Cold Field Emission Research Articles

Influence of gun design on Coulomb interactions in a field emission gun

The authors investigate by simulation the Coulomb effects on brightness and energy spread for cold field emitters. At first, we show that brightness is ultimately limited by Coulomb interactions. The authors analyze the maximum attainable brightness for tip radii ranging from 1 nm to 1 μm. Remarkably, the brightness of a tip of 1 nm is comparable to a tip of 1 μm. Higher brightness can be achieved by adding a plate at the base of the field emitter, and therefore decreasing the shank length. However, the brightness enhancement due to the protrusion is ultimately limited by large extractor voltages and increased energy spread. In particular, the energy spread for larger tip radii grows faster with brightness. The authors conclude that performance enhancement due to the protrusion works best for smaller tip radii.

Fabrication of Gradient Optical Filter Containing Anisotropic Bragg Nanostructure

New gradient optical filters containing asymmetric Bragg structure were prepared from the distributed Bragg reflector (DBR) porous silicon (PSi). Anisotropic DBR PSi displaying a rainbow-colored reflection was generated by using an asymmetric etching configuration. Flexible anisotropic DBR PSi composite films were obtained by casting of polymer solution onto anisotropic DBR PSi thin films. The surface and cross-sectional images images of anisotropic DBR PSi composite films obtained with cold field emission scanning electron microscope indicated that the average pore size and the thickness of porous layer decreased as the lateral distance increased. As lateral distance increased, the reflection resonance shifted to shorter wavelength.

Electron microscopy at a sub-50 pm resolution

An aberration-corrected electron microscope developed in CREST project has been applied for imaging atoms and clusters buried inside crystals. The resolution of the microscope in scanning transmission electron microscopy (STEM) has experimentally proved to be better than 47 pm by use of a cold-field emission gun at 300 kV. The high resolution has given an advantage for imaging light elements such as lithium atoms discriminating one by one. Moreover, a three-dimensional structure imaging has been demonstrated for dopant clusters by a sub-50 pm STEM, using its high depth resolution.

Performance and Application of an Aberration Corrected Analytical Electron Microscope with a Cold Field Emission Gun

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

From Early Days of Cold Field-Emission Electron Gun at Hitachi to Sub-Angstrom Holography

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Ultraviolet resistant/antiwrinkle finishing of cotton fabrics by sol‐gel method

AbstractNano titanium dioxide (TiO2) sols were prepared by sol‐gel method with tetrabutyl orthotitanate (TBOT) as precursors, citric acid (CA) as inhibitors. Ultraviolet resistant capacity of finished cotton fabrics are greatly improved with good wrinkle‐resistance, whiteness and tensile strength as well. The optimum molar ratio for preparing nano‐TiO2 sols in this study is n[C2H5OH] : n[H2O] : n[CA] : n[HCl] : n[TOBT] = 20 : 6 : 1.2 : 0.025 : 1, at ambient temperature. Particle size distribution analysis of the sol reveals that the curve is right tailed with an average diameter of 72.8 nm. Factors affecting the performance of finished cotton fabrics by TiO2 sols, such as concentration of sodium hypophosphite (SHP), triethanolamine (TEA) in finishing bath, curing temperatures and time lengths were investigated by orthogonal experiments. The optimum finishing results can be obtained with 3% SHP, 3% TEA, curing at 165°C for 3 min. Aggregated nano‐TiO2 particles on surfaces of finished cotton fibers both washed and unwashed were investigated by high resolution cold field emission scanning electron microscope and energy dispersive X‐ray spectrometer. Esterification crosslinking between CA and cotton fibers were also demonstrated through infrared spectra. After a five‐time's wash, ultraviolet protection factor of finished cotton fabrics with nano‐TiO2 sols is up to 117.42 and dry crease recovery angles increase by 30.2° with slightly improved whiteness, while the breaking strength decreases by 18.8%. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Dedicated STEM for 200 to 40 keV operation

A dedicated STEM developed for operation at primary energies from 200 keV to 40 keV and lower is described. It has a new cold field emission gun (CFEG) that gives a normalized brightness of 3 × 108 A/(m2 sr V), and excellent short-term and long-term stability. It includes two gun lenses (one electrostatic and one electromagnetic), a fast electrostatic beam blanker, three condenser lenses, a corrector of third- and fifth-order geometric aberrations, an objective lens with low aberration coefficients, a flexible set of projector lenses, an ultra-stable sample stage, and provision for storing up to five samples under high vacuum and loading them into the microscope’s objective lens under remote control. The microscope is enclosed in a magnetically and acoustically shielding enclosure, which allows it to operate at a high performance level even in non-optimal environments. It has reached 53 pm resolution at 200 keV and 123 pm at 40 keV, and an EELS energy resolution of 0.26 eV.

