Basing on our hierarchical equations of motion for time-dependent quantum transport [X. Zheng, G. H. Chen, Y. Mo, S. K. Koo, H. Tian, C. Y. Yam, and Y. J. Yan, J. Chem. Phys. 133, 114101 (2010)], we develop an efficient and accurate numerical algorithm to solve the Liouville-von-Neumann equation. We solve the real-time evolution of the reduced single-electron density matrix at the tight-binding level. Calculations are carried out to simulate the transient current through a linear chain of atoms, with each represented by a single orbital. The self-energy matrix is expanded in terms of multiple Lorentzian functions, and the Fermi distribution function is evaluated via the Padè spectrum decomposition. This Lorentzian-Padè decomposition scheme is employed to simulate the transient current. With sufficient Lorentzian functions used to fit the self-energy matrices, we show that the lead spectral function and the dynamics response can be treated accurately. Compared to the conventional master equation approaches, our method is much more efficient as the computational time scales cubically with the system size and linearly with the simulation time. As a result, the simulations of the transient currents through systems containing up to one hundred of atoms have been carried out. As density functional theory is also an effective one-particle theory, the Lorentzian-Padè decomposition scheme developed here can be generalized for first-principles simulation of realistic systems.

Organometallic halide perovskites have drawn substantial interest due to their outstanding performance in solar energy conversion and optoelectronic applications. The presence of ferroelectric domain walls in these materials has shown to have a profound effect on their electronic structure.Here, we use a density-functional-based tight-binding model, coupled to nonequilibrium Green's function method, to investigate the effects of ferroelectric domain walls on electronic transport properties and charge carrier recombination in methylammonium lead−iodide perovskite, MAPbI 3 . With the presence of ferroelectric domain walls, segregation of transport channels for electrons and holes is observed, and the conductance of perovskites is substantially increased due to the reduced band gap. In addition, by taking into account interactions with photons in the vacuum environment, it is found that electron−hole recombination in perovskites with ferroelectric domain walls is drastically suppressed due to the segregation of carrier transport paths, which could enhance photovoltaic performance.

Time‐dependent quantum transport parameters for graphene nanoribbons (GNR) are calculated by the hierarchical equation of motion (HEOM) method based on the nonequilibrium Green's function (NEGF) theory [Xie et al., J. Chem. Phys. 137, 044113 (2012)]. In this paper, a new initial‐state calculation technique is introduced and accelerated by the contour integration for large systems. Some Lorentzian fitting schemes for the self‐energy matrices are developed to effectively reduce the number of Lorentzians and maintain good fitting results. With these two developments in HEOM, we have calculated the transient quantum transport parameters in GNR. We find a new type of surface state with delta‐function‐like density of states in many semi‐infinite armchair‐type GNR. For zigzag‐type GNR, a large overshooting current and slowly decaying transient charge are observed, which is due to the sharp lead spectra and the “even–odd” effect.

Real-time, local basis-set implementation of time-dependent density functional theory for excited state dynamics simulations A model study of quantum dot polarizability calculations using time-dependent density functional methods Basing on the earlier works on the hierarchical equations of motion for quantum transport, we present in this paper a first principles scheme for time-dependent quantum transport by combining timedependent density functional theory (TDDFT) and Keldysh's non-equilibrium Green's function formalism. This scheme is beyond the wide band limit approximation and is directly applicable to the case of non-orthogonal basis without the need of basis transformation. The overlap between the basis in the lead and the device region is treated properly by including it in the self-energy and it can be shown that this approach is equivalent to a lead-device orthogonalization. This scheme has been implemented at both TDDFT and density functional tight-binding level. Simulation results are presented to demonstrate our method and comparison with wide band limit approximation is made. Finally, the sparsity of the matrices and computational complexity of this method are analyzed.

Based on the complex absorbing potential (CAP) method, a Lorentzian expansion scheme is developed to express the self-energy. The CAP-based Lorentzian expansion of self-energy is employed to solve efficiently the Liouville-von Neumann equation of one-electron density matrix. The resulting method is applicable for both tight-binding and first-principles models, and is used to simulate the transient currents through graphene nanoribbons and a benzene molecule sandwiched between two carbon-atom-chains.

