Predicted Janus monolayer ZrSSe with enhanced n-type thermoelectric properties compared with monolayer $\mathrm{ZrS_2}$

In analogy to transition-metal dichalcogenide (TMD) monolayers, which have wide applications in photoelectricity, piezoelectricity and thermoelectricity, Janus MoSSe monolayer has been successfully synthesized by substituting the top Se atomic layer in $\mathrm{MoSe_2}$ by S atoms. In this work, Janus monolayer ZrSSe is proposed by ab initio calculations. For the electron part, the generalized gradient approximation (GGA) plus spin-orbit coupling (SOC) is used as exchange-correlation potential, while GGA for lattice part. Calculated results show that the ZrSSe monolayer is dynamically and mechanically stable, which exhibits mechanical flexibility due to small Young's modulus. It is found that ZrSSe monolayer is an indirect-gap semiconductors with band gap of 0.60 eV. The electronic and phonon transports of ZrSSe monolayer are investigated by semiclassical Boltzmann transport theory. In n-type doping, the $ZT_e$ between ZrSSe and $\mathrm{ZrS_2}$ monolayers is almost the same due to similar outlines of conduction bands. The p-type $ZT_e$ of ZrSSe monolayer is lower than that of $\mathrm{ZrS_2}$ monolayer, which is due to larger spin-orbit splitting for ZrSSe than $\mathrm{ZrS_2}$ monolayer. The room-temperature sheet thermal conductance is 33.6 $\mathrm{W K^{-1}}$ for ZrSSe monolayer, which is lower than 47.8 $\mathrm{W K^{-1}}$ of $\mathrm{ZrS_2}$ monolayer. Compared to $\mathrm{ZrS_2}$ monolayer, the low sheet thermal conductance of ZrSSe monolayer is mainly due to small group velocities and short phonon lifetimes of ZA mode. Considering their $ZT_e$ and lattice thermal conductivities, the ZrSSe monolayer may have better n-type thermoelectric performance than $\mathrm{ZrS_2}$ monolayer. These results can stimulate further experimental works to synthesize ZrSSe monolayer.