are also shown below each of the overlayer scans. The two overlayers have strikingly different structures. C11SH/C11SH has neither an enhanced specular peak nor the widened specular base clearly apparent in the top pattern. Furthermore, the diffraction scans of other azimuths indicate that this overlayer is a two-dimensional powder even though the underlying ML displays order and orientational alignment. At this point, it is interesting to note the desorption energies of the overlayers from the corresponding supporting SAMs. Despite the fact that C10SH has a bulk heat of vaporization that is about 10% lower than that of C11SH, we measured the desorption energy of the C10SH/C10SH overlayer to be slightly higher than (but within experimental error of) the desorption energy of the C11SH/C11SH overlayer. A tentative explanation of the different behavior of C11SH versus that of C10SH may be that the surface of the (15) Levi, A.; Spadacini, R.; Tommei, G. E. Surf. Sci. 1981, 108, 181. (16) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767. Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (17) Fenter, P.; Schreiber, F.; Berman, L.; Scoles, G.; Eisenberger, P.; Bedzyk, M. J. Surf. Sci. 1998, 412, 213. (18) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216.

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Figure 5. Trace a: specular intensity as a function of time during the exposure of approximately 1 ML/s-1 of dodecane on a C10SH (3 × 2x3) surface while the surface cools. At a crystal temperature of 205 K, the C12H26 begins sticking to the surface, as is indicated by the specular drop. At a surface temperature of about 200 K and an exposure of about 50 MLs of dodecane, complete overlayer coverage is established, indicated by the end of the specular rise. The subsequent specular increase is consistent with Debye-Waller behavior. Trace b: approximately 1 ML of dodecane is deposited on a 40 K C10SH (3 × 2x3) surface, causing the specular intensity to drop from about 73 (point X) to 46 (point Y). The specular intensity is recorded as the surface temperature increases at 0.45 K s-1 through the desorption of the exposed molecules at 205 K.

C11SH substrate is less corrugated than that of the C10SH because of the different orientation of the topmost CH2CH3 bond. This explanation is consistent with recent measurements made of the orientation of a planar-oriented nematic liquid crystal atop an alkanethiol SAM on an oriented gold surface.19 The azimuthal orientation of the nematic liquid crystal is shown to vary by 90° between the SAMs of even and odd numbers of carbon atoms. Dodecane/C11SH and Dodecane/C10SH. To investigate why the C10SH overlayer behaves differently than that of C11SH, we deposited dodecane on the (3 × 2x3) surfaces of both C10SH and C11SH. A single overlayer deposition protocol could not optimize the overlayer surface atop the (3 × 2x3) SAMs of both C10SH and C11SH, so different deposition protocols were followed. This reflects the difference between the (3 × 2x3) surfaces of C10SH and C11SH. The C11SH (3 × 2x3) at a temperature of 196 K was exposed to 50 MLs of dodecane at a rate of 20 MLs/ s. The C10SH (3 × 2x3) surface was exposed to 180 MLs of dodecane at a rate of 1 ML/s as the crystal was cooled through the adsorption/desorption temperature (from 210 to 189 K). Figure 5a is a record of the specularity during the experiment. The specularity initially decreased while passing through the desorption temperature (205 K) as the SAM surface began to accumulate dodecane molecules. However, after an exposure of about 20 MLs at 200 K, the specularity rose dramatically and continued rising until an exposure of about 50 MLs was sustained at 190 K. As the crystal temperature cooled past 190 K, the specular increase is consistent with the Debye-Waller effect, indicating no further ordering due to overlayer augmentation. The diffraction pattern of this surface is shown in curve b of Figure 6. Figure 5b is a record of the specularity during another experiment: a 40 K C10SH (3 × 2x3) surface was exposed to a single ML of dodecane, and then the temperature was increased. The adsorption of the overlayer at 40 K greatly decreases the specularity (from point X to point Y), because the molecules must initially freeze near the random (19) Gupta, V. K.; Abbott, H. L. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1996, 54, R4540.

Schwartz et al.

