Stable Field Emitters Research Articles

Highly Stable Field Emission from a Tungsten Diselenide Monolayer on Zinc Oxide Nanowire by Geometrically Modulating Hot Electrons

AbstractCompared with traditional thermal electron sources, field‐emission electron sources have many advantages, such as high brightness, coherence, and suitability for miniaturization. However, their instability has prevented their widespread use. Stable field emission from a hybrid structure with a tungsten diselenide (WSe2) monolayer on zinc oxide (ZnO) nanowires is achieved by geometrically modulating the WSe2 curvature. It is found that an emitter with a lower WSe2 curvature induces hotter electrons during field emission, which results in a low turn‐on field and high stability. Considering the similarity of 2D materials, a general approach for obtaining a highly stable field emitter using a 2D–1D hybrid structure is provided.

Synthesis of highly dense and vertically aligned array of SWCNTs using a catalyst barrier layer: High performance field emitters for devices

The authors in this research investigated the role of double catalyst layer Fe/Al in the growth of SWCNTs to obtain high current density, low turn on field and high field enhancement factor to act as efficient field emitters. We synthesise SWCNTs by PECVD technique using Fe/Al as a dual catalyst layer on Si substrate. It is seen that during the growth of SWCNTs, Al act as a barrier layer which reduces the diffusion of Fe catalyst in Si substrate resulting in high density vertically aligned growth of SWCNTs. Field emission measurements show high current density 30.5 mA/cm2 at very low turn-on field 1.2 Vμm−1 with a high field enhancement factor of 1.6 × 104. Stability and repeatability parameters of as-grown highly vertically aligned SWCNTs are also studied as a function of current density. Stability results show fairly stable field emitters for 7 h with good repeatability.

Oxygen and nitrogen doping in single wall carbon nanotubes: An efficient stable field emitter

Abstract Vertically aligned single wall Carbon Nanotubes (v-SWCNTs) were synthesized by plasma enhanced chemical vapour deposition system. Oxygen and nitrogen molecule inclusion in v-SWCNTs was performed in ultra high vacuum by radio frequency (RF) sputtering system. The creation of defects induced by oxygen and nitrogen plasma ions also drives the formation of different oxygen and nitrogen species at the SWCNTs sidewall surface as well as SWCNTs tips where incorporation is more efficient. The G/D ratio, radial breathing mode and other oxygen and nitrogen characteristic Raman modes were analyzed from Raman spectra. The type of attachments was also interpreted from fourier transform infrared spectroscopy and the electronic 1s core levels of oxygen and nitrogen with carbon in X-ray photoelectron spectroscopy. The effect of oxygen and nitrogen inclusion on the surface of SWCNTs was investigated in terms of its correlation in the enhancement of field emission characteristics. The analysis was done on the comparison of change in field enhancement factor of as-grown SWCNTs and plasma induced O-&N-SWCNTs. The results show drastic enhancement in current density at low turn on field with long term emission current stability. Our results give very unique improvement in field emission display devices using SWCNTs with simple doping effects.

Vertically aligned self-assembled gold nanorods as low turn-on, stable field emitters

In this work we have investigated field emission from self-assembled, vertically aligned, gold nanorod arrays, which were synthesized via a colloidal growth method. A field emission current density of ∼1mA/cm2 was measured for these gold nanorod arrays using an anode-cathode separation of ∼3.5mm. The field emission investigation of these gold nanorod arrays was carried out at a base pressure of ∼10−8mbar. The turn on field, defined as the electric field required to obtain a current density of 1μA/cm2, is observed to be 1.9V/μm. Assuming a work function value of 5.3eV, the field enhancement factor β is estimated to be ∼2931, which is higher than the reported values for other gold nanostructures/arrays.

High-temperature stable field emission of B-doped SiC nanoneedle arrays.

Current emission stability is one of the key issues for field emitters for them to be practically applied as electron sources. In the present work, large-scale and well-aligned B-doped SiC nanoneedle arrays have been grown on 6H-SiC wafer substrates via pyrolysis of polymeric precursors. The measured field emission (FE) characteristics suggest that the turn-on fields of the as-synthesized SiC nanoneedle arrays are reduced from 1.92 to 0.98 V μm(-1) with temperature increasing from room temperature (RT) to 500 °C, suggesting their excellent FE performances. The slightly changed current fluctuations (only ∼1.3%) between RT and 200 °C confirm that the present SiC nanoarrays with B dopants could be highly stable field emitters to be used in service under harsh conditions of high temperatures.

Double-walled carbon nanotubes: synthesis, structural characterization, and application

Double walled carbon nanotubes (DWCNTs) are considered an ideal model for studying the coupling interactions between different concentric shells in multi-walled CNTs. Due to their intrinsic coaxial structures they are mechanically, thermally, and structurally more stable than single walled CNTs. Geometrically, owing to the buffer-like function of the outer tubes in DWCNTs, the inner tubes exhibit exciting transport and optical properties that lend them promise in the fabrication of field-effect transistors, stable field emitters, and lithium ion batteries. In addition, by utilizing the outer tube chemistry, DWCNTs can be useful for anchoring semiconducting quantum dots and also as effective multifunctional fillers in producing tough, conductive transparent polymer films. The inner tubes meanwhile preserve their excitonic transitions. This article reviews the synthesis of DWCNTs, their electronic structure, transport, and mechanical properties, and their potential uses.

