High and Anomalous Thermal Conductivity in Monolayer MSi2Z4 Semiconductors

The lattice thermal conductivity (kappa) of a newly synthesized two-dimensional (2D) MoSi2N4 family and its associated abnormality are anatomized by ab initio phonon Boltzmann transport calculations. kappa of MoSi2N4 and WSi2N4 is found to be over 400 W m(-1) K-1 at 300 K. kappa of MoSi(2)Z(4) (Z = N, P, As) obeys Slack's rule of thumb, decreasing by 1 order of magnitude from Z = N to Z = As with 46 W m(-1) K-1. However, in MoSi2N4 (M = Mo, Cr, W, Ti, Zr, Hf), the variation of kappa with respect to M is anomalous, that is, deviating from Slack's classic rule. For M in the same group, kappa of MoSi2N4 is insensitive to the average atomic mass, Debye temperature, phonon group velocity, and bond strength owing to the similar phonon structure and scattering rates. MoSi2N4 with heavy group-VIB M even possesses a 3-4 times higher kappa than that with light group-IVB M due to its much stronger M-N and exterior Si-N bonds and thus 1 order of magnitude lower phonon scattering rates. Nevertheless, this abnormality could be traced to an interplay of certain basic vibrational properties including the bunching strength and flatness of acoustic branches and their nearby optical branches, which lie outside of the conventional guidelines by Slack. This work predicts high kappa of 2D MoSi(2)Z(4) for thermal management and provides microscopic insights into deciphering the anomalous kappa of layered 2D structures.

Automated summary

The lattice thermal conductivity of the newly synthesized 2D MoSi2N4 family was analyzed using ab initio phonon Boltzmann transport calculations. It was found that MoSi2N4 showed anomalous behavior in terms of thermal conductivity compared to other elements within the same group, deviating from the classic rule proposed by Slack.