Sharp Emitter Research Articles

Temperature effects on electrospray current from an externally wetted EMI-Im ionic liquid ion source

Ionic liquid ion sources are expected to be used in a wide range of applications such as space electric propulsion and focused ion beam micromachining. It is known that the backstreaming of secondary charged species generated by ion beam impacts can cause unexpected temperature rise and chemical changes in ionic liquids. This paper reports on results of heating experiments using a sharp needle emitter wetted with an ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)amide, at temperatures in a range from room temperature to 120 °C. Current measurements show that positive and negative electrospray currents from the heated emitter increased as the temperature increased. Time-of-flight (TOF) mass spectrometric measurements reveal that the beam composition changed significantly with increasing temperature, indicating that charged droplets as well as ions were emitted from the heated emitter. The TOF data show that a significant fraction of the current is due to droplets at higher temperatures. On the basis of the results obtained, the size and charge of the emitted droplets are discussed. The beam is roughly estimated to contain charged droplets with a diameter of around 20 nm at 120 °C.

Reducing Screening Effect in Close-Packed Si Field Emitter Arrays

Electric field screening effect in close-packed semiconductor field emitter arrays (FEAs) changes the current distribution among emitter tips, such that those on the edges of the array emit much higher current than those in the middle, leading to poor reliability characteristics. Using analysis, numerical simulations, and experimental verification, three different techniques to suppress electric field screening and achieve uniform current distribution are proposed. These techniques are: 1) utilizing very sharp emitter tips; 2) reducing the anode–cathode distance (ACD) to be in the order of emitter pitch; and 3) using extremely low-doped but long field emitter tips. By adopting these techniques together, a more uniform emission from the array and a high emission current density can be achieved. FEAs with 160 K emitter tips with different ACDs and emitter sharpness were fabricated using electron beam lithography. In an array with sharp emitter tips, reliable emission with a maximum current of 0.7 mA (4.4 nA/tip) and a maximum current density of 1.75 A/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{2}}$</tex-math> </inline-formula> was achieved.

General form of the tunneling barrier for nanometrically sharp electron emitters

Field electron emission from nanometer-scale objects deviates from the predictions of the classical emission theory as both the electrostatic potential curves within the tunneling region and the image potential deviates from the planar one. This impels the inclusion of additional correction terms in the potential barrier. At the apex of a tip-like rotationally symmetric surface, these terms are proportional to the (single) local emitter curvature. The present paper generalizes this relation, showing that for any emitter geometry, the coefficient of the correction terms is given by the mean curvature, i.e., the average of the two principal curvatures.

Structure optimization of Spindt-type emitter fabricated by triode high power pulsed magnetron sputtering

Spindt-type emitters were fabricated with cavities made of Al/Mo/SiO2 using the triode high power pulsed magnetron sputtering method. We explored the process parameters (gas pressure and voltage of the additional cap electrode) to optimize the sharpness of the emitter shape. We found that the intermediate pressure and voltage were suited to obtain sharp emitters. Further, we elucidated the crucial effect of the cavity dimensions, such as the cavity depth and hole diameter in the cavity ceiling, on the emitter shape. At a cavity depth of 480 nm, the aspect ratio (AR) of the emitter increased monotonously with an increase in the hole diameter. With a large hole diameter (900 nm) and even shallower cavity (380 nm depth), we attempted to reoptimize the process parameters. Consequently, a very sharp emitter cone structure with an AR exceeding 1.3 was obtained. The cap voltage that produced the optimum AR was found to decrease for the larger-hole and shallower-depth cavities. Finally, the applicability of the process for preparing a working emitter is discussed.

Fabrication of porous emitters for ionic liquid ion source by wire electrical discharge machining combined with electrochemical etching

Ionic liquid ion source (ILIS) is a promising ion source, which can be applied to space propulsion, microfabrication, and surface modification. Fabrication of high-quality ILIS emitters is one of the key technologies for the application of ILIS. A new method is proposed for the fabrication of porous emitters with a designed shape. This method uses wire electrical discharge machining (WEDM) combined with electrochemical etching, and the porous emitter is fabricated by two steps. First, the porous metal is machined by WEDM to get the external geometry of the emitter. Then, electrochemical etching is employed to remove the recast layer. A series of experiments has been conducted to find the appropriate machining parameters. Experiments reveal that sharp porous emitter tips for the ILIS can be fabricated by WEDM combined with electrochemical etching at 5 V etching voltage. Moreover, the apex curvature radius of the emitter is controllable by adjusting the etching time. It is found that the apex curvature radius varies from 4.5 μm to 18.4 μm when increasing etching time from 40 s to 120 s at 5 V etching voltage. Those emitters have been applied to ILIS tests, and their I-V characteristics are investigated. Furthermore, this method has been used to machine dense fields of emitters. A 1 cm2 emitter array chip integrated with 676 emitters has been successfully machined, and the I-V characteristic curve of the emitter array chip is also achieved.

