A typical electronic or photonic device may consist of several materials each one potentially meeting at an interface or terminating with a free-surface boundary. As modern device dimensions reach deeper into the nanoscale regime, interfaces and boundaries become increasingly influential to both electrical and thermal energy transport. While a large majority of the device community focuses on the former, we focus here on the latter issue of thermal transport which is of great importance in implementing nanoscale devices as well as developing solutions for on-chip heat removal and waste heat scavenging. In this document we will discuss how modern performance enhancing techniques (strain, nanostructuring, alloying, etc.) affect thermal transport at boundaries and across interfaces through the avenue of three case studies. We use first-principles Density Functional Perturbation Theory to obtain the phonon spectrum of the materials of interest and then use the dispersion data as input to a phonon Boltzmann Transport model. First, we investigate the combined effects of strain and boundary scattering on the in-plane and cross-plane thermal conductivity of thin-film silicon and germanium. Second, we review a recently developed model for cross-dimensional (2D-3D) phonon transport and apply it to 3D-2D-3D stacked interfaces involving graphene and molybdenum disulfide 2D-layers. Third, we combine relevant models from earlier Chapters to study extrinsic effects, such as line edge roughness and substrate effects, on in-plane and through-plane thermal transport in 1H-phase transition metal dichalcogenide (TMD) alloys. Through these investigations we show that: (1) biaxial strain in Si and Ge thin-films can modulate cross-plane conductivity due to strong boundary scattering, (2) the thermal boundary conductance between 2D-3D materials can be enhanced in the presence of an encapsulating layer, and (3) the thermal conductivity of 1H-phase TMDs can be reduced by an order of magnitude through the combination of nanostructuring, alloying, and substrate effects.

Graphene electronics represent a developing field where many material properties and devices characteristics are still unknown. Researching several possible fabrication processes creates a fabrication process using resources found at Cal Poly a local industry sponsor. The project attempts to produce a graphene network in the shape of a fractal Sierpinski carpet. The fractal geometry proves that PDMS microfluidic channels produce the fine feature dimensions desired during graphene oxide deposit. Thermal reduction then reduces the graphene oxide into a purified state of graphene. Issues arise during thermal reduction because of excessive oxygen content in the furnace. The excess oxygen results in devices burning and additional oxidation of the gate contacts that prevents good electrical contact to the gates. Zero bias testing shows that the graphene oxide resistance decreases after thermal reduction, proving that thermal reduction of the devices occurs. Testing confirms a fabrication process producing graphene electronics; however, revision of processing steps, especially thermal reduction, should greatly improve the yield and functionality of the devices.

We report on the fundamental properties and device applications of semiconductor nanoparticles. ZnO nanowires and CdSe quantum dots were used, prepared, characterized, and assembled into novel light-emitting diodes and solar cells. ZnO nanowire films were grown electrochemically using aqueous soluble chloride-based electrolytes as precursors at temperatures below 90° C. Dopants were added to the electrolyte in the form of chloride compounds, which are AlCl3, CoCl2, CuCl2, and MnCl2. The optical, magnetic, and structural properties of undoped and transition-metal-ion doped ZnO nanowires were explored. Our results indicate that the as-grown nanowire structures have considerable internal strain, resulting in clearly visible lattice distortions in bright and dark-field transmission electron micrographs. Photo and electroluminescence studies indicate that the strain-induced defects strongly dominate any dopant-related effects. However, annealing at moderate temperature as well as laser annealing induces strain relaxation and leads to dopant activation. Hence, the optical and electrical properties of the nanowires significantly improve, allowing these nanowires to become feasible for use in the fabrication of solar cell and LED devices. In addition, the magnetic impurities incorporated into our ZnO nanowires show superparamagnetic behavior at room-temperature, while Al-doped and undoped ZnO nanowires show no magnetic behavior. The electroluminescence (EL) is achieved from a vertical hybrid p-n junction LED arrangement consisting of a hole-conducting polymer and n-type ZnO nanowires, our group was the first to report this vertical nanowire-based LED in Könenkamp et al., 2004 [12]. The observed EL spectra show an ultraviolet excitonic emission peak and a broad defect-related emission band in the visible range. After annealing at 380° C, the defect related EL peak exhibits a characteristic shift to higher wavelengths, where the magnitude of the shift is dependent on the dopant type. Aluminum incorporation exhibited the most improved exciton related-emission, leading to the emergence of a narrow excitonic luminescence peak around 390 nm, which is close to the bandgap of ZnO. The comparison of spectra obtained from temperature-dependent photoluminescence (PL) measurements, before and after thermal annealing, also indicates that the optical activity of impurities changes noticeably upon annealing. The internal quantum efficiency for PL is measured to be as high as 16 percent for Al-doped samples annealed at 380° C. The PL measurements also show that the excitonic luminescence is preferentially guided, while the defect related emission is more isotropically emitted. The nanostructured heterojunction solar cell is designed such that thin CdSe quantum dot films are embedded between a ZnO nanowire film and a hole-conducting polymer layer. This arrangement allows for enhanced light absorption and an efficient collection of photogenerated carriers. Here, we present a detailed analysis of the pyridine solution and 1,2- ethanedithiol ligand exchange processes of the quantum dots, deposition processes of this quantum dot layer, the conformality of this layer on deeply nanostructured samples, and the effect of a surfactant-aided thermal annealing process. Annealing creates a structural conversion of the quantum dot layers into an extremely thin continuous poly-crystalline film, with typical grain diameters of 30-50 nm. This transition is accompanied by a loss of quantum confinement and a significant improvement of the charge transport in the CdSe layer. The combination of the solution and ligand exchange of CdSe quantum dots, as well as the deposition and optimized annealing processes of this quantum dot layer, resulted in solar cells with an open-circuit voltage up to 0.6 V, a short circuit current of ~15 mA/cm2, an external quantum efficiency of 70 percent, and an energy conversion efficiency of 3.4 percent. This 3.4 percent efficiency is presently one of the best efficiencies obtained for this type of device.

