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1
Lancaster, Kelly. "Intramolecular electron transfer in mixed-valence triarylamines". Diss., Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/31709.
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Thesis (Ph.D)--Chemistry and Biochemistry, Georgia Institute of Technology, 2010.Committee Chair: Bredas, Jean-Luc; Committee Member: Kippelen, Bernard; Committee Member: Marder, Seth; Committee Member: Orlando, Thomas; Committee Member: Sherrill, David. Part of the SMARTech Electronic Thesis and Dissertation Collection.
2
唐素明 i So-ming Glenna Tong. "Theoretical studies of transition metal containing diatomics and DNA electron transfer". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2002. http://hub.hku.hk/bib/B31244828.
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Dinte, Bradley Paul, i n/a. "Novel Constraints in the Search for a Van Der Waals Energy Functional". Griffith University. School of Science, 2004. http://www4.gu.edu.au:8080/adt-root/public/adt-QGU20050825.154126.
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In modelling the energetics of molecules and solids, the need for practical electron density functionals that seamlessly include the van der Waals interaction is growing. Such functionals are still in their infancy, and there is yet much experimentation to be performed in the formulation and numerical testing of the requisite approximations. A ground-state density functional approach that uses the exact relations of the adiabatic connection formula and the fluctuation-dissipation theorem to obtain the xc energy from the density-density response function seems promising, though a direct local density approximation for the interacting susceptibility will fail to yield the vdW interaction. Significant nonlocality can be built into the interacting susceptibility by screening a 'bare' susceptibility, for which a carefully chosen constraint-obeying local approximation is sufficient to yield a non-trivial van der Waals energy [6]. The constraints of charge conservation, and no response to a constant potential, are guaranteed by expressing the bare susceptibility in terms of the double gradients of a nonlocal bare polarisability. for which it should be easier to make an approximation based on physical principles than it would be for the susceptibility. The 'no-flow' condition is also deemed important. In this work, a simple delta-function approximation for the nonlocal polarisability is fully constrained by a new version of a recently-discovered force theorem (sum rule), requiring the additional input of the independent-electron Kohn-Sham potential. This constrained polarisability cannot be used as input for the seamless vdW scheme, which requires a non-delta-function bare polarisability, and is instead applied to systems containing spherical fragments in a perturbative/asymptotic fashion for calculation of the widely-separated van der Waals interaction. The main thrust of this work is an investigation of the efficacy of the force theorem to constrain simple approximations for response quantities. Many recent perturbative vdW density functionals are based on response functions that are electron-hydrodynamical approximations to the response of the uniform electron gas. These schemes require their response functions to be 'cut off' at low density and high density-gradient, where the approximation overestimates the true response. The imposition of the cut-off is crucial to the success of such schemes. Here, we replace the cut-off with an exact theorem (the force theorem) which naturally 'ties down' the response, based on the potential- and density-functions of the system. This is the first time that the force theorem has been directly applied as a constraint upon a model response function (its original use, by Vignale and Kohn (7), was as an exact identity in time-dependent DFT). Also new in this work is the orbital-by-orbital Kohn-Sham version of the force theorem, and its proof (differing significantly from Vignale's original derivation (8) of the interacting theorem) by directly appealing to the Kohn-Sham orbitals makes its first appearance here. For quantum dots, our constrained response-approximation exactly recovers the net linear dipole response, due mainly to the force theorem's ideal applicability to harmonically confined systems. For angularly-averaged atoms, reasonable static dipole polarisabilities are obtained for the independent-electron Kohn-Sham (bare) case. The results are poor for the fully-interacting case, attributable to the local nature of the approximation. This lends weight to the assertion that it is better to approximate a bare quantity, then screen it, than it is to directly approximate a fully-interacting quantity. Dynamic net polarisabilities constrained by the force theorem are guaranteed to have the correct high-frequency asymptotic convergence to the free electron response. It is seen that the calculated dynamic polarisabilities for atoms are too small at intermediate frequencies, since the calculated vdW C6 coefficients (Hamaker constants) of atomic dimers are up to an order of magnitude too small, even without the use of a low-density cutoff. It is seen that our constrained local model response is non-analytic along the imaginary-frequency axis, and this is very detrimental to the C6 calculations, even though the integrated net polarisability is analytic. Improvement of the polarisability ansatz is indicated, perhaps to a non-deltafunction uniform-gas-based approximation. The use of pseudopotentials may improve the force theorem results, by softening the extreme nature of the bare Coulomb potential.
4
Worsnop, S. Kent. "Novel tools for studying electron densities, investigation and design of exchange-correlation functionals for density functional theory". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape7/PQDD_0016/NQ49300.pdf.
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Watrous, Mitchell James. "Finite temperature densities via the Green's-function method with application to electron screening in plasmas /". Thesis, Connect to this title online; UW restricted, 1997. http://hdl.handle.net/1773/9705.
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Braden, Dale Andrew. "Part 1--Elucidation of the structure and properties of 19-electron organometallic complexes using density functional theory ; Part 2--Solvent cage effects--identification of solvent and solute characteristics which influence the recombination efficiency of geminate radicals /". view abstract or download file of text, 2000. http://wwwlib.umi.com/cr/uoregon/fullcit?p9963443.
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Thesis (Ph. D.)--University of Oregon, 2000.Typescript. Includes vita and abstract. Includes bibliographical references (leaves 159-176). Also available for download via the World Wide Web; free to University of Oregon users. Address:http://wwwlib.umi.com/cr/uoregon/fullcit?p9963443.
7
Thulasi, Sunita. "Theory of the two-dimensional airy electron gas Hartee-Fock and density-functional studies /". Diss., Columbia, Mo. : University of Missouri-Columbia, 2006. http://hdl.handle.net/10355/4111.
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Thesis (Ph. D.)--University of Missouri-Columbia, 2006.The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (May 17, 2007) Vita. n following parenthesis in formula (LaTiO₃) should be subscript. Includes bibliographical references.
8
D'Acchioli, Jason S. "On the nature of the electronics structure of metal-metal quadruply bonded complexes". Connect to resource, 2005. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1126621699.
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Thesis (Ph. D.)--Ohio State University, 2005.Title from first page of PDF file. Document formatted into pages; contains xii, 286 p.; also includes graphics (some col.). Includes bibliographical references (p. 273-286). Available online via OhioLINK's ETD Center
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Brett, Constance M. "Investigation of the structure and bonding of metal complexes through the use of density functional theory". Connect to this title online, 2005. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1118688725.
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Thesis (Ph. D.)--Ohio State University, 2005.Title from first page of PDF file. Document formatted into pages; contains xxxi, 309 p.; also includes graphics Includes bibliographical references. Available online via OhioLINK's ETD Center
10
Dogbe, John Kofi. "Comparing cluster and slab model geometries from density functional theory calculations of si(100)-2x1 surfaces using low-energy electron diffraction". abstract and full text PDF (free order & download UNR users only), 2007. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3258835.
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Mahler, Andrew. "The One Electron Basis Set: Challenges in Wavefunction and Electron Density Calculations". Thesis, University of North Texas, 2016. https://digital.library.unt.edu/ark:/67531/metadc849642/.
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In the exploration of chemical systems through quantum mechanics, accurate treatment of the electron wavefunction, and the related electron density, is fundamental to extracting information concerning properties of a system. This work examines challenges in achieving accurate chemical information through manipulation of the one-electron basis set.
