Impact of Pollutant Ozone on the Biophysical Properties of Tear Film Lipid Layer Model Membranes

The human tear film lipid layer (TFLL) is a duplex lipid layer film comprising the outermost layer of the tear film, responsible for surface tension reduction of the tear film in the blinking function. Exposure of the tear film lipid layer to increased ground-level concentrations of air pollutants, for example tropospheric ozone, can impact the chemical composition, structure and the function of the tear film lipid layer by impacting its surface activity, stability, respreadability and viscoelasticity, important TFLL characteristics in the blinking function. This compromise in these characteristics, leads to the emergence of dry eye disease (DED). In this research, Langmuir films spread at the air-water interface are used as TFLL mimicking model membranes. In addition to the study of the functional role of each component in the TFLL, the impact of ozone exposure on their biophysical properties is also investigated using Langmuir balance, Brewster angle microscopy, a profile analysis tensiometer, and mass spectrometry. Moreover, the crystallinity, lateral ordering, and vertical structure of the cholesteryl oleate film, as an important cholesteryl ester used in TFLL model membrane studies is investigated using Grazing Incidence X-Ray Diffraction (GIXD), X-ray reflectivity (XR) and Grazing Incidence Off-Specular Scattering (GIXOS), respectively. Crystallinity of the cholesteryl oleate film was attributed to the cholesterol ring packing and a flat, monolayer film was observed at lower surface pressures. Oxidation impacts the phase transition behaviour of the model membranes as well as their multilayer formation. It also leads to expansion of the films to higher molecular areas, fluidization of the films, significant morphological changes, reduction of their respreadability and a composition-driven impact on their viscoelasticity. These findings can help better understand the roles of TFLL components in its function as well as the impact of prolonged ozone exposure on the mechanical and biophysical properties of human TFLL

Reducing Graphene-Metal Contact Resistance via Laser Nano-welding

The large graphene-metal contact resistance is a major limitation for development of graphene electronics. graphene behaves as an insulator for out-of-plane carrier transport to metallic contacts. Laser nano-welding was developed and led to RC reductions of up to 84%. Localized laser irradiation at the edges of graphene led to the formation of chemically active point defects. Precise structural modifications and formation of G-M bonding led to improved carrier efficiency in graphene devices

Advanced Nanofabrication of Carbon-based Materials with Superior Electrical Properties

This document presents a number of nonconventional fabrication/processing techniques developed for the integration of the emerging carbon-based nanomaterials, including carbon nanotubes (CNTs) and graphene, and realization of their superior properties, thereby addressing some of the fundamental challenges in industrial applications. According to recent studies, CNTs possess extraordinary properties that make them promising candidates for improving the performance of a wide range of industrial applications. However, practical realization of these properties and the integration of the carbon-based nanomaterials through conventional fabrication methods has proven to be challenging in terms of quality control, reproducibility, and cost effectiveness. To address these challenges, this dissertation proposes the development and implementation of scalable, cost-effective processing approaches to the integration of carbon-based nanomaterials into industrial applications. The following research topics are covered in this dissertation: 1) suppression of the eddy current loss in laser-processed multi-walled carbon nanotube (MWCNT)-coated copper (Cu) conductors, 2) neutralization of electrical arcing in CNT-implanted rails, and 3) ultralow contact resistance in graphene-based devices via laser-assisted nanowelding. Successful suppression of ac resistance of up to 94% was realized in MWCNT-coated copper (MWCNT-Cu) planar conductors at high operational frequencies (0-15 MHz) through electrophoretic deposition and infrared laser treatment of MWCNT coatings on Cu. In addition, this scalable method was used to reduce the eddy current loss and increase the transmission range in wireless power transfer (WPT) systems. The quality factor of the MWCNT-Cu coils increased by fourfold compared to their metallic counterparts, thereby leading to transmission efficiencies as high as 95.81% at f = 3.45 MHz. A scalable laser drilling/MWCNT implantation method was also developed to neutralize the destructive electrical arcing in third-rail systems. The highly conductive MWCNT composite in the microholes was observed to weaken and divide the arcing current density into harmless magnitudes, thereby minimizing the arc-induced surface damage. Finally, a contact-free, position-selective, and accurate laser-assisted nanowelding technique was developed to reduce the graphene-metal (G-M) contact resistance, leading to values as low as 2.57 Ω-μm. This was achieved through formation of laser-induced defects in order to increase the chemical reactivity of graphene and facilitate the G-M bonding, thereby maximizing the interfacial transportation

