Emission Current Density Research Articles

Enhanced electron field emission from nanocrystalline orthorhombic boron nitride films

Nanocrystalline orthorhombic boron nitride (oBN) thin films with an island-in-honeycomb morphology were prepared on graphite substrate by radio frequency (r.f.) magnetron sputtering. The Field emission (FE) measurement results indicated that the FE properties are significantly enhanced in oBN films compared to high quality cBN films, the turn-on electric field of oBN films is decreased from 17.0 V/μm to 6 V/μm, and the highest emission current density is increased from 2.8 × 10−5 to 3 × 10−4 A/cm2. The enhanced FE properties of the oBN films can be attributed to significant reduction in effective potential barrier caused by both protruded island-in-honeycomb morphology and honeycomb-like interconnected internal structure.

Field emission properties of carbon nitride synthesized by microwave plasma chemical vapor deposition

Crystalline carbon nitride is expected to be an emitter with a long lifetime and high emission stability, because it is composed of carbon and nitrogen with high oxidation resistance and its sharp shape facilitates the occurrence of electron emission. The purpose of this paper is to clarify the field emission properties of crystalline carbon nitride. We studied the improvements in the field emission properties of carbon nitride after surface treatments. The carbon nitride was synthesized using microwave plasma chemical vapor deposition from a CH4-N2 gas system. Si substrates with different electrical properties (p- and n-type of 0.01 and 1500 Ω, respectively) were used. The carbon nitride on the 0.01 Ω n-type substrate showed the highest current density of approximately 0.25 mA/cm2. The maximum current densities of the carbon nitride on the n-type or p-type substrates were higher than those of the undoped type by decreasing the resistivity of the substrate. The threshold field (emission current observed in the minimum electric field) of the carbon nitride on n-type substrates was decreased by decreasing the substrate resistance. In the case of Au-coated deposits on an n-type Si substrate of 0.01 Ω, a threshold field was observed at approximately 1 V/μm. In addition, the electric field emission current density at 20 V/μm was 0.83 mA/cm2. The ultra-thin coatings of Au on the carbon nitride were effective in decreasing the threshold field and increasing the maximum current density. The Fowler-Nordheim plots of the carbon nitride with or without Au coatings formed a straight line, indicating that the Fowler-Nordheim theory perfectly fit the field emission behaviors of samples.

Theoretical Study of a 0.22 THz Backward Wave Oscillator Based on a Dual-Gridded, Carbon-Nanotube Cold Cathode

The carbon nanotube (CNT) cold cathode is an attractive choice for millimeter and terahertz vacuum electronic devices owning to its unique instant switch-on and high emission current density. A novel, dual-gridded, field emission architecture based on a CNT cold cathode is proposed here. CNTs are synthesized directly on the cathode surface. The first separating grid is attached to the CNT cathode surface to shape the CNT cathode array. The second separating grid is responsible for controlled extraction of electrons from the CNT emitters. The cathode surface electric field distribution has been improved drastically compared to conventional planar devices. Furthermore, a high-compression-ratio, dual-gridded, CNT-based electron gun has been designed to further increase the current density, and a 21 kV/50 mA electron beam has been obtained with beam transparency of nearly 100%, along with a compression ratio of 39. A 0.22 THz disk-loaded waveguide backward wave oscillator (BWO) based on this electron gun architecture has been realized theoretically with output power of 32 W. The results indicate that higher output power and higher frequency terahertz BWOs can be made using advanced, nanomaterial-based cold cathodes.

Effect of BaO on the phase composition and properties of aluminates for Ba-W cathodes