12
Stauffert, Oliver [Verfasser], i Michael [Akademischer Betreuer] Walter. "Electron-phonon coupling with density functional theory". Freiburg : Universität, 2019. http://d-nb.info/1191689328/34.
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Zawadzki, Krissia de. "Density-functional theory for single-electron transistors". Universidade de São Paulo, 2018. http://www.teses.usp.br/teses/disponiveis/76/76131/tde-24102018-165237/.
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The study of transport in nano-structured devices and molecular junctions has become a topic of great interest with the recent call for quantum technologies. Most of our knowledge has been guided by experimental and theoretical studies of the single-electron transistor (SET), an elementary device constituted by a quantum dot coupled to two otherwise independent free electron gases. The SET is particularly interesting because its transport properties at low temperatures are governed by the Kondo effect. A methodological difficulty has nonetheless barred theoretical progress in describing accurately realistic devices. On the one hand, Density-Functional Theory (DFT), the most convenient tool to obtain the electronic structure of complex materials, yields only qualitatively descriptions of the low-temperature physical properties of quantum dot devices. On the other hand, a quantitative description of low-temperature transport properties of the SET, such that obtained through the solution of the Anderson model via exact methods, is nonetheless unable to account for realistic features of experimental devices, such as geometry, band structure and electron-electron interactions in the electron gases. DFT describes the electron gases very well, but proves inadequate to treat the electronic correlations introduced by the quantum dot. This thesis proposes a way out of this frustrating dilemma. Our contribution is founded on renormalization-group (RG) concepts. Specifically, we show that, under conditions of experimental interest, the high and low temperatures regimes of a SET corresponds to the weakly-coupling and strongly-coupling fixed points of the Anderson Hamiltonian. Based on an RG analysis, we argue that, at this low-temperature fixed point, the entanglement between impurity and gas-electron spins introduces non-local correlations that lie beyond the reach of local- or quasi-local-density approximations, hence rendering inadequate approximations for the exchange-correlation energy functional. By contrast, the weak-coupling fixed point is within the reach of local-density approximations. With a view to describing realistic properties of quantum dot devices, we therefore propose a hybrid self-consistent procedure that starts with the weak-coupling fixed point and takes advantage of a reliable numerical method to drive the Hamiltonian to the strong-coupling fixed point. Our approach employs traditional DFT to treat the weak-coupling system and the Numerical Renormalization-Group (NRG) method to obtain properties in the strongcoupling regime. As an illustration, we apply the procedure to a single-electron transistor modeled by a generalized one-dimensional Hubbard Hamiltonian. We analyze the thermal dependence of the conductance in the SET and discuss its behavior at low-temperatures, comparing our results with other self-consistent approaches and with experimental data.O estudo de propriedades de transporte em dispositivos nano estruturados e junções moleculares tornou-se um tópico de grande interesse com a recente demanda por novas tecnologias quânticas. Grande parte do nosso conhecimento tem sido guiado por trabalhos experimentais e teóricos de um dispositivo conhecido como transístor de um elétron (SET), o qual é constituído por um ponto quântico acoplado a dois gases de elétrons independentes. O SET é particularmente interessante devido as suas propriedades de transporte a baixas temperaturas, as quais são governadas pelo efeito Kondo. Uma dificuldade metodológica, no entanto, tem barrado novos avanços teóricos para se obter uma descrição precisa de dispositivos realistas. Por um lado, a teoria do funcional da densidade (DFT), uma das ferramentas mais convenientes para calcular a estrutura eletrônica de materiais complexos, provê uma descrição apenas qualitativa das propriedades de transporte de transístores quânticos a baixas temperaturas. Por outro lado, uma descrição quantitativa satisfatória do SET a baixas temperaturas, tal como a modelagem e solução do modelo de Anderson via métodos exatos, é incapaz de levar em conta características realistas de dispositivos complexos, tal como geometria, estrutura de bandas e interações inter eletrônicas nos gases de elétrons. Embora a DFT os descreva bem, ela é inadequada para tratar correlações introduzidas pelo ponto quântico. Na presente tese propomos uma alternativa para este dilema. Nossa contribuição é fundamentada em conceitos de grupo de renormalização (RG). Especificamente, mostramos que, em condições de interesse experimental, os regimes de altas e baixas temperaturas em um SET correspondem aos pontos fixos de acoplamento fraco e forte do Hamiltoniano de Anderson. Baseando-nos em na análise do RG, mostramos que, no ponto fixo de baixas temperaturas, o emaranhamento entre a impureza e os spins dos gases eletrônicos introduz correlações não-locais que não podem ser descritas com abordagens DFT baseadas em aproximações locais ou quase locais para o potencial de troca e correlação. Em contraste, o ponto fixo de acoplamento fraco pode ser descrito por aproximações locais. Com o objetivo de obter uma descrição realista das propriedades de transístores quânticos, propomos um procedimento auto-consistente que começa do ponto fixo de acoplamento fraco e se aproveita de um método numérico eficiente para levar o Hamiltoniano para o ponto fixo de acoplamento forte. Nossa abordagem emprega DFT para tratar o sistema no limite de acoplamento fraco e o método de Grupo de Renormalização Numérico (NRG) para obter propriedades no regime de acoplamento forte. Como ilustração, aplicamos o procedimento para um transístor de um elétron modelado através do Hamiltoniano de Hubbard generalizado. Analisamos a dependência térmica da condutância no SET discutindo seu comportamento a baixas temperatura e comparamos nossos resultados com outras abordagens auto-consistentes e resultados experimentais.
14
Armiento, Rickard. "The many-electron energy in density functional theory : from exchange-correlation functional design to applied electronic structure calculations". Doctoral thesis, Stockholm : AlbaNova Universitetscentrum, Skolan för Teknikvetenskap, Kungliga Tekniska högskolan, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-428.
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Voloshina, Elena, Denis Usvyat, Martin Schütz, Yuriy Dedkov i Beate Paulus. "On the physisorption of water on graphene: a CCSD(T) study". Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-138776.
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The electronic structure of the zero-gap two-dimensional graphene has a charge neutrality point exactly at the Fermi level that limits the practical application of this material. There are several ways to modify the Fermi-level-region of graphene, e.g. adsorption of graphene on different substrates or different molecules on its surface. In all cases the so-called dispersion or van der Waals interactions can play a crucial role in the mechanism, which describes the modification of electronic structure of graphene. The adsorption of water on graphene is not very accurately reproduced in the standard density functional theory (DFT) calculations and highly-accurate quantum-chemical treatments are required. A possibility to apply wavefunction-based methods to extended systems is the use of local correlation schemes. The adsorption energies obtained in the present work by means of CCSD(T) are much higher in magnitude than the values calculated with standard DFT functional although they agree that physisorption is observed. The obtained results are compared with the values available in the literature for binding of water on the graphene-like substratesDieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich
16
Coe, Jeremy Patrick. "Entanglement and density-functional theory in two-electron systems". Thesis, University of York, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.521163.
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Bhandari, Srijana. "AN ELECTRONIC STRUCTURE APPROACH TO UNDERSTAND CHARGE TRANSFERAND TRANSPORT IN ORGANIC SEMICONDUCTING MATERIALS". Kent State University / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=kent1606836665551399.
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Voloshina, Elena, Denis Usvyat, Martin Schütz, Yuriy Dedkov i Beate Paulus. "On the physisorption of water on graphene: a CCSD(T) study". Royal Society of Chemistry, 2011. https://tud.qucosa.de/id/qucosa%3A27779.
