Structure of cadmium and lead sulfide nanoclusters in a matrix of a langmuir-blodgett film

Abstract

Semiconducting materials in the form of nanoclusters distributed in an organic or silicate matrix are of interest to researchers engaged in the field of nanoelectronics and nonlinear optics. These materials involve cadmium, zinc, lead, and mercury chalcogenides in silicate glass or Langmuir-Blodgett (LB) films of fatty acids [1-3]. Investigations in the field of chemical modification of multimolecular LB layers are of great importance for the design of organized molecular assemblies as new nonclassical objects in the physics of low-dimensional systems as well as functional molecular electronics and sensor technology [4]. Formation of lead, zinc, and cadmium sulfides as nanosized clusters by reactions of the corresponding metal behenate films with gaseous hydrogen sulfide was established in [5], where sulfidizing of behenate films was studied at different temperatures and pressures of hydrogen sulfide. In this work we study the structure of the sulfides. Formation of nanoclusters was established, and their size and concentration in the film were determined. Experimental procedure. Cadmium and lead behenate fdms were prepared by the Langmuir-Blodgett procedure. To create monolayers on the surface of a liquid we used hexane solutions of behenic acid and cadmium chloride and lead acetate solutions as a subphase with a concentration of 3-5-10 -4 M, pH 7.0. The monolayers were transferred at a surface pressure of 30-35 mN/m and temperature 22-23~ Conditions of film sulfidizing: H2S pressure 50-100 Torr, temperature 20-5C the holes were subsequently covered with a LB film. These were just the regions investigated by HREM. To obtain an electron diffraction pattern in the reflection mode, the films were applied to polished silicon single crystal plates. Cadmium and lead behenate films of 20 to 30 monolayers (corresponding to a film thickness from 60 to 90 nm) were studied prior to and after sulfidizing. Completeness of sulfidizing was monitored by IR spectra. Experimental results. According to electron diffractometry, cadmium behenate films are characterized by a distinct plate-like texture (Fig. 1). The lattice is hexagonal and has a c axis that is close, on the average, to the normal to the support. Scanning the film showed a 10 ~ change in the inclination of the c axis. The parameters of the hexagonal lattice of the crystal as determined from electron diffractograms are a = 4.8 ~ and c = n 7.5 ~, where n is an integer. Recording by the skewed texture method revealed periodic intervals of 2.2-2.5 A along the c axis, which may be due to periodicity of the arrangement of carbon atoms in aliphatic chains. The crystal structure of lead behenate is analogous