High Emission Current Density Research Articles

A Study on the Field Emission Characteristics of High-Quality Wrinkled Multilayer Graphene Cathodes.

Field emission (FE) necessitates cathode materials with low work function and high thermal and electrical conductivity and stability. To meet these requirements, we developed FE cathodes based on high-quality wrinkled multilayer graphene (MLG) prepared using the bubble-assisted chemical vapor deposition (B-CVD) method and investigated their emission characteristics. The result showed that MLG cathodes prepared using the spin-coating method exhibited a high field emission current density (~7.9 mA/cm2), indicating the excellent intrinsic emission performance of the MLG. However, the weak adhesion between the MLG and the substrate led to the poor stability of the cathode. Screen printing was employed to prepare the cathode to improve stability, and the influence of a silver buffer layer was explored on the cathode's performance. The results demonstrated that these cathodes exhibited better emission stability, and the silver buffer layer further enhanced the comprehensive field emission performance. The optimized cathode possesses low turn-on field strength (~1.5 V/μm), low threshold field strength (~2.65 V/μm), high current density (~10.5 mA/cm2), and good emission uniformity. Moreover, the cathode also exhibits excellent emission stability, with a current fluctuation of only 6.28% during a 4-h test at 1530 V.

Surface modification of a biomass-derived self-supported carbon nano network as an emerging platform for advanced field emitter devices and supercapacitor applications.

Herein, a self-supported carbon network is designed through the sole pyrolysis of Carica papaya seeds (biomass) without any activation agent, demonstrating their field emission and supercapacitor applications. The pyrolysis of seeds in an argon atmosphere leads to the formation of interconnected, rod-like structures. Furthermore, the hydrofluoric acid treatment not only removed impurities, but also resulted in the formation of CaF2 nanocrystals with the addition of F-doping. From the field emission studies, the turn-on field values defined at an emission current density of ∼10 μA cm-2 were found to be ∼2.16 and 1.21 V μm-1 for the as-prepared carbon and F-doped carbon, respectively. Notably, F-doped carbon exhibits a high emission current density of ∼9.49 mA cm-2 and has been drawn at an applied electric field of ∼2.29 V μm-1. Supercapacitor studies were carried out to demonstrate the multi-functionality of the prepared materials. The F-doped carbon electrode material exhibits the highest specific capacitance of 234 F g-1 at 0.5 A g-1. To demonstrate the actual supercapacitor application, the HFC//HFC symmetric coin cell supercapacitor device was assembled. The overall multifunctional applicability of the fabricated hybrid structures provides a futuristic approach to field emission and energy storage applications.

Novel low-macroscopic-field emission cathodes for electron probe spectroscopy systems

The current state of electron probe methods [including energy loss spectroscopy of inelastically scattered electrons (EELS)] is considered. The analysis concerning the issues of their application, challenges, and limitations is performed. Particular attention is paid to the fundamental limitations and the means to overcome those during electron probe methods’ subsequent development for the study of composite nanostructured materials. It is emphasized that the emitted electron energy spectrum dispersion (or electron energy distribution function width) is one of the main factors limiting a further increase in EELS energy resolution, although the use of direct detection sensors and monochromators allows one to approach the physical limit of this method. Novel low-macroscopic-field electron emitters are synthesized and investigated. Their properties are analyzed and compared with previously obtained specimens. Both energy and temporal resolutions of an EEL system utilizing the suggested cathodes are estimated. The comparison of its characteristics with the corresponding parameters of classical facilities is performed. The obtained results indicate the possibility to achieving a significant growth in energy and temporal resolutions as well as a decrease in the detection threshold of chemical elements with trace concentrations while maintaining relatively high emission current density values.

An enhanced field emission for display devices arises from the assembling of ZnO@MOF/MWCNTs

In this study, we propose a new, simple technique for raising the efficiency of field emission devices based on metal organic framework composites. Multi-walled carbon nanotubes (MWCNTs) and ZnO are used in the fabrication of metal-organicframeworks (MOFs) in this technique. This is explained by the composites unique structural structure, which offers a lot of easily available electro active sites, low surface work function, and improved electronic conductivity. One of the less complicated and more economical synthesis methods is used to synthesize the ZnO@MOF/MWCNTs composite. The material's precise morphological and structural features have all been investigated using Raman, FE-SEM, EDX, FTIR, and UV–Visible investigations. The ZnO@MOF/MWCNTs field emitter show impressive and stable field emission properties, such as a low turn-on electric field 0.50 V/μm and a high emission current density 219.15 mA/cm2.Our research revealed an easier way for boosting the electron field emission utilized in different field emission display systems that are based on MOFcomposites.

