Enhanced Electron Emission Research Articles

Nanoplasma formation and expansion ignited by an intense FEL: a study using pump-probe electron spectroscopy

Abstract We demonstrate real-time observations of nanoplasma formation and expansion using intense extreme-ultraviolet (XUV) and near-infrared (NIR) pump--probe electron spectroscopy. We identified the formation of a nanoplasma by the sudden enhancement of low-energy electron emission within a few tens of femtoseconds after XUV excitation, which indicates considerable heating of the clusters by the NIR field. 
We probed the subsequent expansion of the nanoplasma by monitoring the transient resonant enhancement of high-energy electron emission. The dependence of the resonance on the XUV intensity is explained by the expansion speed of the XUV-induced nanoplasma.

Impacting factors and operation thresholds of electron transpiration cooling-based thermal protection system

Electron transpiration cooling (ETC) is a spontaneous endothermic process that occurs during thermionic emission, which shows great potential in ultra-high-temperature thermal protection systems (TPS). Herein, we systematically analyze the main influencing factors on the cooling effect of ETC, such as incoming flow velocity, geometry, and material work function. A two-dimensional finite element model, based on Navier-Stokes equations coupled with the 11-species air reaction model and two-temperature model, is developed to solve the thermal interaction between ETC and the non-equilibrium flow field. Three operational thresholds for ETC are identified. Lower work functions enhance electron emission, thereby reducing wall temperature. With a 2.0 eV work function, ETC significantly outperforms blackbody radiation at 1360 K, and its cooling efficiency increases with temperature. For flow velocities above Mach 9.0, ETC is effective at the leading edge with a 2.4 eV work function and a 5 mm radius. However, it loses effectiveness with a 300 mm leading edge radius, even at Mach 16.0. Notably, at Mach 19.6, with a 2.0 eV work function and a 5 mm radius, ETC reduces surface temperature by up to 48.1 %. These findings highlight the considerable potential of ETC for applications in ultra-high-temperature TPS. These findings highlight the considerable potential of ETC for applications in ultra-high-temperature TPS.

Enhanced Electron Emission Performance and Air-Surface Stability in ScO-Terminated Diamond for Thermionic Energy Converters.

Diamond with negative electron affinity (NEA) and low work function surfaces are suggested as a suitable material for electron-generation applications in vacuum, in particular, as the emitter electrode in thermionic energy converters. Such NEA surfaces can be fabricated by evaporating and then annealing submonolayers of a suitable metal in vacuo onto bare or oxidized diamond. Among the metals studied, scandium termination of bare diamond (100) and (111) surfaces is recently reported to give the largest NEA values reported to date for a metal-diamond system, as well as being thermally stable to 900°C. It is now shown that preoxidized (100) diamond functionalized with 0.25monolayers of Sc also produces a large NEA value of -1.02eV with low work functions (<3.63eV). Moreover, this surface is thermally stable to 700°C and can withstand exposure to air for extended periods. Here, the structural and electronic properties of this Sc─O-functionalized diamond surface are characterized in detail using a variety of surface-science techniques. The results suggest that this material may be the ideal candidate for the fabrication of commercial thermionic energy conversion devices, e.g., for solar-power generation, as well as for various other electronic devices that rely upon electron emission.

Electric field and higher harmonics of RF plasma slit jet measured by antennas and VI probes

The cold atmospheric plasma jets change their character when interacting with the different surfaces. Since such interaction is the primary area of plasma jet applications, it is essential to monitor the process. The non-linearity of the RF plasma slit jet (PSJ) was analyzed using the VI probes and a novel method, the non-intrusive antenna measurements. Regardless of the experimental setup and gas mixture (Ar, Ar/O2, Ar/N2), the PSJ frequency spectrum consisted of the following main features: dominant fundamental frequency peak, relatively strong odd harmonics, and significantly weaker even harmonics. The lowest degree of non-linearity was recorded for the Ar PSJ ignited against a grounded target. Admixing a molecular gas increased the discharge non-linearity. It was attributed to the enhancement of secondary electron emission from the dielectric surfaces. In addition to the non-linearity analysis, the antenna spectra were for the first time used to determine the semi-quantitative values of the PSJ-radiated electric field. The electric fields decreased by a factor of 2 after the admixing of nitrogen and oxygen molecular gases. Out of the studied targets, the highest electric fields were observed when plasma impinged on the grounded targets, followed by the floating target (2× lower) and the PSJ ignited in the open space configuration (4× lower than in the grounded target configuration).

