Field Enhancement Factor Research Articles

Simulation-Based Model of Randomly Distributed Large-Area Field Electron Emitters

With a large-area field electron emitter (LAFE), it is desirable to choose the spacings of individual emitters in such a way that the LAFE-average emission current density and total current are maximised, when the effects of electrostatic depolarization (mutual screening) are taken into account. This paper uses simulations based on a finite element method to investigate how to do this for a LAFE with randomly distributed emitters. The approach is based on finding the apex field enhancement factor and the specific emission current for an emitter, as a function of the average nearest neighbor spacing between emitters. Using electrostatic simulations based on the finite element method, the influence of neighboring emitters on a reference emitter being placed at the LAFE centre is investigated. Arrays with 25 ideal (identical) conical emitters with rounded tops are studied for different emitter densities and applied macroscopic fields. A theoretical average spacing is derived from the Poisson Point Process Theory. An optimum average spacing, and hence optimum emitter density, can be predicted for each macroscopic field.

Effect of Electron Irradiation on the Transport and Field Emission Properties of Few-Layer MoS2 Field-Effect Transistors

Electrical characterization of few-layer MoS2 based field effect transistors with Ti/Au electrodes is performed in the vacuum chamber of a scanning electron microscope in order to study the effects of electron beam irradiation on the transport properties of the device. A negative threshold voltage shift and a carrier mobility enhancement is observed and explained in terms of positive charges trapped in the SiO2 gate oxide, during the irradiation. The transistor channel current is increased up to three order of magnitudes after the exposure to an irradiation dose of 100e-/nm2. Finally, a complete field emission characterization of the MoS2 flake, achieving emission stability for several hours and a minimum turn-on field of about 20 V/um with a field enhancement factor of about 500 at anode-cathode distance of 1.5um, demonstrates the suitability of few-layer MoS2 as two-dimensional emitting surface for cold-cathode applications.

Impact of geometry and magneto-optical properties on field enhancement and optical bistability in core\u2013shell nanoparticles

We study the interaction of electromagnetic field with two of the most tunable nanostructure geometries for nanoplasmonics including the magneto-optical nanoshell structure and the spheroidal geometry. We investigate the effect of combining both geometry and magneto-optical properties within the same nanostructure on the field enhancement factor and optical bistability behaviors. Since the coupling between the inner and outer surface plasmons of the nanoshell is stronger for the elongated spheroidal geometry as compared to that for the spherical case, field enhancement in ellipsoid nanoparticles is much more saleintiant. Moreover, the plasmonic field enhancement is four orders of magnitude larger for the spheroidal nanoshells as compared to spherical nanoshells. In addition to the appearance of optical bistability in this system, it is found that the threshold and window of bistability are strongly dependent on magneto-optical properties and geometry of the core–shell nanoparticle.

Improved field emission from appropriately packed TiO2 nanorods: Designing the miniaturization

An improved electron field emission (FE) property from vertically aligned TiO2 nanorods (TNR) on conducting fluorine doped tin oxide coated glass substrate, prepared by hydrothermal technique, has been reported here. The TNRs having high aspect ratio with appropriate separation were prepared by optimizing the nucleation temperature. The TNR density has been optimized to cease the screening effect in closely packed nanorods. The well aligned rods have been designed so that the applied field is equally distributes on each rod to improve the electron emission. The morphology of the nanorod has been studied using scanning electron microscopy and observed FE current density-field response has been analysed within the frame work of Fowler-Nordheim modelling. Two orders of magnitude enhancement in the electric field has been observed for this kind of geometry giving improved turn on field and field enhancement factor of 0.2 V/μm and 37 thousands respectively.

