Growth And Characterization Of Vertically Aligned Zno Nanorods Synthesized By Chemical Bath Deposition For Uv Photodetector Applications

The objectives of this study are to grow and characterize the structural, optical, and electrical properties of aligned ZnO nanorods, and to develop ultraviolet (UV) photodetectors using these ZnO nanorod arrays grown with optimized growth parameters. The study employed two different substrates for growing ZnO nanorod arrays; first, flexible polyethylene naphthalate (PEN) seeded ZnO and second, porous silicon (PS) seeded ZnO. Well-aligned ZnO nanorod arrays were grown on flexible PEN substrates at a low temperature by using the chemical bath deposition (CBD). The control of the diameter, length, density, and optical and structural properties of the ZnO nanorod arrays were systematically investigated by modifying the growth parameters, including the precursor concentration and growth duration. Vertically aligned ZnO nanorod arrays grown using a 0.050 M precursor concentration and 5 h growth duration had diameters ranging from 10 nm to 40 nm and exhibited the sharpest and most intense UV peak in the room temperature photoluminescence (PL) results compared with other samples. Next, high quality ZnO nanorod arrays were synthesized through the optimization of precursor concentration and growth duration by using CBD method on PS substrates, which were prepared via the photo electrochemical etching (PECE) method. The aligned ZnO nanorods grew perpendicular to the PS substrates and had average diameters and lengths ranging from 13 nm to 69 nm and from 85 nm to 208 nm, respectively

Method to reduce the formation of crystallites in ZnO nanorod thin-films grown via ultra-fast microwave heating

© 2018 This paper discusses the nucleation and growth mechanisms of ZnO nanorod thin-films and larger sized crystallites that form within the solution and on surfaces during an ultra-fast microwave heating growth process. In particular, the work focusses on the elimination of crystallites as this is necessary to improve thin-film uniformity and to prevent electrical short circuits between electrodes in device applications. High microwave power during the early stages of ZnO deposition was found to be a key factor in the formation of unwanted crystallites on substrate surfaces. Once formed, the crystallites, grow at a much faster rate than the nanorods and quickly dominate the thin-film structure. A new two-step microwave heating method was developed that eliminates the onset of crystallite formation, allowing the deposition of large-area nanorod thin-films that are free from crystallites. A dissolution-recrystallization mechanism is proposed to explain why this procedure is successful and we demonstrate the importance of the work in the fabrication of low-cost memristor devices

Facile solution growth of vertically aligned ZnO nanorods sensitized with aqueous CdS and CdSe quantum dots for photovoltaic applications

Vertically aligned single crystalline ZnO nanorod arrays, approximately 3 μm in length and 50-450 nm in diameter are grown by a simple solution approach on a Zn foil substrate. CdS and CdSe colloidal quantum dots are assembled onto ZnO nanorods array using water-soluble nanocrystals capped as-synthesized with a short-chain bifuncional linker thioglycolic acid. The solar cells co-sensitized with both CdS and CdSe quantum dots demonstrate superior efficiency compared with the cells using only one type of quantum dots. A thin Al2O3 layer deposited prior to quantum dot anchoring successfully acts as a barrier inhibiting electron recombination at the Zn/ZnO/electrolyte interface, resulting in power conversion efficiency of approximately 1% with an improved fill factor of 0.55. The in situ growth of ZnO nanorod arrays in a solution containing CdSe quantum dots provides better contact between two materials resulting in enhanced open circuit voltage

