Showing papers on "Field electron emission published in 2022"

High-Performance Cold Cathode X-ray Tubes Using a Carbon Nanotube Field Electron Emitter.

Abstract: A cold cathode X-ray tube was fabricated using a carbon nanotube (CNT) field electron emitter made by a free-standing CNT film which is composed of a highly packed CNT network. A lot of CNT bundles with a sharp tip are vertically aligned at the edge of the thin CNT film with a length of 10 mm and a thickness of 7 μm. The cold cathode X-ray tube using the CNT field emitter presents an extremely high tube current density of 152 A/cm2 (corresponding to tube current of 106.4 mA), the electron beam transmittance of 95.2% and a small focal spot size (FSS) of 0.5 mm. In addition, the cold cathode X-ray tube also shows stable lifetime during 100 000 shots. High emission current density of the cold cathode X-ray tube is mainly attributed to a lot of electron emission sites at an edge of the CNT film. The small FSS is caused by an ensemble of the CNT field electron emitter made by a free-standing thin CNT film and the optimized curve-shape elliptical focusing lens. Based on obtained results, the cold cathode X-ray tube can be widely used for various X-ray applications such as medical diagnosis systems and security check systems in the future.

Attosecond field emission

Abstract: Field-emission of electrons underlies major advances in science and technology, ranging from imaging the atomic-scale structure of matter to signal processing at ever-higher frequencies. The advancement of these applications to their ultimate limits of temporal resolution and frequency calls for techniques that can confine and probe the field emission on the sub-femtosecond time scale. We used intense, sub-cycle transients to induce optical field emission of electron pulses from tungsten nanotips and a weak replica of the same transient to directly probe the emission dynamics in real-time. Access into the temporal profile of the emerging electron pulses, including the duration $\tau$ = (53 as $\pm$ 5 as) and chirp, and the direct probing of nanoscale near-fields, open new prospects for research and applications at the interface of attosecond physics and nanooptics.

Interpretation of field emission current–voltage data: Background theory and detailed simulation testing of a user-friendly webtool

Abstract: In field electron emission (FE) studies, interpretation of measured current–voltage characteristics and extraction of emitter characterization parameters are usually carried out within the framework of “smooth planar metal-like emitter (SPME) methodology”, using a data-analysis plot. This methodology was originally introduced in the 1920s. Three main data-plot types now exist: Millikan–Lauritsen (ML) plots, Fowler–Nordheim (FN) plots, and Murphy–Good (MG) plots. ML plots were commonly used in early FE studies, but most modern analysis uses FN plots. MG plots are a recent introduction. Theoretically, it is now known that ML and FN plots are predicted to be slightly curved in SPME methodology, but a Murphy–Good plot will be very nearly straight. Hence (because 1956 Murphy–Good emission theory is “better physics” than 1928 Fowler–Nordheim emission theory as corrected in 1929), expectation is that parameter extraction using a MG plot will be more precise than extraction using either ML plots or FN plots. In technological FE studies, current–voltage characteristics are often converted into other forms. Thus, measured voltage may be converted to (apparent) macroscopic field, and/or current values may be converted to macroscopic current densities. Thus, four data-input forms can be found in the context of analysing FE current–voltage results. It is also the case that over-simplified models of measurement-system behaviour are very widely assumed, and the question of whether simple use of a data-analysis plot is a valid data-interpretation procedure for the particular system under investigation has often been neglected. Past published studies on field emitter materials development appear to contain a high incidence of spurious values for the emitter characterization parameter “characteristic field enhancement factor”. A procedure (the so-called “Orthodoxy Test”) was described in 2013 that allows a validity check on measurement-system behaviour, and found that around 40% of a small sample of results tested were spuriously high, but has had limited uptake so far. To assist with FE current–voltage data interpretation and validity checks, a simple user-friendly webtool has been under design by the lead author. The webtool needs as user input some system specification data and some “range-limits” data from any of the three forms of data-analysis plot, using any of the four data-input variations. The webtool then applies the Orthodoxy Test, and—if the Test is passed—calculates values of relevant emitter characterization parameters. The present study reports the following: (1) systematic tests of the webtool functionality, using simulated input data prepared using Extended Murphy–Good field electron emission theory; and (2) systematic comparisons of the three different data-plot types, again using simulated input data, in respect of the accuracy with which extracted characterization parameter values match the simulation input values. The paper is introduced by a thorough summary review of the theory on which modern SPME-based current–voltage data-analysis procedures are based. The need in principle to move on (in due course) to data-analysis procedures based on curved-emitter emission theory is noted. An important result is to confirm (by simulations) that, particularly in respect of the extraction of formal emission areas, the performance of the Murphy–Good plot is noticeably better than the performances of Fowler–Nordheim and Millikan–Lauritsen plots. This result is important for field electron emission science because it is now known that differences as between different theories of field electron emission often affect the formal emission area.

