Comparison of Analytical and Numerical Methods of Obtaining Coplanar Capacitance of Microelectrodes for Particulate Matter Detection

This work reports a comparative analysis between the analytical and the finite element modelling, for the calculation of the capacitance of microelectrode-based capacitors in both parallel and coplanar architecture. It shows that the inverse-cosine and the Schwartz-Christoffel methods underestimate the capacitance value of the coplanar plate capacitor. It also shows that the parallel architecture closed form solution and the finite element modelling results agree well in all cases. The work finally compares the effects of design parameter change of the microelectrodes on the capacitance obtained from each technique. It shows that finite element modelling provides the best estimate of the capacitance of a coplanar plate capacitor

Effects of post-deposition vacuum annealing on film characteristics of p-type Cu2_{2}O and its impact on thin film transistor characteristics

Annealing of cuprous oxide (Cu$_{2}$O) thin films in vacuum without phase conversion for subsequent inclusion as the channel layer in p-type thin film transistors (TFTs) has been demonstrated. This is based on a systematic study of vacuum annealing effects on the sputtered p-type Cu$_{2}$O as well as the performance of TFTs on the basis of the crystallographic, optical, and electrical characteristics. It was previously believed that high-temperature annealing of Cu$_{2}$O thin films would lead to phase conversion. In this work, it was observed that an increase in vacuum annealing temperature leads to an improvement in film crystallinity and a reduction in band tail states based on the X-ray diffraction patterns and a reduction in the Urbach tail, respectively. This gave rise to a considerable increase in the Hall mobility from 0.14 cm$^{2}$/V·s of an as-deposited film to 28 cm$^{2}$/V·s. It was also observed that intrinsic carrier density reduces significantly from 1.8 × 1016 to 1.7 × 10$^{13}$ cm$^{-3}$ as annealing temperature increases. It was found that the TFT performance enhanced significantly, resulting from the improvement in the film quality of the Cu$_{2}$O active layer: enhancement in the field-effect mobility and the on/off current ratio, and a reduction in the off-state current. Finally, the bottom-gate staggered p-type TFTs using Cu$_{2}$O annealed at 700 °C showed a field-effect mobility of ∼0.9 cm$^{2}$/V·s and an on/off current ratio of ∼3.4 × 102.This work was supported by the Engineering and Physical Sciences Research Council under Grant No. EP/M013650/1. G.R. acknowledges the support of the Cambridge Trusts

Single-step fabrication of thin-film linear variable bandpass filters based on metal-insulator-metal geometry

A single-step fabrication method is presented for ultra-thin, linearly variable optical bandpass filters (LVBFs) based on a metal–insulator–metal arrangement using modified evaporation deposition techniques. This alternate process methodology offers reduced complexity and cost in comparison to conventional techniques for fabricating LVBFs. We are able to achieve linear variation of insulator thickness across a sample, by adjusting the geometrical parameters of a typical physical vapor deposition process. We demonstrate LVBFs with spectral selectivity from 400 to 850 nm based on Ag (25 nm) and MgF$_{2}$ (75–250 nm). Maximum spectral transmittance is measured at ∼70% with a $\textit{Q}$-factor of ∼20.Engineering and Physical Sciences Research Council (EPSRC) (EP/L015455/1); Cambridge Commonwealth, European and International Trust

Nanostructured plasmonic metapixels

State-of-the-art pixels for high-resolution microdisplays utilize refective surfaces on top of electrical backplanes. Each pixel is a single fxed color and will usually only modulate the amplitude of light. With the rise of nanophotonics, a pixel’s relatively large surface area (~10μm$^2$), is in efect underutilized. Considering the unique optical phenomena associated with plasmonic nanostructures, the scope for use in refective pixel technology for increased functionality is vast. Yet in general, low refectance due to plasmonic losses, and sub-optimal design schemes, have limited the real-world application. Here we demonstrate the $\textit{plasmonic metapixel}$; which permits high refection capability whilst providing vivid, polarization switchable, wide color gamut fltering. Ultra-thin nanostructured metal-insulatormetal geometries result in the excitation of hybridized absorption modes across the visible spectrum. These modes include surface plasmons and quasi-guided modes, and by tailoring the absorption modes to exist either side of target wavelengths, we achieve pixels with polarization dependent multicolor refection on mirror-like surfaces. Because the target wavelength is not part of a plasmonic process, subtractive color fltering and mirror-like refection occurs. We demonstrate wide color-range pixels, RGB pixel designs, and in-plane Gaussian profle pixels that have the potential to enable new functionality beyond that of a conventional ‘square’ pixel.EPSRC Integrated Photonics & Electronic Systems Centre for Doctoral Training (Grant number: EP/L015455/1); Cambridge Commonwealth, European & International Trust

