1,290 research outputs found
Few-Layer C2N: A Promising Metal-free Photocatalyst for Water Splitting
Successful synthesis of the nitrogenated holey two-dimensional structures C2N
(Nat. Commun. 2015, 6, 6486) using simply wet-chemical reaction offer a
cost-effective way to generate other 2D materials with novel optical and
electronic properties. On basis of the density functional theory calculations,
we investigate electronic properties of monolayer and multilayer C2N. We find
that few-layer C2N have a direct bandgap and the direct bandgap of the system
can vary from 2.47 eV for monolayer to 1.84 eV for a five-layer. Besides, for
the few-layer C2N, appropriate band gap, band edge alignments, and strong
visible-light absorption demonstrate it may be a potential metal-free
visible-light driven photocatalyst for water splitting.Comment: 11 pages, 3 figures. arXiv admin note: text overlap with
arXiv:1505.0258
First-principles study of two-dimensional van der Waals heterojunctions
Research on graphene and other two-dimensional (2D) materials, such as
silicene, germanene, phosphorene, hexagonal boron nitride (h-BN), graphitic
carbon nitride (g-C3N4), graphitic zinc oxide (g-ZnO) and molybdenum disulphide
(MoS2), has recently received considerable interest owing to their outstanding
properties and wide applications. Looking beyond this field, combining the
electronic structures of 2D materials in ultrathin van der Waals
heterojunctions has also emerged to widely study theoretically and
experimentally to explore some new properties and potential applications beyond
their single components. Here, this article reviews our recent theoretical
studies on the structural, electronic, electrical and optical properties of 2D
van der Waals heterojunctions using density functional theory calculations,
including the Graphene/Silicene, Graphene/Phosphorene, Graphene/g-ZnO,
Graphene/MoS2 and g-C3N4/MoS2 heterojunctions. Our theoretical simulations,
designs and calculations show that novel 2D van der Waals heterojunctions
provide a promising future for electronic, electrochemical, photovoltaic,
photoresponsive and memory devices in the experiments.Comment: 12 pages, 5 figures in Computational Materials Science (2015). arXiv
admin note: text overlap with arXiv:1411.035
The Moving-Grid Effect in the Harmonic Vibrational Frequency Calculations with Numeric Atom-Centered Orbitals
When using atom-centered integration grids, the portion of the grid that
belongs to a certain atom also moves when this atom is displaced. In the paper,
we investigate the moving-grid effect in the calculation of the harmonic
vibrational frequencies when using all-electron full-potential numeric
atomic-centered orbitals as the basis set. We find that, unlike the first order
derivative (i.e., forces), the moving-grid effect plays an essential role for
the second order derivatives (i.e., vibrational frequencies). Further analysis
reveals that predominantly diagonal force constant terms are affected, which
can be bypassed efficiently by invoking translational symmetry. Our approaches
have been demonstrated in both finite (molecules) and extended (periodic)
systems
Defect in Phosphorene
Defects are inevitably present in materials and always can affect their
properties. Here, first-principles calculations are performed to systematically
study the stability, structural and electronic properties of ten kinds of point
defects in semiconducting phosphorene, including the Stone-Wales (SW-1 and
SW-2) defect, single (SV59 and SV5566) and double vacancy (DV585-1, DV585-2,
DV555777-1, DV555777-2, DV555777-3 and DV4104) defects. We find that these
defects are all much easily created in phosphorene with higher areal density
compared with graphene and silicene. They are easy distinguish each other and
correlate with their defective atomic structures with simulated scanning
tunneling microscopy images at positive bias. The SW, DV585-1, DV555777 and
DV4104 defects have little effect on phosphorene's electronic properties and
defective phosphorene monolayers still show semiconducting with similar band
gap values to perfect phosphorene. The SV59 and DV585-2 defects can introduce
unoccupied localized states into phosphorene's fundamental band gap.
Specifically, the SV59 and 5566 defects can induce hole doping in phosphorene,
and only the stable SV59 defect can result in local magnetic moments in
phosphorene different from all other defects.Comment: 5 pages, 4 figure
New Insight into Electronic Shells of Metal Clusters: Analogues of Simple Molecules
Using Li clusters, a prototype of simple metals, as a test case, we
theoretically find that metal clusters can mimic the behavior of simple
molecules in electronic shells. It is found that Li14, Li10, and Li8 clusters
are exact analogues of F2, N2, and CH4 molecules
Control of Spin in La(Mn,Zn)AsO Alloy by Carrier Doping
The control of spin without magnetic field is one of challenges in developing
spintronic devices. In an attempt to solve this problem, we proposed a novel
hypothetic LaMn0.5Zn0.5AsO alloy from two experimentally synthesized rare earth
element transition metal arsenide oxides, i.e. LaMnAsO and LaZnAsO. On the
basis of the first-principles calculations with strong-correlated correction,
we found that the LaMn0.5Zn0.5AsO alloy is an antiferromagnetic semiconductor
at ground state, while bipolar magnetic semiconductor at ferromagnetic state.
