Nanowires: from Microchip Integrated Synthesis to Application Examples

Both the high demand in the downscaling of electronic devices and the novel properties of nanowires (NWs) as a result of their high surface-to-volume (S/V) ratio and their quantum mechanical properties have driven NWs to be among the favorite research areas in recent years and are expected to remain one of the hot topics in the next decade. Thus the synthesis of NWs which is under intensive study is not only aimed at basic research but also for a wide spectrum of device fabrication and applications.The aim of this research is to explore simple synthesis techniques of metal oxide (semiconductor) and metal-metal oxide hybrid nanostructures with proper contact lines on different substrates and investigate NWs for various device applications. Three novel synthesis routes were developed and utilized in this thesis: Thin Film Fracture (TFF),Flame Transport Synthesis (FTS) and Thermal Annealing Method (TAM). As compared to many other synthesis techniques in the literature, the here used techniques are cost effective, can be easily scaled up, offer a less complex synthesis processes and offer easy integration of NWs into lithographically patterned chips. The three synthesis techniques employed in synthesizing both 1D semiconductor and semiconductor-metal hybrid structures spanning from device fabrication to results will be presented. Both FTS and TFF approaches are implemented for the fabrication of the semiconductor (mainly ZnO) nano-microstructures based UV photodetectors. Comparison of the UV photodetectors performances built from interpenetrating ZnO nano-microstructures fabricated by burner-FTS (B-FTS) and crucible-FTS (C-FTS) techniques will also be presented. Fastest response/recovery time constant (~32 ms) and an on/off ratio ~4.5103 at 2.4 V under 365 nm UV light irradiation for the B-FTS based photodetectors is reported. The growth mechanism, properties and measurements as sensor devices will be presented in detail. Besides that the TFF based ZnO NW specimens showed a promising result as oxygen gas sensors. 1D semiconductor-metal hybrid structures are fabricated using TFF and TAM techniques and offer additional functionalities. A controlled shrinking of Ti NWs (TFF) by electrochemical oxidation, TiO2-Ti hybrid NWs with a promising potential as NWFET have been demonstrated. In addition to the interesting FET characteristics, the hybrid NWFET showed a drastic and fast response in the drain current when it is exposed to different partial pressures of oxygen gas. One of the most important feature of these NWs is that they all exhibit granular structure. This characteristic makes them suitable especially for sensoric applications. The other novel approach for the chip integrated synthesis of iron oxide nanostructures with a variety of unique characteristics is TAM. A thermally initiated diffusion, oxidation and reshaping of Fe thin film in atmospheric air condition driven for the synthesis of single crystalline iron oxide NWs. Detailed investigations on the NWs proved that some specimens are able to form metal-metal oxide hybrid nanostructure with an iron-rich core region, the detailed characteristics are described as well

Low-power techniques for wireless gas sensing network applications: pulsed light excitation with data extraction strategies

