High-endurance micro-engineered LaB6 nanowire electron source for high-resolution electron microscopy

Exploring the Potential of a Thermionic LaB6 Virtual Source Mode Electron Gun for a High Angular Current Density and a Narrow Energy Distribution

To date, lanthanum hexaboride (LaB6) thermionic electron sources have not been able fully to capitalize on their inherent potential, resulting in an ambiguous position within the application area. Although they exhibit higher brightness compared with a tungsten filament source, they still fall short of the performance of Schottky electron sources. This study aims to explore the capabilities of the LaB6 electron source under different operating conditions to bridge the gap, ultimately to realize its untapped potential. Simulations in virtual source mode indicated enhanced beam brightness and a reduced beam half-angle with an increase the extraction voltage, promising up to tenfold times higher beam brightness compared with the crossover mode. The energy distribution measured using a prelens retarding field energy analyzer revealed an energy distribution of 0.55 eV and a high angular current density of 33 mA/sr in the virtual source mode. Therefore, the virtual source mode of LaB6 can provide a narrow energy distribution akin to that of a ZrO/W Schottky electron gun (1600 K) while having an angular current density over 2,000 times higher. In addition, the stability of the virtual source mode is ±0.022%, while that of the crossover mode is ±0.138%.

Bright and ultrafast electron point source made of LaB6 nanotip

The development of time-resolved transmission electron microscopy (TEM), ultrafast electron spectroscopy and pulsed X-ray sources relies on the realization of stable and high brightness sources of ultra-short electron bunches with a long service time. The flat photocathodes implanted in thermionic electron guns have been replaced by Schottky-type or cold-field emission sources driven by ultra-fast laser. Recently, lanthanum hexaboride (LaB₆) nanoneedles have been reported to have high brightness and high emission stability when working in a continuous emission mode. Here, we prepare nano-field emitters from bulk LaB₆ and we report on their use as ultra-fast electron sources. Using a high repetition rate laser in the infrared range, we present different field emission regimes as a function of the extraction voltage and laser intensity. The properties of the electron source (brightness, stability, energy spectrum and emission pattern) are determined for the different regimes. Our results show that LaB₆ nanoneedles can be used as ultrafast and ultra-bright sources for time-resolved TEM, with better performances as compared to metallic ultra-fast field-emitters.

Near-Monochromatic Tuneable Cryogenic Niobium Electron Field Emitter

Creating, manipulating, and detecting coherent electrons is at the heart of future quantum microscopy and spectroscopy technologies. Leveraging and specifically altering the quantum features of an electron beam source at low temperatures can enhance its emission properties. Here, we describe electron field emission from a monocrystalline, superconducting niobium nanotip at a temperature of 5.9 K. The emitted electron energy spectrum reveals an ultranarrow distribution down to 16 meV due to tunable resonant tunneling field emission via localized band states at a nanoprotrusion's apex and a cutoff at the sharp low-temperature Fermi edge. This is an order of magnitude lower than for conventional field emission electron sources. The self-focusing geometry of the tip leads to emission in an angle of 3.7°, a reduced brightness of 3.8×10^{8}  A/(m^{2} sr V), and a stability of hours at 4.1 nA beam current and 69 meV energy width. This source will decrease the impact of lens aberration and enable new modes in low-energy electron microscopy, electron energy loss spectroscopy, and high-resolution vibrational spectroscopy.

High-Resolution Electron Tomography of Ultrathin Boerdijk-Coxeter-Bernal Nanowire Enabled by Superthin Metal Surface Coating

The rapid advancement of transmission electron microscopy has resulted in revolutions in a variety of fields, including physics, chemistry, and materials science. With single-atom resolution, 3D information of each atom in nanoparticles is revealed, while 4D electron tomography is shown to capture the atomic structural kinetics in metal nanoparticles after phase transformation. Quantitative measurements of physical and chemical properties such as chemical coordination, defects, dislocation, and local strain have been made. However, due to the incompatibility of high dose rate with other ultrathin morphologies, such as nanowires, atomic electron tomography has been primarily limited to quasi-spherical nanoparticles. Herein, the 3D atomic structure of a complex core-shell nanowire composed of an ultrathin Boerdijk-Coxeter-Bernal (BCB) core nanowire and a noble metal thin layer shell deposited on the BCB nanowire surface is discovered. Furthermore, it is demonstrated that a new superthin noble metal layer deposition on an ultrathin BCB nanowire could mitigate electron beam damage using an in situ transmission electron microscope and atomic resolution electron tomography. The colloidal coating method developed for electron tomography can be broadly applied to protect the ultrathin nanomaterials from electron beam damage, benefiting both the advanced material characterizations and enabling fundamental in situ mechanistic studies.