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.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/83822 |
| Date | 29 June 2018 |
| Creators | Myers, Kirby |
| Contributors | Physics, Robinson, Hans D., Arav, Nahum, Asryan, Levon V., Heflin, James R. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/x-zip-compressed |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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