21 research outputs found
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)<sub><i>x</i></sub> explain the experimentally observed two-phase coexistence.
The crystal structure of experimentally observed orthorhombic (SN)<sub><i>x</i></sub> is predicted. The high-pressure phase transition
of (SN)<sub><i>x</i></sub> has been studied. Sulfur–sulfur
interactions induced by localized sulfur 3p<sub><i>z</i></sub> electrons are found in the high-pressure phase of (SN)<sub><i>x</i></sub>. 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
SN<sub>2</sub> and SN<sub>4</sub> have been found at high pressure.
SN<sub>4</sub> exhibits a high energy density (2.66 kJ·g<sup>–1</sup>), 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, MnB<sub>2</sub>, 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
Ï€-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 H<sub>3</sub><sup>+</sup> in Solid Halogen Polyhydrides at High Pressures
The
structures of compressed halogen polyhydrides H<sub><i>n</i></sub>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<sub><i>n</i></sub>F (<i>n</i> > 1) are metastable up to 300 GPa,
whereas
for H<sub><i>n</i></sub>Cl (<i>n</i> > 1),
four
new stoichiometries (H<sub>2</sub>Cl, H<sub>3</sub>Cl, H<sub>5</sub>Cl, and H<sub>7</sub>Cl) are predicted to be stable at high pressures.
Interestingly, triangular H<sub>3</sub><sup>+</sup> species are unexpectedly
found in stoichiometries H<sub>2</sub>F with [H<sub>3</sub>]<sup>+</sup>[HF<sub>2</sub>]<sup>−</sup>, H<sub>3</sub>F with [H<sub>3</sub>]<sup>+</sup>[F]<sup>−</sup>, H<sub>5</sub>F with [H<sub>3</sub>]<sup>+</sup>[HF<sub>2</sub>]<sup>−</sup>[H<sub>2</sub>]<sub>3</sub>, and H<sub>5</sub>Cl with [H<sub>3</sub>]<sup>+</sup>[Cl]<sup>−</sup>[H<sub>2</sub>] above 100 GPa. Importantly, formation
processes of H<sub>3</sub><sup>+</sup> 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 H<sub>3</sub><sup>+</sup> 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 (InH<sub>3</sub> and InH<sub>5</sub>) are
predicted to be thermodynamically stable. Particularly, in the two
compounds, all hydrogen atoms exist in the form of H<sub>2</sub> or
H<sub>3</sub> 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><sub>c</sub>) of InH<sub>3</sub> and
InH<sub>5</sub> 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 (N<sub>2</sub>H<sub>4</sub>) 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 N<sub>2</sub>H<sub>4</sub> 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 H<sub>2</sub> production in CdSe
QDs. As a result, the N<sub>2</sub>H<sub>4</sub>-promoted synthesized
CdSe QDs (N-CdSe QDs) produce 44.5 mL (1998 μmol) H<sub>2</sub> 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 TaH<sub>2</sub> were found to be thermodynamically stable over this entire
pressure range, whereas TaH<sub>3</sub>, TaH<sub>4</sub>, and TaH<sub>6</sub> become thermodynamically stable at pressures greater than
50 GPa. The dense <i>Pnma</i> (TaH<sub>2</sub>), <i>R</i>3Ì…<i>m</i> (TaH<sub>4</sub>), and <i>Fdd</i>2 (TaH<sub>6</sub>) compounds possess metallic character
with a strong ionic feature. For the highly hydrogen-rich phase of <i>Fdd</i>2 (TaH<sub>6</sub>), a calculation of electron–phonon
coupling reveals the potential high-<i>T</i><sub>c</sub> 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 WS<sub>2</sub> monolayers on
different substrates, namely, Si/SiO<sub>2</sub> 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 A<sub>1</sub>′ modes are only
observed on Si/SiO<sub>2</sub> substrate due to extra strain imported
from volume decrease in Si and corrugation of SiO<sub>2</sub> 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 WS<sub>2</sub> 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>-Ge<sub>3</sub>N<sub>4</sub> 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 N<sub>2</sub> dimer in <i>Pa</i>-3-GeN<sub>2</sub> phase at 20 GPa, abnormally, weakening the bonding
strength considerably than nitrogen gap (Nî—¼N) 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<sup>–1</sup>, with a similar order of magnitude than polymeric LiN<sub>5</sub> (nonmolecular compound, 2.72 kJ·g<sup>–1</sup>). 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><sup>2</sup> hybridizations that exist
in N<sub>4</sub> units are responsible for the stability of the <i>R</i>-3<i>c</i>-GeN<sub>4</sub> structure and restrict
the delocalization of electrons, exhibiting nonmetallic properties
Coupling-Assisted Renormalization of Excitons and Vibrations in Compressed MoSe<sub>2</sub>–WSe<sub>2</sub> 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 MoSe<sub>2</sub>–WSe<sub>2</sub> 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<sup>–</sup> exciton of
MoSe<sub>2</sub> in HS was found reflecting the band structure transition
in the MoSe<sub>2</sub>–WSe<sub>2</sub> HS. The Raman spectra
reveals that the coupled A<sub>2</sub>″ vibrations of WSe<sub>2</sub> and MoSe<sub>2</sub> in HS was stiffened and out-of-plane
A<sub>1</sub>′ vibrations of WSe<sub>2</sub> and MoSe<sub>2</sub> in HS got coherent upon pressure modulation. This coupling-assisted
renormalization in MoSe<sub>2</sub>–WSe<sub>2</sub> 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