Image Potential Surface States Localized at Chemisorbed Dielectric

Department of Chemistry, Iowa State University, Ames, Iowa 50011, ... Illinois State University, Normal, Illinois 61790-4160, and Department of Physic...
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Langmuir 2002, 18, 4709-4719

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Image Potential Surface States Localized at Chemisorbed Dielectric-Metal Interfaces Albert Avila,† Brian W. Gregory,*,‡ Brian K. Clark,*,§ Jean M. Standard,‡ and Therese M. Cotton†,| Department of Chemistry, Iowa State University, Ames, Iowa 50011, Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160, and Department of Physics, Illinois State University, Normal, Illinois 61790-4560 Received September 10, 2001 The work reported herein involves an extensive examination of alkanethiol self-assembled monolayers (SAMs) on roughened Au and Ag surfaces by surface-enhanced electronic Raman scattering. The observation of a novel resonance Raman-like process in these systems at excitation energies between 1.7 and 2.0 eV is described in detail. At excitation energies between these limits, a series of intense bands appear superimposed upon the normal surface-enhanced Raman vibrational spectrum of the film. The experimental evidence for an electronic Raman scattering process which leads to these bands is presented. The energy level diagram derived from these observed electronic transitions has led to the development of a model based on electronic Raman scattering between image potential surface states (IPSs). While the experimental Raman data indicates that the IPS electron exists within the dielectric film, quantum mechanical modeling of these image potential states for chemisorbed alkanethiol SAMs on roughened metal surfaces strongly argues that the electron is located in close proximity to the headgroup region of the film. Additionally, Raman excitation profiles for CH3(CH2)9SH SAMs on Au have provided evidence for both a filled, lower electronic state within the metal band gap (from which the IPS levels are filled via optical pumping prior to the Raman scattering event) and an upper electronic state; the energetic separation between the two states is 3.70 ( 0.03 eV. Furthermore, an analysis of these excitation profiles places the upper edge of the lower electronic state and the lower edge of the upper electronic state at approximately 3.60 and 7.29 eV above the clean Au(100) Fermi level, respectively. Thus, these states can be located within the surface band diagram of Au(100), and such a diagram is presented for the CH3(CH2)9SH/Au(100) system. The energetic locations of these states within the metal band structure indicate that these states are not intrinsic to the metal, and are consistent with those previously observed by two-photon photoelectron spectroscopy for alkanethiol films on Cu(111).

1. Introduction Monolayers of alkanethiols (R(CH2)mSH, where R ) CH3, HO, HOOC, etc.) chemisorbed on noble metal surfaces1,2 have been touted over the past two decades as useful model systems for the study of a wide variety of important interfacial phenomena, including electron transfer3,4 and corrosion.5 This notion stems from a number of factors: their ease of preparation, their higher degree of structural order relative to that produced by other stateof-the-art techniques (e.g., Langmuir-Blodgett deposition), their supposed structural simplicity, and their stability arising from the metal-sulfur chemisorption interaction.6-8 Compared to other surface modifiers that * To whom correspondence should be addressed. E-mail: [email protected] (B.W.G.), [email protected] (B.K.C.). † Iowa State University. ‡ Department of Chemistry, Illinois State University. § Department of Physcis, Illinois State University. | Deceased. (1) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Cheng, J.; Sa`ghi-Szabo´, G.; Tossell, J. A.; Miller, C. J. J. Am. Chem. Soc. 1996, 118, 680. (4) Richardson, J. N.; Peck, S. R.; Curtin, L. S.; Tender, L. M.; Terrill, R. H.; Carter, M. T.; Murray, R. W.; Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1995, 99, 766. (5) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (6) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; John Wiley: New York, 1978; Vol. 11, p 125. (7) Ju¨ttner, K.; Lorenz, W. J. Z. Phys. Chem. Neue Folge 1980, 122, 163. (8) Conway, B. E. Prog. Surf. Sci. 1984, 16, 1.

are currently employed, alkanethiol self-assembled monolayers (SAMs) are unique in that they embody the combined monolayer-forming features of both underpotential electrochemical deposition (UPD)6-8 and monomolecular lipid films.9,10 Clearly, the ability to modify the chemical and/or physical properties of a surface with molecular-level control has been a major driving force toward understanding both the spatial and electronic structures of these films. The specific affinity of the thiol headgroup for a wide variety of metal (and some nonmetal) surfaces makes these types of SAMs appealing. While the two-dimensional molecular organization and orientation of these films on monocrystalline metal surfaces (e.g., Au, Ag, Cu) have been extensively investigated, information concerning interfacial bonding and electronic structure in these systems has been more difficult to elucidate. Early X-ray photoelectron spectroscopic (XPS) evidence11-14 suggested that the products of alkanethiol or dialkyl disulfide chemisorption on such surfaces were indistinguishable and were consistent with the formation of a metal thiolate.13 These results have (9) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (10) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (11) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (12) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (13) Bain, C. D.; Beibuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (14) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152.

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also been supported by more recent high-resolution XPS investigations.15-18 In addition, using deconvolved XPS spectra, Porter and co-workers have detected the presence of two slightly different sulfur species in these films, and have attributed these to slightly different binding sites (terrace vs step).16 Also using XPS, Castner et al. have demonstrated the presence of both bound and unbound thiol in these films, and their dependence on choice of solvent and rinsing.17 The unraveling of more specific details of the nature of the Au/S electronic structure has been somewhat more elusive. Although the lowest energy electronic transition for n-alkanethiols is expected to occur in the near UV (n f σ*, 4.5-6.2 eV), an optical transition near 2 eV has been previously identified both by ellipsometry19 and by second harmonic generation20-22 for alkanethiol SAMs on Au. Both studies have demonstrated that these transitions are associated with the Au/S interfacial layer. Optical transitions in the same spectral region have also been observed during the in situ UPD of a Te monolayer on Au, and have been attributed to dimer formation during deposition.23 The formation of ordered arrays of Te and Se dimers during UPD on the low-index planes of Au has been studied extensively by low energy electron diffraction (LEED) and Auger electron spectroscopy,24-26 and by scanning tunneling microscopy (STM).26-28 One of the first direct studies of the interfacial electronic structure in these types of SAMs has been the application of two-photon photoelectron (2PPE) spectroscopy to the study of unoccupied states in both aliphatic29 and aromatic30 thiolate SAMs on Cu(111). In these studies, two unoccupied states were located at energies of 3.15 and 6.37 eV above the Fermi level for CH3(CH2)7SH SAMs; unoccupied states at similar energetic locations were also found for thiolates bearing aromatic or other aliphatic substituents. The transitions involving these states were attributed to headgroup-localized (i.e., Cu-S-C) molecular resonances between the HOMO and the LUMO or the LUMO+1, based on the observation that their energetic positions were relatively independent of the nature of the hydrocarbon pendant group. The work reported herein involves an extensive examination of alkanethiol SAMs on roughened Au and Ag surfaces by surface-enhanced Raman scattering (SERS). In this article, the observation of a novel resonance Ramanlike process at excitation energies between 1.7 and 2.0 eV