Aquesta tesi està enfocada en dues línies d'investigació. La primera aborda el desenvolupament d'una metodologia basada en llum polsada per modulació de sensors químic-resistius per a l'extracció d'informació del senyal transitòri, i la segona planteja la implementació d'una xarxa sense fils de sensors (WSN) basada en tecnologia LoRa per al monitoratge de la qualitat de l'aire (AQM) i la detecció d'esdeveniments de fuita de gasos. Aquest document està estructurat en quatre capítols organitzats de la següent manera: el Capítol 1 presenta l'estat de l'art, una introducció als mecanismes de millora de l'comportament dels sensors químic-resistius, així com una introducció a la implementació de xarxes sense fils de sensors per a la monitorització de la qualitat de l'aire; el Capítol 2 està compost pels dos articles publicats relacionats amb la metodologia basada en la modulació utilitzant llum polsada per a l'extracció d'informació del senyal transitòria de sensors químic-resistius; el Capítol 3 presenta l'article publicat relacionat amb la implementació d'una WSN per a AQM; el Capítol 4 presenta les conclusions derivades dels resultats obtinguts durant el desenvolupament de el projecte de tesi i les recomanacions per al treball futur associat a la continuïtat dels principals resultats d'aquesta tesiLa presente tesis está enfocada en dos líneas de investigación, La primera aborda el desarrollo de una metodología basada en luz pulsada para modulación de sensores químico-resistivos para la extracción de información de la señal transitoria; y la segunda plantea la implementación de una red inalámbrica de sensores (WSN) basada en tecnología LoRa para la monitorización de la calidad del aire (AQM) y la detección de eventos de fuga de gases. Este documento está estructurado en cuatro capítulos organizados de la siguiente forma: el Capítulo 1 presenta el estado del arte, una introducción a los mecanismos de mejora del comportamiento de los sensores químico-resistivos, así como una introducción a la implementación de redes inalámbricas de sensores para la monitorización de la calidad del aire; el Capítulo 2 está compuesto por los dos artículos publicados relacionados con la metodología basada en la modulación utilizando luz pulsada para la extracción de información de la señal transitoria de sensores químico-resistivos; el Capítulo 3 presenta el artículo publicado relacionado con la implementación de una WSN para AQM; el Capítulo 4 presenta las conclusiones derivadas de los resultados obtenidos durante el desarrollo de el proyecto de tesis y las recomendaciones para el trabajo futuro asociado a la continuidad de los principales resultados de esta tesis.The present thesis project is focused in two different yet related research lines. The first one addresses the development of a pulsed light-based chemiresistive sensor modulation methodology for transient information extraction. The second research line developed deals with the implementation of a LoRa-based portable, scalable, low-cost, and low power Wireless Sensor Network (WSN) for Air Quality Monitoring (AQM) and gas leakage events detection. This document is structured in four Chapters organized as follows: Chapter 1 presents the state of the art, an introduction to sensing performance enhancement and transient data extraction methods, as well as an introduction to the implementation of WSN for AQM; Chapter 2 is composed of the two published paper related to the pulsed light modulation methodology for transient information extraction; Chapter 3 presents the published paper related to the implementation of a LoRa-based WSN for AQM; Chapter 4 states the conclusions derived from the results obtained during this thesis project and the recommendations for the future work associated to the continuity of this thesis findings

Synthesis and characterization of metal sulfide nanoparticles/polymer nanocomposites

The focus of this project was to synthesize and characterize metal sulfide nanoparticles /polymer nanocomposites. The work involved the synthesis of dithiocarbamato ligands and complexes derived from aniline. Zn(II), Cd(II) and Hg(II) dithiocarbamato complexes were used as single-molecule precursors for the synthesis of the ZnS, CdS and HgS nanoparticles and their optical and structural properties studied. The other focus of this work was to synthesize a combined functionality metal sulfide nanoparticles/polymer nanocomposites by dispersing as-synthesized ZnS, CdS and HgS nanoparticles in polymethyl methacrylate (PMMA) matrix. The characterization of the ligands, complexes, nanoparticles and nanocomposites were investigated using relevant instrumental tools like UV-Vis, photoluminescence (PL), Fourier transform infrared (FTIR), X-ray diffraction (XRD), energy dispersion X-ray (EDX), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM)

Immobilized Non-Precious Electrocatalysts for Advanced Energy Devices

The successful commercialization of advanced energy devices, including fuel cells and solar cells (e.g., dye-sensitized solar cells) is somewhat dependent on the cost, activity and durability of the electrocatalysts. Nowadays, precious metal electrodes are the most widely used. Accordingly, the manufacturing costs are relatively high, which constrains wide application. Recently, some reports have introduced some promising non-precious electrocatalysts to be exploited in both oxidation and reduction reactions. It was concluded that immobilization of the functional material on a proper support can distinctly improve catalytic activity. Moreover, due to the synergetic effect, metallic alloy nanoparticles show very good electrocatalytic activity in this regard. This Special Issue aims to cover the most recent progress and the advances in the field of the immobilized non-precious electrocatalysts. This includes, but is not limited to, non-precious electrocatalysts for alcohol (methanol, ethanol, etc.) oxidation, oxygen reduction reaction and electrolyte reduction in dye-sensitized solar cells

Recent advances in phthalocyanines for chemical sensor, non-linear optics (NLO) and energy storage applications

Phthalocyanines (Pcs) are intensely coloured, robust macrocycles that possess admirable chemical, thermal and photo- stability as a result of its extensive π-network. The Pc family has established their fundamental and technological importance in numerous applications due to structural versatility and unique properties that result from the incorporation of a variety of inorganic and organic components into its framework. The diversity of Pc structures allows for the optimisation of certain properties to obtain functional, high-performance materials. The combination of aromaticity, relatively simple synthesis and structural flexibility makes Pcs a great asset for numerous scientific and industrial advancements. This review focuses on recent advances (2014–2020) of phthalocyanines in the specific technologies of chemical sensors, non-linear optics (NLO), and energy storage applications. Many MPc complexes reported to date favour select metals and ligand derivatives which leaves huge opportunities for further exploration.The National Research Foundation, South Africa and the University of KwaZulu-Natal (UKZN), South Africa.http://www.elsevier.com/locate/ccr2022-06-23hj2021Chemistr