Pełny tekst źródłaStreszczenie:
The electronic structure of the zero-gap two-dimensional graphene has a charge neutrality point exactly at the Fermi level that limits the practical application of this material. There are several ways to modify the Fermi-level-region of graphene, e.g. adsorption of graphene on different substrates or different molecules on its surface. In all cases the so-called dispersion or van der Waals interactions can play a crucial role in the mechanism, which describes the modification of electronic structure of graphene. The adsorption of water on graphene is not very accurately reproduced in the standard density functional theory (DFT) calculations and highly-accurate quantum-chemical treatments are required. A possibility to apply wavefunction-based methods to extended systems is the use of local correlation schemes. The adsorption energies obtained in the present work by means of CCSD(T) are much higher in magnitude than the values calculated with standard DFT functional although they agree that physisorption is observed. The obtained results are compared with the values available in the literature for binding of water on the graphene-like substrates.Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.
19
Ramsden, James. "Properties of exact density functionals for electronic quantum transport". Thesis, University of York, 2013. http://etheses.whiterose.ac.uk/6189/.
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Density functional theory and its extension in the nonequilibrium regime, time-dependent density functional theory, are powerful tools for predicting the structures, energies and dynamics of electronic systems. Their usefulness derives from the Kohn-Sham scheme whereby a system of real, interacting particles is replaced by a fictitious system of non-interacting particles subject to an effective external potential instead of a pairwise particle-particle interaction. The Kohn-Sham universe yields the same observable phenomena as that predicted by standard quantum mechanics so long as the effective external potential is known. However, for the vast majority of systems it is not known, and the usually local (in time and space) functional approximations employed do not capture the physics of true nonlocal interactions. In this thesis, the exact charge and current densities of model quantum transport devices described by nonlocal potentials are studied and methods for reverse-engineering the corresponding exact Kohn-Sham effective external potential for time-dependent and steady-state density functional theory approaches to the same systems are presented, as well as the resulting exact potentials themselves. Features of improved functionals for calculating approximate Kohn-Sham systems are demonstrated. These functionals are suggested to be very different from existing functionals employed, describing not potentials but electric and magnetic fields, and have a strong dependence on the local and semilocal charge and current density.
20
Manoli, Soheil Dimitri. "The generalized exchange local spin density-functional theory /". Thesis, McGill University, 1986. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=75359.
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An orbital dependent local spin density-functional (LSD) scheme with a generated exchange, the LSD GX scheme, has been developed based on the correct normalization conditions of an electron gas. This scheme contains no adjustable parameters; the B$ sb1$, B$ sb2$ and $ alpha sp lim$ are constant for all atoms once the shape of the Fermi hole is chosen. These parameters are rigorously calculated using an unspecified Fermi hole correlation factor and they give an exchange density which reduces exactly to the homogeneous free electron gas one at the high electron density limit.The LSD GX exchange density is corrected for self-interaction (SI) by splitting the total Fermi hole correlation factor into pure-exchange and self-interaction holes.
These new LSD and SI corrected schemes are compared to each other. They also compare very well theoretically and numerically (total energies and eigenvalues) with other local schemes current in the literature.
New equations for the IP and electronegativities of the atoms in these local schemes are derived which give good results.
21
Koivisto, Michael William. "Study of a non-interacting, nonuniform electron gas in two dimensions". Thesis, Kingston, Ont. : [s.n.], 2007. http://hdl.handle.net/1974/905.
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Van, Caillie Carole. "Electronic structure calculations using time-dependent density functional theory". Thesis, University of Cambridge, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.621205.
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Dednam, Wynand. "Atomistic simulations of competing influences on electron transport across metal nanocontacts". Thesis, Universidad de Alicante, 2019. http://hdl.handle.net/10500/26155.
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In our pursuit of ever smaller transistors, with greater computational throughput, many
questions arise about how material properties change with size, and how these properties
may be modelled more accurately. Metallic nanocontacts, especially those for which
magnetic properties are important, are of great interest due to their potential spintronic
applications. Yet, serious challenges remain from the standpoint of theoretical and
computational modelling, particularly with respect to the coupling of the spin and lattice
degrees of freedom in ferromagnetic nanocontacts in emerging spintronic technologies. In
this thesis, an extended method is developed, and applied for the first time, to model the
interplay between magnetism and atomic structure in transition metal nanocontacts. The
dynamic evolution of the model contacts emulates the experimental approaches used in
scanning tunnelling microscopy and mechanically controllable break junctions, and is
realised in this work by classical molecular dynamics and, for the first time, spin-lattice
dynamics. The electronic structure of the model contacts is calculated via plane-wave and
local-atomic orbital density functional theory, at the scalar- and vector-relativistic level of
sophistication. The effects of scalar-relativistic and/or spin-orbit coupling on a number of
emergent properties exhibited by transition metal nanocontacts, in experimental
measurements of conductance, are elucidated by non-equilibrium Green’s Function
quantum transport calculations. The impact of relativistic effects during contact formation
in non-magnetic gold is quantified, and it is found that scalar-relativistic effects enhance the force of attraction between gold atoms much more than between between atoms which
do not have significant relativistic effects, such as silver atoms. The role of non-collinear
magnetism in the electronic transport of iron and nickel nanocontacts is clarified, and it is
found that the most-likely conductance values reported for these metals, at first- and lastcontact,
are determined by geometrical factors, such as the degree of covalent bonding in
iron, and the preference of a certain crystallographic orientation in nickel.Physics
Ph. D. (Physics)
24
Zhang, Lei, i 張磊. "First principle calculation: current density in AC electric field". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2009. http://hub.hku.hk/bib/B43278437.
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Zhang, Lei. "First principle calculation : current density in AC electric field /". Click to view the E-thesis via HKUTO, 2009. http://sunzi.lib.hku.hk/hkuto/record/B43278437.
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Oprea, Corneliu I. "Density Functional Response Theory with Applications to Electron and Nuclear Magnetic Resonance". Doctoral thesis, Stockholm, : Bioteknologi, Kungliga Tekniska högskolan, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4367.
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Gabriel, Margaret A. "Electronic defects in amorphous silicon dioxide /". Thesis, Connect to this title online; UW restricted, 2007. http://hdl.handle.net/1773/8553.
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Cranswick, Matthew A. "Gas-phase Photoelectron Spectroscopy and Computational Studies of Metal-thiolate Interactions: Implications to Biological Electron Transfer". Diss., The University of Arizona, 2008. http://hdl.handle.net/10150/195569.
