First-principles Level Research Articles

Modeling thermoelectric transport in organic materials

Thermoelectric energy converters can directly convert heat to electricity using semiconducting materials via the Seebeck effect and electricity to heat via the Peltier effect. Their efficiency depends on the dimensionless thermoelectric figure of merit of the material, which is defined as zT = S(2)σT/κ with S, σ, κ, and T being the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature respectively. Organic materials for thermoelectric applications have attracted great attention. In this review, we present our recent progress made in developing theories and computational schemes to predict the thermoelectric figure of merit at the first-principles level. The methods have been applied to model thermoelectric transport in closely-packed molecular crystals and one-dimensional conducting polymer chains. The physical insight gained in these studies will help in the design of efficient organic thermoelectric materials.

X-ray spectroscopy of blocked alanine in water solution from supermolecular and supermolecular-continuum solvation models: a first-principles study

The N1s near-edge X-ray absorption fine structure (NEXAFS) and X-ray emission spectra (XES) of blocked alanine in water solution have been investigated at the first-principles level based on cluster models constructed from classical molecular dynamics simulations. The bulk solvent has been described by both supermolecular and combined supermolecular-continuum models. With the former model we show that NEXAFS spectra convergent with respect to system size require at least the inclusion of the second solvation shell and that averaged spectra over several hundreds of snapshots can well represent the statistical effect of different instantaneous configurations of the solvation shells. With the combined model we demonstrate that calculations of a medium-sized peptide-water supermolecule qualitatively predict the NEXAFS spectrum of the solvated peptide even considering a single geometry. Furthermore, sampling over hundreds of snapshots by the combined model, the explicit inclusion of even a few waters yields an averaged spectrum in good quantitative agreement with the discrete model results. In comparison, the XES spectra show little dependence on the structures of either the solvent shell or the peptide itself. The ramifications of these findings are discussed.

Atomic-scale structures of interfaces between phyllosilicate edges and water

We report first-principles molecular dynamics (FPMD) studies on the structures of interfaces between phyllosilicate edges and water. Using FPMD, the substrates and solvents are simulated at the same first-principles level, and the thermal motions are sampled via molecular dynamics. Both the neutral and charged silicate frameworks are considered, and for charged cases, the octahedral (Mg for Al) and tetrahedral (Al for Si) substitutions are taken into account. For all frameworks, we focus on the commonly occurring (010)- and (110)-type edge surfaces.With constrained FPMD, we calculated the free energy of the leaving processes of coordinated water of octahedral cations; therefore, the coordination states of those edge cations are determined. For (010)-type edges, both the 5- and 6-fold coordination states of Al are stable and occur with a similar probability, whereas only the 5-fold coordination is stable for Mg cations. For (110)-type edges, only the 6-fold states of Al cations are stable. However, for Mg cations, both coordination states are stable. In the 5-fold case, the solvent water molecules form H-bonds with the bridging oxygen atoms (i.e., MgOSi). The free energy results indicate that there should be a considerable number of 5-fold coordinated octahedral sites (i.e., MgOH/AlOH) at the interfaces. The interfacial structures and acid/base groups were determined by detailed H-bonding analyses. (1) Bridging oxygen sites. For (010) edges, the bridging oxygen atoms are not effective proton-accepting sites because they are inaccessible from the solvent. For (110) edges, the oxygen atoms of neutral and octahedrally substituted frameworks (i.e., MOSi for MAl/Mg) are proton-accepting sites. For T-sheet substituted cases, the bridging oxygen site becomes a proton-donating group (i.e., AlOHSi) through the capture of a proton. (2) T-sheet groups. All T-sheet edge groups are MOH (MSi/Al) and act as both proton donors and acceptors. (3) O-sheet groups. For (010) edges with 6-fold Al, the active surface groups include Al(OH)(H2O) and Al(OH)2, whereas for Mg cations, the edge group is Mg(OH2)2. For 5-fold coordination, the active groups are AlOH. At (110) edges, proton-donating sites include AlOH, AlOH2 and MgOH2 groups. Overall, our results reveal the significant effects of isomorphic substitutions on the interfacial structures, and the models provide a molecular-level basis for understanding relevant interfacial processes.

