Emission Current Density Research Articles

Fabrication and Emission Performance of Nanostructured Re-W Matrix Impregnated Cathodes.

In this study, rhenium-tungsten mixed particles with different content of rhenium have been prepared by spray-drying method followed by hydrogen reduction. Using such particles, the cathodes have been prepared by powder metallurgy followed by impregnating BaO, CaO, and Al2O3 with 4:1:1 molar ratio. After proper activation, electron emission test is performed in standard parallel-plate diode configuration. The emission results reveal that the Re-W matrix cathode containing 75% rhenium has the highest direct current emission density of 11.67 A/cm2 at 1000 °C. The work function of Re-W matrices has been investigated by density functional theory method in the frame of the generalized gradient approximation (GGA). The theoretical calculation results indicate that the work function of the matrix has limited contribution to the emission current density of Re-W matrix dispenser cathode. The in situ AES, SEM, and XRD were applied and the results reveal that the superior emission property of the 75Re cathode is owing to a plenty of nanoparticles and higher free barium concentration on the cathode surface, which is attributed to the Re3W single phase in 75Re matrix.

Modeling of Barium Coverage and Electron Emission From a Controlled-Porosity Dispenser Cathode

A mathematical model was developed to estimate the dynamic coverage of barium on the surface of a controlled-porosity dispenser (CPD) cathode. This model considers the cumulative barium coverage due to all the neighboring pores and loss of barium due to evaporation. The zero field emission current density and work function obtained from synthesized current–voltage characteristics were derived from barium coverage data. The effect of tungsten wire size on these emission parameters has been studied. The optimum diameter of tungsten wire, used in the fabrication of the CPD cathode, has been determined.

Hydrothermal synthesis of rGO–PbBi2Se4 composite and investigation of its structural, chemical and field emission properties

In the present study we report the one step facile synthesis of pristine lead bismuth selenide (PbBi2Se4) and reduced graphene oxide (rGO) and its composites with PbBi2Se4. Formation of pristine PbBi2Se4 and rGO–PbBi2Se4 composite were confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. The surface morphology and topography investigated by using scanning electron microscopy and transmission electron microscopy revealed the formation of nano-flowers pristine PbBi2Se4. After coupling pristine PbBi2Se4 with rGO the surface morphology shows the formation of sharp vertically protruded nano-sheets/nano-flaks originated from the nano-flowers. Finally, the field emission properties of pristine PbBi2Se4 and rGO–PbBi2Se4 composite have been investigated. It has been observed that the rGO–PbBi2Se4 composite emitter exhibited excellent field emission properties with low turn-on field (~ 2.8 V/µm for 10 µA/cm2), high emission current density (~ 1288 µA/cm2 at 3.9 V/µm) and superior current stability (~ 4.5 h for ~ 1 µA) compare to pristine PbBi2Se4 emitter. Thus, the facile one step synthesis approach and robust nature of rGO–PbBi2Se4 composite emitter can provide prospects for the future development of large-area emitter applications such as flat-panel-display and vacuum micro/nanoelectronics devices.

Fabrication of carbon nanotube on nickel–chromium alloy wire for high-current field emission

The application of field emission (FE) devices requires emitters having a low threshold field (Eth, applied field at 10 mA/cm2) and an excellent FE stability at a high emission current density. Herein, we report on the fabrication and field emission characteristics of a carbon nanotube (CNT) emitter formed as a CNT film grown directly on nickel–chromium (Ni80Cr20) alloy wire using microwave plasma enhanced chemical vapour deposition (PECVD). The FE performance of the emitter has a low threshold field of 0.76 V/µm, a large field enhancement factor (8311.3 ± 53.1) and an extremely large emission current density (Jmax) of 7.65 A/cm2 at a relatively low electric field of 2.13 V/µm. The great increase in Jmax is ascribed to the reinforced adhesion of CNTs to the Ni80Cr20 alloy wire substrates. The direct synthesis of CNTs on the nickel–chromium wire substrate without any catalyst layers and the wrapping of the CNT roots with a carbon nanoflake layer deposited on the surface of the wire substrate are responsible for the enhanced adhesion. The tip-type CNT emitter presents an excellent field emission stability at high emission current densities (3.06 A/cm2), and could potentially be applied to field emission devices.

