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Made available in DSpace on 2014-10-09T12:32:06Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:56Z (GMT). No. of bitstreams: 1 02299.pdf: 1814894 bytes, checksum: cf49ad3b5d16cf422dd5e56dd657a29f (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Thesis advisor: Fazel Tafti / The search for an ideal quantum spin-liquid (QSL) material which can host a QSL ground state as well as exotic excitations has been one of the leading research topics in condensed matter physics over the past few decades. Out of all the proposals to realize the physics of a QSL, the Kitaev model is the most promising proposal with a QSL ground state. The Kitaev Hamiltonian is exactly solvable via fractionalization of its spin degrees of freedom into Majorana excitations, and it can be engineered in real materials. Among all the proposed Kitaev candidates, α-Li2IrO3, Na2IrO3, Li2RhO3, and α-RuCl3 are the most promising candidates. During my Ph.D. research I explored new physics related to Kitaev materials via modification of the symmetry and structural properties of these known Kitaev candidates. First, I studied how modification of the inter-layer chemistry can alter the thermodynamic properties of Kitaev candidate α-Li2IrO3 via an enhancement of the spin-orbit coupling (SOC) effect. The light, octahedrally-coordinated inter-layer Li atoms are replaced with heavier, linearly-coordinated Ag atoms to synthesize Ag3LiIr2O6. In addition to these structural modifications to the parent compound α-Li2IrO3, having heavier elements between the honeycomb layers in the Ag compound increased the effect of SOC in the honeycomb layers and led to a decrease in the long-range ordering temperature in Ag3LiIr2O6 compared to its parent compound. Second, I studied the effect of local crystal distortion in the presence of a weak SOC effect to explore a new spin-orbital state different from the Jeff=1/2 state. Based on theoretical predictions, the ground states of Kitaev materials can be tuned to other exotic spin-orbital states such as an Ising spin-1/2 state. To provide the proper conditions for a competition between the trigonal crystal distortion and the SOC effect, I modified the crystal environment around the magnetic elements in the parent compound Li2RhO3 via a topo-chemical method and synthesized Ag3LiRh2O6. An increase in the strength of trigonal distortion in Ag3LiRh2O6, in the presence of weak SOC, led to a transition from the Jeff=1/2 ground state (Kitaev limit) in the parent compound to an Ising spin-1/2 ground state (Ising limit) in the product. This change in spin-orbital state resulted in a dramatic change in magnetic behavior. Whereas Li2RhO3 shows a spin-freezing transition at 6 K, Ag3LiRh2O6 reveals a robust long-range antiferromagnetic transition at 94 K. This is the first realization of a change of ground state between the Kitaev and Ising limits in the same structural family. Lastly, I studied how the crystal symmetry can be an important factor in the physics of Kitaev materials. Honeycomb layered materials can be crystallized in space groups C2/m, C2/c, and P6_322. However, the crystal symmetry of most Kitaev candidates is described by the C2/m space group. We successfully synthesized a polymorph of a 3d Kitaev candidate, hexagonal Na2Co2TeO6 (P6_322 space group) in space group C2/m. The change in crystal symmetry of this cobalt tellurate replaced three anti-ferromagnetic (AFM) orders at 27, 15, 7 K in the hexagonal polymorph by a single AFM peak at 9.6 K in the monoclinic Na2Co2TeO6. / Thesis (PhD) — Boston College, 2023. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Granular superconductivity at high temperatures in graphite can emerge at certain two-dimensional (2D) stacking faults (SFs) between regions with twisted (around the c-axis) or untwisted crystalline regions with Bernal (ABA…) and/or rhombohedral (ABCABCA…) stacking order. One way to observe experimentally such 2D superconductivity is to measure the frozen magnetic flux produced by a permanent current loop that remains after removing an external magnetic field applied normal to the SFs. Magnetic force microscopy was used to localize and characterize such a permanent current path found in one natural graphite sample out of ∼50 measured graphite samples of different origins. The position of the current path drifts with time and roughly follows a logarithmic time dependence similar to the one for flux creep in type II superconductors. We demonstrate that a ≃10 nm deep scratch on the sample surface at the position of the current path causes a change in its location. A further scratch was enough to irreversibly destroy the remanent state of the sample at room temperature. Our studies clarify some of the reasons for the difficulties of finding a trapped flux in a remanent state at room temperature in graphite samples with SFs.

