MESFET Electronics: Recent Progress on ZnO‐Based Metal‐Semiconductor Field‐Effect Transistors and Their Application in Transparent Integrated Circuits (Adv. Mater. 47/2010)

Metal‐semiconductor field‐effect transistors (MESFETs) are widely known from opaque high‐speed GaAs or high‐power SiC and GaN technology. For the emerging field of transparent electronics, only metal‐insulator‐semiconductor field‐effect transistors (MISFETs) were considered so far. This article reviews the progress of high‐performance MESFETs in oxide electronics and reflects the recent advances of this technique towards transparent MESFET circuitry. We discuss design prospects as well as limitations regarding device performance, reliability and stability. The presented ZnO‐based MESFETs and inverters have superior properties compared to MISFETs, i.e., high channel mobilities and on/off‐ratios, high gain, and low uncertainty level at comparatively low operating voltages. This makes them a promising approach for future low‐cost transparent electronics.     

Directed Self‐Assembly of Templatable Block Copolymers by Easily Accessible Magnetic Control

Magnetic control has been a prosperous and powerful contactless approach in arraying materials into high‐order nanostructures. However, it is tremendously difficult to control organic polymers in this way on account of the weak magnetic response. The preparation of block copolymers (BCPs) with high magnetostatic energy is reported here, relying on an effective electrostatic coupling between paramagnetic ions and polymer side chains. As a result, the BCPs undergo a magnetically directed self‐assembly to form microphase‐segregated nanostructures with long‐range order. It is emphasized that such a precisely controlled alignment of the BCPs is performed upon a single commercial magnet with low‐intensity field (0.35 Tesla). This strategy is profoundly easy‐to‐handle in contrast to routine electromagnetic methods with high‐intensity field (5–10 Tesla). More significantly, the paramagnetic metal component in the BCP samples can be smartly removed, providing a template effect with a preservation of the directed self‐assembled nanofeatures for patterning follow‐up functionalized species through the original binding site.     

Ferroelectric Localized Field–Enhanced ZnO Nanosheet Ultraviolet Photodetector with High Sensitivity and Low Dark Current

Zinc oxide (ZnO) nanosheets have demonstrated outstanding electrical and optical properties, which are well suited for ultraviolet (UV) photodetectors. However, they have a high density of intrinsically unfilled traps, and it is difficult to achieve p‐type doping, leading to the poor performance for low light level switching ratio and a high dark current that limit practical applications in UV photodetection. Here, UV photodetectors based on ZnO nanosheets are demonstrated, whose performance is significantly improved by using a ferroelectric localized field. Specifically, the photodetectors have achieved a responsivity of up to 3.8 × 10⁵ A W^?1, a detectivity of 4.4 × 10¹⁵ Jones, and a photocurrent gain up to 1.24 × 10⁶. These device figures of merit are far beyond those of traditional ZnO ultraviolet photodetectors. In addition, the devices' initial dark current can be easily restored after continuous photocurrent measurement by using a positive gate voltage pulse. This study establishes a new approach to produce high‐sensitivity and low‐dark‐current ultraviolet photodetectors and presents a crucial step for further practical applications.     

Self‐Powered Photoelectrochemical Biosensor Based on CdS/RGO/ZnO Nanowire Array Heterostructure

A CdS/reduced graphene oxide (RGO)/ZnO nanowire array (NWAs) heterostructure is designed, which exhibits enhanced photoelectrochemical (PEC) activity compared to pure ZnO, RGO/ZnO, and CdS/ZnO. The enhancement can be attributed to the synergistic effect of the high electron mobility of ordered 1D ZnO NWAs, extended visible‐light absorption of CdS nanocrystals, and the formed type II band alignment between them. Moreover, the incorporation of RGO further promotes the charge carrier separation and transfer process due to its excellent charge collection and shuttling characteristics. Subsequently, the CdS/RGO/ZnO heterostructure is successfully utilized for the PEC bioanalysis of glutathione at 0 V (vs Ag/AgCl). The self‐powered device demonstrates satisfactory sensing performance with rapid response, a wide detection range from 0.05 mm to 1 mm , an acceptable detection limit of 10 μm , as well as certain selectivity, reproducibility, and stability. Therefore, the CdS/RGO/ZnO heterostructure has opened up a promising channel for the development of PEC biosensors.     

