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Tuning the Response and Stability of Thin Film Mesoporous Silicon Vapor Sensors by Surface Modification Ting Gao, Jun Gao, and Michael J. Sailor* Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093-0358 Received May 31, 2002. In Final Form: October 5, 2002 The effect of chemical surface treatment on the sensitivity, specificity, and stability of mesoporous Si thin film vapor sensors is studied. The vapor sensors operate by measurement of Fabry-Pe´rot interference from the porous Si layer, probed using a diode laser operating at a wavelength of 687 nm. Four chemically distinct surface types are each probed with three different analyte vapors: ethanol, methyl ethyl ketone, and n-hexane, all in a carrier gas of pure nitrogen. The four different surface types include the H-terminated, freshly etched material (Si-H), ozone-treated material (Si-ozone), electrochemically methylated material (Si-CH3), and thermally oxidized samples (Si-O-Si). Surface modification has a pronounced effect on the specificity and stability. It is found that the Si-H material is more sensitive to the hydrophobic analyte relative to either the Si-O-Si or Si-ozone samples. Similarly, the Si-CH3 material is more sensitive to the more hydrophobic analyte, although it is found to be much more stable than the Si-H material.
Introduction Porous silicon has several features that make it an attractive material for use in chemical and biological sensors. The large surface area (typically 200 m2/cm3) can be used to collect and concentrate molecular species.1 Porous Si has flexible surface chemistry that allows it to be chemically modified with specific or nonspecific recognition elements.2-14 Finally, the electronic and optical properties of the material provide several possible signal transduction schemes, including capacitance, photoluminescence, conductivity, and optical interferometry.5 Porous Si is typically made by an electrochemical etch of single-crystal silicon wafers in ethanolic hydrofluoric acid (HF) solutions. Porosity, thickness, optical properties, and morphology can be tuned by appropriate adjustment of electrochemical preparation conditions such as HF concentration, current density, and etch time.1,15,16 Porous Si films can be prepared that exhibit well-resolved Fabry(1) Halimaoui, A. Porous silicon formation by anodisation. In Properties of Porous Silicon; Canham, L., Ed.; Short Run Press Ltd.: London, 1997; Vol. 18, pp 12-22. (2) Janshoff, A.; Dancil, K.-P. S.; Steinem, C.; Greiner, D. P.; Lin, V. S.-Y.; Gurtner, C.; Motesharei, K.; Sailor, M. J.; Ghadiri, M. R. J. Am. Chem. Soc. 1998, 120, 12108-12116. (3) Dancil, K.-P. S.; Greiner, D. P.; Sailor, M. J. J. Am. Chem. Soc. 1999, 121, 7925-7930. (4) Sailor, M. J.; Lee, E. J. Adv. Mater. 1997, 9, 783-793. (5) Sailor, M. J. Sensor Applications of Porous Silicon. In Properties of Porous Silicon; Canham, L., Ed.; Short Run Press Ltd.: London, 1997; Vol. 18, pp 364-370. (6) Tinsley-Bown, A. M.; Canham, L. T.; Hollings, M.; Anderson, M. H.; Reeves, C. L.; Cox, T. I.; Nicklin, S.; Squirrell, D. J.; Perkins, E.; Hutchinson, A.; Sailor, M. J.; Wun, A. Phys. Status Solidi A 2000, 182, 547-53. (7) Chan, S.; Fauchet, P. M.; Li, Y.; Rothberg, L. J.; Miller, B. L. Phys. Status Solidi A 2000, 182, 541-546. (8) Chan, S.; Horner, S. R.; Miller, B. L.; Fauchet, P. M. J. Am. Chem. Soc. 2001, 123, 11797-11798. (9) Snow, P. A.; Squire, E. K.; Russell, P. S. J.; Canham, L. T. J. Appl. Phys. 1999, 86, 1781-1784. (10) Arwin, H.; Gavutis, M.; Gustafsson, J.; Schultzberg, M.; Zangooie, S.; Tengvall, P. Phys. Status Solidi A 2000, 182, 515-20. (11) Buriak, J. M.; Allen, M. J. J. Am. Chem. Soc. 1998, 120, 13391340. (12) Buriak, J. M.; Allen, M. J. J. Luminesc. 1998, 80, 29-35. (13) Stewart, M. P.; Buriak, J. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 3257-3260. (14) Buriak, J. M. Adv. Mater. 1999, 11, 265-267.
