Time-Resolved High-Resolution Electron Microscopy of Clusters, Surfaces, and Interfaces
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electron-microscopy (TRHREM) system is composed of a 200-kV HREM (JEOL: JEM-2010) equipped with a highsensitivity SIT-TV camera, various types of dedicated sample holders, an ultrahighvacuum (UHV) sample-preparation chamber, and a digital videotape recorder, as partly illustrated in Figure 5. The HREM images at 0.8-1.0-million magnification are recorded on the videotape recorder. In the U.S.-Japan TV system, one "frame image" of 1/30-s time resolution is composed of two interlaced "field images" of 1/60-s time resolution. The digital videotape recorder enables us to output the two "field images" separately from two frame memories.6 In this article, we will review the recent development of direct observation of the dynamic phenomena for nanophase materials such as clusters, surfaces, and interfaces using TRHREM.
Direct Observation of the Surface-Diffusion Process The surface-diffusion process of atoms and clusters is one of the important factors for understanding various types of surface-related phenomena such as thinfilm growth and catalysis activity. The diffusion process has been studied as a spatially and temporally averaged flow of atoms over a few millimeters and seconds. Development of field ion microscopy (FIM) was a great breakthrough for
Figure 1. Time-sequential series of high-resolution-electron -microscopy (HREM) images of a tungsten atom diffusing along a step on a MgO(001) surface (a-c) and illustration of the step structure (d). The numbers at the upper right show the duration times.
MRS BULLETIN/AUGUST 1997
Time-Resolved High-Resolution Electron Microscopy of Clusters, Surfaces, and Interfaces
Figure 2. Time-sequential series of HREM images for metal-mediated crystallization of an amorphous germanium film by small gold particles.
the study of surface diffusion at the atomic level.7 Recent development of scanning tunneling microscopy (STM) also enables us to observe directly the movement of atoms on semiconductor surfaces.8'9 The time resolution of ordinary STMs without the tracking technique10 is however not sufficient for studying each jump process. In the present study, we have traced the diffusion process of tungsten atoms on MgO(OOl) surfaces with a time resolution of 1/60 s using TRHREM.61112 The W/ MgO sample was prepared in a UHV sample transfer chamber13 by vacuum deposition of W on a MgO single-crystal film.6 Figures la, lb, and lc show a timesequential series of electron-microscopy images recorded with 1/60-s resolution. Black dots indicated by arrows (w) correspond to a single W atom diffusing along a step on a MgO(OOl) surface at a 218°C substrate temperature as illustrated in Figure Id. Clear observation of the atoms is possible only for a suitable MgO thickness of 18 nm and under off-Bragg diffraction conditions.1415 Detailed inspection of the 1/60-s time-resolved images has given us the following insights: (1) Diffusing W atoms do not prefer any specified atomic sites—for example, Mg sites or O sites of the MgO(001) surface. (2) The diffusion process is sufficiently slow that w
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