Field Emission Characteristics Research Articles

Effect of Crystallinity on the Field Emission Characteristics of Carbon Nanotube Grown on W-Co Bimetallic Catalyst.

Carbon nanotube (CNT) is an excellent field emission material. However, uniformity and stability are the key issues hampering its device application. In this work, a bimetallic W-Co alloy was adopted as the catalyst of CNT in chemical vapor deposition process. The high melting point and stable crystal structure of W-Co helps to increase the grown CNT diameter uniformity and homogeneous crystal structure. High-crystallinity CNTs were grown on the W-Co bimetallic catalyst. Its field emission characteristics demonstrated a low turn-on field, high current density, stable current stability, and uniform emission distribution. The Fowler-Nordheim (FN) and Seppen-Katamuki (SK) analyses revealed that the CNT grown on the W-Co catalyst has a relatively low work function and high field enhancement factor. The high crystallinity and homogeneous crystal structure of CNT also reduce the body resistance and increase the emission current stability and maximum current. The result provides a way to synthesis a high-quality CNT field emitter, which will accelerate the development of cold cathode vacuum electronic device application.

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.

Basic Research on Vacuum Breakdown and Field Emission Characteristics on SUS304 Electrode with Micro-sized Pits

Basic Research on Vacuum Breakdown and Field Emission Characteristics on SUS304 Electrode with Micro-sized Pits

Enhanced field emission characteristics of WS2 nano-films by diamond film and Mo film

WS2 is a type of transition metal chalcogenide materials (TMDCs) that exhibits exceptional photoelectric properties. Due to its atomically sharp edges, WS2 has the capability to enhance the electric field around its edges, thus presenting significant potential in field emission applications. In this study, the electron beam vapor deposition (EBVD) system and microwave plasma chemical vapor deposition (MPCVD) system were used to prepare a series of single-layer WS2 films，single-layer diamond films and diamond/WS2 nano-composite films on N-type highly doped Si substrate, and Mo/WS2 nano-composite films were prepared on Al2O3 ceramic substrate. Based on the characterization and analysis of the microstructure and chemical composition of each thin film layer in the above film devices, the field emission (FE) characteristics of each thin film device are compared and studied. The results demonstrate that the maximum FE current density of diamond/WS2 nano-composite film device is 912 μA/cm2, which is 2.7 times of the maximum FE current density of Mo/WS2 nano-composite film device 330 μA/cm2, and 3.4 times of the maximum FE current density of the single-layer diamond thin film device 267 μA/cm2. The single layer WS2 film did not exhibit measurable FE performance. The working principles of thin film field emission devices with various structures are introduced. It is considered that the diamond layer plays the role of electron accelerating layer, which not only effectively improves the FE characteristics of diamond/WS2 nanocomposite film, but also improves the repeatability and long-term stability of diamond/WS2 nanocomposite film FE devices.

Field emission properties of LIG/ZnO heterojunction prepared by ultrafast laser direct writing

The facile in situ synthesis of graphene-coated metal-oxide nanoparticles and their potential applications in optoelectronic devices are highly anticipated. In this study, we prepared three-dimensional graphene flakes decorated with ZnO nanoparticles (LIG/ZnO) using femtosecond laser-induced zinc salt-containing cork. The LIG/ZnO heterojunction was formed by carbonizing the cork and decomposing the zinc salt in situ to coat LIG during laser irradiation. The correlation between the zinc salt concentration and the morphology, structure, and field-emission characteristics of LIG/ZnO was systematically investigated. The optimized LIG/ZnO-5 emitters demonstrate a decreased threshold electric field (Eth) of 1.07 V/μm, an enhanced field enhancement factor (β) value of 20,559, and a maximum current density of 18.07 mA/cm2 at 1.96 V/μm, representing the most superior performance reported to date. The enhanced field-emission performance of LIG/ZnO can be attributed to the larger number of effective emission sites from ZnO nanoparticles and LIG edges, as well as to the energy band variation induced by the heterostructures. Density functional theory (DFT) calculations show the migration of electrons from the LIG to the ZnO after the formation of the heterojunction, which elevates the Fermi level of emitters. In addition, the accumulation of electrons on the surface of ZnO nanoparticles in the presence of a strong electric field results in a downward shift of the energy band, thereby enhancing the likelihood of electron-vacuum tunneling.

A Study on the Field Emission Characteristics of High-Quality Wrinkled Multilayer Graphene Cathodes.

