High-Performance Ultraviolet Photodetectors Based on Nanoporous GaN with a Ga2O3 Single-Crystal Layer.

The utilization of a nanoporous (NP) GaN fabricated by electrochemical etching has been demonstrated to be effective in the fabrication of a high-performance ultraviolet (UV) photodetector (PD). However, the NP-GaN PD typically exhibits a low light-dark current ratio and slow light response speed. In this study, we present three types of UV PDs based on an unetched GaN, NP-GaN distributed Bragg reflector (DBR), and NP-GaN-DBR with a Ga2O3 single-crystal film (Ga2O3/NP-GaN-DBR). The unetched GaN PD does not exhibit a significant photoresponse. Compared to the NP-GaN-DBR PD device, the Ga2O3/NP-GaN-DBR PD demonstrates a larger light-dark current ratio (6.14 × 103) and higher specific detectivity (8.9 × 1010 Jones) under 365 nm at 5 V bias due to its lower dark current (3.0 × 10-10 A). This reduction in the dark current can be attributed to the insertion of the insulating Ga2O3 between the metal and the NP-GaN-DBR, which provides a thicker barrier thickness and higher barrier height. Additionally, the Ga2O3/NP-GaN-DBR PD device exhibits shorter rise/decay times (0.33/0.23 s) than the NP-GaN-DBR PD, indicating that the growth of a Ga2O3 layer on the DBR effectively reduces the trap density within the NP-GaN DBR structure. Although the device with a Ga2O3 layer presents low photoresponsivity (0.1 A/W), it should be feasible to use Ga2O3 as a dielectric layer based on the above-mentioned reasons.

Tunable nanostructured distributed Bragg reflectors for III-nitride optoelectronic applications.

Highly reflective and conductive distributed Bragg reflectors (DBRs) are the key for high-performance III-nitride optoelectronic devices, such as vertical cavity surface emitting lasers (VCSELs), but they still suffer from lack of lattice-matched conductive DBR and uncontrollable processes. In this work, nanostructured GaN-based DBRs were fabricated and optimized both experimentally and simulatively using electrochemical etching (EC) in different electrolytes using the transfer-matrix method (TMM) to obtain uniform wafer scale, highly reflective and conductive reflectors for the application of GaN-based optoelectronics. The results revealed that a nanostructured GaN-based DBR with high reflectivity (>93%) and broad stopband (∼80 nm) could be achieved in neutral sodium nitrate by EC, and the nanostructured GaN DBR with a full visible spectrum range could be designed by tuning the thickness of the nanostructured GaN DBR layers. The photoluminescence (PL) and light-out power enhancements of the GaN-based micro-LED by incorporating the fabricated nanostructured GaN-based DBR were 6 times and 150% without the degradation of electrical performance, respectively, which contributed to strong light scattering from the DBR layers. We believe that this work will pave a way to obtain high-performance GaN-based optoelectronic devices and guide the applications in the field of flexible devices and biomedical sensors.

Electroluminescence properties of InGaN/GaN multiple quantum well-based LEDs with different indium contents and different well widths.

Two InGaN/GaN multiple quantum well (MQW)-based blue light emitting diodes (LEDs) emitting photons at approximately the same wavelength, with different indium contents and well widths, are prepared, and the temperature-dependences of their electroluminescence (EL) spectra at different fixed injection currents are investigated. The results show that, compared with sample B with its lower indium content and larger well width, sample A with its higher indium content and smaller well width, has a stronger carrier localization effect and higher external quantum efficiency (EQE) at the lower fixed currents; however, upon increasing the injection current, both the localization effect and EQE for sample A decrease at a faster rate. The former is mainly attributed to the deeper potential levels due to the larger indium fluctuations originating from the higher indium content, and to the smaller well width-induced stronger carrier quantum-confine effect (QCE); the latter is mainly attributed to the more significant growing in the electron leakage and/or electron overflow originating from the smaller well width and larger lattice mismatch-induced stronger piezoelectric field, and to the more significant reduction in carrier localization effect originating from the smaller well width-induced smaller density of high-energy localized states.

16S rRNA Gene Amplicon Sequencing Reveals Significant Changes in Microbial Compositions during Cyanobacteria-Laden Drinking Water Sludge Storage.

