Studies of an Electrically Conducting Organic Material: Tetrathiafulvalene-7,7,8,8-Tetracyanoquinodimethane

University essay from Lunds universitet/Fysiska institutionen; Lunds universitet/Fasta tillståndets fysik

Abstract: This thesis aims to investigate the electrical properties of the organic charge transfer complex Tetrathiafulvalene-7,7,8,8-Tetracyanoquinodimethane, TTF-TCNQ. This is done through two different experimental techniques: Hall characterization and terahertz measurements. Before performing the experiments, suitable devices for these techniques were designed and produced. The layer of TTF-TCNQ was evaporated by physical vapour deposition on an insulating contact of parylene-N and on metallic chromium. According to these results the most promising layer sequence is TTF-TCNQ deposited on top of parylene-N. The adhesive skills of TTF-TCNQ deposited on different materials was also tested using the standardized adhesion test ASTM D3359-97. Two types of electrical devices were produced, Hall devices and terahertz devices. A third sample was also produced in order to image the material of interest using a scanning electron microscope and to measure the film thickness of the polycrystalline TTF-TCNQ layer and of parylene-N. The electric measurements provided the charge carrier concentrations, the sheet carrier density and the conductivity of TTF-TCNQ. The charge carrier densities were high but the measured conductivity was very low. This indicates that the measurement techniques used here may not be very suitable for the both hole and electron conducting TTF-TCNQ material, as the effect of the oppositely charged charge carriers cancel each other out. Also, polycrystalline TTF-TCNQ, which was used here, seems to have a much lower mobility than single crystalline TTF-TCNQ. When working with the type of one-dimensional conductivity observed in TTF-TCNQ it is therefore important to grow single crystalline TTF-TCNQ when very high conductivities are desired. The thesis provides an idea of charge carrier transport in molecular materials, electrical measurement techniques and how to produce a suitable device for examining TTF-TCNQ using terahertz measurements and Hall characterization.

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