Au nanocrystal array/silicon nanoantennas as wavelength-selective photoswitches.

Au nanocrystal array/silicon nanoantennas exhibiting wavelength-selective photocurrent enhancement were successfully fabricated by a facile and inexpensive method combining colloidal lithography (CL) and a metal-assisted chemical etching (MaCE) process. The localized surface plasmon resonance (LSPR) response and wavelength-selective photocurrent enhancement characteristics were achieved by tuning the depth of immersion of Au nanocrystal arrays in silicon through a MaCE process. The wavelength selectivity of photocurrent enhancement contributed by LSPR induced local field amplification was confirmed by simulated near-field distribution. In addition, it can be integrated to well-developed Si-based manufacturing process. These characteristics make Au nanocrystal array/Si nanoantennas promising as low power-consumption photoswitches and nano-optoelectronic and photonic communication devices.

Bipolar resistive switching of single gold-in-Ga2O3 nanowire.

We have fabricated single nanowire chips on gold-in-Ga(2)O(3) core-shell nanowires using the electron-beam lithography techniques and realized bipolar resistive switching characteristics having invariable set and reset voltages. We attribute the unique property of invariance to the built-in conduction path of gold core. This invariance allows us to fabricate many resistive switching cells with the same operating voltage by simple depositing repetitive metal electrodes along a single nanowire. Other characteristics of these core-shell resistive switching nanowires include comparable driving electric field with other thin film and nanowire devices and a remarkable on/off ratio more than 3 orders of magnitude at a low driving voltage of 2 V. A smaller but still impressive on/off ratio of 10 can be obtained at an even lower bias of 0.2 V. These characteristics of gold-in-Ga(2)O(3) core-shell nanowires make fabrication of future high-density resistive memory devices possible.

Coaxial metal-silicide Ni2Si/C54-TiSi2 nanowires.

One-dimensional metal silicide nanowires are excellent candidates for interconnect and contact materials in future integrated circuits devices. Novel core-shell Ni(2)Si/C54-TiSi(2) nanowires, 2 µm in length, were grown controllably via a solid-liquid-solid growth mechanism. Their interesting ferromagnetic behaviors and excellent electrical properties have been studied in detail. The coercivities (Hcs) of the core-shell Ni(2)Si/C54-TiSi(2) nanowires was determined to be 200 and 50 Oe at 4 and 300 K, respectively, and the resistivity was measured to be as low as 31 µΩ-cm. The shift of the hysteresis loop with the temperature in zero field cooled (ZFC) and field cooled (FC) studies was found. ZFC and FC curves converge near room temperature at 314 K. The favorable ferromagnetic and electrical properties indicate that the unique core-shell nanowires can be used in penetrative ferromagnetic devices at room temperature simultaneously as a future interconnection in integrated circuits.

Lead-free KNbO3 ferroelectric nanorod based flexible nanogenerators and capacitors.

In spite of high piezoelectricity, only a few one-dimensional ferroelectric nano-materials with perovskite structure have been used for piezoelectric nanogenerator applications. In this paper, we report high output electrical signals, i.e. an open-circuit voltage of 3.2 V and a closed-circuit current of 67.5 nA (current density 9.3 nA cm(-2)) at 0.38% strain and 15.2% s(-1) strain rate, using randomly aligned lead-free KNbO(3) ferroelectric nanorods (~1 µm length) with piezoelectric coefficient (d(33) ~ 55 pm V (-1)). A flexible piezoelectric nanogenerator is mainly composed of KNbO(3)-poly(dimethylsiloxane) (PDMS) composite sandwiched by Au/Cr-coated polymer substrates. We deposit a thin poly(methyl methacrylate) (PMMA) layer between the KNbO(3)-PDMS composite and the Au/Cr electrode to completely prevent dielectric breakdown during electrical poling and to significantly reduce leakage current during excessive straining. The flexible KNbO(3)-PDMS composite device shows a nearly frequency-independent dielectric constant (~3.2) and low dielectric loss (<0.006) for the frequency range of 10(2)-10(5) Hz. These results imply that short and randomly aligned ferroelectric nanorods can be used for a flexible high output nanogenerator as well as high-k capacitor applications by performing electrical poling and further optimizing the device structure.

Self-assembly of an antireflective moth-like structure using antibody-functionalized nanowires and nanopore arrays.

A novel framework to fabricate moth-like nanopillar arrays was proposed. In this scheme, nanowires were first cross-linked with anti-gold nanoparticle (GNP) antibodies and mixed with the nanopore array pre-deposited by GNP, which was then followed by centrifugation. An optimal success rate of 95% was finally obtained by choosing nanorods with an aspect ratio of 5:1 by modifying with 10 ng mL⁻¹ antibodies, and by inserting them into a pore array pre-deposited with 54.4 µM GNP. The nanopillar arrays thus fabricated showed high levels of antireflective efficiency across a broad wavelength. Here we demonstrate the assembly of nanowires and nanopores into nanopillar arrays by the assistance of antibody-antigen binding. The application of bio-nano-interaction provides an economic, time-saving, and throughput approach to manipulating objects on the nanoscale.

