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Metal oxide thin films are of great interest in scientific advancement, particularly semiconductor thin films in transistors and in a wide range of optoelectronic applications. Many metal oxide thin films attract interest for their electronic bandgap, charge carrier mobility, optical opacity, luminescence, low cost, relative abundance and environmentally-friendly production. Additionally, these properties are often tuneable via particle size, film density, surface morphology, film deposition, growth method, hetero-interface engineering or ion-doping. Zinc oxide as a n-type semiconducting metal oxide is material of great interest owing to its intrinsically wide direct bandgap, high electron mobility, relatively high exciton binding energy, high optical transparency, demonstrated metal-ion doping optoelectronic effects, a range of different particle morphologies and deposition methods, photoluminescence ability, low cost and a variety of existing green synthesis methods. Here, these aspects of zinc oxide and some related oxides are reviewed, focusing on how the unique properties of this metal oxide make it suitable for a range of different applications from thin film transistors, high mobility oxide interfaces, transparent conductive oxides, photoanodes photodetectors, chemical sensors, photocatalysts, superlattice electronics and more. The properties and deposition methods and their impact on functionality will be discussed alongside their role in sustainable optoelectronics for future devices., Comment: 72 pages, 22 figures

The influence of thickness gradient and structural order on the spectral response of opal photonic crystals (PhCs) grown by evaporation-induced self-assembly (EISA) are presented. SEM imaging and angle resolved optical transmission spectroscopy are used to investigate the evolution of the PBG along a thickness gradient for opals grown from five different colloidal sphere concentrations at two different evaporation rates. The degradation of structural order along the thickness gradient is demonstrated, the occurrence of which attenuates the PBG with the thinning of the opal film and results in asymmetrical angle-resolved transmission spectra. The asymmetry in transmission intensity becomes more pronounced for opals grown from lower volume fractions, where secondary Bragg reflections also appear at low incident angles., Comment: 9 pages, 4 figures

Innovative new materials are consistently emerging as electrode candidates from lithium-ion battery research, promising high energy densities and high-rate capabilities. Understanding potential structural changes, morphology evolution, degradation mechanisms and side reactions during lithiation is important for designing, optimising and assessing aspiring electrode materials. In-situ and operando analysis techniques provide a means to investigate these material properties under realistic operating conditions. Here, we demonstrate an operando spectroscopic method using photonic crystal-structured electrodes that uses the optical transmission spectrum to monitor changes to the state of charge or discharge during lithiation and the change to electrode structure, in real-time. Photonic crystals possess a signature optical response, with a photonic bandgap (or stopband) presenting as a structural colour reflection from the material. We leverage the presence of this photonic stopband, alongside its intricate relationship to the electrode structure and material phase, to correlate electrode lithiation with changes to the optical spectrum during operation. In this work, we explore the optical and electrochemical behaviour of a TiO2 anode in a lithium-ion battery, structured as an inverse opal photonic crystal. In principle, the operando technique demonstrated here is versatile and applicable to a wide range of electrochemical electrode material candidates when structured with ordered porosity akin to a photonic crystal structure., Comment: 40 pages, 16 figures with Supplementary information

Opals are naturally occurring photonic crystals which can be formed easily using low-cost self-assembly methods. While the optical behaviour of opals has received significant attention over the last number of decades, there is limited information on the effect of crystal thickness on the optical properties they display. Here, the relationship between volume fraction and crystal thickness is established with an evaporation-induced self-assembly (EISA) method of formation. The extent to which thickness can be used to manipulate the optical properties of the crystals is explored, focusing on the change in the photonic band gap (PBG). Microscopical structural characterization and angle-resolved transmission spectroscopy are used to examine the quality of the photonic crystals formed using different volume fractions of polystyrene spheres, with thicknesses up to 37 layers grown from volume fractions of 0.125%. This work provides a direct correlation between sphere solution volume fraction and crystal thickness, and the associated optical fingerprint of opal photonic crystals. Maximum thickness is examined, which is shown to converge to a narrow range over several evaporation rates. We identify the criteria required to achieve thickness control in relatively fast evaporation induced self-assembly while maintaining structural quality, and the change to the spectroscopic signature to the (111) stopband and higher order (220) reflections, under conditions where a less ordered photonic crystals are formed., Comment: 29 pages with Supplementary materials, 9 figures and 1 table

