Quantitative electron microscopy of supported subnanometer clusters
Singhal, Ajay
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https://hdl.handle.net/2142/21300
Description
Title
Quantitative electron microscopy of supported subnanometer clusters
Author(s)
Singhal, Ajay
Issue Date
1996
Doctoral Committee Chair(s)
Gibson, J. Murray
Department of Study
Materials Science and Engineering
Discipline
Materials Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Engineering, Materials Science
Language
eng
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.
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