A ZnO nanorod inorganic/organic heterostructure light-emitting diode emitting at 342 nm

Photoelectrochemical Green Hydrogen Production Utilizing ZnO Nanostructured Photoelectrodes

One of the emerging and environmentally friendly technologies is the photoelectrochemical generation of green hydrogen; however, the cheap cost of production and the need for customizing photoelectrode properties are thought to be the main obstacles to the widespread adoption of this technology. The primary players in hydrogen production by photoelectrochemical (PEC) water splitting, which is becoming more common on a worldwide basis, are solar renewable energy and widely available metal oxide based PEC electrodes. This study attempts to prepare nanoparticulate and nanorod-arrayed films to better understand how nanomorphology can impact structural, optical, and PEC hydrogen production efficiency, as well as electrode stability. Chemical bath deposition (CBD) and spray pyrolysis are used to create ZnO nanostructured photoelectrodes. Various characterization methods are used to investigate morphologies, structures, elemental analysis, and optical characteristics. The crystallite size of the wurtzite hexagonal nanorod arrayed film was 100.8 nm for the (002) orientation, while the crystallite size of nanoparticulate ZnO was 42.1 nm for the favored (101) orientation. The lowest dislocation values for (101) nanoparticulate orientation and (002) nanorod orientation are 5.6 × 10^-4 and 1.0 × 10^-4 dislocation/nm², respectively. By changing the surface morphology from nanoparticulate to hexagonal nanorod arrangement, the band gap is decreased to 2.99 eV. Under white and monochromatic light irradiation, the PEC generation of H₂ is investigated using the proposed photoelectrodes. The solar-to-hydrogen conversion rate of ZnO nanorod-arrayed electrodes was 3.72% and 3.12%, respectively, under 390 and 405 nm monochromatic light, which is higher than previously reported values for other ZnO nanostructures. The output H₂ generation rates for white light and 390 nm monochromatic illuminations were 28.43 and 26.11 mmol.h^-1cm^-2, respectively. The nanorod-arrayed photoelectrode retains 96.6% of its original photocurrent after 10 reusability cycles, compared to 87.4% for the nanoparticulate ZnO photoelectrode. The computation of conversion efficiencies, H₂ output rates, Tafel slope, and corrosion current, as well as the application of low-cost design methods for the photoelectrodes, show how the nanorod-arrayed morphology offers low-cost, high-quality PEC performance and durability.

Recent advances in self-powered and flexible UVC photodetectors

Ultraviolet-C (UVC) radiation is employed in various applications, including irreplaceable applications in military and civil fields, such as missile guidance, flame detection, partial discharge detection, disinfection, and wireless communication. Although most modern electronics are based on Si, UVC detection technology remains a unique exception because the short wavelength of UV radiation makes efficient detection with Si difficult. In this review, recent challenges in obtaining ideal UVC photodetectors with various materials and various forms are introduced. An ideal photodetector must satisfy the following requirements: high sensitivity, fast response speed, high on/off photocurrent ratio, good regional selectivity, outstanding reproducibility, and superior thermal and photo stabilities. UVC detection is still in its infancy compared to the detection of UVA as well as other photon spectra, and recent research has focused on different key components, including the configuration, material, and substrate, to acquire battery-free, super-sensitive, ultra-stable, ultra-small, and portable UVC photodetectors. We introduce and discuss the strategies for fabricating self-powered UVC photodetectors on flexible substrates in terms of the structure, material, and direction of incoming radiation. We also explain the physical mechanisms of self-powered devices with various architectures. Finally, we present a brief outlook that discusses the challenges and future strategies for deep-UVC photodetectors.

