Stable Field Emission Research Articles

DFTB Simulation of Charged Clusters Using Machine Learning Charge Inference.

We present a modification to self-consistent charge density functional-based tight binding (SCC-DFTB), which allows computation based on approximate atomic charges. We obtain these charges by means of a machine learning (ML) process that combines a Coulomb model with a neural network. This allows us to avoid the SCC cycles in the SCC-DFTB calculation while maintaining its accuracy. The main input of the model is the atomic positions characterized by a set of atom-centered symmetry functions. The charge inference from our ML algorithm is as close as 10-2 units of charge from the exact SCC solution. Our ML-DFTB approach provides a good approximation of the density matrix and of the energy and forces with only a single diagonalization. This is a significant computational saving with respect to the complete SCC algorithm, which allows us to investigate a bigger ensemble of atoms. We show the quality of our approach in the case of charged silicon carbide (SiC) clusters. The ML-DFTB potential energy surface (PES) mimics the SCC-DFTB PES rather well, despite its simplicity. This allows us to obtain the same geometric structure ordering with respect to energy for small clusters. The dissociation barriers for ion emission are well-reproduced, which opens the way to investigating ion field emission and charged cluster stability. The ML-DFTB approach is obviously not limited to charged clusters or SiC materials. It opens a new route to investigate larger clusters than those investigated by standard SCC-DFTB, as well as surface and solid-state chemistry at the atomic level.

Nanostructured tungsten trioxide developed from environmentally friendly green process as a promising cathode for excellent field emission

Extensively explored as cathodes for field emission, nanostructured metal oxides play a crucial role in diverse applications such as microwave amplifiers, renewable energy systems, flat panel displays, and micro-electronics. The quest for high current density, low threshold field, and stable field emission persists. In present study, we introduce a green synthesis approach using vegetable oil as a reaction medium to produce hexagonal tungsten trioxide (h-WO3). The structural analysis confirmed the formation of h-WO3 with a nanorod-like morphology. The spin-coated h-WO3 film on a silicon substrate exhibited promising field emission properties, including a field enhancement factor (β) of 1596 and a turn-on field of 4.60 V/μm, comparable to reported values. This research opens avenues for the cost-effective and scalable utilization of h-WO3 as an electron emitter in advanced optical and electrical devices.

Highly Ordered Bamboo-Like 4H-SiC Nanowire Array Enabling Low Turn-On Field and Excellent Field Emission Stability

Highly Ordered Bamboo-Like 4H-SiC Nanowire Array Enabling Low Turn-On Field and Excellent Field Emission Stability

Highly enhanced and stable field emission performance of CNT – Dielectric (Si3N4) hybrids

In the present study, vertically aligned carbon nanotubes (CNTs) were grown onto Si substrates using a simple thermal chemical vapour deposition (TCVD) technique. Subsequently, the dielectric layer of Si3N4 was coated over the pristine CNTs using the Plasma Enhanced CVD (PECVD) method. Various characterizations were carried out on the as-deposited samples to study the structural properties, surface morphology, elemental composition, and electronic structure. The field emission measurements executed on CNT – Si3N4 samples showed an enhanced current density of 8.28 mA/cm2 compared to the 2.85 mA/cm2 current density of the pristine CNTs. A detailed study reveals that, apart from the basic conventional field emission parameters, the electronic structure of the CNTs, along with the presence of disordered carbon atoms and the presence of the dielectric material, are responsible for enhancing the current density and temporal stability of the CNT – Si3N4 hybrids. The study aids in exploring CNT-dielectric hybrids for better field emission properties with improved stability for field emission applications.

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.

Self-joule heating assisted field emission following the Child–Langmuir law

In this study, stable and long-term field emission properties that completely follow the Child–Langmuir law were successfully observed. A tungsten tip covered with a liquid gallium metal was used. The current characteristics showed three phases. The electron emission first began below half of the threshold voltage for the emission from a bare W chip, and the current increased by 20 μA. Then, the field emission pattern showed multiple disordered blinking spots, which originated Ga Taylor cones and the emission current value reached several mAs. Then, emission current began to follow the Child–Langmuir law, and a clear field emission pattern from {011}-oriented tungsten was observed. Electrons emitted from the submicrometer sized area that is heated by itself with joule heating of current densities greater than 107 A/cm2.

