Qing-Yuan Chen

Unexpected electronic structure of the alloyed and doped arsenene sheets: First-Principles calculations.

We study the equilibrium geometry and electronic structure of alloyed and doped arsenene sheets based on the density functional theory calculations. AsN, AsP and SbAs alloys possess indirect band gap and BiAs is direct band gap. Although AsP, SbAs and BiAs alloyed arsenene sheets maintain the semiconducting character of pure arsenene, they have indirect-direct and semiconducting-metallic transitions by applying biaxial strain. We find that B- and N-doped arsenene render p-type semiconducting character, while C- and O-doped arsenene are metallic character. Especially, the C-doped arsenene is spin-polarization asymmetric and can be tuned into the bipolar spin-gapless semiconductor by the external electric field. Moreover, the doping concentration can effectively affect the magnetism of the C-doped system. Finally, we briefly study the chemical molecule adsorbed arsenene. Our results may be valuable for alloyed and doped arsenene sheets applications in mechanical sensors and spintronic devices in the future.

The electronic properties of impurities (N, C, F, Cl, and S) in Ag3PO4: A hybrid functional method study.

The transition energies and formation energies of N, C, F, Cl, and S as substitutional dopants in Ag3PO4 are studied using first-principles calculations based on the hybrid Hartree-Fock density functional, which correctly reproduces the band gap and thus provides the accurate defect states. Our results show that NO and CO act as deep acceptors, FO, ClO, and SP act as shallow donors. NO and CO have high formation energies under O-poor condition therefore they are not suitable for p-type doping Ag3PO4. Though FO, ClO, and SP have shallow transition energies, they have high formation energies, thus FO, ClO, and SP may be compensated by the intrinsic defects (such as Ag vacancy) and they are not possible lead to n-type conductivity in Ag3PO4.

The electronic structure of graphene tuned by hexagonal boron nitrogen layers: Semimetal–semiconductor transition

The electronic structure of graphene and hexagonal boron nitrogen (G/h-BN) systems have been carefully investigated using the pseudo-potential plane-wave within density functional theory (DFT) framework. We find that the stacking geometries and interlayer distances significantly affect the electronic structure of G/h-BN systems. By studying four stacking geometries, we conclude that the monolayer G/h-BN systems should possess metallic electronic properties. The monolayer G/h-BN systems can be transited from metallicity to semiconductor by increasing h-BN layers. It reveals that the alteration of interlayer distances 2.50–3.50 A can obtain the metal–semiconductor–semimetal variation and a tunable band gap for G/h-BN composite systems. The band dispersion along K–H direction is analogous to the band of rhombohedral graphite when the G/h-BN systems are semiconducting.

Emerging novel electronic structure in hydrogen-Arsenene-halogen nanosheets: A computational study

Based on first-principles calculations including spin-orbit coupling, we investigated the stability and electronic structure of unexplored double-side decorated arsenenes. It has been found that these new double-side decorated arsenenes, which we call “hydrogen-arsenene-halogen (H-As-X, X is halogen)”, are dynamically stable via the phonon dispersion calculations except H-As-F sheets. In particular, all of H-As-X nanosheets are direct band gap semiconductors with a strong dispersion near the Fermi level, which is substantially different from the previous works of double-side decorated arsenenes with zero band gaps. Our results reveal a new route to change the band gap of arsenene from indirect to direct. Furthermore, we also studied bilayer, trilayer, and multilayer H-As-Cl sheets to explore the effects of the layer number. The results indicate that bilayer, trilayer, and multilayer H-As-Cl sheets display novel electronic structure, namely multi-Dirac cones character, and the Dirac character depends sensitively on the layer number. It is noted that the frontier states near the Fermi level are dominantly controlled by the top and bottom layers in trilayer and multilayer H-As-Cl sheets. Our findings may provide the valuable information about the new double-side decorated arsenene sheets in various practical applications in the future.

Electronic and magnetic properties of 3D transition-metal atom adsorbed arsenene

To utilize arsenene as the electronic and spintronic material, it is important to enrich its electronic properties and induce useful magnetic properties in it. In this paper, we theoretically studied the electronic and magnetic properties of arsenene functionalized by 3D transition-metal (TM) atoms (TM@As). Although pristine arsenene is a nonmagnetic material, the dilute magnetism can be produced upon TM atoms chemisorption, where the magnetism mainly originates from TM adatoms. We find that the magnetic properties can be tuned by a moderate external strain. The chemisorption of 3D TM atoms also enriches the electronic properties of arsenene, such as metallic, half-metallic, and semiconducting features. Interestingly, we can classify the semiconducting feature into three types according to the band-gap contribution of spin channels. On the other hand, the chemisorption properties can be modified by introducing monovacancy defect in arsenene. Present results suggest that TM-adsorbed arsenene may be a promising candidate for electronic and spintronic applications.

