Thickness shear mode (TSM) sensors, also known as quartz crystal micro-balances (QCM) are a class of acoustic wave sensors that have been used for gas/vapor sensing. Fast and sensitive chemical vapor sensing, specifically of hydrocarbon vapors is an important application for these vapor sensors. The TSM sensors typically used have a lower sensitivity compared with other acoustic wave sensors. This thesis describes the development of high sensitivity organic vapor sensors using thin polymer film coatings on TSM devices. Commercially available AT-quartz TSM devices were milled leaving a thin quartz membrane surrounded by a thicker outer ring. This resulted in an increased frequency and a consequent increase in sensitivity, as described by the Sauerbrey equation. The TSM sensors were then coated with thin sensing films of rubbery polymers. Isothermal experiments at room temperature were conducted. A fully instrumented and automated test bed consisting of a temperature-controlled organic vapor dilution system, a precision impedance analyzer, and computer based data acquisition was developed and used to evaluate the performance of the coated TSM devices. The TSM devices compared in this study were AT cut with fundamental resonant frequencies of 10, 20, and 96 MHz. The results of tests conducted are presented to demonstrate increase in sensitivity for higher fundamental frequency TSM devices. 96 MHz TSM resonators were found to be 8 to 27 times more sensitive than 10 MHz resonators. Sensitivity was limited by the difficulty in coating sensing layers and damping of the resonator. Additionally, each sensor was evaluated and compared in terms of detection limit and noise level. 96 MHz resonators had higher noise levels than 10 MHz or 20 MHz resonators; as a result, 96 MHz resonators did not show significant improvements in LOD. Also, response times for 96 MHz resonators were quicker than 10 MHz or 20 MHz resonators and response times generally decreased with analyte concentration. Several rubbery polymer films as well as copolymers were investigated to determine which sensing film would have the optimal performance in terms of response time, recovery, reproducibility, repeatability, frequency noise, and baseline drift. The organic vapors studied were benzene, toluene, hexane, cyclohexane, heptane, dichloroethane, and chloroform at levels ranging from 0.2 to over 13.7 volume percentage in nitrogen gas. The Butterworth-VanDyke (BVD) equivalent circuit model was used to model both the perturbed and unperturbed TSM resonator. Monitoring the sensor response through the equivalent circuit model allowed for discriminating between the organic vapors. Vapor discrimination, in turn, depended upon the changes in the resistance parameter. Finally, the vapor liquid equilibrium at the polymer solvent interface was utilized to correct for perturbations, due to temperature changes, in the sensor response.

We have investigated the chemical sensitivity of nanoporous (NP) alumina-coated surface acoustic wave (SAW) devices that have been surface-modified with polymeric mediating films. The research in this dissertation covers the refinement of the NP alumina coating, development of dendrimer and/or polymer surface modifications, design of composite ultrathin vapor-phase analyte gates, and preparation of selectively permeable, polymeric films that mediate analyte transport. Nanoporous alumina SAW devices were fabricated from planar Al SAW devices using an anodization process that yields a high-surface-area transduction platform. Refinement of the anodization process results in a homogeneously porous substrate capable of ~40 times the analyte sensitivity of conventional planar SAW devices. Attempts to directly impart selective gas-phase analyte permeation with monolayers of amine-terminated, poly(amidoamine) (PAMAM) dendrimer films were investigated with and without secondary functionalization. We also prepared and characterized pore-bridging polymeric composite ultrathin films (~12 nm) of PAMAM dendrimers and poly(maleic anhydride)-c-poly(methyl vinylether) (Gantrez). Access to the underlying pores of the NP alumina coating can be modulated through the sequential deposition of the composite film. These tailorable ultrathin films result in impermeable surface- modifications which fully gate the analyte response without filling the porous structure. Thin spin-cast films (40 nm) of polydimethylsiloxane (PDMS) were developed to simultaneously provide selective sorption and permeation characteristics towards vapor-phase analytes. The porous nature of the underlying alumina coating provides for this real-time evaluation of sorption and permeation. The results suggest that the thin films offer preferential sorption of non-polar organics and selective permeability towards water vapor.