Multifunctional characterization of Ca-modified new double perovskite for energy harvesting devices

Abstract: The conventional solid-state route method was adopted for the synthesis of the double perovskite Ba1.5Ca0.5FeVO6. The well define spikes that appear in the XRD profile of the investigated sample suggested the formation of a single-phase new compound. The structural refinement carried out through POWD and MAUD software suggested the material crystallize in the monoclinic phase having a space group of P21/m. The morphological investigation of the sample was carried out by field emission scanning electron micrograph (FESEM). The involvement of different vibrational modes such as stretching, bending, etc. as well as functional groups associated with the compounds was studied through Raman and Fourier transform infrared (FTIR) spectroscopic techniques. The electronic configuration and chemical valence state of the involved elements were investigated by incorporating X-ray photoelectron spectroscopy. The optical sensitivity and bandgap of the material were studied through UV–Visible spectroscopy. The dielectric properties of the material were investigated by monitoring the variation of dielectric parameters (dielectric constant and loss tangent) with frequency while non-destructive impedance and modulus techniques were adopted for electrical characterization. Besides, the AC and DC conductivity spectra were studied for a detailed investigation of electrical properties and the type of charge carriers in the material. The identification of ferromagnetism in the material at room temperature was done by M-H loop.

Experimental study of gas breakdown and electron emission in nanoscale gaps at atmospheric pressure

Abstract: While experiment, simulation, and theory all show that the gas breakdown voltage decreases linearly with gap distance for microscale gaps at atmospheric pressure due to the contribution of field emitted electrons, the continuing reduction in device size motivates a more fundamental understanding of gas breakdown scaling for nanoscale gaps. In this study, we measure current–voltage curves for electrodes with different emitter widths for 20–800 nm gaps at atmospheric pressure to measure breakdown voltage and assess electron emission behavior. The breakdown voltage [Formula: see text] depends more strongly on effective gap distance [Formula: see text] than the ratio of the emitter width to the gap distance. For 20 and 800 nm gaps, we measure [Formula: see text] V and [Formula: see text] V. Independent of emitter width, [Formula: see text] decreases linearly with decreasing [Formula: see text] for [Formula: see text] nm; for [Formula: see text] nm, [Formula: see text] decreases less rapidly with decreasing [Formula: see text] which may correspond to a change in the field enhancement factor for smaller gaps. While gas breakdown usually proceeds directly from field emission, as for microscale gaps, some cases exhibit space-charge contribution prior to the transition to breakdown, as demonstrated by orthodoxy tests. Applying nexus theory, we determine that the range of [Formula: see text] studied is close to the transitions between field emission and space-charge-limited current in vacuum and with collisions, necessitating a coupled theoretical solution to more precisely model the electron emission behavior. Implications on device design and an overall assessment of the dependence of emission and breakdown on gap distance are also discussed.

