Excellent Field Emission Performance Research Articles

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.

High performance field emission cathode based on the diamond nanowires prepared by nanocrystalline diamond films annealed in air

This paper reports the field emission (FE) characteristics of a diamond nanowires (DNWs) array. The nanocrystalline diamond (NCD) film was deposited on silicon by microwave plasma chemical vapor deposition (MPCVD) and then annealed in air forming DNWs and hydrogenated at last. A high-field flat-plate emission test structure with a 1.03 μm gap between anode and cathode was prepared and the electrical properties proved it feasible. The FE performance of DNWs array was measured in a vacuum test system and that of NCDs film as a comparison. Finally, their FE parameters were analyzed and extracted based on the Fowler–Nordheim (F-N) theory. The results show that transforming NCDs film into DNWs array can improve the FE characteristics greatly. The turn-on field is as low as 1.36 V μm−1 dropping by one order of magnitude, while the field enhancement factor and FE current density are up to 156.68 and 484.75 mA cm−2 respectively rising both by two orders of magnitude. This excellent FE performance stems from the characteristics of large aspect ratio, very small tip radius and high density of DNWs.

Effect of N2 flux on the field emission properties of synthesized ZnO/ZrN core-shell nanostructures

In this paper, the highly oriented ZnO/ZrN nanoneedle emitters with excellent field emission performance were fabricated by sputtering ZrN particles with different N2 flux to modify ZnO nanoneedle arrays. The effects of different N2 flow on the microscopic morphology, structure and field emission properties of ZnO nanoneedles were investigated. The results shows that the ZnO/ZrN nanoneedles (ZnO/ZrN-1.6) emitter prepared when the flow of N2 gas is 1.6 SCCM exhibits the best field emission performance, with the lowest turn-on electric field Eto at 0.81 V/μm, and a field emission current density of 25 mA/cm2 which was 25 times of pure ZnO. In addition to the improvement of band structure, the improvement of field emission performance can also be attributed to the tunneling effect of electrons through heterostructure. This well-aligned ZnO/ZrN core-shell cathode with excellent emission characteristics grown on carbon cloth substrate has the potential applications in future flexible light source devices.

Low-Pressure CVD Synthesis of Tetragonal Borophene Single-Crystalline Sheets with High Ambient Stability

Being a typical graphene-like two-dimensional (2D) material, borophene has received a lot of attention due to its low density, high electron mobility, large Young’s modulus, and good chemical stability. Compared with the corresponding bulk counterparts, 2D borophene has larger specific surface area, higher conductivity, and smaller work function, finding potential applications in high-speed transistors, optoelectronic detection, field emission (FE), and mechanical enforcement areas. In this paper, 2D tetragonal borophene sheets with high ambient stability have been successfully fabricated on the surface of 1 cm2 copper foil using a developed low-pressure chemical vapor deposition (LPCVD) method. The vapor–solid mechanism was used to comprehend the formation of the tetragonal borophene sheets. The electrical conductivity of individual borophene sheets ranged from 4 to 5 × 10–4 S/cm, and the band gap was about 2.1 eV, suggesting their narrow semiconductive nature. Also, the maximum FE current of individual borophene sheets was close to 1 μA, and their emission properties enhanced with the decreasing sheet thickness, comparable to many other nanomaterials with excellent FE performances. More interestingly, individual borophene sheets were found to retain good enough electrical transport and FE behaviors even if they experienced several days’ atmospheric conservations. Our research results suggest that the LPCVD-grown borophene may be a promising building block for constructing high-performance nanodevices with good ambient stability.