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The research performed aims to develop a deeper understanding and prediction of behaviour of complex chemical and physical systems using density functional theory (DFT) modelling complemented by experimental techniques. We focus on phenomena relevant to practical applications of semiconducting materials. Semiconductor nanowires, produced by the vapor-liquid-solid method are being considered for applications in photo sensors, field effect transistors, light emitting diodes (LEDs) and energy harvesting devices. In particular, semiconductor nanowire based photovoltaic devices show potential for lower cost due to less material utilization and greater energy conversion efficiency arising from enhanced photovoltage or photocurrent due to hot carrier or multiexciton phenomena enhanced light absorption, compared to conventional thin film devices. Further, freedom from lattice matching requirements due to strain accommodation at the nanowire surfaces enable compatibility with a wide variety of substrates including Silicon. Thus understanding and improving the optoelectronic properties of nanowires is of great interest. In the first paper, we study the effect of planar defects on optoelectronic properties of nanowire based semiconductor devices. Specifically, we were interested in investing the origin of various features observed in the photoluminisence (PL) spectrum of GaP nanowire using density functional modelling, which are not well understood. In the second paper, we work to model bonding characteristics during a chemical synthesis. We focus on the synthesis of nanoparticles for supercapacitor application. In the past decade, comprehensive research has been emphasized on manganese oxides for electrochemical supercapacitor (ECS) applications. Mn3O4 has gained significant interest due to its compatibility with capping agents and the unique spinel structure allows for potential modifications with other cations. Many metal oxide synthesis techniques are based on aqueous processing. The synthesized particles are usually dried and redispersed in organic solvents to incorporate water-insoluble additives such as binders to fabricate films and devices. However, during the drying step nano-structures are highly susceptible to agglomeration, which can be attributed to the condensation reactions occurring between particles and reduction in surface energy. Poor electrolyte access due to agglomeration and low intrinsic conductivity of Mn3O4 are detrimental to the performance of Mn 3O4 electrode especially at high active mass loadings. Numerous attempts have focused on controlling size and morphology of Mn3O4 nanostructures using capping agents, which have strong adhesion to particles surface to inhibit agglomeration. Catechol containing molecules have been used for dispersion of metallic nanoparticles and fabrication of composite thin films, resulted in narrow size distribution of nanoparticles and strong adhesion to substrates. Despite the experimental results showing good adsorption of catechol group to metal atoms, the mechanism is unclear since it is highly influenced by synthesis parameters. We use Infrared spectroscopy in conjugation with density functional modelling to understand the binding mechanism of 3,4 dihydroxy benzaldehyde onto Mn3O4 surface. / Thesis / Master of Applied Science (MASc)

Biaxial tensile strain has been shown to greatly enhance the optoelectronic properties of epitaxial germanium (Ge) layers. As a result, tensile-Ge (and#949t-Ge) layers grown on larger lattice constant InGaAs or GeSn have attracted great research interest. However, no previous studies have investigated the plastic relaxation occurring in these and#949t-Ge layers. Here, we experimentally demonstrate that plastic relaxation occurs in nearly all and#949t-Ge epitaxial layers that are of practical interest for optoelectronic applications, even when layers may still exhibit strain-enhanced characteristics. We show arrays of misfit dislocations (MDs), which are mostly disassociated, form at the and#949t-Ge/InGaAs interface for and#949t-Ge layers as thin as 15 nm with less than 1% total mismatch. Wedge geometry of plain view transmission electron microscopy (PV-TEM) foils is utilized to carry out a depth dependent investigation MD spacing for a range of and#949t-Ge/InGaAs heterostructures. MD spacing measured by PV-TEM is correlated to and#949t-Ge layer relaxation measured by high-resolution x-ray diffraction. We confirm very low relaxation (< 10% relaxed) in and#949t-Ge layers does not imply they have been coherently grown. We demonstrate plastic relaxation in the and#949t-Ge layer is acutely sensitive to grown-in threading dislocations (TDs) in the template material, and that reducing TD density is critical for maximizing strain retention. Given that and#949t-Ge layer thicknesses of 150+ nm with greater than 1% tensile strain are desired for optoelectronic devices, this work suggests that MDs may inevitably be present at and#949t-Ge/InGaAs heterointerfaces in practical devices, and that the effect of MDs on optoelectronic performance must be better understood. / Master of Science

Diffusive Molecular Dynamics (DMD) is a class of recently developed computational methods for the simulation of long-term mass transport with a full atomic fidelity. Its basic idea is to couple a discrete kinetic model for the evolution of mass transport process with a non-equilibrium thermodynamics model that governs lattice deformation and supplies the requisite driving forces for kinetics. Compared to previous atomistic models, e.g., accelerated Molecular Dynamics and on-the-fly kinetic Monte Carlo, DMD allows the use of larger time-step sizes and hence has a larger simulation time window for mass transport problems. This dissertation focuses on the development, assessment and application of a DMD computational framework for the long-term, three-dimensional, deformation-diffusion coupled analysis of solute mass transport in nanomaterials. First, a computational framework is presented, which consists mainly of: (1) a computational model for interstitial solute diffusion, which couples a nonlinear optimization problem with a first-order