A ZnO and carbon nanoparticle (NP) mixture has been prepared. The fluorescence color of the mixture changes from yellow to blue when the pH value of the ambient is smaller than 5.5, which accords well with the pH standard value of acid rain (<5.6). This colorimetric change can be observed clearly by the naked eye, and thus the mixture can be employed as a probe to detect acid rain. The mechanism for the colorimetric change can be attributed to the fluorescence quenching of the ZnO NPs in acidic conditions. The results demonstrated in this paper provide a visual detection route for acid rain by the naked eye for the first time, and thus may be promising for the wide application of acid rain detection in the future.

A ZnO and carbon nanoparticle (NP) mixture has been prepared. The fluorescence color of the mixture changes from yellow to blue when the pH value of the ambient is smaller than 5.5, which accords well with the pH standard value of acid rain (<5.6). This colorimetric change can be observed clearly by the naked eye, and thus the mixture can be employed as a probe to detect acid rain. The mechanism for the colorimetric change can be attributed to the fluorescence quenching of the ZnO NPs in acidic conditions. The results demonstrated in this paper provide a visual detection route for acid rain by the naked eye for the first time, and thus may be promising for the wide application of acid rain detection in the future.

A self-powered solar-blind ultraviolet (UV) photodetector was realized on a Ag/ZnMgO/ZnO vertical structure. A ZnO epitaxial layer was employed to serve as both the buffer layer for the growth of ZnMgO and the bottom electrode of the device. Interestingly, the device exhibited an obvious self-powered property owing to the built-in electric fields at the Ag/ZnMgO and ZnMgO/ZnO interfaces. At 0 V, the peak responsivity at 275 nm of our device was 16 mA W−1, which is comparable to that of other ZnMgO self-powered solar-blind UV photodetectors. Moreover, the device also exhibited a very fast response speed (24 μs rise time and 300 μs decay time) at 0 V. This study could provide a much easier and more feasible way to develop ZnMgO self-powered solar-blind UV photodetectors on foreign substrates with high responsivity, fast response speed, and low cost.

Herein, ZnMgO thin film ultraviolet (UV) photodetectors based on metal–semiconductor–metal structure were fabricated on a-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The effect of H2O2 solution treatment on the properties of the ZnMgO thin film and its UV photodetectors was investigated. After immersing the ZnMgO UV photodetector in a H2O2 solution at 100 °C for 3 min, the dark current of the device was reduced by more than one order of magnitude under 1 V bias, whereas the responsivity was slightly decreased. More interestingly, the response speed became much quicker and insensitive to the atmosphere after the treatment of the photodetector with H2O2 solution, which can be attributed to the reduction in the oxygen vacancy defects. Our findings may provide a promising approach for improving the performance of photodetectors.

Oxygen adsorption and desorption processes can strongly slow down the response speed of ZnO-based ultraviolet photodetectors. Surface passivation of ZnO-based films is a common method to increase the response speed, but the effect of the traditional passivation technology is usually not very obvious, and often accompanies a decrease in responsivity. Herein we propose a new method to overcome this problem: treating ZnMgO UV photodetectors with HF solution. Interestingly, after treatment with HF solution, the 90–10% decay time of the device is decreased by more than one order of magnitude (from 4.6 ms to 0.34 ms), which can be attributed to the enhancement of the oxygen adsorption speed on the surface of ZnMgO due to the F-modified surface. Moreover, the dark current is decreased from 0.7 pA to 0.4 pA at a bias of 10 V and the responsivity is increased from 145 A W−1 to 326 A W−1. Thus, chemical treatment with HF solution should be a useful and effective method for improving the performance of ZnMgO UV photodetectors.

One dimensional (1D) zinc oxide (ZnO) nano and microwires have been considered as one of the most promising candidates for the fabrication of novel nano and microscale electronic and optoelectronic devices. In this study, individual Ga heavily doped ZnO microwires (GZO MWs) were successfully synthesized via chemical vapor deposition methods. Bright, stable, and near-infrared light-emission from electrically biased individual GZO MWs has been achieved. Mysteriously, alternating current driven near-infrared electroluminescence (EL) devices based on individual GZO MWs can also be realized. Therefore, individual GZO MWs that can be analogous to incandescent sources, provide promising potential applications in future ultracompact near-infrared electronic and optoelectronic devices or systems.

