Stable Field Electron Emission Research Articles

Stable Field Electron Emission and Plasma Illumination from Boron and Nitrogen Co‐Doped Edge‐Rich Diamond‐Enhanced Carbon Nanowalls

AbstractSuperior field electron emission (FEE) characteristics are achieved in edge‐rich diamond‐enhanced carbon nanowalls (D‐ECNWs) grown in a single‐step chemical vapor deposition process co‐doped with boron and nitrogen. The structure consists of sharp, highly conductive graphene edges supplied by a solid, diamond‐rich bottom. The Raman and transmission electron microscopy studies reveal a hybrid nature of sp3‐diamond and sp2‐graphene in these nanowalls. The ab‐initio calculations were carried out to support the experimental observations of diamond‐graphene hybrid structure. Finally, this hybrid D‐ECNWs is employed as a cathode in an FEE device resulting in a low turn‐on field of 3.1 V µm−1, a large field enhancement factor, a high FEE Je of 2.6 mA cm−2, and long lifetime stability of 438 min. Such an enhancement in the FEE originates from the unique materials combination, resulting in good electron transport from the graphene phases and efficient FEE of electrons from the sharp edges on the nanowalls. The prospective application of these materials is displayed by employing these hybrids as cathodes in a microplasma device ensuing a low threshold voltage of 160 V and high plasma stability of 140 min, which confirms the role of these hybrid structured nanowalls in the enhancement of electron emission.

Stable electron field emission from graphene/hexagonal boron nitride hybrid structure

Graphene/h-BN hybrid structure is constructed using simple and efficient solution process and the field emission properties of the hybrid structure are examined. It is found that the hybrid structure exhibits much more stable electron field emission properties compared to the pure graphene films. A Significantly higher maximum emission current density is obtained from the hybrid structure. In addition, the hybrid structure displays much better emission stability, whose current fluctuation is less than 4.5%. These results indicate that the graphene/h-BN hybrid structure is a promising candidate of reliable electron emission source in vacuum nanoelectronics devices.

Boron-Doped Nanocrystalline Diamond-Carbon Nanospike Hybrid Electron Emission Source.

Electron emission signifies an important mechanism facilitating the enlargement of devices that have modernized large parts of science and technology. Today, the search for innovative electron emission devices for imaging, sensing, electronics, and high-energy physics continues. Integrating two materials with dissimilar electronic properties into a hybrid material is an extremely sought-after synergistic approach, envisioning a superior field electron emission (FEE) material. An innovation is described regarding the fabrication of a nanostructured carbon hybrid, resulting from the one-step growth of boron-doped nanocrystalline diamond (BNCD) and carbon nanospikes (CNSs) by a microwave plasma-enhanced chemical vapor deposition technique. Spectroscopic and microscopic tools are used to investigate the morphological, bonding, and microstructural characteristics related to the growth mechanism of these hybrids. Utilizing the benefits of both the sharp edges of the CNSs and the high stability of BNCD, promising FEE performance with a lower turn-on field of 1.3 V/μm, a higher field enhancement factor of 6780, and a stable FEE current stability lasting for 780 min is obtained. The microplasma devices utilizing these hybrids as a cathode illustrate a superior plasma illumination behavior. Such hybrid carbon nanostructures, with superb electron emission characteristics, can encourage the enlargement of several electron emission device technologies.

Stable electron field emission from carbon nanotubes emitter transferred on graphene films

Carbon nanotubes (CNTs) arrays grown by microwave plasma enhanced chemical vapor deposition (MPCVD) method was transferred onto the substrate covered with graphene layer obtained by thermal chemical vapor deposition (CVD) technology. The graphene buffer layer provides good electrical and thermal contact to the CNTs. The field emission characteristics of this hybrid structure were investigated in this study. Compared with the CNTs arrays directly grown on the silicon substrate, the hybrid emitter shows better field emission performance, such as high emission current and long-term emission stability. The presence of this graphene layer was shown to improve the field emission behavior of CNTs. This work provides an effective way to realize stable field emission from CNTs emitter and similar hybrid structures.