An enhanced field emission for display devices arises from the assembling of ZnO@MOF/MWCNTs

In this study, we propose a new, simple technique for raising the efficiency of field emission devices based on metal organic framework composites. Multi-walled carbon nanotubes (MWCNTs) and ZnO are used in the fabrication of metal-organicframeworks (MOFs) in this technique. This is explained by the composites unique structural structure, which offers a lot of easily available electro active sites, low surface work function, and improved electronic conductivity. One of the less complicated and more economical synthesis methods is used to synthesize the ZnO@MOF/MWCNTs composite. The material's precise morphological and structural features have all been investigated using Raman, FE-SEM, EDX, FTIR, and UV–Visible investigations. The ZnO@MOF/MWCNTs field emitter show impressive and stable field emission properties, such as a low turn-on electric field 0.50 V/μm and a high emission current density 219.15 mA/cm2.Our research revealed an easier way for boosting the electron field emission utilized in different field emission display systems that are based on MOFcomposites.

Tubular Diamond as an Efficient Electron Field Emitter

Herein, we present a straightforward and cost-effective procedure for producing conductive diamond tubes on the surface of porous carbon nanotube hollow fibers using successive 10-pulsed laser annealing shots and 6 h of hot filament chemical vapor deposition techniques. Room-temperature Raman and X-ray diffraction spectra reveal the signature T2g peaks near 1332.4 cm–1 and 111 planes of diamonds near 43.9°, respectively. A low turn-on field (ETO) ∼1.85 V/μm@1 μA/cm2 and a threshold field (ETH) ∼2.54 V/μm@10 μA/cm2 were observed for the tubular diamond structures. The field enhancement factor (β) was calculated at 3594 and highly stable field emission current stability was observed over a long period of 4 h. For the first time, a good insight into the field emission results of the diamond is established with the structural, electronic properties, and the work function (φ) ∼4.84 eV analysis conducted by the density functional theory simulation. Finite electronic states at the Fermi level are observed beyond a band gap, and it demonstrates the wide-band gap (4.4 eV) semiconducting nature of the diamond. The Bader charge analysis and maximum entropy method pattern revealed the negative electron affinity of the diamond, and it is responsible for the emission of electrons from the conduction band of the diamond. Besides, the accumulation of charge carriers, which contributes to the electric field emission, takes place due to the weak π bonds of carbon atoms. The low turn-on field, the high field enhancement factor, and the good field emission current stability of tubular diamond offer great prospects for future efficient and low-cost field emission devices.

Mechanism analysis of field electron emission of titanium

Field electron emission (FEE) is generally considered to be closely correlated with radio frequency (RF) breakdown in accelerating structures and high-power microwave (HPM) devices. In this study, the field emission characteristics of titanium are investigated by using a field emission test system. With the increase of the number of field emission tests and stability tests, the repeatability of the field emission characteristic curve of titanium tends to be better, and the amplitude of the current oscillation in the stability tests gradually decreases, indicating that the field emission performance of titanium gradually becomes stable. Subsequently, the field emission characteristic curve of titanium is analyzed by adopting the field emission theory of metal microprotrusions. Combined with the analysis of the electric field enhancement effects caused by the surface morphology, the dominant effect of the metal microprotrusions on the FEE of titanium is excluded from the two aspects of the electric field enhancement factor and local maximum emission current density. At last, the field emission theory of dielectric micropoints is introduced to analyze the field emission characteristics of titanium. The electric field enhancement factor of 102 ∼ 103 are explained theoretically. Simultaneously, the reasonable effective emission area and local emission current density are given, which can better explain the field emission phenomena of titanium, such as the sharp decrease in emission current and repeatability of the field emission curve. Hence, it is revealed that the key factor that dominates the FEE of titanium is the dielectric impurities on the surface, rather than the metal microprotrusions.

Electric-Field Emission Mechanism in Q-Carbon Field Emitters.

In this paper, we report the excellent field emission properties of Q-carbon and analyze its field emission characteristics through structural, morphological, and electronic property correlations, supported by density functional theory (DFT) simulation studies. The Q-carbon field emitters show impressive and stable field emission properties, such as a low turn-on electric field of ∼2.38 V/μm, a high emission current density of ∼33 μA/cm2, and a critical field of ∼2.44 V/μm for the transition from a linear region to the saturation region in the F-N plot. The outstanding field emission properties of Q-carbon are attributed to (i) a unique sp2/sp3 mixture in Q-carbon, (ii) sp2-bonded highly conductive amorphous carbon-rich channels inside the Q-carbon cluster, (iii) a large local field enhancement due to the local geometry and microstructure of Q-carbon, and (iv) the presence of sp2-bonded amorphous carbon regions in the composite film. The temperature-dependent field emission properties, such as extreme sensitivity and an enhancement in the emission current density with temperature, can be explained by the local density of states near the Fermi level and the excellent thermal stability of the Q-carbon field emitters. From DFT simulation studies, the computed work function and the field-enhancement factor were determined to be 3.62 eV and ∼2300, respectively, which explains the excellent field emission characteristics of Q-carbon. The obtained field emission properties, in most cases, were superior to those from other carbon/diamond-based field emitters, which will open new frontiers in field emission-based electronic applications.

