Structures and field emission properties of silicon nanowire arrays implanted with energetic carbon ion beam

Structures and field emission properties of silicon nanowire arrays (SiNWAs), which were fabricated by using of electroless-chemical etching method and post-implanted by the energetic carbon ion beam with an average energy of 20 keV at various doses, have been investigated. Structural analysis of SEM and XPS shows that SiC compound had been formed at the top of SiNWAs, and Si-C/Si composite nanostructure had been obtained. Compared to as-grown SiNWAs, the C ion implanted SiNWAs have better field emission characteristics. The turn-on field and the applied field at 100 microA/cm2 are reduced from 5.01 V/microm and 5.93 V/microm for as-grown SiNWAs to 4.45 V/microm and 5.40 V/microm for SiNWAs implanted at the dose of 1 x 10(16) cm(-2), respectively. However, large implanting amounts made serious structural damages at the top of nanowires, and impaired the field emission characteristics. The influence of energetic C ion implantation on the structures and field emission properties of SiNWAs has been discussed.     

Direct observation of field emission in a single TaSi2 nanowire

The electric potential change in a single TaSi2 nanowire during field emission was visualized by means of electron holography. During the field emission, the interference fringes of the electron hologram were blurred locally between the TaSi2 nanowire and anode. This phenomenon was interpreted as being due to a change in the electric potential of approximately 1 V in the TaSi2 nanowire after each ballistic emission. The experiments on the single TaSi2 nanowire field emission behavior provide the useful information for understanding the field emission in the nano-field-emitting device.     

Growth, photoluminescence, and field emission of hierarchical ZnO nanostructures

Hierarchical ZnO nanostructures with the morphology of sleeve-fishes have been fabricated on Si substrate through vapor phase transport at 850 degrees C. Studies find that each nansleeve-fish is composed of a screw-shaped microrod and some tapering nanowires grown on the microrod, all of which grow along the [0001] direction. Photoluminescence spectra exhibits strong UV emission around 385 nm without any green emissions detected, indicating that the high-quality ZnO nanostructures with low level of oxygen vacancies are obtained in our experiment. Field-emission measurements on the nanosleeve-fishes show a turn-on field as low as 2.2 V/microm at a current density of 0.1 microA/cm(2) with a anode-sample separation of 600 microm, and the emission current density reached 1 mA/cm(2) at an applied field of about 5.8 V/microm. The excellent field emission properties from such kind of nanostructures make them promising candidates for further applications in FE microelectronic devices.     

Enhanced electron field emission from carbon nanotubes irradiated by energetic C ions

The field emission performance and structure of the vertically aligned multi-walled carbon nanotube arrays irradiated by energetic C ion with average energy of 40 keV have been investigated. During energetic C ion irradiation, the curves of emission current density versus the applied field of samples shift firstly to low applied fields when the irradiation doses are less than 9.6 x 10(16) cm(-2), and further increase of dose makes the curves reversing to a high applied field, which shows that high dose irradiation in carbon nanotube arrays makes their field emission performance worse. After energetic ion irradiation with a dose of 9.6 x 1016 cm(-2), the turn-on electric field and the threshold electric field of samples decreased from 0.80 and 1.13 V/microm to 0.67 and 0.98 V/microm respectively. Structural analysis of scanning electron microscopy, transmission electron microscopy and Raman spectroscopy indicates that the amorphous carbon nanowire/carbon nanotube hetero nano-structures have been fabricated in the C ion irradiated carbon nanotubes. The enhancement of electron field emission is due to the formation of amorphous carbon nanowires at the tip of carbon nanotube arrays, which is an electron emitting material with low work function.     

Enhanced field emission from H2 plasma-treated alpha-Fe2O3 nanowires

Hematite (alpha-Fe2O3) nanowires were synthesized by the thermal oxidation of Fe-Ni alloy grids at 900 degrees C. The effects of hydrogen (H2) plasmas on the morphology and field emission properties of hematite (alpha-Fe2O3) nanowires were investigated. Many nanocrystallites with sharp tips were found to be produced on the surface of the originally smooth nanowires after H2 plasmas treatment. Field emission measurements demonstrated that the treated alpha-Fe2O3 nanowires possessed much better performance with turn-on field of approximately 3.7 V/microm at 0.1 microA/cm2 of current density, compared with the as-grown samples.     

