High Performance Field Emission Research Articles

Enhancing purity and crystallinity of carbon nanotubes by magnetically assisted arc discharge and thermal purification and their field emission characteristics

This study used a magnetically assisted arc discharge (AD) method and thermal purification to enhance the purity and crystallinity of multi-walled carbon nanotubes (MWCNTs). CNT synthesis in the presence of a magnetic field increased the densities of ionic carbon species and the arc plasma temperature, thereby improving CNT production. AD-MWCNTs produced at an optimal magnetic strength exhibited enhanced purity, crystallinity, and thermal stability. Thermal purification was also conducted to increase the purity of the AD-MWCNTs further. Purity was evaluated using Raman spectroscopy. AD-MWCNTs were homogenized to overcome limitations in light-penetration depths to ensure accurate evaluation. The calculated purities were consistent with the analytical results, confirming the reliability of Raman spectroscopy for purity assessment. The number of working CNT emitters on the film surface was directly related to the purity. Highly pure crystalline AD-MWCNTs exhibited high-performance field-emission properties, namely a high current density of 2.7 A/cm2 and long-term emission stability (12 h at a current density of 500 mA/cm2), with no performance reduction.

Utilization of rice straw for parallel production of sugars rich extract and paper

Abstract Complete utilization of different agricultural wastes is a great challenge before human society. The present study reports the valorization of rice straw for simultaneous production of reducing sugars and paper. The rice straw was first pre-extracted with water under different reaction conditions sequentially followed by papermaking. A maximum total reducing sugars (TRS) upto 12.38 % was extracted through aqueous pre-treatment. The pulped biomass was assessed for paper properties viz., tensile index, tear index, burst index, Kappa no. and brightness. For untreated rice straw tensile index, tear index and burst index were recorded within a range of 30.9–35.8 N m/g; 2.25–2.62 mN m2/g and 1.91–2.35 kN/g respectively where as it were ranged from 31.1 to 33.7 N m/g, 2.02–2.51 mN m2/g and 1.78–2.08 kN/g respectively for pretreated rice straw. Moreover, for untreated rice straw, kappa number and brightness were recorded in a range from 20.7 to 26.6 and 32.4 to 36.7 % ISO respectively, whereas for pre-treated rice straw it ranged from 19.1 to 26.3 and 31.6 to 35.5 % ISO respectively. High performance liquid chromatography (HPLC) analysis and field emission gun scanning electron microscopy (FESEM) were further employed during the study. In overall, extraction of underutilized sugars prior to pulping may prove as an efficient route for apt utilization of sugars followed by papermaking in near future.

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.

Controllable synthesis of indium oxide nanorod‐flowers for high field emission performance

AbstractIn this paper, indium oxide (In2O3) nanomaterials are successfully synthesized on the silicon substrates by hydrothermal method and calcination. By changing the ratio of raw materials, In2O3 exhibits three morphologies of nanorods, nanomaces, and nanorod‐flowers. Among the three morphologies of In2O3 nanomaterials, the nanorod‐flowers shaped In2O3 shows a strong field emission property, the turn‐on electric field as low as 0.97 V/µm and the field enhancement factor β up to 1053. The excellent performance is attributed to the higher length to diameter (L/D) ratio of the emitting tips and the better crystal quality for nanorod‐flowers shaped In2O3. The authors also show that for the nanorod‐flowers shaped In2O3, increasing separation distance, the turn‐on electric field increases up to about 3.67 V/µm and β decreases to 573 at d = 900 µm. This work provides new insights to design and synthesize nanomaterials with excellent field emission properties.

Recent Progress of Nanodiamond Film in Controllable Fabrication and Field Emission Properties.