ZnO nanowire optoelectronic synapse for neuromorphic computing

Artificial synapses that integrate functions of sensing, memory and computing are highly desired for developing brain-inspired neuromorphic hardware. In this work, an optoelectronic synapse based on the ZnO nanowire (NW) transistor is achieved, which can be used to emulate both the short-term and long-term synaptic plasticity. Synaptic potentiation is present when the device is stimulated by light pulses, arising from the light-induced O₂desorption and the persistent photoconductivity behavior of the ZnO NW. On the other hand, synaptic depression occurs when the device is stimulated by electrical pulses in dark, which is realized by introducing a charge trapping layer in the gate dielectric to trap carriers. Simulation of a neural network utilizing the ZnO NW synapses is carried out, demonstrating a high recognition accuracy over 90% after only 20 training epochs for recognizing the Modified National Institute of Standards and Technology digits. The present nanoscale optoelectronic synapse has great potential in the development of neuromorphic visual systems.

Broadband Detection Based on 2D Bi2Se3/ZnO Nanowire Heterojunction

The investigation of photodetectors with broadband response and high responsivity is essential. Zinc Oxide (ZnO) nanowire has the potential of application in photodetectors, owing to the great optoelectrical property and good stability in the atmosphere. However, due to a large number of nonradiative centers at interface and the capture of surface state electrons, the photocurrent of ZnO based photodetectors is still low. In this work, 2D Bi2Se3/ZnO NWAs heterojunction with type-I band alignment is established. This heterojunction device shows not only an enhanced photoresponsivity of 0.15 A/W at 377 nm three times of the bare ZnO nanowire (0.046 A/W), but also a broadband photoresponse from UV to near infrared region has been achieved. These results indicate that the Bi2Se3/ZnO NWAs type-I heterojunction is an ideal photodetector in broadband detection.

Floating metal layer as top electrode over vertically aligned nanorod arrays using angle deposition technique

A floating metal layer (FML) is realized over vertically aligned nanorod arrays (NRAs) using a newly developed angle deposition technique (ADT) that utilizes simultaneous metallization from two identical metal sources. The angle of the sources formed with the tip of the nanorod creates a shadow onto adjacent nanorods in the deposition direction. Computational estimation suggests the length of nanorods embedded in FML depends on the length of NRAs and separation distance between them, and normal height and lateral distance of sources from surface of the substrate. A layer of copper (Cu) is metalized using the proposed ADT on top of hydrothermally grown titanium dioxide NRAs (TiO₂-NRAs) formed over fluorine-doped tin oxide (FTO) coated glass substrate (Cu/TiO₂-NRA/FTO). Current-voltage characteristics through the resulting Cu/TiO₂-NRA/FTO vertical device structure in macroscopically large area recorded by sweeping DC-voltage in cycles of [Formula: see text] exhibits resistive switching with transition from high to low resistance state during [Formula: see text] and regaining of the original high resistance state following negative differential resistance behavior during [Formula: see text].

Rapid assessment of nanomaterial homogeneity reveals crosswise structural gradients in zinc-oxide nanowire arrays

In an effort to scale-up nanomaterial growth over large surface areas, we aim to effectively study the structural non-homogeneities within the arrays of zinc oxide nanowires (ZnO-NWs). The assessment of the lateral gradient of the nanowires' characteristics is presented including their height and surface density. To this end, spectroscopic ellipsometry and the rather recently reported technique of spectral domain attenuated reflectometry are used as two fast, simple and non-invasive characterization methods with further capabilities of scanning over the sample surface. Simple models are proposed by considering ZnO-NWs as the equivalent of thin stratified layers based on the effective medium approach. The methodology not only reveals the presence of gradients, but also enables quantitative analysis for all the samples grown using the hydrothermal method with different growth times ranging from 0.5 h up to 4 h. The gradients are confirmed using scanning electron microscopy (SEM) observations taken as a reference. The results also suggest that the sample orientation during the growth influences the NW growth besides the other parameters already known to affect the growth mechanisms.

