Large Area Field Emitters Research Articles

Tungsten oxide nanowires prepared by thermal oxidation for application in cold cathode flat panel x-ray source

Tungsten oxide (WO3−X) nanowire field emitters have important applications in vacuum microelectronic devices, such as cold cathode flat panel x-ray sources. In this study, large-area, high current density, and defect-rich monoclinic WO3−X nanowires were directly synthesized on a glass substrate by thermal oxidation. Field emission measurements from a 4.5 × 4.5 cm2 sample show that a turn-on field of 4.8 MV/m and a high current density of 15.2 mA/cm2 were achieved. Stable emission current with a fluctuation of 1.78% was obtained. Furthermore, a flat panel x-ray source with a reflective anode was fabricated using as-grown WO3−X nanowires as the cold cathode. A radiation dose rate of 1.83 mGy/s was obtained at an anode voltage of 60 kV and a current of 484.1 μA. X-ray imaging experiments were carried out and clear imaging results were obtained. This work is significant for promoting the application of WO3−X nanowires in large-area field emission devices.

Supression of shielding effect of large area field emitter cathode in radio frequency gun environment

Abstract Utilisation of large area field emitters (LAFE) cathodes for rf gun injector hold promise for delivering compact, high power and high brightness electron beam for advanced accelerator technologies. LAFEs subjected to DC electric fields posses significant challenges due to the shielding effect which restricts emission from central emitters and decreases the overall current density. Mitigating the shielding effect of LAFE in rf gun environment is essential for meeting the desired beam quality requirement in an accelerator. The current distribution of LAFE under DC conditions depend on its various geometrical parameters such as emitter height, inter-emitter distance, aspect ratio, number of emitters. Additionally, in rf gunsetup, LAFEs are subjected to variable macroscopic electric field at different emitter position which can potentially alter the current distribution compared to DC fields. In this work, we have systematically studied the shielding effect properties of LAFE in rf gun environment under the influence of various LAFE parameters. A semi-analytical approachhas been adopted to estimate the current distribution which combines the analytically calculated field enhancement factor (γ) and numerically calculated applied rf field values. This new methodology was first validated using COMSOL simulation and then employed for field emission performance estimation of a LAFE cathode integrated in a½ cell S-band (2856 MHz) rf gun. The simulation results reveals that under favourable conditions, a Gaussian spatial distribution of beam can be obtained from LAFE thus countering the shielding effect typical in DC fields. By optimizing the LAFE parameters, the desired current and beam distribution pattern can be achieved. This study highlights the adoption of a promising approach for designing LAFE cathodes suitable for rf gun which can lead to advancement of field emission technologies for accelerator-related applications.

Designing a large area field emitter for uniform electron emission

Obtaining uniform emission from a large area field emitter (LAFE) is a challenge that has proved difficult to overcome, both theoretically and experimentally. We use an approximate analytical formula for the apex field enhancement factor (AFEF) of individual emitters to design a LAFE with uniform electron emission, either by optimizing the location of identical emitters or by optimizing the height of individual emitters placed at fixed locations. The optimized parameters (the location or height) are then fed into COMSOL to check for uniformity in the AFEF, a quantity that determines the emitted current. The AFEFs obtained from COMSOL are found to be nearly identical, thereby validating the semi-analytical design technique. For larger LAFEs containing thousands of emitters, the semi-analytical method can be used for designing the LAFE and estimating the current in the optimized configuration.

Fast and accurate determination of the curvature-corrected field emission current

The curvature-corrected field emission current density, obtained by linearizing at or below the Fermi energy, is investigated. Two special cases, corresponding to the peak of normal energy distribution and mean normal energy, are considered. It is found that the current density evaluated using the mean normal energy results in errors in the net emission current below 3% for apex radius of curvature Ra≥5 nm and for apex fields Ea in the range of 3–10 V/nm for an emitter having a work function of ϕ=4.5 eV. An analytical expression for the net field emission current is also obtained for local parabolic tips using the generalized cosine law. The errors are found to be below 6% for Ra≥5 nm over an identical range of apex field strengths. The benchmark current is obtained by numerically integrating the current density over the emitter surface and the current density itself computed by integrating over the energy states using the exact Gamow factor and the Kemble form for the WKB transmission coefficient. The analytical expression results in a remarkable speed-up in the computation of the net emission current and is especially useful for large area field emitters having tens of thousands of emission sites.

