Enhanced Electron Field Emission Research Articles

Development of diamond cathode materials for enhancing the electron field emission and plasma characteristics using two-step microwave plasma enhanced chemical vapor deposition process

The enhancement on the plasma illumination characteristics of a cylindrical microplasma device due to the utilization of hybrid-diamond (HiD) films as cathode was systematically investigated. The improved plasma illumination behavior was closely related to the enhanced electron field emission (EFE) properties of the diamond films. The HiD films, which possessed better EFE properties, including lower turn-on field for inducing the EFE process [(E0)efe = 2.7 V/μm] and higher EFE current density [(Je)efe = 2.8 mA/cm2, at 10.6 V/μm], resulted in superior illumination performance for the microplasma devices. The plasma can be triggered at a low threshold field of (Eth)pl. = 0.166 V/μm, attaining a large plasma current density of (Je)pl. = 9.6 mA/cm2 at an applied field of 0.266 V/μm (plasma density of ne = 1.70 × 1015 cm−3). The better EFE for the HiD films is ascribed to the unique granular structure of the films. Transmission electron microscopic studies revealed that the HiD films contained large diamond aggregates evenly distributed among the ultrasmall grain matrix. There presents a-few-layer graphite, surrounding the large aggregates that formed electron transport networks and improved the EFE properties for the HiD films. The superior EFE properties for the HiD cathode materials are the prime factor for improving plasma illumination characteristics for the cylindrical microplasma devices.

Enhancing electrical conductivity and electron field emission properties of ultrananocrystalline diamond films by copper ion implantation and annealing

Copper ion implantation and subsequent annealing at 600 °C achieved high electrical conductivity of 95.0 (Ωcm)−1 for ultrananocrystalline diamond (UNCD) films with carrier concentration of 2.8 × 1018 cm−2 and mobility of 6.8 × 102 cm2/V s. Transmission electron microscopy examinations reveal that the implanted Cu ions first formed Cu nanoclusters in UNCD films, which induced the formation of nanographitic grain boundary phases during annealing process. From current imaging tunneling spectroscopy and local current-voltage curves of scanning tunneling spectroscopic measurements, it is observed that the electrons are dominantly emitted from the grain boundaries. Consequently, the nanographitic phases presence in the grain boundaries formed conduction channels for efficient electron transport, ensuing in excellent electron field emission (EFE) properties for copper ion implanted/annealed UNCD films with low turn-on field of 4.80 V/μm and high EFE current density of 3.60 mA/cm2 at an applied field of 8.0 V/μm.

Enhanced field electron emission properties of hierarchically structured MWCNT-based cold cathodes

Hierarchically structured MWCNT (h-MWCNT)-based cold cathodes were successfully achieved by means of a relatively simple and highly effective approach consisting of the appropriate combination of KOH-based pyramidal texturing of Si (100) substrates and PECVD growth of vertically aligned MWCNTs. By controlling the aspect ratio (AR) of the Si pyramids, we were able to tune the field electron emission (FEE) properties of the h-MWCNT cathodes. Indeed, when the AR is increased from 0 (flat Si) to 0.6, not only the emitted current density was found to increase exponentially, but more importantly its associated threshold field (TF) was reduced from 3.52 V/μm to reach a value as low as 1.95 V/μm. The analysis of the J-E emission curves in the light of the conventional Fowler-Nordheim model revealed the existence of two distinct low-field (LF) and high-field (HF) FEE regimes. In both regimes, the hierarchical structuring was found to increase significantly the associated βLF and βHF field enhancement factors of the h-MWCNT cathodes (by a factor of 1.7 and 2.2, respectively). Pyramidal texturing of the cathodes is believed to favor vacuum space charge effects, which could be invoked to account for the significant enhancement of the FEE, particularly in the HF regime where a βHF as high as 6,980 was obtained for the highest AR value of 0.6.

