<p> Due to high specific capacity for lithiation, molybdenum dichalcogenides such as MoO₂ and MoS₂ are potential replacements for graphite anodes in Li-ion batteries. However, in bulk form these materials exhibit poor rate capability and lose capacity with each cycle. While the performance can be improved by changes to morphology, the details of the lithium intercalation mechanism are not fully understood. </p><p> In this work, X-ray absorption spectroscopy (XAS) is employed to investigate this mechanism, including X-ray absorption near-edge spectroscopy (XANES) and X-ray absorption fine-structure spectroscopy (XAFS). For MoS₂, modeling of the local structure supports the metallic conversion reaction model by the second lithiation.</p><p>

The material requirements for optoelectronic devices can vary dramatically depending on the application. Often disparate material systems need to be combined to allow for full device functionality. At the nanometer scale, this can often be challenging because of the inherent chemical and structural incompatibilities of nanofabrication. This dissertation concerns the integration of seemingly dissimilar materials into hybrid optoelectronic devices for photovoltaic, plasmonic, and photonic applications. First, we show that combining a single strip of conjugated polymer and inorganic nanowire can yield a nanoscale solar cell, and modeling of optical absorption and exciton diffusion in this device can provide insight into the efficiency of charge separation. Second, we use an on-chip nanowire light emitting diode to pump a colloidal quantum dot coupled to a silver waveguide. The resulting device is an electro-optic single plasmon source. Finally, we transfer diamond waveguides onto near-field avalanche photodiodes fabricated from GaAs. Embedded in the diamond waveguides are nitrogen vacancy color centers, and the mapping of emission from these single-photon sources is demonstrated using our on-chip detectors, eliminating the need for external photodetectors on an optical table. These studies show the promise of hybrid optoelectronic devices at the nanoscale with applications in alternative energy, optical communication, and quantum optics. / Engineering and Applied Sciences - Applied Physics

Charge carriers in graphene mimic two-dimensional massless Dirac fermions with linear energy dispersion, resulting in unique optical and electronic properties. They exhibit high mobility and strong interaction with electromagnetic radiation over a broad frequency range. Interband transitions in graphene give rise to pronounced optical absorption in the mid-infrared to visible spectral range, where the optical conductivity is close to a universal value $\sigma_0 = \pi e^2/2h$. Free-carrier intraband transitions, on the other hand, cause low-frequency absorption, which varies significantly with charge density and results in strong light extinction at high carrier density. These properties together suggest a rich variety of possible optoelectronic applications for graphene. In this thesis, we investigate the optoelectronic properties of graphene by measuring transient photoconductivity with optical pump-terahertz probe spectroscopy. We demonstrate that graphene exhibits semiconducting positive photoconductivity near zero carrier density, which crosses over to metallic negative photoconductivity at high carrier density. These observations are accounted for by the interplay between photoinduced changes of both the Drude weight and carrier scattering rate. Our findings provide a complete picture to explain the opposite photoconductivity behavior reported in (undoped) graphene grown epitaxially and (doped) graphene grown by chemical vapor deposition. Our measurements also reveal the non-monotonic temperature dependence of the Drude weight in graphene, a unique property of two-dimensional massless Dirac fermions. / Physics

