Pressure-induced insulator–conductor transition in a photoconducting organic liquid-crystal film

Chong-yang Liu and Allen J. Bard ()
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Chong-yang Liu: The University of Texas at Austin
Allen J. Bard: The University of Texas at Austin

Nature, 2002, vol. 418, issue 6894, 162-164

Abstract: Abstract Intermolecular separation determines the extent of orbital overlap and thus the rate of electron transfer between neighbouring molecules in an organic crystal. If such a crystal is compressed, the resistivity decreases owing to a diminishing intermolecular distance1. Metal–insulator transitions have been observed by applying hydrostatic pressure to, for example, Langmuir films of metal nanoparticles2,3. But previous attempts to observe a clear transition point in organic crystals, such as anthracene and tetracene, were not successful owing to difficulties with electrically insulating the high-pressure cell4. Here we report a different approach by using a sample that is photoconductive and forms an organized film. A cylindrical tip (∼100 μm in diameter) was used to compress the sample instead of a piston/cylinder structure, entirely eliminating the problem of electrical insulation. Furthermore, by illuminating the sample with a laser, the conductivity of the sample is increased by several orders of magnitude. By monitoring the photocurrent with sensitivity at the 10-13 A level, changes in resistivity at very low pressure could be monitored. We observe a sharp increase in current that could indicate a transition from hopping to delocalized conduction.

Date: 2002
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DOI: 10.1038/nature00875

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