nonlinear ordinary differential equation; (2) two numerical methods, i.e., mean field approximation and subcycling time integration, for accelerating DMD simulations; and (3) a high-performance computational solver, which is parallelized based on Message Passing Interface (MPI) and the PETSc/TAO library for large-scale simulations. Next, the computational framework is validated and assessed in two groups of numerical experiments that simulate hydrogen mass transport in palladium. Specifically, the framework is validated against a classical lattice random walk model. Its capability to capture the atomic details in nanomaterials over a long diffusive time scale is also demonstrated. In these experiments, the effects of the proposed numerical methods on solution accuracy and computation time are assessed quantitatively. Finally, the computational framework is employed to investigate the long-term hydrogen absorption into palladium nanoparticles with different sizes and shapes. Several significant findings are shown, including the propagation of an atomistically sharp phase boundary, the dynamics of solute-induced lattice deformation and stacking faults, and the effect of lattice crystallinity on absorption rate. Specifically, the two-way interaction between phase boundary propagation and stacking fault dynamics is noteworthy. The effects of particle size and shape on both hydrogen absorption and lattice deformation are also discussed in detail. / Ph. D. / Interstitial diffusion in crystalline solids describes a phenomenon in which the solute constituents (e.g., atoms) move from an interstitial space of the host lattice to a neighboring one that is empty. It is a dominating feature in many important engineering applications, such as metal hydrides, lithium-ion batteries and hydrogen-induced material failures. These applications involve some key problems that might take place over long time periods (e.g., longer than 1 s), while the nanoscale behaviors and mechanisms become significant. The time scale of these problems is beyond the capability of established atomistic models, e.g., accelerated Molecular Dynamics and on-the-fly kinetic Monte Carlo. To this end, this dissertation presents the development and application of a new computational framework, referred to as Diffusive Molecular Dynamics (DMD), for the simulation of long-term interstitial solute diffusion in advanced nanomaterials. The framework includes three key components. Firstly, a DMD computational model is proposed, which accounts for three-dimensional, deformation-diffusion coupled analysis of interstitial solute mass transport. Secondly, nu- merical methods are employed to accelerate the DMD simulations while maintaining a high solution accuracy. Thirdly, a high-performance computational solver is developed to implement the DMD model and the numerical methods. Moreover, regarding its application, the DMD framework is first validated and assessed in the numerical experiments pertaining to hydrogen mass transport in palladium crystals. Then, it is employed to investigate the atomic behaviors and mechanisms involved in the long-term hydrogen absorption by palladium nanoparticles with different sizes and shapes. The two-way interaction between hydrogen absorption and lattice deformation is studied in detail.

In this work cadmium telluride thin film photovoltaic devices have successfully been produced using a novel closed-field magnetron sputtering technique. This technique offers the possibility of producing cells in an all-in-one vacuum process with the potential to provide a new lower cost production route. The sputtered cadmium telluride layers were characterised in detail using a range of advanced microscopy based techniques both in the as deposited and after the cadmium chloride treated state, a treatment that is necessary to produce a working cell. In the as deposited condition the cadmium telluride layer was seen to have a fine-grained columnar structure containing a high density of stacking faults. After the cadmium chloride treatment these grains recrystallized and the new grains were equiaxed with a much lower density of intragranular defects. Similar effects were also observed in samples prepared using close space sublimation. To understand this recrystallization behaviour during the cadmium chloride treatment, the key treatment parameters were systematically varied. Chemical analysis in Scanning Transmission Electron Microscopy (STEM) showed that chlorine travelled down the cadmium telluride grain boundaries and accumulated adjacent to the cadmium telluride/cadmium sulphide interface. This interface is where the cadmium telluride grains were found to recrystallise first during interrupted cadmium chloride treatments. The nature of the stacking faults was examined using High Resolution Transmission Electron Microscopy (HR-TEM). This showed that in localised regions up to one plane of atoms per sequence was missing based on the expected zinc blende structure. This changed the packing of the atoms such that a local change in crystal structure occurred. This local change in phase was successfully mapped using Electron Backscatter Diffraction in planar section produced using Focused Ion Beam milling. This was subsequently studied in more detail using Transmission Electron Backscatter Diffraction in the Scanning Electron Microscope, where the intra-granular arrangement of the phases was observed. HR-TEM was used to quantitatively measure the linear defects in the cadmium telluride layer after thermal annealing with and without the cadmium chloride present. This showed that annealing alone resulted in only a modest reduction in the density of linear defects and grain recrysallisation only occurred in the presence of cadmium chloride. Cadmium magnesium telluride (CMT) was successfully grown epitaxially onto the cadmium telluride as an electron reflector layer to improve cell performance. During deposition the cell experienced high temperatures and this caused the stacking faults to return in a cell that had been previously cadmium chloride treated. This resulted in a reduction in cell efficiency, providing another link between linear defects and a degradation in cell performance.