High Field 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.

A Review of Electron Emitters for High-Power and High-Frequency Vacuum Electron Devices

This work reviews the progress in the area of high current density electron emitters, specifically for use in high frequency and high-power microwave vacuum electron devices. The review is divided into two subsections: 1) thermionic cathodes and 2) field emission cathodes. The thermionic cathode section includes the discussion on the M-type, MM-type, Scandate and the CPD cathodes. CPD cathode promises better usability in a practical device as compared to the Scandate cathodes which are limited by emission nonuniformity, poor resistance to poisoning/ion bombardment and low life. With the increasing demand for miniaturized and high power VEDs in the near future, the development of high current density field emission cathodes is of immediate interest to the VED community. The Spindt, CNT, and graphene-based emitters are the potential candidates in the field emitters section. Graphene-based film cathodes, when compared to the Mo/Si FEAs and the CNTs, offer larger emission area as well as high current carrying capability. This review includes the evolution of the emission capabilities of the emitters, state-of-the-art performances, and possible future developments.

Effect of purification methods on the cross-sectional field emission properties of carbon nanotube and graphene composite films

Abstrace Carbon nanotubes (CNTs) are excellent materials for field emission because of their large aspect ratio, low-threshold field and stable chemical structure. However, the low adhesive bonding strength between CNTs and the supports presents a practical challenge in environments with a high field emission current density. We used a vacuum filtration method to prepare multiwalled CNT (MWCNT)/reduced grapheme oxide (r-GO) composite films in which CNTs were uniformly dispersed in the r-GO layers. Compared with unmodified films, films in which CNTs are dispersed show better field emission performance, and the adhesive bonding strength between the CNTs and the supports is enhanced. At the same time, because MWCNTs grown by chemical vapor deposition (CVD) contain impurities such as carbonaceous materials and catalyst particles, we use different purification methods to treat the MWCNTS, and the effects of these purification methods on the field emission properties of the composite films are analyzed. A comparison of the field emission properties of the unpurified, calcined, calcined/centrifuged and acid-purified composite films shows that the calcined/centrifuged composite films have the best field emission performance.

Highly enhanced field emission from copper oxide nanoparticle decorated vertically aligned carbon nanotubes: Role of interfacial electronic structure

Field emission (FE) properties of Copper (3 nm and 10 nm) decorated vertically aligned carbon nanotubes (VACNTs) grown on Si substrate before and after oxidation are investigated. The current density is found to be the maximum (20 mA/cm2) for 3 nm thick Cu decorated VACNTs after oxidation. The variation in conventionally monitored FE parameters like work function and field enhancement factor cannot explain the experimentally determined high FE current density and its variation. A critical analysis of the electronic structure reveals that the ratio of CuO/Cu2O on VACNTs plays an important role in controlling the FE current density. The highly enhanced FE current density and its temporal stability of 3 nm Cu decorated VACNTs after oxidation indicate that it may be used as a potential material for a FE electron source in future vacuum microelectronic devices.

Fabrication of large-area ZnO nanowire field emitter arrays by thermal oxidation for high-current application

Large-area ZnO nanowire field emitter arrays have important applications in flat-panel displays, light sources, photodetectors and X-ray sources, where it is crucial to realize a high field emission current and current density over a large-area. In this paper, large-area ZnO nanowire arrays with various array spacings were prepared by the thermal oxidation technique from patterned Zn film arrays, and their field emission properties were studied. The results show that the Zn array density is related to the nanowire length and population density. A mechanism is proposed wherein the stress-induced process and mass transport process both play key roles in the growth. Herein, ZnO nanowire arrays in a 4.7 × 4.7 cm2 area exhibited a sufficient quantity of emission sites and diminished field screening effect and a high current of 12.9 mA with a current density of 0.58 mA/cm2. This work provides useful guidance for optimizing ZnO nanowires and enhancing field emission current performance for large-area ZnO nanowire field emission devices.

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 (<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.