Simulation and Optimization of CNTs Cold Cathode Emission Grid Structure

Carbon nanotubes (CNTs) show significant advantages in the development of cold cathode X-ray tubes due to their excellent field emission performance; however, there are still some problems, such as short lifetime and the low emission current of large-area CNTs. In this paper, a front-grid carbon nanotube array model was established, and the electric field intensity near the tip of the CNTs' electric field enhancement factor was analytically calculated. A simulation model of a CNT three-dimensional field emission electron gun was established by using computer simulation technology (CST). The effects of grid wire diameter, grid aperture shape, and the distribution of grid projection on the cathode surface on the cathode current, anode current, and electron transmission efficiency were analyzed. The aperture ratio was used to evaluate the grid performance, and the simulation results show that the ideal aperture ratio should be between 65% and 85%. A grid structure combining a coarse grid and a fine grid was designed, which can make the electric field intensity around the grid evenly distributed, and effectively increased the cathode emission current by 24.2% compared with the structure without the fine grid. The effect of grid aperture ratio on the electron transmission efficiency was tested. The simulation results and optimized structure can provide a reference for the grid design of cold cathode emission X-ray tubes.

Field emission properties of carbon cloth-supported SnO2 with different morphological structures

• SnO 2 nanorods were successfully fabricated on carbon cloth through a facile hydrothermal method. • The nanocone-like SnO 2 /CC sample showed more excellent field emission properties. In this paper, SnO 2 nanorods and SnO 2 nanocone-like structure were synthesized on carbon cloth by facile hydrothermal route. When the carbon cloth (CC) is naturally placed at the bottom of the reaction solution during the process, the SnO 2 nanorods/CC composites were successfully obtained. The turn-on field and field enhancement factor of which were found to be approximately 0.95 V/μm and 21782. However, when the CC was inverted and fixed horizontally in the reaction solution, the top part of the SnO 2 nanorods became a tip structure. The prepared sample has an excellent field emission performance with the lower turn-on field of 0.78 V/μm and higher field enhancement factor of 32268.

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.

Germanium-assisted growth of titanium dioxide nanowires for enhanced photocatalytic and electron emission performance

In this study, free-standing and well-aligned titanium dioxide (TiO2) nanowires were grown on a Ti foil by precisely controlling the growth temperature and the thickness of the Ge catalyst layer. The catalyst layer played an important role in activating the surface of the Ti foil, facilitating the growth of TiO2 nanowires on it in a chemical vapor deposition furnace system. The structure of the TiO2 nanowires was investigated using scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The photocatalytic experiment results showed that the optimum Ge catalyst layer thickness significantly improved the rhodamine B (RhB) degradationefficiencyofthe nanowires. The TiO2 nanowires grown using a 40-nm-thick Ge layer could degrade 74% of RhB after 5 h of ultraviolet irradiation. In addition, the TiO2 nanowires exhibited excellent electron field emission performance as compared to the TiO2 nanocrystals grown without the Ge catalyst layer. The results suggested that TiO2 nanowires grown using a Ge catalyst layer are promising materials for next-generation electronic emitters and photocatalytic systems.

Preparation and Field Emission Properties of Single-Crystal CeB6 Tips

Single-crystal CeB6 tips with different curvature radii are prepared simply and efficiently by using an independently designed electrochemical corrosion device; the average preparation time of a tip was only about 2.75 h. The field emission performance test results show that the single-crystal CeB6 tip exhibits excellent field emission performance. When the applied voltage was 2550 V, the current density of a tip with a curvature radius of about 500 nm is 2.11 × 104 A/cm2, and as the curvature radii increases, the field emission current density drops sharply. The first principle calculation of the electronic structure and work function of single-crystal CeB6 under an electric field show that the free electron energy of single-crystal CeB6 increases and is enriched at the surface under a strong electric field of 0.1 V/A, with a reduced surface barrier. The CeB6 work function under the strong electric field was greatly reduced, which is conducive to electron emission. By calculating the difference between the vacuum level and the Fermi level, a surface work function of 0.42 eV with an applied electric field of 0.1 V/A of CeB6 was obtained. This provides theoretical support for the single-crystal CeB6 tip exhibiting excellent field emission performance.

