High Emission Current Density Research Articles

Field Emission Properties of the Graphene Double‐Walled Carbon Nanotube Hybrid Films Prepared by Vacuum Filtration and Screen Printing

The graphene double‐walled carbon nanotube (DWCNT) hybrid films were prepared by vacuum filtration and screen printing. Their electron field emission properties have been studied systematically. The electron emission properties of the hybrid films are much better than those of pure DWCNT films and pure graphene films. Comparing with the screen printed films, the vacuum filtered films have many advantages, such as lower turn‐on field, higher emission current density, better uniformity, better long‐term stability, and stronger adhesive strength with conductive substrates. The optimized hybrid films with 20% weight ratio of graphene, which were fabricated by vacuum filtration, show the best electron emission performances with a low turn‐on field of 0.50 Vμm−1 (at 1 μAcm−2) and a high field enhancement factor β of 27000.

Optically pumped cesium plasma neutralization of space charge in photon-enhanced thermionic energy converters

Photon-enhanced thermionic emission from semiconducting cathodes is a promising means of increasing cathode current density in low temperature vacuum thermionic energy devices [J. W. Schwede et al., Nature Mater. 9, 762 (2010)]. However, the space charge resulting from high emission current densities prevents emitted electrons from reaching the collector, even with gaps as small as 100 μm and at voltages as high as 1 V. We demonstrate by particle-in-cell simulations that one possible solution to overcoming the space charge is to add cesium filling and generation of a space charge-neutralizing plasma by continuous laser excitation of the cesium resonance level.

Electron field emission from screen-printed graphene/DWCNT composite films

The electron field emission properties of graphene/double-walled carbon nanotube (DWCNT) composite films prepared by screen printing have been systematically studied. Comparing with the pure DWCNT films and pure graphene films, a significant enhancement of electron emission performance of the composite films are observed, such as lower turn-on field, higher emission current density, higher field enhancement factor, and long-term stability. The optimized composite films with 20% weight ratio of graphene show the best electron emission performance with a low turn-on field of 0.62Vμm−1 (at 1μAcm−2) and a high field enhancement factor β of 13,000. A model of the graphene/DWCNT composite films is proposed, which indicate that a certain amount of graphene will contribute the electron transmission in the silver substrate/composite films interface and in the interior of composite films, and finally improve the electron emission performance of the graphene/DWCNT composite films.

Ultra-high current density carbon nanotube field emitter structure on three-dimensional micro-channeled copper

High-current field emitter with extended stability has been focus of many researchers, which can be fulfilled by application of engineered nanomaterials and beneficial design. In the present research, such a field emitter is designed and fabricated, involving growth of interface controlled carbon nanotubes along micro-channels of 3-dimensional copper template. The emitter offers very high emission current density (270 mA/cm2, peak), low turn-on field (1.1 V/μm), moderate threshold field (1.69 V/μm), emission stability for extended operation, and excellent resistance to structural damage. Higher conductivity of Cu, lower interfacial resistance, strong interfacial bonding, and unique 3-dimensional design leads to such 23-27 times higher current density, providing a potential approach to field emission devices.

Metal–oxide–semiconductor field-effect transistor with a vacuum channel

High-speed electronic devices rely on short carrier transport times, which are usually achieved by decreasing the channel length and/or increasing the carrier velocity. Ideally, the carriers enter into a ballistic transport regime in which they are not scattered. However, it is difficult to achieve ballistic transport in a solid-state medium because the high electric fields used to increase the carrier velocity also increase scattering. Vacuum is an ideal medium for ballistic transport, but vacuum electronic devices commonly suffer from low emission currents and high operating voltages. Here, we report the fabrication of a low-voltage field-effect transistor with a vertical vacuum channel (channel length of ~20 nm) etched into a metal-oxide-semiconductor substrate. We measure a transconductance of 20 nS µm(-1), an on/off ratio of 500 and a turn-on gate voltage of 0.5 V under ambient conditions. Coulombic repulsion in the two-dimensional electron system at the interface between the oxide and the metal or the semiconductor reduces the energy barrier to electron emission, leading to a high emission current density (~1 × 10(5) A cm(-2)) under a bias of only 1 V. The emission of two-dimensional electron systems into vacuum channels could enable a new class of low-power, high-speed transistors.

