High Thermoelectric Performance Achieved in Bulk Selenium with Nanostructural Building Blocks

The inherent coupling between transport parameters poses great challenge in improving thermoelectric conversion performance, yet it is mitigated in material with intrinsic nanostructural building blocks by the possibility of exploiting reduced thermal conductivity and enhanced Seebeck coefficient due to strong electronic and thermal anisotropy. By applying first-principles methods combined with Boltzmann transport theory, we report that high thermoelectric performance can be achieved in trigonal and rhombohedral selenium due to its low-dimensional electronic transport and phonon confined features. More specifically, despite atomic chain direction showing relatively large lattice thermal conductivity, its preferable electrical conductivity (σ) and Seebeck coefficient overwhelm the adverse effects, and a large σ/κ ratio for potentially high ZT can be realized in trigonal selenium. On the other hand, benefiting from the delocalized inter-ring bonding nature and impeded thermal transportation, the electron crystal liked electronic performance and ultralow thermal conductivity in turn favor high thermoelectricity of molecular crystal rhombohedral selenium. This work not only confirms the subtle role of nanostructural building blocks in modulating the transport of the electron and phonon in anisotropic crystal structure, but also provides a strategy of searching bulk material that is promising for high performance thermoelectrics.