Advanced Nanofabrication of Carbon-based Materials with Superior Electrical Properties

This document presents a number of nonconventional fabrication/processing techniques developed for the integration of the emerging carbon-based nanomaterials, including carbon nanotubes (CNTs) and graphene, and realization of their superior properties, thereby addressing some of the fundamental challenges in industrial applications. According to recent studies, CNTs possess extraordinary properties that make them promising candidates for improving the performance of a wide range of industrial applications. However, practical realization of these properties and the integration of the carbon-based nanomaterials through conventional fabrication methods has proven to be challenging in terms of quality control, reproducibility, and cost effectiveness. To address these challenges, this dissertation proposes the development and implementation of scalable, cost-effective processing approaches to the integration of carbon-based nanomaterials into industrial applications. The following research topics are covered in this dissertation: 1) suppression of the eddy current loss in laser-processed multi-walled carbon nanotube (MWCNT)-coated copper (Cu) conductors, 2) neutralization of electrical arcing in CNT-implanted rails, and 3) ultralow contact resistance in graphene-based devices via laser-assisted nanowelding. Successful suppression of ac resistance of up to 94% was realized in MWCNT-coated copper (MWCNT-Cu) planar conductors at high operational frequencies (0-15 MHz) through electrophoretic deposition and infrared laser treatment of MWCNT coatings on Cu. In addition, this scalable method was used to reduce the eddy current loss and increase the transmission range in wireless power transfer (WPT) systems. The quality factor of the MWCNT-Cu coils increased by fourfold compared to their metallic counterparts, thereby leading to transmission efficiencies as high as 95.81% at f = 3.45 MHz. A scalable laser drilling/MWCNT implantation method was also developed to neutralize the destructive electrical arcing in third-rail systems. The highly conductive MWCNT composite in the microholes was observed to weaken and divide the arcing current density into harmless magnitudes, thereby minimizing the arc-induced surface damage. Finally, a contact-free, position-selective, and accurate laser-assisted nanowelding technique was developed to reduce the graphene-metal (G-M) contact resistance, leading to values as low as 2.57 Ω-μm. This was achieved through formation of laser-induced defects in order to increase the chemical reactivity of graphene and facilitate the G-M bonding, thereby maximizing the interfacial transportation

Influence of resonant and non-resonant vibrational excitation of ammonia molecules in gallium nitride synthesis

Attempts on the selective promotion of gallium nitride (GaN) growth were investigated by deploying laserassisted vibrational excitation of reactant molecules, which deposits energy selectively into specific molecules and activate the molecules towards the selected reaction pathways. Laser-assisted metal organic chemical vapor deposition (LMOCVD) of GaN was studied using a wavelength-tunable CO2 laser. The NH-wagging modes (υ2) of ammonia (NH3) precursor molecules are strongly infrared active and perfectly match the emission line of the CO2 laser at 9.219, 10.350, and 10.719 μm. On- and off-resonance excitations of molecules were performed via tuning the incident laser wavelengths at on-resonant wavelength 9.219 μm and off-resonant wavelength of 9.201 μm. The on-resonant vibrational excitation allowed a largest fraction of the absorbed laser energy coupled directly into NH3 molecules whereas energy coupling under off-resonant excitations is less efficient in energy coupling and influencing the GaN growth process. The GaN deposition rate was enhanced by a factor of 2.6 accompanied with an improvement of crystalline quality under the on-resonant excitation. Optical emission spectroscopic (OES) studies confirmed that the on-resonant vibrational excitation effectively promotes the dissociation of NH3 molecules and creates N-containing species favoring the GaN growth. This study indicates that the resonant vibrational excitation is an efficient route coupling energy into the reactant molecules to surmount the chemical reaction barrier and steering the growth process

Impact of Pollutant Ozone on the Biophysical Properties of Tear Film Lipid Layer Model Membranes

