Two-dimensional van der Waals materials have been a subject of intense research interest in recent years. High thermal conductivity of graphene can be utilized for many thermal management applications. In spite of possessing very high electron mobility, graphene can’t be used as transistors because of the absence of band gap; however transition metal dichalcogenides are another class of two-dimensional van der Waals materials with inherent band gap and show a great promise for future nanoelectronic applications. But in order to tailor these properties for commercial applications, we should develop a better understanding of the effect of extrinsic factors like size, rough edges, grain boundaries, mass-impurities, interaction with substrate etc. on thermal and electrical transport. Most materials exhibit a smooth ballistic-to-diffusive type of thermal transport in which when the sample size is small as compared to mean-free-path of phonons the transport is ballistic, whereas, when the sample size is large as compared to phonon mean-free-path, phonons undergo multiple scattering events and the thermal transport becomes diffusive in nature. However, graphene exhibits an atypical thermal transport behavior where thermal conductivity shows an increasing logarithmic trend even for samples far greater than the mean-free-path of phonons. We show that this anomalous behavior can be attributed to the significant contribution coming from momentum-conserving normal phonon-phonon scattering. Secondly, graphene grain boundaries have been found to significantly reduce thermal conductivity even in the presence of substrates. In spite of numerous studies on the effect of grain boundaries (GBs) on thermal conductivity in graphene, there lacks a complete correlation between GB resistance and misorientation angle across graphene GBs. We show a direct correlation between thermal GB resistance and mismatch angles with low angle mismatch can be captured only by GB roughness, whereas, large mismatch angles will lead to the formation of a disordered patch at the interface and it could significantly deteriorate the overall thermal conductivity even in the presence of substrates. GBs are found to affect electrical transport in two-dimensional systems as well. Owing to the excellent electronic properties and compactness of these two-dimensional materials, high quality 2D heterojunctions are the subject of intense research interest in recent years. Graphene-MoS2 heterojuctions are found to form ohmic contacts and show great potential for future nanoelectronic applications. We show that the interface resistance in Gr-MoS2 heterojuctions can affect the overall resistance of the device if the channel (MoS2) length is small at low carrier densities, whereas, at high carrier densities interface resistance do not play much role in determining the resistance of the entire device. However, if graphene and MoS2 grains are misorientated then interface resistance can play a crucial role in determining the overall resistance of the device. We also show a weak dependence of misorientation angles on GB resistance across MoS2 grain boundaries.

The aim of this work is to predict the influence of phonon modulations (Kozina et al. 2019 [1]) on the x-ray absorption near-edge fine structure of the Ti-L2,3-edge (Yamaguchi et al. 1982 [2], Thole et al. 1985 [3], De Groot 1990 [4]) in cubic SrTiO3. Employing Density Functional Theory in combination with Multiplet Ligand Field Theory (Haverkort et al. 2012 [5], Luder et al. 2017 [6]), previous experimental and theoretical data on the octahedrally symmetric structure are reproduced with good agreement. Phonon modulations with a maximum atomic displacement of 5% of the lattice parameter are shown to cause polarization-dependent changes in the x-ray absorption spectra just within reach of experimental resolution. This is suggested to reflect the strong susceptibility of the electronic structure to collective lattice excitations in SrTiO3.

The full dielectric tensor of monoclinic Ga2O3 (β-phase) was determined by generalized spectroscopic ellipsometry in the spectral range from 1.0 eV up to 8.5 eV and temperatures in the range from 10K up to 300K. By using the oriented dipole approach, the energies and broadenings of the excitonic transitions are determined as a function of the temperature, and the exciton-phonon coupling properties are deduced.

We present X-ray diffraction and Raman spectroscopy investigations of (InxGa1–x)2O3 thin films and bulk-like ceramics in dependence of their composition. The thin films grown by pulsed laser deposition have a continuous lateral composition spread allowing the determination of phonon mode properties and lattice parameters with high sensitivity to the composition from a single 2-in. wafer. In the regime of low indium concentration, the phonon energies depend linearly on the composition and show a good agreement between both sample types. We determined the slopes of these dependencies for eight different Raman modes. While the lattice parameters of the ceramics follow Vegard’s rule, deviations are observed for the thin films. Further, we found indications of the highpressure phase InGaO3 II in the thin films above a critical indium concentration, its value depending on the type of substrate.