An Experimental Investigation on Enhanced Electron Field Emission from CeO<SUB>2</SUB> Nanoparticles Decorated Graphene Sheets

An Experimental Investigation on Enhanced Electron Field Emission from CeO<SUB>2</SUB> Nanoparticles Decorated Graphene Sheets

Hybrid structure of graphene sheets/ZnO nanorods for enhancing electron field emission properties

A hybrid structure of graphene sheets (GSs)/ZnO nanorods (ZNRs) is developed on glass substrates for applications in field emission (FE) cathodes. ZNRs are grown on glass substrates by a combination of sputtering and hydrothermal processes. Graphene films are synthesized on the Cu foils by the thermal chemical vapor deposition technique, and are transferred onto the ZNRs with various cycles (one, two, and three times) to form GS/ZNR hybrids with different morphology. FESEM, XRD, and Raman spectroscopy are used to analyze the surface morphology, crystallinity, and chemical composition, respectively. It is found that the FE properties of all GS/ZNR cathodes possess a lower turn-on electric field and higher current density than pristine ZNR cathode owing to the presence of the GS. The GSs/ZNRs with two transferred cycle exhibit the lowest turn-on electric field (3.7V/μm, 1μA/cm2) and highest enhancement factor β (8723). These superior properties indicate that the GS/ZNR hybrids are promising field cathodes in FE applications.

Zinc Oxide Nanowire Lateral Field Emission Devices and its Application as Display Pixel Structures

The rational design and fabrication of zinc oxide (ZnO) nanowire (NW) lateral field electron emission device and the possible application as a display pixel structure are reported. In the device, the cathode and anode are ranked side-by-side on the same panel. The NW-clusters were controlled to locally grow on the edges of the electrodes with different tilted status, i.e., in angle range of 75°-110°, 0°-110°, and 0°-57°, respectively. The devices with NWs at different tilt-angle showed distinct field electron emission properties. The device with 0°-57° tilted NWs possess the best performance, i.e., an emission current of 9.3 μA (current density: 6.22 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) was obtained at a low cathode-anode (50 μm in separation) bias of 477 V. Stable cathodoluminescence was observed from the indium titanic oxide anode, suggests a possibility for display application. Mechanisms responsible for the enhanced field electron emission and the related device physics are proposed. Significantly, the low temperature (~ 80°C) solution-phase growth of ZnO NWs enables the fabrication of the devices on flexible polyimide substrate, which has also been demonstrated here. This paper opens up possibilities on developing NW-based lateral field electron emission device for vacuum micro/nanoelectronics applications.

Electron Field Emission Enhancement of Vertically Aligned Ultrananocrystalline Diamond‐Coated ZnO Core–Shell Heterostructured Nanorods

Enhanced electron field emission (EFE) behavior of a core-shell heterostructure, where ZnO nanorods (ZNRs) form the core and ultrananocrystalline diamond needles (UNCDNs) form the shell, is reported. EFE properties of ZNR-UNCDN core-shell heterostructures show a high emission current density of 5.5 mA cm(-2) at an applied field of 4.25 V μm(-1) , and a low turn-on field of 2.08 V μm(-1) compared to the 1.67 mA cm(-2) emission current density (at an applied field of 28.7 V μm(-1) ) and 16.6 V μm(-1) turn-on field for bare ZNRs. Such an enhancement in the field emission originates from the unique materials combination, resulting in good electron transport from ZNRs to UNCDNs and efficient field emission of electrons from the UNCDNs. The potential application of these materials is demonstrated by the plasma illumination measurements that lowering the threshold voltage by 160 V confirms the role of ZNR-UNCDN core-shell heterostructures in the enhancement of electron emission.

Improved electron field emission from metal grafted graphene composites

Metal nanoparticle grafted graphene films (GFs) form a new composite for electron field emission devices. The GFs were deposited on Ni coated Si wafers by microwave plasma enhanced chemical vapour deposition. Graphene-based composites using Ti, Pd, Ag and Au were formed by thermal evaporation. The surface morphology and microstructure were probed by scanning and high resolution transmission electron microscopy. Improvement in the electron field emission and reduction in the turn on and threshold fields were observed in metal grafted GFs as compared to those from pristine films. It was found that among the grafted metals, Ti adsorption contributed more efficiently in enhancing the electron field emission properties by lowering its work function. The enhanced electron field emission characteristics were analyzed using the density functional theory calculations for metal grafted graphene ribbon. Our results indicate increased density of states near the Fermi level for metal grafted graphene ribbon which is responsible for the improvement in electron field emission. We suggest that grafting of metal nanoparticles on GFs could be exploited for the development of efficient field emitters.

