In this thesis, we contribute to the understanding of electronic and optical properties of 2-dimensional materials, with a strong focus on graphene-based nanostructures \cite{graphene_book}. The thesis is structured into eight chapters, starting with an introduction and ending with a conclusion. In chapter 2, we present the methods used throughout this thesis. We start by introducing the tight-binding model to understand the single-particle properties of graphene, bilayer graphene, and graphene quantum dots. We then introduce configuration interaction, the Hubbard model, the Bethe-Salpeter equation, and Hartree-Fock as tools for tackling the interacting problem and correlated electron systems. We also discuss numerical methods, including techniques for addressing the numerical complications that arise when working with the many-body problem such as the calculation of Coulomb matrix elements. In chapter 3, we present a new approach to the energy spectra of $p_z$ electrons in small hexagonal graphene quantum dots. This approach is analytical, and allows us to predict the dependence of the energy gap on size and edge type. In chapter 4, we describe a proposal of a quantum simulator of an extended bipartite highly tunable Hubbard model with broken sublattice symmetry inspired by graphene. We predict the electronic and magnetic properties of a small simulator. The proposed simulator, allows us to study the ground state of the Hubbard Hamiltonian for a broad range of regimes accessible due to the high tunability of the simulator. In chapter 5, we study the electronic properties of quasi 2-dimensional quantum dots made of topological insulators using HgTe. We show that in a square HgTe quantum dot one set of material parameters defines the topologically nontrivial case, in which topologically protected edge states are found, and another set of parameters defines a topologically trivial regime corresponding to a trivial insulator without edge states. In chapter 6, we examine excitons in AB-stacked gated bilayer graphene (BLG) quantum dots (QDs). We confine both electrons and holes using gates and demonstrate that excitons can exist in the BLG QD. We predict absorption to occur in the terahertz regime and find that low-energy excitons are dark. In chapter 7, we determine the many-body states of massive Dirac Fermions confined in a bilayer graphene lateral gated quantum dot. Tuning the strength of Coulomb interactions versus the single-particle level spacing we predict the existence of spontaneously spin and valley symmetry-broken states of interacting massive Dirac Fermions.

No description available.

This work represents a first-principles calculation of the electron tunneling current into quantum dots in the fractional quantum Hall effect regime. The system under consideration consists of an idealized Scanning Tunneling Microscope (STM) tip and a quantum dot with disk geometry and interacting electrons in a transverse magnetic field. Within the context of this model the tunneling current between the STM tip and the dot is examined for spin-polarized electrons at and around a filling factor of 1/3. The current expression is based on a second-quantized Hamiltonian in which electrons in the dot are interacting, confined, and restricted to the lowest Landau level, necessary to capture the physics of the fractional quantum Hall effect. The Hamiltonian includes simple approximations for the STM tip and the tip-dot tunneling. An exact analytic expression for the first-order tunneling current is derived using a Green's function approach. To calculate the tunneling current numerically the infinite Hilbert space of the dot is truncated to have a finite dimension within the lowest Landau level. This simplification is appropriate for a low temperature system in the fractional quantum Hall regime because of the finite size of the quantum dot and the large energy gap between Landau levels. The tunneling current is then solved in two steps. First, many-electron energy eigenstates are calculated from the truncated Hamiltonian by numerical diagonalization. This is carried out for varying numbers of electrons N. The energy eigenstates form a set of complete basis states of the system and are used in the expression for the tunneling current. In the second step, the chemical potential in the dot is chosen to select a desired number of electrons and the tunneling current evaluated. We have carried out this program for filling factors near 1=3 while modulating the system parameters of interest to determine functional dependencies.

The aim of this dissertation is to gain a better understanding of the unique electronic structure of lead salt quantum dots (QDs) and its influences on the nonlinear optical (NLO) properties as well as the time dynamics of the photogenerated charge carriers. A variety of optical techniques such as Z-scan, two-photon excited fluorescence and time-resolved pump probe spectroscopy are used to measure these properties. The one-photon as well as the degenerate and nondegenerate two-photon absorption (2PA) spectra are measured and the electronic wave functions from a four-band envelope function formalism are used to model the results. We observe local maxima in the 2PA spectra for QD samples of many different sizes at energies where only 1PA is predicted by the model. This is similar to the previously measured transitions in the 1PA spectra which are not predicted by the model but accrue at the energies of the two-photon allowed transitions. Most importantly we observe 2PA peaks for all samples at the energy of the first one-photon allowed transition. This result can only be understood in terms of symmetry breaking and therefore is strong evidence that other transitions, not predicted by the model if the selection rules are left intact, also have the origin in the lifted spatial symmetry of the wave functions. On the other hand, the uniquely symmetric eigenenergies of these quantum-confined energy states in the conduction and valance bands explain the observed trend toward larger two-photon cross-sections as the quantum confinement is increased in smaller QDs. Moreover, this unique feature is shown to reduce the possible relaxation channels for photoexcited carriers, which is confirmed experimentally by the reduced carrier relaxation rate as compared to CdSe QDs which lack this symmetry. Carrier multiplication (CM), a process in which several electrons are excited by the iv absorption of a single photon is studied in PbS QDs. We show that for PbS QDs with radius smaller than 2.5 nm the parameters of CM get very close to the theoretical optimum. Nextgeneration solar cells operating under these ideal conditions could potentially have conversion efficiency of up to 42%. This compares favorably to the 30% efficiency limit of a single junction silicon solar cell.