Porous Conjugated Polymer Nanotip Arrays for High Stable Field Emitter

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Porous Conjugated Polymer Nanotip Arrays for Highly Stable Field Emitter

Large area (26.7 cm2) nanotip arrays of porous conducting poly [5, 10, 15, 20-tetra (4-ethynylphenyl) porphyrin] diyne (TEPPD) have been successfully fabricated by an in situ cross-coupling reaction on the surface of the copper foil, which will open a new routine for large-area nanofabrication of porous conducting polymer on a conducting substrate. The surface-area of TEPPD nanotip arrays is up to 146 m2/g. Interestingly, the nanotip arrays of TEPPD display a good field-emission property and exhibit a better stability of field emission than that of organic and polymeric nanostructures because of the good heat radiation of porous, which is comparable to some important nanostructures of inorganic semiconductor. The porous conducting polymer could be used for new field-emission emitter and other molecular electronic devices.

Stable field emission from nanoporous silicon carbide

We report on a new type of stable field emitter capable of electron emission at levels comparable to thermal sources. Such an emitter potentially enables significant advances in several important technologies which currently use thermal electron sources. These include communications through microwave electronics, and more notably imaging for medicine and security where new modalities of detection may arise due to variable-geometry x-ray sources. Stable emission of 6 A cm−2 is demonstrated in a macroscopic array, and lifetime measurements indicate these new emitters are sufficiently robust to be considered for realistic implementation. The emitter is a monolithic structure, and is made in a room-temperature process. It is fabricated from a silicon carbide wafer, which is formed into a highly porous structure resembling an aerogel, and further patterned into an array. The emission properties may be tuned both through control of the nanoscale morphology and the macroscopic shape of the emitter array.

Thin SnO2 Nanowires with Uniform Diameter as Excellent Field Emitters: A Stability of More Than 2400 Minutes

AbstractThe stability of a field‐emission event, i.e., the stability of the emission current over a long period of time, against thermal effects, etc., is one of the key factors for its application in real devices. Although nanostructures have the advantages of high aspect ratios and faster device turn‐on times, the small masses and large surface areas make them vulnerable to both chemical and physical damages and they have a lower melting point compared to bulk materials of same compositions. SnO2, one of the most attractive oxide semiconductors, which has with a relatively low work function of 4.7 eV, has been a perspective candidate for field emitters. A highly stable field emitter based on thin and quasi‐aligned SnO2 nanowire ensembles with uniform diameter is shown. Field‐emission measurements of these SnO2 nanowire ensembles show low turn‐on and threshold voltages of 3.5 V μm−1 and 4.63 V μm−1, respectively, at an anode–sample distance of 200 μm and very long term scale stability of more than 2400 min, acquired at the electric field of 4.65 V μm−1. Such values are not only better than those of the recently developed SnO2 nanostructures with different morphologies and of randomly oriented SnO2 nanowire ensembles with a similar diameter distribution, but also comparable with the most widely studied field‐emission materials, such as carbon nanotubes and ZnO nanostructures. The potential for using these thin SnO2 nanowire ensembles with uniform diameter in field emitters is shown, with particular promise in those operated for long‐term real device applications.

Vertical arrays of SiNWs–ZnO nanostructures as high performance electron field emitters

Multicomponent hybrid materials of nanostructured building blocks are essential for the development of complex devices and advanced applications due to their role as either functional or interconnecting elements. This study introduces a simple and cost effective strategy for the synthesis of vertical arrays of silicon nanowires and ZnO nanostructures (nanorod and multipod structures). Formation of vertical nanostructured arrays is confirmed by SEM and HRTEM imaging as well as XRD and Raman measurements. We have investigated field emission properties of the as-synthesized vertical nanostructured arrays. Our results show that these SiNWs–ZnO nanostructures are highly efficient and stable field emitters.

Field emission studies of carbon nanostructures synthesised over Ni–Cr catalyst layer

Current research on the carbon-based nanotechnology needs progressive methods to control the shape, location and size of the nanostructures. Here, we report significant progress by synthesising the density controlled carbon nanostructures (CNSs) using acetylene and hydrogen in microwave plasma enhanced chemical vapour deposition system. Thin films of Ni–Cr (80 : 20, 60 : 40 and 50 : 50) sputtered over silicon (1 0 0) were used as catalysts. Scanning electron microscopy images of plasma annealed Ni–Cr coated silicon substrates show distributed nanoparticles of varying compositions over plasma annealed substrates. Morphologically and structurally different CNS were obtained when plasma annealed substrates were exposed to carbon vapours present in plasma. Transmission electron microscopy images suggested that the length and tip of CNS were in the range 50–100 nm and 4–6 nm, respectively. High resolution transmission electron microscopy images of the samples confirmed the presence of graphite (0 0 2) and nickel (2 0 0) planes in CNS. The field emission studies and Kelvin probe measurements of CNS grown over 80 : 20 Ni–Cr substrate show turn-on field and corresponding work function as 1.4 V µm−1 and 4.4 eV, respectively. Preliminary results show that these nanostructures could act as stable field emitters.