Both electron and hole doping in the LaMn0.5Zn0.5AsO alloy induces the
transition from antiferromagnetic to ferromagnetic, as well as semiconductor to
half metal. In particular, the spin-polarization direction is switchable
depending on the doped carrier's type. As carrier doping can be realized easily
in experiment by applying a gate voltage, the LaMn0.5Zn0.5AsO alloy stands for
a promising spintronic material to generate and control the spin-polarized
carriers with electric field.Comment: 16 pages, 4 figure
Structure of graphene oxide: thermodynamics versus kinetics
Graphene oxide (GO) is an important intermediate to prepare graphene and it
is also a versatile material with various applications. However, despite its
importance, the detailed structure of GO is still unclear. For example,
previous theoretical studies based on energetics have suggested that hydroxyl
chain is an important structural motif of GO, which, however, is found to be
contrary to nuclear magnetic resonance (NMR) experiment. In this study, we
check both thermodynamic and kinetic aspects missed previously. First
principles thermodynamics gives a free energy based stability ordering similar
to that based on energetics, and hydroxyl chain is thus thermodynamically still
favorable. At the same time, by checking the calculated vibrational
frequencies, we note that hydroxyl chain structure is also inconsistent with
infrared experiment. Therefore, kinetics during GO synthesis is expected to
make an important role in GO structure. Transition state calculations predict
large energy barriers between local minima, which suggests that experimentally
obtained GO has a kinetically constrained structure
Water on Silicene: Hydrogen Bond Autocatalysis Induced Physisorption-Chemisorption-Dissociation Transition
A single water molecule has nothing special. However, macroscopic water
displays many anomalous properties at the interface, such as a high surface
tension, hydrophobicity and hydrophillicity. Although the underlying mechanism
is still elusive, hydrogen bond is expected to have played an important role.
An interesting question is if the few-water molecule clusters will be
qualitatively different from a single molecule. Using adsorption behavior as an
example, by carefully choosing two-dimensional silicene as the substrate
material, we demonstrate that water monomer, dimer and trimer show contrasting
properties. The additional water molecules in dimer and trimer induce a
transition from physisorption to chemisorption then to dissociation on
silicene. Such a hydrogen bond autocatalytic effect is expected to have a broad
application potential in silicene-based water molecule sensor and metal-free
catalyst for water dissociation.Comment: 7 pages, 6 figure
Electronic Structure of Large-Scale Graphene Nanoflakes
With the help of the recently developed SIESTA-PEXSI method [J. Phys.:
Condens. Matter \textbf{26}, 305503 (2014)], we perform Kohn-Sham density
functional theory (DFT) calculations to study the stability and electronic
structure of hexagonal graphene nanoflakes (GNFs) with up to 11,700 atoms. We
find the electronic properties of GNFs, including their cohesive energy,
HOMO-LUMO energy gap, edge states and aromaticity, depend sensitively on the
type of edges (ACGNFs and ZZGNFs), size and the number of electrons. We observe
that, due to the edge-induced strain effect in ACGNFs, large-scale ACGNFs'
cohesive energy decreases as their size increases. This trend does not hold for
ZZGNFs due to the presence of many edge states in ZZGNFs. We find that the
energy gaps of GNFs all decay with respect to , where is the
size of the GNF, in a linear fashion. But as their size increases, ZZGNFs
exhibit more localized edge states. We believe the presence of these states
makes their gap decrease more rapidly. In particular, when is larger than
6.40 , we find that ZZGNFs exhibit metallic characteristics. Furthermore,
we find that the aromatic structures of GNFs appear to depend only on whether
the system has or electrons, where is an integer.Comment: 11 pages, 9 figure
Single layer of MX3 (M=Ti, Zr; X=S, Se, Te): a new platform for nano-electronics and optics
A serial of two dimensional titanium and zirconium trichalcogenides
nanosheets MX3 (M=Ti, Zr; X=S, Se, Te) are investigated based on
first-principles calculations. The evaluated low cleavage energy indicates that
stable two dimensional monolayers can be exfoliated from their bulk crystals in
experiment. Electronic studies reveal very rich electronic properties in these
monolayers, including metallic TiTe3 and ZrTe3, direct band gap semiconductor
TiS3 and indirect band gap semiconductors TiSe3, ZrS3 and ZrSe3. The band gaps
of all the semiconductors are between 0.57~1.90 eV, which implies their
potential applications in nano-electronics. And the calculated effective masses
demonstrate highly anisotropic conduction properties for all the
semiconductors. Optically, TiS3 and TiSe3 monolayers exhibit good light
absorption in the visible and near-infrared region respectively, indicating
their potential applications in optical devices. In particular, the highly
anisotropic optical absorption of TiS3 monolayer suggests it could be used in
designing nano optical waveguide polarizers.Comment: 5 pages, 4 figures, 2 table
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