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School of Physics

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https://hdl.handle.net/1853/70755

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Organizational Unit

College of Sciences

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Center for Relativistic Astrophysics

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Mourigal Lab

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https://finding-aids.library.gatech.edu/agents/corporate_entities/156

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Publication Search Results

Now showing 1 - 10 of 11

Properties of two-dimensional materials grown on metal substrates

(Georgia Institute of Technology, 2019-03-29) Pan, Chi-Ruei

Supercell models are proposed to investigate the properties of three types of heterostructures formed by two-dimensional (2D) materials grown on metal substrates, including (1) silicon (Si) thin films on a silver (Ag) substrate; (2) a single-layer hexagonal boron nitride (h-BN) on ruthenium (Ru) and copper (Cu) substrates; and (3) a stacked combination of lead (Pb) and Ag thin films. Coverage, orbital hybridization, and interface conditions are studied in order to tailor the electronic properties of these heterostructures. For the first system, results show that a Si coverage beyond 2.5 ML is needed for the emergence of the nearly linear energy-momentum relation. This relation is associated with the electronic states induced by the interaction between surface Si and Ag. For the second system, results demonstrate that the nitrogen (N) orbitals can hybridize with the underlying metal orbitals, and thus the regions of the h-BN monolayer with N situated on top of a metal atom will move closer to the substrate, leading to a corrugated h-BN layer. Calculated spatially-periodic modulations of the band profile and the local work function are in agreement with the experimental results. For the third system, results illustrate that the presence of the substrate alters the boundary conditions and thus can change the phase shifts of the quantum well states at the interface. The combination of Pb and Ag films creates a joint potential well that supports combined quantum well states. These findings suggest that in our studied systems, the interaction between the 2D materials and the substrates plays an important role in determining their electronic properties.
Electronic, thermoelectric and vibrational properties of silicon nanowires and copper chalcogenides

(Georgia Institute of Technology, 2015-03-31) Zhuo, Keenan

Silicon nanowires (SiNWs) and the copper chalcogenides, namely copper sulfide (Cu2S) and selenide Cu2Se, have diverse applications in renewable energy technology. For example, SiNWs which have direct band gaps unlike bulk Si, have the potential to radically reduce the cost of Si based photovoltaic cells. However, they degrade quickly under ambient conditions. Various surface passivations have therefore been investigated for enhancing their stability but it is not yet well understood how they affect the electronic structure of SiNWs at a fundamental level. Here, we will explore, from first-principles simulation, how fluorine, methyl and hydrogen surface passivations alter the electronic structures of [111] and [110] SiNWs via strain and quantum confinement. We also show how electronic charge states in [111] and [110] SiNWs can be effectively modelled by simple quantum wells. In addition, we address the issue of why [111] SiNWs are less influenced by their surface passivation than [110] SiNWs. Like SiNWs, Cu2S and Cu2Se also make excellent photovoltaic cells. However, they are most well known for their exceptional thermoelectric performance. This is by virtue of their even more unique solid-liquid hybrid nature which combines the low thermal conductivity and good electrical characteristics required for a high thermoelectric efficiency. We use first-principles molecular dynamics simulations to show that Cu diffusion rates in Cu2S and Cu2Se can be as high as 10-5cm2s-1. We also relate their phonon power spectra to their low thermal conductivities. Furthermore, we evaluate the thermoelectric properties of Cu2S and Cu2Se using a combination of Boltzmann transport theory and first-principles electronic structure calculations. Our results show that both Cu2S and Cu2Se are capable of maintaining high Seebeck coefficients in excess of 200μVK-1 for hole concentrations as high as 3x1020cm-3.
Electronic structure and interlayer coupling in twisted multilayer graphene

(Georgia Institute of Technology, 2014-01-10) Xian, Lede

It has been shown recently that high-quality epitaxial graphene (EPG) can be grown on the SiC substrate that exhibits interesting physical properties and has great advantages for varies device applications. In particular, the multilayer graphene films grown on the C-face show rotational disorder. It is expected that the twisted layers exhibit unique new physics that is distinct from that of either single layer graphene or graphite. In this work, by combining density functional and tight-binding model calculations, we investigate the electric field and doping effects on twisted bilayer graphene (TBG), multiple layer effects on twisted triple-layer graphene, and wave packet propagation properties of TBG. Though these studies, we obtain a comprehensive description of the interesting interlayer interaction in this twisted multilayer graphene system.
First-principles calculations of helium cluster formation in palladium tritides