Electron field emission from Q-carbon

We report field electron emission investigations in Q-carbon composite structures formed by pulsed laser annealing of amorphous carbon layers. Under the optimum fabrication conditions, a dense microstructured morphology of Q-carbon was obtained, which is important for local electric field enhancement in field-emission device applications. The turn-on field required to draw an emission current density of 1 μA/cm2 is found to be 2.4 V/μm. The Q-carbon films show good electron emission stability as a function of time up to 4 h. The microstructure and morphology of the field emitting Q-carbon films was analyzed by a variety of techniques, including field emission scanning electron microscope, Raman spectroscopy, and atomic force microscopy. Our results show a very high emission current density value of ~30 μA/cm2 at an applied electric field of 2.65 V/μm, which is hysteresis-free and stable. The generated emission current has been found to have low fluctuations (<4%) and shows no generation of defects during repeated emission measurements on the sample. Our findings thus hold a great promise for the development of Q-carbon films in applications ranging from field emitters and frictionless motors to heterostructures for novel micro and nano-electronic devices.

Simulation of Non-Uniform Electron Beams in the Gyrotron Electron-Optical System

New calculated data on the effect of emission inhomogeneities on the quality of the electron beam, which is formed in an electron-optical system of a gyrotron, have been obtained. The calculations were based on emission current density distributions, which were measured for the different cathodes in the gyrotron of Peter the Great St. Petersburg Polytechnic University. A satisfactory agreement between the experimental and calculated data on the influence of emission nonuniformities on the velocity spread of electrons has been shown. The necessity of considering the real distribution of the emission current density over the cathode surface to determine the main parameters of the electron beam—the velocity and energy spreads of the electrons, spatial structure of the beam, and coefficient of reflection of electrons from the magnetic mirror—has been demonstrated. The maximum level of emission inhomogeneities, which are permissible for effective work of gyrotrons, has been discussed.

2D Porous ZnO Nanosheets: One Pot Synthesis with Low Turn-on Field

Low turn-on field of 2.3 V/µm was found for the emission current density of 10 µA/cm2 from 2D porous ZnO nanosheets. High current density of 0.76 mA/cm2 was drawn at an applied field of 4.1 V/µm. The observed low turn-on field of porous ZnO nanosheets has been found to be superior to the other ZnO nanostructures reported in the literature. Also, the emission current stability over a period of 3 hr is found to be better. The field emission current density-applied field (J-E) and current-time (I-t) measurements were carried out in all metal field emission microscope by using ‘close proximity’ (also termed as ‘planar diode’). The porous ZnO nanosheets were synthesized by Chemical Bath Deposition (CBD) method at room temperature followed by annealing at 200 oC. The annealed ZnO nanosheets were subjected to structural and morphological analysis prior to the field emission studies. The XRD spectrum of the as-synthesized product reveals formation of crystalline hexagonal phase of ZnO. Simple synthesis route with superior field emission properties indicate the possible use of porous ZnO nanosheets for micro/nanoelectronic devices.

"V" shape curves of physical parameters of field emitters versus applied voltage toward breakdown in vacuum

The objective of this paper is to determine the physical parameters evolution of field emitters with applied voltage increasing toward breakdown of a rod-plane gap in vacuum. Pulses of AC power frequency voltage are applied to vacuum gaps of 1.0, 0.8 and 0.5 mm to breakdown and the field emission current and applied voltage are measured. Through Fowler-Nordheim theory, the physical parameters of field emitters, including the electric field enhancement factor β, effective emission area Ae, microscopic electric field E, and field emission current density jFE are obtained to represent the physical parameters evolution of the emitters up to breakdown. The results show that the electric field enhancement factor β, microscopic electric field E and field emission current density jFE follow a V shape with increasing voltage. When the applied voltage is increased from starting voltage VS to turning voltage VT, the parameters β, E and jFE decreased. When the applied voltage is further increased from the turning voltage VT to the breakdown voltage VB, the trend reverses, i.e. the parameters β, E and jFE increases up to breakdown.