Stable Field Emission from Layered MoS2 Nanosheets in High Vacuum and Observation of 1/f Noise

Field emission and current noise of hydrothermally synthesized MoS2 nanosheets are investigated in ultra-high-vacuum and industrially suited high-vacuum conditions. The study reveals that the emission turn-on field is pressure dependent. Moreover, the MoS2 nanosheets exhibit more stable field-electron emission in high-vacuum than in ultra-high-vacuum conditions. The investigations on field-emission current fluctuations show features of 1/f-type noise in ultra-high-vacuum and high-vacuum conditions, attributed to adsorption and desorption processes. The post-field-emission results indicate the MoS2 nanosheets are a robust field emitter in high-vacuum conditions.

Compact and Energy-Efficient Field Emission Cathodes for Sensor Applications

We report on miniaturized silicon field emitter arrays for the application in compact and energy-saving vacuum-microelectronic devices, e.g. sensors or x-ray tubes. Since standard silicon semiconductor technology has been used for the fabrication, they may be easily integrated with other silicon based circuits and devices on the same chip. The silicon tip geometry and the operating conditions were optimized in order to obtain highly uniform and stable electron field emission from large area cathode arrays. A series of uniform hexagonal tip arrays containing each 547 tips were fabricated and characterized. The electron emission properties of both individual tips as well as of complete emitter arrays were investigated. A saturation level in the voltage-current characteristics was found, which can be explained by the limitation of the supply of electrons due to the p-type silicon wafer material. When operating the arrays in the current saturation regime at an emission current of ~ 1 nA per tip, a highly stable and low noise emission can be observed.

Large and stable emission current from synthesized carbon nanotube/fiber network

In order to obtain a large and stable electron field emission current, the carbon nanotubes have been synthesized on carbon fibers by cold wall chemical vapor deposition method. In the hierarchical nanostructures, carbon fibers are entangled together to form a conductive network, it could provide excellent electron transmission and adhesion property between electrode and emitters, dispersed clusters of carbon nanotubes with smaller diameters have been synthesized on the top of carbon fibers as field emitters, this kind of emitter distribution could alleviate electrostatic shielding effect and protect emitters from being wholly destroyed. Field emission properties of this kind of carbon nanotube/fiber network have been tested, up to 30 mA emission current at an applied electric field of 6.4 V/μm was emitted from as-prepared hierarchical nanostructures. Small current degradation at large emission current output by DC power operation indicated that carbon nanotube/fiber network could be a promising candidate for field emission electron source.

High-density metallic nano-emitter arrays and their field emission characteristics

We report the fabrication and field emission properties of high-density nano-emitter arrays with on-chip electron extraction gate electrodes and up to 106 metallic nanotips that have an apex curvature radius of a few nanometers and a the tip density exceeding 108 cm−2. The gate electrode was fabricated on top of the nano-emitter arrays using a self-aligned polymer mask method. By applying a hot-press step for the polymer planarization, gate–nanotip alignment precision below 10 nm was achieved. Fabricated devices exhibited stable field electron emission with a current density of 0.1 A cm−2, indicating that these are promising for applications that require a miniature high-brightness electron source.

Very Stable Electron Field Emission from Strontium Titanate Coated Carbon Nanotube Matrices with Low Emission Thresholds

Novel PMMA-STO-CNT matrices were created by opened-tip vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) with conformal coatings of strontium titanate (STO) and poly(methyl methacrylate) (PMMA). Emission threshold of 0.8 V/μm was demonstrated, about 5-fold lower than that of the as-grown VA-MWCNTs. This was obtained after considering the related band structures under the perspective of work functions and tunneling width as a function of the STO thickness. We showed that there is an optimum thickness of STO coatings to effectively reduce the work function of CNTs and yet minimize the tunneling width for electron emissions. Furthermore, simulation and modeling suggest that PMMA-STO-CNT matrices have suppressed screening effects and Coulombs' repulsion forces between electrons in adjacent CNTs, leading to low emission threshold, high emission density, and prolonged emission stability. These findings are important for practical application of VA-MWCNTs in field emission devices, X-ray generation, and wave amplification.

