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In a traditional classical force-field model, the atomic point charge that generates the electrostatic potential, and the Born charge induced by atomic movement, are represented by the same charge parameter. But their actual values can be very different, and correct values for both of them are needed in order to yield the correct atomic structure (electrostatic charge) and phonon spectrum (Born charge). This is particularly true for nanostructure calculations. Here, we introduce a double-charge model (DCM) to reconcile the difference between the electrostatic charge and Born charge. The DCM allows us to reproduce the accurate ab initio phonon spectrum not only in bulk systems, but also for nanostructures (slabs and nanowires). This enables the use of classical force fields to study phonon spectra of large nanostructures, which are important for many phenomena from carrier dynamics to thermo conductivities. [ABSTRACT FROM AUTHOR]

Rather than being free carriers or separated single-hole polarons, double holes in anatase TiO2 prefer binding with each other, to form an O-O dimer after large structural distortion. This pushes the hole states upward into the conduction band and traps the holes. Similar double-hole-induced O-O dimerization (a bipolaron) exists also in other transition metal oxides (TMOs) such as V2O5 and MoO3, which have the highest valence bands composed mainly of O 2p states, loose lattices, and short O-O distances. Since the dimerization can happen in impurity-free TMO lattices, independent of any extrinsic dopant, it acts as an intrinsic and general limit to the p-type conductivity in these TMOs. [ABSTRACT FROM AUTHOR]

The dynamic responses of a rhodium catalyst and a graphene sheet are investigated upon random excitation with 80 kV electrons. An extraordinary electron microscope stability and resolution allow studying temporary atom displacements from their equilibrium lattice sites into metastable sites across projected distances as short as 60 pm. In the rhodium catalyst, directed and reversible atom displacements emerge from excitations into metastable interstitial sites and surface states that can be explained by single atom trajectories. Calculated energy barriers of 0.13 eV and 1.05 eV allow capturing single atom trapping events at video rates that are stabilized by the Rh [110] surface corrugation. Molecular dynamics simulations reveal that randomly delivered electrons can also reversibly enhance the sp3 and the sp1 characters of the sp2-bonded carbon atoms in graphene. The underlying collective atom motion can dynamically stabilize characteristic atom displacements that are unpredictable by single atom trajectories. We detect three specific displacements and use two of them to propose a path for the irreversible phase transformation of a graphene nanoribbon into carbene. Collectively stabilized atom displacements greatly exceed the thermal vibration amplitudes described by Debye-Waller factors and their measured dose rate dependence is attributed to tunable phonon contributions to the internal energy of the systems. Our experiments suggest operating electron microscopes with beam currents as small as zepto-amperes/nm2 in a weak-excitation approach to improve on sample integrity and allow for time-resolved studies of conformational object changes that probe for functional behavior of catalytic surfaces or molecules. [ABSTRACT FROM AUTHOR]

Nonradiative carrier recombination is of both applied and fundamental interest. Here a novel algorithm is introduced to calculate such a deep level nonradiative recombination rate using the ab initio density functional theory. This algorithm can calculate the electron-phonon coupling constants all at once. An approximation is presented to calculate the phonon modes for one impurity in a large supercell. The neutral Zn impurity site together with a N vacancy is considered as the carrier-capturing deep impurity level in bulk GaN. Its capture coefficient is calculated as 5.57 ×10-10 cm3/s at 300 K. We found that there is no apparent onset of such a nonradiative process as a function of temperature. [ABSTRACT FROM AUTHOR]

There are materials that exist in unusual solid-liquid hybrid phases, for example, the superionics at high temperatures of 700 °C. Using ab initio molecular dynamics, we show that the intensely studied Cu2S high chalcocite phase is actually a solid-liquid hybrid phase which exists in relatively low temperature (> 105 °C). Its formation mechanism is different from the superionics. We also show that the previously proposed atomic structure for high chalcocite is incorrect, and the low chalcocite to high chalcocite transition should be described as a sublattice solid to liquid transition. [ABSTRACT FROM AUTHOR]

