Emission Current Density Research Articles

Microstructure characterization and thermionic emission performance of barium-tungsten cathode

The effect of the parameters (sintering temperature, pressure, holding time) of spark plasma sintering technique on the W porosity of Ba-W cathode was studied by orthogonal method. When the W porosity varied in the range of 23%−30%, the corresponding process parameters were obtained. On this basis, the spherical and traditional irregular W matrix with different porosity were sintered. The results show that the spherical porous W particles were packed and arranged orderly, and the pore size distribution was concentrated and uniform. When the porosity of spherical W was 26.3%, the size of the median pore was 1.41 μm. The vickers hardness of spherical tungsten matrix was lower than that of conventional irregular tungsten matrix. Under the condition of pulse width 10 μs and frequency 1000 Hz, the impulse current density of Ba-W cathode increased at first and then decreased with the increase in porosity. The maximum current density belonged to the Ba-W cathode with the matrix porosity of 26.3%. At 1050 ℃, the off-point emission current density of Ba-W cathode reached 24.62 A/cm2, the corresponding zero-field current density and workfunction were 7.62 A/cm2 and 1.95 eV, respectively.

Higher order curvature corrections to the field emission current density

A simple expression for the Gamow factor is obtained using a second-order curvature-corrected tunneling potential. Our results show that it approximates accurately the “exact-WKB” transmission coefficient obtained by numerically integrating over the tunneling region to obtain the Gamow factor. The average difference in current density using the respective transmission coefficients is about 1.5%, across a range of work functions ϕ∈[3−5.5] eV, Fermi energy EF∈[5−10] eV, local electric fields El∈[3−9] V/nm, and radius of curvature R≥5 nm. An easy-to-use correction factor λP is also provided to approximately map the “exact-WKB” current density to the “exact” current density in terms of EF/ϕ. The average error on using λP is found to be around 3.5%. This is a vast improvement over the average error of 15% when λP=1. Finally, an analytical expression for the curvature-corrected current density is obtained using the Gamow factor. It is found to compare well with the “exact-WKB” current density even at small values of local electric field and the radius of curvature.

Electrical Characteristics of Vertical GaN Nanowire Vacuum Field Emitter Devices

The electrical characteristics of simulated vertical gallium nitride (GaN) nanowire (NW) vacuum field emission diodes (VFEDs) and GaN NW vacuum field emission transistors (VFETs) with flat- and sharp-tip GaN NW field emitters (FEs) are presented by investigating their dependence on geometry and material parameters. For a flat-tip GaN NW VFED, the turn-on voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TO</sub> ) was decreased by decreasing the GaN NW diameter ( D), the anode to emitter distance (d), the work function of the anode metal (φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</sub> ), or by increasing the GaN NW height ( h). Variations in doping concentration (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> ) of the GaN NW do not impact V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TO</sub> . The sharp-tip GaN NW VFEDs exhibited decreased V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TO</sub> as the degree of sharpness ( d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> ) was increased from 0 to 0.5 μm. The saturation emitter current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> ) was only affected by variations in N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> , and was increased for increased N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> . The transfer (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> - V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GS</sub> ) characteristics of sharp-tip VFETs exhibited reduced threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> ), increased transconductance (G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tm, max</sub> ), and ON-/ OFF-current ratio, compared with the flat-tip ones. The output (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> - V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> ) characteristics of both types of devices showed nonlinear behavior at the low V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> regime.

Highly enhanced field emission properties of a carbon nanotube cathode on a titanium alloy substrate modified by alkali

Field emission electron sources with low turn-on electric fields, high emission current density, and good stability are of paramount importance in widespread applications, and in particular vacuum nanoelectronic equipment. Herein, an alkali modification approach was rationally designed to tailor the morphology of the commonly used compact and planar titanium alloy substrate. As expected, the modified substrate exhibited an interconnected porous network morphology, which offers remarkably strengthened adhesion with the screen-printed carbon nanotube cathode atop. As a result, the devices delivered comprehensively improved performance. Specifically, in the continuous driving mode, the turn-on electric field was as low as 0.79 V/μm, while the emission current of 85.13 mA/cm2 was relatively high. In parallel, in the pulse driving mode, the emission current density reached 1.266 A/cm2 under an electric field of 6.1 V/μm. Encouragingly, the cathode exhibited rather decent emission stability even under continuous operation for hours. This work provides a practical strategy for further enhancement of the field emission performance of carbon nanotube cathodes and highlights the significance of engineering titanium alloy substrates in future development.

