Growth of highly oriented ZnO nanowires on GaN substrates for electronic and optical sensor applications.

In this Paper we present growth and characterization results of highly oriented ZnO nanowires grown on wide bandgap GaN substrates. Experimental results on the ZnO nanowires grown on p-GaN are presented with growth morphology and dimensionality control. We also present experimental results on these nanowire arrays such as I-V measurements and UV sensitivity measurements. The ZnO nanowires can be used for a variety of nanoscale optical and electronics applications.

Large-Area Triboelectric Nanogenerator Mass Spectrometry: Expanded Coverage, Double-Bond Pinpointing, and Supercharging.

Efficient ionization is a necessary condition for mass spectrometric analysis, but many compounds fail to ionize well enough to yield sufficient detection limits. Triboelectric nanogenerators (TENG) coupled to nanoelectrospray ionization (nanoESI) mass spectrometry (MS) are a highly effective approach to high sensitivity MS analysis. Here, we report on new, large-area TENG that constructively leverage the relationship between electrode size, created charges, and open-circuit voltage, leading to wider chemical coverage. Large-area TENG were found to also promote electrospray gas-phase oxidation reactions that enabled double bond position pinpointing for unsaturated lipids and species-specific lipid quantitation. Furthermore, large-area TENG MS of proteins was observed to yield higher charge state distributions (i.e., supercharging) without the need for high surface tension additives.

Beaklike SnO2 nanorods with strong photoluminescent and field-emission properties.

Beaklike SnO2 nanorods were synthesized by a vapor-liquid-solid approach using Au as a catalyst. The nanorods grow along the [10 1] direction and the beak is formed by switching the growth direction to [1 12] through controlling the growth conditions at the end of the synthesis. The photoluminescence (PL) spectrum of the nanorods exhibits visible light emission with a peak at 602 nm. The field-emission (FE) properties of the nanorods have been measured to exhibit a turn-on field of 5.8 V microm(-1). A comparative study of FE measurements between SnO2 nanorods with uniform diameters and these beaklike nanorods suggests that the shape and curved tips are important factors in determining the FE properties.

Lasing mechanism of ZnO nanowires/nanobelts at room temperature.

ZnO has become the focus of photonics and optoelectronic research. We prepared pure Mn(II) doped ZnO nanowires with a controlled reduction reaction by carbon in an asymmetrical tube. Careful time-resolved photoluminescence experimental study indicates three types of lasing mechanisms: exciton-exciton interaction, bipolaronic exciton condensation, and plasma; these exist in different ZnO nanowires, which can be changed by doping Mn in ZnO nanowire. The transformation between varied mechanisms is discussed in detail with their spectral behaviors. These results are important in the design of future violet-blue luminescence and display devices.

Pattern and feature designed growth of ZnO nanowire arrays for vertical devices.

An approach is demonstrated for growing aligned ZnO nanowire/nanorod arrays following a predesigned pattern and feature with controlled site, shape, distribution, and orientation. The technique relies on an integration of atomic force microscopy (AFM) nanomachining with catalytically activated vapor-liquid-solid (VLS) growth. The pattern and growth locations are defined by the catalyst distribution created by AFM, and the orientation is determined by the epitaxial growth on a single-crystal substrate. The technique opens a variety of possibilities of using nanowire arrays as sensor arrays, piezoelectric antenna arrays, nanolasers, photonic band gap crystal, biosensors, and field emitters with controlled density, location, shape, and distribution according to a designed pattern and feature.

Density-controlled growth of aligned ZnO nanowires sharing a common contact: a simple, low-cost, and mask-free technique for large-scale applications.

An effective, low cost, simple, and mask-free pathway is demonstrated for achieving density control of the aligned ZnO nanowires grown for large-scale applications. By a slight variation of the thickness of the thermally evaporated gold catalyst film, a significant change in the density of aligned ZnO nanowires has been controlled. The growth processes of the nanowires on an Al(0.5)Ga(0.5)N substrate has been studied based on the wetting behavior of gold catalyst with or without source vapor, and the results classify the growth processes into three categories: separated dots initiated growth, continuous layer initiated growth, and scattered particle initiated growth. This study presents an approach for growing aligned nanowire arrays on a ceramic substrate with the simultaneous formation of a continuous conducting electrode at the roots, which is important for device applications, such as field emission.

Clinical significance of joint detection of mALB and NAG for early kidney damage in burn patients.