Big pores, small pores, ordered pores, random pores, they all have a function and as is often found, show behaviour in new materials that is not always predicted or obvious at the outset. I started my research journey trying to put extremely thin films onto near-perfect III-V crystals to control (opto)electronic properties and when the first TEM on our campus showed remarkable pore growth and structure in InP almost 21 years ago, the electrochemical modification of the InP made more sense. In this paper, I will summarise a few aspects of research into porous materials and semiconductors, from porous InP that led to studies of other porous semiconductors such as silicon, GaN, ZnO and Indium Tin oxide (ITO), to periodically ordered photonic crystal porous structures and some optical, thermal and electrochemical properties, photocatalysis, studies in batteries and related that were enabled or modified by the porous structure., Comment: 22 pages, 14 figures

The ability to selectively redirect specific wavelengths of light has attracted a lot attention for photonic crystal materials. Presently, there is a wealth of research relating to the fabrication and application of photonic crystal materials. There a number of structures which fallinto the category of a photonic crystal; 1D, 2D and 3D ordered structures can qualify as a photonic crystal, provided there exists ordered repeating lattices of dielectric material with a sufficient refractive index contrast. The optical responses of these structures, namely the associated photonic bandgap or stopband, are of particular interest for any application involving light. The sensitivity of the photonic bandgap to changes in lattice size or refractive index composition creates the possibility foraccurate optical sensors. Optical phenomena involving reduced group velocity at wavelengths on the edge of the photonic bandgap are commonly exploited for photocatalytic applications. The inherent reflectivity of the photonic bandgap has created applications in optical waveguides or as solar cell reflector layers. There are countless examples of research attempting to exploit these facets of photonic crystal behaviorfor improved material design. Here, the role of photonic crystals is reviewed across a wide a variety of disciplines; cataloguing the ways in which these structures have enhanced specific applications. Particular emphasis is placed on providing an understanding of the specific function of the tunable optical response in photonic crystals in relation to their application., Comment: 96 pages, 30 figures

With the importance of Li-ion and emerging alternative batteries to our electric future, predicting new sustainable materials, electrolytes and complete cells that safely provide high performance, long life, energy dense capability is critically important. Understanding interface, microstructure of materials, the nature of electrolytes and factors that affect or limit long term performance are key to new battery chemistries, cell form factors and alternative materials. The electrochemical processes which cause these changes are also difficult to probe because of their metastability and lifetimes, which can be of nanosecond to sub nanosecond time domains. Consequently, developing and adapting high-resolution, non-destructive methods to capture these processes proves challenging, requiring state-of-the-art techniques.Recent progress is very promising, where optical spectroscopies, synchrotron radiation techniques, and energy-specific atom probe tomography and microscopy methods are just some of the approaches that are unravelling the true internal behaviour of battery cells in real-time.In this review, we overview many of the most promising non-destructive methods developed in recent years to assess battery material properties, interfaces, processes,and reactions under operando conditions in electrodes and full cells., Comment: 53 pages, 21 figures

One of the most appealing aspects of photonic crystal structures is the photonic bandgap created in structures with sufficiently high dielectric contrasts between constituent materials. Understanding how specific photonic crystal structures and their associated stopband positions can selectively interfere with incoming light is vital for implementing these structured dielectrics in a range of optical applications. Metallo-dielectric photonic crystals act to incorporate metal particles into the ordered arrangement of these structures. We examined copper, nickel and gold metal infiltration into polystyrene opals and TiO2 inverse opals. We report a consistent optical phenomena directly associated with the creation of metallo-dielectric photonic crystal structures. More pronounced and numerous diffraction resonances emerge in opal photonic crystals with a metal deposited across the top layer. Common to both opal and inverse opal structures, was a blue-shift in the position of the (111) photonic stopband which increased in magnitude with greater metal content in the structure. We investigate the origin of the photonic stopband blue-shift by variation of the metal content and the placement of metal in the photonic crystal structure. Metal introduced to structured dielectric media tunes the photonic stopband by altering the effective dielectric constant of the photonic crystal.

Adding porosity to battery electrodes is believed to be universally useful for adding space to accommodate volumetric expansion, electrolyte access to all active materials, helping to mitigate poor C-rate performance for thicker electrodes and for allowing infilling with other materials. Ordered porous electrode, such as inverse opals that have macroporosity, have been a model system: binder and conductive additive free, interconnected electrically, defined porosity and pore size with thickness, good electrolyte wettability and surprisingly good electrode performance in half cells and Li-battery cells at normal rates. We show that the intrinsic electronic conductivity is important, and at fast rates the intrinsic conductivity ultimately suppresses any charge storage in electrode materials. Using a model system of inverse opal V2O5in a flooded Li battery three-electrode cell, whose Li electrochemistry is very well understood, we show that beyond 10 C, electrodes can store almost no charge, but completely recover once reduced to < 1C. We show how the IO material is modified under lithiation using X-ray diffraction, Raman scattering and electron microscopy, and that little or no reaction occurs to the material at higher rates. We also use chronoamperometry to examine rate behaviour and link the limitations in high rate performance, and complete capacity suppression, to the intrinsic out-of-plane conductivity of the IO network. The data show that even idealized electrodes with nanoscale dimensions, functional porosity and full material interconnectivity, are fundamentally limited for high rate performance when they are less conductive even when fully soaked with electrolyte. While adding so-called functional size reduction, porosity etc. can be useful for some materials, these potential benefits are clearly not universally useful for high rate electrodes in Li-ion batteries., Comment: 30 pages, 10 figures