Self-assembled liquid crystalline nanotemplates and their incorporation in dye-sensitised solar cells

Liquid junction dye-sensitised solar cells (DSSCs) suffer from solvent evaporation and leakage which limit their large-scale production. Here, we have prepared DSSC using a simple and cheap fabrication process with improved photovoltaic parameters and stability. A binary mixture of Smectic A (SmA) and Nematic Liquid Crystal (NLC) was used to provide a self-assembled template for a polymerisable reactive mesogen LC. The layered structure of SmA combined with a low viscosity NLC forms a polygonal structure that provides an ordered and continuous template for reactive mesogens. Once the reactive mesogen is polymerised under UV light, the SmA:NLC mixture is washed away, resulting in a polymer network template containing nanochannels. We demonstrate the incorporation of these templates into DSSCs and find that DSSCs containing these nanochannels show improved open-circuit voltage (V$_{OC}$) (0.705 V) and short-circuit current (J$_{SC}$) (13.25 mA cm$^{-2}$) compared to that of the liquid electrolyte (V$_{OC}$ = 0.694 V and JSC = 10.46 mA cm$^{-2}$). The highest obtained power conversion efficiency with Sm-PE was 5.94% which is higher than that of the reference solar cell (5.51%). These can be attributed to the improved ionic conductivity and ionic diffusion of Sm-PE where the presence of the nanochannels aided the ionic conduction in the polymer electrolyte. In addition, it is hypothesized that the light scattering effect of the polymerised reactive mesogen also contributed to the improved performance of the photovoltaic devices. This finding is important because it is known fact that when a polymer is added to liquid electrolyte, the ionic conductivity will decrease although the stability is improved.A.A.K. and G.R. would like to thank the Cambridge Commonwealth Trust for financial support. A.A.K. would also like to thank the HEC (Pakistan) for financial support. C.W would like to thank EPSRC Integrated Photonics and Electronics Systems funding

Fabrication of nanostructured transmissive optical devices on ITO-glass with UV1116 photoresist using high-energy electron beam lithography

High-energy electron beam lithography for patterning nanostructures on insulating substrates can be challenging. For high resolution, conventional resists require large exposure doses and for reasonable throughput, using typical beam currents leads to charge dissipation problems. Here, we use UV1116 photoresist ($\textit{Dow Chemical Company}$), designed for photolithographic technologies, with a relatively low area dose at a standard operating current (80 kV, 40-50 μC cm$^{-2}$, 1 nAs$^{-1}$) to pattern over large areas on commercially coated ITO-glass cover slips. The minimum linewidth fabricated was ∼33 nm with 80 nm spacing; for isolated structures, ∼45 nm structural width with 50 nm separation. Due to the low beam dose, and nA current, throughput is high. This work highlights the use of UV1116 photoresist as an alternative to conventional e-beam resists on insulating substrates. To evaluate suitability, we fabricate a range of transmissive optical devices, that could find application for customized wire-grid polarisers and spectral filters for imaging, which operate based on the excitation of surface plasmon polaritons in nanosized geometries, with arrays encompassing areas ∼0.25 cm$^{2}$.EPSRC Integrated Photonics and Electronic Systems (Grant number: EP/L015455/1) Centre for Doctoral Training. Commonwealth, European and International trust

Experimental verification of electrostatic boundary conditions in gate-patterned quantum devices

In a model of a gate-patterned quantum device it is important to choose the correct electrostatic boundary conditions (BCs) in order to match experiment. In this study, we model gated-patterned devices in doped and undoped GaAs heterostructures for a variety of BCs. The best match is obtained for an unconstrained surface between the gates, with a dielectric region above it and a frozen layer of surface charge, together with a very deep back boundary. Experimentally, we find a 0.2V offset in pinch-off characteristics of one-dimensional channels in a doped heterostructure before and after etching off a ZnO overlayer, as predicted by the model. Also, we observe a clear quantised current driven by a surface acoustic wave through a lateral induced n-i-n junction in an undoped heterostructure. In the model, the ability to pump electrons in this type of device is highly sensitive to the back BC. Using the improved boundary conditions, it is straightforward to model quantum devices quite accurately using standard software

Nanofabrication of Conductive Metallic Structures on Elastomeric Materials.

