87 research outputs found
Strain Engineering of Antimonene by a First-principles Study: Mechanical and Electronic Properties
In this work, we investigate the mechanical and electronic properties of
monolayer antimonene in its most stable beta-phase using first-principles
calculations. The upper region of its valence band is found to solely consist
of lone pair p-orbital states, which are by nature more delocalized than the
d-orbital states in transition metal dichalcogenides, implying superior
transport performance of antimonene. The Young's and shear moduli of
beta-antimonene are observed to be ~25% higher than those of bulk antimony,
while the hexagonal lattice constant of the monolayer reduces significantly
(~5%) from that in bulk, indicative of strong inter-layer coupling. The ideal
tensile test of beta-antimonene under applied uniaxial strain highlights ideal
strengths of 6 GPa and 8 GPa, corresponding to critical strains of 15% and 17%
in the zigzag and armchair directions, respectively. During the deformation
process, the structural integrity of the material is shown to be better
preserved, albeit moderately, in the armchair direction. Interestingly, the
application of uniaxial strain in the zigzag and armchair directions unveil
direction-dependent trends in the electronic band structure. We find that the
nature of the band gap remains insensitive to strain in the zigzag direction,
while strain in the armchair direction activates an indirect-direct band gap
transition at a critical strain of 4%, owing to a band switching mechanism. The
curvature of the conduction band minimum increases during the transition, which
suggests a lighter effective mass of electrons in the direct-gap configuration
than in the free-standing state of equilibrium. The work function of
free-standing beta-antimonene is 4.59 eV and it attains a maximum value of 5.07
eV under an applied biaxial strain of 4%
Effects of Graphene/BN Encapsulation, Surface Functionalization and Molecular Adsorption on the Electronic Properties of Layered InSe: A First-Principles Study
By using first-principles calculations, we investigated the effects of
graphene/boron nitride (BN) encapsulation, surface functionalization by
metallic elements (K, Al, Mg and typical transition metals) and molecules
(tetracyanoquinodimethane (TCNQ) and tetracyanoethylene (TCNE)) on the
electronic properties of layered indium selenide (InSe). It was found that an
opposite trend of charge transfer is possible for graphene (donor) and BN
(acceptor), which is dramatically different from phosphorene where both
graphene and BN play the same role (donor). For InSe/BN heterostructure, a
change of the interlayer distance due to an out-of-plane compression can
effectively modulate the band gap. Strong acceptor abilities to InSe were found
for the TCNE and TCNQ molecules. For K, Al and Mg-doped monolayer InSe, the
charge transfer from K and Al atoms to the InSe surface was observed, causing
an n-type conduction of InSe, while p-type conduction of InSe observed in case
of the Mg-doping. The atomically thin structure of InSe enables the possible
observation and utilization of the dopant-induced vertical electric field
across the interface. A proper adoption of the n- or p-type dopants allows for
the modulation of the work function, the Fermi level pinning, the band bending,
and the photo-adsorbing efficiency near the InSe surface/interface.
Investigation on the adsorption of transition metal atoms on InSe showed that
Ti-, V-, Cr-, Mn-, Co-adsorbed InSe are spin-polarized, while Ni-, Cu-, Pd-,
Ag- and Au-adsorbed InSe are non-spin-polarized. Our results shed lights on the
possible ways to protect InSe structure and modulate its electronic properties
for nanoelectronics and electrochemical device applications
Exploring the Charge Localization and Band Gap Opening of Borophene: A First-Principles Study
Recently synthesized two-dimensional (2D) boron, borophene, exhibits a novel
metallic behavior rooted in the s-p orbital hybridization, distinctively
different from other 2D materials such as sulfides/selenides and semi-metallic
graphene. This unique feature of borophene implies new routes for charge
delocalization and band gap opening. Herein, using first-principles
calculations, we explore the routes to localize the carriers and open the band
gap of borophene via chemical functionalization, ribbon construction, and
defect engineering. The metallicity of borophene is found to be remarkably
robust against H- and F-functionalization and the presence of vacancies.
