Improved characteristics of near-band-edge and deep-level emissions from ZnO nanorod arrays by atomic-layer-deposited Al2O3 and ZnO shell layers

We report on the characteristics of near-band-edge (NBE) emission and deep-level band from ZnO/Al2O3 and ZnO/ZnO core-shell nanorod arrays (NRAs). Vertically aligned ZnO NRAs were synthesized by an aqueous chemical method, and the Al2O3 and ZnO shell layers were prepared by the highly conformal atomic layer deposition technique. Photoluminescence measurements revealed that the deep-level band was suppressed and the NBE emission was significantly enhanced after the deposition of Al2O3 and ZnO shells, which are attributed to the decrease in oxygen interstitials at the surface and the reduction in surface band bending of ZnO core, respectively. The shift of deep-level emissions from the ZnO/ZnO core-shell NRAs was observed for the first time. Owing to the presence of the ZnO shell layer, the yellow band associated with the oxygen interstitials inside the ZnO core would be prevailed over by the green luminescence, which originates from the recombination of the electrons in the conduction band with the holes trapped by the oxygen vacancies in the ZnO shell

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

Pulsed laser deposition and characterisation of ZnO and aluminium-doped ZnO nanostructures on silicon and flexible plastic substrates

We have developed recipes for the catalyst-free growth of upstanding/vertically aligned ZnO nanorods featuring core/shell or interconnected core/shell architectures on ZnO-seeded Si (100) substrates using the pulsed laser deposition (PLD) technique. The structural, morphological and luminescent properties of these ZnO nanorod samples were established. A ZnO emission band at 3.331 eV was observed in the core/shell and interconnected core/shell nanorod architectures and its origin linked to the defects observed at the crystalline/amorphous interface of the core/shell structure. This particular defect PL emission appears to be a new observation for ZnO. We have grown vertically aligned ZnO nanorods on PLD prepared ZnO-seeded Si substrates by catalyst-free vapour phase transport (VPT). The nanorods featured excellent optical properties and a coverage density higher than previously published data. The structural, morphological and luminescent properties of the seed layers and nanorods were inter-compared. Importantly, we also compared the near band edge emission of such VPT-and PLD-deposits, with a focus on the identification of the origin of the emission feature at 3.331 eV. We have researched the room temperature PLD growth of highly transparent and conductive ZnO and Al-doped ZnO (AZO) nanocrystalline thin films on flexible Zeonor plastic substrates. The trends for the growth rate, surface morphology, hydrophobicity and the structural, optical and electrical properties of 65 nm - 420 nm thick ZnO/AZO films grown on Zeonor substrates were analysed as a function of oxygen growth pressure (1-300 mTorr). The as-grown films showed highly reproducible deposition behaviour, and featured high transmittance, low-electrical resistance, optical smoothness, low residual stress, and hydrophobicity. The results presented in this thesis are discussed in the context of prospectiv

Design and development of hybrid energy harvesters

PhDHybrid energy harvesters (HEHs) targeting multiple energy forms have been drawing increasing interest in recent years. While large scale photovoltaic power plants are capable of providing energy for domestic usage, research has also been focused on kinetic energy harvester with less power output which can be integrated into self-powered electronics such as implantable device, remote wireless sensor, wearables, etc. A number of successful designs of hybrid energy harvesters have been demonstrated which could scavenge solar and kinetic energy simultaneously. However the structures remain complicated; the majority of the designs involve different types of energy harvesters connected in series, which involves complex fabrication processes. Here, a simple structure based on a p-n junction piezoelectric nanogenerator (NG) was designed. The utilization of columnar piezoelectric n-type ZnO nanorods coated with light absorber layer enabled the device to harvest both kinetic and solar energy. This was adapted to either form a N719-based dye-sensitized solar cell (N719-HEH), or a perovskite solar cell (PSC-HEH). To allow high processing temperatures while maintaining mechanical flexibility, Corning© Willow™ (CW) glass substrate was used and compared to the more common ITO/PET. CW showed 56% lower charge transfer resistance and a related 4 times fold increase in power conversion efficiency for N719-HEHs. Oscillation (NG effect) and illumination (PV effect) testing indicated that both N719-HEHS and PSC-HEHs operated as kinetic and solar energy harvesters separately, with the current generated by the photovoltaic orders of magnitude greater than it from mechanical excitation. In addition, under illumination, both N719-HEHs and PSC-HEHs demonstrated further current output enhancement when oscillation was applied. The fact that the current output under NG+PV condition was higher than the summation of current output achieved under NG and PV conditions individually, suggests the piezoelectric potential originated from ZnO affected the charge dynamics within the devices. Thus, HEHs with enhanced output were successfully designed and developed.China Scholar Counci