Individual Sb-doped ZnO (ZnO:Sb) microwires with durable and reproducible p-type conduction have been synthesized, and by increasing the Sb2O3 weight ratios in the precursor mixtures, tunable p-type conduction characteristics can be obtained. Meanwhile, wavelength-tuning electroluminescence (EL) has been observed by applying bias onto individual ZnO:Sb microwires, in which the ZnO:Sb microwires act as emitting filaments. The as-synthesized p-type ZnO:Sb microwires are applied to fabricate homojunction light-emitting devices. The corresponding p–n junction demonstrates excellent diode characteristics, and strong near band edge emissions can be observed with the dominant EL emission wavelengths centered at 400 nm. The results demonstrated the emitting filament characteristics of ZnO:Sb microwires for the first time, and also demonstrated their applicability in homojunction light-emitting diodes, and thus may offer alluring prospects as compact, efficient, reliable building blocks for microscale light sources.

We proposed and demonstrated Ag nanoparticles (NPs)-decorated ZnO photodetectors for UV light sensing. After decoration of their surface with random Ag NPs, the dark current density of ZnO UV photodetectors decreases obviously. Moreover, the device exhibits an obvious increase in peak responsivity at around 380 nm, which can be attributed to the narrow-band quadrupole plasmon resonance of Ag NPs in the UV range. Meanwhile, the responsivity at the other wavelengths decreases a lot. As a result, the response peak becomes more significant, and the response of the devices presents an excellent wavelength selectivity after covering with Ag NPs. The detailed mechanism for this phenomenon was explained. We believe that our findings would open a way to harness the high-order plasmon modes in the field of UV optoelectronic devices.Keywords: quadrupole; semiconductor; solar-blind; surface plasmon resonance; wide band gap;

Because of the direct band gap of 4.9 eV, β-Ga2O3 has been considered as an ideal material for solar-blind photodetection without any bandgap tuning. Practical applications of the photodetectors require fast response speed, high signal-to-noise ratio, low energy consumption and low fabrication cost. Unfortunately, most reported β-Ga2O3-based photodetectors usually possess a relatively long response time. In addition, the β-Ga2O3 photodetectors based on bulk, the individual 1D nanostructure, and the film often suffer from the high cost, the low repeatability, and the relatively large dark current, respectively. In this paper, a Au/β-Ga2O3 nanowires array film vertical Schottky photodiode is successfully fabricated by a simple thermal partial oxidation process. The device exhibits a very low dark current of 10 pA at −30 V with a sharp cutoff at 270 nm. More interestingly, the 90–10% decay time of our device is only around 64 μs, which is much quicker than any other previously reported β-Ga2O3-based photodetectors. Besides, the self-powering, the excellent stability and the good reproducibility of Au/β-Ga2O3 nanowires array film photodetector are helpful to its commercialization and practical applications.Keywords: high-speed; schottky junction; self-powered; solar-blind photodetector; β-Ga2O3

Cubic ZnMgO (c-ZnMgO) with a band gap in the ultraviolet-B (UVB) region was deposited for the first time on the a-face sapphire substrate by molecular beam epitaxy using a thin low-Zn-content ZnMgO film as the buffer layer. The metal–semiconductor–metal photodetectors were demonstrated on this c-ZnMgO with different finger gaps, and their cut-off wavelength is around 320 nm. At 10 V, the device with a finger gap of 2 μm shows a low dark current of 3 pA, and its peak responsivity at 302 nm is ∼5.188 A W−1. A further temporal photoresponse study reveals that the c-ZnMgO UVB photodetectors display a fast response speed (90–10% decay time is around 24 μs) with good reproducibility and stability. Our findings indicate that the present c-ZnMgO UVB photodetectors are very promising for future optoelectronic device applications.

Mixed-phase ZnMgO (m-ZMO) thin films with a single absorption edge tuning from ∼3.9 to ∼4.8 eV were realized on a-face sapphire (a-Al2O3) by plasma-assisted molecular beam epitaxy. The small lattice mismatch of both ZnO and MgO with a-Al2O3 should be responsible for the single and controllable absorption edge. Metal–semiconductor–metal (MSM) photodetectors were fabricated based on these m-ZMO films, and the devices have the single cutoff wavelength, which can be tuned from 335 to 275 nm. These devices possess low dark current (78 pA for m-Z0.67M0.33O, 11 pA for m-Z0.59M0.41O, and 4 pA for m-Z0.39M0.61O at 40 V) and high responsivity (434 A/W for m-Z0.67M0.33O, 89.8 A/W for m-Z0.59M0.41O, and 3.7 A/W for m-Z0.39M0.61O at 40 V). Further response study reveals that the 90–10% decay time of m-Z0.67M0.33O, m-Z0.59M0.41O, and m-Z0.39M0.61O is around 37, 30, and 0.7 ms, respectively. Large amounts of heterojunction interfaces between wurtzite ZMO and cubic rock-salt ZMO could be responsible for the low dark current and high responsivity of our mixed-phase devices. The excellent comprehensive performance of m-ZMO UV photodetectors on a-Al2O3 suggests that m-ZMO UV photodetectors should have great applied potential.Keywords: heterojunction interfaces; high responsivity; high-performance; low dark current; mixed-phase ZnMgO; photodetectors

