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• 1. [Article] Spatio – temporal temperature variations during droplet impingement evaporation : effects of nanofluid and nano-structured surface

  Droplet and spray impingement cooling are typically used in high heat flux thermal management. In this thesis, droplet impingement and evaporation heat transfer characteristics are determined from measured ...

  Citation × Citation Citation Abstract Copy Title: Spatio – temporal temperature variations during droplet impingement evaporation : effects of nanofluid and nano-structured surface Author: Graber, Christof Droplet and spray impingement cooling are typically used in high heat flux thermal management. In this thesis, droplet impingement and evaporation heat transfer characteristics are determined from measured spatially- and temporally- varying fluid and surface temperatures. Unique to this study is the documentation of the effects of using a nanofluid and a nano-structured surface for dropwise cooling by comparison of heat transfer characteristics with that of water droplet impingement on a polished surface. Temperatures were determined using radiation intensities recorded using an Infrared (IR) camera. The impingement surface is either comprised of IR transparent silicon, which permits near-surface fluid temperature measurements, or an IR opaque gold coated surface, which permits surface temperature measurements. A range of surface heat fluxes, resulting in both single-phase and boiling conditions are studied. Three different impingement surfaces have been tested, including polished silicon, nano-structured porous silicon, and gold coated polished silicon. The nanofluid is a water-based carbon nanotube suspension. Five major droplet impingement and evaporation stages have been identified: initial impact, boiling (if the surface temperature was sufficiently high), approximately constant diameter evaporation, stepwise fast receding contact line evaporation, and simultaneously decaying diameter and contact angle final dryout period. The surface temperature spatial distribution shows the lowest temperature values within the contact area bulk region and increasing temperature values toward the contact line region and beyond. The basic temperature trends and evaporation behavior are similar for the polished and nano-structured surface while the nanofluid exhibits some distinction. Evaporation times are reduced up to 20% and 37% using the nanostructured surface and nanofluid, respectively. Considering the evaporation time reduction as a measure of droplet cooling performance, the nano-enhanced surface and nanofluid may improve heat transfer in droplet impingement and spray cooling applications. Title: Spatio – temporal temperature variations during droplet impingement evaporation : effects of nanofluid and nano-structured surface Author: Graber, Christof Droplet and spray impingement cooling are typically used in high heat flux thermal management. In this thesis, droplet impingement and evaporation heat transfer characteristics are determined from measured spatially- and temporally- varying fluid and surface temperatures. Unique to this study is the documentation of the effects of using a nanofluid and a nano-structured surface for dropwise cooling by comparison of heat transfer characteristics with that of water droplet impingement on a polished surface. Temperatures were determined using radiation intensities recorded using an Infrared (IR) camera. The impingement surface is either comprised of IR transparent silicon, which permits near-surface fluid temperature measurements, or an IR opaque gold coated surface, which permits surface temperature measurements. A range of surface heat fluxes, resulting in both single-phase and boiling conditions are studied. Three different impingement surfaces have been tested, including polished silicon, nano-structured porous silicon, and gold coated polished silicon. The nanofluid is a water-based carbon nanotube suspension. Five major droplet impingement and evaporation stages have been identified: initial impact, boiling (if the surface temperature was sufficiently high), approximately constant diameter evaporation, stepwise fast receding contact line evaporation, and simultaneously decaying diameter and contact angle final dryout period. The surface temperature spatial distribution shows the lowest temperature values within the contact area bulk region and increasing temperature values toward the contact line region and beyond. The basic temperature trends and evaporation behavior are similar for the polished and nano-structured surface while the nanofluid exhibits some distinction. Evaporation times are reduced up to 20% and 37% using the nanostructured surface and nanofluid, respectively. Considering the evaporation time reduction as a measure of droplet cooling performance, the nano-enhanced surface and nanofluid may improve heat transfer in droplet impingement and spray cooling applications. Close

• 2. [Article] The surface plasmon enhancement effect on adsorbed molecules at elevated temperatures

  The surface plasmon enhancement effect on adsorbed molecules at elevated substrate temperatures is studied theoretically using surface enhanced Raman scattering (SERS) as an example. The surface structure ...