The interest in the field electron emission cathode nanomaterials is on the rise due to the wide applications, such as electron sources, miniature X-ray devices, display materials, etc. In particular, nanodiamond (ND) film is regarded as an ideal next-generation cathode emitter in the field emission devices, due to the low or negative electron affinity, small grain size, high mechanical hardness, low work function, and high reliability. Increasing efforts are conducted on the investigation of the emission structures, manufacturing cost, and field emission properties improvement of the ND films. This review aims to summarize the recent research, highlight the new findings, and provide a roadmap for future developments in the area of ND film electron field emitter. Specially, the optimizing methods of large-scale, high-quality, and cost-effective synthesis of ND films are discussed to achieve more stable surface structure and optimal physical properties. Additionally, the mainstream strategies applied to produce high field emission performance of ND films are analyzed in detail, including regulating the grain size/boundary, hybrid phase carbon content, and doping element/type of ND films; meanwhile, the problems existing in the related research and the outlook in this area are also discussed.

Superior CoO/SiC nanowire field emitters with substantially increased stable emission sites: ultralow turn-on field, high current density and high stability

We explored high-performance field emitters with numerous CoO nanoparticles surrounding SiC nanowires for increasing emission sites, which exhibited ultralow turn-on field (0.40 V μm−1), high current density (18.6 mA cm−2) and high emission stability.

Superior field emission characteristics of highly crystalline and thermally stable carbon nanotubes grown in N2 and O2 by arc discharge

Multi-walled carbon nanotubes (MWCNTs) grown by chemical vapor deposition (CVD), which are commercially available in large quantities, commonly show poor field emission characteristics due to their low crystallinity. In this study, we fabricated highly crystalline MWCNTs using arc discharge (AD) via a mixture of N2 and O2 gases. The AD-MWCNTs showed a high oxidation temperature and lower threshold (Vth) and turn-on (Vto) voltages in repeated field emission measurements compared to CVD-MWCNTs. In a lifetime evaluation performed at an anode current of 3 mA for 20 and 120 h in diode and triode configurations, respectively, AD-MWCNTs showed better long-term stability than the CVD-MWCNTs. Thus, our findings demonstrate that AD-MWCNTs are promising for use in high-performance field emitters.

Hybrid Carbon Nanotubes–Graphene Nanostructures: Modeling, Formation, Characterization

A technology for the formation and bonding with a substrate of hybrid carbon nanostructures from single-walled carbon nanotubes (SWCNT) and reduced graphene oxide (rGO) by laser radiation is proposed. Molecular dynamics modeling by the real-time time-dependent density functional tight-binding (TD-DFTB) method made it possible to reveal the mechanism of field emission centers formation in carbon nanostructures layers. Laser radiation stimulates the formation of graphene-nanotube covalent contacts and also induces a dipole moment of hybrid nanostructures, which ensures their orientation along the force lines of the radiation field. The main mechanical and emission characteristics of the formed hybrid nanostructures were determined. By Raman spectroscopy, the effect of laser radiation energy on the defectiveness of all types of layers formed from nanostructures was determined. Laser exposure increased the hardness of all samples more than twice. Maximum hardness was obtained for hybrid nanostructure with a buffer layer (bl) of rGO and the main layer of SWCNT—rGO(bl)-SWCNT and was 54.4 GPa. In addition, the adhesion of rGO to the substrate and electron transport between the substrate and rGO(bl)-SWCNT increased. The rGO(bl)-SWCNT cathode with an area of ~1 mm2 showed a field emission current density of 562 mA/cm2 and stability for 9 h at a current of 1 mA. The developed technology for the formation of hybrid nanostructures can be used both to create high-performance and stable field emission cathodes and in other applications where nanomaterials coating with good adhesion, strength, and electrical conductivity is required.

Improving field emission performance of patterned ZnO electron emission source by optimizing array spacing

The field emission (FE) electron source has a wide range of application in the field of vacuum electronics, and the patterning of cathode emitters is one of the key technology. In this paper, a new idea of cathode structure to achieve good luminescence uniformity with the smallest emitter areas is proposed. First, the inkjet printing is applied to precisely position the patterned layer, and the branched ZnO arrays are grown by electrochemical deposition. Then the effect of array spacings on field emission performance is studied. The results show that with the increase of the array spacing, the FE performance first increases and then decreases, among which the arrays with the moderate spacing have the best FE properties, including the lowest turn-on field (Eon) of ∼0.46 V μm−1 and the most uniform luminescence. Finite element calculations using the ANSYS software were performed. Simulations are carried out to analyze the surface electric field distribution and electron movement trajectory, indicating the moderate array spacing is beneficial for FE due to the balance of the field superposition effect and shielding effect. The efficiency of pattern design helps to realize the high-performance field emission electron source, which is of great significance for flexible manufacturing and large-scale integration.