BACKGROUND: The aim of this study was to study the clinical significance of joint detection of mALB and NAG in early kidney damage in burn patients. METHODS: Forty-five burn patients of different degrees were selected and divided into mild, moderate, severe, and heavy burns, and normal healthy controls according to their severity. Their b2- macroglobulin (b2-MG), a1-macroglobulin (a1-MG), mALB and N-acetyl-b-D-NAG were tested for 3 days, 1 week, 2 weeks, 4 weeks after the burn, respectively. RESULTS: The urine concentration change b2-MG, a1-MG, MALB, and NAG are related to the area, depth and degree of the burn. The more serious the burn is, the higher the levels of mALB and NAG (P<0.001 or P<0.01) is. CONCLUSIONS: Early detection of mALB and NAG is helpful for early kidney damage diagnosis in burn patients to prevent further complications.

Systematic study on experimental conditions for large-scale growth of aligned ZnO nanowires on nitrides.

In vapor-liquid-solid (VLS) growth, it is generally believed that nanowires would grow as long as the right catalysts and substrate are supplied as well as the growth temperature is adequate. We show here, however, that oxygen partial pressure plays a key role in determining the quality of the aligned ZnO nanowires. We present a "phase diagram" between the oxygen partial pressure and the growth chamber pressure for synthesizing high quality aligned ZnO nanowires on GaN substrate. This result provides a road map for large-scale, controlled synthesis of ZnO nanowires on nitride semiconductor substrates with the potential to meet the needs of practical applications. The chemical process involved in the growth process is also systematically elaborated based on experimental data received under different conditions.

Measuring the aspect ratios of ZnO nanobelts.

Nanobelts are new materials that have a rectangular cross-section and are characterized by widths and width-to-thickness aspect ratios. In this paper, the thickness and aspect ratios of ZnO nanobelts are measured by a conjunction application of convergent beam electron diffraction (CBED) and electron energy-loss spectroscopy (EELS). The thicknesses of thicker nanobelts are first determined by CBED under two-beam diffracting condition, then they are used to determine the electron inelastic mean-free-path (MFP) length, which is 161+/-15nm for ZnO at 200kV. The thicknesses of the thinner nanobelts are then determined by EELS using the calibrated MFP. The results show that the aspect ratio depends on conditions under which the sample was synthesized.

Patterned polymer nanowire arrays as an effective protein immobilizer for biosensing and HIV detection.

We report an array of polymeric nanowires for effectively immobilizing biomolecules on biochips owing to the large surface area. The nanowires were fabricated in predesigned patterns using an inductively coupled plasma (ICP) etching process. Microfluidic biochips integrated using the substrates with arrays of nanowires and polydimethylsiloxane channels have been demonstrated to be effective for detecting antigens, and a detection limit of antigens at 0.2 µg mL(-1) has been achieved, which is improved by a factor of 50 compared to that based on flat substrates without the nanowires. In addition, the high sensitivity for clinical detection of human immunodeficiency virus (HIV) antibody has also been demonstrated, showing a 20 times enhancement in fluorescent signal intensity between the samples with positive and negative HIV.

The correlation between radiative surface defect states and high color rendering index from ZnO nanotubes.

Combined surface, structural and opto-electrical investigations are drawn from the chemically fashioned ZnO nanotubes and its heterostructure with p-GaN film. A strong correlation has been found between the formation of radiative surface defect states in the nanotubes and the pure cool white light possessing averaged eight color rendering index value of 96 with appropriate color temperature. Highly important deep-red color index value has been realized > 95 which has the capability to render and reproduce natural and vivid colors accurately. Diverse types of deep defect states and their relative contribution to the corresponding wavelengths in the broad emission band is suggested.

Nondestructive in situ identification of crystal orientation of anisotropic ZnO nanostructures.

We present a novel method for direct, fast, nonambiguous, and nondestructive identification of the growth direction and orientation of individual ZnO nanostructures in the device-ready environment by exploiting high-resolution confocal Raman mapping. Various features of the Raman spectrum of ZnO nanostructures, vapor deposition grown nanobelts and peptide-assisted vertical nanorods, were found to be sensitive to the relative orientation of the crystal plane. Furthermore, we discovered that the waveguiding property of the ZnO nanobelt is also orientation dependent and results in either apparent enhancement or suppression of Raman scattering from the underlying substrate. We demonstrate that various features of Raman spectrum of ZnO and the modulation of the substrate signal can be employed for the rapid and nondestructive identification of the crystal growth direction and orientation of these nanostructures even after integration into devices, which is impossible with current electron microscopy and diffraction techniques. We believe that the general features observed here are equally applicable to other wurtzite nanostructures (ZnS, GaN) which are critical in optoelectronics, lasing, and piezotronic applications.

Bulk acoustic resonator based on piezoelectric ZnO belts.

