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.     

ZnO-Sn:ZnO core-shell nanowires and ZnO-Zn2SnO4 comb-like nanocomposites

Novel single-crystalline ZnO-Sn:ZnO (SZO) core-shell nanowires and ZnO-Zn2SnO4 (ZTO) comb-like nanocomposites were synthesized by thermal chemical vapor deposition at a low temperature of 650 degrees C. Scanning electron microscopy and transmission electron microscopy show the diameters and lengths of the core-shell nanowires are in ranges of 25-60 nm and 300-500 nm, respectively. The atomic ratios of Sn to (Zn + Sn) in the central and shell parts of the nanowire are 0.4 at.% and 6.1 at.%, respectively. The ZnO-ZTO comb-like nanocomposites possess ZnO nanocombs with ZTO nano-layers deposited on both sides of them. The ZnO branches and ZTO layers are single-crystalline wurtzite and spinel structures growing along the [0002] and [111] directions, respectively. Room-temperature cathodoluminescence measurements show the nanocomposites exhibit strong ultraviolet (UV) emissions at 300, 384 nm, and a broad green emission. The novel luminescence shows promising singularity for opto-electronic applications.     

Optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures for photovoltaic applications

Although ZnO and ZnS are abundant, stable, and environmentally benign, their band gap energies (3.44, 3.72 eV, respectively) are too large for optimal photovoltaic efficiency. By using band-corrected pseudopotential density functional theory calculations, we study how the band gap, optical absorption, and carrier localization can be controlled by forming quantum-well-like and nanowire-based heterostructures of ZnO/ZnS and ZnO/ZnTe. In the case of ZnO/ZnS core/shell nanowires, which can be synthesized using existing methods, we obtain a band gap of 2.07 eV, which corresponds to a Shockley-Quiesser efficiency limit of 23%. On the basis of these nanowire results, we propose that ZnO/ZnS core/shell nanowires can be used as photovoltaic devices with organic polymer semiconductors as p-channel contacts.     

Fabrication and optical emission of TiO2-sheathed ZnO nanowires

The ZnO nanowires were synthesized by using vapor-liquid-solid mechanism and then the ZnO nanowires were sheathed with TiO2 by metal organic chemical vapor deposition. The coaxial nanowires were 30-200 nm in diameter and up to 0.2 microm in length. Transmission electron microscopy and X-ray diffraction analysis results showed that the ZnO cores and TiO2 shells of the core-shell nanowires had wurtzite and amorphous structures, respectively. Photoluminescence measurement showed that TiO2 coating increased and decreased the near-band edge (NBE) and deep-level emissions of the ZnO nanowires in intensity, respectively. However, it appeared that subsequent annealing was undesirable since it decreased the NBE emission in intensity.     

Synthesis and characterization of ZnO nanowires for nanosensor applications

In this paper we report the synthesis of ZnO nanowires via chemical vapor deposition (CVD) at 650 °C. It will be shown that these nanowires are suitable for sensing applications. ZnO nanowires were grown with diameters ranging from 50 to 200 nm depending on the substrate position in a CVD synthesis reactor and the growth regimes. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Raman spectroscopy (RS) have been used to characterize the ZnO nanowires. To investigate the suitability of the CVD synthesized ZnO nanowires for gas sensing applications, a single ZnO nanowire device (50 nm in diameter) was fabricated using a focused ion beam (FIB). The response to H₂ of a gas nanosensor based on an individual ZnO nanowire is also reported.     

Ordered ZnO/AZO/PAM nanowire arrays prepared by seed-layer-assisted electrochemical deposition

An Al-doped ZnO (AZO) seed layer is prepared on the back side of a porous alumina membrane (PAM) substrate by spin coating followed by annealing in a vacuum at 400 °C. Zinc oxide in ordered arrays mediated by a high aspect ratio and an ordered pore array of AZO/PAM is synthesized. The ZnO nanowire array is prepared via a 3-electrode electrochemical deposition process using ZnSO₄ and H₂O₂ solutions at a potential of − 1 V (versus saturated calomel electrode) and temperatures of 65 and 80 °C. The microstructure and chemical composition of the AZO seed layer and ZnO/AZO/PAM nanowire arrays are characterized by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray spectroscopy (EDS). Results indicate that the ZnO/AZO/PAM nanowire arrays were assembled in the nanochannel of the porous alumina template with diameters of 110–140 nm. The crystallinity of the ZnO nanowires depends on the AZO seed layer during the annealing process. The nucleation and growth process of ZnO/AZO/PAM nanowires are interpreted by the seed-layer-assisted growth mechanism.     

