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|Title:||Quantitative electron microscopy of supported subnanometer clusters|
|Doctoral Committee Chair(s):||Gibson, J. Murray|
|Department / Program:||Materials Science and Engineering|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
Engineering, Materials Science
|Abstract:||Small supported particles (a few angstroms in diameter) are useful as catalysts because of their increased surface area for a given volume of material. Their catalytic activity and selectivity depends upon their size, which determines their electronic structure. Therefore, it is vital to reliably understand particle size distribution, and variation, which occurs due to fragmentation and sintering. In this research, I present a refined electron microscopy approach to study particle size distributions and use this to investigate the stability of very small cluster rhenium supported on graphite.
'Z-contrast' microscopy using a scanning transmission electron microscope (STEM) has had significant influence on the study of ultra-small supported particles. However, the importance of suppressing longitudinal coherence was not fully recognized until recently. Therefore, the Z-contrast (atomic number) method remained less quantitative. Present work fills this gap, by employing very high angles of scattering in annular dark-field STEM imaging of small rhenium particles. At high scattering angles, intra-columnar diffraction, from the atoms in a crystal, is suppressed significantly. This ensures a linear rise of the scattered intensity with the number of atoms in a given metal particle.
Absolute measurements of elastically scattered electron intensity were performed for graphite supported rhenium clusters comprising less than ten metal atoms. Measured cross-sections show good agreement with theoretically calculated values. We use the method to demonstrate exceptional stability of the 6-Re organometallic clusters over other sizes, such as 8-Re particles. These measurements require careful STEM detector calibration. In addition, attention must be paid to preparation of well-dispersed particles suitable for statistical analysis.
|Rights Information:||Copyright 1996 Singhal, Ajay|
|Date Available in IDEALS:||2011-05-07|
|Identifier in Online Catalog:||AAI9712437|
This item appears in the following Collection(s)
Graduate Dissertations and Theses at Illinois
Graduate Theses and Dissertations at Illinois
Dissertations and Theses - Materials Science and Engineering