A Carbon Nanotube-Based Hundred Watt-Level Ka-Band Backward Wave Oscillator

Carbon nanotube (CNT) cold-cathodes hold much promise in a variety of millimeter-wave and terahertz vacuum electronic radiation devices due to their inherent near instantaneous temporal turn-on and near-ideal ideal field electron emission performance. Here we report on the development of a CNT cold-cathode Ka -band backward-wave oscillator (BWO). Using a novel beam compression stage, theoretical studies, simulation results, and empirical findings collectively demonstrate that this device affords an unprecedentedly high output power of 230 W at a technologically important operating frequency of 33.65 GHz. The developed magnetic injection electron gun achieves a high emission current of 265.5 mA (emission current density of 188.3 mA/cm2) and a high focused beam current density of 18.5 A/cm2, which our studies suggest, is essential to the BWOs high output power.

Highly efficient field emission properties of vertically aligned 2D CuSe nanosheets: An experimental and theoretical investigation

We report the synthesis of klockmannite (CuSe) via a three-probe electrochemical set-up (chronoampereometry). The structural properties are examined by X-ray diffraction and Raman spectroscopy. Field emission scanning electron microscopy (FESEM) analysis revealed the formation of vertically aligned CuSe nanosheets with an average thickness of 34 nm and an average lateral size of 700 nm. The CuSe nanosheets exhibit impressive field electron emission characteristics with a turn-on field of 1.4 V/µm for 10 µA/cm2 emission current density. Also, a high current density of 5.8 mA/cm2 is observed at a relatively low applied field of 3.1 V/µm. Complementary first-principles DFT calculations show that CuSe displays metallic conductivity, and the (001) surface has a low work function of 5.12 eV, which is believed to be responsible for the impressive field emission characteristics.

A quantum mechanical model of electron field emission from two dimensional materials. Application to graphene

We implement a new time-independent perturbative quantum method to study quantitatively electron field emission from two dimensional materials and, in particular, from graphene. The Bardeen transfer Hamiltonian formalism is coupled to a detailed description of the electronic structure of the material. This calculation method is first validated on the standard Fowler–Nordheim (FN) model of a three-dimensional (3D) free-electron gas. Then, it is used to study emission from a two-dimensional (2D) free-electron gas and from graphene represented by a tight-binding model. In the case of graphene, we show that a full electronic band model of the material is necessary to obtain reasonable results because emission is not restricted to the vicinity of the Fermi level near the Dirac points. The graphene emitted current density follows a modified FN law with respect to the applied field, with a prefactor exponent for the field n≈1.5 intermediate between the one for the cases of 2D (n=0) and 3D (n=2) free-electron gases. However, the emitted current level is low because the kinetic energy of the electrons corresponds to a motion parallel to the emitting surface, which is not efficient in promoting emission. Our study gives a firm ground to the idea that emission from graphene results almost exclusively from defects.

Controlled decoration of palladium (Pd) nanoparticles on graphene nanosheets and its superior field emission behavior

Development in nanoscience and nanotechnology has inspired scientists to continuously explore new cathode material for field emission applications. In the present article, different weight (wt) % of Pd nanoparticles (Pd NPs) ensemble on graphene (Gr) nanosheets (Pd-Gr) were prepared via one step simple chemical route. The field emission (FE) properties of Pd decorated Gr nanosheets were studied. Structural and morphological analysis of as synthesized Pd-Gr composites were carried out by using X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscope (TEM). The FE properties of the Pd-Gr changed according to the variation in the wt% of Pd NPs. The Pd3-Gr field emitter showed better performance among all the emitters with lower turn on field, threshold field and maximum emission current density (J). The enhanced FE properties of Pd-Gr are due to the decoration of low work function Pd NPs. The FE results demonstrated herein suggest that, Pd-Gr based field emitters can open up many opportunities for their potential utilization in large area field emitters in various vacuum micro-nanoelectronic devices such as flexible field emission displays, portable X-ray and microwave tubes.

