  Topological insulator (TI) represents an unconventional quantum phase of
matter with insulating bulk bandgap and metallic surface states. Recent
theoretical calculations and photoemission spectroscopy measurements show that
Group V-VI materials Bi2Se3, Bi2Te3 and Sb2Te3 are TI with a single Dirac cone
on the surface. These materials have anisotropic, layered structures, in which
five atomic layers are covalently bonded to form a quintuple layer, and
quintuple layers interact weakly through van der Waals interaction to form the
crystal. A few quintuple layers of these materials are predicted to exhibit
interesting surface properties. Different from our previous nanoribbon study,
here we report the synthesis and characterizations of ultrathin Bi2Te3 and
Bi2Se3 nanoplates with thickness down to 3 nm (3 quintuple layers), via
catalyst-free vapor-solid (VS) growth mechanism. Optical images reveal
thickness-dependant color and contrast for nanoplates grown on oxidized silicon
(300nm SiO2/Si). As a new member of TI nanomaterials, ultrathin TI nanoplates
have an extremely large surface-to-volume ratio and can be electrically gated
more effectively than the bulk form, potentially enhancing surface states
effects in transport measurements. Low temperature transport measurements of a
single nanoplate device, with a high-k dielectric top gate, show decrease in
carrier concentration by several times and large tuning of chemical potential.