Quantum dots show great potential for use as spectral converters in solar cells, lighting applications, and biological imaging. These applications require precise control of quantum dot size to maximize performance. The quality, size, and fluorescence of quantum dots depend on parameters that are difficult to control using traditional batch synthesis processes. An alternative, high precision process was developed for the synthesis of cadmium-selenide quantum dots using a microfluidic reactor and fluorescence flow cell. The process required creating separate cadmium and selenium precursors that were then mixed in a nitrogen environment at 17°C. Using an NE-300® syringe pump, the solution was pumped through a microfluidic reactor submerged in a 240°C oil bath. The reactor then fed through a water quench bath at 25°C to terminate the nucleation and growth reaction. The fluorescence profiles of the quantum dot solutions were then characterized with an in-line fluorescence flow cell used in conjunction with an Ocean Optics® USB4000® spectrometer and a ThorLabs® LED UV light source. Flow rates through the reactor were varied from 0.05ml/min to 2ml/min. A central peak wavelength was registered in the fluorescence profiles for each flow rate. Monodisperse Cd-Se quantum dot solutions were synthesized across a broad spectrum of wavelengths ranging from 490nm to 620nm. An empirical relationship between flow rate and center wavelength was determined.

The design and implementation of a micro-force displacement system was completed to test the force-displacement characteristics of square silicon diaphragms with side lengths of 4mm, 5mm, and 7mm with a thickness of 10um. The system utilizes a World Precision Instruments Fort 10g force transducer attached to a World Precession Instruments TBM4M amplifier. A Keithley 2400 source meter provided data acquisition of the force component of the system. A micro prober tip was utilized as the testing probe attached to the force transducer with a tip radius of 5um. The displacement of samples was measured using a Newport M433 linear stage driven by a Newport ESP300 motion controller (force readings at constant displacement intervals). An additional 3 linear stages were used to provide X and Y-axis positioning of samples beneath the probe tip. The system components were mounted to an optical bench to provide stability during testing. C# was used to deliver the code to the individual components of the system. In addition the software provides a graphic user interface for future users that includes a calibration utility (both X/Y and force calibration), live force-displacement graph, motion control, and a live video feed for sample alignment. Calibration of the force transducer was accomplished using an Adam Equipment PGW153e precision balance to assign force values to the voltage data produced from the transducer. Displacement calibration involved the use of a microscope calibration micrometer. The system was characterized with an equipment variability of ±1.02mg at 1.75um, and ±1.86mg at 3.5um with the ability to characterize samples with stiffness less than 279 mg/um. The displacement resolution of the system was determined to be 35 nm per step of the linear stages. The diaphragms created to test the machine were fabricated from 10um thick device layer SOI wafers. An etch consisting of 38g/l silicic acid, 7g/l ammonium persulfate, and 5% TMAH was used to reduce the formation of hillocks, and provide a consistent etch rate. A Gage R&R study was performed on the fabricated diaphragms, indicating that the deflection produced by the 4mm, 5mm, and 7mm diaphragms was resolvable by the machine. A model was developed to correlate theoretical results to the observed measured values.

