High and anomalous thermal conductivity in monolayer MSi$_2$Z$_4$ semiconductors

TL;DR

This study uses ab initio calculations to analyze the thermal conductivity of 2D MSi$_2$Z$_4$ semiconductors, revealing anomalous behaviors and identifying vibrational properties responsible for high and unusual thermal conductivities.

Contribution

It uncovers the microscopic vibrational mechanisms behind the anomalous thermal conductivity in 2D MSi$_2$Z$_4$ semiconductors, challenging conventional rules and guiding thermal management applications.

Findings

MoSi$_2$N$_4$ and WSi$_2$N$_4$ have high thermal conductivity over 400 W/mK.

MSi$_2$N$_4$ with heavy transition metals can have 3-4 times higher conductivity than with lighter metals.

Anomalous thermal conductivity is linked to vibrational properties like acoustic branch bunching and flatness.

Abstract

The lattice thermal conductivity ($\kappa$) of newly synthesized two-dimensional (2D) MoSi$_2$N$_4$ family and its associated abnormality is anatomized by $ab$ $initio$ phonon Boltzmann transport calculations. $\kappa$ of MoSi$_2$N$_4$ and WSi$_2$N$_4$ is found over 400 Wm$^{-1}$K$^{-1}$ at 300 K. $\kappa$ of MoSi$_2$Z$_4$ (Z=N,P,As) obeys Slack's rule of thumb, decreasing by one order of magnitude from Z=N to Z=As with 46 Wm$^{-1}$K$^{-1}$. However, in MSi$_2$N$_4$ (M=Mo,Cr,W,Ti,Zr,Hf), the variation of $\kappa$ with respect to M is anomalous, i.e. deviating from Slack's classic rule. For M in the same group, $\kappa$ of MSi$_2$N$_4$ is insensitive to the average atomic mass, Debye temperature, phonon group velocity, and bond strength owing to the similar phonon structure and scattering rates. MSi$_2$N$_4$ with heavy group-VIB M even possesses a three to four times higher $\kappa$ than that with light group-IVB M, due to its much stronger M-N and exterior Si-N bonds and thus one 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 MSi$_2$Z$_4$ for thermal management and provides microscopic insight into deciphering the anomalous $\kappa$ of layered 2D structures.