Structures of Monolayers of Long-Chain Aliphatic Acids Deposited on

Nov 15, 1997 - Chem. 1982, 86, 2700. ... resolution (one zero-filling yielding an effective resolution of 1 cm-1) ... is approximated using the Beer-L...
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Langmuir 1997, 13, 6769-6779

6769

Structures of Monolayers of Long-Chain Aliphatic Acids Deposited on Metal, Conducting Glass, and Nanocrystalline Semiconductor Substrates Using Langmuir-Blodgett Techniques Xavier Marguerettaz and Donald Fitzmaurice* Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland Received July 14, 1997. In Final Form: September 30, 1997X The structures of monolayers of long-chain aliphatic carboxylic and phosphonic acids deposited, using Langmuir-Blodgett techniques, on metal (silver and gold), conducting glass (fluorine-doped tin oxide), and nanostructured semiconductor (titanium dioxide) substrates have been studied. In the course of these studies, the effects of pH, dissolved ions, and conditioning regime on the structure of the monolayer present at the air/water interface have been examined. Also examined have been the effects of different deposition conditions and different substrate materials on the structure of the deposited monolayer. These findings represent an extension of such studies to molecules, namely, phosphonic acids, and substrates, namely, nanostructured TiO2 films, of increasing scientific and technological importance.

Introduction Organized monolayers of molecules deposited on welldefined substrates are finding increasing numbers of applications in areas such as nonlinear optics, molecular electronics, and sensors.1 Monolayers of molecules are typically prepared and deposited on a substrate using Langmuir-Blodgett (LB) techniques.2 Accordingly, how such techniques may be used to control the properties of a deposited monolayer is a subject of increasing importance. In this context, the effects of pH, dissolved ions, and conditioning regime on the structure of a monolayer at the air/water interface have previously been studied for long-chain aliphatic carboxylic acids.3 Also examined previously have been the effects of different deposition conditions and substrate materials on the structure of the deposited monolayer.4 A large number of techniques have been used to elucidate the structural properties of a monolayer of molecules prior to and following deposition on a solid substrate.5 Of these, infrared reflection-absorption spectroscopy (IRRAS) is a particularly useful nondestructive technique.6 Much of the usefulness of IRRAS arises from the fact that it is possible to simulate the measured spectra * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) (a) Peterson, I. In Molecular Electronics; Ashwell, G. Ed.; Wiley: New York, 1992; pp 117-195. (b) Richardson, T. Chem. Br. 1989, 25, 1218. (c) Zasadzinski, J.; Viswanathan, R.; Madsen, L.; Garnaes, J.; Schwartz, D. Science 1994, 263, 726. (2) (a) Gaines, G. Insoluble Monolayers at Liquid Gas Interfaces; Wiley: New York, 1966. (b) Roberts, G. Langmuir-Blodgett Films; Plenum: New York, 1990. (c) Ulman, A. Ultrathin Organic Films; Academic: San Diego, 1991. (3) (a) Kjaer, K.; Als-Nielsen, J.; Helm, C.; Tippman-Krayer, P.; Mo¨hwald, H. J. Phys. Chem. 1989, 93, 3200. (b) Kenn, R.; Bo¨hm, C.; Peterson, I.; Mo¨hwald, H.; Kjaer, K.; Als-Nielsen, J. J. Phys. Chem. 1991, 95, 2092. (c) Simon-Kutscher, J.; Gericke, A.; Hu¨hnerfuss, H. Langmuir 1996, 12, 1027. (4) (a) Schwartz, D.; Viswanathan, R.; Zasadzinski, J. J. Am. Chem. Soc. 1993, 115, 7374. (b) Fujimoto, Y.; Ozaki, Y.; Kato, T.; Matsumoto, N.; Irriyama, K. Chem Phys. Lett. 1992, 196, 347. (c) Robinson, I.; Sambles, J.; Peterson I. Thin Solid Films 1989, 172, 149. (d) Evenson, S.; Baydal, J.; Pearson, C.; Petty, C. J. Phys. Chem. 1996, 100, 11672. (5) Petty, M. Langmuir-Blodgett Films: An Introduction; Cambridge Universitry: Cambridge, 1996; pp 94-128. (6) (a) Allara, D.; Swalen, J. J. Phys. Chem. 1982, 86, 2700. (b) Dluhy, R.; Cornell, D. J. Phys. Chem. 1985, 89, 3195. (c) Dluhy, R. J. Phys. Chem. 1986, 90, 1373.

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recorded for a monolayer at the air/water interface prior to deposition and at the surface of a reflecting substrate following deposition.7 The above simulations require the thickness of the monolayer, as well as the average tilt, twist, and azimuthal angles as inputs. Consequently, successful simulations yield valuable structural information. Here we report work directed toward extending such studies to a range of molecules and substrates of increasing scientific and technological importance. Specifically, we have studied the monolayers of long-chain aliphatic carboxylic and phosphonic acids prior to and following their deposition on metal, conducting glass, and nanostructured semiconductor substrates. Experimental Section Preparation and Characterization of Long-Chain Aliphatic Acids. Eicosylcarboxylic acid (RC), supplied by Aldrich, was recrystallized from ethanol. Anal. Calcd for RC (C20H40O2): C, 76.84; H, 12.91. Found: C, 77.05; H, 12.85. 1H NMR (chloroform-d): 0.88 (t, J ) 7.1 Hz, 3 H), 1.2-1.3 (unresolved m, 32 H), 1.60 (unresolved m, 2 H), 2.35 (t, J ) 7.3 Hz, 2 H). Eicosylphosphonic acid (RP), prepared as shown in Scheme 1, was recrystallized from ethanol. Anal. Calcd for RP (C20H43O3P): C, 66.26; H, 11.96; P, 8.54. Found: C, 66.13; H, 12.22; P, 8.53. 1H NMR (chloroform-d): 0.88 (t, J ) 7.6 Hz, 3 H), 1.2-1.3 (unresolved m, 34 H), 1.84 (unresolved m, 2 H), 3.43 (t, J ) 7.2 Hz, 2 H). Preparation and Characterization of Substrates. Silver (Ag) and gold (Au) substrates were prepared by thermal evaporation of these metals, at the rate of about 3 Å s-1, onto a previously cleaned microscope slide using a VES LM 300 metal vapor deposition apparatus (Leybold). The freshly prepared substrates (1500 Å thick) were placed in Analar-grade chloroform for 5 min prior to their being mounted in the “dipped”-position in the LB trough. Fluorine-doped tin oxide conducting glass (CG) substrates (Glastron, 0.5 µm thick, 8 Ω) were also placed in Analar-grade ethanol for 5 min and then in Analar-grade chloroform for a further 5 min prior to their being mounted in the dipped position in the LB trough. Nanostructured semiconductor substrates (TiO2-CG) were prepared as described in detail elsewhere.8 Briefly, a close-packed monolayer of surfactant-modified TiO2 (anatase) nanocrystallites (2-nm diameter) was deposited using LB techniques on a CG substrate and fired. (7) (a) Mendelsohn, R.; Brauner, J. W.; Gericke, A. Annu. Rev. Phys. Chem. 1995, 46, 305. (b) Parikh, A.; Allara, D. J. Chem. Phys. 1992, 96, 927. (8) Doherty, S.; Fitzmaurice, D. J. Phys. Chem. 1996, 100, 10732.