Highly Efficient Small Anode Ion Source

We describe some modifications to a Bernas-type ion source that improve the ion beam production efficiency and source operating lifetime. The ionization efficiency of a Bernas type ion source has been improved by using a small anode that is a thin rod, oriented along the magnetic field. The transverse electric field of the small anode causes the plasma to drift in the crossed ExB field to the emission slit. The cathode material recycling was optimized to increase the operating lifetime, and the wall potential optimized to suppress deposition of material and subsequent flake formation. A three-electrode extraction system was optimized for low energy ion beam production and efficient space charge neutralization. An ion beam with emission current density up to 60 mA/cm2 has been extracted from the modified source running on BF3 gas. Space charge neutralization of positive ion beams was improved by injecting electronegative gases.

A primary understanding of field emission from superhigh specific surface area activated carbon with interconnection structure

In this study, activated carbon (AC) with the specific surface area up to 3152 m2 g−1 is synthetized and structural-characterized. The AC powder displays the interconnective framework structure composed of numerous carbon nanosheets and the sharp edges act as the active sites to achieve efficient electron emission. The screen-printed AC cold cathode shows large emission current density of 15 mA cm−2 under relatively low electrical field of 6.0 V μm−1, with stable emission of up to 280 min. In conjunction to its uniform emission pattern, robustness, and vacuum-compatibility test, the feasibility of AC cold cathode in field emission devices have been demonstrated. This work provides a primary understanding of field emission performance for the AC electron emitter and expands the application fields of superhigh specific surface area AC.

The Parameters of the Field Emission Model and the Fabrication of Zinc Oxide Nanorod Arrays/Graphene Film

A large-scale growth of zinc oxide (ZnO) nanorod arrays on graphene sheets was fabricated by a hydrothermal technique, and the Fowler–Nordheim theory was used to build a model to describe the properties of the arrays’ field emission. The results indicated that the morphological characteristics of the ZnO nanorods grown on the graphene sheets can be easily tuned by varying the reaction time and concentrations of the reaction solution. The regular ordered ZnO nanorods arrays on the graphene sheets were obtained under the appropriate experimental conditions. Further, this composite cathode was demonstrated to possess excellent field emission properties due to the outstanding mechanical and electrical properties of graphene. The field emission current density of the composite cathode reached 1,448 μA cm–2 at the electric field of 16.5 V μm–1. The key parameter, field enhancement factor, reached 6,366, while the pure graphene cathode field is about 1,660. These specific nanorod arrays with enhancement of the field emission properties would be useful to sensor or modulator units for accessing networks.

Effects of Antimony Deposition on Field-Emission Current Density of Ge/Si

Abstract To estimate the field-emission current density of a Ge/Si heterosystem, 20-nm germanium/silicon (100) samples were grown by molecular beam epitaxy. The surface of one sample was covered with a layer of antimony, which was removed in vacuum prior to the samples being measured. A second sample of Ge/Si was exposed to room air in the absence of antimony. The current–voltage characteristics of both samples obtained by scanning tunneling microscopy (STM) were discovered to be in agreement with classical Fowler–Nordheim theory. The density of emission current from Ge nanocrystal exceeds the density of emission current from the wetting layer of Ge/Si. The density of emission current of pure Ge nanocrystal is less than the density of emission current of Ge nanocrystal with adsorption layers.

Electrical characterization of pulsed laser deposited high-k HfO2 nanoparticles on tapered Cu2O nanowires: promising cold cathode applications

Pulsed laser deposition (PLD) experiments were employed to deposit a thin adherent coating of HfO2 nanoparticles on electrochemically synthesized Cu2O nanowires. The as-deposited layer of HfO2 on Cu2O nanowires forms a hierarchical assembly of HfO2–Cu2O. The PLD process parameters were optimized to obtain the desired hierarchical morphology and a plausible growth mechanism has been exemplified based on observed SEM results and pertinent literature survey. In addition, field emission (FE) characteristics of the HfO2–Cu2O hierarchical assembly and pristine Cu2O nanowires were carried out at base pressure of 1 × 10–8 mbar. The HfO2–Cu2O nanowires hierarchical emitter exhibits superior FE behavior in terms of lowering in turn-on and threshold fields, delivery of two-fold higher emission current density at relatively lower applied field, as compared to the pristine Cu2O nanowire emitter. HfO2, despite being a wide band gap semiconductor (WBGS) possessing high dielectric constant, the enhanced FE behaviour is mainly attributed to the morphological aspect of hierarchical structure offering high aspect ratio. Furthermore, similar to other WBGS emitters like diamond, in this case too, low electron affinity is speculated to facilitate field emission. The observed results highlight the potential of PLD in fabricating hierarchical structures of WBGs onto other semiconducting oxide nanostructure for novel applications.

