High Emission Current Density Research Articles

Low temperature growth of carbon nanotubes in a magnetic field

We report on the growth of carbon nanotubes on a glass substrate at a low temperature of 450 °C by plasma-enhanced chemical vapor deposition in the presence of a magnetic field. The growth of carbon nanotubes can be realized at 450 °C only when a magnetic field is applied to the substrate. Carbon nanotubes cannot be grown in the absence of a magnetic field at the same temperature. An NH 3 plasma pretreatment significantly improved the uniformity of the grain size of the Ni catalyst under the magnetic field. The enhancement in the growth of CNTs at low temperature can be attributed to the magnetic moment pre-alignment of the ferromagnetic catalyst film under high magnetic field. A high emission current density of 20 mA/cm 2 was obtained at 6 V/μm and a stable emission current was observed. This method permits the growth of carbon nanotubes directly on glass substrate at much more reliable low temperatures for the fabrication of high-density field emitter arrays.

Improve the field emission uniformity of carbon nanotubes treated by ball-milling process

Carbon nanotubes (CNTs) deposited by chemical vapor deposition (CVD) were treated by ball milling. The morphologies and field emission properties of the treated CNTs depending on milling time were studied. The emission turn-on field is increased, and the field emission current density is reduced, when the milling time increased from 0.5 to 3 h. The as-deposited long CNTs were cut to short CNTs (∼1 h) and micro-particles (>1 h) with increasing of the milling time. It is found that the optimized milling time is 0.5–1 h, the treated CNTs showed excellent field emission properties, such as low turn-on field, high emission current density and uniform luminescence spots distribution.

Effect of length, spacing and morphology of vertically alignedRuO2 nanostructures on field-emission properties

The length, spacing and morphology of vertical aligned nanostructuralRuO2 one-dimensional (1D) arrays are varied independently to investigate their effecton the field-emission properties. It is shown that, in general, the lowest turn-onfield, highest emission current density and apparent field enhancement factor canbe achieved in nanocrystals (NCs) with the smallest emitter radius for the tipsand the highest aspect ratio. Moreover, the lower-density NCs exhibit betterfield-emission characteristics, which are related to the screening effect. The resultscould be valuable for the application of field-emission-based devices usingRuO2 NCs arrays as cathode materials.

Synthesis and field emission of four kinds of ZnO nanostructures: nanosleeve-fishes, radialnanowire arrays, nanocombs and nanoflowers

Four different hierarchical ZnO nanostructures, including nanosleeve-fishes, radial nanowirearrays, nanocombs and nanoflowers, have been successfully fabricated throughthermal chemical vapour deposition by adjusting the source temperature andthe gas flow rate. Field emission measurements of these nanostructures showeda high emission current density and a low turn-on field of 1.3, 1.9, 2.5 and3.4 V μm−1 in sequence, which is comparable to that of one-dimensional ZnO nanostructures andcarbon nanotubes. The good performance for field emission makes the hierarchical ZnOnanostructures promising candidates for further applications in field emissionmicroelectronic devices.

Field emission from lotiform-like ZnO nanostructures synthesized via thermal evaporation method

Novel lotiform ZnO nanostructures were synthesized on silicon substrate via simple thermal evaporation. The average diameter of the ZnO nanostructures is ∼1.5 μm. The lotiform-like ZnO structures were formed by nanorods arrays with the average diameter of 70 nm. The as-grown lotiform ZnO nanostructures have excellent field-emission properties such as the low turn-on field of 3.4 V/μm, and very high emission current density of 12.4 mA/cm2 at the field of 9.6 V/μm. These features make the lotiform-like ZnO nanostructures competitive candidates for field-emission-based displays.

Research on the preparation of amorphous diamond nanorod arrays and their excellent field emitting properties

Amorphous diamond nanorod arrays with excellent field emitting have been fabricated firstly on the AAO template by the filtered cathodic arc plasma technique. Microscopic analysis has displayed that the nanorods are very uniformly distributed, and the density is very high up to ∼109 cm−2. The nanorod arrays are found to have an extremely low turn-on field of 0.16 V/μm, which is lower than other reported materials, and a high-emission current density of 180 mA/cm2 under an applied field of 2 V/μm can also be obtained. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and filed emitting tester are employed to characterize the nanorod arrays. The field emission mechanism of the nanorod arrays is also discussed.