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The research outlined in this dissertation focuses on understanding the role of metal-sulfur interactions as applied to bioinorganic and organometallic systems. This metal-sulfur interaction is analyzed using both gas-phase photoelectron spectroscopy (PES) and density functional theory (DFT). Gas-phase photoelectron spectroscopy is the most direct probe of electronic structure and is used in these studies to probe the molecular orbital energy levels of these model compounds, giving rise to an understanding of the metal and sulfur orbital interactions and characters (i.e. is an orbital primarily metal or sulfur based). Using density functional theory, orbital energies, overlap, and characters can be calculated and complement the PES experiments allowing for a detailed understanding of the electronic structure. The first part of my dissertation explains the design and implementation of a dual source gas-phase ultraviolet/X-ray photoelectron spectrometer (UPS/XPS). This gas-phase UPS/XPS can be used to quantify the bonding/antibonding character of frontier molecular orbitals, with specific applications to metal-sulfur interactions, allowing for a thorough analysis of the metal-sulfur interaction. The second part of the dissertation explores using model complexes, of the type Cp₂V(dithiolate) (where Cp is cyclopentadienyl and dithiolate is 1,2-ethenedithiolate or 1,2-benzenedithiolate), along with PES and DFT calculations to investigate the role of the pyranopterindithiolate cofactor and the d¹ electron configuration in modulating the redox potential and electron transfer in the active sites of molybdenum enzymes. This study shows that the d¹ electronic configuration offers a low energy electron transfer pathway for the reoxidation of the active site molybdenum center. The third part of the dissertation explores the use of model compounds that specifically focus on iron-thiolate interactions in biological systems, and the effect of electronic energy matching and sterics on the oxidation potential of this interaction. This study has shown that the metal-sulfur interaction is sensitive to the orientation of the thiolate ligand, and that during oxidation an “electronic-buffering effect” makes assigning a formal oxidation state to the metal center almost meaningless. All of these studies illustrate how the thiolate ligand can modulate the electron density and oxidation potential of the metal-sulfur interaction and the implication of this interaction to biological electron transfer.
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Kowalczyk, Timothy Daniel. "Excited states and electron transfer in solution : models based on density functional theory". Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/73432.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2012.Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (p. 161-185).
Our understanding of organic materials for solar energy conversion stands to benefit greatly from accurate, computationally tractable electronic structure methods for excited states. Here we apply two approaches based on density functional theory (DFT) to predict excitation energies and electron transfer parameters in organic chromophores and semiconductors in solution. First, we apply constrained DFT to characterize charge recombination in a photoexcited donor-acceptor dyad and to understand the photophysical behavior of a fluorescent sensor for aqueous zinc. Second, we discover that the delta-self-consistent-field ([Delta]SCF) approach to excited states in DFT offers accuracy comparable to that of the better-established but more indirect linear-response time-dependent DFT approach, and we offer some justification for the similarity. Finally, we investigate a spin-restricted analog of [Delta]SCF known as restricted open-shell Kohn-Sham (ROKS) theory. We resolve a known ambiguity in the formal solution of the ROKS equations for the singlet excited state by presenting a self-consistent implementation of ROKS with respect to the mixing angle between the two open shells. The excited state methods developed and applied in this work contribute to the expanding toolkit of electronic structure theory for challenging problems in the characterization and design of organic materials.
by Timothy Daniel Kowalczyk.
Ph.D.
30
Wijewardane, Harshani Ovamini. "Nonlinear intersubband dynamics in semiconductor nanostructures". Diss., Columbia, Mo. : University of Missouri-Columbia, 2007. http://hdl.handle.net/10355/4744.
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Thesis (Ph. D.)--University of Missouri-Columbia, 2007.The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on December 17, 2007) Vita. Includes bibliographical references.
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Östlin, Andreas. "Electronic structure studies and method development for complex materials". Doctoral thesis, KTH, Tillämpad materialfysik, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-167109.
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Over the years electronic structure theory has proven to be a powerful method with which one can probe the behaviour of materials, making it possible to describe and predict material properties. The numerical tools needed for these methods are always in need of development, since the desire to calculate more complex materials pushes this field forward. This thesis contains work on both this implementational and developmental aspects. It begins by reviewing density functional theory and dynamical mean field theory, with the aim of merging these two methods. We point out theoretical and technical issues that may occur while doing this. One issue is the Padé approximant, which is used for analytical continuation. We assess the approximant and point out difficulties that can occur, and propose and evaluate methods for their solution. The virial theorem is assessed within the framework of density functional theory merged with many-body methods. We find that the virial theorem is extended from its usual form, and confirm this by performing practical calculations. The unified theory of crystal structure for transition metals has been established a long time ago using early electronic structure calculations. Here we implement the first- principles exact muffin-tin orbitals method to investigate the structural properties of the 6d transition metals. The goal of our study is to verify the existing theory for the mostly unknown 6d series and the performance of the current state-of-the art in the case of heavy d metals. It is found that these elements behave similarly to their lighter counterparts, except for a few deviations. In these cases we argue that it is relativistic effects that cause this anomalous behaviour. Palladium is then studied, taking many-body effects into account. We find that we can reproduce experimental photoemission spectra by these methods, as well as the Fermi surface. The thesis ends with an investigation of the stacking fault energies of the strongly correlated metal cerium. In addition to providing the first ab-initio stacking fault data for the two cubic phases of Ce, we discuss how these results could have an impact on the interpretation of the phase diagram of ceriumQC 20150522
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Jenkins, Anne Ceri. "Applications of spin-polarised relativistic scattering theory to the calculation of the electronic properties of heavy metals and alloys". Thesis, Keele University, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.321408.
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Odavić, Jovan Verfasser], Volker [Akademischer Betreuer] [Meden, Herbert [Akademischer Betreuer] Schoeller i Nicole [Akademischer Betreuer] Helbig. "Density oscillations of one-dimensional correlated electron systems from density functional theory / Jovan Odavić ; Volker Meden, Herbert Schoeller, Nicole Helbig". Aachen : Universitätsbibliothek der RWTH Aachen, 2019. http://d-nb.info/1195151691/34.
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Wood, Hayley Marie. "Density functional studies of relativistic effects on molecular properties". Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/density-functional-studies-of-relativistic-effects-on-molecular-properties(9f362361-5c09-4b35-a296-dec927ce7b7b).html.
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Relativistic effects are extremely important for heavy atoms and heavy atom containing molecules. Therefore, a relativistic treatment is needed when calculating molecular properties of these species. The fully- relativistic Dirac treatment involves electronic and positronic wavefunctions and a very large basis set is required. This leads to calculations that are too costly and time-consuming for larger molecules. The Zeroth-Order Regular Approximation (ZORA) is an approximation to the Dirac approach, which only deals with the electronic wavefunction. However, unfortunately this method is plagued by the gauge-dependence problem. The gauge-independent ZORA (ZORA-GI) and strictly atomic ZORA approaches provide solutions to this problem.In this work, the ZORA-GI and strictly atomic ZORA codes have been successfully implemented into the Gaussian 09 program. They have been used to calculate the bond lengths, harmonic vibrational frequencies and dissociation energies of the I2, Au2 and Pt2 diatomic molecules. The results show good agreement with experiment and previous theoretical studies. The non-relativistic, ZORA-GI, strictly atomic ZORA and pseudopotential approximations have been used to investigate the electronic structure of the actinide monoxides, AnO, and actinide monoxide cations, AnO+ (An = Th – Cm). It was found that the ground state configurations were dependent on the relativistic approximation chosen. The bond lengths, harmonic vibrational frequencies and dissociation energies were also calculated, with the ZORA methods generally outperforming the pseudopotential approximation. The first theoretical g-tensor study of the organouranium(V) complexes [U(C7H7)2]-, [U(η8-C8H8)(NEt2)(THF)]+, [U(η5-C5H5)(NMe2)3(THF)]+, [U(η8-C8H8)(NEt2)3], [U(η5-C5H5)2(NEt2)2]+ and [U(η8-C8H8)(η5-C5H5)(NEt2)2] has been carried out. It was demonstrated that the choice of density functional affects the way in which the g-tensor axes are assigned. The ground state spin density and SOMO are also sensitive to the choice of density functional. It is these factors that determine the value of the g-tensor.
