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Understanding the defect chemistry of lead-halide perovskites and its effects on the hot-carrier lifetime is of significance for both fundamental understanding and applications as solar cell light absorbing materials. In this study, the mechanistic details of hot carrier decay in hybrid perovskites are investigated using a newly developed non-adiabatic molecular dynamics method. In this approach, the nuclear trajectory is based on Born–Oppenheimer ground state molecular dynamics, which is then followed by the evolution of carrier wave function including the detailed balance and decoherence effects. We found the longer decay time for hot electrons due to the incorporation of interstitial iodine in the hybrid lead-halide perovskites (MAPbI3), while the hot hole decay time is not affected significantly by the interstitial iodine. The underlying mechanism for such modulation of hot carrier dynamics is attributed to the changes of carrier density of states and the electron–phonon coupling strength. Hence, iodine interstitial is the necessary condition to create long-lived hot electrons in perovskites, which is further demonstrated by the comparative analysis with the pure MAPbI3.

The separate effects of strain and film thickness on the antiferromagnetic-to-ferromagnetic phase transition temperature of FeRh thin films by both experiment and density functional calculations were determined. Strain was introduced by epitaxial growth onto MgO, SrTiO3, and KTaO3 substrates. Film thicknesses below 15 nm substantially suppress the transition temperature, T*, to below room temperature in unstrained films. For strained films, tensile/compressive strain decreases/increases T*, respectively. KTaO3 (001) substrates produce sufficient compressive strain to increase the transition temperature of 10 nm FeRh films above room temperature, which is useful for many proposed applications previously limited by the stabilization of the ferromagnetic state at small thicknesses. These results demonstrate that a judicious use of film thickness and substrate can be used to manipulate FeRh's transition temperature over a ∼200 K range.

The size dependent deformation potentials of CdSe quantum dots are studied by first principle and semi-empirical pseudopotentials calculations. They find that the amplitude of the quantum dot deformation potential is only slightly larger than the bulk value, and this increase is mostly caused by the off {Lambda} point deformation potentials in the bulk, which are larger in amplitude than the {Lambda} point deformation potential.

The electronic states of core/shell CdSe/CdS and CdSe/CdTe heterostructure quantum dots are studied by large-scale first-principles calculations. According to their natural band-offset alignments CdSe/CdS is a type-I heterostructure and CdSe/CdTe is a type-II heterostructure. We found that, the electron state changes very little, but the hole wave function in CdSe/CdS quantum dots has been localized within the core, while the hole wave function in CdSe/CdTe quantum dots is localized within the shell. The hole state in CdSe/CdTe quantum dots has drastically different characteristics as in CdSe and CdSe/CdS quantum dots. The band alignment, strain effect, and quantum confinement are all important to determine the electronic structures of these systems.

We study the effects of a few types of atomic disorder on the electronic and optical properties of AlAs/GaAs (001) and (111) superlattices: (i) atomic intermixing across the interfaces; (ii) replacing a single monolayer in a superlattice by one containing the opposite atomic type (isoelectronic δ doping); and (iii) random layer‐thickness fluctuations in superlattices (SL). Type (i) is an example of lateral disorder, while types (ii) and (iii) are examples of vertical disorder. Using three‐dimensional empirical pseudopotential theory and a plane‐wave basis, we calculate the band gaps, electronic wave functions, and optical matrix elements for systems containing up to 2000 atoms in the computational unit cell. Spin‐orbit interactions are omitted. Computationally much less costly effective‐mass calculations are used to evaluate the density of states and eigenstates away from the band edges in vertically disordered SLs. Our main findings are: (i) Chemical intermixing across the interface can significantly shi...

We apply a recently developed Wannier-Koopmans method (WKM), based on density functional theory (DFT), to calculate the electronic energy level alignment at an interface between a molecule and metal substrate. We consider two systems: benzenediamine on Au (111), and a bipyridine-Au molecular junction. The WKM calculated level alignment agrees well with the experimental measurements where available, as well as previous GW and DFT + Σ results. Our results suggest that the WKM is a general approach that can be used to correct DFT eigenvalue errors, not only in bulk semiconductors and isolated molecules, but also in hybrid interfaces.

