Solution Synthesis and Additive Manufacturing of Bismuth Telluride Nanoflakes for Flexible Energy Harvesting

Flexible thermoelectric generators (TEGs) are energy harvesting devices which convert temperature differences into electrical power. These devices require no moving parts and offer silent and autonomous operation. The variety of suitable materials covering a broad range of operating temperatures positions TEGs as a promising renewable energy source using waste heat recovery, especially for space and microgravity applications. Conventional manufacturing of flexible electronic and thermoelectric devices requires complicated and relatively expensive processing, which limits the capabilities of in-space manufacturing. Additive manufacturing (AM) expands the use of flexible electronics to new surfaces, applications, and recently, low gravity conditions. Plasma jet printing (PJP) is a new AM modality in which material is deposited via a gravity- independent plasma. This thesis demonstrates solution processing, ink synthesis, and PJP of bismuth telluride (Bi2Te3) nanoflakes for low temperature energy harvesting. Synthesis conditions were tailored to control nanoflake morphology and ink processing was optimized for direct printing on flexible polyimide substrates. The thermoelectric films demonstrate promising thermoelectric properties, material adhesion, and flexibility, with only a 7.2% variation in performance after 10,000 bending cycles over a 16 mm radius of curvature. This advancement of Bi2Te3 solution processing and demonstration of PJP of thermoelectric films marks a significant contribution to in space manufacturing of flexible thermoelectric devices for wearable technology and low temperature energy harvesting

Analyzing the Effects of Gold Reflective Coatings on GaAs Quantum Dot Photoluminescence

Molecular Beam Epitaxy (MBE) is a method for making high purity, tensile-strained GaAs quantum dots (QDs) embedded in solid-state semiconductors. QDs, excited by electricity or lasers, emit photons characteristic of the QDs size and composition, which may be used in tunable optoelectronic devices such as LEDs, lasers and solar cells. Understanding the light emission properties of these QDs is essential for these applications, as well as for continued QD research. Photoluminescence (PL) is a laser-excitation technique used to determine these properties. Occasionally, the PL signals from our samples are too low in intensity to be accurately detected. We will investigate whether the addition of gold coatings on the back of QD samples improves PL emission by reflecting additional photons into the detector. To apply these reflective coatings, we first prepare the samples using a chemical wet etch process and then deposit thin gold films via physical vapor deposition. We will analyze the difference in PL intensity between coated and noncoated samples and gauge the influence of gold deposition thickness

Calibration of Silicon- and Tellurium- Doped Gallium Arsenide Using the Hall Effect

Molecular Beam Epitaxy (MBE) provides a method for growing semiconductor crystals whose electrical properties may be fine-tuned through the addition of impurity (dopant) atoms. The addition of impurity atoms, such as tellurium or silicon, to gallium arsenide allows us to increase its electrical conductivity, which is critical for the production of electronic devices. Characterization using the Hall Effect and van der Pauw technique determines the electrical characteristics of these materials, including mobility and carrier (e.g. electron) concentration. It is the goal of this project to optimize both the MBE growth conditions and characterization methods and to produce a data set identifying the relationship between the temperature of the dopant element during deposition and the resulting carrier concentration. These values were found to have a linear Arrhenius relationship for silicon between 1025 – 1250°C, producing a carrier concentration range of 9.37x1015– 6.14x1018cm-3and a mobility range of 1171 – 4951 cm2/Vs, and for tellurium between the temperatures of 500 – 625°C with a carrier concentration range of 5.12x1016– 1.01x1019cm-3and a mobility range of 1284 – 4632 cm2/Vs. These silicon and tellurium dopant calibrations are essential for yielding reproducible materials and serves as the foundation for continued research into doped semiconductor materials

Using Distributed Bragg Reflectors to Improve Photon Collection From Quantum Dots

Quantum dots (QDs) are novel nanostructures that, when excited by electricity or light, emit photons useful for devices such as LEDs, quantum light sources, and solar cells. QDs emit light in all directions, with wavelengths characteristic of QD size and shape. We are interested in studying how these QDs emit light, but at times the light emission is too low in intensity to accurately detect with photoluminescence (PL) spectroscopy. To improve detection, we use molecular beam epitaxy (MBE) to insert a distributed Bragg reflector (DBR) into the sample structure, with the goal of reflecting additional photons into the PL detector. Theoretical modeling shows that changes in DBR thickness alter the reflected light wavelength, allowing us to tune the reflectivity of the DBR to match the emission spectra from GaAs QDs. Initial results show that we have accurately synthesized two DBR structures tuned to wavelengths of 1000 and 1100 nm. These results will be used to optimize the DBR structure and MBE growth conditions to provide maximum reflectivity. This work will give us a deeper understanding of our QDs, which is essential for device integration and continued QD research

Self-Assembly of Tensile-Strained Ge Quantum Dots on InAlAs(111)A

A recently developed growth technique enables the self-assembly of defect-free quantum dots on (111) surfaces under large tensile strains. We demonstrate the use of this approach to synthesize germanium (Ge) quantum dots on In0.52Al0.48As(111)A with \u3e3% residual tensile strain. We show that the size and areal density of the tensile-strained Ge quantum dots are readily tunable with growth conditions. We also present evidence for an unusual transition in the quantum dot growth mode from Stranski-Krastanov to Volmer-Weber as we adjust the substrate temperature. This work positions Ge quantum dots as a promising starting point for exploring the effects of tensile strain on Ge’s band structure

Defect Engineering of ZnO Nanoparticles for Bioimaging Applications

Many promising attributes of ZnO nanoparticles (nZnO) have led to their utilization in numerous electronic devices and biomedical technologies. nZnO fabrication methods can create a variety of intrinsic defects that modulate the properties of nZnO, which can be exploited for various purposes. Here we developed a new synthesis procedure that controls certain defects in pure nZnO that are theorized to contribute to the n-type conductivity of the material. Interestingly, this procedure created defects that reduced the nanoparticle band gap to ∼3.1 eV and generated strong emissions in the violet to blue region while minimizing the defects responsible for the more commonly observed broad green emissions. Several characterization techniques including thermogravimetric analysis, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, Raman, photoluminescence, and inductively coupled plasma mass spectrometry were employed to verify the sample purity, assess how modifications in the synthesis procedure affect the various defects states, and understand how these alterations impact the physical properties. Since the band gap significantly decreased and a relatively narrow visible emissions band was created by these defects, we investigated utilizing these new nZnO for bioimaging applications using traditional fluorescent microscopy techniques. Although most nZnO generally require UV excitation sources to produce emissions, we demonstrate that reducing the band gap allows for a 405 nm laser to sufficiently excite the nanoparticles to detect their emissions during live-cell imaging experiments using a confocal microscope. This work lays the foundation for the use of these new nZnO in various bioimaging applications and enables researchers to investigate the interactions of pure nZnO with cells through fluorescence-based imaging techniques