Photoelectrical properties of layered GaS single crystals and related structures

The photoconductivity of pure and Cu-doped layered gallium monosulfide (GaS) single crystals, and of Ni-GaS(Cu)-In and GaS(Cu)-ZnO structures, is investigated. The activation energies of surface states were found as 0.10 eV, 0.40 eV and 17 meV, ∼400 meV for GaS and GaS(Cu), respectively. Cu acceptor levels are localized at 0.44 eV and 0.52 eV above the valence band of GaS. ZnO-GaS(Cu) heterojunctions show remarkable photosensitivity in the wavelength range of 250-700 nm.     

One-dimensional germanium nanostructures--formation and their electron field emission properties

Ge nanostructures were synthesized by reduction of GeO(2) in H(2) atmosphere at various temperatures. Entangled and straight Ge nanowires with oxide shells were grown at high temperatures. Ge nanowires with various numbers of nodules were obtained at low temperatures. Ge nanowires without nodules exhibited remarkable field emission properties with a turn-on field of 4.6 V μm(-1) and field enhancement factor of 1242.     

Controlled synthesis and field emission properties of ZnO nanostructures with different morphologies

By simply controlling atmosphere, rods, tetraleg-rods, and arrays of ZnO nanostructures have been fabricated respectively through pure zinc powder evaporation without catalyst at temperature of 650 - 700 degrees C. Investigations through HRTEM and XRD showed that the growth of the synthesized ZnO nanostructures was controlled by vapor-solid mechanism. Field emission measurements revealed that all of the structures, owing to their very low turn-on voltage, sufficient emission current and proper linearity of 1/V - Ln(l/V2), are likely to be potential candidates as a field emitter. The results also indicated that field emission properties are relative to morphology and size of the tips of ZnO nanostructures, and the nanorods with sharp tips possess the first-class FE property.     

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.     

Low-temperature growth of well-aligned ZnO nanorods/nanowires on flexible graphite sheet and their photoluminescence properties

We have grown large-scale well-aligned ZnO nanorods/nanowires on commercial flexible graphite sheet (FGS) at low temperature via chemical vapor deposition method. The products were characterized by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. The effects of the growth temperature and oxygen flow rate on the morphology of ZnO nanostructures have been investigated. The growth mechanism of ZnO is found to be a self-catalytic vapor–solid process assisted by the immiscibility of ZnO with graphite. The as-grown ZnO/FGS products show strong green emission and their photoluminescence properties can be tuned by changing growth condition or annealing treatment.     

Controlled growth of well-aligned hierarchical ZnO arrays by a wet chemical method

A simple synthesis route to hierarchical ZnO arrays on zinc substrate in aqueous solution containing zinc nitrate and ammonia water was reported, without the assistance of any seeds, catalysts and surfactants. The as-prepared product consisted of very thin nanosheets attached onto the top of well-aligned nanorod arrays. A possible growth mechanism was proposed. The room-temperature photoluminescence spectrum showed UV emission around 398 nm and diverse visible emission peaks, indicating there might be a large concentration of oxygen defects in the sample. This method is both controllable and reproducible.     

Effect of catalyst thickness and plasma pretreatment on the growth of carbon nanotubes and their field emission properties

We demonstrated that the diameter and the density of carbon nanotubes (CNTs) which had a close relation to electric-field-screening effect could be easily changed by the control of catalytic Ni thickness combined with NH3 plasma pretreatment. Since the diameter and the density of CNTs had a tremendous impact on the field-emission characteristics, optimized thickness of catalyst and application of plasma pretreatment greatly improved the emission efficiency of CNTs. In the field emission test using diode-type configuration, well-dispersed thinner CNTs exhibited lower turn-on voltage and higher field enhancement factor than the densely-packed CNTs. A CNT film grown using a plasma-pretreated 25 angstroms-thick Ni catalyst showed excellent field emission characteristics with a very low turn-on field of 1.1 V/microm @ 10 microA/cm2 and a high emission current density of 1.9 mA/cm2 @ 4.0 V/microm, respectively.     

Controlled growth of novel doorframe-like ITO nanostructures and their photoluminescence properties

This paper describes a thermal evaporation method that generated large-scale novel doorframe-like ITO nanostructures by regularly switching flow rate of the carried gas. Their morphology and microstructures were determined by scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy and photoluminescence spectroscopy. The as-synthesized doorframe-like nanostructures are single crystal with diameters ranging from hundreds of nm to about 1 μm. The growth direction of the doorframe-like nanostructures are < 100> and < 011> and the growth process follows a self-catalytic vapor-liquid-solid mechanism. The PL spectrum of the doorframe-like nanostructures shows two emission bands around 418 and 505 nm, which is probably resulted from oxygen vacancies, oxygen-indium vacancy pairs and impurity level, respectively.     

