Electron Holographic Study of Semiconductor Light‐Emitting Diodes

Semiconductor light‐emitting diodes (LEDs), especially GaN‐based heterostructures, are widely used in light illumination. The lack of inversion symmetry of wurtzite crystal structures and the lattice mismatch at heterointerfaces cause large polarization fields with contributions from both spontaneous polarization and piezoelectric polarization, which in turn results in obvious quantum confined stark effect. It is possible to alleviate this effect if the local electrostatic fields and band alignment induced charge redistribution can be quantitatively determined across the heterostructures. In this Concept, the applications of electron holography to investigate semiconductor LEDs are summarized. Following the off‐axis electron holography scheme, the GaN‐based LED heterostructures including InGaN/GaN‐based quantum wells, other GaN‐based quantum wells, and other forms of GaN‐based LED materials are discussed, focusing on the local potential drops, polarization fields, and charge distributions. Moreover, GaAs‐based LED heterostructures are briefly discussed. The in‐line electron holography scheme emphasizes the capability of large area strain mapping across LED heterostructures with high spatial resolution and accuracy, which is combined with quantitative electrostatic measurements and other advanced transmission electron microscopy characterizations to provide an overall nanometer scale perspective of LED devices for further improvement in their electric and optical properties.     

Measurement of Local Si‐Nanowire Growth Kinetics Using In situ Transmission Electron Microscopy of Heated Cantilevers

A technique to study nanowire growth processes on locally heated microcantilevers in situ in a transmission electron microscope has been developed. The in situ observations allow the characterization of the nucleation process of silicon wires, as well as the measurement of growth rates of individual nanowires and the ability to observe the formation of nanowire bridges between separate cantilevers to form a complete nanowire device. How well the nanowires can be nucleated controllably on typical cantilever sidewalls is examined, and the measurements of nanowire growth rates are used to calibrate the cantilever‐heater parameters used in finite‐element models of cantilever heating profiles, useful for optimization of the design of devices requiring local growth.     

Dopant-free GaN/AlN/AlGaN radial nanowire heterostructures as high electron mobility transistors

We report the rational synthesis of dopant-free GaN/AlN/AlGaN radial nanowire heterostructures and their implementation as high electron mobility transistors (HEMTs). The radial nanowire heterostructures were prepared by sequential shell growth immediately following nanowire elongation using metal-organic chemical vapor deposition (MOCVD). Transmission electron microscopy (TEM) studies reveal that the GaN/AlN/AlGaN radial nanowire heterostructures are dislocation-free single crystals. In addition, the thicknesses and compositions of the individual AlN and AlGaN shells were unambiguously identified using cross-sectional high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM). Transport measurements carried out on GaN/AlN/AlGaN and GaN nanowires prepared using similar conditions demonstrate the existence of electron gas in the undoped GaN/AlN/AlGaN nanowire heterostructures and also yield an intrinsic electron mobility of 3100 cm(2)/Vs and 21,000 cm(2)/Vs at room temperature and 5 K, respectively, for the heterostructure. Field-effect transistors fabricated with ZrO(2) dielectrics and metal top gates showed excellent gate coupling with near ideal subthreshold slopes of 68 mV/dec, an on/off current ratio of 10(7), and scaled on-current and transconductance values of 500 mA/mm and 420 mS/mm. The ability to control synthetically the electronic properties of nanowires using band structure design in III-nitride radial nanowire heterostructures opens up new opportunities for nanoelectronics and provides a new platform to study the physics of low-dimensional electron gases.     

Growing Oxide Nanowires and Nanowire Networks by Solid State Contact Diffusion into Solution‐Processed Thin Films