  Citation × Citation Citation Abstract Copy Title: The surface plasmon enhancement effect on adsorbed molecules at elevated temperatures Author: Leung, P.T., Chiang, Hai-Pang, Tse, W. S. Year: 1998 The surface plasmon enhancement effect on adsorbed molecules at elevated substrate temperatures is studied theoretically using surface enhanced Raman scattering (SERS) as an example. The surface structure is idealized to be a monodisperse spherical particle with its nonlocal dielectric response accounted for. The temperature effects are modeled using a temperature-dependent collision frequency in the Drude model. Numerical results show that only a small decrease in the SERS enhancement ratio occurs for temperatures up to the melting point of the substrate, even for scattering close to the surface plasmon resonance frequency of the metal. More definitive results are subjected to more realistic modeling as well as systematic experimental studies. The implication of this result to other surface photochemical processes is discussed. Title: The surface plasmon enhancement effect on adsorbed molecules at elevated temperatures Author: Leung, P.T., Chiang, Hai-Pang, Tse, W. S. Year: 1998 The surface plasmon enhancement effect on adsorbed molecules at elevated substrate temperatures is studied theoretically using surface enhanced Raman scattering (SERS) as an example. The surface structure is idealized to be a monodisperse spherical particle with its nonlocal dielectric response accounted for. The temperature effects are modeled using a temperature-dependent collision frequency in the Drude model. Numerical results show that only a small decrease in the SERS enhancement ratio occurs for temperatures up to the melting point of the substrate, even for scattering close to the surface plasmon resonance frequency of the metal. More definitive results are subjected to more realistic modeling as well as systematic experimental studies. The implication of this result to other surface photochemical processes is discussed. Close

• 3. [Article] Nanostructured Semiconductor Heterojunctions from Quantum Dot Layers

  We report the deposition of conformal thin layers of 10?50 nm thickness from II-VI quantum dot suspensions on ZnO nanowire substrates. Smooth polycrystalline films of high electronic quality can be obtained ...

  Citation × Citation Citation Abstract Copy Title: Nanostructured Semiconductor Heterojunctions from Quantum Dot Layers Author: Könenkamp, Rolf, Word, Robert Campbell, Nadarajah, Athavan Year: 2009 We report the deposition of conformal thin layers of 10?50 nm thickness from II-VI quantum dot suspensions on ZnO nanowire substrates. Smooth polycrystalline films of high electronic quality can be obtained from CdSe quantum dots after annealing at moderate temperatures. The electronic properties are adequate for detector and solar cell applications. The growth and annealing temperatures permit deposition on light-weight and flexible substrates. Some elemental diffusion of Se across the CdSe/ZnO interface occurs in the film formation. A comparison with CdS/ZnO junctions indicates that the low Se diffusion rates are essential for efficient charge transfer. Title: Nanostructured Semiconductor Heterojunctions from Quantum Dot Layers Author: Könenkamp, Rolf, Word, Robert Campbell, Nadarajah, Athavan Year: 2009 We report the deposition of conformal thin layers of 10?50 nm thickness from II-VI quantum dot suspensions on ZnO nanowire substrates. Smooth polycrystalline films of high electronic quality can be obtained from CdSe quantum dots after annealing at moderate temperatures. The electronic properties are adequate for detector and solar cell applications. The growth and annealing temperatures permit deposition on light-weight and flexible substrates. Some elemental diffusion of Se across the CdSe/ZnO interface occurs in the film formation. A comparison with CdS/ZnO junctions indicates that the low Se diffusion rates are essential for efficient charge transfer. Close

• 4. [Article] Synthesis, Structure and Contrast Efficacy of Bismuth (III) Chelated with 1,4,7,10-Tetraazacyclododecane-1,4,7,10-Tetramethylene Phosphonate and its Incorporation into Nano-Assembled Capsules

  Medical imaging now plays an integral role in the diagnosis and treatment of illness. The use of contrast agents in molecular imaging has been instrumental in the diagnosis of cancer and similar pathologies. ...