Emission current enhancement from quasi-freestanding epitaxial graphene microstructure electron emitters through surface layered silicon dioxide

Enhanced electron emission from oxide-encapsulated quasi-freestanding bilayer epitaxial graphene devices is reported, including one emission current of 9.4 µA and successful emission even with oxide thicknesses of up to 1.25 µm. The low operating temperature (215 °C), and applied electric fields under which the devices operate indicate electron emission is due to phonon-assisted electron emission, wherein forward-scattering hot phonons impart the necessary energy for the electrons to escape the graphene as an emission current. A suite of device structures and behaviors are cataloged, and various emission behaviors are demonstrated through encapsulating oxide layers. Emission current enhancement due to electron multiplication in the oxide layers is observed across multiple devices and oxide thicknesses.

Boosting electron and x-ray emission from laser produced plasmas by doping liquid droplets

In recent studies, it has been shown that under optimal dual pulse irradiation, it is possible to generate anomalous relativistic temperature (&amp;gt;500 keV) electron emission even with mJ/pulse lasers at 1/100th of the relativistic intensity when micrometer droplets of methanol are used as a high repetition rate target. In this paper, we present the results of doping these droplets with CH3I to bring out a 30-fold enhancement in electron emission. Experiments and simulations presented here find a route to enhance electron emission driven by two plasmon decay instability. Improving compact laser plasma based sources of MeV electrons, x-ray emission schemes that use a high repetition rate, and mJ/pulse laser systems is important not only for basic science but also applications of imaging and radiography.

DC-driven subatmospheric glow discharges in the infrared-stimulated

This paper presents a conceptual framework for experimental research combined with numerical analysis on direct current (DC) glow discharges in microscale planar gas discharge-semiconductor systems (GDSS). In the experimental section, several structural and elemental analyses, including SEM, EDAX, AFM, and near-infrared absorption spectra measurements were carried out for compound semiconductor zinc selenide (ZnSe) cathode sample. Argon (Ar) was charged into the plasma reactor cell of GDSS at pressures of 100 Torr subatmospheric and 760 Torr atmospheric, respectively, by a vacuum pump- gas filling station. Glow discharge light emissions from plasma, excited under three different intensity levels (dark, weak, strong) of infrared beam illumination on ZnSe cathode electrode, were measured by using a phomultiplier tube that is sensitive to UV–Visible wavelengths. In the numerical analysis section, simulation studies were carried out on the two-dimensional gas discharge-semiconductor microplasma system (GDSµPS) cell models using the finite-element method (FEM) solver COMSOL Multiphysics DC plasma program. Calculations and predictions were based on mixture-averaged diffusion drift theory and Maxwellian electron energy distribution function. GDSµPS cell was modeled in a square chamber with planar anode/cathode electrode pair coupled at a 50 μm discharge gap. Single side of ZnSe cathode was finely micro-digitated to increase the effective surface area for enhanced electron emission to the gas discharge cell. The electrical equivalent circuit (EEC) of the proposed model was driven by 1.0 kV DC voltage source. Binary Ar/H2 gas medium in a mixture of 3:2 molar ratio was introduced to the gas discharge chamber at constant 200 Torr subatmospheric pressure. Simulations were run for normal glow discharges to exhibit the electrical fast transient glow discharge behaviours from electron field emission state to self-sustained normal glow discharge state by numerically solving the electron density (ED), electron current density (ECD) and electric potential distribution (EPD) parameters.It is figured out that binary Ar/H2 gas discharge model can undertake a major role in shaping and controlling the spatiotemporal response to transient electro-optical behavior of microplasma-based artificial electromagnetic materials configured for high-efficiency infrared-to-visible wavelength conversion applications.