Low turn-on field and high field emission current density from Ag/TiO2 nanocomposite

High current density of 1.24mA/cm2 was drawn at an applied field of 4.4V/μm from Ag/TiO2 nanocomposite. Also the turn-on field has been reduced from 3.9V/μm to 2.7V/μm for the emission current density of 10μA/cm2. Ag/TiO2 nanocomposite was synthesized by using UV-switchable reducing agent. TiO2 nanotube wall was decorated by Ag nanoparticles with average diameter of 17nm. To the best of our knowledge this is the first report on the field emission studies of Ag/TiO2 nanocomposite. Simple synthesis route coupled with superior field emission properties indicate the possible use of Ag/TiO2 nanocomposite for micro/nanoelectronic devices.

Field emission properties of a-C and a-C:H films deposited on silicon surfaces modified with nickel nanoparticles**Project supported by the National Natural Science Foundation of China (Grant No. 51105186).

The a-C and a-C:H films are deposited on silicon surfaces modified with and without nickel nanoparticles by using mid-frequency magnetron sputtering. The microstructures and morphologies of the films are analyzed by Raman spectroscopy and atomic force microscopy. Field emission behaviors of the deposited films with and without nickel nanoparticles modification are comparatively investigated. It is found that the hydrogen-free carbon film exhibits a high field emission current density and low turn-on electric field compared with the hydrogenated carbon film. Nickel modifying could increase the current density, whereas it has no significant effect on the turn-on electric field. The mechanism of field electron emission of a sample is discussed from the surface morphologies of the films and nickel nanoparticle roles in the interface between film and substrate.

High-Performance Stable Field Emission with Ultralow Turn on Voltage from rGO Conformal Coated TiO2 Nanotubes 3D Arrays.

A facile method to produce conformal coated reduced graphene oxide (rGO) on vertically aligned titanium oxide (TiO2) nanotubes three dimensional (3D) arrays (NTAs) is demonstrated for enhanced field emission display applications. These engineered nano arrays exhibit efficient electron field emission properties such as high field emission current density (80 mA/cm2), low turn-on field (1.0 V/μm) and field enhancement factor (6000) with high emission current stability. Moreover, these enhancements observed in nano arrays attribute to the contribution of low work function with non-rectifying barriers, which allow an easy injection of electrons from the conduction band of TiO2 into the Fermi level of reduced graphene oxide under external electric field. The obtained results are extremely advantageous for its potential application in field emission devices.

High Field Emission Current Density from Patterned Carbon Nanotube Field Emitter Arrays with Random Growth.

High field emission (FE) current density from carbon nanotube (CNT) arrays grown on lithographically patterned silicon substrates is reported. A typical patterned field emitter array consists of bundles of nanotubes separated by a fixed gap and spread over the entire emission area. Emission performance from such an array having randomly oriented nanotube growth within each bundle is reported for different bundle sizes and separations. One typical sample with aligned CNTs within the bundle is also examined for comparison. It is seen that the current density from an array having random nanotube growth within the bundles is appreciably higher as compared to its aligned counterpart. The influence of structure on FE current densities as revealed by Raman spectroscopy is also seen. It is also observed that current density depends on edge length and increases with the same for all samples under study. Highest current density of -100 mA cm(-2) at an applied field of 5 V/μm is achieved from the random growth patterned sample with a bundle size of 2 μm and spacing of 4 μm between the bundles.

纳米金刚石掺混纳米氧化锌的场发射特性

Chrysanthemum-like Zinc Oxide(ZnO)nanorods was fabricated and characterized by hydrothermal route,the diamond was mixed to prepare electrophoretic fluid.After electrophoretic fluid was dispersed by ultrasonic cleaning machine,the coatings were deposited on titanium substrates.Field emission characteristics were tested after heat treatment.Results show that turn-on field of undoped diamond cathode is 7.3V/μm and field emission current density is 81μA/cm2 when electric field is set at20V/μm,while the doped samples show low turn-on field of 4.7~6.0V/μm and high field emission current density of 140~158μA/cm2.The reason is that after the interfusion of nano-ZnO,the electron transport capacity of coating is enhanced,the number of effective emitters increase and field enhancement factorβis improved.Because of diamond,the good bonding between coating and substrate is guaranteed after heat treatment and ohmic contact is formed,which reduces the thermal effect of field emission current.The stability of field emission current declines with the increasing amount of mixed ZnO.Both field emission current density and its stability should be considered,so mixing an appropriate amount of ZnO with nano-diamond can effectively improve field emission characteristics.