We critically examine two nominally equivalent approaches for treating a random alloy: (1) by using one very large supercell as a direct simulation of the alloy and (2) by performing configuration averaging over many smaller supercells; and the common practice using a virtual crystal as the reference for analyzing the alloy band structure and discussing the electronic transport in the alloy. Specifically, (1) we show that, in practice, the size of the "very large" supercell depends on the particular property of interest, and the ideal of configuration averaging is only useful for certain properties. (2) We also examine the assumed equivalency by comparing the results of the two approaches in band-gap energy, energy fluctuation, and intervalley and intravalley scattering, and conclude that the two approaches often lead to nonequivalent physics. (3) We use a generalized moment method that is capable of computing the global electronic structure of a sufficiently large supercell (e.g., ~260 000 atoms) to obtain the intrinsic broadening of a Γ-like electron state caused by the "inelastic" intravalley scattering in a direct-band-gap semiconductor alloy. (4) We demonstrate an efficient way to construct the effective dispersion curves of the alloy with high accuracy for calculating effective masses and examining anisotropy and nonparabolicity of the dispersion curve. (5) Finally, we discuss the limitation of using the virtual-crystal approximation as the reference for evaluating alloy scattering and studying transport properties. [ABSTRACT FROM AUTHOR]

Nonisovalent alloys, also known as aliovalent alloys, are formed by mixing two semiconductors of different valences for both cations and anions. These alloys exhibit many interesting properties and have found applications in optoelectronics, refractory materials, photovoltaics, photocatalytic splitting of water, etc. For example, the alloy of GaN and ZnO has a surprisingly large band gap bowing, enabling visible light absorption and overall water splitting at a record quantum efficiency. The understanding of the properties of nonisovalent alloys, however, is hindered by the lack of knowledge of the detailed atomic structures. We recently developed a charge flow model which can predict the total energy of different atomic configurations of nonisovalent alloys. In this work, we extend this model and apply it to a number of alloy systems-GaN/ZnO, GaAs/ZnSe, InP/CdS, AlN/ZnO, AlN/MgO, and AlN/SiC in wurtzite, zinc blende, and/or rocksalt structures. Good agreement between the model-predicted and ab initio results is found. We also employ the charge flow model in parallel tempering Monte Carlo simulations to calculate the thermodynamic properties of nonisovalent alloys of wurtzite structure. A phase transition between the phase separated and alloying regions is found and a general phase diagram is obtained. This model could be used to guide the design and synthesis of new nonisovalent alloy materials. [ABSTRACT FROM AUTHOR]

We present a first-principles approach to calculate the phonon thermal conductivity based on the Green-Kubo formalism. In this approach, the density functional theory energy is distributed to each atom, and a two-step method in the molecular dynamics is introduced to avoid the atomic position R wrapping problem in a periodic system when the heat current is calculated. We show that this first-principles Green-Kubo approach is particularly suitable for disordered systems like amorphous and liquid, where the thermal conductivities are small due to strong phonon scattering but difficult to be calculated using anharmonic interaction energy. We have applied our method to liquid Ar, liquid Si, and amorphous Si. The calculated thermal conductivities agree well with previous theoretical and experimental results. We have also compared our method to previous works combining first-principles simulations with the Green-Kubo formalism. [ABSTRACT FROM AUTHOR]

Self-consistent solutions of the Dyson equation are obtained using a plane-wave basis set for seven small molecules. Such self-consistent solutions can help to unify the different GW self-consistent schemes, reduce the scatter of results in current GW calculations, and shed light on the true effects of GW self-consistency. Unlike other works of self-consistent GW calculations, in the present work the Green's function is expressed as a matrix under the plane-wave basis set. The algorithmic details which enable such calculations are presented. The ability to solve the full Green's function using a plane-wave basis set may open the door for future beyond-GW many-body perturbation theory calculations. [ABSTRACT FROM AUTHOR]

