C. U. OKUJAGU

department of Physics University of Port Harcourt, P. M.B 5323 Port Harcourt Nigeria.

info@okujagu. com

Abstract

Analysis of photomicrographs and spectral transmittance characteristics of some single metal Suphide and halide films reveal that there is a physical relationship between the material’s structure and the photo-optical properties the nucleation pattern during growth and the area of application of the films. Some important areas of application of such films are:- Spectral splitters, protective coating and selective window coatings. It was observed that sulphides of silver and lead which have very large grain that tends toward continuous films can be used as infrared transmitting spectral splitters for heating plants in conventional green house agriculture (CGHA) and continuous environment agriculture (CEA), since they can absorb all UV-VIS radiation but transmit NIR radiation into the green house. Lead Halide films which have moderate grains that are more uniformly spread show partial UV-VIS transmission and partial VIS and NIR reflection from about 650nm and can be used as photo thermal protective coating for photovoltaic solar cells. Tin and manganese Halide films with smaller compacted continuous structure show high UV absorption at 359nm, high NIR reflection at 850nm and high visible transmission from 350nm to 850nm hence can be used as cooling coating for warm climates.

1. Intriduction

The concept of structure of thin films has different aspects which are varied with respect to their morphology (mono or poly crystals or the so called amorphous structure) [1-3]. It may also indicate the crystalline structure or the geometric exterior of films [1]. In terms of crystalline structure, it may be necessary to define the nature of films i. e whether the films consist of mono or poly — crystals or are amorphous. If materials are crystalline, it is also necessary to determine the size (nanocrystals or microcrystals), shape (oval, circular, or clinical), orientation and grain boundary of the crystallite units, [2, 4-5]. It may also be useful to introduce density n (r) of crystalline atoms as a function of the distance (r) from a chosen atom [4]. For poly-crystals n(r) is not an isotropic function. Hence in the direction along the lattice row (h, k, l) there is an equidistance maxima at the site of the atoms. This indicates high order in the crystal [1]. In a very fine crystal powder with units consisting only one elementary cell, n (r) however is an isotopic function of r. For such crystals “short range” order still persists in the immediate neighborhood of an atom, where as the long range order has disappeared [1]. On the other hand, the exterior geometry of films has to do with the surface microstructure of the films [2, 4]. Such structures include grain size distribution, grain boundaries, grain shape, grain orientations, nucleation patterns during growth of films and the surface smoothness or otherwise of the films. The grain size with all the grain size properties are the most widely used expressions for the geometrical exterior (surface microstructure) [2]. This is because they are somewhat representative of the geometrical features of films and have profound influence on the properties and behavior of materials (such as spectral behaviour), more than the other structural parameters of the film [3-5]. The grain size of films and materials is exceptionally important also because it can be used to analyze or compute the other grain size distribution

properties where direct measurement technique is not available. Although grains are three­dimensional features, their size and properties are also normally estimated by one — or two — dimensional measurements on planar surfaces through the specimen volume. Hence a wide range of parameters can be used to describe grain size [1-8]. These are:

Average grain diameter d,

Average grain area A

Average number of grains per unit area, N A Average intercept length L 3

Average number of grains intercepted by line of a fixed length, N Average number of grains per unit volume, N r, and Average grain volume L v

Of these descriptions of grain size, only the last two spatial parameters provide a measure of the true grain size and grain size distribution [1]. The others are planar parameters and they are based on measurements of various geometrical parameters of the microstructure, which change as the spatial grain size is altered. For example, the first three parameters provided a single number estimate of the grain size, which is useful for structure-property correlation, but give no information about the grain size distribution. Hence grain parameters are actually measured and expressed in terms of the parameters together.