Synthesis, properties and uses of ZnO nanorods: a mini review

Zinc oxide (ZnO) nanorods have been extensively investigated, owing to their extraordinary applications in numerous felds, spatially microchip technology, solar cells, sensors, photodetectors, photocatalysts and many others. Recently, using ZnO nanorods, as photocatalysts, are receiving increasing attention in environmental defense applications. This mini review sum�marizes some remarkable applications for ZnO nanorods. First, the various chemical and physical procedures that were used to produce ZnO nanorods are identifed through symmetric matrices and heterogeneous structures, then the authors explain how to use these methods to produce ZnO nanorods. This mini review, also, discusses the applications of ZnO nanorods in many felds, especially in feld release, emission properties, and electron transference. Last but not least, the appropriate conclusions for future research using ZnO nanorods have been successfully explained.

Metal Oxide Nanomaterials

This is a timely collection of recent diverse work on metal oxide nanomaterials, connecting their fundamental aspects and application perspectives in a concise fashion to give a broad view of the current status of this fascinating field. This book presents eight original research articles and two comprehensive reviews to highlight the recent development and understanding of different types of metal oxide nanoparticles and their use for applications in luminescence, photocatalysis, water–oil separation, optoelectronics, gas sensors, energy-saving smart windows, etc. It presents just the tip of the iceberg of the broad, dynamic, and active fundamental research and applications in the developing field of metal oxide nanomaterials by collecting a few examples of the latest advancements

Structural parameters (size, defect and doping) of ZnO nanostructures and relations with their optical and electrical properties

The performance and properties of the ZnO nanostructure-based devices (mainly including the wire-like and leaf-like structure) are highly dependent on the sizes, defects and doping of ZnO. Therefore, it is necessary to investigate these parameters in the ZnO nanostructure for optimizing its properties, hence providing the motivation of this PhD work. In this thesis, ultralong ZnO nanowires (NWs), indium (In)-doped leaf-like and needle-like ZnO nanostructures, which are fabricated via the chemical vapor deposition (CVD) process and hydrothermal method, have been investigated in details for the relationship of the sizes, defects and doping with their properties. Firstly, a number of analysis techniques are used to understand the correlation between the defects and sizes (diameter and length) of the ZnO NWs. The results show that the concentration of oxygen vacancies (Vo) jointly with zinc interstitials (Zni) defects is observed to be positively correlated with the increasing sizes of the NWs. Importantly, it is found that the variation of the field-enhancement factor (¦Â) of the ZnO NWs in field emission is highly dependent on the concentration of Vo with the length of the NWs. As compared with the ultralong and needle-like ZnO NWs, In-doped ZnO nanostructure has the lowest turn-on and threshold field as well as its relatively high ¦Â value. The reason is ascribed to the specific leaf-like morphology and in doping. Therefore, knowledge of the correlation and inter-relationship between the amount and type of native intrinsic defects or doping present in the NWs as their size varies is a crucial step towards optimizing and tuning the performances of ZnO nanostructure-based devices.Die Eigenschaften und Leistung von Gerätschaften, welche auf ZnO-Nanostrukturen basieren (vornehmlich drahtähnliche und blattähnliche) hängen im Wesentlichen von der Größe der Nanostrukturen, denen in ihnen auftretenden strukturellen Defekten sowie der Dotierung des ZnO ab. Daher ist es nötig diese Parameter in ZnO zu untersuchen um dessen Eigenschaften optimieren zu können, was somit auch die Motivation für diese Dissertationsschrift darstellt. In dieser Arbeit wurden Größen, Defekt- und Dotierungseffekte auf die Eigenschaften von ultralangen ZnO-Nanodrähten, In-dotierten blattähnlichen ZnO Strukturen sowie nadelförmigen ZnO-Nanostrukturen untersucht, welche mittels chemischer Gasphasenabscheidung (CVD) und einer hydrothermalen Abscheidungsmethode hergestellt wurden. Zunächst wurde eine Vielzahl von Analysetechniken angewendet um die Korrelation zwischen den auftretenden Defekten und der Größe, respektive dem Durchmesser und der Länge, der ZnO-Nanodrähte zu ermitteln. Die entsprechenden Resultate zeigen, dass eine steigende Konzentration von Sauerstoffleerstellen (Vo) in Kombination mit einer steigenden Konzentration von Zn Zwischengitterdefekten (Zni) für eine ansteigende Größe der Nanodrähte verantwortlich ist. Besonders erwähnenswert ist, dass die Variation des Feldverstärkungsfaktors (β) der ZnO-Nanodrähte bei Feldemission erheblich von der Konzentration der Sauerstoffleerstellen (Vo) in Kombination mit der Länge der Nanodrähte zusammenhängt. Im Vergleich mit den ultralangen und nadelförmigen ZnO-Nanodrähten, weisen die In-dotierten Nanostrukturen das niedrigste Anschalt- und Grenzfeld sowie den relativ höchsten Feldverstärkungsfaktor β auf. Der Grund hierfür wird der blattähnlichen Morphologie sowie der Dotierung zugesprochen. Daher ist das Wissen um die Korrelation zwischen der Menge und der Art von natürlichen intrinsischen Defektstrukturen sowie der Dotierung in den Nanodrähten mit sich ändernder Größe der Strukturen ein wichtiger Schritt in Richtung einer Optimierung und eines allgemeinen Tuningprozesses von Geräten, welche auf ZnO-Nanostrukturen basieren

