Fabrication and Characterisation of III-Nitride based Nanostructure Devices using Nanosphere Lithography Techniques

In this work, fabrication and characterisation of nanostructure devices has been performed on InGaN/GaN multiple quantum wells (MQW) grown on either c-plane sapphire or (111) silicon substrates. A cost effective nanosphere lithography technique has been employed for the fabrication of a number of nano structures such as nanorod arrays, nanoholes arrays; and single micro-disk lasers. Photonic crystal structures based on nanohole arrays have been designed and then fabricated on InGaN/GaN MQWs with an emission wavelength of 500 nm grown on c-plane sapphire by means of a nanosphere lithography technique, demonstrating a clear photonic crystal effect. Significant suppression of spontaneous emission has been observed when the emission is within the photonic bandgap. Angular dependent measurements show a change in the far-field pattern when the emission lies outside the photonic bandgap compared with the emission which lies inside the photonic bandgap. A coherent nanocavity a two-dimensional (2D) periodic array of nanodisks, was designed and fabricated on an InGaN/GaN MQW structure with an emission wavelength at 510 nm, leading to a significant enhancement in the internal quantum efficiency (IQE) as a result of enhanced spontaneous emission rate. Finite-difference time-domain (FDTD) analysis has performed for the structure design. The coherent nanocavity effect has been confirmed using means of time-resolved photoluminescence measurements, showing a clear enhancement in spontaneous emission rate. Finally, an improvement in IQE of 88 times at 510 nm has been achieved. Optically pumped green lasing has been achieved with thresholds as low as 1 kW/cm2, using an InGaN/GaN based micro-disk with an undercut structure on silicon substrates. The micro-disks with a diameter of around 1 μm were fabricated by means of a combination of a cost-effective silica micro-sphere approach, dry-etching and subsequent a wet-etching. The combination of these techniques both minimises the roughness of the sidewalls of the micro-disks and also produces excellent circular geometry. Utilizing this fabrication process, lasing has been achieved at room temperature under optical pumping from a continuous-wave laser diode. Time–resolved micro-photoluminescence (PL) and confocal PL measurements have been performed in order to further confirm the lasing action in whispering gallery modes and also investigate the excitonic recombination dynamics of the lasing

COVID-19 airborne transmission risk calculation using CO2 concentrations in a 3D office environment

The ongoing COVID-19 pandemic has caused millions of deaths worldwide along with detrimental socioeconomic consequences. Existing evidence suggests that the rate of indoor transmission is directly linked with the Indoor Air Quality (IAQ) conditions. Most of the existing methodologies for virus transmissibility risk estimation are based on the well-known Wells-Riley equation and assume well-mixed, uniform conditions; so spatiotemporal variations within the indoor space are not captured. In this work, a novel fine-grained methodology for real-time virus transmission risk estimation is developed using a 3D model of a real office room with 31 occupants. CONTAM-CFD0 software is used to compute the airflow vectors and the resulting 3D CO2 concentration map (attributed to the exhalations from the occupants). Simulation results are also provided that demonstrate the efficacy of using CO2 sensors for estimating the infection risk in real-time in the 3D office environment

Dataset for "Optical properties and resonant cavity modes in axial InGaN/GaN nanotube microcavities"

