Point Electron Source Research Articles

A novel method for formation of single crystalline tungsten nanotip

A point electron source is desired to improve performance of high brightness electron beam instruments. It is valuable to create nano-sized tungsten (W) tip from sharp ordinary polycrystalline W needle. The sharp W needle, which is manufactured by electrochemical etching, has been practically utilized as a cold field emission electron source. A novel method for formation of single crystalline W nanotip on the top of h-BN coated conventional polycrystalline tungsten, by supplying high voltage, has been found. The W nanotip with an apex radius as small as a few times 10 nm would be grown on the top of the polycrystalline W needle. Field emission characteristics of obtained W nanotip are measured, and the field emission microscopic (FEM) and transmission emission microscopic (TEM) images are observed. The emission current from the W nanotip is measured to exceed 0.1 mA. The FEM image shows significant electron emission from the crystallographic facets of the W single crystal. From these results, the present method for formation of the single crystalline W nanotip would be expected as a key technology to realize a point electron source with a nano-sized apex which makes it possible to improve the performance of high brightness electron beam instruments, especially tiny X-ray tubes for medical use, as well as a cantilever of scanning probe microscope.

Preparation of a miniature carbon nanotube paste emitter for very high resolution X-ray imaging

A miniature carbon nanotube (CNT) paste emitter dot with a diameter of ∼50 μm was fabricated onto the cross-sectional surface of Kovar (nickel-cobalt ferrous alloy) rod without using a sophisticated photolithography. A highly adhesive CNT paste was prepared with silicon carbide and nickel nanoparticles, and then the small CNT emitter was fabricated by a novel and facile technique, called point contact method. The prepared emitter generates very high current density of ∼11.2 A/cm2 when measured using a diode configuration. We further observed for the first time the brightness of electron beam generated from the CNT paste emitter that is a more practical point electron source than the individual CNT emitter. The triode structure fabricated with a miniature CNT paste emitter and a gate aperture (20 μm in diameter) generated a collimated electron beam by which very high resolution (>16 lp/mm) X-ray imaging was achieved.

Effects of packing materials on the sensitivity of RadFET with HfO2 gate dielectric for electron and photon sources

The radiation sensing field effect transistor (RadFET) with SiO2 gate oxide has been commonly used as a device component or dosimetry system in the radiation applications such as space research, radiotherapy, and high-energy physics experiments. However, alternative gate oxides and more suitable packaging materials are still demanded for these dosimeters. HfO2 is one of the most attractive gate oxide materials that are currently under investigation by many researchers. In this study, Monte Carlo simulations of the average deposited energy in RadFET dosimetry systems with different package lid materials for point electron and photon sources were performed with the aim of evaluating the effects of package lids on the sensitivity of the RadFET by using HfO2 as a gate dielectric material. The RadFET geometry was defined in a PENGEOM package and electron–photon transport was simulated by a PENELOPE code. The relatively higher average deposited energies in the sensitive region (HfO2 layer) for electron energies of 250 keV–20 MeV were obtained from the RadFET with the Al2O3 package lid despite of some deviations from the general tendency. For the photon energies of 20–100 keV, the average amount of energy deposited in RadFET with Al2O3 package was higher compared with the other capped devices. The average deposited energy in the sensitive region was quite close to each other at 200 keV for both capped and uncapped devices. The difference in the average deposited energy of the RadFET with different package lid materials was not high for photon energies of 200–1200 keV. The increase in the average deposited energy in the HfO2 layer of the RadFET with Ta package lid was higher compared with the other device configurations above 3 MeV.

Tailoring point electron sources of individual carbon nanotubes

We describe a technique for the fabrication of individual carbon nanotube electron field emitters on silicon substrates, with well-defined tunneling geometries and robust metal contacts. The suspended nanotube emitters have been produced by edge lithography on cleaved silicon substrate in conjunction with edge etching. The I–V curves acquired from the resulting emitters followed the Fowler–Nordheim law and exhibited a low operating voltage in a short cathode-anode distance. The extracted field enhancement factors were an order of magnitude higher than those obtained in an electron microscope but in good agreement with those reported in large-area measurements.

Carbon “Onions” as Point Electron Sources

The novel type of an electron field emitter is demonstrated by welding a single carbon "onion" onto the end of a tungsten tip inside a high-resolution transmission electron microscope. Such merged structure is found to markedly reduce the onset voltage peculiar to a standard tungsten field emitter due to the small size of the onion and its highly curved surface. Similar to short carbon nanotubes, individual C-onion emitters can sustain large emission currents, more than 100 muA, and exhibit good long-term emission stability. Moreover the insertion of a high electrical resistance in series can suppress the current fluctuation to only 1.9%. All these properties make these newly created field emitters promising candidates for the advanced point electron sources.