Ultra-Violet Electroluminescence of ZnO Nanorods/MEH-PPV Heterojunctions by Optimizing Their Thickness and Using AZO as a Transparent Conductive Electrode

In this paper, a series of ITO/ZnO/ZnO nanorods/MEH-PPV/Al were prepared with different thicknesses of MEH-PPV that were changed from 15, 10 to 7 nm. The electric field in the devices was analyzed. An increase in the electric field on ZnO made hole injection easy and the electrons tunnel fast through thinner MEH-PPV to ZnO. This made the carriers prefer to recombine inside the ZnO layer, and the emission of ZnO was predominant under direct current (DC) bias. Furthermore, another device was fabricated with the structure of AZO (Al-doped ZnO)/ZnO/ZnO nanorods/MEH-PPV/Al. Ultra-violet (UV) electroluminescence (EL) at 387 nm from ZnO band edge emission was realized under DC bias. The turn-on voltage of the devices having AZO as the electrode is lower than that of ITO, and the EL power is enhanced. This work also studies the effect of inserting LiF underneath the Al electrode and above the layer of MEH-PPV. The LiF film inserted caused an obvious decrease in turn-on voltage of the devices and a pronounced increase in the EL power. The mechanism of electroluminescence enhancement is also discussed.

Nanowire Length, Density, and Crystalline Quality Retrieved from a Single Optical Spectrum

We propose spectral domain attenuated reflectometry (SDAR) for fast characterization of nanomaterial growth. The method is demonstrated here for zinc oxide (ZnO) nanowires (NWs) which are grown vertically in random forest fashion showing that it is not limited to well-ordered NWs. We show how SDAR can provide, on the basis of a single measured spectrum, simultaneous information on nanowire length, nanowire density (through nanowire/air filling ratio), and crystalline quality (through band gap). The robustness of the proposed method is assessed first through comparison with information obtained from SEM and XRD taken as reference. In SDAR, the process for fast extraction of NW thickness and filling ratio values  makes use of the interference pattern contrast and the spectral periodicity in the reflection response which involve a best fit of the measured spectra with simple theoretical modeling based on the effective medium approach, achieved with a mean square error down to 0.1%. The results also suggest the existence of either 2 or 3 layers of different effective refractive index, hence providing insight on possible growth mechanisms.

Facile fabrication of self-assembled ZnO nanowire network channels and its gate-controlled UV detection

We demonstrate a facile way to fabricate an array of gate-controllable UV sensors based on assembled zinc oxide nanowire (ZnO NW) network field-effect transistor (FET). This was realized by combining both molecular surface programmed patterning and selective NW assembly on the polar regions avoiding the nonpolar regions, followed by heat treatment at 300 °C to ensure stable contact between NWs. The ZnO NW network FET devices showed typical n-type characteristic with an on-off ratio of 10⁵, transconductance around 47 nS, and mobility around 0.175 cm² V^- 1 s^- 1. In addition, the devices showed photoresponsive behavior to UV light that can be controlled by the applied gate voltage. The photoresponsivity was found to be linearly proportional to the channel voltage V_ds, showing maximum photoresponsivity at V_ds = 7 V.

Effect of structural evolution of ZnO/HfO2 nanocrystals on Eu2+/Eu3+ emission in glass-ceramic waveguides for photonic applications

Eu-doped 70SiO₂-23HfO₂-7ZnO (mol%) glass-ceramic waveguides have been fabricated by sol-gel method as a function of heat-treatment temperatures for on-chip blue-light emitting source applications. Structural evolution of spherical ZnO and spherical as well as rod-like HfO₂ nanocrystalline structures have been observed with heat-treatments at different temperatures. Initially, in the as-prepared samples at 900 ^◦C, both, Eu²⁺ as well as Eu³⁺ ions are found to be present in the ternary matrix. With controlled heat-treatments of up to 1000 ^◦C for 2 h, local environment of Eu-ions become more crystalline in nature and the reduction of Eu³⁺ to Eu²⁺ takes place in such ZnO/HfO₂ crystalline environments. In these ternary glass-ceramic waveguides, heat-treated at higher temperatures, the blue-light emission characteristic, which is the signature of 4f ⁶5d [Formula: see text] 4f ⁷ energy level transition of Eu²⁺ ions is found to be greatly enhanced. The as-prepared glass-ceramic waveguides exhibit a propagation loss of 0.4 ± 0.2 dB cm^-1 at 632.8 nm. Though the propagation losses increase with the growth of nanocrystals, the added functionalities achieved in the optimally heat-treated Eu-doped 70SiO₂-23HfO₂-7ZnO (mol%) waveguides, make them a viable functional optical material for the fabrication of on-chip blue-light emitting sources for integrated optic applications.