Seed‐Induced Growth of Flower‐Like Au–Ni–ZnO Metal–Semiconductor Hybrid Nanocrystals for Photocatalytic Applications

The combination of metal and semiconductor components in nanoscale to form a hybrid nanocrystal provides an important approach for achieving advanced functional materials with special optical, magnetic and photocatalytic functionalities. Here, a facile solution method is reported for the synthesis of Au–Ni–ZnO metal–semiconductor hybrid nanocrystals with a flower‐like morphology and multifunctional properties. This synthetic strategy uses noble and magnetic metal Au@Ni nanocrystal seeds formed in situ to induce the heteroepitaxial growth of semiconducting ZnO nanopyramids onto the surface of metal cores. Evidence of epitaxial growth of ZnO{0001} facets on Ni {111} facets is observed on the heterojunction, even though there is a large lattice mismatch between the semiconducting and magnetic components. Adjustment of the amount of Au and Ni precursors can control the size and composition of the metal core, and consequently modify the surface plasmon resonance (SPR) and magnetic properties. Room‐temperature superparamagnetic properties can be achieved by tuning the size of Ni core. The as‐prepared Au–Ni–ZnO nanocrystals are strongly photocatalytic and can be separated and re‐cycled by virtue of their magnetic properties. The simultaneous combination of plasmonic, semiconducting and magnetic components within a single hybrid nanocrystal furnishes it multifunctionalities that may find wide potential applications.     

Detection of Nitric Oxide from Living Cells Using Polymeric Zinc Organic Framework‐Derived Zinc Oxide Composite with Conducting Polymer

Sensitive and selective detection of nitric oxide (NO) in the human body is crucial since it has the vital roles in the physiological and pathological processes. This study reports a new type of electrochemical NO biosensor based on zinc‐dithiooxamide framework derived porous ZnO nanoparticles and polyterthiophene‐rGO composite. By taking advantage of the synergetic effect between ZnO and poly(TTBA‐rGO) (TTBA = 3′‐(p‐benzoic acid)‐2,2′:5′,2″‐terthiophene, rGO = reduced graphene oxide) nanocomposite layer, the poly(TTBA‐rGO)/ZnO sensor probe displays excellent electrocatalytic activity and explores to detect NO released from normal and cancer cell lines. The ZnO is immobilized on a composite layer of poly(TTBA‐rGO). The highly porous ZnO offers a high electrolyte accessible surface area and high ion–electron transport rates that efficiently catalyze the NO reduction reaction. Amperometry with the modified electrode displays highly sensitive response and wide dynamic range of 0.019–76 × 10^?6m with the detection limit of 7.7 ± 0.43 × 10^?9m . The sensor probe is demonstrated to detect NO released from living cells by drug stimulation. The proposed sensor provides a powerful platform for the low detection limit that is feasible for real‐time analysis of NO in a biological system.     

AFM‐Based Spin‐Exchange Microscopy Using Chiral Molecules

Local magnetic imaging at nanoscale resolution is desirable for basic studies of magnetic materials and for magnetic logic and memories. However, such local imaging is hard to achieve by means of standard magnetic force microscopy. Other techniques require low temperatures, high vacuum, or strict limitations on the sample conditions. A simple and robust method is presented for locally resolved magnetic imaging based on short‐range spin‐exchange interactions that can be scaled down to atomic resolution. The presented method requires a conventional AFM tip functionalized with a chiral molecule. In proximity to the measured magnetic sample, charge redistribution in the chiral molecule leads to a transient spin state, caused by the chiral‐induced spin‐selectivity effect, followed by the exchange interaction with the imaged sample. While magnetic force microscopy imaging strongly depends on a large working distance, an accurate image is achieved using the molecular tip in proximity to the sample. The chiral molecules' spin‐exchange interaction is found to be 150 meV. Using the tip with the adsorbed chiral molecules, two oppositely magnetized samples are characterized, and a magnetic imaging is performed. This method is simple to perform at room temperature and does not require high‐vacuum conditions.     

