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Carrier heating in disordered conjugated polymers in electric field

Description: The electric field dependence of charge carrier transport and the effect of carrier heating in disordered conjugated polymers were investigated. A parameter-free multiscale methodology consisting of classical molecular dynamics simulation for the generation of the atomic structure, large system electronic structure and electron-phonon coupling constants calculations and the procedure for extracting the bulk polymer mobility, was used. The results suggested that the mobility of a fully disordered poly(3-hexylthiophene) (P3HT) polymer increases with electric field which is consistent with the experimental results on samples of regiorandom P3HT and different from the results on more ordered regioregular P3HT polymers, where the opposite trend is often observed at low electric fields. We calculated the electric field dependence of the effective carrier temperature and showed however that the effective temperature cannot be used to replace the joint effect of temperature and electric field, in contrast to previous theoretical results from phenomenological models. Such a difference was traced to originate from the use of simplified Miller-Abrahams hopping rates in phenomenological models in contrast to our considerations that explicitly take into account the electronic state wave functions and the interaction with all phonon modes.
Date: January 26, 2010
Creator: Vukmirovic, Nenad & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Carrier hopping in disordered semiconducting polymers: How accurate is the Miller-Abrahams model?

Description: 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 wavefunctions are the quantities necessary to properly describe carrier hopping in disordered conjugated polymers.
Date: July 30, 2010
Creator: Vukmirovic, Nenad & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

A divide-and-conquer linear scaling three dimensional fragment method for large scale electronic structure calculations

Description: We present a new linear scaling ab initio total energy electronic structure calculation method based on the divide-and-conquer strategy. This method is simple to implement, easily to parallelize, and produces very accurate results when compared with the direct ab initio method. The method has been tested using up to 8,000 processors, and has been used to calculate nanosystems up to 15,000 atoms.
Date: July 11, 2008
Creator: Wang, Lin-Wang; Zhao, Zhengji; Meza, Juan & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Effects of d-electrons in pseudopotential screened-exchange density functional calculations

Description: 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 the inaccurate pseudo-wavefunctions can lead to (1) an overestimation of the screened-exchange interaction between the 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 the d state binding energy with good agreements with experiments and the full potential linearized augmented planewave (FLAPW) 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 energy. 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.
Date: August 11, 2008
Creator: Wang, Lin-Wang; Lee, Byounghak; Canning, Andrew & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Non-Bloch nature of alloy states in a conventional semiconductor alloy - GaxIn1-xP as an example

Description: Contrary to the conventional wisdom, electronic states in a 'well behaved' semiconductor alloy such as Ga{sub x}In{sub 1-x}P may drastically deviate from a Bloch state, which can be true even for band edge states if they are derived from degenerate critical points. For Ga{sub x}In{sub 1-x}P in the entire composition range, k-space spectral analyses are performed for the important critical points, revealing the significance of the (near) resonant inter-and intra-valley scatterings of the fluctuation potential in the alloy. The non-trivial implications of such scatterings on the transport and strain effect are discussed.
Date: July 11, 2008
Creator: Wang, Lin-Wang; Zhang, Yong; Mascarenhas, Angelo & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Synthesis of cadmium telluride quantum wires and the similarity of their band gaps to those of equidiameter cadmium telluride quantum dots

Description: High-quality colloidal CdTe quantum wires having purposefully controlled diameters in the range of 5-11 nm are grown by the solution-liquid-solid (SLS) method, using Bi-nanoparticle catalysts, cadmium octadecylphosphonate and trioctylphosphine telluride as precursors, and a TOPO solvent. The wires adopt the wurtzite structure, and grow along the [002] direction (parallel to the c axis). The size dependence of the band gaps in the wires are determined from the absorption spectra, and compared to the experimental results for high-quality CdTe quantum dots. In contrast to the predictions of an effective-mass approximation, particle-in-a-box model, and previous experimental results from CdSe and InP dot-wire comparisons, the band gaps of CdTe dots and wires of like diameter are found to be experimentally indistinguishable. The present results are analyzed using density functional theory under the local-density approximation by implementing a charge-patching method. The higher-level theoretical analysis finds the general existence of a threshold diameter, above which dot and wire band gaps converge. The origin and magnitude of this threshold diameter is discussed.
Date: July 11, 2008
Creator: Wang, Lin-Wang; Sun, Jianwei; Wang, Lin-Wang & Buhro, William E.
Partner: UNT Libraries Government Documents Department

Linear scaling 3D fragment method for large-scale electronic structure calculations

