Electron Microscope Gun Research Articles

An ultrafast electron microscope gun driven by two-photon photoemission from a nanotip cathode

We experimentally and numerically investigate the performance of an advanced ultrafast electron source, based on two-photon photoemission from a tungsten needle cathode incorporated in an electron microscope gun geometry. Emission properties are characterized as a function of the electrostatic gun settings, and operating conditions leading to laser-triggered electron beams of very low emittance (below 20 nm mrad) are identified. The results highlight the excellent suitability of optically driven nano-cathodes for the further development of ultrafast transmission electron microscopy.

Nanocarbon hybrids of graphene-based materials and ultradispersed diamond: investigating structure and hierarchical defects evolution with electron-beam irradiation

We report the influence of electron-beam (E-beam) irradiation on the structural and physical properties modification of monolayer graphene (Gr), reduced graphene oxide (rGO) and graphene oxide (GO) with ultradispersed diamond (UDD) forming novel hybrid composite ensembles. The films were subjected to a constant energy of 200 keV (40 nA over 100 nm region or electron flux of 3.9 × 1019 cm−2s−1) from a transmission electron microscope gun for 0 (pristine) to 20 min with an interval of 2.5 min continuously – such conditions resemble increased temperature and/or pressure regime, enabling a degree of structural fluidity. To assess the modifications induced by E-beam, the films were analyzed prior to and post-irradiation. We focus on the characterization of hierarchical defects evolution using in situ transmission electron microscopy combined with selected area electron diffraction, Raman spectroscopy (RS) and Raman mapping techniques. The experiments showed that the E-beam irradiation generates microscopic defects (most likely, interstitials and vacancies) in a hierarchical manner much below the amorphization threshold and hybrids stabilized with UDD becomes radiation resilient, elucidated through the intensity, bandwidth, and position variation in prominent RS signatures and mapping, revealing the defects density distribution. The graphene sheet edges start bending, shrinking, and generating gaps (holes) at ~10–12.5 min owing to E-beam surface sputtering and primary knock-on damage mechanisms that suffer catastrophic destruction at ~20 min. The microscopic point defects are stabilized by UDD for hybrids in the order of GO > rGO ≥ Gr besides geometric influence, i.e. the int erplay of curvature-induced (planar vs curved) energy dispersion/absorption effects. Furthermore, an attempt was made to identify the nature of defects (charged vs residual) through inter-defect distance (i.e. LD). The trends of LD for graphene-based hybrids with E-beam irradiation implies charged defects described in terms of dangling bonds in contrast to passivated residual or neutral defects. More importantly, they provided a contrasting comparison among variants of graphene and their hybrids with UDD. Copyright © 2015 John Wiley & Sons, Ltd.

Room temperature dc electrical conductivity studies of electron-beam irradiated carbon nanotubes

The influence of electron-beam (E-beam) irradiation on the electrical (electronic) properties of single- (SW) and multi-walled (MW) carbon nanotube grown by microwave chemical vapor deposition is investigated. These films were subjected to a constant energy of 50 keV (50 A/cm 2) from a scanning electron microscope gun for 2.5, 5.5, 8.0, and 15 h continuously — such conditions resemble increased temperature and/or pressure regime, enabling a degree of structural fluidity. To assess the structural modifications and electrical properties, the films were analyzed before and after irradiation. The experiments show that with increased exposure to ≥ 8–9 h, occasionally found individual bundles of single-wall nanotubes tend to collapse or pinch, graphitize/amorphize, and oxidize within the area of the electron-beam focus. Dramatic improvement in the I– V properties for single-walled (from semiconducting to quasi-metallic) and relatively small but systematic behavior for multi-walled with increasing exposure is discussed in terms of the critical role of controlled introduction of defects. The contact resistance decreases by orders of magnitude when exposed to electron beam and for all of the measurements the values ranged between 80 Ω and 10 kΩ at room temperature. These results also indicated that multi-walled nanotubes tend to reach a state of saturation degradation assessed by four-probe conductivity measurements. It is suggestive that there may be local gradual re-organization, i.e. sp 2 + δ , sp 3 C ⇔ sp 2 C. More importantly, they provided a contrasting comparison between metallic/semiconducting ( single/ double- wall) and invariably metallic ( multi- wall) carbon nanotubes.