MoS2 nanosheet arrays grown on Mo foils for high-performance field emission

Free-standing molybdenum disulfide (MoS2) nanosheets with different morphologies on molybdenum (Mo) foil were synthesized through hydrothermal method using thiourea as the sulphur source and Mo foil as the Mo source. The effect of the concentration of thiourea in the reaction solution on the morphology and field emission (FE) properties of MoS2 nanosheets were investigated. The vertically aligned MoS2 nanosheets exhibit a low turn-on (∼1.90 V/μm) and threshold electric field (∼2.51 V/μm), along with good emission stability. The results are attributed to the high geometrical field enhancement factor (∼1979) and the sturdy structure of the MoS2 grown on highly conductive Mo substrate.

Multi-layer thin-film deposition for high-performance X-ray field-emission characteristics

In this study, we developed a nanoscale emitter having a multi-layer thin-film nanostructure in an effort to maximize the field-emission effect with a low voltage difference. The emitter was a sapphire board on which tungsten–DLC multi-player thin film was deposited using PVD and CVD processes. This multi-layer thin-film emitter was examined in a high-vacuum X-ray tube system. Its field-emission efficiency according to the applied voltage was then analyzed.

High-Performance Field Emitters Based on SiC Nanowires with Designed Electron Emission Sites.

Making field emitters with both low turn-on field (Eto) and high current emission stability is one of the keys to push forward their practical applications. In the present work, we report the exploration of high-performance field emitters with designed sharp corners around SiC nanowires for fundamentally enhanced electron emission sites. The sharp corners with tailored densities are rationally created based on a facile etching technique. Accordingly, the emission sites and nanowires are integrated into a single-crystalline configuration without interfaces, which could offer the emitters with a robust structure to avoid the structural damage induced by the generated Joule heat and electrostatic forces over long-term field emission (FE) operation. Consequently, the Eto of the as-fabricated SiC field emitter is low down to 0.52 V/μm, which is comparable to the state-of-the-art one ever reported. Moreover, they have high electron emission stability with a current fluctuation of just 2% over 10 h, representing their promising applications in FE-based electronic units.

Facile Fabrication and High Field Emission Performance of 2-D Ti₃C₂Tₓ MXene Nanosheets for Vacuum Electronic Devices

Although Ti3C2T x has been widely studied in applications such as electrical storage devices, there are very few reports about the field emission properties of Ti3C2T x . In this work, 2-D Ti3C2T x MXene was fabricated by simple etching Al layer from Ti3AlC2 in hydrofluoric acid (HF) at room temperature and employed as a cold cathode for field emission devices. The device presents a high field emission performance with a low turn-on field of 2.7 V/ $\mu \text{m}$ and excellent stability, much lower than the reported value of the former work (around 5 V/ $\mu \text{m}$ ), its relative 1-D TiC nanowires (7.1 V/ $\mu \text{m}$ ), and most of other 2-D cathode materials, such as MoS2 and graphene, which is found due to its unique accordion structure with evenly distributed sharp edges and enlarged layer spacing reducing the field screening effect. The results confirmed by numerical simulation demonstrate that the local electric intensity at sharp edges indeed significantly higher than that of elsewhere. In other words, more sharp edges owning to the increased interlayer spacing exposed, which can considerably boost field emission performance. Based on experimental data and simulation analysis, the edge effect was found to correspond well with other former reports. Hence, 2-D MXene can be a promising candidate for vacuum electronic applications other than energy storage and conversion devices.