Fabrication, Densification and Thermionic Emission Property of Lanthanum Hexaboride

An effective way to improve sintering densification of LaB6 was proposed and confirmed experimentally. Firstly, LaB6 nanopowders with a cube-like shape of ~ 94.7 nm were fabricated by molten salt synthesis route at 800 °C for 1 h. Then, LaB6 bulk material of 98% density was prepared by hot pressing sintering of as-synthesized LaB6 nanopowders under 1800 °C/50 MPa/30 min. The acquired LaB6 bulk material had a work function of 2.87 eV and exhibited an excellent thermionic emission property. The saturation emission current density at 1500 and 1600 °C reached 37.4 and 44.3 A/cm2, respectively.

Floating potential of emitting surfaces in plasmas with respect to the space potential

The potential difference between a floating emitting surface and the plasma surrounding it has been described by several sheath models, including the space-charge-limited sheath, the electron sheath with high emission current, and the inverse sheath produced by charge-exchange ion trapping. Our measurements reveal that each of these models has its own regime of validity. We determine the potential of an emissive filament relative to the plasma potential, emphasizing variations in emitted current density and neutral particle density. The potential of a filament in a diffuse plasma is first shown to vanish, consistent with the electron sheath model and increasing electron emission. In a denser plasma with ample neutral pressure, the floating filament potential is positive, as predicted by a derived ion trapping condition. Lastly, the filament floated negatively in a third plasma, where flowing ions and electrons and nonnegligible electric fields may have disrupted ion trapping. Depending on the regime chosen, emitting surfaces can float positively or negatively with respect to the plasma potential.

Comment on “Water-processed carbon nanotube/graphene hybrids with enhanced field emission properties” [AIP Advances 5, 097130 (2015)

In their research paper, M. Song et al. [AIP ADVANCES 5, 097130 (2015)] have claimed to have achieved enhanced field emission (FE) characteristics of carbon nanotubes (CNT)/graphene hybrids experimentally, exhibiting improved FE parameters e.g. turn-on electric field of 0.79 V/μm, threshold electric field of 1.05 V/μm, maximum emission current density (Jmax) of 5.76 mA/cm2, and field enhancement factor (β) of ∼1.3 × 104. The authors have emphasized on the surprisingly high value of β to be the basis of their claim of achieving superior FE performance which is further attributed to the optimized mass ratio CNT/ graphene, which is 5:1 in the present case. However, the claim based upon high value of β is misleading because it does not corroborate with the obtained Jmax parameter. Also, the obtained value of J is quite low in the mentioned study as compared to the reported values. For an instance, Sameera et al. [J. Appl. Phys. 111, 044307 (2012) &amp; Appl. Phys. Lett. 102, 033102 (2013)] have reported FE properties of CNT composites and reduced graphene oxide with Jmax and β values of the order of ∼102 mA/cm2 and 6 × 103, respectively. Therefore, the conclusions drawn by M. Song et al. [AIP ADVANCES 5, 097130 (2015)] in their paper do no hold.

The effect of water on the electronic and field emission properties of inorganic AlN nanocones: Computational study

Using density functional theory calculations, we explored the effect of water adsorption on the electronic and field emission properties of AlN nanocones (AlNN) at the presence and absence of a uniform electric field (EF) about ±0.02a.u. in direction of Z-axis. At zero EF, the H2O molecule tends to be adsorbed from its O atom at the apex of AlNN with adsorption energy about −23.2kcal/mol so that its effect on the electronic and field emission properties of AlNN is negligible. Both EFs increase the electrical conductivity of AlNN. At the + and −EFs, the adsorption energy changes to −40.6 and −4.1kcal/mol, respectively. At the +EF, the HOMO level of AlNN largely shifts up from −3.69 to −3.08eV, and thus, the gap significantly decreases by about 31.6%. As a result, the Fermi level (FL) shifts to higher energies, and the work function (Φ) of AlNN decreases by about 12.0%. Therefore, it can be concluded that H2O molecules can increase the field emission current density (EECD) from the surface of AlNN. But at the −EF, after the H2O adsorption, the LUMO level of AlNN largely shifts down and the gap is meaningfully decreased by about 59.4%. As a result, the FL shifts to lower energies and the Φ of AlNN increases by about 14.0%. Thus, H2O molecules decrease the EECD from the surface of AlNN at −EF.