We present a simple, linear‐in‐size method that enables calculation of the eigensolutions of a Schrodinger equation in a desired energy window. We illustrate this method by studying the near‐gap electronic structure of Si quantum dots with size up to Si1315H460(≊37 A in diameter) using a plane wave pseudopotential representation.

An empirical pseudopotential method is used for calculating the band structure of partially CuPt ordered GaxIn1−xP alloy with order parameter η varying from 0 to 1. Because the relative band alignments between the binaries (GaAs, GaP, and InP) are taken into account in the pseudopotential fitting, such a calculation naturally yields the conduction and valence band alignment between the GaInP alloy and GaAs, as well as shows how the alignments change with the order parameter. The band alignment is found to change from type I to type II at η=0.46 (0.54) for x=0.50 (0.52), which is in good agreement with experimental data.

The electronic structure of a GaAsN alloy is calculated using a 4096 atom supercell, with a 70 Ry plane wave basis cutoff and Ga atom 3d electrons as valence electrons. The charge density of this supercell is generated by patching the charge density of a small unit cell with the charge density of bulk GaAs. The local-density-approximation band gap error is corrected by modifying the nonlocal pseudopotentials. A localized nitrogen state [a1(N)] is obtained,and it plays an important role in the band gap reduction of GaAsN.

It is shown that the standard (decoupled) 6×6 k⋅p effective-mass approach for semiconductor quantum dots overestimates significantly the hole and electron confinement energies, and, for dots made of materials with small spin-orbit coupling (e.g., phosphides, sulphides) produces a reverse order of s- and p-like valence states. By contrasting the electronic structures of dots as obtained by a direct diagonalization (multiband) pseudopotential approach and by its k⋅p approximation, we are able to trace the systematic errors of k⋅p in dots to the k⋅p errors in the underlying bulk solids. This suggests a “diagnostic tool” and a strategy for improving the k⋅p.

BiVO4 is one of the most promising photoanodes for water-splitting applications. Similar to many d10 materials, where the full-shell d electrons are not directly involved in the bonding, the Bi 6s electrons form isolated low-energy bands in BiVO4. By systematically altering the energy of the Bi 6s states, we find direct evidences that the isolated s states, through the s-p coupling, affect the BiVO4 properties, including valence band maximum position, charge density, and atomic structural distortion. We find that many good properties of BiVO4 for water splitting are related to the s-p coupling due to the existence of Bi 6s states. Based on this understanding, we propose that alloying Bi with Sb can enhance these properties, and hence improve the water-splitting efficiency.

Heterojunction interfaces between metal oxides are often highly lattice mismatched. The atomic and electronic structures of such interfaces, however, are not well understood. We have synthesized Cu 2O/TiO2 heterojunction thin films with 13% lattice mismatch and studied the interface via experimental methods and large-scale density function theory calculations of supercells containing ∼1300 atoms. We find that an interface of epitaxial quality is formed via a coincidence site lattice of 8 Cu 2O unit cells matching 9 TiO2 unit cells. Calculations reveal the existence of a dislocation core of the O sublattices at the interface and a random arrangement of one layer of interfacial Cu atoms. The interfacial electronic structure is found to be mostly determined by the interfacial Cu distribution, rather than by the O dislocation core. The conduction band minimum and valence band maximum states are spatially separated, and there is no strongly localized state near the core.

We have presented a fully atomistic quantum mechanical simulation method on band-to-band tunneling (BTBT) field-effect transistors (FETs). Our simulation approach is based on the linear combination of bulk band method with empirical pseudopotentials, which is an atomist method beyond the effective-mass approximation or k.p perturbation method, and can be used to simulate real-size devices (∼105 atoms) efficiently (∼5 h on a few computational cores). Using this approach, we studied the InAs dual-gate BTBT FETs. The I-V characteristics from our approach agree very well with the tight-binding non-equilibrium Green's function results, yet our method costs much less computationally. In addition, we have studied ways to increase the tunneling current and analyzed the effects of different mechanisms for that purpose.