Description: We present a new linearly scaling three-dimensional fragment (LS3DF) method for large scale ab initio electronic structure calculations. LS3DF is based on a divide-and-conquer approach, which incorporates a novel patching scheme that effectively cancels out the artificial boundary effects due to the subdivision of the system. As a consequence, the LS3DF program yields essentially the same results as direct density functional theory (DFT) calculations. The fragments of the LS3DF algorithm can be calculated separately with different groups of processors. This leads to almost perfect parallelization on tens of thousands of processors. After code optimization, we were able to achieve 35.1 Tflop/s, which is 39% of the theoretical speed on 17,280 Cray XT4 processor cores. Our 13,824-atom ZnTeO alloy calculation runs 400 times faster than a direct DFT calculation, even presuming that the direct DFT calculation can scale well up to 17,280 processor cores. These results demonstrate the applicability of the LS3DF method to material simulations, the advantage of using linearly scaling algorithms over conventional O(N{sup 3}) methods, and the potential for petascale computation using the LS3DF method.
Date: July 11, 2008
Creator: Wang, Lin-Wang; Wang, Lin-Wang; Lee, Byounghak; Shan, HongZhang; Zhao, Zhengji; Meza, Juan et al.
Partner: UNT Libraries Government Documents Department

Multiple Valley Couplings in Nanometer Si MOSFETs

Description: We investigate the couplings between different energy band valleys in a MOSFET device using self-consistent calculations of million-atom Schroedinger-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 2V, 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.
Date: July 11, 2008
Creator: Wang, Lin-Wang; Deng, Hui-Xiong; Jiang, Xiang-Wei; Luo, Jun-Wei; Li, Shu-Shen; Xia, Jian-Bai et al.
Partner: UNT Libraries Government Documents Department

A fully 3D atomistic quantum mechanical study on random dopant induced effects in 25nm MOSFETs

Description: We present a fully 3D atomistic quantum mechanical simulation for nanometered MOSFET using a coupled Schroedinger equation and Poisson equation approach. Empirical pseudopotential is used to represent the single particle Hamiltonian and linear combination of bulk band (LCBB) method is used to solve the million atom Schroedinger's equation. We studied gate threshold fluctuations and threshold lowering due to the discrete dopant configurations. We compared our results with semiclassical simulation results. We found quantum mechanical effects increase the threshold fluctuation while decreases the threshold lowering. The increase of threshold fluctuation is in agreement with previous study based on approximated density gradient approach to represent the quantum mechanical effect. However, the decrease in threshold lowering is in contrast with the previous density gradient calculations.
Date: July 11, 2008
Creator: Wang, Lin-Wang; Jiang, Xiang-Wei; Deng, Hui-Xiong; Luo, Jun-Wei; Li, Shu-Shen; Wang, Lin-Wang et al.
Partner: UNT Libraries Government Documents Department

Energy levels of isoelectronic impurities by large scale LDA calculations

Description: Isoelectronic impurity states are localized states induced by stoichiometric single atom substitution in bulk semiconductor. Photoluminescence spectra indicate deep impurity levels of 0.5 to 0.9eV above the top of valence band for systems like: GaN:As, GaN:P, CdS:Te, ZnS:Te. Previous calculations based on small supercells seemingly confirmed these experimental results. However, the current ab initio calculations based on thousand atom supercells indicate that the impurity levels of the above systems are actually much shallower(0.04 to 0.23 eV), and these impurity levels should be compared with photoluminescence excitation spectra, not photoluminescence spectra.
Date: November 22, 2002
Creator: Li, Jingbo & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Comparison of quantum confinement effects between quantum wires and dots

Description: Dimensionality is an important factor to govern the electronic structures of semiconductor nanocrystals. The quantum confinement energies in one-dimensional quantum wires and zero-dimensional quantum dots are quite different. Using large-scale first-principles calculations, we systematically study the electronic structures of semiconductor (including group IV, III-V, and II-VI) surface-passivated quantum wires and dots. The band-gap energies of quantum wires and dots have the same scaling with diameter for a given material. The ratio of band-gap-increases between quantum wires and dots is material-dependent, and slightly deviates from 0.586 predicted by effective-mass approximation. Highly linear polarization of photoluminescence in quantum wires is found. The degree of polarization decreases with the increasing temperature and size.
Date: March 30, 2004
Creator: Li, Jingbo & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

First principle thousand atom quantum dot calculations

Description: A charge patching method and an idealized surface passivation are used to calculate the single electronic states of IV-IV, III-V, II-VI semiconductor quantum dots up to a thousand atoms. This approach scales linearly and has a 1000 fold speed-up compared to direct first principle methods with a cost of eigen energy error of about 20 meV. The calculated quantum dot band gaps are parametrized for future references.
Date: March 30, 2004
Creator: Wang, Lin-Wang & Li, Jingbo
Partner: UNT Libraries Government Documents Department