Ozone exposure from environmental smog has been implicated as a risk factor for developing dry eye disease (DED). The tear film lipid layer (TFLL), which is the outermost layer of the tear film and responsible for surface tension reduction while blinking, is in direct contact with the environment and serves as the first line of defense against external aggressors such as environmental pollution. The impact of exposure to ozone on the biophysical properties of three TFLL model membranes was investigated. These model membranes include a binary mixture of cholesteryl oleate (CO) and L-α-phosphatidylcholine (egg PC), a ternary mixture of CO, glyceryl trioleate (GT) and PC, as well as a quaternary mixture of CO, GT, a mixture of free fatty acids palmitic acid and stearic acid (FFAs) and PC. Biophysical impacts were evaluated as changes to the surface activity, respreadability, morphology and viscoelastic properties of the films. Expansion to higher molecular areas was observed in all the TFLL model membrane films which is attributable to the accommodation of the cleaved chains in the film. Significant morphological changes were observed, namely fluidization and the disruption of the phase transition behaviour of GT, and multilayer formation of CO. This fluidization reduces the hysteresis loops for the model membranes. On the other hand, the viscoelastic properties of the films exhibited differential impacts from ozone exposure as a function of composition. These findings are correlated to chemical changes to the lipids determined using ESI-MS

Skin Effect Suppression in Infrared-laser Irradiated Planar Multi-walled Carbon Nanotube/ Cu Conductors

Skin effect suppression in planar multi-walled carbon nanotube (MWCNT)/Copper (Cu) conductors was realized at the 0-10 MHz frequency range through infrared laser irradiation of MWCNTs, which were coated on the surface of the Cu substrate via the electrophoretic deposition (EPD) method. The effect of laser irradiation and its power density on electrical and structural properties of the MWCNT/Cu conductors was investigated using a wavelength-tunable CO2 laser and then comparing the performance of the samples prepared at different conditions with that of pristine Cu. The irradiation at λ=9.219 μm proved to be effective in selective delivery of energy towards depths close to the interface, compared to the conventional rapid thermal processing (RTP) annealing method. At f=10 MHz, the ac resistance of the laser irradiated MWCNT/Cu conductors was reduced by more than one order of magnitude compared to its original value for the pristine Cu. Impedance measurements and structural characterizations indicate that this technique results in successful implementation of the nanotubes on the surface of the metallic substrate to operate as current channels with saturated skin depths and reduced contact resistance at the interface. Therefore, the limited performance of Cu conductors at high frequencies can be modified. Additionally, it was further demonstrated that the impedance reduction and the suppression of the skin effect in MWCNT/Cu conductors are in direct relation with the irradiated power density and can thus be easily controlled by this parameter

EFFECT OF LASER-ASSISTED RESONANT EXCITATION ON THE GROWTH OF GaN FILMS

Gallium nitride (GaN) films were grown using laser-assisted metal organic chemical vapor deposition (LMOCVD). The vibrational mode (1084.63 cm-1) of ammonia (NH3) molecules was resonantly excited using a wavelength-tunable CO2 laser at a laser wavelength of 9.219 μm due to its high absorption cross-section. Through wavelength-matched resonant excitation of the NH3 molecules, highly c-axis oriented GaN films were successfully deposited on sapphire (α-Al2O3) substrates at low temperatures (250 to 600oC). The strong (0001) GaN peak in Xray diffraction spectra confirmed the good crystalline quality of GaN films. Additionally, the resonant vibrational excitation of NH3 in LMOCVD promoted the GaN growth rate considerably compared to that synthesized by MOCVD without resonant vibrational excitation of NH3 molecules

Laser-Assisted Nanowelding of Graphene to Metals: An Optical Approach Toward Ultralow Contact Resistance

The electrical performance of graphene-based devices is largely limited by substantial contact resistance at the heterodimensional graphene-metal junctions. A laserassisted nanowelding technique was developed to reduce graphene-metal (G-M) contact resistance and improve carrier injection in suspended graphene devices. Selective breakdown of C-C bonds and formation of structural defects were realized through laser irradiation at the edges of graphene within the G-M contact regions in order to increase the chemical reactivity of graphene, facilitate G-M bonding and, therefore, maximize interfacial carrier transportation. Through this method, significantly reduced G-M contact resistances, as low as 2.57 Ω-μm were obtained. In addition, it was demonstrated that the location of laser-induced defects within the contact areas significantly impacts the interfacial properties and the carrier mobility of graphene devices. A fourfold increase in photocurrent was observed in the suspended graphene photodetectors with treated G-M interfaces as compared to ordinary ones. This contact-free and position-selective technique minimizes the degradation of the graphene channels and maintains the superior performance of graphene devices, making it a promising approach for reducing G-M resistance in the fabrication of graphene-based devices