Reducing Screening Effect in Close-Packed Si Field Emitter Arrays

Electric field screening effect in close-packed semiconductor field emitter arrays (FEAs) changes the current distribution among emitter tips, such that those on the edges of the array emit much higher current than those in the middle, leading to poor reliability characteristics. Using analysis, numerical simulations, and experimental verification, three different techniques to suppress electric field screening and achieve uniform current distribution are proposed. These techniques are: 1) utilizing very sharp emitter tips; 2) reducing the anode–cathode distance (ACD) to be in the order of emitter pitch; and 3) using extremely low-doped but long field emitter tips. By adopting these techniques together, a more uniform emission from the array and a high emission current density can be achieved. FEAs with 160 K emitter tips with different ACDs and emitter sharpness were fabricated using electron beam lithography. In an array with sharp emitter tips, reliable emission with a maximum current of 0.7 mA (4.4 nA/tip) and a maximum current density of 1.75 A/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{2}}$</tex-math> </inline-formula> was achieved.

Electric-Field Emission Mechanism in Q-Carbon Field Emitters.

In this paper, we report the excellent field emission properties of Q-carbon and analyze its field emission characteristics through structural, morphological, and electronic property correlations, supported by density functional theory (DFT) simulation studies. The Q-carbon field emitters show impressive and stable field emission properties, such as a low turn-on electric field of ∼2.38 V/μm, a high emission current density of ∼33 μA/cm2, and a critical field of ∼2.44 V/μm for the transition from a linear region to the saturation region in the F-N plot. The outstanding field emission properties of Q-carbon are attributed to (i) a unique sp2/sp3 mixture in Q-carbon, (ii) sp2-bonded highly conductive amorphous carbon-rich channels inside the Q-carbon cluster, (iii) a large local field enhancement due to the local geometry and microstructure of Q-carbon, and (iv) the presence of sp2-bonded amorphous carbon regions in the composite film. The temperature-dependent field emission properties, such as extreme sensitivity and an enhancement in the emission current density with temperature, can be explained by the local density of states near the Fermi level and the excellent thermal stability of the Q-carbon field emitters. From DFT simulation studies, the computed work function and the field-enhancement factor were determined to be 3.62 eV and ∼2300, respectively, which explains the excellent field emission characteristics of Q-carbon. The obtained field emission properties, in most cases, were superior to those from other carbon/diamond-based field emitters, which will open new frontiers in field emission-based electronic applications.

Directly tuning the surface morphologies and electron pathway of graphite/diamond composite films for enhanced electron field emission

Cathode electron field emitters are widely used in high-power and high-frequency vacuum electron devices. The development of novel technique to fabricate the emitters with high electron field emission performance has stimulated extensive research. Here we present a unique one-step microwave plasma chemical vapor deposition process to fabricate the graphite/diamond composite films. The gaseous methane is replaced by liquid n-butylamine, which acts as the single reactive carbon source. The components and surface morphologies of the graphite/diamond composite films can be effectively tuned upon changing the deposition temperatures. The vertically oriented graphite/diamond composite film with a graphite-diamond-graphite nanostructure is obtained and shows superior electron field emission performance of low turn-on field (E0 = 4.0 V/µm) and high emission current density (J@8.8 V/µm = 13.0 mA/cm2). Our study reveal that the sharp edges, appropriate distribution density of the nanosheets, as well as good electron pathway from the bulk to the surface are the key ingredients for the electron field emission performance. This work on graphite/diamond composite films provides a new carbon source instead of methane to fabricate the emitters with high electron field emission performance, which may promote the industrial-scale application.

Physics based model of an AlGaN/GaN vacuum field effect transistor

A vacuum field effect transistor (VacFET) is proposed that consists of a modification of a conventional AlGaN/GaN high electron mobility transistor to include a nanogap near the gate on either the source (cathode) or drain (anode) side of the device. The current flowing through the two-dimensional electron gas (2DEG) under the gate is obtained using a charge-control model, which is forced to be equal to the tunneling current across the nanogap. The latter is modeled using a modified version of Simmons tunneling theory of a metal–insulator–metal junction to include the effect of barrier lowering across the nanogap. When compared to other recently fabricated VacFETs, the proposed device has potential for much higher emission current densities and transconductance levels, of the order of several hundreds of mA/mm and tens of mS/mm, respectively. For similar material parameters and physical dimensions, the proposed VacFET has a turn-on voltage that depends on the location of the nanogap on either the source or drain side of the gate. It is shown that the current–voltage characteristics of VacFETs with a nanogap either on the drain or source side of the gate are highly sensitive to their physical parameters and biasing conditions, making them a very strong candidate for chemical or gas sensing applications. This is due to the sensitivity of the tunneling current to the effective barrier height and field enhancement factor of the nanogap.

