Controlled Growth of ZnO Nanowires and Their Optical Properties

This article surveys recent developments in the rational synthesis of single‐crystalline zinc oxide nanowires and their unique optical properties. The growth of ZnO nanowires was carried out in a simple chemical vapor transport and condensation (CVTC) system. Based on our fundamental understanding of the vapor–liquid–solid (VLS) nanowire growth mechanism, different levels of growth controls (including positional, orientational, diameter, and density control) have been achieved. Power‐dependent emission has been examined and lasing action was observed in these ZnO nanowires when the excitation intensity exceeds a threshold (∼40 kW cm^–2). These short‐wavelength nanolasers operate at room temperature and the areal density of these nanolasers on substrate readily reaches 1 × 10¹⁰ cm^–2. The observation of lasing action in these nanowire arrays without any fabricated mirrors indicates these single‐crystalline, well‐facetted nanowires can function as self‐contained optical resonance cavities. This argument is further supported by our recent near‐field scanning optical microscopy (NSOM) studies on single nanowires.     

Novel Polygonal Vanadium Oxide Nanoscrolls as Stable Cathode for Lithium Storage

Scroll‐shape structures with adjustable space provide interlayer sliding to accommodate the volume changes, which are promising candidates for increasing the stability of lithium batteries (LBs). In this work, for the first time, novel vanadium oxide polygonal nanoscrolls (PNSs) are synthesized in solution through self‐rolling, Ostwald ripening, and scroll‐by‐scroll processes. The PNSs are of various shapes (including triangle, quadrangle, pentagon, and so forth) and spiral‐wrapped multiwall. When evaluated as cathode for LB, the vanadium oxide PNSs cathode exhibits largely enhanced cycling stability (capacity retention of 91.7% after 150 cycles at 0.1 A g^–1 in 2.0–4.0 V) compared with those of nonscrolled nanobelts (40.0%) and nanowires (35.8%). Even at 1.0 A g^–1, the PNSs cathode delivers high‐rate long‐life performance with capacity retention of 80.6% after 500 cycles. The unique polygonal nanoscroll structure is favorable for improving the cyclability and rate capability in energy storage applications as demonstrated here, and it will be interesting and has great potential for other related applications.     

Interfacial Reaction‐Directed Synthesis of Ce–Mn Binary Oxide Nanotubes and Their Applications in CO Oxidation and Water Treatment

Interfacial oxidation–reduction reaction is herein developed to prepare hollow binary oxide nanostructures. Ce–Mn nanotubes are fabricated by treating Ce(OH)CO₃ templates with KMnO₄ aqueous solution, where MnO₄^? is reduced to manganese oxide and the Ce³⁺ in Ce(OH)CO₃ is simultaneously oxidized to form cerium oxide, followed by selective wash with HNO₃. The resulting Ce–Mn binary oxide nanotubes exhibit high catalytic activity towards CO oxidation and show significant adsorption capacity of Congo red. Moreover, guided by the same interfacial‐reaction principle, binary oxide hollow nanostructures with different shapes and compositions are synthesized. Specifically, hollow Ce–Mn binary oxide cubes, and Co‐Mn and Ce‐Fe binary oxide hollow nanostructures are achieved by changing the shape of the Ce(OH)CO₃ templates from rods to cubes, by changing the tempates from Ce(OH)CO₃ nanorods to Co(CO₃)_0.35Cl_0.20(OH)_1.10 nanowires, and by replacing the oxidant of KMnO₄ with another strong one, K₂FeO₄, respectively. This work is expected to open a new, simple avenue for the general synthesis of hollow binary oxide nanostructures.     

Heteroepitaxial Patterned Growth of Vertically Aligned and Periodically Distributed ZnO Nanowires on GaN Using Laser Interference Ablation

A simple two‐step method of fabricating vertically aligned and periodically distributed ZnO nanowires on gallium nitride (GaN) substrates is described. The method combines laser interference ablation (LIA) and low temperature hydrothermal decomposition. The ZnO nanowires grow heteroepitaxially on unablated regions of GaN over areas spanning 1 cm², with a high degree of control over size, orientation, uniformity, and periodicity. High resolution transmission electron microscopy and scanning electron microscopy are utilized to study the structural characteristics of the LIA‐patterned GaN substrate in detail. These studies reveal the possible mechanism for the preferential, site‐selective growth of the ZnO nanowires. The method demonstrates high application potential for wafer‐scale integration into sensor arrays, piezoelectric devices, and optoelectronic devices.     