We have developed an efficient real-time time-dependent density functional theory (TDDFT) method that can increase the effective time step from <1 as in traditional methods to 0.1-0.5 fs. With this algorithm, the TDDFT simulation can have comparable speed to the Born-Oppenheimer (BO) ab initio molecular dynamics (MD). As an application, we simulated the process of an energetic Cl particle colliding onto a monolayer of MoSe2. Our simulations show a significant energy transfer from the kinetic energy of the Cl particle to the electronic energy of MoSe2, and the result of TDDFT is very different from that of BO-MD simulations. [ABSTRACT FROM AUTHOR]

We have studied the electron hopping in a two-CdSe quantum dot (QD) system linked by an azobenzene-derived light-switching molecule. This system can be considered as a prototype of a QD supercrystal. Following the computational strategies given in our recent work [I.-H. Chu et al., J. Phys. Chem. C 115, 21409 (2011)], we have investigated the effects of molecular attachment, molecular isomer (trans and cis), and QD size on the electron hopping rate using Marcus theory. Our results indicate that molecular attachment has a large impact on the system for both isomers. In the most energetically favorable attachment, the cis isomer provides significantly greater coupling between the two QDs and hence the electron hopping rate is greater compared to the trans isomer. As a result, the carrier mobility of the QD array in the low carrier density, weak external electric-field regime is several orders of magnitude higher in the cis compared to the trans configuration. This demonstration of mobility modulation using QDs and azobenzene could lead to an alternative type of switching device. [ABSTRACT FROM AUTHOR]

We present a large-scale nonadiabatic molecular dynamics simulation to study carrier transport in an organic monolayer. This simulation calculates a 4802-atom system for 825 fs in about 3 h using 51 744 computer cores, while deploying a plane-wave pseudopotential density-functional theory Hamiltonian. A new approach is developed that makes such large-scale calculation possible. Our simulation on the pentathiophene butyric acid monolayer reveals the mechanism for the carrier transport in the system: the hole wave functions are localized by thermal fluctuation-induced disorder, while the hole transport is via charge transfer during state energy crossing. The simulation also shows that the system is never in a thermodynamic equilibrium in terms of adiabatic-state populations according to Boltzmann distribution. [ABSTRACT FROM AUTHOR]

We propose a systematic approach that can empirically correct three major errors typically found in a density functional theory (DFT) calculation within the local density approximation (LDA) simultaneously for a set of common cation binary semiconductors, such as III-V compounds, (Ga or In)X with X=N,P,As,Sb, and II-VI compounds, (Zn or Cd)X, with X=O,S,Se,Te. By correcting (1) the binary band gaps at high-symmetry points Γ, L, X, (2) the separation of p-and d-orbital-derived valence bands, and (3) conduction band effective masses to experimental values and doing so simultaneously for common cation binaries, the resulting DFT-LDA-based quasi-first-principles method can be used to predict the electronic structure of complex materials involving multiple binaries with comparable accuracy but much less computational cost than a GW level theory. This approach provides an efficient way to evaluate the electronic structures and other material properties of complex systems, much needed for material discovery and design. [ABSTRACT FROM AUTHOR]

Orbital-free (OF) density functional theory (DFT) directly solves for the electron density rather than the wave function of many electron systems, greatly simplifying and enabling large scale first principles simulations. However, the required approximate noninteracting kinetic energy density functionals and local electron-ion pseudopotentials severely restrict the general applicability of conventional OFDFT. Here, we present a new generation of OFDFT called angular-momentum-dependent (AMD)-OFDFT to harness the accuracy of Kohn-Sham DFT and the simplicity of OFDFT. The angular momenta of electrons are explicitly introduced within atom-centered spheres so that the important ionic core region can be accurately described. In addition to conventional OF total energy functionals, we introduce a crucial nonlocal energy term with a set of AMD energies to correct errors due to the kinetic energy density functional and the local pseudopotential. We find that our AMD-OFDFT formalism offers substantial improvements over conventional OFDFT, as we show for various properties of the transition metal titanium. [ABSTRACT FROM AUTHOR]

