Composite Emitter Research Articles

Effect of traps and conductive pathways on electron emission from copper broad-area composite emitters

This study investigates electron emission from copper broad-area emitters (CBAEs) and copper broad-area composite emitters (CBACEs) based on the principles of trapping and conductive pathways. Emission current measurements were conducted on two CBACEs, which consisted of copper coated with a 300–400 μm epoxy resin and subjected to high voltages up to 15 kV. The research specifically examines the ‘switch-on’ and collapse phenomena occurring within the epoxy layer. Field emission microscopy (FEM) was utilized in a high-vacuum environment ( 10−6 mbar) to observe these effects. A comprehensive model is developed to explain the formation of conductive pathways within the epoxy layer, allowing electrons transfer from traps to the surface. This model treats the composite emitter as a trap-rich capacitor. The study also clarifies the effects of trap density and epoxy layer thickness on the collapse process. To gain a deeper understanding of the model, changes in the I-V curve were examined. Simulations, scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-EDX) images, and Fourier transform infrared spectroscopy (FTIR) analysis were employed to understand the collapse mechanism of the epoxy collapse. Additionally, Nyquist and Cole–Cole plots were analyzed across frequencies ranging from 1 to 106 Hz before and after applying a high electric field on the samples, revealing changes in the capacitive component and the role of diodes in the formation of conductive channels.

Fabrication of three dimensional graphene/carbon nanotube/copper foam field emitter by electrophoretic deposition and its emission properties

In this paper, the graphene/carbon nanotubes/copper foam (GR/CNTs/CuF) three-dimensional field emitters were prepared by electrophoretic deposition (EPD). For comparison, the graphene/copper foam (GR/CuF) and carbon nanotubes copper foam (CNTs/CuF) field emitters were also prepared by the same way and characterized by the several different methods. The turn on electrical field (Eon) of composite materials of GR/CNTs/CuF was obtained to be 0.85 V/μm. Threshold electric field (Eth) is 1.52 V/μm. The composite of GR/CNTs/CuF have a large contact area, a lower volume resistance and lower contact resistance, which is more easy to electron transport. The root of the CNTs or the fold tip of GR is fixed in the interlaced composite, and the tip or fold forms a stable micro tip emission structure, which enhanced the field strength factor. The field emission performances are significantly better than that of individual GR/CuF or CNTs/CuF field emitter. The GR/CNTs/CuF as 3D field emitters were prepared by EPD method with low cost and easy manipulation, which provided a good way for a large-scale preparation of composite field emitters.

Electron Flows Formed by Electron-Optical Systems Using Composite Field Emitters Made of Thermally Expanded Graphite and Diamond–Graphite Mixtures

Electron Flows Formed by Electron-Optical Systems Using Composite Field Emitters Made of Thermally Expanded Graphite and Diamond–Graphite Mixtures

Field Electron Emission Characteristics of Tungsten–Polyethylene Composite Material As a Source of Electron Emission

Abstract: This work provides an experimental study on the effects of polyethylene coating on the field electron emission characteristics of clean (uncoated) tungsten tips. Several tungsten tips, with different apex radii, have been prepared, coated with different thicknesses of polyethylene layers and then examined using a standard field electron emission microscope. Various field electron emission characteristics have been measured under high-vacuum conditions. These include current-voltage characteristics, Fowler-Nordheim plots, scanning electron micrographs and spatial current distributions (electron-emission images). Based on this work, it is proved that coating tungsten tips with polyethylene layers has caused dramatic improvements on the tip emission properties. In particular, coating the tips improves the current-voltage characteristics, which is reflected in lowering the extraction voltage, getting more stable emission currents, expressing the current-voltage characteristics using Fowler-Nordheim plots and finally, the spatial distributions of the emitted electrons for the coated tips are more stable and uniform. Keywords: Field electron emitter, Polyethylene dielectric layer, Composite emitter, Field electron microscope, Scanning electron microscope.