Study on electron field emission enhancement from nitrogenated carbon nanotips synthesized by plasma-enhanced hot filament chemical vapor deposition

Nitrogenated carbon nanotips (NCNTPs) with different structures were synthesized by plasma-enhanced hot filament chemical vapor deposition using methane, hydrogen and nitrogen as the reactive gases. The structures and compositions of the NCNTPs were studied by field emission scanning electron microscopy (FESEM), micro-Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The XPS spectra reveal that nitrogen is incorporated into the carbon nanotips to form the NCNTPs under plasma condition. The Raman spectra and FESEM images show that the NCNTPs are amorphous structure and their morphologies change with the change in deposition conditions, respectively. The electron field emission (EFE) from the NCNTPs was measured and the EFE results indicate that the NCNTPs with the smooth surfaces and high density can emit a current density of 3 × 103 μA/cm2 at an electric field of 7.2 V/μm, which exhibits better EFE characteristic than the NCNTPs with the carbon nanowires on their surfaces due to small amount of oxygen adsorbed on the smooth surfaces of NCNTPs. According to the possible structures of nitrogen in sp2 cluster in rings, the EFE enhancement of the NCNTPs compared with pure carbon nanotips was studied. The high emission current density (3 × 103 μA/cm2) at low field (7.2 V/μm) suggests that the NCNTPs can serve as effective electron emission sources for numerous applications.

Advanced nanodiamond emitter with pyramidal tip-on-pole structure for emission self-regulation

In this paper, we report an innovative nanodiamond field emitter structure consisting of an individual pyramidal tip sitting on top of a ballast resistor “pole.” The tip-on-pole nanodiamond structures are fabricated by a new mold transfer process that is comprised of reactive-ion-etching of 3.5 μm-thick thermal oxide on Si substrate, anisotropic etching of Si, tip sharpening by thermal oxidation and chemical vapor deposition of nanodiamond. The fabricated tip-on-pole nitrogen-incorporated nanodiamond emitter exhibits a low turn-on electric field of 3.5 V/um and a very high emission current density of ∼1.7 A/cm2 at an electric field of ∼7.5 V/um. Analysis of the emission current based on Fowler–Nordheim theory indicates a current regulated regime due to the pole-structured ballast resistor with the resistance value of ∼140 kΩ. Thus, the diamond pole ballast resistor has proven to provide self-limiting of emission current that improves the total current density as well as the emission current stability of the pyramidal nanodiamond emitters. Therefore, the proposed tip-on-pole nanodiamond emitters show great promise for high current and power applications.

Field emission characteristics of carbon nanotube films fabricated on a metal mesh by filtration

A novel-structured field emitter was fabricated by vacuum-filtrating in sequence an aqueous suspension of thin, highly crystalline multiwalled carbon nanotubes (MWCNTs) and an aqueous suspension of thick, defective MWCNTs through a metal mesh placed on a polymer membrane. We could form a mechanically strong CNT film by weaving carbon nanotubes (CNTs) through the metal mesh residing inside the film. On drying at 70°C, the polymer membrane was spontaneously separated from the CNT-mesh hybrid structure due to a large difference between their thermal expansion coefficients, producing vertically aligned bushes of thin CNTs on the CNT film. On top of the bushes, sharp CNT tips were well developed and worked excellently as field emitters. This structure was designed in such a way that the thin CNTs worked as field emitters while the thick CNTs supported the emitter CNTs. Our CNT emitters showed a high emission current density of 220mA/cm2 at 4.3V/μm, and its emission life span, measured at 40mA/cm2 in a DC bias mode, was ∼171h. We expect that the CNT emitters with a novel structure are promising for the applications to field emission sources requiring small area but high current, for example, X-ray generators, microwave amplifiers, etc.

High-current field emission of point-type carbon nanotube emitters on Ni-coated metal wires

The fabrication and field emission characteristics are reported for point-type carbon nanotube (CNT) emitters formed by transferring a CNT film onto a Ni-coated Cu wire with a diameter of 1.24mm. A Ni layer plays a role in enhancing the adhesion of CNTs to the substrate and improving their field emission characteristics. On firing at 400°C, CNTs appear to directly bonded to a Ni layer. With a Ni layer introduced, a turn-on electric field of CNT emitters decreases from 1.73 to 0.81V/μm by firing. The CNT film on the Ni-coated wire produces a high emission current density of 667mA/cm2 at quite a low electric field of 2.87V/μm. This CNT film shows no degradation of emission current over 40h for a current density of 60mA/cm2 at electric field of 6.7V/μm. X-ray imaging of a printed circuit board with fine features is demonstrated by using our point-type CNT emitters.

Enhanced electron field emission properties of high aspect ratio silicon nanowire–zinc oxide core–shell arrays

The enhanced electron field emission (EFE) properties of high aspect ratio, vertically aligned SiNW-ZnO core-shell arrays are presented. These core-shell arrays are prepared by a thin, controlled, highly crystalline and conformal coating of zinc oxide as shell using the plasma assisted-atomic layer deposition (PA-ALD) route on vertically aligned silicon nanowire arrays core. The core-shell nanostuctures are confirmed by HRTEM imaging along with the individual elemental mapping demonstrating the conformal deposition of 10 nm ZnO on the SiNWs. EFE properties of va-SiNW-ZnO core-shell arrays showed a high emission current density of 51 μA cm(-2) and a low turn on field of 7.6 V μm(-1) (defined at a current density of 1 μA cm(-2)) compared to the 3.2 μA cm(-2) emission current density and 9.1 V μm(-1) turn on field for SiNWs. The field enhancement factor (β) of 4227 for the devices demonstrates that these core-shell nanowire arrays are excellent field-emitters. Such an enhancement in the field emission originates from the details of the band structure of this peculiar material combination resulting in good electron transport from SiNW to ZnO as evident from the band diagram of the core-shell material. This is further supported by the conducting AFM studies where lowering in threshold voltage by 1 eV confirms the role of ZnO coating in the enhancement of the emission characteristics.