Field Emission in Emerging Two-Dimensional and Topological Materials: A Perspective

Abstract: Since the first field emission model or the well-known Fowler-Nordheim (FN) law was formulated about a century ago (in 1928), it remains an active topic to discover different aspects of field emission due to new materials and structures, geometrical effects, high current space charge effects, short pulse regime, and analytical models. Despite its simplicity, the original FN law, or its improvement, the Murphy-Good (MG) model remains as the important equations to characterize different field emitters. With the emergence of two-dimensional (2-D) materials such as graphene, transition-metal dichalcogenide (TMD) materials, and topological materials (such as topological insulators and semimetals) in the past two decades, new experiments and theoretical models are produced to study the validity of FN law and MG model on field emission from these quantum materials and to explore their applications as compact field emitters operating at low turn-on field. In this short perspective, after a very brief introduction of FN law and some recent (selected) improvements, we proceed to review the experimental works measuring field emission from the abovementioned quantum materials. The key performances, such as experimental growth methods, and emission characteristics, such as turn-on field, field enhancement factor, and field emission current density, are summarized and compared. During the discussion, we also highlight some recent models proposed to account for the effects of nonparabolic dispersion and topologically nontrivial bandstructures within these quantum materials. We suggest the importance of having consistent physics-based models to understand the field emission from these quantum materials and reinforce the presence of a non-FN scaling law due to their unique properties absent from the traditional bulk materials. Finally, we conclude by highlighting some possible new directions that may be further explored in future.

Field emission: calculations supporting a new methodology of comparing theory with experiment

Abstract: This paper provides a demonstration-of-concept of a new methodology for comparing field electron emission (FE) theory and experiment. It uses the parameter κ in the mathematical equation Im = CVmκ exp[–B/Vm] (where B and C are weakly varying or constants) that is taken to describe how measured current Im depends on measured voltage Vm for electronically ideal FE systems (i.e. systems that (i) have constant configuration during voltage application and (ii) have Im(Vm) given by the emission physics alone). Experimental parameter values (κm) are used to compare two alternative FE theories, for which allowable (but different) κ ranges have been established. At present, contributions to the ‘total theoretical κ’ made by voltage dependence of notional emission area are not well known: simulations reported here provide data about four commonly investigated emitter shapes. The methodology is then applied to compare 1928/1929 Fowler–Nordheim (FN) FE theory and 1956 Murphy–Good (MG) FE theory. It is theoretically certain that the 1956 theory is ‘better physics’ than the 1928/1929 theory. As in previous attempts to reach known correct theoretical conclusions by experimentally based argument, the new methodology tends to favour MG FE theory, but is formally indecisive at this stage. Further progress needs better methods of establishing error limits and of measuring κm.

Ag nanowires based SERS substrates with very high enhancement factor

Abstract: Silver nanowires (Ag NWs) are synthesized using hydrothermal method by maintaining a constant polyvinyl pyrrolidone (PVP): silver nitrate (AgNO 3 ) molar ratio. The effect of concentration of AgNO 3 under constant PVP: AgNO 3 ratio on the formation of Ag NWs is examined. The morphological, structural, and optical properties of the obtained Ag NWs are studied using field emission scanning electron microscope , transmission electron microscope, and ultraviolet–visible spectroscopy, respectively. The elemental analysis of the synthesized samples is carried out using energy dispersive X-ray (EDX) spectroscopy. Ag NWs with an average length of 5 μm are obtained at a higher concentration of AgNO 3 . The potential of the grown Ag NWs as SERS substrates is explored and a significantly high enhancement factor is acheived.