Semi-analytical modeling of large area field emitters having non-identical pins

The Line Charge Model (LCM) is an excellent analytical tool to model vertically aligned nano-tips in large area field emitters (LAFE). The linear line charge model is exact for isolated hemi-ellipsoidal nano-tips placed in a uniform external electric field. It has recently been used to model a LAFE with randomly placed identical emitters. The results are accurate when the mean spacing c is moderate to large compared to the emitter height h. In a closely packed LAFE (c⪅0.75h), the LCM underpredicts the apex enhancement factor. We introduce a heuristic correction in the LCM result that yields a better accuracy in predicting the apex enhancement factor over a wider range of mean spacing. The corrected LCM model is then used to simulate emitter shapes having a distribution in the height of emitters and apex radius of curvature Ra. A hybrid approach is adopted for non-ellipsoidal shapes where the line charge density is nonlinear and, hence, harder to implement. Predictions for the apex enhancement factor and the net emission current are found to be reasonably accurate for a LAFE with a wide variation in h and Ra values.

Influence of NiO ALD Coatings on the Field Emission Characteristic of CNT Arrays.

The paper presents a study of a large-area field emitter based on a composite of vertically aligned carbon nanotubes covered with a continuous and conformal layer of nickel oxide by the atomic layer deposition method. The arrays of carbon nanotubes were grown by direct current plasma-enhanced chemical vapor deposition on a pure Si substrate using a nickel oxide catalyst which was also deposited by atomic layer deposition. The emission characteristics of an array of pure vertically oriented carbon nanotubes with a structure identical in morphology, covered with a layer of thin nickel oxide, are compared using the data from a unique computerized field emission projector. The deposition of an oxide coating favorably affected the emission current fluctuations, reducing them from 40% to 15% for a pristine carbon nanotube and carbon nanotube/nickel oxide, respectively. However, the 7.5 nm nickel oxide layer coating leads to an increase in the turn-on field from 6.2 to 9.7 V/µm.

Semi-analytical theory of emission and transport in a LAFE-based diode

A large area field emitter (LAFE) typically consists of several thousands of nanoscale emitting tips. These are difficult to simulate using purely numerical methods based on finite/boundary element or finite difference methods. We show here that a semi-analytically obtained electrostatic field allows tracking of field emitted electrons of a LAFE fairly accurately using the knowledge of only the LAFE geometry. Using a single and a nine-emitter configuration, the beam parameters calculated using this method are compared with the results of tracking using fields generated by COMSOL. The net emission current, energy conservation, and the transverse trace-emittance are found to be reproduced with reasonable accuracy.

The Fowler–Nordheim plot behaviour from virtual field emitters arrays of zinc oxide nanorods mixed with carbon nanotubes

ABSTRACT The Fowler–Nordheim plot behaviour is investigated for large area field emitters arrays composed of mixtures of ZnO nanorods and single-wall carbon nanotubes (CNTs). The field emission current–voltage characteristics are calculated using the Murphy-Good equation based on the Fowler–Nordheim model. The influencing factors taken into consideration are the radii of the ZnO rods, the heights of the CNTs, the variation of the onset voltage of the saturation of the ZnO conduction band (CB) emission current and the degradation of the CNTs at high applied voltages. The results show that the FN plots of the mixed arrays exhibit nonlinear behaviour near the high-field region which could be related to the FN plot behaviour of the constituting emitters arrays. The distribution of the emitters geometrical parameters and the onset of saturation of the ZnO CB current are found to affect the FN plot behaviour of the pure ZnO, CNTs arrays as well as mixed arrays. The comparisons between the FN plots from the present work and those measured experimentally are found useful to shed light on the field emission mechanism. This may in turn aid the development of complex field emission arrays for future technological applications. The work also shows the possibility to apply the standard FN model to complex large area field emitters arrays.

Outgassing during large area field emitter operation in the diode system

We report peculiar emission behavior of large-area cathode based on a multiwalled carbon nanotube/polymer nanocomposite. Mass spectra and kinetics of the partial pressure of the main volatile products released from the surface of the electrodes were obtained when a constant voltage of different levels was applied to the cathode. The main volatile products during field emission diode operation are H2, H2O, CO/C2H4, and CO2. The behavior of H2O peak intensity is characterized by increased inertia relative to sudden voltage changes.