A facile pulse laser ablation approach for preparing black titanium oxide nanospheres, which could be used as photocatalysts under visible light, is proposed. The black titanium oxide nanospheres are prepared by pulsed-laser irradiation of pure titanium oxide in suspended aqueous solution. The crystalline phases, morphology, and optical properties of the obtained nanospheres are characterized by means of X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), and UV–vis–NIR diffuse reflectance spectroscopy. It is shown that high-energy laser ablation of titanium oxide suspended solution benefited the formation of Ti3+ species and surface disorder on the surface of the titanium oxide nanospheres. The laser-modified black titanium oxide nanospheres could absorb the full spectrum of visible light, thus exhibiting good photocatalytic performance under visible light.Keywords: laser; nanospheres; photocatalytic; titanium oxides; visible light;

Well-aligned ZnO nanowire arrays have been prepared, and p-MgZnO has been deposited onto the nanowires to form core–shell heterostructures. Transmission electron microscopy confirms the formation of n-ZnO/p-MgZnO core–shell nanowire heterostructures. Under injection of a continuous current, random lasing with a threshold current of around 15 mA has been observed from the heterostructures. The low threshold may be due to the relatively high crystalline quality of the ZnO nanowires as well as the carrier confinement in the heterostructures.

In this work, photoinduced charge separation behaviors in non-long-chain-molecule-functionalized carbon nanodots (CDs) with visible intrinsic absorption (CDs-V) and TiO2 composites were investigated. Efficient photoinduced electron injection from CDs-V to TiO2 with a rate of 8.8 × 108 s−1 and efficiency of 91% was achieved in the CDs-V/TiO2 composites. The CDs-V/TiO2 composites exhibited excellent photocatalytic activity under visible light irradiation, superior to pure TiO2 and the CDs with the main absorption band in the ultraviolet region and TiO2 composites, which indicated that visible photoinduced electrons and holes in such CDs-V/TiO2 composites could be effectively separated. The incident photon-to-current conversion efficiency (IPCE) results for the CD-sensitized TiO2 solar cells also agreed with efficient photoinduced charge separation between CDs-V and the TiO2 electrode in the visible range. These results demonstrate that non-long-chain-molecule-functionlized CDs with a visible intrinsic absorption band could be appropriate candidates for photosensitizers and offer a new possibility for the development of a well performing CD-based photovoltaic system.

We propose a kind of hybrid plasmonic Fabry–Perot (F–P) microcavity consisting of ZnO microwires with quadrate cross-section and planar multilayer metal–insulator–metal (MIM) homostructures with a nanoscale SiO2 gap in between. MIM homostructures can be used to create a micro-resonator that simultaneously provides feedback for laser action and supports the coupling between the plasmonic waveguide modes and microwire modes across the gap. The hybridization of ZnO microwire modes and surface plasmons across the gap forms hybrid plasmonic F–P microcavity modes, which are highly confined to the low-loss SiO2 gap region. By comparing with bare ZnO microwires, an enhancement in photoluminescence (PL) intensity of two orders of magnitude is realized experimentally due to the coupling between plasmonic MIM homostructures and ZnO microwires. The controllability and miniaturization emission properties of this type of microcavity are potentially important for designing laser cavity applications and information transmission.

We demonstrate a novel type of ZnO self-powered photodetector based on the asymmetric metal-semiconductor-metal (MSM) structure: one Au interdigitated electrode with wide fingers and the other one with narrow fingers. These ZnO photodetectors exhibit attractive photovoltaic characteristics at 0 V bias. More interestingly, with increasing the asymmetric ratio (the width of wide fingers:the width of narrow fingers) of the interdigitated electrodes, the responsivity of the ZnO self-powered UV photodetectors was enhanced obviously, reaching as high as 20 mA W−1 when the asymmetric ratio was 20:1. A physical model based on band energy theory was developed to illustrate the origin of the photoresponse at 0 V in our device. Our findings provide a new route to realizing self-powered photodetectors.