Carbon nanotube based 3-dimensional hierarchical field emitter structure

3-dimensional hierarchical structure of carbon nanotubes on CuO nanotubes offering high field emission current density.

Observation of high thermionic field emission current density from spun carbon fibers fabricated from multiwall carbon nanotubes

The spun carbon fibers were fabricated from multi-walled carbon nanotubes by a drawing and twisting method. The thermionic field emission behavior of the carbon fibers was investigated as a function of applied heating power P-H and field emission voltage V-F. The thermal field emission turn-on voltage and threshold voltage are found at electric field E = 2.6 V/mu m and E = 3.0 V/mu m, respectively. It is found that the emission current density, dominated by the field emission mechanism, reached up to J(0) = 1.3 A/cm(2) (P-H = 0 W) and J = 3 A/cm(2) (P-H = 2.0 W) at an emission field of E = 5.0 V/mu m. The lifetime with respect to heating power P-H, exhibiting about 45 h continuous luminescence under a constant P-H = 0.8 W, was investigated in the spun carbon fibers. Our results indicate that high-strength carbon fibers show good performance as a thermionic field emission source.

Low-temperature ammonia annealed TiO2 nanotube arrays: Synergy of morphology improvement and nitrogen doping for enhanced field emission

The field emission (FE) properties of TiO2 nanotube arrays (TNAs) fabricated by anodization are dramatically enhanced by ammonia annealing at a low temperature of 90°C with various annealing durations of 5–15h, which can spontaneously tailor the as-anodized amorphous TNAs to be an anatase hierarchical structure of worm-like TiO2 decorated TNAs, and simultaneously implant nitrogen into the hierarchical structure. Compared to the as-anodized TNAs, the turn-on field of ammonia annealed TNAs are significantly dropped from 23.11 to 2.94V/μm, and related to the annealing time. Importantly, the optimized sample of ammonia annealed TNAs treated for 10h exhibits excellent FE properties involving both a low turn-on field of 2.94V/μm, a high FE current density of about 3.35mA/cm2 at 7.35V/μm, and a remarkable FE stability. The substantially enhanced FE properties can be attributed to the combination of the optimized morphology, the improved conductivity, and the presence of localized oxygen vacancy states and N 2p states created by N-doping of ammonia annealed TNAs.

Field Emission Behavior of Carbon Nanotube Yarn for Micro-Resolution X-Ray Tube Cathode

Carbon nanotube (CNT) has excellent electrical and thermal conductivity and high aspect ratio for X-ray tube cathode. However, CNT field emission cathode has been shown unstable field emission and short life time due to field evaporation by high current density and detachment by electrostatic force. An alternative approach in this direction is the introduction of CNT yarn, which is a one dimensional assembly of individual carbon nanotubes bonded by the Van der Waals force. Because CNT yarn is composed with many CNTs, CNT yarns are expected to increase current density and life time for X-ray tube applications. In this research, CNT yarn was fabricated by spinning of a super-aligned CNT forest and was characterized for application to an X-ray tube cathode. CNT yarn showed a high field emission current density and a long lifetime of over 450 hours. Applying the CNT yarn field emitter to the X-ray tube cathode, it was possible to obtain micro-scale resolution images. The relationship between the field emission properties and the microstructure evolution was investigated and the unraveling effect of the CNT yarn was discussed.

