  Recent theoretical calculations and photoemission spectroscopy measurements
on the bulk Bi2Se3 material show that it is a three-dimensional topological
insulator possessing conductive surface states with nondegenerate spins,
attractive for dissipationless electronics and spintronics applications.
Nanoscale topological insulator materials have a large surface-to-volume ratio
that can manifest the conductive surface states and are promising candidates
for devices. Here we report the synthesis and characterization of high quality
single crystalline Bi2Se3 nanomaterials with a variety of morphologies. The
synthesis of Bi2Se3 nanowires and nanoribbons employs Au-catalyzed
vapor-liquid-solid (VLS) mechanism. Nanowires, which exhibit rough surfaces,
are formed by stacking nanoplatelets along the axial direction of the wires.
Nanoribbons are grown along [11-20] direction with a rectangular cross-section
and have diverse morphologies, including quasi-one-dimensional, sheetlike,
zigzag and sawtooth shapes. Scanning tunneling microscopy (STM) studies on
nanoribbons show atomically smooth surfaces with ~ 1 nm step edges, indicating
single Se-Bi-Se-Bi-Se quintuple layers. STM measurements reveal a honeycomb
atomic lattice, suggesting that the STM tip couples not only to the top Se
atomic layer, but also to the Bi atomic layer underneath, which opens up the
possibility to investigate the contribution of different atomic orbitals to the
topological surface states. Transport measurements of a single nanoribbon
device (four terminal resistance and Hall resistance) show great promise for
nanoribbons as candidates to study topological surface states.