Fully Coupled Finite Element Analysis for Ion Flow Field Under HVDC Transmission Lines Employing Field Enhancement Factor

A fully coupled finite element analysis (FEA) technique was proposed for solving the ion-flow field due to corona discharge under a high-voltage direct-current (HVDC) bipolar transmission line employing a field enhancement factor. To cover the shape and roughness effects for the stranded wire conductor, the field enhancement factor was introduced in the charge generation mechanism. The employed fully coupled FEA can solve the corona electric field with space charge propagation without the Deutsch assumption and is stable and fully convergent regardless of the possible higher values of wind speed. First, we simulated a dc ±500 kV transmission line tower consisting of six subconductor bundles in a bipolar configuration. Furthermore, the accuracy and the reliability of the simulated results were validated using the experimental results from the literature for a dc ±800 kV one-bipolar transmission line with six subconductor bundles. Moreover, the electric fields were assessed with and without consideration of the space charge, including the wind effect. Overall, the simulated results agree with the experimental results from the literature.

Modeling of stochastic processes in the emission characteristics of multitips electron sources

In this work, the modeling of the fluctuations in the current-voltage characteristics of the multi-tip field emitter was produced using adsorption/desorption theory principles. The model is based on two-component emitter with emission sites, which have two different work functions: with adsorbed particles and without ones. As a result, the cloud type fluctuations in the SK-diagram were generated and the histogram of the effective field enhancement factor was plotted. Obtained dependences was similar to the experimental ones.

Investigation of explosive electron emission sites on surface of polished cathodes in vacuum

The field enhancement effect of the surface microstructures of electrodes with two different surface treatments is evaluated, on the basis of observations made by a 3-D laser microscope. The maximum field enhancement factor of the micro-protrusions on the surface after turning or mirror polishing is about 1–5.8. The vacuum gap breakdown strength of the mirror polished electrodes is 1–1.4 times higher than that of the turned electrodes, because the surface grooves are removed. Experiments to test the breakdown strength of 2.5 cm vacuum gaps with the electrodes after the treatments were carried out and the experimental results were found to correspond to the analysis. The results also revealed that it is not enough to induce explosive electron emission and vacuum breakdown just by the field enhancement at the micro-protrusions and grooves. Moreover, the emission sites of mirror polished cathode made of titanium alloy and stainless steel after vacuum breakdown were observed by a scanning electron microscope and an energy dispersive spectrometer. The results show that the proportion of carbon atoms in the damaged area of the stainless steel cathode surface is as high as 40–70%, and most of the pits located at the grain boundaries, which suggests that grain boundaries have important effects on the electron emission and vacuum breakdown.

Direct patterned growth of intrinsic/doped vertical graphene nanosheets on stainless steel via heating solid precursor films for field emission application

Vertical graphene nanosheets (VGNs), normally consisting of one to several graphene layers vertically aligned on substrates, are promising in a variety of applications including field electron emitters, gas sensors and energy storage devices. Herein, we report a simple, green, easily scalable and cost-effective strategy of growing both intrinsic and nitrogen (N)-doped VGNs on stainless steel (SS) just by heating the solid thin layers of glucose and/or urea in a resistance-heating furnace. It is interesting that VGNs mainly grow on the roughened regions, which can be attributed to the more nucleation and catalyzing sites on such regions than smooth SS. Meanwhile, the N doping concentration can be adjusted by varying the urea addition. Field electron emission measurement indicates that the obtained N-doped VGNs exhibit excellent field emission with a relatively low turn-on electric field strength (~2.6 V μm−1 at the current density of 10 μA cm−2), large field enhancement factor (~9428) and high stability.

Field emission from graphene sheets and its application in floating gate memories

The potential performance of 2D NAND flash with a graphene floating gate (FG) layer is presented. The field enhancement factor for patterned CVD graphene sheets atop a tunneling dielectric is experimentally extracted and used to drive higher-level circuit simulations on 64-bit NAND strings. The average field enhancement factor at a barrier height of 3.1 eV was found to be ∼2.85 with a maximum value of 4. Our modest extracted β value explains the contradiction in prior experiments that reported a field enhancement factor of few thousands but only 30%–40% improvement in the write voltage of FG memory devices. Design and operational tradeoffs are benchmarked based on these experimental values and it is shown that 2D NAND programming time and/or programming voltage can be suppressed to 10 ns and 5 V, respectively, based on a 65 nm process node. The onset of read disturbs from the more efficient FG layer are identified and shown to be easily mitigated through error correction code.