Investigation of initial growth of ZnO nanowires and their growth mechanism

ZnO nanowires were synthesized on Si substrates by a simple metal vapor deposition method without any catalysts. The initial growth and the growth mechanism of the ZnO nanowires were studied using scanning and transmission electron microscopy. We found that the ZnO nanowires grew on the Si substrate via a self-seeding vapor-solid mechanism. The growth process of the ZnO nanowires consisted of four steps: self-seeding, one-dimensional epitaxial growth of the nanowires on the seeds by a base-growth mode, further acceleration of nanowire growth with additional seeding, and active formation of the nanowires.     

Controlled growth of ternary alloy nanowires using metalorganic chemical vapor deposition

We report the growth and characterization of ternary AlxGa1- xAs nanowires by metalorganic chemical vapor deposition as a function of temperature and V/III ratio. Transmission electron microscopy and energy dispersive X-ray spectroscopy show that, at high temperatures and high V/III ratios, the nanowires form a core-shell structure with higher Al composition in the nanowire core than in the shell. We develop a growth model that takes into account diffusion of reactants and decomposition rates at the nanowire catalyst and stem to describe the compositional difference and the shell growth rate. Utilizing this model, we have successfully grown compositionally uniform Al0.16Ga0.84As nanowires. The ability to rationally tune the composition of ternary alloy nanowires broadens the application range of nanowires by enabling more complex nanowire heterostructures.     

In-situ phosphrous doping in ZnTe nanowires with enhanced p-type conductivity

Single-crystalline undoped and phosphrous-doped (P-doped) p-type ZnTe nanowires (NWs) were synthesized via a simple vapor transport and deposition method. Both undoped and P-doped ZnTe nanowires have zinc blende structure and uniform geometry. X-ray diffraction peaks of the P-doped ZnTe nanowires show an obvious shift toward higher diffraction angle as compared with the undoped ZnTe nanowires. X-ray photoelectron spectroscopy confirms the existence of P-dopant in the ZnTe nanowires. Field-effect transistors based on both undoped and P-doped ZnTe nanowires were fabricated and characterized. Electrical measurements demonstrated that P-doping led to an enhancement in ptype conductivity of ZnTe nanowires. A defect reaction mechanism was proposed to explain the p-type behaviors of both undoped and P-doped ZnTe nanowires.     

Room temperature electroluminescence from arsenic doped p-type ZnO nanowires/n-ZnO thin film homojunction light-emitting diode

The highly arrayed arsenic doped p-ZnO nanowires/n-ZnO thin film homojunction light-emitting diode was fabricated on semi-insulated Si substrate. The homojunction was consisted of high-quality n-ZnO thin film grown by metal–organic chemical vapor deposition technology following arsenic doped ZnO nanowires grown by chemical vapor deposition. The device shows good rectification characteristic with a turn-on voltage of ~4.8 V and reverse breakdown voltage of ~18 V. Moreover, two distinct electroluminescence bands centered at 2.35 and 3.18 eV are detected from this device under forward bias at room temperature.     

Focused ion beam fabrication of novel core-shell nanowire structures

A novel method of indirect deposition by means of a focused ion beam (FIB) is utilized to develop metal/insulator/semiconductor nanowire core-shell structures. This method is based upon depositing an annular pattern centered on a nanowire, with secondary deposition then coating the wire. Typical cross-sectional deposition area increments as a function of ion doses are 1.3 × 10(-2)?μm(2)?nC(-1) for Pt and 3.5 × 10(-2)?μm(2)?nC(-1) for SiO(2). The structures are examined with a transmission electron microscope (TEM) using a new nanowire TEM sample preparation method that allows direct examinations of individually selected core-shell nanowires fabricated under different indirect FIB deposition conditions. Elemental analyses by means of energy dispersive x-ray spectroscopy and electron energy filtered TEM imaging verify the deposition of SiO(2) and Pt layers. Relatively uniform Pt and SiO(2) coatings on individual GaP nanowires can be achieved with overall thickness deviation of about 10% for deposition up to 25-30?nm thick Pt or SiO(2) shells. It should be possible to extend this approach to any nanowire/nanotube system, and to a wide range of coatings in any desired layer sequences.     