Interpreting the field emission equation for large area field emitters

Both single emitters and large area field emitters (LAFEs) are generally characterized using the slope and intercept of a Murphy–Good (or Fowler–Nordheim) plot, which are used to extract the field enhancement factor and the emission area. Using a shielding model that has been developed recently for a LAFE, the validity of the underlying assumption is investigated. It is found that in the case of a LAFE, the slope has contributions from the enhancement factor as well as the rate at which the effective number of superemitters changes with the applied field. As a consequence, the emission area is related to both the slope and intercept in a LAFE. When the mean spacing in a LAFE is much larger than the height of the emitter, the usual interpretation of the slope and intercept is recovered.

Behavior of the field enhancement due to mutual depolarization on a pair of triangular emitters at short and large separations

The Schwarz-Christoffel transformation is used to analytically evaluate the Field Enhancement Factor (FEF) in the vicinity of the apex of two isosceles triangular emitters close to each other. It is shown that the fractional reduction between the apex-FEF of a single triangular emitter and this same FEF evaluated when another identical emitter is placed close to it, −δ, may be well described by an exponential or a power-law behavior involving the ratio between the distance and the height of the emitters and/or their aspect-ratios at different regimes. This way the analytical model presented here intends to investigate the existence of universal depolarization laws for non-cylindrical emitters used in different scientific and technological applications involving large area field emitters. The results obtained here show a quadratic power-law decay of the fractional reduction with respect to the ratio between the distance and the height of the emitters for asymptotically large distances between them. This differs from the cubic decay frequently verified in the literature for three-dimensional structures but the robustness of this exponent, which remains independent of the aspect-ratio of the emitters, is also reinforced.

Predicting space-charge affected field emission current from curved tips

Field-emission studies incorporating the effect of space charge reveal that for planar emitters, the steady-state field EP, after initial transients, settles down to a value lower than the vacuum field EL. The ratio ϑ=EP/EL is a measure of the severity of space-charge effect, with ϑ=0 being the most severe and ϑ≃1 denoting the lack of significant effect. While EL can be determined from a single numerical evaluation of the Laplace equation, EP is largely an unknown quantity whose value can be approximately found using physical models or can be determined “exactly” by particle-in-cell or molecular dynamics codes. We propose here a simple model that applies to planar as well as curved emitters based on an application of Gauss’s law. The model is then refined using simple approximations for the magnitude of the anode field and the spread of the beam when it reaches the anode. The predictions are compared with existing molecular dynamics results for the planar case and particle-in-cell simulation results using PASUPAT for curved emitters. In both cases, the agreement is good. The method may also be applied to large area field emitters if the individual enhancement factors are known, for instance, using the hybrid model [D. Biswas, J. Vac. Sci. Technol. B 38, 063201 (2020)].

Fabrication and complex investigation of LAFE based on CNT by PECVD with island catalyst

This paper presents a study of large area field emitter based on carbon nanotubes grown by PECVD method on Si/SiO2 substrate with Fe catalyst. The catalyst was deposited by CVD on the substrate from ferrocene in the form of islands. The sample creation technology was described and results of the emission properties study were presented. Current-voltage characteristics were registered and tested for compliance with the cold field emission regime. The fluctuation statistic of effective microscopic parameters was constructed. Using data from a computerized field projector, the emission profile of the sample was calculated.

Low-threshold field electron emission from graphene nanostructures

This paper presents the results of a study of field electron emission from two-dimensional graphene-like nanostructures synthesized by the method of self-propagating high-temperature synthesis. It is shown that this technology makes it possible to create efficient large-area field emitters. It is found that such structures exhibit the effect of low-threshold field electron emission. The possibility of obtaining high-current electron beams with currents up to several hundred amperes has been confirmed in pulsed electric fields.

Scaling in large area field emitters and the emission dimension

Electrostatic shielding is an important consideration for large area field emitters (LAFEs) and results in a distribution of field enhancement factors even when the constituent emitters are identical. Ideally, the mean and variance together with the nature of the distribution should characterize a LAFE. In practice, however, it is generally characterized by an effective field enhancement factor obtained from a linear fit to a Fowler–Nordheim plot of the I−V data. An alternate characterization is proposed here based on the observation that for a dense packing of emitters, shielding is large and LAFE emission occurs largely from the periphery, while well separated emitter tips show a more uniform or two-dimensional emission. This observation naturally leads to the question of the existence of an emission dimension, De, for characterizing LAFEs. We show here that the number of patches of size LP in the ON-state (above average emission) scales as N(LP)∼LP−De in a given LAFE. The exponent De is found to depend on the applied field (or voltage) and approaches De=2 asymptotically.

Recent Progress on ZnO Nanowires Cold Cathode and Its Applications.