Pe´rot fringes in their optical reflection spectra due to thin film interference.17 Reflection maxima occur at wavelengths satisfying the following equation (for light at normal incidence):
mλ ) 2nd
(1)
where λ is the wavelength of incident light, n is the average refractive index of the porous Si film, m is an integer, and d is the film thickness. Upon exposure to organic vapors, the average refractive index of the porous Si layer changes, resulting in a shift of the wavelength of the Fabry-Pe´rot fringes.17 Although vapor sensing can be achieved by measuring this shift using a dispersive spectrometer, a more sensitive method is to measure the change of reflectance at a fixed wavelength.18,19 A porous Si vapor sensor based on laser interferometry has been demonstrated with detection limits in the parts per billion range.18 The high sensitivity of porous Si vapor sensors relies on surface adsorption and capillary condensation effects.9,19 Moderate selectivity has been demonstrated for these systems,20,21 although, presumably with the appropriate surface chemistry, greater selectivity can be achieved.22 In addition, the long-term stability of porous Si sensors in ambient conditions is an issue. In this paper, we explore the effect of surface chemical modification on the performance of a porous Si vapor sensor, focusing on specificity and stability. The response of four different surface types to each of three different analytes is probed. (15) He´rino, R. Pore Size Distribution in Porous Silicon. In Properties of Porous Silicon; Canham, L., Ed.; Short Run Press Ltd.: London, 1997; Vol. 18, pp 89-96. (16) Thonissen, M.; Berger, M. G. Multilayer structures of porous silicon. In Properties of Porous Silicon; Canham, L., Ed.; Short Run Press Ltd.: London, 1997; Vol. 18, pp 30-37. (17) Curtis, C. L.; Doan, V. V.; Credo, G. M.; Sailor, M. J. J. Electrochem. Soc. 1993, 140, 3492-3494. (18) Gao, J.; Gao, T.; Sailor, M. J. Appl. Phys. Lett. 2000, 77, 901-3. (19) Gao, J.; Gao, T.; Li, Y.; Sailor, M. J. Langmuir 2002, 18, 22292233. (20) Le´tant, S.; Sailor, M. J. Adv. Mater. 2001, 13, 335-338. (21) Allcock, P.; Snow, P. A. J. Appl. Phys. 2001, 90, 5052-7. (22) Lauerhaas, J. M.; Sailor, M. J. Science 1993, 261, 1567-1568.