Ultra-high brightness electron beams from very-high field cryogenic radiofrequency photocathode sources

Recent investigations of RF copper structures operated at cryogenic temperatures performed by a SLAC-UCLA collaboration have shown a dramatic increase in the maximum surface electric field, to 500 MV/m. We examine use of these fields to enable very high field cryogenic photoinjectors that can attain over an order of magnitude increase in peak electron beam brightness. We present beam dynamics studies relevant to X-ray FEL injectors, using start-to-end simulations that show the high brightness and low emittance of this source enables operation of a compact FEL reaching a photon energy of 80 keV. The preservation of beam brightness in compression, exploiting micro-bunching techniques is discussed. While the gain in brightness at high field is due to increase of the emission current density, further increases in brightness due to lowering of the intrinsic cathode emittance in cryogenic operation are also enabled. While the original proposal for this type of cryogenic, ultra-high field photoinjector has emphasized S-band designs, there are numerous potential advantages that may be conferred by operation in C-band. We examine issues related to experimental implementation in C-band, and expected performance of this type of device in a future hard X-ray FEL such as MaRIE.

Field emission study of urchin like nanostructured cobalt oxide films prepared by pulsed laser deposition

Cobalt oxide (Co3O4) urchin structures have been grown via Pulsed Laser Deposition (PLD) technique followed by thermal treatment in air. Morphological and structural analysis of as synthesized nanostructures has been carried out using Field Emission Scanning Electron Microscopy (FESEM) and X-ray Diffraction (XRD) technique. Chemical nature of the films have been revealed using X-ray Photoelectron Spectroscopy (XPS). Field emission measurements were carried out at a base pressure of 1 × 10−8 mbar. Turn-on field corresponding to an emission current density of 10 μA/cm2 was found to be 3 V/μm and a high field enhancement factor of ∼3900 was estimated, both of which can be attributed to radially grown nanowires of long length with sharp tips. Overall good field emission current stability was observed for Co3O4 urchin structured films. Dependence of FE properties such as, current density and current stability on morphology and density of emitting sites has been observed and analyzed systematically. The observed FE results demonstrate Co3O4 urchin structures as potential candidate for applications in various vacuum micro/nanoelectronic devices.

Maximum field emission current density of CuO nanowires: theoretical study using a defect-related semiconductor field emission model and in situ measurements

In this study, we proposed a theoretical model for one-dimensional semiconductor nanowires (NWs), taking account of the defect-related electrical transport process. The maximum emission current density was calculated by considering the influence of Joule heating, using a one-dimensional heat equation. The field emission properties of individual CuO NWs with different electrical properties were studied using an in situ experimental technique. The experimental results for maximum emission current density agreed well with the theoretical predictions and suggested that multiple conduction mechanisms were active. These may be induced by the concentration of defects in the CuO NW. The concentration of defects and the transport mechanisms were found to be key factors influencing the maximum field emission current density of the semiconductor NW. As is limited by the change of resistivity with temperature, only thermal runaway can trigger breakdown in CuO NWs.

A computational study on the electronic and field emission properties of Mg and Si doped AlN nanocones

Following an experimental work, we explored the effect of replacing an Al atom of an AlN nanocone by Si or Mg atom on its electronic and field emission properties using density functional theory calculations. We found that both Si-doping and Mg-doping increase the electrical conductivity of AlN nanocone, but their influences on the filed emission properties are significantly different. The Si-doping increases the electron concentration of AlN nanocone and results in a large electron mobility and a low work function, whereas Mg-doping leads to a high hole concentration below the conduction level and increases the work function in agreement with the experimental results. It is predicted that Si-doped AlN nanocones show excellent filed emission performance with higher emitted electron current density compared to the pristine AlN nanocone. But the Mg-doping meaningfully decreases the emitted electron current density from the surface of AlN nanocone. The Mg-doping can increase the work function about 41.9% and the Si-doping can decrease it about 6.3%. The Mg-doping and Si-doping convert the AlN nanocone to a p-type and n-type semiconductors, respectively. Our results explain in a molecular level what observed in the experiment.