A simulation study on the radiosensitization properties of gold nanorods

Objective. Gold nanorods (GNRs) have emerged as versatile nanoparticles with unique properties, holding promise in various modalities of cancer treatment through drug delivery and photothermal therapy. In the rapidly evolving field of nanoparticle radiosensitization (NPRS) for cancer therapy, this study assessed the potential of gold nanorods as radiosensitizing agents by quantifying the key features of NPRS, such as secondary electron emission and dose enhancement, using Monte Carlo simulations. Approach. Employing the TOPAS track structure code, we conducted a comprehensive evaluation of the radiosensitization behavior of spherical gold nanoparticles and gold nanorods. We systematically explored the impact of nanorod geometry (in particular size and aspect ratio) and orientation on secondary electron emission and deposited energy ratio, providing validated results against previously published simulations. Main results. Our findings demonstrate that gold nanorods exhibit comparable secondary electron emission to their spherical counterparts. Notably, nanorods with smaller surface-area-to-volume ratios (SA:V) and alignment with the incident photon beam proved to be more efficient radiosensitizing agents, showing superiority in emitted electron fluence. However, in the microscale, the deposited energy ratio (DER) was not markedly influenced by the SA:V of the nanorod. Additionally, our findings revealed that the geometry of gold nanoparticles has a more significant impact on the emission of M-shell Auger electrons (with energies below 3.5 keV) than on higher-energy electrons. Significance. This research investigated the radiosensitization properties of gold nanorods, positioning them as promising alternatives to the more conventionally studied spherical gold nanoparticles in the context of cancer research. With increasing interest in multimodal cancer therapy, our findings have the potential to contribute valuable insights into the perspective of gold nanorods as effective multipurpose agents for synergistic photothermal therapy and radiotherapy. Future directions may involve exploring alternative metallic nanorods as well as further optimizing the geometry and coating materials, opening new possibilities for more effective cancer treatments.

Two-color laser driven enhanced electron emission from carbon nanotubes with long carrier lifetime

Ultrafast electron pulses generated using femtosecond lasers offer a direct means for investigating ultrafast processes, but the yield of electron emission remains a significant practical limitation. This study demonstrates a substantial enhancement in yield of electron emission from carbon nanotubes induced by two-color (ω + 2ω) pulses with an 80 MHz repetition rate and this enhancement is non-delay-dependent. Compared to the sum of photocurrents induced by each color pulse individually, the combined two-color pulses result in a remarkable enhancement in yield of electron emission by more than 25 times. We attribute this enhanced yield to carrier-assisted optical field emission: carbon nanotubes sustain high carrier concentration state under laser radiation thereby enhancing yield of electron emission. We propose the carrier migration emission model to provide a semi-quantitative description for the carrier-assisted optical field emission. This phenomenon introduces a new method to increase the yield of ultrafast electron pulses from CNTs and paves the way for next-generation ultrafast electron emitters and on chip ultrafast optoelectronic device.

Secondary electron emission enhancement of MgO-Au composite film by SiO2 doping in MgO surface layer

The secondary electron emission (SEE) properties of MgO-Au composite film deposited by magnetron sputtering was improved by SiO2 doping with a Si/Mg ratio of 14 % in its MgO surface layer. Compared to the conventional electronic material MgO/(MgO-Au) film, the SiO2-doped MgO/(MgO-Au) film achieved a secondary electron yield (SEY) of 7.69 with an increase of 9.2 % at the primary electron energy (Ep) of 300 eV and the maximum SEY of 11.4 eV with an increase of 12.9 % at the Ep of 700 eV, and it kept relatively high SEY consistently under the continuous bombardment of the incident electrons with the Ep of 200 eV. This SEE enhancement is attributed to the reductions of both the band gap and work function of MgO crystal as well as the grain enlargement and surface roughness decrease of MgO surface layer induced by SiO2 doping. This scheme of SiO2 doping in the MgO surface layer of MgO-Au film provides a new idea for the preparation of high performance MgO-based SEE material.