Characterization and Field‐Emission Properties of Needle‐like Zinc Oxide Nanowires Grown Vertically on Conductive Zinc Oxide Films

Needle‐like ZnO nanowires with high density are grown uniformly and vertically over an entire Ga‐doped conductive ZnO film at 550 °C. The nanowires are grown preferentially in the c‐axis direction. The X‐ray diffraction (XRD) θ‐scan curve shows a full width at half maximum (FWHM) value of 2°. This indicates that the c‐axes of the nanorods are along the normal direction of the substrate surface. The investigation using high‐resolution transmission electron microscopy (HRTEM) confirmed that each nanowire is a single crystal. A room‐temperature photoluminescence (PL) spectrum of the wires consists of a strong and sharp UV emission band at 380 nm and a weak and broad green–yellow band. It reveals a low concentration of oxygen vacancies in the ZnO nanowires and their high optical quality. Field electron emission from the wires was also investigated. The turn‐on field for the ZnO nanowires was found to be about 18 V μm^–1 at a current density of 0.01 μA cm^–2. The emission current density from the ZnO nanowires reached 0.1 mA cm^–2 at a bias field of 24 V μm^–1.     

The Relationship Between Nanoscale Structure and Electrochemical Properties of Vanadium Oxide Nanorolls

In this paper, we explore the relationship between the nanoscale structure and electrochemical performance of nanoscale scrolls of vanadium oxides (vanadium oxide nanorolls). The vanadium oxide nanorolls, which are synthesized through a ligand‐assisted templating method, exhibit different morphologies and properties depending upon the synthetic conditions. Under highly reducing conditions, nearly perfect scrolls can be produced which have essentially no cracks in the walls (well‐ordered nanorolls). If the materials are produced under less reducing conditions, nanorolls with many cracks in the oxide walls can be generated (defect‐rich nanorolls). Both types of samples were examined by X‐ray diffraction (XRD), transmission electron microscopy (TEM), and X‐ray photoemission spectroscopy (XPS) to characterize their local structure, local redox state, and nanoscale structure. After ion‐exchange to replace the templating ammonium ions with Na⁺, the ability of these materials to electrochemically intercalate lithium reversibly was investigated. In sweep voltammetry experiments, the well‐ordered nanorolls showed responses similar to those seen in crystalline orthorhombic V₂O₅. In contrast, the defect‐rich vanadium oxide nanorolls behaved electrochemically more like sol–gel‐prepared vanadium oxide materials. Moreover, the specific capacity of the well‐ordered nanorolls was about 240 mA h g^–1 while that of the defect rich nanorolls was found to be as much as 340 mA h g^–1 under these same conditions. Disorders on both the atomic and nanometer length scales are believed to contribute to this difference.     

A Site‐Specific Charge Carrier Control in Monolithic Integrated Amorphous Oxide Semiconductors and Circuits with Locally Induced Optical‐Doping Process

Amorphous oxide semiconductor (AOS) thin film transistors (TFTs) have found cutting‐edge applications in sensor technologies. To reduce manufacturing costs, sensors, analog front end, and digital signal processing circuits need to be integrated on the identical substrate. Unlike traditional silicon‐based devices, optimizations for locally controllable electrical parameters of the AOSs have rarely been investigated. Here, photoactivated combustion reduction is utilized as doping motivation for solution‐processed amorphous indium–gallium–zinc oxide (a‐IGZO) to tune their electrical performance. By controlling parameters of a‐IGZO TFTs, which can be partly doped with covering the desired area of the identical substrate, it is possible to match the particular threshold voltage for various circuits. For circuit optimization, automatic integrated circuit modeling spice is carried out to find the best match of the complementary metal–oxide semiconductor circuits. Finally, the site‐specific performance of switching TFTs, amplifiers, and ring oscillators implemented with low‐temperature solution‐processed a‐IGZO and p‐type single‐walled carbon nanotube TFTs is demonstrated. The optical‐doped a‐IGZO TFTs exhibiting a saturation mobility of >9.15 cm² V^?1 s^?1 with a locally tunable threshold voltage of ?5 – 1.5 V are realized, enabling monolithic integration of functional devices. The resultant circuits demonstrate excellent amplification of 24 dB and an oscillation frequency of 12 kHz for 7‐stage ring oscillators.     

