Field Emission Performance Research Articles

Supression of shielding effect of large area field emitter cathode in radio frequency gun environment

Abstract Utilisation of large area field emitters (LAFE) cathodes for rf gun injector hold promise for delivering compact, high power and high brightness electron beam for advanced accelerator technologies. LAFEs subjected to DC electric fields posses significant challenges due to the shielding effect which restricts emission from central emitters and decreases the overall current density. Mitigating the shielding effect of LAFE in rf gun environment is essential for meeting the desired beam quality requirement in an accelerator. The current distribution of LAFE under DC conditions depend on its various geometrical parameters such as emitter height, inter-emitter distance, aspect ratio, number of emitters. Additionally, in rf gunsetup, LAFEs are subjected to variable macroscopic electric field at different emitter position which can potentially alter the current distribution compared to DC fields. In this work, we have systematically studied the shielding effect properties of LAFE in rf gun environment under the influence of various LAFE parameters. A semi-analytical approachhas been adopted to estimate the current distribution which combines the analytically calculated field enhancement factor (γ) and numerically calculated applied rf field values. This new methodology was first validated using COMSOL simulation and then employed for field emission performance estimation of a LAFE cathode integrated in a½ cell S-band (2856 MHz) rf gun. The simulation results reveals that under favourable conditions, a Gaussian spatial distribution of beam can be obtained from LAFE thus countering the shielding effect typical in DC fields. By optimizing the LAFE parameters, the desired current and beam distribution pattern can be achieved. This study highlights the adoption of a promising approach for designing LAFE cathodes suitable for rf gun which can lead to advancement of field emission technologies for accelerator-related applications.

Field emission and thermo-electrochemical cell performances of various types of carbon nanotubes

Field emission and thermo-electrochemical cell performances of various types of carbon nanotubes

Silver Oxide Nanoparticle Decorated Carbon Nanotube as Efficient Electron Gun

AbstractAlthough there have been numerous great field emission reports of pure and hybrid carbon nanotubes (CNTs) with ultra‐low turn‐on field and ultra‐high emission stability, practically all of the reports indicate concern about CNT manufacture on a large scale. This is due to the fact that CNT experiments require high pressure, temperature, metal catalysis, and an inert environment, and even after meeting all of these perfect conditions, the yield is quite low. Furthermore, as CNT is relatively inert in nature, it is nearly hard to make it reactive with any other metal without adequate functionalization. Keeping this in mind, this work reports the synthesis of CNT in amorphous form by a simple low‐temperature solid‐state reaction between ammonium chloride and ferrocene. The as‐synthesized CNT is further functionalized by silver oxide nanoparticle without additional functionalization. X‐ray diffraction (XRD) confirms the proper phase formation as well as the successful functionalization of the pure and hybrid sample, electron microscopic images confirm the successful functionalization of the as‐prepared CNT whereas Fourier transformed infrared (FTIR) spectroscopic analysis gives the ideas about different bonding present in all the samples. It has been found that the hybrid sample gives much better field emission performance compare to the pure CNT with betterment in both turn‐on field and enhancement factor more than 100%. The betterment is believed to be due to the favorable band bending, larger number of emission sites, and sharp curvature of the silver particles.

Electron field emission property of NiO-decorated ZnO nanorods grown on diamond films

ZnO/micron diamond (MCD) composite structure combines two wide-band gap semiconductor materials, providing stable performance suitable for high-performance electron field emission (EFE) devices operating in various complex environments. Nonetheless, the optical and electrochemical limitations of ZnO constrain its effectiveness. Typically, NiO can compensate these inherent defects in ZnO, thereby enhancing the performance of field-emitting devices. In this research, NiO-decorated ZnO thin films were fabricated on diamond surfaces using hydrothermal/sol-gel two-step method. The impact of varying Ni doping concentrations caused by nickel acetate solutions on the field emission properties was thoroughly investigated. Furthermore, this study involved the fabrication of NiO-decorated ZnO/micron diamond heterojunction composite structures, exploring the impact of the structure on the optoelectronic performance of the devices. The Ni doping concentration increases in the NiO films formed between the ZnO nanorods, the doped Ni exists in the ZnO/MCD composite structure as both Ni2+ and Ni3+, which are important materials for semiconductor electronic devices. The NiO decoration process induced the creation of defect levels within the bandgap of the nanorod array structure, consequently enhancing the photoluminescence performance of ZnO. Furthermore, the interaction between NiO and ZnO facilitated the formation of a p-n junction at the interface, generating an internal electric field. This electric field significantly improved the current conduction field and maximum current density of ZnO, thereby enhancing its electric field emission performance. The optical and electrical properties of ZnO nanorods doped at 0.1M exhibited the most favorable characteristics among all tested samples. At a doping concentration of 0.1 M, the turn-on electric field reaches a minimum value of 0.96 V/μm and the maximum current density J reaches a maximum value of 2.54 mA/cm2. These results offer novel insights for advancing the development of integrated broadband optical devices.