© 1997 American Chemical Society

6770 Langmuir, Vol. 13, No. 25, 1997

Marguerettaz and Fitzmaurice Scheme 1

Freshly prepared TiO2-CG substrates were mounted in the dipped position in the LB trough. Preparation and Deposition of Molecular Monolayers. Monolayers of RC and RP were deposited on Ag, Au, CG, and TiO2-CG substrates using a JL automation langmuir minitrough following standard procedures. Briefly, solutions of RC (1.0 × 10-3 mol dm-3) and RP (5.0 × 10-4 mol dm-3) were prepared in Analar-grade chloroform. A known volume of one of the above stock solutions, typically 180 µL of RC and 360 µL of RP, was added to the trough using a precision syringe held 5 mm above the surface of the aqueous subphase and 30 min allowed for solvent evaporation. Generally, monolayers were conditioned by successive compression-expansion cycles at a barrier rate of 30 mm min-1. For a fully conditioned monolayer of RC and RP at a surface pressure of 40 mN m-1, a molecule occupied an area of 20.5 and 22.0 Å2, respectively. Finally, a previously conditioned monolayer was maintained at the required dipping pressure for 15 min and the previously immersed substrate (Ag, Au, CG, or TiO2-CG) raised at a rate of 15 mm min-1. Unless otherwise stated, a transfer ratio of 1 was observed. Infrared Reflection-Absorption Spectroscopy. IRRAS was used to characterize monolayers following their deposition on Ag, Au, CG, and TiO2-CG substrates. Specifically, a variableangle reflection accessory (Graseby-Specac) was placed in the sample compartment of an FT-IR spectrometer (Mattson Infinity) equipped with a MCT detector (liquid nitrogen cooled). The sample compartment was purged with nitrogen prior to and during data acquisition. Spectra were recorded at 2-cm-1 resolution (one zero-filling yielding an effective resolution of 1 cm-1) using p-polarized light at an incident angle of 84° for Ag and Au and of 73° for CG and TiO2-CG.9 All spectra reported are a ratio of 1000 sample scans to 1000 background scans, the latter recorded for unmodified Ag, Au, CG, and TiO2-CG substrates. IRRAS was also used to characterize monolayers prior to their deposition. Specifically, a variable-angle reflection accessory (Graseby-Specac), fitted with a LB minitrough (Graseby-Specac), was placed in the sample compartment of the above FT-IR spectrometer. The surface area per molecule was calculated from the volume (18 µL) of added RC or RP stock solution (both 5 × 10-4 mol dm-3 in chloroform) and the known area of minitrough (between 9 and 19 cm2). The sample compartment was left open during data acquisition. Spectra were recorded at 2-cm-1 resolution (one zero-filling yielding an effective resolution of 1 cm-1) using p-polarized light at an incident angle of 30°. All spectra reported are a ratio of 2000 sample scans to 2000 background scans, the latter recorded for the unmodified aqueous subphase. Infrared Transmission Spectroscopy. Also recorded were infrared transmission spectra of RC and RP in the crystalline state. The final spectrum was obtained by coaddition of 100 scans recorded under a nitrogen purge at 1-cm-1 resolution for a known concentration of the crystalline compound in a KBr pellet (0.05 mg in 100.00 mg of KBr). The optical path length was determined by measurement of the thickness of the KBr pellet. All spectra reported are a ratio of 100 sample scans to 100 background scans, the latter recorded for a blank KBr pellet of the same thickness. Ellipsometry. The average thickness of a deposited monolayer was determined by ellipsometry. Specifically, a S. A. Jobin Yvon UVISEL variable-angle spectroscopic phase modulated ellipsometer was used at angles of incidence of 65°, 70°, and 75° and over the wavelength range 275-775 nm.10 The complex refractive indexes for a given substrate were calculated, from (9) Udagawa, A.; Matsui, T.; Tanaka, S. Appl. Spectrosc. 1986, 40, 794. (10) We thank the staff of the National Microelectronics Research Centre, in particular Dr. Patrick Kelly, for their assistance in undertaking the reported ellipsometric studies.

classical electromagnetic theory, using a two-phase parallel-layer model. After adsorption of a monolayer of RC or RP, the sample was again analyzed and the film thickness determined from a three-phase parallel-layer model. An initial value of 1.46 for the refractive index was used for the monolayer since the imaginary part of the complex refractive index is zero at over the spectral range studied; i.e., both RC and RP are transparent at these wavelengths.

Theory The isotropic optical constants, n and k, were obtained for RC and RP in the crystalline state from the infrared transmission spectra measured as described above. The method described by Allara and Nuzzo is summarized below.11

nˆ ) n + ik

(1)

The imaginary part of the complex refractive index k(νj) is approximated using the Beer-Lambert law:

T ) exp - [4πk′(νj)dzνj] T0

(2)

T and T0 are the intensities of the light transmitted by a KBr disk containing either RC or RP and a KBr disk containing neither RC or RP, respectively, dz is the pellet thickness, and νj the wavenumber. k′(νj) is given by eq 3,

k′(νj) ) k(νj)