Single-crystal SnO2 nanoshuttles: shape-controlled synthesis, perfect flexibility and high-performance fieldemission

Vertically aligned single-crystalSnO2 nanoshuttle arrays with uniform morphology and a relatively high aspect ratio weresynthesized by a simple hot-wall chemical vapor deposition (CVD) method. It was foundthat regulating the growth temperature gradient could change the shape of theSnO2 nanostructure from nanoshuttles to nanochisels and nanoneedles, and aself-catalyzing growth process was responsible for tunable morphologies ofSnO2 nanostructures. Theas-synthesized SnO2 nanoshuttles showed ultrahigh flexibility and strong toughness with a large elastic strain of ∼ 6.2, which is much higher than reported for Si and ZnO nanowire as wellas most crystalline metallic materials. The field emitter fabricated usingSnO2 nanoshuttle arrays has a low turn-on electric field of around0.6 V µm − 1, and a high field emission current density of above10 mA cm − 2, which is comparable with the highest emission current density of carbon nanotube andnanowire field emitters.

Flexible Field Emission from Thermally Welded Chemically Doped Graphene Thin Films

Flexible field-emission devices (FEDs) based on reduced graphene oxide (RGO) emitters are fabricated by the thermal welding of RGO thin films onto a polymeric substrate. The RGO edges are vertically aligned relative to the substrate as a result of cohesive failure in the RGO layer after thermal welding. Even at large bending angles, excellent electron emission properties, such as low turn-on and threshold fields, a high emission current density, a high field enhancement factor, and long-term stability of the emission properties of RGO emitters, arise from the uniform distribution and high density of the extremely sharp RGO edges, as well as the high interfacial strength between the RGO emitters and the substrate. Al- and Au-doped RGO emitters are fabricated by introducing a dopant solution to the RGO emitters, and the resulting field-emission characteristics are discussed. The proposed approach is straightforward and enables the practical use of high-performance RGO flexible FEDs.

Synthesis of ZnO films with a special texture and enhanced field emission properties

ZnO films with special textures are fabricated on Mo-coated Al2O3 ceramic substrates by the catalyst-free electron beam evaporation method, and the as-deposited films are treated by hydrogen plasma. It is found that the surface morphologies of the films are changed significantly after hydrogen plasma treatment and that the films consist of vertically standing and intersecting nanosheets. A lower turn-on field of 1.2 V/μm and an enhanced current density ∼0.11 mA/cm2 at 2.47 V/μm are achieved. The low threshold field and the high emission current density are attributed primarily to the unique shape and smaller resistivity of the ZnO nanosheet films.

Free standing graphene-diamond hybrid films and their electron emission properties

Free standing graphene-diamond hybrid films have been fabricated using saturated hydrocarbon polymers as seeding material by hot filament chemical vapor deposition technique. The films are characterized with x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). The XRD shows the characteristic diffraction peaks of both diamond and graphene. The Raman spectrum shows the characteristic band of diamond at 1332 cm−1 and D, G, and 2D bands of graphene at 1349, 1592, and 2687 cm−1, respectively. Both SEM and TEM depict the presence of diamond and graphene in the films. The EELS recorded in the carbon K-edge region also shows the signature peaks of diamond and graphene. The free standing hybrid films exhibit a remarkably low turn-on field of about 2.4 V/μm and a high emission current density of 0.1 mA/cm2. Furthermore, emission currents are stable over the period of 7 days. The superior field emission characteristics of the free standing graphene-diamond hybrid films are attributed to the heat sink capability of diamond and high electrical conductivity of graphene.

Adhesive Enhancement Improved Field Emission Characteristics of Carbon Nanotube Arrays on Energetic Ion Pre-Bombarded Si Substrates

Field emission (FE) characteristics of well-aligned multiwall carbon nanotube arrays (CNTAs) grown on originally polished and energetic iron ion bombarded Si substrates were investigated. It was found that the FE characteristics have been improved remarkably by the pretreatment of iron ion bombardment, an evident promotion of the highest emission current density from 4.05 mA/cm2to 54.45 mA/cm2was as an expression of this enhancement, this enhancement in characteristics is attributed to the improved adhesion between CNTs and Si substrate for the existence of iron buffer layer. The relationship between adhesive force and emission current density has been introduced, and the calculation reveals that the adhesion has been enhanced by 14.4 times due to the energetic ion pre-bombardment on Si substrate.