We present X-ray diffraction and Raman spectroscopy investigations of a (100)-oriented (AlxGa1–x)2O3 thin film on MgO (100) and bulk-like ceramics in dependence on their composition. The thin film grown by pulsed laser deposition has a continuous lateral composition spread allowing to determine precisely the dependence of the phonon mode properties and lattice parameters on the chemical composition. For x<0.4, we observe the single-phase b-modification. Its lattice parameters and phonon energies depend linearly on the composition. We determined the slopes of these dependencies for the individual lattice parameters and for nine Raman lines, respectively. While the lattice parameters of the ceramics follow Vegard’s rule, deviations are observed for the thin film. This deviation has only a small effect on the phonon energies, which show a reasonably good agreement between thin film and ceramics.

No description available.

<p>  </p> <p>This dissertation focuses on predictive modeling of phonon-mediated thermal transport in thorium dioxide (ThO2) with defects. ThO2 has lately gained attention as it is a suitable model system for more complex nuclear reactor materials such as uranium dioxide and its mixed oxides. The reduction in thermal conductivity of the fuel as a result of irradiation-induced lattice defects is arguably the most important fuel performance metric in regard to reactor efficiency and safety. For this reason, the present work presents a theoretical investigation of thermal conductivity reduction seen in defect-bearing ThO2 and compares directly with experimental measurements. Thermal transport in irradiated ThO2 is first modeled here by a non-transport solution of the linearized Boltzmann transport equation (BTE) within the single-mode relaxation time approximation. Classic models for phonon-defect scattering rates are used to model point defects, voids, and dislocation loops in irradiated ThO2, and the resultant thermal conductivity is directly compared to experimental measurements of irradiated specimens. Our predicted conductivity values agree well with measured values near room temperature. However, discrepancy between our predictions and experimental values exist at lower temperatures where experimentally measured conductivity values seem to reach a saturation level while the model predicts further reduction in thermal conductivity. This discrepancy is most notable in higher irradiation dose samples where the thermal conductivity is almost completely controlled by the dislocation loop density. This hints at the conclusion that classic models for phonon-defect scattering rates which integrate out local variation of the defect strain field and replace this by a defect density may not be adequate to capture all physics of phonon-defect scattering, especially for dislocation loops at low temperatures. This motivated us to model defects through their spatially resolved lattice distortion fields and investigate phonon scattering in those fields in an explicit fashion. A transport solution of the phonon BTE is implemented based upon the Monte Carlo (MC) method, which explicitly tracks the phonon population as it evolves in space and time according to phonon group velocities and scattering rates. An expression for the scattering rate of phonons from an arbitrary strain field is derived from a generalized form of Grüneisen’s law of thermal expansion, and applied to the case of dislocations in ThO2. It is found that the localized strain in the material, resulting from the presence of a crystal defect, leads to a net heat flux into the strained region. This provides evidence for thermal fluxes in the absence of a temperature gradient, a phenomenon that cannot be captured via Fourier’s law. This evidence for material heating owing to the imposed strain of material defects would be immediately applicable to the field of thermoelectrics and defect engineering where large temperature gradients are desirable to improve the thermoelectric efficiency.  Although the model is applied specifically to the case of dislocations in ThO2, the derived phonon scattering rate expression is general and may be applied to any defect for which a strain field may be generated.</p>

The main focus of the thesis is to have better understanding of the atomic and electronic structures, vibrational dynamics and thermodynamics of metallic surfaces and bi-metallic nanoparticles (NPs) via a multi-scale simulational approach. The research presented here involves the study of the physical and chemical properties of metallic surfaces and NPs that are useful to determine their functionality in building novel materials. The study follows the 'bottom-up' approach for which the knowledge gathered at the scale of atoms and NPs serves as a base to build, at the macroscopic scale, materials with desired physical and chemical properties. We use a variety of theoretical and computational tools with different degrees of accuracy to study problems in different time and length scales. Interactions between the atoms are derived using both Density Functional Theory (DFT) and Embedded Atom Method (EAM), depending on the scale of the problem at hand. For some cases, both methods are used for the purpose of comparison. For revealing the local contributions to the vibrational dynamics and thermodynamics for the systems possessing site-specific environments, a local approach in real-space is used, namely Real Space Green's Function method (RSGF). For simulating diffusion of atoms/clusters and growth on metal surfaces, Molecular Statics (MS) and Molecular Dynamics (MD) methods are employed.