C C0

(3)

where C and C0 are the concentrations of RC or RP in the spectroscopic sample and in the pure crystalline state, respectively. The real part n(νj) was calculated using the Kramer-Kronig transform given by eq 4.12

n(νji) ) n∞ +

k(νj)νj dνj

∫νjνj νj2 - ν 2

1 π

2

1

(4)

i

This initial set of values for n(νj) and k(νj) were used in a Fresnel model for multiple layers to evaluate the transmittance for RC and RP.13 This yields sets of values for k(νj) almost identical to the initial values. The selfconsistency of these data sets was taken as confirming the validity of the Beer-Lambert approximation. To construct the required optical tensor, the k(νj) spectrum was resolved into eight separate contributions from isolated excited modes with a prior knowledge of the individual modes contributing to a given peak. The characteristic frequencies and band shapes reported in the literature were used initially.14 Each mode was arbitrarily assumed to be a 60/40 mixture of the Gaussian/ Lorentzian band shapes. A least-squares routine was used to resolve the different modes, with the free parameters (11) Allara, D.; Nuzzo, R. Langmuir 1985, 1, 52-65. (12) Arfken, G. Mathematical Methods for Physicists; Academic: San Diego, 1985; pp 421-424. (13) Heavens O. Optical Properties of Thin Films; Dover: New York, 1991; pp 74-77. (14) MacPhail, R.; Strauss, H.; Snyder, R.; Elliger, C. J. Phys. Chem. 1984, 88, 334.

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Chart 1

being the intensity of each mode and the full width at half-maximum (FWHM). To take into account the specific orientation of the adsorbate on the surface, the scalar function nˆ (νj) was transformed into a tensor n˜ (νj) as described by Parikh and Allara.7b Briefly, the resolved k(νj) values are scaled by a factor which represents the direction cosine of a given oscillator in the experimental coordinate system. This was achieved using a rotational matrix representing the tilt (R), twist (β), and azimuthal (ψ) angles in the experimental coordinate system; see Chart 1.15 The simulation of the IRRAS for a given orientation of a monolayer of RC or RP on a substrate is based on the assumption that the above monolayer is a collection of uniformly oriented molecules forming a crystalline thin film. Specifically, a FORTRAN algorithm is used to calculate the propagation of an electromagnetic wave through an anisotropic (biaxial) three-layer medium (air/ organic monolayer/substrate). The above FORTRAN code, based on that developed by Parikh and Allara, in turn based on the 4 × 4 matrix formalism developed by Yeh’s,7b,16 has been adapted for our own use.17 The thickness of each layer (determined from ellipsometric studies), the optical tensor for an assumed orientation of the organic monolayer, and the tensors for the substrate and air layers were all used as input parameters for the calculation. The optical tensor for an assumed orientation of the organic monolayer was subsequently adjusted until the best fit between the recorded spectra and the simulated one was obtained. Results and Discussion Surface Pressure vs Area Isotherms. The surface pressure vs area isotherm measured for a monolayer of RC, conditioned by successive compression-expansion cycles on an aqueous subphase at pH 5.5, is shown in Figure 1a. The above isotherm shows a gas to liquidcondensed phase transition at about 1 mN m-1 and 25 Å2 per molecule, a liquid-condensed to solid phase transition at about 27 mN m-1 and 20 Å2 per molecule, and a collapse transition at about 55 mN m-1. The reported transitions (15) Wilson, E.; Decius, J.; Cross, P. Molecular Vibrations; McGraw-Hill: New York, 1955; pp 285-286. (16) (a) Yeh, P. Surf. Sci. 1980, 96, 41. (b) Yeh, P. J. Opt. Soc. Am. 1979, 69, 742. (17) We thank Drs. Allara, Heitpas, and Parikh for their assistance in modifying their algorithm and the corresponding program for use in the studies reported here.

Figure 1. (a) Surface pressure vs area isotherm for RC conditioned by successive compression-expansion cycles on an aqueous subphase at pH 5.5 and 20 °C. (b) As in a for RP (solid line). Also shown is the isotherm measured during the initial compression (dashed line).

and the extrapolated surface area per molecule for a fully compressed monolayer, 20.5 Å2 per molecule, are in good agreement with previously reported findings for long-chain aliphatic carboxylic acids.2a,18 These findings are summarized in Table 1. The surface pressure vs area isotherm measured for a monolayer of RP, conditioned by successive compressionexpansion cycles on an aqueous subphase at pH 5.5, is shown in Figure 1b. The above isotherm shows a gas to liquid-condensed phase transition at about 1 mN m-1 and 23 Å2 per molecule, no discrete liquid-condensed to solid phase transition, and a collapse transition at about 55 mN m-1. The reported transitions and the extrapolated surface area per molecule for the fully compressed monolayer, 22.0 Å2 per molecule, are in good agreement with previously reported findings for long-chain aliphatic phosphonic acids.18 These findings are summarized in Table 1. It is noted that a recently reported isotherm for octadecylphosphonic acid shows a discrete gas to liquidcondensed phase transition at about 1 mN m-1 and 22 Å2 per molecule, a liquid-condensed to solid phase transition at about 15 mN m-1 and 19 Å2 per molecule, and a significantly lower collapse transition at about 40 mN m-1.19 An extrapolated surface area of 19.8 Å2 per molecule was also reported. It is also noted that, on occasion, two liquid-condensed phases have been observed for aliphatic acids, although there are an equal number of reports in which only a single (18) (a) Ries, H.; Cook, H J. Colloid. Sci. 1954, 9, 535. (b) Peterson, I.; Brzezinski, V.; Kenn, R.; Steitz, R. Langmuir 1992, 8, 2995. (19) Woodward, J.; Ulman, A.; Schwartz, D. Langmuir 1996, 12, 3626.

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Table 1. Conditioning of Aliphatic Carboxylic and Phosphonic Acid Monolayers on an Aqueous Subphase RCa gas-liquid transition surf press, mN m-1 surf area, Å2 molecule-1 liquid-solid transition surf press, mN m-1 surf area, Å2 molecule-1 extrapolated surf area,f Å2 molecule-1 collapse transn, mN m-1

RC (added Cd2+)b

RPa,c

RP (added Cd2+)d

1 25

e e

e (2) e (25)

e e

27 20 20.5 55

2 20 20.5 55

1 (23) 23 (21) 22.0 (23.0) 55 (55)

1 23 22.0 55

a pH 5.5. b pH 8.0, 2 × 10-4 mol dm-3 Cd2+ (added CdCl ). c The values in parentheses are those observed for the first compression cycle. 2 pH 3.0, 2 × 10-4 mol dm-3 Cd2+ (added CdCl2). e No transition observed. f Surface area per molecule determined by extrapolation of the solid region of isotherms in Figures 1 and 2 to zero surface pressure.