Co‐synthesis of ZnO–CuO Nanostructures by Directly Heating Brass in Air

ZnO–CuO nanostructures have been simultaneously synthesized by directly heating a CuZn alloy (brass) on a hotplate in ambient conditions. Depending on the Zn concentrations in the brasses, the dominant products transition from CuO nanowires to ZnO nanostructures. By changing the growth temperature and local Zn contents, 1D ZnO nanowires/nanoflakes, 2D ZnO nanosheets, and complicated 3D ZnO networks are obtained. Electron microscopy studies show that the as‐synthesized ZnO nanoflakes and nanosheets are single crystalline. Based on “self‐catalytic” growth, a tip‐growth mechanism for ZnO nanostructures is discussed, in which the Cu in brass plays an important role to confine the lateral growth of ZnO. Finally, the electron field emission from the ZnO–CuO hybrid systems is tested for the demonstration of potential applications.     

Long K‐Doped Titania and Titanate Nanowires on Ti Foil and FTO/Quartz Substrates for Solar‐Cell Applications

Potassium‐doped titania and titanate nanowires are fabricated by moisture‐assisted direct oxidation of titanium. The influence of the fabrication conditions on nanowire structure and morphology is investigated. It is shown that the presence of potassium is essential for nanowire formation, while the nanowire structure and morphology are strongly dependent on the fabrication temperature. The longest nanowires (ca. 10 μm) are obtained at 650 °C. At this substrate temperature, nanowires could be produced over a large substrate area both by oxidation of the Ti foil as well as by depositing a Ti film on the substrate (quartz or fluorine‐doped tin oxide (FTO)/quartz). Photovoltaic cells based on these nanowires are fabricated. The cell performance is dependent on the nanowire fabrication temperature and the substrate used, as well as on the annealing environment. Short‐circuit current densities of I_sc = 3.05 mA cm^–2 and I_sc = 4.97 mA cm^–2 could be obtained for Ti foil and FTO/quartz substrates, respectively, while the corresponding power‐conversion efficiencies are η = 0.93 % and η = 1.88 % (under AM 1.5 illumination, 100 mW cm^–2; AM: air mass).     

Large‐Area Schottky Barrier Transistors Based on Vertically Stacked Graphene–Metal Oxide Heterostructures

The fabrication of all‐transparent flexible vertical Schottky barrier (SB) transistors and logic gates based on graphene–metal oxide–metal heterostructures and ion gel gate dielectrics is demonstrated. The vertical SB transistor structure is formed by (i) vertically sandwiching a solution‐processed indium‐gallium‐zinc‐oxide (IGZO) semiconductor layer between graphene (source) and metallic (drain) electrodes and (ii) employing a separate coplanar gate electrode bridged with a vertical channel through an ion gel. The channel current is modulated by tuning the Schottky barrier height across the graphene–IGZO junction under an applied external gate bias. The ion gel gate dielectric with high specific capacitance enables modulation of the Schottky barrier height at the graphene–IGZO junction over 0.87 eV using a voltage below 2 V. The resulting vertical devices show high current densities (18.9 A cm^?2) and on–off current ratios (>10⁴) at low voltages. The simple structure of the unit transistor enables the successful fabrication of low‐power logic gates based on device assemblies, such as the NOT, NAND, and NOR gates, prepared on a flexible substrate. The facile, large‐area, and room‐temperature deposition of both semiconducting metal oxide and gate insulators integrates with transparent and flexible graphene opens up new opportunities for realizing graphene‐based future electronics.     

