111 research outputs found
Two-Dimensional Phosphorus Porous Polymorphs with Tunable Band Gaps
Exploring stable two-dimensional
(2D) crystalline structures of
phosphorus with tunable properties is of considerable importance partly
due to the novel anisotropic behavior in phosphorene and potential
applications in high-performance devices. Here, 21 new 2D phosphorus
allotropes with porous structure are reported based on topological
modeling method and first-principles calculations. We establish that
stable 2D phosphorus crystals can be obtained by topologically assembling
selected phosphorus monomer, dimer, trimer, tetramer, and hexamer.
Nine of reported structures are predicted to be more stable than white
phosphorus. Their dynamic and thermal stabilities are confirmed by
the calculated vibration spectra and Born–Oppenheimer molecular
dynamic simulation at temperatures up to 1500 K. These phosphorus
porous polymorphs have isotropic mechanic properties that are significantly
softer than phosphorene. The electronic band structures calculated
with the HSE06 method indicate that new structures are semiconductors
with band gaps ranging widely from 0.15 to 3.42 eV, which are tuned
by the basic units assembled in the network. Of particular importance
is that the position of both conduction and valence band edges of
some allotropes matches well with the chemical reaction potential
of H<sub>2</sub>/H<sup>+</sup> and O<sub>2</sub>/H<sub>2</sub>O, which
can be used as element photocatalysts for visible-light-driven water
splitting
Three-Dimensional Covalently Linked Allotropic Structures of Phosphorus
The discovery of
new phosphorus allotropes has attracted continuous
attention over recent decades, partly due to the importance of phosphorus
in life and their fantastic structural diversity. Generally, phosphorus
allotropes consist of covalently linked substructures, stacked together
with van der Waals interactions, and a few phosphorus allotropes possess
three-dimensional covalently linked structures only at high pressure.
On the basis of first-principles calculations, five new phosphorus
allotropes with three-dimensional covalently linked structures are
predicted by assembling phosphorus units at ambient pressure, which
are energetically more favorable than white phosphorus. Particularly,
three of them share the same structures as those of previously reported
three-dimensional nitrogen allotropes. These new allotropes are semiconductors
with band gaps ranging from 0.52 to 2.39 eV, and the Young’s
modulus varies from 39 to 72 GPa. The structural stability of the
new phosphorus allotropes are confirmed with a phonon spectrum and
Born–Oppenheimer molecular dynamic simulation at temperatures
up to 700 K. Our findings enrich the phosphorus allotrope family with
three-dimensional covalently linked structures at ambient pressure
and versatile electronic properties
Room-Temperature Half-Metallicity in La(Mn,Zn)AsO Alloy via Element Substitutions
Exploring half-metallic
materials with high Curie temperature,
wide half-metallic gap, and large magnetic anisotropy energy is one
of the effective solutions to develop high-performance spintronic
devices. Using first-principles calculations, we design a practicable
half-metal based on a layered LaÂ(Mn<sub>0.5</sub>Zn<sub>0.5</sub>)ÂAsO
alloy via element substitutions. At its ground state, the pristine
LaÂ(Mn<sub>0.5</sub>Zn<sub>0.5</sub>)ÂAsO alloy is an antiferromagnetic
semiconductor. Either hole doping via (Ca<sup>2+</sup>/Sr<sup>2+</sup>,La<sup>3+</sup>) substitutions or electron doping via (H<sup>–</sup>/F<sup>–</sup>,O<sup>2–</sup>) substitutions in the
[LaO]<sup>+</sup> layer induce half-metallicity in the LaÂ(Mn<sub>0.5</sub>Zn<sub>0.5</sub>)ÂAsO alloy. The half-metallic gap is as large as
0.74 eV. Monte Carlo simulations based on the Ising model predict
a Curie temperature of 475 K for 25% Ca doping and 600 K for 50% H
doping, respectively. Moreover, the quasi two-dimensional structure
endows the doped LaÂ(Mn,Zn)ÂAsO alloy a sizable magnetic anisotropy
energy with the magnitude of at least one order larger than those
of Fe, Co, and Ni bulks
Fluorination and Conjugation Engineering Synergistically Enhance the Optoelectronic Properties of Two-Dimensional Hybrid Organic–Inorganic Perovskites
Two-dimensional (2D) hybrid organic–inorganic
perovskites
(HOIPs) are expected to be a viable alternative to three-dimensional
(3D) analogs in solar cells (SCs) and optoelectronic devices due to
their high stability, diverse composition, and physical properties.
However, unsuitable band alignment and large bandgaps limit the power
conversion efficiency (PCE) improvement of SCs based on 2D HOIPs.