  Citation × Citation Citation Abstract Copy Title: Synthesis, Structure and Contrast Efficacy of Bismuth (III) Chelated with 1,4,7,10-Tetraazacyclododecane-1,4,7,10-Tetramethylene Phosphonate and its Incorporation into Nano-Assembled Capsules Author: Maier, Karley B. Year: 2016 Medical imaging now plays an integral role in the diagnosis and treatment of illness. The use of contrast agents in molecular imaging has been instrumental in the diagnosis of cancer and similar pathologies. The incorporation of lanthanide-based contrast agents employed in magnetic resonance imaging (MRI) into nano-assembled capsules (NACs) have shown potential utility for improved contrast in MRI by slowing the tumbling of the polyanionic paramagnetic metal ions. The utility of NACs was herein extended to computed tomography (CT) imaging by the incorporation of electron dense bismuth (III)-based chelates into NACs in order to provide a high payload of agent in relatively small volumes. Bismuth (III) was chelated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethylene phosphonate (DOTP) and incorporated into a NAC encapsulated with a spherical silica nanoparticle shell at varied concentrations, solvent systems and temperature. The size distribution of these NACs was characterized and compared to gadolinium-based NACs. The formation of bismuth containing NACs was confirmed; the reproducibility in size distribution was not as reproducible as expected, and did not adhere to the same trends as lanthanide containing capsules. Although aging time may be directly related to size distribution of the capsules, the solvent system and temperature studies did not yield a consistent relationship. Title: Synthesis, Structure and Contrast Efficacy of Bismuth (III) Chelated with 1,4,7,10-Tetraazacyclododecane-1,4,7,10-Tetramethylene Phosphonate and its Incorporation into Nano-Assembled Capsules Author: Maier, Karley B. Year: 2016 Medical imaging now plays an integral role in the diagnosis and treatment of illness. The use of contrast agents in molecular imaging has been instrumental in the diagnosis of cancer and similar pathologies. The incorporation of lanthanide-based contrast agents employed in magnetic resonance imaging (MRI) into nano-assembled capsules (NACs) have shown potential utility for improved contrast in MRI by slowing the tumbling of the polyanionic paramagnetic metal ions. The utility of NACs was herein extended to computed tomography (CT) imaging by the incorporation of electron dense bismuth (III)-based chelates into NACs in order to provide a high payload of agent in relatively small volumes. Bismuth (III) was chelated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethylene phosphonate (DOTP) and incorporated into a NAC encapsulated with a spherical silica nanoparticle shell at varied concentrations, solvent systems and temperature. The size distribution of these NACs was characterized and compared to gadolinium-based NACs. The formation of bismuth containing NACs was confirmed; the reproducibility in size distribution was not as reproducible as expected, and did not adhere to the same trends as lanthanide containing capsules. Although aging time may be directly related to size distribution of the capsules, the solvent system and temperature studies did not yield a consistent relationship. Close

• 5. [Article] Remote Plasma Assisted Growth of Graphene Films

  Single and multiple layers of graphene films were grown on 111 oriented single crystals of nickel and polycrystalline nickel films using remote plasma assisted chemical vapor deposition. Remote plasma ...

