Single ZnO nanowire/p-type GaN heterojunctions for photovoltaic devices and UV light-emitting diodes

Enhanced UV Emission from ZnO on Silver Nanoparticle Arrays by the Surface Plasmon Resonance Effect

Periodical silver nanoparticle (NP) arrays were fabricated by magnetron sputtering method with anodic aluminum oxide templates to enhance the UV light emission from ZnO by the surface plasmon resonance effect. Theoretical simulations indicated that the surface plasmon resonance wavelength depended on the diameter and space of Ag NP arrays. By introducing Ag NP arrays with the diameter of 40 nm and space of 100 nm, the photoluminescence intensity of the near band-edge emission from ZnO was twofold enhanced. Time-resolved photoluminescence measurement and energy band analysis indicated that the UV light emission enhancement was attributed to the coupling between the surface plasmons in Ag NP arrays and the excitons in ZnO with the improved spontaneous emission rate and enhanced local electromagnetic fields.

Graphene/ZnO Nanowire/p-GaN Vertical Junction for a High-Performance Nanoscale Light Source

We report on a high-brightness ultraviolet (UV) nanoscale light source. The light emission diodes are constructed with graphene/ZnO nanowire/p-GaN vertical junctions, which exhibit strong UV electroluminescence (EL) emissions centered at a wavelength of 397 nm at one end of the ZnO nanowire. Compared to the horizontal heterojunction, the vertical junction based on the ZnO nanowire increases the interface area of the heterojunction along with a high-quality interface, thus making the device robust under a large excitation current. In this structure, transparent flexible graphene is used as the top electrode, which can effectively improve performance by increasing the carrier injection area. Moreover, by analyzing the relationship between the integrated light intensity and applied bias, a superlinear dependency with a slope of 3.99 is observed, which means high electrical-to-optical conversion efficiency. Three electron-hole irradiation recombination processes are distinguished according to the EL emission spectra.

Near unity charge separation efficiency leads to pure ultraviolet emission in few layer graphene nanosheets

Two-dimensional materials with van der Waals structure attract intense interest due to their high performance in ultrathin optoelectronic devices. In particular, the high efficiency charge separation between the two-dimensional materials can significantly improve the photo-response of a given device. Here we report the discovery of pure ultraviolet (UV) emission from few layer graphene nanosheets (GNS). Near unity charge separation efficiency is key to pure UV emission. The dynamics of an excited electron were analyzed using femtosecond transient absorption techniques. Electron transfer is observed from surface defect states induced by oxygen-containing functional groups to intrinsic sp² domain states in few layer GNS. Moreover, a solar blind response device based on few layer GNS with a high on-off ratio was successfully fabricated.

Polar-Induced Selective Epitaxial Growth of Multijunction Nanoribbons for High-Performance Optoelectronics

Semiconductor heterostructures are basic building blocks for modern electronics and optoelectronics. However, it still remains a great challenge to combine different semiconductor materials in single nanostructures with tailored geometry and chemical composition. Here, a polar-induced selective epitaxial growth method is reported to alternately grow CdS and CdS _xSe_{1- x} heterostructure nanoribbons (NRs) side by side in the lateral direction, with the heterointerface (junction) number to be well controlled. Transmission electron microscopy (TEM) and spatial-resolved μ-PL spectra are employed to characterize the heterostructure NRs, which indicate that the achieved NRs are high-quality heterostructures with sharp interfaces. Kelvin probe force microscopy (KPFM) and femtosecond pump-probe characterizations further confirm the efficient charge-transfer process across the interfaces in the multijunction NRs. Photodetectors based on the achieved NRs are realized and systematically investigated, demonstrating junction number-dependent optoelectronic response behaviors. NRs with more junctions exhibit more superior device performances, reflecting the important roles of the high-quality interface regions. Based on this multijunction NRs device, high on-off ratio (10⁷) and remarkable responsivity (1.5 × 10⁵ A/W) are demonstrated, both of which represent the best results compared to the reported CdS, CdSe, and their heterostructures. These novel multijunction NRs may find broad applications in future integrated photonics and optoelectronics devices and systems.