Crystallization of Amorphous Tris(8‐hydroxyquinoline)aluminum Nanoparticles and Transformation to Nanowires

Amorphous tris(8‐hydroxyquinoline)aluminum (AlQ₃) nanoparticles can be grown directly into α‐phase crystalline nanowires in a one‐step heat treatment. At the most appropriate Ar pressure, heating time, and heating temperatures (between 150 and 190 °C), fine and long nanowires are obtained. The growth of the nanowires is dictated by the anisotropic bonding in α‐AlQ₃ crystals. The growth mechanism is illustrated by the concept of nucleation and molecular migration. Two exotherms are revealed, from differential scanning calorimetry analyses, in the transformation process of AlQ₃ amorphous nanoparticles to crystalline nanowires. The first exotherm is the transition from amorphous nanoparticles to the γ‐phase, and the second exotherm is the transition from the γ‐ to the α‐phase. By means of Kissinger plots, the activation energies for the crystallization of the γ‐phase and the transition from the γ‐ to the α‐phase are calculated, for the first time, to be 9.7 and 12.1 kJ mol^–1, respectively. A blue‐shift and higher intensity of photoluminescence after heat treatment are also demonstrated.     

Shape Anisotropy and Magnetization Modulation in Hexagonal Cobalt Nanowires

Ferromagnetic cobalt nanowires with high‐crystalline quality are synthesized using a low‐voltage electrodeposition method. High‐resolution transmission electron microscopy (HRTEM) and X‐ray diffraction (XRD) results show that the nanowires are uniform in size, and consist of predominantly hexagonal close‐packed (hcp) structure with the magnetocrystalline easy axis (c‐axis) perpendicular to the wire axis. Superconducting quantum interference device (SQUID) measurements illustrate the dominance of shape anisotropy, manifested by the weak temperature dependence of the enhanced coercive field along the wire axis. Furthermore, the magnetic structures of individual, segmented, or intersected nanowires are studied using magnetic force microscopy. This reveals a strong dipole at the two ends of the wire, together with a spatial magnetization modulation along the wire. Based on theoretical modeling, such intrinsic modulation is attributed to magnetization frustration due to the competition between the magnetocrystalline polarization along the easy axis and the shape anisotropy along the wire axis.     

Sn‐catalyzed silicon nanowire solar cells with 4.9% efficiency grown on glass

We present a single pump‐down process to texture hydrogenated amorphous silicon solar cells. Mats of p‐type crystalline silicon nanowires were grown to lengths of 1 µm on glass covered with flat ZnO using a plasma‐assisted Sn‐catalyzed vapor‐liquid‐solid process. The nanowires were covered with conformal layers of intrinsic and n‐type hydrogenated amorphous silicon and a sputtered layer of indium tin oxide. Each cell connects in excess of 10⁷ radial junctions over areas of 0.126 cm². Devices reach open‐circuit voltages of 0.8 V and short‐circuit current densities of 12.4 mA cm⁻², matching those of hydrogenated amorphous silicon cells deposited on textured substrates. Copyright © 2012 John Wiley & Sons, Ltd.     

One‐Step Synthesis of Highly Ordered Mesoporous Silica Monoliths with Metal Oxide Nanocrystals in their Channels

A simple, one‐step synthetic route to prepare ordered mesoporous silica monoliths with controllable quantities of metal oxide nanocrystals in their channels is presented. The method is based on the assisted assembly effect for mesostructure‐directing of the metal complexes formed by the interaction of metal ions with the –O– groups of copolymers. Highly ordered hexagonal silica monoliths, loaded with various metal oxide nanocrystals, including those of Cr₂O₃, MnO, Fe₂O₃, Co₃O₄, NiO, CuO, ZnO, CdO, SnO₂, and In₂O₃, can be obtained by this one‐step pathway. In the NiO/SiO₂ nanocomposite, nickel oxide nanorods with face‐centered cubic lattices are formed at low doping ratios, and they can be transformed into nanowires by increasing the quantities of the precursors. In the Fe₂O₃/SiO₂ nanocomposites, a one‐dimensional assembly of iron oxide nanoparticles is observed. In the In₂O₃/SiO₂ nanocomposites, single crystal nanowires with high aspect ratios are obtained. For the other metal oxide nanocomposites, including Cr₂O₃, MnO, Co₃O₄, CuO, ZnO, CdO, and SnO, only crystalline nanorods are obtained. N₂ sorption results of the metal oxide/SiO₂ mesostructured nanocomposites reveal that nanocrystals inside the pores do not severely decrease the pore volume or the Brunauer–Emmett–Teller (BET) surface area of the mesoporous silica host. The bandgaps of SnO₂ and In₂O₃ nanocrystals, calculated from UV‐vis spectra, are much larger than the corresponding bulk materials, implying the quantum confinement effect in the small particles. Co₃O₄/SiO₂ mesostructured nanocomposites catalyze the complete combustion of CH₄. These studies provide a new and simple method for templating synthesis of metal oxide nanostructures.     