Field emission from geometrically modulated tungsten-nickel sulfide / graphitic carbon nanobelts on Si microchannel plates

The low efficiency and instability of electron emission from two-dimensional (2D) materials such as WS2 and NiS hamper application to vacuum microelectronic devices. Although nickel-based alloys have many favorable physicochemical properties suitable for nanoelectronics, electron field emission (FE) from heterostructures comprising tungsten-nickel sulfide alloys and graphite carbon (WNi–S/C) has been seldom studied because it is difficult to fabricate the nanostructures in situ. In this work, field emitters composed of the WNi–S/C nanobelt composite are produced on Si microchannel plates (WNi–S/C@Si) hydrothermally. The structure has a porous and vertically aligned 3D morphology in addition to excellent adhesion strength with the substrate. Geometrical modulation of the WNi–S/C@Si is performed by changing the hydrothermal reaction temperature and an ultralow turn-on electric field of 0.51 V μm−1 for a current density 0.1 mA cm−2, threshold electric field of 0.65 V μm−1 for a current density of 1 mA cm−2, and stability of 97.1% for 8 h are accomplished. The related mechanisms are investigated and discussed. Moreover, finite element simulation shows that not only the shape of the nanostructures but also the high aspect plays a key role in reducing screen effects. These outstanding properties suggest large potential in high-performance FE and vacuum nanodevices.

High-performance x-ray source based on graphene oxide-coated Cu2S nanowires grown on copper film

Full static x-ray computed tomography (CT) technology has enabled higher precision and resolution imaging and has been applied in many applications such as diagnostic medical imaging, industrial inspection and security screening. In this technique, the x-ray source section is mainly composed of a thermionic cathode and electron beam scanning system. However, they have several shortcomings such as limited scanning angle, long response time and large volume. Distributed and programmable cold cathode (i.e. carbon nanotubes, ZnO nanowires (NWs)) field-emission x-ray sources are expected to solve these problems. However, there have been several long-standing challenges to the application of such cold field emitters for x-ray sources, such as the short lifetime and rigorous fabrication process, which have fundamentally prevented their widespread use. Here, we propose and demonstrate a cold field-emission x-ray source based on a graphene oxide (GO)-coated cuprous sulfide nanowire (Cu2S NW/GO) cathode. The proposed Cu2S NW/GO x-ray source provides stable emission (>18 h at a direct voltage of 2600 V) and has a low threshold (4.5 MV m−1 for obtaining a current density of 1 μA cm−2), benefiting from the demonstrated key features such as in situ epitaxy growth of Cu2S NWs on Cu, nanometer-scale sharp protrusions within GO and charge transfer between the Cu2S NWs and GO layer. Our research provides a simple and robust method to obtain a high-performance cold field emitter, leading to great potential for the next generation of x-ray source and CT.

High current density electron emission from an electrodeposited metal nanowire array

High-current electron field emitters are of interest for many applications, but state-of-the-art devices suffer from limitations such as high turn-on macroscopic field, low macroscopic current density, poor emission stability, and short lifetime. Field emitter arrays with a high spatial density of uniform emitters have the potential to address these problems. This work presents the process development, fabrication, and testing of a novel field emitter array. The authors employed electron beam lithography and templated electrodeposition to fabricate a high spatial density array of metallic nanowires, resulting in electron emission with high macroscopic current density (2 A/cm2) and low turn-on macroscopic field (4.35 V/μm). Templated electrodeposition of metallic nanowire arrays is a promising method for producing high performance field emitters.

Well-aligned periodic germanium nanoisland arrays with large areas and improved field emission performance induced by femtosecond laser

Si-based field emitters with high and stable current densities have gained increasing attention owing to their relevant compatibility with other silicon-based microelectronic and photonic devices. Currently, well-aligned Ge nanostructure arrays can be used in fabrication of Si-based field emitters with excellent performance. In this study, large-area well-aligned Ge nanoisland arrays are induced by femtosecond laser (120 fs, 800 nm, 1 kHz). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman analyses, and X-ray photoelectron spectroscopy (XPS) reveal that well-aligned Ge nanoisland arrays with good quality. Field emission (FE) measurements indicate Ge nanoisland arrays with high emission current densities and good stability. Overall, the excellent FE properties of the proposed Ge nanoisland arrays will make them potential for applications in high-performance field emitters integrated with Si nanodevices.