Surfaces of metal oxides play a vital role in many technologically important applications. The surfaces of titanium dioxide, in particular, show quite promising properties that can be utilized in solid-state gas sensing and photocatalysis applications. In the first part of this dissertation we investigate these properties of TiO2 surfaces through a vigorous surface scientific approach. In the second part, we investigate the possibilities of modifying the TiO2 surfaces by depositing multi-component transition metal oxide monolayers so that the properties of bare TiO2 surface can be influenced in a beneficial way. For instance, via formation of new surface sites or cations that have different valance states, the chemisorption and catalytic properties can be modified. We use sophisticated experimental surface science techniques that are compatible with ultra-high vacuum technology for surface characterization. All the experimental results, except for the photocatalysis experiments, were compared to and verified by supporting DFT-based theoretical results produced by our theory collaborators. TiO2 based solid-state gas sensors have been used before for detecting trace amounts of explosives such as 2,4-dinitrololuene (DNT), a toxic decomposition product of the explosive 2,4,6-trinitrotoluene (TNT) that have very low vapor pressure. However, the adsorption, desorption and reaction mechanism were not well- understood. Here, we investigate 2,4-DNT adsorption on rutile-TiO2(110) surface in order to gain insight about these mechanisms in an atomistic level and we propose an efficient way of desorbing DNT from the surface through UV-light induced photoreactions. TiO2 exists in different polymorphs and the photocatalytic activity differs from one polymorph to another. Rutile and anatase are the most famous forms of TiO2 in photocatalysis and anatase is known to show higher activity than rutile. The photoactivity also varies depending on the surface orientation for the same polymorph. So far, a reasonable explanation as to why these differences exist was not reported. In our studies, we used high quality epitaxial rutile and anatase thin films which enabled isolating the surface effects from the bulk effects and show that it is the difference between the charge carrier diffusion lengths that causes this difference in activities. In addition to that, using different surface orientations of rutile-TiO2, we show that the anisotropic bulk charge carrier mobility may contribute to the orientation dependent photoactivity. Moreover, we show that different surface preparation methods also affect the activity of the sample and vacuum reduction results in an enhanced activity. In an effort to modify the TiO2 surfaces with monolayer/mixed monolayer oxides, we carried out experiments on (011) orientation of single crystal rutile TiO2 with few of the selected transition metal oxides namely Fe, V, Cr and Ni. We found that for specific oxidation conditions a monolayer mixed oxide is formed for all M (M= Fe, V, Cr, Ni), with one common structure with the composition MTi2O5. For small amounts of M the surface segregates into pure TiO2(011)-2×1 and into domains of MTi2O5indicating that this mixed monolayer oxide is a low energy line phase in a compositional surface phase diagram. The oxygen pressure required for the formation of this unique monolayer structure increases in the order of V2O5 mixed monolayer oxide by DFT-based simulations was verified by X-ray photoemission diffraction measurements performed at a synchrotron facility.

Thesis (M. Sc. (Physics)) -- University of Limpopo, 2016 / Titania is a cheap and nontoxic polymorphic material of current interest for a variety of technological applications like in gas sensing and photovoltaic cells. Generally, TiO2, with a band gap of 3.2 eV, can only be excited by a small UV fraction of solar light, which accounts for only 3-5% of the solar energy. Various strategies have been pursued including doping with metallic elements (e.g. Fe) or nonmetallic elements (e.g. N) with the aim of shifting the absorption into the visible range. Since the properties and performance of devices, particularly for high-temperature applications, may be affected by the transformation from one phase to another, it is of significant interest to understand the conditions that affect phase transitions. In the present work TiO2 was doped with platinum (Pt), palladium (Pd), silver (Ag) and gold (Au) at doping levels of 5% weight, following the standard sol-gel methods. Structural characterization was carried out using scanning electron microscopy, Raman Spectroscopy and X-ray diffraction. Optical properties were studied using the Diffused reflectance Spectroscopy (DRS). Doping with Pt and Pd resulted in a lower anatase to rutile phase transformation temperature while doping with Au and Ag did not affect the transformation temperature. SEM micrographs show that the surface contains irregular shaped particles which are the aggregation of tiny crystals at lower temperature range, whereas at higher temperatures (900 °C), spheroids are observe.The reflectance spectra of the metal loaded TiO2 reveal substantial strong spectral cut-off starting from roughly 400 nm to the entire visible region (i.e. they show enhanced absorption).