d

transition is observed.5 This variability has been attributed to differences in the sensitivity of the LB trough used, compression rates, and compression regimes. The differences between the isotherms for RC and RP in Figure 1 have been attributed to the excellent filmforming properties of RP, a consequence of hydrogen bonding between the phosphonic acid headgroups and the larger cross-sectional area of the phosphonic acid headgroups themselves.18a The differences between the isotherm for RP in Figure 1b and the isotherm reported for octadecylphosphonic acid by Woodward et al.19 are accounted for as follows: The isotherm in Figure 1b is that for a monolayer of RP which has undergone successive compression-expansion cycles. The isotherm reported by Woodward et al., although not stated to be the case, is likely that for a monolayer of octadecylphosphonic acid being compressed for the first time.19 This suggestion is supported by the isotherm in Figure 1b measured for a monolayer of RP during its initial compression cycle. The latter shows a pronounced liquidcondensed to solid phase transition at 23 mN m-1 and 21 Å2 per molecule. This view is further supported by the fact that during subsequent compression-expansion cycles, no pronounced liquid-condensed to solid phase transition is observed. RC generally exhibits good film-forming properties at low and medium pHs and poor film-forming properties at high pHs.20 This behavior is attributed to deprotonation of the carboxylic acid headgroup of RC at medium and high pHs and disruption, due to electrostatic repulsion, of film formation.20b Consequently, addition of Cd2+ ions has no effect on the isotherm measured at pH 3.0, a small effect on the isotherm measured at pH 5.5, and a pronounced effect on the isotherm measured at pH 8.0.4a,20a The effect of added Cd2+ ions on the isotherms measured at medium and high pHs, namely, the complete suppression of the liquid region, is accounted for by the ability of the added ions to bridge two adjacent carboxylate anions and to increase the rigidity and stability of the compressed monolayer.20 These findings are summarized in Table 1. RP generally exhibits good film-forming properties at low and medium pHs and poor film-forming properties at high pHs. This behavior is attributed to complete deprotonation of the phosphonic acid headgroup of RP at high pHs and disruption, due to electrostatic repulsion, of film formation. Consequently, addition of Cd2+ ions has no effect on the isotherms measured at pH 3.0, a small effect on the isotherm measured at pH 5.5, and a pronounced effect on the isotherm measured at pH 8.0. The effect of added Cd2+ ions on the isotherms measured at high pHs, namely, the appearance of an extended nonideal gas to (20) (a) Gericke, A.; Hu¨hnerfuss, H. Thin Solid Films 1994, 245, 74. (b) Betts, J.; Pethica, B. Trans. Faraday Soc. 1956, 52, 1581. (c) Yang, H.; Katsunori, A.; Hong, H.-G.; Sackett, D.; Arendt, M.; Yau, S.-L.; Bell, C.; Mallouk, T. J. Am. Chem. Soc. 1993, 115, 11855.

Figure 2. IRRAS at the air/water interface of RC at surface pressures corresponding to points I-III in Figure 1. As in a for RP. All spectra were obtained using a p-polarized beam incident at 30°.

solid transition, is accounted for by the ability of the added ions to bridge two adjacent phosphonate anions. It is noted that in the case of RC, addition of Cd2+ ions led to improved film-forming properties. In the case of RP, however, which possesses good film-forming properties in the absence of added Cd2+ ions, the partially or fully deprotonated phosphonic acid headgroups interact sufficiently strongly with the added Cd2+ ions to prevent closepacking of the alkyl chain and give rise to a nonideal gas to solid transition. In support of this view, it has been observed that in the presence of divalent metal cations, the alkyl chains of self-assembled monolayers of phosphonic acids are not close-packed.20c These findings are summarized in Table 1. Structures of Monolayers at the Air/Water Interface. Shown in Figure 2 are IRRAS recorded at the air/ water interface for monolayers of RC and RP, conditioned by successive compression-expansion cycles on an aqueous subphase at pH 5.5, at approximately the points labeled I, II, and III on the isotherms in Figure 1.

Long-Chain Aliphatic Acids Deposited on Metal

Langmuir, Vol. 13, No. 25, 1997 6773

Table 2. Structure of Aliphatic Carboxylic and Phosphonic Acid Monolayers on Aqueous Subphase

RCc I II III RC (added Cd2+)e I III RPc,f I II III

surface pressure,a mN mn-1

surface area,b Å2 molecule-1

νas(CH2), cm-1

νs CH2), cm-1

tilt, deg

twist, deg

thickness, Å

2 15 40

25 23 20

2918 2917 2916

2850 2849 2849

10 8 0

44 43 43

22d (22.8) 22 (22.9) 22 (23.2)

1 40

20 20

2916 2916

2849 2849

0 0

43 43

23 (23.2) 23 (23.2)

2 15 40

25 23 20

2917 2916 2916

2849 2849 2849

10 3 1

43 43 43

24 (22.8) 24 (23.1) 24 (23.1)

a Surface pressures estimated from isotherms shown in Figures 1 and 2. b Surface area calculated from volume of added RC or RP stock solution in chloroform (typically 18 µL of 5 × 10-4 mol dm-3) and known area of minitrough (typically between 9 and 17 cm2). c pH 5.5. d Effective thickness obtained by adjusting calculated thickness for a good fit between experimental and simulated spectra. Thicknesses obtained by molecular modeling are given in parentheses and have been corrected for the measured tilt angle. e pH 8.0, 2 × 10-4 mol dm-3 Cd2+ (added CdCl2). f Findings upon addition of 2 × 10-4 mol dm-3 Cd2+ (added CdCl2) are not significantly different at pH 3.0. At pH 5.5 and pH 8.0, an extended nonideal gas-to-solid region is observed.

Figure 3. (a) Simulation (dashed line) of IRRAS in Figure 2 of RC compressed to III at the air/water interface. (b) As in a for RP.

