AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Low Pressure Differential Mobility Analysis for Online Characterization of Silicon Nanocrystals Synthesized in a Nonthermal Plasma
XIAOSHUANG CHEN, Jasmine Johnson, Takafumi Seto, Uwe R. Kortshagen, Christopher Hogan Jr., University of Minnesota
Abstract Number: 240 Working Group: Nanoparticles and Materials Synthesis
Abstract Low pressure nonthermal plasmas are used extensively in the synthesis of semiconductor nanocrystals. However, online characterization of reactor output has previously been challenging, as such plasmas are typically operated below 20 Torr, and because the nanocrystals are produced at higher number concentrations than are encountered in atmospheric pressure, non-plasma synthesis systems. We have developed a system for online size distribution measurement of nonthermal plasma synthesized nanoparticles using a low pressure differential mobility analyzer (LPDMA) and faraday cage electrometer (FCE). LPDMA calibration is carried out with a tandem DMA technique with a conventional nanoDMA upstream of the LPDMA. A calibration coefficient is introduced, which is dependent upon the LPDMA dimensions, molecular mass of the sheath gas, and LPDMA temperature; this coefficient enables direct inference of the mobility selected by the LPDMA at any operating pressure. The minimum pressure in the LPDMA is bounded by the sheath and aerosol flow rate and at any pressure the accessible size/mobility range of the LPDMA is governed by the Paschen’s law.
Following calibration the LPDMA-FCE system has been used to examine the size distribution function of Silicon nanocrystals produced in an Ar-silane non-thermal plasma at operating pressures below 20 Torr. The number concentration of particles existing the plasma was modulated (reduced) using a metal mesh screen at the plasma outlet; concentration reduction mitigated the influences of post-plasma coagulation on the size distribution. With coagulation minimized, it was confirmed that the reactor produces low polydispersity, sub 10 nm diameter nanocrystals, with the nanocrystal size influenced by plasma reactor residence time.