New techniques for electron microscopy characterization of small metal particles

This chapter extends various studies to the case of Pt supported in γ-Al₂O₃ with the particles being in a very highly dispersed state. A new technique is described that allows the observation of small metal clusters down to a diameter of 5 Å. This technique demonstrates that weak beam dark field images can yield valuable information on the particle shape. This technique can be very valuable in obtaining size distributions that can be correlated with chemisorption data. By combining this technique with thickness fringes and dynamical theory of diffraction calculations, the chapter demonstrates that the particle shape correspond to that described as “rafts” in but is around three layers thick. The method described requires the use of a high resolution goniometer stage and low contamination rate to allow a complete tilting sequence to be made. The stability of the microscope to produce long exposure pictures is an additional requirement.