In this study, we prepared (4.8+x)BaO·CaO·2Al2O3 (0 ≤ x ≤ 1.6) aluminates by calcining the precursors under static air at 1500 °C for 120 min. The precursor powders were prepared using a liquid phase co-precipitation method. The effects of the molar content of BaO on the phase composition (before and after melting), melting properties, environmental stability, evaporation, and emission properties of the aluminates was investigated systematically The results showed that the phase of the aluminates completely transformed from Ba5CaAl4O12 to Ba3CaAl2O7 with an increase in the BaO content. After melting, the phase changed from Ba5CaAl4O12 to a mixed phase of Ba5CaAl4O12 and Ba3CaAl2O7. In the high-temperature molten state, the aluminates were in the ionic state, which generated a relatively low-energy Ba5CaAl4O12 phase during cooling crystallization. With every 0.4 mol increase in the BaO content, the initial melting temperature of the aluminates decreased by 10–20 °C, while the environmental stability deteriorated gradually. When the aluminates reacted with H2O and CO2 in the air, the original phase still existed and the characteristic peaks gradually broadened, but with the formation of Ca(OH)2, CaCO3, and BaCO3. At 1050 °C, with an increase in the BaO content, the evaporation rate of the Ba-W cathodes increased and the emission current density first increased and then decreased. The main components of the Ba-W cathode evaporation were Ba and BaO. At n(BaO):n(CaO):n(Al2O3) = 6:1:2, the Ba-W cathode showed the best emission performance, and its pulse emission current density at 1050 °C was as high as 35.31 A/cm2.

Enhanced field emission properties of V2O5/MWCNTs nanocomposite

A simple solution chemistry route is employed for the synthesis of V2O5 nanoflakes and V2O5/MWCNTs nanocomposite. An attempt for enhanced field emission properties is carried out by nanocomposite formation of V2O5/MWCNTs. The as-synthesized product is characterized further by SEM, XRD, TEM, etc. The field emission characteristics are measured at the base pressure of 1 × 10−8 mbar. The turn-on field defined as the field required to draw an emission current density of10 µA/cm2 is found to be 1.75, 1.3 and 0.66 V/µm for V2O5nanoflakes, MWCNTs and V2O5/MWCNTs nanocomposite, respectively. Herein, more than 2.5-fold reduction in turn-on field for V2O5/MWCNTs nanocomposite is reported for the first time. The observed value of lowturn-on field ofV2O5/MWCNTs nanocomposite is found to be quite remarkable than the values reported for other V2O5 nanostructures. In addition, high current density of 977 µA/cm2 is recorded at an applied field of 1.15 V/µm. Further, the emission current stability is recorded for preset of 10 µA emission current for the duration of 2 h. A detail discussion on correlation between morphology of the emitter and obtained field emission results is presented in paper. Here in, more than 2.5-fold reduction in turn-on field for V2O5/MWCNTs nanocomposite is reported. The observed value of low turn-on field of V2O5/MWCNTs nanocomposite is found to be quite superior than the values reported for V2O5 nanostructures. Exploration of field emission properties of present emitter is important for scientific and technological advancement.

Transition metal (Ti, Cr, Fe, Ni, and Zn) decorated porphyrin-like porous [60]-fullerenes: DFT study

We investigated the energetic, structural and electronic properties of a N4 cavity doped C60 fullerene (C54N4), and its transition metal (TM = Ti, Cr, Fe, Ni, and Zn) decorated complexes using density functional theory calculations. The computed formation energy for C54N4 is about −156.1 kcal/mol, which is slightly less negative than that of C60 (−161.0 kcal/mol). The electrical conductivity, and work function of C60 are increased by N4 doping. The TM-C54N4 bond includes a synergistic interaction between a σ-type donation, and π-type back-bonding. The TM@C54N4 complexes possess a ground state electron distribution with a spin multiplicity which equals that of the free TM atom. The stability of the complexes are as follows: Cr > Ti > Ni > Fe > Zn depending on the effective nuclear potential, and electronic configuration of the TM atoms. The decoration energy is inversely related to the experimental first ionization potential energy of the TM atoms. All TM@C54N4 indicate higher electrical conductivity compared to the bare C54N4 and by increasing the atomic number of TM, the electrical conductivity is increased. Also, the work function of C54N4 nanocluster significantly decreases by TM decoration which reduces the emitted electron current density from the surface of C54N4 nanocluster.

