ULTIMATE OBSERVATION OF TUNGSTEN ATOMS AND CLUSTERS ADSORBED ON SINGLE-CRYSTALLINE MGO FILMS

Tungsten single atoms and clusters composed of four and five atoms dep osited on MgO (001) thin films were observed by high-resolution transm ission electron microscopy at R.T. The observation was realized by opt imizing the thickness of the MgO films and by using special imaging te chniques such as an off-Bragg HREM method. The multislice simulation f or the interpretation of the image contrast showed a possibility of di scrimination 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 w ith the f.c.c. configuration. The reason for the difference was clarif ied through the multislice image simulation, suggesting that the latti ce mismatch between the clusters and the MgO lattice was a key factor in determining the intensity of the center of the clusters. (C) 1988 W iley-Liss, Inc.