Molecular composite films of MoS 2 and styrene
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I. INTRODUCTION
Transition metal dichalcogenides are a very interesting class of compounds and are of some technological importance. MoS2, a member of this class of compounds, shows interesting electrical, optical, and chemical properties due to its layered structure.1"3 Unlike many other layered materials, MoS2 shows a pronounced resistance to intercalation with organic molecules. However, we have described a technique of including almost any organic molecule between the layers of MoS2 and other such compounds by first preparing a suspension of single molecular layers in water4'5 and modifying them by mixing with a water immiscible organic liquid whose molecules are to be included. The different chemical nature of the basal and edge sites of the layers makes it possible to modify the single molecular layers into a unique amphiphilic macromolecule where the nonpolar basal planes readily adsorb the organic molecules while the edge planes remain polar with hydoxylation. The resulting two-dimensional amphiphile migrates to the water/organic interface where it finds its lowest energy configuration. The material collected at the interface can be transferred onto a substrate by either dipping the substrate through the interface and slowly withdrawing or by bringing the wet substrate to the interface upon which a uniform film of the collected material spontaneously spreads on it due to the Marangoni effect. Details of the process can be found elsewhere.4'5 Instead of a liquid, one can use a solid compound dissolved in a volatile organic solvent so that after film preparation a monolayer of the solute will remain between the layers of MoS2. Thus, a large class of novel composite films can be formed by incorporating various organic molecules like organic semiconductors, monomers, dyes, liquid crystals, and organometallic materials between the MoS2 layers. Films of composite materials are of current interest. Conventional composites are prepared using plain mixing methods whereas the above technique allows one to "mix" the constituents orderly at a molecular J. Mater. Res., Vol. 6, No. 5, May 1991 http://journals.cambridge.org
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level. In this report we describe the preliminary work on preparing such a novel composite film formed by including styrene, a monomer, in MoS2 and subsequently polymerizing thermally. Styrene was selected because it can be very easily polymerized to form polystyrene. It is found that the styrene-included MoS2 system is sensitive to humidity where a reversible change in the electrical resistivity was observed with varying humidity. In situ x-ray diffraction measurements will be discussed. After polymerization, a stable composite film with a large electrical conductivity anisotropy is obtained. The emphasis of this report will be on the behavior of the styrene-included system. II. EXPERIMENTAL
General details of the film preparation are reported elsewhere.4'5 In the experiments to be described in this report the styrene used was commercially obtained and and distilled once. B
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