High Emission Current Density Research Articles

High current density and longtime stable field electron transfer from large-area densely arrayed graphene nanosheet-carbon nanotube hybrids.

Achieving high current and longtime stable field emission from large area (larger than 1 mm(2)), densely arrayed emitters is of great importance in applications for vacuum electron sources. We report here the preparation of graphene nanosheet-carbon nanotube (GNS-CNT) hybrids by following a process of iron ion prebombardment on Si wafers, catalyst-free growth of GNSs on CNTs, and high-temperature annealing. Structural observations indicate that the iron ion prebombardment influences the growth of CNTs quite limitedly, and the self-assembled GNSs sparsely distributed on the tips of CNTs with their sharp edges unfolded outside. The field emission study indicates that the maximum emission current density (Jmax) is gradually promoted after these treatments, and the composition with GNSs is helpful for decreasing the operation fields of CNTs. An optimal Jmax up to 85.10 mA/cm(2) is achieved from a 4.65 mm(2) GNS-CNT sample, far larger than 7.41 mA/cm(2) for the as-grown CNTs. This great increase of Jmax is ascribed to the reinforced adhesion of GNS-CNT hybrids to substrates. We propose a rough calculation and find that this adhesion is promoted by 7.37 times after the three-step processing. We consider that both the ion prebombardment produced rough surface and the wrapping of CNT foot by catalyst residuals during thermal processing are responsible for this enhanced adhesion. Furthermore, the three-step prepared GNS-CNT hybrids present excellent field emission stability at high emission current densities (larger than 20 mA/cm(2)) after being perfectly aged.

Radial AlN nanotips on carbon fibers as flexible electron emitters

A facile catalyst-free approach using a simple thermal transport method has been developed to fabricate high-density AlN nanotips on flexible carbon cloth at large scales for use as field emission (FE) emitters. The AlN nanotips exhibit good performance as flexible cold-cathode electron emitters, with a very low turn-on electric field of 1.1–2.3Vμm−1, a low threshold electric field of 1.5–2.5Vμm−1, and a high emission current density. The excellent field emission properties of the AlN nanotips are attributed to the large field enhancement factor of 6895 as well as the combined effect of the tip profile of the AlN nanostructures and the excellent electron transport path of the conductive carbon cloth substrate.

Graphene field emission devices

Graphene field emission devices are fabricated using a scalable process. The field enhancement factors, determined from the Fowler-Nordheim plots, are within few hundreds and match the theoretical predictions. The devices show high emission current density of ∼10 nA μm−1 at modest voltages of tens of volts. The emission is stable with time and repeatable over long term, whereas the noise in the emission current is comparable to that from individual carbon nanotubes emitting under similar conditions. We demonstrate a power law dependence of emission current on pressure which can be utilized for sensing. The excellent characteristics and relative ease of making the devices promise their great potential for sensing and electronic applications.

V2O5 precursor-templated synthesis of textured nanoparticles based VN nanofibers and their exploration as efficient field emitter

Textured Vanadium Nitride (VN) nanofibers are prepared from hydrothermally derived V2O5 nanofibers by NH3 reduction and nitridation approach. The reaction temperature is seen to be a crucial parameter to prepare pure VN phase. VN produced via this route has distinctive structural characteristics. The crystal structure, morphology, and oxidation states of the elements of VN nanofibers are characterized by XRD, FESEM, TEM, HRTEM, and XPS studies. Field emission properties for these textured VN nanofibers have been explored. The field emission properties show the reproducible turn-on field of 3.4 V/μm for the emission current density of 10 μA/cm2. The observed turn-on field is found to be quite superior compare to carbide, nitride and oxide nanostructures. High emission current density of 44 μA/cm2 has been found at a field of 4 V/μm. Emission current stability for the preset current of 5 μA has been recorded for the period of 2 h. Present results indicate the considerable potential for the utilizing VN nanofibers as a cold cathode in the field emitter devices.

Life Test Studies on Dispenser Cathode With Dual-Layer Porous Tungsten

This paper introduces a newly developed dual-layer porous tungsten dispenser cathode, which can meet the growing requirements on both life and current density. In comparison with the conventional dispenser cathodes, the new-type dispenser cathodes have higher emission current density, lower evaporation rate and longer lifetime. According to the results of the life tests and the related analysis, their longest lifetime at a 1.5 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> current density under 974 °C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> is estimated to be over 1.9 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> h.