Stable Field Emitters for a Miniature X-ray Tube Using Carbon Nanotube Drop Drying on a Flat Metal Tip.

Stable carbon nanotube (CNT) field emitters for a vacuum-sealed miniature X-ray tube have been fabricated. The field emitters with a uniform CNT coating are prepared by a simple drop drying of a CNT mixture solution that is composed of chemically modified multi-walled CNTs, silver nanoparticles, and isopropyl alcohol on flat tungsten tips. A highly thermal- and electrical-conductive silver layer strongly attaches CNTs to the tungsten tips. Consequently, the field emitters exhibit good electron emission stability: continuous electron emission of around 100 μA at 2.3 V/μm has stably lasted over 40 h even at non-high vacuum ambient (~10−3 Pa).

Field emission from carbon nanotube/tin composite

Powder metallurgy was used to fabricate carbon nanotube (CNT) field emission cathodes. CNTs and tin (Sn) powder were blended, compacted and sintered. After polishing and etching, CNTs were exposed and protruded from the metal surface. CNTs were embedded into the Sn matrix, which acted as stable field emitters. The J-E curves show excellent field emission properties, such as low turn-on field of 2.8 V/μm, high emission current density and good current stability.

Improvement of carbon nanotube field emission properties by ultrasonic nanowelding

Ultrasonic nanowelding was used to improve the field emission properties of carbon nanotube (CNT) cathodes. The CNTs were deposited on the Ti-coated glass substrate by electrophoretic deposition. By pressing CNTs against metal (Ti) substrate under a vibrating force at ultrasonic frequency, a reliable and low resistance contact was obtained between CNTs and Ti. The scanning electron microscopy results show that CNTs are embedded into the metal substrate and act as stable field emitters. The welded cathode demonstrates an excellent field emission with high emission current density and good current stability.

Sb-doped SnO 2 wire: Highly stable field emitter

Field emission investigations of a single Sb-doped SnO 2 wire synthesized by thermal evaporation have been carried out in conventional field emission geometry under ultrahigh vacuum. An onset field required to draw a current of 1 nA has been reproducibly observed to be 9.5×10 3 V/μm. A current density of 5.88×10 3 A/cm 2 has been drawn with an applied field of 2×10 4 V/μm. The Fowler–Nordheim plot, obtained from the current–voltage ( I– V) measurements, shows linear nature in accordance with the quantum mechanical tunneling phenomenon. The field enhancement factor has been estimated to be 26,500 cm −1, indicating that the emission is from the nanometric features of the Sb-doped SnO 2 wire. The current–time ( I– t) measurement shows high current stability without severe deviations from the initial set value of 1 μA.

Large-scale aligned silicon carbonitride nanotube arrays: Synthesis, characterization, and field emission property

Large-scale aligned silicon carbonitride (SiCN) nanotube arrays have been synthesized by microwave-plasma-assisted chemical vapor deposition using SiH4, CH4, and N2 as precursors. The three elements of Si, C, and N are chemically bonded with each other and the nanotube composition can be adjusted by varying the SiH4 concentration, as revealed by electron energy loss spectroscopy and x-ray photoelectron spectroscopy. The evolution of microstructure of the SiCN nanotubes with different Si concentrations was characterized by high-resolution transmission electron microscopy and Raman spectroscopy. The dependence of field emission characteristics of the SiCN nanotubes on the composition has been investigated. With the increasing Si concentration, the SiCN nanotube exhibits more favorable oxidation resistance, which suggests that SiCN nanotube is a promising candidate as stable field emitter.

Multiple-tip liquid-metal field emitter

A stable field emitter with a large tip density (up to 108 cm−2) has been created using track membrane technology. The emitter has been tested in various current collection regimes from a constant current to short pulses of nanosecond duration with a repetition frequency of several kilohertz. The high stability of the emission current at moderate electronic current densities up to 100 mA/cm2 is attributed to the presence of a large number of emitting tips and the dynamic equilibrium between the repulsive forces of the electric field and the forces due to surface tension.

Field Emission Cathodes

The vacuum requirements to obtain stable field emission cathodes are discussed. To keep the average work function of a tungsten emitter constant, the total pressure of all chemically active gases should not exceed 10−14 Torr. To minimize ion bombardment of the emitter surface the helium pressure should be reduced by ion pumping even with tubes made of an alumino-silicate glass. The relationship between the current density and the slope of a Fowler-Nordheim plot has been calculated. Experimental evidence is given that variations in the work function can be compensated by changes in the field strength in such a way that stable field emitters are obtained. A qualitative explanation of such a mechanism is suggested.