The bands assigned to asymmetric and symmetric CH2 stretches of RC at I are at 2918 and 2850 cm-1, respectively; see Figure 2a. These bands are at lower frequencies for RC at III, i.e., 2916 and 2849 cm-1, respectively. This finding is consistent with the increased ordering of the film and agrees well with previously published observations.21 It should be noted that the frequencies of the above bands are sensitive to the presence of gauche rotamers.22 These findings are summarized in Table 2. The bands assigned to asymmetric and symmetric CH2 stretches of RP at I are at 2917 and 2849 cm-1, respectively; (21) Sakai, H.; Umemura, J. Chem. Lett. 1993, 12, 2167. (22) Snyder, R.; Aljibury, A.; Strauss, H.; Casal, H.; Gough, K.; Murphy, W. J. Chem. Phys. 1984, 81, 5352.

see Figure 2b. These bands are at only slightly lower frequencies for RP at III, i.e., 2916 and 2849 cm-1, respectively. This finding is consistent with the view that RP possesses better film-forming properties than RC but that compression still results in greater ordering of the monolayer. These findings are summarized in Table 2. Shown in Figure 3 are simulations of the IRRAS for RC and RP at III from which the orientation of the alkyl chain at the air/water interface may be deduced. The IRRAS for RC and RP at I and II have been similarly analyzed. Not unexpectedly, the average tilt angle decreases from 10° to 0° for RC and from 10° to 1° for RP on compressing a monolayer from I to III. It should be noted that the average tilt and twist angles, 0° and 43°, respectively, determined for RC at III agree well with the values reported recently, also for RC, by Flach et al.23 The corresponding values for RP were 1° and 43°, respectively. These findings are summarized in Table 2. The bands assigned to asymmetric and symmetric CH2 stretches of RC at I-III are at 2916 and 2849 cm-1, respectively, following the addition of Cd2+ ions to the aqueous subphase at pH 8.0. Simulation of the above IRRAS yields average tilt and twist angles of 0° and 43°, respectively, for RC at I-III. The lower CH2 stretching frequencies and average tilt angles, and the fact that both are independent of the degree of compression, support the view that addition of Cd2+ ions at medium and high pHs improves the film-forming properties of RC. Addition of Cd2+ ions to the aqueous subphase at pH 3.0 has no measurable effect on the IRRAS measured for a RP at I-III. This observation is consistent with the purportedly excellent film-forming properties of this molecule. These findings are summarized in Table 2. Structure of the Monolayer Deposited on a Substrate. Silver. Shown in Figure 4a are the IRRAS of a monolayer of RC deposited from an aqueous subphase at pH 5.5 onto a freshly evaporated Ag substrate at dipping pressures corresponding to the points labeled I-III on the isotherm in Figure 1a. Similar spectra for RP are shown in Figure 4b. The bands assigned to the asymmetric and symmetric CH2 stretches of RC deposited at I-III are at 2916 and 2850 cm-1, respectively. It is noted that the above spectra agree well with those previously reported for long-chain aliphatic carboxylic acids deposited, using LB techniques, on a Ag substrate.6a It is noted also that these bands are at frequencies characteristic of a crystalline monolayer.22 (23) Flach, C.; Gericke, A.; Mendelsohn, R. J. Phys. Chem. B 1997, 101, 58.

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Table 3. Structure of Aliphatic Carboxylic and Phosphonic Acid Monolayers on Silver and Gold Substrates surface pressure, mN m-1

surface area, Å2 molecule-1

νas(CH2), cm-1

νs(CH2), cm-1

νas(CH3), cm-1

νs(CH3), cm-1

νs(CH3), cm-1

tilt,b deg

twist,b deg

thickness, Å

Silver Substratea RCc I II III RC (added Cd2+)d I II III RPc,e I II III

1 15 40

25 22 20

2916 2916 2916

2850 2850 2850

2961 2963 2964

2937 2937 2937

2877 2877 2877

13 11 10

39 40 40

26.7 28.3 29.5

1 15 40

21 20 20

2918 2918 2917

2850 2850 2850

2965 2965 2965

2939 2939 2939

2878 2878 2878

6 5 5

39 40 40

28.5 28.9 29.1

1 15 40

23 21 20

2918 2917 2916

2850 2850 2850

2964 2964 2964

2938 2938 2938

2878 2878 2878

16 14 12

48 47 46

30.0 30.7 32.5

2878

22

45

30.0

Gold Substratea RCc,f III RC (added Cd2+)d I II III RPc,e I II III

40

20

2917

2849

2964

2 15 40

25 22 20

2918 2918 2917

2850 2850 2850

2963 2964 2964

2938 2938 2938

2878 2878 2878

8 8 6

40 40 40

29.1 29.3 30.1

2 15 40

23 21 20

2918 2917 2917

2850 2850 2850

2967 2967 2967

2938 2938 2938

2878 2878 2878

12 10 9

45 45 45

30.2 30.7 32.0

a Metal substrate evaporated onto glass microscope slides at a rate of 3 Å s-1 to a thickness of greater than 1500 Å. b Spectra deposited at I and II were normalized to the surface concentration determined for a monolayer deposited at III prior to simulation of the tilt and twist angles reported. c pH 5.5. d pH 8.0, 2 × 10-4 mol dm-3 Cd2+ (added CdCl2). e Findings upon addition of 2 × 10-4 mol dm-3 Cd2+ (added CdCl2) are not significantly different at pH 3.0. At pH 5.5 and pH 8.0, an extended nonideal gas-to-solid region is observed. f These values are those determined for the initially deposited monolayer.

Figure 4. (a) IRRAS of RC deposited on a Ag substrate at surface pressures corresponding to points I-III in Figure 1. (b) As in a for RP. All spectra were obtained using a p-polarized beam incident at 84°.

The spectra for RP deposited at I-III agree well with those measured for RC. These findings are summarized in Table 3.

As the dipping pressure increases from I to III and the surface area per molecule decreases, the bands assigned to the CH2 stretches of RC are expected to increase in intensity. Previous studies have established, however, that the tilt angle of the alkyl chains in a monolayer of an aliphatic carboxylic acid, deposited on a variety of substrates, decreases as the dipping pressure is increased.4b-d Due to the metal-selection rule, a decrease in the tilt angle will be accompanied by a decrease in the intensity, i.e., in -log(R/R0), of the bands assigned to the CH2 stretches.24 Therefore, if the change in tilt angle accompanying compression of a monolayer is sufficiently large, the intensity of the bands assigned to the CH2 stretch may, in fact, decrease. As can be seen from the IRRAS in Figure 4a, this is the case for RC deposited at I-III on Ag. Due to the better film-forming properties of RP, characterized principally by a steeper pressure vs area isotherm, a smaller decrease in the surface area per molecule is accompanied by a greater increase in the average tilt angle as the dipping pressure is increased from I to III. Consequently, as can be seen from the IRRAS in Figure 4b, there is a correspondingly larger increase in intensity of the bands assigned to CH2 stretches of RP as the dipping pressure is increased from I to III. More quantitatively, the IRRAS of RC and RP deposited at I-III have been simulated and values obtained for the average tilt and twist angles. In the case of RC, the average tilt angle decreases from 13° to 10° as the dipping pressure increases from I to III and the area per molecule decreases from 25 to 20 Å2. At the same time, the ellipsometrically determined average film thickness increases from 26.7 to 29.5 Å. In the case of RP, the average tilt angle decreases from 16° to 12° as the dipping pressure increases from I to III and the area per molecule decreases from 23 to 20 Å2. At the same time, the ellipsometrically determined average film thickness increases from 30.0 to 32.5 Å. These findings are summarized in Table 3. (24) Greenler, R. J. Chem Phys. 1966, 44, 310.

