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

Abstract 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 ₂ and 2H‐3R‐NbSe ₂ polymorphs by conducting experimental measurements and theoretical analysis are comparatively studied. Owing to its 1.8 times larger unit cell, 2H‐3R‐NbSe ₂ has a considerably greater number of optical phonon branches than does 2H‐NbSe ₂ , suggesting that 2H‐3R‐NbSe ₂ absorbs thermal energy rather than transporting it. In addition, scattering is more likely to occur in 2H‐3R‐NbSe ₂ because a far greater number of states satisfy the selection rule. As a result of these, the 2H‐3R‐NbSe ₂ has considerably lower thermal conductivity than that of the 2H‐NbSe ₂ . The results highlight how the size of the unit cell affects the thermal conductivities of polymorphs.