Vertically aligned Zn2SiO4 nanotube/ZnO nanowire heterojunction arrays

Bimetallic Nanoparticle Oxidation in Three Dimensions by Chemically Sensitive Electron Tomography and in Situ Transmission Electron Microscopy

The formation of hollow-structured oxide nanoparticles is primarily governed by the Kirkendall effect. However, the degree of complexity of the oxidation process multiplies in the bimetallic system because of the incorporation of more than one element. Spatially dependent oxidation kinetics controls the final morphology of the hollow nanoparticles, and the process is highly dependent on the elemental composition. Currently, a theoretical framework that can predict how different metal elements result in different oxide morphologies remains elusive. In this work, utilizing a combination of state-of-the-art in situ environmental transmission electron microscopy and three-dimensional (3D) chemically sensitive electron tomography, we provide an in situ and 3D investigation of the oxidation mechanism of the Ni-Fe nanoparticles. The direct measurements allow us to correlate the 3D elemental segregation in the particles with the oxidation morphologies, that is, single-cavity or dual-cavity hollow structure, and multicavity porous structures. Our findings in conjunction with theoretical calculations show that metal concentration, diffusivity, and particle size are important parameters that dictate the mechanical and phase stabilities of the hollow oxide shell, which in turn determine its barrier properties and the final hollow oxide morphology. It sheds light on how to use multielemental oxidation to control morphology in nanomaterials and demonstrates the power of 3D chemical imaging.

Synthesis, properties, and formation mechanism of Mn-doped Zn2SiO4 nanowires and associated heterostructures

Charge transfer and energy transfer phenomena were observed and analyzed in heterostructure systems composed of CdSe QDs immobilized onto Mn-doped Zn₂SiO₄ nanowire host materials.

Synthesis of Hollow Nanotubes of Zn2SiO4 or SiO2: Mechanistic Understanding and Uranium Adsorption Behavior

We report a facile synthesis of Zn2SiO4 nanotubes using a two-step process consisting of a wet-chemical synthesis of core-shell ZnO@SiO2 nanorods followed by thermal annealing. While annealing in air leads to the formation of hollow Zn2SiO4, annealing under reducing atmosphere leads to the formation of SiO2 nanotubes. We rationalize the formation of the silicate phase at temperatures much lower than the temperatures reported in the literature based on the porous nature of the silica shell on the ZnO nanorods. We present results from in situ transmission electron microscopy experiments to clearly show void nucleation at the interface between ZnO and the silica shell and the growth of the silicate phase by the Kirkendall effect. The porous nature of the silica shell is also responsible for the etching of the ZnO leading to the formation of silica nanotubes under reducing conditions. Both the hollow silica and silicate nanotubes exhibit good uranium sorption at different ranges of pH making them possible candidates for nuclear waste management.

Anomalous optical and magnetic behavior of multi-phase Mn doped Zn(2)SiO(4) nanowires: a new class of dilute magnetic semiconductors

We present the synthesis of Mn doped Zn(2)SiO(4) dense nanowire bundles using the VLS mode of growth with unusual optical and magnetic properties. The synthesized Zn(2)SiO(4) nanowires were identified with two phases, α-Zn(2)SiO(4) as the major phase and β-Zn(2)SiO(4) as the minor phase. XPS studies confirmed that Zn(2)SiO(4) nanowires were Zn rich and Mn doped. Temperature dependent photoluminescence (PL) measurements showed three distinct emission bands: green, yellow and red due to Mn doping in the α-phase, β-phase and the substitution of Si with Mn in the α-phase, respectively. The PL analysis showed that these emission bands followed anomalous Berthelot-type behavior. The carrier escape energies were 70 ± 3 meV, 49 ± 2 meV and 65 ± 4 meV for the 530, 570 and 660 nm bands, respectively, while the radiation rates (Er =) were 1.0 ± 0.4 meV, 3.10 ± 1.10 meV and 1.4 ± 0.4 meV corresponding to the three respective bands. Mn doping of Zn(2)SiO(4) nanowires induced ferromagnetism, which was observed above room temperature, with a Curie temperature well above 380 K. The observation of magnetic behavior in this class of semiconductors has potential applications in high temperature spintronics and magneto-optical devices.