Composite Multiferroic Terahertz Emitter: Polarization Control via an Electric Field

Electrical control of conjugate degrees of freedom in multiferroics provides the advantage of reducing energy consumption to femto- and even attojoules per switch in spintronics and memory devices. This is achieved through the development of technologies that make it possible to fabricate artificial materials with constantly improving properties. Here, we present the design, physics, and characteristics of a composite multiferroic spintronic emitter, which provides electrical control of the emitted terahertz (THz) wave polarization. The effect is due to electrical control of the magnetization in a high-quality magnetostrictive superlattice, ${\mathrm{Tb}\mathrm{Co}}_{2}/\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Co},$ deposited on an anisotropic piezoelectric substrate. In our approach, several mechanisms are realized in the system simultaneously: the strain-mediated coupling of the magnetic and piezoelectric subsystems, which operate in the range of the spin-reorientation transition of the magnetic superlattice, and THz-wave generation in the superlattice by an optical femtosecond pulse. This provides flexibility and control of the set of parameters. We determine the magnetoelectric parameter, which is responsible for THz polarization control. Our results offer a significant fundamental insight into the physics of composite multiferroic systems that can be used for applications of multiferroicity, primarily for THz spintronic emitters. We believe that our findings represent a decisive step towards technologies for other types of spintronic and memory devices.

Entangled photons from composite cascade emitters.

One of the most versatile sources for entangled photons are emitters that interact via more than one tunable mechanism. Here, we demonstrate how hybridization and dipole-dipole interactions-potentially simultaneously available in colloidal quantum dots and molecular aggregates-leveraged in conjunction can couple simple, well understood emitters into composite emitters with flexible control over the level structure. We show that cascade decay through carefully designed level structures can result in emission of frequency-entangled photons with Bell states and three-photon GHZ states as example cases. These results pave the way toward rational design of quantum optical emitters of entangled photons.

Enhanced field emission performance of MXene-TiO2 composite films.

A facile method to produce an MXene-TiO2 composite is demonstrated for enhanced field emission display applications. The field emission performance of two-dimensional free-standing and linear-shaped field emitters has been systematically investigated and enhanced electron emission behaviors (e.g. emission current, stability and emission patterns) are achieved by compositing MXene and TiO2 nanowires. The relationship between the emission current density, electric field and anode-cathode gap distance is studied and the emitters, especially the cross-section of the composite film, show good performance. The emission current from the cross-section of the composite film can reach 289 mA cm-2, which is the best result of the state of the art compared to single MXene and TiO2 nanowires. We have also reported a triboelectric nanogenerator powered by free-standing MXene-TiO2 composite emitters, implying the feasibility of the self-powering field emission devices and possibly enlarging the applications of cold emitters in various fields.

Synthesis, characterization and field emission studies of NiO-rGO nanocomposite

In this work, NiO-rGO nanocomposite was synthesized by hydrothermal/solvothermal method on nickel foil. The as-synthesized product was subjected to various characterizations such as X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Raman spectroscopy, in order to reveal its phase, morphology and structure. The SEM analysis reveals formation of nearly vertically oriented and randomly distributed nanoflakes of NiO-rGO nanocomposite on entire substrate surface. The Raman spectrum of the NiO-rGO composite showed characteristic D and G bands of rGO, confirming its presence in the composite. The NiO-rGO composite emitter showed promising field emission behaviour. The values of turn-on and threshold fields, defined at emission current density of 1 and 10 µA/cm2, were observed to be 1.8 and 2.25 V/µm, respectively, which are comparable to alike composite emitters. Furthermore, from the NiO-rGO composite emitter, maximum current density of 719 µA/cm2 has been extracted at an applied field of 4.5 V/µm. The observed results imply potential of NiO-rGO composite emitter for application in vacuum microelectronic devices.

Simultaneous enhancement of luminescence and stability of lead halide perovskites by a diatomite microcavity for light-emitting diodes

Lead halide perovskite quantum dots (Pe QDs) are revolutionizing the photoelectric field and hold considerable potential for future applications. However, the simultaneous promotion of both high luminescence performance and excellent stability is still a major challenge for its practical application. Herein, we presented a diatomite mineral with naturally optical microcavity is used to construct polyacrylic acid-grafted diatomite (DE-g-PAA) mediate reactors for in-situ crafting CsPbX3 QDs (X = Cl, Br and I). The as-prepared CPB (CsPbBr3) -DE-g-PAA composite emitters not only preserve a higher photoluminescence quantum yield about 76.4% ± 0.7% compared to 33.1% ± 0.5% for CPB but also present outstanding thermal stability and water-resistance property. This luminescence improvement further be confirmed as the electromagnetic field enhancement from resonance effect of cavities with uniform size in the diatomite via electromagnetic field simulations. This study provides new insights for achieving stable and highly efficient CsPbX3 QDs-based multifunctional materials using natural clay minerals with quasi-photonic crystal structure.