(∼720-620 nm) is described in detail. No corresponding electronic transition is observed that far in the red in the bulk solids of these materials. Between these energy limits, a series of intense bands appears superimposed upon the normal SERS vibrational spectrum of the film; in many cases, this set tends to obscure the normal SERS bands. These bands appear only for the long chain homologues (m g 9) at room temperature, and also for shorter chains (e.g., CH3(CH2)5SH SAMs) when the spectra are acquired for samples at liquid N2 temperatures, indicating that they are associated with the phase behavior of crystalline portions of the film. Extensive isotopic-labeling experiments (including 2H, 13C, and 34S substitutions), and comparisons between SERS obtained from alkanethiol and alkaneselenol SAMs, have revealed that these resonantly enhanced Raman bands are not of vibrational origin, but involve electronic Raman transitions. The experimental SERS evidence leading to these conclusions is demonstrated in their entirety. The energy level diagram derived from the observed transitions has previously led to the development of a model based on electronic Raman scattering between image potential surface states (IPSs).31 While the experimental Raman data indicate that the IPS electron exists within the dielectric film, quantum mechanical modeling of these image potential states for chemisorbed alkanethiol SAMs on roughened metal surfaces has shown that the electron is located in close proximity to the headgroup region of the film.32-34 Additionally, Raman excitation profiles for CH3(CH2)9SH SAMs on Au have provided evidence for both a filled, lower electronic state within the metal band gap (from which the IPS levels are filled via optical pumping prior to the Raman scattering event) and an upper electronic state; the energetic separation between the two states is 3.70 ( 0.03 eV. Furthermore, an analysis of these excitation profiles places the upper edge of the lower electronic state and the lower edge of the upper electronic state at approximately 3.60 and 7.29 eV above the clean Au(100) Fermi level, respectively. The energetic locations of these states within the metal band structure indicate that these states are not intrinsic to the metal and are consistent with those previously observed by 2PPE for alkanethiol films on Cu(111),29 which have been attributed to molecular states near the substrate-headgroup region.

(15) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323. (16) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (17) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (18) Bourg, M.-C.; Badia, A.; Lennox, R. B. J. Phys. Chem. B 2000, 104, 6562. (19) Shi, J.; Hong, B.; Parikh, A. N.; Collins, R. W.; Allara, D. L. Chem. Phys. Lett. 1995, 246, 90. (20) Buck, M.; Eisert, F. J. Electron Spectrosc. Relat. Phenom. 1993, 64/65, 159. (21) Buck, M.; Eisert, F.; Fischer, J.; Grunze, M.; Tra¨ger, F. J. Vac. Sci. Technol., A 1992, 10, 926. (22) Buck, M. Appl. Phys. A 1992, 55, 395. (23) Yagi, I.; Lantz, J. M.; Nakabayashi, S.; Corn, R. M.; Uosaki, K. J. Electroanal. Chem. 1996, 401, 95. (24) Suggs, D. W.; Stickney, J. L. J. Phys. Chem. 1991, 95, 10056. (25) Suggs, D. W.; Stickney, J. L. Surf. Sci. 1993, 290, 362. (26) Lister, T. E.; Huang, B. M.; Herrick, R. D., II; Stickney, J. L. J. Vac. Sci. Technol., B 1995, 13, 1268. (27) Suggs, D. W.; Stickney, J. L. Surf. Sci. 1993, 290, 375. (28) Goetting, L. B.; Huang, B. M.; Lister, T. E.; Stickney, J. L. Electrochim. Acta 1995, 40, 143. (29) Vondrak, T.; Wang, H.; Winget, P.; Cramer, C. J.; Zhu, X.-Y. J. Am. Chem. Soc. 2000, 122, 4700. (30) Vondrak, T.; Cramer, C. J.; Zhu, X.-Y. J. Phys. Chem. B 1999, 103, 8915.

2.1. Materials. All alkanethiol solutions were freshly prepared in HPLC grade methanol (Fisher). A wide variety of alkanethiols were chosen for study, which differed both in chain length and in identity of the terminal moeity. Hydrophobic monolayers were constructed from methyl-terminated alkanethiols (including n-butanethiol (99%, Aldrich), n-hexanethiol (95%, Aldrich), n-octanethiol (97+%, Aldrich), n-decanethiol (96%, Aldrich), n-dodecanethiol (98%, Aldrich), n-tetradecanethiol (97%, Pfaltz and Bauer), n-hexadecanethiol (92%, Aldrich) and n-octadecanethiol (98%, Aldrich)) and were used as received. Hydrophilic monolayer films were constructed from hydroxyl-terminated alkanethiols (either 11-mercapto-1-undecanol or 12-mercapto1-dodecanol), both of which were prepared in our laboratory according to standard literature procedures.35 The reagents used for their preparation (thiourea (99+%, Sigma) and either 11bromo-1-undecanol (98%, Aldrich) or 12-bromo-1-dodecanol (99%,

2. Experimental Section

(31) Clark, B. K.; Gregory, B. W.; Avila, A.; Cotton, T. M.; Standard, J. M. J. Phys. Chem. B 1999, 103, 8201. (32) Clark, B. K.; Gregory, B. W.; Standard, J. M. Phys. Rev. B 2000, 62, 17084. (33) Gregory, B. W.; Clark, B. K.; Standard, J. M.; Avila, A. J. Phys. Chem. B 2001, 105, 4684. (34) Clark, B. K.; Standard, J. M.; Gregory, B. W.; Hall, A. D. Surf. Sci. 2002, 498, 285.

IPSs at Dielectric-Metal Interfaces Aldrich)) were also used as received. Briefly, equimolar amounts of the ω-bromo-1-alcohol and thiourea were refluxed in 95% ethanol for 3 h, followed by the addition of aqueous base and refluxed for an additional 2 h. The solutions were cooled and neutralized with dilute H2SO4, and the ω-mercapto alcohols were extracted from the aqueous solutions. The synthesized mercapto alcohols were washed with HPLC grade methanol and ultrapure water (Millipore Milli-Q water system, nominal resistivity 18 MΩ‚cm) to remove impurities, and were subsequently vacuumdried. Gas chromatograph/mass spectral (GC/MS) analysis revealed mercapto alcohols of >98% purity (see below). The synthesis of a selenium analogue, didecyl diselenide, was also performed, and was achieved via a modification of the alkanethiol procedure. Equimolar amounts of 1-bromodecane (98%, Aldrich) and selenourea (99.9+%, Aldrich) were used as received and refluxed in 95% ethanol for approximately 4 h in a nitrogen atmosphere. Aqueous base was added and the solution was refluxed for approximately three additional hours. Despite the nitrogen atmosphere employed during the synthesis, the formation of the diselenide instead of 1-decaneselenol occurred due to trace amounts of oxygen in the reaction vessel; this product was verified via GC/MS analysis (see below). The didecyl diselenide formed as a yellow oil and then separated from the aqueous reaction mixture. The aqueous fraction of this reaction mixture was then acidified with dilute H2SO4 and extracted with hexane to yield additional diselenide. The diselenide fractions were added together and placed on a Buchner Rota-vap to remove remaining hexane and ethanol. GC/MS analysis revealed a didecyl diselenide purity of >98%. Solutions prepared for self-assembly typically contained total alkanethiol or alkanediselenide concentrations of approximately 1-5 mM. SAM formation was achieved by exposing either the roughened Au or Ag electrodes to the deposition solutions overnight at room temperature, unless otherwise noted. 2.2. Sample Purity Determination. The purities of the synthesized mercapto alcohols and diselenides were determined using a Hewlett-Packard (HP) 5890 Series II Plus gas chromatograph/5972 Series mass spectrometer (GC/MS) employing a HP-5MS GC column (30 m length, 0.25 mm i.d., 0.25 µm film thickness) with He as the carrier gas. Data analysis was subsequently carried out on a HP Vectra 486/66XM computer using G1034B HP Chemstation software. 1H NMR data were collected using a Bruker V22520 NMR spectrometer operating at 300 MHz with a Varian VXR-300 controller for data collection, and subsequent data analysis was performed using Varian NMR software. Experiments were conducted in either CDCl3 or CD3OD solvents (Aldrich, used as received); no tetramethylsilane (TMS) was used for NMR calibration. For samples in CD3OD, the center band of the solvent (quintet, with center located at 3.30 ppm) was used for spectral calibration; for samples in CDCl3, the solvent singlet band at 7.25 ppm was used. The number of sample scans varied depending on individual sample concentrations. 2.3. Electrode Preparation. Electrodes were prepared from either polycrystalline Au foil or polycrystalline Au or Ag rod electrodes press fitted into Teflon shrouds (Bioanalytical Systems, West Lafayette, IN). The Au foil electrodes were initially placed in warm concentrated nitric acid for approximately 2 h to remove surface impurities and then sonicated several times in ultrapure water (nominal resistivity 18 MΩ‚cm). They were subsequently flame-annealed to achieve a smoother, more uniform surface. The Teflon-shrouded electrodes were initially polished with successively finer gradations of alumina paste until they had a mirrorlike optical finish, and were subsequently sonicated in ultrapure water. Afterward, the electrodes were immersed in 9 M nitric acid for approximately 3-5 s, followed by sonication in ultrapure water again. Teflon-shrouded Au electrodes were electrochemically roughened in 0.1 M KCl solution using 40 oxidation-reduction cycles (ORCs), which consisted of potential ramps (at 500 mV/s) between +1.3 and -0.3 V followed by rinsing and storing in ultrapure water until time for monolayer deposition. Polycrystalline Au foil electrodes were roughened using (35) Urquhart, G. G.; Gates, J. W.; Connor, R. In Organic Syntheses; Horning, E. C., Ed.; John Wiley & Sons: New York, 1964; Collective Vol. 3, p 363.