Direct observation of Au/Ga2O3 peapodded nanowires and their plasmonic behaviors.

Gold-peapodded Ga(2)O(3) nanowires were fabricated successfully in a well-controlled manner by thermal annealing of core-shell gold-Ga(2)O(3) nanowires. During the heating process, the core gold nanowires were broken up into chains of nanoparticles at sufficiently high temperature by the mechanism of Rayleigh instability. In addition, the size, shape, and interspacing between the particles can be manipulated by varying the annealing time and/or the forming gas. The plasmonic behaviors of these nanostructures are investigated by optical spectroscopy. A single nanowire optical device was designed, and its photonic characteristics were investigated. A remarkably high on/off photocurrent ratio in response to a 532 nm Nd:YAG laser light was found. As the size of the particle (pea) increases, the corresponding spectra are red-shifted. In addition, morphological changes of the peas lead to a distinct spectral response. The results may usher in the diverse applications in optoelectronics and biosensing devices with peapod nanostructures.

Near UV LEDs made with in situ doped p-n homojunction ZnO nanowire arrays.

Catalyst-free p-n homojunction ZnO nanowire (NW) arrays in which the phosphorus (P) and zinc (Zn) served as p- and n-type dopants, respectively, have been synthesized for the first time by a controlled in situ doping process for fabricating efficient ultraviolet light-emitting devices. The doping transition region defined as the width for P atoms gradually occupying Zn sites along the growth direction can be narrowed down to sub-50 nm. The cathodoluminescence emission peak at 340 nm emitted from n-type ZnO:Zn NW arrays is likely due to the Burstein-Moss effect in the high electron carrier concentration regime. Further, the electroluminescence spectra from the p-n ZnO NW arrays distinctively exhibit the short-wavelength emission at 342 nm and the blue shift from 342 to 325 nm is observed as the operating voltage further increasing. The ZnO NW p-n homojunctions comprising p-type segment with high electron concentration are promising building blocks for short-wavelength lighting device and photoelectronics.

Nitrogen-doped tungsten oxide nanowires: low-temperature synthesis on Si, and electrical, optical, and field-emission properties.

Very dense and uniformly distributed nitrogen-doped tungsten oxide (WO(3)) nanowires were synthesized successfully on a 4-inch Si(100) wafer at low temperature. The nanowires were of lengths extending up to 5 mum and diameters ranging from 25 to 35 nm. The highest aspect ratio was estimated to be about 200. An emission peak at 470 nm was found by photoluminescence measurement at room temperature. The suggested growth mechanism of the nanowires is vapor-solid growth, in which gaseous ammonia plays a significant role to reduce the formation temperature. The approach has proved to be a reliable way to produce nitrogen-doped WO(3) nanowires on Si in large quantities. The direct fabrication of WO(3)-based nanodevices on Si has been demonstrated.

Localized CO2 laser annealing induced dehydrogenation/ablation and optical refinement of silicon-rich silicon dioxide film with embedded si nanocrystals.

CO2 laser annealing induced effects of dehydrogenation, Si nanocrystal precipitation, ablation, and optical refinement in PECVD grown SiO1.25 film are investigated. Dehydrogenation shrinks SiO1.25 thickness by 40 nm after annealing at laser intensity (Plaser) of 4 kW/cm(2) for 1.4 ms. As Plaser increases to 6 kW/cm(2), the photoluminescence (PL) red-shifts to 806 nm due to the size enlargement of Si nanocrystals, while a reduced optical bandgap energy from 3.3 to 2.43 eV and an enlarged refractive index from 1.57 to 1.87 are also observed. Transmission electron microscopy analysis reveals that the randomly oriented Si nanocrystals exhibit an average diameter of 5.3 nm and a volume density of 1.9 x 10(18) cm(-3). CO2 Laser ablation initiates at intensity higher than 7 kW/cm(2), which introduces numerous structural defects with a strong PL at 410 nm. Such an ablation inevitably leads to a blue-shifted optical bandgap energy from 2.43 to 2.76 eV as Plaser enlarges from 6 to 12 kW/cm(2) are concluded.

Doped spiral alumina nanowires.

Spiral alumina nanowires, doped with Cr and Si, are directly and reliably produced in bulk quantities via annealing of high entropy alloys.

Direct assessment of the mechanical modulus of graphene co-doped with low concentrations of boron-nitrogen by a non-contact approach.

Boron and nitrogen co-doping has been shown to be an effective way to induce a band gap in graphene for electrical applications but only a few theoretical studies have been done to understand the elastic and mechanical properties of the modified graphene. Until now, no experimental assessment of the mechanical modulus of boron-nitrogen-doped graphene (BNG) has been reported in the literature. Here, we demonstrate a novel non-contact approach to determine the in-plane stiffness of BNG at low BN concentrations. The in-plane stiffness of BNG with 2 at% BN concentration was estimated to be about 309 N m(-1), which is lower than that of pristine graphene, in good agreement with some theoretical studies. Moreover, we correlated the conductivity of BNG with induced strain and found the BNG to be more sensitive than pristine graphene in response to externally applied strain. This result indicates that BNG is a more suitable material than graphene for strain sensor applications.