Zinc Oxide Nanostructures for Flexible and Transparent Electronics

As a multifunctional material, ZnO possesses remarkable and unique properties and has attracted much research interest for use in a variety of applications. Especially, it has been regarded as a leading material for flexible and transparent electronics, which is a promising emerging technology in electronics. This dissertation studies doping behavior of Ga in ZnO for transparent electrode applications and presents new approaches to ZnO nanostructures for next-generation flexible and transparent electronics. These approaches include developing techniques that enable multiple stacked ZnO nanoflowers and thermal treatment processes at high temperature. Transparent conductive oxides have been extensively studied for the use as a transparent electrode, which is one of the most fundamental and essential parts in transparent electronic devices. In this study, Ga-doped ZnO nanorods were grown on glass substrates, and the effects of Ga doping concentration on the physical properties of ZnO nanorods were investigated using various characterization tools. ZnO nanoflower is a highly preferred nanostructure for solar cells, sensors, and photodetectors due to its high surface area to volume ratio. To-date, ZnO nanoflowers have mostly been synthesized in the form of nanopowders without a substrate, and ZnO nanoflowers grown on substrates have only been single-stacks. Atmospheric pressure plasma jet treatment was used to increase the surface area to volume ratio of ZnO nanoflowers. The plasma treatment induced a significant increase in the height and density of the ZnO nanoflowers/nanorods because the plasma effectively increased the surface energy and roughness of the seed layers while barely affecting the crystal shape and phase of the ZnO nanoflowers/nanorods. Flexible and transparent mica substrates were used for the growth of vertically well-aligned ZnO nanorods. The adoption of mica as a substrate material permitted high temperature annealing processes, which improved the structural and optical properties of ZnO nanorods with uniform surface coverage and excellent adhesion. A practical application for the synthesized ZnO nanorods is also presented in this dissertation. ZnO nanorod-based flexible and transparent dye-sensitized solar cells (DSSCs) and piezoelectric nanogenerators (NGs) were fabricated and the device performances were investigated. Although only two kinds of energy-harvesting devices (DSSCs and NGs) are presented as examples of applications in this dissertation, it is expected that this new approach will provide a breakthrough for overcoming the limited process temperature on plastic and cellulose nanopaper substrates because mica can be extensively used as a flexible and transparent substrate material for electronics, optoelectronics, energy/environmental, and biomedical applications where high temperature processes are required

Three-dimensional nanotube arrays for solar energy harvesting and production of solar fuels

Over the past decade extensive research has been carried out on photovoltaic semiconductors to provide a solution towards a renewable energy future. Fabricating high-efficiency photovoltaic devices largely rely on nanostructuring the photoabsorber layers due to the ability of improving photoabsorption, photocurrent generation and transport in nanometer scale. Vertically aligned, highly uniform nanorods and nanowire arrays for solar energy conversion have been explored as potential candidates for solar energy conversion and solar-fuel generation owing to their enhanced photoconversion efficiencies. However, controlled fabrication of nanorod and especially nanotube arrays with uniform size and shape and a pre-determined distribution density is still a significant challenge. In this research work, we demonstrate how to address this issue by fabricating nanotube arrays by confined electrodeposition on lithographically patterned nanoelectrodes defined through electron beam as well as nanosphere photolithography. This simple technique can lay a strong foundation for the study of novel photovoltaic devices because successful fabrication of these devices will enhance the ability to control structure-property relationships. The nanotube patterns fabricated by this method could produce an equivalent amount of photocurrent density produced by a thin film like device while having less than 10% of semiconducting material coverage. This project also focused on solar fuel generation through photoelectrocatalytic water splitting for which efficient electrocatalysts were developed from non-precious elements. Lastly, a protocol was developed to disperse these electrocatalysts into a butadiene based polymeric catalytic ink and further processing to yield free-standing catalytic film applicable for water electrolysis”--Abstract, page iv

Investigation Of ZnO Nanostructures On Glass Substrate Deposited By Thermal Evaporation Method For UV Detection

In this work, ZnO nanostructures with different morphologies were fabricated on a cost-effective glass substrate using a simple thermal evaporation in a conventional horizontal tube furnace via vapor–solid (VS) and vapor–liquid–solid (VLS) mechanisms. This study aims to provide a new approach for synthesis of one-dimensional (1D) and three-dimensional (3D) ZnO nanostructures using a low cost substrate by thermal evaporation of Zn powder in the presence of O2 gas, and determine optimal parameters to control high crystal quality and optical properties, thereby improving the performance of ultraviolet (UV) photodetectors (PDs) based on the produced ZnO