A cold cathode has many applications in high frequency and high power electronic devices, X-ray source, vacuum microelectronic devices and vacuum nanoelectronic devices. After decades of exploration on the cold cathode materials, ZnO nanowire has been regarded as one of the most promising candidates, in particular for large area field emitter arrays (FEAs). Numerous works on the fundamental field emission properties of ZnO nanowire, as well as demonstrations of varieties of large area vacuum microelectronic applications, have been reported. Moreover, techniques such as modifying the geometrical structure, surface decoration and element doping were also proposed for optimizing the field emissions. This paper aims to provide a comprehensive review on recent progress on the ZnO nanowire cold cathode and its applications. We will begin with a brief introduction on the synthesis methods and discuss their advantages/disadvantages for cold cathode applications. After that, the field emission properties, mechanism and optimization will be introduced in detail. Then, the development for applications of large-area ZnO nanowire FEAs will also be covered. Finally, some future perspectives are provided.

Controlled decoration of palladium (Pd) nanoparticles on graphene nanosheets and its superior field emission behavior

Development in nanoscience and nanotechnology has inspired scientists to continuously explore new cathode material for field emission applications. In the present article, different weight (wt) % of Pd nanoparticles (Pd NPs) ensemble on graphene (Gr) nanosheets (Pd-Gr) were prepared via one step simple chemical route. The field emission (FE) properties of Pd decorated Gr nanosheets were studied. Structural and morphological analysis of as synthesized Pd-Gr composites were carried out by using X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscope (TEM). The FE properties of the Pd-Gr changed according to the variation in the wt% of Pd NPs. The Pd3-Gr field emitter showed better performance among all the emitters with lower turn on field, threshold field and maximum emission current density (J). The enhanced FE properties of Pd-Gr are due to the decoration of low work function Pd NPs. The FE results demonstrated herein suggest that, Pd-Gr based field emitters can open up many opportunities for their potential utilization in large area field emitters in various vacuum micro-nanoelectronic devices such as flexible field emission displays, portable X-ray and microwave tubes.

Hybrid approach to modeling large area field emitters

Large area field electron emitters, typically consisting of several thousands of nanotips, pose a major challenge since numerical modeling requires enormous computational resources. We propose a hybrid approach where the local electrostatic field enhancement parameters of an individual emitter are determined numerically while electrostatic shielding and anode-proximity effects are incorporated using recent analytical advances. The hybrid model is tested numerically on an ordered arrangement of emitters and then applied to recent experimental results on randomly distributed gold nanocones. Using the current-voltage data of two samples with vastly different emitter densities but having similar nanocone sizes, we show that an appropriate modeling of the emitter apex together with the analytical results on shielding and anode-proximity effects leads to consistent results for the apex radius of curvature. In both cases, the I−V data are approximately reproduced for Ra≃9 nm. Importantly, it is found that anode-proximity plays a significant role in counter-balancing electrostatic shielding, and ignoring this effect results in the requirement of a much smaller value of Ra.

The behavior of Fowler-Nordheim plot from carbon nanotubes-based large area field emitters arrays

The Fowler – Nordheim (FN) plot behavior is investigated for field emitted electrons from virtual carbon nanotubes (CNTs)-based large area field emitters (LAFEs) arrays. The field emission currents are calculated using the Murphy-Good field emission equation assuming emission from two sets of geometrically-different CNTs. No screening effects are considered in the calculations. The FN plots nonlinear behavior, in the form of an upward bend, is observed, analyzed and attributed to the emitters' geometrical features and their numbers in the arrays. The calculations emphasize that the nonlinear characteristic depends, not only on the two-class geometries of the emitters but also on the statistical distribution of these emitters in the arrays. The calculations adopted in the present work allow fitting the experimental data of the LAFEs for any desirable range of applied voltages with minimal adjustable parameters. The present investigation is believed to help further development of the LAFEs for future applications.

Comparison of macroscopic and microscopic emission characteristics of large area field emitters based on carbon nanotubes and graphene

Nanostructured multitip surfaces have sufficient potential to obtain the high emission currents necessary to develop stable and noninertial sources of free electrons with increased levels of permissible currents. The key to understanding the processes of formation and stability of macroscopic emission currents from these large area field emitters (LAFEs) is assessing the local characteristics of individual emission sites. Herein, a method for determining the local emission characteristics of nanoscale emission sites is developed via processing the glow pattern data and a system for rapidly recording the current–voltage characteristics of LAFEs.