Field emission properties of tetrapod-shaped ZnO/TiN point light source with semicircle reflecting anode

A novel point light source composed of tetrapod-shaped ZnO/TiN (T-ZnO/TiN) cathode and semi-circular groove reflective anode is demonstrated. The T-ZnO/TiN cathode is fabricated on nickel wire by spraying T-ZnO NRs powder and subsequent sputtering of TiN film. Semicircular reflective anodes with different opening sizes were prepared by wire electro-discharge machine. The field-emission properties of point light sources could be dramatically enhanced under semicircle reflecting anode by decreasing the threshold field from 2.61 to 2.10 V/μm. The point light source of D1500 has an extremely high emission current density of 132.73 mA/cm2 at 3.53 V/μm. The value of Jmax is about 2.7 times to that of D3000 (∼47.89 mA/m2 at 4.18 V/μm). Experiments and ANSYS electric field simulation show that the combination of low work function hybrid cathode and semicircular reflective anode structure leads to the increase of local field concentration and the enhancement of field emission characteristics. As far as we know, this point light source with semicircular reflective anode is prepared and applied for the first time. The results show that the point light source structure proposed here can not only effectively reduce the threshold field, but also improve the emission current density and luminescence uniformity.

Influence of different temperatures on morphology and chemical state of the surface-active layer of scandium‑tungsten cathode

Scandium‑tungsten (ScW) cathodes have garnered a lot of research attention due to their high emission current density. Herein, the cathode working environment is simulated by heating in a vacuum chamber, and scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) are used to study the surface morphology and chemical oxidation states in the temperature range of 25 °C to 1300 °C. The results reveal that the surface of the tungsten matrix is obviously recrystallized with increasing temperature. Moreover, the scandium oxide becomes globular and clear depressions or holes occur in the aggregation region of the rare-earth oxide at the grain boundaries. The work function is measured by the Richardson method at room temperature, 1000 °C, and 1300 °C and is found to be 2.77, 2.72, and 2.88 eV, respectively. The work function of the cathode initially decreases with increasing temperature, followed by a gradual increase. This is due to the fact that active substances such as scandium oxide are severely lost at high temperatures, destroying the surface structure of the sample. As the Sc/W value decreases, the rate of depletion of active material from the surface is greater than the rate of replenishment of material to the surface.

High-Performance Cold Cathode X-ray Tubes Using a Carbon Nanotube Field Electron Emitter.

A cold cathode X-ray tube was fabricated using a carbon nanotube (CNT) field electron emitter made by a free-standing CNT film which is composed of a highly packed CNT network. A lot of CNT bundles with a sharp tip are vertically aligned at the edge of the thin CNT film with a length of 10 mm and a thickness of 7 μm. The cold cathode X-ray tube using the CNT field emitter presents an extremely high tube current density of 152 A/cm2 (corresponding to tube current of 106.4 mA), the electron beam transmittance of 95.2% and a small focal spot size (FSS) of 0.5 mm. In addition, the cold cathode X-ray tube also shows stable lifetime during 100 000 shots. High emission current density of the cold cathode X-ray tube is mainly attributed to a lot of electron emission sites at an edge of the CNT film. The small FSS is caused by an ensemble of the CNT field electron emitter made by a free-standing thin CNT film and the optimized curve-shape elliptical focusing lens. Based on obtained results, the cold cathode X-ray tube can be widely used for various X-ray applications such as medical diagnosis systems and security check systems in the future.

Nanocrystalline high-entropy hexaboride ceramics enable remarkable performance as thermionic emission cathodes

The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential, yet relevant studies have been rarely reported. In this work, we report nanocrystalline (La0.25Ce0.25Nd0.25Eu0.25)B6 high-entropy rare-earth hexaboride (HEReB6-1) ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time. The as-fabricated samples exhibited a highly dense (96.3%) nanocrystalline (94 nm) microstructure with major (001) fiber textures and good grain boundaries without any impurities, resulting in a remarkable mechanical, electrical, and thermionic emission performance. The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures; these were greater than the average values of corresponding binary rare-earth hexaborides, such as a Vickers hardness of 23.4 ± 0.6 GPa and a fracture toughness of 3.0 ± 0.4 MPa•m1/2 at room temperature. More importantly, they showed high emission current densities at elevated temperatures, which were higher than the average values of the corresponding binary rare-earth hexaborides. For instance, the maximum emission current density reached 48.3 A•cm−2 at 1873 K. Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes.