(Georgia Institute of Technology, 2010-05-20) Lin, Pei

The accumulation of helium atoms in metals or metal tritides is known to result in the formation of helium bubbles in the lattice and to cause degradation of the material. Helium is introduced either through neutron transmutation reaction or via the radioactive decay of tritium. We have performed first-principles calculations of interstitial helium inside Pd and Pd tritide using density functional theory (DFT) and the projector augmented-wave (PAW) method within the generalized gradient approximation (GGA). We model the growth process of an interstitial helium cluster and find that when the size of the cluster reaches to five atoms, the cluster can induce an energetically favorable vacancy with a self-trapping mechanism. The cluster growth mechanism of interstitial helium is addressed by investigating the associated energetics, cluster configurations, and electronic structural properties. In addition, we study the diffusion properties of helium in palladium-based compounds by performing the nudged elastic band (NEB) calculations. Our computational models propose that by loading the lattice with hydrogen atoms at certain concentration, or substituting with alloying metals can modify the diffusivity by increasing its migration barrier, which may impede the cluster formation in the beginning stage.
First-principles study of hydrogen storage materials

(Georgia Institute of Technology, 2008-03-24) Ma, Zhu

In this thesis, we use first-principles calculations to study the structural, electronic, and thermal properties of several complex hydrides. We investigate structural and electronic properties of Na-Li alanates. Although Na alanate can reversibly store H with Ti catalyst, its weight capacity needs to be improved. This can be accomplished by partial replacement of Na with lighter elements. We explore the structures of possible Na-Li alloy alanates, and study their phase stability. We also study the structural and thermal properties of Li/Mg/Li-Mg Amides/Imides. Current experimental results give a disordered model about the structure of Li-Mg Imide, in which the positions of Li and Mg are not specified. In addition the model gives a controversial composition stoichiometry. We try to resolve this controversy by searching for low-energy ordered phases. In the last part, we study the structural, energetic, and electronic properties of the La-Mg-Pd-H system. This quaternary system is another example of hydrogenation-induced metal-nonmetal transition without major reconstruction of metal host structure, and it is also with partial reversible H capacity. Experiment gives partially disordered H occupancy on two Wyckoff positions. Our calculation explains the structural and bonding characteristics observed in experiment.
Confinement effect on semiconductor nanowires properties

(Georgia Institute of Technology, 2007-11-02) Nduwimana, Alexis

Confinement effect on semiconductor nanowires properties. Alexis Nduwimana 100 pages Directed by Dr. Mei-Yin Chou We study the effect of confinement on various properties of semiconductor nanowires. First, we study the size and direction dependence of the band gap of germanium nanowires. We use the density functional theory in the local density approximation. Results shows that the band gap decreases with the diameter The susceptibility of these nanowires is also computed. Second, we look at the confinement effect on the piezoelectric coefficients of ZnO and AlN nanowires. The Berry phase method is used. It is found that depending on passivation, thepiezoelectric effect can decrease or increase. Finally, we study the size and direction dependence of the melting temperature of silicon nanowires. We use the molecular dynamics with the Stillinger Weber potential. Results indicate that the melting temperature increases with the nanowire diameter and that it is direction dependent.
First-principles Calculations on the Electronic, Vibrational, and Optical Properties of Semiconductor Nanowires

(Georgia Institute of Technology, 2006-08-15) Yang, Li

The first part of my PhD work is about the lattice vibrations in silicon nanowires. First-principles calculations based on the linear response are performed to investigate the quantum confinement effect in lattice vibrations of silicon nanowires (SiNW). The radial breathing modes (RBM) are found in our calculations, which have a different size-dependent frequency shift compared with the optical modes. They are well explained by the elastic model. Finally, the relative activity of the Raman scattering in the smallest SiNW is calculated. The RBM can be clearly identified in the Raman spectrum, which can be used to estimate the size of nanowires in experiment. In the second part of my PhD work, we focus on the electron-hole pair (exciton) in semiconductor nanowires and its influence on the optical absorption spectra. First-principles calculations are performed for a hydrogen-passivated silicon nanowire with a diameter of 1.2 nm. Using plane wave and pseudopotentials, the quasiparticle states are calculated within the so-called GW approximation, and the electron-hole interaction is evaluated with the Bethe-Salpeter Equation (BSE). The enhanced excitonic effect is found in the absorption spectrum. The third part of my work is about the electronic structure in Si/Ge core-shell nanowires. The electronic band structure is studied with first-principles methods. Individual conduction and valence bands are found in the core part and the shell part, respectively. The band offsets are determined, which give rise to the spatial separation of electron and hole charge carriers in different regions of the nanowires. This allows for a novel-doping scheme that supplies the carriers into a separate region in order to avoid the scattering problem. This is the key factor to create high-speed devices. With the confinement effect, our results show important correction in the band offset compared with the bulk heterostructure. Finally, an optimum doping strategy is proposed based on our band-offset data.
Quasiparticle calculations for metal hydrides

(Georgia Institute of Technology, 2002-12) Alford, John Ashley, II
A quantum Monte Carlo study of exchange and correlation in the silicon pseudo atom

(Georgia Institute of Technology, 2000-12) Puzder, Aaron
A first-principles study of the niobium-hydrogen system

(Georgia Institute of Technology, 2000-12) Li, Changlin