This is the first study to systematically investigate the microbial community structure in cyanobacteria-laden drinking water sludge generated by different types of coagulants (including AlCl3, FeCl3, and polymeric aluminum ferric chloride (PAFC)) using Illumina 16S rRNA gene MiSeq sequencing. Results show that Cyanobacteria, Proteobacteria, Firmicutes, Bacteroidetes, Verrucomicrobia, and Planctomycetes were the most dominant phyla in sludge, and because of the toxicity of high Al and Fe level in AlCl3 and FeCl3 sludges, respectively, the PAFC sludge exhibited greater microbial richness than that in AlCl3 and FeCl3 sludges. Due to lack of light and oxygen in sludge, relative abundance of the dominant genera Microcystis, Rhodobacter, Phenylobacterium, and Hydrogenophaga clearly decreased, especially after 4 days storage, and the amounts of extracellular microcystin and organic matter rose. As a result, the relative abundance of microcystin and organic degradation bacteria increased significantly, including pathogens such as Bacillus cereus, in particular after 4 days storage. Hence, sludge should be disposed of within 4 days to prevent massive growth of pathogens. In addition, because the increase of extracellular microcystins, organic matter, and pathogens in AlCl3 sludge was higher than that in FeCl3 and PAFC sludges, FeCl3 and PAFC may be ideal coagulants in drinking water treatment plants.

Fabrication and improved photoelectrochemical properties of a transferred GaN-based thin film with InGaN/GaN layers.

Herein, a lift-off mesoporous GaN-based thin film, which consisted of a strong phase-separated InGaN/GaN layer and an n-GaN layer, was fabricated via an electrochemical etching method in a hydrofluoric acid (HF) solution for the first time and then transferred onto quartz or n-Si substrates, acting as photoanodes during photoelectrochemical (PEC) water splitting in a 1 M NaCl aqueous solution. Compared to the as-grown GaN-based film, the transferred GaN-based thin films possess higher and blue-shifted light emission, presumably resulting from an increase in the surface area and stress relaxation in the InGaN/GaN layer embedded on the mesoporous n-GaN. The properties such as (i) high photoconversion efficiency, (ii) low turn-on voltage (-0.79 V versus Ag/AgCl), and (iii) outstanding stability enable the transferred films to have excellent PEC water splitting ability. Furthermore, as compared to the film transferred onto the quartz substrate, the film transferred onto the n-Si substrate exhibits higher photoconversion efficiency (2.99% at -0.10 V) due to holes (h+) in the mesoporous n-GaN layer that originate from the n-Si substrate.

Preparation and photovoltaic properties of CdS quantum dot-sensitized solar cell based on zinc tin mixed metal oxides.

The present study reports a new type of quantum dot sensitized solar cells (QDSSCs) using the zinc tin mixed metal oxides (MMO) as the anode materials, which were obtained from the layered double hydroxide (LDH) precursor. The successive ionic layer adsorption and reaction (SILAR) method is applied to deposit CdS quantum dots. The effects of sensitizing cycles on the performance of CdS QDSSC are studied. Scanning electron microscopy (SEM), Transmission electron microscope (TEM) and X-ray diffraction (XRD) are used to identify the surface profile and crystal structure of the mixed metal oxides anode. The photovoltaic performance of the QDSSC is studied by the electrochemical method. The new CdS QDSSC exhibits power conversion efficiency (PCE) up to 0.48% when the anode was sensitized for eight cycles.

Characteristics of water obtained by dewatering cyanobacteria-containing sludge formed during drinking water treatment, including C-, N-disinfection byproduct formation.

