Abnormal in-plane thermal conductivity anisotropy in bilayer α-phase tellurene

Abstract

In layered materials, it is well known that the cross-plane or cross-chain (CC) thermal conductivity is lower than that of the in-plane or along-chain direction due to weaker bonding in the cross-plane or CC directions. In this work, surprisingly, we predict using first-principles that the CC thermal conductivity κ⊥ is higher than the along-chain thermal conductivity κ∥ with a κ⊥/κ∥ ratio of 1.174 in bilayer α-phase tellurene (2L-αTe), an emerging 2D material that has recently drawn extensive interest due to its intriguing electronic, thermoelectronic, and piezoelectronic properties. Also, the room-temperature κ⊥ and κ∥ of 2L-αTe are 5.69 W/mK and 4.85 W/mK, which are 251% and 31% enhanced from that of bulk Te, respectively. A detailed analysis of lattice structure, phonon spectra, and electron properties of 2L-αTe and bulk Te is provided. We find that the larger lattice shrink in CC direction and the change in band angle lead to stiffened phonons and weaker anharmonicity, which in turn yield higher phonon group velocity and longer phonon lifetime in CC direction for 2L-αTe, resulting in the enhanced thermal conductivity and the anomolus thermal anisotropy. Crystal orbital Hamiltan population (COHP) and electron localization function (ELF) analysis demonstrate the enhancement of the strength of “covalent-like quasi-bonding” (CLQB) in CC direction of 2L-αTe, which is accompanied by the lattice shrink in CC direction and the change of band angle. Our study identifies a case of unexpected anisotropic thermal transport in 2D materials, and sheds light on the improvement of thermal conductivity isotropy in 2D materials for the thermal management of electronic devices.