Doctor of Philosophy

dissertationTiO2 is an extensively studied material due to its nontoxic, environmental friendly, corrosion-resistant nature and wide band gap (~3 eV). TiO2 nanotubes (T-NTs) synthesized via electrochemical anodization have been studied extensively, with particular focus on their electrical and optical properties. The advantage of T-NT is the large surface area to volume ratio. T-NT has been used to demonstrate many applications such as sensors and energy harvesting. These applications have traditionally been demonstrated via T-NT synthesized on Ti foil. However, there is currently no commercially available T-NT- based device, which may be due to a lack of fabrication techniques, to make such devices on a large scale. One of the requirements for fabricating compact T-NT- based devices is the need for a stable and planar substrate. The titanium foils commonly used for T-NT synthesis are mechanically flexible, making them more prone to bending, limiting the integration of T-NT with microfabrication techniques. Here, we present the synthesis of T-NT on Si wafer at room temperature from direct current (D.C.) sputtered as well as e-beam evaporated thin Ti film. Hundred nm SiO2 was used to electrically isolate the T-NT from the substrate. We demonstrate integration of the synthesis of T-NT with photolithography, which is one of the most important requirements for scaling up a T-NT-based device. The T-NT was stable up to 500oC, which is required for improved charge transport. The T-NT was 1.4 times longer than the thickness of the Ti film, showing selective electric field-assisted etching of Ti by the electrolyte. We also report site-specific and patterned growth of the T-NT. The effect of properties of thin films such as grain size, residual stress and density on the morphology of T-NT was studied to improve the stability and quality of the T-NT. We demonstrate the synthesis of TiO2-WO3 composite nanotubes for photoelectrochemical cells with up to a 40% increase in photocurrent in comparison to plain T-NT. The T-NT was extensively studied and characterized using SEM, AFM, UV-Vis spectroscopy, XRD, and XPS

TiO2-WO3 composite nanotubes from co-sputtered thin films on si substrate for enhanced photoelectrochemical water splitting

pre-printElectrochemical anodization of a Ti-W nano-composite thin films deposited on a Si substrate by simultaneous magnetron sputtering of Ti and W resulted in the formation of TiO2-WO3 nanotubular arrays. A change in the morphology of TiO2-WO3 composite nanotubes with varying percentage of W in Ti-W composite thin films was observed. With a W density of less than or equal to 1.75 × 1019 W atoms per cm3 (after anodization), the morphology of the composite nanotubes were similar to that of plain TiO2 nanotubes. Whereas with further increase in W density resulted in a nanoporous morphology. Ti-W composite films were also deposited on Si substrates with a 100 nm thick layer of tin doped indium oxide (ITO) to examine the PEC activity of the formed oxide composites. The TiO2-WO3 composite nanotubes with 1.05 × 1019 W atoms per cm3 (3.15 × 1018 W atoms per cm3 before anodization) demonstrated to be an optimal W density for this system, giving rise to 40% increase in photocurrent at 0.5 V (vs. Ag/AgCl) compared to plain TiO2 nanotubes

Growth and characterization of TiO2 nanotubes from sputtered Ti film on Si substrate

In this paper, we present the synthesis of self-organized TiO(2) nanotube arrays formed by anodization of thin Ti film deposited on Si wafers by direct current (D.C.) sputtering. Organic electrolyte was used to demonstrate the growth of stable nanotubes at room temperature with voltages varying from 10 to 60 V (D.C.). The tubes were about 1.4 times longer than the thickness of the sputtered Ti film, showing little undesired dissolution of the metal in the electrolyte during anodization. By varying the thickness of the deposited Ti film, the length of the nanotubes could be controlled precisely irrespective of longer anodization time and/or anodization voltage. Scanning electron microscopy, atomic force microscopy, diffuse-reflectance UV–vis spectroscopy, and X-ray diffraction were used to characterize the thin film nanotubes. The tubes exhibited good adhesion to the wafer and did not peel off after annealing in air at 350 °C to form anatase TiO(2). With TiO(2) nanotubes on planar/stable Si substrates, one can envision their integration with the current micro-fabrication technique large-scale fabrication of TiO(2) nanotube-based devices

MoS₂ Quantum Dot Modified Electrode: An Efficient Probe for Electrochemical Detection of Hydrazine