Patterned Synthesis of ZnO Nanorod Arrays for Nanoplasmonic Waveguide Applications

We report the patterned synthesis of ZnO nanorod arrays of diameters between 50 nm and 130 nm and various spacings. This was achieved by patterning hole arrays in a polymethyl methacrylate layer with electron beam lithography, followed by chemical synthesis of ZnO nanorods in the patterned holes using the hydrothermal method. The fabrication of ZnO nanorod waveguide arrays is also demonstrated by embedding the nanorods in a silver film using the electroplating process. Optical transmission measurement through the nanorod waveguide arrays is performed and strong resonant transmission of visible light is observed. We have found the resonance shifts to a longer wavelength with increasing nanorod diameter. Furthermore, the resonance wavelength is independent of the nanowaveguide array period, indicating the observed resonant transmission is the effect of a single ZnO nanorod waveguide. These nanorod waveguides may be used in single-molecule imaging and sensing as a result of the nanoscopic profile of the light transmitted through the nanorods and the controlled locations of these nanoscale light sources.

A nanowire array with two types of bromoplumbate chains and high anisotropic conductance

Crystalline nanowire arrays would create great opportunity for novel electrical application. Herein, we report a metal halide-based crystalline nanowire array, which was the first example constructed from two types of bromoplumbate anion chains. The single crystal shows high anisotropic conductivity with semiconducting transport along the c axis and an insulating feature perpendicular to the c direction.

Toward near-white-light electroluminescence from n-ZnO nanocrystals/n-Si isotype heterojunctions via an AZO spectral scissor

A strategy to realize ZnO-based near-white-light electroluminescence (EL) was proposed by utilizing and regulating the intrinsic defect-related emissions of solution-processed ZnO nanocrystals (NCs). Prototype near-white light-emitting diodes (LEDs) based upon this strategy were demonstrated by using n-ZnO NCs/n-Si isotype heterojunctions. The emission color of the n-ZnO NCs/n-Si isotype heterojunction LEDs was tuned toward near white by using an Al-doped ZnO (AZO) spectral "scissor" which can tailor the green light more severely, rather than the blue or red light. Moreover, quantum size effect was clearly observed in both the photoluminescence (PL) and EL spectra via the redshift of the near-band-edge UV emission of the ZnO NCs. The strategy using AZO spectral "scissors" to regulate the V_O-related green emission of ZnO may present a promising pathway to realize ZnO-based white-light LEDs.

Flexible cellulose and ZnO hybrid nanocomposite and its UV sensing characteristics

This paper reports the synthesis and UV sensing characteristics of a cellulose and ZnO hybrid nanocomposite (CEZOHN) prepared by exploiting the synergetic effects of ZnO functionality and the renewability of cellulose. Vertically aligned ZnO nanorods were grown well on a flexible cellulose film by direct ZnO seeding and hydrothermal growing processes. The ZnO nanorods have the wurtzite structure and an aspect ratio of 9 ~ 11. Photoresponse of the prepared CEZOHN was evaluated by measuring photocurrent under UV illumination. CEZOHN shows bi-directional, linear and fast photoresponse as a function of UV intensity. Electrode materials, light sources, repeatability, durability and flexibility of the prepared CEZOHN were tested and the photocurrent generation mechanism is discussed. The silver nanowire coating used for electrodes on CEZOHN is compatible with a transparent UV sensor. The prepared CEZOHN is flexible, transparent and biocompatible, and hence can be used for flexible and wearable UV sensors.

