Effects of Unit Cell Size on Thermal Conductivity in Two Different Polymorphs of Niobium Diselenide.

2D materials possess weak inter-layer van der Waals bonding, allowing them to exist as different polymorphs depending on the stacking sequence of the layers. Herein, the thermal conductivities of the 2H-NbSe and 2H-3R-NbSe polymorphs by conducting experimental measurements and theoretical analysis are comparatively studied. Owing to its 1.8 times larger unit cell, 2H-3R-NbSe has a considerably greater number of optical phonon branches than does 2H-NbSe, suggesting that 2H-3R-NbSe absorbs thermal energy rather than transporting it. In addition, scattering is more likely to occur in 2H-3R-NbSe because a far greater number of states satisfy the selection rule. As a result of these, the 2H-3R-NbSe has considerably lower thermal conductivity than that of the 2H-NbSe. The results highlight how the size of the unit cell affects the thermal conductivities of polymorphs.