Figure 6. Comparison of dodecane/C11SH and dodecane/C10SH overlayers. Curves a and a′ are the overlayer and corresponding underlying C11SH (3 × 2x3) surface, respectively. Curves b and b′ are the overlayer and corresponding underlying C10SH (3 × 2x3) surface, respectively.

locations where they are deposited. The surface order improves from about 60 to 100 K20 and then forms a completely disordered state until it desorbs at about 205 K, and the specularity increases to that of the original (3 × 2x3) surface. The specular intensity at 80 K is the same as that of the original (3 × 2x3) surface at that temperature. The amount of dodecane exposure was chosen to optimize the specular intensity. It is likely that the resulting coverage is that of a multilayer, as is judged by both the specular response discussed above and the diffraction scans from lesser exposures. Single ML exposures did not result in diffraction patterns such as curve b in Figure 6 that are different from that of the original (3 × 2x3) surface. Figure 6 is a comparison of the diffraction scans from overlayers of dodecane on the (3 × 2x3) surface of C10SH and C11SH SAMs. Diffraction scans produced by the corresponding bare (3 × 2x3) surfaces are also shown. The overlayer surfaces (curves a and b) are not of the same quality but seem to have the same structural features. Although a more systematic study would be necessary (and warranted) to clarify the structural issues opened by these results, we initially infer that the increased specularity is a characteristic of the overlayers. It is likely that the attraction between the molecules in the overlayer itself is stronger than the attraction to the (3 × 2x3) surface. Layers of Dodecane on Bare Au(111). It is useful to compare the overlayers discussed above to the physisorbed ML phases of C10SH, C11SH, and dodecane on the bare gold surface in terms of both structure as well as stiffness (discussed in the next section). The striped phases of C10SH and C11SH are well-known to consist of the molecules lying in rows, 5 Å apart (x3 times the 2.88 Å gold-gold spacing), sulfur to sulfur, aligned in the gold 〈11 h 0〉 direction.10 For dodecane/Au(111), we observed two phases. Figure 7a displays the diffraction patterns obtained from dodecane/Au(111), resulting from a ML exposure on a bare gold surface at 270 K. The diffraction peaks in the 〈11 h 0〉 direction are located at integer multiples of 0.364 Å-1 (marked with vertical solid lines), indicating a periodicity of 17.3 Å. The diffraction peak in the 〈112 h 〉 direction at ∆k| ) 2.6 Å-1 indicates a periodicity of 2.5 Å. Figure 7b displays (20) Similar annealing behavior has been reported for the SAMs of alkyl halides on alkali halides: Herna´ndez, J. J.; Li, J. A.; Baker, J.; Safron, S. A.; Skofronick, J. G. J. Vac. Sci. Technol., A 1996, 14, 1788.

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Figure 7. Diffraction scans of dodecane/Au(111). Part a shows a long periodicity (17.3 Å) phase, where the major periodicity is in the 〈11 h 0〉 direction. Part b shows a shorter periodicity (14.65 Å) phase, where the major periodicity is in the 〈112 h 〉 direction. The large peak in the 〈112 h 〉 direction at ∆k| ) -2.54 Å-1 for both scans indicates that, in both phases, the molecules are aligned in the 〈11 h 0〉 direction of the substrate. The dashed lines mark the off-azimuth peaks, shifted to smaller values of ∆k| by cos 30°, which result from the poor azimuthal resolution of the diffractometer. For possible real-space configurations, see Figure 8.

the diffraction pattern resulting from subsequent exposure of the surface to about 1200 MLs at a surface temperature of 200 K (approximately the desorption temperature of the second layer of dodecane). The diffraction peaks in the 〈112 h 〉 direction are located at integer multiples of 0.429 Å-1 (marked with vertical solid lines), indicating a periodicity of 14.65 Å (∼17.3 Å cos 30°). The length of the molecule is calculated to be 18.3 Å. Interdigitation of the terminal methyl groups could result in an inter-row spacing of 17.3 Å. Parts a and b of Figure 8 are real space diagrams of the two phases that are consistent with the corresponding diffraction patterns. The 5 Å chain-chain separation is greater than the 4.5 Å interchain distance required for the closest-packing hydrocarbon configuration. This implies that the corrugation energy of the hydrocarbons on the gold surface is stronger than the chain-chain van der Waals attraction. Interestingly, Xe/Pt(111) initially exhibits a commensurate (x3 × x3)R30° phase but becomes a higher density, incommensurate phase with increased exposure.21 4. Debye-Waller Measurements The process by which the intensities of the diffraction peaks are attenuated with increased surface temperature (the Debye-Waller effect) yields important information on the intermolecular constraining forces. The amount of the thermally induced surface disorder is inversely related to the stiffness of the surface. Peak intensities decrease with increased crystal temperature, according to the equation22,23

I ) I0e-2w

(3)

where I0 is the intensity of the peaks at absolute zero and 2w is the attenuation factor due to the thermal disorder of the surface. Specifically

2w ) ∆kz2〈Uz2〉 + ∆k|2〈U|2〉

(4)

where ∆kz and ∆k| are the helium atom momentum transfers perpendicular and parallel to the surface, respectively. 〈Uz2〉 and 〈U|2〉 are the mean square deviations from equilibrium position of the surface atoms perpen(21) Kern, K.; David, R.; Palmer, R. S.; Comsa, G. Phys. Rev. Lett. 1986, 56, 620. (22) Hautman, J.; Klein, M. L. J. Chem. Phys. 1990, 93, 7483. (23) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493.

