Quantitative evaluation of molecular orientation in thin Langmuir-Blodgett films by FT-IR transmission and reflection-absorption spectroscopy

Abstract

By a comparison of infrared reflection-absorption (RA) and transmission intensities, a method for the quantitative evaluation of molecular orientation in thin Langmuir-Blodgett (LB) films has been developed. The enhancement factors for the RA to transmission absorption intensities of hypothetical isotropic films were theoretically calculated by using Hansen's optical formulas for thin multilayer films. By combination of these values with the experimentally determined intensity ratios of the RA to transmission spectra for uniaxially oriented LB films, the orientation angles of the transition moments of major infrared bands were evaluated. Application of this method to the 7-monolayer LB film of cadmium stearate deposited on Ag (for RA measurements) and on ZnSe (for transmission measurements) substrates gave reasonable tilt angles for the molecular chain as compared to those reported by other investigators and to the data from the X-ray analysis. This method will be discussed in detail and the various factors which influence the accuracy of the orientation analysis will also be covered.

In Situ Detection of Surface SiHn in Synchrotron-Radiation-Induced Chemical Vapor Deposition of a-Si on an SiO2 Substrate

“…The LB film thickness was five monolayers. The observed peaks are assigned to asymmetric CH3 stretching (2957.7 cm -1), asymmetric CH2 stretching (2917.3cm-1), symmetric CH3 stretching (2872.4cm-1), symmetric CH2 stretching (2850.0 cm-~ ), asymmetric COO-stretching (1512.8cm-1), and symmetric COOstretching (1446.2 cm -1) modes (Umemura, Kamata, Kawai & Takenaka, 1990). The improved performance of BML…”

“…LB films were deposited on the surface of an SiO2 BML substrate and an n-type Si(100) substrate of resistivity larger than 1 f~ cm, without removing the surface native oxide, by a conventional method using a solution of stearic acid in benzene spread on the surface of purified water containing 5 x 10 -4 M BaCI2. By comparing the transmission spectra of LB films deposited on the Si(100) substrate and the IRAS spectrum of the BML substrate, it is concluded that LB films are deposited with their hydrocarbon chain axis almost perpendicular to the substrate surface (Umemura, Kamata, Kawai & Takenaka, 1990). The spectra with the LB films were measured by using a Fourier-transformed infrared (FFIR) spectrometer (Perkin-Elmer model 1600) and a TGS detector (4cm -1 resolution and 100 scans).…”

Infrared Reflection-Absorption Spectra of Calcium Icosanate Langmuir-Blodgett Films Observed by Using Zinc Oxide-Based Transparent Electrode and Gold

“…Besides the signal enhancement effect, this orientation of the optical electric field is another advantage of the RAS method, because it can be quite helpful in evaluating the molecular orientation in the organized molecular films such as LB films. That is, comparison of the absorption spectra observed with the RAS and transmission method, in the latter of which the optical electric field is parallel to the substrate surface (with the s-polarization) or oblique (with the p-polarization), provides information about the orientation of the transition dipole moments [3]. However, this comparison may be useless if the structure of the films deposited on the substrates for the RAS and transmission methods are largely different.…”

“…For example, mono-and multilayered Langmuir-Blodgett (LB) films of cadmium octadecanate were intensively studied by this method with silver substrates [3].…”

Different Adsorption Behavior of Rare Earth and Metallic Ion Complexes on Langmuir Monolayers Probed by Sum-Frequency Generation Spectroscopy

“…For example, Brewster-angle microscopy (BAM) images a monolayer by measuring the refractive index difference between water and organic molecules at the surface [9]. Infrared reflection absorption spectroscopy (IRRAS) is able to detect vibrational modes of molecules at the surface monolayer [10]. But BAM gives only information of overall refractive index Different Adsorption Behavior of Rare Earth and Metallic Ion Complexes on … -Woongmo Sung et al change at the surface thus is not suitable to monitor the structure of a Langmuir monolayer and the interfacial water separately.…”