Graphene Photonics and Optoelectronics

The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential to be in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultra-wide-band tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light emitting devices, to touch screens, photodetectors and ultrafast lasers. Here we review the state of the art in this emerging field.Comment: Review Nature Photonics, in pres

Nanostructured Metal Oxide-Based Acetone Gas Sensors: A Review.

Acetone is a well-known volatile organic compound that is widely used in different industrial and domestic areas. However, it can have dangerous effects on human life and health. Thus, the realization of sensitive and selective sensors for recognition of acetone is highly important. Among different gas sensors, resistive gas sensors based on nanostructured metal oxide with high surface area, have been widely reported for successful detection of acetone gas, owing to their high sensitivity, fast dynamics, high stability, and low price. Herein, we discuss different aspects of metal oxide-based acetone gas sensors in pristine, composite, doped, and noble metal functionalized forms. Gas sensing mechanisms are also discussed. This review is an informative document for those who are working in the field of gas sensors

High Photoelectric Conversion Efficiency of Metal Phthalocyanine/Fullerene Heterojunction Photovoltaic Device

This paper introduces the fundamental physical characteristics of organic photovoltaic (OPV) devices. Photoelectric conversion efficiency is crucial to the evaluation of quality in OPV devices, and enhancing efficiency has been spurring on researchers to seek alternatives to this problem. In this paper, we focus on organic photovoltaic (OPV) devices and review several approaches to enhance the energy conversion efficiency of small molecular heterojunction OPV devices based on an optimal metal-phthalocyanine/fullerene (C60) planar heterojunction thin film structure. For the sake of discussion, these mechanisms have been divided into electrical and optical sections: (1) Electrical: Modification on electrodes or active regions to benefit carrier injection, charge transport and exciton dissociation; (2) Optical: Optional architectures or infilling to promote photon confinement and enhance absorption

Continuous Hydrothermal Flow Synthesis of Optimised Transparent Conducting Oxide Nanoparticles and Thin Films

This thesis focuses on the synthesis of a variety of different transparent conducting oxide (TCO) nanomaterials using a continuous hydrothermal flow synthesis process, wherein aqueous solutions of chemical precursors were mixed with heated, pressurised water to facilitate nanoparticle formation. In Chapter 3, a screening investigation was carried out by doping zinc oxide with a number of different elements in order to highlight the most promising systems with regards to electronic conductivity. Of the twenty-four materials tested, zinc oxides doped with aluminium (AZO), gallium (GZO), and silicon (SiZO), Chapters 4 and 5, respectively, were selected for compositional optimisation and further testing. Aluminium and gallium doping and co-doping (AGZO) optimisation resulted in materials of similar conductivity to indium tin oxide (ITO), the industry standard TCO material. Upon completion of compositional optimisation, ITO and AGZO were synthesised with a citrate coating added in-process (Chapter 6). This aided in the dispersion of the nanoparticles for deposition into thin films by inkjet printing and spin coating; the latter was also carried out with un-coated GZO, AGZO, and SiZO. Preliminary inkjet printed films demonstrated very high conductivity (ITO) or very high transparency (AGZO), but never both in the same film, indicating the promise of the deposition method while requiring further investigation to be carried out. The spin coated films of all four materials were highly transparent and conductive, competitive with the best performing materials so far reported in literature. The AGZO spin coated films in particular, were the most conductive ever reported, superior even to those deposited by the sputtering methods currently used in industry

Transition metal oxides - Thermoelectric properties

Transition metal oxides (TMOs) are a fascinating class of materials due to their wide ranging electronic, chemical and mechanical properties. Additionally, they are gaining increasing attention for their thermoelectric (TE) properties due to their high temperature stability, tunable electronic and phonon transport properties and well established synthesis techniques. In this article, we review TE TMOs at cryogenic, ambient and high temperatures. An overview of strategies used for morphological, compositing and stoichiometric tuning of their key TE parameters is presented. This article also provides an outlook on the current and future prospects of implementing TMOs for a wide range of TE applications

Applications of MXenes in human-like sensors and actuators

Human beings perceive the world through the senses of sight, hearing, smell, taste, touch, space, and balance. The first five senses are prerequisites for people to live. The sensing organs upload information to the nervous systems, including the brain, for interpreting the surrounding environment. Then, the brain sends commands to muscles reflexively to react to stimuli, including light, gas, chemicals, sound, and pressure. MXene, as an emerging two-dimensional material, has been intensively adopted in the applications of various sensors and actuators. In this review, we update the sensors to mimic five primary senses and actuators for stimulating muscles, which employ MXene-based film, membrane, and composite with other functional materials. First, a brief introduction is delivered for the structure, properties, and synthesis methods of MXenes. Then, we feed the readers the recent reports on the MXene-derived image sensors as artificial retinas, gas sensors, chemical biosensors, acoustic devices, and tactile sensors for electronic skin. Besides, the actuators of MXene-based composite are introduced. Eventually, future opportunities are given to MXene research based on the requirements of artificial intelligence and humanoid robot, which may induce prospects in accompanying healthcare and biomedical engineering applications. [Figure not available: see fulltext.