A solid-state wind-energy transformer

We show that a solid-state apparatus with no moving parts can harvest electrical power from the wind. This apparatus, a Solid-state Wind-Energy Transformer (SWET), uses coronal discharge to create negative air ions, which the wind carries away from the SWET. The SWET harnesses the wind-induced currents and voltages to produce electrical power. We report on the operation of a low-power, proof-of-concept SWET. This device consists of a number of parallel electrical wires: “emitter wires,” which have numerous, sharp coronal emitters attached to it, and bare “attractor wires.” When a negative bias voltage is applied to the emitter wires relative to the attractor wires, the coronal emitters generate negative ions. The wind carries off these ions, which eventually settle to ground. The power imparted to the ions by the wind is extracted from the current returning to the SWET from the ground. This proof-of-concept SWET demonstrates that it is possible to generate net electrical power from the wind using only air ions. We estimate that SWET can be scaled up to commercially interesting powers by increasing the number and length of emitter and attractor wires and by controlling the bias voltage. SWETs have the potential to produce large amounts of electrical power at low costs with little negative environmental impact.

Enhanced field emission properties of aligned sharp graphene emitter arrays prepared by freeze-drying and hydrothermal reduction

Reduced graphene oxide bundles with sharp tips were fabricated by a simple approach with freeze-drying and hydrothermal reduction, and this approach could modify the surface topography of the field emission cathode and make more graphene bundles sharpen and aligned. This field emission cathode material displayed excellent field emission properties decreased from 2.9 V μm−1 to a low turn-on field of 0.8 V μm−1 at the emission current density of 10 μA cm−2. Sharp graphene emitter arrays have sharp protruded tips in abundance, leading to an enlarged field enhancement factor from 1683 to 10182, which was much better than those films prepared by other methods we adopted. It also revealed a good field emission stability with no degradation in the current for 3 h. We believe that this approach can enhance the field emission performance of graphene emitters, which could be widely candidate for various field emission applications.

Electrostatic field imaged in 3D around an electron nano-emitter

Nanoscale electro-static fields of a sharp tungsten electron emitter were measured and showed almost perfect agreement with simulated data.

Determination of 3D electrostatic field at an electron nano-emitter

Differential phase contrast in scanning transmission electron microscopy has been applied to image nanoscale electrostatic fields of a sharp tungsten electron emitter with an apex radius of about 20 nm and under field emission conditions. Assuming axial symmetry of the nano-emitter, we derived a method based on the inverse Abel transform to quantitatively reconstruct an axial slice of the 3D electrostatic field from a single projection measurement. The highest field strength of 2.92 V/nm is measured at the nano-emitter apex under the condition of a bias voltage of −140 V with respect to the grounded counter electrode located at about 650 nm from the apex, resulting in an emission current of more than 2 μA. The experimental results are compared with simulations based on a finite element numerical Maxwell equation solver. Quantitative agreement between experiment and simulation has been achieved.

Novel Nanofabricated Mo Field-Emitter Array for Low-Cost and Large-Area Application

This paper demonstrates a novel nanofabrication technology for producing high-density gated molybdenum (Mo) field-emitter array (FEA) based on modified nanosphere lithography (NSL). The arc-shaped side wall of gate apertures introduced by the conventional NSL process put obstacles in the formation of sharp cone-type emitters. In this paper, a two-method solution was employed to facilitate the realization of gate apertures with sharper cutting side wall which are beneficial for the formation of sharper gated Mo emitters. One method was softening nanosphere to enlarge the contact area between nanosphere and substrate. The other was optimizing gate film deposition conditions to dramatically reduce the diffraction effects. Due to the improvements in the geometrical shape of gated Mo emitters, a low-voltage working high-density gated Mo FEA was demonstrated in this paper. This is the first observation of remarkable electron emission from high-density gated Mo FEA fabricated using such as low-cost, high-throughput, and high-efficiency technique which is expected to substantially overcome the challenges in the traditional fabrication process and promotes the low-cost and large-area application of high-density gated field-emitter devices. In addition, the novel nanofabricating method is expected to offer a simple and scalable route which allows fabrication of many gated field-emission devices.

