For the limitation of experimental methods in crystal characterization, in this study, the mechanical, thermodynamic and electronic properties of wurtzite and zinc-blende GaN crystals were investigated by first-principles calculations based on density functional theory. Firstly, bulk moduli, shear moduli, elastic moduli and Poisson’s ratios of the two GaN polycrystals were calculated using Voigt and Hill approximations, and the results show wurtzite GaN has larger shear and elastic moduli and exhibits more obvious brittleness. Moreover, both wurtzite and zinc-blende GaN monocrystals present obvious mechanical anisotropic behavior. For wurtzite GaN monocrystal, the maximum and minimum elastic moduli are located at orientations [001] and , respectively, while they are in the orientations and for zinc-blende GaN monocrystal, respectively. Compared to the elastic modulus, the shear moduli of the two GaN monocrystals have completely opposite direction dependences. However, different from elastic and shear moduli, the bulk moduli of the two monocrystals are nearly isotropic, especially for the zinc-blende GaN. Besides, in the wurtzite GaN, Poisson’s ratios at the planes containing [001] axis are anisotropic, and the maximum value is 0.31 which is located at the directions vertical to [001] axis. For zinc-blende GaN, Poisson’s ratios at planes (100) and (111) are isotropic, while the Poisson’s ratio at plane (110) exhibits dramatically anisotropic phenomenon. Additionally, the calculated Debye temperatures of wurtzite and zinc-blende GaN are 641.8 and 620.2 K, respectively. At 300 K, the calculated heat capacities of wurtzite and zinc-blende are 33.6 and 33.5 J mol⁻¹ K⁻¹, respectively. Finally, the band gap is located at the G point for the two crystals, and the band gaps of wurtzite and zinc-blende GaN are 3.62 eV and 3.06 eV, respectively. At the G point, the lowest energy of conduction band in the wurtzite GaN is larger, resulting in a wider band gap. Densities of states in the orbital hybridization between Ga and N atoms of wurtzite GaN are much higher, indicating more electrons participate in forming Ga-N ionic bonds in the wurtzite GaN.

The gas-adsorption behaviors of small molecules CO, H2O, H2S, NH3, SO2, and NO on pristine penta-graphene (PG) were investigated using first-principles calculations to explore their potential for use as advanced gas-sensing materials. Results show that, except for CO, H2O, H2S, NH3, and SO2 are physically adsorbed on the surface of penta-graphene with considerable adsorption energy and moderate charge transfer, while NO is prone to be chemically adsorbed on the surface of penta-graphene. Moreover, the electronic properties of PG can be effectively modified after H2O, H2S, NH3, SO2, and NO are adsorbed, and penta-graphene has potential for using in gas sensors via the charge-transfer mechanism.

CH₂O is a common toxic gas molecule that can cause asthma and dermatitis in humans. In this study the adsorption behaviors of the CH₂O adsorbed on the boron nitride (BN), aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), boron phosphide (BP), and phosphorus (P) monolayers were investigated using the first-principles method, and potential materials that could be used for detecting CH₂O were identified. The gas adsorption energies, charge transfers and electronic properties of the gas adsorption systems have been calculated to study the gas adsorption behaviors of CH₂O on these single-layer materials. The electronic characteristics of these materials, except for the BP monolayer, were observed to change after CH₂O adsorption. For CH₂O on the BN, GaN, BP, and P surfaces, the gas adsorption behaviors were considered to follow a physical trend, whereas CH₂O was chemically adsorbed on the AlN and InN monolayers. Given their large gas adsorption energies and high charge transfers, the AlN, GaN, and InN monolayers are potential materials for CH₂O detection using the charge transfer mechanism.