School of Physics - Research Publications
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ItemVacuum-Free Liquid-Metal-Printed 2D Semiconducting Tin Dioxide: The Effect of Annealing(American Chemical Society, 2024)Thin film transistors (TFTs) offer unparalleled opportunities for the fabrication of multifunctional electronic and optoelectronic devices. In this work, we report a vacuum-free liquid metal exfoliation technique for rapidly printing ∼2 nm-thick layer of oxide from molten tin. We explore the effect of rapid thermal annealing at 450 °C on the stoichiometry, morphology, and crystal structure of the resulting tin oxide nanosheets. The annealed samples exhibit a dominant SnO2 phase and a high degree of transparency (>99%) in the visible spectra. Field-effect transistors based on the two-dimensional (2D) SnO2 films show typical n-channel conduction with a field-effect mobility of ∼7.5 cm2 V–1 s–1. Photodetectors utilizing annealed tin dioxide demonstrate significant improvement in photoresponsivity reaching a value of 5.2 × 103 A W–1 compared to that found in an unannealed sample at an ultraviolet wavelength of 285 nm. We demonstrate that the improvement in device performance is due to nanocrystalline changes within the oxide layers during the annealing process. This work offers a straightforward and ambient air-compatible method for depositing ultrathin, large-area semiconducting oxides as potential candidates for enabling emerging applications in transparent nanoelectronics and optoelectronics.
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ItemNo Preview AvailableMeta-optics for new approaches to all-optical image processing(SPIE, 2023-08-12)Meta-optical devices have emerged as promising candidates for all-optical image processing. These devices are of subwavelength size and have the potential to address limitations of current image processing methods including processing speed, energy requirements as well as form factor. We present experimental results demonstrating the use of thin-film absorbers and optical metasurfaces to real-time detection of edges in images and the visualisation of phase objects including human cancer cells. Furthermore, we discuss progress towards the use of meta-optics for ultra-compact wavefront recovery. The findings to be presented have potential for applications in biological live-cell imaging, ultra-compact medical diagnostic tools, and wavefront correction methods.
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ItemHigh resolution bio-imaging via inverse design of metasurfaces(SPIE, 2024-03-08)Metasurfaces with angular sensitivity have been shown to provide a platform for developing an ultra-compact phase imaging system. Their performance, however, is often limited to a narrow range of spatial frequencies. Here, we apply inverse design to design and fabricate a metasurface an asymmetric optical transfer function across a numerical aperture (NA) of 0.6. The engineered response of this device enables phase imaging of microscopic transparent objects.
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ItemInstant-in-Air Liquid Metal Printed Ultrathin Tin Oxide for High-Performance Ammonia Sensors(Wiley, 2024)Liquid metal-based printing techniques are emerging as an exemplary platform for harvesting non-layered 2D materials with a thickness down to a few nanometres, leading to an ultra-large surface-area-to-volume ratio that is ideal for sensing applications. In this work, the synthesis of 2D tin dioxide (SnO2) by exfoliating the surface oxide of molten tin is reported which highlights the enhanced sensing capability of the obtained materials to ammonia (NH3) gas is reported. It is demonstrated that amperometric gas sensors based on liquid metal-derived 2D SnO2 nanosheets can achieve excellent NH3 sensing performance at low temperature (150 °C) with and without UV light assistance. Detection over a wide range of NH3 concentrations (5–500 ppm) is observed, revealing a limit of detection at the parts per billion (ppb) level. The 2D SnO2 nanosheets also feature excellent cross-interference performance toward different organic and inorganic gas species, showcasing a high selectivity. Further, ab initio DFT calculations reveal the NH3 adsorption mechanism is dominated by chemisorption with a charge transfer into 2D SnO2 nanosheets. In addition, a proof of concept for prototype flexible ammonia sensors is demonstrated by depositing 2D SnO2 on a polyimide substrate, signifying the high potential of employing liquid metal printed SnO2 for realizing wearable gas sensors.
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ItemNo Preview AvailableModelling structural colour from helicoidal multi-layer thin films with natural disorder(Optica Publishing Group, 2023-10-23)A coupled mode theory based on Takagi-Taupin equations describing electromagnetic scattering from distorted periodic arrays is applied to the problem of light scattering from beetles. We extend the method to include perturbations in the permittivity tensor to helicoidal arrays seen in many species of scarab beetle and optically anisotropic layered materials more generally. This extension permits analysis of typical dislocations arising from the biological assembly process and the presence of other structures in the elytra. We show that by extracting structural information from transmission electron microscopy data, including characteristic disorder parameters, good agreement with spectral specular and non-specular reflectance measurements is obtained.