<p>With people today spending most of their time in indoor environments, it is important to monitor indoor air quality (IAQ) to better serve human health and safety protocols. Therefore, developing sensing devices to better monitor the potential of hazardous gases in the air is critical. There are various types of indoor pollutants that can be harmful to a person’s health, such as volatile organic compounds (VOCs) and flammable refrigerants. These gases originate from sources such as the decomposition of building materials; damaged heating, ventilation, and air conditioning (HVAC) units; cellular respiration of an overpopulated building; and leaks from gas powered homes. As such, researchers have developed small, lightweight sensors such as microelectromechanical systems (MEMS) that allow for real-time detection of gases to be implemented in IAQ monitoring. MEMS sensors have shown great promise as they are a highly sensitive and selective for a target gas analyte, are able to detect at room temperature, and are inexpensive to manufacture. However, most MEMS are highly dependent on the chemical recognition layer used. The thesis will highlight my work in the development of next-generation chemical recognition layers for gravimetric MEMS that are utilized in the detection carbon dioxide, formaldehyde, and hydrogen. For carbon dioxide, a polymer blend of polyethylene imine (PEI) and polyethylene oxide (PEO) was implemented as the chemical recognition layer. The blending of semicrystalline PEO into PEI caused a phase separation in which the materials morphology became highly porous. This microstructure allowed for detection of carbon dioxide down to 5 ppm. For formaldehyde sensing, a blend of poly-5-carboxyindole (P5C) and beta-cyclodextrin (β-CD) nanoparticles were used as a chemical recognition layer. The β-CD moieties helped to buffer the P5C to enhance the hydrogen bonding of the carboxylic acid associated with P5C. This allowed for detection of formaldehyde to concentrations as low as 25 ppm. In the hydrogen sensing devices, ultrathin palladium nanosheets (PdNS) were employed as a chemical recognition layer. The nanosheets were composed of monoatomic layers with 0.23 nm spacing. The tight uniformity and small pore size of the PdNS allowed for the detection of hydrogen as low as 1% in concentration. Furthermore, this document will briefly discuss areas where MEMS could have improvements to their sensitivity and selectivity towards a target gas analyte. These areas include improvements to material processing, filter encapsulation, and device modification.</p>