Long-Chain Aliphatic Acids Deposited on Metal

Figure 5. (a) IRRAS of RC at indicated times following deposition on a Au substrate at surface pressures corresponding to point III in Figure 1. (b) IRRAS of RP deposited on a Au substrate at surface pressures corresponding to points I-III in Figure 1. All spectra were obtained using a p-polarized beam incident at 84°.

The relatively small average tilt angles for RC suggest a specific interaction between the carboxylic acid headgroup and the native oxide layer on the surface of the Ag film.25 Consistent with this view is the fact that Ag is a reactive substrate at which long-chain aliphatic carboxylic acids are known to self-assemble.26 The significance of the average tilt angles for RP is discussed below. Finally, addition of Cd2+ ions to the aqueous subphase improves the film-forming properties of RC. This is reflected in a decrease in the average tilt angle for RC deposited from an aqueous subphase at pH 8.0 containing added Cd2+ ions. As expected, there is no corresponding decrease in average tilt angle for RP deposited from an aqueous subphase at pH 3.0 containing added Cd2+ ions. These findings are summarized in Table 3. Gold. Shown in Figure 5a are the IRRAS of a monolayer of RC deposited from an aqueous subphase at pH 5.5 onto a freshly evaporated Au substrate at a dipping pressure corresponding to the point labeled III on the isotherm in Figure 1a. Also shown are the IRRAS recorded at the indicated times after deposition. Shown in Figure 5b are the IRRAS of a monolayer of RP deposited from an aqueous subphase at pH 5.5 onto a freshly evaporated Au substrate at dipping pressures corresponding to the points labeled I-III on the isotherm in Figure 1b. The IRRAS of RC deposited at III differs significantly from that of RC deposited on Ag under similar conditions; see Figure 4a and Figure 5a. Further, the intensities of the bands assigned to the asymmetric and symmetric CH2 (25) Tao, Y.-T. J. Am. Chem. Soc. 1993, 115, 4350. (26) Tao, Y.-T.; Hietpas, G.; Allara, D. J. Am. Chem. Soc. 1996, 118, 6724.

Langmuir, Vol. 13, No. 25, 1997 6775

stretches increase with time. Simulation of the IRRAS measured immediately following deposition yields average tilt and twist angles of greater than 22° and of 45°, respectively. The ellipsometrically determined film thickness was 30.0 Å. It should be noted that the increase in the intensity of the bands assigned to the asymmetric and symmetric CH2 stretches after 24 h is such that the IRRAS can no longer be simulated, even assuming a monolayer structure in which the alkyl chains are parallel to the surface. These findings are summarized in Table 3. Similar observations have been reported by other workers for aliphatic carboxylic acid monolayers on a gold substrate.27 These were interpreted as indicating a rearrangement of the monolayer into a three-dimensional crystal with the constituent molecules arranged as hydrogen-bonded dimers in an orthorhombic structure.27 For example, Allara et al. have reported an increase in the intensity of the bands assigned to the asymmetric and symmetric CH2 stretches of monolayers of aliphatic carboxylic acids self-assembled on an Ag substrate in the presence of HCl.26 These workers concluded that the presence of HCl was partially protonating the carboxylic acid headgroups and promoting their reorganization into hydrogen-bonded dimers and subsequently into threedimensional crystalline microdomains. This view was supported by simulation, using an effective medium approximation for the hydrocarbon layer, of the corresponding IRRAS. Therefore, the IRRAS in Figure 5a suggest that a specific interaction between the carboxylic acid group of RC and the Au substrate is absent, i.e., nonreactive adsorption. The IRRAS of RP deposited at I-III agree well with those for RP on Ag and exhibit no time evolution. Accordingly, simulation of the IRRAS for RP yields similar values for the average tilt and twist angles. The ellipsometrically determined film thickness is also unchanged. These findings are summarized in Table 3. Clearly, the structure of RP on Ag and Au is determined principally by headgroup-headgroup interactions that are the basis of the excellent film-forming properties of this molecule. To the extent that adsorption is reactive, it does not appear to measurably affect the structure of the deposited monolayer. Finally, addition of Cd2+ ions to the aqueous subphase at pH 8.0 stabilizes RC deposited on Au and prevents the formation of microcrystalline domains. Specifically, no time evolution of the IRRAS for RC deposited at I-III is observed. Simulation of these spectra indicates a corresponding decrease in the average tilt angle for RC. Due to the superior film-forming properties of RP, no effect of added Cd2+ ions is observed for RP deposited at I-III from an aqueous subphase at pH 3.0. Conducting Glass. To permit simulation of the IRRAS of RC or RP deposited on CG, it was first necessary to determine the optical constants of the above substrate. To this end, p-polarized IRRAS were recorded for CG over a range of reflection angles against a background recorded for a freshly evaporated Au substrate. The values for the optical constants of CG were determined using a leastsquares routine to fit the measured reflectances to a theoretical model;28 see Figure 6a. The values of n ) 1.09 and k ) 4.35 were obtained. These values are similar to those previously reported for indium tin oxide and F-doped (27) (a) Dote, J.; Mowery R. J. Phys. Chem. 1988, 92, 41571. (b) Fujimoto, Y.; Higashi, A.; Ozaki, Y.; Kato, T.; Matsumoto, N.; Iriyama, K. SPIE 1993, 1921, 304. (28) Ward, L. The Optical Constants of Bulk Material and Films, 2nd ed.; IOP: London, 1994; Chapters 1-2.