Tuning Optoelectrical Properties of ZnO Nanorods with Excitonic Defects via Submerged Illumination

When applied in optoelectronic devices, a ZnO semiconductor dominantly absorbs or emits ultraviolet light because of its direct electron transition through a wide energy bandgap. On the contrary, crystal defects and nanostructure morphology are the chief key factors for indirect, interband transitions of ZnO optoelectronic devices in the visible light range. By ultraviolet illumination in ultrapure water, we demonstrate here a conceptually unique approach to tune the shape of ZnO nanorods from tapered to capped-end via apical surface morphology control. We show that oxygen vacancy point defects activated by excitonic effects near the tip-edge of a nanorod serve as an optoelectrical hotspot for the light-driven formation and tunability of the optoelectrical properties. A double increase of electron energy absorption on near band edge energy of ZnO was observed near the tip-edge of the tapered nanorod. The optoelectrical hotspot explanation rivals that of conventional electrostatics, impurity control, and alkaline pH control-associated mechanisms. Thus, it highlights a new perspective to understanding light-driven nanorod formation in pure neutral water.

A retrospect on the role of piezoelectric nanogenerators in the development of the green world

This paper gives a detailed report of the evolution and potential applications of piezoelectric nanogenerators (PENGs).

Effects of GaxZn1−xO nanorods on the photoelectric properties of n-ZnO nanorods/p-GaN heterojunction light-emitting diodes

A Ga-doped ZnO nanorod array has been synthesized on a p-GaN/Al₂O₃ substrate by a hydrothermal method at low temperature.

Nanostructured ZnO thin films for self-cleaning applications

Nanostructured ZnO films were deposited by wet chemical methods on glass substrates for self-cleaning applications.

Solution-Grown ZnO Films toward Transparent and Smart Dual-Color Light-Emitting Diode

An individual light-emitting diode (LED) capable of emitting different colors of light under different bias conditions not only allows for compact device integration but also extends the functionality of the LED beyond traditional illumination and display. Herein, we report a color-switchable LED based on solution-grown n-type ZnO on p-GaN/n-GaN heterojunction. The LED emits red light with a peak centered at ∼692 nm and a full width at half-maximum of ∼90 nm under forward bias, while it emits green light under reverse bias. These two lighting colors can be switched repeatedly by reversing the bias polarity. The bias-polarity-switched dual-color LED enables independent control over the lighting color and brightness of each emission with two-terminal operation. The results offer a promising strategy toward transparent, miniaturized, and smart LEDs, which hold great potential in optoelectronics and optical communication.

Plasmon-enhanced Electrically Light-emitting from ZnO Nanorod Arrays/p-GaN Heterostructure Devices

Effective and bright light-emitting-diodes (LEDs) have attracted broad interests in fundamental research and industrial application, especially on short wavelength LEDs. In this paper, a well aligned ZnO nanorod arrays grown on the p-GaN substrate to form a heterostructured light-emitting diode and Al nanoparticles (NPs) were decorated to improve the electroluminescence performance. More than 30-folds enhancement of the electroluminescence intensity was obtained compared with the device without Al NPs decoration. The investigation on the stable and transient photoluminescence spectraof the ZnO nanorod arrays before and after Al NPs decoration demonstrated that the metal surface plasmon resonance coupling with excitons of ZnO leads to the enhancement of the internal quantum efficiency (IQE). Our results provide aneffective approach to design novel optoelectronic devices such as light-emitting diodes and plasmonic nanolasers.

Charge transfer and surface defect healing within ZnO nanoparticle decorated graphene hybrid materials

To harness the unique properties of graphene and ZnO nanoparticles (NPs) for novel applications, the development of graphene-ZnO nanoparticle hybrid materials has attracted great attention and is the subject of ongoing research. For this contribution, graphene-oxide-ZnO (GO-ZnO) and thiol-functionalized reduced graphene oxide-ZnO (TrGO-ZnO) nanohybrid materials were prepared by novel self-assembly processes. Based on electron paramagnetic resonance (EPR) and photoluminescence (PL) investigations on bare ZnO NPs, GO-ZnO and TrGO-ZnO hybrid materials, we found that several physical phenomena were occurring when ZnO NPs were hybridized with GO and TrGO. The electrons trapped in Zn vacancy defects (VZn(-)) within the core of ZnO NPs vanished by transfer to GO and TrGO in the hybrid materials, thus leading to the disappearance of the core signals in the EPR spectra of ZnO NPs. The thiol groups of TrGO and sulfur can effectively "heal" the oxygen vacancy (VO(+)) related surface defects of ZnO NPs while oxygen-containing functionalities have low healing ability at a synthesis temperature of 100 °C. Photoexcited electron transfer from the conduction band of ZnO NPs to graphene leads to photoluminescence (PL) quenching of near band gap emission (NBE) of both GO-ZnO and TrGO-ZnO. Simultaneously, electron transfer from graphene to defect states of ZnO NPs is the origin of enhanced green defect emission from GO-ZnO. This observation is consistent with the energy level diagram model of hybrid materials.