Langmuir, Vol. 18, No. 12, 2002 4711 1000-1500 ORCs (at the same potentials as above), and were also stored in ultrapure water. The Ag electrodes were electrochemically roughened via a series of ORCs in 0.1 M NaSO4 solution, also followed by rinsing and storing in ultrapure water. For Ag, the roughening procedure consisted of 1000-1500 ORCs consisting of potential steps between +1.3 and -0.3 V, followed by a 2 s rest at -0.3 V. Immediately before exposure to the alkanethiol solutions, the electrodes were briefly dipped in HPLC grade methanol to remove excess water. All electrodes were exposed to the alkanethiol solutions within 1 h of preparation. 2.4. Raman Instrumentation and Acquisition Conditions. Excitation was provided by a number of laser sources. Discrete wavelengths throughout the visible region were obtained from either a Coherent Innova 100 Kr+ laser (1.648, 1.833, 1.916, and 2.182 eV) or a Coherent Innova 200 Ar+ laser (2.410 eV). Wavelength tunability within the 1.653-2.066 eV range was accomplished by using the latter to pump either a Coherent 599 dye laser (charged with 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), wavelength range ) 1.7512.053 eV) or a Spectra Physics Model 3900 CW Ti-sapphire laser (wavelength range ) 1.653-1.722 eV). Plasma line removal from either the Ar+ or Kr+ laser lines was achieved with an Anaspec premonochromator. The laser was focused onto the sample surface to a spot size of ∼1 mm2, and the power measured at the samples for all experiments was 20 mW unless otherwise noted. Raman scattered radiation was collected with a Nikkor-S lens and focused onto the entrance slit of a 1877 Spex Triplemate monochromator equipped with a 1200 grooves/mm grating. A Princeton Applied Research CCD (Model LN/CCD-1152) cooled to -120 °C was used for Raman signal detection. Signal integration periods were typically 200 s unless otherwise specified. To examine the temperature dependence, a heating/cooling system was constructed to control the electrode surface temperature. A Teflon diaphragm pump was used to circulate the heating/cooling fluids through a Pyrex flow cell containing the electrodes. All components of the heating/cooling system were connected exclusively with Teflon tubing. A glass-jacketed thermocouple submerged in bath oil and placed immediately following the flow cell was used to monitor the fluid temperature. For the heating experiments, ultrapure water was heated in a round-bottom flask using a heating mantle, and was mechanically agitated to promote temperature uniformity. The useful temperature range for electrode heating was approximately 20-80 °C. For cooling experiments, an ethanol/liquid nitrogen slurry was produced in a Dewar flask and agitated by a stirring rod. The effective temperature range for the cooling the electrode was approximately -30 to 0 °C. A stream of N2 gas was directed at the flow cell during cooling experiments to avoid atmospheric water condensation on the flow cell window. Flow rates varied according to the desired temperature of the electrodes and the viscosity of the heating/cooling fluids. For cooling below -30 °C, samples were immersed directly in liquid nitrogen (-196 °C) prior to spectral acquisition. 2.5. Au Colloidal Solution Preparation and Alkanethiol Exposure. Au colloidal solutions were prepared according to published literature methods36 from hydrogen tetrachloroaurate(III) hydrate (HAuCl4‚xH2O, 99.999%, Aldrich) and sodium citrate (Fisher, certified grade), both of which were used as received. Ultrapure water (nominal resistivity ) 18 MΩ‚cm) was used for all preparations, and all HAuCl4 transfers were conducted quickly in air. Briefly, a 50.0 mL solution of 0.01% (by weight) HAuCl4 was brought to a reflux, upon which 0.50 mL of a 1% (by weight) sodium citrate solution was added, yielding particles of approximately 40 nm diameter.36 The solution was allowed to reflux for 30 min to ensure complete reduction of the gold. After cooling, the Au colloidal solutions were stored in sterile bottles. To achieve alkanethiol monolayer formation on the Au colloid, the colloids were first transferred to methanol. Typically, a 5 mL aliquot of the colloid solution was sedimented by centrifugation and 4.5 mL of H2O was removed. Methanol (Fisher, HPLC grade) was added to bring the volume back to 5 mL, and the colloid was redispersed into solution by brief sonication. The sedimentationdispersion cycle was completed 3-4 times to remove most of the H2O. Following the last sedimentation step, the Au colloid was (36) Frens, G. Nat. Phys. Sci. 1973, 241, 20.

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resuspended in methanolic alkanethiol solution (1 mM) for time periods ranging from 12 to 24 h. The SAM-modified colloids were subsequently transferred back to aqueous solution by sedimentation-dispersion cycles similar to those described above.