Interestingly, a strong odd-even oscillation of the electronic structure with
width is revealed for H-functionalized borophene nanoribbons, while an
ultra-high work function (~ 7.83 eV) is found for the F-functionalized
borophene due to its strong charge transfer to the atomic adsorbates
Large Electronic Anisotropy and Enhanced Chemical Activity of Highly Rippled Phosphorene
We investigate the electronic structure and chemical activity of rippled
phosphorene induced by large compressive strains via first-principles
calculation. It is found that phosphorene is extraordinarily bendable, enabling
the accommodation of ripples with large curvatures. Such highly rippled
phosphorene shows a strong anisotropy in electronic properties. For ripples
along the armchair direction, the band gap changes from 0.84 to 0.51 eV for the
compressive strain up to -20% and further compression shows no significant
effect, for ripples along the zigzag direction, semiconductor to metal
transition occurs. Within the rippled phosphorene, the local electronic
properties, such as the modulated band gap and the alignments of frontier
orbitals, are found to be highly spatially dependent, which may be used for
modulating the injection and confinement of carriers for optical and
photovoltaic applications. The examination of the interaction of a physisorbed
NO molecule with the rippled phosphorene under different compressive strains
shows that the chemical activities of the phosphorene are significantly
enhanced at the top and bottom peaks of the ripples, indicated by the enhanced
adsorption and charge transfer between them. All these features can be ascribed
to the effect of curvatures, which modifies the orbital coupling between atoms
at the ripple peaks
Exploring Mechanisms of Hydration and Carbonation of MgO and Mg(OH)2 in Reactive Magnesium Oxide-based Cements
Reactive magnesium oxide (MgO)-based cement (RMC) can play a key role in
carbon capture processes. However, knowledge on the driving forces that control
the degree of carbonation and hydration and rate of reactions in this system
remains limited. In this work, density functional theory-based simulations are
used to investigate the physical nature of the reactions taking place during
the fabrication of RMCs under ambient conditions. Parametric indicators such as
adsorption energies, charge transfer, electron localization function,
adsorption/dissociation energy barriers and the mechanisms of interaction of
H2O and CO2 molecules with MgO and brucite (Mg(OH)2) clusters are considered.
The following hydration and carbonation interactions relevant to RMCs are
evaluated i) carbonation of MgO, ii) hydration of MgO, carbonation of hydrated
MgO, iii) carbonation of Mg(OH)2, iv) hydration of Mg(OH)2 and v) hydration of
carbonated Mg(OH)2. A comparison of the energy barriers and reaction pathways
of these mechanisms shows that the carbonation of MgO is hindered by presence
of H2O molecules, while the carbonation of Mg(OH)2 is hindered by the formation
of initial carbonate and hydrate layers as well as presence of excessed H2O
molecules. To compare these finding to bulk mineral surfaces, the interactions
of the CO2 and H2O molecules with the MgO(001) and Mg(OH)2 (001) surfaces are
studied. Therefore, this work presents deep insights into the physical nature
of the reactions and the mechanisms involved in hydrated magnesium carbonates
production that can be beneficial for its development
Atomic-scale mechanisms of defect- and light-induced oxidation and degradation of InSe
Layered indium selenide (InSe), a new two-dimensional (2D) material with a
hexagonal structure and semiconducting characteristic, is gaining increasing
attention owing to its intriguing electronic properties. Here, by using
first-principles calculations, we reveal that perfect InSe possesses a high
chemical stability against oxidation, superior to MoS2. However, the presence
of intrinsic Se vacancy (VSe) and light illumination can markedly affect the
surface activity. In particular, the excess electrons associated with the
exposed In atoms at the VSe site under illumination are able to remarkably
reduce the dissociation barrier of O2 to ~0.2 eV. Moreover, at ambient
conditions, the splitting of O2 enables the formation of substitutional
(apical) oxygen atomic species, which further cause the trapping and subsequent
rapid splitting of H2O molecules and ultimately the formation of hydroxyl
groups. Our findings uncover the causes and underlying mechanisms of InSe
surface degradation via the defect-photo promoted oxidations. Such results will
be beneficial in developing strategies for the storage of InSe material and its
applications for surface passivation with boron nitride, graphene or In-based
oxide layers
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