High—Performance Solar‐Blind Deep Ultraviolet Photodetector Based on Individual Single‐Crystalline Zn2GeO4 Nanowire

Solar‐blind deep ultraviolet (DUV) photodetectors have been a hot topic in recent years because of their wide commercial and military applications. A wide bandgap (4.68 eV) of ternary oxide Zn₂GeO₄ makes it an ideal material for the solar‐blind DUV detection. Unfortunately, the sensing performance of previously reported photodetectors based on Zn₂GeO₄ nanowires has been unsatisfactory for practical applications, because they suffer from long response and decay times, low responsivity, and quantum efficiency. Here, high‐performance solar‐blind DUV photodetectors are developed based on individual single‐crystalline Zn₂GeO₄ nanowires. The transport mechanism is discussed in the frame of the small polaron theory. In situ electrical characterization of individual Zn₂GeO₄ nanowires reveals a high gain under high energy electron beam. The devices demonstrate outstanding solar‐blind light sensing performances: a responsivity of 5.11 × 10³ A W^?1, external quantum efficiency of 2.45 × 10⁶%, detectivity of ≈2.91 × 10¹¹ Jones, τ_rise ≈ 10 ms, and τ_decay ≈ 13 ms, which are superior to all reported Zn₂GeO₄ and other ternary oxide nanowire photodetectors. These results render the Zn₂GeO₄ nanowires particularly valuable for optoelectronic devices.     

A Roadmap for Controlled and Efficient n‐Type Doping of Self‐Assisted GaAs Nanowires Grown by Molecular Beam Epitaxy

N‐type doping of GaAs nanowires has proven to be difficult because the amphoteric character of silicon impurities is enhanced by the nanowire growth mechanism and growth conditions. The controllable growth of n‐type GaAs nanowires with carrier density as high as 10²⁰ electron cm^?3 by self‐assisted molecular beam epitaxy using Te donors is demonstrated here. Carrier density and electron mobility of highly doped nanowires are extracted through a combination of transport measurement and Kelvin probe force microscopy analysis in single‐wire field‐effect devices. Low‐temperature photoluminescence is used to characterize the Te‐doped nanowires over several orders of magnitude of the impurity concentration. The combined use of those techniques allows the precise definition of the growth conditions required for effective Te incorporation.     

Templateless Synthesis of Polygonal Gold Nanoparticles: An Unsupported and Reusable Catalyst with Superior Activity

We report the synthesis of polygonal gold nanoparticles (GNPs) by an in situ reduction technique using ferric ammonium citrate as reducing agent in absence of any surfactant or polymeric template. Transmission electron microscopic analysis and selected area electron diffraction patterns confirmed the formation of well‐crystalline polygonal GNPs grown preferentially along the (111) direction, which is consistent with the results of X‐ray diffractometry analysis. The results of control experiments of HAuCl₄ with tri‐ammonium citrate in presence of different externally added metal ions like Fe³⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Al³⁺ suggested the ion‐induced growth mechanism in the formation of polygonal GNPs. The purified polygonal GNPs were then successfully used as catalyst in the borohydride reduction of three isomeric nitrophenols and also in the aerobic oxidation of different D ‐hexoses (e.g., D ‐glucose, D ‐mannose, D ‐fructose). The catalytic activity of these polygonal GNPs is higher by a factor of 300–1000, depending on the GNP's sample type, in nitrophenol reduction compared to that of spherical GNPs. Similar activity enhancement was also observed in the aerobic oxidation of different D ‐hexoses. These polygonal GNPs catalyst are very stable and could be reused several times in the borohydride reduction of nitrophenols without much losing in their virgin catalytic activity.     