A Core-Shell MWCNT-Pt Nanowire Electron Source with Anomalously Long-Term Stable Field Emission.

A hybrid core-shell structured nanowire is proposed for a long-term stable electron source based on an isolated platinum/multi-walled carbon nanotube (Pt/MWCNT). This hybrid nanowire is prepared by growing a Pt shell on a metallic MWCNT through a field-emission-induced deposition (FEID) method. An in situ field emission (FE) platform was constructed inside a scanning electron microscope (SEM) equipped with two nanorobotic manipulators (NRMs) for the preparation and testing of the hybrid nanowire. An in situ fatigue test was conducted with high current intensity (500 nA) to show the influence of the Pt shell. Compared with the pristine bare MWCNT, our hybrid-nanowire-based electron source has a lifetime of hundreds of times longer and can work continuously for up to 48 h under relatively high pressure (3.6×10-3 Pa) without having an apparent change in its structure or emission currents, demonstrating good stability and tolerance to poor working conditions. The anomalous long-term stability is attributed mainly to the shielding of oxygen by Pt from the carbon shells and less heating due to the work function lowering by Pt.

Influence of the growth temperature on electron field-emission stability of the carbon nanotubes’ field emitters

Influence of the growth temperature on electron field-emission stability of the carbon nanotubes’ field emitters

Improved field emission stability with a high current density of decorated CNTs for electron emission devices

Improved field emission stability with a high current density of decorated CNTs for electron emission devices

Enhanced field emission stability of vertically aligned carbon nanotubes through anchoring for X-ray imaging applications

We reported the anchored carbon nanotube emitters shows enhanced field emission stability.

Correction: Enhanced field emission stability of vertically aligned carbon nanotubes through anchoring for X-ray imaging applications

Correction for ‘Enhanced field emission stability of vertically aligned carbon nanotubes through anchoring for X-ray imaging applications’ by Xinchuan Liu et al., J. Mater. Chem. C, 2023, 11, 2505–2513, https://doi.org/10.1039/d2tc04363g.

Hybrid Carbon Nanotubes-Graphene Nanostructures: Modeling, Formation, Characterization.

A technology for the formation and bonding with a substrate of hybrid carbon nanostructures from single-walled carbon nanotubes (SWCNT) and reduced graphene oxide (rGO) by laser radiation is proposed. Molecular dynamics modeling by the real-time time-dependent density functional tight-binding (TD-DFTB) method made it possible to reveal the mechanism of field emission centers formation in carbon nanostructures layers. Laser radiation stimulates the formation of graphene-nanotube covalent contacts and also induces a dipole moment of hybrid nanostructures, which ensures their orientation along the force lines of the radiation field. The main mechanical and emission characteristics of the formed hybrid nanostructures were determined. By Raman spectroscopy, the effect of laser radiation energy on the defectiveness of all types of layers formed from nanostructures was determined. Laser exposure increased the hardness of all samples more than twice. Maximum hardness was obtained for hybrid nanostructure with a buffer layer (bl) of rGO and the main layer of SWCNT—rGO(bl)-SWCNT and was 54.4 GPa. In addition, the adhesion of rGO to the substrate and electron transport between the substrate and rGO(bl)-SWCNT increased. The rGO(bl)-SWCNT cathode with an area of ~1 mm2 showed a field emission current density of 562 mA/cm2 and stability for 9 h at a current of 1 mA. The developed technology for the formation of hybrid nanostructures can be used both to create high-performance and stable field emission cathodes and in other applications where nanomaterials coating with good adhesion, strength, and electrical conductivity is required.