  Citation × Citation Citation Abstract Copy Title: Remote Plasma Assisted Growth of Graphene Films Author: Nandamuri, Gopichand, Roumimov, Sergei, Solanki, Raj Year: 2010 Single and multiple layers of graphene films were grown on 111 oriented single crystals of nickel and polycrystalline nickel films using remote plasma assisted chemical vapor deposition. Remote plasma was employed to eliminate the effect of the plasma electrical field on the orientation of the grown graphene films, as well as to reduce the growth temperature compared to conventional chemical vapor deposition. The electrical and optical properties, including high resolution transmission electron microscopy of these films, suggest that this approach is both versatile and scalable for potential large area optoelectronic applications. Title: Remote Plasma Assisted Growth of Graphene Films Author: Nandamuri, Gopichand, Roumimov, Sergei, Solanki, Raj Year: 2010 Single and multiple layers of graphene films were grown on 111 oriented single crystals of nickel and polycrystalline nickel films using remote plasma assisted chemical vapor deposition. Remote plasma was employed to eliminate the effect of the plasma electrical field on the orientation of the grown graphene films, as well as to reduce the growth temperature compared to conventional chemical vapor deposition. The electrical and optical properties, including high resolution transmission electron microscopy of these films, suggest that this approach is both versatile and scalable for potential large area optoelectronic applications. Close

• 6. [Article] Transmission studies of explosive vaporization of a transparent liquid film on an opaque solid surface induced by excimer-laser-pulsed irradiation

  Examines the dynamics of the explosion of a liquid film by an ultraviolet excimer pulsed laser studied experimentally on top of an amorphous silicon film deposited on fused quartz. Background on thermal ...

  Citation × Citation Citation Abstract Copy Title: Transmission studies of explosive vaporization of a transparent liquid film on an opaque solid surface induced by excimer-laser-pulsed irradiation Author: Leung, P.T., Do, Nhan, Klees, Leander, Leung, Wing P., Tong, Frank Year: 1992 Examines the dynamics of the explosion of a liquid film by an ultraviolet excimer pulsed laser studied experimentally on top of an amorphous silicon film deposited on fused quartz. Background on thermal physics of superheated pure or mixed liquids; Techniques for surface temperature measurement; Experimental of probe laser and pulsed excimer laser. Title: Transmission studies of explosive vaporization of a transparent liquid film on an opaque solid surface induced by excimer-laser-pulsed irradiation Author: Leung, P.T., Do, Nhan, Klees, Leander, Leung, Wing P., Tong, Frank Year: 1992 Examines the dynamics of the explosion of a liquid film by an ultraviolet excimer pulsed laser studied experimentally on top of an amorphous silicon film deposited on fused quartz. Background on thermal physics of superheated pure or mixed liquids; Techniques for surface temperature measurement; Experimental of probe laser and pulsed excimer laser. Close

• 7. [Article] Synthesis of titanium dioxide nanorods using a continuous flow microreactor

  The primary goal of this thesis is to synthesize titanium dioxide nanorods using a continuous flow microreactor. Titania powder, one of the most important particulate materials is used for many purposes ...