Wavelength-Tunable Ultraviolet Electroluminescence from Ga-Doped ZnO Microwires

The usage of ZnO as active layers to fabricate hybrid heterojunction light-emitting diodes is expected to be an effective approach for ultraviolet light sources. Individual ZnO microwires with controlled gallium (Ga) incorporation (ZnO/Ga MWs) have been fabricated via a chemical vapor deposition method. It is found that with the increasing Ga-incorporated concentration, the near-band-edge (NBE) photoluminescence of the ZnO MWs blue-shifted gradually from 390 to 370 nm. Heterojunction diodes comprising single ZnO/Ga MWs and p-GaN have been fabricated. With increasing injection currents, the interfacial emissions can be suppressed effectively and the typical NBE emission dominates the electroluminescence (EL). In particular, with increasing Ga-doping concentration, the dominant EL emission wavelengths of the ZnO/Ga MW-based heterojunction diodes blue-shifted from 384 to 372 nm, and the blue shift can be ascribed to the Burstein-Moss effect induced by the Ga incorporation. The present work demonstrates the feasibility of optical band gap engineering of ZnO MWs and the potential application for wavelength-tuning ultraviolet light sources.

Direct Vapor Growth of Perovskite CsPbBr3 Nanoplate Electroluminescence Devices

Metal halide perovskite nanostructures hold great promises as nanoscale light sources for integrated photonics due to their excellent optoelectronic properties. However, it remains a great challenge to fabricate halide perovskite nanodevices using traditional lithographic methods because the halide perovskites can be dissolved in polar solvents that are required in the traditional device fabrication process. Herein, we report single CsPbBr₃ nanoplate electroluminescence (EL) devices fabricated by directly growing CsPbBr₃ nanoplates on prepatterned indium tin oxide (ITO) electrodes via a vapor-phase deposition. Bright EL occurs in the region near the negatively biased contact, with a turn-on voltage of ∼3 V, a narrow full width at half-maximum of 22 nm, and an external quantum efficiency of ∼0.2%. Moreover, through scanning photocurrent microscopy and surface electrostatic potential measurements, we found that the formation of ITO/p-type CsPbBr₃ Schottky barriers with highly efficient carrier injection is essential in realizing the EL. The formation of the ITO/p-type CsPbBr₃ Schottky diode is also confirmed by the corresponding transistor characteristics. The achievement of EL nanodevices enabled by directly grown perovskite nanostructures could find applications in on-chip integrated photonics circuits and systems.

A Self-Powered Fast-Response Ultraviolet Detector of p-n Homojunction Assembled from Two ZnO-Based Nanowires

ABSTRACT

Nowadays, fabrication of micro/nano-scale electronic devices with bottom-up approach is paid much research attention. Here, we provide a novel micro/nano-assembling method, which is accurate and efficient, especially suitable for the fabrication of micro/nano-scale electronic devices. Using this method, a self-powered ZnO/Sb-doped ZnO nanowire p-n homojunction ultraviolet detector (UVD) was fabricated, and the detailed photoelectric properties were tested. At a reverse bias of -0.1 V under UV light illumination, the photoresponse sensitivity of the UVD was 26.5 and the rise/decay time of the UVD was as short as 30 ms. The micro/nano-assembling method has wide potential applications in the fabrication of specific micro/nano-scale electronic devices.

GRAPHICAL ABSTRACT

A self-powered ZnO/Sb-doped ZnO nanowire p-n homojunction ultraviolet detector (UVD) was fabricated by using a novel micro/nano-assembling method with bottom-up approach. At reverse bias of -0.1 V under UV light illumination, the photoresponse sensitivity of the UVD was 26.5, and the rise time and decay time of the UVD were as short as 30 ms.