Electron field emission enhancement of vertically aligned ultrananocrystalline diamond-coated ZnO core-shell heterostructured nanorods

Enhanced electron field emission (EFE) behavior of a core-shell heterostructure, where ZnO nanorods (ZNRs) form the core and ultrananocrystalline diamond needles (UNCDNs) form the shell, is reported. EFE properties of ZNR-UNCDN core-shell heterostructures show a high emission current density of 5.5 mA cm(-2) at an applied field of 4.25 V μm(-1) , and a low turn-on field of 2.08 V μm(-1) compared to the 1.67 mA cm(-2) emission current density (at an applied field of 28.7 V μm(-1) ) and 16.6 V μm(-1) turn-on field for bare ZNRs. Such an enhancement in the field emission originates from the unique materials combination, resulting in good electron transport from ZNRs to UNCDNs and efficient field emission of electrons from the UNCDNs. The potential application of these materials is demonstrated by the plasma illumination measurements that lowering the threshold voltage by 160 V confirms the role of ZNR-UNCDN core-shell heterostructures in the enhancement of electron emission.

Assembly of three-dimensional hetero-epitaxial ZnO/ZnS core/shell nanorod and single crystalline hollow ZnS nanotube arrays

Hetero-epitaxial growth along three-dimensional (3D) interfaces from materials with an intrinsic large lattice mismatch is a key challenge today. In this work we report, for the first time, the controlled synthesis of vertically aligned ZnO/ZnS core/shell nanorod arrays composed of single crystalline wurtzite (WZ) ZnS conformally grown on ZnO rods along 3D interfaces through a simple two-step thermal evaporation method. Structural characterization reveals a "(01-10)(ZnO)//(01-10)(ZnS) and [0001](ZnO)//[0001](ZnS)" epitaxial relationship between the ZnO core and the ZnS shell. It is exciting that arrays of single crystalline hollow ZnS nanotubes are also innovatively obtained by simply etching away the inner ZnO cores. On the basis of systematic structural analysis, a rational growth mechanism for the formation of hetero-epitaxial core/shell nanorods is proposed. Optical properties are also investigated via cathodoluminescence and photoluminescence measurements. Remarkably, the synthesized ZnO/ZnS core/shell heterostructures exhibit a greatly reduced ultraviolet emission and dramatically enhanced green emission compared to the pure ZnO nanorods. The present single-crystalline heterostructure and hollow nanotube arrays are envisaged to be highly promising for applications in novel nanoscale optoelectronic devices, such as UV-A photodetectors, lasers, solar cells, and nanogenerators.

In situ growth, structure characterization, and enhanced photocatalysis of high-quality, single-crystalline ZnTe/ZnO branched nanoheterostructures

Single-crystalline, high-quality branched ZnTe-core/ZnO-branch nanoheterostructures were synthesized by an in situ strategy in an environmental scanning electron microscope. Composition and structure characterization confirmed that ZnO nanowires were perfectly epitaxially grown on ZnTe nanowires as branches. Noticeably, growth temperature plays a crucial role in determining the density and diameter of the ZnO nanobranches on ZnTe nanowires: a higher growth temperature leads to ZnO nanowires with higher density and smaller diameter. It was demonstrated that ZnO nanobranches exhibited a selective nucleation behavior on distinct side facets of ZnTe nanowires. Highly ordered ZnO nanobranches were found epitaxially grown on {211} facet of ZnTe nanowires, while there was no ZnO nanowire growth on {110} facet of ZnTe nanowires. Using first-principles calculation, we found that surface energy of distinct side facets has a strong impact on ZnO nucleation, and confirm that {211} facet of ZnTe nanowires is energetically more favorable for ZnO nanowire growth than {110} facet, which is in good agreement with our experimental findings. Remarkably, such unique ZnTe/ZnO 3D branched nanowire heterostructures exhibited improved photocatalytic abilities, superior to ZnO nanowires and ZnTe nanowires, due to the much enhanced effective surface area of their unique architecture and effective electron-hole separation at the ZnTe/ZnO interfaces.