Electron field emission from graphene nanosheets grown on Si nanoporous pillar array

Graphene nanosheet (GNS) thin film was grown flat lying on Si nanoporous pillar array (Si-NPA) substrate using Ni nanocrystallites as catalyst by a chemical vapor deposition method, and its field electron emission characteristics were studied. The thin film was proved to be composed of high-quality few-layer GNSs with a typical size of ~6 nm. With a turn-on field of ~2.85 V/μm, an emission current density of ~53.9 μA/cm2 was obtained at an electric field of 4.2 V/μm. Based on the experimental data, the enhancement factor of few-layer GNS/Si-NPA was calculated to be ~2700 according to the Fowler–Nordheim theory. The cold cathode also showed higher emission stability than vertically standing graphene at low operating voltages. For comparison, GNS/Si-NPA with multi-layer GNSs was prepared and its turn-on field was obtained as high as 8.5 V/μm. The origin of the high emission performance was attributed to numerous emission sites formed at the edges of GNSs, unique structure and morphology of GNS/Si-NPA, and low electric resistance of GNSs. Our results might have provided an alternative approach for fabricating Si-based low-voltage cold cathodes with high device performances.

Facile Hydrothermal Synthesis of WO3 Nanoconifer Thin Film: Multifunctional Behavior for Gas Sensing and Field Emission Applications

Well aligned tungsten trioxide (WO3) nanoconifer films were synthesized on tungsten substrates employing a facile hydrothermal treatment. The structural properties of as-synthesized WO3 nanostructure thin films were confirmed by X-ray diffraction micro Raman spectroscopy, whereas surface morphology and elemental composition was revealed using scanning electron microscopy and energy dispersive X-ray spectroscopy. Gas sensing characteristics of the WO3 nanoconifer thin film towards NH3 (ammonia) gas were investigated. The WO3 nanoconifer thin film exhibited maximum sensor response of ~ 51.9% for 300 ppm of NH3 at operating temperature of ~ 175 °C. Interestingly the response and recover times were observed to be 35 and 62 s respectively, which are superior to the reported ones. Further, field emission study of WO3 nanoconifer thin film was carried at the base pressure of ~ 1 × 10−8 mbar. The values of turn-on and threshold fields (corresponding to emission current densities of 1 and 10 µA cm−2) are found to be 2.43 and 3.08 V µm−1, respectively. The WO3 nanoconifer thin film emitter delivered maximum current density of ~ 118.57 µA cm−2 at an applied field of 4.29 V µm−1. The emission current was observed to be fairly stable over duration four hours at a preset value ~ 2 µA, characterized with standard deviation around 7.86%. The observed FE characteristics are comparable to other semiconducting metal oxide nanostructures thin film emitters. The present results imply WO3 nanoconifer as a promising candidate for potential applications in gas sensing as well as field emission based applications in vacuum microelectronic devices.

Coplanar-gate ZnO nanowire field emitter arrays with enhanced gate-control performance using a ring-shaped cathode

Nanowire field emitters have great potential for use as large-area gated field emitter arrays (FEAs). However, the micrometer-scale cathode patterns in gated FEA devices will reduce regulation of the gate voltage and limit the field emission currents of these devices as a result of field-screening effect among the neighboring nanowires. In this article, a ring-shaped ZnO nanowire pad is proposed to overcome this problem. Diode measurements show that the prepared ring-shaped ZnO nanowire pad arrays shows uniform emission with a turn-on field of 5.9 V/µm and a field emission current density of 4.6 mA/cm2 under an applied field of 9 V/µm. The ZnO nanowire pad arrays were integrated into coplanar-gate FEAs and enhanced gate-controlled device characteristics were obtained. The gate-controlled capability was studied via microscopic in-situ measurements of the field emission from the ZnO nanowires in the coplanar-gate FEAs. Based on the results of both simulations and experiments, we attributed the enhanced gate-controlled device capabilities to more efficient emission of electrons from the ZnO nanowires as a result of the increase edge area by designing ring-shaped ZnO nanowire pad. The results are important to the realization of large-area gate-controlled FEAs based on nanowire emitters for use in vacuum electronic devices.

Effect of CaO on Phase Composition and Properties of Aluminates for Barium Tungsten Cathode.