New techniques to directly grow metal oxide nanowire networks without the need for initial nanoparticle seed deposition or postsynthesis nanowire casting will bridge the gap between bottom‐up formation and top‐down processing for many electronic, photonic, energy storage, and conversion technologies. Whether etched top‐down, or grown from catalyst nanoparticles bottom‐up, nanowire growth relies on heterogeneous material seeds. Converting surface oxide films, ubiquitous in the microelectronics industry, to nanowires and nanowire networks by the incorporation of extra species through interdiffusion can provide an alternative deposition method. It is shown that solution‐processed thin films of oxides can be converted and recrystallized into nanowires and networks of nanowires by solid‐state interdiffusion of ionic species from a mechanically contacted donor substrate. NaVO₃ nanowire networks on smooth Si/SiO₂ and granular fluorine‐doped tin oxide surfaces can be formed by low‐temperature annealing of a Na diffusion species‐containing donor glass to a solution‐processed V₂O₅ thin film, where recrystallization drives nanowire growth according to the crystal habit of the new oxide phase. This technique illustrates a new method for the direct formation of complex metal oxide nanowires on technologically relevant substrates, from smooth semiconductors, to transparent conducting materials and interdigitated device structures.     

A Transparent Nanowire‐Based Cell Impalement Device Suitable for Detailed Cell–Nanowire Interaction Studies

A method to fabricate inexpensive and transparent nanowire impalement devices is invented based on CuO nanowire arrays grown by thermal oxidation. By employing a novel process the nanowires are transferred to a transparent, cell‐compatible epoxy membrane. Cargo delivery and detailed cell‐nanowire interaction studies are performed, revealing that the cell plasma membrane tightly wraps the nanowires, while cell membrane penetration is not observed. The presented device offers an efficient investigation platform for further optimization, leading towards a simple and versatile impalement delivery system.     

Ordered Arrays of Nanostructures and Applications in High‐Efficient Nano‐Generators

Large‐scale ordered nanostructure arrays on substrates, including nanowires, nanotubes, nanodots, and nano‐holes, can be fabricated using template fabrication processes. The controllable structural parameters and properties of the ordered nanostructure arrays make them quite suitable to be used in many device‐related application areas. It is shown that large‐scale nanowire arrays are good candidates for the realization of a nano‐generator based on the piezoelectric effect of ZnO nanowires. The mechanism of a proposed high‐efficient nano‐generator based on an assembled nanowire/nanohole embedded structure shows high application potentials for biological and nanometer‐sized devices.     

Progress in Piezo‐Phototronic‐Effect‐Enhanced Light‐Emitting Diodes and Pressure Imaging

Wurtzite materials exhibit both semiconductor and piezoelectric properties under strains due to the non‐central symmetric crystal structures. The three‐way coupling of semiconductor properties, piezoelectric polarization and optical excitation in ZnO, GaN, CdS and other piezoelectric semiconductors leads to the emerging field of piezo‐phototronics. This effect can efficiently manipulate the emission intensity of light‐emitting diodes (LEDs) by utilizing the piezo‐polarization charges created at the junction upon straining to modulate the energy band diagrams and the optoelectronic processes, such as generation, separation, recombination and/or transport of charge carriers. Starting from fundamental physics principles, recent progress in piezo‐phototronic‐effect‐enhanced LEDs is reviewed; following their development from single‐nanowire pressure‐sensitive devices to high‐resolution array matrices for pressure‐distribution mapping applications. The piezo‐phototronic effect provides a promising method to enhance the light emission of LEDs based on piezoelectric semiconductors through applying static strains, and may find perspective applications in various optoelectronic devices and integrated systems.     

GaN纳米线材料的特性和制备技术

GaN是一种具有优越热稳定性和化学性质的宽禁带半导体材料，这种材料及相关器件可以工作在高温、高辐射等恶劣环境中，并可用于大功率微波器件．最近几年，由于GaN蓝光二极管的成功研制，使GaN成为了化合物半导体领域中最热门的研究课题．简要介绍了GaN纳米线材料的制备技术；综述了GaN纳米线材料的制备结果和特性．用CVD法研制的GaN纳米线的直径已经达到5～12nm，长度达到几百个微米．纳米线具有GaN的六方纤锌矿结构，其PL谱具有宽的发射峰，谱峰中心在420nm．GaN纳米线已经在肖特基二极管的研制中得到应用．     

Full-color InGaN/GaN dot-in-a-wire light emitting diodes on silicon

We report on the achievement of a new class of nanowire light emitting diodes (LEDs), incorporating InGaN/GaN dot-in-a-wire nanoscale heterostructures grown directly on Si(111) substrates. Strong emission across nearly the entire visible wavelength range can be realized by varying the dot composition. Moreover, we have demonstrated phosphor-free white LEDs by controlling the indium content in the dots in a single epitaxial growth step. Such devices can exhibit relatively high internal quantum efficiency (>20%) and no apparent efficiency droop for current densities up to ~ 200 A cm(-2).     

