Ultimate observation of tungsten atoms and clusters adsorbed on single crystalline MgO films.

Tungsten single atoms and clusters composed of four and five atoms deposited on MgO (001) thin films were observed by high-resolution transmission electron microscopy at R.T. The observation was realized by optimizing the thickness of the MgO films and by using special imaging techniques such as an off-Bragg HREM method. The multislice simulation for the interpretation of the image contrast showed a possibility of discrimination of the atomic configuration of clusters, such as "b.c.c.," "on-top," or "f.c.c." type clusters, from details of the image-contrast. The image intensity at the center of the clusters with the b.c.c. configuration was evidently smaller than that of the clusters with the f.c.c. configuration. The reason for the difference was clarified through the multislice image simulation, suggesting that the lattice mismatch between the clusters and the MgO lattice was a key factor in determining the intensity of the center of the clusters.