Emission Current Increases Research Articles

Numerical investigations of spatiotemporal dynamics of space-charge limited collisional sheaths

Electrostatic particle-in-cell (PIC) and direct simulation Monte Carlo (DSMC) methods are used to compare the plasma dynamics of collisionless with collisional emissive sheaths in partially ionized environments. Space-charge limited emissive sheaths submersed in a plasma with a density of ∼1017 m−3 are examined using a PIC-DSMC solver, CHAOS. Collisionless emissive sheaths with plasma domains sufficiently long (30 and 60 Debye lengths, λD) are subject to strong oscillations due to two-stream electron instability, whereas emissive sheaths in weakly collisional conditions with a short domain (15 λD) exhibit self-spike (sawtooth) oscillations in the plasma field due to the trapped charge-exchange (CEX) ion population within the virtual cathode (VC) region. The two-stream electron instability leads to strong temporal fluctuations in the total emission current, with maximum deviations of 60% and 100% from the time-averaged current for the long plasma domains, whereas CEX collisions cause strong spikes in the emission current if the domain size is short. Our PIC-DSMC simulations show for the first time that the interaction of the two types of instabilities causes the strength of the self-spike to be weakened due to the strong fluctuations caused by the two-stream instability when a sufficiently long computational domain with ion-neutral collisions is employed. By conducting a two-dimensional Fast Fourier Transform (FFT) on the collisional and collisionless sheaths with long domains, we show that the transient evolution of CEX entrapment in the VC increases frequency of sheath oscillations up to two times the ion-acoustic frequencies observed in the collisionless sheath. CEX collisions weaken the VC region and result in a total emission current more than that obtained from the collisionless case for the same domain length. With a more rarefied neutral environment of 1019 m−3 in the plasma sheath, the total emission current increases only 4% in comparison with 14% for one order of magnitude denser environment, within 20 μs. In addition, the spike period is tested with different neutral temperatures and densities. While we do not observe any self-spike in the more rarefied environment, the spike period increased from 5 to 7.5 μs when the neutral temperature is increased from 300 to 2000 K in the denser environment with the simulation time of 20 μs.

Emission characteristics of a forevacuum plasma-cathode source of wide-aperture pulsed electron beam with layer (mesh) stabilization of emission plasma

Research of emission characteristics of a pulsed forevacuum plasma electron source with layer (mesh) stabilization of emission plasma boundary has been carried out. Generation of a pulsed wide-aperture electron beam is provided by DC accelerating voltage (1–10 kV) and pulse-periodic formation of emission plasma by a cathodic arc with current 5–80 A, pulse duration 500–600 μs. Pressure (4–30 Pa) and the type of working gas (N2, O2, Ar, He, Kr) significantly influence on the emission characteristics of the forevacuum plasma electron source. An increase in the gas pressure and the use of gas with larger ionization cross-section provide an increase in the electron emission current. At low pressure (below a certain threshold value, which determined by the type of gas) an increase in the discharge current leads to an increase in the efficiency of electron emission. But at pressures more than the threshold value the emission efficiency decreases as the discharge current increases. The influence of pressure and type of gas on the emission characteristics are caused by the ion flux from the beam plasma which is formed due to ionization of gas by accelerated beam's electrons.

Regulating the aspect ratio of bulk few-layer graphene to improve the field emission performance

Owing to its excellent electrical conductivity and local electric field enhancement capability from abundant electron tunneling edges, graphene is a promising material for field emission. However, graphene thin-film emitters mostly emit in planar structures, and due to the flexibility of the thin film, it is commonly challenging to achieve vertical structure emission with large aspect ratios. Therefore, the emission performance of current graphene field emitters is difficult to meet the requirements of high current applications. In the present investigation, bulk few-layer graphene cathodes are formed by cold pressing to achieve upright structural field emission, and the emission performance is enhanced by appropriately adjusting the aspect ratio. The obtained bulk few-layer graphene emitters exhibit a higher conductivity of 2498 S/cm and tensile strength of 6.02 MPa. The turn-on electric field (at 1 mA/cm2) and threshold electric field (at 10 mA/cm2) in order are determined to be 1.78 V/μm and 2.66 V/μm, with a maximum current density of 986 mA/cm2 (at 6.65 V/μm) and a maximum field enhancement factor of 14,891. Excellent emission stability is achieved over a continuous period of 110 h at 5 mA @ 526 mA/cm2. With the increase of emission current, the bulk few-layer graphene with large aspect ratios exhibits better emission stability. This work provides valuable insights into the application of graphene-based field emitters at high currents.

