| AVS 66th International Symposium & Exhibition | |
| Plasma Science and Technology Division | Tuesday Sessions |
| Session PS-TuM |
| Session: | Plasma Diagnostics and Sources I |
| Presenter: | Xiaoshuang Chen, University of Minnesota, Minneapolis |
| Authors: | X. Chen, University of Minnesota, Minneapolis S. Ghosh, University of Minnesota, Minneapolis D. Buckley, University of Minnesota, Minneapolis R.M. Sankaran, Case Western Reserve University T. Seto, Kanazawa University, Japan U.R. Kortshagen, University of Minnesota, Minneapolis C.J. Hogan, University of Minnesota, Minneapolis |
| Correspondent: | Click to Email |
Non-thermal plasmas (NTPs) have been shown to be capable of producing chemically-pure, size-controlled, low polydispersity, crystalline nanoparticles (NPs). Vapor-phase precursors are dissociated in the NTP reactor, typically by high electron energy impact, leading to nucleation and NP growth in the gas phase. A prevailing thought is that the NPs are unipolarly charged negative in the plasma, which mitigates coagulation and promotes surface growth, yielding low polydispersity NPs. In this study, we apply ion mobility spectrometry (IMS) as an online diagnostic to NTPs to address particle charging and coagulation.
In the first part of the study, we present an IMS system developed for low pressure to characterize Si nanocrystals synthesized in a radio-frequency (RF), capacitively-coupled NTP operating at 2 Torr. A uniquely designed low-pressure differential mobility analyzer (LPDMA) coupled with an electrical detector was utilized to measure NP size distributions in real time. Via the Twomey-Markowsi inversion approach, we have demonstrated, for the first time, that DMAs can be utilized to analyze NPs synthesized in low-pressure NTPs. Excellent agreement was found between the size distributions measured and inverted by LPDMA system and inferred from TEM images. Importantly, we found that at the outlet of the NTP flow tube reactor, Si NPs are bipolarly charged with nearly identical size distribution functions for both negatively and positively charged NPs. Furthermore, NPs are modestly aggregated, implying that the decharging of NPs exiting the plasma reactor from highly negative charge states to a bipolar charge distribution likely drives aggregation on the plasma boundary.
In the second part, ion-mobility mass-spectrometry (IM-MS) method was implemented to study the morphology of as-synthesized carbon-coated Ni NPs generated in an atmospheric-pressure DC microplasma. Sequentially, NPs were sampled by a DMA that classifies NPs by their mobilities, and an aerosol particle mass analyzer (APM), which separates NPs by masses. The concentration of size-mass classified NPs was measured by a condensation particle counter (CPC) downstream of the DMA-APM system, yielding two-dimensional (2D) size-mass distribution function. The shape and location of the sampled NPs on the 2D contour plot reveal their morphologies and extent of aggregation. Utilizing fractal theory to describe particle mobility, particle morphologies were described quantitatively. Our results demonstrate that Ni NPs leaving the plasma reactor are aggregated in chain-like, low fractal dimension aggregates, which are also verified by TEM images.