Field emission of carbon-nanotube point electron source

Carbon nanotube (CNT)-based point electron emitter was fabricated using a cavity-confined dielectrophoresis. The emission current of an individual multi-walled CNT (MWCNT) was stable up to 10 μA and reached ~ 2 mA (1.7 × 10 8 A/cm 2), about three orders of magnitude higher than the threshold current of the existing single MWCNT tip. At low electric field, the current fluctuated in a stepwise manner. On the other hand, above critical field, CNT point emitter started to disintegrate so that the current fluctuated rapidly and gradually diminished. These anomalous behaviors were explained from the cap opening and field-induced unraveling of tip edges.

Field‐Emission Characteristics of Individual Carbon Nanotubes with a Conical Tip: The Validity of the Fowler–Nordheim Theory and Maximum Emission Current

Carbon nanotubes (CNTs) are widely considered as one of the most promising candidates for building field emission (FE) devices. These devices include X-ray tubes, scanning X-ray sources, and flat panel displays. In addition to its high aspect ratio, good electrical conduction, and superb chemical stability, the tip structure of the CNT is a crucial factor in determining its excellent performance as a field emitter. In particular, for a cappedCNT, the closed apex with each carbon atom bound to three neighbor atoms via covalent bonds shows much higher FE stability than that of an opened end, and capped CNTs have therefore become the focus of scientific and industrial interest for their potential uses as, for example, high-brightness point-electron sources. However, even for a capped CNT with a closed termination built by strong C–C bonds, the emission can be unstable. Its cap structure can be damaged at extremely strong local field levels and high temperatures resulting from current heating, and so these factors define a upper limit for the FE current from a CNT. While technically this maximum FE current (Imax) is a very important parameter, this quantity has not been investigated by directly correlating it to the CNT cap structure. The CNT emitter was found to deviate from Fowler–Nordheim (F–N) behavior in many experiments, but some recent results show that the F–N theory might still give a good description for capped CNTs. So a question arises: to what extent does the conventional F–N theory applies for an extremely curved CNT cap surface? Recently we demonstrated an in situ cap–engineering technique inside a transmission electron microscope (TEM) for fabricating conical tips with controllable size on a single CNT. This cone-shaped CNT, in fact, provides a good experimental object for us to investigate the two fundamental FE issues raised above: what is the applicability of the F–N theory, and what is themaximum stable FE current that can be extracted from a single CNT?

Scaled fabrication of single-nanotube-tipped ends from carbon nanotube micro-yarns andtheir field emission applications

Joule-heating-induced electrical breakdown was applied to break suspendedcarbon nanotube (CNT) micro-yarns. The yarn ends at the breaking pointswere well-shaped sharp tips and mostly terminated by a single nanotube. Theuppermost CNT was approximately 5 nm in diameter and was firmly compactedwith the CNTs below it, yielding better mechanical, electrical and thermalcontacts. An individual end could provide an emission current of approximately25 µA, with potential application as a point electron source. In addition, we developed a pixelstructure for a field emission display using oppositely aligned ends as the cathode and gate,respectively.

Atomic force microscopy measurement of the Young’s modulus and hardness of single LaB6 nanowires

We have employed the atomic force microscopy based (a) three-point bending and (b) nanoindentation methods to obtain the Young’s modulus and hardness of single LaB6 nanowires. The Young’s modulus, E=467.1±15.8GPa, is the same as that of the LaB6 single crystals but larger than the sintered polycrystalline LaB6 samples. The nanoindentation hardness of the LaB6 nanowire is H=70.6±2.1GPa at an indent depth of 4.6nm, which is higher than that of the LaB6 single crystals, LaB6 polycrystals, and W metals. A superior resistance against thermal vibration, field modification, and ion bombardment is expected for the LaB6 nanowires as a field-emission point electron source.

Mounting multi-walled carbon nanotubes on probes by dielectrophoresis

We developed a dielectrophoresis method by employing an AC voltage to align, assemble and bridge multi-walled carbon nanotubes (MWNTs) between two oppositely aligned probes. The opposite probes with a gap less than 20 µm were submerged into MWNT suspension solution and an alternative electric field applied between the two probes made MWNTs bridge them. By breaking the bridge, we got MWNT tips, which can find applications in scanning probe microscopes and point electron sources. Field emission properties of the as-prepared MWNT tips were further studied and the maximal emission current was about 25 µA.

Local field-emission characteristic of individual AlN cone fabricated by focused ion-beam etching method

Highly (0 0 2)-oriented AlN film was deposited on n-type (1 0 0)-oriented silicon substrates by the radio frequency magnetron sputtering method. An individual AlN cone with high aspect ratio was fabricated by the focused ion-beam (FIB) etching process in the surface of an as-formed AlN film. This etching method can easily control the tip radius and height to obtain AlN cones with different aspect ratios. The field-emission property of the individual AlN cone was measured in a scanning electron microscopy system equipped with a movable probe as the anode above the AlN tip. The results indicated that the as-formed single AlN cone with high aspect ratio possessed good field-emission ability although it only had a tiny emission area. Compared with a single Si tip fabricated by the same method, a single AlN cone exhibits better field-emission ability, and hence, has great potential as a promising candidate of point electron source for application in vacuum electronic devices.