Site‐Specific Self‐Assembled Liquid‐Gated ZnO Nanowire Transistors for Sensing Applications

A scalable bottom‐up solution‐based approach for the site‐specific realization of ZnO nanowire (ZnO‐NW)‐based field‐effect transistors for sensing applications in liquids is reported. The nanowires are grown across predefined electrodes patterned by photolithography. Site specificity is attained by the use of nanoparticles acting as seeds. Using integrated on‐chip microchannels and microfabricated gate electrodes, electrochemically gated ZnO‐NW network transistors functioning in liquids are demonstrated. The optimized devices are rendered sensitive to pH through chemical functionalization. The unique combination of the sensitivity, site specificity, scalability, and cost effectiveness of the technique opens up avenues for the routine realization of one‐dimensional nanostructure‐based chemical and biosensors for analytical and diagnostic applications.     

Controlled Growth of High‐Quality ZnO‐Based Films and Fabrication of Visible‐Blind and Solar‐Blind Ultra‐Violet Detectors

ZnO is a wide‐bandgap (3.37 eV at room temperature) oxide semiconductor that is attractive for its great potential in short‐wavelength optoelectronic devices, in which high quality films and heterostructures are essential for high performance. In this study, controlled growth of ZnO‐based thin films and heterostructures by molecular beam epitaxy (MBE) is demonstrated on different substrates with emphasis on interface engineering. It is revealed that ultrathin AlN or MgO interfacial layers play a key role in establishing structural and chemical compatibility between ZnO and substrates. Furthermore, a quasi‐homo buffer is introduced prior to growth of a wurtzite MgZnO epilayer to suppress the phase segregation of rock‐salt MgO, achieving wide‐range bandgap tuning from 3.3 to 4.55 eV. Finally, a visible‐blind UV detector exploiting a double heterojunction of n‐ZnO/insulator‐MgO/p‐Si and a solar‐blind UV detector using MgZnO as an active layer are fabricated by using the growth techniques discussed here.     

Additive Manufacturing of 3D‐Architected Multifunctional Metal Oxides

Additive manufacturing (AM) of complex three‐dimensional (3D) metal oxides at the micro‐ and nanoscales has attracted considerable attention in recent years. State‐of‐the‐art techniques that use slurry‐based or organic–inorganic photoresins are often hampered by challenges in resin preparation and synthesis, and/or by the limited resolution of patterned features. A facile process for fabricating 3D‐architected metal oxides via the use of an aqueous metal‐ion‐containing photoresin is presented. The efficacy of this process, which is termed photopolymer complex synthesis, is demonstrated by creating nanoarchitected zinc oxide (ZnO) architectures with feature sizes of 250 nm, by first patterning a zinc‐ion‐containing aqueous photoresin using two‐photon lithography and subsequently calcining them at 500 ºC. Transmission electron microscopy (TEM) analysis reveals their microstructure to be nanocrystalline ZnO with grain sizes of 5.1 ± 1.6 nm. In situ compression experiments conducted in a scanning electron microscope show an emergent electromechanical response: a 200 nm mechanical compression of an architected ZnO structure results in a voltage drop of 0.52 mV. This photopolymer complex synthesis provides a pathway to easily create arbitrarily shaped 3D metal oxides that could enable previously impossible devices and smart materials.     