Selective homoepitaxial growth and luminescent properties of ZnO nanopillars

High spatial density ZnO nanopillars (NPs) have been fabricated on catalyst- and pattern-free Si wafers using atmospheric pressure metal organic chemical vapor deposition (APMOCVD) at a moderate temperature (500 °C). The nanopillar diameter is ∼ 35 nm and the length is ∼ 150 nm, with a density of ∼ 2 × 10(9) cm( - 2). The growth evolution of the nanopillars, providing the (0001)(NP) ∐ (0001)(ZNO grain) ∐ (100)(Si surface) epitaxial relationship, is extensively studied by scanning and high resolution transmission microscopy. The approach to obtaining the ZnO 1D structures is explained in terms of selective homoepitaxial growth via the crystallographic anisotropy of the seeding layer. The advanced PL properties of ZnO NPs, e.g. indications of free excitonic and absence of defect emission, are related to their single crystalline nature within one pillar and most probably better stoichiometry and less contamination. The observed efficient monochromatic UV emission from the ZnO NPs at room temperature points toward their potential application as building blocks for nanoscale optoelectronic devices.

Nanoscale Kirkendall effect for the synthesis of Bi2MoO6 boxes via a facile solution-phase method

A one-pot surfactant-free method has been successfully developed to synthesize Bi(2)MoO(6) boxes using MoO(3) nanorods as templates. A formation mechanism involving the nanoscale Kirkendall effect has been proposed. Our work demonstrates the generic feature of a mild solution-phase-mediated nanoscale Kirkendall effect for the synthesis of multicomponent materials with box-like structures.

Fabrication and characterization of flower-like CuO-ZnO heterostructure nanowire arrays by photochemical deposition

A simple two step solution-based method was applied to fabricate CuO-ZnO heterostructured nanowire (NW) arrays. First, ZnO nanowires were grown on a Si substrate using the ammonia solution hydrothermal reaction. Afterwards, flower-like CuO crystals were photochemically deposited on the tip of the ZnO NWs, using ultraviolet (UV) light (312 nm wavelength) irradiation at room temperature. The morphology of the CuO was controlled by reaction time, density of ZnO NWs, and concentration of the solution. Because the deposited CuO is p-type and has narrow band gap properties, CuO-ZnO heterostructured NWs exhibited a stable p-n junction property and good ability to absorb visible light. Through investigation of UV light-triggered reaction phenomena, we found that the production of OH(-) from the photocatalytic process on the surface of ZnO NWs plays a critical role in the CuO deposition mechanism.

Vertically aligned ZnO/amorphous-Si core-shell heterostructured nanowire arrays

We report the synthesis of vertically aligned ZnO/a-Si core-shell nanowire arrays (ZnO nanowires coated with amorphous silicon) through chemical vapor deposition. The core-shell heterostructured nanowires possessed uniform morphology and the thickness of the amorphous silicon shells could be controlled easily by tuning the deposition duration and temperature. The core-shell heterostructured nanowires exhibited enhanced antireflection and absorption performance as well as tunable PL properties. Because the individual ZnO/a-Si nanowires showed p-type characteristics and the ZnO cores were n-type semiconductors, the core-shell nanowires formed p-n junctions naturally.

Selective synthesis of Zn(1 - x)Mn(x)Se nanobelts and nanotubes from [Zn(1 - x)Mn(x)Se](DETA)0.5 nanbelts in solution (x = 0-0.15) and their EPR and optical properties

Zn(1 - x)Mn(x)Se (x = 0-0.15) nanobelts and nanotubes can be synthesized via the removal of diethylenetriamine (DETA) in 1-octadecene (ODE) and ethylene glycol (EG), respectively, using [Zn(1 - x)Mn(x)Se](DETA)(0.5) nanobelts as a template. The as-prepared ZnSe nanobelts are single-crystalline and grown along the [001] direction, and the ZnSe nanotubes consist of nanoparticles assembled along the [001] direction. In addition, Mn(2+)-doped Zn(1 - x)Mn(x)Se (x = 0.05, 0.10, 0.15) nanotubes are prepared for the first time if doped [Zn(1 - x)Mn(x)Se](DETA)(0.5) nanobelts are used as the template. The formation process of Zn(1 - x)Mn(x)Se nanobelts and nanotubes has been studied, and a plausible mechanism is proposed. Photoluminescence (PL) and electron paramagnetic resonance (EPR) spectra of Zn(1 - x)Mn(x)Se nanobelts and Zn(1 - x)Mn(x)Se nanotubes have been investigated.