Quantum dots are semiconductor nanocrystals that have tunable emission through changes in their size. Producing bright, efficient quantum dots with stable fluorescence is important for using them in applications in lighting, photovoltaics, and biological imaging. This study aimed to optimize the process for coating CdSe quantum dots (which are colloidally suspended in octadecene) with a ZnS shell through the pyrolysis of organometallic precursors to increase their fluorescence and stability. This process was optimized by determining the ZnS shell thickness between 0.53 and 5.47 monolayers and the Zn:S ratio in the precursor solution between 0.23:1 and 1.6:1 that maximized the relative photoluminescence quantum yield (PLQY) while maintaining a small size dispersion and minimizing the shift in the center wavelength (CWL) of the fluorescence curve. The process that was developed introduced a greater amount of control in the coating procedure than previously available at Cal Poly. Quantum yield was observed to increase with increasing shell thickness until 3 monolayers, after which quantum yield decreased and the likelihood of flocculation of the colloid increased. The quantum yield also increased with increasing Zn:S ratio, possibly indicating that zinc atoms may substitute for missing cadmium atoms at the CdSe surface. The full-width at half-maximum (FWHM) of the fluorescence spectrum did not change more than ±5 nm due to the coating process, indicating that a small size dispersion was maintained. The center wavelength (CWL) of the fluorescence spectrum red shifted less than 35 nm on average, with CWL shifts tending to decrease with increasing Zn:S ratio and larger CdSe particle size. The highest quantum yield was achieved by using a Zn:S ratio of 1.37:1 in the precursor solution and a ZnS shell thickness of approximately 3 monolayers, which had a red shift of less than 30 nm and a change in FWHM of ±3 nm. Photostability increased with ZnS coating as well. Intense UV irradiation over 12 hours caused dissolution of CdSe samples, while ZnS coated samples flocculated but remained fluorescent. Atomic absorption spectroscopy was investigated as a method for determining the thickness of the ZnS shell, and it was concluded that improved sample preparation techniques, such as further purification and complete removal of unreacted precursors, could make this testing method viable for obtaining quantitative results in conjunction with other methods. However, the ZnS coating process is subject to variations due to factors that were not controlled, such as slight variations in temperature, injection speed, and rate and degree of precursor decomposition, resulting in standard deviations in quantum yield of up to half of the mean and flocculation of some samples, indicating a need for as much process control as possible.

Hybrid quantum dot /organic semiconductor systems are of great interest in optoelectronic and photovoltaic applications, because they combine the robust and tunable optical properties of inorganic semiconductors with the processability of organic thin films. In particular, cadmium selenide (CdSe) quantum dots coordinated with oligo-(phenylene vinylene) ligands have displayed a number of hybrid optical properties that make them particularly well-suited to these applications. When probed on an individual particle level, these so-called CdSe-OPV nanostructures display a number of surprising photophysical characteristics, including strong quenching of fluorescence from coordinating ligands, enhanced emission from the CdSe quantum dot core, suppression of fluorescence intermittency, and photon antibunching, all of which make them attractive in the applications described above. By correlating fluorescence properties with atomic force microscopy, the effects of ligands on quantum dot luminescence are elucidated. In addition, recent studies on individual CdSe-OPV nanostructures have revealed a strong electronic coupling between the coordinating ligands and the nanocrystal core. These studies have shown that excitations in the organic ligands can strongly affect the electronic properties of the quantum dot, leading to linearly polarized optical transitions (both in absorption and emission) and polarization-modulated shifts in band edge emission frequency. These polarization effects suggest exciting new uses for these nanostructures in applications that demand the robust optical properties of quantum dots combined with polarization-switchable control of photon emission.

Today, amphibian extinction rates are rising at an alarming rate. Captive assurance colonies have become a hedge against extinction, and often must employ assisted reproductive technologies (ART) in species that do not readily breed in captivity. One technique that can be utilized in assisted breeding is hormone therapy, which involves the treatment of individuals with exogenous reproductive hormones. The primary delivery method used in most breeding programs is intraperitoneal injection, but many institutions either lack the training necessary to conduct this invasive procedure, or require veterinary staff to perform them, thus delaying breeding events. Therefore, there is interest in alternate means of hormone delivery. In particular, the use of intranasaladministration. The following studies were conducted to determine the efficacy of hormones administered via alternate delivery routes, and to investigate the pathways taken by both intraperitoneal and intranasal delivery methods. Through these studies, wefound that intranasal administration gonadotropin releasing hormone (GnRH), is effective at eliciting sperm production in male anurans. In order to investigate the paths taken by intraperitoneal and intranasal GnRH, I used a treatment of hormone-conjugated quantum dot nanoparticles and employed both in-vivo fluorescence imaging techniques and histological imaging. The evidence presented here suggests that the route traveled by nasally-delivered GnRH is largely swallowed and accumulates in the GI tract, buteventually diffuses into the bloodstream in large enough concentrations to exact a reproductive response. The other hormone investigated here was arginine vasotocin (AVT), a hormone known to elicit calling and amplexus behaviors in amphibians. Though limited reproductive behaviors were observed in these studies, I found that both intranasal and intraperitoneal delivery of AVT resulted in water uptake and retention in males. Fluorescence imaging revealed that AVT, when administered nasally, is largely swallowed, similarly to GnRH. Intraperitoneally-injected AVT, however, was found to accumulate in large concentrations within the interrenal gland and kidney, where it likely stimulated the observed osmoregulatory effects. This study therefore offers insight into an effective alternate hormone delivery method (nasal) and provides compelling evidence into the organs wherein GnRH and AVT act following two different delivery routes.

No description available.

No description available.

No description available.