Conductive and paintable inks of 2D layered MoS2 with aspect ratio-dependent conductivity are demonstrated. Using ultrasonically assisted solvent-exfoliation of MoS2, high concentration 2D and few-layer suspensions become inks that provide coherent films when painted. Conductivity of paintable 2D MoS2 inks can be modulated by length and width, where the conductivity is linked to the painting direction. Reducing the painted film width, increases conductivity for similar length, and the films conductivity is aspect ratio-dependent. Inks of solvent-exfoliated 2D MoS2 can be painted without polymeric additives., Comment: 11 pages, 4 figures

Porous metallic structures are regularly used in electrochemical energy storage devices as supports, current collectors or active electrode materials. Bulk metal porosification, dealloying, welding or chemical synthesis routes involving crystal growth or self-assembly for example, can sometimes provide limited control of porous length scale, ordering, periodicity, reproducibility, porosity and surface area. Additive manufacturing and 3D printing has shown the potential to revolutionize the fabrication of architected metals many forms, allowing complex geometries not usually possible by traditional methods, but enabling complete design freedom of a porous metal based on the required physical or chemical property to be exploited. We discuss properties of porous metal structures in EES devices and provide some opinions on how architected metals may alleviate issues with electrochemically active porous metal current collectors, and provide opportunities for optimum design based on electrochemical characteristics required by batteries, supercapacitors or other electrochemical devices., Comment: 11 pages, 2 figures

Plasmonic photocatalysis has facilitated rapid progress in enhancing photocatalytic efficiency under visible light irradiation. Poor visible-light-responsive photocatalytic materials and low photocatalytic efficiency remain major challenges. Plasmonic metal-semiconductor heterostructures where both the metal and semiconductor are photosensitive are promising for light harvesting catalysis, as both components can absorb solar light. Efficiency of photon capture can be further improved by structuring the catalyst as a photonic crystal. Here we report the synthesis of photonic crystal plasmonic photocatalyst materials using Au nanoparticle-functionalized inverse opal (IO) photonic crystals. A catalyst prepared using a visible light responsive semiconductor (V2O5) displayed over an order of magnitude increase in reaction rate under green light excitation ($\lambda$=532 nm) compared to no illumination. The superior performance of Au-V2O5 IO was attributed to spectral overlap of the electronic band gap, localized surface plasmon resonance and incident light source. Comparing the photocatalytic performance of Au-V2O5 IO with a conventional Au-TiO2 IO catalyst, where the semiconductor band gap is in the UV, revealed that optimal photocatalytic activity is observed under different illumination conditions depending on the nature of the semiconductor. For the Au-TiO2 catalyst, despite coupling of the LSPR and excitation source at $\lambda$=532 nm, this was not as effective in enhancing photocatalytic activity compared to carrying out the reaction under broadband visible light, which is attributed to improved photon adsorption in the visible by the presence of a photonic band gap, and exploiting slow light in the photonic crystal to enhance photon absorption to create this synergistic type of photocatalyst., Comment: 19 pages, 8 figures, + Supporting information

Understanding the nature of light transmission and the photonic bandgap in inverse opal photonic crystals is essential for linking their optical characteristics to any application. This is especially important when these structures are examined in liquids or solvents. We examined TiO2 and SnO2 IOs in a range of common solvents to solve the conflict between Bragg-Snell theory, optical and physical measurements by a comprehensive angle-resolved light transmission study coupled to microscopy examination of the IO structure. Tuning the position of the photonic bandgap and index contrast by solvent infiltration of each inverse opal requires a modification to the Bragg-Snell theory and the photonic crystal unit cell definition. We also demonstrate experimentally and theroetically that low fill factors are caused by less desne material infilling all interstitial vancancies in the opal template to form an IO. By also including an optical interference condition for inverse opals with an effective refractive index greater than its substrate, and an alternative internal refraction angle in the substrate, angle-resolved transmission spectra for inverse opals are now consistent with physical measurements. This work now allows an accurate correlation between the true response of an IO to the index contrast with a solvent, how an IO is infilled, and the directionality and bandwidth of the photonic bandgap. As control in functional photonic materials becomes more prevalent outside of optics and photonics, such as biosensing and energy storage, for example, a comprehensive and consistent correlation between photonic crystals structures and their primary optical signatures is a fundamental requirement for application., Comment: 25 pages, 5 figures and full supplementary materials