Advances in Parallel-Detection EELS

Parallel-detection electron energy-loss spectrometers (PEELS) now routinely provide spectra in which the main source of noise is the statistical fluctuation in the number of arriving electrons (i.e., they have DQE > 0.5), achieve an energy resolution which is more than 90% limited by the electron microscope gun, and are fairly easy to operate. They have pushed the minimum detectable mass (MDM) obtainable by PEELS close to the single atom level, and have improved the minimum detectable mass fraction (MDF) so that it is now comparable or better than MDF detectable by EDXS even for elements as heavy as Fe. The attainable energy resolution is now 0.3-0.5 eV on a routine basis when the spectrometer is mounted on a cold field-emission gun (S)TEM operating at 100 kV (Fig. 1). This impressive progress has opened up an important question: where next?Our answer is three-fold: towards greater integration of EELS with other techniques of electron microscopy, towards new applications of the technique, and towards more quantitative and yet more user-friendly analysis of the results.

Optimisation of low-perveance electron gun by experimental and numerical methods

The paper presents a proposed method for improving the electron gun used for microscopes or similar electron beam devices. The electron gun has been divided into two zones: the high perveance cathode zone and the low perveance lens zone. The final virtual crossover of the gun is thus the image of the exit pupil of the cathode zone. This image is produced by the action of the lens zone. The basic assumption of this approach is that the electron brightness does not depend significantly on the lens zone electron-optical parameters. Based on this assumption the optimisation of the electrodes has been carried out. This has been done experimentally in the cathode zone and by means of numerical calculations in the lens zone. Results obtained for the scanning electron microscope gun are presented as an example of this approach.

The Mean Brightness of the Electron Microscope Gun

As the brightness obtained in the measurement is always the mean brightness averaged over a certain aperture angle and a certain area, it is not equal to the Langmuir's theoretical value except for a special operating condition of the electron gun and in general becomes lower than the theoretical value. The amount of decrease of the measured brightness depends on the sizes of the apertures used in the measurement and the electron optical characteristics of the electron gun. When the bias voltage is much reduced, the electron gun may be considered approximately as the plane, spherical or cylindrical diode system. From this point of view, the expressions for the mean brightness in these diode systems are derived both for thermionic and field emissions. It is also shown that these results will be able to apply to the calculation of the mean brightness of the triode electron gun.

Resistance bias characteristic of the electron microscope gun

It has been shown in a previous paper that the theoretical value of current density per unit solid angle (brightness) could be obtained from the hairpin cathode electron microscope gun for a wide range of geometrical design, provided an optimum bias voltage, which depended on the geometrical design, was applied between shield and cathode. When using a self-biasing arrangement, the total beam current tends to become stabilized by negative feed-back action. This is a valuable advantage but may lead to the impossibility of operating the gun at the optimum bias voltage. The present investigation shows that for a reasonably wide range of geometrical design, conditions can be established, with the correct choice of bias resistance, whereby the gun can be operated at theoretical brightness and with current stabilization, for a reasonable choice of filament temperature, giving a range for choice in beam angle or beam current.

Characteristics of the hot cathode electron microscope gun

Experiments have been carried out on hot cathode, high-voltage three-electrode electron guns. The brightness has been measured and found under optimum operating conditions to approach closely the value expected from the theoretical considerations developed by Langmuir. The effect of varying geometry over a fairly wide range is shown to be of secondary importance, except in as far as it influences the bias conditions and current efficiency. The mechanism of electron beam formation is discussed briefly.