The tunneling potential for field emission from nanotips

In the quasi-planar approximation of field emission, the potential energy due to an external electrostatic field E0 is expressed as −eγE0Δs, where Δs is the perpendicular distance from the emission site and γ is the local field enhancement factor on the surface of the emitter. We show that for curved emitter tips, the current density can be accurately computed if terms involving (Δs/R2)2 and (Δs/R2)3 are incorporated in the potential where R2 is the second (smaller) principle radius of curvature. The result is established analytically for the hemiellipsoid and hyperboloid emitters, and it is found that for sharply curved emitters, the expansion coefficients are equal and coincide with that of a sphere. The expansion seems to be applicable to generic emitters as demonstrated numerically for an emitter with a conical base and quadratic tip. The correction terms in the potential are adequate for Ra⪆2 nm for local field strengths of 5 V/nm or higher. The result can also be used for nano-tipped emitter arrays or even a randomly placed bunch of sharp emitters.

Enhanced Field-Emission Properties of Sol–Gel-Derived Nanostructured $$\hbox {SnO}_{2}$$ SnO 2 :F Thin Film for Vacuum Microelectronics

Nanocrystalline fluorine-doped tin oxide thin film is synthesized by sol–gel-dip-coating technique. Hydrofluoric acid is used as the F-source, which has performed a simultaneous dual activity: (1) it has improved the conductivity by F-doping and (2) has increased the surface roughness via mild etching, both of which have improved the field-emission performance of the doped film over the undoped film. Electron microscopic image of $$\hbox {SnO}_{2}$$ :F film has revealed highly sharp emitter tips at the film surface, which has provided a very high geometrical field enhancement factor ( $$\beta \sim 760$$ ) and a very low turn-on field ( $$\sim $$ 4.8 V $$\upmu \hbox {m}^{-1})$$ . The field-emission process follows the Fowler–Nordheim tunnelling of cold-electron emission. This simple and cost-effective approach can be very useful for surface nanostructuring of thin film-based field emitters with improved field-emission characteristics for vacuum microelectronics applications.

A general computational method for electron emission and thermal effects in field emitting nanotips

Electron emission from nanometric size emitters becomes of increasing interest due to its involvement to sharp electron sources, vacuum breakdown phenomena and various other vacuum nanoelectronics applications. The most commonly used theoretical tools for the calculation of electron emission are still nowadays the Fowler-Nordheim and the Richardson-Laue-Dushman equations although it has been shown since the 1990s that they are inadequate for nanometrically sharp emitters or in the intermediate thermal-field regime. In this paper we develop a computational method for the calculation of emission currents and Nottingham heat, which automatically distinguishes among different emission regimes, and implements the appropriate calculation method for each. Our method covers all electron emission regimes (thermal, field and intermediate), aiming to maximize the calculation accuracy while minimizing the computational time. As an example, we implemented it in atomistic simulations of the thermal evolution of Cu nanotips under strong electric fields and found that the predicted behavior of such nanotips by the developed technique differs significantly from estimations obtained based on the Fowler-Nordheim equation. Finally, we show that our tool can be also successfully applied in the analysis of experimental I-V data.

Coulomb interactions in sharp tip pulsed photo field emitters

Photofield emitters show great potential for many single electron pulsed applications. However, for the brightest pulses &amp;gt;1011A/(m2 sr V), our simulations show that Poisson statistics and stochastic Coulomb interactions limit the brightness and increase the energy spread even with an average of a single electron per pulse. For the systems, we study we find that the energy spread is probably the limiting factor for most applications.

Effective field enhancement factor and the influence of emitted space charge

Although Fowler and Nordheim developed the basics of field emission nearly one century ago with their introduction of the Fowler-Nordheim equation (FNE), the topic continues to attract research interest particularly with the development of new materials that have been proposed as field emitters. The first order analysis of experiments typically relies upon the FNE for at minimum a basic understand of the physical emission process and its parameters of emission. The three key parameters in the FNE are the work function, emission area, and field enhancement factor, all of which can be difficult to determine under experimental conditions. This paper focuses in particular, on the field enhancement factor β. It is generally understood that β provides an indication of the surface roughness or sharpness of a field emitter cathode. However, in this paper, we experimentally and computationally demonstrate that cathodes with highly similar surface morphologies can manifest quite different field enhancements solely through having different emission regions. This fact can cause one to re-interpret results in which a single sharp emitter is proposed to dominate the emission from a field emitting cathode.