Synthesis and field emission characterization of La-doped SiC nanowires using graphite powder as carbon source

Lanthanum (La)-doped SiC nanowires (NMs) were synthesized via a carbon thermal reduction process using different graphite powders, while milled Si-SiO2 mixed powders were employed as the silicon source. The identification of the products as β-SiC were supported by Select-area electron diffraction (SAED) and X-ray diffraction (XRD) analysis. The field emission results demonstrated that the turn-on field reached a minimum value of approximately 2.3 V/μm when the graphite content ranged from 2.5 g–3 g. The product exhibited higher density at this stage, accompanied by an increase in nanowire diameter and a tendency toward straightness. The energy spectrum analysis revealed a significant increase in the lanthanum content within the nanowires, with the atomic percentage rising from 0.05 to 0.26–0.27. The synergistic effect of morphology and La improved the field emission performance of the product. The findings may offer valuable insights for enhancing the field emission performance of one-dimensional nanomaterials.

Enhanced field emission performance of holey expanded graphite by heat treatment

Expanded graphite (EG) is a promising material in field emission due to its high conductivity, thermal stability, and low cost. In this paper, the field emission performance of EG is enhanced by introducing hole defects through partial air oxidation. First, EG was prepared using electrochemical intercalation and microwave-assisted expansion methods. Subsequently, holey expanded graphite (HEG) was obtained by air-heating treatment of EG at different temperatures. Finally, bulk HEG cathodes were prepared by the cold pressing method, and the holes' effect on field emission performance was investigated. The results showed that EG's conductivity and tensile properties significantly improved after heat treatment. Optimal conditions of heat treatment were established at 400 °C providing material with the best field emission performance. The turn-on field (at 1 mA/cm2) and threshold field (at 10 mA/cm2) are 3.01 V/μm and 3.42 V/μm, respectively, with a maximum current density of 1359.17 mA/cm2 (at 5.83 V/μm) and a maximum field enhancement factor of 2407. The voltage fluctuation is only 1.1 % over 24 h at 3 mA @ 440.33 mA/cm2, indicating good emission stability and operation life.

Electric field emission properties of ZnO/microcrystalline diamond composite structures modified by Cu nanoparticles decoration

ZnO nanorods hold promise as cathode materials for field electron emission (FEE) applications, yet their high work function and wide bandgap can hinder performance. Recent research has focused on utilizing Cu nanoparticles (NPs) to decorate and modify the FEE properties of these nanorods, leveraging Cu's low work function and excellent conductivity. In this study, ZnO nanorods were synthesized on diamond substrates via the sol method to create ZnO/microcrystalline diamond (MCD) composite structures. Subsequently, Cu NPs were decorated onto the nanorods using a gel method. The influence of Cu NP concentration on the organization and FEE performance of the composites was investigated. The decorated of Cu NPs can improve the Fermi energy level of the nanorods, replenish electrons lost on the surface due to oxygen adsorption, and improve the field emission performance. With the increase of doping concentration, Cu NPs increase, and when the doping concentration is 0.5 M, NPs appear agglomeration phenomenon, resulting in that the surface of ZnO nanorods is almost completely covered by bulk Cu NPs, and the field emission performance decreases. The minimum value of the turn-on electric field is 0.48 V/μm and the maximum value of the current density is 1.60 mA/cm2, when the decorated concentration is 0.25 M.

Vertical tellurium nano-tower arrays: Topological shape engineering to excellent field emission and optical performance

Vertically aligned tellurium nano-towers (TNTs) building blocks comprising sharp edges have been attained by Au assisted controlled CVD method. The density, size and novel TNTs structures have been optimized in a comprehensive systematic way. The measured base diameter of the TNTs is nearly 150 nm which slowly decreases towards their tip (∼20 nm) in the form of steps realizing their sharp edges. The high density growth of TNTs is estimated to be approximately 2.5 × 108mm2. The two prominent binding energy peaks of TNTs positioned at 583.8 eV and 573.6 eV in XPS graphs are credited to Te-3d3/2 and Te-3d5/2 states of Te along with their XRD lattice parameters a = 4.14 Å and c = 5.25 Å. Owing to distinctive morphology of TNTs which is supportive to easy electron emission, have displayed incredible low turn-on value (3.15 Vμm−1 with excellent stability, exhibiting insignificant (<5 %) FE current fluctuation for 120 min. The intense photoluminescence (PL) peaks displayed by TNTs at 368 nm and comparatively weak shoulder peak at 428 nm (2.897 eV) evidences their potential application in light emitting devices (LEDs). Hence, the incredible PL properties and exclusive FE characteristics revealed by unique TNTs provides the potential applications in opto-electronics and FE display devices.