Effect of water vapor desorption on the performance of gallium nitride field emitter array

We are exploring the potential of gallium nitride (GaN) field emitter arrays in vacuum channel transistors. This study investigated the impact of ultraviolet (UV) light on the emission properties of large arrays of GaN field emitters. Arrays of 150 × 150 emitters were analyzed before and after UV exposure. With a constant collector voltage of 200 V DC, gate voltage sweeps from 0 to 60 V were applied. The initial I–V measurements showed a rapid increase in emission current, indicating a conditioning effect, settling at a stable value of 1.25 μA after three to five sweeps. Remarkably, exposure to UV light resulted in a fivefold increase in the maximum field emission current, reaching an impressive 6 μA. This significant enhancement highlights the potential of UV treatment for improving the performance of GaN-based field emitters. This surge in current can be attributed to the desorption of water vapor caused by the UV light. To compare with the heating-based water desorption technique, another array of 150 × 150 emitters was characterized before and after heating at 400 °C. While the collector voltage remained constant at 200 V DC, the gate voltage was systematically increased from 0 to 75 V in this experiment. This controlled sweeping of the gate voltage provided a precise method for characterizing the field emission properties of the GaN emitters. The I–V measurements revealed that, similar to the UV exposure case, collector current increased by approximately four times after heat treatment at 400 °C for 10 min. This resulted in a maximum field emission current of around 10 μA at 75 V. As with the UV case, this increase can also be attributed to surface desorption, primarily of water.

Emission measurements and in-situ observation of ionic liquid electrospray thrusters with longitudinally grooved emitters

An externally wetted emitter array with longitudinally grooved structures for ionic liquid electrospray thrusters was fabricated to improve ionic liquid transport to the emitter tips. Two grooved emitter shapes with different groove depths were successfully fabricated using microelectromechanical system processing techniques. We evaluated the current–voltage characteristics, measured the mass spectra using time-of-flight (ToF) spectrometry, and conducted in-situ observations using a high-speed microscope. The experimental results of ion emission show that the absolute emission current increases compared with that of our previous emitter without grooves. This tendency is strengthened with deeper grooves. Moreover, the slope of the current–voltage curve for the grooved emitters does not decrease even when high voltages are applied, indicating that the grooved structure improves the ionic liquid transport to the emitter tips. This improvement is attributed to the low hydraulic impedance of the emitter structure. However, deeper grooving also increases the percentage of current intercepted by the extractor electrode, and electrochemical reactions are not avoided at an alternation frequency of 1 Hz. Although the first current–voltage measurement tended to have unstable characteristics, the ToF results indicated that the emission in the center line was in the pure-ion regime, composed mostly of monomer and dimer ions, under all the measured conditions. High-speed microscope observations showed that too much ionic liquid deposited on the extractor causes ion emission from the extractor to the emitter, known as backspray, and implies that no large droplets are emitted for either grooved emitter structure, which is consistent with the ToF results.

The synthesis of ReS2 flakes and its application in photodetectors

ReS2 is attracting much attention because of its stable trion state. This kind of stable trion state arises on account of weak interlayer coupling as well as anisotropic crystal structure. In this research, we have synthesized ReS2 flakes successfully by using chemical vapor deposition (CVD) method. Stable ionic states in hexagonal wafers are observed by photoluminescence spectroscopy (PL). This substance is stable at room temperature. The HRTEM image from the single ReS2 hexagon reveals that the individual hexagon is single crystal. EDS spectroscopy indicates the purity of the synthesized product. We find that the Re and S atoms ratio in pure ReS2 is 1:2. Then we fabricate a photo detector on individual ReS2 flakes and test its performance. We compare the photocurrent in dark current and under a 500 nm incident light for two media (air and 100 ppm H2). Emission current increases from 1.15 μA to 1.67 μA (forward) and from 7.9 μA to 13.8 μA (reverse). Therefore, the ReS2 hexagonal wafer is an ideal choice for stable and reliable room temperature optical gas sensor. And the material can also be used for fast switch.