Structures and field emission characteristics of ion irradiated silicon nanowire arrays

Silicon nanowire (SiNW) arrays irradiated by energetic Si ions were fabricated by metal vapor vacuum arc (MEVVA) ion implantation method. Hetero-structure of amorphous/crystalline nanowire was formed in which structure of the implanted region on the top of the nanowires was amorphous while the structure of unimplanted region on the bottom remained crystal. Field emission (FE) properties of the SiNW arrays could be improved and modulated by different implantation doses. A low turn-on field of 4.63 V/microm was observed in the SiNWs irradiated by 21 keV Si ion with a dose of 7.86 x 10(16)/cm2, and the applied field for the emission current density reaching 100 microA/cm2 is only 5.52 V/microm. The main reason for the efficient emission is attributed to the formation of amorphous SiNWs and structure defects after implantation. The ion irradiated SiNWs after post-annealing at high temperature had better FE property due to eliminating the restrain effect to electrons.     

Photocatalytic degradation of rhodamine B by anchored TiO2 nanowires

Rutile TiO2 nanowires anchored on silica were fabricated by annealing TiO2 nanoparticles dispersed on silicon or quartz substrate by means of a polystyrene nanosphere monolayer template at 1000 degrees C for 1 h without any catalyst. The diameter and length of the nanowires were 30-80 nm and 1-3 microm, respectively. The growth direction of the nanowires is [112]. The photocatalytic activities of TiO2 nanoparticles and anchored nanowires were evaluated. TiO2 nanowires had higher photocatalytic activity for rhodamine B than TiO2 nanoparticles.     

Dielectrophoretic and electrothermal effects at alternating current heated disk microelectrodes

When a disk microelectrode is polarized with an alternating potential of very high frequency (0.1-2 GHz) and a high amplitude (up to 2.8 V rms), the electrode is heated up, and at the same time, a very intense electric field is created around the electrode (>10(6) V/m for electrodes 1 microm in radius). This strong electric field gives rise to positive or negative dielectrophoretic effects. Positive dielectrophoretic effects can be used to assemble nanowires from nanoparticles at the electrode edge. On the other hand, a negative dielectrophoretic effect is probably responsible for "jet boiling" observed at overheated microelectrodes. In addition, a combination of a high temperature gradient and a high potential gradient generates an intense electrothermal flow of solution which very strongly enhances the mass transport and is responsible for intense convection in such systems. The electrothermal flow and dielectrophoretic forces can be generated directly on a microelectrode employed in electrochemical detection because the high frequency ac polarization of the electrode does not interfere with the acquisition of analytical signals.     

Tin-doped rutile titanium dioxide nanowires: luminescence, gas sensor, and field emission properties

Sn-doped rutile TiO2 nanowires were synthesized by a thermal reactive evaporation route. Field emission scanning electron microscopy (FESEM) imaging reveals that the Sn-doped TiO2 nanowires exhibited diameters of 80-150 nm and 2-3 microns in length. High-resolution transmission electron microscopy (HRTEM) imaging makes it possible to observe that Sn-doped TiO2 nanowires show a certain lattices fringe of approximately 0.32 nm, which demonstrates that the nanowires are single crystalline with rutile structure and grow along the [110] axis. Cathodoluminescence (CL) reflected that on the surface of Sn-doped TiO2 nanowires, many oxygen vacancies and defect states were formed during the crystal growth. These defect states raised a broad emission peak around the red-orange band. The ethanol sensing properties of Sn-doped rutile TiO2 nanowires at a temperature of 190 degrees C for the ethanol concentrations of 50, 100, 150, 200, 400, 500, and 600 ppm, correspond to the sensor' sensitivity of 7, 12, 18, 19, 23, and 26%, respectively. The sensitivity increased with an increase in the ethanol concentration. As-synthesized TiO2 nanowires revealed a turn-on field, approximately 5.1 V/microm, at a current density of 1 microAcm(-2).     

Field emission from few-layer graphene nanosheets produced by liquid phase exfoliation of graphite

Graphene nanosheets have been synthesized from commercial expandable graphite by heating in a microwave oven and dispersing in ethanol by ultrasonication. Scanning and transmission electron microscopy and electron energy-loss spectroscopy and atomic force microscope showed that the nanosheets were about 2 nm in thickness and 10 microm in diameter. The field emission of the graphene sheets has been investigated. An emission current density of 1 mA/cm2 has been achieved at an electric field of 3.7 V/microm with a turn-on field of 1.7 V/microm at 0.01 mA/cm2. The annealing of the samples at 400 degrees C in vacuum greatly improved the field emission performance.     