10.1021/la026024w CCC: $22.00 © 2002 American Chemical Society Published on Web 11/09/2002
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Results and Discussion Preparation of Si-H, Si-Ozone, Si-CH3, and SiO-Si Samples. Four types of porous Si samples were investigated in this study, corresponding to four different surface chemistries. All were prepared using identical electrochemical etching conditions, from 〈100〉 p++-type Si wafers with mesoporous (2-20 nm) pore morphologies. Approximate pore dimensions were confirmed by tapping mode atomic force microscopy. Porosity and thickness were determined by gravimetric measurements1 to be 82.6 ( 0.4% and 4.7 ( 0.2 µm, respectively. The first type, designated Si-H, represents the H-terminated, freshly etched sample. This material has been found to be unstable in air and aqueous media, generating a surface oxide within a few minutes to a few hours.23,24 Early attempts to improve the stability of porous Si focused on preoxidation of the material by either chemical,22,25,26 thermal,27 or electrochemical28,29 means. The other three surface types were prepared from this starting material (same wafer resistivity and etch conditions). Two of the surface chemistries involve oxidized material, one using ozone oxidation30,31 and the other employing a thermal treatment in air. Material designated Si-ozone represents the ozonetreated (at room temperature) samples, which have been shown to present a hydrophilic surface containing a significant number of surface OH groups.3 Samples designated as Si-O-Si represent thermally oxidized material (600 °C in air for 90 min, after the procedure of Petrova-Koch et al.27). The thermal treatment tends to produce a surface oxide containing more Si-O-Si groups and fewer surface Si-OH species, as indicated by FTIR (Figure 1). Samples designated Si-CH3 represent material that has been methylated using a previously described electrochemical treatment.32,33 The sensor experiments performed on the Si-CH3, Si-ozone, and Si-O-Si samples, described in more detail below, yielded stable and reproducible sensor readings over several hours and multiple analyte dosing cycles. Infrared Spectroscopic Characterization. Figure 1 presents FTIR spectra representative of the samples used in the present study. Superimposed on all samples are broad bands that are characteristic of optical interference fringes (see below). The Si-H material displays infrared absorption features characteristic of surface Si-H bending and stretching modes and Si-Si lattice modes.34 The FTIR spectrum of the Si-ozone sample shows two features at 1070 and 1160 cm-1. The band at 1070 cm-1 is assigned to a TO asymmetrical Si-O-Si stretching mode, consistent with previous analyses.35 The band at (23) Fauchet, P. M. J. Luminesc. 1996, 70, 294-309. (24) Batstone, J. L.; Tischier, M. A.; Collins, R. T. Appl. Phys. Lett. 1993, 62, 2667-2669. (25) Lee, E. J.; Ha, J. S.; Sailor, M. J. J. Am. Chem. Soc. 1995, 117, 8295-8296. (26) Song, J. H.; Sailor, M. J. Inorg. Chem. 1998, 37, 3355-3360. (27) Petrova-Koch, V.; Muschik, T.; Kux, A.; Meyer, B. K.; Koch, F.; Lehmann, V. Appl. Phys. Lett. 1992, 61, 943-945. (28) Gelloz, B.; Nakagawa, T.; Koshida, N. Appl. Phys. Lett. 1998, 73, 2021-2023. (29) Le´tant, S. E.; Content, S.; Tan, T. T.; Zenhausern, F.; Sailor, M. J. Sens. Actuators B 2000, 69, 193-198. (30) Kurokawa, A.; Ichimura, S. Appl. Surf. Sci. 1996, 100/101, 436439. (31) Thompson, W. H.; Yamani, Z.; Abu Hassan, L. H.; Green, J.; Nayfeh, M.; Hasan, M.-A. J. Appl. Phys. 1996, 80, 5415-21. (32) Allongue, P.; deVilleneuve, C. H.; Pinson, J.; Ozanam, F.; Chazalviel, J. N.; Wallart, X. Electrochim. Acta 1998, 43, 2791-2798. (33) Gurtner, C.; Wun, A. W.; Sailor, M. J. Angew. Chem., Int. Ed. 1999, 38, 1966-1968. (34) Gupta, P.; Dillon, A. C.; Bracker, A. S.; George, S. M. Surf. Sci. 1991, 245, 360-372. (35) Kirk, C. T. Phys. Rev. B 1988, 38, 1255-1273.
Figure 1. Diffuse reflectance Fourier transform infrared spectra of porous Si samples used in this study. From the top, Si-H represents the H-terminated, freshly etched sample; Siozone represents the ozone-treated material; Si-CH3 represents the electrochemically methylated material; and Si-O-Si is the thermally oxidized material. Spectra are offset along the y axis for clarity. Fabry-Pe´rot interference effects characteristic of the films are also observed in the infrared spectrum as the broad, evenly spaced bands.