Current rise phenomena due to electron emission from CTJ in vacuum

AbstractIn general, cathode triple junctions (CTJ) are regarded as electrical weakness points because of the occurrence of electron emission. It's believed that vacuum surface flashover starts from the initiation of electron emission at CTJ. Therefore, it is important to investigate electron emission phenomenon around the CTJ region. However, such a study which is direct measurement of the electron emission from CTJ has not been almost done because it is difficult to measure it directly. So, the purpose of this study is to elucidate the electron emission phenomenon from CTJ by investigating field electron emission current characteristics and charge distribution at CTJ region. Thus, we measured the electron emission current and the surface potential for the samples which CTJ was formed artificially. In addition, we simulated the electric lines of force near CTJ region. The results show that electron emission current increases due to the effect of the insulator surface being charged by the electrons emitted from the CTJ.

Self-consistent charging of PMMA thin film induced by a penetrating electron beam in electron microscopy.

The charging of the polymer thin film irradiated by penetrating electron beam (e-beam) is investigated, in parallel with the numerical simulation and experiment. The simulation is performed by combining scattering, drift, diffusion, trapping and recombination. Results show that, due to the electron emission the net charge near the surface is distribution positively, but negatively inside the film because of low electron mobility. The surface potential is positive near surface and accordingly forces some of secondary electrons to return surface. As irradiation proceeds, currents flowing into and out of the film can tend to equilibrium. In the equilibrium state, with increasing beam energy, the surface potential and the efficient emission current decrease, and the electron beam-induced current and the transmission current tend to zero and the beam current valuable, respectively. E-beams of 10-30keV cause positive charging on PMMA film of 2 µm, which means the high-energy e-beam can effectively discharge a thin film that has been charged negatively by irradiation with low-energy e-beam. With the increase of the film thickness from 1 to 3 µm, the positive surface potential and the emission current decreases and increases, respectively, and the transmission current tends to zero.

イオンエンジンにおける複数台の電界放出型電子が中和性能に及ぼす影響

In order to improve ion engine's performance, the neutralization performance with two field emission cathodes in an ion engine is investigated. The neutralization performance is evaluated by the potential difference between cathode and ground, using a 100μN class ion thruster developed at Kyushu University and two 50×50mm2 field emission cathodes with carbon nanotube emitter. The potential difference between cathode and ground is not only determined by cathode electron supply capacity and the position of the cathodes but also foot print of the neutralizers, it would be due to the space charge limitation. That is, the potential difference between cathode and ground is improved with increase in total emission current, and that with a single field emission cathode at emission current of 6mA is -20V, on the contrary, that with two field emission cathodes is -12V.

Experimental investigation of sheath effects on I–V traces of strongly electron emitting probes

I–V traces of strongly emitting emissive probes are investigated in a multidiople filament discharge. It is found that at sufficiently high neutral pressure and emitting current, the variation of the I–V traces and their associated inflection points no longer follow the previous predictions of space charge limited (SCL) models. A new, steep slope region of the I–V trace appears near the plasma potential when the probe is strongly emitting, causing the inflection point and the floating potential to increase towards the plasma potential as emission current increases, rather than staying constant. This is, to our knowledge, the first experimental evidence that the effects predicted by Campanell et al’s inverse sheath theory (2017 Physics of Plasmas 24 057101) not only affect the floating potential but also a region in the I–V trace of an emissive probe. It is also found that the double inflection point structure when the probe is biased below the ionization energy of the working gas is highly likely to be an emission retardation effect from enhanced virtual cathode formation due to the increased local electron density. The implications of these findings on hot cathode sources are briefly discussed.

Multifunctional ZnO nanorods decorated with plasmonic gold nanoparticles for enhanced room temperature field emission, photo-luminescence and catalytic properties

We report the enhancement of room temperature photoluminescence (PL), field emission and photocatalytic properties of ZnO nanorods (NRs) decorated by gold NPs (Au-ZnO NRs) prepared using hydrothermal process. The results of X-ray diffraction (XRD) analysis confirmed formation of gold NPs (NPs) and wurtzite phase of ZnO in the final nanostructures. Microscopic studies revealed that uniform ZnO NRs were formed and gold NPs are well decorated on the surface of the NRs. It was shown that both reaction time and concentration of HAuCl4 play a crucial role in photocatalytic, field emission and PL properties of Au-ZnO NRs. We suggest that there is a correlation between the shape of the NRs and concentration of HAuCl4 which causes the maximum PL intensity and stronger field emission properties. The ultraviolet (UV) and visible-light photocatalytic performance was successfully demonstrated for the photodegradation of Methylene blue (MB) as a model dye. Field emission studies revealed that by introducing gold NPs into the ZnO NRs the emission current increases. We have found that the shape of the nanostructure has a crucial role in the field emission properties and at higher reaction times, where the aspect ratio of the ZnO NRs decreases, the emission current also decreases.

