Langmuir-Blodgett (LB) films are prepared by transferring floating organic monolayers onto solid substrates. A combination of innovative chemistry and a carefully engineered instrument (a Langmuir trough) can result in high quality monomolecular assemblies displaying a high degree of structural order. The technique was first reported about fifty years ago and since then, LB films have been widely used as model systems in fundamental research. However, it is only during the past decade that the extensive applied potential has been recognised. Their precise thickness, coupled with the degree of control over the molecular architecture, has now firmly established a role for these layers in thin film technology. Following a historical introduction, this review describes the preparation and characterization of LB films and summarizes their importance in basic science. Particular emphasis is then placed on their potential applications, especially in fields related to electronics. The article concludes w...

An applied science perspective of Langmuir-Blodgett films

Fourier transform IR measurements of Langmuir–Blodgett monolayers have been made with both grazing incidence reflection and transmission techniques. The reflection arrangement at incident angles from 85° to 88° has the electric field component perpendicular to the substrate surface while the standard transmission arrangement has the electric field component in the plane of the film. These techniques were used to explore the anisotropic arrangement of cadmium arachidate molecules perpendicular and parallel to the monolayer plane. The implication of these results for characterizing the orientational anisotropy of chemical groups in thin polymer films is also discussed. In addition, the question of uniaxial and biaxial orientation in the monolayer plane was addressed because of the importance of intermolecular packing on both the extent and rate of polymerization in unsaturated fatty acid monolayer components which form synthetic membranes.

Anisotropic orientation in molecular monolayers by infrared spectroscopy

A semitheoretical formalism based on classical electromagnetic wave theory has been developed for application to the quantitative treatment of reflection spectra from multilayered anisotropic films on both metallic and nonmetallic substrates. Both internal and external reflection experiments as well as transmission can be handled. The theory is valid for all wavelengths and is appropriate, therefore, for such experiments as x‐ray reflectivity, uv–visible spectroscopic ellipsometry, and infrared reflection spectroscopy. Further, the theory is applicable to multilayered film structures of variable number of layers, each with any degree of anisotropy up to and including full biaxial symmetry. The reflectivities (and transmissivities) are obtained at each frequency by solving the wave propagation equations using a rigorous 4×4 transfer matrix method developed by Yeh in which the optical functions of each medium are described in the form of second rank (3×3) tensors. In order to obtain optical tensors for materials not readily available in single crystal form, a method has been developed to evaluate tensor elements from the complex scalar optical functions (n) obtained from the isotropic material with the limitations that the molecular excitations are well characterized and obey photon–dipole selection rules.This method is intended primarily for infrared vibrational spectroscopy and involves quantitative decomposition of the isotropic imaginary optical function (k) spectrum into a sum of contributions from fundamental modes, the assignment of a direction in molecular coordinates to the transition dipole matrix elements for each mode, the appropriate scaling of each k vector component in surface coordinates according to a selected surface orientation of the molecule to give a diagonal im(n) tensor, and the calculation of the real(n) spectrum tensor elements by the Kramers–Kronig transformation. Tensors for other surface orientations are generated by an appropriate rotation matrix operation. To test the viability of this approach, three sets of experimentally derived infrared spectra of oriented monolayer assemblies on quite distinctively different substrates were chosen for simulation: (1) n‐alkanethiols self‐ assembled onto gold, (2) n‐alkanoic acid salt Langmuir–Blodgett (LB) monolayers on carbon, and (3) n‐alkanoic acid salt LB monolayers on silica glass. The formalism developed was used to simulate the spectral response and to derive structural features of the monolayers. Good agreement was found where comparisons with independent studies could be made and, in general, the method appears quite useful for structural studies of highly organized thin films.

Quantitative determination of molecular structure in multilayered thin films of biaxial and lower symmetry from photon spectroscopies. I. Reflection infrared vibrational spectroscopy

Langmuir-Blodgett films

A two step melting process in Langmuir–Blodgett (LB) monolayer assemblies of cadmium arachidate (CdA) has been studied by infrared spectroscopy in conjunction with thermal measurements. These techniques have allowed, for the first time, the investigation of detailed structural changes which occur during heating, melting, and annealing cycles. Prior to the melting point, a pretransitional disordering of the hydrocarbon tails was observed. This was followed by a catastrophic breakup of the cadmium lattice at a temperature corresponding to the melting point as measured by differential scanning calorimetry (DSC).

H. Ishida

Papers

X-ray photoelectron spectra and Fourier transform infrared spectra of mono- and multilayer films of cadmium arachidate