New ZnO-based core-shell nanostructures for perovskite solar cells

Perovskite solar cells are the emerging thin-film photovoltaics that has been most studied in the last decade, reaching record power conversion efficiencies close to those exhibited by silicon solar cells, which means a considerable breakthrough in photovoltaic technology. However, it has been found that in methylammonium lead halide perovskite devices, MAPbX₃, either the electron transport material or hole transport material, affects the perovskite material’s stability, compromising the perovskite device performance. Therefore, other alternatives should be considered to avoid the perovskite layer’s degradation, namely, the electron transport material employed in the perovskite solar cells. This PhD project aimed to develop nanostructures alternatives to the standard titanium dioxide (TiO₂) through ZnO-based nanostructures, using a low-cost and versatile technique such as pulsed electrodeposition, and applying them as the electron transport layer within perovskite solar cells. Well-aligned arrays of ZnO nanorods were produced by pulsed potentiostatic electrodeposition in aqueous media, under mild reaction conditions. Several modified substrates were evaluated for the growth of nanorods to optimise the nanorod diameter and vertical orientation. Using a TiO₂ intermediate layer as a template for the ZnO nanorods growth successfully allowed a decrease of the nanorod diameter, increased their spatial density, and increased the ZnO films’ chemical stability with time and under illumination. Also, it was verified that the pulsed electrodeposition conditions at which the nanorods grow, namely the pulse operational parameters, had a very appreciable impact on its optoelectronic properties. Several ZnO thin films prepared using different deposition media were studied to assess the influence of ZnO nature on the thermal stability of the MAPbI₃ perovskite. Some chemical groups attached to the ZnO surface affected the crystallization of the perovskite layer and accelerated its thermal degradation at the ZnO/perovskite interface. The ZnO@TiO₂ core-shell nanostructures were considered to prevent the perovskite instability issues when in intimate contact with ZnO. Despite the slight improvement in device performance using ZnO@TiO₂ core-shell nanorods, compared to ZnO nanorods, the morphological reproducibility of a TiO₂ shell that completely covers the ZnO surface is crucial to obtain higher photovoltaic performances

Growth of spatially ordered ZnO nanowire arrays for field emission applications

In this work the growth of spatially ordered and vertically aligned ZnO nanowires is examined. Nanowire arrays are grown using chemical bath deposition (CBD) and carbothermal reduction vapour phase transport (CTR-VPT) techniques. Nanosphere lithography (NSL) was used to achieve spatial ordering of these arrays; arrays with inter-wire distances of 500 nm, 1.0 μm, and 1.5 μm were grown using both CBD and CTR-VPT techniques. Two distinct implementations of the NSL technique are investigated, one which relied on the deposition of a catalyst material and one which involved the deposition of a secondary mask which prevents ZnO deposition from occurring in undesired areas. The field emission (FE) characteristics of these arrays were examined, revealing a significant dependence of the FE properties on both nanowire morphology and array density. A geometric factor is calculated which is dependent on both nanowire aspect ratio and the density of nanowires in an array and this factor has been found to correlate with other indicators of FE properties. The results presented in this work may be useful in informing the design of ZnO nanowire arrays in order to maximise their FE efficiency and uniformity