Long-term Stable Field Electron Transfer from Carbon Nanotube Arrays at High Emission Current Densities

The stability behaviors of multiwalled carbon nanotube arrays during field electron emission are studied. The results indicate that the stability, even at a high emission current density, has been greatly improved by an aging process, and a degradation of about 0.66% in the emission current density at 21.86 mA/cm 2 during a 10-hour stability test has been obtained. A detailed analysis of the deterioration of the field electron emission characteristics is given, and the generation of Joule heat during field emission is found to be able to burn o the extruded carbon nanotubes, which will directly reduce the number of emission sites. On the other hand, the Joule heating eect may induce an annealing of the defects existing in the carbon nanotubes and may influence the distribution of electron energy states, both having a bad influence on the field emission characteristics. An aging process, especially aging at high emission current densities, can greatly reduce the influence of Jouleheating induced current degradation. Hence, an aging process at high emission current densities provides an eective way to realize long-term stable field electron emission from carbon nanotube arrays.

Stable Electron Field Emission from PMMA−CNT Matrices

We have created PMMA-CNT matrices by embedding opened-tip vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) with poly(methyl methacrylate) (PMMA). These PMMA-CNT matrices are excellent electron field emitters with an emission threshold field of 1.675 V/μm, more than 2-fold lower that that of the as-grown sample. In addition, the emission site density from these matrices is high, merely filling up the entire sample surface. Emission stability test at ∼1.35 mA/cm(2) was performed continuously for 40 h with no significant degradation. On the basis of our theoretical simulation and hypothetical modeling, we attribute these performances to the reduced screening effect and fewer Joule heatings due to the shorter effective transport distance of the electrons in MWCNTs.

A study of control growth of three-dimensional nanowire networks of tungsten oxides: From aligned nanowires through hybrid nanostructures to 3D networks

Three-dimensional (3D) nanowire network is a potential building block for nanodevices. Films of 3D tungsten nanowire networks were found early, and the present study is to develop a technical procedure for achieving very high percentage of 3D tungsten nanowire networks in a film. We demonstrate that the content of 3D tungsten nanowire networks in a film, prepared by thermal vapor deposition, may be adjusted by controlling the temperature of substrate, and also that films of very high percentage of 3D tungsten nanowire networks may be prepared. It is found that all films exhibit stable field electron emission, but that the performance varies depending on content of 3D nanowire networks.

Stable electron field emission from triangular-shaped ZnO nanoplate arrays with low localheating effects

We show here successful large-area growth of triangular-shaped ZnO nanoplatearrays on silicon substrates and the investigation of their electron field emissionproperties. The special geometric-shaped nanoplate emitters have a very largesurface area for heat dissipation and large field enhancement factors. These lead tovery stable and low-threshold field emission. The fluctuations of the macroscopiccurrent density are less than 1.5%. The threshold macroscopic field is about4.3 V µm−1 at an emitter–anodegap of 525 µm. The stable electron field emission is ascribed to low local heating effects, originating fromthe superior heat dissipation ability and the low Joule heating of the sheet-like emitters.Plotting Fowler–Nordheim curves yields straight lines with small deviations at lowand high macroscopic fields. The origins of these deviations are discussed. Ourexperiments demonstrate a promising design for stable and efficient emitters.

Stability of field emission current from various types of carbon nanotube films

A series of emission current measurements were taken from various types of multiwalled carbon nanotube (MWCNT) films in order to examine their stability for electron field emission. We found that the MWCNTs films grown by the catalytic thermal chemical vapor deposition (CVD) method exhibited much improved emission stability as compared to MWCNT films grown by the plasma-enhanced CVD (PECVD) method. We explain this difference of performance by referring to the graphitic order of these MWCNTs as detected by transmission electron microscopy and Raman spectroscopy. Results indicate that MWCNTs with high-order tubular structures demonstrate stable electron field emission. The best performing sample exhibits a constant current degradation of ∼3% per hour at an emission current density of ∼1 mJ/cm 2.

