High-resolution transmission electron microscopy study on the anomalous structure of lead nanoparticles with UHV-MBE-TEM system

The structure of lead nanoparticles ranging from 2 to 10 nm in diameter was studied with high-resolution transmission electron microscopy (HRTEM). Lead nanoparticles were deposited on a silicon substrate at room temperature and observed without exposure to the air. In addition to face-centered cubic (fcc), we have found a body-centered tetragonal (bct) structure that has a space group of I4/mmm with lattice parameters a and c being about 0.33 nm and 0.62 nm, respectively. This bct structure existed in not only the single crystals and single-twinned particles, but also multiply-twinned particles (MTPs). It was found that coexistence of the fcc and bct units forms a perfect MTP structure with fivefold axis. MTPs with sixfold axis were also observed, which is explained properly by the combination of bct structure units with different orientations.