The coupling interaction among metal nanoparticles can cause asymmetric distribution of the surface charges, which can lead to the cleavage of surface plasmon resonance. Higher order resonance modes in plasmonic nanostructures exhibit sharp resonance and dramatic line shape shifts. These unique resonant behaviours existing in nanostructures can be used to improve the performance of wide band gap semiconductor devices, such as increasing the output power of lasers. According to the theoretical calculation and simulation, silver nanoparticles have been prepared experimentally, and the corresponding extinction spectra were also tested to verify the simulation results. It is found that the hybrid quadrupolar resonance can indeed occur in the short wavelength region.

By taking semiconductors with different band-gap energies as the active layers and controlling the electron–hole recombination region through the electric field, bias-polarity dependent ultraviolet/visible switchable light-emitting devices have been realized in Au/MgO/Mg0.49Zn0.51O/MgxZn1–xO/n-ZnO structures, of which the emission bands can be switched from the ultraviolet region to the orange region by changing the polarity of the applied bias. The results reported here may provide a feasible idea to multicolor-switchable light-emitting devices.Keywords: accelerated electrons; color-switchable; light-emitting devices; magnesium zinc oxide; ultraviolet; wide-band-gap semiconductors;

Hybrid plasmonic waveguides have achieved rapid advancement in plasmonics, which has given rise to remarkable field enhancement, light harvest, light-transport capabilities, bridging the gap between electronics and photonics by routing and manipulating light at sub-wavelength regions and so on. However, the development of plasmonic waveguides is hindered by lack of devices that can adjust coherent plasmonic fields. In this letter, hybridized planar multilayer insulator metal insulator metal insulator heterostructures are proposed, and it is demonstrated that their unique capabilities can be used to adjust the mode characteristics by means of varying the thickness of the insulator spacer layer inserted between two metal films, such as the shift of the surface plasmon resonance wavelength. This type of hybrid plasmonic waveguides opens up opportunities for the tunability of mode characteristics, adjustment of resonant energy transfer processes, that have a potential for designing novel optical micro/nano resonance cavities.

Double hetero-structured n-Mg0.13Zn0.87O/i-ZnO/p-Mg0.13Zn0.87O light-emitting devices (LEDs) have been fabricated, and the p-type Mg0.13Zn0.87O layer was obtained via a lithium–nitrogen codoping method. Obvious emission at around 400 nm has been observed from the LEDs under forward bias. To increase the light extraction from the LEDs, a distributed Bragg reflector whose reflectivity is 98% at 400 nm was bonded on the back side of the device, and the emission of the device was enhanced by around 1.6 times with the reflector.

The tunable hybridized quadrupole plasmons and their strong coupling with excitons have been demonstrated in ZnMgO/Ag nanoclusters (NCs) system. By adjusting the density of the random Ag NCs, the hybridized quadrupole resonance with strong intensity can be obtained due to the asymmetry Fano-like interference. Owing to a good energy match between hybridized quadrupole plasmons and excitons of ZnMgO, a giant near-band-edge UV emission enhancement was realized. The finite difference time-domain method was used to demonstrate the formation of the tunable hybridized quadrupole resonance in the short wavelength range. This novel method for enhancing the UV emission, associated with the hybridized quadrupole plasmons, may pave the way for the further development of high-efficiency UV light-emitting diodes and laser diodes.

In this paper, localized surface plasmon enhanced n-ZnO/i-ZnO/MgO/p-GaN structured light-emitting devices have been designed and constructed. It is found that the electroluminescence of the devices can be enhanced at selective wavelengths that match the localized surface plasmon extinction spectra of the metal nanoparticles.

Vertically aligned ZnO nanowires have been prepared, and structural characterization shows that the nanowires have relatively high crystalline quality. The dominant free exciton emission and the appearance of B-type exciton emissions at low temperatures reveal the high optical quality of the nanowires. Au–MgO–ZnO nanowire structures have been constructed, and random lasing has been observed from the structure under the injection of continuous current.

Hybrid plasmonic waveguides have achieved rapid advancement in plasmonics, which has given rise to remarkable field enhancement, light harvest, light-transport capabilities, bridging the gap between electronics and photonics by routing and manipulating light at sub-wavelength regions and so on. However, the development of plasmonic waveguides is hindered by lack of devices that can adjust coherent plasmonic fields. In this letter, hybridized planar multilayer insulator metal insulator metal insulator heterostructures are proposed, and it is demonstrated that their unique capabilities can be used to adjust the mode characteristics by means of varying the thickness of the insulator spacer layer inserted between two metal films, such as the shift of the surface plasmon resonance wavelength. This type of hybrid plasmonic waveguides opens up opportunities for the tunability of mode characteristics, adjustment of resonant energy transfer processes, that have a potential for designing novel optical micro/nano resonance cavities.

