Quantum Transport Simulations of Nanoscale Materials

Handle URI:
http://hdl.handle.net/10754/593720
Title:
Quantum Transport Simulations of Nanoscale Materials
Authors:
Obodo, Tobechukwu Joshua ( 0000-0003-1511-0918 )
Abstract:
Nanoscale materials have many potential advantages because of their quantum confinement, cost and producibility by low-temperature chemical methods. Advancement of theoretical methods as well as the availability of modern high-performance supercomputers allow us to control and exploit their microscopic properties at the atomic scale, hence making it possible to design novel nanoscale molecular devices with interesting features (e.g switches, rectifiers, negative differential conductance, and high magnetoresistance). In this thesis, state-of-the-art theoretical calculations have been performed for the quantum transport properties of nano-structured materials within the framework of Density Functional Theory (DFT) and the Nonequilibrium Green's Function (NEGF) formalism. The switching behavior of a dithiolated phenylene-vinylene oligomer sandwiched between Au(111) electrodes is investigated. The molecule presents a configurational bistability, which can be exploited in constructing molecular memories, switches, and sensors. We find that protonation of the terminating thiol groups is at the origin of the change in conductance. H bonding at the thiol group weakens the S-Au bond, and thus lowers the conductance. Our results allow us to re-interpret the experimental data originally attributing the conductance reduction to H dissociation. Also examined is current-induced migration of atoms in nanoscale devices that plays an important role for device operation and breakdown. We studied the migration of adatoms and defects in graphene and carbon nanotubes under finite bias. We demonstrate that current-induced forces within DFT are non-conservative, which so far has only been shown for model systems, and can lower migration barrier heights. Further, we investigated the quantum transport behavior of an experimentally observed diblock molecule by varying the amounts of phenyl (donor) and pyrimidinyl (acceptor) rings under finite bias. We show that a tandem configuration of two dipyrimidinyl-diphenyl molecules improves the rectification ratio, and tuning the asymmetry of the tandem set-up by rearranging the molecular blocks greatly enhances it. It has been recently demonstrated that the large band gap of boronitrene can be significantly reduced by carbon functionalization. We show that specific defect configurations can result in metallicity, raising interest in the material for electronic applications. In particular, we demonstrate negative differential conductance with high peak-to-valley ratios, depending on the details of the material, and identify the finite bias effects that are responsible for this behavior. Also, we studied the spin polarized transport through Mn-decorated topological line defects in graphene. Strong preferential bonding is found, which overcomes the high mobility of transition metal atoms on graphene and results in stable structures. Despite a large distance between the magnetic centers, we find a high magnetoresistance and attribute this unexpected property to very strong induced π magnetism. Finally, the results obtained herein advance the field of quantum electronic transport and provide significant insight on switches, rectification, negative differential conductance, magnetoresistance, and current-induced forces of novel nanoscale materials.
Advisors:
Schwingenschlögl, Udo ( 0000-0003-4179-7231 )
Committee Member:
Manchon, Aurelien ( 0000-0002-4768-293X ) ; Eppinger, Jörg ( 0000-0001-7886-7059 ) ; Sanvito, Stefano
KAUST Department:
Physical Sciences and Engineering (PSE) Division; Materials Science and Engineering Program
Program:
Materials Science and Engineering
Issue Date:
7-Jan-2016
Type:
Dissertation
Appears in Collections:
Dissertations; Physical Sciences and Engineering (PSE) Division; Materials Science and Engineering Program

Full metadata record

DC FieldValue Language
dc.contributor.advisorSchwingenschlögl, Udoen
dc.contributor.authorObodo, Tobechukwu Joshuaen
dc.date.accessioned2016-01-18T14:01:46Zen
dc.date.available2016-01-18T14:01:46Zen
dc.date.issued2016-01-07en
dc.identifier.urihttp://hdl.handle.net/10754/593720en
dc.description.abstractNanoscale materials have many potential advantages because of their quantum confinement, cost and producibility by low-temperature chemical methods. Advancement of theoretical methods as well as the availability of modern high-performance supercomputers allow us to control and exploit their microscopic properties at the atomic scale, hence making it possible to design novel nanoscale molecular devices with interesting features (e.g switches, rectifiers, negative differential conductance, and high magnetoresistance). In this thesis, state-of-the-art theoretical calculations have been performed for the quantum transport properties of nano-structured materials within the framework of Density Functional Theory (DFT) and the Nonequilibrium Green's Function (NEGF) formalism. The switching behavior of a dithiolated phenylene-vinylene oligomer sandwiched between Au(111) electrodes is investigated. The molecule presents a configurational bistability, which can be exploited in constructing molecular memories, switches, and sensors. We find that protonation of the terminating thiol groups is at the origin of the change in conductance. H bonding at the thiol group weakens the S-Au bond, and thus lowers the conductance. Our results allow us to re-interpret the experimental data originally attributing the conductance reduction to H dissociation. Also examined is current-induced migration of atoms in nanoscale devices that plays an important role for device operation and breakdown. We studied the migration of adatoms and defects in graphene and carbon nanotubes under finite bias. We demonstrate that current-induced forces within DFT are non-conservative, which so far has only been shown for model systems, and can lower migration barrier heights. Further, we investigated the quantum transport behavior of an experimentally observed diblock molecule by varying the amounts of phenyl (donor) and pyrimidinyl (acceptor) rings under finite bias. We show that a tandem configuration of two dipyrimidinyl-diphenyl molecules improves the rectification ratio, and tuning the asymmetry of the tandem set-up by rearranging the molecular blocks greatly enhances it. It has been recently demonstrated that the large band gap of boronitrene can be significantly reduced by carbon functionalization. We show that specific defect configurations can result in metallicity, raising interest in the material for electronic applications. In particular, we demonstrate negative differential conductance with high peak-to-valley ratios, depending on the details of the material, and identify the finite bias effects that are responsible for this behavior. Also, we studied the spin polarized transport through Mn-decorated topological line defects in graphene. Strong preferential bonding is found, which overcomes the high mobility of transition metal atoms on graphene and results in stable structures. Despite a large distance between the magnetic centers, we find a high magnetoresistance and attribute this unexpected property to very strong induced π magnetism. Finally, the results obtained herein advance the field of quantum electronic transport and provide significant insight on switches, rectification, negative differential conductance, magnetoresistance, and current-induced forces of novel nanoscale materials.en
dc.language.isoenen
dc.subjectDFT/NEGFen
dc.subjectSingle moleculesen
dc.subjectSwitchesen
dc.subjectRectifiersen
dc.subjectQuantum Transporten
dc.subjectGrapheneen
dc.titleQuantum Transport Simulations of Nanoscale Materialsen
dc.typeDissertationen
dc.contributor.departmentPhysical Sciences and Engineering (PSE) Divisionen
dc.contributor.departmentMaterials Science and Engineering Programen
thesis.degree.grantorKing Abdullah University of Science and Technologyen_GB
dc.contributor.committeememberManchon, Aurelienen
dc.contributor.committeememberEppinger, Jörgen
dc.contributor.committeememberSanvito, Stefanoen
thesis.degree.disciplineMaterials Science and Engineeringen
thesis.degree.nameDoctor of Philosophyen
dc.person.id116780en
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