Energy distribution and angular distribution of the photoelectrons from InP photocathodes are investigated using a precise Monte Carlo model. It is found that Γ-valley electrons contribute to the first peak of the energy distribution curve, but the second peak is contributed by both Γ-valley and L-valley electrons rather than only L-valley electrons. L valley electrons are shown to have a smaller angular spread than Γ-valley electrons, which is attributed to the much higher potential energy of L-valley minimum. The further simulation indicates that the performance of InP photocathodes can be improved by increasing the hole concentration or decreasing the temperature, but the activation layer thickness variation only has very slight influence on either energy or angular distribution.

A precise Monte Carlo model is developed to investigate the electron energy and angle distribution of the transmission-mode GaAs (100) photocathode at room temperature. Both distributions are important for high-quality electron sources. The results show that the energy loss (0.1309 eV) and the angle-dependent energy distribution curves fit well with experimental data. It is found that 65.24% of the emission electrons come from Γ valley, 33.62% from L valley, and 1.15% from X valley. The peak of the energy distribution curve is contributed by both Γ and L-valley electrons, while the high-energy part is contributed by Γ-valley electrons rather than L electrons, which is different from previous inference and can be attributed to the narrow energy range of L-valley electrons. However, L-valley electrons have a larger angular spread than Γ-valley electrons and lead to the spread of the emission cone. The further simulation indicates that increasing the hole concentration or the thickness of the first activatio...

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 $O_N$ antisite is the dominant defect, which determines its intrinsic n-type conductivity and the p-type doping difficulty; (ii) the $O_N$ antisite has a shallower donor level than O or N vacancies, with a delocalized distribution composed mainly of the Ta $5d$ 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.

A molecular transistor based on torsion-angle conformation change driven by gate electric field is designed and studied using ab initio calculations. This transistor consists of a SH–C6H2F(CH3)C6H2(CH3)F–SH molecule sandwiched between two Au(111) electrodes, where the interaction between the molecular dipole and a gate voltage induced electric field will cause the molecule to twist along its c-axis, changing the quantum conductivity of the molecule. The effect of thermal fluctuation on the molecular conformation is studied, so is the ability of the transistor to shut off its current. The advantages and challenges of using such molecular conformation change as a mechanism for transistor gating are discussed.

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 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 wave functions are the quantities necessary to properly describe carrier hopping in disordered conjugated polymers.

The atomistic pseudopotential quantum mechanical calculations are used to study the transport in million atom nanosized metal-oxide-semiconductor field-effect transistors. In the charge self-consistent calculation, the quantum mechanical eigenstates of closed systems instead of scattering states of open systems are calculated. The question of how to use these eigenstates to simulate a nonequilibrium system, and how to calculate the electric currents, is addressed. Two methods to occupy the electron eigenstates to yield the charge density in a nonequilibrium condition are tested and compared. One is a partition method and another is a quasi-Fermi level method. Two methods are also used to evaluate the current: one uses the ballistic and tunneling current approximation, another uses the drift-diffusion method.

We present a comprehensive study of the binding energies of B, Al, Ga, In, Tl shallow acceptors in bulk Si using density functional theory. Two approaches are used to calculate the binding energies. One is based on the eigenenergy of the single particle Kohn–Sham equation, and another is based on the total energy change during the impurity ionization process. Planewave pseudopotential Hamiltonian under local density approximation is used. A special potential patching method is presented which allows the calculation of 64 000 atom supercells needed for converging the eigenenergies. We found that the calculated impurity eigenenergies reproduce correctly the trend of the element dependence of the binding energy. But the calculated binding energies for In and Tl are much smaller than the experimental values. A linear response formula is derived which relates the total energy difference between the systems with occupied and unoccupied impurity to the impurity state eigenenergy and the impurity state self-interaction. However, the total energy difference gives much worse binding energies when compared to experiment due to the self-interaction error in the local density approximation. We conclude that one must go beyond the usual approximations of the density functional theory in order to predict accurately the binding energies of these shallow impurities.