Flexible White Light Emitting Diodes Based on Nitride Nanowires and Nanophosphors

We report the first demonstration of flexible white phosphor-converted light emitting diodes (LEDs) based on p-n junction core/shell nitride nanowires. GaN nanowires containing seven radial In_0.2Ga_0.8N/GaN quantum wells were grown by metal-organic chemical vapor deposition on a sapphire substrate by a catalyst-free approach. To fabricate the flexible LED, the nanowires are embedded into a phosphor-doped polymer matrix, peeled off from the growth substrate, and contacted using a flexible and transparent silver nanowire mesh. The electroluminescence of a flexible device presents a cool-white color with a spectral distribution covering a broad spectral range from 400 to 700 nm. Mechanical bending stress down to a curvature radius of 5 mm does not yield any degradation of the LED performance. The maximal measured external quantum efficiency of the white LED is 9.3%, and the wall plug efficiency is 2.4%.

Highly conductive air-stable ZnO thin film formation under in situ UV illumination for an indium-free transparent electrode

In situ UV irradiation during ALD cycles generates oxygen-vacancies, partially removes O–H bonds, and thereby produces a highly transparent and highly conductive air-stable ZnO film.

An extreme high-performance ultraviolet photovoltaic detector based on a ZnO nanorods/phenanthrene heterojunction

ZnO nanorods/Phen heterojunction based UV photovoltaic detector is fabricated which shows an extremely high performance with a detectivity up to ∼9.0 × 10¹³ Jones.

Enhanced Field Emission from a Carbon Nanotube Array Coated with a Hexagonal Boron Nitride Thin Film

A high-quality field emission electron source made of a highly ordered array of carbon nanotubes (CNTs) coated with a thin film of hexagonal boron nitride (h-BN) is fabricated using a simple and scalable method. This method offers the benefit of reproducibility, as well as the simplicity, safety, and low cost inherent in using B(2)O(3) as the boron precursor. Results measured using h-BN-coated CNT arrays are compared with uncoated control arrays. The optimal thickness of the h-BN film is found to be 3 nm. As a result of the incorporation of h-BN, the turn-on field is found to decrease from 4.11 to 1.36 V μm(-1), which can be explained by the significantly lower emission barrier that is achieved due to the negative electron affinity of h-BN. Meanwhile, the total emission current is observed to increase from 1.6 to 3.7 mA, due to a mechanism that limits the self-current of any individual emitting tip. This phenomenon also leads to improved emission stability and uniformity. In addition, the lifetime of the arrays is improved as well. The h-BN-coated CNT array-based field emitters proposed in this work may open new paths for the development of future high-performance vacuum electronic devices.

Hydrothermally Grown In-doped ZnO Nanorods on p-GaN Films for Color-tunable Heterojunction Light-emitting-diodes

The incorporation of doping elements in ZnO nanostructures plays an important role in adjusting the optical and electrical properties in optoelectronic devices. In the present study, we fabricated 1-D ZnO nanorods (NRs) doped with different In contents (0% ~ 5%) on p-GaN films using a facile hydrothermal method, and investigated the effect of the In doping on the morphology and electronic structure of the NRs and the electrical and optical performances of the n-ZnO NRs/p-GaN heterojunction light emitting diodes (LEDs). As the In content increased, the size (diameter and length) of the NRs increased, and the electrical performance of the LEDs improved. From the electroluminescence (EL) spectra, it was found that the broad green-yellow-orange emission band significantly increased with increasing In content due to the increased defect states (oxygen vacancies) in the ZnO NRs, and consequently, the superposition of the emission bands centered at 415 nm and 570 nm led to the generation of white-light. These results suggest that In doping is an effective way to tailor the morphology and the optical, electronic, and electrical properties of ZnO NRs, as well as the EL emission property of heterojunction LEDs.