The Research on Backward Wave Oscillator with Wide Tunable Bandwidth and High Power

A continuous wave backward wave oscillator is studied in this paper. The design of the slowing wave system is completed through theoretical calculation and numerical simulation, and the slowing wave system is manufactured through precision machining. In order to improve the output power of the electron tube, a high resistance diode electron gun is used to produce a sheet electron beam with high emission current density. The test shows that the developed BWO can work in the frequency range of 250 ~ 310 GHz, and the output power is 20 mW to 42 mW.

Electron emission mechanism of scandium‑tungsten cathodes

Scandium‑tungsten (Sc-W) cathodes have garnered a lot of research attention due to their high emission current density. This study investigates the electronic properties of Sc-W cathodes using the VASP code to explain the operating mechanism of hot electron emission. The results reveal that the work function is decreased by 0.58 eV, whereas the charge density in the crystal structure, the number of high valence electrons and the energy of electrons increase when Sc content is increased from 1% to 2%. Moreover, the bandgap and energy required for electrons to jump from the valence band to the conduction band decreased with the increase of Sc content, enhancing the current emission density and reducing the work function of the cathode.

Preparations of lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6)

AbstractDue to strong anti‐poisoning ability, good emission stability, high emission current density, lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) have been maturely applied in electron emission emitter. In this paper, a new manufacturing method for LaB6 (or CeB6) powder was proposed by using La2O3 (or CeO2), B4C, and Al as raw materials. After high‐temperature reaction in the range of 1673–1773 K and the following alkaline leaching at 90°C, LaB6 or CeB6 powder with particle size of about 10 μm was obtained. Furthermore, by Al metal flux method, the obtained powder was used to manufacture single crystal block with size of several millimeters.

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.

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.

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.

Electrohydrodynamic emitters of ion beams

We discuss physical processes underlying the generation of ion beams with high emission current density in electrohydrodynamic (EHD) emitters based on liquid metals and alloys and with room temperature ionic liquids. We consider EHD effects that influence the emission of ions (ion production mechanisms) and the kinetics of ion interactions in high-density beams. We analyze the factors determining the emission zone size, sustainability of emission at high and low currents, generation of clusters, increase in energy spread, decrease in brightness, and other features of ion beams. We consider the specific design features of EHD emitters and the problems of practically ensuring their stable operation. Discussed in detail are modern application areas for ion sources with EHD emitters, including technological installations for ion beam lithography, micro- and nanopatterning, ion microscopes and tools for local mass spectrometry of secondary ions, and systems to control and neutralize the potential of spacecraft as well as electrostatic rocket engines (microthrusters). We analyze prospects for further development of EHD emitters themselves and instruments based on them.

Direct synthesis of micropillars of vertically aligned carbon nanotubes on stainless-steel and their excellent field emission properties

Vacuum micro/nano-electronics can greatly benefit from the improved emission current density and temporal stability of the electron beam due to the morphological control and direct contact of cold field-emitters on a conducting substrate. In this work, high-performance vertically aligned carbon nanotube (VACNT) pillar-based field-emitter arrays (VACNT-P-FEAs) were synthesized directly on stainless-steel (SS) via plasma-enhanced chemical vapor deposition. Nanosphere lithography was employed to limit the growth sites of VACNTs and form VACNT-P-FEAs of different sizes on the SS substrate. The field emission (FE) properties of the VACNT-P-FEAs were dependent on the size of the pillar emitters. Remarkable FE properties, such as low turn-on electric field (ETo = 1.57 V/μm), low threshold electric field (ETh = 2.94 V/μm), high field enhancement factor (β = 4977), and high emission current density (∼33 mA/cm2 at a field of 4 V/μm) were observed from the VACNT-P-FEAs grown using polystyrene microspheres (450 nm diameter) for lithography. The excellent FE performance can be ascribed to the inherent properties of the sample, such as the high conductivity of the SS substrate, low contact resistance between the SS substrate and VACNTs, the suitable number of active VACNT pillar field-emitters, and the reduced screening effect between the VACNT pillars.