Surface-plasmon mediated photoluminescence enhancement of Pt-coated ZnO nanowires by inserting an atomic-layer-deposited Al₂O₃ spacer layer

Surface-plasmon mediated photoluminescence emission enhancement has been investigated for ZnO nanowire (NW)/Pt nanoparticle (NP) nanostructures by inserting an Al2O3 spacer layer. The thickness of the Al2O3 spacer layer and of the Pt NPs capped on the ZnO NWs are well controlled by atomic layer deposition. It is found that the photoluminescence property of the ZnO NW/Al2O3/Pt hybrid structure is highly tunable with respect to the thickness of the inserted Al2O3 spacer layer. The highest enhancement (∼14 times) of the near band emission of ZnO NWs is obtained with an optimized Al2O3 spacer layer thickness of 10 nm leading to a ultraviolet-visible emission ratio of 271.2 compared to 18.8 for bare ZnO NWs. The enhancement of emission is influenced by a Förster-type non-radiative energy transfer process of the exciton energy from ZnO NWs to Pt NPs as well as the coupling effect between excitons of ZnO NWs and surface plasmons of Pt NPs. The highly versatile and tunable photoluminescence properties of Pt-coated ZnO NWs achieved by introducing an Al2O3 spacer layer demonstrate their potential application in highly efficient optoelectronic devices.

Enhancing UV-emissions through optical and electronic dual-function tuning of Ag nanoparticles hybridized with n-ZnO nanorods/p-GaN heterojunction light-emitting diodes

ZnO nanorods (NRs) and Ag nanoparticles (NPs) are known to enhance the luminescence of light-emitting diodes (LEDs) through the high directionality of waveguide mode transmission and efficient energy transfer of localized surface plasmon (LSP) resonances, respectively. In this work, we have demonstrated Ag NP-incorporated n-ZnO NRs/p-GaN heterojunctions by facilely hydrothermally growing ZnO NRs on Ag NP-covered GaN, in which the Ag NPs were introduced and randomly distributed on the p-GaN surface to excite the LSP resonances. Compared with the reference LED, the light-output power of the near-band-edge (NBE) emission (ZnO, λ = 380 nm) of our hybridized structure is increased almost 1.5-2 times and can be further modified in a controlled manner by varying the surface morphology of the surrounding medium of the Ag NPs. The improved light-output power is mainly attributed to the LSP resonance between the NBE emission of ZnO NRs and LSPs in Ag NPs. We also observed different behaviors in the electroluminescence (EL) spectra as the injection current increases for the treatment and reference LEDs. This observation might be attributed to the modification of the energy band diagram for introducing Ag NPs at the interface between n-ZnO NRs and p-GaN. Our results pave the way for developing advanced nanostructured LED devices with high luminescence efficiency in the UV emission regime.

Strain Loading Mode Dependent Bandgap Deformation Potential in ZnO Micro/Nanowires

The electronic-mechanical coupling in semiconductor nanostructures under different strain loading modes can modulate their photoelectric properties in different manners. Here, we report the systematic investigation on the strain mode dependent bandgap deformation potential of ZnO micro/nanowires under both uniaxial tensile and bending strains at room temperature. Uniaxial stretching-photoluminescence results show that the deformation potential of the smaller ZnO nanowire (with diameter d = 260 nm) is -30.6 meV/%, and is close to the bulk value, whereas it deviates the bulk value and becomes to be -10.6 meV/% when the wire diameter is increased to d = 2 μm. This unconventional size dependence stems from surface effect induced inhomogeneous strain in the surface layer and the core of the ZnO micro/nanowires under uniaxial tension. For bending load mode, the in situ high-resolution transmission electron microscope analysis reveals that the local strain distributes linearly in the bending cross section. Further cathodoluminescence measurements on a bending ZnO microwire (d = 1.8 μm) demonstrate that the deformation potential is -27 meV/%, whose absolute value is much larger than that of the ZnO microwire under uniaxial tension. Further analysis reveals that the distinct deformation potentials originate from the different deforming modes in ZnO micro/nanowires under bending or uniaxial tensile strains. Our results should facilitate the design of flexible optoelectronic nanodevices.

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.