1160 cm-1 is assigned to a disordered Si-O surface species.36 Note that in these samples we were unable to observe the Si-O (775 cm-1) or O-H (ca. 3400 cm-1) stretching modes usually associated with surface hydroxyl groups,34 although previous studies have indicated that the ozone treatment produces a significant number of these species.37 To generate a stable, hydrophobic porous Si surface, CH3 groups were attached by galvanostatic electrochemical reduction of CH3I at a porous Si cathode.33 The FTIR spectrum of the Si-CH3 material displays the mode at 762 cm-1 characteristic of a CH3 group bound to a Si surface.33,38,39 The electrochemical process is reported to (36) Tsai, C.; Li, K.-H.; Sarathy, J.; Shih, S.; Campbell, J. C.; Hance, B. K.; White, J. M. Appl. Phys. Lett. 1991, 59, 2814-2816. (37) Le´tant, S.; Sailor, M. J. Adv. Mater. 2000, 12, 355-359. (38) Glass, J. A. J.; Wovchko, E. A.; Yates, J. T. J. Surf. Sci. 1995, 338, 125-137. (39) Canaria, C. A.; Lees, I. N.; Wun, A. W.; Miskelly, G. M.; Sailor, M. J. Inorg. Chem. Commun. 2002, 5, 560-564.
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remove some, but not all, of the Si-H surface species,33 although the quality of the spectra in this work are not sufficient to discern these Si-H bands. The small Si-O band located at 1020 cm-1 in the FTIR spectrum is attributed to air oxidation that occurs with age. The FTIR spectrum of the thermally oxidized porous Si sample, Si-O-Si, displays three strong modes associated with Si-O-Si vibrations at 817, 1020, and 1210 cm-1. The band around 810 cm-1 has previously been assigned to symmetrical stretching along a line bisecting the Si-Si vector.35 The two bands at 1020 and 1210 cm-1 are assigned to asymmetrical stretching along a line parallel to the Si-Si vector, either in phase (1020 cm-1) or out of phase (1210 cm-1) with respect to the asymmetrical vibration of a neighboring Si-O-Si species.35 For pure silica, the inphase Si-O-Si asymmetrical vibration is reported at a slightly higher frequency (1076 cm-1).35 The lower frequency value observed in the present work is attributed to the presence of Si-Si bonded species in close proximity to the Si-O-Si oscillator.40 Compared to the ozone oxidized (Si-ozone) sample, the thermally oxidized sample is more completely oxidized. Reflectivity Characteristics. All of the porous Si samples show well-resolved Fabry-Pe´rot fringes in their reflection spectra even after chemical modification. The freshly etched Si-H samples used in these experiments typically display an optical thickness (value of nd from eq 1) of 7623 ( 5 nm (95% C.I., five replicate measurements on a single sample). The reproducibility of measured optical thickness from sample to sample (preparation conditions held constant) is on the order of 20 nm. Typical values of the optical thickness of ozone-oxidized (Siozone), methylated (Si-CH3), and thermally treated (SiO-Si) samples are 8873 ( 8 nm, 10706 ( 7 nm, and 7330 ( 2 nm, respectively. The increase in optical thickness observed for the Si-ozone and Si-CH3 samples is attributed to an expansion of the film volume upon chemical modification, which translates to an increase in the value of d (eq 1). The thermally oxidized Si-O-Si samples display a decrease in optical thickness, presumably due to conversion of a large fraction of Si to SiO2. SiO2 has a smaller refractive index (n ) 1.46 at visible light wavelengths) compared to Si (n ) 3.5). Stability of n of Si-CH3 Samples Relative to SiH. Freshly etched porous Si (Si-H) is moderately oxidized by air. While ozone oxidization (Si-ozone) or thermal oxidization (Si-O-Si) yields more stable sensors, these treatments also change the surface characteristics from hydrophobic to hydrophilic. Methylated porous Si (SiCH3) provides a surface that is both hydrophobic and stable in air.33 In addition, Si-CH3 is more stable to the vapors that are corrosive to the Si-H material. Figure 