This dataset contains scanning electron microscopy (SEM) images, Catodoluminescence (CL), MicroPL and Confocal PL measurements and Finite-Difference Time-Domain (FDTD) simulations carried out on InGaN/GaN nanotube microcavities. The samples were fabricated via a combination of Displacement Talbot Lithography for patterning and inductively coupled plasma top-down dry-etching. SEM imaging were used to assess first, the patterning of SiNx mask, and second, the InGaN/GaN nanotube morphology and dimensions. CL were used to assess the optical properties of individual InGaN/GaN nanotube. MicroPL and Confocal PL were used to carry out continuous excitation at room temperature of isolated InGaN/GaN nanotube. FDTD simulations were used to investigate the nature of the resonant modes. Correlation between PL techniques and FDTD simulation suggests that both mixed whispering gallery – Fabry-Perot cavity modes are observed within the single nanotube.Secondary electron images were captured using a Hitachi S-4300 scanning electron microscope (SEM). An accelerating voltage of 5 kV was used to collect the images. Cathodoluminescence hyperspectral imaging measurements were carried out at room temperature in a modified FEI Quanta 250 field emission SEM using electron energies of 5.0 keV and beam currents of approximately 1 nA. Light was collected using an NA 0.28 reflecting objective with its axis perpendicular to the electron beam and focused directly on the entrance of the spectrograph using an off-axis paraboloidal mirror. We used a 125 mm focal length spectrograph with a 600 lines/mm grating and 50 μm entrance slit, coupled to a cooled electron multiplying charge-coupled device (EMCCD) detector. µPL characterization was carried out on single nanotube cavities in an in-house made micro-PL system. A 375 nm continuous wave (CW) laser diode was used to selectively excite the InGaN/GaN SQW. A 50x magnification, 0.43 NA objective was used to focus the excitation laser down to ~ 2μm and collect the emission through a 1 mm fibre bundle. A 0.55 m Jobin Yvon spectrometer (iHR550) was used to disperse the emission and the emission was detected with an air-cooled charge coupled device (CCD). The system was equipped with X-Y-Z motorized stages allowing to selectively excite single nanotubes. All the measurements were performed at room temperature. Confocal PL characterization was performed using a commercial WITec confocal microscope. A 375 nm continuous wave (CW) diode laser was used as an optical pumping source and the system was equipped with a 300 mm Princeton instruments monochromator (SP2300i) and an air-cooled charge coupled device (CCD). An objective lens (100×, NA=0.95) was used to focus the laser beam down to a spot with a diameter of ~400 nm. The emission was dispersed by the monochromator with a resolution of ~0.1 nm. An optical fiber with a diameter of 10 µm acted as a pinhole, thus allowing the emission to be collected from only where the sample was excited. The system was equipped with a high-resolution x-y-z piezo-stage to individually address and examine single nanotubes. The spatial resolution of the system was ~ 160 nm. All the measurements were performed at room temperature. A three-dimensional finite difference time domain (FDTD) approach has been used to simulate the nanotube structure using commercial-grade software [Lumerical Solutions, Inc. http://www.lumerical.com/tcad-products/fdtd/]. A dipole source with emission wavelengths from 370 to 720 nm was vertically positioned in the plane of the nanotube quantum wells and centred within the width of the walls. All the geometrical data used for the simulation are from the SEM measurement of our nanotube. The n-GaN, p-GaN and GaN-substrate layers are given a refractive index defined by ellipsometry measurements performed on the as-grown structure at the University of Strathclyde. The InGaN quantum well layer is 3nm thick (n=2.6). There is a minimum of fifteen mesh cells per wavelength in the simulation that is run for 2000fs. Frequency-domain power monitors have been used to record the emission profile over the simulation region, which is surrounded by perfectly matched layer absorbing boundaries on all sides. The simulated spectra are collected by a grid of 12 time-monitors placed at various points inside the nanotube structure, accurately calculating the envelope of the time-domain field signal. For the perfectly-circular higher-Q examples, each resonant peak is isolated in the frequency domain using a Gaussian filter, and by taking the inverse Fourier transforms the time decay have been calculated separately for each peak. The slope of the time decay is used to calculate the Q-factor of each resonance. In the DTL fabricated structures, the electromagnetic fields decay completely from the simulation in a time that can be simulated reasonably by FDTD. Therefore, the resonant frequencies are found from the time-monitors along with the full-width half-maximum (FWHM) of the corresponding peaks. We can then use Q=fR/Δf where fR is the resonant frequency and Δf is the FWHM.The nanotube investigated by µPL and Confocal PL were mechanically removed from the substrate in an ultrasonic bath and then dispersed on a host sapphire substrat

Flexible, Free-Standing Polymer Membranes Sensitized by CsPbX3 Nanocrystals as Gain Media for Low Threshold, Multicolor Light Amplification