Abstract-We establish the dependence of the permittivity of oxidized ultra-thin silicon films on the film thickness by means of atomistic simulations within the density-functional-based tight-binding theory (DFTB). This is of utmost importance for modeling ultra-and extremely-thin silicon-on-insulator MOSFETs, and for evaluating their scaling potential. We demonstrate that electronic contribution to the dielectric response naturally emerges from the DFTB Hamiltonian when coupled to Poisson equation solved in vacuum, without phenomenological parameters, and obtain good agreement with available experimental data. Comparison to calculations of H-passivated Si films reveals much weaker dependence of permittivity on film thickness for the SiO2-passivated Si, with less than 18% reduction in the case of 0.9 nm silicon-on-insulator. Index Terms-permittivity, atomistic modeling, oxide interface, density-functional tight binding, silicon-on-insulatorIt is well known that the dramatic reduction of the dimensions of the Si channel, e.g. in ultra-thin-body silicon-on-insulators devices, leads to a significant change in the electronic and dielectric properties of Si, particularly at channel thickness below 6 nm [1-3]. A number density-functional theory (DFT) studies applied to hydrogen-passivated Si films suggest that the decrease in permittivity with the decrease of Si-film thickness becomes significant even earlier than the corresponding widening of the fundamental band-gap, and predict 35-45% reduction at around 1 nm [4][5][6]. On the experimental side however, we are aware of only one study of oxidized Si films down to 3.3 nm, and while a qualitative trend is evident, the scatter of the results precludes us from establishing an accurate quantitative picture towards sub-nm Si thickness [7]. We note further that to the best of our knowledge, the permittivity dependence of oxidized Si films has not been modeled ab initio, most likely due to the complexity and cost associated with including the oxide on each side of Si in DFT. However, very recently we A. Pecchia is with University of Rome "Tor Vergata", Rome, Italy. demonstrated that density-functional tight-binding theory (DFTB), with an accurate parameterization, can give us very good description of the electronic properties not only of bulk Si and SiO2, but also of their interface [3], and allows us to explore substantially larger systems, including transport through ETSOI devices [8]. The purpose of this paper is therefore twofold: 1) to evaluate a way of calculating the dielectric constant of thin-films within the framework of DFTB; and 2) to establish the permittivity dependence on the thickness of oxidized ultra-thin Si films.We employ the DFTB+ computer code [9], implementing the self-consistent-charge DFTB, coupled self-consistently to a Poisson solver [10][11][12]. This permits us to apply bias ( ) and find the distribution of potential ( ) and electric field ( ) in the model atomic structures. The atomic models are the SiO2/Si/SiO2/vacuum super-cells with varying ...

Photovoltaic devices, electrochemical cells, catalysis processes, light emitting diodes, scanning tunneling microscopes, molecular electronics, and related devices have one thing in common: open quantum systems where energy and matter are not conserved. Traditionally quantum chemistry is confined to isolated and closed systems, while quantum dissipation theory studies open quantum systems. The key quantity in quantum dissipation theory is the reduced system density matrix. As the reduced system density matrix is an O(M! × M!) matrix, where M is the number of the particles of the system of interest, quantum dissipation theory can only be employed to simulate systems of a few particles or degrees of freedom. It is thus important to combine quantum chemistry and quantum dissipation theory so that realistic open quantum systems can be simulated from first-principles. We have developed a first-principles method to simulate the dynamics of open electronic systems, the time-dependent density functional theory for open systems (TDDFT-OS). Instead of the reduced system density matrix, the key quantity is the reduced single-electron density matrix, which is an N × N matrix where N is the number of the atomic bases of the system of interest. As the dimension of the key quantity is drastically reduced, the TDDFT-OS can thus be used to simulate the dynamics of realistic open electronic systems and efficient numerical algorithms have been developed. As an application, we apply the method to study how quantum interference develops in a molecular transistor in time domain. We include electron-phonon interaction in our simulation and show that quantum interference in the given system is robust against nuclear vibration not only in the steady state but also in the transient dynamics. As another application, by combining TDDFT-OS with Ehrenfest dynamics, we study current-induced dissociation of water molecules under scanning tunneling microscopy and follow its time dependent dynamics. Given the rapid development in ultrafast experiments with atomic resolution in recent years, time dependent simulation of open electronic systems will be useful to gain insight and understanding of such experiments. This Account will mainly focus on the practical aspects of the TDDFT-OS method, describing the numerical implementation and demonstrating the method with applications.

Quantum interference in cross-conjugated molecules can be utilized to construct molecular quantum interference effect transistors. However, whether its application can be achieved depends on the survivability of the quantum interference under real conditions such as nuclear vibration. We use two simulation methods to investigate the effects of nuclear vibration on quantum interference in a meta-linked benzene system. The simulation results suggest that the quantum interference is robust against nuclear vibration not only in the steady state but also in its transient dynamics, and thus the molecular quantum interference effect transistors can be realized.