The study of channeled vacuum GaAs photocathodes for enhanced electron emission properties

In this paper, we design and analyze the channeled vacuum photocathodes with novel structures towards low operating temperature, high current density, and miniaturized cathode electron sources. We simulate the characteristics of the electron emission of the channeled vacuum GaAs photocathodes in detail with the commercial CST software. Simulation results indicate that the emission current of the photocathode can increase with the optimal gate and anode voltage choices, and the emission efficiency can reach up to 63.4 %. Using this optimized structure as the benchmark, the quantum efficiency and emission efficiency are also investigated. Furthermore, we have altered the channel morphology, i.e., with the curved vacuum channel structures, to enhance the current density. We have found that the emission current density can increase significantly and reach up to 2.68 times of the regular rectangular channel. The simulation results can lay the foundation for the development of new type of micro-structured GaAs photocathodes.

Field electron emission behavior of pristine and Au nanoparticles decorated Sb2Te3-rGO heterostructure emitters

A facile one-pot hydrothermal method was used to synthesize pristine Sb2Te3 and its heterostructure with reduced graphene oxide (rGO). A novel approach dealing with high energy (6 MeV) electron irradiation has been efficiently used for synthesis of gold nanoparticles (Au- NPs) and their in-situ decoration on Sb2Te3-rGO heterostructure. From an application point of view, we have investigated the field electron emission characteristics of pristine Sb2Te3 and Sb2Te3-rGO heterostructures, with and without Au-NPs decoration. The Au-NPs decorated Sb2Te3-rGO heterostructure emitter showed enhanced electron emission behavior in terms of reduced value of turn-on field, delivery of large emission current density (∼1171 µA/cm2) at relatively lower applied field (∼3.1 V/µm), and excellent emission stability. The improvement in emission characteristics upon decoration of Au-NPs is attributed to noticeable increase in the number of emission sites, higher value of field enhancement factor (β = 4562), and betterment in the electronic properties due to Au-NPs-Sb2Te3-rGO interface effects. The obtained results suggest that Au-NPs decorated Sb2Te3-rGO heterostructure emitter has potential towards practical applications in various vacuum microelectronic devices. Furthermore, the present approach dealing with high energy electron irradiation for synthesis and in-situ decoration of nanoparticles can extended to other inorganic heterostructures towards improving their functionality.

Effects of environment on the electron-impact ionization dynamics of argon clusters

Ionization of the $3p$ orbital of argon (Ar) monomers, dimers (${\mathrm{Ar}}_{2}$), and small-size Ar clusters (${\mathrm{Ar}}_{n},\phantom{\rule{0.16em}{0ex}}\ensuremath{\langle}n\ensuremath{\rangle}\ensuremath{\approx}15$) is investigated for 90-eV electron impact. Experimentally, the three-dimensional momentum vectors of the two outgoing electrons and the residual ion are measured in triple coincidence, as is the mass-over-charge ratio of the ion, using a reaction microscope that covers a large part of the final-state phase space. The triple-differential cross sections (TDCS) are obtained for almost the full solid angle of the ejected electron, in which the data for Ar monomers are well reproduced by $B$-spline $R$-matrix calculations for ejected electron energies of 3 eV, 5 eV, and 10 eV, and projectile scattering angles of ${10}^{\ensuremath{\circ}}$ and ${20}^{\ensuremath{\circ}}$. Compared to the Ar monomer, the cross sections for ${\mathrm{Ar}}_{2}$ and ${\mathrm{Ar}}_{n}$ exhibit effects due to the neighboring atoms by the suppression of binary and recoil lobes and the enhanced electron emission out of the projectile scattering plane. These environmental effects are further investigated using a multicenter three-distorted-wave theory, which can generate the TDCSs for electron-impact ionization of ${\mathrm{Ar}}_{2}$. These studies reveal an important role of multicenter scattering reactions on the ionization dynamics of Ar clusters. Moreover, a two-center interference is not found in the TDCS of ${\mathrm{Ar}}_{2}$. This is interpreted as due to the involvement of orbitals with both gerade and ungerade symmetries in the ionization process.