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Botti, Silvana. "Electronic excitations in complex systems: beyond density functional theory for real materials". Habilitation à diriger des recherches, Université Claude Bernard - Lyon I, 2010. http://tel.archives-ouvertes.fr/tel-00520068.
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Aujourd'hui il est possible d'étudier à partir des premier principes la réponse sous excitation de matériaux utilisés dans des applications modernes très variés. En effet, grâce à de récents développements théoriques, ainsi qu'à l'optimisation des algorithmes de calcul, les simulations ab initio ne sont plus seulement limitées à des systèmes idéaux simplifiés, mais elles ont finalement l'ambition de capturer toute la complexité de l'échantillon testé dans l'expérience. Dans ce contexte, ce mémoire porte sur l'étude, à l'aide de différentes approches ab initio, des excitations électroniques dans une gamme de matériaux complexes et nanostructurés. Pour accéder aux excitations électroniques, la connaissance de la densité de l'état fondamental du système n'est plus suffisante, ce qui signifie que l'on doit trouver le moyen approprié d'aller au-delà de la théorie de la fonctionnelle de la densité (DFT) standard. Deux voies ont été intensivement explorées: l'une est basée sur la densité dépendante du temps et l'autre sur les fonctions de Green. La théorie de la fonctionnelle de la densité dépendante du temps (TDDFT) a été proposée en 1984 par Runge et Gross, qui ont dérivé un théorème du type Hohenberg-Kohn pour l'équation de Schrödinger en fonction du temps. Le champ d'application de cette généralisation de la théorie de la fonctionnelle de la densité inclut le calcul des spectres de photo-absorption ou, plus généralement, l'étude de l'interaction de la matière avec des champs électromagnétiques ou des particules qui la perturbent. À présent, l'application la plus populaire de cette théorie est l'extraction des propriétés de l'état électronique excité, et en particulier des fréquences d'excitation électroniques. En appliquant la TDDFT, après avoir déterminé l'état fondamental d'une molécule ou un agrégat, nous pouvons explorer et comprendre son spectre d'absorption, ayant en même temps des informations extrêmement détaillées sur le comportement du système excité. La complexité du problème à plusieurs corps en TDDFT est cachée dans le potentiel d'échange et de corrélation dépendant du temps qui apparaît dans les équations de Kohn- Sham et pour lequel il est primordial de trouver une bonne approximation. Beaucoup d'approximations ont été proposées et testées pour les systèmes finis, où même la très simple approximation TDLDA a souvent donné de très bons résultats. En général, les approximations existantes pour la fonctionnelle d'échange et corrélation fonctionnent assez bien pour certaines propriétés, mais elles se montrent insuffisantes pour d'autres. Dans le cas des matériaux solides, la TDDLA ne parvient pas à reproduire les spectres d'absorption optique, qui sont par contre bien décrits par la résolution de l'équation de Bethe-Salpeter en combinaison avec l'approximation GW pour les états de quasi-électron. D'autre part, la TDLDA peut déjà conduire à des résultats excellents pour la fonction de perte d'énergie d'un solide. La solution de l'équation de Bethe-Salpeter est beaucoup plus onéreuse du point de vue numérique. Ainsi, on poursuit encore la recherche d'approximations fiables en TDDFT, et au fil du temps, on espère atteindre la même maturité qu'on trouve maintenant dans la DFT pour l'état fondamental. En particulier, de nouvelles perspectives (et ses limites) ont étés révélées pendant ces dernières années grâce à la combinaison de deux théories distinctes : la TDDFT et l'approche des fonctions de Green (dont l'approximation GW et l'équation de Bethe- Salpeter font partie). Ces deux approches peuvent partager dans la pratique le point de départ commun de la théorie de la fonctionnelle de la densité pour le calcul de l'état fondamental électronique. Leur combinaison permet d'allier la simplicité de l'une (TDDFT) avec la précision de l'autre (GW et Bethe-Salpeter), afin d'en déduire des noyaux d'échange et de corrélation pour les solides. À partir de ces noyaux nous avons aussi travaillé sur le développement de noyaux modèles pour des applications efficaces à des systèmes de grande taille. Le présent mémoire contient une vue d'ensemble relativement condensée de la TDDFT et des approches basées sur la théorie des fonctions de Green, avec des applications aux domaines des nanotechnologies, aux matériaux photovoltaïques et au stockage de données. Ces applications ont constitué notre principal sujet de recherche au cours des dernières années. Ce mémoire est organisée comme suit. Avant d'entrer dans le domaine des approches pour les états excités, nous donnons dans le chapitre 1 un bref aperçu des idées de base de la DFT pour l'état fondamental, ce qui nous permet d'expliquer pourquoi il faut aller au-delà de la DFT standard, d'introduire quelques concepts-clés et de fixer la notation de base qui sera utilisée dans ce mémoire. Les chapitres suivants font un point sur la théorie formelle, avec une brève présentation des approches théoriques utilisées pour étudier les excitations électroniques: le chapitre 2 est dédié aux approches GW et à l'équation de Bethe-Salpeter, tandis que la TDDFT et la théorie de la réponse linéaire sont décrites dans le chapitre 3. Les noyaux dérivés à partir de l'équation de Bethe-Salpeter et notre travail sur les noyaux modèles sont discutés dans le chapitre 4. Le chapitre 5 contient des applications de la TDDFT dans le domaine de la réponse linéaire aux nanostructures. L'objectif principal est d'obtenir des spectres fiables (en général des spectres d'absorption) à partir de calculs de premiers principes. En comparant ces spectres avec des courbes expérimentales, on peut normalement déduire des informations importantes qui ne sont pas directement accessibles dans les expériences. D'autre part, la connaissance détaillée des propriétés d'excitation électronique contribue à une meilleure compréhension de la physique de ces systèmes dans leur généralité. Le chapitre 6 présente des applications à des matériaux solides d'intérêt technologique. En particulier, nous nous sommes intéressé aux propriétés optiques des matériaux à changement de phase, utilisés dans le DVD re-inscriptibles, ainsi que aux états électroniques des absorbeurs et des oxydes transparents conducteurs pour les cellules solaires à couches minces. Le chapitre 7 est dédié aux cruciales interactions de van der Waals et au calcul – via la TDDFT – des paramètres qui les décrivent. Nous discutons à la fois des interactions entre deux agrégats, et entre un agrégat et une surface semi-conductrice. Le dernier chapitre 8 fait le point sur les résultats de notre réflexion.
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Sena, A. M. P. "Density functional theory studies of surface interactions and electron transfer in porphyrins and other molecules". Thesis, University College London (University of London), 2010. http://discovery.ucl.ac.uk/147572/.