3. Results and Discussion 3.1. Previous SERS Studies. The first SERS investigations of alkanethiol SAMs (R(CH2)mSH, where R ) CH3) on roughened Au and Ag electrode surfaces were reported by Pemberton and co-workers.37,38 In these studies, excitation was performed at 2.410 eV for monolayers on Ag37 and at 2.066 and 1.722 eV for those on Au.38 The authors rationalized from surface selection rule arguments that differences in the spectra between films on the two substrates originated from significant differences in molecular orientation within the film. This model was particularly relevant for the ν(C-S) stretching region, where the large disparity in intensities for this mode between the two substrates reflected considerable differences in metal-sulfur bonding character (since the interatomic distances in Au (2.8894 Å) and Ag (2.8841 Å) differ by 2600 cm-1) appear superimposed upon the normal vibrational spectrum of the film, and often obscure the vibrational spectrum. The 10 most intense bands occur between 590 and 1000 cm-1 and are routinely observed for most samples, whereas the higher frequency modes tend to be somewhat weaker and appear infrequently. An optical transition in the same energy range reported here (i.e., near 2 eV) has been previously identified both by ellipsometry19 and by second harmonic generation (SHG)20-22 for alkanethiol SAMs on Au. Discrete optical transitions in this region have been observed by optical frequency ellipsometry, and have been shown to be independent of alkanethiol chain length (for chain lengths 9 e m e 21) and associated with the presence of a 1.2 Å thick interfacial layer near the gold-sulfur region.19 SHG studies of CH3(CH2)15SH on polycrystalline Au have demonstrated a significant wavelength dependence in SHG signal intensity for fundamental excitation energies between 1.88 and 2.02 eV.20 Such results have indicated the presence of an optical transition associated with the Au/S region, since the SHG signal arises entirely from the metal/headgroup interface.20 Similar optical transitions in the same spectral region (2.12 eV) have also been observed by SHG during the in situ UPD of Te monolayers on Au.23 The observed resonance has been attributed to the formation of Te dimers and chain structures, which (37) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 3629. (38) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (39) Hill, W.; Wehling, B.; Klockow, D. Sens. Actuators, B 1994, 18/ 19, 188. (40) Yamamoto, Y.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1993, 140, 436.

Figure 1. SERS spectrum of a CH3(CH2)9SH SAM on electrochemically roughened Au, acquired at Eexc ) 1.92 eV (25 mW power), with most of the first-order SEERS features labeled. Virtually all the observed bands in this spectrum are due to electronic Raman scattering between image potential surface states. Due to the intensities of these bands, none of the wellknown vibrational SERS bands associated with the SAM are visible. Inset: Image state energy level diagram derived from the SEERS spectrum (in eV).

were thought to modify the interfacial electronic properties and hence its nonlinear susceptibility. Such polychalcogenide structures are known to form during UPD of both Te and Se on Au.24,25,27 Evidence for Electronic Raman Scattering from Alkanethiol/Au SAMs. Evidence that these resonance Raman-like features arise from an electronic Raman scattering process was acquired via the use of isotopically labeled n-alkanethiol precursors. These precursors included perdeuterated dodecanethiol (CD3(CD2)11SH), R-13Cdodecanethiol (CH3(CH2)1013CH2SH), and CH3(CH2)934SH. Originally, these isotopic species were chosen to examine the overall dependence of the band positions and intensities on mass changes in the tail region, in the methylene subunit nearest the sulfur headgroup, and in the headgroup itself, respectively. Band positions for those SERS modes that have been previously assigned to specific vibrational group frequencies (section 3.1) shifted upon isotopic substitution, as expected. Quite surprisingly, band positions for all the resonantly enhanced SERS bands remained unchanged (within (1 cm-1) for all isotopically labeled species (Figure 2A-E). Furthermore, substitution of selenium for sulfur as the headgroup species resulted in no changes to the observed resonance SERS modes either; monolayers of didecyl diselenide (CH3(CH2)10Se)2 exhibited spectra identical (within (1 cm-1) to those observed for the alkanethiols and dialkyl disulfides (Figure 2F). Additionally, virtually identical bands are observed for CH3(CH2)9SH and other longer chain alkanethiol SAMs

IPSs at Dielectric-Metal Interfaces

Figure 2. SERS spectra of various isotopically labeled alkanethiol SAMs on electrochemically roughened Au (or Ag, as noted), acquired at Eexc ) 1.92 eV: (A) CH3(CH2)11SH, (B) CH3(CH2)1013CH2SH, (C) CD3(CD2)11SH, (D) CH3(CH2)9SH, (E) CH3(CH2)934SH, (F) (CH3(CH2)9Se)2, and (G) CH3(CH2)9SH on Ag. Asterisks denote the most intense resonantly enhanced SERS bands.

on Ag (Figure 2G). Clearly, the insensitivity of the peak frequencies for these bands with both isotopic substitution and the use of different substrates strongly indicates that these Raman modes are not associated with vibrational excitations at all, but arise from a surface electronic Raman scattering process. Such a process is also supported by an analysis of the relative intensities of the Stokes and antiStokes bands (Figure 3). The negligibly small anti-Stokes bands suggest upper state initial populations which are nearly too low to be detected, an observation which is also consistent with a surface electronic Raman scattering mechanism. In fact, the anti-Stokes intensities are 1-2 orders of magnitude smaller than that expected based on a simple Boltzmann population analysis. Presently, it is not clear what role(s) both the state populations and the transition probabilities play in dictating this significant difference in intensities. These isotopic substitution studies and Stokes/anti-Stokes observations have led to the acronym SEERS, for “surface-enhanced electronic Raman scattering,” which has been used to describe these spectra.31-33 3.2.2. Effect of SAM Film Crystallinity on SEERS Features. Alkyl Chain Length Dependence of Observed Bands. Figure 4 demonstrates some of the observed changes in the SEERS spectra between 500 and 1200 cm-1 that accompany increasing chain length in these alkanethiol SAMs (CH3(CH2)mSH) on Au. These spectra were obtained at room temperature on electrochemically roughened, alkanethiol-covered polycrystalline Au electrodes31-33 at an excitation energy of 1.916 eV using a Kr+ laser operating at low laser powers (20 mW); the same SEERS spectra were also observed for these compounds on Au colloidal films.31,41,42 In these spectra, the SEERS bands appear for all chain lengths having m g 8, although they

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Figure 3. Stokes and anti-Stokes SERS spectra of a CH3(CH2)9SH SAM on electrochemically roughened Au, acquired at Eexc ) 1.92 eV (50 mW power).

tend to be weakest for m ) 8. These bands have not been observed at room temperature for alkanethiol monolayers with m e 8 (data not shown). However, they do appear in the SEERS spectra of shorter chain alkanethiol films (e.g., m ) 5) upon cooling the samples to liquid nitrogen temperatures (Figure 5). These resonantly enhanced bands have also been observed for other ω-functionalized long chain alkanethiols, including those containing R ) HO and HOOC (data not shown). Peak positions for these bands are remarkably insensitive (within (1 cm-1) to variations in alkyl chain length for all chain lengths studied (m ) 8-17). The onset of these peaks as a function of alkyl chain length clearly coincides with established structural models for these films.2 Pronounced structural differences have been observed between short (m < 9) and long (m > 9) chain alkanethiolate films, due to increased interchain dispersion interactions which result in more highly ordered and crystalline films for the longer chain homologues.2,43-46 In fact, the chain density for long chain alkanethiolate SAMs has been shown to be comparable to that for polymethylene chains in bulk crystalline n-alkanes.1 Therefore, the alkyl chain length dependence of these (41) Avila, A.; Gregory, B. W.; Sokolov, K.; Cotton, T. M. In Fifteenth International Conference on Raman Spectroscopy; Asher, S. A., Stein, P., Eds.; John Wiley & Sons: Chichester, England, 1996; p 720. (42) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466. (43) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (44) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (45) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (46) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Gu¨ntherodt, J.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869.