Here, we report a molecular design strategy that combines fluorination
and conjugation engineering to tune the electronic structure and optimize
the PCE of 2D HOIPs. Our results show that type IIa band
alignment and tunable bandgaps can be achieved in 2D Dion–Jacobson
(DJ) HOIPs by H/F substitution of organic cations with different degrees
of conjugation. In general, the bandgap of 2D DJ-HOIPs decreases monotonously
with the increase of the number of F atoms, which is due to the gradual
decrease of the lowest unoccupied molecular orbitals (LUMO) of organic
cations. In addition, the enhanced interlayer charge transfer and
higher dielectric constant suggest that the fluorination-induced dielectric
limitations are weakened. The estimated PCE of 2D DJ-HOIPs is exponentially
increased and positively correlated with the degree of conjugation
and fluorination of organic cations, with a PCE approaching 29% under
their synergistic effect. Our results not only provide promising candidates
for photovoltaic device applications but also provide an effective
method for PCE optimization
Turning Nonmagnetic Two-Dimensional Molybdenum Disulfides into Room-Temperature Ferromagnets by the Synergistic Effect of Lattice Stretching and Charge Injection
Exploring two-dimensional (2D) room-temperature
magnetic materials
in the field of 2D spintronics remains a formidable challenge. The
vast array of nonmagnetic 2D materials provides abundant resources
for exploration, but the strategy to convert them into intrinsic room-temperature
magnets remains elusive. To address this challenge, we present a general
strategy based on surface halogenation for the transition from nonmagnetism
to intrinsic room-temperature ferromagnetism in 2D MoS2 based on first-principles calculations. The derived 2D halogenated
MoS2 are half-semimetals with a high Curie temperature
(TC) of 430–589 K and excellent
stability. In-depth mechanistic studies revealed that this marvelous
nonmagnetism-to-ferromagnetism transition originates from the modulation
of the splitting as well as the occupation of the Mo d orbitals by
the synergy of lattice stretching and charge injection induced by
the surface halogenation. This work establishes a promising route
for exploring 2D room-temperature magnetic materials from the abundant
pool of 2D nonmagnetic counterparts
Fluorination and Conjugation Engineering Synergistically Enhance the Optoelectronic Properties of Two-Dimensional Hybrid Organic–Inorganic Perovskites
Two-dimensional (2D) hybrid organic–inorganic
perovskites
(HOIPs) are expected to be a viable alternative to three-dimensional
(3D) analogs in solar cells (SCs) and optoelectronic devices due to
their high stability, diverse composition, and physical properties.
However, unsuitable band alignment and large bandgaps limit the power
conversion efficiency (PCE) improvement of SCs based on 2D HOIPs.
Here, we report a molecular design strategy that combines fluorination
and conjugation engineering to tune the electronic structure and optimize
the PCE of 2D HOIPs. Our results show that type IIa band
alignment and tunable bandgaps can be achieved in 2D Dion–Jacobson
(DJ) HOIPs by H/F substitution of organic cations with different degrees
of conjugation. In general, the bandgap of 2D DJ-HOIPs decreases monotonously
with the increase of the number of F atoms, which is due to the gradual
decrease of the lowest unoccupied molecular orbitals (LUMO) of organic
cations. In addition, the enhanced interlayer charge transfer and
higher dielectric constant suggest that the fluorination-induced dielectric
limitations are weakened. The estimated PCE of 2D DJ-HOIPs is exponentially
increased and positively correlated with the degree of conjugation
and fluorination of organic cations, with a PCE approaching 29% under
their synergistic effect. Our results not only provide promising candidates
for photovoltaic device applications but also provide an effective
method for PCE optimization
Room-Temperature Ferromagnetism in Two-Dimensional Fe<sub>2</sub>Si Nanosheet with Enhanced Spin-Polarization Ratio
Searching experimental feasible two-dimensional (2D) ferromagnetic
crystals with large spin-polarization ratio, high Curie temperature
and large magnetic anisotropic energy is one key to develop next-generation
spintronic nanodevices. Here, 2D Fe<sub>2</sub>Si nanosheet, one counterpart
of Hapkeite mineral discovered in meteorite with novel magnetism is
proposed on the basis of first-principles calculations. The 2D Fe<sub>2</sub>Si crystal has a slightly buckled triangular lattice with
planar hexacoordinated Si and Fe atoms. The spin-polarized calculations
with hybrid HSE06 function method indicate that 2D Fe<sub>2</sub>Si
is a ferromagnetic half metal at its ground state with 100% spin-polarization
ratio at Fermi energy level. The phonon spectrum calculation and ab
initio molecular dynamic simulation shows that 2D Fe<sub>2</sub>Si