Figure 8. Dodecane MLs physisorbed on Au(111). The possible molecular configurations that are consistent with the diffraction patterns (Figure 7) are shown. (a) A long periodicity phase and (b) shorter periodicity phase are shown. (c) The 30° cant measured for the shorter periodicity phase is very close to that resulting from staggering the dodecane chains one notch per 5 Å of separation.

dicular and parallel to the surface. ∆k| is found from eq 1, and

∆kz ) ki[(cos2 θi + /E)1/2 + (cos2 θf + /E)1/2] (5) where /E is the Beeby correction for the additional perpendicular momentum gained by the helium atoms as they fall into the attractive helium surface potential.21 E is the kinetic energy of the helium atoms, and  is the depth of the helium surface potential. The potential, ,

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Schwartz et al. Table 1. Values of d〈Uz2〉/dT and d〈U|2〉/dT and the Corresponding Incident Anglesa surface Au(111) dodecane/Au(111) dodecane/C11SH (3 × 2x3) C11SH (3 × 2x3) C10SH/C10SH (3 × 2x3) C10SH (3 × 2x3) C11SH stripes/Au(111)

d〈Uz2〉/dT d〈U|2〉/dT 0.295 0.94 1.93 1.93 1.95 2.18 1.40

0.2 0 2.0 0 1.8 0

incident angle (deg) 54.2 54.6 71.5 71.5 54.6 56.8 61.5

a The incident wave vector is 5.33 Å-1 for all experiments. All the multilayer organic surfaces are on the Au(111) surface. The units for the derivatives of 〈U2〉 are 10-4 Å2 K-1. The C10SH/C10SH overlayer is that on top of the disordered standing-up phase depicted in Figure 2.

Figure 9. Dodecane/C11SH Debye-Waller thermal attenuation of diffraction peaks. The vertical lines indicate the ∆k| ) 1.45 Å-1 expected positions of the (3 × 2x3) hexagonal spacing.

Figure 10. Integrated peak intensities of selected peaks from Figure 9. The lines indicate a least-squares fit.

has been reported to be 6.8 meV for the helium-organic surface interaction for the standing-up SAMs of even carbon-numbered thiols,23 8.0 meV for the SAMs of odd carbon-numbered thiols,23 and 8.0 meV for the heliumAu(111) interaction.24 From eq 4, we can obtain the temperature derivatives of 〈Uz2〉 and 〈U|2〉. Analysis of the specular peak alone (∆k| ) 0) yields d〈Uz2〉/dT, but the lack of sensitivity of the measurements to 〈U|2〉 makes the determination of d〈U|2〉/ dT difficult. Because of the variety of organic surfaces that this study presents, we approximate the potential, , to be 8.0 meV for all surfaces studied. An increase of the potential, , from 6.8 to 8.0 meV results in a 10% decrease of the values of d〈Uz2〉/dT obtained by fitting eq 4 to the data. Figure 9 shows the Debye-Waller attenuation of the diffraction-peak intensities from the overlayer of dodecane on a (3 × 2x3) of C11SH. The solid vertical lines indicate the positions of the first-, second-, and third-order hexagonal peaks. Figure 10 shows a semilog plot of the integrated peak intensities as a function of the crystal temperature. The slope of this graph is the derivative of 2w (eq 4). Table 1 gives the values of d〈Uz2〉/dT and d〈U|2〉/ dT obtained from eqs 4 and 5, along with the incident angles. The values for d〈Uz2〉/dT are accurate to about 10% (assuming that no error is contributed by the Beeby correction).21 The data for C10SH/C10SH are from the overlayer grown on top of the disordered C10SH standing(24) Vidali, G.; Ihm, G.; Kim, H.-Y.; Cole, M. Surf. Sci. Rep. 1991, 12, 133.