Field emission (FE) necessitates cathode materials with low work function and high thermal and electrical conductivity and stability. To meet these requirements, we developed FE cathodes based on high-quality wrinkled multilayer graphene (MLG) prepared using the bubble-assisted chemical vapor deposition (B-CVD) method and investigated their emission characteristics. The result showed that MLG cathodes prepared using the spin-coating method exhibited a high field emission current density (~7.9 mA/cm2), indicating the excellent intrinsic emission performance of the MLG. However, the weak adhesion between the MLG and the substrate led to the poor stability of the cathode. Screen printing was employed to prepare the cathode to improve stability, and the influence of a silver buffer layer was explored on the cathode's performance. The results demonstrated that these cathodes exhibited better emission stability, and the silver buffer layer further enhanced the comprehensive field emission performance. The optimized cathode possesses low turn-on field strength (~1.5 V/μm), low threshold field strength (~2.65 V/μm), high current density (~10.5 mA/cm2), and good emission uniformity. Moreover, the cathode also exhibits excellent emission stability, with a current fluctuation of only 6.28% during a 4-h test at 1530 V.

Effect of Metal Nanoparticles in the Field Emission of Silicon Nanowires

In this work, an efficient method is reported for creating a metal nanoparticle (silver) / Si composite structure consisting of a vertical array of silicon nanowires (SiNWs) decorated with silver metal nanoparticles. A two-stage metal-assisted etching method is employed to obtain SiNWs and Silver (Ag) metal nanoparticles are decorated on the SiNWs using the electroless deposition method. It allows the good coverage of silver metal nanoparticles over SiNWs. Scanning Electron Microscopy (SEM) analysis revealed that Ag was covered with SiNWs. High-work function metal nanoparticles such as Ag nanoparticles on SiNWs have been utilized in different applications such as photovoltaics and sensors. The size of SiNWs is determined through the Raman shift. The silicon optical phonon peak showed an increase in redshift and a decrease of full-width at half maxima with a decrease in diameter due to the quantum confinement. The Electron Field Emission (EFE) characteristics of the Agdecorated SiNW films were studied based on the current-voltage measurements and analyzed using the Fowler-Nordheim (F-N) equation. The low turn-on field is obtained through the Ag metal nanoparticles which have wider applications in lowpower operational devices.

Innovative multiwall carbon nanotubes synthesis on 3D nickel surface: a comparative study

The article presents the synthesis of multi-walled carbon nanotubes (MWCNTs) on the porous nickel foam (3D-Ni) surface using the chemical vapor deposition (CVD) method, followed by the field emission characteristics of the resulting 3D-Ni/MWCNT nanocomposite structures. The characterization of the 3D-Ni/MWCNT morphology showed a uniform distribution of the “moss-like” CNT coating on the walls of the porous 3D-Ni. TEM images showed a high number of walls in MWCNTs, which made such a material resistant to destruction under conditions of strong electric fields during the field emission process. The studies also showed a high degree of chemical purity of the synthesized MWCNTs under the presented technological conditions. An increase in the emission current due to the introduction of CNTs into the structure of porous 3D-Ni is shown. 3D-Ni/MWCNTs field emitter showed a low turn-on field of 5.41 V/μm; a field enhancement factor of 915; and high operating stability. For the first time, the current mode of 3D-Ni-MWCNTs cathode was tested for compliance with the classical field emission law using the Forbes “orthodoxy” test.

Role of Surface Chemistry of Ta Metal Foil on the Growth of GaN Nanorods by Laser Molecular Beam Epitaxy and Their Field Emission Characteristics.

This study investigates the influence of surface nitridation of Ta metal foil substrates on the growth of GaN nanorods using the laser molecular beam epitaxy (LMBE) technique and the field emission characteristics of the grown GaN nanorod ensemble. Surface morphology examinations underscore the pivotal role of Ta foil nitridation in shaping the dimensions and densities of GaN nanorods. Bare Ta foil fosters the formation of high-density, vertically self-aligned GaN nanorods at a growth temperature of 700 °C. Furthermore, the density of these nanorods is directly related to the duration of Ta foil nitridation, with increased duration leading to a reduced nanorod density. X-ray Photoelectron Spectroscopy (XPS) studies reveal that the transition of the Ta foil surface from tantalum oxide to tantalum nitride during nitridation emerges as a crucial factor influencing GaN nanorod growth. Photoluminescence (PL) spectroscopy at ambient temperature reveals a strong near-band-edge (NBE) emission peak with negligible defect-related peaks, displaying the high optical quality of the GaN nanorods. The highly dense vertically aligned GaN nanorod ensemble growth without Ta prenitridation exhibits the most favorable field emission performance, featuring a turn-on field of 2.1 V/μm, a field enhancement factor of 2480, and a stable long-term operation at the emission current density of 2.26 mA/cm2. This study advances the understanding of the role of the surface chemistry of metal foil in determining GaN nanorod growth and opens up exciting possibilities for tailoring advanced optoelectronic devices for specific application requirements.