Scalable growth of SiC quasi-aligned nanoneedle arrays and their electron emission behaviors under high temperatures

Scalable growth of SiC quasi-aligned nanoarrays on SiC wafer substrate was reported through the pyrolysis of polymer precursor by using Au as the catalysts. Differently to the conventional one-dimensional nanostructures often with cylindrical shape, the as-grown SiC nanowires possessed tapered configurations with sharp tips, enabling their desired formation with typical nanoneedle morphology. Their turn-on fields manifested a decline from 3.4 to 1.4 V/μm, when the temperatures were adjusted from room temperature to 400 °C, evidencing their excellent field emission performance. Furthermore, they behaved a high stability with no obvious current emission degradation when subjected to high temperature of 200 °C over 10 h, representing their promising applications in vacuum electronics with robust stability under high-temperature harsh conditions.

Ge-doped ZnO nanowire arrays as cold field emitters with excellent performance

Elemental doping is an efficient strategy to modify the electronic and optoelectronic properties of semiconductor materials. In this work, a high-quality Ge-doped tapered ZnO nanowire array was prepared via a simple chemical vapor deposition approach. The I–V and light emission behaviors were investigated based on a single nanowire with good electrical conductivity. The Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy characterizations indicate that the doped nanowires possess a largely reduced work function relative to that of the pure ZnO nanostructures. These advantages allow Ge-doped ZnO nanowire arrays to deliver excellent field emission performance, including low turn-on and threshold fields, high emission current and long-term stability.

Fabrication and Field-Emission Properties of Vertically-Aligned Tapered [110]Si Nanowire Arrays Prepared by Nanosphere Lithography and Electroless Ag-Catalyzed Etching

In this study, the controllable fabrication of a variety of vertically aligned, single-crystalline [110]-oriented Si nanowire arrays with sharp tips on (110)Si substrates is achieved using a combined self-assembled nanosphere lithography and multiple electroless Ag-catalyzed Si etching processes. All of the experiments were performed at room temperature. The morphological evolution and formation mechanism of long tapered [110]Si nanowire arrays during the multiple tip-sharpening cycle processes have been investigated by scanning electron microscopy, transmission electron microscopy and water contact angle measurements. Field emission measurements demonstrate that the field-emission behaviors of all nanowire samples produced in this study agree well with the Fowler–Nordheim theory, and the produced long tapered [110]Si nanowire array possesses superior electron emission characteristics, with a very low turn-on field of 1.4[Formula: see text]V/[Formula: see text]m and a high field enhancement factor of 3816. The simple and room temperature fabrication of the well-ordered long tapered [110]Si nanowire array and its excellent electron field emission performance suggest that it can serve as a good candidate for applications in high-performance Si-based vacuum electronic nanodevices.

Al doping influences on fabricating ZnO nanowire arrays: Enhanced field emission property

Increase of free electrons concentration and decrease of defects density are the most desirable solution for stimulating the field emission property in semiconductor nanostructures. To implement this, herein we study the field emission efficiency of Al-doped ZnO nanowire arrays with different Al doping concentration, which were prepared by a simple facile hydrothermal method. The Al doping concentration plays a very important role on the structure, morphology, photoluminescence and field emission properties of the as-assembled samples. The results indicate that Al-doped ZnO nanowire arrays with Al doping concentration of 7at% exhibit the highest quality crystalline structure and lowest defect density relative to others samples thanks to the suitable modification of Al doping, and this led to the increase of free electrons concentration and decrease of defects density, which have an excellent field emission performance with the lower turn on field of 1.03V/μm and higher field enhancement factor of 20658. This remarkable field emission performances of the Al-doped ZnO nanowire arrays may provide promising applications for different field emission devices.

Enhanced field emission of ZnO nanowire arrays by the control of their structures

Four kinds of ZnO nanostructrues, i.e. nanorods, nanopencils, nanotowers, and nanoneedles, were successfully synthesized by chemical vapor deposition method, and their field emission properties were comparatively investigated. The results showed that field emission performance was strongly affected by nanowire morphology, and the nanotower-like ZnO exhibited the lowest turn-on field of 4.36 V/μm. The classical F-N theory was employed to analyze the enhancement mechanism. It was found that only the optimized combination of field enhancement factors and geometrical factors could assure excellent field emission performance.