Composite Metallic Nano Emitters Coated with a Layer of Insulator Covered by Au Layer

Abstract: In this work, the differences in the behavior and properties of the emitted electron beam from tungsten (W) tips were studied before and after coating these tips with a thin layer of dielectric material followed by a thin layer of gold, to improve the emission current density, stability and emission current pattern concentration. The core of the composite cathode is made of high-purity tungsten (W). Measurements have been made with clean W emitters before and after coating these tips with two types of epoxy resins (epoxy 478 resins or epoxy UPR- 4 resins) followed by a thin layer of gold. For critical comparison, several tungsten tips with various apex-radii have been prepared using electrochemical etching techniques. The emitters have been coated by dielectric thin films of various thicknesses and the layer of Au used for coating the Epoxy layer has the same thicknesses. Their behavior has been recorded before and after the process of coating. These measurements include the current-voltage (I-V) characteristics and Fowler-Nordheim (F-N) plots. Imaging has also been done using a visible light microscope (VLM), along with a scanning electron microscope (SEM) to help in characterizing the epoxy layer thickness on the tip surface after coating. Besides, the emission patterns have been recorded from the phosphorescent screen of a field electron emission microscope (FEM). Having two types of composite systems tested under similar conditions provided several advantages. These measurements helped in producing a new type of emitters that have more suitable features with each of the two resins. Keywords: Cold field emission, Nano emitter, Dielectric coating, Au layer.

Solution-Processed Quasi-Two-Dimensional/Nanoscrystals Perovskite Composite Film Enhances the Efficiency and Stability of Perovskite Light-Emitting Diodes.

Solution-processed quasi-two-dimensional (Q-2D)/colloidal perovskite nanocrystals (PNCs) perovskite composite films are first prepared as the emitting layers of perovskite light-emitting diodes (PeLEDs). The subsequent multi-spin-coating of PNCs not only fills the gully-like fluctuations of the nanocrystal pinning-prepared Q-2D perovskite films and decreases their surface roughness but also transforms the bilayer perovskite nanosheets into multilayer ones, thus improving the charge transport and reducing the hole-injection barrier in the composite films. More importantly, the bromide vacancies and Pb defects in the Q-2D perovskites are removed via Br- supply and Pb-OOC-R interaction, in which the Br ions and COO- groups (from oleic acid) come from the PNC solution, and the radiation recombination is significantly enhanced. Based on the Q-2D/PNCs perovskite composite emitter, the PeLEDs achieve a maximum luminescence of ∼2.0 × 104 cd/m2 and a peak current efficiency of 27.5 cd/A, showing 175 and 337% enhancements compared to the control device with the pristine Q-2D perovskite emitter. The lifetime for the luminance decaying to 50% of the initial intensity increases by a factor of 13.8, demonstrating that the device stability is also improved by the Q-2D/PNCs perovskite composite film.

Thermal metamaterials for radiative plus conductive heat flow control

In this Letter, we show that the heat transferred from a thermal composite hot body emitter may be selectively directed in the far field toward a cold body receiver of choice through the enhanced design of both the anisotropic thermal conductivity of the emitter body and its surface emissivity. Specifically, focused radiosity of a representative cylindrical emitter in a preferential direction is attained by optimizing the layout of high and low thermally conductive materials within the solid in combination with an angularly varying emissivity surface profile. The relationship between the multi-body view factor scene and the thermal metamaterial design is clarified by way of numerical experiments. Subsequent gradient-based co-optimization of the thermal composite confirms the working principles of the device and reveals non-intuitive material and surface coating layouts that further enhance the directional radiative intensity of the emitter. The principles are extendable to the engineering of arbitrarily shaped advanced composites for thermal protection systems, energy conservation, or spacecraft low-energy deep-space maneuvering by way of radiation/conduction heat flow control.