In this work, photoinduced charge separation behaviors in non-long-chain-molecule-functionalized carbon nanodots (CDs) with visible intrinsic absorption (CDs-V) and TiO2 composites were investigated. Efficient photoinduced electron injection from CDs-V to TiO2 with a rate of 8.8 × 108 s−1 and efficiency of 91% was achieved in the CDs-V/TiO2 composites. The CDs-V/TiO2 composites exhibited excellent photocatalytic activity under visible light irradiation, superior to pure TiO2 and the CDs with the main absorption band in the ultraviolet region and TiO2 composites, which indicated that visible photoinduced electrons and holes in such CDs-V/TiO2 composites could be effectively separated. The incident photon-to-current conversion efficiency (IPCE) results for the CD-sensitized TiO2 solar cells also agreed with efficient photoinduced charge separation between CDs-V and the TiO2 electrode in the visible range. These results demonstrate that non-long-chain-molecule-functionlized CDs with a visible intrinsic absorption band could be appropriate candidates for photosensitizers and offer a new possibility for the development of a well performing CD-based photovoltaic system.

In this paper, localized surface plasmon enhanced n-ZnO/i-ZnO/MgO/p-GaN structured light-emitting devices have been designed and constructed. It is found that the electroluminescence of the devices can be enhanced at selective wavelengths that match the localized surface plasmon extinction spectra of the metal nanoparticles.

We demonstrate a novel type of ZnO self-powered photodetector based on the asymmetric metal-semiconductor-metal (MSM) structure: one Au interdigitated electrode with wide fingers and the other one with narrow fingers. These ZnO photodetectors exhibit attractive photovoltaic characteristics at 0 V bias. More interestingly, with increasing the asymmetric ratio (the width of wide fingers:the width of narrow fingers) of the interdigitated electrodes, the responsivity of the ZnO self-powered UV photodetectors was enhanced obviously, reaching as high as 20 mA W−1 when the asymmetric ratio was 20:1. A physical model based on band energy theory was developed to illustrate the origin of the photoresponse at 0 V in our device. Our findings provide a new route to realizing self-powered photodetectors.

The coupling interaction among metal nanoparticles can cause asymmetric distribution of the surface charges, which can lead to the cleavage of surface plasmon resonance. Higher order resonance modes in plasmonic nanostructures exhibit sharp resonance and dramatic line shape shifts. These unique resonant behaviours existing in nanostructures can be used to improve the performance of wide band gap semiconductor devices, such as increasing the output power of lasers. According to the theoretical calculation and simulation, silver nanoparticles have been prepared experimentally, and the corresponding extinction spectra were also tested to verify the simulation results. It is found that the hybrid quadrupolar resonance can indeed occur in the short wavelength region.

Cubic ZnMgO (c-ZnMgO) with a band gap in the ultraviolet-B (UVB) region was deposited for the first time on the a-face sapphire substrate by molecular beam epitaxy using a thin low-Zn-content ZnMgO film as the buffer layer. The metal–semiconductor–metal photodetectors were demonstrated on this c-ZnMgO with different finger gaps, and their cut-off wavelength is around 320 nm. At 10 V, the device with a finger gap of 2 μm shows a low dark current of 3 pA, and its peak responsivity at 302 nm is ∼5.188 A W−1. A further temporal photoresponse study reveals that the c-ZnMgO UVB photodetectors display a fast response speed (90–10% decay time is around 24 μs) with good reproducibility and stability. Our findings indicate that the present c-ZnMgO UVB photodetectors are very promising for future optoelectronic device applications.

One dimensional (1D) zinc oxide (ZnO) nano and microwires have been considered as one of the most promising candidates for the fabrication of novel nano and microscale electronic and optoelectronic devices. In this study, individual Ga heavily doped ZnO microwires (GZO MWs) were successfully synthesized via chemical vapor deposition methods. Bright, stable, and near-infrared light-emission from electrically biased individual GZO MWs has been achieved. Mysteriously, alternating current driven near-infrared electroluminescence (EL) devices based on individual GZO MWs can also be realized. Therefore, individual GZO MWs that can be analogous to incandescent sources, provide promising potential applications in future ultracompact near-infrared electronic and optoelectronic devices or systems.