High Emission Current Density Research Articles

Theoretical Study of a 0.22 THz Backward Wave Oscillator Based on a Dual-Gridded, Carbon-Nanotube Cold Cathode

The carbon nanotube (CNT) cold cathode is an attractive choice for millimeter and terahertz vacuum electronic devices owning to its unique instant switch-on and high emission current density. A novel, dual-gridded, field emission architecture based on a CNT cold cathode is proposed here. CNTs are synthesized directly on the cathode surface. The first separating grid is attached to the CNT cathode surface to shape the CNT cathode array. The second separating grid is responsible for controlled extraction of electrons from the CNT emitters. The cathode surface electric field distribution has been improved drastically compared to conventional planar devices. Furthermore, a high-compression-ratio, dual-gridded, CNT-based electron gun has been designed to further increase the current density, and a 21 kV/50 mA electron beam has been obtained with beam transparency of nearly 100%, along with a compression ratio of 39. A 0.22 THz disk-loaded waveguide backward wave oscillator (BWO) based on this electron gun architecture has been realized theoretically with output power of 32 W. The results indicate that higher output power and higher frequency terahertz BWOs can be made using advanced, nanomaterial-based cold cathodes.

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.

Field emission properties of nano-structured cobalt ferrite (CoFe2O4) synthesized by low-temperature chemical method

We report on the field-emission properties of structure-morphology controlled nano-CoFe2O4 (CFO) synthesized via a simple and low-temperature chemical method. Structural analyses indicate that the spongy-CFO (approximately, 2.96 nm) is nano-structured, spherical, uniformly-distributed, cubic-structured and porous. Field emission studies reveal that CFO exhibit low turn-on field (4.27 V/μm) and high emission current-density (775 μA/cm2) at a lower applied electric field of 6.80 V/μm. In addition, extremely good emission current stability is obtained at a pre-set value of 1 μA and high emission spot-density over large area (2 × 2 cm2) suggesting the applicability of these materials for practical applications in vacuum micro-/nano-electronics.

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.

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.

High-current field emission from “flower-like” few-layer graphene grown on tip of nichrome (8020) wire

ABSTRACTWe report a novel tip-type field emission (FE) emitter by synthesizing the few-layer graphene (FLG) flakes on tip of nichrome (8020) wire (ϕ≈80 μm) by microwave plasma enhanced chemical vapor deposition(PECVD). These resultant random arrays of free-standing FLG flakes are aligned vertically to the substrate surface in a high-density and stacked to each other to form several larger “flower-like” agglomerates in spherical shapes. The FE performance of the tip-type FLG flakes emitter shows a low threshold field of 0.55 V/μm, a large field enhancement factor of 9455 ± 46, a large field emission current density of 22.18 A/cm2 at 2.70 V/μm, and an excellent field emission stability at high emission current densities (6.93 A/cm2). It can be used in variety of applications that include cathode-ray tube monitors, X-ray sources, electron microscopes, and other vacuum electronic applications.

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.

High current density and low emission field of carbon nanotube array microbundle

Field electron emission from carbon nanotubes shows promising application potential in devices. Low adhesive bonding strength between the carbon nanotubes and the substrate presents a practical challenge in environments such as high field emission current density. In this paper, we report on the performance of a carbon nanotube microbundle attached to a tungsten needle by a uniform glue layer. The device is easily fabricated without complex fixture and possesses a complete array structure. After curing the glue in air, the adhesive strength between the carbon nanotubes and the needle is over 2000 N/cm2. Field emission measurements demonstrate that the maximum current density of the emitters is over 20 A/cm2 under a low applied field (&amp;lt;0.4 V/μm). The emitter with high current density, low emission field, and good stability has a wide range of applications in flat displays, X-ray tubes, and lamps.