Existing techniques for patterning metallic structures on elastomers are limited in terms of resolution, yield and scalability. The primary constraint is the incompatibility of their physical properties with conventional cleanroom techniques. We demonstrate a reliable fabrication strategy to transfer high resolution metallic structures of <500 nm in dimension on elastomers. The proposed method consists of producing a metallic pattern using conventional lithographic techniques on silicon coated with a thin sacrificial aluminium layer. Subsequent wet etching of the sacrificial layer releases the elastomer with the embedded metallic pattern. Using this method, a nano-resistor with minimum feature size of 400 nm is fabricated on polydimethylsiloxane (PDMS) and applied in gas sensing. Adsorption of solvents in the PDMS causes swelling and increases the device resistance, which therefore enables the detection of volatile organic compounds (VOCs). Sensitivity to chloroform and toluene vapor with a rapid response (~30 s) and recovery (~200 s) is demonstrated using this PDMS nano-resistor at room temperature

Gravimetric sensors operating at 1.1 GHz based on inclined c-axis ZnO grown on textured Al electrodes

Shear mode solidly mounted resonators (SMRs) are fabricated using an inclined c-axis ZnO grown on a rough Al electrode. The roughness of the Al surface is controlled by changing the substrate temperature during the deposition process to promote the growth of inclined ZnO microcrystals. The optimum substrate temperature to obtain homogeneously inclined c-axis grains in ZnO films is achieved by depositing Al at 100 °C with a surface roughness ~9.2 nm, which caused an inclination angle of ~25° of the ZnO c-axis with respect to the surface normal. Shear mode devices with quality-factors at resonance, Q r and effective electromechanical coupling factors, [Formula: see text], as high as 180 and 3.4% are respectively measured. Mass sensitivities, S m of (4.9 ± 0.1) kHz · cm(2)/ng and temperature coefficient of frequency (TCF) of ~-67 ppm/K are obtained using this shear mode. The performance of the devices as viscosity sensors and biosensors is demonstrated by determining the frequency shifts of water-ethanol mixtures and detection of Rabbit immunoglobin G (IgG) whole molecule (H&L) respectively.The research leading to these results has received funding from the European Community’s Horizon 2020 Programme under Grant Agreement No. SPIRE-01-2014-636820, the IC1208 Cost action, and from the Ministerio de Economía y Competitividad del Gobierno de España through the project MAT2013-45957-R. Financial support from these institutions is therefore gratefully acknowledged. G.R. also wishes to acknowledge funding from the Cambridge Commonwealth, European and International Trust

Design, Modeling and Simulation of a Capacitive Size-Discriminating Particulate Matter Sensor for Personal Air Quality Monitoring

We applied the well-established thermophoretic effect to air quality monitoring. We developed a novel method for particulate matter distribution analysis in an in-flow capacitive detection device. The proposed Multiphysics model combines fluid dynamics of particulate matter influenced by thermophoresis with electric field variations in the active volume space of a charged coplanar interdigitated electrodes. The model allows to anticipate the effect of thermophoresis in separating particles of PM10 and PM2.5 size ranges into different streams from a single particle-entrained flow and provides an estimated value of sensitivity for capacitive PM detection. The model is described through the Finite Element Method from the main equations to the simulation run using COMSOL Multiphysics and validated by comparing the results with literature. We obtained high sensor sensitivity of up to 0.48 zF/particle as far as $18~\mu \text{m}$ from coplanar electrode surface using the Computational Fluid Dynamics and Heat Transfer, Electrostatics and Particle tracing modules. We compare results of the simulations for different particle positions, electrode width and inter-electrode spacing, then we use the results to identify optimal design parameters for a novel architecture of a PM detection system with high sensitivity down to PM2.5 single particles and embedded particle size discrimination by using $10~\mu \text{m}$ electrode width and \mu \text{m}$ inter-electrode spacing