Enhanced field electron emission properties of graphite nanoplatelet paste emitters using graphite binder and graphite nanoparticle filler

Graphite nanoplatelet (GNP) paste field emitters fabricated using all-carbon-based materials, such as GNPs, graphite nanoparticles, graphite binders, and a graphite rod substrate have been demonstrated. The GNP field emitters on the graphite rod substrate showed an emission current density of 84.3 mA/cm2 and good emission stability with a degradation rate of less than 10%, indicating higher emission performance than other GNP field emitters. The enhanced field emission performance of the GNP field emitter is attributed to a low work function barrier between carbon materials, low contact resistance between GNPs and the graphite rod substrate, and good GNP paste bulk electrical conductivity. The new approach to fabricate GNP field emitters using all-carbon-based materials can be useful for various field emission devices.

A simple method to synthesize worm-like AlN nanowires and its field emission studies* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11774017 and 51761135129).

The worm-like AlN nanowires are fabricated by the plasma-enhanced chemical vapor deposition (PECVD) on Si substrates through using Al powder and N2 as precursors, CaF2 as fluxing medium, Au as catalyst, respectively. The as-grown worm-like AlN nanowires each have a polycrystalline and hexagonal wurtzite structure. Their diameters are about 300 nm, and the lengths are over 10 μm. The growth mechanism of worm-like AlN nanowires is discussed. Hydrogen plasma plays a very important role in forming the polycrystalline structure and rough surfaces of worm-like AlN nanowires. The worm-like AlN nanowires exhibit an excellent field-emission (FE) property with a low turn-on field of 4.5 V/μm at a current density of 0.01 mA/cm2 and low threshold field of 9.9 V/μm at 1 mA/cm2. The emission current densities of worm-like AlN nanowires each have a good stability. The enhanced FE properties of worm-like AlN nanowires may be due to their polycrystalline and rough structure with nanosize and high aspect ratio. The excellent FE properties of worm-like AlN nanowires can be explained by a grain boundary conduction mechanism. The results demonstrate that the worm-like AlN nanowires prepared by the proposed simple and the PECVD method possesses the potential applications in photoelectric and field-emission devices.

Arc erosion resistance of hybrid copper matrix composites reinforced with CNTs and micro-TiB2 particles

Synergistic strengthening is increasingly explored for developing high-performance copper matrix composite. In this work, the hybrid copper matrix composites reinforced with carbon nanotubes (CNTs) and micro- TiB2 particles（（CNTs-TiB2）/Cu composites）were prepared by electroless copper plating, powder metallurgy and hot extrusion. The arc erosion resistance of the hybrid copper matrix composites was studied in JF04C electrical contact testing system. And the synergistic effect of CNTs and TiB2 particles in different content ratios on the arc erosion resistance of composites was analyzed emphatically. The action mechanism of CNTs and TiB2 particles in arc erosion process is discussed in detail. Results show that the arc duration and arc energy of the hybrid copper matrix composites are lower than that of copper matrix composites single reinforced with CNTs or TiB2 particles. When the volume fraction of CNTs is 4.2% (The content of TiB2 particles is 1.6%), the arc erosion resistance of the hybrid copper matrix composites is optimal. In arc erosion process, CNTs can form a network structure parallel to the extrusion direction in copper matrix, and the continuous CNTs network structure can effectively disperse cathode arc, promote arc movement, increase work function of copper matrix composites, inhibit electron emission and reduce total emission current density. And the micro- TiB2 particles are helpful to improve the viscosity of molten pool, which can reduce the material mass loss and erosion pit on the surface of composites. Under the combined actions of CNTs and TiB2 particles (4.2CNTs-1.6TiB2)/Cu composite presents better arc erosion resistance stability, lower arc duration and energy.