Enhanced electrochemiluminescence of luminol based on Cu2O–Au heterostructure enabled multiple-amplification strategy

Herein, a credible construction strategy to improve electrochemiluminescence (ECL) of luminol was developed based on Cu2O–Au heterostructures. Summarily, gold nanoparticles (AuNPs) were anchored on surface of Cu2O nanocube (Cu2O@AuNPs) by spontaneous reduction reaction. Then, luminol molecules were concentrated on Cu2O@AuNPs using L-Cysteine (Cys) as covalent linkage to build the composite emitter (Cu2O@AuNPs-Cys-luminol). The enhancement mechanism was realized by following aspects: (I) Cu2O@AuNPs worked as electrocatalyst for glucose to generate coreactant of H2O2 in situ, avoiding the instability of direct addition of H2O2. (II) luminol molecules were firmly attached on Cu2O@AuNPs to achieve centralized and strong luminescence at low consumption. (III) Cys acted as an intramolecular coreactant and directly linked to luminol to increase luminous efficiency. To validate the effectiveness, a sandwiched immunoassay was built using concanavalinA (ConA) as analyte. Electroreduced graphene film as substrate provided phenoxy-derivatized dextran (DexP) with abundant binding sites and improved conductivity. To improve the specificity, DexP was used to identify ConA via the specific carbohydrate-ConA interaction. Then, Cu2O@AuNPs-Cys-luminol was modified on electrode as ECL signal indicator. The ECL immunosensor achieved determination of ConA with low detection limit of 2.9 × 10−5 ng/mL and excellent stability of continuous potential scan for 8 cycles. Experimental results demonstrated that the proposed construction strategy made considerable progress in ECL efficiency and stability of luminol. The creational pattern of construction strategy achieves high detection capabilities to ConA and expands the applicability of luminol in ECL system. It is expected to have more potential application value in immunoassay with universality.

Effects on Addition of Metal Oxides with Low Workfunctions on the Ca-Sr-Ba Oxide Cathodes for VUV Ionizers

There are several manufacturing techniques for developing thermionic cathodes for vacuum ultraviolet(VUV) ionizers. The triple alkaline earth metal emitters(Ca-Sr-Ba) are formulated as efficient and reliable thermo-electron sources with a great many different compositions for the ionizing devices. We prepare two basic suspensions with different compositions: calcium, strontium and barium. After evaluating the electron-emitting performance for europium, gadolinium, and yttrium-based cathodes mixed with these suspensions, we selected the yttrium for its better performance. Next, another transition metal indium and a lanthanide metal neodymium salt is introduced to two base emitters. These final composite metal emitters are coated on the tungsten filament and then activated to the oxide cathodes by an intentionally programmed calcination process under an ultra-high vacuum(~10-6 torr). The performance of electron emission of the cathodes is characterized by their anode currents with respect to the addition of each element, In and Nd, and their concentration of cathodes. Compared to both the base cathodes, the electron emission performance of the cathodes containing indium and neodymium decreases. The anode current of the Nd cathode is more markedly degraded than that with In.

Analysis of the Behavior of Individual Emission Sites on the Surface of a Multi-Tip Field Cathode

A method of analysis of the surface of multi-tip field cathodes using data obtained with the aid of a computer-controlled field emission projector and a system of fast high-voltage scanning is described. An investigation of a polymer/carbon nanotube composite field electron emitter by this method was used to determine the distribution of individual emission sites over the surface of a multi-tip field cathode and to estimate variation of the work function of separate emission sites depending on the applied voltage. The contributions of separate groups of emission sites to a change of the shape of current–voltage characteristic of a multi-tip cathode have been numerically estimated.

Анализ поведения индивидуальных эмиссионных центров на поверхности многоострийного полевого катода

AbstractA method of analysis of the surface of multi-tip field cathodes using data obtained with the aid of a computer-controlled field emission projector and a system of fast high-voltage scanning is described. An investigation of a polymer/carbon nanotube composite field electron emitter by this method was used to determine the distribution of individual emission sites over the surface of a multi-tip field cathode and to estimate variation of the work function of separate emission sites depending on the applied voltage. The contributions of separate groups of emission sites to a change of the shape of current–voltage characteristic of a multi-tip cathode have been numerically estimated.

Hydrothermal synthesis of rGO–PbBi2Se4 composite and investigation of its structural, chemical and field emission properties

