Ten It Wong

Nanocrystal engineering of sputter grown CuO photocathode for visible light driven electrochemical water splitting

Cupric oxide (CuO) thin film was sputtered onto fluorine-doped tin oxide (FTO) coated glass substrate and incorporated into a photoelectrochemical (PEC) cell as a photocathode. Through in situ nanocrystal engineering, sputtered CuO film shows an improvement in its stability and photocurrent generation capability. For the same CuO film thickness (150 nm), films deposited at a sputtering power of 300 W exhibit a photocurrent of ∼0.92 mAcm(-2) (0 V vs RHE), which is significantly higher than those deposited at 30 W (∼0.58 mAcm(-2)). By increasing the film thickness to 500 nm, the photocurrent is further enhanced to 2.5 mAcm(-2), which represents a photocurrent conversion efficiency of 3.1%. Systematic characterization using Raman, XRD, and HR-TEM reveals that the high sputtering power results in an improvement in CuO film crystallinity, which enhances its charge transport property and, hence, its photocurrent generation capabilities.

Rapid thermal annealing assisted stability and efficiency enhancement in a sputter deposited CuO photocathode

We designed a stable and efficient CuO based photocathode by tuning the crystallinity and surface morphology of films by rapid thermal treatment. The role of the annealing temperature on film crystallinity, optical absorption and grain size is studied. The impact of these parameters upon the photocatalytic water splitting performance of CuO films is investigated. We observed that a higher annealing temperature improves the film crystallinity and increases the grain size of CuO film, which significantly enhance the photocurrent generation capability. Rapid thermal annealing at 550 °C is found the best temperature to achieve the highest PEC performance. The thickness of the CuO photocathodes is also optimized and we observed that 550 nm thick films results in the highest photocurrent of 1.68 mA cm−2. Our optimized CuO photocathode has shown better stability against photo-corrosion and a 30% decrease in the initial value of photocurrent is measured after 15 min, while a 60% decrease in the photocurrent is noticed in case of the as-deposited film.

Color tunable low cost transparent heat reflector using copper and titanium oxide for energy saving application.

Multilayer coating structure comprising a copper (Cu) layer sandwiched between titanium dioxide (TiO2) were demonstrated as a transparent heat reflecting (THR) coating on glass for energy-saving window application. The main highlight is the utilization of Cu, a low-cost material, in-lieu of silver which is widely used in current commercial heat reflecting coating on glass. Color tunable transparent heat reflecting coating was realized through the design of multilayer structure and process optimization. The impact of thermal treatment on the overall performance of sputter deposited TiO2/Cu/TiO2 multilayer thin film on glass substrate is investigated in detail. Significant enhancement of transmittance in the visible range and reflectance in the infra-red (IR) region has been observed after thermal treatment of TiO2/Cu/TiO2 multilayer thin film at 500 °C due to the improvement of crystal quality of TiO2. Highest visible transmittance of 90% and IR reflectance of 85% at a wavelength of 1200 nm are demonstrated for the TiO2/Cu/TiO2 multilayer thin film after annealing at 500 °C. Performance of TiO2/Cu/TiO2 heat reflector coating decreases after thermal treatment at 600 °C. The wear performance of the TiO2/Cu/TiO2 multilayer structure has been evaluated through scratch hardness test. The present work shows promising characteristics of Cu-based THR coating for energy-saving building industry.

Optical bandgap widening and phase transformation of nitrogen doped cupric oxide

The structural and optical properties of sputter deposited nitrogen (N) doped CuO (CuO(N)) thin films are systematically investigated. It is found that the incorporation of N into CuO causes an enlargement of optical bandgap and reduction in resistivity of the CuO(N) films. Furthermore, a gradual phase transformation from CuO to Cu2O is observed with the increase in N concentration. The effects of annealing temperature on the structural properties of CuO (N) and its dependence on N concentration are also investigated. It is observed that the phase transformation process from CuO to Cu2O significantly depends on the N concentration and the annealing temperature. Heterojunction solar cells of p-type CuO(N) on n-type silicon (Si) substrate, p-CuO(N)/n-Si, are fabricated to investigate the impact of N doping on its photovoltaic properties.

