  We have investigated the transport properties of topological insulator Bi2Se3
thin films grown using magnetron sputtering with an emphasis on understanding
the behavior as a function of thickness. We show that thickness has a strong
influence on all aspects of transport as the two-dimensional limit is
approached. Bulk resistivity and Hall mobility show disproportionately large
changes below 6 quintuple layer which we directly correlate to an increase in
the bulk band gap of few-layer Bi2Se3, an effect that is concomitant with
surface gap opening. A tendency to crossover from a metallic to an insulating
behavior in temperature-dependent resistivity measurements in ultra-thin Bi2Se3
is also consistent with an increase in the bulk band gap along with enhanced
disorder at the film-substrate interface. Our work highlights that the
properties of few-layer Bi2Se3 are tunable that may be attractive for a variety
of device applications in areas such as optoelectronics, nanoelectronics and
spintronics.