Spectroscopy of transmission resonances through a C₆₀ junction

Electron transport through a single C60 molecule on Cu(1 1 1) has been investigated with a scanning tunnelling microscope in tunnelling and contact ranges. Single-C60 junctions have been fabricated by establishing a contact between the molecule and the tip, which is reflected by a down-shift in the lowest unoccupied molecular orbital resonance. These junctions are stable even at elevated bias voltages enabling conductance measurements at high voltages and nonlinear conductance spectroscopy in tunnelling and contact ranges. Spectroscopy and first principles transport calculations clarify the relation between molecular orbital resonances and the junction conductance. Due to the strong molecule-electrode coupling the simple picture of electron transport through individual orbitals does not hold.

Effects of various hybrid nanostructures on antireflective performance of poly-Si solar cells

The comparative of three kinds of hybrid nanostructures (flat film, column and cone) as antireflection layers to reduce reflectivity for solar cells.

Heterojunction of ZnO nanoparticle/PMMA and its ultraviolet electroluminescence

A ZnO nanoparticle layer was synthesized by the sol-gel method on ITO glass. Devices with a ZnO nanoparticle layer/polymethylmethacrylate (PMMA) heterojunction structure were then fabricated and the room temperature electroluminescence (EL) spectra were studied. Under DC bias, ultraviolet (UV) EL at 390 nm from the ZnO band edge emission was observed, and defect-related emission of ZnO at 600 and 760 nm were also detected. The EL mechanisms are discussed in terms of the carrier tunneling process.

Cl-doped ZnO nanowires with metallic conductivity and their application for high-performance photoelectrochemical electrodes

Doping semiconductor nanowires (NWs) for altering their electrical and optical properties is a critical strategy for tailoring the performance of nanodevices. ZnO NWs grown by hydrothermal method are pervasively used in optoelectronic, photovoltaic, and piezoelectric energy-harvesting devices. We synthesized in situ Cl-doped ZnO NWs with metallic conductivity that would fit seamlessly with these devices and improve their performance. Possible Cl doping mechanisms were discussed. UV-visible absorption spectroscopy confirmed the visible light transparency of Cl-doped ZnO NWs. Cl-doped ZnO NW/TiO2 core/shell-structured photoelectrochemical (PEC) anode was fabricated to demonstrate the application potential of highly conductive ZnO NWs. Higher photocurrent density and overall PEC efficiency compared with the undoped ZnO NW-based device were achieved. The successful doping and low resistivity of ZnO could unlock the potential of ZnO NWs for applications in low-cost flexible transparent electrodes.

Unusual electroluminescence from n-ZnO@i-MgO core–shell nanowire color-tunable light-emitting diode at reverse bias

Light-emitting diodes based on n-ZnO@i-MgO core–shell nanowire/p-NiO heterojunction only demonstrated reverse-bias electroluminescence.

Piezotronics and piezo-phototronics: fundamentals and applications

Technology advancement that can provide new solutions and enable augmented capabilities to complementary metal–oxide–semiconductor (CMOS)-based technology, such as active and adaptive interaction between machine and human/ambient, is highly desired. Piezotronic nanodevices and integrated systems exhibit potential in achieving these application goals. Utilizing the gating effect of piezopotential over carrier behaviors in piezoelectric semiconductor materials under externally applied deformation, the piezoelectric and semiconducting properties together with optoelectronic excitation processes can be coupled in these materials for the investigation of novel fundamental physics and the implementation of unprecedented applications. Piezopotential is created by the strain-induced ionic polarization in the piezoelectric semiconducting crystal. Piezotronics deal with the devices fabricated using the piezopotential as a ‘gate’ voltage to tune/control charge-carrier transport across the metal–semiconductor contact or the p–n junction. Piezo-phototronics is to use the piezopotential for controlling the carrier generation, transport, separation and/or recombination for improving the performance of optoelectronic devices. This review intends to provide an overview of the rapid progress in the emerging fields of piezotronics and piezo-phototronics. The concepts and results presented in this review show promises for implementing novel nano-electromechanical devices and integrating with micro/nano-electromechanical system technology to achieve augmented functionalities to the state-of-the-art CMOS technology that may find applications in the human–machine interfacing, active flexible/stretchable electronics, sensing, energy harvesting, biomedical diagnosis/therapy, and prosthetics.