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This thesis contains a series of density functional studies on porphyrins, surfaces and other molecules, that are of relevance to surface science and electron transfer. In chapter 1 the main concepts of the thesis and how they fit together, are outlined. Chapter 2 describes density functional theory (DFT), the principle theoretical technique used throughout. The thesis then considers two main aspects. Chapters 3, 4 and 5 look at how systems interact with surfaces and compare and contrast situations of differing interaction strengths. Chapter 3 investigates the weak interaction of a haem molecule with the Si(111):H surface and studies how this interaction can be tuned by desorbing hydrogen atoms from the surface. In chapter 4, the structure of experimentally observed Mn nanolines on the Si(001) surface is studied. How these lines self assemble and interact strongly with the surface is discussed. Elements of these two studies are then combined in chapter 5 with a study of manganese porphyrin on the Si(001) surface displaying some features common to both previous systems. In chapter 6, 7 and 8 the focus switches to electron transfer. The basics of electron transfer theory are outlined in chapter 6. Then, the difficulties faced by DFT when studying electron transfer in large systems, such as the self-interaction error and cubic scaling, are described. Chapter 7 describes the constrained DFT formalism and its implementation into the linear scaling DFT code CONQUEST. In chapter 8, this implementation is used to perform some electron transfer calculations on small organic molecules, with systems demonstrating both charge localization and charge separation investigated. Chapter 9 concludes the thesis indicating how, following this thesis, large scale electron transfer calculations of organic molecules on surfaces can be performed with some confidence and giving suggestions for future calculations.
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Hellström, Matti. "Chemistry and Physics of Cu and H2O on ZnO Surfaces : Electron Transfer, Surface Triangles, and Theory". Doctoral thesis, Uppsala universitet, Strukturkemi, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-236302.
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This thesis discusses the chemistry and physics of Cu and H2O on ZnO surfaces, based primarily on results from quantum chemical calculations. The underlying context is heterogeneous catalysis, where Cu/ZnO-mixtures are used in the industrial synthesis of methanol and in the water gas shift reaction. Electron transfer between small Cu clusters and ZnO is central to this thesis, as are the design and use of models that can describe realistic and very large-scale ZnO surface structures while still retaining the electronic nature of the system. Method and model enhancements as well as tests and validations constitute a large part of this thesis. The thesis demonstrates that the charges of small Cu clusters, adsorbed on the non-polar ZnO(10-10) surface, depend on whether the Cu clusters contain an even or odd number of atoms, and whether water is present (water can induce electron transfer from Cu to ZnO). On the polar Zn-terminated ZnO(0001) surface, Cu becomes negatively charged, which causes it to attract positively charged subsurface defects and to wet the ZnO(0001) surface at elevated temperatures. When a Cu cluster on a ZnO surface becomes positively charged, this happens because it donates an electron to the ZnO conduction band. Hence, it is necessary to use a method which describes the ZnO band gap correctly, and we show that a hybrid density functional, which includes a fraction of Hartree-Fock exchange, fulfills this requirement. When the ZnO conduction band becomes populated by electrons from Cu, band-filling occurs, which affects the adsorption energy. The band-filling correction is presented as a means to extrapolate the calculated adsorption energy under periodic boundary conditions to the zero coverage (isolated adsorbate, infinite supercell) limit. A part of this thesis concerns the parameterization of the computationally very efficient SCC-DFTB method (density functional based tight binding with self-consistent charges), in a multi-scale modeling approach. Our findings suggest that the SCC-DFTB method satisfactorily describes the interaction between ZnO surfaces and water, as well as the stabilities of different surface reconstructions (such as triangularly and hexagonally shaped pits) at the polar ZnO(0001) and ZnO(000-1) surfaces.
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Iusan, Diana Mihaela. "Density Functional Theory Applied to Materials for Spintronics". Doctoral thesis, Uppsala universitet, Materialteori, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-119887.
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The properties of dilute magnetic semiconductors have been studied by combined ab initio, Monte Carlo, and experimental techniques. This class of materials could be very important for future spintronic devices, that offer enriched functionality by making use of both the spin and the charge of the electrons. The main part of the thesis concerns the transition metal doped ZnO. The role of defects on the magnetic interactions in Mn-doped ZnO was investigated. In the presence of acceptor defects such as zinc vacancies and oxygen substitution by nitrogen, the magnetic interactions are ferromagnetic. For dilute concentrations of Mn (~ 5%) the ordering temperature of the system is low, due to the short ranged character of the exchange interactions and disorder effects. The clustering tendency of the Co atoms in a ZnO matrix was also studied. The electronic structure, and in turn the magnetic interactions among the Co atoms, is strongly dependent on the exchange-correlation functional used. It is found that Co impurities tend to form nanoclusters and that the interactions among these atoms are antiferromagnetic within the local spin density approximation + Hubbard U approach. The electronic structure, as well as the chemical and magnetic interactions in Co and (Co,Al)-doped ZnO, was investigated by joined experimental and theoretical techniques. For a good agreement between the two, approximations beyond the local density approximation must be used. It is found that the Co atoms prefer to cluster within the semiconducting matrix, a tendency which is increased with Al co-doping. We envision that it is best to describe the system as superparamagnetic due to the formation of Co nanoclusters within which the interactions are antiferromagnetic. The magnetic anisotropy and evolution of magnetic domains in Fe81Ni19/Co(001) superlattices were investigated both experimentally, as well as using model spin dynamics. A magnetic reorientation transition was found.
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Zhan, Yuxuan. "Model-based high-density functional diffuse optical tomography of human brain". Thesis, University of Birmingham, 2013. http://etheses.bham.ac.uk//id/eprint/4450/.
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Functional diffuse optical tomography (fDOT) is an emerging functional neuroimaging technology that allows non-invasive imaging of human brain functions. The aims of this thesis are to enhance current understandings and knowledge of fDOT image quality and to improve on its imaging performance using a model-based approach. Specifically we have established a computationally efficient finite element method (FEM)-based routine to conduct MRI-guided fDOT simulation studies. Based on this framework, we have demonstrated that HD-fDOT is capable of imaging focal haemodynamic response up to 18 mm depth below the human scalp surface at 10 mm image resolution and localisation accuracy, allowing distinguishability of gyri. In addition, we also investigated the effects of uncertainty in the background tissue optical property on HD-fDOT image quality. Our multi-model comparative study has concluded that the use of a proposed homogeneous background absorption fitting scheme in HD-fDOT can minimise the chances of obtaining sub-optimal image quality due to uncertainty in background tissue optical properties. Finally we have addressed and resolved a regularisation problem in spectral fDOT that was previously not understood. Our proposed singular-decomposition-based regularisation method has been shown to reduce imaging crosstalk observed in both spectral and non-spectral fDOT.
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Odell, Anders. "Quantum transport in photoswitching molecules : An investigation based on ab initio calculations and Non Equilibrium Green Function theory". Licentiate thesis, KTH, Materials Science and Engineering, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4790.
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Molecular electronics is envisioned as a possible next step in device miniaturization. It is usually taken to mean the design and manufacturing of electronic devices and applications where organic molecules work as the fundamental functioning unit. It involves the easurement and manipulation of electronic response and transport in molecules attached to conducting leads. Organic molecules have the advantages over conventional solid state electronics of inherent small sizes, endless chemical diversity and ambient temperature low cost manufacturing.
In this thesis we investigate the switching and conducting properties of photochromic dithienylethene derivatives. Such molecules change their conformation in solution when acted upon by light. Photochromic molecules are attractive candidates for use in molecular electronics because of the switching between different states with different conducting properties. The possibility of optically controlling the conductance of the molecule attached to leads may lead to new device implementations.
The switching reaction is investigated with potential energy calculations for different values of the reaction coordinate between the closed and the open isomer. The electronic and atomic structure calculations are performed with density functional theory (DFT). It is concluded that there is a large potential energy barrier separating the open and closed isomer and that switching between open and closed forms must involve excited states.
