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IR Reflection Spectra of Monolayer Films Sandwiched between Two High Refractive Index Materials† Thomas Lummerstorfer and Helmuth Hoffmann* Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9, A-1060 Wien, Austria Received March 18, 2004. In Final Form: June 18, 2004 Monolayer films adsorbed on substrates with high refractive indices such as metals or semiconductors yield strongly enhanced infrared reflection spectra when they are contacted with a transparent, high refractive index ambient medium and are probed with p-polarized light at high incidence angles. The sensitivity increase arises from the enhancement of the perpendicular electric field within a thin, low refractive index layer sandwiched between two high refractive index materials and gives rise to signal intensity gains up to 2 orders of magnitude in combination with an essentially exclusive detection of only perpendicular surface vibrations. Experimental spectra of ordered monolayer films of octadecanethiol on gold and of octadecylsiloxane on silicon in this sandwich configuration yield enhancement factors between 15 (on Si) and 30 (on gold) compared to conventional grazing incidence external reflection spectra and are governed by a common, simple surface selection rule, which allows immediate quantitative evaluation and comparison of the film structures on different substrates.
Introduction Over the past decades, infrared spectroscopy has contributed substantially to the knowledge of the composition and structure of surfaces and interfaces and has established itself as one of the most versatile surface analytical methods, applicable for a variety of different samples ranging from single-crystal surfaces in ultrahigh vacuum to buried interfaces within a bulk, solid material.1 Despite substantial instrumental improvements and the utilization of surface electric field enhancements on plane metal surfaces (grazing incidence reflection),2 on metal island substrates (surface enhanced IR),3 or in various resonant structures (optical cavities),4 sensitivity is still a limiting factor for the observation of monolayer and submonolayer films. One exceptionally strong sensitivity enhancement arises in a thin film sandwiched between two high refractive index materials (substrate/film/ambient), when the infrared light strikes the sample surface from the ambient phase at high incidence angles.5-12 Due to the continuity of the perpendicular component, Dz ) Ez, of the dielectric displacement vector (D) upon transmission from the ambient phase (dielectric function 1) to the sample layer (dielectric function 2), the perpendicular electric field (Ez) in the sample layer is enhanced by a factor of 1/2 ) nˇ 12/nˇ 22 (nˇ : complex refractive * Corresponding author. Email:
[email protected]. Fax: +43 1 58801 16299. Phone: +43 1 58801 15330. † This paper is dedicated to Prof. Peter Stanetty on the occasion of his 60th birthday. (1) Griffiths, P. R., Chalmers, J. M., Eds. Handbook of Vibrational Spectroscopy; Wiley: New York, 2002; Vol. 2. (2) Greenler, R. G. J. Chem. Phys. 1966, 44, 310-315. (3) Osawa, M. Top. Appl. Phys. 2001, 81, 163-187. (4) Harrick, N. J. Internal Reflection Spectroscopy, 3rd ed.; Harrick Scientific Company: Ossining, NY, 1987; Chapter 5. (5) Ishino, Y.; Ishida, H. Appl. Spectrosc. 1988, 42, 1296-1302. (6) Olsen, J. E.; Shimura, F. J. Appl. Phys. 1989, 66, 1353-1358. (7) Brendel, R. J. Appl. Phys. 1992, 72, 794-796. (8) Chabal, Y. J.; Hines, M. A.; Feijoo, D. J. Vac. Sci. Technol. 1995, A13, 1719-1727. (9) Khoo, C. G. L.; Ishida, H. Appl. Spectrosc. 1990, 44, 512-518. (10) Rochat, N.; Chabli, A.; Bertin, F.; Olivier, M.; Vergnaud, C.; Mur, P. J. Appl. Phys. 2002, 91, 5029-5034. (11) Milosevic, M.; Berets, S. L.; Fadeev, A. Y. Appl. Spectrosc. 2003, 57, 724-727. (12) Mulcahy, M. E.; Berets, S. L.; Milosevic, M.; Michl, J. J. Phys. Chem. B 2004, 108, 1519-1521.
