Non Contact Atomic Force Microscopy Investigation of Silicon Nanoparticles Deposited on HOPG

The aim of this thesis was to further our understanding on the surface dynamics of silicon nanoparticles deposited on Highly Oriented Pyrolytic Graphite (HOPG) using non-contact Atomic Force Microscopy (NC-AFM) with the longer term aim of developing practical applications. Silicon nanoparticles in liquid were produced using sputtering gas aggregation by co-depositing with water in an ultrahigh vacuum (UHV) environment (von Haeften 2009). Water was sprayed into the system with custom designed apparatus onto a cold target cooled with liquid Nitrogen (LN2) to form an ice matrix. The melt from this ice matrix was then collected and diluted with isopropanol (IPA) and drop cast onto HOPG. Analysis using X-ray Photoelecron Spectroscopy (XPS) confirms the existence of silicon in liquid suspension, and that the silicon exists in high oxidization states. AFM investigations suggest the minimum size of these nanoparticles to be 0.8 nm; however it was seen that the size of the nanoparticles shifts depending on which layer on the HOPG substrate they have been deposited on. Formation of the nanoparticle film layers was also observed when changing the dilution of the nanoparticle solution using IPA. The growth mode of the nanoparticles was determined to be the Stransky- Krastanov mode. NC-AFM measurements related to the surface dynamics of the nanoparticles revealed that silicon nanoparticles on HOPG surfaces show rich surface dynamics. The diffusion coefficient of many active nanoparticles undergoing Brownian-type motion was calculated. Besides this, the change in shape and size of very active circular nanostructures on the HOPG surface are detected and tracked. Strong evidence were found to suggest that the dynamical changes are related to the effect(s) of the AFM tip, due to induced impulsion upon nanoparticles. Many other examples related to the tip impulsion effect upon nanoparticles are presented. Self-assembly mechanisms of nanoparticles are observed and exemplified. In addition, evidence for the stability of the nanoparticles – demonstrated by example – is presented