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

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.