Nanocomposite Films for Gas Sensing

Nanocomposite films are thin films formed by mixing two or more dissimilar materials having nano-dimensional phase(s) in order to control and develop new and improved structures and properties. The properties of nanocomposite films depend not only on the individual components used but also on the morphology and the interfacial characteristics. Nanocomposite films that combine materials with synergetic or complementary behaviours possess unique physical, chemical, optical, mechanical, magnetic and electrical properties unavailable from that of the component materials and have attracted much attention for a wide range of device applications such as gas sensors.NRC publication: Ye

Development of a low-cost zinc oxide-based material hybrid memristor

In recent years, there has been a resurgence of interest in two-terminal resistive devices as a new universal memory, which has the speed of static random access memory (SRAM), simplicity of dynamic random access memory (DRAM) and non-volatile storage of Flash memory. This has arisen from a report in 2008 that links switching in TiO2 memristor (dubbed the HP memristor) with Leon Chua’s memristor theory from 1970. The work importantly helped establish a framework for understanding and developing the field forward, which combined with the potential to scale devices down beyond the limits of complementary metal-oxide semiconductors has stimulated progress, enabling significant improvement in the technology.In this thesis, we have developed a new material-hybrid zinc oxide (ZnO) nanorod- polymethyl methacrylate (PMMA) memristor fabricated using a novel microwave-based technique that enables devices to be made in a simple, quick and low-cost manner. A study into the growth of the nanowires identifies the significance of a uniform and aligned seed layer in producing dense distributions of aligned and homogeneous nanowire arrays, which is necessary for controlling the properties of the spacing layer and avoiding short-circuits. The rate of heating affects the nanowire growth significantly by inducing the formation of larger crystallites when heating at high power for very short durations, while low-power heating over larger durations reduces the formation of these larger particles.We also find that the transport mechanisms are dependent on the device configurations. Through electrical I-V measurements, we observe that devices with gold (Au) top and bottom electrodes produce on/off resistance ratios (referring to the ratio of the low resistance state to the high resistance state) of typically around 5 - 8 and exhibit electrical behaviour typical of Poole-Frenkel emission and space charge limited conduction (SCLC). Devices with an aluminium (Al) top electrode and Au bottom electrode typically exhibit on/off ratios of approximately 10 (one order of magnitude), and their on/off ratios are larger than the best achieved with both Au electrodes; these devices typically exhibit Schottky emission behaviour, but do not exhibit clear Poole-Frenkel or SCLC behaviour. Devices using indium tin oxide (ITO) as a bottom electrode typically have on/off ratios of ~5 and appear to be dominated by Schottky emission behaviour.The introduction of a PMMA layer affects the behaviour of all configurations. None of the transport models clearly fit the data for the Au/ZnO/PMMA/Al configuration; Poole-Frenkel emission and SCLC behaviour can be observed in ITO/ZnO/PMMA/Al devices, with many SCLC regimes being clearly identifiable that were not present in the configuration without PMMA. The additional PMMA layer is observed to affect the SCLC behaviour in Au/ZnO/PMMA/Au configurations, as the device no longer produces the higher-order trap charge limited conduction regime observed in the device without PMMA; the Poole-Frenkel behaviour is also affected, as the change in gradient for the Poole-Frenkel plot indicates that the data fits the “modified Poole-Frenkel effect” model. This suggests that additional trap centres may be created with the addition of a PMMA layer.This thesis concludes that, while the devices require further optimization (particularly in terms of endurance and retention) to become commercially viable, the technique has much potential for future application