In this paper, a bulk acoustic resonator based on ZnO belts is demonstrated. This device shows a great deal of promise in applications as an electronic filter and as a mass sensor. The fabricated device was characterized using vector network analysis, and both the first and third harmonics of resonance were observed at approximately 247 and 754 MHz, respectively. A one-dimensional Krimholt-Leedom-Matthaei model was utilized to predict the resonant frequency of the device and confirm the observed behavior.

Piezoelectric field effect transistor and nanoforce sensor based on a single ZnO nanowire.

Utilizing the coupled piezoelectric and semiconducting dual properties of ZnO, we demonstrate a piezoelectric field effect transistor (PE-FET) that is composed of a ZnO nanowire (NW) (or nanobelt) bridging across two Ohmic contacts, in which the source to drain current is controlled by the bending of the NW. A possible mechanism for the PE-FET is suggested to be associated with the carrier trapping effect and the creation of a charge depletion zone under elastic deformatioin. This PE-FET has been applied as a force/pressure sensor for measuring forces in the nanonewton range and even smaller with the use of smaller NWs. An almost linear relationship between the bending force and the conductance was found at small bending regions, demonstrating the principle of nanowire-based nanoforce and nanopressure sensors.

ZnO nanobelt/nanowire Schottky diodes formed by dielectrophoresis alignment across au electrodes.

Rectifying diodes of single nanobelt/nanowire-based devices have been fabricated by aligning single ZnO nanobelts/nanowires across paired Au electrodes using dielectrophoresis. A current of 0.5 microA at 1.5 V forward bias has been received, and the diode can bear an applied voltage of up to 10 V. The ideality factor of the diode is approximately 3, and the on-to-off current ratio is as high as 2,000. The detailed IV characteristics of the Schottky diodes have been investigated at low temperatures. The formation of the Schottky diodes is suggested due to the asymmetric contacts formed in the dielectrophoresis aligning process.

Elastic property of vertically aligned nanowires.

An atomic force microscopy (AFM) based technique is demonstrated for measuring the elastic modulus of individual nanowires/nanotubes aligned on a solid substrate without destructing or manipulating the sample. By simultaneously acquiring the topography and lateral force image of the aligned nanowires in the AFM contacting mode, the elastic modulus of the individual nanowires in the image has been derived. The measurement is based on quantifying the lateral force required to induce the maximal deflection of the nanowire where the AFM tip was scanning over the surface in contact mode. For the [0001] ZnO nanowires/nanorods grown on a sapphire surface with an average diameter of 45 nm, the elastic modulus is measured to be 29 +/- 8 GPa.

Large-scale Ni-doped ZnO nanowire arrays and electrical and optical properties.

Large-scale Ni-doped ZnO nanowire (NW) arrays are grown. The electrical conductivity of a single Ni-doped ZnO NW has been increased for 30 times. The photoluminescence (PL) spectrum of the doped ZnO NWs has a red shift, suggesting possible doping induced band edge bending. The doped NW arrays could be the basis for building integrated nanoscale transistors, sensors, and photodetectors.

Large-size liftable inverted-nanobowl sheets as reusable masks for nanolithiography.

A low-cost procedure is introduced for fabricating large-area, liftable, ordered TiO2 nanobowl sheets. The sheet is made using the template of self-assembled polystyrene spheres, followed by atomic layer deposition (ALD), ion milling, and etching. By introducing a thin organic layer between the nanobowls and the substrate, the whole sheet can be lifted-off in full size. The dimension of the holes at the bottom of the nanobowls is controlled by additional ALD; thus, the sheet has been applied as a reusable mask for producing nanodot patterns with designed sizes. This technique demonstrates a simple and economic nanolithiography approach for producing various designed patterns without using a clean room, and it has a great potential for scale-up, mass production, and commercial applications.

Growth of uniformly aligned ZnO nanowire heterojunction arrays on GaN, AlN, and Al0.5Ga0.5N substrates.

Vertically aligned single-crystal ZnO nanorods have been successfully fabricated on semiconducting GaN, Al0.5Ga0.5N, and AlN substrates through a vapor-liquid-solid process. Near-perfect alignment was observed for all substrates without lateral growth. Room-temperature photoluminescence measurements revealed a strong luminescence peak at approximately 378 nm. This work demonstrates the possibility of growing heterojunction arrays of ZnO nanorods on AlxGa1-xN, which has a tunable band gap from 3.44 to 6.20 eV by changing the Al composition from 0 to 1, and opens a new channel for building vertically aligned heterojunction device arrays with tunable optical properties and the realization of a new class of nanoheterojunction devices.