Enhanced field electron emission of single-walled carbon nanotubes prepared by imprinting technique

In this work, single-walled carbon nanotubes (SWCNTs) field electron emitters were fabricated by imprinting SWCNTs on substrates with a silver buffer layer. It was found that the field emission properties of imprinted SWCNTs were significantly improved with turn-on field at 0.62 V/μm and threshold field at 1.04 V/μm, comparing to screen-printed SWCNTs emitters who had turn-on field at 1 V/μm and threshold field at 2 V/μm. These improvements of SWCNTs emitters were owing to the directly imprinting method which made SWCNTs have clean surface and more emitters on the substrates. The lower turn-on electric field of field emission was attributed to no impurities encapsulate on the emitters and the large aspect ratio of SWCNTs resulting in high field enhancement factor of 15800. The emission performance of the imprinted cathode was almost unchanged after many times repeated experiments. This simple and low cost technique can fabricate high performance field emission cathodes efficiently and also demonstrates potential applications in many electronic devices.

Graphene nanosheets assisted carbon hollow cylinder for high-performance field emission applications

Electron sources are critical constituents for myriad many electronic applications including electron imaging, mass spectroscopy etc. Here, we report the realization of high-performance field emission from graphene assisted carbon hollow cylinders (Gr-CHCs) composed of multi-stacked radially aligned CNTs. These CHCs have built-in synthesis related Fe nanoparticles (NPs). This engineered nano-structure exhibits excellent field emission properties such as one of the ultra-low turn-on field (0.64 V μm−1 at 10 μA cm−2), low threshold field (0.74 V μm−1 at 100 μA cm−2), very high current density (15.49 mA cm−2 at 1.32 V μm−1) which is nearly double the current density obtained in our previous study. High field enhancement factor (0.72 × 104) with highly stable emission current at 100 μA was observed for more than 3 hrs at the base pressure of ∼1 × 10−8 mbar. This study suggests an approach to enhance the current density using the proposed innovative nanostructure and forms the basic theme of this communication. Highly efficient and stable field emissions observed are attributed to the geometry of cylinder and the production of the high density of sharp protrusions within the graphene sheets which enhance the local electric field and dramatically enhance field emission. This innovative cylindrical geometry associated with graphene assisted on Fe bearing aligned CNTs along with the periphery of the bulk cylinder provides an easy injection of electrons from the conduction band of CHCs into the vacuum in the presence of an external electric field. The tunability of field emission properties of these CHCs can be easily achieved by tailoring their diameter (10 and 20 mm) and different concentrations of the precursor. This new approach of the graphene assisted cylindrical geometry-based field emitter source provides enormous prospects and demand for next-generation high resolutions display devices.

One-step synthesis of tunable nitrogen-doped graphene from graphene oxide and its high performance field emission properties

Simultaneous reduction, repairing and doping of graphene oxide has been realized by the chemical vapor deposition method, using acetonitrile as a nitrogen source. A step-wise increase of acetonitrile partial pressure from 15 to 90 Pa results in nitrogen doped graphene (NG) with gradually tuning N-doping concentration from 0.38 to 0.66 at% and systematically rising graphite-N ratio from 23.25 to 45.39%, which in turn modulates field emission performance and enhance the stability. Raman spectroscopy and X-ray photoelectron spectroscopy suggest pyrrolic-N and pyridinic-N doping bring defects, while defects decrease as N-doping concentration increases due to the rising of graphite-N ratio. Proper defects may increase emission site density and N-doping can reduce work function. When N-doping concentration is controlled at 0.42 at%, NG exhibits the considerable decreasing of turn-on field from 3.35 to 2.18 V/μm at the emission current of 10 μA/cm2 and increasing of field enhancement factor from 1835 to 2967. It also reveals a good field emission stability with no degradation, which is superior to pristine reduced GO emitters. It is suggested the NG with tuning concentration of three type nitrogen emitter is a widely candidate for various field emission devices and applications.