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Figure 4. SERS spectra of CH3(CH2)mSH SAMs of varying chain lengths on electrochemically roughened Au, acquired at Eexc ) 1.92 eV: (A) CH3(CH2)17SH, (B) CH3(CH2)11SH, (C) CH3(CH2)9SH, and (D) CH3(CH2)8SH. Asterisks denote the most intense SEERS bands.

SEERS modes strongly suggests that their appearance is intimately connected with the formation of a highly crystalline film. Surface-Active Contamination/Photodecomposition Studies. The possibility of adventitious surface contamination from trace components in the deposition solutions was exhaustively explored, with an emphasis directed toward analysis for components exhibiting optical absorption in the red (e.g., a thiol-containing dye). Accordingly, extensive examination of these solutions (and of their constituent pure reagents) using GC/MS, 1H NMR, and fluorescence spectroscopy yielded no obvious source of such contamination. A number of different sources of alkanethiol reagent were analyzed in this manner, including those synthesized directly by reaction of the corresponding n-bromoalkane with thiourea.35 Despite the lack of evidence pointing toward such trace contaminants, all long-chain alkanethiol samples produced SEERS spectra similar to those shown in Figures 1-5. Au electrodes were also exposed to solutions containing the reagents used in the alkanethiol syntheses, none of which yielded the characteristic spectra. In addition, exposure of a roughened Au electrode to 1 mM solutions of dioctadecyl disulfide (CH3(CH2)17S)2 (which was produced directly from CH3(CH2)17SH by oxidation with molecular iodine47 as a means of sample prepurification) resulted in spectra identical with that from its thiol analogue. This observation correlates with the well-known fact that indistinguishable films result from the deposition of either alkanethiols or dialkyl disulfides.1,2 The possibility of photodecomposition of the alkanethiol SAMs was examined by monitoring changes in the SEERS

Avila et al.

Figure 5. SERS spectra of a CH3(CH2)5SH SAM on electrochemically roughened Au, acquired at Eexc ) 1.92 eV and taken both at room temperature (20 °C) and at liquid nitrogen (-196 °C) temperatures. Note the appearance of the SEERS bands (marked by asterisks) at low temperature.

spectra on both Au and Ag electrodes as a function of laser power. Studies employing incident powers ranging from 10 to 200 mW, however, demonstrated no changes in any of the relative band intensities of the alkanethiols in this spectral region on either Ag or Au, indicating that little or no photodecomposition occurs in this energy regime. Kinetic Studies. Previous kinetic studies2 of alkanethiol monolayer formation on Au(111) surfaces from moderately concentrated solutions (mM) have demonstrated the presence of two distinct steps in the adsorption process: an initial fast step which lasts at most a few minutes, after which nearly all the surface adsorption sites have been saturated; a second slower step lasting several hours, which has been characterized as a two-dimensional crystallization process. The kinetics of the first step is controlled both by the solution concentration and by the reactivity of the headgroup with the substrate surface atoms, whereas the second step is dominated by intermolecular interactions and the surface mobility of the adsorbed species. Interchain dispersion forces are therefore primarily responsible for dictating the kinetics associated with this second step. This notion has been supported by the observation that these kinetics are faster for the longer chain homologues, presumably due to the larger intermolecular dispersion forces compared to the shorter chain varieties.2,48 In light of these previous investigations, the kinetics of alkanethiol monolayer formation on Au were monitored by measuring the Raman intensities of a number of bands (47) Reid, E. E. Organic Chemistry of Bivalent Sulfur; Chemical Publishing Co.: New York, 1960; Vol. 1. (48) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.

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intimately connected to the two-dimensional organization of the SAM. 3.3. Synopsis of Prior Modeling of Observed Spectral Transitions Based on Electronic Raman Scattering between Image Potential Surface States. The observed SEERS line positions have led to the development of an energy level diagram, and consequently to a preliminary model based on electronic Raman scattering between image potential surface states (IPSs).31 These bound surface electronic states arise from the attractive force between an electron existing outside a metal surface and its induced image charge in the metal. The abrupt change in polarizability at the interface gives rise to these quantized bound states, and results in the localization of the IPS electron in a direction normal to the surface. The physical description and modeling of these states at a single, atomically flat interface is usually accomplished through the method of images.49 This treatment leads to a 1/z Coulomb potential (z is the distance normal to the metal surface) and a series of Rydberg-like bound energy levels En

En )

Figure 6. Plot of the relative intensity of various SEERS and vibrational SERS bands for a CH3(CH2)17SH SAM on electrochemically roughened Au as a function of deposition time, taken at the same spot on the electrode surface and acquired at Eexc ) 1.92 eV.

(including both SEERS and normal vibrational modes) as a function of deposition time. The intention of these studies was to determine if the two sets of bands exhibited distinctly different kinetics, and if so, whether either could be attributed to one of the two regimes in the adsorption process (i.e., the initial fast adsorption or slow twodimensional crystallization). Figure 6 displays a plot of the relative intensity for particular bands observed in the Raman spectra of CH3(CH2)17SH on electrochemically roughened Au (at 1.916 eV excitation) as a function of deposition time. These spectra were acquired in air following the labeled deposition periods; the substrates were emersed, rinsed with copious amounts of blank solvent (HPLC grade methanol), and dried in a stream of nitrogen prior to spectral acquisition. After data collection, the substrates were immersed back into the deposition solution. Since both inter- and intrasample variation in SERS/SEERS intensities are often substantial on these types of roughened substrates, considerable care was exercised in obtaining data from the exact same spot for all the spectra from a particular sample. The data clearly demonstrate that the observed Raman modes attributed to well-established group vibrational frequencies (e.g., νa(C-C), 1063 cm-1; Fr(CH3), 890 cm-1) reach maximum intensity well within the first 0.5 h of deposition and change very little afterward. This indicates that, at these concentrations, monolayer formation is virtually complete within this time frame, and therefore supports the first step in the adsorption model. The SEERS modes (e.g., 680 and 747 cm-1), however, exhibit pronounced growth over a much longer time period, consistent with the second step involving a much slower, two-dimensional crystallization process. Both growth regimes however appear to be easily differentiated in the data, and the slower emergence of the resonantly enhanced modes again strongly suggests that the origin of the SEERS bands is

-0.85 eV 2(n + δ)2

(1)