Electron Emission Regimes of Planar Nano Vacuum Emitters

Abstract: Recent advancements in nanofabrication have enabled the creation of vacuum electronic devices with nanoscale free-space gaps. These nanoelectronic devices promise the benefits of cold-field emission and transport through free space, such as high nonlinearity and relative insensitivity to temperature and ionizing radiation, all while drastically reducing the footprint, increasing the operating bandwidth, and reducing the power consumption of each device. Furthermore, planarized vacuum nanoelectronics could easily be integrated at scale similar to typical microscale and nanoscale semiconductor electronics. However, the interplay between different electron emission mechanisms from these devices is not well understood, and inconsistencies with pure Fowler–Nordheim (FN) emission have been noted by others. In this work, we systematically study the current–voltage characteristics of planar vacuum nanodevices having few-nanometer radii of curvature and free-space gaps between the emitter and the collector. By investigating the current–voltage characteristics of nearly identical devices fabricated from two different materials and under various environmental conditions, such as temperature and atmospheric pressure, we are able to clearly isolate three distinct emission regimes within a single device: Schottky, FN, and saturation. Our work will enable robust and accurate modeling of vacuum nanoelectronics, which will be critical for future applications requiring high-speed and low-power electronics capable of operation in extreme conditions.

Fabrication and field emission properties of vertical, tapered GaN nanowires etched via phosphoric acid.

Abstract: The controlled fabrication of vertical, tapered, and high-aspect ratio GaN nanowires via a two-step top-down process consisting of an inductively coupled plasma reactive ion etch followed by a hot, 85% H3PO4crystallographic wet etch is explored. The vertical nanowires are oriented in the[0001]direction and are bound by sidewalls comprising of{336¯2}semipolar planes which are at a 12° angle from the [0001] axis. High temperature H3PO4etching between 60 °C and 95 °C result in smooth semipolar faceting with no visible micro-faceting, whereas a 50 °C etch reveals a micro-faceted etch evolution. High-angle annular dark-field scanning transmission electron microscopy imaging confirms nanowire tip dimensions down to 8-12 nanometers. The activation energy associated with the etch process is 0.90 ± 0.09 eV, which is consistent with a reaction-rate limited dissolution process. The exposure of the{336¯2}type planes is consistent with etching barrier index calculations. The field emission properties of the nanowires were investigated via a nanoprobe in a scanning electron microscope as well as by a vacuum field emission electron microscope. The measurements show a gap size dependent turn-on voltage, with a maximum current of 33 nA and turn-on field of 1.92 V nm-1for a 50 nm gap, and uniform emission across the array.

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

Abstract: 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.

Critical electric field for transition of thermionic field emission/field emission transport in heavily doped SiC Schottky barrier diodes

Abstract: In this study, n-type SiC Schottky barrier diodes (SBDs) with various doping concentrations ([Formula: see text]–[Formula: see text]) were fabricated, and their forward and reverse current–voltage ( I– V) characteristics were analyzed focusing on tunneling current. Numerical calculation with the fundamental formula of tunneling current gives good agreement with experimental forward and reverse I– V curves in the heavily doped SiC SBDs ([Formula: see text]). The analysis of the energy where electron tunneling most frequently occurs revealed that field emission (FE) tunneling dominates conduction instead of thermionic field emission (TFE) under a higher electric field in reverse-biased heavily doped SiC SBDs, while forward I– V characteristics are described only by TFE. In addition, the critical electric field for the TFE–FE transition is quantitatively clarified by carefully considering the sharply changing electric field distribution in SiC with a high donor concentration.

Field emission microscope for a single fullerene molecule

Abstract: Abstract Applying strong direct current (DC) electric fields on the apex of a sharp metallic tip, electrons can be radially emitted from the apex to vacuum. Subsequently, they magnify the nanoscopic information on the apex, which serves as a field emission microscope (FEM). When depositing molecules on such a tip, peculiar electron emission patterns such as clover leaves appear. These phenomena were first observed seventy years ago. However, the source of these emission patterns has not yet been identified owing to the limited experimental information about molecular configurations on a tip. Here, we used fullerene molecules and characterized the molecule-covered tip by an FEM. In addition to the experiments, simulations were performed to obtain optimized molecular configurations on a tip. Both results indicate that the molecules, the source of the peculiar emission patterns, appear on a molecule layer formed on the tip under strong DC electric fields. Furthermore, the simulations revealed that these molecules are mostly isolated single molecules forming single-molecule-terminated protrusions. Upon the excellent agreements in both results, we concluded that each emission pattern originates from a single molecule. Our work should pave the way to revive old-fashioned electron microscopy as a powerful tool for investigating a single molecule.