Flexible,Printable Soft‐X‐Ray Detectors Based on All‐Inorganic Perovskite Quantum Dots

Metal halide perovskites represent a family of the most promising materials for fascinating photovoltaic and photodetector applications due to their unique optoelectronic properties and much needed simple and low‐cost fabrication process. The high atomic number (Z) of their constituents and significantly higher carrier mobility also make perovskite semiconductors suitable for the detection of ionizing radiation. By taking advantage of that, the direct detection of soft‐X‐ray‐induced photocurrent is demonstrated in both rigid and flexible detectors based on all‐inorganic halide perovskite quantum dots (QDs) synthesized via a solution process. Utilizing a synchrotron soft‐X‐ray beamline, high sensitivities of up to 1450 µC Gy_air^?1 cm^?2 are achieved under an X‐ray dose rate of 0.0172 mGy_air s^?1 with only 0.1 V bias voltage, which is about 70‐fold more sensitive than conventional α‐Se devices. Furthermore, the perovskite film is printed homogeneously on various substrates by the inexpensive inkjet printing method to demonstrate large‐scale fabrication of arrays of multichannel detectors. These results suggest that the perovskite QDs are ideal candidates for the detection of soft X‐rays and for large‐area flat or flexible panels with tremendous application potential in multidimensional and different architectures imaging technologies.     

Plasmonic ZnO/Ag Embedded Structures as Collecting Layers for Photogenerating Electrons in Solar Hydrogen Generation Photoelectrodes

A new fabrication strategy in which Ag plasmonics are embedded in the interface between ZnO nanorods and a conducting substrate is experimentally demonstrated using a femtosecond‐laser (fs‐laser)‐induced plasmonic ZnO/Ag photoelectrodes. This fs‐laser fabrication technique can be applied to generate patternable plasmonic nanostructures for improving their effectiveness in hydrogen generation. Plasmonic ZnO/Ag nanostructure photoelectrodes show an increase in the photocurrent of a ZnO nanorod photoelectrodes by higher than 85% at 0.5 V. Both localized surface plasmon resonance in metal nanoparticles and plasmon polaritons propagating at the metal/semiconductor interface are available for improving the capture of sunlight and collecting charge carriers. Furthermore, in‐situ X‐ray absorption spectroscopy is performed to monitor the plasmonic‐generating electromagnetic field upon the interface between ZnO/Ag nanostructures. This can reveal induced vacancies on the conduction band of ZnO, which allow effective separation of charge carriers and improves the efficiency of hydrogen generation. Plasmon‐induced effects enhance the photoresponse simultaneously, by improving optical absorbance and facilitating the separation of charge carriers.     

Velocity‐based formulations for standard and quasi‐incompressible hypoelastic‐plastic solids

We present three velocity‐based updated Lagrangian formulations for standard and quasi‐incompressible hypoelastic‐plastic solids. Three low‐order finite elements are derived and tested for non‐linear solid mechanics problems. The so‐called V‐element is based on a standard velocity approach, while a mixed velocity–pressure formulation is used for the VP and the VPS elements. The two‐field problem is solved via a two‐step Gauss–Seidel partitioned iterative scheme. First, the momentum equations are solved in terms of velocity increments, as for the V‐element. Then, the constitutive relation for the pressure is solved using the updated velocities obtained at the previous step. For the VPS‐element, the formulation is stabilized using the finite calculus method in order to solve problems involving quasi‐incompressible materials. All the solid elements are validated by solving two‐dimensional and three‐dimensional benchmark problems in statics as in dynamics. Copyright © 2016 John Wiley & Sons, Ltd.     

ZnO Nanostructures for Dye‐Sensitized Solar Cells

This Review focuses on recent developments in the use of ZnO nanostructures for dye‐sensitized solar cell (DSC) applications. It is shown that carefully designed and fabricated nanostructured ZnO films are advantageous for use as a DSC photoelectrode as they offer larger surface areas than bulk film material, direct electron pathways, or effective light‐scattering centers, and, when combined with TiO₂, produce a core–shell structure that reduces the combination rate. The limitations of ZnO‐based DSCs are also discussed and several possible methods are proposed so as to expand the knowledge of ZnO to TiO₂, motivating further improvement in the power‐conversion efficiency of DSCs.     