where n is the traditional principal quantum number,  is the dielectric constant of the overlayer (if present), and δ is the quantum defect due to elastic scattering.50-52 The constant 0.85 eV (13.6 eV/16) indicates that the binding energy is reduced by a factor of 16 from the hydrogen atom case because the IPS electron (located at distance z from the metal surface) is separated from its image charge by 2z.50 Since these bound states are pinned to the vacuum level, changes in the observed binding energies with respect to the Fermi level reflect changes in the local work function;53 such changes have been used to monitor the growth modes of various overlayers on metal substrates.54-56 While qualitative agreement with experimental image state binding energies can be obtained via eq 1, this relationship does not account for penetration of the electron wave function into the metal. Two-photon photoemission (2PPE) spectroscopy has become the preferred technique for IPS studies because of its energy resolution (20-50 meV) and surface sensitivity.50,57 In addition, 2PPE via ultrafast laser excitation has provided a means by which electron dynamics of the image states can be investigated.58,59 As discussed previously, Figure 1 displays the SEERS spectrum of a CH3(CH2)9SH SAM on a roughened polycrystalline Au electrode. The inset in this figure is the IPS energy level diagram derived from the observed SEERS spectra (in eV) and from fitting the n > 1 states with the n ) 1 state at -0.121 eV to eq 1.31 Most of the first-order transition SEERS scattering features are labeled (in cm-1) in this figure; the corresponding assignments for the main spectral transitions attributable (49) Jennings, P. J.; Jones, R. O. Adv. Phys. 1988, 37, 341. (50) Fauster, Th. Appl. Phys. A 1994, 59, 479. (51) Echenique, P. M.; Pendry, J. B. Prog. Surf. Sci. 1990, 32, 111. (52) Straub, D.; Himpsel, F. J. Phys. Rev. Lett. 1984, 52, 1922. (53) Fischer, R.; Schuppler, S.; Fischer, N.; Fauster, Th.; Steinmann, W. Phys. Rev. Lett. 1993, 70, 654. (54) Fischer, R.; Fauster, Th.; Steinmann, W. Phys. Rev. B 1993, 48, 15496. (55) Fischer, R.; Fauster, Th. Phys. Rev. B 1995, 51, 7112. (56) Wallauer, W.; Fauster, Th. Surf. Sci. 1995, 333, 731. (57) Steinmann, W. Phys. Status Solidi B 1995, 192, 339. (58) Ho¨fer, U.; Shumay, I. L.; Reuss, Ch.; Thomann, U.; Wallauer, W.; Fauster, Th. Science 1997, 277, 1480. (59) Ge, N.-H.; Wong, C. M.; Lingle, R. L., Jr.; McNeill, J. D.; Gaffney, K. J.; Harris, C. B. Science 1998, 279, 202.

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Avila et al.

to SEERS up to ∼1600 cm-1 have been previously discussed.31 The lowest four observed energy levels in the Figure 1 inset have been collectively attributed to the n ) 1 image state. The strongest SEERS transition (680 cm-1) originates from the energy level at -0.121 eV; consequently it is considered the most significant n ) 1 energy level. The multiple energy levels attributed to the n ) 1 image state manifold are not addressed by eq 1, but may reflect local variations in the image plane position due to surface corrugation effects.34 No scattering originating from any of the n g 2 levels has been observed. Several observations have indicated that the energetic positions of the bound levels are dominated by the portion of the electron wave function located within the dielectric film. First, the fit to eq 1 assumes that the wave function exists exclusively within the dielectric. The dielectric constants  obtained by fitting in turn the four levels within the n ) 1 energy manifold with the n > 1 energy levels range from 1.9 to 2.26. These values closely agree with the bulk value of  ) 2.25 for close-packed alkyl chains, and are also consistent with those derived from surface potential studies of alkanethiols on Au.60 Second, as discussed above, identical SEERS spectra have been observed for alkanethiol films in both air and aqueous solution, and even at liquid nitrogen temperature (Figure 5), indicating localization of the electron within the film. Third, with the exception of intensities, identical SEERS spectra are observed for all chain lengths studied (i.e., from m ) 8 to 17), and even for m ) 5 at low temperature (Figure 4). These alkyl chain lengths correspond to film thicknesses from ∼6 to 20 Å. This is in contrast to 2PPE studies of image states at Ag(111)/n-alkane interfaces, wherein a 2-fold decrease in the IPS binding energies occurs as the alkane film thickness is increased.61-65 In the physisorbed Ag(111)/n-alkane systems, results indicate that the n ) 1 IPS electron resides at the alkane/ vacuum interface, and that the electron-image charge separation increases with increasing film thickness. In contrast, our SEERS data for the strongly chemisorbed alkanethiol systems point to localization of the IPS electron within the dielectric film and an electron-image charge separation that is independent of film thickness. Additionally, surfaces that give rise to SEERS contain roughness features on many different length scales (including those of optical wavelengths). Consequently, a more complex surface model has been developed to account both for the types of gross surface roughness features expected at these surfaces and for the penetration of the electron wave function into the substrate.32-34 In short, a simple model potential simulating step edge/terrace features was developed that allows an IPS electron to interact with two intersecting orthogonal surface planes, but is confined to the image plane of the terrace and moves orthogonally to the step edge.32,33 The potential orthogonal to the step edge is given by nearly free electron (NFE) theory for the electron wave function in both the substrate and thiol headgroup layer.34 External to the substrate, the potential is dictated by the Coulomb interaction between the electron and its image charge in the step

edge. Based on the observed n ) 1 IPS line widths observed experimentally by SEERS, Clark et al. have shown that the observed image states most likely occur at Au and Ag step edges of (100) crystallinity.34 The relevant bulk substrate parameters required for the model potential include the bulk potential V0, the metal half-band gap VG, the energetic position of the lower edge of the band gap EL, the lattice constant a, and the work function Φ0. The work function Φ0 and bulk substrate parameters V0, VG, EL, and a are well-known for both the Au(100)66,67 and Ag(100)68 surfaces. The manner in which the periodic potential inside the bulk substrate is joined to the image potential outside the surface at the interfacial region is critical to developing a suitable model. Since a constant potential barrier in this region is clearly unphysical, a more accurate representation (based on the method of Chulkov et al.69) was included by allowing some potential variation due to the thiol headgroup region.34 This is reasonable in light of the fact that these SAMs are strongly bound to the substrate surface and result in a significant redistribution of the interfacial electron density upon chemisorption. Allowed energy levels for the electronmetal system were then determined by solving the timeindependent Schro¨dinger equation for the resulting potential using the Cooley-Cashion-Zare method.70,71 Results from these quantum mechanical calculations have yielded excellent agreement with the experimental SEERS spectra. From the modeling, a value of  ) 2.20 ( 0.04 for the film dielectric constant on both Au(100) and Ag(100) has yielded bound state energies in excellent quantitative agreement with experiment;34 this value of  is also consistent with those extracted from nonlinear fitting of eq 1.31 Similarities in the calculated energies on both Au(100) and Ag(100) are consistent with the experimental observation that the IPS SEERS spectra are identical within experimental resolution for both surfaces.32-34 The resulting electron wave functions demonstrate that the location of maximum probability for finding the n ) 1 IPS electron is ∼10 Å outside the step edge surface.34 As previously mentioned, the IPS states observed by SEERS arise from the electron/step edge system, and the motion of the IPS electron that is being probed is with respect to the step edge at the terrace image plane (which is also the approximate location of the sulfur headgroup layer).32,33 Since it is unlikely that all sulfur headgroups occupy equivalent positions with respect to the step edge, potential variations due to surface corrugation along the step edge may give rise to the multiple states belonging to the n ) 1 manifold. Consequently, the observation of four n ) 1 IPS levels may arise due to the presence of four inequivalent headgroups located at terrace/step edge junctions, resulting in surface corrugation of the potential both parallel and perpendicular to the terrace.34 The occurrence of multiple headgroup binding sites in alkanethiol SAMs on Au is supported by recent experimental studies, which have indicated the presence of at least two inequivalent headgroup binding sites on both the (111) and (100) surfaces.72 Since the wave functions for higher order image state levels (n g 2) exhibit substantially less

(60) Evans, S. D.; Ulman, A. Chem. Phys. Lett. 1990, 170, 462. (61) Ge, H.-H.; Wong, C. M.; Harris, C. B. Acc. Chem. Res. 2000, 33, 111. (62) Harris, C. B.; Ge, N.-H.; Lingle, R. L., Jr.; McNeill, J. D.; Wong, C. M. Annu. Rev. Phys. Chem. 1997, 48, 711. (63) Lingle, R. L., Jr.; Padowitz, D. F.; Jordan, R. E.; McNeill, J. D.; Harris, C. B. Phys. Rev. Lett. 1994, 72, 2243. (64) Lingle, R. L., Jr.; Ge, N.-H.; Jordan, R. E.; McNeill, J. D.; Harris, C. B. Chem. Phys. 1996, 205, 191. (65) Lingle, R. L., Jr.; Ge, N.-H.; Jordan, R. E.; McNeill, J. D.; Harris, C. B. Chem. Phys. 1996, 208, 297.

