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A study was conducted of CuₓS and its interaction with the substrate and ambient. The goals of these CuₓS on CdS and Zn_yCd_1-yS substrates were to find the differences in materials related properties, if any. Cadmium and zinc compositions in CuₓS formed on Zn_yCd_1-yS films (O < y < 0.25) by means of ion exchange were measured using Auger Electron Spectroscopy (AES), Atomic Absorption Spectroscopy (AAS), and Electron Spectroscopy for Chemical Analysis (ESCA). Net concentrations of Cd and Zn in as-formed Cu₂S are generally in the 10¹⁸-10¹⁹ cm⁻³ range. Heat treatments in both oxidizing and reducing ambients raise the concentrations by over an order of magnitude, with the Zn concentration increasing more so than those of Cd. Large increases in Zn at or near the CuₓS surface were measured subsequent to heat treatment, accompanied by increased oxygen. Following heat treatments, Cd and Zn concentrations in the CuₓS"bulk" are found to be less than 10¹⁹ and 10²⁰ cm.⁻³, respectively, for all substrate compositions used. It is proposed that the presence of Cd and Zn can adversely effect the minority carrier lifetimes. These effects would tend to reduce the light generated current. The effects of heat treating CuₓS/Zn_yCd_1-yS and CuₓS/CdS in reducing and various oxidizing ambients are also reported. Structural changes taking place in CuₓS as a result of these heat treatments were monitored by using x-ray diffraction. The principal physical mechanism responsible for phase changes in CuₓS appears to x be copper diffusion through the copper sulfide layer to the top surface as well as into the substrate. Changes in CuₓS stoichiometry were correlated with the sheet resistance of the CuₓS layer. Results indicate that heat treatment in a hydrogen atmosphere causes an increase in resistivity (corresponding to an increase in stoichiometry) while heat treatment in air causes the reverse effect. Wet air heat treatment tended to decrease the resistivity much more as compared to dry air. It was observed that CuₓS formed on Zn_yCd_1-yS tended to degrade in stoichiometry much faster as compared to CuₓS formed on CdS. The resistivity of evaporated CuₓS on plain glass seemed to be linked to the amount of free copper and sulfur present in the as-deposited film. Argon heat treatment tended to decrease the resistivity by an order of magnitude. Heat treatment tended to react the free copper and sulfur, giving CuₓS. Free copper and sulfur can increase the resistivity by acting as neutral impurity scattering centers. As-deposited films were always Cu rich as evidenced by x-ray diffraction and EDAX. Argon heat treatment tended to decrease the amount of free copper present. X-ray photoelectron spectroscopy (XPS) was applied to the surface chemical characterization of chemiplated CuₓS on Zn_yCd_1-yS and CdS. CuₓS was also vacuum evaporated onto glass substrates for this purpose. The effects of ambient (oxygen and water vapor in particular) on chemical species present at or near the CuₓS surface were investigated. Subsequent heat treatments of CuₓS/Zn_yCd_1-yS and CuₓS/CdS promoted migration of Zn and Cd toward the Cu₂S surface. When formed on CdS, the CuₓS surface was found to contain CdO (or Cd (OH)₂) CuO, CuSO₄.nH₂O and CdSO₄.nH₂). Cu₂S formed on Zn_yCd_1-yS was found to contain ZnO as the predominant chemical species, with the Cu and Cd compounds present in lesser amounts. Some interesting characteristics of powder standards used in the XPS studies, some of which have not appeared in the literature, are presented in Appendix 2. The above effects can account for key differences in the properties of CuₓS formed on Zn_yCd_1-yS and CdS films. This provides information on the possible degradation mechanisms for these types of junctions. / Ph. D.

