Growth and properties of aligned ZnO nanowires and their applications to n-ZnO/p-Si heterojunction diodes

Aligned n-ZnO nanowires were synthesized via simple thermal evaporation process by using metallic zinc powder in the presence of oxygen on p-silicon (Si) substrate. The as-synthesized aligned ZnO nanowires were characterized in terms of their structural and optical properties by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction process (XRD) and room-temperature photoluminescence (PL) properties. The detailed structural and optical studies revealed that the as-grown nanowires are single crystalline with the wurtzite hexagonal phase and exhibit good optical properties. From application point of view, the as-grown aligned n-ZnO nanowires on p-Si substrates were used to fabricate heterojunction diodes. The fabricated heterojunction diodes exhibited good electrical (I-V) properties with the turn-on voltage of approximately 1.0 V. A temperature-dependant (from 25 degrees C approximately 130 degrees C), I-V characteristics for the fabricated device was also demonstrated in this paper. The presented results demonstrate that the simply grown aligned n-ZnO nanowires on p-Si substrate can be efficiently used for the fabrication of efficient heterojunction devices.     

Temperature dependant characteristics of zinc oxide nanorods based heterojunction diode

Temperature-dependant characteristics of heterojunction diode made by n-ZnO nanorods grown on p-silicon substrates has been characterized and demonstrated in this paper. ZnO nanorods were grown onto the silicon substrate via simple thermal evaporation process by using metallic zinc powder in the presence of oxygen at approximately 550 degrees C without the use of any metal catalysts or additives. The as-grown ZnO nanorods were characterized in terms of their structural and optical properties. The detailed structural studies by XRD, TEM, HRTEM and SAED revealed that the grown nanorods are well-crystalline with the wurtzite hexagonal phase and preferentially grown along the [0001] direction. The as-grown n-ZnO nanorods grown on p-Si substrate were used to fabricate p-n heterojunction diode. The fabricated p-n junction diode attained almost similar turn-on voltage of approximately 0.6 V. The values of turn-on voltage and least current are same with the variations of temperature (i.e., 27 degrees C, 70 degrees C and 130 degrees C).     

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.     

Playing with dimensions: rational design for heteroepitaxial p-n junctions

A design for a heteroepitaxial junction by the way of one-dimensional wurzite on a two-dimensional spinel structure in a low-temperature solution process was introduced, and it's capability was confirmed by successful fabrication of a diode consisting of p-type cobalt oxide (Co(3)O(4)) nanoplate/n-type zinc oxide (ZnO) nanorods, showing reasonable electrical performance. During thermal decomposition, the 30° rotated lattice orientation of Co(3)O(4) nanoplates from the orientation of β-Co(OH)(2) nanoplates was directly observed using high-resolution transmission electron microscopy. The epitaxial relations and the surface stress-induced ZnO nanowire growth on Co(3)O(4) were well supported using the first-principles calculations. Over the large area, (0001) preferred oriented ZnO nanorods epitaxially grown on the (111) plane of Co(3)O(4) nanoplates were experimentally obtained. Using this epitaxial p-n junction, a diode was fabricated. The ideality factor, turn-on voltage, and rectifying ratio of the diode were measured to be 2.38, 2.5 V and 10(4), respectively.     

Single-crystalline branched zinc phosphide nanostructures: synthesis, properties, and optoelectronic devices

Hierarchical tree-shaped nanostructures, nanobelts, and nanowires of Zn3P2 were synthesized in a thermal assisted laser ablation process. All nanostructures are tetragonal phased Zn3P2 with excellent crystallinity and are free from an oxidization layer according to electron microscopy and X-ray diffraction analyses. Optical measurement revealed a strong absorption from the ultraviolet to near-infrared regions. Optoelectronic devices fabricated using individual nanowires demonstrate a high sensitivity and rapid response to impinging light. A crossed heterojunction of an n-type ZnO nanowire and a p-type Zn3P2 nanowire has been characterized, and it offers a great potential for a high efficient spatial resolved photon detector.     

Effects of post annealing temperature on formation of alumina core shell on zinc oxide nanowire: optoelectronic properties and dye sensitized solar cell applications

Alumina coated zinc oxide (ZnO) nanowires were synthesized through the combination of hydrothermal growth and sol–gel deposition method. The effects of post annealing temperature on structural and optoelectronic properties of the deposited samples were evaluated using scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy, photoluminescence spectroscopy and Raman spectroscopy. It was found that the ZnO nanowire can be used as template for further functionalization. As a demonstration, dye sensitized solar cells were fabricated with the alumina coated ZnO nanowires.     