The influence of emitter conditioning on the performance of a tungsten cold field emission gun operating at 300 kV

In this contribution, we examine the influence of emitter conditioning for a <111> tungsten cold field emission gun on the emission and beam characteristics of a double aberration corrected electron microscope. By varying the post flash build-up parameters we can control the effective emitter tip radius. A sharp emitter yields an energy resolution of 0.31eV but relatively low beam current whereas an increased tip radius results in a reduction in energy resolution to 0.4eV but much higher potential beam current. Consequently, careful control of the build-up parameters can be used as a means of tailoring the emission to suit specific instrumental requirements.

Analysis of nonuniform field emission from a sharp tip emitter of Lorentzian or hyperboloid shape

For a sharp tip emitter, due to the non-uniform emission feature and the electron beam expansion in the vacuum, it is difficult to precisely determine the average field enhancement factor βc as well as the effective emission area Seff for a single field emitter. In this paper, we conduct a numerical experiment to simulate the electron field emission from a sharp tip emitter (Lorentzian or hyperboloid shape). By collecting the emission current Itot at the finite anode area Stot, we establish the criteria in using Fowler-Nordheim plot to estimate both βc and Seff, which agree well with our initial emission condition. It is found that the values of βc and Seff depend on the emitter's properties as well as the size of the anode area Stot. In order to determine the precise value of βc, Stot must be large enough to collect all the emitted electrons from the sharp tip (e.g., Itot reaches maximum). As an example, a Lorentzian type emitter with an aspect ratio of 10 (height over width), the effective enhancement factor is about βc=33 as compared to the maximal enhancement of 35 at the apex. At similar maximal enhancement factor at the apex (=360), both types of emitters will give different average field enhancement dependent on the collecting area. The extension of this simple model to a statistical more complicated model to simulate field emission from a cathode consisting of many field emitters is also briefly discussed. This paper should be useful to analyze and characterize field emission data together with experimental measurement.

Integration of Fully Microfabricated, Three-Dimensionally Sharp Electrospray Ionization Tips with Microfluidic Glass Chips

This paper presents parallel microfabrication of three-dimensionally sharp electrospray ionization emitters made out of glass. For the first time, the fabrication of glass emitters relies only on standard microfabrication techniques (i.e., deposition, photolithography, and wet etching), and all manual machining steps are omitted. We also demonstrate a straightforward integration of the three-dimensionally sharp emitter tip with a microfluidic separation channel, which has been one of the major challenges of micro total chemical analysis systems for the past 15 years. As a result, our microfabrication approach provides glass ESI emitters that allow robust performance from run to run and tip to tip and do not suffer from sample spreading at the microchannel outlet. The repeatability of the signal intensity for parallel tips was shown to be within 8.0% RSD (n = 6), and the migration time repeatability for repeated injections was within 6.2% RSD (n = 6). At best, separation plates of up to 2.7 × 10(5)/m were obtained. Since the microfabrication process readily yields three-dimensionally sharp emitter tips, very low ESI voltages (typically 1.4-1.75 kV) suffice for stable ESI, which eventually allows for the use of a variety of different solvent compositions from purely aqueous to high organic content. Here, the advantage of using aqueous conditions is demonstrated in protein analysis.

Controlling the shape and gap width of silicon electrodes using local anodic oxidation and anisotropic TMAH wet etching

A simple method for fabricating silicon electrodes with various shapes and gap widths was designed using the special properties of anisotropic tetramethylammonium hydroxide (TMAH) wet etching and local anodic oxidation (LAO). A statistical system was used for the optimization of the parameters of the LAO process to facilitate a better understanding and precise analysis of the process. Analyses of the interaction effects among the significant factors of LAO showed that the relative humidity and applied voltage were interdependent. They had the strongest interaction effect on the dimensions of the oxide mask. TMAH with a concentration of 25% was used as an etchant solution in (1 0 0) silicon with a rectangular oxide mask. The observed undercutting at convex corners, tip shape of emitters and gap widths of electrodes were exactly consistent with theoretical studies. Combination of the LAO method and anisotropic TMAH wet etching was successfully used to fabricate Si nano-gap electrodes. This fabrication method of sharp and round tip emitters was simple, controllable and faster than common techniques. These results indicate that the method can be a new approach for studying the electrical properties of nano-gap electrodes.

New-Type Planar Field Emission Display with Superaligned Carbon Nanotube Yarn Emitter

With the superaligned carbon nanotube yarn as emitter, we have fabricated a 16 × 16 pixel field emission display prototype by adopting screen printing and laser cutting technologies. A planar diode field emission structure has been adopted. A very sharp carbon nanotube yarn tip emitter can be formed by laser cutting. Low voltage phosphor was coated on the anode electrodes also by screen printing. With a specially designed circuit, we have demonstrated the dynamic character display with the field emission display prototype. The emitter material and fabrication technologies in this paper are both easy to scale up to large areas.