J. Phys. Chem. B 1998, 102, 99-105
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Infrared Reflection-Absorption Spectroscopy of Thin Organic Films on Nonmetallic Substrates: Optimal Angle of Incidence Daniel Blaudez,† Thierry Buffeteau,*,‡ Bernard Desbat,‡ Patrice Fournier,§ Anna-Marie Ritcey,§ and Michel Pe´ zolet§ Centre de Physique Mole´ culaire Optique et Hertzienne, URA 283 du CNRS, UniVersite´ Bordeaux I, 33405 Talence, France, Laboratoire de Spectroscopie Mole´ culaire et Cristalline, URA 124 du CNRS, UniVersite´ Bordeaux I, 33405 Talence, France, and CERSIM, De´ partement de Chimie, UniVersite´ LaVal, Cite´ UniVersitaire, Que´ bec G1K 7P4, Canada ReceiVed: March 21, 1997; In Final Form: October 17, 1997X
Infrared external reflectance spectra of a Langmuir-Blodgett film of cadmium arachidate deposited on a glass substrate were recorded for s- and p-polarized radiations at different angles of incidence from 20 to 80°. The peak intensity of the asymmetric CH2 and COO- stretching bands was measured on the difference (∆R) and the normalized (∆R/R) spectra and compared quantitatively with that simulated using a three-phase model system consisting of air, a uniaxial film, and a semi-infinite glass substrate. The maximum intensity in the ∆R and ∆R/R spectra occurs at different angles of incidence. However, when the signal-to-noise ratio is taken into account, the optimal incident angle for both the difference and the normalized spectra is at about 75° for the p- and s-polarizations. Reflectance spectra at different angles of incidence have also been studied as a function of the orientation of the hydrocarbon tail relative to the surface normal. The results show that the accuracy of the determination of the molecular orientation is at least as good at high angles of incidence than near the Brewster angle.
Introduction In recent years, there has been considerable interest in the characterization of the conformation and orientation of amphiphilic molecules in Langmuir monolayers.1,2 The sensitivity of modern Fourier transform infrared spectrometers is now sufficiently good to record reliable spectra of single monolayers supported on solid substrates by either transmission, attenuated total reflectance, or external reflection-absorption infrared spectroscopy.3-9 Furthermore, infrared reflection-absorption spectroscopy (IRRAS) has been shown to be a valuable tool for the in situ characterization of monolayers at the air/water interface.10-20 In the case of monolayers deposited on metal surfaces, the best experimental conditions for obtaining IRRAS spectra are governed by the selectivity of metal surfaces and the spectra have to be recorded at grazing incidence (around 80°) with the radiation polarized parallel to the plane of incidence (ppolarization).21,22 Under such conditions, the surface electric field of the resulting stationary wave is normal to the metal plane and presents an enhanced intensity due to constructive interferences. IRRAS on metal surfaces is thus a very sensitive technique for investigating vibrations with transition moments oriented perpendicular to the film surface. However, this technique is insensitive for transition moments oriented in the plane of the film because the electric field perpendicular to the plane of incidence (s-polarization) is almost zero. * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (33) 5 56 84 84 02. † Centre de Physique Mole ´ culaire Optique et Hertzienne, Universite´ Bordeaux I ‡ Laboratoire de Spectroscopie Mole ´ culaire et Cristalline, Universite´ Bordeaux I. § Universite ´ Laval. X Abstract published in AdVance ACS Abstracts, December 1, 1997.
The case of IRRAS measurements on nonmetallic substrates (semiconductors and dielectrics) is much more complex. It has been shown, both experimentally and theoretically, that the sign and the intensity of the IRRAS bands of organic films on such surfaces depend strongly on the angle of incidence, the polarization of the infrared radiation, and the orientation of the transition moments.7,17,23-30 In the case of s-polarization, the bands have the same sign throughout the entire range of incident angles. If the symmetry of orientation is cylindrical around the normal to the film (uniaxial orientation), the s-polarization does not provide direct information on the orientation of the molecules (i.e., film anisotropy). In the case of p-polarization, the sign of the IRRAS bands changes by crossing the Brewster angle and their intensity varies with the angle of incidence. This polarization provides information on film anisotropy because the electric field is spread both in and out of the plane of the film. IRRAS of thin films on nonmetallic substrates is thus a convenient technique for studying vibrations with transition moments in the plane of the film as opposed to IRRAS on metal surfaces. Owing to the complex behavior of reflection-absorption spectra recorded on nonmetallic substrates, it is particularly important to find the best experimental conditions to obtain results with the highest accuracy. To determine the molecular orientation from IRRAS spectra of thin films deposited on nonmetallic substrates, it has been suggested from model calculations of the normalized reflectivity ∆R/R that the optimal sensitivity is obtained for p-polarized radiation at an angle of incidence that approaches the Brewster angle.26,28 However, owing to the reflectivity minimum for p-polarized radiation upon approaching the Brewster angle, the signal-to-noise ratio in the spectra becomes very poor. Consequently, most studies published so far were carried out at angles of incidence between
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100 J. Phys. Chem. B, Vol. 102, No. 1, 1998
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25 and 45°. On the other hand, it has also been suggested that the best results should be obtained near grazing incidence (7080°)16,23,30 as predicted from the behavior of the difference reflectivity ∆R with the angle of incidence. It thus appears from model calculations that ∆R/R and ∆R do not yield the same optimal angle of incidence for IRRAS measurements on nonmetallic substrates. To solve this problem and to determine the best conditions with respect to both the signal-to-noise ratio and the sensitivity to film anisotropy for IRRAS measurements of thin films on dielectric substrates, we have recorded p- and s-polarized spectra of Langmuir-Blodgett films of cadmium arachidate at different angles of incidence from 20 to 80°. From the reflectance measurements we have calculated the difference (∆R) and the normalized (∆R/R) spectra and the observed intensities of the strongest bands were compared with the theoretical values calculated using thin film optics. The results show that if the spectrum noise is taken into account in the determination of the best conditions, similar optimal angles of incidence are predicted for both ∆R/R and ∆R representations. Experimental Section A film composed of 11 monolayers of cadmium arachidate (CdAr) was prepared by the conventional Langmuir-Blodgett (LB) technique using a KSV 3000 trough. Arachidic acid (Aldrich) was dissolved in chloroform at a concentration of 5 × 10-3 M and spread onto Nanopure (Barnstead) ultrapure water containing 3 × 10-4 M CdCl2 in order to obtain the arachidate salt. The pH of the subphase was adjusted to 6.7 by the dropwise addition of a small amount of NaHCO3. During deposition, the surface pressure was maintained at a constant value of 30 mN/m corresponding to a molecular area of 21 Å2. The monolayers were transferred onto a hydrophilic vitreous silica flat glass plate (4 mm thickness) from Saint Gobain Recherche Company, cleaned with 2-propanol, and stored in pure water. The surface roughness of the glass in contact with the monolayers was very low (