Gold ion implantation induced high conductivity and enhanced electron field emission properties in ultrananocrystalline diamond films

We report high conductivity of 185 (Ω cm)−1 and superior electron field emission (EFE) properties, viz. low turn-on field of 4.88 V/μm with high EFE current density of 6.52 mA/cm2 at an applied field of 8.0 V/μm in ultrananocrystalline diamond (UNCD) films due to gold ion implantation. Transmission electron microscopy examinations reveal the presence of Au nanoparticles in films, which result in the induction of nanographitic phases in grain boundaries, forming conduction channels for electron transport. Highly conducting Au ion implanted UNCD films overwhelms that of nitrogen doped ones and will create a remarkable impact to diamond-based electronics.

Edge effect enhanced electron field emission in top assembled reduced graphene oxide assisted by amorphous CNT-coated carbon cloth substrate

In this work a hybrid structure assembly of amorphous carbon nanotubes (a-CNTs) -reduced graphene oxide (RGO) has been fabricated on carbon cloth/PET substrates for enhanced edge effect assisted flexible field emission device application. The carbon nanostructures prepared by chemical processes were finally deposited one over the other by a simple electrophoretic deposition (EPD) method on carbon cloth (CC) fabric. The thin films were then characterized by X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscope (HRTEM). Field assisted electron emission measurement was performed on this hybrid structure. It was observed that the hybrid carbon nanostructure showed exceptional field emission properties with outstanding low turn-on and threshold field (Eto∼ 0.26 Vμm−1, Eth ∼ 0.55 Vμm1). These observed results are far better compared to standalone and plasma etched edge enhanced RGO systems due to the bottom layer a-CNTs bed which assisted in significant enhancement of edge effect in RGO sheets.

Microplasma enhancement via the formation of a graphite-like phase on diamond cathodes

Enhanced electron field emission (EFE) properties in microcrystalline diamond (MCD) films that have been Fe-coated and postannealed are observed. Additionally, improved microplasma characteristics are also observed when these materials are used as cathodes. The turn-on field for inducing the EFE process decreases from 4.7 V/μm for pristine MCD films to 2.2 V/μm for the Fe-coated/postannealed ones, whereas the EFE current density at an applied field of 8.8 V/μm increases from 36.5 to 5327.1 μA/cm2. Transmission electron microscopy, in conjunction with high-angle annular dark field and 3D-tomography studies, reveals that enhanced EFE in the Fe-coated/postannealed MCD films is due to the graphite-like phase on the surface of diamond films. The authors infer that the Fe-coating interacts with the diamond in the postannealing process to dissolve carbons and reprecipitate them in nanographite networks. This process is similar to the formation of carbon nanotubes by the dissolution and reprecipitation of carbon species at the presence of nanosized Fe catalysts. The utilization of high EFE diamond films as cathode materials enhances the microplasma, as the ignition field for initiating the plasma is lowered and a high plasma current density is attainable.

Synthesis of diamond nanotips for enhancing the plasma illumination characteristics of capacitive-type plasma devices

The enhancement of the plasma illumination characteristics of capacitive-type plasma devices (CP-devices) utilizing diamond-coated Si-nanotips as cathodes was systematically investigated. The enhanced electron field emission (EFE) properties of the diamond films resulted in improved plasma illumination characteristics of the devices. Microcrystalline diamond films grown using ultrananocrystalline diamond as a nucleation layer (MCD/UNCD) possessed a lower turn-on field for inducing the EFE process with a higher EFE current density and resulted in a better plasma illumination performance for the CP-devices compared with those made from MCD films grown directly on Si-substrates without the nucleation layer. Transmission electron microscopy revealed that, in a two-step microwave plasma enhanced chemical vapor deposition process, the second step altered the granular structure of the UNCD nuclear layer instead of growing a layer of large-grain diamond film on top of the UNCD nucleation layer, resulting in a duplex microstructure. The MCD/UNCD films contained large diamond aggregates evenly distributed among the ultrasmall-grain matrix, with the induction of a few layers of graphite, surrounding the large aggregates. The presence of the graphene-like phase is presumed to be the prime factor resulting in the superior EFE properties of the MCD/UNCD films and the better plasma illumination characteristics of the CP-devices.