Synthesis of Sn doped CuO nanotubes from core-shell Cu/SnO(2) nanowires by the Kirkendall effect

Sn doped CuO nanotubes were synthesized by thermal oxidization of Cu/SnO(2) core-shell nanowires in air through the Kirkendall effect. The Cu/SnO(2) core-shell nanowires were sequentially electrodeposited by forming a SnO(2) shell followed by electrodeposition of the Cu core. After thermal treatment in air, the core-shell Cu/SnO(2) (13 +/- 2 nm thick shell on 128 +/- 15 nm in diameter core) nanowires were oxidized to form Sn doped CuO nanotubes with an average wall thickness and outer diameter of 54 nm and 176 nm, respectively. Room temperature I-V characterization indicated that the electrical resistivity of the nanostructures was 870 +/- 85 Omega cm. The methodology that was demonstrated is very general and could be used to synthesize coaxial SnO(2) shells with a variety of electrodeposited cores. In addition, doped metal oxide nanotubes can be readily synthesized by thermal oxidization of core-shell nanowires in air where the dopant content can be tuned by controlling the shell thickness through adjusting the deposition time.

Effect of Size-Dependent Thermal Instability on Synthesis of Zn2 SiO4-SiOx Core-Shell Nanotube Arrays and Their Cathodoluminescence Properties

Vertically aligned Zn2SiO4-SiOx(x < 2) core-shell nanotube arrays consisting of Zn2SiO4-nanoparticle chains encapsulated into SiOx nanotubes and SiOx-coated Zn2SiO4 coaxial nanotubes were synthesized via one-step thermal annealing process using ZnO nanowire (ZNW) arrays as templates. The appearance of different nanotube morphologies was due to size-dependent thermal instability and specific melting of ZNWs. With an increase in ZNW diameter, the formation mechanism changed from decomposition of "etching" to Rayleigh instability and then to Kirkendall effect, consequently resulting in polycrystalline Zn2SiO4-SiOx coaxial nanotubes, single-crystalline Zn2SiO4-nanoparticle-chain-embedded SiOx nanotubes, and single-crystalline Zn2SiO4-SiOx coaxial nanotubes. The difference in spatially resolved optical properties related to a particular morphology was efficiently documented by means of cathodoluminescence (CL) spectroscopy using a middle-ultraviolet emission at 310 nm from the Zn2SiO4 phase.

Modified Kirkendall effect for fabrication of magnetic nanotubes

The iron hydroxide nanotubes are fabricated through the reaction of Fe³⁺ ions and ZnO nanorods involving the Kirkendall effect. Depending on the calcination conditions, both haematite and magnetite nanotubes are produced.

Synthesis of Novel Double-Layer Nanostructures of SiC-WO(x) by a Two Step Thermal Evaporation Process

A novel double-layer nanostructure of silicon carbide and tungsten oxide is synthesized by a two-step thermal evaporation process using NiO as the catalyst. First, SiC nanowires are grown on Si substrate and then high density W(18)O(49) nanorods are grown on these SiC nanowires to form a double-layer nanostructure. XRD and TEM analysis revealed that the synthesized nanostructures are well crystalline. The growth of W(18)O(49) nanorods on SiC nanowires is explained on the basis of vapor-solid (VS) mechanism. The reasonably better turn-on field (5.4 V/mum) measured from the field emission measurements suggest that the synthesized nanostructures could be used as potential field emitters.

