  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.