Fabrication of free-standing highly conducting ultrananocrystalline diamond films with enhanced electron field emission properties

Fabrication of free-standing/highly conducting ultrananocrystalline diamond (fc-UNCD) films at low growth temperature (&amp;lt;475 °C) is demonstrated. The fc-UNCD films show high conductivity of σ = 146 (Ω cm)−1 with superior electron field emission (EFE) properties, viz. low turn-on field of 4.35 V/μm and high EFE current density of 3.76 mA/cm2 at an applied field of 12.5 V/μm. Transmission electron microscopy examinations reveal the presence of Au/Cu clusters in film-to-substrate interface, which consequences in the induction of nanographite phases, surrounding the diamond grains that form conduction channels for electrons transport, ensuing in marvelous EFE properties of fc-UNCD films.

Electrospun MgO-loaded carbon nanofibers: Enhanced field electron emission from the fibers in vacuum

MgO-loaded electrospun carbon nanofibers (MgO/CNFs) were prepared by electrospinning a magnesium acetate containing polyacrylonitrile composite followed by stabilization under an air atmosphere at 280°C and carbonization under a nitrogen atmosphere at 800°C. In addition to investigating the morphological and material features of the nanofibers, the field emission (FE) characteristics of the carbonized NFs (CNFs), performed in an ultra-high vacuum chamber utilizing scanning electron microscopy (SEM), were determined. The results of the investigation show that the MgO/CNFs (195.5% enhancement) display enhanced field electron emission as compared to that of pure CNFs as a result of the existence of a MgO phase. Consequently, it appears that the graphitic structures of CNFs can be tuned, a finding that has significance in studies aimed at developing new field electron emission devices.

Enhanced Electron Field Emission from Carbon Nanotubes Irradiated by Energetic C Ions

The field emission performance and structure of the vertically aligned multi-walled carbon nanotube arrays irradiated by energetic C ion with average energy of 40 keV have been investigated. During energetic C ion irradiation, the curves of emission current density versus the applied field of samples shift firstly to low applied fields when the irradiation doses are less than 9.6 x 10(16) cm(-2), and further increase of dose makes the curves reversing to a high applied field, which shows that high dose irradiation in carbon nanotube arrays makes their field emission performance worse. After energetic ion irradiation with a dose of 9.6 x 1016 cm(-2), the turn-on electric field and the threshold electric field of samples decreased from 0.80 and 1.13 V/microm to 0.67 and 0.98 V/microm respectively. Structural analysis of scanning electron microscopy, transmission electron microscopy and Raman spectroscopy indicates that the amorphous carbon nanowire/carbon nanotube hetero nano-structures have been fabricated in the C ion irradiated carbon nanotubes. The enhancement of electron field emission is due to the formation of amorphous carbon nanowires at the tip of carbon nanotube arrays, which is an electron emitting material with low work function.

Engineering the Interface Characteristics of Ultrananocrystalline Diamond Films Grown on Au-Coated Si Substrates

Enhanced electron field emission (EFE) properties have been observed for ultrananocrystalline diamond (UNCD) films grown on Au-coated Si (UNCD/Au-Si) substrates. The EFE properties of UNCD/Au-Si could be turned on at a low field of 8.9 V/μm, attaining EFE current density of 4.5 mA/cm(2) at an applied field of 10.5 V/μm, which is superior to that of UNCD films grown on Si (UNCD/Si) substrates with the same chemical vapor deposition process. Moreover, a significant difference in current-voltage curves from scanning tunneling spectroscopic measurements at the grain and the grain boundary has been observed. From the variation of normalized conductance (dI/dV)/(I/V) versus V, bandgap of UNCD/Au-Si is measured to be 2.8 eV at the grain and nearly metallic at the grain boundary. Current imaging tunneling spectroscopy measurements show that the grain boundaries have higher electron field emission capacity than the grains. The diffusion of Au into the interface layer that results in the induction of graphite and converts the metal-to-Si interface from Schottky to Ohmic contact is believed to be the authentic factors, resulting in marvelous EFE properties of UNCD/Au-Si.

