Cold Field Emission Research Articles

Resolving 45-pm-separated Si-Si atomic columns with an aberration-corrected STEM.

Si-Si atomic columns separated by 45 pm were successfully resolved with a 300-kV aberration-corrected scanning transmission electron microscope (STEM) equipped with a cold-field emission gun. Using a sufficiently small Gaussian effective source size and a 0.4-eV energy spread at 300 kV, the focused electron probe on the specimen was simulated to be sub-50 pm. Image simulation showed that the present probe condition was sufficient to resolve 45 pm Si-Si dumbbells. A silicon crystalline specimen was observed from the [114] direction with a high-angle annular dark field STEM and the intensity profile showed 45 pm separation. A spot corresponding to (45 pm)(-1) was confirmed in the power spectrum of the Fourier transform.

Rotation axes analysis of deformed magnesium based on rotation contour contrast in a scanning electron microscope

A crystallographic orientation contrast in the form of cross-shaped and intersecting contours was observed in a backscattered electron (BSE) micrograph of deformed magnesium (Mg) grains in a cold field emission scanning electron microscope (CFE-SEM). This contrast was identified as rotation contour contrast (RCC). A model is presented to link the RCC in the BSE micrograph to the channeling contrast in the corresponding channeling pattern. Based on this model, the appearance of the cross-shaped RCC in the BSE micrograph was attributed to the rotation of the crystal about two rotation axes and the RCC was related to a two dimensional angular scan of the corresponding channeling pattern. This model was experimentally validated using the selected area channeling pattern (SACP) technique. The crystallographic directions of the rotation axes were identified using the electron backscatter diffraction (EBSD) technique.

Aberration corrected 1.2-MV cold field-emission transmission electron microscope with a sub-50-pm resolution

Atomic-resolution electromagnetic field observation is critical to the development of advanced materials and to the unveiling of their fundamental physics. For this purpose, a spherical-aberration corrected 1.2-MV cold field-emission transmission electron microscope has been developed. The microscope has the following superior properties: stabilized accelerating voltage, minimized electrical and mechanical fluctuation, and coherent electron emission. These properties have enabled to obtain 43-pm information transfer. On the bases of these performances, a 43-pm resolution has been obtained by correcting lens aberrations up to the third order. Observations of GaN [411] thin crystal showed a projected atomic locations with a separation of 44 pm.

Detection of Human Ig G Using Photoluminescent Porous Silicon Interferometer.

Photoluminescent porous silicon (PSi) interferometers having dual optical properties, both Fabry-Pérot fringe and photolumincence (PL), have been developed and used as biosensors for detection of Human Immunoglobin G (Ig G). PSi samples were prepared by electrochemical etching of p-type silicon under white light exposure. The surface of PSi was characterized using a cold field emission scanning electron microscope. The sensor system studied consisted of a single layer of porous silicon modified with Protein A. The system was probed with various fragments of aqueous human immunoglobin G (Ig G) analyte. Both reflectivity and PL were simultaneously measured under the exposure of human Ig G. An increase of optical thickness and decrease of PL were obtained under the exposure of human Ig G. Detection limit of 500 fM was observed for the human Ig G.

Morphological optimization of large-area arrays of TiO2 nanowires & nanotubes for enhanced cold field emission: experiment and theory

Designed by finite elemental modelling, large-area arrays of TiO2 nanowires and nanotubes with differentiated heights mixed together are synthesized on a planar Ti wafer via hydrothermal methods.

Development of TEM and SEM high brightness electron guns using cold-field emission from a carbon nanotip

A newly developed carbon cone nanotip (CCnT) has been used as field emission cathode both in low voltage SEM (30kV) electron source and high voltage TEM (200kV) electron source. The results clearly show, for both technologies, an unprecedented stability of the emission and the probe current with almost no decay during 1h, as well as a very small noise (rms less than 0.5%) compared to standard sources which use tungsten tips as emitting cathode. In addition, quantitative electric field mapping around the FE tip have been performed using in situ electron holography experiments during the emission of the new tip. These results show the advantage of the very high aspect ratio of the new CCnT which induces a strong enhancement of the electric field at the apex of the tip, leading to very small extraction voltage (some hundred of volts) for which the field emission will start. The combination of these experiments with emission current measurements has also allowed to extract an exit work function value of 4.8eV.