Field emission properties from flexible field emitters using carbon nanotube film

Flexible carbon nanotube (CNT) field emitters are fabricated using CNT films on polyethylene terephthalate films. The flexible CNT emitters, which are made using double-walled CNTs, show high emission performance and also indicate stable field emission properties under several bending conditions. The flexible CNT emitters have a low turn-on field of about 0.82 V/μm and a high emission current density of about 2.0 mA/cm2 at an electric field of 1.6 V/μm. During stability tests, the flexible CNT emitters initially degrade over the first 4 h but exhibit no further significant degradation over the next 16 h testing while being continually bent. A flexible lamp made using the flexible CNT emitter displays uniform and bright emission patterns in a convex mode.

Enhancement of the stability of electron field emission behavior and the related microplasma devices of carbon nanotubes by coating diamond films.

The enhanced lifetime stability for the carbon nanotubes (CNTs) by coating hybrid granular structured diamond (HiD) films on Au-decorated CNTs/Si using a two-step microwave plasma enhanced chemical vapor deposition process was reported. Electron field emission (EFE) properties of HiD/Au/CNTs emitters show a low turn-on field (E0) of 3.50 V/μm and a high emission current density (Je) of 0.64 mA/cm(2) at an applied field of 5.0 V/μm. There is no notable current degradation or fluctuation over a period of τ(HiD/Au/CNTs) = 360 min for HiD/CNTs EFE emitters tested under a constant current of 4.5 μA. The robustness of the HiD/CNTs EFE emitter is overwhelmingly superior to that of bare CNTs EFE emitters (τ(CNTs) = 30 min), even though the HiD/Au/CNTs do not show the same good EFE properties as CNTs, which are E0 = 0.73 V/μm and Je = 1.10 mA/cm(2) at 1.05 V/μm. Furthermore, the plasma illumination (PI) property of a parallel-plate microplasma device fabricated using the HiD/Au/CNTs as a cathode shows a high Ar plasma current density of 1.76 mA/cm(2) at an applied field of 5600 V/cm with a lifetime of plasma stability of about 209 min, which is markedly better than the devices utilizing bare CNTs as a cathode. The CNT emitters coated with diamond films possessing marvelous EFE and PI properties with improved lifetime stability have great potential for the applications as cathodes in flat-panel displays and microplasma display devices.

The effect of cathode geometry on barium transport in hollow cathode plasmas

The effect of barium transport on the operation of dispenser hollow cathodes was investigated in numerical modeling of a cathode with two different orifice sizes. Despite large differences in cathode emitter temperature, emitted electron current density, internal xenon neutral and plasma densities, and size of the plasma-surface interaction region, the barium transport in the two geometries is qualitatively very similar. Barium is produced in the insert and flows to the surface through the porous structure. A buildup of neutral Ba pressure in the plasma over the emitter surface can suppress the reactions supplying the Ba, restricting the net production rate. Neutral Ba flows into the dense Xe plasma and has a high probability of being ionized at the periphery of this zone. The steady state neutral Ba density distribution is determined by a balance between pressure gradient forces and the drag force associated with collisions between neutral Ba and neutral Xe atoms. A small fraction of the neutral Ba is lost upstream. The majority of the neutral Ba is ionized in the high temperature Xe plasma and is pushed back to the emitter surface by the electric field. The steady state Ba+ ion density distribution results from a balance between electrostatic and pressure forces, neutral Xe drag and Xe+ ion drag with the dominant forces dependent on location in the discharge. These results indicate that hollow cathodes are very effective at recycling Ba within the discharge and therefore maintain a high coverage of Ba on the emitter surface, which reduces the work function and sustains high electron emission current densities at moderate temperatures. Barium recycling is more effective in the cathode with the smaller orifice because the Ba is ionized in the dense Xe plasma concentrated just upstream of the orifice and pushed back into the hollow cathode. Despite a lower emitter temperature, the large orifice cathode has a higher Ba loss rate through the orifice because the Xe plasma density peaks further upstream.

Excellent oxidation endurance of boron nitride nanotube field electron emitters

Boron nitride nanotubes (BNNTs) are considered as a promising cold electron emission material owing to their negative electron affinity. BNNT field emitters show excellent oxidation endurance after high temperature thermal annealing of 600 °C in air ambient. There is no damage to the BNNTs after thermal annealing at a temperature of 600 °C and also no degradation of field emission properties. The thermally annealed BNNTs exhibit a high maximum emission current density of 8.39 mA/cm2 and show very robust emission stability. The BNNTs can be a promising emitter material for field emission devices under harsh oxygen environments.