Self‐Assembled Ordered Three‐Phase Au–BaTiO3–ZnO Vertically Aligned Nanocomposites Achieved by a Templating Method

Complex multiphase nanocomposite designs present enormous opportunities for developing next‐generation integrated photonic and electronic devices. Here, a unique three‐phase nanostructure combining a ferroelectric BaTiO₃, a wide‐bandgap semiconductor of ZnO, and a plasmonic metal of Au toward multifunctionalities is demonstrated. By a novel two‐step templated growth, a highly ordered Au–BaTiO₃–ZnO nanocomposite in a unique “nanoman”‐like form, i.e., self‐assembled ZnO nanopillars and Au nanopillars in a BaTiO₃ matrix, is realized, and is very different from the random three‐phase ones with randomly arranged Au nanoparticles and ZnO nanopillars in the BaTiO₃ matrix. The ordered three‐phase “nanoman”‐like structure provides unique functionalities such as obvious hyperbolic dispersion in the visible and near‐infrared regime enabled by the highly anisotropic nanostructures compared to other random structures. Such a self‐assembled and ordered three‐phase nanocomposite is obtained through a combination of vapor–liquid–solid (VLS) and two‐phase epitaxy growth mechanisms. The study opens up new possibilities in the design, growth, and application of multiphase structures and provides a new approach to engineer the ordering of complex nanocomposite systems with unprecedented control over electron–light–matter interactions at the nanoscale.     

High‐Quality,Ultraconformal Aluminum‐Doped Zinc Oxide Nanoplasmonic and Hyperbolic Metamaterials

Aluminum‐doped zinc oxide (AZO) is a tunable low‐loss plasmonic material capable of supporting dopant concentrations high enough to operate at telecommunication wavelengths. Due to its ultrahigh conformality and compatibility with semiconductor processing, atomic layer deposition (ALD) is a powerful tool for many plasmonic applications. However, despite many attempts, high‐quality AZO with a plasma frequency below 1550 nm has not yet been realized by ALD. Here a simple procedure is devised to tune the optical constants of AZO and enable plasmonic activity at 1550 nm with low loss. The highly conformal nature of ALD is also exploited to coat silicon nanopillars to create localized surface plasmon resonances that are tunable by adjusting the aluminum concentration, thermal conditions, and the use of a ZnO buffer layer. The high‐quality AZO is then used to make a layered AZO/ZnO structure that displays negative refraction in the telecommunication wavelength region due to hyperbolic dispersion. Finally, a novel synthetic scheme is demonstrated to create AZO embedded nanowires in ZnO, which also exhibits hyperbolic dispersion.     

Cellulose‐Based Microparticles for Magnetically Controlled Optical Modulation and Sensing

Responsive materials with birefringent optical properties have been exploited for the manipulation of light in several modern electronic devices. While electrical fields are often utilized to achieve optical modulation, magnetic stimuli may offer an enticing complementary approach for controlling and manipulating light remotely. Here, the synthesis and characterization of magnetically responsive birefringent microparticles with unusual magneto‐optical properties are reported. These functional microparticles are prepared via a microfluidic emulsification process, in which water‐based droplets are generated in a flow‐focusing device and stretched into anisotropic shapes before conversion into particles via photopolymerization. Birefringence properties are achieved by aligning cellulose nanocrystals within the microparticles during droplet stretching, whereas magnetic responsiveness results from the addition of superparamagnetic nanoparticles to the initial droplet template. When suspended in a fluid, the microparticles can be controllably manipulated via an external magnetic field to result in unique magneto‐optical coupling effects. Using a remotely actuated magnetic field coupled to a polarized optical microscope, these microparticles can be employed to convert magnetic into optical signals or to estimate the viscosity of the suspending fluid through magnetically driven microrheology.     