6776 Langmuir, Vol. 13, No. 25, 1997

Figure 6. (a) Experimental reflectivities at 2900 cm-1 of a CG substrate at the indicated angles of incidence. Background reflectivities of a Au substrate have been subtracted. The indicated optical constants for a CG substrate are obtained by a best fit to the experimental data (solid line). (b) Electric field intensity at the air/substrate interface plotted as a function of the angle of incidence for a Au and CG substrate.

tin oxide conducting glasses.9,29 However, as has been shown that the optical constants for conducting glass substrates depend strongly on the dopant concentration, a prediction of classical Drude theory,29,30 the optical constants need to be determined for a given conducting glass substrate. Shown in Figure 6b is the amplitude of the p-polarized standing wave at the air/substrate interface as a function of the angle of incidence.31 For a Au substrate, the standing wave has significant amplitude only in the vertical direction (z axis) and, therefore, only oscillating dipoles with a component in the vertical direction (metalselection rule) will absorb light.24 Consequently, for a monolayer deposited at a metal substrate, only changes in the average tilt and twist angles of the molecules constituting the monolayer will alter the amplitude of oscillation of the dipole component in the vertical direction. In short, the measured spectrum is insensitive to changes in the azimuthal angle of the molecules constituting the monolayer. In the case of a CG substrate, however, the standing wave has significant amplitude both in the vertical (z-axis) and in-plane (x-axis and y-axis) directions. Consequently, for a monolayer deposited at a CG substrate, changes in the average tilt, twist, and azimuthal angles of the molecules constituting the monolayer will (29) Shanti, E.; Banerjee, A.; Dutta, V.; Chopra, K. J. Appl. Phys. 1982, 53, 1615. (30) Shanti, E.; Dutta, V.; Banerjee, A.; Chopra, K. J. Appl. Phys. 1980, 51, 6243. (31) McIntyre, J. Advances in Electrochemistry and Electrochemical Engineering; Wiley: New York, 1973; Vol. 9, p 86.

Marguerettaz and Fitzmaurice

Figure 7. (a) IRRAS of RC deposited on a CG substrate at surface pressures corresponding to points I-III in Figure 1. (b) As in a for RP monolayer. All spectra were obtained using a p-polarized beam incident at 73°.

alter the amplitude of oscillation of the dipole components in the vertical and in-plane directions. Shown in Figure 7a are the IRRAS of a monolayer of RC deposited from an aqueous subphase at pH 5.5 onto a CG substrate at dipping pressures corresponding to the points labeled I-III on the isotherm in Figure 1a. Similar spectra for RP are shown in Figure 7b. The frequencies of the bands assigned to the symmetric and asymmetric CH2 stretches of RC and RP deposited at I-III on CG are characteristic of a crystalline monolayer. The above have been simulated and values obtained for average tilt and twist angles; see Figure 8. In the case of RC, the average tilt angle decreases from 27° to 26° as the dipping pressure increases and the area per molecule decreases from 25 to 20 Å2. In the case of RP, the average tilt angle decreases from 17° to 16° as the dipping pressure increases and the area per molecule decreases from 23 to 20 Å2. These findings are summarized in Table 4. On the basis of these findings and on those reported above for Ag and Au, we conclude the following: firstly, that the carboxylic acid headgroup of RC is less strongly adsorbed at CG than at Ag and that this results in a greater tilt angle in the deposited monolayer; and secondly, that the carboxylic acid headgroup of RC is more strongly adsorbed at CG than at Au32 and it is this that prevents reorganization of the constituent molecules of the initially deposited monolayer into three-dimensional microcrystalline domains. In this context, it is noted that the IRRAS measured 1 week after deposition showed no evidence of reorganization of the deposited monolayer. As expected, (32) Gardner, T.; Frisbie, C.; Wrighton, M. J. Am. Chem. Soc. 1995, 117, 6927.

Long-Chain Aliphatic Acids Deposited on Metal

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Table 4. Structure of Aliphatic Carboxylic and Phosphonic Acid Monolayers on Conducting Glass and Nanocrystalline Semiconductor Substrates surface pressure, mN m-1

surface area, Å2 molecule-1

νas(CH2), cm-1

νs(CH2), cm-1

νas(CH3), cm-1

tilt, deg

twist, deg

thickness,c Å

Conducting Glass Substratea RCb I II III RC (added Cd2+) d I II III RPb,e I II III

2 15 40

25 22 20

2916 2916 2916

2849 2949 2948

2964 2964 2964

27 27 26

45 45 45

2 15 40

21 20 20

2916 2916 2915

2849 2849 2848

2965 2965 2965

16 16 15

43 43 43

2 15 40

23 21 20

2917 2916 2916

2849 2849 2849

2965 2965 2965

17 17 16

48 48 48

Nanocrystalline Semiconductor Substratef RCb I II III RC (added Cd2+)d I II III RPb,e I II III

2 15 40

25 22 20

2916 2916 2915

2849 2949 2948

2965 2965 2965

18 19 16

48 48 48

2 15 40

21 20 20

2916 2916 2915

2849 2849 2849

2965 2965 2965

17 17 15

47 47 47

2 15 40

23 21 20

2917 2917 2916

2850 2850 2849

2966 2966 2966

16 15 14

47 48 48

a F-doped tin oxide glass (0.5 µm thick and 8 Ω cm-2 resistance) supplied by Glastron. b pH 5.5. c Thickness may not be determined ellipsometrically as the conducting glass substrate is not totally reflecting. d pH 8.0, 2 × 10-4 mol dm-3 Cd2+ (added CdCl2). e Findings upon addition of 2 × 10-4 mol dm-3 Cd2+ (added CdCl2) are not significantly different at pH 3.0. At pH 5.5 and pH 8.0, an extended nonideal gas-to-solid region is observed. f Close-packed monolayer of 2-nm TiO2 nanocrystallites deposited on F-doped tin oxide glass (0.5 µm thick and 8 Ω cm-2 resistance) supplied by Glastron.

Figure 8. (a) Simulation (dashed line) of IRRAS in Figure 7 of RC deposited at III on a CG substrate. (b) As in a for RP.

due to the superior film-forming properties of RP, the monolayer deposited initially on CG has similar properties to the films deposited on Ag and Au.

Figure 9. (a) IRRAS of RC deposited on a TiO2-CG substrate at surface pressures corresponding to points I-III in Figure 1. (b) As in a for RP monolayer. All spectra were obtained using a p-polarized beam incident at 73°.

