STUDIES OF THE STATIC DIELECTRIC CONSTANT OF SOME MONO AND MULTILAYER LANGMUIR FILMS

Abstract

Dielectric and optical behaviour of thin insulator films have been the subject of intensive theoretical and experimental investigations from the last many years as thin films are being currently used for the development of a variety of miniaturized solid- state devices such as fdiodestlitriodest, capacitors and optical devices such as interference filters etc. Particularly, interesting is the dielectric as well as optical behaviour of 'built-up' Langmuir films of metallic salts of fatty acids such as barium stearate etc. which have been chosen for the present studies. Incidentally, such 'built-upt films of barium stearate have been shown to be promising for making thin film devices like, tunneling sandwich between superconductors etc. and antireflection coatings etc. These films have great advantages that their thicknesses are very accurately known and. are closely controllable in a small range. The added advantages are the smoothness, reproducibility, high dielectric strength and well defined structure etc. These films have also been shown to have high structural perfection and practically free from porosity and gross defects, unlike evaporated film systems. The quest for devices application of these thin insulating molecular films, has greatly enhanced the interest of workers towards an understanding of their dielectric and optical behaviour. Some experimental work has been done on this behaviour of films but there is no• theoretical inter-pretation of the results. In the present work)a theory has been formulated for the first time for the calculation of static dielectric constant and refractive index of 'built-up' films of barium palmitate, margarate, stearate and behenate. Measurements of the dielectric constant have also been carried out here and a good agreement is found between the calculated and measured values., The conventional dielectric theory can not be applied as such in the case of 'built-up' molecular(Langmuir). films, as these constitute a crystal with long hydrocarbon chain of molecules as entities and require the calculation of inter-and intra-molecular interactions to be made.. For present calculations, these interactions have been calculated in the presence of static eleetric field by considering the inter-action between different parts of the molecules. Electron diffraction studies have shown that the hydrocarbon chains of the molecules form a hexagonal array with their axes normal to the plane of film. It is known that the molecules of Ba-salts of the fatty acids consist of two parallel hydrocarbon chains with polar group (000)2 Ba at their one extremity. However, the author has made a reasonable assumption that the molecule is equivalent to the two ieffectivet molecules each consisting of one hydrocarbon chain and half of the end group. These teffective' molecules thus forM the hexagonal array with their axes normal to the supporting surface. The extent of the molecules is taken into account by regarding these hydrocarbon chains as divided into small identical groups which can be assumed to be isotropic and, to behave like point dipoles. The calculation of dielectric constant has been made on mono- and multimolecular 'built-upt films of Ba-salts of fatty acids, along the symmetry hexagonal axis (which is the direction of electric field) by calculating the local field, The local field inside the monolayer at one molecule due to other molecules is calculated by considering the interaction between the small units of the molecules and following the assumption of local, additivity, For calculating intermolecular contribution to the local field, the interaction between the . hydrocarbon chains and the end groups are calculated separately. The intramolecular contribution to the local field is calculated by the interaction between the units of the same molecule. The local field inside the*multilayer film is calculated, following Mailer treatment, that the molecules are continuously distributed in all layers except the one in which the molecule is situated. The refractive index of these tbuilt-up' Langmuir films is calculated along the symmetry hexagonal axis which corresponds to the semi-major axis of the index ellipsadof positive uniaxial crystal film. The refractive index which is given by the dielectric constant at very high frequency has been calculated for mono-and multimolecular 'built-up' films essentially on the same lines as static dielectric constant (chapter IV). The calculations have been made using Maxwell's equation, The results obtained are compared with the available experimental values and an agreement is found. The tbuilt-upt films have been deposited by transferring the monolayers on the aluminized glass slide by a repeated dipping and withdrawal process (chapter II) across the spread monolayer on water surface. This technique of Blodgett- Langmuir enabled the deposition of films having required number of layers and hence desired film thickness. For capacitor fabrication of the type Al-Film-Al, properly selected and cleaned smooth glass slides were used (chapter V), The caPacitarce of these capacitors has been measured using the universal bridge and dielectric constant is calculated from the knowledge of accurate film thickness and capacitor area. A systematic study of the thickness dependence of capacitance and hence_ the dielectric constant, has also been made and a definite and unmistakable thickness dependence of the dielectric constant has been found in low thickness range. The results for monolayer and thick films are in complete quantitative agreement with the calculated ones. To the author's knowledge the present work provides the first experimental evidence for the expected thickness dependence of the dielectric constant on a well controlled and structurally well defined thin film system.