Insight into TaSi2 Nanostructures with Different Morphologies and Their Field Emission Properties

AbstractRecognizing the strong potential of cold cathodes for important commercial applications in fields such as electronics, there is a growing interest in the exploration of novel 1D nanomaterials. Among various cold cathode materials, TaSi2 is of great interest for its outstanding field emission performance. In this work, the Si‐TaSi2 eutectic composite with nano‐sized highly oriented TaSi2 fibers and semi‐coherent phase interfaces is prepared by the laser floating zone melting technique with a very high‐temperature gradient of 6000 K cm−1 at a solidification rate of 200 µm s−1. On the basis of directionally solidified Si‐TaSi2 eutectic composite, well‐aligned TaSi2 nanorod and nanotip arrays are fabricated by inductively coupling plasma (ICP) etching process and HNO3/HF wet etching process, respectively. The field emission measurements show that the field enhancement factor, turn‐on electric field, and effective work function are strongly affected by tip morphologies. The TaSi2 array with regular nanotip structure possesses the best field emission characteristic among all TaSi2 nanostructures, with a relatively low turn‐on field of 4.8 V µm−1 and a high current density of 733 µA cm−2. These findings preliminarily establish a clear relationship between the performance and structure of the array, providing technical guidance for the application of this material in electronic devices.

Transcritical CO2 system with two-stage throttling for supermarkets: Field tests, energy and emission performance assessment in China

Transcritical CO2 system is eco-friendly for the application of refrigeration and cooling in supermarket. To evaluate the applicability of transcritical CO2 supermarket refrigeration system in China with different climate regions, this study focuses on the field tests of a transcritical CO2 two-stage throttling system in a supermarket in Suzhou, Jiangsu Province, China. Furthermore, based on the tested data, the system energy and emissions performance models of four system configurations are also developed and validated, including the system integrated with an internal heat exchanger, a parallel compression, and mechanically assisted subcooling. In order to improve the usability of the system in different climatic zones, 40 cities were selected for calculation in China. Five representative cities (Beijing, Harbin, Xiamen, Chongqing, and Kunming) located in different climate regions of China are selected, and the annual performance factor and life cycle climate performance (LCCP) are determined to evaluate the application potential of the CO2 refrigeration systems in different climate conditions. The tested results show the operation mode of transcritical and subcritical changes occurs at an ambient temperature of about 27 °C, at which the coefficient of performance (COP), compressor discharge pressure and discharge temperature change dramatically. The simulation results demonstrate that the mechanical subcooling system shows the best performance. The annual performance factor can be improved by 18.69 % when the mechanical subcooling is adopted. For the case of the system used in Xiamen by using mechanical subcooling, the carbon emissions can be reduced by 19.3 %.

The role of Ni on the structure and related properties of Ni/a-C:H film

Ni target sputtered by CH4 and Ar was used to deposit Ni/a-C:H composite films and thus improve its mechanical, field-emission and tribological properties. The results showed that the composite film exhibited graphite-like C structure with nanocrystal Ni dispersed in it and no bond formed between them. Ni crystals changed from (200) to (111) planes with increasing Ni content, accompanied with increasing surface roughness. Furthermore, the content of sp2C, hardness, maximum current density and frictional coefficient of the film increased firstly and then decreased. Especially, the proper content of sp2C/sp3C and lower surface roughness kept the A2 film (Ni: 14.7 at.%) better field emission performance and higher hardness, and herein better resistance to elastic failure strain and plastic deformation.

Formation of Diamond Films with Different Grain Sizes on TiO2 Nanotubes and Its Field Emission Properties

Diamond films with different grain sizes are deposited on TiO2 nanotube (TNT) arrays prepared by anodic oxidation using a microwave plasma chemical vapor deposition reactor. The scanning electron microscope, X‐Ray diffractometer, and Raman results indicate that the diamond film is successfully deposited and that the substrate undergoes a phase transition due to the diamond deposition temperature, resulting in the rutile phase becoming dominant with lower energy bandgap and work function. Notably, after 5 h of deposition, a relatively continuous microcrystalline film is formed on the TNTs, and the diamond grains change from spherical to pyramidal and show excellent electron field emission behavior, with a low turn‐on field of 0.5 V μm−1 and a current density of 85.9 μA cm−2. In addition, the deposition environment of the nanocrystalline diamond has a large impact on the substrate morphology, resulting in a blocked electron transport path, which reduces the overall field emission performance. The enhanced performance is attributed to the synergistic effect between the highly efficient 1D electron path of the TNTs, the negative electron affinity of the diamond surface, and the lower work function of rutile TiO2.