6BaO·xCaO·2Al2O3 (x = 0.8, 1.2, 1.6, 2, and 2.2) aluminates were synthesized via a liquid phase co-precipitation method. Effects of the molar amount of CaO on the phase of aluminates before and after melting and their hygroscopic phase, melting properties, environmental stability, evaporation, and emission properties were systematically studied. The results show that with the increase of the molar amount of CaO, the aluminates change from a mixture phase to a single phase of Ba3CaAl2O7, and the diffraction peak shifts to a higher angle. The melted phase of the aluminates changed from a single phase to a mixed phase of Ba5CaAl4O12 and Ba3CaAl2O7. Meanwhile, the comprehensive properties of the aluminates are improved. The weight gain of 6BaO·2CaO·2Al2O3 aluminates is only 10.88% after exposure to air for 48 h; the pulse emission current density of barium tungsten cathodes impregnated with 6BaO·2CaO·2Al2O3 aluminates in the porous tungsten matrix can reach 28.60 A/cm2 at 1050 °C, and the evaporation rate is 2.52 × 10−10 g/(cm2·s).

High-quality LaxCeyPr1-x-yB6 single crystal with excellent thermionic emission properties grown by optical floating zone melting method

The high-quality LaxCeyPr1-x-yB6 (x = 0.6–0.8, y = 0.1–0.3) single crystals have been firstly grown by optical floating zone melting method. The crystal structure, mechanical and thermionic emission properties of the as-grown single crystals were characterized by X-ray single crystal diffractometer, SEM, XRD, X-ray Laue diffraction. Subgrain-boundary-free [100] LaxCeyPr1-x-yB6 single crystal with a good crystalline integrity and homogeneous element distribution is obtained. The Lattice constant of LaxCeyPr1-x-yB6 single crystal is lower than that of the LaB6 single crystal. The lowest FWHM of 162.0 arcsec belongs to the La0.6Ce0.3Pr0.1B6 single crystal. The highest thermionic emission current density of 105.10 A/cm2 and the lowest work function of 2.562 eV indicate that La0.6Ce0.3Pr0.1B6 single crystal is a very promising thermionic cathode material.

Application of the transferred matrix method to a unified evaluation of the cathodic electron emission

The work is concerned with the Transfer Matrix Method for solving the steady-state Schrödinger equation applied for a unified evaluation of the emission current density from non-refractory cathodes. The method is applicable to arbitrary shapes of the potential barrier and its transmission probability is obtained without any analytical approximations. The Fermi-Dirac distribution for the free electrons in the metal is considered as a supply function. The results, obtained for a work function of the cathode material of 4.5 eV over a wide range of values of the surface temperature and the electric field strength, clearly show a growing deviation from those obtained by the classical Jeffreys-Wentzel-Kramers-Brillouin approximation with the increase of the electric field strength. Preliminary results are obtained to demonstrate the applicability of the Transfer Matrix method to the evaluation of the ion-assisted electron emission. A significant local enhancement of the emission current density is obtained as a result of the presence of an ion at a fixed position near the metal surface. The effect becomes very strongly pronounced at an appropriate value of the electric field strength, for which a resonant ion contribution appears.

Controlled Hetero‐Architectures of Au‐Nanoparticles‐Decorated ZnO Nanowires for Enhanced Field Electron Emission Displays

AbstractThe spitzer shaped pointed ZnO nanowires of diameter 120 nm and length ∼5‐6 μm synthesized via hydrothermal techniques were decorated with Au nanoparticles of diameter < 20 nm along their textural boundaries. The turn‐on field (Eon) of 1.56 V/μm required to draw current density of 10 μA/cm2 from ZnO nanowires was reduced further to 0.96 V/μm after controlled decoration of Au nanoparticle, is significantly lower than pristine/doped ZnO nanostructures, Ag@ZnO, Au@ZnO, Au@CuO, and MoS2@TiO2 heterostructure based field emission (FE) devices. The orthodoxy test confirmed the feasibility of field‐enhancement factor (βFE) of 4477 for Au/ZnO/ITO emitters. The enhanced FE behavior are attributed to pointed nanotips, individual dispersion of Au nanoparticles along the textural boundaries of ZnO nanowires, and enhanced energy well formation at their interface. Moreover, the density of state increases enormously due to the hetero‐structured interface of Au@ZnO and a significant amount of electrons from both Au and ZnO contribute for enhanced emission current density (2.1 mA/cm2 @ 1.92 V/μm), and greatly stable electron emission. Our experiments suggest that the tunable hetero‐architectures of Au nanoparticles@ZnO nanowires hold the promises for applications in display screens and intense electron source.