Edge‐Epitaxial Growth of InSe Nanowires toward High‐Performance Photodetectors

Semiconducting nanowires offer many opportunities for electronic and optoelectronic device applications due to their unique geometries and physical properties. However, it is challenging to synthesize semiconducting nanowires directly on a SiO₂/Si substrate due to lattice mismatch. Here, a catalysis‐free approach is developed to achieve direct synthesis of long and straight InSe nanowires on SiO₂/Si substrates through edge‐homoepitaxial growth. Parallel InSe nanowires are achieved further on SiO₂/Si substrates through controlling growth conditions. The underlying growth mechanism is attributed to a selenium self‐driven vapor–liquid–solid process, which is distinct from the conventional metal‐catalytic vapor–liquid–solid method widely used for growing Si and III–V nanowires. Furthermore, it is demonstrated that the as‐grown InSe nanowire‐based visible light photodetector simultaneously possesses an extraordinary photoresponsivity of 271 A W^?1, ultrahigh detectivity of 1.57 × 10¹⁴ Jones, and a fast response speed of microsecond scale. The excellent performance of the photodetector indicates that as‐grown InSe nanowires are promising in future optoelectronic applications. More importantly, the proposed edge‐homoepitaxial approach may open up a novel avenue for direct synthesis of semiconducting nanowire arrays on SiO₂/Si substrates.     

High‐Brightness Blue Light‐Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer

Single‐crystalline GaN‐based light‐emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid‐state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high‐quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high‐brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal–organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and In_xGa_1?xN/GaN multiple quantum well structures. The as‐fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one‐step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high‐brightness LEDs.     

Metal Nanowire Networks: The Next Generation of Transparent Conductors

There is an ongoing drive to replace the most common transparent conductor, indium tin oxide (ITO), with a material that gives comparable performance, but can be coated from solution at speeds orders of magnitude faster than the sputtering processes used to deposit ITO. Metal nanowires are currently the only alternative to ITO that meets these requirements. This Progress Report summarizes recent advances toward understanding the relationship between the structure of metal nanowires, the electrical and optical properties of metal nanowires, and the properties of a network of metal nanowires. Using the structure–property relationship of metal nanowire networks as a roadmap, this Progress Report describes different synthetic strategies to produce metal nanowires with the desired properties. Practical aspects of processing metal nanowires into high‐performance transparent conducting films are discussed, as well as the use of nanowire films in a variety of applications.     

Submicrometre resolved optical characterization of green nanowire-based light emitting diodes

The electroluminescent properties of InGaN/GaN nanowire-based light emitting diodes (LEDs) are studied at different resolution scales. Axial one-dimensional heterostructures were grown by plasma-assisted molecular beam epitaxy (PAMBE) directly on a silicon (111) substrate and consist of the following sequentially deposited layers: n-type GaN, three undoped InGaN/GaN quantum wells, p-type AlGaN electron blocking layer and p-type GaN. From the macroscopic point of view, the devices emit light in the green spectral range (around 550 nm) under electrical injection. At 100 mA DC current, a 1 mm2 chip that integrates around 10(7) nanowires emits an output power on the order of 10 μW. However, the emission of the nanowire-based LED shows a spotty and polychromatic emission. By using a confocal microscope, we have been able to improve the spatial resolution of the optical characterizations down to the submicrometre scale that can be assessed to a single nanowire. Detailed μ-electroluminescent characterization (emission wavelength and output power) over a representative number of single nanowires provides new insights into the vertically integrated nanowire-based LED operation. By combining both μ-electroluminescent and μ-photoluminescent excitation, we have experimentally shown that electrical injection failure is the major source of losses in these nanowire-based LEDs.     

