Stability of Sulfur Nitrides: A First-Principles Study

A systematic computational study on the structural, electronic, and bonding properties of binary sulfur nitrides has been performed using the projector augmented wave method based on density functional theory. The pressure–composition phase diagram of the S–N system has been established. The simulated pressure–temperature phase diagram and X-ray diffraction pattern of (SN)_<i>x</i> explain the experimentally observed two-phase coexistence. The crystal structure of experimentally observed orthorhombic (SN)_<i>x</i> is predicted. The high-pressure phase transition of (SN)_<i>x</i> has been studied. Sulfur–sulfur interactions induced by localized sulfur 3p_<i>z</i> electrons are found in the high-pressure phase of (SN)_<i>x</i>. With increasing nitrogen composition, the coordination number of sulfur atoms increases from two to six in the S–N system. Furthermore, two nitrogen-rich sulfur nitrides SN₂ and SN₄ have been found at high pressure. SN₄ exhibits a high energy density (2.66 kJ·g^–1), which makes it potentially interesting for industrial applications as a high energy density material

Investigating Robust Honeycomb Borophenes Sandwiching Manganese Layers in Manganese Diboride

We report a robust honeycomb boron layers sandwiching manganese layers compound, MnB₂, synthesized by high pressure and high temperature. First-principle calculation combined with X-ray photoelectron spectrum unravel that the honeycomb boron structure was stabilized by filling the empty π-band via grabbing electrons from manganese layers. Honeycomb boron layers sandwiching manganese layers is an extraordinary prototype of this type of sandwiched structure exhibiting electronic conductivity and ferromagnetism. Hydrostatic compression of the crystal structure, thermal expansion, and the hardness testing reveal that the crystal structure is of strong anisotropy. The strong anisotropy and first-principle calculation suggests that B–B bonds in the honeycomb boron structure are a strong directional covalent feature, while the Mn–B bonds are soft ionic nature. Sandwiching honeycomb boron layers with manganese layers that combine p-block elements with magnetic transition metal elements could endow its novel physical and chemical properties

Predicted Formation of H₃⁺ in Solid Halogen Polyhydrides at High Pressures

The structures of compressed halogen polyhydrides H_<i>n</i>X (X = F, Cl and <i>n</i> > 1) and their evolution under pressure are studied using <i>ab initio</i> calculation based on density functional theory. H_<i>n</i>F (<i>n</i> > 1) are metastable up to 300 GPa, whereas for H_<i>n</i>Cl (<i>n</i> > 1), four new stoichiometries (H₂Cl, H₃Cl, H₅Cl, and H₇Cl) are predicted to be stable at high pressures. Interestingly, triangular H₃⁺ species are unexpectedly found in stoichiometries H₂F with [H₃]⁺[HF₂]⁻, H₃F with [H₃]⁺[F]⁻, H₅F with [H₃]⁺[HF₂]⁻[H₂]₃, and H₅Cl with [H₃]⁺[Cl]⁻[H₂] above 100 GPa. Importantly, formation processes of H₃⁺ species are clearly seen on the basis of comparing bond lengths, bond overlap populations, electron localization functions, and Bader charges as a functions of pressure. Further analysis reveals that the formation of H₃⁺ species is attributed to the pressure-induced charge transfer from hydrogen atoms to halogen atoms

Pressure-Induced Structures and Properties in Indium Hydrides

The structures, electron properties, and potential superconductivity of indium hydrides are systematically studied under high pressure by first-principles density functional calculations. Upon compression, two stable stoichiometries (InH₃ and InH₅) are predicted to be thermodynamically stable. Particularly, in the two compounds, all hydrogen atoms exist in the form of H₂ or H₃ units. The stable phases present metallic features with the overlap between the conduction and the valence bands. The Bader analysis indicates that charges transfer from In atoms to H atoms. Electron–phonon calculations show that the estimated transition temperatures (<i>T</i>_c) of InH₃ and InH₅ are 34.1–40.5 and 22.4–27.1 K at 200 and 150 GPa, respectively

Effect of Surface Trap States on Photocatalytic Activity of Semiconductor Quantum Dots

Semiconductor quantum dots (QDs) are promising photocatalysts for water splitting due to the large specific area, but the influence of surface trap states on the photocatalytic activity of QDs is still not fully understood yet. To answer this question, CdSe QDs with the same morphology, diameter, crystal structure, and energy level are prepared following a hydrazine hydrate (N₂H₄) promoted synthesis strategy and conventional hydrothermal synthesis method. Through various characterizations and analysis, it is found that the conventional hydrothermal synthesized CdSe QDs (H-CdSe QDs) have a high concentration of Cd-involved shallow electron trap states, which seriously hinder the charge separation and transfer between CdSe and cocatalysts. In contrast, the N₂H₄ promoted synthesis strategy provides an energy-saving, low-cost, and facile pathway to eliminate the surface shallow electron traps, ensuring an efficient charge separation and H₂ production in CdSe QDs. As a result, the N₂H₄-promoted synthesized CdSe QDs (N-CdSe QDs) produce 44.5 mL (1998 μmol) H₂ in 7 h, roughly 1.6 times higher than that of H-CdSe QDs (27.5 mL, 1236 μmol). Because the surface trap states are widespread in semiconductor QDs, it is believed that our study provides valuable guidance on the design and preparation of QDs for photocatalysis

Pressure-Stabilized Superconductive Ionic Tantalum Hydrides

High-pressure structures of tantalum hydrides were investigated over a wide pressure range of 0–300 GPa by utilizing evolutionary structure searches. TaH and TaH₂ were found to be thermodynamically stable over this entire pressure range, whereas TaH₃, TaH₄, and TaH₆ become thermodynamically stable at pressures greater than 50 GPa. The dense <i>Pnma</i> (TaH₂), <i>R</i>3̅<i>m</i> (TaH₄), and <i>Fdd</i>2 (TaH₆) compounds possess metallic character with a strong ionic feature. For the highly hydrogen-rich phase of <i>Fdd</i>2 (TaH₆), a calculation of electron–phonon coupling reveals the potential high-<i>T</i>_c superconductivity with an estimated value of 124.2–135.8 K