Formation of WS2 nanosheets and its field emission studies

The present study describes the results on the formation of WS2 nanosheets using the nanosecond laser ablation in a liquid environment. The laser ablation in liquid is a well-known method and is widely used for the generation of nanostructure. The nanosecond laser ablation is used to irradiate WS2 in a isopropyl alcohol to generate WS2 nanosheets. The laser ablation time was 120 min. The field emission measurement of irradiated WS2 nanostructures was performed. The Bulk and WS2 were used to characterize the Transmission Electron Microscopy (TEM) and Raman spectroscopy to reveal their surface and structural morphology. It is found that the laser ablation time is an important parameter to transform the bulk WS2 into WS2 nanosheets. The generation of WS2 nanosheets is revealed by the Raman spectra. Moreover, the TEM images indicate that the size of WS2 microsheet has been reduced to 400 nm after the laser ablation treatment. It is also found that the field emission current density of WS2 is increased after the laser ablation. The improvement in the field emission characteristics of WS2 is due to the formation of WS2 as well as the separation of layered WS2 structure into few layers of WS2. The formation of WS2 nanosheets can be used for future optoelectronic devices.

Investigation of the temperature dependent field emission from individual ZnO nanowires for evidence of field-induced hot electrons emission

ZnO nanowires as field emitters have important applications in flat panel display and x-ray source. Understanding the intrinsic field emission mechanism is crucial for further improving the performance of ZnO nanowire field emitters. In this article, the temperature dependent field emission from individual ZnO nanowires was investigated by an in situ measurement in ultra-high vacuum. The divergent temperature-dependent Fowler–Nordheim plots is found in the low field region. A field-induced hot electrons emission model, that takes into account penetration length, is proposed to explain the results. The carrier density and temperature dependence of the field-induced hot electrons emission current are derived theoretically. The obtained results are consistent with the experimental results, which could be attributed to the variation of effective electron temperature. All of these are important for a better understanding on the field emission process of semiconductor nanostructures.

Study on the preparation and optoelectronic properties of nano-ZnO arrays

ABSTRACTNano-ZnO arrays with high degree of orientation and good dispersion were prepared by sol-gel method and hydrothermal method. The morphology, structure and photoluminescence properties of samples were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) and fluorescence spectrometer. The results showed that the prepared samples are hexagonal wurtzite structure and the growth of the (002) crystal surface is preferred. The samples have a strong characteristic absorption and UV absorption capacity at 371.5 nm, and there is a high UV emission peak at 387 nm, besides, there is a relatively weak blue emission peak at 468 nm. Moreover, The field emission characteristics tests of samples were conducted and revealed the low turn-on field is 1 V/μm, in addition,the field emission current density is 210 μA/cm2 under the condition of electric field of 7.5 V/μm. Finally, field emission mechanism of nano-ZnO array is discussed.

Microstructural modification of diamond-graphite nanohybrid films via adjusting deposition temperatures for enhanced electron field emission

To achieve enhanced electron field emission, diamond-graphite nanohybrid films are deposited on Si substrates by means of a microwave plasma chemical vapor deposition (MPCVD) reactor. Liquid diethylamine is used as a sole reactive source. An increase of deposition temperature from 730 °C to 830 °C leads to remarkable changes of morphology and microstructure of diamond-graphite nanohybrid films, namely from a typical planar ultrananocrystalline diamond (UNCD) film at 730 °C to a novel nanocrystalline diamond/graphene nanowall (ND/GNW) film at 830 °C. The electrical conductivity of a ND/GNW film is measured to be 762.4 Ω−1·cm−1, while for a UNCD film it is 18.0 Ω−1·cm−1. Moreover, the electron field emission is significantly enhanced on a ND/GNW film. With an applied electrical field of 9.9 V μm−1 its emission current density is 1.6 mA cm−2, 8-time larger than that for a UNCD film. The possible electron emission mechanism is then discussed based on the Fowler-Nordheim theory. This work offers a simple way to alter effectively the microstructures of sp2/sp3 nanohybrid films that benefit greatly to develop carbon-based field emission cold cathodes featuring high performance.

