Two-dimensional (2D) materials with graphene as a representative have been intensively studied for a long time. Recently, monolayer gallium nitride (ML GaN) with honeycomb structure was successfully fabricated in experiments, generating enormous research interest for its promising applications in nano- and opto-electronics. Considering all these applications are inevitably involved with thermal transport, systematic investigation of the phonon transport properties of 2D GaN is in demand. In this paper, by solving the Boltzmann transport equation (BTE) based on first-principles calculations, we performed a comprehensive study of the phonon transport properties of ML GaN, with detailed comparison to bulk GaN, 2D graphene, silicene and ML BN with similar honeycomb structure. Considering the similar planar structure of ML GaN to graphene, it is quite intriguing to find that the thermal conductivity (κ) of ML GaN (14.93 W mK⁻¹) is more than two orders of magnitude lower than that of graphene and is even lower than that of silicene with a buckled structure. Systematic analysis is performed based on the study of the contribution from phonon branches, comparison among the mode level phonon group velocity and lifetime, the detailed process and channels of phonon–phonon scattering, and phonon anharmonicity with potential energy well. We found that, different from graphene and ML BN, the phonon–phonon scattering selection rule in 2D GaN is slightly broken by the lowered symmetry due to the large difference in the atomic radius and mass between Ga and N atoms. Further deep insight is gained from the electronic structure. Resulting from the special sp orbital hybridization mediated by the Ga-d orbital in ML GaN, the strongly polarized Ga–N bond, localized charge density, and its inhomogeneous distribution induce large phonon anharmonicity and lead to the intrinsic low κ of ML GaN. The orbitally driven low κ of ML GaN unraveled in this work would make 2D GaN prospective for applications in energy conversion such as thermoelectrics. Our study offers fundamental understanding of phonon transport in ML GaN within the framework of BTE and further electronic structure, which will enrich the studies of nanoscale phonon transport in 2D materials and shed light on further studies.

The ferromagnetism induced by the intrinsic point defects in wurtzite zinc oxide is studied by using ab initio calculation based on density functional theory. The calculations show that both oxygen interstitial and zinc vacancy may induce ferromagnetism into this material. The calculations also show that zinc oxide with oxygen interstitial may be a ferromagnetic semiconductor. Based on the simplified electronic configuration of the defect molecules, we explain the total magnetic moment, electronic structure, and ferromagnetism.

${\text{Zn}}_x{\left(\text{ZnO}\right)}_{1-x}$ granular films with nominal atomic concentration of $x=0\sim 1$ were prepared by magnetron cosputtering method. Ferromagnetism is observed in films with $0.04\le x<0.60$. The room-temperature saturated magnetization increases with increasing $x$ and reaches its maximum value of about 3.34 emu/cc at $x=0.31$. The temperature-dependent magnetization curve could be fitted within the framework of Stoner model in a large temperature range from 50 to 800 K. The obtained Curie temperature is higher than $500°\text{C}$. It is found that the main point defects in ZnO are Zn interstatial and oxygen vacancy. Room-temperature photoluminescence analysis and high-temperature x-ray diffraction measurement show conclusive evidence that the native point defect of Zn interstitial plays a crucial role in the observed magnetic behaviors. By implicating the shallow donor related carriers and/or extending the charge-transfer mechanism to metal/semiconductor heterostructure, the result could be qualitatively explained based on the Stoner theory of band magnetism. These findings may help to get further insight into the ferromagnetic origin in nonmagnetic ion doped ZnO systems.

Magnetic properties and electronic structure of copper ferrites in both normal and inverse spinel structures are studied using a principle spin-polarized band structure calculation method with a modified Becke’s three-parameter exchange correlation. The calculated exchange constants show that the Néel configuration may be unstable for both normal and inverse structures. The local magnetic moments are calculated using Mülliken population analysis and show that the normal structure may achieve very high magnetization. The calculated density of states show that copper ferrite in both normal and inverse spinel structure may be half metallic.