Bringing order to the world of nanowire devices by phase shift lithography

Semiconductor nanowire devices have several properties which match future requirements of scaling down the size of electronics. In typical microelectronics production, a number of microstructures are aligned precisely on top of each other during the fabrication process. In the case of nanowires, this mandatory condition is still hard to achieve. A technological breakthrough is needed to accurately place nanowires at any specific position and then form devices in mass production. In this article, an upscalable process combining conventional micromachining with phase shift lithography will be demonstrated as a suitable tool for nanowire device technology. Vertical Si and ZnO nanowires are demonstrated on very large (several cm(2)) areas. We demonstrate how the nanowire positions can be controlled, and the resulting nanowires are used for device fabrication. As an example Si/ZnO heterojunction diode arrays are fabricated. The electrical characterization of the produced devices has also been performed to confirm the functionality of the fabricated diodes.     

Combination of surface texturing and nanostructure coating for reduction of light reflection in ZnO/Si heterojunction thin film solar cell

In this study, we report a single heterojunction solar cell based on n-type zinc oxide/p-type silicon. Three different solar cells were fabricated based on ZnO thin film on Si substrate, ZnO nanorods on Si substrate, and ZnO nanorods on micro-pyramidal structure of Si substrate. The comparison between these three kinds of solar cells was studied. Pyramidal structure of silicon was fabricated using chemical etching technique of p-type Si (100). The chemical solution consists of NaOH, isopropyl alcohol and hydrazine hydrate. The results showed that Si micro-pyramids can enhance optical absorption of Si substrates by increasing surface area and entrapping of incident light. For fabrication of uniform ZnO nanorods, a seed layer of ZnO was deposited on Si substrates via radio frequency magnetron sputtering technique. This layer can be used as an active n-type material in heterojunction solar cells as well. ZnO nanostructures can increase light absorption due to their high specific surface area. The combination of ZnO nanorods and Si micro-pyramids can enhance light trapping effect and increase the efficiency of solar cells. The structural and morphology of samples were studied using field-emission scanning electron microscopy, atomic force microscopy and X-ray diffractometry while the optical properties were investigated using photoluminescence and reflectance spectrometry. The efficiency and fill factor of solar cells were obtained from current–voltage characteristics using a solar simulator and a source-meter. The results showed that the efficiency of solar cell based on nanostructures of ZnO/micropyramids of Si is highly increased due to high anti-reflective behavior of this sample.     

Zigzag zinc blende ZnS nanowires: Large scale synthesis and their structure evolution induced by electron irradiation

Large scale zigzag zinc blende single crystal ZnS nanowires have been successfully synthesized during a vapor phase growth process together with a small yield of straight wurtzite single crystal ZnS nanowires. AuPd alloy nanoparticles were utilized to catalyze a vapor-solid-solid growth process of both types of ZnS nanowires, instead of the more common vapor-liquid-solid growth process. Surprisingly, the vapor-phase grown zigzag zinc blende ZnS nanowires are metastable under high-energy electron irradiation in a transmission electron microscope, with straight wurtzite nanowires being much more stable. Upon exposure to electron irradiation, a wurtzite ZnO nanoparticle layer formed on the zigzag zinc blende ZnS nanowire surface with concomitant displacement damage. Both electron inelastic scattering and surface oxidation as a result of electron-beam heating occur during this structure evolution process. When prolonged higher-voltage electron irradiation was applied, local zinc blende ZnS nanowire bodies evolved into ZnS-ZnO nanocables, and dispersed ZnS-ZnO nanoparticle networks. Random AuPd nanoparticles were observed distributed on zigzag ZnS nanowire surfaces, which might be responsible for a catalytic oxidation effect and speed up the surface oxidation-induced structure evolution.      