Role of alkali metal adsorption and defect position on the work function of a (5, 5) capped single-walled carbon nanotube

The authors used first-principles calculations to investigate the influence of alkali metal (Li/Na/Cs) adsorption and defect position on the work function of a (5, 5) armchair single-walled carbon nanotube (CNT) with a capped edge. The atomic Cs adsorption can more effectively reduce the work function of the CNT than the atomic Li/Na adsorption. Adsorption positions have a measurable impact on the work function of the CNT. Any vacancy defect on the tip can raise the work function of the CNT regardless of whether or not an alkali metal atom is absorbed. The variations of work functions are mainly attributed to the change of Fermi levels induced by charge redistributions. The alkali metal adsorption can also transform the semiconducting CNT into a metallic tube, which is significant for the CNTs as a promising field emission cold cathode material.

Cold field emission from hydrogen exfoliated graphene composites

In the present letter, field emission property of highly wrinkled hydrogen exfoliated graphene (HEG) was studied using an indigenously fabricated setup under ∼10−6 mbar pressure. Graphene was coated on a flexible carbon cloth by spin coating for the field emission study. The turn on voltage obtained for a HEG field emitter is around 1.18 V/μm. Further, the field emission property of HEG was improved by decorating metal oxide over HEG. The stability of the field emitter was tested and field enhancement factor was calculated using Fowler–Nordheim plot.

Quantitative annular dark-field STEM images of a silicon crystal using a large-angle convergent electron probe with a 300-kV cold field-emission gun

Annular dark-field scanning transmission electron microscope (ADF-STEM) images of an Si (001) crystal were obtained by using an aberration-corrected electron microscope, at 30-mrad convergent probe and cold field-emission gun at 300kV. The intensity of ADF-STEM images, that is, the number of scattered electrons relative to the incident electrons, obtained for specimen thickness from 10 to 50nm was compared quantitatively with absorptive multi-slice simulation. The column and background intensities were analyzed by column-by-column two-dimensional Gaussian fitting. These intensities were found to increase linearly with the sample thicknesses. However, the simulated image gave higher column intensity and lower background intensity for all the sample thickness. We found that experimental images were reproduced by the simulation with Gaussian convolution of 70pm full-width at half-maximum for all the sample thicknesses from 10 to 50nm. The possible factors accounted for this Gaussian convolution is discussed.

1G1424 冷陰極電界放出銃を用いた、クライオ電子顕微鏡法による小さい生体分子複合体構造解析への挑戦(蛋白質_構造 1,第49回日本生物物理学会年会)

1G1424 冷陰極電界放出銃を用いた、クライオ電子顕微鏡法による小さい生体分子複合体構造解析への挑戦(蛋白質_構造 1,第49回日本生物物理学会年会)

Large field enhancement at electrochemically grown quasi-1D Ni nanostructures withlow-threshold cold-field electron emission

Ni nanorod arrays have been vertically grown on a Ta-coated Si substrate via anelectrodeposition process through the nanopores of a porous alumina membrane. Field emissionstudies of the samples are performed which show a considerable low-threshold field around5 V µm − 1. The field emission mechanism followed Fowler–Nordheim tunneling due to large fieldenhancement at the emitter tips. Low-dimensional structures of the nanorod tips providedthe large geometrical field enhancement and thus produce a high enough local or barrierfield for low-threshold cold-field electron emission. The cost-effective synthesis of verticallyaligned Ni nanorods on an Si substrate and low-threshold field emission properties canprovide a potential alternative to conventional carbon-based field emitters for low powerpanel applications.

Brightness limitations of cold field emitters caused by Coulomb interactions

Emission theory predicts that high brightness cold field emitters can enhance imaging in the electron microscope. This (neglecting chromatic aberration) is because of the large (coherent) probe current available from a high brightness source and is based on theoretically determined values of reduced brightnesses up to 1014 A/(m2 sr V). However, in their analysis, the authors find that statistical Coulomb interactions limit the reduced brightness of even atomically sharp cold field emitters to 1011 A/(m2 sr V) and regular tungsten cold field emitters to around 2×108 A/(m2 sr V). The authors also find that for tip radii in the range from 5 nm to 1 μm, cold field emitters do not outperform larger Schottky (thermal field) emitters. Although this is applied to only one geometry, they expect that similar results will occur for most other cases due to a distinct difference in the behavior of different beam regimes.

Characteristics of a Triode Field Emission Display Panel with the Suspension Gate Structure

With the effective screen-printing technique and high-temperature sintering process, the suspersion gate structure was developed. The silver slurry was printed on the gate substrate to form the gate electrode. Using carbon nanotube as cold field emitter, the triode field emission display (FED) panel was fabricated, and the detailed manufacture process was also presented. The anode back plane, the cathode back plane and spacer combined to device room, in which the suspension gate structure would be included. The distance between the gate electrode and carbon nanotube cathode could be reduced, which could decrease the device manufacture cost because of the small gate voltage. The modulation of emitted electron by the gate voltage would be confirmed, and the field emission characteristics was measured. The sealed FED panel with simple fabrication process and designed structure possessed better field emission uniformity, high display brightness and field emission perofrmance.