Field emission of carbon nanotubes grown on nickel substrate

Carbon nanotubes (CNTs) have been synthesized directly on the electrically conducting nickel substrate without additional catalyst. Field emission properties of the as-prepared sample were characterized using parallel plate diode configurations. It was observed that the field emission qualitatively follows the conventional Fowler–Nordheim (F–N) theory from the straight line of ln( I/ V 2) versus 1/ V plot at the high applied field region. The uniformity and stability of the electron emission have also been examined. The low electron turn-on field ( E to) and high emission current density indicates the potential applications of this new CNT-based emitter.

Non-catalytic and template-free growth of aligned CdS nanowires exhibiting high field emission current densities

Various CdS nanostructures, including nanoparticle film, bundles of quasi-aligned and well-aligned nanowires, were fabricated with a non-catalytic and template-free MOCVD process. The well-aligned CdS nanowires exhibit unusually high field emission current densities of 225 mA cm(-2) at the applied electric field of 20 V microm(-1).

High emission current density microwave-plasma-grown carbon nanotube arrays by postdepositional radio-frequency oxygen plasma treatment

Highly stable field emission current densities of more than 6A∕cm2 along with scalable total field emission currents of ∼300μA per 70μm diameter carbon nanotube (CNT)-covered electron emitter dot are reported. Microwave-plasma chemical vapor deposition, along with a novel catalyst sandwich structure and postdepositional radio-frequency (rf) oxygen plasma treatment lead to well-structured vertically aligned CNTs with excellent and scalable emission properties. Scanning electron and transmission electron microscope investigations reveal that postdepositional treatment reduces not only the number but modifies the structure of the CNTs. Well-structured microwave-plasma-grown nanotubes become amorphous during rf oxygen plasma treatment and the measured work functions of CNTs change from 4.6eVto4.0eV before and after treatment, respectively. Our experiments outline a novel fabrication route for structured CNT arrays with improved and scalable field emission characteristics.

Optical and Field Emission Properties of Zinc Oxide Nanostructures

Zinc Oxide (ZnO) nano-pikes were produced by oxidative evaporation and condensation of Zn powders. The crystalline structure and optical properties of the ZnO nanostructures (ZnONs) greatly depend on the deposition position of the ZnONs. TEM and XRD indicated that the ZnONs close to the reactor center, ZnON-A, has better crystalline structure than the ZnONs away from the center, ZnON-B. ZnON-A showed the PL and Raman spectra characteristic of perfect ZnO crystals, whereas ZnON-B produced very strong green emission band at 500 nm in the photoluminescence (PL) spectrum and very strong Raman scattering peak at 560 cm(-1), both related to the oxygen deficiency due to insufficient oxidation of zinc vapor. ZnON-B exhibited better field emission properties with higher emission current density and lower turn-on field than ZnON-A.

Scandate cathode for TWT

Because of its low work function, a scandate cathode can have a high emission current density at a low operating temperature. However, this cathode has got limited application because of its lack of emission uniformity and poor stability in operation. By modifying both the fabrication of the tungsten sponge and the cathode structure, the emission uniformity and the emission current density of these cathodes have been improved at well-chosen operating temperatures. If the variation of the heater power is kept within ±5%, then the corresponding change of cathode temperature will be less than 25–30 °C, and the fluctuation in the emission current will be less than ±2% in the application of traveling wave tubes (TWTs) in our company.

Efficient field emission from α-Fe2O3 nanoflakes on an atomic force microscope tip

Aligned arrays of flake-shaped hematite (α-Fe2O3) nanostructure have been fabricated on an atomic force microscope (AFM) tip. They are created by simply heating an iron-coated AFM tip in ambience on a hot plate. These nanoflakes are characterized as α-Fe2O3 single crystalline structures with tip radii as small as several nanometers and are highly effective as electron field emitters. With a vacuum gap of about 150μm, field emission measurements of α-Fe2O3 nanoflakes on AFM tips show a low turn-on voltage of about 400–600V and a high current density of 1.6Acm−2 under 900V. Such high emission current density is attributed to the nanoscale sharp tips of the as-grown nanoflakes. Based on the Fowler–Nordheim theory, it is demonstrated the enhancement factor of α-Fe2O3 nanoflakes on AFM tips is comparable to that of carbon nanotubes. Our findings suggest that α-Fe2O3 nanoflakes are potentially useful as candidates for future electron field emission devices.

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.