Flexible three-dimensional CeB6 nanowire arrays and excellent field emission emitters

We have developed an effective chemical-vapor-deposition method to fabricate large-scale cerium hexaboride nanowire arrays on flexible carbon substates. The nanowires show diameters of 80–200 nm and lengths of 5–30 μm. The high resolution transmission electron microcopy (HRTEM) indicates that the nanowire is of high crystalline with the growth orientation of [001]. The field emission of these nanowires shows favorable electron emission properties, including the turn-on fields of 1.76 V μm−1, threshold fields of 3.60 V μm−1 and high field emission enhancement factors of 2203 ± 27, respectively. The field emission properties depending on the gap between the anode and cathode are also investigated. The results show that the turn-on fields and threshold fields decrease from 1.76 and 3.60 V μm−1 to 1.28 and 2.77 V μm−1, respectively; whereas the field emission enhancement factors increase from 2203 ± 27 to 4012 ± 66 with the anode-cathode distance increasing from 500 to 700 μm. The excellent field emission performances indicate the promising application prospects of this material in flexible electronics and optoelectronics, such as high light electron sources, flat panel displays and thermal electrical power converters.

Field emission of graphene oxide decorated ZnO nanorods grown on Fe alloy substrates

Graphene oxides (GO) are decorated on the top surface of ZnO nanorods (NRs) grown on Fe alloy substrates for efficient field emission. The GO decorated ZnO NRs acting as cold electron emitters exhibit excellent field emission performance with the turn-on field Eto as low as 1.63 V/μm and the threshold field Ethr down to 3.12 V/μm. ZnO NRs grown on the alloy substrates have a low interfacial resistance and intend to enhance electrical conduction. A schottky contact of Fe-ZnO and matched Fermi levels of ZnO-GO interface contribute to the enhanced current emission efficiency. Besides, some nanometer-scaled sharp protrusions have been formed in the GO sheets, and GO itself owns abundant C-O-C oxygen functional groups that also help to improve the field emission current. A straight line Fowler-Nordheim plot of the field emission current from the emitter is obtained and the effective work function for the decorated GO sheets is calculated from the slope with a value below 1.5 eV. Finally, field emission mechanism of the GO decorated ZnO NRs has been proposed. This work may help the development of the practical electron sources and advanced optronic devices based on GO field emitters.

Synthesis and Properties of 2D Carbon-Graphdiyne.

Graphdiyne (GDY) is a flat material comprising sp2- and sp-hybridized carbon atoms with high degrees of π conjugation that features uniformly distributed pores. It is interesting not only from a structural point of view but also from the perspective of its electronic, chemical, mechanical, and magnetic properties. We have developed an in situ homocoupling reaction of hexaethynylbenzene on Cu foil for the fabrication of large-area ordered films of graphdiyne. These films are uniform and composed of graphdiyne multilayers. The conductivity of graphdiyne films, calculated at 2.52 × 10-4 S m-1, is comparable to that of Si, suggesting excellent semiconducting properties. Through morphology-controlled syntheses, we have prepared several well-defined graphdiyne structures (e.g., nanotubes, nanowires, and nanowalls) having distinct properties. The graphdiyne nanotube arrays and graphdiyne nanowalls exhibited excellent field emission performance, higher than that of some other semiconductors such as graphite and carbon nanotubes. These structures have several promising applications, for example, as energy storage materials and as anode materials in batteries. The unique atomic arrangement and electronic structure of graphdiyne also inspired us to use it to develop highly efficient catalysts; indeed, its low reduction potential and highly conjugated electronic structure allow graphdiyne to be used as a reducing agent and stabilizer for the electroless deposition of highly dispersed and surfactant-free Pd clusters. GDY-based three-dimensional (3D) nanoarchitectures featuring well-defined porous network structures can function as highly active cathodes for H2 evolution. Heteroatom-doped GDY structures are excellent metal-free electrocatalysts for the oxygen reduction reaction (ORR). Its excellent electrocatalytic activity and inexpensive, convenient, and scalable preparation make GDY a promising candidate for practical and efficient energy applications; indeed, we have explored the application of GDY as a highly efficient lithium storage material and have elucidated the method through which lithium storage occurs in multilayer GDY. Lithium-ion batteries featuring GDY-based electrodes display excellent electrochemical performance, including high specific capacity, outstanding rate performance, and long cycle life. We have also explored the application of GDY in energy conversion and found that it exhibits excellent conductivity. In this Account, we summarize the relationships between the functions of graphdiyne and its well-defined nanostructures. Our results suggest that GDY is a novel 2D carbon material possessing many attractive properties. It can be designed into new nanostructures and materials across a range of compositions, sizes, shapes, and functionalities and can be applied in the fields of electronics, optics, energy, and optoelectronics.

