Wurtzite Zinc-Oxide (w-ZnO) is a wide bandgap semiconductor that holds promise in power electronics applications, where heat dissipation is of critical importance. However, large discrepancies exist in the literature on the thermal conductivity of w-ZnO. In this paper, we determine the thermal conductivity of w-ZnO using first-principles lattice dynamics and compare it to that of wurtzite Gallium-Nitride (w-GaN) – another important wide bandgap semiconductor with the same crystal structure and similar atomic masses as w-ZnO. However, the thermal conductivity values show large differences (400 W/mK of w-GaN vs. 50 W/mK of w-ZnO at room temperature). It is found that the much lower thermal conductivity of ZnO originates from the smaller phonon group velocities, larger three-phonon scattering phase space and larger anharmonicity. Compared to w-GaN, w-ZnO has a smaller frequency gap in phonon dispersion, which is responsible for the stronger anharmonic phonon scattering, and the weaker interatomic bonds in w-ZnO leads to smaller phonon group velocities. The thermal conductivity of w-ZnO also shows strong size effect with nano-sized grains or structures. The results from this work help identify the cause of large discrepancies in w-ZnO thermal conductivity and will provide in-depth understanding of phonon dynamics for the design of w-ZnO-based electronics.

Penta-graphene (PG) has been identified as a novel 2D material with an intrinsic bandgap, which makes it especially promising for electronics applications. In this work, we use first-principles lattice dynamics and iterative solution of the phonon Boltzmann transport equation (BTE) to determine the thermal conductivity of PG and its more stable derivative -hydrogenated pentagraphene (HPG). As a comparison, we also studied the effect of hydrogenation on graphene thermal conductivity. In contrast to hydrogenation of graphene, which leads to a dramatic decrease in thermal conductivity (from 3590 to 1328 W/mK -a 63% reduction), HPG shows a notable increase in thermal conductivity (615 W/mK), which is 76% higher than that of PG (350 W/mK). The high thermal conductivity of HPG makes it more thermally conductive than most other semi-conducting 2D materials, such as the transition metal chalcogenides. Our detailed analyses show that the primary reason for the counter-intuitive hydrogenation-induced thermal conductivity enhancement is the weaker bond anharmonicity in HPG than PG. This leads to weaker phonon scattering after hydrogenation, despite the increase in the phonon scattering 2 phase space. The high thermal conductivity of HPG may inspire intensive research around HPG and other derivatives of PG as potential materials for future nanoelectronic devices. The fundamental physics understood from this study may open up a new strategy to engineer thermal transport properties of other 2D materials by controlling bond anharmonicity via functionalization. 3In the context of nanoelectronics, heat dissipation is considered to be one of the most crucial phenomena. As the electrons flow under applied voltages, current generation leads to dissipative heating, which could be further augmented in 2D material-based nanoelectronics due to the small heat capacity of single layer devices. Such Joule heating problems can eventually affect device performance, reliability, and in turn, shorten their effective lifetime. Hence, it is desired to have highly thermally conductive material components for effective dissipation of the generated heat during device operation.In the context of materials for nanoelectronics applications, graphene is known to be among the most thermally conductive material in nature with the reported thermal conductivity, κ, ranging from 1500 to 5000 W/mK.1-4 Besides its attractive applications, 2,5,6 the unique physics behind the high thermal conductivity is even more fascinating 7-9 -light carbon atoms, strong sp 2 bonds and the unusual quadratic out-of-plane phonon modes lead to fast traveling (large group velocity) and hard-to-decay (long lifetime) phonon waves. These not only lead to high thermal conductivity, 1,2,7-9 but research has also shown the divergence in graphene thermal conductivity 7 if there is no boundary limiting how far phonons can travel before they are scattered. Despite its fascinating electrical and thermal properties, graphene is also known to be a semi-metal without a bandgap, whic...