Conduction mechanisms in lanthanum manganite nanofibers

Lanthanum manganites are being constantly investigated for their interesting magnetic and transport properties. In this study, polycrystalline, single phase lanthanum manganite (LaMnO3) nanofibers were synthesized using an electrospinning technique, and characterized in terms of phase, microstructure, ac and dc electrical properties. X-ray diffraction and scanning electron microscopy revealed the formation of uniform composite nanofibers (average dia.∼ 600 nm) at 100 °C and polycrystalline single phase porous LaMnO3 nanofibers (average dia.∼ 400 nm) when calcined at 600 °C. Temperature dependent impedance and current-voltage (IV) characteristics of LaMnO3 nanofibers deposited on an interdigitated electrode type device, were investigated at temperatures ranging from 298 to 378 K. The observed decrease in the dielectric constant at high frequencies may be due to decrease in the contribution from dipolar polarization. Analysis of ac conductivity indicated thermally activated type conduction mechanism in these nanofibers which could be explained by a correlated barrier hopping (CBH) model. The observed decreasing trend in activation energy with increasing frequency also supported the CBH model. Furthermore, the analysis of log (J) vs. E0.5 curves at different temperatures showed a large difference of ~5.82 between the experimental and theoretical field lowering coefficient with an internal field enhancement factor of α2 = 33.91. This result excluded the possibility of Pool-Frenkel type conduction and the observed variation might be due to the presence of some localized electric fields. Further investigation of IV characteristics revealed that the ‘space-charge-limited current (SCLC) with traps’ may be the possible conduction mechanism.

Enhanced field emission from copper nanowires synthesized using ion track-etch membranes as scaffolds

Copper nanowires have been synthesized at different pH values through the template assisted electrodeposition technique using polycarbonate track-etch membranes as scaffolds. The effect of pH (0.8–2.8) of the electrolyte on structure, morphology, composition and deposition rate of copper into the pores of the template, while keeping other electrochemical conditions same, was investigated. X-ray diffraction analysis confirmed the face centered cubic phase of synthesized nanowires. With the change in pH, no shift in peaks was observed except the inclusion of an additional peak of copper oxide in nanowires synthesized at pH 2.8. The nanocrystallite size, strain, lattice stress and energy density were evaluated by X-ray analysis. Field emission scanning electron microscopy images revealed that nanowires obtained at pH 0.8, 1.1 and 1.4 showed incomplete deposition in the pores of the membrane whereas, the nanowires obtained at pH 1.7 were densely stacked, vertically aligned and uniform along the diameter and that obtained from pH 2.0–2.8 had overdeposition on their top. An increase in deposition rate was observed with the increase in pH value. The average diameter of Cu nanowires was found to be ~ 105 nm. The electrical conductivity of as-grown nanowires was observed to decrease 13-fold as the transition from bulk values to the nanosystem. Nanowires prepared at pH of 1.7 were characterized for their field-emission properties. A very large field-enhancement factor of ~ 10,855 was obtained indicating that Cu nanowires grown by reported technique shows outstanding potential as efficient field-emitters for flat panel displays.

Evaluating microgap breakdown mode transition with electric field non-uniformity

This paper evaluates the transitions between the breakdown modes that are dominated by field emission (FE) and by secondary electron emission (SEE) in argon microgaps, based on a mathematical model originally proposed for the modified Paschen’s curve (Go and Pohlman 2010 J. Appl. Phys. 107 103303). In the present model, instead of assuming a constant electric field across the gap, the spatial distribution of the electric field due to the presence of a parabolic cathode protrusion is implemented, and a modification of the first Townsend ionization coefficient for microscale gaps from particle-in-cell/Monte Carlo collision simulations is employed (Venkattraman and Alexeenko 2012 Phys. Plasmas 19 123515). The breakdown mode transitions are quantified by the variations of gas pressure and gap distance. It is found that assuming a constant electric field across the gap will overestimate the electron avalanche, which underestimates the breakdown voltage and shifts the breakdown mode transitions to a smaller gap. By varying the field enhancement factor, the critical point of the breakdown mode transition can be significantly adjusted. Increasing the field enhancement factor will lower the breakdown voltage in the FE regime and increase the breakdown voltage in the SEE regime.