Synthesis and characterization of nanostructured materials

In addition to technological motivations, nanomaterials are interesting for basic scientific investigation because their properties reside in the largely unexplored realm between molecules and bulk solids. The controlled synthesis of these materials, by methods that permit their assembly into functional nanoscale structures, lies at the core of nanoscience and nanotechnology. Here, controlled synthesis refers to a process of collective nanostructure growth where the pertinent attributes such as location, size, orientation, and composition as well as the electrical, mechanical, and chemical properties of the individual elements can be predetermined by the choice of the growth conditions and the preparation of the growth substrate. This dissertation work furthers the understanding of the mechanisms by which synthesis conditions affect the morphology, composition, and crystal structure of nanostructured materials with the objective of achieving greater control over the synthesis process. Three types of systems are investigated in depth: vertically aligned carbon nanofibers (grown by plasma-enhanced chemical vapor deposition), catalytic alloy nanoparticles (sputter-deposited, carbon-encapsulated), and tungsten nanowires (grown by electron-beam-induced deposition). The effects of growth parameters on the resulting nanostructure properties are characterized by methods including high-resolution transmission electron microscopy, electron diffraction, and chemical spectroscopy

Recent advances in multistep solution nanosynthesis of nanostructured three-dimensional complexes of semiconductive materials

AbstractConstructing simply nanostructured zero-, one-, and two-dimensional crystallites into three-dimensional multifunctional assemblies and systems at low-cost is essential and highly challenging in materials science and engineering. Compared to the simply nanostructured components, a three-dimensional (3D) complex made with a precisely controlled spatial organization of all structural nanocomponents can enable us to concert functionalities from all the nanocomponents. Methodologically, so doing in nm-scales via a solution chemistry route may be much easier and less expensive than via other mechanisms. Hence, we discuss herein some recent advances in multistep solution syntheses of nanostructured 3D complexes of semiconductors with a focus mainly on their synthetic strategies and detailed mechanisms

Materials Processing for Production of Nanostructured Thin Films

Thin films are important in many of the technologies used every day, impacting major markets for energy, medicine, and coatings. Scientists and engineers have been producing thin films on a wide range of surfaces for many decades but now have begun to explore giving these films new and controlled structures at the nanometer scale. These efforts are part of the new horizons opened by the field of nanoscience and impart novel structures and properties to these thin films. This book covers some of the methods for making these nanostructured thin films and their applications in areas impacting on health and energy usage

FABRICATION OF ZINC OXIDE MICRO-NANOSTRUCTURES AND THEIR APPLICATIONS IN GAS SENSING AND NANOCOMPOSITES

To date, one-dimensional ZnO micro/nanostructures have been attracting much attention for wide potential applications due to their unique electrical, piezoelectric, optoelectronic, and photochemical properties. The overall objective of this dissertation is to grow various ZnO micro and nanostructures using a novel microwave thermal evaporation-deposition approach, to explore the application of ZnO nanostructures in gas sensing, and to fabricate and characterize multifunctional ZnO nanowires-polyimide nanocomposite. Therefore, three parts were included in this study: (1) A novel thermal evaporation-deposition method using microwave energy was investigated. Batch of ZnO structures including microtubes, microrods, nanotubes, nanowires and nanobelts have been successfully synthesized in the microwave system with a unique source materials-substrate configuration and a desirable temperature profile. These products are pure, structurally uniform, and single crystalline. The photoluminescence (PL) exhibits strong ultraviolet emission at room temperature, indicating potential applications for short-wave light-emitting photonic devices. (2) Piezoelectric crystal langasite bulk acoustic wave (LGS) resonator based high temperature gas sensor was fabricated. Ordered ZnO nanowire arrays were grown on the langasite resonator as the sensitive layer by two-step hydrothermal method at low temperature. The gas sensor coated with ZnO nanowire arrays exhibited good sensitivity to NO2 and NH3. The response of the sensor is fast due to the large surface area of ZnO nanowires. In addition, this work demonstrates that the combination of nanowire arrays with langasite thickness shear mode resonators could provide a promising high temperature gas sensing platform with both high sensitivity and enhanced response speed. (3) The nanocomposite with controlled alignment of ZnO nanowires in the polyimide matrix was achieved using self-alignment method and external electric field assisted method. For the the self-alignment process, the morphologies of the designed nanocomposites were dramatically influenced by the viscosity of the polymer and the geometrical structure of ZnO nanowires. For the nanocomposite prepared by the electric field assisted alignment, the density and the alignment degree of ordered ZnO nanowires significantly depended on the magnitude and the frequency of the applied ac electric field. The DC offset voltage had strong effect on the deposition sites of nanowires. The resultant nanocomposite devices exhibited great dielectric constant and conductivity enhancement at room temperature due to the interfacial effect between ZnO nanowires and the polymer matrix. These nanocomposites combining the superb properties of ZnO nanowires with the polyimide matrix provide a smart material candidate for multifunctional applications that require self-sensing and self-actuation capabilities. The self-alignment method and electric field assisted alignment method also provide a bright route to combine superb properties of nanomaterials with the lightweight, flexibility, and manufacturability of dielectric polymers for future generations of multifunctional materials

