Topologically nontrivial phase and tunable Rashba effect in half-oxidized bismuthene

Predicting band gaps of ABN3 perovskites: an account from machine learning and first-principle DFT studies

The present paper is primarily focused on predicting the band gaps of nitride perovskites from machine learning (ML) models. The ML models have been framed from the feature descriptors and band gap values of 1563 inorganic nitride perovskites having formation energies <-0.026 eV and band gaps ranging from ∼1.0 to 3.1 eV. Four supervised ML models such as multi-layer perceptron (MLP), gradient boosted decision tree (GBDT), support vector regression (SVR) and random forest regression (RFR) have been considered to predict the band gaps of the said systems. The accuracy of each model has been tested from mean absolute error, root-mean-square error and determination coefficient R2 values. The bivariate plots between the predicted and input band gaps of the compounds for both the training and test datasets have also been estimated. Additionally, two ABN3-type nitride perovskites CeBN3 (B = Mo, W) have been selected and their electronic band structures and optoelectronic properties have been studied from density functional theory (DFT) calculations. The band gap values of the said compounds have been estimated from DFT calculations at PBE, HSE06, G0W0@PBE, G0W0@HSE06 level of theories. The present study will be helpful in exploring the ML models in predicting the band gaps of nitride perovskites which in turn may bear potential applications in photovoltaic cells and optical luminescent devices.

Toward Planar Iodine 2D Crystal Materials

Usually, the octet rule determines whether an elemental 2D material can only be set by one of the elements in groups IIIA-VA, whose outmost electrons can form hybridized orbits from an s-wave and a p-wave. The hybridized orbits can accommodate all of the outermost electrons and form robust σ bonds. As for the elements in VIA-VIIA, the outermost electrons seem too abundant to be accommodated in hybridized orbits. Here, we show a spd² hybridization rule, accommodating all of the outermost electrons of halogen elements. Each atom can be connected to a contiguous atom by a robust σ bond and carries one dangling unpaired electron, implying that the formation of a π bond is possible. One iodine atomic layer can be robustly locked by the σ bond, forming an iodiene sheet by spd² hybridized orbits. With application of compression strain, the π bond forms, and further compression drives the band inversion successively at the valence band and the conduction band. The appearance of Dirac points (arc or hoop) suggests that the transformation of a normal semimetal into a Dirac semimetal occurs.

Topological semimetal state with triply degenerate nodal points in a stable Cu2Te structure

Cu2Te is commonly used as the backside contact of CdTe-based solar cells. We predict a stable topological semimetal structure of Cu2Te(R3m) with triply degenerate nodal points near the Fermi energy. Triply degenerate nodal points are formed by the band crossing between two states with angular momentum j equal to 3/2 and 1/2 along the unique C3 axis. The anisotropic strain breaking C3 symmetry opens the energy gap, and transforms semimetal Cu2Te(R3m) into a topological insulator. It provides strong evidence for understanding the unconventional large linear magnetoresistance in Cu2-xTe. The band crossing of Cu2Te(R3m) strongly depends on the orbital on-site energy difference and the SOC strength. Crystal structures with the space group R3m (no. 160) are a good platform to obtain topological semimetals with triply degenerate nodal points. Compounds X2Y (X = Cu, Ag, Au, Y = O, S, Se, Te) except for Au2S and Cu2O are topological semimetals with triply degenerate nodal points around the Fermi energy.

Ideal strength and strain engineering of the Rashba effect in two-dimensional BiTeBr

Strain engineering can lead to enhanced charge transfer and therefore, can effectively tune Rashba effect.

Exploring charge density distribution and electronic properties of hybrid organic-germanium layers

Band gap tuning and dielectric properties of small organic ligands adsorbed on bidimensional germanium monolayers (germanene) have been investigated using first-principles calculations. We show that the adsorption of these small groups retains the initially stable free-standing pristine buckled structure form. Charge density and chemical bonding analyses show that the ligands are chemisorbed on the germanium layers. Finally, we demonstrate that the dielectric properties of the bare and ligand adsorbed germanene have a large anisotropy. Our findings of a finite gap open a path for the rational design of nanostructures with possible applications in biosensors and solar cells.

Stable halogen 2D materials: the case of iodine and astatine

Two-dimensional (2D) materials have applications towards electronic devices, energy storages, and catalysis,et al. So far, most of elemental 2D materials are composed based on groups IIIA, IVA or VA. To expand the 2D material family, the orbital hybridization becomes a key factor to determine stability. Here we predict that sp2d3hybridization of the outmost electrons in iodine and astatine can build up 2D triangle lattices, delta-iodiene and delta-astatiene, using first-principles calculations. Each atom is connected by σ bonds with nearest 6 atoms and the π bonds are thus introduced. The band gaps can approach zero because of interaction of unpaired single electron between each atom, if the identical bond length is reduced. By inducing compression strain, the Dirac points or topological nontrivial points can be created in the delta-iodiene and delta-astatiene. Our discovery paves a new way to construction of 2D materials.