In situ codoping of a CuO absorber layer with aluminum and titanium: the impact of codoping and interface engineering on the performance of a CuO-based heterojunction solar cell

Aluminum-doped cupric oxide (CuO:Al) was prepared via an out-diffusion process of Al from an Al-coated substrate into the deposited CuO thin film upon thermal treatment. The effect of the annealing temperature on the structural and optical properties of CuO:Al was investigated in detail. The influence of Al incorporation on the photovoltaic properties was then investigated by preparing a p-CuO:Al/n-Si heterojunction solar cell. A significant improvement in the performance of the solar cell was achieved by controlling the out-diffusion of Al. A novel in situ method to co-dope CuO with Al and titanium (Ti) has been proposed to demonstrate CuO-based solar cells with the front surface field (FSF) design. The FSF design was created by depositing a CuO:Al layer followed by a Ti-doped CuO (CuO:Ti) layer. This is the first successful experimental demonstration of the codoping of a CuO thin film and CuO thin film solar cells with the FSF design. The open circuit voltage (V oc), short circuit current density (J sc) and fill factor (FF) of the fabricated solar cells were significantly higher for the FSF device compared to devices without FSF. The FF of this device improved by 68% through the FSF design and a record efficiency ɳ of 2% was achieved. The improvement of the solar cell properties is mainly attributed to the reduction of surface recombination, which influences the charge carrier collection.

Tunable Plasmonic Nanohole Arrays Actuated by a Thermoresponsive Hydrogel Cushion

New plasmonic structure with actively tunable optical characteristics based on thermoresponsive hydrogel is reported. It consists of a thin, template-stripped Au film with arrays of nanoholes that is tethered to a transparent support by a cross-linked poly(N-isopropylacrylamide) (pNIPAAm)-based polymer network. Upon a contact of the porous Au surface with an aqueous environment, a rapid flow of water through the pores enables swelling and collapsing of the underlying pNIPAAm network. The swelling and collapsing could be triggered by small temperature changes around the lower critical solution temperature (LCST) of the hydrogel. The process is reversible, and it is associated with strong refractive index changes of Δn ∼ 0.1, which characteristically alters the spectrum of surface plasmon modes supported by the porous Au film. This approach can offer new attractive means for optical biosensors with flow-through architecture and actively tunable plasmonic transmission optical filters.

Development of Localized Surface Plasmon Resonance-Based Point-of-Care System

This paper describes our point-of-care system development based on localized surface plasmon resonance (LSPR). Although LSPR has been a hot research area for a few decades, there are several bottlenecks which hampered its application for point-of-care (POC) medical diagnostics. The first is the detection sensitivity shortage when the direct LSPR wavelength shift is used for sensing, the second is the mass fabrication of durable metal nanostructures on the substrate, and the third is the microfluidic chip and the POC system which have to be combined with LSPR chips in a seamless but cost-effective way. To solve the above challenges, several novel technologies are initiated and successfully implemented in this work. To increase the sensitivity of the LSPR detection, we use plasmonic field to excite the fluorescence dyes conjugated to the analyte rather than directly detecting the LSPR wavelength shift upon analyte bonding. This method can enhance the biomarker detection sensitivity 10 to 100 times upon careful design of the metal nanostructures and the location of the fluorescence dyes in the bioassay. To mass fabricate the metal nanostructures, a 4″ nickel mold is fabricated by electroplating and employed for UV nanoimprinting lithography. Our technology achieves high yield on wafer-level mass fabrication of the designed metal nanostructures. In terms of the surface modification of the bioassay, the orientation of the capturing antibody is controlled to enhance the sensitivity of biomarker detections, and an antifouling polymer is synthesized inside the gold nanoholes. To accomplish the cost-effective point-of-care system, a plastic multichamber microfluidic chip is fabricated, which contains the metal nanostructures, microfluidic channels, and trenches for controlling the sample flow. The microfluidic chip is inserted into a point-of-care system which consists of micropumps to control the microfluidic flow, a light source for fluorescence excitation, a camera system for fluorescence detection, and software to automate the POC system and to analyze the results. We believe this highly sensitive LSPR point-of-care system has ample applications on medical diagnostics.

