  The experimental studies about monolayer transition metal dichalcogenides in
the recent year reveal this kind of compounds have many metastable phases with
unique physical properties, not just 1H phases. Here, we focus on the 1T''
phase and systematically investigate the electronic structures and transport
properties of MX2 (M=Mo, W; X=S, Se, Te) using the first-principles
calculations with Boltzmann transport theory. And among them, only three
molybdenum compounds has small direct bandgap at K point, which derive from the
distortion of octahedral-coordination [MoS6]. For these three cases, hole
carrier mobility of MoSe2 is estimated as 690 cm^2/Vs at room temperature, far
more high than that of other two MoX2. For the reason, the combination of the
modest carrier effective mass and weak electron-phonon coupling lead to the
outstanding transport performance of MoSe2. The Seebeck coefficient of MoSe2 is
also evaluated as high as 300 10^-6 V/K at room temperature. Due to the
temperature dependent mobility of T^-1.9 and higher Seebeck coefficient at low
temperature, it is found that MoSe2 has a large thermoelectric power factor
around 6 10^-3 W/mK^2 in the low to intermediate temperature range. The present
results suggests 1T'' MoSe2 maybe a excellent candidate for thermoelectric
material.
