  Thermoelectric device is a promising next-generation energy solution owing to
its capability to transform waste heat into useful electric energy, which can
be realized in materials with high elec- tric conductivities and low thermal
conductivities. A recently synthesized silicon allotrope of Si$_{24}$ features
highly anisotropic crystal structure with nanometre-sized regular pores. Here,
based on first-principles study without any empirical parameter, we show that
the slightly doped Si$_{24}$ can pro- vide an order-of-magnitude enhanced
thermoelectric figure of merit at room temperature, compared with the cubic
diamond phase of silicon. We ascribe the enhancement to the intrinsic
nanostructure formed by the nanopore array, which effectively hinders heat
conduction while electric conductivity is maintained. This can be a viable
option to enhance the thermoelectric figure of merit without further forming an
extrinsic nanostructure. In addition, we propose a practical strategy to
further diminish the thermal conductivity without affecting electric
conductivity by confining rattling guest atoms in the pores.