Field emission characterization of field-aligned carbon nanotubes synthesized in an environmental transmission electron microscope

Optimizing the synthesis of carbon nanotubes (CNTs) for applications like field emission (FE) sources requires a fundamental understanding of the growth kinetics of individual CNTs. In this article, we explore how applying electric fields during CNT synthesis influences the as-grown nanotubes and their FE performance. We observe growth and undertake FE measurements in real time using an environmental transmission electron microscope. This is achieved through a polarizable capacitor gap within a microchip sample heater specifically designed for this purpose. Individual nanotubes are easily resolved and are predominantly single-wall CNTs. At low-applied fields, the growing nanotubes can span the gap and link with the opposite electrode, albeit with some loss due to mechanical failure. With a high-applied field and positive bias for FE, we continue to observe the oriented growth of nanotubes. However, this growth is constrained within the gap due to the possibility of FE occurring during the growth process, which can result in either saturation or damage. At any given time, we have the flexibility to halt the growth process and conduct in situ FE experiments. This approach enables us to comprehensively track the complete development of the CNTs and gain insights into the various mechanisms responsible for limiting the performance of CNT cathodes. Interestingly, we report an original self-oscillation induced destruction mechanism that has not been reported before.

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.

Size- and temperature-dependent optical, and electron field emission characteristics on SnO2 nanocrystals

Size-dependent SnO2 nanocrystals (NCs)) were synthesized using a co-precipitation technique. The particle size was varied by annealing the as-prepared SnO2 NCs with the estimated size of 2.3 nm at different temperatures. The particle size obtained was 4.7, 9.0, 15.4, 19.6, and 48.0 nm at annealing temperatures of 400, 600, 800, 1000, and 1200 °C, respectively. The XRD data revealed the tetragonal rutile structured for all SnO2 NCs. The temperature-dependent photoluminescence (PL) studies showed that the sub-bridging oxygen vacancy contributed dominantly compared to the in-plane and bridging oxygen vacancies at 5K. The function of nanocrystals’ size on field emission properties is also reported. The current density was high (1.9 μA/cm2) with modest the turn-on field (8.4 V/μm) for as-prepared SnO2 QDs with the lowest size (2.3 nm) compared to the high temperature annealed SnO2 nanocrystals.

Investigation of Changes in the Field Emission Characteristics of the Eroded Cathode Surface after Submicrosecond Vacuum Arc Discharges with Different Current Amplitudes

Investigation of Changes in the Field Emission Characteristics of the Eroded Cathode Surface after Submicrosecond Vacuum Arc Discharges with Different Current Amplitudes

Boosting field electron emission of carbon nanotubes through small-hole-patterning design of the substrate

High field emission of carbon nanotube (CNT) cold cathodes is realized by printing modified CNT paste on small-hole-patterned substrates. The field emission characteristics and stability of samples under DC continuous and pulse driving modes have been investigated. The results show that the maximum emission current of CNT emitters can be up to 45 mA at an electric field of 2100 V (7.0 V μm−1), corresponding to a high current density of 643 mA cm−2 under continuous mode. The cathodes also demonstrate stable electron emission without obvious attenuation. In pulse (10 μs and 200 Hz) mode, the peak current can reach 250 mA and the corresponding current density is 3.57 A cm−2 under an electric field of 14.0 V μm−1. The hole-patterned CNT cathode presents unique advantages in field emission current, stability and especially endurance of high electric field. This work makes it possible to fabricate highly efficient emission CNT cold electron sources, which have broad application prospects in vacuum electronic devices requiring both large current and high current density.

Densification effect on field emission characteristics of CNT film emitters for electron emission devices

Field electron emission characteristics of the carbon nanotube (CNT) film emitters were investigated according to densification conditions such as nitric acid, acetic acid, and salicylic acid. The emission performance of the CNT film emitters was strongly affected by the densification conditions. Salicylic acid exhibits the best field electron emission properties of the CNT film emitters, followed by nitric acid and acetic acid. The efficient densification of the CNT film emitter by salicylic acid is caused by the role of polarity and p orbitals, nitric acid by hydrogen ions, and acetic acid by weak polarity. After the densification with salicylic acid, the turn-on field of the CNT film emitter decreases from 1.94 V μm−1 to 1.86 V μm−1, the threshold field decreases from 3.41 V μm−1 to 2.95 V μm−1, the emission current significantly increases from 20.92 mA to 43.98 mA, and the degradation rate from the long-term emission stability decreases from 49.9% to 21%. The improved emission characteristics are attributed to the increased emission sites at the CNT film and the increased electrical conductivity of the CNT film. The densification is a useful way to enhance the field electron emission properties of CNT film emitters.