Квантовая теория эмиссии электронов из структуры "металл-диэлектрик" в сильных электрических полях

A generalized formula is derived for the electron emission current in relation to the temperature, the electric field, and the electronic work function for a “metal–dielectric” system. The formula takes into account the quantum nature of the image forces. In deriving it, the Fermi–Dirac distribution and the quantum image potential obtained in terms of the electron–polaron theory are used. In the limit of the classical potential of image forces, the well-known Richardson–Schottky and Fowler–Nordheim formulas are obtained for thermionic emission and field emission, respectively. It is shown that at high temperatures and electric fields E ≥ 10 MV/cm, the polaron contribution to the electron emission current increases with increasing field and decreases with increasing temperature. The decrease in current is related to an increase in effective electronic work function due to the electron-polaron effect. Extrapolation formulas convenient to obtain theoretical estimates are derived for the thermionic and the field emission current.

X-ray intensity, stability, and source size characterizations for the 10 kV/5 mA water cooled upgraded Manson source at the Atomic Energy Commission for Laser MégaJoule Facility.

Calibration of x-ray cameras integrated within plasma diagnostics for Laser MégaJoule's experiment is of great interest for interpretations of physical phenomena. At the Atomic Energy and Alternative Energy Commission, an x-ray calibration laboratory has been trying for many years to provide the most relevant calibrations. In the soft x-ray range (<1.5 keV), a model 5 Manson source is used in combination with a concave grating monochromator equipped permanently with an absolute calibrated silicon drift detector. To reduce the calibration time achieved with this experimental setup, the Manson source has been modified to reach an emission current of up to 5 mA (100 µA for a standard source model) while keeping the x-ray monochromatic intensity constant within ∼2% for several operating hours. A water cooling system has been integrated within the anode carousel and an emission current regulation system inside a completely redesigned power supply. In this work, we present characterization of the source size and x-ray intensity as a function of both source voltage and emission current, as well as the stability of this new source. Both arms of the source have been used: one for monitoring the monochromatic spectra and x-ray intensity and the other to measure the source size. An x-ray spot was observed to be evolved from the oval shape toward the annular one as the emission current increases. For fixed emission current, the source size is reduced as the voltage increases. The x-ray intensity has significantly been increased to a maximum factor of ∼20 (depending on the anode) while being constant within around 2% as expected.

Zero bias emission current in laser heated emissive probe and proper choice of probe-tip material

Emissive probes are considered as one of the best diagnostic tools for the direct measurement of plasma potential in low temperature plasma systems. Laser-heated emissive probes (LHEPs), due to their higher lifetime, are becoming more popular than their current-heated conventional counterparts. Graphite due to its higher absorption coefficient at CO2 laser wavelength and LaB6 due to its lower work function are commonly used as the probe materials for laser heated probes. Experiments with a probe-tip made of a graphite material have revealed that the emission processes other than thermionic emission also occur on heating the material with the laser, which can affect the plasma potential measurements and change the I-V characteristics of the LHEP. This additional emission due to the laser irradiation on the graphite-tip explains the measurements of lower values of plasma potentials and also describes the observation of an increase in electron saturation current with the increase in thermionic emission current due to heating. The possible mechanisms of this negatively charged-emission other than the thermionic emission from the graphite-tip have been thoroughly characterized in the absence of plasma. This extra emission is not observed in the probe-tip made up of LaB6, and a good comparison of I-V characteristics could be established with a conventional filament emissive probe in the case of LaB6.

