  The quest for high-efficiency heat-to-electricity conversion has been one of
the major driving forces towards renewable energy production for the future.
Efficient thermoelectric devices require high voltage generation from a
temperature gradient and a large electrical conductivity, while maintaining a
low thermal conductivity. For a given thermal conductivity and temperature, the
thermoelectric powerfactor is determined by the electronic structure of the
material. Low dimensionality (1D and 2D) opens new routes to high powerfactor
due to the unique density of states (DOS) of confined electrons and holes. 2D
transition metal dichalcogenide (TMDC) semiconductors represent a new class of
thermoelectric materials not only due to such confinement effects, but
especially due to their large effective masses and valley degeneracies. Here we
report a powerfactor of $MoS_2$ as large as $8.5 mWm^{-1}K^{-2}$ at room
temperature, which is amongst the highest measured in traditional, gapped
thermoelectric materials. To obtain these high powerfactors, we perform
thermoelectric measurements on few-layer $MoS_2$ in the metallic regime, which
allows us to access the 2D DOS near the conduction band edge and exploit the
effect of 2D confinement on electron scattering rates, which result in a large
Seebeck coefficient. The demonstrated high, electronically modulated
powerfactor in 2D TMDCs holds promise for efficient thermoelectric energy
conversion.