Addition of Cd2+ ions to the aqueous subphase at pH 8.0 improves the film-forming properties of a RC mono-

6778 Langmuir, Vol. 13, No. 25, 1997

Marguerettaz and Fitzmaurice

It is not possible to conclude, on the basis of the data presented here, that RP is reactively adsorbed at TiO2CG. This is because the measured properties of the deposited monolayer are not strongly substrate-dependent, a consequence of the excellent film-forming properties of this monomer. However, recent studies showing that a sensitizer molecule incorporating phosphonic acid groups as linkers are more strongly adsorbed at nanostructured semiconductor electrodes than the same sensitizer incorporating carboxylic acid groups as linkers would suggest this is the case.34 As expected, addition of Cd2+ ions to the aqueous subphase at pH 8.0 improves the film-forming properties of a RC monolayer. This is reflected in the average tilt angle of between 17° and 15° obtained by simulation of the IRRAS measured for RC. No effect is observed for RP due to its superior film-forming properties. These findings are summarized in Table 4. Conclusions

Figure 10. (a) Simulation (dashed line) of IRRAS in Figure 9 of RC deposited at III on a TiO2-CG substrate. (b) As in a for RP.

layer. This is reflected in the average tilt angles of between 16° and 15° obtained by simulation of the IRRAS measured for RC. No effect is observed for RP due to its superior film-forming properties. These findings are summarized in Table 4. Nanostructured Semiconductor. Shown in Figure 9a are the IRRAS of a monolayer of RC deposited from an aqueous subphase at pH 5.5 onto a TiO2-CG substrate at dipping pressures corresponding to the points labeled I-III on the isotherm in Figure 1a. Similar spectra for RP are shown in Figure 9b. The bands assigned to the asymmetric and symmetric CH2 stretches of RC deposited at I-III are observed at frequencies characteristic of a crystalline monolayer.22 The spectra for RP deposited at I-III agree well with those for RC. The above have been simulated and values obtained for average tilt and twist angles; see Figure 10. In the case of RC, the average tilt angle decreases from 18° to 16° as the dipping pressure increases and the area per molecule decreases from 25 to 20 Å2. In the case of RP, the average tilt angle decreases from 16° to 14° as the dipping pressure increases and the area per molecule decreases from 23 to 20 Å2. These findings are summarized in Table 4. These findings suggest there is reactive adsorption of RC on TiO2-CG at I-III. Specifically, the average tilt angle above (18-16°) is greater than observed for RC on Ag (13-10°), where there is known to be reactive adsorption, but less than that observed for RC on either Au (>22°) and CG (26-27°), where it is known there is not. This suggestion is consistent with findings that sensitizer molecules incorporating carboxylic acid groups as linkers are strongly adsorbed at nanostructured TiO2 films.33

The structures of monolayers of long-chain aliphatic carboxylic and phosphonic acids deposited, using Langmuir-Blodgett techniques, on metal (silver and gold), conducting glass (fluorine-doped tin oxide), and nanostructured semiconductor (titanium dioxide) substrates have been studied in detail. In the course of these studies, the effects of pH, dissolved ions, and conditioning regime on the structure of the monolayer present at the air/water interface and subsequently deposited on the above substrates have been examined. The principal findings of these studies are summarized below. Firstly, it is not possible to infer the structure of a monolayer deposited on a substrate from the structure of the monolayer at the air/water interface prior to deposition. For example, the average tilt angle of RC and RP at the air/water interface decreases from 13° to 0° on decreasing the surface area per molecule from 25 to 20 Å2. However, the average tilt angle for RC or RP subsequently deposited on a Ag, Au, CG, or TiO2-CG substrate is significantly larger. Secondly, the extent to which the structure of a deposited monolayer is determined by the nature of the substrate is largely dependent on the film-forming properties of the constituent monomer. For example, in the case of RC, a relatively poor film-forming monomer, the structure of the monolayer deposited on Ag (reactive adsorption and an average tilt angle of between 13° and 10°), Au (nonreactive adsorption leading to the formation of microcrystalline domains), CG (reactive adsorption and an average tilt angle of between 27° and 26°), and TiO2CG(reactive adsorption and an average tilt angle of 18° and 16°) is determined principally by the strength of the interaction between the carboxylic acid headgroup of RC and the substrate. In the case of RP, a good film-forming monomer, the structure of the monolayer deposited on Ag (average tilt angle of between 16° and 12°), Au (average tilt angle of between 12° and 19°), CG (average tilt angle of between 17° and 16°), and TiO2-CG (average tilt angle of 16° and 14°) is determined principally by the strength (33) (a) Ko¨lle, U.; Moser, J.; Gra¨tzel, M. Inorg. Chem. 1985, 24, 2253. (b) Frei, H.; Fitzmaurice, D.; Gra¨tzel, M. Langmuir 1990, 6, 198. (c) Moser, J.; Punchihewa, S.; Infelta, P.; Gra¨tzel, M. Langmuir 1991, 7, 3012. (d) Redmond, G.; Fitzmaurice, D.; Gra¨tzel, M. J. Phys. Chem. 1993, 97, 6951. (e) Moser, J.; Gra¨tzel, M. Chem. Phys. 1993, 176, 493. (34) (a) Pe´chy, P.; Rotzinger, F.; Nazeeruddin, M. K.; Kohle, O.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. J. Chem. Soc., Chem. Commun. 1995, 65. (b) Yan, S. G.; Hupp, J. J. Phys. Chem. 1996, 100, 6867.

Long-Chain Aliphatic Acids Deposited on Metal

of the interaction between the adjacent phosphonic acid headgroups of RP. As a consequence, it is not possible to say whether adsorption of RP is nonreactive or reactive, although, for the reasons outlined above, it is likely that adsorption of RP at TiO2-CG is reactive. Finally, these findings represent an extension of such studies to molecules, namely, phosphonic acids, and substrates, namely, nanostructured TiO2 films, of increasing scientific and technological importance. These findings, and those of future studies, will be used to optimize nanocrystalline solar cells and electrochromic windows based on nanostructured TiO2 films modified by molecules adsorbed at the surface using phosphonic acid linker groups.

Langmuir, Vol. 13, No. 25, 1997 6779

Acknowledgment. We thank the staff of the National Microelectronics Research Centre, in particular Dr. Patrick Kelly, for their assistance in undertaking the reported ellipsometric studies. We also thank Drs. Allara, Heitpas, and Parikh for their assistance in porting their code and its use in simulating the reported surface reflectionabsorption infrared spectra. The studies performed at UCD were supported by a grant from the Commission of the European Union under the Joule III programme (Contract JOR3-CT96-0107). LA9707944