crystal has a high thermodynamic stability and its 2D lattice can
be retained at the temperature up to 1200 K. Monte Carlo simulations
based on the Ising model predict a Curie temperature over 780 K in
2D Fe<sub>2</sub>Si crystal, which can be further tuned by applying
a biaxial strain. Moreover, 2D structure and strong in-plane Fe–Fe
interaction endow Fe<sub>2</sub>Si nanosheet sizable magnetocrystalline
anisotropy energy with the magnitude of at least two orders larger
than those of Fe, Co and Ni bulks. These novel magnetic properties
render the 2D Fe<sub>2</sub>Si crystal a very promising material for
developing practical spintronic nanodevice
Band-Gap Engineering <i>via</i> Tailored Line Defects in Boron-Nitride Nanoribbons, Sheets, and Nanotubes
We perform a comprehensive study of the effects of line defects on electronic and magnetic properties of monolayer boron-nitride (BN) sheets, nanoribbons, and single-walled BN nanotubes using first-principles calculations and Born–Oppenheimer quantum molecular dynamic simulation. Although line defects divide the BN sheet (or nanotube) into domains, we show that certain line defects can lead to tailor-made edges on BN sheets (or imperfect nanotube) that can significantly reduce the band gap of the BN sheet or nanotube. In particular, we find that the line-defect-embedded zigzag BN nanoribbons (LD-zBNNRs) with chemically homogeneous edges such as B- or N-terminated edges can be realized by introducing a B<sub>2</sub>, N<sub>2</sub>, or C<sub>2</sub> pentagon–octagon–pentagon (5–8–5) line defect or through the creation of the antisite line defect. The LD-zBNNRs with only B-terminated edges are predicted to be antiferromagnetic semiconductors at the ground state, whereas the LD-zBNNRs with only N-terminated edges are metallic with degenerated antiferromagnetic and ferromagnetic states. In addition, we find that the hydrogen-passivated LD-zBNNRs as well as line-defect-embedded BN sheets (and nanotubes) are nonmagnetic semiconductors with markedly reduced band gap. The band gap reduction is attributed to the line-defect-induced impurity states. Potential applications of line-defect-embedded BN nanomaterials include nanoelectronic and spintronic devices
Band-Gap Engineering <i>via</i> Tailored Line Defects in Boron-Nitride Nanoribbons, Sheets, and Nanotubes
We perform a comprehensive study of the effects of line defects on electronic and magnetic properties of monolayer boron-nitride (BN) sheets, nanoribbons, and single-walled BN nanotubes using first-principles calculations and Born–Oppenheimer quantum molecular dynamic simulation. Although line defects divide the BN sheet (or nanotube) into domains, we show that certain line defects can lead to tailor-made edges on BN sheets (or imperfect nanotube) that can significantly reduce the band gap of the BN sheet or nanotube. In particular, we find that the line-defect-embedded zigzag BN nanoribbons (LD-zBNNRs) with chemically homogeneous edges such as B- or N-terminated edges can be realized by introducing a B<sub>2</sub>, N<sub>2</sub>, or C<sub>2</sub> pentagon–octagon–pentagon (5–8–5) line defect or through the creation of the antisite line defect. The LD-zBNNRs with only B-terminated edges are predicted to be antiferromagnetic semiconductors at the ground state, whereas the LD-zBNNRs with only N-terminated edges are metallic with degenerated antiferromagnetic and ferromagnetic states. In addition, we find that the hydrogen-passivated LD-zBNNRs as well as line-defect-embedded BN sheets (and nanotubes) are nonmagnetic semiconductors with markedly reduced band gap. The band gap reduction is attributed to the line-defect-induced impurity states. Potential applications of line-defect-embedded BN nanomaterials include nanoelectronic and spintronic devices
Unusual Metallic Microporous Boron Nitride Networks
Two metallic zeolite-like microporous
BN crystals with all-sp<sup>2</sup> bonding networks are predicted
from an unbiased structure
search based on the particle-swarm optimization (PSO) algorithm in
combination with first-principles density functional theory (DFT)
calculations. The stabilities of both microporous structures are confirmed
via the phonon spectrum analysis and Born–Oppenheimer molecular
dynamics simulations with temperature control at 1000 K. The unusual
metallicity for the microporous BN allotropes stems from the delocalized
p electrons along the axial direction of the micropores. Both microporous
BN structures entail large surface areas, ranging from 3200 to 3400
m<sup>2</sup>/g. Moreover, the microporous BN structures show a preference
toward organic molecule adsorption (e.g., the computed adsorption
energy for CH<sub>3</sub>CH<sub>2</sub>OH is much more negative than
that of H<sub>2</sub>O). This preferential adsorption can be exploited
for water cleaning, as demonstrated recently using porous boron BN
nanosheets (Nat. Commun. 2013, 4, 1777)
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