  Citation × Citation Citation Abstract Copy Title: Synthesis of titanium dioxide nanorods using a continuous flow microreactor Author: Yen, Wei-Liang The primary goal of this thesis is to synthesize titanium dioxide nanorods using a continuous flow microreactor. Titania powder, one of the most important particulate materials is used for many purposes owing to its excellent optical properties of a high refractive index leading to high hiding power and whiteness of titania paint, high chemical stability, and relatively low production cost. Particularly, one-dimensional inorganic nanostructures (rods, wires and tubes) of TiO₂ attract both relevant theoretical and technological interest, because they reveal a wide range of electrical and optical properties that depend on both size and shape. In our research, the growth of anatase TiO₂ nanorods was carefully controlled at low reaction temperature (around 100 °C) with oleic acid as a surfactant preventing hydrolysis of titania nuclei in undesired sites. Hinder hydrolysis of titanium alkoxide has been engaged and manipulated by means of a strategy that relies on the chemical modification of the titania precursor (Titanium tetraisopropoxide and Oleic acid) by the carboxylic acid, and on the use of suitable catalysts (trimethylamino-N-oxide dehydrate, TMAO) to promote crystallization in a continues flow microreactor. The X-ray diffraction and raman spectroscopy analysis show that TiO₂ produced from the microreactor matched with the products from the batch synthesis. Scanning Transmission electron Microscope (TEM) and High Resolution TEM images show that TiO₂ nanoparticles were rod shape, and Fourier transform of the HRTEM image indicates that mineral form of TiO₂ corresponds to the standard TiO2 anatase diffraction pattern. Thermal Gravimetric Analysis indicates that TiO₂ nanorods produced from the microreactor have more ligands attached to their surface than nanorods made by the batch reactor. The continuous flow microreactor can produce TiO₂ nanorods much faster (about 40 to 50 min of residence time) than a batch reactor (with reaction time around 24 hr), and the quality of the TiO₂ nanorods from the microreactor is as good as the nanorods from the conventional batch reactor. Title: Synthesis of titanium dioxide nanorods using a continuous flow microreactor Author: Yen, Wei-Liang The primary goal of this thesis is to synthesize titanium dioxide nanorods using a continuous flow microreactor. Titania powder, one of the most important particulate materials is used for many purposes owing to its excellent optical properties of a high refractive index leading to high hiding power and whiteness of titania paint, high chemical stability, and relatively low production cost. Particularly, one-dimensional inorganic nanostructures (rods, wires and tubes) of TiO₂ attract both relevant theoretical and technological interest, because they reveal a wide range of electrical and optical properties that depend on both size and shape. In our research, the growth of anatase TiO₂ nanorods was carefully controlled at low reaction temperature (around 100 °C) with oleic acid as a surfactant preventing hydrolysis of titania nuclei in undesired sites. Hinder hydrolysis of titanium alkoxide has been engaged and manipulated by means of a strategy that relies on the chemical modification of the titania precursor (Titanium tetraisopropoxide and Oleic acid) by the carboxylic acid, and on the use of suitable catalysts (trimethylamino-N-oxide dehydrate, TMAO) to promote crystallization in a continues flow microreactor. The X-ray diffraction and raman spectroscopy analysis show that TiO₂ produced from the microreactor matched with the products from the batch synthesis. Scanning Transmission electron Microscope (TEM) and High Resolution TEM images show that TiO₂ nanoparticles were rod shape, and Fourier transform of the HRTEM image indicates that mineral form of TiO₂ corresponds to the standard TiO2 anatase diffraction pattern. Thermal Gravimetric Analysis indicates that TiO₂ nanorods produced from the microreactor have more ligands attached to their surface than nanorods made by the batch reactor. The continuous flow microreactor can produce TiO₂ nanorods much faster (about 40 to 50 min of residence time) than a batch reactor (with reaction time around 24 hr), and the quality of the TiO₂ nanorods from the microreactor is as good as the nanorods from the conventional batch reactor. Close

• 8. [Article] Investigation of oxide semiconductor based thin films : deposition, characterization, functionalization, and electronic applications

  Nanostructured ZnO films were obtained via thermal oxidation of thin films formed with metallic Zn-nanoparticle dispersions. Commercial zinc nanoparticles used for this work were characterized by microscopic ...