Strain and electric field tunable electronic structure of buckled bismuthene

Based on first-principles density functional theory calculations, we systemically study the properties of two-dimensional buckled single-layer bismuth (b-bismuthene). The structure, stability, and electronic properties are mainly discussed by PBE + SOC method and the hybrid functional HSE06 method is used to further revise the band gap. The optimized b-bismuthene is determined to be dynamically and thermally stable with an indirect band gap. In particular, there is a peculiar Rashba spin-splitting emerging in the valence band maximum (VBM) states. Interestingly, the Rashba energy could be effectively modulated by in-layer biaxial strain. By applying in-layer biaxial strain, one can find that b-bismuthene has indirect–direct band gap and semiconductor–semimetal transitions. Moreover, we also study the electronic structure of bilayer b-bismuthene that is sensitively dependent on the interlayer distance. We demonstrate that the electric field (E-field) leads to a breaking of the Rashba-type splitting near the VBM of single b-bismuthene. More importantly, there is a synergistic effect when both strain and electric field are applied at the same time. The E-field induced band splitting character could be modified by the strain strength. Thus, this study indicates that b-bismuthene may be a potential material in both electronic and spintronic devices.

The atomic size effect on hybrid inorganic–organic perovskite CH3NH3BI3 (B = Pb, Sn) from first-principles study

The inorganic–organic perovskite CH3NH3PbI3 is a hot research material owing to its outstanding performances as one light absorbing layer of solid-state dye-sensitized solar cells. In this study, we focused on the atomic size effect on CH3NH3BI3 (B = Sn, Pb), provided the best atomic size with which CH3NH3BI3 absorbs widest range of different wavelengths of light, by first-principles calculation. We found that the halogen I–p states are mainly composed of the valence band maximum (VBM) of CH3NH3BI3, and the cation B–p states are primarily composed of the conduction band minimum (CBM). Besides, the bandgap of CH3NH3BI3 decreases and absorptive capacities of different wavelengths of light expand when we reduced the size of the atom and changed B atom from Pb to Sn during the change of suitable range. From all of the above, it is discovered that when the atomic size is 20% less than the normal size, CH3NH3PbI3 has the best optical properties, and its light-absorption range is the widest among all sizes of CH3NH3BI3 compounds. All these results reveal that the stress and strain on CH3NH3BI3 change the atomic size which leads to alteration of bandgap and optical properties in high-efficiency solar cells among all CH3NH3BI3 compounds, namely we can enhance the efficiency of the inorganic–organic perovskite solar cells by setting up suitable pressure on the material in future.

Electronegativity explanation on the efficiency-enhancing mechanism of the hybrid inorganic–organic perovskite ABX3 from first-principles study*

Organic–inorganic hybrid perovskites play an important role in improving the efficiency of solid-state dye-sensitized solar cells. In this paper, we systematically explore the efficiency-enhancing mechanism of ABX3 (A = CH3NH3; B = Sn, Pb; X = Cl, Br, I) and provide the best absorber among ABX3 when the organic framework A is CH3NH3 by first-principles calculations. The results reveal that the valence band maximum (VBM) of the ABX3 is mainly composed of anion X p states and that conduction band minimum (CBM) of the ABX3 is primarily composed of cation B p states. The bandgap of the ABX3 decreases and the absorptive capacities of different wavelengths of light expand when reducing the size of the organic framework A, changing the B atom from Pb to Sn, and changing the X atom from Cl to Br to I. Finally, based on our calculations, it is discovered that CH3NH3SnI3 has the best optical properties and its light-adsorption range is the widest among all the ABX3 compounds when A is CH3NH3. All these results indicate that the electronegativity difference between X and B plays a fundamental role in changing the energy gap and optical properties among ABX3 compounds when A remains the same and that CH3NH3SnI3 is a promising perovskite absorber in the high efficiency solar batteries among all the CH3NH3BX3 compounds.