Electron Field Emission Enhancement Based on Pt-Adsorbed ZnO Nanorods With UV Irradiation

Zinc oxide, a low-cost and practical II–VI chemical material, is utilized to absorb platinum nanoparticle on ZnO nanorods (Pt/ZnO NRs). This material can improve the electrical characteristics of ZnO. Samples were prepared for different prerequisites of Pt precursors and named as ZnO and Pt/ZnO NRs with 0 and 30 s sputter deposition times, respectively. Results indicated that ZnO and Pt/ZnO NRs presented an average diameter and length of ∼50 nm and ∼1.9 μm, respectively. Moreover, the topography surfaces of ZnO and Pt/ZnO NRs showed no evident changes. ZnO NRs were structurally uniform and vertically grown on glass substrates and well oriented with pure wurtzite structure by hydrothermal method. The turn-on field values of ZnO and Pt/ZnO NR samples in the dark were 2.85 and 2.52 V/ μ m, and the field enhancement factor β values were 702.19 and 1924.98, respectively.

Fabrication and Field-Emission Properties of Vertically-Aligned Tapered [110]Si Nanowire Arrays Prepared by Nanosphere Lithography and Electroless Ag-Catalyzed Etching

In this study, the controllable fabrication of a variety of vertically aligned, single-crystalline [110]-oriented Si nanowire arrays with sharp tips on (110)Si substrates is achieved using a combined self-assembled nanosphere lithography and multiple electroless Ag-catalyzed Si etching processes. All of the experiments were performed at room temperature. The morphological evolution and formation mechanism of long tapered [110]Si nanowire arrays during the multiple tip-sharpening cycle processes have been investigated by scanning electron microscopy, transmission electron microscopy and water contact angle measurements. Field emission measurements demonstrate that the field-emission behaviors of all nanowire samples produced in this study agree well with the Fowler–Nordheim theory, and the produced long tapered [110]Si nanowire array possesses superior electron emission characteristics, with a very low turn-on field of 1.4[Formula: see text]V/[Formula: see text]m and a high field enhancement factor of 3816. The simple and room temperature fabrication of the well-ordered long tapered [110]Si nanowire array and its excellent electron field emission performance suggest that it can serve as a good candidate for applications in high-performance Si-based vacuum electronic nanodevices.

Low Temperature Synthesis of Lithium-Doped Nanocrystalline Diamond Films with Enhanced Field Electron Emission Properties.

Low temperature (350 °C) grown conductive nanocrystalline diamond (NCD) films were realized by lithium diffusion from Cr-coated lithium niobate substrates (Cr/LNO). The NCD/Cr/LNO films showed a low resistivity of 0.01 Ω·cm and excellent field electron emission characteristics, viz. a low turn-on field of 2.3 V/µm, a high-current density of 11.0 mA/cm2 (at 4.9 V/m), a large field enhancement factor of 1670, and a life-time stability of 445 min (at 3.0 mA/cm2). The low temperature deposition process combined with the excellent electrical characteristics offers a new prospective for applications based on temperature sensitive materials.

Physics-based derivation of a formula for the mutual depolarization of two post-like field emitters

Recent analyses of the apex field enhancement factor (FEF) for many forms of field emitter have revealed that the depolarization effect is more persistent with respect to the separation between the emitters than originally assumed. It has been shown that, at sufficiently large separations, the fractional reduction of the FEF decays with the inverse cube power of separation, rather than exponentially. The behavior of the fractional reduction of the FEF encompassing both the range of technological interest (c being the separation and h is the height of the emitters) and large separations () has not been predicted by the existing formulas in field emission literature, for post-like emitters of any shape. In this work, we use first principles to derive a simple two-parameter formula for fractional reduction that can be useful for experimentalists for modeling and interpreting the FEFs for small clusters of emitters or arrays at separations of interest. For the structures tested, the agreement between numerical and analytical data is ∼1%.