Deformation potentials of CdSe quantum dots

Description: 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.
Date: June 2, 2004
Creator: Li, Jingbo & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Quantum transport calculations using periodic boundaryconditions

Description: 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 groundstate calculations, thus is makes accurate quantum transport calculations for large systems possible.
Date: June 15, 2004
Creator: Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Effects of stacking faults on the electronic structures of quantum rods

Description: Atomistic semiempirical pseudopotential method is used to study the effects of stacking faults in a wurtzite structure quantum rod. It is found that a single stacking fault can cause a 10-50 meV change in the conduction state eigen energy, and a localization in the electron wave function. However, the effects on the hole eigen energies and wave functions are very small.
Date: March 30, 2004
Creator: Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

A survey of codes and algorithms used in NERSC material scienceallocations

Description: We have carried out a survey of codes and algorithms used on NERSC computers within the science category of material science. This is part of the effort to track the usage of different algorithms in NERSC community. This survey is based on the data provided in the ERCAP application of FY06. To figure out the usage of each code in one account, we have multiplied the total high performance computer (HPC) time allocation (MPP hours) of this account with the percentage usage of this code as estimated by the users in the ERCAP application. This is not the actual usage time, but should be a good estimation of it, and it represents the intention of the users.
Date: June 1, 2006
Creator: Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Electronic structure of Calcium hexaborides

Description: 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.
Date: June 15, 2005
Creator: Lee, Byounghak & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Band gap bowing and electron localization of (GaxIn1-x)N

Description: 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.
Date: May 9, 2006
Creator: Lee, Byounghak & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Motif based Hessian matrixfor ab initio geometry optimization ofnanostructures

Description: A simple method to estimate the atomic degree Hessian matrixof a nanosystem is presented. The estimated Hessian matrix, based on themotif decomposition of the nanosystem, can be used to accelerate abinitio atomic relaxations with speedups of 2 to 4 depending on the sizeof the system. In addition, the programing implementation for using thismethod in a standard ab initio package is trivial.
Date: April 5, 2006
Creator: Zhao, Zhengji; Wang, Lin-Wang & Meza, Juan
Partner: UNT Libraries Government Documents Department

A brief comparison between grid based real space algorithms andspectrum algorithms for electronic structure calculations

Description: Quantum mechanical ab initio calculation constitutes the biggest portion of the computer time in material science and chemical science simulations. As a computer center like NERSC, to better serve these communities, it will be very useful to have a prediction for the future trends of ab initio calculations in these areas. Such prediction can help us to decide what future computer architecture can be most useful for these communities, and what should be emphasized on in future supercomputer procurement. As the size of the computer and the size of the simulated physical systems increase, there is a renewed interest in using the real space grid method in electronic structure calculations. This is fueled by two factors. First, it is generally assumed that the real space grid method is more suitable for parallel computation for its limited communication requirement, compared with spectrum method where a global FFT is required. Second, as the size N of the calculated system increases together with the computer power, O(N) scaling approaches become more favorable than the traditional direct O(N{sup 3}) scaling methods. These O(N) methods are usually based on localized orbital in real space, which can be described more naturally by the real space basis. In this report, the author compares the real space methods versus the traditional plane wave (PW) spectrum methods, for their technical pros and cons, and the possible of future trends. For the real space method, the author focuses on the regular grid finite different (FD) method and the finite element (FE) method. These are the methods used mostly in material science simulation. As for chemical science, the predominant methods are still Gaussian basis method, and sometime the atomic orbital basis method. These two basis sets are localized in real space, and there is no indication that their roles in quantum ...
Date: December 1, 2006
Creator: Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Electronic Structure of zinc-blende AlxGa1-xN; Screened-ExchangeStudy

Description: We present a first principle investigation of the electronicstructure and the band gap bowing parameter of zinc-blende \AlGaN usingboth local density approximation and screened-exchange density functionalmethod. The calculated sX-LDA band gaps for GaN and AlN are 95 percentand 90 percent of the experimentally observed values, respectively, whileLDA under estimates the gaps to 62 percent and 70 percent. In contrast tothe gap itself, the band gap bowing parameter is found to be very similarin sX-LDA and LDA. Because of the difference in the conduction bandstructure, the direct to indirect band gap crossover is predicted tooccur at different Al concentration.
Date: January 3, 2006
Creator: Lee, Byounghak & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department