ZnO nanowires hydrothermally grown on PET polymer substrates and their characteristics

Zinc oxide nanowires (ZnO NWs) were successfully synthesized on the ITO/PET polymer substrates by a hydrothermal method. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy investigations were carried out to characterize the crystallinity, surface morphologies, and orientations of these NWs, respectively. The influence of NW surface morphologies on the optical and electrical properties of ZnO NWs was studied. The hydrothermally grown ZnO NWs with direct band gap of 3.21 eV emitted ultraviolet photoluminescence of 406 nm at room temperature. Field emission measurements revealed that the threshold electric fields (Eth, current density of 1 mA/cm2) of ZnO NWs/ITO/PET and ZnO NWs/ZnO/ITO/PET are 1.6 and 2.2 V/microm with the enhancement factors, beta values, of 3275 and 4502, respectively. Furthermore, the field emission performance of ZnO NWs deposited on the ITO/PET substrate can be enhanced by illumination with Eth of 1.3 V/microm and displays a maximum emission current density of 18 mA/cm2. The ZnO NWs successfully grown on polymer substrate with high transmittance, low threshold electric field, and high emission current density may be applied to a flexible field emission display in the future.     

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.     

Flexible cold cathode with ultralow threshold field designed through wet chemical route

A flexible cold cathode based on a uniform array of ZnO nanowires over carbon fabrics was designed via a simple wet chemical route. The structural parameters of the nanowires (i.e. length, diameter) as well as their arrangement over the carbon fibers were tailored by adjusting nutrient solution composition and growth duration. The optimized arrays of ZnO nanowires exhibit excellent electron emission performance with ultralow turn-on as well as threshold fields of 0.27 and 0.56 V μm(-1). This threshold field value is the lowest compared to any of the previous zinc-oxide-based cold cathodes realized through either chemical or vapor phase processes. In addition, the current density can reach an exceptionally high value of ～ 11 mA cm(-2) at an applied electric field of only 0.8 V μm(-1). Flexible electronic devices based on a field emitter cold cathode may thus be realized through chemical processing at low budget but having high efficiency.     

Direction-tunable nanotwins in copper nanowires by laser-assisted electrochemical deposition

Nanotwins can improve mechanical strength and maintain high electrical conductivity in metallic nanowires. We demonstrated a method of pulsed-laser-assisted electrodeposition, which could form dense nanotwins with tunable directions in copper nanowires of uniform sizes. Transmission electron microscopy characterization showed with a growth potential of -0:2 V, nanotwins tend to align along the longitudinal direction of the nanowires, whereas at a larger potential of -0:8 V, nanotwins of {111}g/<112> type perpendicular to the longitudinal direction of the wire were formed. The two types of nanotwins were investigated by comparing the microstructures under different electrochemical conditions and laser irradiation energies. Two different mechanisms are proposed—annealing twins and growth twins.     

Field-emission characteristics of Al-doped ZnO nanostructures hydrothermally synthesized at low temperature

The aluminum-doped ZnO (AZO) nanostructures with different Al concentrations were synthesized on AZO/glass substrate via a simple hydrothermal growth method at a temperature as low as 85 degrees C. The morphologies, crystallinity, optical emission properties, and chemical bonding states of AZO nanostructures show evident dependence on the aluminum dosage. The morphologies of AZO nanostructures were changed from vertically aligned nanowires (NWs), and NWs coexisted with nanosheets (NSs), to complete NSs in respect of the Al-dosages of 0-3 at.%, 5 at.%, and 7 at.%, correspondingly. The undoped ZnO and lightly Al-doped AZO (< or = 3 at.%) NWs are single-crystalline wurtzite structure. In contrast, heavily Al-doped AZO sample is polycrystalline. The AZO nanostructure with 3 at.% Al-dosages reveals the optimal crystallinity and less structural defects, reflecting the longest carrier lifetime and highest conductivity. Consequently, the field-emission characteristics of such an AZO emitter can exhibit the higher current density, larger field-enhancement factor (beta) of 3131, lower turn-on field of 2.17 V/microm, and lower threshold field of 3.43 V/microm.     

Crabwise ZnO nanowires: growth and field emission properties

Crabwise ZnO nanowires with an average length of 5 microm and an average diameter of 30 nm were selectively grown on ZnO:Ga/glass templates. Cathodoluminescence measurement indicated that the crystal quality of the crabwise ZnO nanowires was good. With an applied voltage of 120 V, the crabwise ZnO nanowire field emitters gave an emission current of 0.1 mA/cm2. Moreover, the field enhancement factor, beta, of the crabwise ZnO nanowires was approximately 980.     

