We investigate the atomistic details of a single atom-extraction process realized by using the scanning tunneling microscope tip-cluster interaction on a Ag(111) surface at 6 K. Single atoms are extracted from a silver cluster one atom at a time using small tunneling biases less than 35 mV. Combined total energy calculations and molecular dynamics simulations show a lowering of the atom-extraction barrier upon approaching the tip to the cluster. Thus, a mere tuning of the proximity between the tip and the cluster governs the extraction process. The atomic precision and reproducibility of this procedure are demonstrated by repeatedly extracting single atoms from a silver cluster on an atom-by-atom basis.

No abstract

No abstract

Model thermionic cathodes with different oxide coatings have been surveyed using a combination of photoelectron emission microscopy and thermionic electron emission microscopy. The model cathodes are imaged directly at operating temperatures and the electron yield is measured with a Faraday cup located in the image plane. Cathodes with combinations of Ba, Sc, BaO, Sc2O3, and pulsed-laser-deposited Sc2O3 on W substrates have been examined. BaO deposited on Sc2O3 is the most stable cathode combination. Measurement of the W–Sc2O3–BaO cathode shows that the electron yield enhancement produced by BaO and Sc2O3 is a two-step process which requires that the BaO be on top of the Sc2O3. BaO does not need to be in direct contact with the W substrate.