The unifying themes connecting the chapters in this dissertation are the profound and often surprising effects of disorder in classical and quantum systems and the tremendous insight gained from a classical perspective, even in quantum systems. In particular, we investigate disorder in the form of weak, spatially correlated random potentials, i.e. far from the Anderson Localization regime. We present a new scar-like phenomenon in quantum wells. With the introduction of local impurities to the oscillator, the eigenstates localize onto classical periodic orbits of the unperturbed system. Compared to traditional scars in chaotic billiards, these scars are both more common and stronger. Though the unperturbed system has circular symmetry, the random perturbation selects a small number of orientations which are shared by many scarred states -- dozens or even hundreds -- over a range of energies. We show, via degenerate perturbation theory, that the cause of the new scars is the combination of an underlying classical resonance of the unperturbed system and a perturbation induced coupling that is strongly local in action space. Next we examine the same type of local perturbation applied to an open system: branched flow. Caustics in the manifold of trajectories have been implicated in the formation of strong branches. We show that caustic formation is intimately tied to compression of manifolds of trajectories in phase space, which has important implications for the position space density. We introduce the "Kick and Drift" model, a generalization of the standard map. The model is a good approximation to the full two dimensional dynamics of a wave propagating over a weak random potential, but it provides a simpler framework for studying branched flow. Next we develop a classical model for electrons executing cyclotron motion in a graphene flake and implement it numerically. We derive classical equations of motion for electrons moving through the graphene flake with a position dependent effective mass due to fluctuations in the background carrier density. I apply these methods to an experiment performed by the Westervelt group. They imaged the flow of electrons in a graphene flake by measuring the transresistance as they rastered a charged scanning probe microscope tip over the surface. My simulations show that the regions with the greatest change in transresistance do always coincide with the regions with the highest current density. Furthermore I show that the experimental results can qualitatively reproduced by treating the system classically. Finally, we extend Heller's thawed Gaussian approximation from second order in the classical action to third order, in order to capture curvature in phase space. Such phase space dynamics are ubiquitous in systems with weak random potentials, such as those discussed above. We derive a closed form solution, but find that more work needs to be done to make it numerically tractable and competitive with other methods. A semiclassical method capturing phase space curvature could provide insight into the behavior of scars away from the hbar goes to zero limit. / Physics

Strongly-correlated electron systems generate some of the richest phenomena and most challenging theoretical problems studied in physics. One approach to understanding these systems is with ultracold fermionic atoms in optical lattices, which can provide a level of control and ways of observing strongly-correlated fermionic systems that are not accessible with conventional materials. This thesis describes the development of an experimental technique where a quantum gas of fermionic 6Li atoms is prepared in a two-dimensional optical lattice and each atom can be frozen in place and imaged with single-site resolution. Combining a vacuum-compatible large numerical aperture microscope with Raman sideband cooling enables site-resolved fluorescence imaging with high fidelity. We observe several phases of the Hubbard model, including band and Mott insulators. The observed in-situ occupation distributions of atoms in the lattice are compared to theory with unprecedented detail and are used to determine the thermodynamic properties of the system. By combining site-resolved imaging with a spin-removal technique, we observe antiferromagnetic correlations in the Hubbard model with single-site resolution. We observe, for the first time in cold atom systems, beyond-nearest-neighbor magnetic correlations, which provide a direct measurement of the correlation length. We also present detailed measurements of the formation of correlations during lattice loading. / Physics

In this thesis, we present a thorough investigation into the rigidity of a polymer melt above and below its glass transition (GT) at a temperature TG = 0.465 (in reduced Lennard-Jones units). We use an isothermal-compression method to enter the glassy phase and NPT ensemble is realized through molecular dynamics simulations. We monitor such quantities as the mean-square displacement, the heat capacity CP, the volume and time-dependent shear modulus G(t). Whenever possible, these quantities are monitored below the GT as well. We also compute the shear modulus mu via external deformations and, in the zero-shear limit, find reasonably good agreement with G(t &rarr; infinity). The rigidity transition (RT) in the system is found to occur slightly below the GT at a temperature TR = 0.44. The results are explained in terms of sufficient free volume above TR allowing collective motion and local stress relaxation. This is seen through dynamics which are not only heterogeneous, but also spatially correlated. We appeal to notions such as "jamming" within the system and the presence of floppy modes (which allow for deformations without energy cost) to interpret the RT phenomenon. We also characterize the response to external deformation: small and large deformation regimes can be identified, the latter type causing a non-negligible reconfiguration, an over-stretching of the chains and a move to a more shallow potential energy "well." Furthermore, we analyze the "aging" phenomenon as a series of intermittent collective rearrangements and show that the two types of instantaneous shear deformations both induce "overaging," but in two completely different manners.