Development of a New Quantitative X-Ray Microanalysis Method for Electron Microscopy

Quantitative X-ray microanalysis of thick samples is usually performed by measuring the characteristic X-ray intensities of each element in a sample and in corresponding standards. The ratio of the measured intensities from the unknown material to that from the standard is related to the concentration using the ZAF or ϕ(ρz) equations. Under optimal conditions, accuracies approaching 1% are possible. However, all the experimental conditions must remain the same during the sample and standard measurements. This is not possible with cold field emission scanning electron microscopes (FE-SEMs) where beam current can fluctuate around 5% in its stable regime. Very little work has been done on variable beam current conditions (Griffin, B.J. & Nockolds, C.E., Scanning 13, 307-312, 1991), and none relating to cold FE-SEM applications. To address this issue, a new method was developed using a single spectral measurement. It is similar in approach to the Cliff-Lorimer method developed for the analytical transmission electron microscope. However, corrections are made for X rays generated from thick specimens using the ratio of the characteristic X-ray intensities of two elements in the same material. The proposed method utilizes the ratio of the intensity of a characteristic X-ray normalized by the sum of X-ray intensities of all the elements measured for the sample, which should also reduce the amplitude of error propagation. Uncertainties in the physical parameters of X-ray generation are corrected using a calibration factor that must be previously acquired or calculated. As an example, when this method was applied to the calculation of the composition of Au-Cu National Institute of Standards and Technology standards measured with a cold field emission source SEM, relative accuracies better than 5% were obtained.

Stabilization of a tungsten ⟨310⟩ cold field emitter

Cold-field-emission current from a tungsten ⟨310⟩ emitter in a scanning electron microscope (SEM) gun, evacuated by an ion pump and a supplementary nonevaporative getter pump, was stabilized. It was verified that the probe current from a local (310) crystal plane exhibits different time variations in comparison to that of total current. As for the probe current under a pressure of 2×10−9 Pa, a stable plateau region—which lasted about 4 h—appeared just after flashing of the emitter. By observing emission patterns, it was verified that these different emission characteristics are originated from the anisotropy of current decay in accordance with crystal planes. With low-temperature “mild flashings” at 700 °C, the plateau region was extended to 12 h, which is long enough for practical SEM application. The superior properties of the plateau region, namely, high current, low noise, and small current variation, will enhance the performance and usability of electron microscopes.

DESIGN AND CONSTRUCTION OF A DIAGNOSTIC PULSED X-RAY TUBE

A pulsed X-ray Tube based on the field emission cold cathode principle has been designed and constructed. The device produced X- ray pulses of duration of (1.2) µ sec and intensities of (90) mR at a distance of (10) cm from the window, when a (150) KV is discharged across the electrodes of the tube. The intensity of the x ray radiation at various distances was determined at various cathode anode spacing to determine its optimum value. Special efforts were made in the design to be demountable, easy to maintain, and offering a practical possibility for future industrial and medical application.

Nanostructured LaB6 Field Emitter with Lowest Apical Work Function

LaB(6) nanowires are ideal for applications as an electrical field-induced ion and electron point source due to their miniature dimensions, low work function, as well as excellent electrical, thermal, and mechanical properties. We present here a reliable method to fabricate and assemble single LaB(6) nanowire-based field emitters of different crystal orientations. The atomic arrangement, emission brightness from each crystal plane, and field emission stability have been characterized using field ion microscopy (FIM) and field emission microscopy (FEM). It is found that the 001 oriented LaB(6) nanowire emitter has the highest field emission symmetry while the 012 oriented LaB(6) nanowire has the lowest apical work function. The field emission stability from the single LaB(6) nanowire emitter is significantly better than either the LaB(6) needle-type emitter or W cold field emitters.

Investigation of carbon nanotube field emitter geometry for increased current density

In this work we present field emission characteristics of four geometrically distinct carbon nanotube pillar arrays. Each cathode has a unique geometric configuration with different structural parameters such as number of edges and vertices. We present experimental data demonstrating a carbon nanotube cold field emitter with an emitted current density of 31.8 mA/cm 2 at an applied field of 11 V/μm. The performance of these cathodes can be directly attributed to the electric field being enhanced along the edges and vertices of the structures. We investigated this phenomenon experimentally by changing the geometry of the carbon nanotube pillar structure. We show that by increasing the number of edges and vertices of a structure to keep the electric field screening to a minimum, the emission current can be increased.

Combinatorial Evaluation for Field Emission Properties of Carbon Nanotubes Part II: High Growth Rate System

Carbon nanotube (CNT) emitters are of interest for inclusion in cold cathodes and field emission displays. CNT field electron emitters self-organized on substrates with an Fe/Al2O3 catalytic/supporting layer, which accelerates CNT growth, are characterized using combinatorial libraries. A variety of morphologies are formed on single substrates by C2H2 thermal chemical vapor deposition for 10 s at ambient pressure. Degradation of field emission decreases upon prolonged operation. Raman signals from thinner single-walled CNTs predominantly degrade during operation. Controlling the number of protruding thin CNTs is crucial to extracting current and ensuring sustainability. Thin CNTs protruding from CNT ensembles formed on a substrate with a multimodal distribution of catalyst particles show good field emission (FE) properties with practical sustainability. A potential design for self-organized thin CNTs fabricated by the current process is discussed on the basis of the combinatorial evaluation for field emis...