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We present a new empirical pseudopotential (EPM) calculation approach to simulate the million atom nanostructured semiconductor devices under potential bias using periodic boundary conditions. To treat the nonequilibrium condition, instead of directly calculating the scattering states from the source and drain, we calculate the stationary states by the linear combination of bulk band method and then decompose the stationary wave function into source and drain injecting scattering states according to an approximated top of the barrier splitting (TBS) scheme based on physical insight of ballistic and tunneling transports. The decomposed electronic scattering states are then occupied according to the source/drain Fermi-Levels to yield the occupied electron density which is then used to solve the potential, forming a self-consistent loop. The TBS is tested in a one-dimensional effective mass model by comparing with the direct scattering state calculation results. It is also tested in a three-dimensional 22 nm double gate ultra-thin-body field-effect transistor study, by comparing the TBS-EPM result with the nonequilibrium Green's function tight-binding result. We expected the TBS scheme will work whenever the potential in the barrier region is smoother than the wave function oscillations and it does not have local minimum, thus there is no multiple scattering as in a resonant tunneling diode, and when a three-dimensional problem can be represented as a quasi-one-dimensional problem, e.g., in a variable separation approximation. Using our approach, a million atom nonequilibrium nanostructure device can be simulated with EPM on a single processor computer. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Semiconductor p-n junctions are fundamental building blocks for modern optical and electronic devices. The p- and n-type regions are typically created by chemical doping process. Here we show that in the new class of halide perovskite semiconductors, the p-n junctions can be readily induced through a localized thermaldriven phase transition. We demonstrate this p-n junction formation in a single-crystalline halide perovskite CsSnI3 nanowire (NW). This material undergoes a phase transition from a double-chain yellow (Y) phase to an orthorhombic black (B) phase. The formation energies of the cation and anion vacancies in these two phases are significantly different, which leads to n- and p- type electrical characteristics for Y and B phases, respectively. Interface formation between these two phases and directional interface propagation within a single NW are directly observed under cathodoluminescence (CL) microscopy. Current rectification is demonstrated for the p-n junction formed with this localized thermal-driven phase transition. [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]

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]

The electronic structure of nanowires in contact with metallic electrodes of experimentally relevant sizes is calculated by incorporating the electrostatic image potential into the atomistic single particle Schrödinger equation. We show that the presence of an electrode produces localized electron/hole states near the electrode. We found a strong nanowire size dependence of this localization effect. We calculate several electrode/nanowire geometries, with varying contact depths and nanowire radii. We demonstrate the change in the band gap of up to 0.5 eV in 3 nm diameter CdSe nanowires and calculate the magnitude of the applied electric field necessary to overcome the localization. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The article presents the discussion on providing fresh insights into photoinduced ultrafast nonthermal melting. Topics include creating a local self-trapping center for the excited carriers and yielding the random nucleation seeds; and lattice disordering starts and finishing well before the completion of carrier-lattice thermalization through electron-phonon coupling.

In dye-sensitized solar cells (DSCs), the electron transfer from photoexcited dye molecules to semiconductor substrates remains a major bottleneck. Replacing TiO2 with ZnO is expected to enhance the efficiency of DSCs, owing to the latter possesses a much larger electron mobility, but similar bandgap and band positions as TiO2 remain. However, the record efficiency of ZnO-based DSCs is only 7% compared with 13% of TiO2-based DSCs due to the even slower electron-transfer rate in ZnO-based DSCs, which becomes a long-standing puzzle. Here, we computationally investigate the electron transfer from the dye molecule into ZnO and TiO2, respectively, by performing the first-principles calculations within the frame of the Marcus theory. The predicted electron-transfer rate in the TiO2-based DSC is about 1.15 x 109 s-1, a factor of 15 faster than that of the ZnO-based DSC, which is in good agreement with experimental data. We find that the much larger density of states of the TiO2 compared with ZnO near the conduction band edge is the dominant factor, which is responsible for the faster electron-transfer rate in TiO2-based DSCs. These denser states provide additional efficient channels for the electron transfer. We also provide design principles to boost the efficiency of DSCs through surface engineering of high mobility photoanode semiconductors. [ABSTRACT FROM AUTHOR]

Hole transfer between a CdSe/CdS core/shell semiconductor nanorod and a surface-ligated alkyl ferrocene is investigated by a combination of ab initio quantum chemistry calculations and electrochemical and time-resolved photoluminescence measurements. The calculated driving force for hole transfer corresponds well with electrochemical measurements of nanorods partially ligated by 6-ferrocenylhexanethiolate. The calculations and the experiments suggest that single step hole transfer from the valence band to ferrocene is in the Marcus inverted region. Additionally, time-resolved photoluminescence data suggest that two-step hole transfer to ferrocene mediated by a deep trap state is unlikely. However, the calculations also suggest that shallow surface states of the CdS shell could play a significant role in mediating hole transfer as long as their energies are close enough to the nanorod highest occupied molecular orbital energy. Regardless of the detailed mechanism of hole transfer, our results suggest that holes may be extracted more efficiently from well-passivated nanocrystals by reducing the energetic driving force for hole transfer, thus minimizing energetic losses. [ABSTRACT FROM AUTHOR]