Liquid-gas boundary catalysis by gold/polystyrene-coated hollow titania and the effective location of active sites in liquid-liquid phase-boundary catalyst

The research described in this thesis is an attempt to synthesize floating catalyst to be used in phase-boundary catalysis for liquid-gas reaction. It is also an attempt to determine the effective location of catalytic active sites in phase-boundary catalyst for immiscible liquid-liquid reaction. Phase-boundary catalysis (PBC) is a heterogeneous catalytic reaction in which the catalyst particles are located at the interphase of either immiscible liquid-liquid or liquid-gas phases. In this research, gold/polystyrene-coated hollow titania was successfully synthesized. The synthesis steps involved hydrothermal synthesis of carbon sphere from sucrose as the template, coating of the carbon sphere with titania, removal of the carbon sphere to produce hollow titania, followed by coating of polystyrene on the surface of hollow titania and the attachment of gold nanoparticles. The results showed that the size of the carbon spheres increased when the concentration of the sucrose increased. The hollow titania obtained by carbon sphere template was found to have smaller diameter than its template and was in the anatase phase. Polystyrene was coated on hollow titania by in-situ polymerization of styrene with aqueous H2O2, while gold was deposited by sputtering deposition technique on the surface of polystyrene-coated hollow titania. It has been demonstrated that gold/polystyrene-coated hollow titania can float on water due to its low density and it is a potential catalyst for liquid–gas boundary catalysis in the oxidation of benzyl alcohol by using molecular oxygen. In this study, some aspects in the determination of the effective location of active sites of PBC in immiscible liquid-liquid system were also studied using NaY, HZSM-5 and TS-1 zeolites as the catalysts in the oxidation of 1-octene and hydroxylation of cyclohexene using aqueous H2O2. Based on experimental results, it is concluded that the effective location of active sites is located on the external surface of zeolites

Plasmonic core-multi-shell nanomaterials for improving energy efficiency and sensing

In recent times, plasmonics has been a hallmark in improving optoelectronic device performance as well as in improving sensing. Confining light in dimensions below the diffraction limit and subsequently converting the incident photons into localized charge-density oscillations called localized surface plasmons, optical enhancements of the local fields by many orders of magnitude is possible. This dissertation explores the use of such surface plasmon resonances in core multishell nanostructures and demonstrates the values of such structures in energy harvesting and sensing. Additionally, it also shows the use of emerging plasmonic materials like metal nitrides (TiN, ZrN) instead of traditional plasmonic materials (Au, Ag) in the nanostructure designs. Utilizing the localized surface plasmon resonance (LSPR) in metallic components of core multishell nanowires, calculations of the local density of states as a measure of emission were made using a Green’s function method, while the absorption and scattering were simulated using the Mie formalism. Combining both the absorption and the emission, the quantum efficiency of white LEDs was calculated and the optimal material/dimensions for maximal performance was determined for different phosphor components in a white LED. Additionally, the use of ZrN as a plasmonic cloak for noise cancellation in Si photodetectors is shown and the performance is compared with an Au cloak. Using the developed methodology, it is proved that ZrN cloaks can outperform Au cloaks in a certain region of the visible spectrum, showing the benefit of using such plasmonic systems in place of traditional materials. The fabrication of the different components of the core multishell nanowires is also presented, and in particular, fabrication of ultra-thin (sub-10 nm) plasmonic TiN is achieved. Utilizing plasmon hybridization, a tunable double resonance feature is observed in Au/SiO2/Au core shell shell (CSS) nanoparticles, which have been then demonstrated to improve the photocatalytic performance in hematite. In particular, the double resonance peak allows absorption of light beyond the band gap of hematite and subsequent conversion into photocurrent through hot electron injection. Comparison has been made with Au nanoparticles, and it has been shown that the CSS nanoparticles outperform Au nanoparticles significantly. These CSS nanoparticles have also been used for bioimaging, in particular for Raman spectroscopy, with strong results at high densities of the nanoparticles. Utilizing stronger scattering SiO2/Au Nanoshells, it has been possible to work towards single particle imaging of molecules and demonstration of this phenomenon has been shown here through the use of coherent Raman scattering spectroscopy