Damage to few-layer graphene sheets during electron field emission

Graphene has demonstrated excellent field emission performance, however, the emitted-current shows irreversible decline. The cause of this decline has not been clearly studied or thoroughly identified, which limits application of graphene as field emitters. In this work, accompanied by the decay of emission currents, the damage to few-layer graphene sheets was observed for the first time by introducing a plate-needle-type setup. The low turn-on field show excellent emission ability. The thinnest regions at the edges of graphene sheets, which have high emission efficiency, are the first to be destroyed. Thus, the graphene sheets become shortened and show thicker serrated edges because of thermally assisted field evaporation induced by the local heating effect of the currents, resulting in the degradation of emission ability. In contrast to the damage, changes in the Raman features of the graphene sheets are not visible, indicating that the graphene structure is retained in the remaining graphene sheets and only the morphology is changed, which renders the ϕ unchanged. This work provides the first direct and credible evidence that the degradation of emission performance is derived from damage to the graphene sheets, which leads to changes in their geometry and determines the β.

Facile growth of vertically-aligned graphene nanosheets via thermal CVD: The experimental and theoretical investigations

Owing to the distinctively morphological and structural features, vertically-aligned graphene nanosheets (VGs) possess many unique properties and hold great promise for applications in various fields. For controllable preparation and wide application of VGs, the establishing reliable growth method and profound understanding of the growth mechanism are of vital significance. Up to date, VGs are normally produced by plasma-enhanced chemical vapor deposition (PECVD) and it's considered that plasma is an indispensible factor for the vertical alignment of graphene sheets. Herein, for the first time, we report the facile and controllable VGs growth via a thermal CVD by precisely tuning growth parameters. Experimental observations in combination with detailed energy calculations reveal that the flow rate of carbon precursor determines the growth dynamics of graphene in CVD. This work offers a novel and reliable technique for VGs preparation and provides new insights into the intrinsic mechanism of vertical graphene growth. Furthermore, benefiting from the ultra-high density of edge sites, thin thickness, and outstanding electrical conductivity of VGs, the as-prepared VGs exhibit excellent field-emission performance such as ultra-low turn-on electric field and threshold field down to 1.07 and 1.65 V μm−1, respectively.

Defect modification to improve field emission of ZnO NRs by coating ultrathin Pt films

Ultrathin platinum (Pt) films are coated on the surface of ZnO NRs grown on Fe alloy substrates and display excellent field emission performance with the turn-on field Eto as low as 0.40 V/μm and the threshold field Ethr down to 2.62 V/μm. By coating ultrathin Pt films on the surface of the ZnO NRs, this not only helps to modify the linear defects on the outer surface of ZnO NRs, but also suppresses random electron emission along the defect directions. At an external electric field, a large amount of electrons are transported to the top of ZnO NRs where a highly localized electric field is caused, which results in enhanced field emission. Besides, ZnO NRs grown on Fe alloy substrates have a low interfacial contact resistance and intend to reduce the barrier between the substrate and the ZnO. Meanwhile, metal Pt films can help to obtain emitting centers that ensure highly conductive paths for the electrons from the ZnO NRs towards the vacuum, which effectively decrease the barrier between the ZnO and the vacuum. Finally, a schottky contact of Fe-ZnO and matched Fermi levels of ZnO-Pt contribute to the enhanced current emission efficiency. This work may help the development of the practical electron sources and advanced devices based on ZnO field emitters.