Ion Mobility Spectrometry for the Characterization of a Low Pressure Nanocrystal Synthesis System
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). 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, we can confirm that the reactor produces low polydispersity, sub 10 nm diameter nanocrystals, with the nanocrystal size influenced by plasma reactor residence time.