Hexagonally packed zinc oxide nanorod bundles on hydrotalcite sheets

A hexagonal-shaped, hydrotalcite-like [Zn(II)(1−x)Al(III)x(OH)2)]·mH2O interface that is very effective in promoting the self-assembly of ZnO nanorods has been developed on aluminium-coated substrates in alkaline hydrothermal conditions. The growth of the hydrotalcite interface is due to the reaction between zinc and aluminium ions and it provides a surface of low interfacial energy for the heterogeneous growth of ZnO nanorods at supersaturation levels that do not sustain homogeneous nucleation. The ZnO nanorods self-assemble to form a tightly packed “nail-on-bed” hexagonal structure on the hydrotalcite template. The size of the nanorods can be controlled by controlling the pH of the solution. Delamination of the thin hydrotalcite template at high temperatures produces sheets of ZnO nanorod bundles. Oriented growth of ZnO nanorods can be achieved on a wide range of substrates mediated by the thin hydrotalcite-precoated interface. Stable electron field emission characteristics can be obtained from the ZnO nanorods at a turn-on voltage of 2.73 V µm−1.

Field electron emission from flat metal cathodes covered by thin polymer films

We have studied the physical properties of a new class of field electron emitters representing metal cathodes covered by thin polymer films based on a soluble imide-siloxane copolymer. Polymer coating leads to an increase in the emission current and provides for a quite stable field electron emission of flat polished molybdenum and niobium cathodes.

Pretreatment of Ni-carboxylates metal-organics for growing carbon nanotubes on silicon substrates

A special pretreatment process was adopted for converting the Ni-carboxylates, Ni(C 7H 15COO) 2, into Ni-clusters, such that they were agglomerated but were not coalesced even after experiencing high temperature process. The growth of carbon nanotubes (CNTs) was thus facilitated. The CNTs obtained are very small in diameter (approx. 30 nm) and are straight, indicating that the CNTs contain very few defects. They exhibit large and stable electron field emission properties, that is, the field emission can be turned on at E o =3.3–4.2 V/μm, with emission current capacity of J e =2.2 mA/cm 2, at 7.0 V/μm applied field. These results imply that spin coating of Ni(C 7H 15COO) 2 is a simple and inexpensive method to form catalysts for the growth of CNTs and has the potential for scaling up.

Field and photo-field electron emission from self-assembled Ge?Si nanostructures with quantum dots

Monolayer and multilayer Ge nanocluster structures were prepared on Si(1 0 0) using molecular beam epitaxy. The cluster size was ⩽10 nm and cluster density was ∼1010 cm−2. A stable field electron emission was obtained from these structures, showing current peaks in the current–voltage characteristics, which may be attributed to the resonant electron tunneling via the energy levels of the nanocluster potential well. For cluster multilayers, the current–voltage curves also showed current peaks with a complex shape. The cluster multilayer structures had a considerable temperature sensitivity, as well as photosensitivity, in the wavelength range from 0.4 to 10 μm.

Field and photo-field electron emission from self-assembled Ge–Si nanostructures with quantum dots

Monolayer and multilayer Ge nanocluster structures were prepared on Si(1 0 0) using molecular beam epitaxy. The cluster size was ⩽10 nm and cluster density was ∼10 10 cm −2. A stable field electron emission was obtained from these structures, showing current peaks in the current–voltage characteristics, which may be attributed to the resonant electron tunneling via the energy levels of the nanocluster potential well. For cluster multilayers, the current–voltage curves also showed current peaks with a complex shape. The cluster multilayer structures had a considerable temperature sensitivity, as well as photosensitivity, in the wavelength range from 0.4 to 10 μm.

Preparation and characterization of nanostructured film of graphitized diamond crystallites for field electron emission

The details are given of the preparation technique that has been developed for depositing crystalline-nanodiamond particles on large-area silicon substrates, based on an dielectrophoresis process. A patented technique was adapted to prepare the substrate surface with high density of uniformly distributed nanoprotrusions that provide local field enhancement necessary for dielectrophoresis deposition. The nanodiamond crystallites were treated at elevated temperatures to have their surfaces graphitized. High-resolution transmission microscopy and electron diffraction techniques as well as micro Raman analysis were used to study the properties of the so-prepared nanostructured film. Stable field electron emission from these films with current density of >5 mA/cm2 may be regularly obtained at fields as low as ∼3 V/μm. The current–voltage characteristics and the corresponding Fowler–Nordheim plots, emission uniformity, and current stability were studied. The findings suggest that this type of film is a potential candidate as a large-area cold cathode. A model based on emission associated nanoscale graphite network and nanotriple junction is presented.