An ab initio method is presented to calculate shallow impurity levels in bulk semiconductors. This method combines the GW calculation for the treatment of the central-cell potential with a potential patching method for large systems (with 64000 atoms) to describe the impurity state wave functions. The calculated acceptor levels in Si, GaAs, and an isovalent bound state of GaP are in excellent agreement with experiments with a root-mean-square error of 8.4 meV. [ABSTRACT FROM AUTHOR]

As Moore's law approaches its end, two-dimensional (2D) materials are intensely studied for their potentials as one of the "More than Moore' (MM) devices. However, the ultimate performance limits and the optimal design parameters for such devices are still unknown. One common problem for the 2D-material-based device is the relative weak on-current. In this study, two-dimensional Schottky-barrier field-effect transistors (SBFETs) consisting of in-plane heterojunctions of 1T metallic-phase and 2H semiconducting-phase transition-metal dichalcogenides (TMDs) are studied following the recent experimental synthesis of such devices at a much larger scale. Our ab initio simulation reveals the ultimate performance limits of such devices and offers suggestions for better TMD materials. Our study shows that the Schottky-barrier heights (SBHs) of the in-plane 1T/2H contacts are smaller than the SBHs of out-of-plane contacts, and the contact coupling is also stronger in the in-plane contact. Due to the atomic thickness of the monolayer TMD, the average subthreshold swing of the in-plane TMD-SBFETs is found to be close to the limit of 60 mV/dec, and smaller than that of the out-of-plane TMD-SBFET device. Different TMDs are considered and it is found that the in-plane WTe2- SBFET provides the best performance and can satisfy the performance requirement of the sub-10-nm high-performance transistor outlined by the International Technology Roadmap for Semiconductors, and thus could be developed into a viable sub-10-nm MM device in the future. [ABSTRACT FROM AUTHOR]

Ab initio atomic relaxations often take large numbers of steps and long times to converge, especially when the initial atomic configurations are far from the local minimum or there are curved and narrow valleys in the multidimensional potentials. An atomic relaxation method based on on-the-flight force learning and a corresponding curved-line search algorithm is presented to accelerate this process. Results demonstrate the superior performance of this method for metal and magnetic clusters when compared with the conventional conjugate-gradient method. [ABSTRACT FROM AUTHOR]

The GW approximation is a well-known method to obtain the quasiparticle and spectral properties of systems ranging from molecules to solids. In practice, GW calculations are often employed with many different approximations and truncations. In this work, we describe the implementation of a fully self-consistent GW approach based on the solution of the Dyson equation using a plane wave basis set. Algorithmic, numerical, and technical details of the self-consistent GW approach are presented. The fully self-consistent GW calculations are performed for GaAs, ZnO, and CdS including semicores in the pseudopotentials. No further approximations and truncations apart from the truncation on the plane wave basis set are made in our implementation of the GW calculation. After adopting a special potential technique, a ~100Ry energy cutoff can be used without the loss of accuracy. We found that the self-consistent GW (sc-GW) significantly overestimates the bulk band gaps, and this overestimation is likely due to the underestimation of the macroscopic dielectric constants. On the other hand, the sc-GW accurately predicts the d-state positions, most likely because the d-state screening does not sensitively depend on the macroscopic dielectric constant. Our work indicates the need to include the high-order vertex term in order for the many-body perturbation theory to accurately predict the semiconductor band gaps. It also sheds some light on why, in some cases, the G0W0 bulk calculation is more accurate than the fully self-consistent GW calculation, because the initial density-functional theory has a better dielectric constant compared to experiments. [ABSTRACT FROM AUTHOR]