A Microribbon Hybrid Structure of CoOx‐MoC Encapsulated in N‐Doped Carbon Nanowire Derived from MOF as Efficient Oxygen Evolution Electrocatalysts

Developing highly efficient electrocatalysts for oxygen evolution is vital for renewable and sustainable energy production and storage. Herein, nitrogen‐doped carbon encapsulated CoOx‐MoC heterostructures are reported for the first time as high performance oxygen evolution electrocatalysts. The composition can be tuned by the addition of a Mo source to form a nanowire‐assembled hierarchically porous microstructure, which can enlarge the specific surface area, thus exposing more active sites, facilitating mass transport and charge transfer. Moreover, it is demonstrated that the formation of CoOx‐MoC heterostructures and the resulting synergistic effect between MoC and Co facilitate the reaction kinetics, leading to significantly improved oxygen evolution reaction (OER) activity with an onset overpotential of merely 290 mV, and a low overpotential of 330 mV to afford a current density of 10 mA cm^?2. The well‐constructed microarchitecture contributes to superior long term stability electrochemical behaviors. This work provides a facile strategy via composition tuning and structure optimization for the development of next‐generation nonprecious metal‐based OER electrocatalysts.     

Multi-quantum-well nanowire heterostructures for wavelength-controlled lasers

Rational design and synthesis of nanowires with increasingly complex structures can yield enhanced and/or novel electronic and photonic functions. For example, Ge/Si core/shell nanowires have exhibited substantially higher performance as field-effect transistors and low-temperature quantum devices compared with homogeneous materials, and nano-roughened Si nanowires were recently shown to have an unusually high thermoelectric figure of merit. Here, we report the first multi-quantum-well (MQW) core/shell nanowire heterostructures based on well-defined III-nitride materials that enable lasing over a broad range of wavelengths at room temperature. Transmission electron microscopy studies show that the triangular GaN nanowire cores enable epitaxial and dislocation-free growth of highly uniform (InGaN/GaN)n quantum wells with n=3, 13 and 26 and InGaN well thicknesses of 1-3 nm. Optical excitation of individual MQW nanowire structures yielded lasing with InGaN quantum-well composition-dependent emission from 365 to 494 nm, and threshold dependent on quantum well number, n. Our work demonstrates a new level of complexity in nanowire structures, which potentially can yield free-standing injection nanolasers.     

Doping GaP Core–Shell Nanowire pn‐Junctions: A Study by Off‐Axis Electron Holography

The doping process in GaP core–shell nanowire pn‐junctions using different precursors is evaluated by mapping the nanowires' electrostatic potential distribution by means of off‐axis electron holography. Three precursors, triethyltin (TESn), ditertiarybutylselenide, and silane are investigated for n‐type doping of nanowire shells; among them, TESn is shown to be the most efficient precursor. Off‐axis electron holography reveals higher electrostatic potentials in the regions of nanowire cores grown by the vapor–liquid–solid (VLS) mechanism (axial growth) than the regions grown parasitically by the vapor–solid (VS) mechanism (radial growth), attributed to different incorporation efficiency between VLS and VS of unintentional p‐type carbon doping originating from the trimethylgallium precursor. This study shows that off‐axis electron holography of doped nanowires is unique in terms of the ability to map the electrostatic potential and thereby the active dopant distribution with high spatial resolution.     

Molecular Nanowire Bonding to Epitaxial Single‐Layer MoS2 by an On‐Surface Ullmann Coupling Reaction

Lateral heterostructures consisting of 2D transition metal dichalcogenides (TMDCs) directly interfaced with molecular networks or nanowires can be used to construct new hybrid materials with interesting electronic and spintronic properties. However, chemical methods for selective and controllable bond formation between 2D materials and organic molecular networks need to be developed. As a demonstration of a self‐assembled organic nanowire‐TMDC system, a method to link and interconnect epitaxial single‐layer MoS₂ flakes with organic molecules is demonstrated. Whereas pristine epitaxial single‐layer MoS₂ has no affinity for molecular attachment, it is found that single‐layer MoS₂ will selectively bind the organic molecule 2,8‐dibromodibenzothiophene (DBDBT) in a surface‐assisted Ullmann coupling reaction when the MoS₂ has been activated by pre‐exposing it to hydrogen. Atom‐resolved scanning tunneling microscopy (STM) imaging is used to analyze the bonding of the nanowires, and thereby it is revealed that selective bonding takes place on a specific S atom at the corner site between the two types of zig‐zag edges available in a hexagonal single layer MoS₂ sheet. The method reported here successfully combining synthesis of epitaxial TMDCs and Ullmann coupling reactions on surfaces may open up new synthesis routes for 2D organic‐TMDC hybrid materials.     

