First-principles prediction of infrared phonon and dielectric function in biaxial hyperbolic van der Waals crystal α-MoO3

Layered biaxial hyperbolic molybdenum trioxide (α-MoO₃) with weak van der Waals (vdW) interlayer bonding recently received extensive attention due to its anisotropic dielectric response to infrared (IR) radiation, which couples to the lattice vibrations and allows for manipulating the radiative energy transport. However, the understanding of IR-active phonon modes and dielectric function of it has not yet been fully achieved. Here, by utilizing mode-level first-principles analysis based on density functional theory (DFT), the phonon modes contributing to the IR dielectric response of α-MoO₃ are fully determined. The anisotropic IR-active modes are identified from lattice vibration analysis, allowing for a clear evaluation of the IR absorption contribution from the weak or strong IR phonon modes. By further employing anharmonic-lattice dynamics calculations, the damping of the corresponding IR modes is directly obtained. This approach enables predictions of IR optical properties without any fitting or assumed parameters. Our predictions bridge the scientific gap of comprehensively understanding the unreported IR-active phonon modes of α-MoO₃ and overall agree well with available experimental data, placing our DFT-based method at a privileged stage for accurately predicting the IR optical properties of α-MoO₃. These comprehensive understandings of the IR phonons and dielectric properties of α-MoO₃ pave the way for nanophotonic devices with tunable functionalities and enable design of α-MoO₃ for advanced optical devices.

Assessing the performance of self-consistent hybrid functional for band gap calculation in oxide semiconductors

In this paper we assess the predictive power of the self-consistent hybrid functional scPBE0 in calculating the band gap of oxide semiconductors. The computational procedure is based on the self-consistent evaluation of the mixing parameter α by means of an iterative calculation of the static dielectric constant using the perturbation expansion after discretization method and making use of the relation [Formula: see text]. Our materials dataset is formed by 30 compounds covering a wide range of band gaps and dielectric properties, and includes materials with a wide spectrum of applications such as thermoelectrics, photocatalysis, photovoltaics, transparent conducting oxides, and refractory materials. Our results show that the scPBE0 functional provides better band gaps than the non self-consistent hybrids PBE0 and HSE06, but scPBE0 does not show significant improvement on the description of the static dielectric constants. Overall, the scPBE0 data exhibit a mean absolute percentage error of 14% (band gaps) and 10% ([Formula: see text]). For materials with weak dielectric screening and large excitonic biding energies scPBE0, unlike PBE0 and HSE06, overestimates the band gaps, but the value of the gap becomes very close to the experimental value when excitonic effects are included (e.g. for SiO₂). However, special caution must be given to the compounds with small band gaps due to the tendency of scPBE0 to overestimate the dielectric constant in the proximity of the metallic limit.

Stability and electronic properties of phosphorene oxides: from 0-dimensional to amorphous 2-dimensional structures

Combining the screening by first-principles calculations and Born-Oppenheimer molecular dynamics simulations, we fully reconsider phosphorene oxidation and the formation of low-dimensional phosphorus oxides (P_xO_y). It is found that the previously reported 2-dimensional P_xO_y (2d-P_xO_y) structures cannot provide a full understanding of 2d-P_xO_y properties. We show that the P-O interaction can result in highly stable 0d-P_xO_y and 2d-P_xO_y structures with close energetics, but a noticeable difference in band-gap energies. Here, the possibility of the formation of amorphous 2d-P_xO_y structures and their unique electronic properties are also studied in detail.