Review on one-dimensional ZnO nanostructures for electron field emitters

The emission of electrons from the surface of a solid caused by a high electric field is called field emission (FE). Electron sources based on FE are used today in a wide range of applications, such as microwave traveling wave tubes, e-beam evaporators, mass spectrometers, flat panel of field emission displays (FEDs), and highly efficient lamps. Since the discovery of carbon nanotubes (CNTs) in 1991, much attention has been paid to explore the usage of these ideal one-dimensional (1D) nanomaterials as field emitters achieving high FE current density at a low electric field because of their high aspect ratio and “whisker-like” shape for optimum geometrical field enhancement. 1D metal oxide semiconductors, such as ZnO and WO3 possess high melting point and chemical stability, thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications. In addition, unlike CNTs, in which both semiconductor and metallic CNTs can co-exist in the as-synthesized products, it is possible to prepare 1D semiconductor nanostructures with a unique electronic property. Moreover, 1D semiconductor nanostructures generally have the advantage of a lower surface potential barrier than that of CNTs due to lower electron affinity and the conductivity could be enhanced by doping with certain elements. As a consequence, there has been increasing interest in the investigation of 1D metal oxide nanostructure as an appropriate alternative FE electron source to CNT for FE devices in the past few years. This paper provides a comprehensive review of the state-of-the-art research activities in the field. It mainly focuses on FE properties and applications of the most widely studied 1D ZnO nanostructures, such as nanowires (NWs), nanobelts, nanoneedles and nanotubes (NTs). We begin with the growth mechanism, and then systematically discuss the recent progresses on several kinds of important nanostructures and their FE characteristics and applications in details. Finally, it is concluded with the outlook and future research tendency in the area.

Formation and field emission characteristics of nitrogen plasma-treated titanium dioxide clusters

Nitrogen plasma treatment was used to enhance the field emission (FE) properties of titanium dioxide (TiO2) clusters and form nitrogen doped TiO2 (N-doped TiO2) on the TiO2 clusters. The TiO2 clusters were grown directly onto the Fe/Al/Ti sheet using atmospheric annealing. The N-doped TiO2 content was controlled using the nitrogen plasma treatment time. Because continuous nitrogen plasma treatment damages the N-doped TiO2/TiO2 cluster surface, increasing the N-doped TiO2 content could not continually improve the FE current density. A balance between the N-doped TiO2 content and surface damage must be reached to attain a relatively higher FE current density. The FE characteristics are discussed with regard to the N-doped TiO2 content and the cluster surface morphologies. An optimal nitrogen plasma treatment time of 60min was developed in this study to attain the highest FE current density.

Single-crystal SnO2 nanoshuttles: shape-controlled synthesis, perfect flexibility and high-performance fieldemission

Vertically aligned single-crystalSnO2 nanoshuttle arrays with uniform morphology and a relatively high aspect ratio weresynthesized by a simple hot-wall chemical vapor deposition (CVD) method. It was foundthat regulating the growth temperature gradient could change the shape of theSnO2 nanostructure from nanoshuttles to nanochisels and nanoneedles, and aself-catalyzing growth process was responsible for tunable morphologies ofSnO2 nanostructures. Theas-synthesized SnO2 nanoshuttles showed ultrahigh flexibility and strong toughness with a large elastic strain of ∼ 6.2, which is much higher than reported for Si and ZnO nanowire as wellas most crystalline metallic materials. The field emitter fabricated usingSnO2 nanoshuttle arrays has a low turn-on electric field of around0.6 V µm − 1, and a high field emission current density of above10 mA cm − 2, which is comparable with the highest emission current density of carbon nanotube andnanowire field emitters.

Field Emission Properies of CNTs Treated by Helium Plasma

This paper presented a novel post-treatment method of He plasma,which could effectively improve the field emission characteristics of screen-printed CNTs cathodes. Notable changes in the surface morphologies of screen-printed CNTs cathodes were investigated by scanning electron micro scope (SEM) as the f unction of treatment by He plasma.The results showed the post-treated CNTs cathodes hold lower turn-on electric field,higher field emission current density,more emission sites,and better uniformity than those of untreated ones.