Fabrication of Conducting Thin Films on the Surfaces of 7,7,8,8-Tetracyanoquinodimethane Single-Component and Charge-Transfer Complex Single Crystals: Nucleation, Crystal Growth, Morphology, and Charge Transport

摘要

Electrically conducting TTF-TCNQ thin films are fabricated on various molecular crystals containing 7,7,8,8-tetracyanoquinodimethane (TCNQ) by exposing a tetrathiafulvalene (TTF) vapor under ambient conditions. To systematically investigate the properties of the films, mixed stack TCNQ charge-transfer (CT) complex crystals with nine kinds of donors have been prepared as the substrates, and the morphology change of the films on the surfaces at the initial stage of the TTF vapor contact has been observed. When the substrate is a TCNQ single-component crystal, randomly oriented TTF TCNQ nanometer-size needle crystals are grown by the reaction with a TTF vapor. However, when the substrate is a TCNQ CT complex crystal, TTF TCNQ crystals are grown with alignment of their needle axis along the mixed-stack direction of the substrate. The surface roughness, the size of the needle crystals, and the degree of the dense packing of the needles have been found to systematically depend on the strength of the CT interactions in the substrate, and the sheet resistance also exhibits a systematic change. The resistance drop is rapid and remarkable when the donor of the substrate CT complex is weak. The difference in the morphology and the properties is considered to arise from the difference in the ease of nucleus formation and the rate of crystal growth of the TTF TCNQ nanocrystals.