Features of Transport in Ultrathin Gold Nanowire Structures

The origin of the interface formation appearing due to the realization of contacts to ultrathin gold nanowire devices is revealed. Such interfaces play an important role in transport mechanisms in nanowire structures and can determine the electrical and operating parameters of a nanodevice. Based on experimental results, the specific electrical properties of bundles of ultrathin gold nanowires fabricated by wet chemical synthesis and subsequently assembled and contacted with gold electrodes are reported. It is demonstrated that these properties are strongly affected by the monolayers of organic molecules inevitably present on the surface of the nanowires due to synthetic conditions. In particular, such layers form a potential barrier to tunneling of the electrons from contacts to the nanowires. The electric transport behavior of the investigated nanowire structures in the temperature range from 500 mK to 300 K obeys the model of thermal fluctuation‐induced tunneling conduction through the nanowire‐metal electrode molecular junction. Application of this model allows calculation of the parameters of the molecular potential barrier. The formation of such a molecular barrier is verified by scanning tunneling microscope (STM) and transmission electron microscope (TEM) measurements performed using a supporting graphene layer. These findings are important for designing novel nanodevices for molecular electronics on the basis of ultrathin nanowires.     

Fabrication of Nanowire Channels with Unidirectional Alignment and Controlled Length by a Simple,Gas‐Blowing‐Assisted,Selective‐Transfer‐Printing Technique

A printing‐based lithographic technique for the patterning of V₂O₅ nanowire channels with unidirectional orientation and controlled length is introduced. The simple, directional blowing of a patterned polymer stamp with N₂ gas, inked with randomly distributed V₂O₅ nanowires, induces alignment of the nanowires perpendicular to the long axis of the line patterns. Subsequent stamping on the amine‐terminated surface results in the selective transfer of the aligned nanowires with a controlled length corresponding to the width of the relief region of the polymer stamp. By employing such a gas‐blowing‐assisted, selective‐transfer‐printing technique, two kinds of device structures consisting of nanowire channels and two metal electrodes with top contact, whereby the nanowires were aligned either parallel (parallel device) or perpendicular (serial device) to the current flow in the conduction channel, are fabricated. The electrical properties demonstrate a noticeable difference between the two devices, with a large hysteresis in the parallel device but none in the serial device. Systematic analysis of the hysteresis and the electrical stability account for the observed hysteresis in terms of the proton diffusion in the water layer of the V₂O₅ nanowires, induced by the application of an external bias voltage higher than a certain threshold voltage.     

Temperature-dependent nonradiative recombination processes in GaN-based nanowire white-light-emitting diodes on silicon

In this paper, we have performed a detailed investigation of the temperature- and current-dependent emission characteristics of nanowire light-emitting diodes, wherein InGaN/GaN dot-in-a-wire nanoscale heterostructures and a p-doped AlGaN electron blocking layer are incorporated in the device's active region to achieve white-light emission and to prevent electron overflow, respectively. Through these studies, the Auger coefficient is estimated to be in the range of ～10(-34) cm(6) s(-1) or less, which is nearly four orders of magnitude smaller than the commonly reported values of planar InGaN/GaN heterostructures, suggesting Auger recombination plays an essentially negligible role in the performance of GaN-based nanowire light-emitting diodes. It is observed, however, that the performance of such nanowire LEDs suffers severely from Shockley-Read-Hall recombination, which can account for nearly 40% of the total carrier recombination under moderate injection conditions (～100 A cm(-2)) at room temperature. The Shockley-Read-Hall nonradiative lifetime is estimated to be in the range of a few nanoseconds at room temperature, which correlates well with the surface recombination velocity of GaN and the wire diameters used in this experiment.