  The structural stability, electronic structure, and thermal transport
properties of one to six quintuple layers (QLs) of Bi2Se3 are investigated by
van der Waals density functional theory and semi-classical Boltzmann theory.
The bandgap amounts to 0.41 eV for a single QL and reduces to 0.23 eV when the
number of QLs increases to six. A single QL has a significantly higher
thermoelectric figure of merit (0.27) than the bulk material (0.10), which can
be further enhanced to 0.30 by introducing 2.5% compressive strain. Positive
phonon frequencies under strain indicate that the structural stability is
maintained.
