  Many applications for topological insulators (TIs) as well as new phenomena
require devices with reduced dimensions. While much progress has been made to
realize thin films of TIs with low bulk carrier density, nanostructures have
not yet been reported with similar properties, despite the fact that size
confinement should help reduce contributions from bulk carriers. Here we
demonstrate that Bi2Se3 nanoribbons, grown by a simple catalyst-free
physical-vapour deposition, have inherently low bulk carrier densities, and can
be further made bulk-free by size confinement, thus revealing the high mobility
topological surface states. Magneto transport and Hall conductance
measurements, in single nanoribbons, show that at thicknesses below 30nm the
bulk transport is completely suppressed which is supported by self-consistent
band-bending calculations. The results highlight the importance of material
growth and geometrical confinement to properly exploit the unique properties of
the topological surface states.