2 presents the response of hydrogen-terminated and methylated porous Si sensor films (Si-CH3) to successive doses of 2.4% methyl ethyl ketone (2-butanone, MEK), in N2. As previously reported, freshly etched porous Si reacts with aldehydes,41 and silanes are known to react with ketones.42 Repeated exposure of Si-H to MEK results in a drift in the baseline of the response curve of Figure 2 and a corresponding increase in the features associated with surface oxide species in the FTIR spectrum. These observations are attributed to reaction of the hydrides on porous Si with residual air, water, or the ketone. By (40) Hollenstein, C.; Howling, A. A.; Courteille, C.; Magni, D.; Scholz, S. M.; Kroesen, G. M. W.; Simons, N.; de Zeeuw, W.; Schwarzenbach, W. J. Phys. D 1998, 31, 74-84. (41) Boukherroub, R.; Morin, S.; Wayner, D. D. M.; Lockwood, D. J. Phys. Status Solidi A 2000, 182, 117-21. (42) Kulicke, K. J.; Giese, B. Synlett 1990, 2, 91-92.
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Figure 2. Response of hydrogen-terminated (Si-H) and methylated (Si-CH3) porous Si sensor films to methyl ethyl ketone (MEK), showing the relative stability of the films as a function of time. Regions marked 1 and 2 correspond to pure N2 and MEK (2.4% in N2) exposure, respectively. The y axis represents the voltage change (Va - VN2), where Va is the voltage measured on the photodiode in the presence of analyte and VN2 is the voltage measured in the presence of pure N2. The data for Si-H are offset along the y axis by 0.6 V for clarity.
contrast, Si-CH3 shows good reversibility and reproducibility to successive doses of all the analytes studied. In addition, it was found that the Si-CH3 material retains its hydrophilic surface properties even after several months of storage in air. Interpretation of Response of Samples to Analyte Vapors. Dose-response curves were obtained on the four types of samples using a laser interferometric vapor sensor configuration previously described.19 The response curves are presented in Figure 3 for the analytes ethanol, MEK, and n-hexane in a pure nitrogen carrier stream. Some physical properties of these three analytes are presented in Table 1. All have similar refractive indices, although they vary dramatically in their compatibility with water. While ethanol is completely miscible with water, MEK has limited solubility and n-hexane is insoluble. The differences in the dose-response curves tend to track this measure of hydrogen bonding capability of the analytes. There are two operative mechanisms of adsorption of condensable analytes in the mesoporous domains of porous Si samples: physisorption and capillary condensation, as has been discussed in several previous reports.9,19,21,43,44 At the intermediate to high analyte pressures used in this study, capillary condensation is expected to be the predominant mechanism, and the pressure-pore size relationship is described by the Kelvin equation (eq 2).45 The tranduction mechanism of the porous Si sensors then relies on a change in the effective refractive index that occurs on condensation of analyte into the pores. The Kelvin equation relates the relative vapor pressure at which condensation occurs to a characteristic pore radius:
()
ln
γVL P )Ps RTr
(2)
where r is the pore radius, γ is the surface tension of the liquid, VL is the molar volume of the liquid, R is the gas constant, T is temperature, Ps is the saturation vapor pressure of the liquid, and P is the observed pressure of (43) Zangooie, S.; Bjorklund, R.; Arwin, H. Sens. Actuators B 1997, 43, 168-174. (44) Zangooie, S.; Jansson, R.; Arwin, H. J. Appl. Phys. 1999, 86, 850-8. (45) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press Inc.: London, 1982; p 112. (46) CRC Handbook of Chemistry and Physics, 60th ed.; CRC Press, Inc.: Boca Raton, FL, 1980. (47) ChemInfo Database on CCINFO CD-ROM disc 89-2, Canadian Centre for Occupational Health and Safety.