High-performance field emission device utilizing vertically aligned carbon nanotubes-based pillar architectures

The vertical aligned carbon nanotubes (CNTs)-based pillar architectures were created on laminated silicon oxide/silicon (SiO2/Si) wafer substrate at 775 °C by using water-assisted chemical vapor deposition under low pressure process condition. The lamination was carried out by aluminum (Al, 10.0 nm thickness) as a barrier layer and iron (Fe, 1.5 nm thickness) as a catalyst precursor layer sequentially on a silicon wafer substrate. Scanning electron microscope (SEM) images show that synthesized CNTs are vertically aligned and uniformly distributed with a high density. The CNTs have approximately 2–30 walls with an inner diameter of 3–8 nm. Raman spectrum analysis shows G-band at 1580 cm−1 and D-band at 1340 cm−1. The G-band is higher than D-band, which indicates that CNTs are highly graphitized. The field emission analysis of the CNTs revealed high field emission current density (4mA/cm2 at 1.2V/μm), low turn-on field (0.6 V/μm) and field enhancement factor (6917) with better stability and longer lifetime. Emitter morphology resulting in improved promising field emission performances, which is a crucial factor for the fabrication of pillared shaped vertical aligned CNTs bundles as practical electron sources.

Self-organized multi-layered graphene-boron-doped diamond hybrid nanowalls for high-performance electron emission devices.

Carbon nanomaterials such as nanotubes, nanoflakes/nanowalls, and graphene have been used as electron sources due to their superior field electron emission (FEE) characteristics. However, these materials show poor stability and short lifetimes, which prevent their use in practical device applications. The aim of this study was to find an innovative nanomaterial possessing both high robustness and reliable FEE behavior. Herein, a hybrid structure of self-organized multi-layered graphene (MLG)-boron doped diamond (BDD) nanowall materials with superior FEE characteristics was successfully synthesized using a microwave plasma enhanced chemical vapor deposition process. Transmission electron microscopy reveals that the as-prepared carbon clusters have a uniform, dense, and sharp nanowall morphology with sp3 diamond cores encased by an sp2 MLG shell. Detailed nanoscale investigations conducted using peak force-controlled tunneling atomic force microscopy show that each of the core-shell structured carbon cluster fields emits electrons equally well. The MLG-BDD nanowall materials show a low turn-on field of 2.4 V μm-1, a high emission current density of 4.2 mA cm-2 at an applied field of 4.0 V μm-1, a large field enhancement factor of 4500, and prominently high lifetime stability (lasting for 700 min), which demonstrate the superiority of these materials over other hybrid nanostructured materials. The potential of these MLG-BDD hybrid nanowall materials in practical device applications was further illustrated by the plasma illumination behavior of a microplasma device with these materials as the cathode, where a low threshold voltage of 330 V (low threshold field of 330 V mm-1) and long plasma stability of 358 min were demonstrated. The fabrication of these hybrid nanowalls is straight forward and thereby opens up a pathway for the advancement of next-generation cathode materials for high brightness electron emission and microplasma-based display devices.

Synthesis and field electron emission properties of multi-walled carbon nanotube films directly grown on catalytic stainless steel substrate

The multi-walled CNT films were directly grown on stainless steel substrates using an oxidation-reduction treatment with chemical vapor deposition. Microstructure and field emission properties of the as-received CNT films were detailedly investigated. The results indicate that the CNTs have originally random orientation with relative high density, good crystalline perfection and large diameter. Besides, the CNT films exhibit superior field electron emission performances with low threshold electric field (∼4.2 V/μm) and very high emission current density (∼24 mA/cm2). Importantly, the CNTs directly grown on stainless steel substrates have remarkable long-term stability with ∼3.4% fluctuation over 50 h continuous measurement under high current density, ∼10 mA/cm2. The perfect graphitization, large field enhancement factor, low contact resistance, good adhesion and high thermal conductivity are believed to be principally responsible for their superior field emission properties, which make them very promising application as an electron source in microthrust space electric propulsion neutralizer.

Highly improved field emission from vertical graphene–carbon nanotube composites

Few-layer vertical graphene (VG) was prepared on arrayed carbon nanotubes (CNTs) without a catalyst by using radio frequency sputtering deposition and microwave plasma enhanced chemical vapor deposition, and the field emission (FE) performance of VG–CNT composites was tested and optimized. The FE performance is found to be strongly dependent on the heights (but not the actual heights) and densities of CNTs, the shapes of VG, and the diameters of VG–CNT composites. The optimal shapes for FE are demonstrated to be CNTs with middle densities and small diameters and VG with high densities and small sizes. The optimal FE performance from our large area VG–CNT composites is found to have a low threshold field of 1.28 V/μm (at 10 mA/cm2), a high maximum emission current density of 87.63 mA/cm2 (4.28 times higher than that from our previously prepared graphene–CNT composites), and excellent stability at an extremely high mean emission current density of 36.28 mA/cm2 over a period of 50 h, far better than those of pristine CNTs, pure VG, and shape non-optimized VG–CNT composites.