First results on laser-induced field emission from a CNT-based nanotip

We present the first demonstration of ultrafast laser-induced field emission and measurement of the energy distribution of electrons from a nanotip based on a carbon nanotube (CNT). Our experimental setup extends the studies performed on conventional tungsten or gold tips by using this new innovative tip. The carbon tip consists of concentric carbon layers in the shape of a cone, and has been previously studied as a very good candidate for cold field emission. The first laser-induced field emission from a CNT-based nanotip has been observed and we measured the energy spectrum as well as the polarization dependance of the emission. We also characterize the damage threshold of the tip, when illuminated by a high repetition rate femtosecond laser. These first results are encouraging further studies of electron emission from CNT-based carbon nanotips.

Correction: Corrigendum: Atomically resolved imaging of highly ordered alternating fluorinated graphene

Nature Communications 5:4902 doi: (2014); Published 16 September 2014; Updated 12 November 2014 In the Methods section of this Article, the details of the STEM equipment and operating parameters used for the EELS studies were provided incorrectly. The following are the correct details: ‘EELS studieswere also performed on partially fluorinated graphene samples at the SuperSTEM Laboratory on a Nion UltraSTEM100 dedicated ultrahigh vacuum scanning transmission electron microscope equipped with a cold field emission gun with a native energy spread of 0.

The quantum mechanics of ion-enhanced field emission and how it influences microscale gas breakdown

The presence of a positive gas ion can enhance cold electron field emission by deforming the potential barrier and increasing the tunneling probability of electrons—a process known as ion-enhanced field emission. In microscale gas discharges, ion-enhanced field emission produces additional emission from the cathode and effectively reduces the voltage required to breakdown a gaseous medium at the microscale (<10 μm). In this work, we enhance classic field emission theory by determining the impact of a gaseous ion on electron tunneling and compute the effect of ion-enhanced field emission on the breakdown voltage. We reveal that the current density for ion-enhanced field emission retains the same scaling as vacuum cold field emission and that this leads to deviations from traditional breakdown theory at microscale dimensions.

Studies on obtaining of aluminium phosphates modified with ammonium, calcium and molybdenum

Studies on obtaining of aluminium phosphates modified with ammonium, calcium and molybdenum were implemented. Reagent grade substrates (NH4)2HPO4, CaCO3, (NH4)6Mo7O24·4H2O, aqueous ammonia, and amorphous Al(OH)3 were prepared and used in the experiments. An influence of process parameters, i.e. reaction time (60±30min) and total salt concentration (50±10wt%) on the phase composition and product properties was determined. Statistical software STATISTICA 10 for planning and evaluation of the experiments was used. Based on the statistical evaluation of planned research by the plan fractional factorial design at three levels 3(k−p), process parameters allowing to obtain the material of with expected physicochemical properties were determined. The content of aluminium, calcium and molybdenum in the products was determined using ICP-AES technique. The phosphates and ammonium content was determined by means of a spectrophotometric method and ion selective electrode Orion 11–35 type, respectively. The phase composition of the obtained materials was studied using XRD analysis. The specific surface area was calculated using the BET method and the particle size was determined by LSM. The Tafel experiments for an uncoated mild steel (immersed in an aqueous phosphate extract in 3.5wt% NaCl solution) were performed. The steel corrosion products as well as tested phosphates were examined by means of scanning electron microscope with a cold field emission coupled with the energy dispersive X-ray analyzer. The content of the individual components was in the range of 10.52–16.33wt% (Al), 10.59–18.54wt% (Ca), 0.59–5.28wt% (Mo), 0.1–4.91wt% (NH3), and 6.97–23.4wt% (P2O5). The products were characterized by different physicochemical parameters. The surface area was in the range from 6 to 63m2/g and the oil absorption was 36–71g of oil/100g of product, whereas the average particle size of products reached 221–319nm. The Tafel tests revealed markedly higher anticorrosive properties of aluminium phosphates modified with ammonium, calcium, molybdenum in comparison to commercial aluminium phosphate and zinc phosphate.

Use of HfC(310) as a High Brightness Electron Sources for Advanced Imaging Applications

This paper continues our work to demonstrate a new electron source for applications where high brightness is required. Thermionic emission and cold field emission are ends in a continuum of electron emission processes; between lays extended Schottky emission (ESE) and thermal-field emission (TFE). Presently, commercial electron sources operate in the extended Schottky or cold field emission (CFE) modes both using tungsten as the base material. It is known that surface tension and field forces contribute to blunting or build-up on these W based emitters. However, HfC sources have activation energy for surface migration much large than for W. This coupled with loosely bound surface contaminants mean operation at elevated temperatures can keep the surface clean but not trigger geometric changes.