Dislocation-induced nanoparticle decoration on a GaN nanowire

GaN nanowires with homoepitaxial decorated GaN nanoparticles on their surface along the radial direction have been synthesized by means of a chemical vapor deposition method. The growth of GaN nanowires is catalyzed by Au particles via the vapor-liquid-solid (VLS) mechanism. Screw dislocations are generated along the radial direction of the nanowires under slight Zn doping. In contrast to the metal-catalyst-assisted VLS growth, GaN nanoparticles are found to prefer to nucleate and grow at these dislocation sites. High-resolution transmission electron microscopy (HRTEM) analysis demonstrates that the GaN nanoparticles possess two types of epitaxial orientation with respect to the corresponding GaN nanowire: (I) [1̅21̅0]np//[1̅21̅0]nw, (0001)np//(0001)nw; (II) [1̅21̅3]np//[12̅10]nw, (101̅0)np//(101̅0)nw. An increased Ga signal in the energy-dispersive spectroscopy (EDS) profile lines of the nanowires suggests GaN nanoparticle growth at the edge surface of the wires. All the crystallographic results confirm the importance of the dislocations with respect to the homoepitaxial growth of the GaN nanoparticles. Here, screw dislocations situated on the (0001) plane provide the self-step source to enable nucleation of the GaN nanoparticles.

A comprehensive review on synthesis methods for transition-metal oxide nanostructures

Recent developments of transition-metal oxide nanostructures with designed shape and dimensionality, including various synthesis methods and applications, are presented.

Optical and Sensing Properties of Cu Doped ZnO Nanocrystalline Thin Films

Undoped and Cu doped ZnO films of two different molarities deposited by spray pyrolysis using zinc nitrate and cupric chloride as precursors show polycrystalline nature and hexagonal wurtzite structure of ZnO. The crystallite size varies between 10 and 21 nm. Doping increases the transmittance of the films whereas the optical band gap of ZnO is reduced from 3.28 to 3.18 eV. With increment in doping the surface morphology changes from irregular shaped grains to netted structure with holes and then to net making needle-like structures which lends gas sensing characteristics to the films. Undoped ZnO shows maximum sensitivity at 400°C for higher concentration of CO2. The sensitivity of Cu doped sample is maximum at 200°C for all CO2concentrations from 500 to 4000 ppm.

Doped ZnO 1D nanostructures: synthesis, properties, and photodetector application

In the past decades, the doping of ZnO one-dimensional nanostructures has attracted a great deal of attention due to the variety of possible morphologies, large surface-to-volume ratios, simple and low cost processing, and excellent physical properties for fabricating high-performance electronic, magnetic, and optoelectronic devices. This article mainly concentrates on recent advances regarding the doping of ZnO one-dimensional nanostructures, including a brief overview of the vapor phase transport method and hydrothermal method, as well as the fabrication process for photodetectors. The dopant elements include B, Al, Ga, In, N, P, As, Sb, Ag, Cu, Ti, Na, K, Li, La, C, F, Cl, H, Mg, Mn, S, and Sn. The various dopants which act as acceptors or donors to realize either p-type or n-type are discussed. Doping to alter optical properties is also considered. Lastly, the perspectives and future research outlook of doped ZnO nanostructures are summarized.

Effect of annealing on the structural, morphological and photoluminescence properties of ZnO thin films prepared by spin coating

Zinc oxide (ZnO) thin films were deposited on silicon substrates by a sol-gel method using the spin coating technique. The ZnO films were annealed at 700°C in an oxygen environment using different annealing times ranging from 1 to 4 h. It was observed that all the annealed films exhibited a hexagonal wurtzite structure. The particle size increased from 65 to 160 nm with the increase in annealing time, while the roughness of the films increased from 2.3 to 10.6 nm with the increase in the annealing time. Si diffusion from the substrate into the ZnO layer occurred during the annealing process. It is likely that the Si and O2 influenced the emission of the ZnO by reducing the amount of Zn defects and the creation of new oxygen related defects during annealing in the O2 atmosphere. The emission intensity was found to be dependent on the reflectance of the thin films.