Development of multi-pixel x-ray source using oxide-coated cathodes

Multiple pixel x-ray sources facilitate new designs of imaging modalities that may result in faster imaging speed, improved image quality, and more compact geometry. We are developing a high-brightness multiple-pixel thermionic emission x-ray (MPTEX) source based on oxide-coated cathodes. Oxide cathodes have high emission efficiency and, thereby, produce high emission current density at low temperature when compared to traditional tungsten filaments. Indirectly heated micro-rectangular oxide cathodes were developed using carbonates, which were converted to semiconductor oxides of barium, strontium, and calcium after activation. Each cathode produces a focal spot on an elongated fixed anode. The x-ray beam ON and OFF control is performed by source-switching electronics, which supplies bias voltage to the cathode emitters. In this paper, we report the initial performance of the oxide-coated cathodes and the MPTEX source.

Pulsed laser deposition of tin oxide thin films for field emission studies

A comparative study of Pulsed Laser Deposition (PLD) based Tin Oxide (SnO2) thin films deposited at various substrate deposition temperature (Ts) has been performed. Surface morphology of the films was studied by Field Emission Scanning Electron Microscopy (FESEM) and surface composition of the films by X-ray PhotoelectronSpectroscopy (XPS) technique. X-ray diffraction (XRD) technique has been used to investigate crystalline nature of the deposited films. Field Emission (FE) properties of the SnO2 films were measured and a significantly low turn on field (2.1V/μm) (field necessary to draw an emission current density of 10μA/cm2) for films deposited at high substrate temperature (700°C) was observed. Field enhancement factor estimated from FE studies was found to strongly depend on the surface morphology of the films. Overall good field emission current stability was observed for all SnO2 films. Dependence of FE properties on surface morphology, surface composition and deposition environment has been observed and analyzed systematically. Significantly low turn on field with high emission current density and field enhancement factor exhibited by films deposited when substrate was maintained at 700°C has been mainly correlated to surface morphology and surface composition.

Architecture of 3D ZnCo2O4 marigold flowers: Influence of annealing on cold emission and photocatalytic behavior

The present work demonstrates the field emission characteristics and photocatalytic behavior of ZnCo2O4 marigold flowers synthesized via a facile hydrothermal method. The effect of annealing of these 3D porous hierarchical nanostructures on field emission and photocatalytic performances is studied. When compared with the as-synthesized sample, annealed ZnCo2O4 exhibits a ∼2-fold improvement in photocatalytic activity for methylene blue (MB) degradation under visible light irradiation. The turn-on, threshold fields and high emission current densities are also strongly influenced as a result of annealing. The turn on field required to draw an emission current density of ∼1 μA/cm2 is found to be ∼2.4 and ∼1.8 V/μm for as-synthesized and annealed ZnCo2O4 marigold flowers, respectively. Field-emission measurements demonstrate remarkably large field enhancement and better stability for annealed samples. The X-ray diffraction and Raman analysis reveal that annealing improves the crystallinity and also help to remove the structural defects in ZnCo2O4. The enhancement in the field emission and photocatalytic activity of annealed ZnCo2O4 marigold flowers is attributed to the modification of electronic properties as a result of dehydration, crystallite growth and reduced surface defects/impurity phases.

Enhanced Field Emission Characteristics of a 3D Hierarchical HfO2‐ZnO Heteroarchitecture

AbstractThree dimensional (3D) HfO2‐ZnO heteroarchitecture comprised of thin coating of HfO2 on self assembled 3D ZnO urchins with pointed apex has been synthesized using hydrothermal route followed by Pulsed Laser Deposition (PLD). The as‐synthesized HfO2‐ZnO heteroarchitecture was characterized using XRD, SEM, EDS, and (HR) TEM, in order to reveal its structural, morphological, and chemical properties. The HfO2‐ZnO heteroarchitecture emitter exhibits superior field emission (FE) behaviour in contrast to the pristine ZnO urchins, demonstrated by delivery of high emission current density of ∼ 885 μA/cm2 at an applied field of ∼ 3.35 V/μm, against ∼383 μA/cm2 at an applied field of ∼ 4.32 V/μm for the pristine ZnO urchins emitter. Interestingly, the HfO2‐ZnO heteroarchitecture emitter exhibits excellent emission current stability characterized with fewer fluctuations, owing to very good ion‐bombardment resistance offered by the HfO2 coating. Furthermore, the heteroarchitecture thus obtained facilitates tailoring of the morphology with high aspect ratio and modulation of electronic properties as well, thereby enhancing the FE behaviour.Despite HfO2 being wide band gap and high‐k material, the HfO2‐ZnO heteroarchitecture exhibits potential as promising candidate for fabrication of high current density cold cathode