Field Emission from Carbon Nanotubes on Titanium Nitride-Coated Planar and 3D-Printed Substrates.

Carbon nanotubes (CNTs) are well known for their outstanding field emission (FE) performance, facilitated by their unique combination of electrical, mechanical, and thermal properties. However, if the substrate of choice is a poor conductor, the electron supply towards the CNTs can be limited, restricting the FE current. Furthermore, ineffective heat dissipation can lead to emitter-substrate bond degradation, shortening the field emitters' lifetime. Herein, temperature-stable titanium nitride (TiN) was deposited by plasma-enhanced atomic layer deposition (PEALD) on different substrate types prior to the CNT growth. A turn-on field reduction of up to 59% was found for the emitters that were generated on TiN-coated bulk substrates instead of on pristine ones. This observation was attributed exclusively to the TiN layer as no significant change in the emitter morphology could be identified. The fabrication route and, consequently, improved FE properties were transferred from bulk substrates to free-standing, electrically insulating nanomembranes. Moreover, 3D-printed, polymeric microstructures were overcoated by atomic layer deposition (ALD) employing its high conformality. The results of our approach by combining ALD with CNT growth could assist the future fabrication of highly efficient field emitters on 3D scaffold structures regardless of the substrate material.

Optimized Field Emission from Graphene Sheets with Rare Earth Oxides

This paper demonstrates a simple method to improve the field emission of graphene sheets (GSs) by coating them with thin films of rare earth oxides. The rare earth oxide films are coated on GS using drop coating, without changing the surface morphology, resulting in a remarkable improvement in the field emission properties of GSs. The field emission property of GSs is tunable and can be optimized by applying various rare earth oxide films at the appropriate level. It is found that the turn-on field of GSs is reduced from 4.2 V/mm to 1.7 V/mm by Gd2O3 and to 2.2 V/mm by La2O3. The threshold field of GS is also reduced from 7.8 V/mm to 3.4 V/mm and 4.8 V/mm, respectively. Field emission results indicate that the improvement is due to the low work function surface and more effective emission sites generated around the GS surface after coating. The field emission test and the emission pattern suggest that the field emission performance of GS can be significantly enhanced through the application of La2O3 and Gd2O3 coating, as well as by optimizing the concentration of rare earth oxides in the coating. Hence, the rare earth-coated GS can serve as a potential field emitter.

Influence of Grid Aperture Ratio on Electron Transmittance and Electron Beam Spot Size in Field Emission Processes of Carbon Nanotubes

Field emission is an important work mode for electron sources, and carbon nanotubes (CNTs) have been extensively studied for their good emission properties. It is well known that the parameters of the grid deeply influence the field emission performance of CNTs, a relationship that requires further elucidation. Therefore, in this study, the relationship between the grid aperture ratio and electron transmittance was studied through simulations and experiments. This study’s results indicate that the electron transmittance improved as the grid aperture ratio increased. Meanwhile, electron beam spot simulations and imaging experiments indicate that an increased grid aperture ratio will expand the cathode electron divergence, leading to a larger electron beam spot size. These results demonstrate that there is a trade-off in maintaining the grid aperture ratio between high electron transmittance and relatively small electron beam spot size, and the optimum grid aperture ratio is between 75% and 85%. These results will provide a reference for the design and optimization of X-ray tubes and other electron sources.

Field emission properties of LIG/ZnO heterojunction prepared by ultrafast laser direct writing

The facile in situ synthesis of graphene-coated metal-oxide nanoparticles and their potential applications in optoelectronic devices are highly anticipated. In this study, we prepared three-dimensional graphene flakes decorated with ZnO nanoparticles (LIG/ZnO) using femtosecond laser-induced zinc salt-containing cork. The LIG/ZnO heterojunction was formed by carbonizing the cork and decomposing the zinc salt in situ to coat LIG during laser irradiation. The correlation between the zinc salt concentration and the morphology, structure, and field-emission characteristics of LIG/ZnO was systematically investigated. The optimized LIG/ZnO-5 emitters demonstrate a decreased threshold electric field (Eth) of 1.07 V/μm, an enhanced field enhancement factor (β) value of 20,559, and a maximum current density of 18.07 mA/cm2 at 1.96 V/μm, representing the most superior performance reported to date. The enhanced field-emission performance of LIG/ZnO can be attributed to the larger number of effective emission sites from ZnO nanoparticles and LIG edges, as well as to the energy band variation induced by the heterostructures. Density functional theory (DFT) calculations show the migration of electrons from the LIG to the ZnO after the formation of the heterojunction, which elevates the Fermi level of emitters. In addition, the accumulation of electrons on the surface of ZnO nanoparticles in the presence of a strong electric field results in a downward shift of the energy band, thereby enhancing the likelihood of electron-vacuum tunneling.