The conducting properties of the molecule inserted between gold leads is calculated within the Non Equilibrium Green Function theory. The transmission function is calculated for the two isomers with different basis sizes for the gold contacts, as well as the electrostatic potential, for finite applied bias voltages. We conclude that a Au 6s basis give qualitatively the same result as a Au spd basis close to the Fermi level. The transmission coefficient at the Fermi energy is around 10 times larger in the closed molecule compared to the open. This will result in a large difference in conductivity. It is also found that the large difference in conductivity will remain for small applied bias voltages. The results are consistent with earlier work.
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Morgan, Stewart H. "Density functional studies of nanomaterials with applications in electronic devices and hydrogen storage". Thesis, University College London (University of London), 2016. http://discovery.ucl.ac.uk/1508444/.
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We present density functional calculations investigating two different nanomaterials: a titanium carbide nanocluster and few-layered black phosphorus. The titanium carbide nanocluster, Ti₈C₁₂, has properties that are well suited to applications in hydrogen storage, while few-layered black phosphorus has recently been used in the fabrication of novel field effect transistors. Chapter 1 provides some background information regarding hydrogen storage and electronic devices, with Chapter 2 introducing the computational methods used throughout subsequent chapters. In Chapter 3, we investigate the thermodynamic and kinetic profile of H₂ dissociation by Ti₈C₁₂ under a range of conditions. Our results show that that Ti₈C₁₂ is able to reversibly dissociate H₂ with an unusually low activation barrier. In Chapter 4, we introduce few-layered black phosphorus, dubbed phosphorene. The use of black-phosphorus exfoliates in FETs is potentially important given the fast approaching limits of transistor miniaturization using current technologies. Phosphorene appears to have properties necessary for use in next generation FETs, and has therefore attracted enormous experimental and theoretical attention. Our work on phosphorene contributes to an ever growing body of information, with Chapter 5 investigating the effects of deforming monolayer and bilayer phosphorene and Chapter 6 investigating the properties of phosphorene nanoribbons. In Chapter 5, we show that compressing bilayer phosphorene normal to its surface dramatically increases n-type mobility and modulates the band gap. The compressions required to increase n-type mobility by a factor of 10² are modest, meaning that our results are experimentally relevant. We also investigate the effects of bending of phosphorene, which has a highly anisotropic bending modulus. Our work on phosphorene nanoribbons in Chapter 6 shows that in-plane quantum confinement effects lead to a significant increase in the band gap. We replicate this effect by applying periodic boundary conditions to the bulk and derive a formula relating the band gap of phosphorene nanoribbons to phosphorene's band edge effective masses. We also show that the band gap and mobility of phosphorene nanoribbons can be modified through the application of linear strain. II Chapter 7 concludes the main body of this thesis, summarising its outcomes and giving a direction for future work. We also include a brief investigation into a family of semiconducting quaternary oxynitride compounds in the Appendix. These compounds are of interest given that their band gaps fall within the visible light region.
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Casadei, Marco [Verfasser]. "Density-Functional Theory for f-Electron Systems: The alpha-gamma Phase Transition in Cerium / Marco Casadei". Berlin : Freie Universität Berlin, 2013. http://d-nb.info/104429423X/34.
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Fisher, Harry. "First-principles investigation of electron-phonon interactions in novel superconductors". Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:1f2fee8c-28fa-4d2f-998f-ec6abf81de3e.
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Despite over 100 years of scientific research, a full understanding of superconductivity remains elusive. While it is known that the electron-phonon interaction is responsible for the formation of Cooper pairs in conventional superconductors, many superconductors exhibit behaviour suggestive of more exotic pairing mechanisms. In this thesis, two novel superconducting materials are considered, monolayer transition metal dichalcogenide, MoS2, and iron-based superconductor, LaFeAsO1−xFx. The former is ideal for the study of the electron-phonon interaction, as it not only has potential applications as an atomically thin transistor, but also displays a dome-shaped superconductive state as a function of electron doping. In the latter, the superconductive state emerges from a magnetic parent compound upon flourine doping. Its high critical temperature is thought to be enhanced by magnetic fluctuation rather than being purely phonon-mediated. By using novel first-principles techniques, the electron-phonon interaction in electron doped single-layer MoS2 is investigated. The superconducting gap is calculated using the Migdal-Eliashberg theory, and by considering the electronic structure and lattice dynamics in this material, an explanation is provided for the experimentally observed doping-dependent critical temperature in this material. The origin of the doping-induced transition from a magnetic phase to a nonmagnetic phase in LaFeAsO1−xFx is determined. A new model to capture the effects of the fluorine dopants is developed, which has implications for the electron-phonon interaction in this material.
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Löfås, Henrik. "Computational Studies of Electron Transport in Nanoscale Devices". Doctoral thesis, Uppsala universitet, Materialteori, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-209261.
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In this thesis, a combination of density functional theory (DFT) based calculations and nonequilibrium Green’s functions are employed to investigate electron transport in molecular switches, molecular cords and nanoscale devices. Molecular electronic devices have been proposed as an approach to complement today’s silicon based electronic devices. However, engineering of such miniature devices and design of functional molecular components still present significant challenges. First, the way to connect a molecule to conductive electrodes has to be controlled. We study, in a nanoelectrode-nanoparticle platform, how structural changes affect the measured conductance and how current fluctuations due to these structural changes can be decreased. We find that, for reproducible measurements, it is important to have the molecules chemically bonded to the surfaces of adjacent nanoparticles. Furthermore, we show by a combination of DFT and theoretical modeling that we can identify signals from single-molecules in inelastic electron spectroscopy measurements on these devices. Second, active elements based on molecules, some examples being switches, rectifiers or memory devices, have to be designed. We study molecular conductance switches that can be operated by light and/or temperature. By tuning the substituents on the molecules, we can optimize the shift of the most conducting molecular orbital and increase the effective coupling between the molecule and the electrodes when going from the OFF to the ON-state of the switches, giving high switching ratio (up to three orders of magnitude). We also study so called mechanoswitches that are activated by a mechanical force elongating the molecules, which means that these switches could operate as sensors. Furthermore, we have studied two different classes of compounds that may function either as rigid molecular spacers with a well-defined conductance or as molecular cords. In both cases, we find that it is of great importance to match the conjugation of the anchoring groups with the molecular backbone for high conductance. The last part of the thesis is devoted to another interesting semiconductor material, diamond. We have accurately calculated the band structure and effective masses for this material. Furthermore, these results have been used to calculate the Hall coefficient, the resistivity and the Seebeck coefficient.
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Muscenti, Thomas Michael. "Density Functional Theory Study of Rutile SiO₂ Stishovite: An Electron Pair Description of Bulk and Surface Properties". Thesis, Virginia Tech, 2004. http://hdl.handle.net/10919/10179.
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The bulk structure and the nonpolar, stoichiometric (110) surface of stishovite, rutile structure type SiO₂, has been studied using a first principles, density functional method. The geometric and electronic structure, including the density of states, charge density, and electron localization function for both the bulk and the surface have been examined. The electron pair properties of both bulk and surface-layer atoms were found to be similar to molecular analogs. The analogs allowed for the description of surface electronic structure using simple molecular models. The adsorption of hydrogen fluoride was studied on the (110) surface. The geometry optimized and electronic structure have been found for various initial geometries. Relaxed structures of certain initial geometries give dissociated hydrogen fluoride upon geometry optimization.Master of Science
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Wang, Baochang. "Electronic Structure and Optical Properties of Solar Energy Materials". Doctoral thesis, KTH, Flerskalig materialmodellering, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-145625.