Research and Development Aspects on Chemical Preparation Techniques of Photoanodes for Dye Sensitized Solar Cells

The importance of dye sensitized solar cells (DSSCs) as a low-cost and environmentally friendly photovoltaic (PV) technology has prompted many researchers to improve its efficiency and durability. The realization of these goals is impossible without taking into account the importance of the materials in DSSCs, so the focus on the preparation/deposition methods is essential. These methods can be either chemical or physical. In this study, the chemical applied methods that utilize chemical reaction to synthesize and deposit the materials are covered and categorized according to their gas phase and liquid phase precursors. Film processing techniques that can be used to enhance the materials' properties postpreparation are also included for further evaluation in this study. However, there is a variety of consideration, and certain criteria must be taken into account when selecting a specific deposition method, due to the fact that the fabrication conditions vary and are unoptimized

1D TiO2 nanostructures probed by 2D transmission electron microscopy

Hybrid solar cells based on nanoparticulate TiO2, dye and poly(3-hexylthiophene) are a common benchmark in the field of solid-state dye-sensitized solar cells. One-dimensionally nanostructured titanium dioxide is expected to enhance power-conversion efficiency (PCE) due to a high surface area combined with a direct path for electrons from the active interface to the back electrode. However, current devices do not meet those expectations and cannot surpass their mesoporous counterparts. This work approaches the problem by detailed investigation of diverse nanostructures on a nanoscale by advanced transmission electron microscopy (TEM). Anodized TiO2 nanotubes are analyzed concerning their crystallinity. An unexpectedly large grain size is found, and its implication is shown by corresponding solar cell characteristics which feature an above-average fill factor. Quasi-single crystalline rutile nanowires are grown hydrothermally, and a peculiar defect structure consisting of free internal surfaces is revealed. A growth model based on Coulombic repulsion and steric hindrance is developed to explain the resulting V-shaped defect cascade. The influence of the defects on solar cell performance is investigated and interpreted by a combination of TEM, electronic device characterization and photoluminescence spectroscopy, including lifetime measurements. A specific annealing treatment is proposed to counter the defects, suppressing several loss mechanisms and resulting in an improvement of PCEs by 35 %. Simultaneously, a process is developed to streamline electron-tomographic reconstruction of complex nanoparticles. Its suitability is demonstrated by the reconstruction of a gold nanostar and a number of iron-based particles distributed on few-layered graphene

ZnO Nanostructures: Low-Temperature Synthesis, Characterisation, Their Potential Application as Gene-Delivery Tools