Enhanced Field Electron Emission from Electrospun Co-Loaded Activated Porous Carbon Nanofibers

Highly porous, Co-loaded, activated carbon nanofibers (Co/AP-CNFs) were prepared by electrospinning a CoCl2-containing polyacrylonitrile composite, followed by thermal treatment processes under air and inert atmospheres. Observations show that carbon nanofibers (CNFs) generated in this fashion have a dramatically large porosity that results in an increase in the specific surface area from 193.5 to 417.3 m(2) g(-1)as a consequence of the presence of CoCl2 in PAN/CoCl2 precursor nanofibers. The nanofibers have a larger graphitic structure, which is enhanced by the addition of the cobaltous phase during the carbonization process. Besides evaluating the morphological and material features of the fibers, we have also carried out a field electron emission investigation of the fibers. The results show that an enhancement in the field electron emission of Co/AP-CNFs occurs as a result of the existence of cobalt in the carbon nanofibers, which results in a greater graphitization, increased specific total surface area and porosity of the carbon nanofibers. Overall, the Co/AP-CNFs are prepared in a facile manner and exhibit an enhanced field electron emission (54.79%) compared to that of pure CNFs, a feature that suggests their potential application to field electron emission devices.

Enhanced Field Electron Emission From Zinc-Doped CuO Nanowires

Zinc-doped copper oxide (CuO:Zn) nanowires (NWs) with Cu and Zn layers were grown by thermal oxidation on a glass template in ambient air. The Zn content in the CuO NWs was approximated 9.9%. Field emitters using these CuO:Zn NWs were also fabricated on the glass substrate and compared with those using NWs composed of CuO alone. The threshold fields of the CuO:Zn NW and CuO NW field emitters can be significantly decreased from 8.3 to 4.1 V/mm, and the work function can also be reduced from 4.5 to 1.54 eV by introducing Zn atoms into the CuO NWs.

Study on electron field emission enhancement from nitrogenated carbon nanotips synthesized by plasma-enhanced hot filament chemical vapor deposition

Nitrogenated carbon nanotips (NCNTPs) with different structures were synthesized by plasma-enhanced hot filament chemical vapor deposition using methane, hydrogen and nitrogen as the reactive gases. The structures and compositions of the NCNTPs were studied by field emission scanning electron microscopy (FESEM), micro-Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The XPS spectra reveal that nitrogen is incorporated into the carbon nanotips to form the NCNTPs under plasma condition. The Raman spectra and FESEM images show that the NCNTPs are amorphous structure and their morphologies change with the change in deposition conditions, respectively. The electron field emission (EFE) from the NCNTPs was measured and the EFE results indicate that the NCNTPs with the smooth surfaces and high density can emit a current density of 3 × 103 μA/cm2 at an electric field of 7.2 V/μm, which exhibits better EFE characteristic than the NCNTPs with the carbon nanowires on their surfaces due to small amount of oxygen adsorbed on the smooth surfaces of NCNTPs. According to the possible structures of nitrogen in sp2 cluster in rings, the EFE enhancement of the NCNTPs compared with pure carbon nanotips was studied. The high emission current density (3 × 103 μA/cm2) at low field (7.2 V/μm) suggests that the NCNTPs can serve as effective electron emission sources for numerous applications.

Enhanced electron field emission from ZnO nanoparticles-embedded DLC films prepared by electrochemical deposition

ZnO nanoparticles-embedded diamond-like amorphous (DLC) carbon films have been prepared by electrochemical deposition. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) results confirm that the embedded ZnO nanoparticles are in the wurtzite structure with diameters of around 4 nm. Based on Raman measurements and atomic force microscope (AFM) results, it has been found that ZnO nanoparticles embedding could enhance both graphitization and surface roughness of DLC matrix. Also, the field electron emission (FEE) properties of the ZnO nanoparticles-embedded DLC film were improved by both lowering the turn-on field and increasing the current density. The enhancement of the FEE properties of the ZnO-embedded DLC film has been analyzed in the context of microstructure and chemical composition.