Growth of high quality ZnO nanowires on graphene

We report growth of the ZnO nanowires on graphene/SiO2/Si substrates using a chemical vapor deposition method. The length of nanowires varies from 1 microm to 10 microm with increasing the growth time from 30 min to 90 min. X-ray diffraction and high-resolution transmission electron microscopy investigations predict the high structural quality of the c-axis grown single crystalline ZnO nanowires. Temperature dependent photoluminescence spectra from the nanowires reveal excellent optical quality and excitonic behavior in the single crystalline ZnO nanowires. A well-resolved free exciton emission at 3.375 eV, indicates high crystalline quality nanowires and a strong PL peak at 3.370 eV is assigned to neutral-donor bound excitons (D0X).     

Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting

We report the first demonstration of hydrogen treatment as a simple and effective strategy to fundamentally improve the performance of TiO(2) nanowires for photoelectrochemical (PEC) water splitting. Hydrogen-treated rutile TiO(2) (H:TiO(2)) nanowires were prepared by annealing the pristine TiO(2) nanowires in hydrogen atmosphere at various temperatures in a range of 200-550 °C. In comparison to pristine TiO(2) nanowires, H:TiO(2) samples show substantially enhanced photocurrent in the entire potential window. More importantly, H:TiO(2) samples have exceptionally low photocurrent saturation potentials of -0.6 V vs Ag/AgCl (0.4 V vs RHE), indicating very efficient charge separation and transportation. The optimized H:TiO(2) nanowire sample yields a photocurrent density of ～1.97 mA/cm(2) at -0.6 V vs Ag/AgCl, in 1 M NaOH solution under the illumination of simulated solar light (100 mW/cm(2) from 150 W xenon lamp coupled with an AM 1.5G filter). This photocurrent density corresponds to a solar-to-hydrogen (STH) efficiency of ～1.63%. After eliminating the discrepancy between the irradiance of the xenon lamp and solar light, by integrating the incident-photon-to-current-conversion efficiency (IPCE) spectrum of the H:TiO(2) nanowire sample with a standard AM 1.5G solar spectrum, the STH efficiency is calculated to be ～1.1%, which is the best value for a TiO(2) photoanode. IPCE analyses confirm the photocurrent enhancement is mainly due to the improved photoactivity of TiO(2) in the UV region. Hydrogen treatment increases the donor density of TiO(2) nanowires by 3 orders of magnitudes, via creating a high density of oxygen vacancies that serve as electron donors. Similar enhancements in photocurrent were also observed in anatase H:TiO(2) nanotubes. The capability of making highly photoactive H:TiO(2) nanowires and nanotubes opens up new opportunities in various areas, including PEC water splitting, dye-sensitized solar cells, and photocatalysis.     

Field emission properties of AIN nanostructures

AIN with different morphologies, including thin film, nanowires, nanoneedles and nanochilies, depending on the controlled growth parameters, have been successfully synthesized by chemical vapor deposition technique. Field emission properties have been systematically studied. The experimental results show that all AIN nanostructures have good field emission properties. In comparison, AIN nano-chilies possess the best field emission properties with a low turn-on and threshold fields of 1.8 V/microm and 3.1 V/microm, respectively. The results suggest that the morphological modulation is an effective way to optimize field emission performance of nanostructures.     

Ferromagnetism driven by oxygen vacancies in SnO2 nanowires

Room temperature ferromagnetism has been observed in SnO2 nanowires synthesized by a chemical vapor deposition using Au layers as catalyst. The nanowires are homogeneous and single-crystalline grown along the [101] direction, with diameters ranging from 25 to 100 nm and length greater than 20 microm. The special magnetization reaches 0.114 emu/g for the nanowires with diameter of approximately 25 nm and reduces with increasing diameters. Branched SnO2 nanowires were prepared via a two-step vapor-liquid-solid approach, and an enhanced magnetization was obtained. To the contrary, the nanowires annealed at 1300 degrees C in air were completely transformed into the particles and exhibit weakened magnetization. These results demonstrate that the ferromagnetic properties of the samples depend on the surface-to-volume ratio of nanowires. With a combined study of photoluminescence, our results reveal that the oxygen vacancies at the surface of nanowires contribute to the ferromagnetism of SnO2 nanowires. This argument is further confirmed by a sequential annealing in a rich-oxygen atmosphere, then in a low vacuum condition.