318,454 research outputs found
Intrinsic Defects and Electronic Conductivity of TaON: First-Principles Insights
As a compound in between the tantalum oxide and nitride, the tantalum
oxynitride TaON is expected to combine their advantages and act as an efficient
visible-light-driven photocatalyst. In this letter, using hybrid functional
calculations we show that TaON has different defect properties from the binary
tantalum oxide and nitride: (i) instead of O or N vacancies or Ta
interstitials, the antisite is the dominant defect, which determines its
intrinsic n-type conductivity and the p-type doping difficulty; (ii) the
antisite has a shallower donor level than O or N vacancies, with a delocalized
distribution composed mainly of the Ta orbitals, which gives rise to
better electronic conductivity in the oxynitride than in the oxide and nitride.
The phase stability analysis reveals that the easy oxidation of TaON is
inevitable under O rich conditions, and a relatively O poor condition is
required to synthesize stoichiometric TaON samples
Carrier hopping in disordered semiconducting polymers: How accurate is the Miller-Abrahams model?
We performed direct calculations of carrier hopping rates in strongly
disordered conjugated polymers based on the atomic structure of the system, the
corresponding electronic states and their coupling to all phonon modes. We
found that the dependence of hopping rates on distance and the dependence of
the mobility on temperature are significantly different than the ones stemming
from the simple Miller-Abrahams model, regardless of the choice of the
parameters in the model. A new model that satisfactorily describes the hopping
rates in the system and avoids the explicit calculation of electron-phonon
coupling constants was then proposed and verified. Our results indicate that,
in addition to electronic density of states, the phonon density of states and
the spatial overlap of the wavefunctions are the quantities necessary to
properly describe carrier hopping in disordered conjugated polymers.Comment: the final version accepted for publication in Appl. Phys. Let
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Role of initial magnetic disorder: A time-dependent ab initio study of ultrafast demagnetization mechanisms.
Despite more than 20 years of development, the underlying physics of the laser-induced demagnetization process is still debated. We present a fast, real-time time-dependent density functional theory (rt-TDDFT) algorithm together with the phenomenological atomic Landau-Lifshitz-Gilbert model to investigate this problem. Our Hamiltonian considers noncollinear magnetic moment, spin-orbit coupling (SOC), electron-electron, electron-phonon, and electron-light interactions. The algorithm for time evolution achieves hundreds of times of speedup enabling calculation of large systems. Our simulations yield a demagnetization rate similar to experiments. We found that (i) the angular momentum flow from light to the system is not essential and the spin Zeeman effect is negligible. (ii) The phonon can play a role but is not essential. (iii) The initial spin disorder and the self-consistent update of the electron-electron interaction play dominant roles and enhance the demagnetization to the experimentally observed rate. The spin disorder connects the electronic structure theory with the phenomenological three-temperature model
Thermodynamic Oxidation and Reduction Potentials of Photocatalytic Semiconductors in Aqueous Solution
We introduce an approach to calculate the thermodynamic oxidation and
reduction potentials of semiconductors in aqueous solution. By combining a
newly-developed ab initio calculation for compound formation energy and band
alignment with electrochemistry experimental data, this approach can be used to
predict the stability of almost any compound semiconductor in aqueous solution.
30 photocatalytic semiconductors have been studied, and a graph (a simplified
Pourbaix diagram) showing their valence/conduction band levels and
oxidation/reduction potentials is produced. Based on this graph, we have
studied the stabilities and trends against the oxidative and reductive
photocorrosion for compound semiconductors. We found that, only metal oxides
can be thermodynamically stable when used as the n-type photoanodes. All the
non-oxides are unstable due to easy oxidation by the photogenerated holes, but
they can be resistant to the reduction by electrons, thus stable as the p-type
photocathodes
Quantum Transport Calculations Using Periodic Boundary Conditions
An efficient new method is presented to calculate the quantum transports
using periodic boundary conditions. This method allows the use of conventional
ground state ab initio programs without big changes. The computational effort
is only a few times of a normal ground state calculation, thus it makes
accurate quantum transport calculations for large systems possible.Comment: 9 pages, 6 figure
Overlapping fragments method for electronic structure calculation of large systems
We present a method for the calculation of electronic structure of systems
that contain tens of thousands of atoms. The method is based on the division of
the system into mutually overlapping fragments and the representation of the
single-particle Hamiltonian in the basis of eigenstates of these fragments. In
practice, for the range of system size that we studied (up to tens of thousands
of atoms), {the dominant part of the calculation scales} linearly with the size
of the system when all the states within a fixed energy interval are required.
The method is highly suitable for making good use of parallel computing
architectures. We illustrate the method by applying it to diagonalize the
single-particle Hamiltonian obtained using the density functional theory based
charge patching method in the case of amorphous alkane and polythiophene
polymers.Comment: 9 pages, 10 figures, the version accepted in J. Chem. Phy
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