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ItemNo Preview AvailableTunable nonlocal metasurfaces based on graphene for analogue optical computation(Optica Publishing Group, 2023-05-01)Meta-optical devices have recently emerged as ultra-compact candidates for real-time computation in the spatial domain. The use of meta-optics for applications in image processing and wavefront sensing could enable an order of magnitude increase in processing speed and data throughput, while simultaneously drastically reducing the footprint of currently available solutions to enable miniaturisation. Most research to date has focused on static devices that can perform a single operation. Dynamically tunable devices, however, offer increased versatility. Here we propose graphene covered subwavelength silicon carbide gratings as electrically tunable optical computation and image processing devices at mid-infrared wavelengths.
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ItemAn investigation of evanescent wave-induced fluorescence spectroscopy for exploring high refractive index media(IOP Publishing Ltd, 2023-01-01)Abstract Evanescent wave-induced fluorescence spectroscopy (EWIFS) is a widely used technique for probing the interfacial behavior of different complex media in investigations of samples in the physical, chemical, and biological sciences. This technique takes advantage of the sharply decaying evanescent field, established following total internal reflection (TIR) at the interface of two media, for spatially identifying the photoluminescence characteristics of the sample. The generation of the evanescent field requires the refractive index of the second medium to be lower than that of the first, so a major disadvantage of this increasingly widely used spectroscopic technique is the inability to exploit the advantages of EWIFS to image a sample with a higher refractive index than the incident substrate medium. A proposed configuration in which a thin, low refractive index intermediate layer is established between the TIR substrate and a high refractive index sample is investigated. We illustrate that this arrangement does not afford the desired advantages of evanescent field-induced fluorescence measurements for investigating high refractive index media.
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Item2-nm-Thick Indium Oxide Featuring High Mobility(WILEY, 2023-03)Abstract Thin film transistors (TFTs) are key components for the fabrication of electronic and optoelectronic devices, resulting in a push for the wider exploration of semiconducting materials and cost‐effective synthesis processes. In this report, a simple approach is proposed to achieve 2‐nm‐thick indium oxide nanosheets from liquid metal surfaces by employing a squeeze printing technique and thermal annealing at 250 °C in air. The resulting materials exhibit a high degree of transparency (>99 %) and an excellent electron mobility of ≈96 cm2 V−1 s−1, surpassing that of pristine printed 2D In2O3 and many other reported 2D semiconductors. UV‐detectors based on annealed 2D In2O3 also benefit from this process step, with the photoresponsivity reaching 5.2 × 104 and 9.4 × 103 A W−1 at the wavelengths of 285 and 365 nm, respectively. These values are an order of magnitude higher than for as‐synthesized 2D In2O3. Utilizing transmission electron microscopy with in situ annealing, it is demonstrated that the improvement in device performances is due to nanostructural changes within the oxide layers during annealing process. This work highlights a facile and ambient air compatible method for fabricating high‐quality semiconducting oxides, which will find application in emerging transparent electronics and optoelectronics.
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ItemThin film notch filters as platforms for biological image processing(NATURE PORTFOLIO, 2023-03-18)Many image processing operations involve the modification of the spatial frequency content of images. Here we demonstrate object-plane spatial frequency filtering utilizing the angular sensitivity of a commercial spectral bandstop filter. This approach to all-optical image processing is shown to generate real-time pseudo-3D images of transparent biological and other samples, such as human cervical cancer cells. This work demonstrates the potential of non-local, non-interferometric approaches to image processing for uses in label-free biological cell imaging and dynamical monitoring.
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ItemDeconstructed beetles: Bilayered composite materials produce green coloration with remarkably high near-infrared reflectance(ELSEVIER, 2023-06)Beetle elytra (hardened forewings) are a promising source of inspiration to develop or enhance the performance of human-fabricated composite materials. The structures responsible for optical properties in the ultra-violet to visible spectrum (300–700 nm) have been extensively characterised, but we have limited knowledge of optical properties and their physical origin in the near-infrared (NIR; 700–1700 nm). We examined the elytra of three species of green scarab beetles (Xylonichus eucalypti, Anoplognathus prasinus and Paraschizognathus olivaceus) with very high NIR reflectance. We manually separated layers in the elytra to disambiguate their contributions to the overall optical response. We show that unlike other scarabs, nanostructures within the cuticle layer do not produce notable reflectance. Instead, the cuticle resembles a pigment-based filter with 50% transmittance in the NIR and absorption in the visible spectrum contributing to the green appearance. Each species has a layer below the cuticle that appears white to the naked eye and produces broadband reflectance, particularly in the near-infrared; however, the structure of the white underlay differs markedly between the three species. In A. prasinus and P. olivaceus, the structure is disordered (no regular, repeated elements at optical length scales); whereas in Xylonichus eucalypti, the white underlay was notably thinner and comprised quasi-ordered hollow cylindrical structures embedded in a chitin matrix. We modelled the coherent scattering produced by this structure to demonstrate that it is responsible for broadband visible and NIR reflectance. We discuss biological implications and technological applications of the composite structure of beetle elytra.
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