Ultraviolet electroluminescence of light-emitting diodes based on single n-ZnO/p-AlGaN heterojunction nanowires

We present successful fabrication of single n-ZnO/p-AlGaN heterojunction nanowires with excellent optoelectronic properties. Because of the formation of high-quality interfacial structure, heterojunction nanowire showed a diodelike rectification behavior and an electroluminescence (EL) ultraviolet (UV) emission centered at 394 nm from a single nanowire was observed when the injection current is 4 μA due to high exciton efficiency in the interfacial layer between ZnO and AlGaN. With the increase of the applied current, the EL peak at 5 μA becomes weaker revealing an optimal injection current of less than 5 μA. These results are expected to open up new application possibilities in nanoscale UV light-emitting devices based on single ZnO heterostructure.

Intense emission from ZnO nanocolumn Schottky diodes

Zinc oxide (ZnO) nanocolumns have been prepared by a metal-organic chemical vapor deposition technique, and structural and optical characterization reveal that the nanocolumns have high crystalline and luminescent qualities. Au/MgO/ZnO/In structured Schottky diodes have been fabricated from the nanocolumns. An intense emission can be detected from the diodes under the drive of bias voltage, and the output power can reach 3.7 μW. The intense emission comes from both the high crystalline and luminescent qualities of the ZnO nanocolumns, and the ideal Schottky contact formed in the Au/MgO/ZnO/In structures.

Highly efficient, spectrally pure 340 nm ultraviolet emission from AlxGa₁-xN nanowire based light emitting diodes

High crystal quality, vertically aligned AlxGa1-xN nanowire based double heterojunction light emitting diodes (LEDs) are grown on Si substrate by molecular beam epitaxy. Such AlxGa1-xN nanowires exhibit unique core-shell structures, which can significantly suppress surface nonradiative recombination. We successfully demonstrate highly efficient AlxGa1-xN nanowire array based LEDs operating at ∼340 nm. Such nanowire devices exhibit superior electrical and optical performance, including an internal quantum efficiency of ∼59% at room temperature, a relatively small series resistance, highly stable emission characteristics, and the absence of efficiency droop under pulsed biasing conditions.

CuInSe2 nanowires from facile chemical transformation of In2Se3 and their integration in single-nanowire devices

Nanowire solar cells are receiving a significant amount of attention for their potential to improve light absorption and charge collection in photovoltaics. Single-nanowire solar cells offer the ability to investigate performance limits for macroscale devices, as well as the opportunity for in-depth structural characterization and property measurement in small working devices. Copper indium selenide (CIS) is a material uniquely suited to these investigations. Not only could nanowire solar cells of CIS perhaps allow efficient macroscale photovoltaics to be fabricated while reducing the amount of CIS required, important for a system with possible resource limitations, but it is also a photovoltaic material for which fundamental understanding has been elusive. We here present a recipe for a scaled up vapor liquid solid based synthesis of CIS nanowires, in-depth material and property correlation of single crystalline CIS nanowires, and the first report of a single CIS nanowire solar cell. The synthesis was accomplished by annealing copper-coated In2Se3 nanowires at a moderate temperature of 350 °C, leading to solid-state reaction forming CIS nanowires. These nanowires are p-type with a resitivity of 6.5 Ωcm. Evidence is observed for a strong diameter dependence on the nanowire transport properties. The single-nanowire solar cells have an open-circuit voltage of 500 mV and a short-circuit current of 2 pA under AM 1.5 illumination.

Fabrication of lateral electrodes on semiconductor nanowires through structurally matched insulation for functional optoelectronics

A strategy of using structurally matched alumina insulation to produce lateral electrodes on semiconductor nanowires is presented. Nanowires in the architecture are structurally matched with alumina insulation using selective anodic oxidation. Lateral electrodes are fabricated by directly evaporating metallic atoms onto the opposite sides of the nanowires. The integrated architecture with lateral electrodes propels carriers to transport them across nanowires and is crucially beneficial to the injection/extraction in optoelectronics. The matched architecture and the insulating properties of the alumina layer are investigated experimentally. ZnO nanowires are functionalized into an ultraviolet photodiode as an example. The present strategy successfully implements an advantageous architecture and is significant in developing diverse semiconductor nanowires in optoelectronic applications.

