Surface effects in layered semiconductors ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$

Scanning tunneling spectroscopy of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ layered narrow gap semiconductors reveals finite in-gap density of states and suppressed conduction in the energy range of high valence-band states. Electronic structure calculations suggest that the surface effects are responsible for these properties. Conversely, the interlayer coupling has a strong effect on the bulk near-gap electronic structure. These properties may prove to be important for the thermoelectric performance of these and other related chalcogenides.