Synthesis of highly dense and vertically aligned array of SWCNTs using a catalyst barrier layer: High performance field emitters for devices

The authors in this research investigated the role of double catalyst layer Fe/Al in the growth of SWCNTs to obtain high current density, low turn on field and high field enhancement factor to act as efficient field emitters. We synthesise SWCNTs by PECVD technique using Fe/Al as a dual catalyst layer on Si substrate. It is seen that during the growth of SWCNTs, Al act as a barrier layer which reduces the diffusion of Fe catalyst in Si substrate resulting in high density vertically aligned growth of SWCNTs. Field emission measurements show high current density 30.5 mA/cm2 at very low turn-on field 1.2 Vμm−1 with a high field enhancement factor of 1.6 × 104. Stability and repeatability parameters of as-grown highly vertically aligned SWCNTs are also studied as a function of current density. Stability results show fairly stable field emitters for 7 h with good repeatability.

Shielding effects in random large area field emitters, the field enhancement factor distribution, and current calculation

A finite-size uniform random distribution of vertically aligned field emitters on a planar surface is studied under the assumption that the asymptotic field is uniform and parallel to the emitter axis. A formula for field enhancement factor is first derived for a 2-emitter system and this is then generalized for N-emitters placed arbitrarily (line, array, or random). It is found that geometric effects dominate the shielding of field lines. The distribution of field enhancement factor for a uniform random distribution of emitter locations is found to be closely approximated by an extreme value (Gumbel-minimum) distribution when the mean separation is greater than the emitter height but is better approximated by a Gaussian for mean separations close to the emitter height. It is shown that these distributions can be used to accurately predict the current emitted from a large area field emitter.

Modeling of the in-situ nitrogen (N) doping of graphene-carbon nanotube (CNT) hybrids in a plasma medium and their field emission properties

A numerical formalism for investigating the effect of in-situ nitrogen doping (N-doping) on the plasma-assisted growth of graphene-carbon nanotube (CNT) hybrids is established. The formalism includes the energy balance on the catalyst particle and the kinetics of plasma species with contribution from hydrogen, hydrocarbon, and ammonia that aids in N-doping, for the growth of in-situ N-doped CNT, graphene, and graphene-CNT hybrids. The growth rate equations for the N-doped CNT, graphene, and graphene-CNT hybrids are also set up as a part of the model. With N-doping, the hydrogen ionic species density initially increases and then falls, promoting the growth of higher order hydrocarbons in plasma. The electron density also increases with N-doping such that the electron-mediated ionization and dissociation processes, increase eventually affecting the availability of growth precursors. The cumulative effect of the variation in the plasma species density with N-doping leads to the growth of nanostructures with a high aspect ratio. In the present work, the field enhancement factor (β) of the graphene-CNT hybrids is approximated as the ratio of its dimension along the y-axis to that along the z-axis. The β of graphene-CNT hybrids is found to be larger than that of the undoped and N-doped CNT and graphene. Moreover, N-doping further enhances the β of graphene-CNT hybrids.

Field emission study from tantalum surfaces micro-structured with femtosecond pulsed laser irradiation

This paper presents our results on surface micro-structuring of tantalum samples using femtosecond (fs) pulsed laser irradiation and its effect on field emission performance. Micro-structuring of Ta targets has been carried by varying laser fluence in the range 0.35–0.55 J/cm2 while keeping target scan speed constant at 25 µm/s. Laser-treated surfaces have shown the formation of high density micro-protrusions and an oxide phase mainly consisting of crystalline Ta2O5. Our results showed substantial improvement in field emission performance of laser micro-structured surfaces in comparison to pristine Ta surface. Areal number density of micro-protrusions decreases from 8.8 × 105 to 3.5 × 105 protrusions/cm2 as incident laser fluence varied from 0.35 to 0.55 J/cm2. Ta sample micro-structured with 0.55 J/cm2 laser fluence has shown low turn on field (~ 4.0 ± 0.6 Vµm− 1) and high-field enhancement factor (4500 ± 500).