Improved thermoelectric properties of WS₂-WSe₂ phononic crystals: insights from first-principles calculations

Recently, two-dimensional transition metal dichalcogenide (TMDC) monolayers have attracted much attention owing to their excellent physical properties. In the present study, we systematically investigate the thermoelectric properties of different WS₂-WSe₂ phononic crystals by utilizing first-principles calculations. First, the thermal properties of all phononic crystals with superlattices (SL1 and SL2) and their individual components (WS₂ and WSe₂) are evaluated, in which the lattice thermal conductivities (k_ph) of WS₂ and WSe₂ monolayers present isotropic behaviors, while the values of SL1 and SL2 monolayers reveal weak anisotropic behaviors. It can be observed that the k_ph values of WS₂ and WSe₂ monolayers are larger than those of SL1 and SL2 monolayers, which can be attributed to the decreasing phonon group velocity and phonon lifetime. Moreover, we calculate the electronic band structures of all monolayers, indicating that all monolayers are semiconductors. Afterwards, the electrical conductivities, the Seebeck coefficients, the power factors, the electronic thermal conductivities, and the ZT values at different temperatures are evaluated. The ZT_max values of WS₂, WSe₂, SL1, and SL2 monolayers with p-type doping are 0.43, 0.37, 0.95, and 0.66 at 1000 K. It can be proved that the SL1 monolayer possesses the largest ZT, which is at least two times higher than those of the WS₂ and WSe₂ monolayer. Finally, we build two kinds of phononic crystals with periodic holes (PCH1 and PCH2) and evaluate the thermoelectric properties. It can be observed that the PCH2 structure shows the best thermoelectric performance. The ZT_max values of the PCH2 structure can reach 2.53 and 4.54 with p-type doping along the x and y directions, which are 2.66 and 6.75 times higher than those of the SL1 monolayer. This work provides a new strategy to obtain higher thermoelectric performance and demonstrates the potential applications of phononic crystals in TMDC-based nanoelectronic devices.