Exploring the electronic and optical anisotropy of quasi-one-dimensional ternary chalcogenide CrSbSe3: a DFT study

The anisotropy of the electronic and optical properties of the quasi-one-dimensional ternary transition metal chalcogenide CrSbSe3 is investigated using density functional theory (DFT) including the on-site Coulomb self-interaction potential (DFT + U) and spin-orbit interaction. The compound is an indirect bandgap semiconductor with bandgap of 0.58 eV. Effective masses and charge recombination rates along different directions show appreciable anisotropy, indicating the possibility of sensor applications. CrSbSe3 exhibits satisfactorily high carrier mobility and charge recombination rates along all directions, making it suitable for solar and photovoltaic applications. The Se-p and Cr-d orbitals contribute to the DOS in the valence band, while DOS contribution to the conduction band bottom comes mainly from Sb-p and Cr-d orbitals. Simulated XPS and HAXPES show that for soft x-rays, the peak in the low binding energy is due to the electron emission from Se-p and Cr-d orbitals while the peak in the high binding energy is due to the electron emission from Cr-d and Se-s orbitals. For hard x-rays, the relative intensity of the low binding energy peak decreases in comparison to the high binding energy peak, indicating enhanced electron emission from Se-s orbitals at higher energy x-rays. CrSbSe3 possesses large anisotropy in optical parameters such as refractive index, optical absorption, reflectivity, optical conductivity and optical extinction. The birefringence and dichroism are large in the IR, visible and near UV regions, with a giant birefringence equal to 0.56 at 650 nm and dichroism equal to 0.57 at 550 nm. The electron energy loss function is anisotropic for high energetic electrons, while for electrons below 4 eV it is isotropic. CrSbSe3 is an ideal material for photovoltaic, sensor and other optoelectronic applications as it possesses a bandgap of 0.58 eV, large optical anisotropy, high values of dielectric constant, refractive index, absorption coefficient and extinction coefficient.

Pulsed voltage driving enhanced electron emission in ZnO nanowire cold cathode flat-panel X-ray source

A fully vacuum-sealed flat-panel X-ray source was fabricated composed of molybdenum metal anode and ZnO nanowire field emitters array cathode. The cathode current and X-ray radiation dose rate of the device were measured when the anode is applied with direct current (DC) and pulsed voltage, respectively. It is found that the peak emission current and corresponding dose rate under pulsed voltage driving are much higher than those under DC voltage driving. The phenomena may be explained by a plasma induced field emission enhancement related to the electron bombardment induced gas desorption. Clear projection X-ray imaging of moving objects was obtained using pulsed voltage driving. The result shows that owing to the enhanced emission current driven by the pulsed voltage, the flat-panel X-ray source can achieve fast and clear imaging of high-speed moving objects. Our results are of significance for the application of large area flat-panel X-ray source in dynamic imaging.

Microparticle‐initiated electron emission and subsequent gas breakdown

AbstractMicroparticle‐initiated breakdown phenomena in gas gap have obtained extensive research interest and micro‐gap discharge is convinced to play a key role. To illustrate this, this paper focused on microparticle‐initiated electron emission and subsequent gas breakdown process in a nitrogen gap. Analytic results of the electric field distribution between the charged microparticle and electrode surface are obtained by using the image method first. Results indicate that for the typical microparticles (R &lt; 20 μm) from electrode erosion, when the micro‐gap distance h is lower than 2 μm, the maximum enhanced field strength could exceed 3 × 107 V/m. Based on this, a two‐dimensional axis‐symmetry numerical model is calculated by using the particle‐in‐cell coupling with Monte Carlo method and the charged microparticle is positioned near anode and cathode separately. In both cases, the local enhanced electron emission between microparticle and electrode surface is the key to initiate the final gas breakdown. The breakdown near cathode is electron‐dominated, which could accomplish in several nanoseconds. The breakdown near anode is ion‐dominated, which needs stronger field enhancement factor or longer breakdown time and would be harder to occur in reality.

