Direct identification of the charge state in a single platinum nanoparticle on titanium oxide.

A goal in the characterization of supported metal catalysts is to achieve particle-by-particle analysis of the charge state strongly correlated with the catalytic activity. Here, we demonstrate the direct identification of the charge state of individual platinum nanoparticles (NPs) supported on titanium dioxide using ultrahigh sensitivity and precision electron holography. Sophisticated phase-shift analysis for the part of the NPs protruding into the vacuum visualized slight potential changes around individual platinum NPs. The analysis revealed the number (only one to six electrons) and sense (positive or negative) of the charge per platinum NP. The underlying mechanism of platinum charging is explained by the work function differences between platinum and titanium dioxide (depending on the orientation relationship and lattice distortion) and by first-principles calculations in terms of the charge transfer processes.

Improved efficiency in automated acquisition of ultra-high-resolution electron holograms using automated target detection.

An automated hologram acquisition system for big-data analysis and for improving the statistical precision of phase analysis has been upgraded with automated particle detection technology. The coordinates of objects in low-magnification images are automatically detected using zero-mean normalized cross-correlation with preselected reference images. In contrast with the conventional scanning acquisitions from the whole area of a microgrid and/or a thin specimen, the new method allows efficient data collections only from the desired fields of view including the particles. The acquisition time of the cubic/triangular nanoparticles that were observed was shortened by about one-fifty eighth that of the conventional scanning acquisition method because of efficient data collections. The developed technology can improve statistical precision in electron holography with shorter acquisition time and is applicable to the analysis of electromagnetic fields for various kinds of nanoparticles.

Development of angle-resolved spectroscopy system of electrons emitted from a surface with negative electron affinity state.

We developed an angle-resolved photoemission spectroscopy system for the analysis of conduction-band electrons. By forming a negative electron affinity surface on a semiconductor surface, electrons in conduction bands are emitted into a vacuum and measured by using an analyzer. This method enables us to determine the energy and momentum of the conduction electrons. Furthermore, it can be used to determine unoccupied conduction band structures. The main challenges of this method are that the energies of the emitted electrons are extremely low and the trajectories of the electrons change due to various influences. We overcame these problems by placing the shielding mesh close to the sample and parallel to the sample surface. The entire chambers, including the shielding mesh, were grounded, and a negative bias voltage was applied only to the sample. This configuration realizes the acceleration of electrons while preserving the momentum component parallel to the sample surface. Another problem is the establishment of a method for converting a detected angle into the corresponding wavevector. We focused on the emission angle of electrons emitted from a sample and their minimum energy and then established an analytical method for converting detected angles into corresponding wavevectors on the basis of the minimum energy.