Traditional imaging methods fall flat when dealing with objects embedded in heavily scattering media because information necessary to image formation is lost in the scattered imaging signal. The nitrogen-vacancy (NV) center in fluorescent nanodiamonds (FNDs) is one promising system to realize high-resolution imaging through heavily scattering media. A novel high-resolution imaging method that is largely impervious to scattering difficulties can be achieved by exploiting the magnetic-field dependence of the NV center’s fluorescence. A single NV center is a magnetic quantum system with spin energy level splitting that depends on the value of the magnetic field at the location of the NV center. Using optically detected magnetic resonance (ODMR), we can determine the splitting between two spin energy levels and thus the value of the magnetic field at the FND. It follows that if FNDs are placed in a magnetic field whose value is known at all points, the FND positions can be determined through application of ODMR, potentially allowing for spatial resolution in the nanoscale. In this work, we characterize the optical and spin properties of FNDs created with a very high density of NV centers in order to determine their suitability for diffusive imaging. Optical brightness, NV center density, and the sensitivity of the fluorescence dependence on magnetic field are reported. Using these properties, we estimate the expected performance of a diffusive imaging microscope based on the FNDs.

Self-rolled nano and micro-structures have been fabricated by standard microfabrication techniques including photolithography, physical vapor deposition, and wet chemical processing. The rolling characteristics of functionalizable (gold-coated) magnetic tubes has been explored as a function of microstructure, size, and processing. Magnetization parallel to the transverse axis has been examined as a method to prevent agglomeration of particles in solution.

Defect characterization of a completed Photovoltaic (PV) device is an important and often tricky step of improving the efficiency of PV devices. Typical techniques include photocurrent versus voltage (JV) and external quantum efficiency (EQE) measurements. When preforming these measurements the device is taken to be homogenous over the entirety of the device. However, PV devices tend to vary greatly in efficiency over the device. Laser beam induced current (LBIC) is a method where a local photocurrent is generated in the device with a laser beam. Using this method of local excitation, a high density LBIC map can be generated to give insight into the non-homogenous features of the device by taking a large number of LBIC measurements and recording the XY position of each point. A Horiba LabRam controls the XY position and a Stanford Lockin Amplifier are used to measure the current. In CZTS photovoltaic devices it is suspected that a major loss of efficiency happens at the grain boundaries. μ-LBIC is done using a 100x magnification to achieve very high resolution on a small area while 10x LBIC maps are done to map the entire device. Using both of these measurements with 785nm and 500 nm wavelength lasers, the effects of current loss at grain boundaries are visible. Using this methodology in conjunction with further XRD, PL, and Ramen measurements offers the potential to improve our understanding of the mechanism.

Thermoelectric materials produce a voltage when placed under a temperature gradient (or vice versa), making them an appealing clean energy technology. However, thermoelectric efficiency is dependent on three intertwined yet competing properties; the Seebeck coefficient and electrical conductivity must be increased while keeping thermal conductivity low. Unfortunately, the Seebeck coefficient tends to be inversely related to electrical conductivity while decreasing thermal conductivity also decreases electrical conductivity in traditional semiconductors. In recent years, careful control of nanostructure has enabled researchers to decouple these conflicting properties in order to optimize thermoelectric efficiency. Carrier filtering is one mechanism for enhancing the Seebeck coefficient with minimal reduction of electrical conductivity. This can be accomplished by creating a composite of two materials with slightly offset band gaps, resulting in low energy carriers being filtered out. The goal of my research has been to create novel core/shell pnictogen chalcogenide nanoparticles in order to study the effects of the unique nanoarchitecture on transport properties. Bi₂Te₃/Bi₂Se₃ core/shell particles were created using a solvothermal synthesis. Current research is focused on gaining more fine control of the shell size, which will have direct impact on carrier filtration. Careful manipulation of synthetic conditions, such as reaction time and temperature, has been shown to direct the size and shape of the core/shell growth. Once the desired core/shell nanoparticle architectures have been synthesized, nanoparticles will be sintered into a bulk material in order to test thermoelectric properties. Future research includes applying the core/shell synthesis to similar pnictogen chalcogenide systems such as Sb₂Te₃/Sb₂Se₃ and Bi₂Se₃/Bi₂Te₃. Seeing the effects of core/shell architecture on thermoelectric properties will help give insight into the poorly understood electrical transport mechanisms that must be controlled in order to optimize thermoelectric efficiency.

Hybrid composites of inorganic quantum dots, with organic semiconducting polymers offer a potential means of producing low-cost, solution-processable photovoltaics. The synthesis of quantum dots typically involves the use of large surfactant molecules to facilitate particle growth and solubility. However, these native ligands are electrically insulating and must be exchanged with smaller ligands to achieve efficient charge carrier photogeneration and transport via reduced inter-dot spacing and enhanced electronic coupling. In this research, we examine the effect of different ligand treatments on electronic properties and morphology in bulk heterojunction blends of low band gap PbS quantum dots with the conjugated polymer poly((4,8-bis(octyloxy)benzo(1,2-b:4,5-b')dithiophene-2,6-diyl)(2-((dodecyloxy)carbonyl)thieno(3,4-b)thiophenediyl)) (PTB1). These ligands include halide ions and small bidentate organic molecules. Using photoinduced absorption (PIA) spectroscopy and transient photovoltage (TPV), we have studied long-lived charge generation and recombination kinetics of our devices, and demonstrated that ligand exchanges exhibiting higher device performance correlate to longer free carrier recombination lifetimes. We now seek to understand how the different ligand exchanges influence charge generation at fast (ps-ns) timescales using transient absorption (TA) spectroscopy. Furthermore, understanding the film morphology has been a critical missing variable in these devices. By synthesizing PbS quantum dots and preparing blends with different ligand treatments for characterization using high angle annular dark field electron tomography (HAADF-ET) in collaboration with the Moule group at UC Davis, we aim to obtain a detailed three-dimensional tomography images. This project will give us a better understanding of the relationship between the electronic and morphological properties induced by chemical modification of the quantum dot surfaces in hybrid polymer/quantum dot solar cells.