index) and the intensities of the film absorptions with perpendicular dipole moment orientation increase by (nˇ 1/nˇ 2).4 Different configurations have been suggested in the literature to take advantage of this signal enhancement (metal overlayer attenuated total reflection (ATR),5,9 grazing internal transmission (GIT),7 and multiple internal transmission (MIT)8), all of which require prefabricated sandwich structures and/or specific types of samples with rather tedious sample preparation procedures. To utilize this effect without further manipulations for a standard type of sample, that is, an adsorbate layer on a given substrate, one must contact the sample with an IR transparent, high refractive index crystal and direct the IR beam through the high index crystal to the film/ substrate interface at large incidence angles. The problem with this setup is that (i) an essentially perfect optical contact must be made between the film and the crystal, because the signal enhancement decreases sharply with increasing thickness of the residual air gap5,9 and (ii) any contaminants in the sandwiched sample layer (surface oxide layers, hydrocarbon contaminants, etc.) are equally enhanced and might cover up the sample signals. Rochat et al.10 have used oxide-covered silicon wafers with oxide thicknesses from 0.4 to 50 nm, which were pressed against a germanium prism, and measured the Si-O stretching bands between 1000 and 1300 cm-1 with p-polarized IR light at 71° incidence. Oxide films as thin as 0.4 nm could be detected, but an air gap of ∼30 nm reduced the possible signal enhancement by more than 1 order of magnitude. Using a similar experimental setup, Milosevic et al.11 and Mulcahy et al.12 have measured organic monolayers on silicon and gold. They observed an enhancement of about a factor of 10 for the ν(CD2) absorptions of an octadecanethiol-d37 monolayer on gold as compared to a conventional grazing angle reflection spectrum, although broad regions of their spectra showed strong artifacts or miscanceled background features and also the ν(CD) peaks did not show the expected band profile of an octadecanethiolate monolayer film.13 Despite these limitations discernible in previous studies, this method has a large potential for the characterization of monolayer films because (i) it requires only minor modifications to a standard infrared reflection optical system, (ii) it promises
10.1021/la0492916 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/07/2004
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Langmuir, Vol. 20, No. 16, 2004 6543
signal enhancements up to 2 orders of magnitude for a perfect optical contact,10 and (iii) electromagnetic theory predicts simple, “metal-like” surface selection rules9,10 independent of the optical properties of the substrate. The IR spectra of monolayer films on dielectric substrates, for example, which show very complex band profiles measured in the conventional external reflection mode,14 should be converted into simple, easily interpretable absorption spectra, as if the substrate was a metal. To test this predicted performance, we have chosen two well-studied model systemssmonolayers of octadecylsiloxane on silicon and of octadecanethiol on goldsand have compared their grazing incidence external reflection spectra measured in air as the ambient medium with internal reflection spectra using a germanium crystal as the incidence medium. Spectral simulations based on classical electromagnetic theory applied to a stratified layer system were used to confirm the predicted surface selection rules for the sandwiched sample layers and to evaluate the effective air gap thickness between the sample and the germanium crystal.
Then, the sample was brought into contact with the crystal and the sample spectrum was obtained and ratioed against the background spectrum. For both the background and the sample single beam spectra, 256 scans at 4 cm-1 were collected. IR spectral simulations were carried out using a customized computer program based on a semiempirical matrix method described in detail elsewhere.14 A three-phase model (air/film/ substrate) was used for external reflection spectra and a fourphase model (germanium/air/film/substrate) for internal reflection spectra of the sandwiched sample layer. The following optical constants (refractive index (n), absorption index (k)) were used for the 3000 cm-1 frequency range: air (n ) 1, k ) 0), germanium (n ) 4.0, k ) 0), silicon (n ) 3.42, k ) 0), and gold (n ) 1.958, k ) 20.7). For the adsorbate films ODS and ODT, a simplified geometrical model of an all-trans octadecyl group with a chainaxis tilt angle (R) toward the surface normal and a C-atom-plane twist angle (β) has been assumed.14 R and β were varied in the simulations until the best fit was obtained for both external reflection and internal reflection spectra. Optical parameters for the CH stretching absorptions and transition dipole moment orientations needed for the simulations were taken from the literature.16
Experimental Details