The development of an effective sensor system that can detect carcinogenic hydrazine is of prime scientific interest for the protection of human health and the environment. In the present study, MoS2 quantum dots (QDs) with an average diameter of ~5 nm were synthesized using a facile one-step, bottom-up hydrothermal method using cysteine as reducing as well as capping agents. The presence of cysteine was evaluated by FTIR spectroscopy. The synthesized MoS2 QDs were applied to modify the conventional glassy carbon electrode (GCE) in order to detect hydrazine electrochemically in neutral pH conditions. In the cyclic voltammetry (CV) study, the MoS2 QDs-modified electrode revealed much better catalytic activities for hydrazine electro-oxidation compared to the bare GCE surface. The smaller size of the QDs with high surface area and the presence of carboxylic acid containing cysteine on the surface of the QDs enhanced the adsorption as well as the electrocatalytic activity. The amperometric response of MoS2-QD-modified GCE unveiled excellent electrocatalytic sensing properties towards neurotoxic hydrazine with a very high sensitivity of 990 μAmM−1cm−2 (R2 = 0.998), low LOD of 34.8 μM, and a broad linear range. Moreover, this high-sensitive, binder and conducting filler-free MoS2-QD-based sensing system is very promising in agile amperometric detection of neurotoxic hydrazine for environmental monitoring in industrial sectors

Miniature pressure sensor based on suspended MWCNT

Conventional pressure sensors rely on diaphragms with large surface areas, which deform in response to pressure. Down scalability of these devices is one of the major challenges of the technology along with reducing the overall actuation voltage and achieving ultra-high sensitivity. We present a sensitive miniature pressure sensor based on the change in the physisorbed gases with the pressure of the surrounding air. The sensor consists of a suspended individual multiwall carbon nanotube (MWCNT) clamped on Au electrodes by electron-beam-induced deposition (EBID) of Pt. The variation in the surrounding pressure is shown to be tracked by monitoring the change in the resistivity, hence resistance, of the MWCNT bridge structure due to the change in percentage of oxygen and humidity in the surrounding medium with pressure. The experimental data reveal the practicability and simplicity of the proposed pressure sensor. (C) 2019 Elsevier B.V. All rights reserved

An experimental and theoretical investigation of electrostatically coupled cantilever microbeams

We present an experimental and theoretical investigation of the static and dynamic behavior of electrostatically coupled laterally actuated silicon microbeams. The coupled beam resonators are composed of two almost identical flexible cantilever beams forming the two sides of a capacitor. The experimental and theoretical analysis of the coupled system is carried out and compared against the results of beams actuated with fixed electrodes individually. The pull-in characteristics of the electrostatically coupled beams are studied, including the pull-in time. The dynamics of the coupled dual beams are explored via frequency sweeps around the neighborhood of the natural frequencies of the system for different input voltages. Good agreement is reported among the simulation results and the experimental data. The results show considerable drop in the pull-in values as compared to single microbeam resonators. The dynamics of the coupled beam resonators are demonstrated as a way to increase the bandwidth of the resonator near primary resonance as well as a way to introduce increased frequency shift, which can be promising for resonant sensing applications. Moreover the dynamic pull-in characteristics are also studied and proposed as a way to sense the shift in resonance frequency. (C) 2016 Elsevier B.V. All rights reserved

In-Plane MEMS Shallow Arch Beam for Mechanical Memory

We demonstrate a memory device based on the nonlinear dynamics of an in-plane microelectromechanical systems (MEMS) clamped-clamped beam resonator, which is deliberately fabricated as a shallow arch. The arch beam is made of silicon, and is electrostatically actuated. The concept relies on the inherent quadratic nonlinearity originating from the arch curvature, which results in a softening behavior that creates hysteresis and co-existing states of motion. Since it is independent of the electrostatic force, this nonlinearity gives more flexibility in the operating conditions and allows for lower actuation voltages. Experimental results are generated through electrical characterization setup. Results are shown demonstrating the switching between the two vibrational states with the change of the direct current (DC) bias voltage, thereby proving the memory concept

A Comparative Study of Interdigitated Electrode and Quartz Crystal Microbalance Transduction Techniques for Metal–Organic Framework-Based Acetone Sensors

We present a comparative study of two types of sensor with different transduction techniques but coated with the same sensing material to determine the effect of the transduction mechanism on the sensing performance of sensing a target analyte. For this purpose, interdigitated electrode (IDE)-based capacitors and quartz crystal microbalance (QCM)-based resonators were coated with a zeolitic⁻imidazolate framework (ZIF-8) metal⁻organic framework thin films as the sensing material and applied to the sensing of the volatile organic compound acetone. Cyclic immersion in methanolic precursor solutions technique was used for depositing the ZIF-8 thin films. The sensors were exposed to various acetone concentrations ranging from 5.3 to 26.5 vol % in N2 and characterized/compared for their sensitivity, hysteresis, long-term and short-term stability, selectivity, detection limit, and effect of temperature. Furthermore, the IDE substrates were used for resistive transduction and compared using capacitive transduction