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Figure 3. Response curves for the four types of porous Si films exposed to the three analytes (ethanol, MEK, and n-hexane vapors) used in this study. Si-H represents the H-terminated, freshly etched sample; Si-ozone represents the ozone-treated material; Si-CH3 represents the electrochemically methylated material; and Si-O-Si is the thermally oxidized material. The y axis represents the voltage change (Va - VN2), where Va is the voltage measured on the photodiode in the presence of analyte and VN2 is the voltage measured in the presence of pure N2. The values of P/Ps are plotted along the x axis, where P is the pressure of analyte in the N2 stream and Ps is the saturation vapor pressure for that particular compound. For each sample, analyte pressure vs photodiode voltage measurements were obtained in three separate runs, and all three measurements are plotted. The data were acquired using illumination from a diode laser operating at 687 nm. Table 1. Physical Propertiesa of Ethanol, Methyl Ethyl Ketone (MEK), and n-Hexane46,47 mol wt (g/mol) nD solubility (H2O) σ or γ (dyn/cm) vapor pressure (Torr) V (mL/mol)
ethanol
MEK
n-hexane
46.07 1.3611 infinite 22.75 50 58.4
72.11 1.3788 27.5% 24.6 90.7 89.5
86.18 1.3751 not soluble 18.43 124 130.5
a Here, n is refractive index, solubility is given as a volume D percent, σ or γ is surface energy or surface tension of the liquid phase (relative to air), vapor pressure is the saturated vapor pressure of the liquid at room temperature, and V is the molar volume of the liquid.
the vapor. The data are presented as a function of P/Ps. Thus, at a given value of P/Ps, the response measured along the y axis in Figure 3 corresponds to condensation occurring in pores with a given value of r according to eq 2. The porous Si films contain a distribution of pore radii, and so the response curves of Figure 3 probe the range of r values in the ensemble. The value of γ is essentially a
chemical interaction term, specific to a given analyte, and it is this variable that changes when the surface of porous Si is chemically modified. The main conclusion that can be drawn from the data of Figure 3 is that surface chemistry has a relatively small effect on the specificity of the sensor. The response curves are presented as a function of the relative pressure P/Ps, which has the effect of correcting the data for the different analyte vapor pressures. Thus, the data reflect the values of r and γ from eq 2. Although the mean and width of the distribution of pore radii (r) are not expected to be the same for each of the four sample types, they are the same for a given sample. Therefore, it is appropriate to compare the response curve for one analyte relative to the other analytes on a given sample. The trends observed in the plots of Figure 3 were reproduced on at least three separate samples for each chemical preparation. The response curves for all three analytes tend to fall fairly close to each other, though there are distinct differences that can be ascribed to the different surface chemistries. The most pronounced effect is seen with the Si-ozone material. This surface tends to have a high concentration of Si-OH species,37 and hydrogen bonding effects are readily observed. The data on Si-ozone show a strong response to the ketone (MEK) and the alcohol (ethanol), and a very weak response to n-hexane. By contrast, the hydrophobic surfaces Si-H and Si-CH3 display a more pronounced response to hexane, in particular at higher analyte pressures. For porous Si with hydrophobic properties such as with the Si-H and Si-CH3 samples, γ is expected to have a larger value for hexane than for ethanol or MEK, and it is expected that the response to hexane should be larger than that for the Si-ozone and Si-O-Si samples. The Si-ozone and Si-O-Si surfaces possess less hydrophobic nature and greater hydrogen bonding capability, resulting in a slightly greater (Si-O-Si) to much greater (Si-ozone) response to the polar, oxygenated analytes. The greater sensitivity of the Si-ozone samples is attributed to a greater hydrogen bonding capability of this material relative to the Si-O-Si material. The infrared data (Figure 1) clearly