STEM-based mass spectroscopy of supported Re clusters

Annular dark-field imaging in a scanning transmission electron microscope (STEM) is known to enhance contrast of small supported particles of heavy elements. At high scattering angles (∼ 100 mrad), this imaging technique can be used quantitatively to measure absolute particle size by comparing measured and theoretical elastic scattering cross-sections. Here, we report quantitative size measurement of small rhenium particles supported on thin graphite films. Statistical measurement of absolute cross-sections of prepared Re-6 clusters show good agreement with theoretical cross-sections, within a ±2 atom random experimental error. This is the first report of such measurements using angles sufficiently high that coherent effects can be neglected. Our experiments confirm the exceptional stability of the Re-6 organometallic compound relative to other compounds with 7, and 8-Re atoms. Details of the high angle annular dark-field STEM experiments are presented including image processing/analysis, annular dark-field detector calibration, and sources of error in the absolute intensity measurements. We discuss the future possibility of individual atomic sensititity.