Field Emission from One-Dimensional Submicron Diamond Rods

One-dimensional boron-doped submicron diamond rods (SDRs) were fabricated on diamond/Si substrates by oxygen plasma etching. The SDRs are ∼4.5 µm in height and ∼383 nm in diameter. Iron oxide coated on SDRs is essential in the formation of one-dimensional SDRs. However, the as-etched SDRs suffer with high turn-on field (ETO) and low field emission current density (JFE) due to the iron oxide. A huge improvement in the field emission characteristics can be achieved by removing iron oxide using a wet-etch process in a diluted HCl (37%). After the wet-etch, the SDRs exhibit a low ETO value of4.5 V/µm (at 10 µA/cm2) and a high JFE value of 30 mA/cm2 (at 8.5 V/µm). The FE emitter is only stable for a short period of time at high current stress owing to the rounding of the tips of SDRs.

Carbon Nanoparticles on Carbon Fabric for Flexible and High‐Performance Field Emitters

AbstractCarbon nanoparticles (CNPs) are grown on flexible carbon fabric via a simple and low‐cost flame synthesis process. The entire struture of the carbon fabric substrate retains its high flexibility after the growth of CNPs and can even be rolled‐up and twisted to a large degree without affecting the electric characteristics. No appreciable changes in the conductance can be observed under different bending curvatures after hundreds of bending cycles. The thermal conductivity of the carbon fabric with CNPs is about 2.34 W m−1 K−1, about one order of magnitude higher than that of most polymer substrates. The field emitter fabricated using the structure has a low threshold electric field of around 2.8 V μm−1, and a high field emission current density of 108 mA cm−2, which is about two to four orders of magnitude higher than that of most polymer substrate‐based flexible CNT field emitters. These results indicate that CNPs on carbon fabric have potential applications in flexible electronics devices and displays.

Long-term Stable Field Electron Transfer from Carbon Nanotube Arrays at High Emission Current Densities

The stability behaviors of multiwalled carbon nanotube arrays during field electron emission are studied. The results indicate that the stability, even at a high emission current density, has been greatly improved by an aging process, and a degradation of about 0.66% in the emission current density at 21.86 mA/cm 2 during a 10-hour stability test has been obtained. A detailed analysis of the deterioration of the field electron emission characteristics is given, and the generation of Joule heat during field emission is found to be able to burn o the extruded carbon nanotubes, which will directly reduce the number of emission sites. On the other hand, the Joule heating eect may induce an annealing of the defects existing in the carbon nanotubes and may influence the distribution of electron energy states, both having a bad influence on the field emission characteristics. An aging process, especially aging at high emission current densities, can greatly reduce the influence of Jouleheating induced current degradation. Hence, an aging process at high emission current densities provides an eective way to realize long-term stable field electron emission from carbon nanotube arrays.

Synthesis and field emission property of carbon nanotubes with sharp tips

Carbon nanotubes (CNTs) with sharp tips were synthesized by a hydrogen arc discharge method. Three unusual morpholo- gies, i.e., a cone-shaped tip, a suddenly-shrinking tip, and a pencil point-like tip were observed. These novel tip structures are considered to be related to the addition of a small amount of silicon powder in the raw material, which may introduce structural defects in the CNTs. The field emission property of the sharp-tip CNTs was investigated, and a low threshold electric field of 3.75 V/m, a high field emission current density of ~1.6×10 5 A/cm 2 , and a good emission stability were demonstrated. The superior field emission performance of the CNTs can be attributed to their good crystallinity and unique tip structures.

High-current electron emission characteristics of cathodes based on diamond films

Using different gas source, four types of diamond thin films were prepared on silicon substrate by microwave plasma chemical vapor deposition (MPCVD) technology, and characterized in detail through scanning electron microscopy (SEM), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. High-current pulsed emission characteristics, tested with a 2 MeV line-inducing injector, showed that all of CVD diamond films had high emission current density (> 70 A/cm 2) and [100] textured B-doped microcrystalline diamond film possessed the largest emission current density of 115.1 A/cm 2. No obvious bright light and luminescent zones from side view CCD images indicated a possible pure field-emission mechanism of these diamond cathodes. Simultaneously, large decrease in the electron emission capability, above 15%, could be observed after several pulsed measurements, but this decrease could be completely recovered through the treatment of surface re-hydrogenation for emitted diamond cathodes, suggesting that emission performance of CVD diamond cathodes was closely relevant to hydrogen coverage ratio. The present data indicated that as-deposited CVD diamond films could be a potential candidate as cold cathode for the application in high-current electron emission field.