Laser welding of a single tungsten oxide nanotip on a handleable tungsten wire: A demonstration of laser-weld nanoassembly

The authors demonstrate that individual nanotips (W18O49) may be laser welded onto the supporting microtip. The nanotip-microtip assembly can be handheld or ready for further manual manipulation, and thus is very useful for individual nanowire’s characterization, selection, and applications in nanoprobe analysis, nano-optical and nanoelectronic devices, and biostudy. Well mechanical and electrical connections are shown between the nanotip-microtip. Field emission characterization shows that the welded nanotip is a promising candidate for point electron source application.

Fabrication and characterization of single carbon nanotube emitters as point electron sources

Individual carbon nanotube electron field emitters with good controllability have been fabricated in a two-step process involving (a) producing micron-size carbon fibers which contain single carbon nanotubes at their cores by a chemical vapor deposition method and (b) exposing the nanotubes by fracturing the fiber with mechanical forces and mounting the fiber to a copper wire. These fiber-nanotube electron emitters show low operating voltage, generate high field enhancement, and produce fine electron beams. The field emission characteristics and durability of this structure offer promising applications for analytical instruments where high performance point electron sources are required.

Comparison of parameters for Schottky and cold field emission sources

Total energy distribution (TED) measurements were carried out for point electron sources operating in the cold field (T=300K) and Schottky (T=1800K) emission regimes. The full width at half maximum (FWHM) values of the TED’s for both emission regimes were found to increase significantly above the respective theoretical values as the emitter radius (a) was decreased and as the angular current density (I′) was increased. This increase in the FWHM arises from the stochastic electron-electron interactions in the beam commonly known as the Boersch [Z. Phys. 139, 115 (1954)] effect. A method was devised to extract the magnitude of the Boersch effect from the experimental TED’s. The TED’s were investigated as a function of I′ and a. In addition, the reduced brightness for both emitters was calculated from the virtual source size and I′ values as a function of the FWHM values.

Fabrication and characterization of single carbon nanotube emitters as point electron sources

Fabrication and characterization of single carbon nanotube emitters as point electron sources

LaB6 nanowires and their field emission properties

AbstractFor field-induced electron emission, the two factors that enable a high emission current density at low applied voltages are (a) low work function of the emitter and (b) sharpness of the emitter tip. We have developed and applied a chemical vapor deposition method to synthesize single-crystalline LaB6 nanowires for applications as point electron emitters. The crystallographic orientation of the grown nanowires can be controlled by the catalysts used in synthesis and their typical diameter is ranged from below 20 nm to over 100 nm. The nanowires’ tip is either hemispherical or flat top with rectangular cross-section depending on the catalyst being utilized. The field emission properties have also been measured from the single nanowire emitters and the results are discussed for applications as point electron sources used in high performance electron optical instruments such as the transmission and scanning electron microscopes.

Fabrication and Test of Single Nanotube Emitter as Point Electron Source

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

High-efficiency, high-power, stable 172 nm xenon excimer light source

Stable, continuous-wave light sources at 172 nm, based on the Xe2* excimer molecule, with conversion efficiency of electrical energy to vacuum ultraviolet (VUV) light greater than 50%, are reported. In high-pressure xenon gas, “Saint Elmo’s Fire” corona discharges serve as localized point electron sources with a metal grid at a few kilovolts providing an accelerating electric field. An extended VUV light-emitting region with high-energy conversion efficiency indicates that electron energy loss is predominantly by excitation of Xe atoms rather than by ionization. A room-temperature prototype lamp with variable VUV power to 35 mW/cm2 has been demonstrated.

High brightness electron beam from a multi-walled carbon nanotube.

Carbon nanotubes can act as electron sources with very rigid structures, making them particularly interesting for use as point electron sources in high-resolution electron-beam instruments. Promising results have been reported with respect to some important requirements for such applications: a stable emitted current and a long lifetime. Two parameters of an electron source affect the resolution of these instruments: the energy spread of the emitted electrons and a parameter called the reduced brightness, which depends on the angular current density and the virtual source size. Several authors have measured a low energy spread associated with electron emission. Here we measure the reduced brightness, and find a value that is more than a factor of ten larger than provided by state-of-the-art electron sources in electron microscopes. In addition, we show that an individual multi-walled carbon nanotube emits most current into a single narrow beam. On the basis of these results, we expect that carbon nanotube electron sources will lead to a significant improvement in the performance of high-resolution electron-beam instruments.

An automated device for manufacturing microneedles of iterated shape

A computer-controlled device allows for obtaining metallic microneedles with a vertex radius of ∼10 nm from wire bars by using the electropolishing technique. The microneedles have a highly reproducible shape and a nondisturbed interior microstructure of bars. The essence of the method consists in terminating the process at an optimal instant determined from the time dependence of the electric current by using a computer program. The microneedles obtained are utilized as point electron sources, samples in field-emission microscopy, and indenters in tunneling microscopy.