Study on particle size and field effect with sp2/sp3 ratio of hydrogenated diamond-like carbon

Hydrogenated diamond-like carbon (HDLC) films were synthesized through a reactive gas-plasma process employing methane (CH4) and hydrogen (H2) as precursor gases on a silicon (100) wafer substrate, conducted at room temperature. The deposition process utilized a biased enhanced nucleation technique, varying the flow rate ratio of hydrogen and methane. The investigations revealed that increasing the methane flow rate led to a reduction in grain size and an augmented nucleation density of HDLC, as evidenced by contact-mode atomic force microscopy (AFM) images. This study demonstrated the effective control of diamond grain growth by introducing high-methane-concentration pulses during deposition. The field emission characteristics of HDLC samples were analyzed, revealing threshold fields of 12.2 V/μm for nanocrystalline films, 8.5 V/μm for subcrystalline films and 4.1 V/μm for microcrystalline films, corroborated by Raman spectra. Surface energy measurements indicated hydrophobic behavior in the samples. Notably, a decrease in the hydrogen/methane ratio was found to increase the sp2 character, which correlated with the emission field. AFM analysis of HDLC samples yielded surface roughness values ranging from 0.2 nm to approximately 0.01 nm, confirming the continuous, non-porous and smooth nature of the surfaces.

Experimental study on the influence of grain boundary on breakdown in relativistic backward wave oscillator

The presence of radio frequency (RF) breakdown seriously degrades the performance of relativistic backward wave oscillator (RBWO) and affects its lifetime. In this paper, the influence of grain boundary on RF breakdown in an X-band RBWO has been experimentally investigated. Firstly, titanium with different grain sizes has been prepared using a high-temperature annealing method before its field emission characteristics are studied by a field emission test system. The experimental results indicate that the segregation effect of the impurities at the position of the grain boundary may become more pronounced as the grain size increases, which results in a significant improvement in the field emission performance of titanium. A high-power microwave experimental platform is then set up, and the influence of the grain boundary of titanium on RF breakdown has been revealed. The shortening of the microwave pulses is more serious with the increase of the grain size, and the corresponding breakdown traces appearing in the high-frequency structures become more apparent. Therefore, developing fine-grained and single-crystal metallic materials may be an important method for suppressing RF breakdown in RBWOs.

Influence of the strength of rock-like models on the local deformation field and acoustic emission characteristics

Influence of the strength of rock-like models on the local deformation field and acoustic emission characteristics

Self-consistent numerical solution of quantum regime with exchange-correlation effects of space charges for electron field emission in a nano-gap

The quantum effects of space charge on electron field emission have been widely investigated since the last century. When electrons energy and their mean spacing approach the Hartree level and the de Broglie wavelength respectively, the influence of the quantum effects on the field emission current becomes significant. In this work, by developing an in-house software, we self-consistently solve the one-dimensional Poisson-Schrödinger equation together with the Wentzel-Kramers-Brillouin-Jeffreys (WKBJ) model for metal-vacuum-metal nanogaps, after considering the anode screening effect, space charge Coulomb potential and exchange-correlation effects simultaneously. Employing the method, the electron field emission characteristics were studied by varying the nanogap spacing (D) and the electric field strength (F), and four different emission regimes including quantum regime (QR), space charge limited regime (SCLR), direct tunnelling regime (DTR) and field emission regime (FER) are defined. The influences of space charge field components on the field emission characteristics and space charge distribution are analyzed for different emission regimes in nanogap. In addition, the impact of using different exchange-correlation functionals (LDA, GGA and meta-GGA) on Jacob’s ladder for describing the quantum effects of space charge on the electron emission current density was analyzed. Finally, electron field emission properties of one-dimensional (1-D) nanogaps consisting of refractory metals (W and Mo) as well as the three-dimensional (3-D) nano-tip are discussed to elucidate the impact of the exchange-correlation effects on the enhanced field emission process at nanoscale.

Demonstration of gallium oxide nano-pillar field emitter arrays

We demonstrate field emission characteristics of β-Ga2O3 nano-pillar arrays fabricated using a damage-free etching technique. The technique utilizes Ga flux in an ultra-high vacuum environment (molecular beam epitaxy) to form high aspect ratio Ga2O3 nano-pillars with atomic-scale etching precision. Electrically conductive Ga2O3 nano-pillars with uniform widths of ∼200–300 nm were realized without the use of e-beam lithography. Furthermore, field emission characteristics on the nano-pillars displayed an emission current of 19 nA per tip at an electric field of 385 kV/cm. The field emission characteristics were modeled using the Murphy–Good model, and the measurements were validated with a field emission orthodoxy test.