Electroluminescence and rectifying properties of heterojunction LEDs based on ZnO nanorods

n-ZnO NR/p-Si and n-ZnO NR/p-PEDOT/PSS heterojunction light-emitting diodes (LEDs) have been fabricated with ZnO nanorods (NRs) grown by a low-temperature method as well as by employing pulsed laser deposition (PLD). The low-temperature method involves growing the ZnO nanorods by the reaction of water with zinc metal. The current-voltage (I-V) characteristics of the heterojunctions show good rectifying diode characteristics. The electroluminescence (EL) spectra of the nanorods show an emission band at around 390?nm and defect related bands in the 400-550?nm region. Room-temperature electroluminescence is detected under forward bias for both the heterostructures. With the low-temperature grown nanorods, the defect related bands in the 400-550?nm range are more intense in the EL spectra, whereas with the PLD grown nanorods, only the 390?nm band is prominent.     

Vertical nanowire array-based light emitting diodes

Electroluminescence from a nanowire array-based light emitting diode is reported. The junction consists of a p-type GaN thin film grown by metal organic chemical vapor deposition (MOCVD) and a vertical n-type ZnO nanowire array grown epitaxially from the thin film through a simple low temperature solution method. The fabricated devices exhibit diode like current voltage behavior. Electroluminescence is visible to the human eye at a forward bias of 10 V and spectroscopy reveals that emission is dominated by acceptor to band transitions in the p-GaN thin film. It is suggested that the vertical nanowire architecture of the device leads to waveguided emission from the thin film through the nanowire array.     

Epitaxial growth of the zinc oxide nanorods, their characterization and in vitro biocompatibility studies

Here, we have synthesized Zinc Oxide (ZnO) nanorods at room temperature using zinc acetate and hexamethylenetetramine as precursors followed by characterization using X-ray diffraction (XRD), fourier transform infra red spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy. The growth of the synthesized ZnO was found to be very close to its hexagonal nature, which is confirmed by XRD. The nanorods were grown perpendicular to the long-axis and grew along the [001] direction, which is the nature of ZnO growth. The morphology of the synthesized ZnO nanorods was also confirmed by SEM. The size of the nanorod was estimated to be around 20-25?nm in diameter and approximately 50-60?nm in length. Our biocompatibility studies using synthesized ZnO showed no significant dose- or time-dependent increase in the formation of free radicals, accumulation of peroxidative products, antioxidant depletion or loss of cell viability on lung epithelial cells.     

Direct synthesis of ZnO nanowire arrays on Zn foil by a simple thermal evaporation process

ZnO nanowire arrays were synthesized on zinc foil by a simple thermal evaporation process at relatively low temperature. Morphology and size controlled synthesis of the ZnO nanostructures was achieved by variation of the synthesis temperature, reaction time and the surface roughness of the substrate. A gas-solid and self-catalytic liquid-solid mechanism is proposed for the growth of nanowires at different temperatures. High-resolution transmission electron microscopy (HRTEM) showed that the as-grown nanowires were of single crystal hexagonal wurtzite structure, growing along the [101] direction. Photoluminescence exhibited strong UV emission at ～382?nm and a broad green emission at ～513?nm with 325?nm excitation. Raman spectroscopy revealed a phonon confinement effect when compared with results from bulk ZnO. The nanowire arrays also exhibited a field emission property.     

Flash synthesis of flower-like ZnO nanostructures by microwave-induced combustion process

Flower-like ZnO particles were fabricated from zinc nitrate and urea via the microwave-induced combustion technique (MICT). The structure and morphology of the products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The effects of growth conditions such as microwave power, radiation time, and molar ratio of urea/Zn²⁺ were investigated. Results show that the morphologies and particle sizes of the final products depend on the molar ratio of urea/Zn²⁺. The SEM images of ZnO flowers grown in different periods were employed to explain the formation mechanism. This study provides a simple and efficient approach for synthesizing flower-like ZnO heterostructures.     

Growth of high-aspect ratio horizontally-aligned ZnO nanowire arrays

A method of fabricating horizontally-aligned zinc-oxide (ZnO) nanowire (NW) arrays with full control over the width and length is demonstrated. SEM images reveal the hexagonal structure typical of zinc oxide NWs. Arrays of high-aspect ratio horizontal ZnO NWs are fabricated by making use of the lateral overgrowth from dot patterns created by electron beam lithography (EBL). An array of patterned wires are lifted off and transferred to a flexible PDMS substrate with possible applications in several key nanotechnology areas.     