Evaluation of Charge Mobility in Organic Materials: From Localized to Delocalized Descriptions at a First‐Principles Level

The carrier mobility for carbon electronic materials is an important parameter for optoelectronics. We report here some recently developed theoretical tools to predict the mobility without any free parameters. Carrier scatterings with phonons and traps are the key factors in evaluating the mobility. We consider three major scattering regimes: i) where the molecular internal vibration severely induces charge self-trapping and, thus, the hopping mechanism dominates; ii) where both intermolecular and intramolecular scatterings come to play roles, so the Holstein-Peierls polaron model is applied; and, iii) where charge is well delocalized with coherence length comparable with acoustic phonon wavelength, so that a deformation potential approach is more appropriate. We develop computational methods at the first-principles level for the three different cases that have extensive potential application in rationalizing material design.

Computational methods for design of organic materials with high charge mobility

Charge carrier mobility is at the center of organic electronic devices. The strong couplings between electrons and nuclear motions lead to complexities in theoretical description of charge transport, which pose a major challenge for the fundamental understanding and computational design of transport organic materials. This tutorial review describes recent progresses in developing computational tools to assess the carrier mobility in organic molecular semiconductors at the first-principles level. Some rational molecular design strategies for high mobility organic materials are outlined.

Theory of defects in Si and Ge: Past, present and recent developments

Over the past few decades, considerable progress has been achieved in the theoretical predictions of a wide range of properties of defects in semiconductors. In addition to structures, energetics, spin and charge densities, theory now routinely predicts accurate vibrational properties of defects, and thus connects to the optical characterization of light impurities. However, the positions of gap levels have yet to be predicted with systemically reliable accuracy. Today, supercells much larger than in the past are being used to describe defect centers from first principles. Systems large enough to study the dynamics of extended defects can be handled near the first-principles level. This paper contains a brief review of the key developments that have rendered theory quantitatively useful to experimentalists and an overview of the current ‘state-of-the-art’ and ongoing developments. Some of the remaining challenges are discussed, with examples in Si and Ge.

A Hierarchical Approach to Study the Thermal Behavior of Protonated Water Clusters H(+)(H2O)n.

The energy landscape of protonated water clusters H(+)(H2O)n is thoroughly explored at the first-principle level using a hierarchical search methodology. In particular, the distinct configurational isomers of OSS2 empirical potential for n = 5-9 are uncovered and archived systematically using an asynchronous genetic algorithm and are subsequently refined with first-principle calculations. Using the OSS2 model, quantitative agreements in the thermal properties between Monte Carlo and harmonic superposition approximation (HSA) highlighted the reliability of the latter approach for the study of small- to medium-sized protonated water clusters. From the large sets of collected isomers, finite temperature behavior of the clusters can be efficiently examined at first-principle accuracy with the use of HSA. From the results obtained, evidence of structural changes from single-ring to treelike (n = 5-7) and multi-ring to single-ring structures (n = 7-9) is observed, as expected for the empirical model. Finally, the relevance of these findings to recent experimental data is discussed.

Optimal Feedback Control: Foundations, Examples, and Experimental Results for a New Approach

Typical optimal feedback controls are nonsmooth functions. Nonsmooth controls raise fundamental theoretical problems on the existence and uniqueness of state trajectories. Many of these problems are frequently addressed in control applications through the concept of a Filippov solution. In recent years, the simpler concept of a π solution has emerged as a practical and powerful means to address these theoretical issues. In this paper, we advance the notion of Caratheodory-π- solutions that stem from the equivalence between closed-loop and feedback trajectories. In recognizing that feedback controls are not necessarily closed-form expressions, we develop a sampling theorem that indicates that the Lipschitz constant of the dynamics is a fundamental sampling frequency. These ideas lead to a new set of foundations for achieving feedback wherein optimality principles are interwoven to achieve stability and system performance, whereas the computation of optimal controls is at the level of first principles. We demonstrate these principles by way of pseudospectral methods because these techniques can generate Caratheodory-π solutions at a sufficiently fast sampling rate even when implemented in a MATLAB® environment running on legacy computer hardware. To facilitate an exposition of the proposed ideas to a wide audience, we introduce the core principles only and relegate the intricate details to numerous recent references. These principles are then applied to generate pseudospectral feedback controls for the slew maneuvering of NPSAT1, a spacecraft conceived, designed, and built at the Naval Postgraduate School and scheduled to be launched in fall 2007.