Bottom-up growth of fully transparent contact layers of indium tin oxide nanowires for light-emitting devices

Thin layers of indium tin oxide are widely used as transparent coatings and electrodes in solar energy cells, flat-panel displays, antireflection coatings, radiation protection and lithium-ion battery materials, because they have the characteristics of low resistivity, strong absorption at ultraviolet wavelengths, high transmission in the visible, high reflectivity in the far-infrared and strong attenuation in the microwave region. However, there is often a trade-off between electrical conductivity and transparency at visible wavelengths for indium tin oxide and other transparent conducting oxides. Here, we report the growth of layers of indium tin oxide nanowires that show optimum electronic and photonic properties and demonstrate their use as fully transparent top contacts in the visible to near-infrared region for light-emitting devices.

3D ordered assemblies of semiconductive micro/nanowires using microscale fibrous building blocks

Three-dimensional (3D) ordered assemblies of semiconductive micro/nanowires are made by match-stick assembly of fibrous building blocks (FBBs). A glass tube filled with powders of starting material is processed drawn into centimeter-long, micrometer-diameter FBBs with controlled diameter and spacing. By repeating a draw-cut-stack process, the diameter and spacing of filling material can be programmably reduced from millimeters to hundreds of nanometers. The FBBs are densely packed into 3D ordered structures such that wires in one layer are at defined angle (theta) relative to those in the adjacent layers, where theta is between 0 and 180 degrees. The electrical measurements at bundled wires and single wire level confirm semiconducting behavior of wires. By directly manipulating microscale FBBs, the method allows high yield production of 3D ordered micro/nanowires with controlled position and orientation, enabling the construction of a new class of micro/nanomaterials.

The prominent photoinduced voltage effect of as-prepared macroscopically long Ag core/Ni shell nanoheterojunctions

Macroscopically long Ag core/Ni shell nanoheterojunctions have been well prepared by a dynamic growth approach. The structure characterized in detail by scanning electron microscopy reveals that the Ag nanowire bundles are wrapped in Ni nanoshields and form multicore coaxial cable frames. Notable photoinduced voltage with a fine repeatability, for irradiation with a laser, is exhibited compared with the case for bulk Ag pole/Ni shell heterojunctions and Ag nanowire bundle/bulk Ni heterojunctions. The prominent photoinduced voltage and the substantial metal nanoscale Ohmic interconnects provided by this kind of nanoheterojunction may have a wide range of applications in the future.

Highly conductive coaxial SnO(2)-In(2)O(3) heterostructured nanowires for Li ion battery electrodes

Novel SnO(2)-In(2)O(3) heterostructured nanowires were produced via a thermal evaporation method, and their possible nucleation/growth mechanism is proposed. We found that the electronic conductivity of the individual SnO(2)-In(2)O(3) nanowires was 2 orders of magnitude better than that of the pure SnO(2) nanowires, due to the formation of Sn-doped In(2)O(3) caused by the incorporation of Sn into the In(2)O(3) lattice during the nucleation and growth of the In(2)O(3) shell nanostructures. This provides the SnO(2)-In(2)O(3) nanowires with an outstanding lithium storage capacity, making them suitable for promising Li ion battery electrodes.

Formation of nanotubes and hollow nanoparticles based on Kirkendall and diffusion processes: a review

The Kirkendall effect is a consequence of the different diffusivities of atoms in a diffusion couple causing a supersaturation of lattice vacancies. This supersaturation may lead to a condensation of extra vacancies in the form of so-called "Kirkendall voids" close to the interface. On the macroscopic and micrometer scale these Kirkendall voids are generally considered as a nuisance because they deteriorate the properties of the interface. In contrast, in the nanoworld the Kirkendall effect has been positively used as a new fabrication route to designed hollow nano-objects. In this Review we summarize and discuss the demonstrated examples of hollow nanoparticles and nanotubes induced by the Kirkendall effect. Merits of this route are compared with other general methods for nanotube fabrication. Theories of the kinetics and thermodynamics are also reviewed and evaluated in terms of their relevance to experiments. Moreover, nanotube fabrication by solid-state reactions and non-Kirkendall type diffusion processes are covered.