Investigations on the transformation of vertically aligned CNTs to intramolecular junctions by atmospheric pressure PECVD

Atmospheric pressure glow discharge plasma assisted chemical vapor deposition process is used for the growth of carbon nanotubes (CNTs) on Inconel without using any external catalyst. Four different sets of samples are prepared by varying the growth time. The grown nanostructures are then characterized by field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. Field emission measurements are also done on the grown structures. Our investigations revealed that vertically aligned CNTs transformed into intramolecular junctions of CNTs with an increase in deposition time from 4 to 10 min. This time dependence for the transformation is investigated and correlated with other characterization results. It is seen that field emission behavior is the best for the vertically aligned CNTs but it also improves when the junctions of CNTs are present in the matrix after increasing growth time to 15 min. The investigations reveal that branching brings favorable defects in the grown CNTs that can give rise to impressive field emission current density. A schematic model has been prepared based on the experimental findings to find out the reason behind the growth of these junctions and how did they help in improving field emission.

Band-to-Band Tunneling-Dominated Thermo-Enhanced Field Electron Emission from p-Si/ZnO Nanoemitters.

Thermo-enhancement is an effective way to achieve high performance field electron emitters, and enables the individually tuning on the emission current by temperature and the electron energy by voltage. The field emission current from metal or n-doped semiconductor emitter at a relatively lower temperature (i.e., < 1000 K) is less temperature sensitive due to the weak dependence of free electron density on temperature, while that from p-doped semiconductor emitter is restricted by its limited free electron density. Here, we developed full array of uniform individual p-Si/ZnO nanoemitters and demonstrated the strong thermo-enhanced field emission. The mechanism of forming uniform nanoemitters with well Si/ZnO mechanical joint in the nanotemplates was elucidated. No current saturation was observed in the thermo-enhanced field emission measurements. The emission current density showed about ten-time enhancement (from 1.31 to 12.11 mA/cm2 at 60.6 MV/m) by increasing the temperature from 323 to 623 K. The distinctive performance did not agree with the interband excitation mechanism but well-fit to the band-to-band tunneling model. The strong thermo-enhancement was proposed to be benefit from the increase of band-to-band tunneling probability at the surface portion of the p-Si/ZnO nanojunction. This work provides promising cathode for portable X-ray tubes/panel, ionization vacuum gauges and low energy electron beam lithography, in where electron-dose control at a fixed energy is needed.

Theoretical Study and Monte Carlo Simulation on the Dynamic Process of a Novel Multipacting Cathode for High-Current Diode

The prototype and configuration design of a novel multipacting cathode producing a ring-like electron beam for high-current diode are presented. The whole dynamic process of the moving secondary electron on the multipacting cathode surface is theoretically investigated and numerically simulated using the Monte Carlo method. The results indicate that the prototype of the novel multipacting cathode for high-current diode is feasible. Under the interactions of the applied electric field (~MV/m) in axial and radial directions and the guiding magnetic field (~T) in axial direction, the seed electrons from the triple junctions move with spiral advancing trajectories along axial direction. Electron number can be effectively increased by each impact on the coating surface with high secondary electron emission yield coefficient. This phenomenon will achieve electron current amplification with the ring-like electron beam output till multipacting enters into saturation. The simulated characteristic parameters of the secondary electron and the working range of the multipacting cathode agree with the theoretical results. Under the typical condition, the moving distance of the secondary electron is on the order of submillimeters, the fight time is on the order of picoseconds, and the spread speed is on the order of 106 to 107 m/s. In addition, the multipacting saturation mechanism is analyzed, and the emission current density is roughly estimated to ~kA/cm2. Enhancing the magnitude of applied electric field in radial direction can effectively improve the emission current density.