Ultralow lattice thermal conductivity and promising thermoelectric properties of a new 2D MoW3Te8 membrane

Abstract

Assembling new van der Waals (vdW) materials is challenging for the development of two-dimensional (2D) function devices. The MoW3X8 membrane (X = Se, Te) is a new 2D TMDs membrane molecule composed of one WX2 monolayer and one WX2-MoX2-WX2 sandwich trilayer. The presence of Mo/W atoms endows the new structure with the bridges between X atoms that connect pairs of MoX2/WX2 monolayers and the terminal sites that produce the van der Waals gap in these layers. The mirror symmetry is broken and the phonon dispersion is suppressed by reducing the dimensionality of the MoW3X8 membrane. In this work, the phonon transport and thermoelectric properties of the MoW3X8 membrane are investigated using first-principles method combined with the semi-classical Boltzmann transport and relaxation time approximation (RTA) theories. It is found that the larger gap between low-frequency and high-frequency optical branches in the membrane prevents atomic vibrations and drastically reduces the phonon velocity in a mid-frequency range below the gap. The low-lying optical and acoustic phonon modes are closely linked in MoW3Te8 membranes, enhancing the phonon–phonon scattering and thereby shortening the phonon relaxation time. These characteristics allow the MoW3Te8 membrane to achieve an extremely low lattice thermal conductivity of 0.49 Wm-1K−1 relative to that of the MoW3Se8 membrane (3.25 Wm-1K−1), which also leads to the improvement in thermoelectric performance of the former one. Besides, the maximum ZT values of 4 (4.5) at 900 K and the carrier concentration of 4 × 1011 cm−2 in the n-type (p-type) MoW3Te8 membrane could be enriched because of the low lattice thermal conductivity.