Piezotronic effects on the optical properties of ZnO nanowires

We report the piezotronic effects on the photoluminescence (PL) properties of bent ZnO nanowires (NWs). We find that the piezoelectric field largely modifies the spatial distribution of the photoexcited carriers in a bent ZnO NW. This effect, together with strain-induced changes in the energy band structure due to the piezoresistive effects, results in a net redshift of free exciton PL emission from a bent ZnO NW. At the large-size limit, this net redshift depends only on the strain parameter, but it is size-dependent if the diameter of the NW is comparable to that of the depletion layer. The experimental data obtained using the near-field scanning optical microscopy technique at low temperatures support our theoretical model.

Photo-response of a nanopore device with a single embedded ZnO nanoparticle

The photo-response of a ZnO nanoparticle embedded in a nanopore made on a silicon nitride membrane is investigated. The ZnO nanoparticle is manipulated onto the nanopore and sandwiched between aluminum contact electrodes from both the top and bottom. The asymmetric device structure facilitates current-voltage rectification that enables photovoltaic capacity. Under illumination, the device shows open-circuit voltage as well as short-circuit current. The fill factor is found to increase at low temperatures and reaches 48.6% at 100 K. The nanopore structure and the manipulation technique provide a solid platform for exploring the electrical properties of single nanoparticles.

Photovoltaic device on a single ZnO nanowire p-n homojunction

A photovoltaic device was successfully grown solely based on the single ZnO p-n homojunction nanowire. The ZnO nanowire p-n diode consists of an as-grown n-type segment and an in situ arsenic-doped p-type segment. This p-n homojunction acts as a good photovoltaic cell, producing a photocurrent almost 45 times larger than the dark current under reverse-biased conditions. Our results demonstrate that the present ZnO p-n homojunction nanowire can be used as a self-powered ultraviolet photodetector as well as a photovoltaic cell, which can also be used as an ultralow electrical power source for nanoscale electronic, optoelectronic and medical devices.

Giant optical anisotropy of oblique-aligned ZnO nanowire arrays

A combined method of modified oblique-angle deposition and hydrothermal growth was adopted to grow an optically anisotropic nanomaterial based on single crystalline ZnO nanowire arrays (NWAs) with highly oblique angles (75°-85°), exhibiting giant in-plane birefringence and optical polarization degree in emission. The in-plane birefringence of oblique-aligned ZnO NWAs is almost one order of magnitude higher than that of natural quartz. The strong optical anisotropy in emission due to the optical confinement was observed. The oblique-aligned NWAs not only allow important technological applications in passive photonic components but also benefit the development of the optoelectronic devices in polarized light sensing and emission.

Improved performance of ZnO nanowire field-effect transistors via focused ion beam treatment

A seven orders of magnitude increase in the current on/off ratio of ZnO nanowire field-effect transistors (FETs) after Ga( + ) irradiation was observed. Transmission electron microscopy characterization revealed that the surface crystal quality of the ZnO nanowire was improved via the Ga( + ) treatment. The Ga( + ) irradiation efficiently reduces chemisorption effects and decreases oxygen vacancies in the surface layer. The enhanced performance of the nanowire FET was attributed to the decrease of surface trapped electrons and the decrease in carrier concentration.

Self-powered, ultrafast, visible-blind UV detection and optical logical operation based on ZnO/GaN nanoscale p-n junctions

Ultrafast-response (20 μs) UV detectors, which are visible-blind and self-powered, in devices where an n-type ZnO nanowire partially lies on a p-type GaN film, are demonstrated. Moreover, a CdSe-nanowire red-light detector powered by a nanoscale ZnO/GaN photovoltaic cell is also demonstrated, which extends the device function to a selective multiwavelength photodetector and shows the function of an optical logical AND gate.

Characterizations of low-temperature electroluminescence from ZnO nanowire light-emitting arrays on the p-GaN layer

Low-temperature electroluminescence from ZnO nanowire light-emitting arrays is reported. By inserting a thin MgO current blocking layer in between ZnO nanowire and p-GaN, high-purity UV light emission at wavelength 398 nm was obtained. As the temperature is decreased, contrary to the typical GaN-based light emitting diodes, our device shows a decrease of optical output intensity. The results are associated with various carrier tunneling processes and frozen MgO defects.