Enhanced photocatalytic activity of quantum-dot-sensitized one-dimensionally-ordered ZnO nanorod photocatalyst

An efficient photocatalyst was fabricated by assembling quantum dots (QDs) onto one-dimensionally-ordered ZnO nanorods, and the photocatalytic properties for Methyl Orange degradation were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV-Vis-NIR absorption spectroscopy and photoluminescence. The results indicate that the catalyst with assembled QDs is more favorable for the degradation than the pristine ZnO nanorods. The QDs with core-shell structure lower the photocatalytic ability due to the higher carrier transport barrier of the ZnS shell layer. Besides its degradation efficiency, the photocatalyst has several advantages given that the one-dimensionally-ordered ZnO nanorods have been grown directly on indium tin oxide substrates. The article provides a new method to design an effective and easily recyclable photocatalyst.

Passivation of surface states in the ZnO nanowire with thermally evaporated copper phthalocyanine for hybrid photodetectors

The adsorption of O2/H2O molecules on the ZnO nanowire (NW) surface results in the long lifetime of photo-generated carriers and thus benefits ZnO NW-based ultraviolet photodetectors by suppressing the dark current and improving the photocurrent gain, but the slow adsorption process also leads to slow detector response time. Here we show that a thermally evaporated copper phthalocyanine film is effective in passivating surface trap states of ZnO NWs. As a result, the organic/inorganic hybrid photodetector devices exhibit simultaneously improved photosensitivity and response time. This work suggests that it could be an effective way in interfacial passivation using organic/inorganic hybrid structures.

Largely enhanced efficiency in ZnO nanowire/p-polymer hybridized inorganic/organic ultraviolet light-emitting diode by piezo-phototronic effect

ZnO nanowire inorganic/organic hybrid ultraviolet (UV) light-emitting diodes (LEDs) have attracted considerable attention as they not only combine the high flexibility of polymers with the structural and chemical stability of inorganic nanostructures but also have a higher light extraction efficiency than thin film structures. However, up to date, the external quantum efficiency of UV LED based on ZnO nanostructures has been limited by a lack of efficient methods to achieve a balance between electron contributed current and hole contributed current that reduces the nonradiative recombination at interface. Here we demonstrate that the piezo-phototronic effect can largely enhance the efficiency of a hybridized inorganic/organic LED made of a ZnO nanowire/p-polymer structure, by trimming the electron current to match the hole current and increasing the localized hole density near the interface through a carrier channel created by piezoelectric polarization charges on the ZnO side. The external efficiency of the hybrid LED was enhanced by at least a factor of 2 after applying a proper strain, reaching 5.92%. This study offers a new concept for increasing organic LED efficiency and has a great potential for a wide variety of high-performance flexible optoelectronic devices.

Optical cavity modes of a single crystalline zinc oxide microsphere

A detailed study on the optical cavity modes of zinc oxide microspheres under the optical excitation is presented. The zinc oxide microspheres with diameters ranging from 1.5 to 3.0 µm are prepared using hydrothermal growth technique. The photoluminescence measurement of a single microsphere shows prominent resonances of whispering gallery modes at room temperature. The experimentally observed whispering gallery modes in the photoluminescence spectrum are compared with theoretical calculations using analytical and finite element methods in order to clarify resonance properties of these modes. The comparison between theoretical analysis and experiment suggests that the dielectric constant of the ZnO microsphere is somewhat different from that for bulk ZnO. The sharp resonances of whispering gallery modes in zinc oxide microspheres cover the entire visible window. They may be utilized in realizations of optical resonators, light emitting devices, and lasers for future chip integrations with micro/nano optoelectronic circuits, and developments of optical biosensors.