Techno functional characterization of green-extracted soluble fibre from orange by-product

This work studies the techno-functional properties of the spray dried water-soluble extracts from orange pomace, obtained from different green extraction technologies. Orange pomace, a by-product from the juice production industry, was used for soluble fibre extraction. Starting from pomace, four extraction procedures were assayed: hot water, extrusion + hot water, jet cooker and jet cooker + hot water. After treatments, the supernatant was spray dried with whey protein isolate to obtain soluble fibre enriched powders. Microstructure by field emission scanning electron microscopy, color, particle size, moisture content, hydration properties, foaming capacity and stability, rheology, and glass transition temperatures were assayed. All powders exhibited an orange shade. In microstructure, particle size, and glass transition temperature, the powder obtained from extrusion + hot water presented a different behavior from the other powders: less caking and agglomeration, and smaller particles. Also, this powder showed the highest foaming stability and viscosity, as well as glass transition temperature, properties that make it interesting for the food industry. Through valorization of orange pomace fiber-enriched powders could be obtained; these powders can be used as ingredients in the industry to address the fibre gap and provide technological functionalities.

A stable LaB6 nanoneedle field-emission electron source for atomic resolution imaging with a transmission electron microscope

Field-induced electron emission offers the best performance point electron source for electron microscopes. Herewith we report a new cold field-emission point electron source utilizing a nanoneedle made of lanthanum hexaboride (LaB6) and its implementation in a spherical aberration-corrected transmission electron microscope (TEM). A sub-ångstrom resolution of 0.96 Å has been obtained with the TEM operated at an acceleration voltage of 60 kV and a smaller energy spread was also observed compared with the contemporary W(310) electron source. In particular, the LaB6 nanoneedle cold field-emission electron source required no thermal flashing for continuous operation of extended hours (>100 hours) while exhibiting and sustaining a high stability in emission current (<1%). We attribute the excellent performance of the LaB6 nanoneedle electron source to (i) a low work function of LaB6; (ii) excellent alignment of the nanoneedle emitter by the electron and ion dual-beam processing with nanoscale precision and (iii) robustness of the nanometric structure that suppressed mechanical vibrations of the LaB6 emitter even in a high electric field. This new LaB6 nanoneedle cold field-emission electron source enables stable high-resolution imaging in TEM and it will also benefit cryogenic electron microscopy, scanning electron microscopy, electron beam lithography, and other electron beam technologies.

Enhanced Field Emission from Ultrananocrystalline Diamond-Decorated Carbon Nanowalls Prepared by a Self-Assembly Seeding Technique.

Vertically aligned nanographite structures, the so-called carbon nanowalls (CNWs), are decorated with ultrananocrystalline diamond particles by an electrostatic self-assembly seeding technique, followed by short-term growth in plasma chemical vapor deposition, to enhance field emission efficiency and stability. A nanodiamond suspension diluted with a dispersion medium with high wettability on CNWs enables seeding of diamond nanograins consisting of nanoparticles of 3-5 nm in diameter on CNWs with high uniformity and minimal aggregation and control of their number density. The field emission turn-on field depends upon the density of diamond nanograins and decreases from 3.0 V μm-1 for bare CNWs to 1.8 V μm-1 for diamond-decorated CNWs together with about an order of magnitude increase in current density. Finite element modeling indicates that only a part of decorating diamond located at the top of nanowalls actually contributes to field amplification and emission. The diamond-decorated CNWs show also higher emission stability with much larger time constants of current degradation than the bare CNWs for long-term duration. The enhanced emission efficiency is explained by larger field amplification rather than lowering of the tunneling barrier, while the enhanced emission stability is attributed to the higher robustness of diamond.

Field emission and electron beam transmission characteristics of microtips array on the (100) plane of single-crystal Gadolinium hexaboride ceramic

A field-emission microtips array with a radius of curvature, height, and separation distance of 0.5, 5, and 4 μm, respectively, was processed on the single-crystal Gadolinium hexaboride ceramic (GdB6) (100) plane using a femtosecond laser. The height and morphology of the microtips array were uniform, and the crystal structure and phase composition of GdB6 did not change after the processing. According to the field-emission performance test of a GdB6 microtips array, the turn-on electric field intensity was 1.9 V/μm, and the field-emission current density reached 0.49 A/cm2 when the applied electric field strength was 6.7 V/μm; meanwhile, it had outstanding field-emission stability. Subsequently, the transmission process of the field-emission current of the GdB6 between the cathode and the anode was simulated using COMSOL Multiphysics 5.6 software. The transmission process of the electron beam from emitters to the anode surface was divided into three stages due to the interaction between emission electrons and a magnetic field generated by its motion, including divergence, pinch, and drift. The increase of electric field strength reduced the radial displacement of the electron beam while moving the beam waist further away from the anode surface. The simulation results provide theoretical guidance for regulating the shape of the field-emission electron beam, size, and energy.