Ultrahigh-current field emission from sandwich-grown well-aligned uniform multi-walled carbon nanotube arrays with high adherence strength

We describe a new method to grow multi-walled carbon nanotube (MWCNT) arrays, which enable very high and stable macroscopic emission current density of 3.55?A?cm(-2) along with a scalable total emission current of more than 710?mA. A sandwich-growth technology was employed to synthesize vertically well-aligned MWCNT arrays in large areas and patterned uniformly by using microwave plasma chemical vapour deposition. A thick nickel layer was inserted between the silicon substrate and catalyst layer to achieve good adhesion between the MWCNTs and the substrate. Scanning electron microscope and transmission electron microscope investigations showed that well-structured, vertically aligned and uniform MWCNTs with perfect crystal lattices had been grown on lithographically predetermined sites. The root ends of MWCNTs adhered firmly to the nickel layer, establishing high electrical and thermal conductance of the MWCNTs to the substrate. This feature largely explains the large and stable emission current density of the MWCNT arrays.     

C基薄膜电子场发射的一般特性及发射模型

介绍了几种碳基材料的场发射特性及其发射模型.金刚石表面具有较低的或负的电子亲和势,因无法实现N型掺杂,难以用作电子发射材料.类金刚石膜及非晶碳膜材料经过"激活"后在表面形成具有较大场增强因子的熔坑,在几～几十V/μm的低阈值电场下得到非本征的电子发射,纳米结构的碳和碳纳米管本身具有较大的场增强因子,是较有前途的平面阴极场发射材料.碳基材料的导电性不同,遵循的发射模型不同.     

Controlled synthesis of AlN/GaN multiple quantum well nanowire structures and their optical properties

We report the controlled synthesis of AlN/GaN multi-quantum well (MQW) radial nanowire heterostructures by metal-organic chemical vapor deposition. The structure consists of a single-crystal GaN nanowire core and an epitaxially grown (AlN/GaN)(m) (m = 3, 13) MQW shell. Optical excitation of individual MQW nanowires yielded strong, blue-shifted photoluminescence in the range 340-360 nm, with respect to the GaN near band-edge emission at 368.8 nm. Cathodoluminescence analysis on the cross-sectional MQW nanowire samples showed that the blue-shifted ultraviolet luminescence originated from the GaN quantum wells, while the defect-associated yellow luminescence was emitted from the GaN core. Computational simulation provided a quantitative analysis of the mini-band energies in the AlN/GaN superlattices and suggested the observed blue-shifted emission corresponds to the interband transitions between the second subbands of GaN, as a result of quantum confinement and strain effect in these AlN/GaN MQW nanowire structures.     

Synthesis of carbon-nitrogen nanostructures by hot isostatic pressure apparatus and their field emission properties

Carbon-nitrogen (CN) nanofibers were synthesized in argon-nitrogen gas mixture at 75 MPa by high isostatic pressure (HIP) apparatus using a graphite resistive heater. The CN nanofibers were grown in random with the diameter of about 200 nm and the length over 5 microm. The structures obtained can be divided bamboo-like, spring-like, and bead necklace-like CN nanofibers. The nitrogen content of up to 8.4% was found in CN nanofibers by EELS analysis. Field emission results showed that the density of field emitters and the field enhancement factors changed by surface treatments and that CN nanofibers contained glass frit. The screen-printed CN nanofiber had a turn-on field of 2 V/microm.     

Controlled growth of quasibicrystal zinc oxide structures

The first results on the controlled growth of quasibicrystal structures containing interblock boundaries in epitaxial zinc oxide layers on sapphire (α-Al₂O₃) are reported. The structures with boundaries oriented in a preset direction can be used as a base for submicron microelectronic devices. Using the method of buffer layers, it is possible to obtain highly oriented layers of (11[`2]0)ZnO(11\bar 20)ZnO and (0001)ZnO with clear boundaries between blocks on the same (10[`1]2)Al₂ O₃(10\bar 12)Al_2 O_3 substrate surface. Data on the features of structure and morphology of these layers are presented.     

Electronic structures of well-aligned Er-doped ZnO nanorod arrays

Electronic structures of well-aligned Er-doped ZnO (ZnO:Er) nanorod arrays (NRAs) synthesized by a solution-based hydrothermal process were characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray absorption fine structure (XAFS). HRTEM and angular dependent X-ray absorption near-edge structure analysis at O K and Zn L3 edges indicates that the spontaneous polarization is along the [0001] direction. The analysis of Er L3-edge XAFS demonstrates that the local structure around Er in the ZnO:Er NRAs was transformed from O(h) to C(4v), after annealing.     

