Jean Leloup

An automatic trough to make alternate layers

Abstract The Langmuir trough described is specially designed to fabricate alternate multilayers automatically by the Langmuir-Blodgett (LB) technique. Its working principle consists of: (i) making two different films at the surface of a double Langmuir trough; (ii) programming the path of the substrate so that it passes in turn through each film. The apparatus consists principally of a two-compartment Langmuir trough and a system to transfer the substrate from one compartment to the other underneath water. The Langmuir trough contains a single subphase. Its surface is divided into two compartments by a fixed surface barrier located above the central well. Each compartment is equipped with a mobile barrier, so that it can be operated independently for film fabrication and compression. Each compartment is also equipped with an independently programmable dipping arm. Substrate transfer is carried out in the following way: while the two dipping arms are at rest, an auxiliary rotating arm picks up the substrate from one of the arms, transfers it to the other compartment underneath the central barrier and hangs it on the other dipping arm. This sample transferrer is described in detail, together with the way it operates. When the films in the compartments are made of different molecules, this trough builds up alternate LB layers. According to the programme imposed on the substrate transferrer, either centrosymmetric, or non-centrosymmetric, or paired alternate layers can be built. Examples of such alternate layers are given. Their actual structure is discussed: in some cases the films are found to reorganize on or after transfer.

Microdefect decoration and visualization in Langmuir-Blodgett films☆

Abstract Advantage is taken of the chemical reactivity in Langmuir-Blodgett (LB) films to introduce heavy ions (silver) into the polar planes of the layers and thus to decorate these planes for direct transmission electron microscopy observation. This technique of in situ decoration is used to visualize microcollapses which take place during behenic acid film fabrication and transfer. These perturbed regions had been suspected for long to be the origin of the so-called “slow collapse” of fatty acids on water but had not been observed so far. In these regions arrays of upright parallel silver dense planes can be seen with a distance between neighbouring planes of about 45–50 A. This technique can be used to visualize the arrangement and nature of defects in LB monolayers or multilayers other than those of fatty acids, provided that one of the compounds in the film can react with a heavy atom or ion.

XPS study of copper sulphides inserted into a Langmuir-Blodgett matrix

Abstract X-ray photoelectron spectroscopy (XPS) is used to characterize copper sulphides inserted into Langmuir-Blodgett (LB) films of behenic acid. Comparisons are made between samples exhibiting very different electrical conductivity but synthesized by the same method. Sulphur atoms with two different environments are found: divalent sulphur S 2- and dimer sulphur (S 2 ) 2- , higher concentrations of dimer sulphur (S 2 ) 2- being present in insulating samples. Chemical bonding of copper in LB films is compared with CuS and Cu 2 S powders. The copper to sulphur ratio and the divalent to dimer sulphur ratio are obtained from various samples and lead to the formula: Cu 2 S, x S 2 with x ranging from 0.5 to 0.8.

Electrical properties of copper sulfide semiconductors inserted in Langmuir-Blodgett films

Abstract We describe optical and electrical porperties of semi-conducting copper sulfide layers inserted in the polar planes of a LB matrix. The most current resistivity values are around 103 ωcm. The samples are also photoconducting. In some cases, under vacuum, uncontrolled doping drops the resistivity down below 1 ω cm. The temperature behaviour of these samples proves the continuity of the semi-conducting film. Micrometer scale resistivity measurements show two sorts of samples: thin, continuous semi-conducting layers and insulated aggregates.