up ML shown in Figure 2. Because the values for 2w depend very weakly on 〈U|2〉, d〈U|2〉/dT is very difficult to determine. Bare Au is the stiffest surface, with dodecane/Au being the next stiffest surface. Adding a layer of C10SH or dodecane to the top of a (3 × 2x3) produces a surface of greater stiffness than that of the underlying SAM (especially to lateral movement). Three factors may contribute to this: (1) In the case of the SAMs, the terminal groups are oscillating thermally at the end of a series of oscillators. (2) In the case of the overlayers, the horizontal orientation of the carbon-carbon backbone of the overlayer strongly constrains the vibration of the individual methylene groups. (3) The larger effective mass of the long, horizontal molecules results in a collision that is more elastic. 5. Adsorption-Energy Dependence on the Distance from the Gold Surface This section focuses on the use of helium atom reflectivity to probe the overlayer enthalpy of desorption as a function of the distance of the overlayer from the surface. The latter quantity was set by creating various types of MLs of different thicknesses on a clean Au(111) surface. For example, a dodecane “striped” layer establishes a ML as thick as the molecule is wide, approximately 3 Å. C10SH (3 × 2x3) SAMs establish a ML that is approximately 13 Å thick, and C11SH (3 × 2x3) SAMs create MLs that are approximately 14 Å thick. Adsorbates are bound to these surfaces by a total energy equal to at least the attractive potential of the underlying substrate and the attractive potential of the underlying ML. Alkanes were used in this study to simplify the analysis. Specifically, the alkane enthalpies of desorption from the MLs were compared with the respective alkane’s bulk heats of vaporization at 25 °C. Previous temperature-programmed desorption (TPD) studies6,7 showed that alkane and alkanethiol adsorbates on atomically clean Au(111) desorb with enthalpies substantially greater than their respective bulk heats of vaporization. In these studies, the helium atom specular reflectivity from an adsorbate-covered Au(111) surface was monitored while the surface temperature was increased. When the adsorbates desorbed from the surface at their particular desorption temperature, the specular reflection increased to a maximum, corresponding to a bare gold surface. Analysis of the specular reflection as a function of the temperature yields the adsorbate’s desorption temperature, which was converted to the enthalpy of desorption using the Redhead25 equation.

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Langmuir, Vol. 19, No. 12, 2003 4975

Figure 11. Desorption enthalpies of dodecane from Au(111) surfaces, as was measured by TPD. The data point at 0 Å is the desorption from a bare Au(111) surface. The data point at 3 Å is the desorption from a lying-down dodecane ML on Au(111). The data points at 13 and 14 Å are the desorptions from C10SH and C11SH (3 × 2x3) SAMs, respectively. The dotted line represents the dodecane bulk heat of vaporization at 25 °C. Table 2. Alkane Enthalpy of Desorption from Various MLs on Au(111) Compared to the Bulk Enthalpies of Vaporization surface

distance (Å)

hexane (kJ mol-1)

octane (kJ mol-1)

dodecane (kJ mol-1)

Au(111) dodecane C10 (3 × 2x3) C11 (3 × 2x3) ∆Hvap (25 °C)

0 3 12 14.7 bulk

55.9 36.1 34.1

69.7 45.2 44.5

31.6

41.5

93.6 58.3 58.6 62.0 61.5

The atomic-beam specular reflectivity of each organic surface is often unique. When an overlayer desorbs from the supporting underlying ML surface, there is a change in the observed specular reflectivity. Thus, we were able to use the above TPD technique to establish a series of desorption enthalpies of various overlayer molecules at different distances to the gold substrate (provided by different underlying supporting MLs). Figure 11 shows the enthalpy of desorption of dodecane as a function of the distance from the Au(111) surface. The latter quantity is determined by the type of supporting ML adsorbed to the Au(111) surface. The minimum distance is the adsorption to bare Au(111), followed by the thickness of a dodecane pinstripe layer on the Au(111) surface, approximately 3 Å. The final two distances are established by the thickness of an upright C10SH (3 × 2x3) layer and an C11SH (3 × 2x3) layer, respectively. The figure clearly shows that the dodecane enthalpy of desorption is greatest on the bare gold surface and then approaches the dodecane bulk heat of vaporization (shown as the dotted line) at only 3 Å (desorption from the dodecane pinstripe layer). Increasing the distance from the surface to 13 Å has no appreciable effect on the enthalpy of desorption. This trend implies that the attractive potential of the Au(111) surface is short-range enough to be considered negligible across a separation equal to the thickness of a ML of hydrocarbon chains lying flat on the surface, about 3 Å. Table 2 shows the full set of data. Hexane, octane, and dodecane desorption enthalpies are shown as a function of the separation (in Å) from the surface. The bottom row of the table shows the respective bulk heats of vaporization (at 25 °C) for the respective alkanes. In each case, the confidence in the desorption enthalpies is (5 kJ mol-1, (25) Redhead, P. Vacuum 1965, 12, 203.