High Energy and Spatial Resolution EELS Band Gap Measurements Using a Nion Monochromated Cold Field Emission HERMES Dedicated STEM

Local measurement of band gaps in nanomaterials is an intriguing experimental possibility, but until recently not possible because both high spatial resolution and high EELS energy resolution with small zero loss peak (ZLP) tails are required. The addition of a monochromator to a Nion probe corrected, cold FEG source D-STEM microscope (denoted HERMES) fitted with a Gatan Enfinium spectrometer has produced the EELS performance required for band gap measurements down less than 1 eV. The microscope, and its ultrastable installation site have been described [1,2]. ZLPs (normalized to the same intensity) from this microscope (in red) and another probe corrected Schottky FEG source but nonmonochromated AEM located in our laboratory are compared in Figure 1 and the data are summarized in Table 1. The 37 meV energy resolution of the monochromated cold FEG source Nion is significantly better than the typical Schottky FEG 950 meV resolution as shown in Table 1 and the energy tails at lower intensity are reduced. The resolution and beam tails depend somewhat on microscope alignment and collection time. The Nion ZLP in Figure 1 is the sum of 15 one second spectra. Shorter collection times (0.1 second, 1 spectrum) result in higher energy resolution (14 meV at FWHM) and smaller extended tails (54 meV FW at 1% maximum intensity). Kimoto et al have shown that both CFEG and a monochromated Schottky source are useful for measuring band gaps, but both of these have more extended low loss tails than the HERMES monochromated CFEG, particularly the tunneling tail for the CFEG, which complicates small gap measurement [3]. Some first measurements are shown below.

Monochromator for Aberration-Corrected STEM

In a scanning transmission electron microscope (STEM), the elemental analysis at atomic resolution is realized by the combination with electron energy-loss spectroscopy (EELS), since an aberration corrected probe forming lens system enables us to obtain an electron probe sized about 0.1 nm [1]. In EELS, the energy resolution is mainly limited by the energy spread of the electron source, which is approximately 0.7 ~ 1.0 eV for a Schottky source and 0.3 ~ 0.5 eV for a cold field emission source. For the analysis of a detailed electronic state at an atomic scale at higher energy resolution, we developed a monochromator for an aberration-corrected STEM, which integrates a double Wien-filter system. [2]

Quantitative Assessment of Lower-Voltage TEM Performance Using 3D Fourier Transform of Through-Focus Series

The spherical aberration corrector was a quantum leap in high-resolution transmission electron microscopy (TEM) [1]. It brought about a breakthrough in high-resolution lower-voltage TEM, and a variety of novel nanostructured materials have been explored utilizing its high spatial resolution, high image contrast and low knock-on damage [2,3]. The performance of an aberration-corrected TEM is determined by the information limit that is often evaluated using Young’s fringe method, in which the diffractogram of a double-exposure image with a given image shift, is analyzed. It was already reported, however, that Young’s fringe method shows unexpected high frequency information [4] due to the nonlinear terms, which correspond to interference between diffracted waves. The presence of the non-linear terms is a major difficulty in the existing procedures for evaluating TEM performance as pointed out by a few researchers [5,6]. The three-dimensional (3D) Fourier transform (FT) of through-focus TEM images allows us to discriminate between the linear imaging terms and the non-linear imaging terms [7– 9]. The linear imaging terms are observed on twin Ewald spheres in the 3D FT using an amorphous specimen. Here, we use the 3D FT of through-focus TEM images for the assessment of two TEM systems. Two spherical-aberration-corrected microscopes were assessed and compared at a relatively lower acceleration voltage. One transmission electron microscope was a Titan 3 (FEI) equipped with a monochromator and a spherical aberration corrector for image forming (CEOS, CETCOR) operated at an acceleration voltage of 80 kV. The energy spread of the electron source was 0.1 eV under monochromated condition. The other microscope, the TripleC microscope, was equipped with a cold field-emission gun (CFEG) and the spherical aberration corrector developed for the TripleC project. This microscope was operated at 60 and 30 kV [10], and the energy spread was 0.3-0.4eV.