Correlatively Dependent Lattice and Electronic Structural Evolutions in Compressed Monolayer Tungsten Disulfide

Transition-metal dichalcogenides (TMDs) are promising materials for optoelectronic devices. Their lattice and electronic structural evolutions under high strain conditions and their relations remain open questions. We exert pressure on WS₂ monolayers on different substrates, namely, Si/SiO₂ substrate and diamond anvil surface up to ∼25 GPa. Structural distortions in various degree are disclosed based on the emergence of Raman-inactive B mode. Splits of out-of-plane B and A₁′ modes are only observed on Si/SiO₂ substrate due to extra strain imported from volume decrease in Si and corrugation of SiO₂ surface, and its photoluminescence (PL) quenches quickly because of decreased K–K transition by conspicuous distortion of Brillouin zone. While diamond anvil surface provides better hydrostatic environment, combined analysis of PL and absorption proves that pressure effectively tunes PL emission energy and enhances Coulomb interactions. Knowledge of these distinct pressure tunable characteristics of monolayer WS₂ improves further understanding of structural and optical properties of TMDs

Size-Controlled Synthesis of Bifunctional Magnetic and Ultraviolet Optical Rock-Salt MnS Nanocube Superlattices

Wide-band-gap rock-salt (RS) MnS nanocubes were synthesized by the one-pot solvent thermal approach. The edge length of the nanocubes can be easily controlled by prolonging the reaction time (or aging time). We systematically explored the formation of RS-MnS nanocubes and found that the present synthetic method is virtually a combination of oriented aggregation and intraparticle ripening processes. Furthermore, these RS-MnS nanocubes could spontaneously assemble into ordered superlattices via the natural cooling process. The optical and magnetic properties were investigated using measured by UV–vis absorption, photoluminescence spectra, and a magnetometer. The obtained RS-MnS nanocubes exhibit good ultraviolet optical properties depending on the size of the samples. The magnetic measurements suggest that RS-MnS nanocubes consist of an antiferromagnetic core and a ferromagnetic shell below the blocking temperatures. Furthermore, the hysteresis measurements indicate these RS-MnS nanocubes have large coercive fields (e.g., 1265 Oe for 40 nm nanocubes), which is attributed to the size and self-assembly of the samples

Insights into Antibonding Induced Energy Density Enhancement and Exotic Electronic Properties for Germanium Nitrides at Modest Pressures

Here, the electronic and bonding features in ground-state structures of germanium nitrides under different components that not accessible at ambient conditions have been systematically studied. The forming essence of weak covalent bonds between the Ge and N atom in high-pressure ionic crystal <i>Fd</i>-3<i>m</i>-Ge₃N₄ is induced by the binding effect of electronic clouds originated from the Ge_<i>p</i> orbitals. Hence, it helps us to understand the essence of covalent bond under high pressure, profoundly. As an excellent reducing agent, germanium transfer electrons to the antibonding state of the N₂ dimer in <i>Pa</i>-3-GeN₂ phase at 20 GPa, abnormally, weakening the bonding strength considerably than nitrogen gap (NN) at ambient pressure. Furthermore, the common cognition that the atomic distance will be shortened under the high pressures has been broken. Amazingly, with a lower range of synthetic pressure (∼15 GPa) and nitrogen contents (28%), its energy density is up to 2.32 kJ·g^–1, with a similar order of magnitude than polymeric LiN₅ (nonmolecular compound, 2.72 kJ·g^–1). It breaks the universal recognition once again that nitrides just containing polymeric nitrogen were regarded as high energy density materials. Hence, antibonding induced energy density enhancement mechanism for low nitrogen content and pressure has been exposed in view of electrons. Both the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are usually the separated orbitals of N_π* and N_σ*, which are the key to stabilization. Besides, the <i>sp</i>² hybridizations that exist in N₄ units are responsible for the stability of the <i>R</i>-3<i>c</i>-GeN₄ structure and restrict the delocalization of electrons, exhibiting nonmetallic properties

Coupling-Assisted Renormalization of Excitons and Vibrations in Compressed MoSe₂–WSe₂ Heterostructure

Vertical heterostructures (HSs) constructed with two-dimensional (2D) materials is expected to generate fascinating properties due to interlayer coupling between neighboring layers. However, interlayer coupling can be easily obscured by cross-contamination during transfer processes, rendering their experimental demonstration challenging. Here, we explore the coupling-assisted renormalization of excitons and vibrations in a mechanically fabricated MoSe₂–WSe₂ HS through high-pressure photoluminescence, Raman spectra, and density functional theory calculations. Accompanied by the interlayer coupling enhancement, the excitonic and vibrational renormalizations involving dimensionality and composition variations were achieved. A cycle of 2D–3D–2D excitonic evolution was disclosed and pressure-induced emergence of X^– exciton of MoSe₂ in HS was found reflecting the band structure transition in the MoSe₂–WSe₂ HS. The Raman spectra reveals that the coupled A₂″ vibrations of WSe₂ and MoSe₂ in HS was stiffened and out-of-plane A₁′ vibrations of WSe₂ and MoSe₂ in HS got coherent upon pressure modulation. This coupling-assisted renormalization in MoSe₂–WSe₂ HS can be extended to other 2D layered HSs, which indicates the possibility to design a flexible HS with controlled excitonic and vibrational system for light-emitting diodes, excitonic, and photovoltaic devices