This thesis covers the design, fabrication, modeling and characterization of electrostatically actuated silicon membranes, with applications to microelectromechanical systems (MEMS). A microfabrication process was designed to realize thin membranes etched into a silicon wafer using a wet anisotropic etching process. These flexible membranes were bonded to a rigid counterelectrode using a photo-patterned gap layer. The membranes were actuated electrostatically by applying a voltage bias across the electrode gap formed by the membrane and the counterelectrode, causing the membrane to deflect towards the counterelectrode. This deflection was characterized for a range of actuating voltages and these results were compared to the deflections predicted by calculations and Finite Element Analysis (FEA). This thesis demonstrates the first electrostatically actuated MEMS device fabricated in the Cal Poly, San Luis Obispo Microfabrication Facility. Furthermore, this thesis should serve as groundwork for students who wish to improve upon the microfabrication processes presented herein, or who wish to fabricate thin silicon structures or electrostatically actuated MEMS structures of their own.

This paper studies the optical properties of a GaN Laser Diode (LD). Through simulation, the GaN LD is optimized for the best optical confinement factor. It is found that there are optimal thicknesses of each layer in the diode that yield the highest optical confinement factor. There is a strong relationship between the optical confinement factor and lasing threshold—a higher optical confinement factor results in a lower lasing threshold. Increasing optical confinement improves lasing efficiency. Blue LDs are important to the future of lighting sources as they represent the final color in the RGB spectrum that does not have a high efficiency solution. The modeled GaN LD emits blue light at around ~450nm. Each layer of the GaN LD is drawn in a model simulation program called LaserMOD created by RSOFT Design Group, Inc. By properly modifying the properties of each layer, an accurate model of the GaN LD is created and then simulated. This paper describes the steps taken to properly model and optimize the GaN LD in the 1D and 2D models.

Research to increase the efficiency of conventional solar cells is constantly underway. The goal of this work is to increase the efficiency of conventional solar cells by incorporating quantum dot (QD) nanoparticles in the absorption mechanism. The strategy is to have the QDs absorb UV and fluoresce photons in the visible region that are more readily absorbed by the cells. The outcome is that the cells have more visible photons to absorb and have increased power output. The QDs, having a CdSe core and a ZnS shell, were applied to the solar cells as follows. First, the QDs were synthesized in an octadecene solution, then they were removed from the solution and finally they were dried and deposited into polydimethylsiloxane (PDMS) and the PDMS/QD composite is allowed to cure. The cured sample is applied to a silicon solar panel. The panel with the PDMS/QD application outputs 2.5% more power than the one without, under identical illumination by a tungsten halogen lamp, using QDs that fluoresce in the orange region. This work demonstrates the feasibility of incorporating QDs to increase the efficiency of conventional solar cells. Because the solar cells absorb better in the red region, future effort will be to use QDs that fluoresce in that region to further boost cell output.

This report proposes a novel technique for measuring of acoustic fields in water. A Laser Hydrophone is proposed taking advantage of the properties of Total Internal Reflection. A theoretical analysis of the idea is presented followed by a prediction of the operating characteristics of an actual system. Actual data were taken with the proposed system and it is compared to the predicted.

<p>Nanomaterials have been heavily studied in the past two decades. Previous findings have demonstrated that the characteristics of nanocomposites and the performance of nanomaterial-based devices are both determined by the interfacial characteristics of the nanomaterials. However, there are still some remaining challenges from interfacial characteristics to device applications, which are specified as follows: the difficulty in identifying the interfacial contacts of nanostructured surfaces, the instability of nanocomposite surfaces, and the under-researched mechanism of the correlation between interfacial characteristics and the performance of devices.</p> <p>Therefore, the main theme of this thesis is to investigate the interfacial contacts of nanostructured solid-liquid interfaces by direct observation, and to develop a stable nanocomposite based on which the direct observation of the interfacial contact can be better conducted, and to eventually investigate the effect of interfacial contacts on the performance of organic solar cells.</p> <p>As the previous identification of the solid-liquid interface is limited to a microscale range, a direct method of tracing the different wetting states of water was developed, on nanostructured surfaces. This method provided an answer to a long standing question of, whether there is a transition from Wenzel to Cassie states in the sliding angle drop on nanocomposite thin films. In order to complete the observation of the wetting states of water, a stable superhydrophobic nanocomposite thin film with hierarchical structure was developed.</p> <p>Furthermore, with the knowledge of identifying the wetting states and the preparing procedures of the nanocomposites, a surfactant-free small-molecule nanoparticle organic solar cell with a much improved fill factor was developed by spin coating. The inverse correlation of series resistance and parallel resistance was discovered, due to the morphology change and the variation of the charge carrier concentration near the donor-acceptor interface in small-molecule organic solar cells.</p> / Doctor of Philosophy (PhD)