Investigation of field emission properties of laser irradiated tungsten

Nd:YAG laser irradiation of Tungsten (W) has been performed in air at atmospheric pressure for four laser fluences ranging from 130 to 500 J/cm2. Scanning electron microscope analysis revealed the formation of micro and nanoscale surface features including cones, grains, mounds and pores. Field emission (FE) studies have been performed in a planar diode configuration under ultra-high vacuum conditions by recording I–V characteristics and plotting corresponding electric field (E) versus emission current density (J). The Fowler–Nordheim (FN) plots are found to be linear confirming the quantum mechanical tunneling phenomena for the structured targets. The irradiated samples at different fluences exhibit a turn-on field, field enhancement factor β and a maximum current density ranging from 5 to 8.5 V/µm, 1300 to 3490 and 107 to 350 µA/cm2, respectively. The difference in the FE properties is attributed to the variation in the nature and density of the grown structures at different fluences.

Improved field emission properties of α-Fe2O3 nanoflakes with current aging treatment and morphology optimization

α-Fe2O3 nanomaterials were synthesized by thermal oxidation of pure iron foil and the effects of current aging treatment and morphology on their field emission properties were systematically investigated. The current aging treatment was found to be an efficient method to improve the field emission properties of α-Fe2O3 nanoflakes. The emission current density was largely enhanced from 0.05–5.70 mA cm−2 under an applied electrical field of 7.8 MV m−1, and their threshold field decreased from than 11.0–6.6 MV m−1 after the current aging treatment. The mechanism of the improvement in the field emission performance of α-Fe2O3 nanoflakes induced by the current aging treatment is discussed. α-Fe2O3 nanostructures with various morphologies were synthesized by adjusting the growth temperatures between 300 °C–450 °C to optimize their morphologies. α-Fe2O3 nanoflakes synthesized at 350 °C were superior field emitters with a low threshold field of 5.1 MV m−1, high current density of 63.4 mA cm−2, and stable emission, which demonstrated that α-Fe2O3 nanoflakes could be a promising material for application as field emitters.

Solvothermal synthesis of tin sulfide (SnS) nanorods and investigation of its field emission properties

In the present study, we report synthesis of tin sulfide (SnS) nano-rods using a simple solvothermal method at different reaction time period. The formation of single phase SnS has been confirmed by X-ray diffraction (XRD) and Raman analysis. The XRD analysis revealed that the predominant phase in all prepared samples is orthorhombic SnS. The formation of nano-rods of SnS was confirmed by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) analysis. To investigate the optical properties of SnS nano-rods UV–visible spectroscopy analysis was carried out. We observed that the band gap of SnS nano-rods decreases with increase in reaction time and can be attributed to the quantum confinement effect. Finally, field emission investigations on the SnS nano-rods at the base pressure of 1 × 10− 8 mbar were carried out and found to be superior to the other chalcogenide nanostructures. As-synthesized SnS nano-rods emitter exhibits excellent field emission properties such as low turn-on field (~ 2.5 V/µm for 10 µA/ cm2), high emission current density (~ 647 µA/cm2 at 3.9 V/µm) and superior current stability (~ 5 h for ~ 1 µA). Thus, the facile one-step synthesis approach and robust nature of SnS nano-rods emitter can provide prospects for the future development of large-area emitter applications such as flat-panel-display devices.

Enhanced field emission properties of graphene-based cathodes fabricated by ultrasonic atomization spray.

Two types of graphene cathodes were constructed by doctor blade and ultrasonic atomization spray respectively. The effects of cathodic film morphology and thickness on graphene cathodes' field emission performance were investigated. Ultrasonic atomization spray coated graphene cathodes possess a much lower turn-on and threshold field but much higher emission current density than graphene cathodes coated by doctor blade. The enhanced field emission properties can be ascribed to the suppression of field-screening effect by roughened surface geometry rendered by ultrasonic atomization spray. For ultrasonic atomization sprayed graphene cathodes at a film thickness of 116 μm, an emission current density as large as 29.6 mA cm−2 was achieved at 5 V μm−1, along with a turn-on field and threshold field as low as 1.52 V μm−1 and 2.65 V μm−1, respectively. Emission stability examination shows no visible emission current density fluctuation or decline over a 10 hour operation at ∼3.72 mA cm−2, demonstrating excellent field emission stability for ultrasonic atomization sprayed graphene cathodes. The luminance test also indicates good uniformity of electron emission from ultrasonic atomization sprayed graphene cathodes. The above experiment results indicate that the ultrasonic atomization spray method is suitable for large-area fabrication of high-performance graphene field emitters and holds great potential for applications in field emission displays.