Figure 1 compares experimental and calculated spectra of an octadecylsiloxane (ODS) monolayer on native silicon (Si/SiO2) in a conventional external reflection configuration (air/ODS/Si, bottom spectra) with a series of internal reflection spectra using a germanium hemisphere as the incidence medium and increasing pressure (from bottom to top) by which the sample is pressed against the Ge crystal.19 For the simulated spectra, an assumed surface geometry of 15° tilt and 45° twist for the ODS hydrocarbon chains gave the best overall fit for the experimental spectra, in good agreement with previous results from sum frequency generation20 (R e 15°), internal reflection IR21 (10° < R < 18°), external reflection IR16 (R ) 8 ( 3°), and transmission IR22 (R ) 10 ( 2°). The external reflection spectrum of an ODS monolayer on Si has been discussed in detail previously16,23 and shows four major CH stretching absorptions, νas(CH3) at 2968 cm-1, νas(CH2) at 2919 cm-1, νs(CH3) at 2879 cm-1, and νs(CH2) at 2851 cm-1. νas(CH2) and νs(CH2) are oriented close to parallel to the surface for a 15° chain-axis tilt and point in the positive, upward direction, whereas νas(CH3) and νs(CH3) have larger perpendicular components and point therefore in the negative, downward direction.23 The internal reflection spectra of the same monolayer film in the sandwich configuration look completely different; only the peak positions remain the same. The ν(CH3) absorptions become strong, positive peaks of approximately equal intensity to the intrinsically much stronger ν(CH2) absorptions as a consequence of the metal-like surface selection rules for this Ge/ODS/Si sandwich,10 where vibrations with a substantial perpendicular dipole component such as the ν(CH3) absorptions are strongly enhanced, whereas largely parallel vibrations such as the ν(CH2) peaks gain only little in intensity. The overall appearance of these spectra
Results and Discussion Monolayer films of octadecanethiol (ODT) on gold and of octadecylsiloxane (ODS) on silicon were used as samples. Details of the substrate pretreatment and the monolayer preparation are described in previous publications.15-17 Briefly, ODT monolayers on gold were prepared by immersing Au-coated glass slides (25 × 15 mm2) in 1 mmol/L solutions of octadecanthiol in n-hexane for 1 h. ODS monolayers on native silicon (Si/SiO2) were prepared by immersion of Si(100) wafers (25 × 15 mm2) covered with 1.0-1.5 nm of native oxide in 1 mmol/L solutions of octadecyltrichlorosilane in toluene. After removal of the substrates from the adsorbate solutions, they were thoroughly rinsed with toluene, acetone, and ethanol, they were blow-dried in a high-purity N2 stream, and then the film thickness was measured ellipsometrically.15 Values of 2.6 nm for ODS on Si and 2.4 nm for ODT on Au were reproducibly measured in close agreement to reported literature values. Infrared spectra were measured on a Mattson RS1 Fourier transform infrared (FT-IR) spectrometer. A custommade optical system with a fixed incidence angle of 80° was used for external reflection spectra.18 Internal reflection spectra were measured with p-polarized light at 65° incidence in the internal sample compartment of the spectrometer, using the Seagull optics (Harrick Scientific) with a germanium hemisphere (d ) 12.5 mm) as the reflection element. A vertical-translation sample stage driven by a micrometer screw was added to the standard sample holder of the Seagull unit, which allowed the samples to be raised vertically and pressed against the lower, flat, circular surface of the Ge hemisphere. Prior to each measurement, the Ge crystal was cleaned in a UV-ozone chamber to remove hydrocarbon contaminants. After mounting the crystal in the spectrometer, a background spectrum of the clean Ge surface was measured. (13) The spectra shown in Figure 3 of ref 12 show two absorptions at 2194 and 2221 cm-1, which are assigned to the ν(CD2) vibrations. According to several consistent literature spectra of long-chain deuterated hydrocarbon compounds (see, for example: Basia, A.; Cuccia, L.; Demers, L.; Morin, F.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682-2692), only one of them is a ν(CD2) vibration (νas(CD2) at 2194 cm-1), whereas νs(CD2) absorbs around 2090 cm-1 and is not detected in the spectra of ref 12. The peak at 2221 cm-1 is probably a νas(CD3) absorption, whose counterpart νs(CD3), which is of comparable intensity in the nondeuterated octadecanethiol monolayer spectra (see ref 23), is apparently also not observed in ref 12. (14) Kattner, J.; Hoffmann, H. In Handbook of Vibrational Spectroscopy; Griffiths, P. R., Chalmers, J. M., Eds.; Wiley: New York, 2002; Vol. 2, pp 1009-1027. (15) Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H. Surf. Sci. 1996, 368, 279-291. (16) Hoffmann, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304-1312. (17) Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H.; Leitner, T.; Resch, R.; Friedbacher, G. J. Phys. Chem. B 1998, 102, 7190. (18) Hoffmann, H.; Mayer, U.; Brunner, H.; Krischanitz, A. Vib. Spectrosc. 1995, 8, 151.
(19) Presently, we have no knowledge about the pressure exerted on the sample or the thickness of the residual air gap between the sample and the Ge crystal. However, the ν(SiO) intensity of the native SiO2 layer can be used as an internal parameter for the air gap thickness, which can be adjusted to a certain peak height with a reproducibility of