Flexible electron field emitters fabricated using conducting ultrananocrystalline diamond pyramidal microtips on polynorbornene films

High performance flexible field emitters made of aligned pyramidal shaped conducting ultrananocrystalline diamond (C-UNCD) microtips on polynorbornene substrates is demonstrated. Flexible C-UNCD pyramidal microtips show a low turn-on field of 1.80 V/μm with a field enhancement factor of 4580 and a high emission current density of 5.8 mA/cm2 (at an applied field of 4.20 V/μm) with life-time stability of 210 min. Such an enhancement in the field emission is due to the presence of sp2-graphitic sheath with a nanowire-like diamond core. This high performance flexible C-UNCD field emitter is potentially useful for the fabrication of diverse, flexible electronic devices.

Field Electron Emission from Electrophoretic Deposited MWCNT/ZnO Hybrid Film

Hybrid film of MWCNT/ZnO was prepared on the silver electrodes using electrophoresis deposition. A constant dc voltage of about 20V was applied to the electrodes and the MWCNT/ZnO was deposited on the surface of the anode electrode as a result. Field Emission Scanning Electron Microscopy was used to investigate the structure MWCNT/ZnO composite. The optical characterization was investigated by Raman Spectroscopy. The MWCNT film exhibits excellent field electron emission properties with high emission current densities, low threshold electric fields and good field emission stability.

Room-Temperature Fabrication of Au- and Ag-Incorporated Carbon Nanofibers by Ion Irradiation and Their Field Emission Properties

We have demonstrated the growth of Au- and Ag-incorporated carbon nanofibers (CNFs) at room temperature by Ar+ bombardment on graphite surfaces with simultaneous Au and Ag supply. The evolution of their morphology and its effects on field emission properties were investigated. The structure and density of the grown CNFs depended on the metal supply rate. The ion-irradiated surfaces with excess metal supply featured sparsely distributed conical protrusions and a wall-like structure, while the surfaces irradiated with appropriate metal supply produced densely distributed CNF-tipped cones and a needlelike structure. Compared with Ag supply, Au supply yielded fewer CNFs in terms of number density. Thus, the CNF number density was controllable by adjusting the metal supply rate and metal species. A lower threshold field and a higher emission current density were achieved in the field emission of both metal-incorporated CNFs than of pristine CNFs (without metal incorporation). Thus, it is believed that metal-incorporated CNFs are promising for practical field emission device applications.

Field Emission Behavior of Carbon Nanotube Yarn for Micro-Resolution X-Ray Tube Cathode

Carbon nanotube (CNT) has excellent electrical and thermal conductivity and high aspect ratio for X-ray tube cathode. However, CNT field emission cathode has been shown unstable field emission and short life time due to field evaporation by high current density and detachment by electrostatic force. An alternative approach in this direction is the introduction of CNT yarn, which is a one dimensional assembly of individual carbon nanotubes bonded by the Van der Waals force. Because CNT yarn is composed with many CNTs, CNT yarns are expected to increase current density and life time for X-ray tube applications. In this research, CNT yarn was fabricated by spinning of a super-aligned CNT forest and was characterized for application to an X-ray tube cathode. CNT yarn showed a high field emission current density and a long lifetime of over 450 hours. Applying the CNT yarn field emitter to the X-ray tube cathode, it was possible to obtain micro-scale resolution images. The relationship between the field emission properties and the microstructure evolution was investigated and the unraveling effect of the CNT yarn was discussed.

Electron Field Emission Enhancement of Vertically Aligned Ultrananocrystalline Diamond‐Coated ZnO Core–Shell Heterostructured Nanorods

Enhanced electron field emission (EFE) behavior of a core-shell heterostructure, where ZnO nanorods (ZNRs) form the core and ultrananocrystalline diamond needles (UNCDNs) form the shell, is reported. EFE properties of ZNR-UNCDN core-shell heterostructures show a high emission current density of 5.5 mA cm(-2) at an applied field of 4.25 V μm(-1) , and a low turn-on field of 2.08 V μm(-1) compared to the 1.67 mA cm(-2) emission current density (at an applied field of 28.7 V μm(-1) ) and 16.6 V μm(-1) turn-on field for bare ZNRs. Such an enhancement in the field emission originates from the unique materials combination, resulting in good electron transport from ZNRs to UNCDNs and efficient field emission of electrons from the UNCDNs. The potential application of these materials is demonstrated by the plasma illumination measurements that lowering the threshold voltage by 160 V confirms the role of ZNR-UNCDN core-shell heterostructures in the enhancement of electron emission.

