Chemical vapor deposition of zirconium oxide on aerosolized silicon nanoparticles

Nanoscale shells of ZrO2 were deposited onto size- selected, aerosolized Si nanoparticles by chemical vapor deposition ( CVD). The CVD precursor was nitronium pentanitratozirconate ( ZN, [ NO2][ Zr( NO3) 5]), which has been used previously to deposit ZrO2 films on silicon wafers. Silicon nanocrystals were synthesized from silane in a low- pressure nonthermal plasma and were directly extracted from the plasma growth chamber into an atmospheric- pressure aerosol flow tube reactor. The particle streams flowed first through a preheating furnace, which could be set between room temperature and 1000 degrees C, and then through a differential mobility diameter ( DMA) that was set to transmit particles having a mobility diameter of 12 nm. After size selection, ZN was mixed into the carrier gas stream, and the resulting nanocrystal/ ZN/ carrier gas mixtures passed through a heated reaction zone ( T approximate to 100 degrees C), where deposition took place. The residence time in the reaction zone was approximately 8 s. The extent of deposition was determined by measuring the size distribution function of the postreaction aerosol with a second DMA. Particles were also analyzed by transmission electron microscopy ( TEM). The measured changes in peak particle mobility diameter imply that at low- to- moderate ZN partial pressures (< 100 Pa), the deposition rate is first- order in ZN partial pressure. At higher partial pressures, the rate increases more slowly with increasing ZN pressure. Possible reasons for these behaviors are discussed. The deposition rate increases with particle preheating temperature; the rate of particles that were preheated to 500 C is roughly twice that of particles that are not preheated. This behavior is attributed to desorption of hydrogen from the particle surface leading to a more reactive substrate for CVD. We believe that these results are among the first in which CVD is used to coat size- selected, aerosolized nanoparticles and also among the first in which tandem differential mobility analysis ( TDMA) is used to measure the rate of a CVD reaction.