Lead halide perovskite nanocrystals (NCs) are highly suitable active media for solution-processed lasers in the visible spectrum, owing to the wide tunability of their emission from blue to red via facile ion-exchange reactions. Their outstanding optical gain properties and the suppressed nonradiative recombination losses stem from their defect-tolerant nature. In this work, we demonstrate flexible waveguides combining the transparent, bioplastic, polymer cellulose acetate with green CsPbBr3 or red-emitting CsPb(Br,I)3 NCs in simple solution-processed architectures based on polymer-NC multilayers deposited on polymer micro-slabs. Experiments and simulations indicate that the employment of the thin, free-standing membranes results in confined electrical fields, enhanced by 2 orders of magnitude compared to identical multilayer stacks deposited on conventional, rigid quartz substrates. As a result, the polymer structures exhibit improved amplified emission characteristics under nanosecond excitation, with amplified spontaneous emission (ASE) thresholds down to ∼95 μJ cm–2 and ∼70 μJ cm–2 and high net modal gain up to ∼450 and ∼630 cm–1 in the green and red parts of the spectrum, respectively. The optimized gain properties are accompanied by a notable improvement of the ASE operational stability due to the low thermal resistance of the substrate-less membranes and the intimate thermal contact between the polymer and the NCs. Their application potential is further highlighted by the membrane’s ability to sustain dual-color ASE in the green and red parts of the spectrum through excitation by a single UV source, activate underwater stimulated emission, and operate as efficient white light downconverters of commercial blue LEDs, producing high-quality white light emission, 115% of the NTSC color gamut.ISSN:2330-402

Antimony-Doped Tin Oxide Hole Injection Interlayer Improving the Efficiency of Perovskite Nanocrystal Light Emitting Diodes

We present significant performance enhancement of CsPbBr₃ perovskite nanocrystal (NC) light emitting diodes (PNC LEDs) by incorporation of a solution processed doped metal oxide hole injection interlayer consisting of 10 atom % doped antimony tin oxide (ATO) within the PNC LEDs device architecture. The incorporation of an ATO interlayer between ITO and poly-TPD improves the bottom electrode hole injection properties and provides charge balanced PNC LEDs that show three and half times increased luminance, lower turn-on voltage, and improved maximum current and power efficiency compared to reference CsPbBr₃ PNC LEDs incorporating the commonly used PEDOT:PSS hole injection interlayer.ISSN:2771-985

Surface Functionalization of CsPbBr3 Νanocrystals for Photonic Applications

The primary obstacle to the use of lead halide perovskite nanocrystals (NCs) in optoelectronics is the inability of traditional ligand engineering approaches to provide robust surface passivation. The structural lability can be mitigated by employing different ligands such as long-chain quaternary ammonium and zwitterionic surfactants. Here, we report a comprehensive study that probes the impact of such surface passivation routes on the optoelectronic properties of weakly confined CsPbBr3 NCs. Spectroscopy unravels clear correlations of various photophysical figures of merit with the ligand type used. Compared to NCs decorated by conventional oleic acid/oleylamine ligands, passivation with the quaternary ammonium or zwitterionic surfactants increases the NC solid-state emission yield by up to 40% by halving the average trap depth and increasing by 1.5 times the exciton binding energy. Furthermore, the aforementioned ligands better preserve the size of NCs in thin films, as shown by the absence of significant NC aggregation and the confinement-induced increase by a factor of 2 of the Fröhlich interaction between excitons and optical phonons. The suitability of ligands for photonics is finally assessed by probing metrics, such as the amplified spontaneous emission threshold, the moisture tolerance, and the photoconductivity and electroluminescent performance of lateral and vertical devices, respectively

Dataset for "Optical properties and resonant cavity modes in axial InGaN/GaN nanotube microcavities"