In situ Raman spectroscopy was used to track the growth of carbon nanotubes grown by chemical vapor deposition. The dynamic evolution of three kinds of Raman bands, namely the G, D, and RBM bands, was analyzed. The evolution of nanotube diameter and crystallinity was analyzed from the RBM and D/G band evolution, respectively. A characteristic growth sequence consisting of four distinct stages of growth was consistently observed. The growth rate of each stage was found to decrease with increasing temperature, possibly due to parasitic, competing reactions, and energy scales for each stage are extracted. The evolution and nanotube distribution of samples grown with and without alumina support layers is contrasted and the role of alumina is discussed.

Recently, InAs/lnP quantum dot lasers operating in the telecommunications wavelength range showed promising properties such as low threshold, high external efficiency, high bandwidth, multiwavelength emission and passive mode-locking. The photoluminescence of quantum dot layers is investigated, in the scope of improving InAs/InP quantum dot lasers performance operating in the telecommunications range. Stacking of layers with GaP underlayer reduces the full width at half maximum of the emission, improving the laser performances to a competitive alternative to quantum well lasers. Polarization photoluminescence of sample edge emission is investigated to aim for the development of a polarization insensitive semiconductor optical amplifier operating at 1.55 mu m. Polarization properties are studied for a stack period between 5 and 30 nm and for single quantum dot layers rapid thermal annealed from 600 to 700&deg;0. Decreasing the period lowers the degree of polarization of side emission from 80 to 40%, suggesting modification of polarization properties by coupling between the layers.

The minimum spacing of a plasma waveguide was calculated and applied to the formation of periodic nanocracks. The minimum spacing decreased with decreasing plasma frequency but was found to have limited effect on the spacing of the nanocracks. An extension to a standard Finite-Difference Time-Domain method was created to include nonlinear processes and the dynamic build up of the electron plasma. The ionized area produced in the simulation agrees with experiment. The existence of a self-limited absorption effect on a Gaussian pulse in time was verified in the simulations. The region was elongated along the direction parallel to the polarization of the light. The multiphoton absorption was found to be the main cause of the distinct shape of the damaged area. Plasma dispersion and self-focusing create larger electron densities and a shift in the location of the electron density peak, but did not affect the general shape.

This thesis presents a study of two basic modes of deformation of a thin sheet: wrinkling and crumpling, viewed primarily in the context of an elastic sheet confined by capillary forces on a drop of liquid. First, it provides a brief conceptual background in the relevant physics of thin sheet mechanics and capillarity and introduces the general principles of wrinkling and crumpling. The problem of confining a circular sheet on an increasingly curved spherical drop is presented as a vehicle to explore these principles. At finite curvature, the sheet is seen to wrinkle around its outer edge. At large confinement, characteristic features of crumpling gradually dominate the pattern. The experimental observations in both regimes are analyzed separately. Analysis of images of the sheet in the wrinkled regime yield data for the number and length of the wrinkled zone, as a function of the experimental control parameter, the pressure. The length of the wrinkles is correctly described by a far-from-threshold theory, which describes a limiting regime in thin-sheet mechanics, distinguished by high 'bendability'. The validity of this theory is verified by the data for highly bendable, ultrathin sheets for the first time. The theory is based on the assumption that the wrinkles completely relax compressive stresses and therefore preserve the cylindrical symmetry of the stress field. The emergence of crumpling from the wrinkled shape is explored via evolution of visible features in the sheet as well as gaussian curvature measurements obtained by analyzing height maps from optical profilometry. The emergence of several length scales, increasing asymmetry in curvature distribution, the failure of wrinkle extent prediction and formation of d-cones associated with crumpling are all measured to locate the transition to a crumpled state. The value of gaussian curvature at the center of the sheet appears to follow the cylindrically symmetric prediction over the whole range of the experiment, suggesting that the onset of crumpling events does not affect the global shape of the sheet. Finally, analogous wrinkling and crumpling behavior of particle-laden interfaces is discussed. The spontaneous formation of conical defects in a curved 2D crystal is compared to the crumpling of a sheet on a drop, and insight from thin sheet mechanics is applied to the mysterious wrinkling of particle rafts. Some future directions for measuring wrinkling of sheets on negative curvature surfaces and deformations of fluid interfaces are proposed.