Increased field-emission site density from regrown carbon nanotube films

Electron field-emission properties of as-grown, etched, and regrown carbon nanotube thin films were investigated. The aligned carbon nanotube films were deposited by the microwave plasma-assisted chemical vapor deposition technique. The surface of the as-grown film contained a carbon nanotube mat of amorphous carbon and entangled nanotubes with some tubes protruding from the surface. Hydrogen plasma etching resulted in the removal of the surface layer, and regrowth on the etched surface displayed the formation of a new carbon nanotube mat. The emission site density and the current–voltage dependence of the field emission from all of the samples were analyzed. The results showed that the as-grown sample had a few strong emission spots and a relatively high emission current density (∼20μA∕cm2 at 1V∕μm), while the regrown sample exhibited a significantly increased emission site density.

An improved reservoir oxide cathode

A new type of reservoir oxide cathode has been developed in IECAS. The emission characteristics of the cathode are tested. The results show the new cathode has higher emission current density and better resistance to poisoning at same operating condition compared with those of conventional reservoir oxide cathode.

Development of new types of oxide cathodes

High reliability, long lifetime and low temperature, high emission current density oxide cathodes are presented in this paper. The characteristics of these cathodes, such as dc and pulse emission, evaporation, resistance to poisoning and lifetime, are discussed. They have been widely applied to TWTs and klystrons with high power, long pulse and high duty ratio. The results showed that high emission current density and long lifetime can be realized.

Polycrystalline GaN for light emitter and field electron emitter applications

We have grown polycrystalline GaN on quartz, refractory metal (W, Mo, Ta and Nb) and Si substrates by using plasma-assisted molecular beam epitaxy. It has been found that polycrystalline GaN grown on quartz and refractory metal substrates shows a strong band-edge emission without yellow emission. GaN growth on Si with native oxide produces well c-orientated nanorods exhibiting a low field emission threshold and high emission current density. We will review the growth of polycrystalline GaN films and the evaluation of their optical properties and electron field emission characteristics.

Efficient field emission from tetrapod-like zinc oxide nanoneedles

Tetrapod-like zinc oxide (ZnO) nanoneedles were fabricated using a simple and economical method of rapid heating high purity zinc powders at 900 °C. No catalyst and vacuum were employed in the experiment. Field-emission measurements showed that the turn-on field of the synthesized tetrapod-like ZnO nanoneedles was as low as 1.8 V/μm at the emission current density of 1.0 μA/cm 2 and the emission current density reached 1.0 mA/cm 2 under an applied field of about 3.9 V/μm. The low turn-on field, high emission current density, and good electron emission stability make the ZnO nanoneedles one of the promising candidates for field-emission displays.

Characterization of an advanced high-efficiency electron emission device

As previously reported, the emission characteristics of metal-insulator-semiconductor (MIS) electron emission devices could be significantly improved by introducing microdimple structures into the emission sites. In this paper, we describe the characteristics of efficient cold electron emitters from a viewpoint of application to flat panel display (FPD). First, the forming operation for the device to activate the dynamic emission is discussed on a basis of the experimental analyses of the electrical properties and output electron energy spectra. Next it is demonstrated that the device has some desirable features as an excitation source of FPD: a high emission current density, a low operation voltage, emission uniformity, and stable emission with small angle dispersion. It is also shown from the result of lifetime evaluation that the device shows a half-life of 3000 h, and that the deteriorated emission can be recovered by the dc reactivation treatment.

P-42: Development of CNT Cathodes for Field-Emission FPDs by Liquid-Phase Fabrication

Fabrication of CNT pattern by liquid-phase based deposition technique has been studied in Xintek for several years. By using this technique, we successfully fabricated sealed CNT field emission display prototypes with diode structures. The CNT panel size has been scaled up from 4 inch to 8 inch. Patterned CNT cathode was made by incorporating photolithography techniques into the liquid-phase deposition process. CNT pattern size of 50 um was achieved on our standard (unpolished) display substrates. A simple substrate polishing process could reduce the CNT pattern size down to 20 um. It indicates that high-resolution displays are possible by improving the surface smoothness of the substrates. The patterned CNT cathodes demonstrated good field emission characteristics including low turn-on field and high emission current density. However, the emission uniformity remains to be a major concern. The global emission uniformity of the CNT panels can be improved through more engineering efforts. We believe that the emission uniformity in pixel can be enhanced by optimization of the liquid-phase deposition process and CNTs concentration, distribution as well as morphology in the cathodes. Liquid-phase deposition technique can also deposit CNTs into preformed triode structures with the aid of photolithography techniques.