Microstructural Characterization of Mg–0.3Al–0.2Ca Alloy Using Ion Milling Surface Preparation Technique

The Vickers microindentation and spherical nanoindentation tests were carried out on a polycrystalline Mg–0.3Al–0.2Ca alloy specimen. Specimens were prepared with chemical polishing and ion milling techniques. The electron channeling contrast imaging (ECCI) and electron backscattered diffraction (EBSD) were used to study the plastic deformation inside and around the indents with a cold-field emission scanning electron microscope. The distribution of strain fields, slip lines, and twins was imaged on an indented surface. A crystallographic orientation mapping was performed to map the local crystallographic misorientation associated with channeling contrast variations. The chemical polishing technique was used to remove the material from the surface and allow the study of deformed microstructure at a maximum depth of the indent. The combination of ECCI and EBSD with chemical polishing and ion milling surface preparation techniques proved to be beneficial to study the indentation-induced plastic deformation in a polycrystalline Mg–0.3Al–0.2Ca alloy specimen.

Application of Low Energy STEM with the In-lens Cold FE-SEM

We have developed a high resolution STEM imaging method using accelerating beam voltages of 30 kV and less. In this study, we examined and determined the low accelerating voltage parameters necessary for obtaining lattice resolution in STEM images, using the cold field emission scanning electron microscope (CFE-SEM), Hitachi SU9000. We also investigated and optimized the objective lens conditions for optimum lattice fringe imaging. The STEM images and associated Fourier transform image obtained at 30 kV show the Si (222) lattice fringes and reflection spots, corresponding to the d-lattice spacing of 0.157 nm. The STEM images and associated Fourier transform image at 15 kV show the Si (111) lattice fringes and reflection spot, corresponding to the d-lattice spacing of 0.314 nm.

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.

Toward single mode, atomic size electron vortex beams.

We propose a practical method of producing a single mode electron vortex beam suitable for use in a scanning transmission electron microscope (STEM). The method involves using a holographic "fork" aperture to produce a row of beams of different orbital angular momenta, as is now well established, magnifying the row so that neighboring beams are separated by about 1 µm, selecting the desired beam with a narrow slit, and demagnifying the selected beam down to 1-2 Å in size. We show that the method can be implemented by adding two condenser lenses plus a selection slit to a straight-column cold-field emission STEM. It can also be carried out in an existing instrument, the monochromated Nion high-energy-resolution monochromated electron energy-loss spectroscopy-STEM, by using its monochromator in a novel way. We estimate that atom-sized vortex beams with ≥ 20 pA of current should be attainable at 100-200 keV in either instrument.

Aberration-corrected STEM/TEM imaging at 15kV.

The performance of aberration-corrected (scanning) transmission electron microscopy (S/TEM) at an accelerating voltage of 15kV was evaluated in a low-voltage microscope equipped with a cold-field emission gun and a higher-order aberration corrector. Aberrations up to the fifth order were corrected by the aberration measurement and auto-correction system using the diffractogram tableau method in TEM and Ronchigram analysis in STEM. TEM observation of nanometer-sized particles demonstrated that aberrations up to an angle of 50mrad were compensated. A TEM image of Si[110] exhibited lattice fringes with a spacing of 0.192nm, and the power spectrum of the image showed spots corresponding to distances of 0.111nm. An annular dark-field STEM image of Si[110] showed lattice fringes of (111) and (22¯0) planes corresponding to lattice distances of 0.314nm and 0.192nm, respectively. At an accelerating voltage of 15kV, the developed low-voltage microscope achieved atomic-resolution imaging with a small chromatic aberration and a large uniform phase.

Enhanced cold field emission of large-area arrays of vertically aligned ZnO-nanotapers via sharpening: experiment and theory.

Large-area arrays of vertically aligned ZnO-nanotapers with tailored taper angle and height are electrodeposited on planar Zn-plate via continuously tuning the Zn(NH3)4(NO3)2 concentration in the electrolyte. Experimental measurements reveal that the field-emission performance of the ZnO-nanotaper arrays is enhanced with the sharpness and height of the ZnO-nanotapers. Theoretically, the ZnO-nanotaper is simplified to a “charge disc” model, based on which the characteristic macroscopic field enhancement factor (γC) is quantified. The theoretically calculated γC values are in good agreement with the experimental ones measured from arrays of ZnO-nanotapers with a series of geometrical parameters. The ZnO-nanotaper arrays have promising potentials in field-emission. The electrochemical synthetic strategy we developed may be extended to nanotaper arrays of other materials that are amenable to electrodeposition, and the “charge disc” model can be used for quasi-one-dimensional field emitters of other materials with nano-sized diameters.