Nanofabrication techniques continue to advance and are rapidly becoming the primary route to enhancement for the electrical, thermal, and optical properties of materials. The work presented in this dissertation details fabrication and characterization techniques of thin films and nanoparticles for these purposes. The four primary areas of research presented here are thermoelectric enhancement through nanostructured thin films, an alternative frequency-domain thermoreflectance method for thin film thermal conductivity measurement, thermal rectification in nanodendritic porous silicon, and plasmonic enhancement in silver nanospheroids as a reverse photolithography technique. Nanostructured thermoelectrics have been proposed to greatly increase thermopower efficiency and to bring thermoelectrics to mainstream power generation and cooling applications. In our work, thermoelectric thin films of SbTe, BiTe, and PbTe grown by atomic layer deposition and electrochemical atomic layer deposition were characterized for enhanced performance over corresponding bulk materials. Seebeck coefficient measurements were performed at temperatures ranging from 77 K to 380 K. Atomic composition was verified by energy-dispersive X-ray spectroscopy and structures were imaged by scanning electron microscopy. All thin films measured were ultimately found to have a comparable or smaller Seebeck coefficient to corresponding materials made by conventional techniques, likely due to issues with the growth process. Frequency-domain thermoreflectance offers a minimally invasive optical pump-probe technique for measuring thermal conductivity. Like time-domain thermoreflectance, the version of frequency-domain thermoreflectance demonstrated here relies on a non-zero thermo-optic coefficient in the sample, but uses moderate cost continuous wave lasers modulated at kHz or MHz frequencies rather than a more expensive ultrafast laser system. The longer timescales of these frequency ranges enables this technique to take measurements of films with thicknesses ranging from 100 nm to 10 um, complimentary to time-domain thermoreflectance. This method differentiates itself from other frequency-domain methods in that it is also capable of simultaneous independent measurements of both the in plane and out of plane values of the thermal conductivity in anisotropic samples through relative reflective magnitude, rather than phase, measurements. We validated this alternate technique by measuring the thermal conductivity of Al2O3 and soda-lime and found agreement both with literature values and with separate measurements obtained with a conventional time-domain thermoreflectance setup. Thermal rectification has the potential to enhance microcircuit performance, improve thermoelectric efficiency, and enable the creation of thermal logic circuits. Passive thermal rectification has been proposed to occur in geometrically asymmetric nanostructures when heat conduction is dominated by ballistic phonons. Here, nanodendritic structures with branch widths of ~ 10 nm and lengths of ~ 20 nm connected to ~ 50 um long trunks were electrochemically etched from <111> silicon wafers. Thermal rectification measurements were performed at temperatures ranging from 80 K to 250 K by symmetric thermal conductivity measurements. No thermal rectification was ultimately found in these samples within the margin of thermal conductivity measurement error 1%. This result is consistent with another study which found thermal rectification with greater conduction in the direction opposite to what ballistic phonon heat conduction theories predicted. Plasmonic resonance concentrates incident photon energy and enables channeling of that energy into sub-wavelength volumes where it can be used for nanoscale applications. We demonstrated that surface plasmon polaritons induced in silver nanosphereoid films by 532 nm light defunctionalize previously photocleaved ligands adsorbed onto the films, to yield a reverse photolithographic technique. In this method, gold nanosphere conjugation were conjugated to a photocleaved ligand, however conjugation could be inhibited by exposing the cleaved ligand to 532 nm light and consequently yield a reversal technique. This defunctionalizion effect did not occur on gold films or nanoparticles conjugated with the ligand in IR spectroscopy, and was observed to have a reduced effect in silver films relative to silver nanospheroid film. As silver nanospheroid films and gold nanospheres of the size used in this study are known to have plasmon resonance in the green wavelengths, while gold and silver continuous films do not, this defunctionalization likely results from plasmonic effects. / Ph. D. / The increasing trend toward smaller and more efficient electronic devices requires continuous refinement of manufacturing and materials technology. From communication devices to temperature management, miniaturization in electronic components allows for greater versatility in applications. In battery powered devices, increasing efficiency both extends operational lifetime and reduces operational costs in terms of kilowatt hours as well as carbon footprint resulting from powering the devices. Through the application of miniaturization, conventional fabrication techniques are rapidly approaching the physical limits of their applicability, and newer techniques must be developed. Nanofabrication methods involve working with materials at scales where quantum mechanical effects can dominate over classical effects. Some examples of these effects are unique heat and electrical conduction properties in, effectively, one or two dimensional materials as in the case of quantum dots or thin films. This size regime not only allows for construction at smaller scales, but also enables the manipulation of quantum mechanical effects to produce different types of devices which were not possible to make previously. For example, materials can be built up one atomic layer at a time, enabling the creation of a material with perfect atomic ordering, as opposed to common methods which yield many imperfections. This dissertation details fabrication and characterization techniques of nanoscale devices focusing on thermoelectrics, thin film thermal conductivity, thermal rectification, and plasmonic enhancement. Thermoelectrics are devices that use temperature differences across the device to produce electric power or, conversely, create a temperature difference across the device when