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.

Study on field emission performance of SrTiO3 film enhanced by LiF film

A series of monolayer SrTiO3 films and LiF/SrTiO3 bilayer films were prepared on n-Si substrates by using vacuum electron beam vapor deposition (EBVD) technology. The morphologies and thicknesses of the SrTiO3 film and LiF film were controlled through adjustments and optimization of the growth conditions as well as deposition time. The experimental results show that the field emission performance of SrTiO3 film is significantly improved after LiF film is added. Under the research conditions in our laboratory, a large number of LiF/SrTiO3 bilayer film field emission devices have been tested and analyzed in the wide range of LiF film thickness (10–300 nm), and the effective field emission LiF film thickness (50–90 nm) was initially determined. Further study shows that the LiF/SrTiO3 bilayer film has the best field emission performance when the optimal LiF film thickness is 70 nm, and its maximum field emission current density can reach 305.48 μA/cm2. The field emission current density of the LiF/SrTiO3 bilayer thin film device is nearly 4 times compared to that of the monolayer SrTiO3 film when the applied electric field intensity reaches 9.05 V/μm. All the field emission devices exhibit excellent functioning stability and experimental reproducibility.

Regulating the aspect ratio of bulk few-layer graphene to improve the field emission performance

Owing to its excellent electrical conductivity and local electric field enhancement capability from abundant electron tunneling edges, graphene is a promising material for field emission. However, graphene thin-film emitters mostly emit in planar structures, and due to the flexibility of the thin film, it is commonly challenging to achieve vertical structure emission with large aspect ratios. Therefore, the emission performance of current graphene field emitters is difficult to meet the requirements of high current applications. In the present investigation, bulk few-layer graphene cathodes are formed by cold pressing to achieve upright structural field emission, and the emission performance is enhanced by appropriately adjusting the aspect ratio. The obtained bulk few-layer graphene emitters exhibit a higher conductivity of 2498 S/cm and tensile strength of 6.02 MPa. The turn-on electric field (at 1 mA/cm2) and threshold electric field (at 10 mA/cm2) in order are determined to be 1.78 V/μm and 2.66 V/μm, with a maximum current density of 986 mA/cm2 (at 6.65 V/μm) and a maximum field enhancement factor of 14,891. Excellent emission stability is achieved over a continuous period of 110 h at 5 mA @ 526 mA/cm2. With the increase of emission current, the bulk few-layer graphene with large aspect ratios exhibits better emission stability. This work provides valuable insights into the application of graphene-based field emitters at high currents.

Role of Surface Chemistry of Ta Metal Foil on the Growth of GaN Nanorods by Laser Molecular Beam Epitaxy and Their Field Emission Characteristics.

This study investigates the influence of surface nitridation of Ta metal foil substrates on the growth of GaN nanorods using the laser molecular beam epitaxy (LMBE) technique and the field emission characteristics of the grown GaN nanorod ensemble. Surface morphology examinations underscore the pivotal role of Ta foil nitridation in shaping the dimensions and densities of GaN nanorods. Bare Ta foil fosters the formation of high-density, vertically self-aligned GaN nanorods at a growth temperature of 700 °C. Furthermore, the density of these nanorods is directly related to the duration of Ta foil nitridation, with increased duration leading to a reduced nanorod density. X-ray Photoelectron Spectroscopy (XPS) studies reveal that the transition of the Ta foil surface from tantalum oxide to tantalum nitride during nitridation emerges as a crucial factor influencing GaN nanorod growth. Photoluminescence (PL) spectroscopy at ambient temperature reveals a strong near-band-edge (NBE) emission peak with negligible defect-related peaks, displaying the high optical quality of the GaN nanorods. The highly dense vertically aligned GaN nanorod ensemble growth without Ta prenitridation exhibits the most favorable field emission performance, featuring a turn-on field of 2.1 V/μm, a field enhancement factor of 2480, and a stable long-term operation at the emission current density of 2.26 mA/cm2. This study advances the understanding of the role of the surface chemistry of metal foil in determining GaN nanorod growth and opens up exciting possibilities for tailoring advanced optoelectronic devices for specific application requirements.