A wide variety of tunable physio-chemical properties make ZnO nanowires a promising candidate for functional device applications. Although bottom-up grown nanowires are producible in volume, their high-throughput device integration requires control over dimensions and, more importantly, of precise placement. Thus development of top-down fabrication routes with accurate device positioning is imperative and hence pursued in this thesis. ZnO thin film transistors (TFT) were fabricated using solution based precursor zinc neodecanoate. A range of ZnO thin films were prepared by varying process parameters, such as precursor concentrations and annealing temperatures, and then analysed for their optical and electrical characteristics. ZnO TFTs prepared from a 15 % precursor concentration and annealing at 700 $^\circ$C exhibited best device performance with a saturation mobility of 0.1 cm$^2$/V.s and an on/off ratio of 10$^7$. Trap limited conduction (TLC) transport was found to be dominant in these devices. A direct-write electron beam lithography (EBL) process was developed using zinc naphthenate and zinc neodecanoate precursors for the top-down synthesis of ZnO nanowires. Nanoscale ZnO patterns with a resolution of 50 nm and lengths up to 25 $\mu$m were fabricated. A linear mobility of 0.5 cm$^2$/V.s and an on/off ratio of $\sim$10$^5$ was achieved in the micro-FETs with 50 $\mu$m channel width. Interestingly, on scaling down the ZnO channel width down to 100 nm, almost two orders of magnitude enhancement in the linear mobility was observed, which reached $\sim$33.75 cm$^2$/V.s. Such increment in the device performance was attributed to the formation of larger grains and thus reduction in the grain-boundary scattering. Six volatile organic compounds (VOCs) were sensed at room temperature using the direct-write EBL fabricated ZnO devices under UV sensitisation. As the surface-to-volume ratio increases with the decreasing channel width (from 50 $\mu$m to 100 nm), sensing response of the ZnO devices becomes more significant. Ppm level detection of various VOCs was observed; with a 25 ppm level Anisole detection being the lowest concentration. Additionally, using 100 nm device, detection of 10 ppm NO$_2$ was achieved at room temperature. The sensing response towards NO$_2$ was found to be increased with UV illumination and sensor temperature. This led to exhibit $\sim$171 % sensing response for a 2.5 ppm level of NO$_2$.

<p>Chemical gas sensors can be used in a variety of applications such as process control, security systems and medical diagnosis. In the research for new functions and new sensing materials a “breadboard” would be useful. A technique that has been investigated for such a purpose is the grid-gate device which is a metal-oxide-semiconductor (MOS) based gas sensor. It is a MOS capacitor consisting of a passive grid-gate with depositions of sensing materials overlapping the grid. The measuring is carried out with a light addressable method called scanning light pulse technique (SLPT) which enables the detection of spatially distributed gas response.</p><p>A development of the grid-gate sensor would be to separate the sensing materials from the chip. In this thesis the aim was to see if this was possible by depositing the sensing material on a slide of micro pillars which was put on top of a biased grid-gate chip.</p><p>The test was made with palladium depositions in an ambient of synthetic air and 2500 ppm hydrogen, and the measuring technique was SLPT as for the preceding device.</p><p>The result of the test was that the new device showed a combined gas response of both charge content shift at flat-band voltage and at inversion voltages. The conclusion is therefore that the sensing material can be separated from the grid-gate chip and that the response will be caused by several mechanisms. The two-dimensional image response utilized for the preceding grid-gate device will instead be a multi-dimensional response consisting of the curve for the charge content shift at every measuring position.</p>

<p>Nanoparticles of iron, cobalt and tungsten oxide were synthesised by photolytic laser assisted chemical vapour deposition (LCVD). An excimer laser (operating at 193 nm) was used as an excitation source. The LCVD process, was monitored <i>in situ</i> by optical emission spectroscopy (OES). The synthesised particles were further analysed using transmission electron spectroscopy (TEM), X-ray diffraction (XRD), high resolution scanning electron microscopy (HRSEM), X-ray fluorescence spectroscopy (XRF), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.</p><p>Iron and cobalt single crystalline nanoparticles were synthesized using ferrocene and cobaltocene precursors. The diameter of the particles could be tailored by the experimental parameters (e.g., partial pressure and laser power) and were in the range 1 - 50 nm in diameter. In both cases, the particles were covered by a carbon shell, typically 7 nm thick. A thin graphitic layer was observed at the interface metal-carbon. Amorphous carbon was deposited on top of the graphitic carbon. Particle temperature, reaching the boiling point of the respective metal, was observed by OES of the thermal emission during the laser-induced particle formation process (and subsequent heating). Both bcc and fcc Fe phases were formed, both hcp and fcc for the Co phases. Size dependent magnetic properties were observed using superconducting quantum interference device (SQUID) measurements, where super-paramagnetic magnetic domains dominated for <i>d</i> < 10 nm. The iron particles were further processed, whereby the amorphous shell was removed by refluxing in nitric acid. In a subsequent step, the graphitic surface was functionalized by attaching an octyl ester, rendering the particles hydrophobic.</p><p>Tungsten oxides were synthesized from combinations of WF₆/H₂/O₂ as precursors. No particles could be deposited if H₂ was removed from the gas-mixture. The as-deposited oxide nanoparticle film was amorphous. A monoclinic WO₃ particle film could be achieved by annealing the amorphous oxide. Above 400°C, the oxide particles increased in size from ca. 20 nm to 60 nm through coalescence. The gas-sensing properties of the tungsten oxide were tested by conductance measurements using H₂S as analyte. The sensitivity of the amorphous oxide nanoparticle film was found to be superior to that of a crystalline oxide nanoparticle film. </p>