  Citation × Citation Citation Abstract Copy Title: Investigation of oxide semiconductor based thin films : deposition, characterization, functionalization, and electronic applications Author: Rajachidambaram, Meena Suhanya Nanostructured ZnO films were obtained via thermal oxidation of thin films formed with metallic Zn-nanoparticle dispersions. Commercial zinc nanoparticles used for this work were characterized by microscopic and thermal analysis methods to analyze the Zn-ZnO core shell structure, surface morphology and oxidation characteristics. These dispersions were spin-coated on SiO₂/Si substrates and then annealed in air between 100 and 600 °C. Significant nanostructural changes were observed for the resulting films, particularly those from larger Zn nanoparticles. These nanostructures, including nanoneedles and nanorods, were likely formed due to fracturing of ZnO outer shell due to differential thermal expansion between the Zn core and the ZnO shell. At temperatures above 227 °C, the metallic Zn has a high vapor pressure leading to high mass transport through these defects. Ultimately the Zn vapor rapidly oxidizes in air to form the ZnO nanostructures. We have found that the resulting films annealed above 400 °C had high electrical resistivity. The zinc nanoparticles were incorporated into zinc indium oxide solution and spin-coated to form thin film transistor (TFT) test structures to evaluate the potential of forming nanostructured field effect sensors using simple solution processing. The functionalization of zinc tin oxide (ZTO) films with self-assembled monolayers (SAMs) of n-hexylphosphonic acid (n-HPA) was investigated. The n-HPA modified ZTO surfaces were characterized using contact angle measurement, x-ray photoelectron spectroscopy (XPS) and electrical measurements. High contact angles were obtained suggesting high surface coverage of n-HPA on the ZTO films, which was also confirmed using XPS. The impact of n-HPA functionalization on the stability of ZTO TFTs was investigated. The n-HPA functionalized ZTO TFTs were either measured directly after drying or after post-annealing at 140 °C for 48 hours in flowing nitrogen. Their electrical characteristics were compared with that of non-functionalized ZTO reference TFTs fabricated using identical conditions. We found that the non-functionalized devices had a significant turn-on voltage (V[subscript ON]) shift of ~0.9 V and ~1.5 V for the non-annealed and the post-annealed conditions under positive gate bias stress for 10,000 seconds. The n-HPA modified devices showed very minimal shift in V[subscript ON] (0.1 V), regardless of post-thermal treatment. The VON instabilities were attributed to the interaction of species from the ambient atmosphere with the exposed ZTO back channel during gate voltage stress. These species can either accept or donate electrons resulting in changes in the channel conductance with respect to the applied stress. Title: Investigation of oxide semiconductor based thin films : deposition, characterization, functionalization, and electronic applications Author: Rajachidambaram, Meena Suhanya Nanostructured ZnO films were obtained via thermal oxidation of thin films formed with metallic Zn-nanoparticle dispersions. Commercial zinc nanoparticles used for this work were characterized by microscopic and thermal analysis methods to analyze the Zn-ZnO core shell structure, surface morphology and oxidation characteristics. These dispersions were spin-coated on SiO₂/Si substrates and then annealed in air between 100 and 600 °C. Significant nanostructural changes were observed for the resulting films, particularly those from larger Zn nanoparticles. These nanostructures, including nanoneedles and nanorods, were likely formed due to fracturing of ZnO outer shell due to differential thermal expansion between the Zn core and the ZnO shell. At temperatures above 227 °C, the metallic Zn has a high vapor pressure leading to high mass transport through these defects. Ultimately the Zn vapor rapidly oxidizes in air to form the ZnO nanostructures. We have found that the resulting films annealed above 400 °C had high electrical resistivity. The zinc nanoparticles were incorporated into zinc indium oxide solution and spin-coated to form thin film transistor (TFT) test structures to evaluate the potential of forming nanostructured field effect sensors using simple solution processing. The functionalization of zinc tin oxide (ZTO) films with self-assembled monolayers (SAMs) of n-hexylphosphonic acid (n-HPA) was investigated. The n-HPA modified ZTO surfaces were characterized using contact angle measurement, x-ray photoelectron spectroscopy (XPS) and electrical measurements. High contact angles were obtained suggesting high surface coverage of n-HPA on the ZTO films, which was also confirmed using XPS. The impact of n-HPA functionalization on the stability of ZTO TFTs was investigated. The n-HPA functionalized ZTO TFTs were either measured directly after drying or after post-annealing at 140 °C for 48 hours in flowing nitrogen. Their electrical characteristics were compared with that of non-functionalized ZTO reference TFTs fabricated using identical conditions. We found that the non-functionalized devices had a significant turn-on voltage (V[subscript ON]) shift of ~0.9 V and ~1.5 V for the non-annealed and the post-annealed conditions under positive gate bias stress for 10,000 seconds. The n-HPA modified devices showed very minimal shift in V[subscript ON] (0.1 V), regardless of post-thermal treatment. The VON instabilities were attributed to the interaction of species from the ambient atmosphere with the exposed ZTO back channel during gate voltage stress. These species can either accept or donate electrons resulting in changes in the channel conductance with respect to the applied stress. Close