Zinc Oxide Nanorods (ZnO NRs) for photovoltaic applications

Zinc oxide (ZnO) is a material that has highly attractive cost-effective features and can be grown using different methods leading to a wide range of nanostructured morphologies. In this work, zinc oxide nanorods (ZnO NRs) with various surface distribution density were sensitised using aqueous solutions based on zinc salt at 40°C85°C. Systematic investigations were carried out on the influences of zinc salt, different ZnO seed layer thickness and growth temperature (Tgrowth). The grown ZnO NRs were used in two different aspects. The first aspect was to grow the NRs on a releasable layer and here Omnicaot was used as a sacrificial layer and SU-8 photoresist as a support structural layer to reach the desired aim. Also, ZnO NRs were grown on polydimethylsiloxane (PDMS). This was used to improve the performance of different types of solar cells by mounting the full structure on top with help of optical gel. The results obtained showed that wet lift-off showed an increasing ɳ from 1.56% to 2.05% when a GaAs solar cell was used, whereas the same solar cell showed an efficiency ɳ increasing to about 2.03% when using dry peel-off nanostructures. CZTSSe (Cu2ZnSn(S,Se)4) solar cells showed an increase in ɳ from 1.3% to (1.79%, 1.65% and 2.15%) for ZnO NRs/SU-8, ZnO NRs/PDMS and flower-like/diluted PDMS, respectively. The second aspect was to fabricate extremely thin absorber solar cells. For lift-off process, the work presented herein provides a cost-effective, simple novel combination of lift-off processing with hydrothermally grown ZnO NRs on SU-8 or PDMS, in a low temperature range Tgrowth from 40°C- 85°C. This study addresses a controllable release/transfer of ZnO NRs when high growth temperature represents a barrier to carrying out an immediate growth on flexible substrates for example wearable electronics applications. The evolution of ZnO NRs with Tgrowth were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and UV-Vis spectroscopy. The ZnO NRs/SU-8 and ZnO NRs/PDMS structures were successfully detached from the glass. The SEM images confirmed that, the ZnO NRs formed different diameters and lengths as Tgrowth increased. Transmitted/scattered light characteristics showed different trends depending on the Tgrowth and structure stack used. The findings of this study offer an easy method of lift-off ZnO NRs (and subsequent stack of layers) using the low-cost facilities and at low temperature. Current-voltage (I-V) and external quantum efficiency (EQE) measurements showed an affective influence of mounting the released ZnO NRs on CZTS and GaAs solar cells. Moreover, a study on ETA cell fabrication using CZTS is presented. The fabricated ETA cell structure was as the following: glass/ITO/ZnO seed/ZnO NRs/CdS/CZTS/P3HT/gold and the aim is for the CdS/CZTS to conformally coat the ZnO NRs, while P3HT acts as a hole transport layer which in turn helps avoiding shunting. ETA cell showed about 0.02% of efficiency (ɳ), 0.05V (Voc), 0.15mA/cm2 (Jsc), 27.46% (FF) and about 20% EQE in the 300nm-600nm spectra region. These results are clear indication of promising sight for ETA solar cells based on CZTS nanoparticles which can be more improved

Vertically Ordered Nanostructures for Energy Harvesting and Storage

Vertically ordered (1-D) nanostructures provide a promising alternative to conventional nanoparticle films used as electrode materials for energy conversion and storage devices. These 1-D nanostructures, in forms of nanowires or nanotubes, promote mass transfer and accessibility of the electrodes while providing a direct conduction path for electrons. Our work has been focused on synthesis and application of novel 1-D nanostructures for dye-sensitized solar cells (DSCs) and lithium-ion batteries (LIBs). The vertically aligned 1-D nanostructures are employed in DSCs to overcome the limitation of nanoparticle-based DSCs. Much longer electron life time has been observed in DSCs based on 1-D nanostructures compared to the nanoparticle-based ones, which allows us to use thicker sensitized film to improve the efficiency. We have developed a facile low-temperature hydrothermal method to synthesize vertically aligned ZnO nanowire arrays directly on transparent conductive oxide, and to use the ZnO nanowire arrays as a template to synthesize SnO2 nanotube arrays. In addition, we have developed a convenient approach that involves alternate cycles of nanowire growth and self-assembled monolayer coating processes for synthesizing multilayer assemblies of 1-D nanostructures with ultrahigh internal surface areas. The vertically aligned nanostructure also enables us to fabricate high-efficiency solid-state DSCs by replacing the liquid electrolyte with a solid hole transporting material. The vertically aligned nanostructures provide straight channels for filling the solid electrolyte, enabling the use of thicker photoanodes for solid-state DSCs. Significantly, by using vertically aligned multilayer arrays of TiO2-coated ZnO nanowires, liquid-electrolyte DSCs with power conversion efficiency up to 7.0% and solid-state DSCs with efficiency up to 5.65% have been obtained. Vertically ordered 1-D nanostructures also offer remarkable advantages for rechargeable LIBs including fast electron transport/collection and ion diffusion, enhanced electrode-electrolyte contact area, and facile accommodation of strains caused during the charge and discharge cycles. We have developed a method to fabricate SnO2 nanotube arrays and hybrid Sn-based nanotube arrays directly on current collecting substrate (Ti) and have evaluated their performance as anodes in rechargeable LIBs. The hybrid Sn-based nanotube arrays synthesized by us delivered a capacity of 710 mAh/g after 80 cycles with a low capacity fade