Influence of seed layers on the vertical growth of ZnO nanowires

We synthesized vertically aligned ZnO nanowires on SiO₂ wafer <100> using the Au, ZnO and Au/ZnO seed layers through the physical vapor deposition process. The growth direction of ZnO nanowire was controlled by using the three different seed layers. From the XRD results, we observed the highest intensity of the (002) peak on the Au/ZnO seed layer among the three seed layers. The SEM images show that all of the ZnO nanowires have an average diameter of about 100 ~ 200 nm and a length of about 5 μm, and the nanowires grown on the Au/ZnO seed layer are oriented the most perpendicularly to the substrate surface. From the PL analysis, we observed that the intensity of broad emissions at 400-600 nm relating the green emission for the ZnO nanowires on the Au/ZnO seed layer was much weaker than that for the ZnO nanowires on the ZnO seed layer. The experiment results indicate that the selection of seed layers is important to grow nanowires vertically for the application of nanoscale devices.     

Low-temperature growth of well-aligned ZnO nanowire arrays by chemical bath deposition for hybrid solar cell application

Well-aligned ZnO nanowire arrays were grown on indium tin oxide coated glass substrates by a facile chemical bath deposition technique. Morphologies, crystalline structure and optical transmission were investigated by field-emission scanning electron microscope, X-ray diffraction and UV–visible transmission spectrum, respectively. The results showed that ZnO nanowires were aligned in a dense array approximately perpendicular to substrate surface, they were wurtzite-structured (hexagonal) ZnO. In addition, the nanowire arrays exhibited high optical transmission (>85 %) in the visible region. Furthermore, an inverted inorganic/polymer hybrid solar cell was built using as-grown well-aligned ZnO nanowire arrays as inorganic layer, under the AM 1.5 illumination with a light intensity of 80 mW/cm², the device showed an open circuit voltage (V_oc) of 0.44 V, a short circuit current (J_sc) of 3.23 mA/cm², a fill-factor of 38 %, and a power conversion efficiency of 0.68 %.     

Dramatically enhanced ultraviolet photosensing mechanism in a n-ZnO nanowires/i-MgO/n-Si structure with highly dense nanowires and ultrathin MgO layers

This study reports that the visible-blind ultraviolet (UV) photodetecting properties of ZnO nanowire based photodetectors were remarkably improved by introducing ultrathin insulating MgO layers between the ZnO nanowires and Si substrates. All layers were grown without pause by metal organic chemical vapor deposition and the density and vertical arrangement of the ZnO nanowires were strongly dependent on the thickness of the MgO layers. The sample in which an MgO layer with a thickness of 8 nm was inserted had high density nanowires with a vertical alignment and showed dramatically improved UV photosensing performance (photo-to-dark current ratio = 1344.5 and recovery time = 350 ms). The photoresponse spectrum revealed good visible-blind UV detectivity with a sharp cut off at 378 nm and a high UV/visible rejection ratio. A detailed discussion regarding the developed UV photosensing mechanism from the introduction of the i-MgO layers and highly dense nanowires in the n-ZnO nanowires/i-MgO/n-Si substrates structure is presented in this work.     

Superior Functionality by Design: Selective Ozone Sensing Realized by Rationally Constructed High‐Index ZnO Surfaces

A new technique is reported for the transformation of smooth nonpolar ZnO nanowire surfaces to zigzagged high‐index polar surfaces using polycrystalline ZnO thin films deposited by atomic layer deposition (ALD). The c‐axis‐oriented ZnO nanowires with smooth nonpolar surfaces are fabricated using vapor deposition method and subsequently coated by ALD with a ZnO particulate thin film. The synthesized ZnO–ZnO core–shell nanostructures are annealed at 800 °C to transform the smooth ZnO nanowires to zigzagged nanowires with high‐index polar surfaces. Ozone sensing response is compared for all three types of fabricated nanowire morphologies, namely nanowires with smooth surfaces, ZnO–ZnO core–shell nanowires, and zigzagged ZnO nanowires to determine the role of crystallographic surface planes on gas response. While the smooth and core–shell nanowires are largely non‐responsive to varying O₃ concentrations in the experiments, zigzagged nanowires show a significantly higher sensitivity (ppb level) owing to inherent defect‐rich high‐index polar surfaces.     

