Enormous suppression of phonon transport in silicon nanowires with five-fold twin boundary
Si nanowires (NWs) have been experimentally proven to be better candidates for thermoelectric applications than other Si-based nanomaterials due to their extremely low thermal conductivity (TC). Designing and manufacturing high-performance Si NW-based thermoelectrics require a systematic and robust understanding of their thermal transport properties. Therefore, various experimentally observed Si NWs, i.e., pure Si NWs, five-fold twinned Si NWs (5T-Si NWs) and alloyed Si NWs are systematically studied here. Our results show that TC of circular-section Si NWs is clearly lower than that of rectangular-section Si NWs, which is caused by the coarser external boundary (EB). Introducing a five-fold twin boundary (TB) in Si NWs can lead to 3–4 times reduction of the TC. Furthermore, Ge-doping can result in further enormous TC reduction, and the TC can be lowered to an extreme value (1.4 W mK−1); this value approaches the lowest limit of the TC of Si-based nanomaterials (1 W mK−1) and is lower than that of pure amorphous Si NWs (2.4 W mK−1). After combining with lattice dynamics analysis, we notice that compared to EB, five-fold TB not only leads to the usual boundary-phonon scattering, but also results in resonance to reduce the phonon group velocity of low-frequency phonons due to its symmetrical structure, i.e., the symmetrical TB can weaken TCs in two ways: vibration and scattering, which enable it to show greater potential in the field of thermoelectrics. There are three major reasons for the extremely low TC of 5T alloyed Si NWs: (1) the hybridization effect caused by the resonance of the five symmetrical independent twin domains. (2) The greater reduction of TC contributed by TB when compared to that of EB. (3) The appearance of non-propagating phonons caused by Ge-doping. These results reveal that the combined implementation of five-fold TB and Ge-doping, which can enormously block the phonon transport in the entire frequency range, is an effective method to reduce TC and consequently to enhance the figure-of-merit (ZT). In general, our investigations can be expected to offer some useful guidance for the enhancement of ZT of conventional silicon materials.