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In this thesis, we have studied the electronic and optical properties of solar energy m-terials. The studies are performed in the framework of density functional theory (DFT), GW, Bethe-Salpeter equation (BSE) approaches and Kinetic Monte Carlo (KMC). We present four sets of results. In the first part, we report our results on the band gap engineering issues for BiNbO4and NaTaO3, both of which are good photocatalysts. The band gap tuning is required for these materials in order to achieve the maximum solar to hydrogen conversion efficiency. The most common method for the band gap reduction is an introduction of foreign elements. The mono-doping in the system generates electrons or holes states near band edges, which reduce the efficiency of photocatalytic process. Co-doping with anion and cation or anion and anion can provide a clean band gap. We have shown that further band gap reduction can be achieved by double-hole mediated coupling between two anionic dopants. In the second part, the structure and optical properties of (CdSxSe1x)42nanoclusters have been studied. Within this study, the structures of the (CdS)42, (CdSe)42, Cd42Se32S10, Cd42Se22S20, and Cd42Se10S32 clusters have been determined using the simulated annealing method. Factors influencing the band gap value have been analyzed. We show that the gap is most significantly reduced when strongly under coordinated atoms are present on the surface of the nanoclusters. In addition, the band gap depends on the S concentration as well as on the distribution of the S and Se atoms in the clusters. We present the optical absorption spectra calculated with BSE and random phase approximation (RPA) methods based on the GW corrected quasiparticle energies. In the third part, we have employed the state-of-art computational methods to investigate the electronic structure and optical properties of TiO2high pressure polymorphs. GW and BSE methods have been used in these calculations. Our calculations suggest that the band gap of fluorite and pyrite phases have optimal values for the photocatalytic process of decomposing water in the visible light range. In the fourth part we have built a kinetic model of the first water monolayer growth on TiO2(110) using the kinetic Monte Carlo (KMC) method based on parameters describing water diffusion and dissociation obtained from first principle calculations. Our simulations reproduce the experimental trends and rationalize these observations in terms of a competition between different elementary processes. At high temperatures our simulation shows that the structure is well equilibrated, while at lower temperatures adsorbed water molecules are trapped in hydrogen-bonded chains around pairs of hydroxyl groups, causing the observed higher number of molecularly adsorbed species at lower temperature.QC 20140603
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Aryal, Sita Ram Ching Wai-Yim. "Density functional calculations of the electronic structure of aluminosilicate polymorphs (Al₂SiO₅) and mullite". Diss., UMK access, 2007.
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Thesis (M.S.)--Dept. of Physics. University of Missouri--Kansas City, 2007."A thesis in physics." Typescript. Advisor: Wai-Yim Ching. Vita. Title from "catalog record" of the print edition Description based on contents viewed Sept. 12, 2008. Includes bibliographical references (leaves 53-58). Online version of the print edition.
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Gunasinghe, Rosi Najeela. "Structural and electronic properties of boron nano structures: A dispersion-corrected density functional study". DigitalCommons@Robert W. Woodruff Library, Atlanta University Center, 2012. http://digitalcommons.auctr.edu/dissertations/341.
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We have revisited the general constructing schemes for a large family of stable hollowboron fullerenes with 80 + 8n (n = 0,2,3,...) atoms. In contrast to the hollow pentagon boron fullerenes with 12 hollow pentagons, the stable boron fullerenes constitute 12 filled pentagons and 12 additional hollow hexagons, which are more stable than the empty pentagon boron fullerenes including the “magic” B80 buckyball. Based on results from first-principles density-functional calculations, an empirical rule for filled pentagons is proposed along with a revised electron counting scheme to account for the improved stability and the associated electronic bonding feature. We have also studied the relative stability of various boron fullerene structures and structural and electronic properties of B80 bucky ball and boron nanotubes via dispersion-corrected density-functional calculations. Our results reveal that the energy order of fullerenes strongly depends on the exchange-correlation functional employed in the calculation and the vibrational stability for the icosahedral B80 with the inclusion of dispersion corrections, in contrast to the instability to a tetrahedral B80 with puckered capping atoms from preceding density functional theory calculations. Similarly, the dispersion-corrected density-functional calculations yield non-puckered boron nanotube conformations and an associated metallic state for zigzag tubes. A systematic study elucidates the importance of incorporating dispersion forces to account for the intricate interplay of two and three centered bonding in boron nanostructures.
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Martin, Claudia. "Density functional study of the electronic and magnetic properties of selected transition metal complexes". Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2014. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-134958.
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Die vorliegende Promotionsarbeit “Density functional study of the electronic and magnetic properties of selected transition metal complexes” beschäftigt sich mit dem Zusammenhang zwischen strukturellen Merkmalen sowie elektronischen und magnetischen Eigenschaften von Einzelmolekül-Magneten. Im Wesentlichen konnte dabei gezeigt werden, dass die magnetischen Eigenschaften sowohl von strukturellen Merkmalen als auch von den elektronischen Eigenschaften bestimmt werden. Des Weiteren ergab sich, dass verschiedene Kenngrößen der magnetischen Eigenschaften (im speziellen der magnetische Grundzustand S sowie die magnetische Anisotropie D) miteinander korreliert sind. Dies ist im Besonderen für eine mögliche Anwendung von Einzelmolekül-Magneten im Bereich der Datenspeicherung von Bedeutung.
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Ghosh, Swarnava Ghosh. "Orbital-free density functional theory using higher-order finite differences". Thesis, Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/53603.
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Density functional theory (DFT) is not only an accurate but also a widely used theory for describing the quantum-mechanical electronic structure of matter. In this approach, the intractable problem of interacting electrons is simplified to a tractable
problem of non-interacting electrons moving in an effective potential. Even with this simplification, DFT remains extremely computationally expensive. In particular, DFT scales cubically with respect to the number of atoms, which restricts the size of systems that can be studied. Orbital free density functional theory (OF-DFT)
represents a simplification of DFT applicable to metallic systems that behave like a free-electron gas.
Current implementations of OF-DFT employ the plane-wave basis, the global nature of the basis prevents the efficient use of modern high-performance computer archi-
tectures. We present a real-space formulation and higher-order finite-difference implementation of periodic Orbital-free Density Functional Theory (OF-DFT). Specifically, utilizing a local reformulation of the electrostatic and kernel terms, we develop a gener-
alized framework suitable for performing OF-DFT simulations with different variants of the electronic kinetic energy. In particular, we develop a self-consistent field (SCF)
type fixed-point method for calculations involving linear-response kinetic energy functionals. In doing so, we make the calculation of the electronic ground-state and forces
on the nuclei amenable to computations that altogether scale linearly with the number
of atoms. We develop a parallel implementation of our method using Portable, Extensible Toolkit for scientific computations (PETSc) suite of data structures and routines.
The communication between processors is handled via the Message Passing Interface(MPI). We implement this formulation using the finite-difference discretization, us-
ing which we demonstrate that higher-order finite-differences can achieve relatively large convergence rates with respect to mesh-size in both the energies and forces.
Additionally, we establish that the fixed-point iteration converges rapidly, and that it can be further accelerated using extrapolation techniques like Anderson mixing. We verify the accuracy of our results by comparing the energies and forces with
plane-wave methods for selected examples, one of which is the vacancy formation energy in Aluminum. Overall, we demonstrate that the proposed formulation and
implementation is an attractive choice for performing OF-DFT calculations.