At the high-temperature conditions of the Earth's interior, lattice anharmonic effects in crystalline mineral phases can become pronounced. Anharmonicity, i.e., deviations of vibrations from harmonic oscillations, is caused by phonon-phonon interactions. Knowledge of lattice anharmonicity is essential to elucidate distinctive thermal properties in solids. Yet, accurate investigations of anharmonicity encounter difficulties owing to cumbersome computations. Here we present anharmonic property calculations with the phonon quasiparticle approach for various solids. The phonon quasiparticle approach efficiently and reliably addresses lattice anharmonicity by combining molecular dynamics and lattice dynamics calculations. It characterizes anharmonic phonons by extracting renormalized frequency and phonon lifetime from the mode-projected velocity autocorrelation function without explicitly computing higher-order interatomic force constants. In principle, it accounts for full anharmonic effects and overcomes finite-size effects typical of molecular dynamics. The validity and effectiveness of the current approach are demonstrated in computations of temperature-induced frequency shifts, anharmonic thermodynamics, phase boundaries, and lattice thermal conductivities of both weakly and strongly anharmonic, both insulating and metallic, and both simple and complex systems. These materials include a simple model crystal, Si with diamond structure, minerals of geophysical significance, MgSiO₃ perovskite and postperovskite, cubic CaSiO₃ perovskite, and B8 and B2 phases of FeO. Accurate anharmonic thermodynamic properties, phase boundaries, and lattice thermal conductivities presented in this thesis are important for geodynamic modeling. The theoretical framework validated in this thesis also enables predictive studies of various anharmonic materials which could not be previously addressed by conventional approaches, such as quasiharmonic approximation for thermodynamics calculations and finite displacement method for anharmonic lattice dynamics calculations.

Vibrations of nuclei in crystals govern various properties such as thermal expansion, phase transitions, and elasticity, and the quasiharmonic approximation (QHA) is the simplest nontrivial approximation which includes the effects of vibrational anharmonicity into temperature dependent observables. Nonetheless, the QHA is often implemented with additional approximations due to the complexity of computing phonons under arbitrary strains, and the generalized QHA, which employs constant stress boundary conditions, has not been completely developed. Here we formulate the generalized QHA, providing a practical algorithm for computing the strain and other observables as a function of temperature and true stress. We circumvent the complexity of computing phonons under arbitrary strains by employing irreducible second order displacement derivatives of the Born-Oppenheimer potential and their strain dependence, which are efficiently and precisely computed using the lone irreducible derivative approach. We formulate two complementary strain parametrizations: a discretized strain grid interpolation and a Taylor series expansion in symmetrized strain. We illustrate the quasiharmonic approximation by evaluating the temperature and pressure dependence of select elastic constants and the thermal expansion in thoria (ThO₂) using density functional theory with three exchange-correlation functionals. The convergence of the two complementary strain parametrizations is evaluated for the computed thermal expansion. The temperature dependent lattice parameter and thermal expansion computed within the QHA is compared with experimental measurements. The QHA results are compared to measurements of the elastic constant tensor using time domain Brillouin scattering and inelastic neutron scattering. We then demonstrate the generalized quasiharmonic approximation in a non-cubic material, ferroelectric lead titanate, computing the temperature and stress dependence of the full elastic constant tensor. The irreducible derivative approach is employed for computing strain dependent phonons using finite difference, explicitly including dipole-quadrupole contributions. We use density functional theory, computing all independent elastic constants and piezoelectric strain coefficients at finite temperature and stress. There is good agreement between the quasiharmonic approximation and the experimentally measured lattice parameters close to 0 K. The quasiharmonic approximation overestimates the measured temperature dependence of the lattice parameters and elastic constant tensor, demonstrating that a higher level of strain dependent anharmonic vibrational theory is needed. The next material we study is zirconium nitride, employing the quasiharmonic approximation with the irreducible derivative approach to compute the phonons and thermal expansion. Density functional theory is used with two exchange-correlation functionals. We investigate the difference between the measured and computed optical phonon branches, showing that volume effects, two-phonon scattering, and nitrogen vacancies do not explain the discrepancy between the measurement and computation. The temperature dependent lattice parameter is computed within the QHA, where the thermal expansion is overestimated as compared with existing experimental measurements.