  Two-dimensional (2D) transition metal dinitride (HfN2) has been studied for
their optoelectronic, piezoelectric, and thermoelectric properties. Both
monolayer and bilayer of HfN2 were studied using density functional theory
(DFT) and Boltzmann transport equation (BTE). The bilayer of HfN2 with
different stacking layers (AA and AB) showed different electronic properties.
The optical property of the material suggests that it is a very good absorber
in the ultraviolet (UV) region thus, can be used as a UV-photodetector and as
an absorber layer in photovoltaic devices. The piezoelectric properties of the
material also showed promising behavior as the piezoelectric stress and strain
tensors have highest value of 8.97*(10)^(-10) C/m and 12.59 pm/V respectively
for the bilayer. The piezoelectric tensors have highest value for AB stacked
bilayer. The ZT value of 0.8 at 900 K is also highest for bilayer AB stacked
HfN2. These high values of piezoelectric and thermoelectric parameters of the
material suggest that the material would be an excellent choice as
thermoelectric energy harvesting devices as well as mechanical stress sensor or
actuator.