Effect of surface and defect chemistry on the photo-catalytic properties of intentionally defect-rich ZnO nanorod arrays

Due to the abundance of intrinsic defects in zinc oxide (ZnO) the material properties are often governed by same. Knowledge of the defect chemistry has proven to be highly important, especially in terms of the photo-catalytic degradation of pollutants. Given the fact that defect-free materials or structures exhibiting only one type of defect are extremely difficult to produce, it is necessary to evaluate what influence various defects may have when present together in the material. In this study, intentionally defect-rich ZnO nanorod (NR) arrays are grown using a simple low-temperature solution-based growth technique. Upon changing the defect chemistry using rapid thermal annealing (RTA) the material properties are carefully assessed and correlated to the resulting photo-catalytic properties. Special focus is put on the investigation of these properties for samples showing strong orange photoluminescence (PL). It is shown that intense orange emitting NR arrays exhibit improved dye-degradation rates under UV-light irradiation. Furthermore strong dye-adsorption has been observed for some samples. This behavior is found to stem from a graphitic surface structure (e.g. shell) formed during RTA in vacuum. Since orange-luminescent samples also exhibit an enhancement of the dye-adsorption a possible interplay and synergy of these two defects is elucidated. Additionally, evidence is presented suggesting that in annealed ZnO NRs structural defects may be responsible for the often observed PL emission at 3.31 eV. However, a clear correlation with the photo-catalytic properties could not be established for these defects. Building on the specific findings presented here, this study also presents some more general guidelines which it is suggested, should be employed when assessing the photo-catalytic properties of defect-rich ZnO

Growth and Characterisation of ZnO Nanostructures: Excitonic Properties and Morphology

The growth mechanism of aligned ZnO nanostructures, grown by the vapour phase transport (VPT) growth method, specifically with nanorod and nanowall morphologies has been studied. The thesis begins with an introductory chapter on ZnO nanostructures and related topics, and the second chapter introduces the various experimental techniques used. The main thesis work involves five distinct studies. Firstly, the conditions to grow nanorod and nanorod/nanowall structures on sapphire using a ZnO/graphite powder mixture as a growth source are studied and the optimum conditions for each morphology identified. Secondly, the effects on ZnO nanostructure growth on sapphire of using activated carbon and carbon black powders, rather than graphite powder are studied. Nanostructures can be grown at significantly lower temperatures with carbon black and activated carbon, though with different morphologies, compared to graphite. Thirdly, low temperature cathodoluminescence spectroscopy measurements of ZnO nanostructures grown on Si substrate are presented. These data show significant inhomogeneity in the spatial distribution of emission throughout the sample for the Al-related donor bound exciton emission at 3.3605 eV and the Al-related emissions are compared to the other spectral features seen for these samples. The possible origin of this inhomogeneity is discussed. Fourthly, the microscopic origin of a unique photoluminescence peak at ~3.367 eV, which is known as surface exciton peak, has been studied in detail and its behaviour is studied after samples have been subjected to various post-growth treatments such as plasma treatment, UV exposure in vacuum and exposure to high voltages. Finally, post-growth passivation of nanostructures has been done using PVP and HF on ZnO nanostructure samples. The effects of these chemicals on the optical emission from these samples are studied and the potential for these to act as effective passivation agents is discussed. The thesis concludes with a summary of the work done, some general conclusions and comments on possible future directions