Zn-VI/Cu2O Heterojunctions for Earth-Abundant Photovoltaics

The need for sustainable energy production motivates the study of photovoltaic materials, which convert energy from sunlight directly into electricity. This work has focused on the development of Cu2O as an earth-abundant solar absorber due to the abundance of its constituent elements in the earth's crust, its suitable band gap, and its potential for low cost processing. Crystalline wafers of Cu2O with minority carrier diffusion lengths on the order of microns can be manufactured in a uniquely simple fashion — directly from copper foils by thermal oxidation. Furthermore, Cu2O has an optical band gap of 1.9 eV, which gives it a detailed balance energy conversion efficiency of 24.7% and the possibility for an independently connected Si/Cu2O dual junction with a detailed balance efficiency of 44.3%. However, the highest energy conversion efficiency achieved in a photovoltaic device with a Cu2O absorber layer is currently only 5.38% despite the favorable optical and electronic properties listed above. There are several challenges to making a Cu2O photovoltaic device, including an inability to dope the material, its relatively low chemical stability compared to other oxides, and a lack of suitable heterojunction partners due to an unusually small electron affinity. We have addressed the low chemical stability, namely the fact that Cu2O is an especially reactive oxide due to its low enthalpy of formation (ΔHf0 = -168.7 kJ/mol), by developing a novel surface preparation technique. We have addressed the lack of suitable heterojunction partners by investigating the heterojunction band alignment of several Zn-VI materials with Cu2O. Finally, We have addressed the typically high series resistance of Cu2O wafers by developing methods to make very thin, bulk Cu2O, including devices on Cu2O wafers as thin as 20 microns. Using these methods we have been able to achieve photovoltages over 1 V, and have demonstrated the potential of a new heterojunction material, Zn(O,S).</p

Processing, microstructure, properties and performance of thermal sprayed ceramic coatings from powder, suspension and solution precursor

Ceramic coatings are applied in many sectors, ranging from functional coatings to high temperature coatings for aerospace engines. Typically, the deposition technique chosen represents a compromise between the required properties and the economic restrains. Over the years, thermal spraying has proven itself as a reliable and cost-efficient method for the deposition of a multitude of compositions. For instance, thermal sprayed ceramic coatings are essential for the current and future generations of aerospace engines. Nickel-based superalloys are typically coated with thermal sprayed thermal barrier coatings (TBCs). Even the potential replacement, ceramic matrix composites (CMCs), require thermal sprayed environmental barrier coatings (EBCs) to protect them. In this context, this Ph.D. thesis is framed, working with the study of thermal sprayed coatings from powder, suspension and solution precursor. The introduction of liquid feedstocks was developed to overcome the limitations presented by powder, such as flowability with sub-micron particles, and the complexity of introducing doping elements. This thesis first presents a thorough study of the use of a novel solution precursor feedstock to produce coatings where the composition can be modified to produce functional coatings with improved properties. Taking niobium doped TiO2 as an example material, homogenously doped coatings were produced, proving the capabilities of this solution precursor route. In addition to the chemical modification, porosity was controlled through the spraying parameters, showing that microstructure can be easily tailored to the final application. In order to provide a comprehensive view accounting for the differences observed, a model detailing the evolution of the liquid feedstock as it turns into solid material during its transformation in-flight was presented. The importance of porosity, already realised in the previous stage, is the centre of the next step in the research. Conventional powder feedstock tends to produce coatings with porosity with a minimum range of tens of microns. However, this limitation is not present when suspension thermal spraying is used. To properly study this new range of pores, techniques such as image analysis are ill suited. Instead, this thesis proposes the use of more advanced techniques, such as neutron scattering techniques performed at large neutron facilities. Using yttria-stabilised zirconia, a standard TBC (where porosity has severe implications on lifetime and thermal conductivity), neutron scattering was proven as a powerful, non-destructive technique capable of accessing pores with a size of 1 nm. As a proof of concept, the evolution of porosity during heat treatment was studied and a detailed evolution study was conducted. Finally, the work in this thesis focuses on the performance of EBCs under simulated corrosive environments. As these protective coatings are expected to limit the ingress of corrosive species into the CMC substrates, porosity again plays a key role. In order to investigate this scenario, low and high porosity content EBCs were exposed to molten calcium magnesium alumino-silicates (CMAS) and superheated steam (1350 °C and 1400 °C) to assess their performance. The results show that both the low and high porosity EBCs behaved similarly, particularly during longer exposures (48 h), where inter-splat boundaries become the preferential path for access, instead of the porosity, as suggested by the literature review here presented. No evidence of failure could be detected in any of the EBCs, presenting a promising result for the development of abradable environmental barrier coatings. The work presented in this thesis represents the foundation for further research into solution precursor and suspension thermal sprayed EBCs, with both composition and porosity levels easily tailored to the end application. The new framework here discussed will ensure adequate measurement of the porosity in such coatings, accounting for sub-micron pores if using a suitable technique. Finally, these novel EBCs would require performance assessment under simulated service conditions. CMC coated samples, including high porosity abradable EBCs, would represent the natural progression of this research