Evelyne Salançon

In this article, iridium tips produced by electrochemical etching are evaluated for use as field emitters in electron sources for electron microscopy. They prove to have a much larger radius of curvature than predicted by Fowler–Nordheim curves for field emission. The source’s brightness is estimated at B > 1012 A m−2 sr−1 by projection microscopy. The position of the emitting region appears stable, although the current itself may be affected when pressure exceeds 10−9 mbar. The robustness of iridium tips is demonstrated by their ability to withstand such high pressures without significant optical or electrical changes.

We investigated the pressure dependence of the gas flow and the field ion intensity of a coaxial ion source operating at room temperature over a wide pressure range, testing various gases and ionization voltages. Flow conductance measurements taking into account the different gases’ viscosity and molecular mass consistently exhibit a generic pattern. Three different flow regimes appear with increasing upstream pressure. Since the coaxial ion source supplies the gas locally, very near the apex of the tip where ionization occurs, large ionization currents can be obtained without degrading the propagation conditions of the beam. Compared with field ionization in a partial pressure chamber, using the coaxial ion source increases the ion current a hundredfold for the same residual low pressure. We also show that the gas flow regime does not impact the ionization yield. Although a fuller characterization remains to be performed, brightness reaches 3×1011 A/m2/sr at 12 kV extracting voltage.

The emission of electrons from an insulating crystal deposited on a conductor occurs at a macroscopic electric field of a few volts per micrometer, 3 orders of magnitude below the field emission from a clean metal. This is due to the local field enhancement induced by the presence of the insulating crystal. The emission profiles depend on the shape of the conductive substrate; analyzing these profiles enables the local difference in electric potential and the opening angle to be traced. Given the thickness of the crystal, the local difference in potential indicates the local field enhancement of a few volts per nanometer applied to the conductor.

The development of bright sources is allowing technological breakthroughs, especially in the field of microscopy. This requires a very advanced control and understanding of the emission mechanisms. For bright electron sources, a projection microscope with a field emission tip provides an interference image that corresponds to a holographic recording. Image reconstruction can be performed digitally to form a “real” image of the object. However, interference images can only be obtained with a bright source that is small: often, an ultra-thin tip of tungsten whose radius of curvature is of the order of 10nm. The contrast and ultimate resolution of this image-projecting microscope depend only on the size of the apparent source. Thus, a projection microscope can be used to characterize source brightness: for example, analyzing the interference contrast enables the size of the source to be estimated. Ultra-thin W tips are not the only way to obtain bright sources: field emission can also be achieved by applying voltages leading to a weak macroscopic electric field (< 1V∕μm) to insulating micron crystals deposited on conductors with a large radius of curvature (> 10 μm). Moreover, analyzing the holograms reveals the source size, and the brightness of these new emitters equals that of traditional field emission sources.

The electron celadonite source described here performs well in a low-energy electron point-source projection microscope in long-range imaging. It presents major advantages compared to sharp metal tips. Its robustness affords a lifetime of months and it can be used under relatively high pressure. The celadonite crystal is deposited at the apex of a carbon fiber, maintained itself in a coaxial structure ensuring a spherical beam shape and easy mechanical positioning to align the source, the object and the electron-optical system axis. There is a single crystal deposition via generation of celadonite-containing water droplets with a micropipette. Scanning electron microscopy observation can be performed to verify the deposition. However, this adds steps and therefore increases the risk of damaging the source. Thus, after preparation, the source is usually inserted directly under vacuum in the projection microscope. A first high voltage supply provides the kick-off needed to start the electron emission. The field emission process involved is then measured: it has already been observed for dozens of electron sources prepared in this way. The brightness is under-estimated through an over-estimation of source size, intensity at one energy and cone angle measured in a projection system.

This paper points out the strong didactic potential of an experiment originally intended for electron source research. Using a dual stage micro-channel plate in a low-energy electron point projection microscope, electrons are spatially detected one by one. Field electron emission affords wide-ranging intensity, enabling the electron charge to be determined by comparing counting and analog measurements. The same setup is used to illustrate the buildup of an interference pattern from separate low-energy electron detection, thereby enabling the Planck constant to be determined. Both granular and continuous signals are directly observable in this educational approach.

Following the success of the pedagogic tool for monochromatic holography realized for popular science purposes a few years ago (Voslion and Escarguel 2012 Eur. J. Phys . 33 1803), an upgrade has been developed. The resulting device includes all the necessary equipment to produce far more spectacular 4″ × 5″ color holograms for science outreach purposes. Graduate schools are also offered several experiments such as holographic diffraction gratings. Optimized to be extremely robust to vibrations, particularly by using the weight of the object to avoid unwanted vibrations during hologram exposure, the robustness of this device is demonstrated through optical microscopy measurements.

Using nanopipettes with very small apertures (<10 nm) is a good way to improve the spatial resolution in scanning conductance experiments, to monitor single-molecule delivery and to strain long molecules stretching during translocation. However, such nanopipettes can be difficult to fill. Here we describe a dynamic microdistillation technique that successfully fills all nanopipettes, whatever their shape or tip radius. Even elongated or bent nanopipettes with a small-angle tip are completely filled using this new technique. The nanopipettes are first filled with pure water, which is later replaced with the desired electrolyte via electromigration. Electrical measurements are used to check that filling is complete.