Applications of In-Situ UHV and High Resolution Tem to the Study of Small Particles
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APPLICATIONS OF IN-SITU UHV AND HIGH RESOLUTION TEM TO THE STUDY OF SMALL PARTICLES M. AVALOS-BORJA,t** D. SUt F. A. PONCE,* J. C. TRAMONTANA,* Q.-H. GUO,t K.HEINEMANN,tt H. POPPAt and M. BOUDARTt tDepartment of Chemical Engineering, Stanford University, Stanford, CA 94305 *Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 ttEloret Institute, 1178 Maraschino Drive, Sunnyvale, CA 94087 ABSTRACT In-situ electron microscopy is a powerful tool for the study of small particles. Since most of the interesting phenomena take place in particles smaller than -5 nm, high resolution is highly desirable. In-situ and high resolution conditions are difficult to achieve in a single instrument. We have combined the in-situ UHV capabilities of a modified microscope at Stanford University with the high resolution capabilities of a 200 kV and a UHV-400 kV microscopes at Xerox PARC. Examples are presented, pointing out the advantages of in-situ deposition and treatment, and post deposition ex-situ observation at atomic resolution. Advantages and limitations of this approach are discussed. INTRODUCTION Small particles, especially those smaller than 5nm, are fascinating subjects of research, because they possess unique physical and chemical properties not found in bulk crystals or single atoms. Even though many different approaches can be used to study small particles, transmission electron microscopy for direct imaging is highly desirable. An ideal microscope should allow the researcher to follow not only changes in sizes and number density of particles, but should also provide detailed structural information regarding morphology and defect structure following different preparation conditions and treatments. Such ideal experimental system should satisfy some basic requirements: clean environment for preparation and treatment, capability for real time observations, and adequate resolution. In-situ TEM has been used successfully to investigate problems such as nucleation and growth [1-3] and catalytic reactions [4,5]. Instrumental limitations are primarily associated with the control of the environment. It is not sufficient to be able to perform experiments in real time, cleanliness is of utmost importance. Trace amounts of contaminants can influence, for example, the equilibrium shape of small particles [6-8]. The near-atomic resolution of present day microscopes allows the study of the orientation, crystallography, and the shape of particles [9]. The ability to identify very small particles supported on substrate films is especially important in the study of the early stages of nucleation and growth [10]. Questions such as whether twinning is a nucleation or a growth phenomenon are examples of the necessity of atomic resolution for the study of the early stages of nucleation and growth. Our group has been involved in the study of small particles for many years, and what follows is a discussion of our recent efforts in trying to use a combination of different electron microscopy systems to elucidate some of the
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