In the present study we report the one step facile synthesis of pristine lead bismuth selenide (PbBi2Se4) and reduced graphene oxide (rGO) and its composites with PbBi2Se4. Formation of pristine PbBi2Se4 and rGO–PbBi2Se4 composite were confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. The surface morphology and topography investigated by using scanning electron microscopy and transmission electron microscopy revealed the formation of nano-flowers pristine PbBi2Se4. After coupling pristine PbBi2Se4 with rGO the surface morphology shows the formation of sharp vertically protruded nano-sheets/nano-flaks originated from the nano-flowers. Finally, the field emission properties of pristine PbBi2Se4 and rGO–PbBi2Se4 composite have been investigated. It has been observed that the rGO–PbBi2Se4 composite emitter exhibited excellent field emission properties with low turn-on field (~ 2.8 V/µm for 10 µA/cm2), high emission current density (~ 1288 µA/cm2 at 3.9 V/µm) and superior current stability (~ 4.5 h for ~ 1 µA) compare to pristine PbBi2Se4 emitter. Thus, the facile one step synthesis approach and robust nature of rGO–PbBi2Se4 composite emitter can provide prospects for the future development of large-area emitter applications such as flat-panel-display and vacuum micro/nanoelectronics devices.

Stretchable Light‐Emitting Diodes with Organometal‐Halide‐Perovskite–Polymer Composite Emitters

Intrinsically stretchable light-emitting diodes (LEDs) are demonstrated using organometal-halide-perovskite/polymer composite emitters. The polymer matrix serves as a microscale elastic connector for the rigid and brittle perovskite and induces stretchability to the composite emissive layers. The stretchable LEDs consist of poly(ethylene oxide)-modified poly(3,4-ethylenedioxythiophene) polystyrene sulfonate as a transparent and stretchable anode, a perovskite/polymer composite emissive layer, and eutectic indium-gallium as the cathode. The devices exhibit a turn-on voltage of 2.4 V, and a maximum luminance intensity of 15 960 cd m-2 at 8.5 V. Such performance far exceeds all reported intrinsically stretchable LEDs based on electroluminescent polymers. The stretchable perovskite LEDs are mechanically robust and can be reversibly stretched up to 40% strain for 100 cycles without failure.

Relationship of the Distribution Thickness of Dielectric Layer on the Nano-Tip Apex and Distribution of Emitted Electrons 

Field electron emission is the emission of electrons from the surface of a cathode under the influence of a high applied electrostatic field (typically about 3 V/nm) (Forbes et al., 2004; Mousa, 1996). The field electron emitter is particularly attractive as an electron source, due to its suitable emission properties and simple operating principle (Fischer et al., 2013). Tungsten is still one of the materials that are most frequently used for manufacturing field emitter tips (Latham, 1981; Marrese, 2000; Marulanda, 2010). Theoretically, cold field electron emission is the regime where (i) the electrons in the emitting region are effectively in local thermodynamic equilibrium (Mousa et al., 2012), and (ii) most electrons escape by deep tunneling from states close to the emitter’s Fermi level (Forbes et al., 2013). The first scientist to attempt to develop theory for cold field emission was Schottky (1923). Fowler and Nordheim (1928) developed the first appropriate theory for explaining field emission related phenomena. The first observation of what appears to be a field electron emission initiated electric discharge had been made by Winkler long before (Winkler & Gedanken, 1744). Electron emitter fabrication technology based on electrolytic etching (Melmed, 1991) has long been investigated and enhanced. This technology makes it possible to prepare electron emitters with a diameter in the nanometre range (Golubev & Shrednik, 2003). A wide range of composite micropoint cathodes has been manufactured in the authors’ group. The role of epoxylite resin in manufacturing composite emitters (Tungsten-Clark Electromedical Instruments Epoxylite resin [Tungsten/CEI-resin emitter]) have been to avoid degradation of the electron emitter tip due to ion sputtering processes during emission, to obtain an electron emitter with long lifetime, and improve the emission characteristics for the emitter (Mousa & Share, 1999; Mousa et al., 2001, 2012). In this work, various composite microemitters (Tungsten/ CEI-resin emitter) with different apex radii ranging from 112 to 287 nm were prepared. Scanning electron microscope (SEM) images were used to extract tip profiles (i.e., apex

Facile synthesis and enhanced field emission properties of Cu nanoparticles decorated graphene-based emitters

Cu nanoparticles (Cu NPs) were decorated on the surface of graphene films to form composite emitters. The graphene films were deposited by electrophoretic deposition and subsequently coated with Cu NPs via electrochemical reduction. The Cu NPs decorated graphene films exhibited lower turn-on field, lower threshold field and larger field enhancement factor compared with those of the pristine films. The Cu NPs decorated graphene films also showed good emission stability. The field emission properties of Cu NPs decorated graphene films were tunable through adjusting decoration time. This graphene-based composite could be used as a possible candidate for vacuum electron source.