indicate that the oxide on the Si-ozone samples is different than the oxide on the thermally oxidized Si-O-Si samples, containing more disordered Si-O species and less silica-like Si-O-Si species. At lower values of P/Ps, the samples display lower sensitivities (smaller slopes in the dose-response curves). The concentration of analyte is too low in this regime to exhibit capillary condensation effects, and the response of the sensors in this low-pressure region presumably arises from physisorption. The observation of an anomalously high sensitivity of the Si-ozone surface toward the oxygenated analytes in the low-pressure regime is consistent with this interpretation; the presence of surface Si-OH or disordered Si-O species enhances adsorption of analytes capable of interacting via hydrogen bonds. Conclusions The surface affinity of porous Si can be modified by oxidation or electrochemical alkylation. These treatments improve the chemical stability of porous Si relative to freshly etched, hydrogen-terminated porous Si. The optical reflectivity properties of thin Fabry-Pe´rot layers of the surface-modified materials provide reproducible sensing of condensable organic vapors. A degree of analyte selectivity is provided by the different surfaces. In the case of the ozone-oxidized samples, a clear improvement
Thin Film Mesoporous Silicon Vapor Sensors
in sensitivity toward polar oxygenated hydrocarbon analytes is observed. The data are interpreted using a capillary condensation model that follows the Kelvin equation at relatively high analyte pressures and a surface affinitymediated adsorption model (hydrogen bonding and physisorption processes) at lower pressures. Experimental Section Preparation of Porous Si Samples. All porous Si samples were prepared by anodic etch of highly doped p++ 〈100〉 silicon wafers (Silicon Quest International) with resistivities of approximately 1 mΩ‚cm. The etching solution consisted of an equal volume of aqueous HF (48%) (Quantum Chemicals) and absolute ethanol (Aldrich Chemicals). Galvanostatic etching was carried out in a Teflon cell using a two-electrode configuration with an applied current density of 5 mA/cm2 for 30 min. Chemical Modification of Porous Si Samples. Ozone oxidation was performed by placing the as-prepared H-terminated porous Si samples in a flowing stream of ozone (OZO 2HD, OZOmax Ltd., flux of 8 g/h) for 1 h. Methylated porous Si samples were obtained by electrochemical reduction of CH3I at a freshly etched porous Si cathode.33 The reaction was carried out in 1-2 mL of a solution of 0.2 M CH3I (Aldrich Chemicals) and LiI (Aldrich Chemicals) in dry, deoxygenated acetonitrile in an electrochemical cell equipped with a platinum auxiliary electrode. Methyl derivatization was performed by passing a cathodic current of 10 mA/cm2 for 30 s.
Langmuir, Vol. 18, No. 25, 2002 9957 During modification, the samples were illuminated with a tungsten filament lamp at 170 mW/cm2. After derivatization the solution was removed and the sample rinsed sequentially with glacial acetic acid, acetonitrile, and ethanol and then dried under a stream of N2. Thermally oxidized porous Si samples were prepared by heating the as-prepared, H-terminated porous Si samples in a tube furnace under air at 600 °C for 90 min. Characterization of Porous Si Samples. Atomic-force microscopy was performed with a Digital Instruments Nanoscope III, operated in tapping mode. Fourier transform infrared (FTIR) spectra were recorded with a Nicolet MAGNA 550 spectrometer equipped with a CsI beam splitter, using a spectral resolution of 4 cm-1 and an average of 64 scans per spectrum. The sample chamber was purged with nitrogen during spectral acquisition. Optical reflection spectra were recorded using a CCD (charge coupled device) spectrometer (Ocean Optics). The value of the optical thickness of the films was obtained from a Fourier transform of the reflectance spectra as described previously.37 The laser interferometer setup used to acquire the dose-response data has been described previously.19
Acknowledgment. The authors thank Mr. Haohao Lin for help with the surface modification chemistry. This work was supported by the National Science Foundation (Grant DMR-97-00202). LA026024W