Integrated optical waveguide and photodetector arrays based on comb-like ZnO structures.

On-chip integrations of photonic waveguides and high-performance electrically-driven devices, by combining different active or passive optical components, are imperative towards the advancement of nanophotonic circuitry systems. We experimentally demonstrate the collective optical functionalities of ZnO microstructures towards designing an integrated photonic system by combining the optical waveguiding and detection properties. Comb-like microstructures composed of periodic arrays of smooth, single-crystalline ZnO nanowires are synthesized for these purposes. We demonstrate that ZnO comb structures could be used as optical waveguides, which can manipulate the blue, green, and red laser beams to an interconnected waveguide array. These results are substantiated by extensive investigation of waveguiding properties of single, stacked or crossbar nanowires, and different branched microstructures. These waveguide arrays can be successfully coupled with another ZnO comb-based photodetector and the collective performances of the integrated optical micro-device units are investigated in detail. This study shows that ZnO comb-based optical waveguide arrays have the great potential to be used as a bottom-up strategy for the construction of various miniaturized photonic demultiplexer systems.

Realizing high-efficiency omnidirectional n-type Si solar cells via the hierarchical architecture concept with radial junctions.

Hierarchical structures combining micropyramids and nanowires with appropriate control of surface carrier recombination represent a class of architectures for radial p-n junction solar cells that synergizes the advantageous features including excellent broad-band, omnidirectional light-harvesting and efficient separation/collection of photoexcited carriers. The heterojunction solar cells fabricated with hierarchical structures exhibit the efficiency of 15.14% using cost-effective as-cut Czochralski n-type Si substrates, which is the highest reported efficiency among all n-type Si nanostructured solar cells. We also demonstrate the omnidirectional solar cell that exhibits the daily generated power enhancement of 44.2% by using hierarchical structures, as compared to conventional micropyramid control cells. The concurrent improvement in optical and electrical properties for realizing high-efficiency omnidirectional solar cells using as-cut Czochralski n-type Si substrates demonstrated here makes a hierarchical architecture concept promising for large-area and cost-effective mass production.

A hybrid piezoelectric structure for wearable nanogenerators.

A hybrid-fiber nanogenerator comprising a ZnO nanowire array, PVDF polymer and two electrodes is presented. Depending on the bending or spreading action of the human arm, at an angle of ∼90°, the hybrid fiber reaches electrical outputs of ∼0.1 V and ∼10 nA cm(-2) . The unique structure of the hybrid fiber may inspire future research in wearable energy-harvesting technology.

Gallium nitride nanowire based nanogenerators and light-emitting diodes.

Single-crystal n-type GaN nanowires have been grown epitaxially on a Mg-doped p-type GaN substrate. Piezoelectric nanognerators based on GaN nanowires are investigated by conductive AFM, and the results showed an output power density of nearly 12.5 mW/m(2). Luminous LED modules based on n-GaN nanowires/p-GaN substrate have been fabricated. CCD images of the lighted LED and the corresponding electroluminescence spectra are recorded at a forward bias. Moreover, the GaN nanowire LED can be lighted up by the power provided by a ZnO nanowire based nanogenerator, demonstrating a self-powered LED using wurtzite-structured nanomaterials.

Lead-free NaNbO3 nanowires for a high output piezoelectric nanogenerator.

Perovskite ferroelectric nanowires have rarely been used for the conversion of tiny mechanical vibrations into electricity, in spite of their large piezoelectricity. Here we present a lead-free NaNbO(3) nanowire-based piezoelectric device as a high output and cost-effective flexible nanogenerator. The device consists of a NaNbO(3) nanowire-poly(dimethylsiloxane) (PDMS) polymer composite and Au/Cr-coated polymer films. High-quality NaNbO(3) nanowires can be grown by hydrothermal method at low temperature and can be poled by an electric field at room temperature. The NaNbO(3) nanowire-PDMS polymer composite device shows an output voltage of 3.2 V and output current of 72 nA (current density of 16 nA/cm(2)) under a compressive strain of 0.23%. These results imply that NaNbO(3) nanowires should be quite useful for large-scale lead-free piezoelectric nanogenerator applications.

Plasmon resonance spectroscopy of gold-in-gallium oxide peapod and core/shell nanowires.

Light-scattering properties of individual gold-in-Ga(2)O(3) peapod nanowires and gold-in-Ga(2)O(3) core/shell nanowires were investigated by optical dark-field microscopy. The observed scattering peaks are suggested to result from plasmonic resonance of the gold nanopeas and nanorods in the Ga(2)O(3) nanowires. As the diameter of gold peapod increases, the resonance peak of the optical scattering spectra showed a red-shift. In addition, as the gold peas are elongated along one axis, the corresponding plasmon resonance peak splits into two peaks at shorter and longer wavelengths. The cladding Ga(2)O(3) dielectric layer also plays a key role in light-scattering behavior. The observed modulation in the p-polarized spectrum revealed that future vertical nanoplasmonic devices made by plasmonic nanocavity due to the short propagation length of surface plasmons can be realized and optimized.