Ultralow lattice thermal conductivity and promising thermoelectric properties of a new 2D MoW 3 Te 8 membrane

Assembling new van der Waals (vdW) materials is challenging for the development of two-dimensional (2D) function devices. The MoW 3 X 8 membrane (X = Se, Te) is a new 2D TMDs membrane molecule composed of one WX 2 monolayer and one WX 2 -MoX 2 -WX 2 sandwich trilayer. The presence of Mo/W atoms endows the new structure with the bridges between X atoms that connect pairs of MoX 2 /WX 2 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 MoW 3 X 8 membrane. In this work, the phonon transport and thermoelectric properties of the MoW 3 X 8 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 MoW 3 Te 8 membranes, enhancing the phonon–phonon scattering and thereby shortening the phonon relaxation time. These characteristics allow the MoW 3 Te 8 membrane to achieve an extremely low lattice thermal conductivity of 0.49 Wm -1 K −1 relative to that of the MoW 3 Se 8 membrane (3.25 Wm -1 K −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 × 10 11 cm −2 in the n -type (p-type) MoW 3 Te 8 membrane could be enriched because of the low lattice thermal conductivity.