Artificial Synapse Based on Oxygen Vacancy Migration in Ferroelectric-Like C-Axis-Aligned Crystalline InGaSnO Semiconductor Thin-Film Transistors for Highly Integrated Neuromorphic Electronics

Artificial synapses are a key component of neuromorphic computing systems. To achieve high-performance neuromorphic computing ability, a huge number of artificial synapses should be integrated because the human brain has a huge number of synapses (≈10¹⁵). In this study, a coplanar synaptic, thin-film transistor (TFT) made of c-axis-aligned crystalline indium gallium tin oxide (CAAC–IGTO) is developed. The electrical characteristics of the biological synapses such as inhibitory postsynaptic current (IPSC), paired-pulse depression (PPD), short-term plasticity (STP), and long-term plasticity at V_DS = 0.1 V, are demonstrated. The measured synaptic behavior can be explained by the migration of positively charged oxygen vacancies (V_o⁺/V_o⁺⁺) in the CAAC–IGTO layer. The mechanism of implementing synaptic behavior is completely new, compared to previous reports using electrolytes or ferroelectric gate insulators. The advantage of this device is to use conventional gate insulators such as SiO₂ for synaptic behavior. Previous studies use chitosan, Ta₂O₃, SiO₂ nanoparticles , Gd₂O₃, and HfZrO_x for gate insulators, which cannot be used for high integration of synaptic devices. The metal–oxide TFTs, widely used in the display industry, can be applied to the synaptic transistors. Therefore, CAAC–IGTO synaptic TFT can be a good candidate for application as an artificial synapse for highly integrated neuromorphic chips.     

Stoichiometry Controlled,Single‐Crystalline Bi2Te3 Nanowires for Transport in the Basal Plane

Thermoelectric Bi₂Te₃ based bulk materials are widely used for solid‐state refrigeration and power‐generation at room temperature. For low‐dimensional and nanostructured thermoelectric materials an increase of the thermoelectric figure of merit ZT is predicted due to quantum confinement and phonon scattering at interfaces. Therefore, the fabrication of Bi₂Te₃ nanowires, thin films, and nanostructured bulk materials has become an important and active field of research. Stoichiometric Bi₂Te₃ nanowires with diameters of 50–80 nm and a length of 56 μm are grown by a potential‐pulsed electrochemical deposition in a nanostructured Al₂O₃ matrix. By transmission electron microscopy (TEM), dark‐field images together with electron diffraction reveal single‐crystalline wires, no grain boundaries can be detected. The stoichiometry control of the wires by high‐accuracy, quantitative enegy‐dispersive X‐ray spectroscopy (EDX) in the TEM instrument is of paramount importance for successfully implementing the growth technology. Combined electron diffraction and EDX spectroscopy in the TEM unambiguously prove the correct crystal structure and stoichiometry of the Bi₂Te₃ nanowires. X‐ray and electron diffraction reveal growth along the [110] and [210] directions and the c axis of the Bi₂Te₃ structure lies perpendicular to the wire axis. For the first time single crystalline, stoichiometric Bi₂Te₃ nanowires are grown that allow transport in the basal plane without being affected by grain boundaries.     

Design and Synthesis of Hierarchical Nanowire Composites for Electrochemical Energy Storage

Nanocomposites of interpenetrating carbon nanotubes and vanadium pentoxide (V₂O₅) nanowires networks are synthesized via a simple in situ hydrothermal process. These fibrous nanocomposites are hierarchically porous with high surface area and good electric conductivity, which makes them excellent material candidates for supercapacitors with high energy density and power density. Nanocomposites with a capacitance up to 440 and 200 F g^?1 are achieved at current densities of 0.25 and 10 A g^?1, respectively. Asymmetric devices based on these nanocomposites and aqueous electrolyte exhibit an excellent charge/discharge capability, and high energy densities of 16 W h kg^?1 at a power density of 75 W kg^?1 and 5.5 W h kg^?1 at a high power density of 3 750 W kg^?1. This performance is a significant improvement over current electrochemical capacitors and is highly competetive with Ni–MH batteries. This work provides a new platform for high‐density electrical‐energy storage for electric vehicles and other applications.