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Si nanocrystals (NCs) are synthesizable in low-pressure plasmas with narrow diameter distributions because of unipolar negative charging in the plasma. However, NCs must pass into a spatial afterglow, where NCs may decharge and aggregate. We use both low-pressure differential mobility analysis and quantitative TEM image analysis to examine Si NCs sampled from a 2 Torr Ar-SiH4 plasma reactor. We find that Si NCs are incorporated into aggregates largely composed of 20 and fewer primary particles with nearly equal concentrations of negatively and positively charged NCs. The projected areas of aggregates scale with aggregate volume in similar manner to the scaling observed for aggregates from atmospheric pressure systems. Higher aggregate concentration is observed with increasing flow rate, while larger aggregate size is found with increasing precursor concentration. Measurements show that the spatial afterglow has a significant influence on the charge and extent of NC aggregation.
Bibliographical noteFunding Information:
The manufacture of the low pressure differential mobility analyzer was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013 . TEM characterization was carried out at the University of Minnesota Characterization facility, which is also supported by the University of Minnesota MRSEC. X.C. was additionally supported by Department of Energy Award SC0018202 .
© 2020 Elsevier B.V.
- Differential mobility analysis
- Nonthermal plasma
- Spatial afterglow
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