electrically driven. Theoretical studies have proposed that the efficiency of thermoelectric materials can be greatly increased through nanofabrication. Here, thin film thermoelectric devices made from commonly employed bulk materials such as SbTe, BiTe, and PbTe produced by atomic layer deposition and electrochemical atomic layer deposition, were characterized to test these theories. Ultimately, no notable enhancement was found in our samples over conventionally produced materials, but this may have been due to difficulties in the fabrication process of the thin films. Thermoreflectance is a purely optical technique for thermal conductivity (the measure of how well a material conducts heat) measurement which can measure thin film materials. Other benefits of the technique are its speed and that samples measured by it are not damaged, unlike other methods which effectively ruin the sample for any purpose beyond the measurement. Cost, however, is a major downside to conventional thermoreflectance, as it requires pricey ultrafast laser systems. Presented here is an alternative method of thermoreflectance which used much more economical diode lasers to achieve thermal conductivity measurements. This system costs approximately a tenth of what a conventional system would, while also being capable of measuring in-plane and cross-plane thermal conductivity simultaneously. The drawbacks of this method are thicker film requirements and the necessity of having well-defined control samples of similar thermal conductivity to the sample of interest. Management of waste heat is one of the major design limitations in modern circuitry. Removal of waste heat is most often performed by adhering large surface area heat sinks to heat generating areas and/or mechanical fans to aid in heat radiation. One proposed method of reducing the amount of space required for heat management is through the development and implementation of thermal rectifiers, which are materials that conduct heat more efficiently in one direction than the opposite. The thermal rectification properties of nanodendritic porous silicon is explored here. This material is made by electrochemically etching silicon wafers such that the surface is formed into an array of pine-tree-like structures on the nanoscale. While it was proposed that these structures would manifest thermal rectification under the right conditions, no rectification was observed. This result is consistent with previous experimental work which observed preferential heat conduction in the opposite direction to that proposed by this theory, likely caused by a different effect. Plasmonic enhancement enables absorption and manipulation of light energy in structures far smaller than conventional techniques permit. In the case of photolithography, a go-to method of commercial microfabrication, the smallest feature size is a function of the wavelength of light used and is typically around 100 nm. Plasmonic techniques enable optical manipulation in structures of sizes down to a few nm. The plasmonic enhancement technique demonstrated here is a photolithography technique in which selective nanosphere-to-nanosphere binding occurs This technique offers another method of directed self-assembly, where nanoparticles can come together to form larger structures. A benefit of this method is that large quantities of nanoparticle assemblies can occur simultaneously, allowing for rapid production of assembled nanostructures.

Adsorption of xyloglucan (XG) onto thin desulfated nanocrystalline cellulose (DNC) films was studied by surface plasmon resonance spectroscopy (SPR), quartz crystal microbalance with dissipation monitoring (QCM-D), and atomic force microscopy (AFM) measurements. These studies were compared to adsorption studies of XG onto thin sulfated nanocrystalline cellulose (SNC) films and regenerated cellulose (RC) films performed by others. Collectively, these studies show the accessible surface area is the key factor for the differences in surface concentrations observed for XG adsorbed onto the three cellulose surfaces. XG penetrated into the porous nanocrystalline cellulose films. In contrast, XG was confined to the surfaces of the smooth, non-porous RC films. Surprisingly surface charge and cellulose morphology played a limited role on XG adsorption. The effect of the non-ionic surfactant Tween 80 on the adsorption of cellulase onto alkane thiol self-assembled monolayers (SAMs) on gold was also studied. Methyl (-CH3), hydroxyl (-OH) and carboxyl (-COOH) terminated SAMs were prepared. Adsorption of cellulase onto untreated and Tween 80-treated SAMs were monitored by SPR, QCM-D and AFM. The results indicated cellulase adsorption onto SAM-CH3 and SAM-COOH were driven by strong hydrophobic and electrostatic interactions, however, hydrogen bonding between cellulase and SAM-OH was weak. Tween 80 effectively hindered the adsorption of cellulase onto hydrophobic SAM-CH3 substrates. In contrast, it had almost no effect on the adsorption of cellulase onto SAM-OH and SAM-COOH substrates because of its reversible adsorption on these substrates. / Master of Science

Delamination along thin film interfaces is a prevalent failure mechanism in microelectronic, photonic, MEMS, and other engineering applications. Current interfacial fracture test techniques specific to thin films are limited by either sophisticated mechanical fixturing, physical contact near the crack tip, non-representative test specimens, or complicated stress fields. Moreover, these techniques are generally not suitable for investigating fatigue crack propagation under cyclical loading. A fixtureless and noncontact experimental test technique is thus proposed and implemented to study interfacial fracture for thin film systems. The proposed test incorporates permanent magnets surface mounted onto micro-fabricated released thin film structures. An applied external magnetic field induces noncontact monotonic or fatigue loading to initiate delamination along the interface between the thin film and underlying substrate. Characterization of the film deflection, peel angle, and delamination propagation is accomplished through in situ optical techniques. Analytical and finite-element models are used to extract fracture parameters from the experimental data using thin-film peel mechanics. The developed interfacial fracture test has been demonstrated for Cu thin films on a SiO₂/Si substrate.

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