Theoretical investigation of nuclear quadrupole interactions in DNA at first-principles level

We have studied the nuclear quadrupole interactions (NQI) of the 14N, 17O and 2H nuclei in the nucleobases cytosine, adenine, guanine and thymine in the free state as well as when they are bonded to the sugar ring in DNA, simulated through a CH3 group attached to the nucleobases. The nucleobase uracil, which replaces thymine in RNA, has also been studied. Our results show that there are substantial indirect effects of the bonding with the sugar group in the nucleic acids on the NQI parameters e 2 qQ/h and η. It is hoped that measurements of these NQI parameters in DNA will be available in the future to compare with our predictions. Our results provide the conclusion that for any property dependent on the electronic structures of the nucleic acids, the effects of the bonding between the nucleobases and the nucleic acid backbones have to be included.

Phase stabilities of monoclinic oxoborates LaB 3O 6 and GdB 3O 6 in C121 and I12 /a1 phase—Energetics and chemical bonds derived from first-principles calculations

The computational study on the relative phase stabilities between two monoclinic polymorphs of the lanthanide-containing oxoborates LaB 3O 6 and GdB 3O 6 is presented at the first-principles density functional theory gradient-corrected B3PW level. The hypothetical monoclinic α-BiB 3O 6-like C121 non-centrosymmetric crystal structures were assumed for LaB 3O 6 and GdB 3O 6 and the corresponding geometries were calculated and compared with their monoclinic I12 /a1 centrosymmetric structures. The enthalpy–pressure correlations were calculated and the first-principles chemical bonds based on the crystal orbital overlapping population were quantitatively analyzed for LaB 3O 6 and GdB 3O 6. The chemical bonds between the central cation (La (III)/Gd (III)) and coordinated oxygen atoms rather than the B–O bonds in the borate units are found to stabilize the I12 /a1 centrosymmetric LaB 3O 6 and GdB 3O 6 structures with respect to the C121 non-centrosymmetric counterparts.

Understanding of nuclear quadrupole interactions of 35 Cl, 79 Br and 129 I and binding energies of solid halogens at first-principles level

This paper deals with the understanding at a first-principles level of the nuclear quadrupole interaction (NQI) parameters of solid chlorine, bromine and iodine as well as the intermolecular binding of these molecules in the solid. The electronic structure investigations that we have carried out to study these properties of the solid halogens are based on the Hartree-Fock Cluster approach using the Roothaan variational procedure with electron correlation effects included using many-body perturbation theory with the empty orbitals used in the perturbation theory investigations for the excited states. The results of our investigations provide good agreement with the measured NQI parameters primarily from the Hartree-Fock one electron wave-functions with many-body effects making minor contributions. The binding (dissociation) energies for the molecules with the solid state environment on the other hand arises from intermolecular many body effects identified as the Van der Waals attraction with one-electron Hartree-Fock contribution being repulsive in nature.

Advancing Science for Water Resources Management

Despite the major advances in science to underpin water resources and river management that have taken place over the past two decades, a need remains to establish a unifying framework that will lead to new, appropriate tools for water resources management. In Europe, this need has been highlighted by the promotion of the Water Framework Directive. From a scientific perspective, key questions focus on the ecological significance of flow variability over a range of timescales and the linkage between flow variability, habitat variability and biological population responses, and the biological interactions among these populations. Creation of scientifically sound tools requires development of knowledge at the level of first principles to realize sustainable developments within the context of adaptive management. Similitude analyses provide a mechanism for upscaling from fine ‘research’ scales to the coarser scales of water resource managers. Lack of appropriate data is the major obstacle to the development of these tools, especially those concerned with large rivers.

Perturbational calculations of parity-violating effects in nuclear-magnetic-resonance parameters

We investigate the effects of the parity-violating electroweak interaction in the spectral parameters of nuclear magnetic resonance. Perturbational theory of parity-violating effects in the nuclear magnetic shielding is presented to the order of G(F)alpha, and in the indirect spin-spin coupling, to the order of G(F)alpha3. These leading-order parity-violating corrections are evaluated using analytical linear-response theory methods based on Hartree-Fock and density-functional theory reference states. Parity-violating contributions to spin-spin couplings are evaluated for the first time at the first-principles level. Calculations are carried out for two chiral halomethanes, bromochlorofluoromethane and bromofluoroiodomethane.