Systematic growth of carbon nanotubes on aluminum substrate for enhanced field emission performance

For more than two decades, carbon nanotubes (CNTs) have shown great potential for a wide range of applications. Several methods are known to synthesize CNTs, though only a few of them are able to produce good quality and economically available CNTs. Chemical vapor deposition (CVD) is one of those methods that produce economically feasible and good quality CNTs onto specific substrates, even with nanopatterning. However, growing CNTs by CVD at temperatures below 700 °C remained a long-time challenge, as this meant keeping a host of low-melting materials out of bounds for direct CNT growth on them. In this work, CNTs have been synthesized directly onto a low-melting, conducting substrate, aluminum, by thermal CVD, at a temperature as low as 550 °C and up to as high as 650 °C (just below the melting point of aluminum). The diameters of the grown CNTs were observed to be influenced by process parameters, e.g., temperature and pressure. The effect of synthesis parameters on CNT diameters was verified by scanning electron microscopy and transmission electron microscopy. The quality of the CNTs was checked by Raman spectroscopy, selected area electron diffraction pattern of transmission electron microscopy, and XPS. It was observed that an increase in temperature and pressure had a significant effect on the diameters of the CNTs. Randomly entangled CNTs were measured to have an average diameter of 28 nm at 550 °C and one atmospheric (760 Torr) pressure, whereas it was observed to be 78 nm at a temperature of 650 °C and pressure of 0.01 Torr. The field emission response, i.e., the turn-on field (2.5 V/μm) and the maximum emission current density (2.17 mA/cm2) of the CNTs synthesized at the temperature of 550 °C and pressure of 1 atm (760 Torr) was found to be excellent.

Origin of Monochromatic Electron Emission From Planar-Type Graphene/ h -BN/ n -Si Devices

We previously reported highly monochromatic electron emission from the planar-type electron emission devices based on a graphene/hexagonal boron nitride (h-BN) heterostructure. In this paper, the electron energy distribution (EED) of these devices is examined to clarify the mechanism of monochromatic electron emission. We find that the monochromaticity of the electron beam depends significantly on the electronic structure of the substrate material; for the devices with an n-type silicon substrate, the narrowest FWHM of the electron beam is 0.18 eV, whereas that of devices with a metallic (Nb) substrate is 0.33 eV. At the same time, simulations considering the electron scattering by phonons acceptably reproduced the shape of each EED spectrum considering the small energy loss due to out-of-plane acoustic phonon modes in h-BN. Thus, the monochromatic electron emission from the $\mathrm{graphene}/h$-$\mathrm{BN}/n$-$\mathrm{Si}$ device is ascribed to a combination of the narrow energy distribution of electrons at the conduction band of the n-$\mathrm{Si}$ substrate and small phonon energy of the h-BN insulating layer. These features also realize the excellent emission properties in addition to the monochromaticity of the beam, such as a high emission current density of $9.3{\mathrm{A}/\mathrm{cm}}^{2}$, insensitivity to environmental pressure up to 10 Pa, and long lifetime of more than 7 days with little decay.

Enhanced field emission performance of MXene-TiO2 composite films.

A facile method to produce an MXene-TiO2 composite is demonstrated for enhanced field emission display applications. The field emission performance of two-dimensional free-standing and linear-shaped field emitters has been systematically investigated and enhanced electron emission behaviors (e.g. emission current, stability and emission patterns) are achieved by compositing MXene and TiO2 nanowires. The relationship between the emission current density, electric field and anode-cathode gap distance is studied and the emitters, especially the cross-section of the composite film, show good performance. The emission current from the cross-section of the composite film can reach 289 mA cm-2, which is the best result of the state of the art compared to single MXene and TiO2 nanowires. We have also reported a triboelectric nanogenerator powered by free-standing MXene-TiO2 composite emitters, implying the feasibility of the self-powering field emission devices and possibly enlarging the applications of cold emitters in various fields.

Surface modification via silver nanoparticles attachment: An ex-situ approach for enhancing the electron field emission properties of CNT field emitters

A novel one-dimensional carbon material Known as carbon nanotubes (CNTs), having their high aspects ratio and as well as advance electrical properties which make them best candidate for cathode field emitters. In this work, we have introduced the easy and cost-effective approach to improving the CNT field emitters emission properties. Here we have modified the surface of CNT field emitter via decorating with silver nanoparticles (Ag NPs). Attachment of Ag NPs on CNT surface via thermal evaporation technique. The as-prepared samples of pristine CNTs and decorated CNTs with Ag NPs were characterized by the scanning electron microscope for morphology analysis, quality and qualitative analysis done by Raman spectroscopy. Here we have applied the Fowler Nordheim hypothesis for the analysis of electron field emission of as-prepared cathode field emitters. The turn-on (Eto) and threshold (Eth) field reduced after CNT field emitters coated with Ag NPs. The field enhancement factor (β) and emission current density (J) enhanced after coating. Ag NPs decorated/coated CNT field emitters can be utilized in field emission device applications.