Selective plasma etching treatment of the screen-printed carbon nanotube cold cathode

A high-precision printing and patterning carbon nanotube (CNT) cathode was prepared using the screen-printing method. Selective plasma etchings were introduced to improve field emission properties of the CNT cathode through the reactive ion etching (RIE) system. The field emission characteristics and mechanism of the cathode after etching treatment were studied. It was found that the reactive ion etching could effectively expose plentiful CNTs inside the cathode and protrude them from the surface. Moreover, with the increase of RIE operation pressure, CNT cathode field emission behavior changed from metal-insulation medium-vacuum (MIV) to a classical metallic-microprotrusion (MM) structure field emission model. The results showed that only the cathode with appropriate RIE operation pressure etching has a low operation field and a high emission current density.

Effectively improved field emission for graphene film by mechanical surface modification

A mechanical surface treatment is presented by using adhesive tape to modify the surface morphology of graphene cathodes and to improve their field emission properties. The treated graphene cathodes have numerous vertical sharp edges, which act as excellent field emission sites. Effectively improved field emission properties with low turn on field, low threshold field, high emission current density and high field enhancement factor are obtained due to the full utilization of the unique structure and excellent properties of graphene.

The synthesis of vertically oriented carbon nanosheet–carbon nanotube hybrid films and their excellent field emission properties

Vertically oriented carbon nanosheet (VCNS)–carbon nanotube (CNT) hybrid films with a large area (∼4cm2) were fabricated on Mo layers by using microwave plasma-assisted chemical vapor deposition technique. The Mo layers, which were deposited on Al2O3 ceramic substrates through electron beam evaporation deposition, were pretreated by a laser-grooving technology (4lines/mm). The films were characterized by scanning electron microscopy, Raman spectroscopy and field emission I–V measurements. A pure VCNS film was found in the laser-etched area, while in the un-etched area a hybrid film of VCNS–CNT was formed. The growth mechanisms of the films were discussed. Effects of laser-grooving pretreatment on surface morphologies and microstructure of the hybrid films were investigated. Field electron emission results showed a turn on field as low as 1.0V/μm and a high current density of 3.2mA/cm2 at an electric field of 5.8V/μm. The low threshold field and high emission current density were attributed primarily to the unique shape and small resistivity of the films.

Electron field emission from reduced graphene oxide on polymer film

Field emission of reduced graphene oxide coated on polystyrene film is studied in both parallel and perpendicular configurations. Low turn-on field of 0.6 V/μm and high emission current density of 200 mA/cm2 are observed in perpendicular configuration (along the cross section), whereas a turn-on field of 6 V/μm and current density of 20 μA/cm2 are obtained in parallel configuration (top surface). The emission characteristics follow Fowler–Nordheim (FN) tunneling and the values of enhancement factor estimated from FN plots are 5818 (perpendicular) and 741 (parallel). Furthermore, stability and repeatability of the field emission characteristics in perpendicular configuration are presented.

大电流碳纳米管场发射阴极研究

A large current and a high emission current density from printed carbon nanotube (CNT) are reported. To enhance the field-emission current, a small number of metal nano-particles were added to the CNT paste. The metal nano-particles enhanced the emitters-substrate contact and improved the CNTs adhesion properties. A large current of 68.0 mA is obtained from a cathode area of 1.1 mm2, and the high emission current density is about 6.2 A/cm2. The CNT cathode is applied to field-emission vacuum device prototype successfully. Experiments show that current and current density can meet the requirements for large current field-emission electron devices.

Controlled synthesis of aligned Bi2S3nanowires, sharp apex nanowires and nanobelts with its morphology dependent field emission investigations

Well-aligned ultra-long Bi2S3 nanowire arrays with three kinds of apex morphology – abruptly sharpened apex, thin belt and flat – have been systematically fabricated on tungsten (W) foil by a facile hydrothermal method. The structural and morphological studies reveal formation of distinct tip morphologies, possessing high aspect ratio, single crystalline nature and growth along the orthorhombic [001] axis. A plausible growth mechanism has been proposed on the basis of observed experimental results. The field emission properties of Bi2S3 nanowires and sharp apex Bi2S3 nanowires are investigated. From the field emission studies, the values of the turn-on field, required to draw emission current density of ∼0.1 μA cm−2, are observed to be ∼2.01 and 1.21 V μm−1 for nanowires and sharp apex nanowires, respectively. Furthermore, ultra-long Bi2S3 nanowires are also grown on the W microtip (brush-like) from which very high emission current density ∼11 mA cm−2 has been drawn. These results are helpful for the design, fabrication and optimization of integrated field emitters using 1D nanostructures as cold cathode material.