Cl‐Doped ZnO Nanowire Arrays on 3D Graphene Foam with Highly Efficient Field Emission and Photocatalytic Properties

An environmentally friendly, low‐cost, and large‐scale method is developed for fabrication of Cl‐doped ZnO nanowire arrays (NWAs) on 3D graphene foam (Cl‐ZnO NWAs/GF), and investigates its applications as a highly efficient field emitter and photocatalyst. The introduction of Cl‐dopant in ZnO increases free electrons in the conduction band of ZnO and also leads to the rough surface of ZnO NWAs, which greatly improves the field emission properties of the Cl‐ZnO NWAs/GF. The Cl‐ZnO NWAs/GF demonstrates a low turn‐on field (≈1.6 V μm⁻¹), a high field enhancement factor (≈12844), and excellent field emission stability. Also, the Cl‐ZnO NWAs/GF shows high photocatalytic efficiency under UV irradiation, enabling photodegradation of organic dyes such as RhB within ≈75 min, with excellent recyclability. The excellent photocatalytic performance of the Cl‐ZnO NWAs/GF originates from the highly efficient charge separation efficiency at the heterointerface of Cl‐ZnO and GF, as well as improved electron transport efficiency due to the doping of Cl. These results open up new possibilities of using Cl‐ZnO and graphene‐based hybrid nanostructures for various functional devices.     

Large‐Scale,Long‐Range‐Ordered Patterning of Nanocrystals via Capillary‐Bridge Manipulation

Deterministic assembly of nanoparticles with programmable patterns is a core opportunity for property‐by‐design fabrication and large‐scale integration of functional materials and devices. The wet‐chemical‐synthesized colloidal nanocrystals are compatible with solution assembly techniques, thus possessing advantages of high efficiency, low cost, and large scale. However, conventional solution process suffers from tradeoffs between spatial precision and long‐range order of nanocrystal assembly arising from the uncontrollable dewetting dynamics and fluid flow. Here, a capillary‐bridge manipulation method is demonstrated for directing the dewetting of nanocrystal inks and deterministically patterning long‐range‐ordered superlattice structures. This is achieved by employing micropillars with programmable size, arrangement, and shape, which permits deterministic manipulation of geometry, position, and dewetting dynamics of capillary bridges. Various superlattice structures, including one‐dimensional (1D), circle, square, pentagon, hexagon, pentagram, cross arrays, are fabricated. Compared to the glassy thin films, long‐range‐ordered superlattice arrays exhibit improved ferroelectric polarization. Coassembly of nanocrystal superlattice and organic functional molecule is further demonstrated. Through introducing azobenzene into superlattice arrays, a switchable ferroelectric polarization is realized, which is triggered by order–disorder transition of nanocrystal stacking in reversible isomerization process of azobenzene. This method offers a platform for patterning nanocrystal superlattices and fabricating microdevices with functionalities for multiferroics, electronics, and photonics.     

A p‐adaptive method for electromagnetic wave propagation

The discontinuous Galerkin FEM is used for the numerical solution of the three‐dimensional Maxwell equations. Control of errors in the numerical level for the divergence‐free constraint of the magnetic field can be obtained through the use of divergence‐free vector bases. In this work, the so‐called perfectly hyperbolic formulation of the Maxwell equations is used to retain both divergence‐free magnetic field and in the presence of charges to satisfy the Gauss constraint for the electric field at the numerical level. For both approaches, it is found that higher‐order approximations have favorable effect on the preservation of the divergence constraints and that the perfectly hyperbolic formulations retains these errors to a lower level. It is shown that high‐order accuracy in space and time is achieved in unstructured meshes using implicit time marching. For nonuniform meshes, local resolution refinement is used using p‐type adaptivity to ensure accurate electromagnetic wave propagation. Thus, the potential of the method to reach the required higher resolution in anisotropic meshes and obtain accurate electromagnetic wave propagation with reduced computational effort is demonstrated.