This dataset contains scanning electron microscopy (SEM) images, Catodoluminescence (CL), MicroPL and Confocal PL measurements and Finite-Difference Time-Domain (FDTD) simulations carried out on InGaN/GaN nanotube microcavities. The samples were fabricated via a combination of Displacement Talbot Lithography for patterning and inductively coupled plasma top-down dry-etching. SEM imaging were used to assess first, the patterning of SiNx mask, and second, the InGaN/GaN nanotube morphology and dimensions. CL were used to assess the optical properties of individual InGaN/GaN nanotube. MicroPL and Confocal PL were used to carry out continuous excitation at room temperature of isolated InGaN/GaN nanotube. FDTD simulations were used to investigate the nature of the resonant modes. Correlation between PL techniques and FDTD simulation suggests that both mixed whispering gallery – Fabry-Perot cavity modes are observed within the single nanotube

Dataset for "Optical properties and resonant cavity modes in axial InGaN/GaN nanotube microcavities"

This dataset contains scanning electron microscopy (SEM) images, Catodoluminescence (CL), MicroPL and Confocal PL measurements and Finite-Difference Time-Domain (FDTD) simulations carried out on InGaN/GaN nanotube microcavities. The samples were fabricated via a combination of Displacement Talbot Lithography for patterning and inductively coupled plasma top-down dry-etching. SEM imaging were used to assess first, the patterning of SiNx mask, and second, the InGaN/GaN nanotube morphology and dimensions. CL were used to assess the optical properties of individual InGaN/GaN nanotube. MicroPL and Confocal PL were used to carry out continuous excitation at room temperature of isolated InGaN/GaN nanotube. FDTD simulations were used to investigate the nature of the resonant modes. Correlation between PL techniques and FDTD simulation suggests that both mixed whispering gallery – Fabry-Perot cavity modes are observed within the single nanotube

Dataset for "Optical properties and resonant cavity modes in axial InGaN/GaN nanotube microcavities"

This dataset contains scanning electron microscopy (SEM) images, Catodoluminescence (CL), MicroPL and Confocal PL measurements and Finite-Difference Time-Domain (FDTD) simulations carried out on InGaN/GaN nanotube microcavities. The samples were fabricated via a combination of Displacement Talbot Lithography for patterning and inductively coupled plasma top-down dry-etching. SEM imaging were used to assess first, the patterning of SiNx mask, and second, the InGaN/GaN nanotube morphology and dimensions. CL were used to assess the optical properties of individual InGaN/GaN nanotube. MicroPL and Confocal PL were used to carry out continuous excitation at room temperature of isolated InGaN/GaN nanotube. FDTD simulations were used to investigate the nature of the resonant modes. Correlation between PL techniques and FDTD simulation suggests that both mixed whispering gallery – Fabry-Perot cavity modes are observed within the single nanotube

All-Perovskite Multicomponent Nanocrystal Superlattices

Nanocrystal superlattices (NC SLs) have long been sought as promising metamaterials, with nanoscale-engineered properties arising from collective and synergistic effects among the constituent building blocks. Lead halide perovskite (LHP) NCs come across as outstanding candidates for SL design, as they demonstrate collective light emission, known as superfluorescence, in single- and multicomponent SLs. Thus far, LHP NCs have only been assembled in single-component SLs or coassembled with dielectric NC building blocks acting solely as spacers between luminescent NCs. Here, we report the formation of multicomponent LHP NC-only SLs, i.e., using only CsPbBr₃ NCs of different sizes as building blocks. The structural diversity of the obtained SLs encompasses the ABO(₆), ABO(₃), and NaCl structure types, all of which contain orientationally and positionally locked NCs. For the selected model system, the ABO(₆)-type SL, we observed efficient NC coupling and F & ouml;rster-like energy transfer from strongly confined 5.3 nm CsPbBr₃ NCs to weakly confined 17.6 nm CsPbBr₃ NCs, along with characteristic superfluorescence features at cryogenic temperatures. Spatiotemporal exciton dynamics measurements reveal that binary SLs exhibit enhanced exciton diffusivity compared to single-component NC assemblies across the entire temperature range (from 5 to 298 K). The observed coherent and incoherent NC coupling and controllable excitonic transport within the solid NC SLs hold promise for applications in quantum optoelectronic devices.ISSN:1936-0851ISSN:1936-086