Nanoparticles of iron, cobalt and tungsten oxide were synthesised by photolytic laser assisted chemical vapour deposition (LCVD). An excimer laser (operating at 193 nm) was used as an excitation source. The LCVD process, was monitored in situ by optical emission spectroscopy (OES). The synthesised particles were further analysed using transmission electron spectroscopy (TEM), X-ray diffraction (XRD), high resolution scanning electron microscopy (HRSEM), X-ray fluorescence spectroscopy (XRF), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Iron and cobalt single crystalline nanoparticles were synthesized using ferrocene and cobaltocene precursors. The diameter of the particles could be tailored by the experimental parameters (e.g., partial pressure and laser power) and were in the range 1 - 50 nm in diameter. In both cases, the particles were covered by a carbon shell, typically 7 nm thick. A thin graphitic layer was observed at the interface metal-carbon. Amorphous carbon was deposited on top of the graphitic carbon. Particle temperature, reaching the boiling point of the respective metal, was observed by OES of the thermal emission during the laser-induced particle formation process (and subsequent heating). Both bcc and fcc Fe phases were formed, both hcp and fcc for the Co phases. Size dependent magnetic properties were observed using superconducting quantum interference device (SQUID) measurements, where super-paramagnetic magnetic domains dominated for d &lt; 10 nm. The iron particles were further processed, whereby the amorphous shell was removed by refluxing in nitric acid. In a subsequent step, the graphitic surface was functionalized by attaching an octyl ester, rendering the particles hydrophobic. Tungsten oxides were synthesized from combinations of WF6/H2/O2 as precursors. No particles could be deposited if H2 was removed from the gas-mixture. The as-deposited oxide nanoparticle film was amorphous. A monoclinic WO3 particle film could be achieved by annealing the amorphous oxide. Above 400°C, the oxide particles increased in size from ca. 20 nm to 60 nm through coalescence. The gas-sensing properties of the tungsten oxide were tested by conductance measurements using H2S as analyte. The sensitivity of the amorphous oxide nanoparticle film was found to be superior to that of a crystalline oxide nanoparticle film.

Chemical gas sensors can be used in a variety of applications such as process control, security systems and medical diagnosis. In the research for new functions and new sensing materials a “breadboard” would be useful. A technique that has been investigated for such a purpose is the grid-gate device which is a metal-oxide-semiconductor (MOS) based gas sensor. It is a MOS capacitor consisting of a passive grid-gate with depositions of sensing materials overlapping the grid. The measuring is carried out with a light addressable method called scanning light pulse technique (SLPT) which enables the detection of spatially distributed gas response. A development of the grid-gate sensor would be to separate the sensing materials from the chip. In this thesis the aim was to see if this was possible by depositing the sensing material on a slide of micro pillars which was put on top of a biased grid-gate chip. The test was made with palladium depositions in an ambient of synthetic air and 2500 ppm hydrogen, and the measuring technique was SLPT as for the preceding device. The result of the test was that the new device showed a combined gas response of both charge content shift at flat-band voltage and at inversion voltages. The conclusion is therefore that the sensing material can be separated from the grid-gate chip and that the response will be caused by several mechanisms. The two-dimensional image response utilized for the preceding grid-gate device will instead be a multi-dimensional response consisting of the curve for the charge content shift at every measuring position.