stemming from errors in the heating rate and analysis of the peak desorption temperature. As can be seen in Table 2, the alkane’s desorption enthalpy on the bare Au(111) surface is greater than its bulk heat of vaporization by 76% for hexane, 68% for octane, and 52% for dodecane. However, for the desorption from the pinstripe layer (3 Å) and the (3 × 2x3) layers (13 and 14 Å), the desorption energy for each molecule is essentially the bulk value. There does not seem to be a difference among the alkanes for this behavior. The desorption enthalpy seems to be restricted to interactions between the alkanes and the local environment of the ML, which itself is a hydrocarbon. Within the context of self-assembly, the interaction potential of the substrate is sufficiently weak so that the dominant interaction is between the hydrocarbon tails stemming from the adsorbed thiol. Adsorbates on the surface of the (3 × 2x3) structure are controlled mainly by three factors in the local environment: the corrugation of the methyl surface, interaction of functional groups at the surface, and interaction between the adsorbates. Recent studies on the orientation of liquid crystals atop an alkanethiol ML on gold8 show that the orientation of a liquid crystal on the surface of a C17SH/Au(111) ML is parallel to the ML surface, whereas an alkylsilane ML on glass or germanium orients the same liquid crystal perpendicular to the surface. These studies claim that the reason for the structural difference is due to a difference in adsorption energy of the liquid crystal resulting from the contribution of the underlying substrate, approximately 20 Å away. The results of the present TPD measurements would imply that the contribution of the substrate surface to the adsorption energy of the liquid-crystal overlayer at a distance of 20 Å is negligible. Instead, we propose that the change in the liquid-crystal orientation is mainly the result of a structural difference of the underlying ML surface. Although it is well-known that alkanethiols on Au(111) form densely packed MLs resulting in a smooth surface,1,3 aliphatic chains supported on substrates of silica and germanium do not form such smooth surfaces. A recent study26 shows that the silicon-oxygen cross linking that most likely takes place during alkylsilane ML formation sterically hinders the formation of close-packed, all-trans hydrocarbon packing. The gaps left between the hydrocarbon chains would allow interdigitation of the liquidcrystal overlayers. This would promote a perpendicular liquid-crystal orientation much in the same way as that of a mixture of short- and long-chain alkanethiols used to created a SAM on gold.27 6. Summary We were able to grow well-ordered, crystalline MLs of thiols and multilayers of dodecane on the surface of the alkanethiol (3 × 2x3). This was done by exposing the (3 × 2x3) surface to the appropriate amount of flux at a surface temperature below that required for the bulk desorption of the overlayer. Except for the C11SH/C11SH system, the overlayer surfaces proved to be considerably more ordered than those of the underlying (3 × 2x3), as was judged by the specular and diffraction-peak intensities. Even though the surfaces of the dodecane multilayers have the same structure whether grown on a C11SH or C10SH (3 × 2x3), the C10SH/C10SH surface was distinctly different from that of C11SH/C11SH. Debye-Waller measurements revealed that the surface of an overlayer is also stiffer than that of the underlying (3 × 2x3). (26) Stevens, J. Langmuir 1999, 15, 2773. (27) Abbott, N. L.; Gupta, V. K. Langmuir 1996, 12, 2587.

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TPD experiments have shown that, although the adsorption energy of organic molecules on a gold surface is much greater than that of the bulk heat of vaporization, this adsorption energy drops to that of the bulk value with only a separation from the gold surface equal to the thickness of a single lying-down layer of dodecane (about 3 Å). We propose that the order and energetics of the molecules adsorbed onto an organic ML on gold depend primarily on the compatibility of the adsorbed molecules

Schwartz et al.

with the underlying, supporting organic surface and not on the distance to the gold surface. Acknowledgment. Alexander Tsuji is thanked for editing. This work has been supported by the Materials Science Program of the Office of Basic Energy Sciences of Doe under Grant DE-FG02-93ER45503 and, in part, the National Science Foundation under the Princeton MRSEC Grant. LA034159B