Fabrication of patterned boron carbide nanowires and their electrical, field emission, and flexibility properties

Large-area patterned boron carbide nanowires (B₄C NWs) have been synthesized using chemical vapor deposition (CVD). The average diameter of B₄C NWs is about 50 nm, with a mean length of 20 ??m. The B₄C NWs have a single-crystal structure and conductivities around 5.1 × 10^?2 ??^?1·cm^?1. Field emission measurements of patterned B₄C NWs films show that their turn-on electric field is 2.7 V/??m, lower than that of continuous B₄C NWs films. A single nanowire also exhibits excellent flexibility under high-strain bending cycles without deformation or failure. All together, this suggests that B₄C NWs are a promising candidate for flexible cold cathode materials.      

Synthesis of straight multi-walled carbon nanotubes by arc discharge in air and their field emission properties

The possibility of preparing straight multi-walled carbon nanotubes (MWCNTs) on a large scale is demonstrated using direct current arc discharge with a rotating graphite anode in low pressure air. The process is time-saving, economical, and non-hazardous. It is found that the optimum air pressure for the highest yield of MWCNTs is about 60 Torr. Investigation of the internal organization of the cathode deposit reveals that many columns about 40 μm in diameter are closely packed and mechanically stable. The highest content of MWCNTs is found in the intercolumnar spaces between columns. Emitters made of the cathode deposits that contain a large number of straight nanotubes exhibit outstanding field emission properties. The turn-on electric field decreases from 1.44 to 0.93 V/μm and the field enhancement factor β increases from about 3,190 to 7,830 only after simple burning at 750 °C for 30 min in air. The results indicate that MWCNTs prepared by arc discharge in air are promising for field emission application.     

Synthesis of large-area single-walled carbon nanotube films on glass substrate and their field electron emission properties

Large-area and homogeneous single-walled carbon nanotube (SWCNT) films have been deposited via arc discharge directly on glass substrate coated with a layer of indium tin oxide film. The characterization, by means of electron microscopy and Raman spectroscopy, shows that the as-grown films are uniformly woven and consist of SWCNT with diameters ranging from 0.82 to 1.15 nm. As a cathode material, the field emission test indicates the films have low turn-on field of ∼1.2 V/μm at 10 μA/cm² emission current, and high emission intensity causing luminance of about 7000 cd/cm² with fine uniformity. The best performing sample exhibits a constant degradation of less than 3% per hour at an emission current of around 1 mA. Measuring with the high voltage (2000 V) on the films for 2.0 h increased the field enhancement factor from 4500 to 5400 at the high field region. The results are of significance to the development of field emission display using nanoemitters.     

Selective growth of well-aligned carbon nanotubes by APCVD

Well-aligned good-quality carbon nanotube (CNT) array was grown on silicon substrate by atmospheric pressure chemical vapor deposition (APCVD) through SiO₂ masking. First, the patterned substrate was pretreated with NH₃ and then CNTs were synthesized at 800 °C using Ni as the catalyst, acetylene (C₂H₂) as the carbon source material and N₂ as the carrier gas. Effects of the NH₃-pretreatment time, the flow ratio of [C₂H₂]/[NH₃] and the CNT growth time on the qualities of CNT array were analyzed in detail. It was found that good-quality CNTs with an average length of around 15 μm could be grown by pretreating the Si substrate with NH₃ for 10 min and then conducting the CNT growth with a flow ratio of [C₂H₂]/[NH₃] = 30/100. Furthermore, the field emission property of CNT array was investigated using a diode structure. It was found that the turn-on electric field decreased with increasing CNT length. The turn-on electric field as low as about 2 V/μm with an emission current density of 10 μA/cm² was achieved for a CNT-array diode with the tube length near 18 μm. For the same device, the emission current density could be elevated to 10 mA/cm² with the applied voltage of 3.26 V/μm.     

金刚石微米聚晶结构的生长及其场发射特性研究

在覆盖金属钛层的陶瓷上,利用微波等离子体化学气相沉积(MPCVD)法制备出类球状微米金刚石聚晶薄膜.利用扫描电子显微镜、拉曼光谱、X射线衍射,分析了薄膜的结构和表面形貌.测试了类球状微米金刚石聚晶膜的场致电子发射特性.开启电场仅为0.55V/μm,在2.18V/μm的电场下,其场发射电、流密度高达11mA/cm2.对类球状微米金刚石聚晶阵列形成机理和场发射机理进行了研究.