Among metal oxide nanomaterials, zinc oxide (ZnO) nanostructures are one of the most important nanomaterials in today’s nanotechnology research. Over the past several decades, ZnO nanostructures have been extensively investigated for their extraordinary physical and chemical characteristics and also for their prominent performance in various novel applications such as photonics, optics, electronics, drug delivery, cancer treatment, bio-imaging, etc. However, the functionality of theses nanomaterials is eventually dictated by the capability to govern their properties including shape, size, position, and crystalline structure on the nanosized scale. This thesis investigates the solution-based synthesis of ZnO nanostructures and their morphological and structural properties. Importantly, in order to achieve the promising structure of ZnO, a systematic investigation of the influence of processing parameters, including solution concentration, time and temperature of growth reaction on the resultant materials was addressed. The other main point for this work is not only to effectively control the morphology, size, uniformly distribution, and orientation of ZnO nanomaterials, but also to build a good comprehension of the mechanism of the fabrication process to raise their performance in future nanoscale applications. Furthermore, the catalytic effect of RF sputtered gold (Au) thin layer on Si substrate prior to ZnO growth was investigated to demonstrate the contributory for the remarkable catalytic activity of Au nanoparticles in the formation of high-quality ZnO nanostructures. Furthermore, we introduce an effective, inexpensive lithographic patterning method to consistently control the position of solution-processed ZnO nanowires. Nanosphere lithography technique (NSL) utilizes a catalyst-assisted pattern generated by employing colloidal self-assembled crystal of polystyrene spheres (PS) on the substrate surface to guide the hydrothermal growth of ZnO nanowires. Further, we fabricate 3D NFs and branched NFs of ZnO on a silicon substrate via a simple and cost-effective solution growth method, incorporating with seed ZnO nanoparticles deposition. The synthesis of 3D branched ZnO nanostructure could potentially exploit for applications in optoelectronics, catalysis, sensing, and photovoltaics. In addition to the synthesis of 1D and 3D ZnO nanostructures, their morphology and distribution have been analysed via scanning electron microscopy (SEM) while the surface topography was analysed by atomic force microscopy (AFM). The crystalline structure, phase purity, and particle size of ZnO nanomaterials have been investigated using X-ray diffraction (XRD). The outcomes from all these efforts have been integrated for cellular investigation via fluorescence microscopy technique (FM) to demonstrate the potential application of ZnO nanostructures as a gene delivery/-tissue engineering tool in different expression systems.Libyan Governmen

Dry-transfer of chemical vapour deposited nanocarbon thin films

This thesis presents the development of chemical vapour deposited (CVD) graphene and multi-walled carbon nanotubes (MWCNTs) as enabling technologies for flexible transparent conductors offering enhanced functionality. The technologies developed could be employed as thin film field emission sources, optical sensors and substrate-free wideband optical polarisers. Detailed studies were performed on CVD Fe and Ni catalysed carbon nanotubes and nanofibres on indium tin oxide, aluminium and alumina diffusion barriers. Activations energies of 0.5 and 1.5 eV were extracted supporting surface diffusion limited catalysis for CNTs and CNFs. For the first time an activation energy of 2.4 eV has been determined for Cu-catalysed growth of CVD graphene. Graphene was shown to deviate significantly from the more traditional rate-limited surface diffusion and suggests carbon-atom-latticeintegration limited catalysis. An aligned dry-transferred MWCNT thin film fabrication technique was developed using MWCNTs of varied lengths to control the optical transparency and conductivity. A process based on the hot-press lamination of bilayer CVD graphene (HPLG) was also developed. Transport studies revealed that these thin films behave, in a macroscopic sense, similar to traditional c-axis conductive graphite and deviate toward tunnel dominated conduction with increasing degrees of network disorder. Various MWCNT-based thin film field emitters were considered. Solution processing was shown to augment the surface work function of the MWCNTs resulting in reduced turn-on electric fields. Integrated zinc oxide nanowires were investigated and were shown to ballast the emission, thereby preventing tip burn out, and offered lower than expected turn-on fields due to the excitation of a hot electron population. To obviate nearest neighbour electrostatic shielding effects an electrochemical catalyst activation procedure was developed to directly deposit highly aligned sparse carbon nanofibres on stainless steel mesh. Highly-aligned free-standing MWCNT membranes were fabricated through a solid-state peeling technique. Membranes were spanned across large distances thereby offering an ideal platform to investigate the unambiguous photoresponse of MWCNTs by removing all extraneous substrate interfaces, charge traps and nanotube-electrode Shottky barriers as well as using pure, chemically untreated material. Oxygen physisorbtion was repeatedly implicated through in-situ lasing and in-situ heated EDX measurements, FT-IR and lowtemperature transport and transfer measurements. A MWCNT membrane absorptive polariser was fabricated. Polarisers showed wideband operation from 400 nm to 1.1 μm and offered operation over greater spectral windows than commercially available polymer and glass-support dichroic films. Ab-initio simulations showed excellent agreement with the measured polarisation attributing the effect to long-axis selective absorption