Synthesis and Surface Modification of Group IV Nanoparticles Using Non-thermal Plasmas

Doctor of Philosophy, Graduate Program in Mechanical Engineering
University of California, Riverside, June 2015
Dr. Lorenzo Mangolini, Chairperson

The rapidly increasing interest in silicon nanostructures is motivated by important advantages of this material   compared   to   other  semiconductors   commonly   investigated   in  the   broad   field   of nanotechnology.  Silicon  nanoparticles  are  promising  materials  for  many  applications  such  as photovoltaics, transistors, light emitting devices, and energy storage devices.

Commonly  used  nanoparticle  synthesis  techniques  have  many  challenges  such  as  high  cost,  long processing  time,  and  wide  particle  size  distribution.  In  this  dissertation,  non-thermal  plasma technique  is  used  to  overcome  these  challenges.  In  this  study,  in  order  to  produce  high  quality nanoparticles, the direct comparison of the use of a chlorinated and hydrogenated precursor and its consequences on both the process parameters and material properties are initially investigated. The analysis results show that the chlorinated precursor yields nanoparticles vulnerable against oxidation in  air  compared  to  the  hydrogenated  precursor.  In  addition,  it  is  found  that  in  the  chlorinated precursor  case  the  gas  composition  needs  to  be  modified  and  hydrogen  needs  to  be  added  to  the mixture to enable the nucleation and growth of the powder. Silicon nanocrystals with sizes between 5
and 10 nm have been produced in a non-thermal plasma reactor using chlorinated precursor. 

The properties of the silicon nanoparticles can be tuned through post-processing steps to optimize for targeted  applications.  In  particular  surface  modification  is  generally  necessary  to  both  tune dispersibility  of  the  particles  in  desired  solvents  to  achieve  optimal  coating  conditions,  and  to interface the particles with other materials to realize functional heterostructures. In this contribution a non-thermal plasma-based process for the synthesis of silicon nanoparticles and their in-flight coating with a plasma polymerized shell (silicon/polymer core/shell) has been developed. It is found that it is possible  to  tune  the  chemistry  of  the  shell  by  modifying  the  gas-phase  composition  during  the polymerization step.

These nanoparticles are used as an anode material for lithium-ion batteries. The coating of the silicon particle  with  a  polymer  shell  offers  a  way  to  uniformly  disperse  the  particles  into  a  carbon  matrix after  high-temperature  treatment,  which  provides  an  improvement  in  the  stability  of  an  anode  for lithium-ion batteries, compared to the case of uncoated silicon particles.