Quantification of a cardiac biomarker in human serum using extraordinary optical transmission (EOT).

Nanoimprinting lithography (NIL) is a manufacturing process that can produce macroscale surface areas with nanoscale features. In this paper, this technique is used to solve three fundamental issues for the application of localized surface plasmonic resonance (LSPR) in practical clinical measurements: assay sensitivity, chip-to-chip variance, and the ability to perform assays in human serum. Using NIL, arrays of 140 nm square features were fabricated on a sensing area of 1.5 mm x 1.5 mm with low cost. The high reproducibility of NIL allowed for the use of a one-chip, one-measurement approach with 12 individually manufactured surfaces with minimal chip-to-chip variations. To better approximate a real world setting, all chips were modified with a biocompatible, multi-component monolayer and inter-chip variability was assessed by measuring a bioanalyte standard (2.5−75 ng/ml) in the presence of a complex biofluid, human serum. In this setting, nanoimprinted LSPR chips were able to provide sufficient characteristics for a ‘low-tech’ approach to laboratory-based bioanalyte measurement, including: 1) sufficient size to interface with a common laboratory light source and detector without the need for a microscope, 2) high sensitivity in serum with a cardiac troponin limit of detection of 0.55 ng/ml, and 3) very low variability in chip manufacturing to produce a figure of merit (FOM) of 10.5. These findings drive LSPR closer to technical comparability with ELISA-based assays while preserving the unique particularities of a LSPR based sensor, suitability for multiplexing and miniaturization, and point-of-care detections.

Epitaxial growth of ZnO film on Si(1?1?1) with CeO2(1?1?1) as buffer layer

ZnO(0 0 2) epitaxial films have been successfully grown on Si(1 1 1) with CeO2 as a buffer layer by pulsed laser deposition. In spite of large lattice mismatch between ZnO and CeO2, good film quality was achieved, as proven by Fourier filtered high-resolution transmission electron microscopy (HRTEM) image, due to reduction in interface strain by domain matching epitaxy. The epitaxial relationship of ZnO and CeO2 on the Si substrate was determined to be (0 0 2)[2 1 0]ZnO||(1 1 1)[1 1 2] (1 1 1)[1 1 2]Si. The HRTEM images show low defect concentrations in both the deposited ZnO film and CeO2 layer. Ordered crack lines are observed on the surface of the ZnO film which are due to A-type and B-type stackings of CeO2 on Si(1 1 1). Sharp near-band edge emission at 3.253 eV was detected for the ZnO film through photoluminiscence measurement at room temperature.

Mechanism of insulator-to-metal transition in heavily Nb doped anatase TiO2

Heavily Nb-doped anatase TiO2 (TNO) thin films were prepared by pulsed dc magnetron sputtering using an Nb-doped TiO2 target. The as-grown films exhibit high resistivity whose resistance decreases by ~2 × 104-fold upon vacuum annealing. The ~40% Nb-doped anatase TiO2 film shows a low resistivity of 5.7 × 10−4 Ω cm and a high electron concentration of 3.07 × 1021 cm−3. Combined in situ x-ray photoelectron spectroscopy (XPS) measurement and density-functional theory (DFT) calculations show that oxygen interstitial (Oint) and Nb interstitial (Nbint) defect clusters introduce localized shallow p-type accepter states that trap the electrons and reduce the conductivity. These defect clusters can be eliminated by vacuum annealing which is companied by outward diffusion of Nb. As a result, the trapped electrons backfill the Ti sites which are delocalized to promote conductivity.