(66) Christensen, N. E.; Seraphin, B. O. Phys. Rev. B 1971, 4, 3321. (67) Ciccacci, F.; De Rossi, S.; Taglia, A.; Crampin, S. J. Phys.: Condens. Matter 1994, 6, 7227. (68) Schuppler, S.; Fischer, R.; Fischer, N.; Fauster, Th.; Steinmann, W. Appl. Phys. A: Solids Surf. 1990, 51, 322. (69) Chulkov, E. V.; Silkin, V. M.; Echenique, P. M. Surf. Sci. 1999, 437, 330. (70) Cooley, J. W. Math. Comp. 1961, 15, 363. (71) Cashion, J. K. J. Chem. Phys. 1963, 39, 1872. (72) Fenter, P. In Self-Assembled Monolayers of Thiols; Ulman, A. Ed.; Academic Press: San Diego, 1998; Chapter 4 and references therein.

IPSs at Dielectric-Metal Interfaces

substrate penetration than that for the n ) 1 level, the n g 2 levels will likely be less sensitive to the surface corrugation. Given our experimental resolution, splitting of these higher n levels may not be measurable. 3.4. Raman Excitation Profiles. As discussed previously, the resonance Raman-like behavior of the IPS SEERS bands is unusual since no optical transitions are expected for the alkanethiol precursors in the range of excitation energies at which SEERS is observed (1.7-2.0 eV). In fact, for sufficiently short chain length alkanethiol SAMs (m < 8) at room temperature, no SEERS bands are observed whatsoever (Figure 5). The SEERS bands do appear for the short chain homologues simply by reducing the sample temperature (Figure 5), demonstrating that changes in the electronic structure occur as the film becomes more highly ordered. The observation that the SEERS line widths are not significantly temperature dependent shows that other relaxation mechanisms (e.g., electron-phonon coupling, scattering) are not preventing their appearance for the short chain homologues at room temperature. The abrupt appearance of SEERS for alkanethiol SAMs with increasing chain length (Figure 4) is not a direct consequence of changes in the overlayer dielectric constant , as  for the alkyl chain is expected to increase smoothly and monotonically between ∼1.9 and 2.2 across the range of chain lengths investigated.73 Therefore, the experimental SEERS and IPS modeling results strongly indicate that changes in the electronic structure of the overlayer occur when the SAMs become highly crystalline (for m g 8 at room temperature), and that these electronic changes reside in the headgroup region of the film. Consequently, these results are consistent with changes in headgroup-substrate and/or headgroup-headgroup interactions. Preliminary information as to the nature of the optical transition associated with the IPS SEERS has been obtained by measuring SEERS intensities as a function of excitation energy, which are known as Raman excitation profiles (REPs). Figure 7 displays REPs for the five most intense IPS SEERS bands, acquired from the same spot on a CH3(CH2)9SH-covered roughened Au substrate: 593 cm-1 {n ) 1 (-0.110 eV) f n ) 2 (-0.0366 eV)}, 680 cm-1 {n ) 1 (-0.121 eV) f n ) 2 (-0.0366 eV)}, 748 cm-1 {n ) 1 (-0.110 eV) f n ) 3 (-0.0175 eV)}, 833 cm-1 {n ) 1 (-0.121 eV) f n ) 3 (-0.0175 eV)}, and 954 cm-1 {n ) 1 (-0.155 eV) f n ) 2 (-0.0366 eV)}. These REPs were acquired at 2 nm step intervals across the excitation wavelength range of 630-740 nm (i.e., 4.5-6.2 meV step intervals across excitation energies of 1.675-1.968 eV). SEERS REPs obtained from different spots on the electrode surface exhibited the same superstructure of peaks as shown in Figure 7, although variations in absolute intensities between REPs of the same SEERS mode at different spots were often significant. Note that as the excitation energy is tuned into resonance at the low energy end of the absorption envelope, all five SEERS bands appear simultaneously and undergo concurrent increases in scattering intensity, which indicates that all four n ) 1 image state levels are simultaneously populated. This observation is consistent with optical pumping of electrons from a lower lying energetically broad reservoir state directly to an image state level (or levels) within the n ) 1 manifold (Figure 8A). The population among the n ) 1 states is likely mixed by thermal effects or quantum mechanical mixing of the n ) 1 wave functions. Laser radiation is further Raman scattered via a nonresonant virtual level to produce the observed spectra. (73) CRC Handbook of Chemistry and Physics; Weast, R. C., Astle, M. J., Eds.; CRC Press: Boca Raton, FL, 1981.

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Figure 7. Raman excitation profiles (REPs) correponding to labeled SEERS bands. The Raman intensities for these bands were acquired at 2 nm step intervals across the range 630-740 nm (1.675-1.968 eV). For clarity, data points are shown only for the 680 cm-1 REP. The 593, 680, and 954 cm-1 REPs correspond to three of the observed n ) 1 f n ) 2 transitions, whereas the 747 and 833 cm-1 REPs correspond to two of the observed n ) 1 f n ) 3 transitions. (See the inset of Figure 1 for more information).

An alternative model in which an initially populated n ) 1 manifold of image states undergoes resonance Ramanlike scattering via a higher energy electronic state (Figure 8B) is not supported by the data. In such a process, one would expect that the three n ) 1 f n ) 2 Raman emissions listed above would appear sequentially as the excitation energy is increased, in order of increasing energy (i.e., 594, 680, and 954 cm-1). Consequently, these excitation profiles would be separated energetically by their respective energy differences (e.g., the excitation profiles for the 594 and 954 cm-1 bands would be separated by ∼360 cm-1, or 45 meV), which would be easily discernible. Furthermore, the observation that the peak superstructure for different SEERS REPs occurs at the same energies differs from that typically observed in resonance Raman spectra of molecular systems exhibiting long-time wave packet dynamics.74-76 In such molecular systems, where the resonance Raman process occurs via an upper molecular electronic state, the vibronic-like features of different REPs are often offset energetically such that the ratios of the Raman intensities oscillate as the excitation energy is (74) Heller, E. J. Acc. Chem. Res. 1981, 14, 368. (75) Heller, E. J.; Sundberg, R. L.; Tannor, D. J. Phys. Chem. 1982, 86, 1822. (76) Wootton, J. L.; Zink, J. I. J. Am. Chem. Soc. 1997, 119, 1895.

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Figure 8. (A) Model in which optical pumping of the image states occurs prior to SEERS. Population mixing among the n ) 1 states results in the simultaneous appearance of all the SEERS bands. (B) Model in which an initially populated n ) 1 manifold of image states undergoes resonance Raman-like scattering via a higher energy electronic state. This scenario is inconsistent with the simultaneous appearance of the SEERS bands shown in the REPs of Figure 8. (C) Energy level diagram for CH3(CH2)9SH/Au(100) system based upon SEERS REPs. Three of the n ) 1 IPS levels are shown in expanded form below Evac for clarity. Based upon analysis of the SEERS REPs, the energetic separation between the upper and lower electronic states for this system is (1.72 + 0.155 - 0.110 + 1.93 eV )) 3.70 ( 0.03 eV.