Extraction of the Very High Tunneling Current and Extremely Stable Emission Current from GdB6/W‐Tip Source Synthesized Using Arc Plasma

AbstractHerein, we report the one step arc plasma synthesis of the GdB6 (Gadolinium hexaboride) nanoparticles and its field emission (FE) characteristics on tungsten point substrate (GdB6/W). The SEM / TEM analysis revealed the GdB6 nanoparticles on W point substrate exhibit irregular shaped, grainy, dense, course morphology, i. e. uniformly covering the entire tip substrate surface. For GdB6/W point source, the values of the turn‐on and threshold fields, defined as field required to draw an emission current density ∼1 mA/cm2 and ∼100 mA/cm2, respectively are found to be ∼ 2.2 and ∼2.7 V/μm, for anode‐cathode separation of ∼1 mm. Interestingly, a very high emission current density of ∼3.5 A/cm2 has been drawn from the GdB6/W point emitter at relatively lower applied field of ∼6.4 V/μm. The field enhancement factor found to be ∼10,250. The GdB6/W point electron source exhibits a good emission current stability at ∼10 mA for a period of 6 hr. The emission current stability is enumerated in terms of standard deviation and its magnitude has been measured to be only 1.72 % with respect to the average value. The superlative field emission characteristics signify the GdB6/W point electron source as potential candidates for vacuum micro/nanoelectronics device applications.

Pulsed laser-deposited nanocrystalline GdB6 thin films on W and Re as field emitters

Gadolinium hexaboride (GdB6) nanocrystalline thin films were grown on tungsten (W), rhenium (Re) tips and foil substrates using optimized pulsed laser deposition (PLD) technique. The X-ray diffraction analysis reveals formation of pure, crystalline cubic phase of GdB6 on W and Re substrates, under the prevailing PLD conditions. The field emission (FE) studies of GdB6/W and GdB6/Re emitters were performed in a planar diode configuration at the base pressure ~10−8 mbar. The GdB6/W and GdB6/Re tip emitters deliver high emission current densities of ~1.4 and 0.811 mA/cm2 at an applied field of ~6.0 and 7.0 V/µm, respectively. The Fowler–Nordheim (F–N) plots were found to be nearly linear showing metallic nature of the emitters. The noticeably high values of field enhancement factor (β) estimated using the slopes of the F–N plots indicate that the PLD GdB6 coating on W and Re substrates comprises of high-aspect-ratio nanostructures. Interestingly, the GdB6/W and GdB6/Re planar emitters exhibit excellent current stability at the preset values over a long-term operation, as compared to the tip emitters. Furthermore, the values of workfunction of the GdB6/W and GdB6/Re emitters, experimentally measured using ultraviolet photoelectron spectroscopy, are found to be same, ~1.6 ± 0.1 eV. Despite possessing same workfunction value, the FE characteristics of the GdB6/W emitter are markedly different from that of GdB6/Re emitter, which can be attributed to the growth of GdB6 films on W and Re substrates.

Field emission properties and strong localization effect in conduction mechanism of nanostructured perovskite LaNiO3

We report the potential field emission of highly conducting metallic perovskite lanthanum nickelate (LaNiO3) from the nanostructured pyramidal and whisker shaped tips as electron emitters. Nano particles of lanthanum nickelate (LNO) were prepared by sol-gel route. Structural and morphological studies have been carried out. Field emission of LNO exhibited high emission current density, J = 3.37 mA/cm2 at a low threshold electric field, Eth = 16.91 V/μm, obeying Fowler–Nordheim tunneling. The DC electrical resistivity exhibited upturn at 11.6 K indicating localization of electron at low temperature. Magnetoresistance measurement at different temperatures confirmed strong localization in nanostructured LNO obeying Anderson localization effect at low temperature.