Photo tuned electron field emission from vertically aligned CH3NH3PbI3 nanorods grown in AAO template

Among the several untapped aspects of CH3NH3PbI3 i.e. Methylammonium lead iodide perovskite, electron field emission is an important one. Motivated by the scarcity of reports available in this segment, this work carries out a detail investigation of the field emission properties of vertically aligned CH3NH3PbI3 nanorods (NRs) grown within AAO (Anodic aluminum oxide) template. The emission produced considerable current density of 187 μA/cm2 with turn on field 3.4 V/μm and enhancement factor 5500. Accompanied by the simulation results of local electric field distribution, examination of incident photons impact upon emission behaviour under varying light intensities revealed enhanced electron emission under increased light intensities due to improved number of photo-excited carriers.

Stable Field Electron Emission and Plasma Illumination from Boron and Nitrogen Co‐Doped Edge‐Rich Diamond‐Enhanced Carbon Nanowalls

AbstractSuperior field electron emission (FEE) characteristics are achieved in edge‐rich diamond‐enhanced carbon nanowalls (D‐ECNWs) grown in a single‐step chemical vapor deposition process co‐doped with boron and nitrogen. The structure consists of sharp, highly conductive graphene edges supplied by a solid, diamond‐rich bottom. The Raman and transmission electron microscopy studies reveal a hybrid nature of sp3‐diamond and sp2‐graphene in these nanowalls. The ab‐initio calculations were carried out to support the experimental observations of diamond‐graphene hybrid structure. Finally, this hybrid D‐ECNWs is employed as a cathode in an FEE device resulting in a low turn‐on field of 3.1 V µm−1, a large field enhancement factor, a high FEE Je of 2.6 mA cm−2, and long lifetime stability of 438 min. Such an enhancement in the FEE originates from the unique materials combination, resulting in good electron transport from the graphene phases and efficient FEE of electrons from the sharp edges on the nanowalls. The prospective application of these materials is displayed by employing these hybrids as cathodes in a microplasma device ensuing a low threshold voltage of 160 V and high plasma stability of 140 min, which confirms the role of these hybrid structured nanowalls in the enhancement of electron emission.

Enhanced electron emission from ternary solid solution-MWCNT hybrid with theoretical validation

Coalescence of unique properties of semiconductor and carbon nanotubes (CNTs) produces new advanced materials with exciting properties, which unwraps several new fields. Here, finite element modeling is employed to optimize the diameter of semiconducting zinc cadmium sulfide (Zn0.8Cd0.2S) microspheres on the wall of metallic CNTs to get the improved field emission property. Then, Zn0.8Cd0.2S microspheres are attached on COOH functionalized multiwalled CNTs (MWCNTs) by a simple one step solvothermal method for experimental corroboration. Field emission experiment shows a considerable enhancement of current density with respect to bare microspheres and functionalized CNTs. The turn-on field is found to be as low as 3V/μm at a current density of 10 μΑ/cm2 for the hybrid system. Density functional theory (DFT) calculation is carried out for the model structures of Zn0.8Cd0.2S and CNT-Zn0.8Cd0.2S to calculate the work functions. This improvement in field emission property of the hybrid system is demonstrated in terms of band bending, geometry, high field enhancement factor, low work function and electron affinity. Thus, non-field emitting microspheres with less toxicity have been transformed into a decent field emitter by the contribution of few carbon nanotubes.

High-Performance Field Emitters Based on SiC Nanowires with Designed Electron Emission Sites.

Making field emitters with both low turn-on field (Eto) and high current emission stability is one of the keys to push forward their practical applications. In the present work, we report the exploration of high-performance field emitters with designed sharp corners around SiC nanowires for fundamentally enhanced electron emission sites. The sharp corners with tailored densities are rationally created based on a facile etching technique. Accordingly, the emission sites and nanowires are integrated into a single-crystalline configuration without interfaces, which could offer the emitters with a robust structure to avoid the structural damage induced by the generated Joule heat and electrostatic forces over long-term field emission (FE) operation. Consequently, the Eto of the as-fabricated SiC field emitter is low down to 0.52 V/μm, which is comparable to the state-of-the-art one ever reported. Moreover, they have high electron emission stability with a current fluctuation of just 2% over 10 h, representing their promising applications in FE-based electronic units.