Additive manufacturing and 3D printing in particular have the potential to revolutionize existing fabrication processes where objects with complex structures and shapes can be built with multifunctional material systems. For electrochemical energy storage devices such as batteries and supercapacitors, 3D printing methods allows alternative form factors to be conceived based on the end use application need in mind at the design stage. Additively manufactured energy storage devices require active materials and composites that are printable and this is influenced by performance requirements and the basic electrochemistry. The interplay between electrochemical response, stability, material type, object complexity and end use application are key to realising 3D printing for electrochemical energy storage. Here, we summarise recent advances and highlight the important role of methods, designs and material selection for energy storage devices made by 3D printing, which is general to the majority of methods in use currently., Comment: 11 pages, 2 figures, 1 table

Additive manufacturing has revolutionized the building of materials direct from design, allowing high resolution rapid prototyping in complex 3D designs with many materials. 3D printing hasenabled high strength damage-tolerant structures, bioprinted artificial organs and tissues, ultralight metals, medicine, education, prosthetics, architecture, consumer electronics,and as a prototyping tool for engineers and hobbyists alike. 3D printing has emerged as a useful tool for complex electrode and material assembly method for batteries and supercapacitors in recent years. The field initially grew from extrusion-based methods such as fused deposition modelling, and evolved to photopolymerization printing of intricate composites, while supercapacitor technologies less sensitive to solvents more often involved material jetting processes. Underpinning every part of a 3D printable battery and many other devices is the printing method and the nature of the feed material. Material purity, printing fidelity, accuracy, complexity, and the ability to form conductive, ceramic, glassy, or solvent-stable plastics relies on the nature of the feed material or composite to such an extent, that the future of 3D printable batteries and electrochemical energy storage devices will depend on materials and printing methods that are co-operatively informed by the requirements of the device and how it is fabricated. In this Perspective, we address the materials and methods requirements in 3D printable batteries and supercapacitors and outline requirements for the future of the field by linking existing performance limitations to the requirements of printable energy storage materials, casing materials and the direct printing of electrodes and electrolytes. We also look to the future by taking inspiration from additive manufacturing, to posit links between materials and printing methods to allow new form factor cells., Comment: 50 pages, 9 figures, 204 references

Energy Autonomous Wearable Sensors (EAWS) have attracted a large interest due to their potential to provide reliable measurements and continuous bioelectric signals, which help to reduce health risk factors early on, ongoing assessment for disease prevention, and maintaining optimum, lifelong health quality. This review paper presents recent developments and state-of-the-art research related to three critical elements that enable an EAWS. The first element is wearable sensors, which monitor human body physiological signals and activities. Emphasis is given on explaining different types of transduction mechanisms presented, and emerging materials and fabrication techniques. The second element is the flexible and wearable energy storage device to drive low-power electronics and the software needed for automatic detection of unstable physiological parameters. The third is the flexible and stretchable energy harvesting module to recharge batteries for continuous operation of wearable sensors. We conclude by discussing some of the technical challenges in realizing energy-autonomous wearable sensing technologies and possible solutions for overcoming them.

In this report, future performance demands of batteries for various vehicular applications are modeled. Vehicles ranging in size from electric bikes to heavy trucks are assessed using driving cycle data which allows key performance parameters such as desired range (km), specific energy of the battery (Wh/Kg), cycle life requirement and expected price per unit capacity (Euro/kWh) to be calculated. These projected performance requirements are compared with the outputs for three existing Li-ion batteries (namely (a) Kokam based high specific energy source (b) A123 based high power energy source and (c) Winston low cost system). The theoretical, current state of the art and projected performance parameters for 'beyond Li-ion' technologies (Li-S and Li-O2) are also compared to the modeled battery performance demands. The analysis indicates that current battery technologies are unlikely to meet future requirements in terms of required specific energies and will likely be too costly. In comparison, fully realized beyond Li-ion alternatives may deliver the required specific energy for the full range of vehicles examined. However, scale-up of these systems is a daunting challenge and their successful implementation will depend on improvements in terms of cycle life, electrode and electrolyte stability, rate performance and development of practical battery architectures., Comment: 14 pages, 4 figures, 9 tables