Growth of aligned hexagonal ZnO nanorods on FTO substrate for dye-sensitized solar cells (DSSCs) application

This article reports a facile growth of well-crystalline aligned hexagonal ZnO nanorods on fluorine-doped tin-oxide (FTO) substrate via non-catalytic thermal evaporation process. The morphological investigations done by field-emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM) reveal that the grown products are aligned hexagonal ZnO nanorods which are grown in a very high density over the whole substrate surface. The detailed structural properties observed by high-resolution TEM equipped with selected area electron diffraction (SAED) and X-ray diffraction (XRD) pattern confirmed that the synthesized nanorods are well-crystalline possessing wurtzite hexagonal phase and preferentially grown along the c-axis direction. A sharp and strong UV emission at 381 nm in room-temperature photoluminescence (PL) spectrum showed that the as-grown ZnO nanorods possess excellent optical properties. The as-grown nanorods were used as photo-anode for the fabrication of dye-sensitized solar cells (DSSCs) which exhibits an overall light-to-electricity conversion efficiency (ECE) of 0.7% with V(oc) of 0.571 V, J(sc) of 2.02 mA/cm2 and FF of 0.58.     

A practical vacuum sensor based on a ZnO nanowire array

We report a practical vacuum pressure sensor based on a ZnO nanowire array (NWA). An oriented single-crystal ZnO NWA was synthesized by electrodeposition. The device consists of two ITO glass plates coated with a ZnO NWA. Scanning electron microscopy (SEM) and the x-ray diffraction (XRD) pattern show that the as-grown ZnO NWAs are single-crystal and roughly oriented with the ZnO(002) plane parallel to the substrate. Through measuring the pressure dependent resistance of the sensor at different gas species and temperatures, we discovered that the resistance increases monotonically with vacuum pressure. This demonstrates that a practical vacuum sensor could be fabricated since measurements were carried out with a normal multimeter, with no need for the high sensitivity and costly equipment as routinely required in nanotechnology for extremely weak signals. Measurement at elevated temperature (300?°C) showed that the vacuum sensor is much stabler and more sensitive to O(2) pressure. The principle of the device relates to the adsorbed oxygen species on the large surface area of a ZnO NWA to form a resistive depletion layer at the nanowire (NW) surface.     

Electrical Characteristics of Hybrid Nanoparticle–Nanowire Devices

Gold nanoparticles synthesized by a colloidal method were deposited in an Al₂O₃ dielectric layer of an omega-gated single ZnO nanowire FET. These gold nanoparticles were utilized as localized trap sites. The adsorption of the gold nanoparticles on an Al₂O₃-coated ZnO nanowire was confirmed by high-resolution transmission electron microscopy. In this study, a hybrid nanoparticle-nanowire device was fabricated by conventional Si processing. Its electrical characteristics indicated that electrons in the conduction band of the ZnO nanowire can be transported to the localized trap sites by gold nanoparticles for gate voltages greater than 1 V, through the 10-nm-thick Al ₂O₃ tunneling oxide layer.     

Fabrication and characterization of high quality n-ZnO/p-GaN heterojunction light emission diodes

High quality single crystalline n-type ZnO film was grown on p-type GaN substrate using molecular beam epitaxy. Transmission electron microscopy reveals a sharp ZnO/GaN interface. Light-emitting diode was fabricated from this heterostructure, and a turn-on voltage of ~ 3.4 V was demonstrated. We found that the emission peak shifts from violet (430 nm) to near-ultraviolet (375 nm) when the driving current increases from 0.38 mA to 3.08 mA. This intriguing phenomenon can be understood by charged carrier's radical recombination occurring at both sides of the device, and the current enhancement of ZnO emission efficiency.     

Fabrication of ordered flower-like ZnO nanostructures by a microwave and ultrasonic combined technique and their enhanced photocatalytic activity

Ordered flower-like zinc oxide (ZnO) nanostructures were fabricated via a facile microwave and ultrasonic combined technique. The product was characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction pattern (SAED). The flower-like ZnO nanostructures were assembled by a central petal and six symmetrical petals which grew radially from the center. The flower-like ZnO sample showed an enhanced photocatalytic performance compared with the ZnO microrods for the methylene blue (MB) degradation, which could be attributed to its special structure feature. Au/ZnO and Ag/ZnO nanocomposites were also synthesized and exhibited enhanced photocatalytic efficiency after decorating noble metal nanoparticles on the surface of flower-like ZnO nanostructures.