Field emission properties of TiO2 nanotubes fabricated on Ti wire

The present work reports the field emission (FE) properties of well-aligned TiO2 nanotubes (TNTs) grown on Ti wire. Vertically-oriented, and highly-ordered TNTs with high aspect ratio are synthesized on Ti wire by electrochemical anodization in ethylene glycol (EG) electrolyte containing a small amount of ammonium fluoride (NH4F) and de-ionized (DI) water. The as-anodized TNTs have been characterized using FESEM, HRTEM, EDAX, XRD, and XPS. FESEM study shows that the average diameter of the nanotubes is ∼90 nm and the wall thickness is ∼25 nm. GAXRD and HRTEM analyses indicate that the as-anodized TNTs are amorphous. EDX analysis confirms that TNTs are composed of Ti and O along with considerable amount of F. XPS results reveal the oxide state of TiO2. FE studies of the nanotubes showed that the turn-on field is 6 V/μm with corresponding emission current density of ∼10 μA/cm2 and field enhancement factor (β) of ∼1000. The emission current increases exponentially with increase in applied field and reaches to the maximum value of ∼150 μm at 15 V/μm. The FE properties of TNTs on Ti wire show their potential applications in the future display devices as a point electron source with small optical size. It can also be a good candidate for the fabrication of miniature X-rays tubes for industrial and medical applications.

Photoassisted and multiphoton emission from single-crystal diamond needles.

Practical realization of stable and high brightness sources of ultra-short electron pulses is an important issue in the development of time-resolved electron microscopy for the study of ultra-fast dynamics in materials. Here, we report on the experimental investigation of static (in the dark) and pulsed (under illumination by sub-picosecond laser pulses at 1040 nm) electron emission from single-crystal diamond needles. A significant increase of electron emission current was detected under laser illumination. The nonlinear dependence of the emission current on the laser intensity and on the angle between the needle and the laser beam polarization axis suggests multi-photon emission processes. This interpretation is in agreement with electron spectroscopy measurements performed for electrons emitted at different bias voltages and different laser power levels and repetition rates. The remarkable feature of the diamond emitters is their stability under high average power of laser radiation. This provides a new highly efficient source of photoemitted electrons based on single-crystal diamond.

Enhanced field emission performance of lanthanum hexaboride coated on graphene film

Lanthanum hexaboride (LaB6) is coated on electrophoretically deposited-graphite film by electron beam evaporation method. Surface morphology, Raman spectra and field emission properties are discussed by varying LaB6 coating thickness. It is demonstrated that this film cathode always shows much better field emission performance than pristine graphite cathode. Especially when LaB6 film thickness is 90 nm, the emission current increases from 153 μA to 850 μA with the applied electric field of 6.7 V μm−1, and the turn-on field decreases from 2.3 V μm−1 to 1.0 V μm−1. The results suggest that the graphene-based composite is an excellent field emission cathode, especially suitable for vacuum devices such as field emission display.

Low pressure hydrogen sensing based on carbon nanotube field emission: Mechanism of atomic adsorption induced work function effects

The mechanism of the innovative adsorption based field emission hydrogen sensing technique is investigated from experimental and first-principles investigations. Hydrogen pressure from 10−3 to 10−7 Pa range could be detected. High carbon nanotube (CNT) crystallinity plays key role for the reduction of the effective work function from the dissociated hydrogen adsorption, leading to the field emission current increase. For CNT emitters with high dangling-bond (DB) type of defects/structures, the dissociated H adsorption occurs preferentially on the DB site with negligible work function variation. Joule heating & temperature rising is a core factor to assist the dissociated hydrogen adsorption with enhanced sensing effect. For work function sensitive CNT devices, operations under hydrogen & high temperature conditions may produce performance stability issue.

A plasma electron source for generating beam plasma at low gas pressures

A plasma electron source design for generating beam plasma at low gas pressures is reported. The designed plasma electron source can run stably in the pressure range between 0.1 Pa and 4 Pa. The electron source emission current increases as the working pressure increases. The DC electron beam can reach 11 A at an extraction voltage of 400 V and a gas pressure of 4.0 Pa, which is experimentally verified in this article. This low energy and high current electron beam interacts with gas molecules to generate a beam plasma, which increased the plasma electron source output current. As the working pressure and emission current increased, the beam plasma emission spectrum intensity increased.

Photoinduced effects in field electron emission from diamond needles

We report an explicit experimental observation of photostimulated electron field emission from diamond. The electron emission properties of monocrystalline diamond needles were investigated in the dark and under illumination by nanosecond laser pulses. A prominent increase in the electron emission current was detected under illumination by light with photon energies above 5.0 eV. The linear dependence of the photoinduced emission current on the light intensity was observed in the spectral range of 5.0 to 5.9 eV, while its field dependence demonstrated saturation behavior. The remarkable feature of the observed phenomenon was the fact that illuminated and field emission areas of the diamond needles were spatially separated by about 100 μm in the used experimental setup. Possible mechanisms for the observed effects are discussed.