Pressure Sensitivity of Electron Emission from SiOx Tunneling Diodes and their Outstanding Emission Performance under Rough Vacuum

Abstract: On‐chip electron sources with low pressure‐sensitivity and outstanding emission performances under rough vacuum are highly desired for developing miniature fully encapsulated vacuum electronic devices. Here, the sensitivity of electron emission from SiOx tunneling diodes formed in electroformed SiOx to the variation of ambient vacuum pressure from ≈10−4 to ≈102 Pa is explored. Electron emission from SiOx tunneling diodes is found to be insensitive to pressure variation when the pressure is lower than ≈10−1 Pa and exhibits fast degradation above the pressure, showing much lower pressure sensitivity than field emission sources. Despite emission current degradation above ≈10−1 Pa, SiOx tunneling diodes exhibit stable and reproducible electron emission under fixed rough vacuum up to 8 Pa, and good restorability after being repeatedly exposed to air pressure. By analyzing transport current of SiOx tunneling diodes under varying ambient vacuum pressure, the degradation of emission current above ≈10−1 Pa is attributed to the widening of insulating SiOx channel in the diode due to the transformation of Si conducting filament to SiOx with increasing oxygen partial pressure.

Hybrid Carbon Nanotubes–Graphene Nanostructures: Modeling, Formation, Characterization

Abstract: 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.

Nanoscale Vacuum Field Emission Triode With a Double Gate Structure

Abstract: A planar lateral Vacuum Field Emission Triode (VFET) with a double gate structure is proposed to improve the gate modulation characteristics in this letter. The preparation of this double gate structure is obtained by using the mature bottom gate VFET manufacturing process without any additional dedicated masks. The emission characteristics of the fabricated VFET under vacuum and atmosphere condition are measured. The experimental results demonstrate that both the bottom gate and the top gate can effectively control the emission electrons in the vacuum channel with width of about 110–200 nm which is successfully prepared by the electro-forming process. The operating mechanism of the device is basically the same as that of the plane-bottom gate VFET, and the device has achieved good field electron emission characteristics and double-gate modulation characteristics. Meanwhile, the vacuum channel is covered by the top gate insulator layer, which relaxing the vacuum requirement and improving the collection efficiency of emitted electrons by drain electrode about 50% in air.

Optical-field-induced Electron Emission in a dc-Biased Nanogap

Abstract: We construct an analytical formulation for nonlinear photoelectron emission in a dc-biased metallic nanovacuum gap triggered by a laser field, by exactly solving the one-dimensional time-dependent Schr\"odinger equation. We theoretically investigate the photoelectron energy spectra and emission current from left- and right-side surfaces of the asymmetric nanojunction with various dc biases, laser fields, and gap distances. The underlying photoemission mechanisms transitioning form multiphoton over-barrier emission to photon-assisted field tunneling, and the spatiotemporal dynamics of electron transport inside the gap are analyzed in detail. Our calculation shows applying a dc field could greatly reduce the interference oscillation in the transmission current in the nanogap, due to the shift of dominant emission away from the multiphoton over-barrier regime. Our results demonstrate that, besides the dc bias, varying the gap spacing could strongly influence the rectification on the photoelectron emission in a dc-biased metal-vacuum-metal gap. Our study provides useful guideline to the design of ultrafast nanogap-based signal rectification devices, such as photoelectron emitters and photodetectors, by choosing an optimal combination of dc bias, gap spacing, and material properties.