First-principles energy and stress fields in defectedmaterials

New methods are presented for microscopically characterizingdefects in materials in terms of local energy and stress fieldscalculated at the first-principles level of theory. Thesefields provide a quantitative measure of the local disturbancecreated by defect-induced electronic and atomic inhomogeneitiesin a solid. The local stress density {σαβ}(r) is computed by explicitly evaluating the strain derivative of a suitably defined energydensity field, ε(r). Althoughε(r) and {σαβ}(r) are defined only up to a gauge transformation, they yield thecorrect total energy and the average macroscopic stress tensor,respectively, when integrated over the entire volume of theunderlying unit cell. In systems with defects, it is shown thatwell-defined averages of ε(r) and{σαβ}(r) can be constructed byrestricting the domain of integration to smaller volumes, whichare integral multiples of the Wigner-Seitz cell for thesupercell containing the defect. These fields can provideimportant insights into the nature of atomic-scale defects. Explicit expressions for ε(r) and{σαβ}(r) are derived within the densityfunctional plane-wave pseudopotential formalism. For the testcases of bulk Al with a vacancy and a Al(001) surface, it isshown that the averaged forms of ε(r) and{σαβ}(r) help characterize the defectsin a physically meaningful manner. Potential applications ofε(r) and {σαβ}(r) tothe characterization of surface relaxations and to multi-scalestudies of materials are suggested.

Scanning the potential energy surface of iron clusters: A novel search strategy

A new methodology for finding the low-energy structures of transition metal clusters is developed. A two-step strategy of successive density functional tight binding (DFTB) and density functional theory (DFT) investigations is employed. The cluster configuration space is impartially searched for candidate ground-state structures using a new single-parent genetic algorithm [I. Rata et al., Phys. Rev. Lett. 85, 546 (2000)] combined with DFTB. Separate searches are conducted for different total spin states. The ten lowest energy structures for each spin state in DFTB are optimized further at a first-principles level in DFT, yielding the optimal structures and optimal spin states for the clusters. The methodology is applied to investigate the structures of Fe4, Fe7, Fe10, and Fe19 clusters. Our results demonstrate the applicability of DFTB as an efficient tool in generating the possible candidates for the ground state and higher energy structures of iron clusters. Trends in the physical properties of iron clusters are also studied by approximating the structures of iron clusters in the size range n=2–26 by Lennard-Jones-type structures. We find that the magnetic moment of the clusters remains in the vicinity of 3μB/atom over this entire size range.

Comprehensive modeling of bulk-damage effects in silicon radiation detectors

In this paper, the issue of numerical modeling of radiation-damaged silicon devices is discussed, with reference to radiation detectors employed in high-energy physics experiments. Since the actual physical picture is far too complex to be accounted for at a first-principle (i.e., defect kinetics) level and not yet fully understood, a hierarchical approach has been followed looking for a suitable approximation of macroscopic changes of the electrical behavior of silicon device induced by radiation damage. In particular, a three deep-level trapping mechanism is accounted for by means of Shockley-Read-Hall theory, whereas the shallow-level sensitivity on the radiation is considered by means of a donor-removal model.

A simple, continuous, analytical charge/capacitance model for the short-channel MOSFET

A charge/capacitance model of a simple form that is continuous across the linear and saturation regimes is developed. The model is based on a conductance analysis of the MOSFET which incorporates velocity saturation at a first-principles level. By relating charge layers within the device to characteristics of the conductance, the charge model not only is able to characterize C-V behavior but to also incorporate velocity saturation. Since the basic conductance form is a hyperbola, the model is mathematically simple and robust and yields MOSFET capacitances and charges which are continuous and of infinite differentiability over the linear and saturation regimes of device operation.

One-electron excitations, correlation effects, and the plasmon in cesium metal

We study the dynamical electronic response of Cs at a first-principles level. The spatially localized 5p semicore shell induces a physical interplay between crystal local fields and electron correlations, leading to a novel, and relatively large, many-body shift of the plasmon energy. This effect, combined with that of oneelectron transitions into empty states near the plasmon energy, yields a plasmon dispersion curve which is in excellent agreement with experiment for small wave vectors. In addition, our results feature a flat dispersion for large wave vectors, in qualitative agreement with experiment. @S0163-1829~97!07504-8#

Introducing undergraduates to the moment method

Four application problems are presented which have been found to facilitate introductory instruction on numerical methods for undergraduate electrical engineering students. The method of moments (MOM) is introduced at the level of first principles, preparing students for subsequent advanced topics in matrix methods and developments through linear vector space theory. The problems are such that the associated computer programming assignments are straightforward, minimizing distraction from the mathematical concepts. Numerical solutions may be obtained, or verified, with a pocket calculator in some cases. Computer programs to obtain more sophisticated graphics output and to expedite solution of equations with large matrices can be progressively added as supplementary student exercises.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>