We investigate the couplings between different energy band valleys in a metal-oxide-semiconductor field-effect transistor (MOSFET) device using self-consistent calculations of million-atom Schrodinger-Poisson equations. Atomistic empirical pseudopotentials are used to describe the device Hamiltonian and the underlying bulk band structure. The MOSFET device is under nonequilibrium condition with a source-drain bias up to 2 V and a gate potential close to the threshold potential. We find that all the intervalley couplings are small, with the coupling constants less than 3 meV. As a result, the system eigenstates derived from different bulk valleys can be calculated separately. This will significantly reduce the simulation time because the diagonalization of the Hamiltonian matrix scales as the third power of the total number of basis functions. (C) 2008 American Institute of Physics.

We report a theoretical study on the role of shallow d states in the screened-exchange local density approximation (sX-LDA) band structure of binary semiconductor systems.We found that inaccurate pseudo-wavefunctions can lead to 1) an overestimation of the screened-exchange interaction betweenthe localized d states and the delocalized higher energy s and p states and 2) an underestimation of the screened-exchange interaction between the d states. The resulting sX-LDA band structures have substantially smaller band gaps compared with experiments. We correct the pseudo-wavefunctions of d states by including the semicore s and p states of the same shell in the valence states. The correction of pseudo-wavefunctions yields band gaps and d state binding energies in good agreement with experiments and the full potential linearized augmented plane wave sX-LDA calculations. Compared with the quasi-particle GW method, our sX-LDA results shows not only similar quality on the band gaps but also much better d state binding energies. Combined with its capability of ground state structure calculation, the sX-LDA is expected to be a valuable theoretical tool for the II-VI and III-V (especially the III-N) bulk semiconductors and nanostructure studies.

The development of the charge patching method for the calculation of the electronic structure of organic systems containing a large number of atoms was presented. The method was tested on a range of systems including alkane and alkene chains, polyacenes, polythiophenes, polypyrroles, polyfuranes, polyphenylene vinylene, and poly(amidoamine) dendrimers. The results obtained by the method are in very good agreement with direct calculations based on density functional theory, since the eigenstate errors are typically of the order of a few tens of meV.

The atomistic pseudopotential quantum mechanical calculations for million atom nanosized metal-oxide-semiconductor field-effect transistors (MOSFETs) are presented. When compared with semiclassical Thomas-Fermi simulation results, there are significant differences in I-V curve, electron threshold voltage, and gate capacitance. In many aspects, the quantum mechanical effects exacerbate the problems encountered during device minimization, and it also presents different mechanisms in controlling the behaviors of a nanometer device than the classical one.

The band gap bowing and the electron localization ofGaxIn1-xN are calculated using both the local density approximation (LDA)and screened-exchange local density functional (sX-LDA) methods. Thecalculated sX-LDA band gaps are in good agreement with the experimentallyobserved values, with errors of -0.26 and 0.09 eV for bulk GaN and InN,respectively. The LDA band gap errors are 1.33 and 0.81 eV for GaN andInN, in order. In contrast to the gap itself, the band gap bowingparameter is found to be very similar in sX-LDA and LDA. We identify thelocalization of hole states in GaxIn1-xN alloys along In-N-In chains. Thepredicted localizationis stronger in sX-LDA.

By comparing the results of some well-controlled calculation methods, we analyze the relative importance of bulk band structure, multi-bulk-band coupling, and boundary conditions in determining colloidal quantum dot conduction band eigenenergies. We find that while the bulk band structure and correct boundary conditions are important, the effects of multi-bulk-band coupling are small.

We present a theoretical study of crystal and electronic structures of CaB6 within a screened-exchange local density approximation (sX-LDA). Our ab initio total energy calculations show that CaB6 is a semiconductor with a gap of > 1.2 eV, in agreement with recent experimental observations. We show a very sensitive band gap dependence on the crystal internal parameter, which might partially explain the scatter of previous theoretical results. Our calculation demonstrates that it is essential to study this system simultaneously for both crystal structures and electronic properties, and that the sX-LDA provides an ideal method for this problem.

We present an unconventional scheme that is able to dramatically modify single-electron states as well as their couplings in semiconductor nanostructures. The approach consists in perturbing the wave-function core (rather than the insignificant tail) of nanostructure states. We demonstrate this approach using a structure of silicon quantum rings. Anomalous interstate mixing, large tunability of orbital energy, and uncommon s∕p level crossing are predicted.