Avila et al.

changed. The similarities in peak superstructure between the different SEERS REPs therefore indicate that the Raman scattering process does not occur via an upper electronic state, and that these features likely reflect local density of state information of the lower reservoir level. Although the SEERS REPs indicate that Raman scattering does not occur through an upper electronic state, an analysis of the high energy end of the REPs (Figure 7) does provide evidence of the presence of such an upper state. This information can be found by examining the energies at which the respective REPs terminate. One can see from Figure 7 that the 593 and 748 cm-1 REPs, which originate from the same n ) 1 level but terminate at n ) 2 and n ) 3, respectively (see Figure 1 inset), undergo a significant reduction in intensity and disappear altogether prior to the other REPs. (In fact, the reduction in intensity for the 748 cm-1 REP closely follows that for the 593 cm-1 REP.) This observation becomes more apparent when one notes that the 593 cm-1 mode is the second most intense SEERS feature observed throughout most of the excitation range, and that its intensity drops off as there is an intensity increase in both the 833 and 954 cm-1 modes. Since the 593 and 748 cm-1 bands originate from the least bound n ) 1 IPS level at -0.110 eV (Figure 1 inset), the fact that they terminate prior to the other bands strongly indicates that their disappearance must be the result of a direct optical transition to an upper state when the excitation energy becomes sufficiently great. This conclusion is further supported by the observation that the 680 cm-1 band disappears at slightly higher excitation energies, since that band originates from the more strongly bound n ) 1 IPS level at -0.121 eV (Figure 1 inset). Consequently, the energetic separation between the intensity falloffs of these REPs (∼0.015-0.025 eV) is roughly equal to the difference in energy between the two n ) 1 IPS levels (0.011 eV). The observation that the intensities of the 833 and 954 cm-1 bands disappear at the same energies as those for the 680 cm-1 (Figure 7) is not unusual since the latter is the strongest SEERS transition; when its intensity is driven downward as a result of direct optical pumping to an upper state, the other bands follow. In fact, this observation also substantiates the earlier notion that population mixing is occurring among all the IPS states within the n ) 1 manifold. From the preceding discussion, an analysis of the appearance and termination points of the five SEERS REPs allows the construction of an energy band diagram for the various electronic states for the CH3(CH2)9SH SAM within the Au(100) band structure (Figure 8C). (Recall from section 3.3 that the n ) 1 IPS line widths observed experimentally by SEERS indicate that the observed image states occur at Au and Ag surfaces of (100) crystallinity.) The bulk band structure projected onto the Au(100) surface (Figure 8C) can be determined from the values for the average bulk potential V0 (measured relative to the vacuum level Evac), the metal half-band gap VG, the energetic position of the lower edge of the band gap EL, the lattice constant a, and the work function Φ0, all of which are connected via the relation

V0 ) EG + Φ0 - VG - EL

(2)

where EG is the average bulk potential measured relative to the band gap midpoint and is given by EG ) (pG/2)2/2m, and G is the reciprocal lattice vector (G ) 2π/a).32,33 These bulk parameters are well-known for clean Au(100),32,33,66,67 and therefore the energetic positions of Evac, EL, and the upper edge of the band gap EU ()EL + 2VG) relative to the

IPSs at Dielectric-Metal Interfaces

Fermi level EF are easily located (Au(100): EG ) 9.05 eV, Φ0 ) 5.47 eV, VG ) 2.15 eV, and EL ) 2.37 eV, Figure 8C). From analyses of the REPs, placement of both the lower electronic state (i.e., the reservoir level from which electrons are pumped prior to electronic Raman scattering) and the upper electronic state within the Au(100) band diagram is relatively straightforward since the energetic positions of the image state levels are referenced with respect to Evac. Given that the IPS SEERS bands first appear at approximately 1.72 eV, and that the 593 cm-1 band disappears at 1.93 eV (due to optical pumping into the upper electronic state), the energetic separation between the two electronic states for the CH3(CH2)9SH/ Au(100) system is calculated to be 3.70 ( 0.03 eV ()1.72 + 0.155 - 0.110 + 1.93 eV, Figure 8C). This places the upper edge of the lower electronic state and the lower edge of the upper electronic state at approximately 3.60 and 7.29 eV above the clean Au(100) Fermi level, respectively. The energetic positions of these levels are similar to those observed by 2PPE for CH3(CH2)7SH SAMs on Cu(111) (E1 ) 3.15 eV, E2 ) 6.37 eV), which have been attributed to empty C-S-Cu molecular states within the metal band gap.29 A number of observations indicate that the upper and lower electronic states observed in these SEERS studies may in fact be localized molecular states, and are hence not intrinsic to the metal. Based on the energies required for optical excitation (∼1.7 eV), it is unlikely that the IPS levels could be populated by direct photoexcitation from the metal Fermi level on Au(100) since such a process would necessitate energies in excess of 3 eV (even if one accounts for the expected reduction in Φ0 of ∼1 eV).29 Thus, it is clear that the lower reservoir level resides well within the Au(100) band gap, and that it likely corresponds to a filled (at least partially), perhaps localized, molecular state. Additionally, the fact that the lower edge of the upper electronic state is approximately 0.62 eV above EU argues that it also must be a distinctly different state from those allowed by the metal band structure. This observation by SEERS of two electronic states separated by 3.5-4.0 eV, with the IPS levels positioned roughly midway between them, is consistent

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with previous optical ellipsometry results for alkanethiol SAMs on Au,19 which have revealed the occurrence of optical transitions at 2.2 and 4.3 eV in these systems. Further experiments and modeling are presently underway to examine the effect of chain length, headgroup (sulfur vs selenium), and tail group (CH3 vs OH) on the band gap. 4. Summary Electronic Raman scattering between image potential surface states has been observed from n-alkanethiol SAMs on roughened Au and Ag surfaces. Experimental Raman data have demonstrated that the appearance of these transitions is intimately tied to the high degree of film crystallinity. While the experimental Raman data indicates that the IPS electrons exist within the dielectric film, quantum mechanical modeling of these image potential states for chemisorbed alkanethiol SAMs on roughened metal surfaces strongly argues that the IPS electrons must be confined to move in close proximity to the headgroup layer, parallel to terraces near terrace/ step edge junctions. The motion of the IPS electron that is being probed by SEERS is with respect to the step edge. Additionally, Raman excitation profiles for CH3(CH2)9SH SAMs on Au have provided evidence for both a filled, lower electronic state within the metal band gap (from which the IPS levels are filled via optical pumping prior to the Raman scattering event) and an upper electronic state; the energetic separation between the two states is 3.70 ( 0.03 eV. Furthermore, an analysis of these excitation profiles places the upper edge of the lower electronic state and the lower edge of the upper electronic state at approximately 3.60 and 7.29 eV above the clean Au(100) Fermi level, respectively. The energetic locations of these states within the metal band structure indicate that these states are not intrinsic to the metal, and are consistent with those previously observed by 2PPE for alkanethiol films on Cu(111),29 which have been attributed to molecular states near the substrate-headgroup region. LA011413W