Origin of the High Thermoelectric Performance in Si Nanowires: A First-Principle Study

First-principles calculations on the electronic, transport, and lattice vibrational behaviors of Si nanowires (SiNWs) are performed to reveal the origin of the high thermoelectric performance in SiNWs. Our results show that the power factor (S-2 sigma) of SiNWs deviate only slightly from those of bulk Si originating from the similar bonding characters between SiNWs and bulk Si. In contrast, lattice heat transport of SiNWs has been largely revised by the spatial confinement, leading to a significant reduction of group velocity and phonon scattering time, thus reducing dramatically the lattice thermal conductivity. The extremely low lattice thermal conductivities together with the invariant power factor are responsible for the greatly enhanced figure of merit (ZT) of SiNWs, in excellent agreement with experiments. We suggest that our exploration on the physical principle of the high thermoelectric efficiency in SiNWs can be helpful for the deep understanding of SiNWs and sheds strong light on the search or even design of new low dimensional thermoelectric materials through phonon engineering.