Layer-by-layer assembly synthesis of ZnO/SnO2 composite nanowire arrays as high-performance anode for lithium-ion batteries

A layer-by-layer approach has been developed to synthesize ZnO/SnO₂ composite nanowire arrays on copper substrate. ZnO nanowire arrays have been first prepared on copper substrate through seed-assisted method, and then, the surface of ZnO nanowires have been modified by the polyelectrolyte. After oxidation-reduction reaction, SnO₂ layer has been deposited onto the surface of ZnO nanowires. The as-synthesized ZnO/SnO₂ composite nanowire arrays have been applied as anode for lithium-ion batteries, which show high reversible capacity and good cycling stability compared to pure ZnO nanowire arrays and SnO₂ nanoparticles. It is believed that the improved performance may be attributed to the high capacity of SnO₂ and the good cycling stability of the array structure on current collector.     

High resolution, two-dimensional image mapping of ZnO nanowires by confocal microphotoluminescence and microraman spectroscopy

High-quality ZnO nanowires were synthesized using both Au catalysts and ZnO seeds by chemical vapor depositionon basal plane sapphire substrates. The nanowires were hexagonal and aligned with their c-axis closely perpendicular to the sapphire substrate surface. The structural characteristics of the nanowiresgrown using the different catalysts/seeds were compared using scanning electron microscopyand X-ray diffraction. Their optical properties were assessed using microphoto-luminescence and confocal microRaman spectroscopy and compared. The nanowires exhibited a strong near band-edge related UV luminescence emission along with a defect related visible emission. The dependence of the luminescence as a function of incident excitation power and depth along the axis of the nanowires was studied. The wurtzite structure of the ZnO was confirmed from the Raman measurements. Two-dimensional mappings of the microphotoluminescence emission at different wavelengths and microRaman scattering from the nanowire samples were carried out using a confocal laser scanning microscope. This enabled the ability to correlate the near band-edge UV and visible emissions over the mapped area.     

Chemical vapor deposition-based growth of aligned ZnO nanowires on polycrystalline Zn<Subscript>2</Subscript>GeO<Subscript>4</Subscript>:Mn substrates

ZnO nanowires have been grown on polycrystalline Zn₂GeO₄:Mn substrates for the first time using a chemical vapor deposition method. Both Zn and ZnO sources were used to supply Zn vapor in the growth process of ZnO nanowires. The Zn₂GeO₄:Mn substrates were prepared using solid-state ceramic synthesis methods, and average grain sizes of ~1 μm were achieved. The nanowires of diameters in the range of 100–200 nm and length of ~30 μm were observed. In addition to nanowires, other morphologies of ZnO nanostructures, such as ZnO tetrapods, were observed when Zn powder was used as the source for the CVD growth. The results reveal that polycrystalline substrates are also promising as novel alternative substrates for growth of ZnO nanostructures. The as-synthesized ZnO nanowire/Zn₂GeO₄:Mn composites are being developed for future electroluminescent devices.     

Regular arrays of monodisperse platinum/erbium disilicide core-shell nanowires and nanoparticles on Si(001) via a self-assembled template

We have developed a process for fabricating monodisperse noble metal/rare earth disilicide core-shell nanoparticles and nanowires in regular arrays on Si(001) with a density of 5 x 10(10) / cm2, and over areas > 1 mm2. Pt deposited via physical vapor deposition on a self-assembled rare earth disilicide nanowire template combined with reactive ion etching produces arrays of nanostructures. SEM images demonstrate the ability to select nanowires or nanoparticles as a function of Pt coverage. Statistical analysis of images of Pt nanoparticle arrays yield a mean feature size of 8 nm with a size variation of +/- 0.9 nm and interparticle spacing of approximately 15 nm.     

Efficient field emission properties of ZnO (core)/graphite (shell) nanowires

In this work the field emission studies of a new type of field emitter, zinc oxide (ZnO) core/graphitic (g-C) shell nanowires are presented. The nanowires are synthesized by chemical vapor deposition of zinc acetate at 1300 °C Scanning and transmission electron microscopy characterization confirm high aspect ratio and novel core–shell morphology of the nanowires. Raman spectrum of the nanowires mat represents the characteristic Raman modes from g-C shell as well as from the ZnO core. A low turn on field of 2.75 V/μm and a high current density of 1.0 mA/cm² at 4.5 V/μm for ZnO/g-C nanowires ensure the superior field emission behavior compared to the bare ZnO nanowires.