This is the first study to systematically investigate the characteristics of the water obtained by dewatering cyanobacteria-containing sludge generated in the drinking water treatment plant, including formation of C- and N-disinfection by-products (DBPs). Results showed that this 'dewatering water' (DW) had different properties when the sludge was stored at different times. The content of dissolved organic matter (DOM) and microcystins (MCs) in the DW were low when the sludge was treated or disposed of within 4 days; correspondingly, the C-, N-DBP production was also low. However, due to the damage of algal cells to some extent, the DOM and MC levels increased significantly for storage time longer than 4 days; the production of C-, N-DBPs also increased. There were also obvious differences in the characteristics of the DW from sludges generated with different coagulant species. Due to the better protective effect of FeCl3 and polymeric aluminium ferric chloride (PAFC) flocs, the DOM and MC levels and the production of C-, N-DBPs in the DW with FeCl3 and PAFC coagulation were lower than those with AlCl3 coagulation, even though the sludges were stored for the same amount of time. Furthermore, because of the formation of Al and Fe hydroxides, precipitated onto the surface of flocs, the soluble Al and Fe in the DW decreased with increased storage time, especially in the first four days. Overall, this study revealed the trends in variation of DW quality for cyanobacteria-containing sludges formed with different coagulants, then FeCl3 and PAFC coagulants are recommended and sludge should be treated or disposed of within 4 days.

Behaviors of Microcystis aeruginosa cells during floc storage in drinking water treatment process.

This is the first study to systematically investigate the different behaviors of Microcystis aeruginosa in the sludges formed by AlCl3, FeCl3, and polymeric aluminium ferric chloride (PAFC) coagulants during storage. Results show that the viability of Microcystis aeruginosa in PAFC sludge was stronger than that of cells in either AlCl3 or FeCl3 sludge after the same storage time, while the cells' viability in the latter two systems stayed at almost the same level. In AlCl3 and FeCl3 sludges high concentrations of Al and Fe were toxic to Microcystis aeruginosa, whereas in PAFC sludge low levels of Al showed little toxic effect on Microcystis aeruginosa growth and moderate amounts of Fe were beneficial to growth. The lysis of Microcystis aeruginosa in AlCl3 sludge was more serious than that in PAFC sludge, for the same storage time. Although the cell viability in FeCl3 sludge was low (similar to AlCl3 sludge), the Microcystis aeruginosa cells remained basically intact after 10 d storage (similar to PAFC sludge). The maintenance of cellular integrity in FeCl3 sludge might be due to the large floc size and high density, which had a protective effect for Microcystis aeruginosa.

Inactivation of Microcystis aeruginosa by hydrogen-terminated porous Si wafer: Performance and mechanisms.

We proposed a method to inactivate Microcystis aeruginosa by using hydrogen-terminated porous Si (H-PSi) wafer. The influences of oxidation time on the removal of M. aeruginosa were investigated. Samples oxidized by H-PSi wafer were subsequently grown under illuminated culture conditions. The results demonstrated that the optimal oxidation time was about 1h, which could control the growth of M. aeruginosa about 65%, after 3days culture. Simultaneously, extracellular microcystins was decreased from 14.65 to 7.06µgL(-1) and remain relative integrity of M. aeruginosa cells which could avoid secretion of large amounts of organic material. Multiple analysis techniques including fluorescence excitation-emission matrix (EEM) and fluorescence microscope were used to reveal the inhibition mechanisms of M. aeruginosa. Meanwhile, analyses of reactive oxygen level, malondialdehyde content, and superoxide dismutase activity indicated that the damage and inactivate of M. aeruginosa cells are mainly due to accumulation of lipid peroxidation and inhibition of normal physiological metabolism by free radicals produced by H-PSi wafer under visible light irradiation. In conclusion, these results suggest that H-PSi wafer may be useful in controlling growth and survival of M. aeruginosa in many large lakes and reservoirs, thus mitigating many of the economic, esthetic ecological impacts of the invasive alga.

Preparation of single phase Zn2TiO4 spinel from a new ZnTi layered double hydroxide precursor.

A single-source ZnTi-layered double hydroxide precursor was used to prepare single phase Zn2TiO4. This approach involves two steps: the calcination of a ZnTi-layered double hydroxide precursor and selective leaching zinc oxide from the resultant calcined products. The materials were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) thermogravimetric and differential thermal analysis (TG-DTA), transmission electron microscope (TEM), surface area measurement and UVvis diffuse reflection spectroscopy. The results indicated that a single phase Zn2TiO4 could be successfully obtained from a ZnTi-layered double hydroxide precursor at a relatively low temperature in short calcination time. The TEM and SEM show that the diameter of Zn2TiO4 particles prepared at 900 degrees C is in the range of 20-100 nm and smaller than that prepared by the solid-state method. UV-Vis diffuse reflection spectroscopy demonstrates that the material has an energy bandgap around 3.7 eV.