  Si-SiO2 multilayer nanocomposite (NCp) films, grown using pulsed laser
deposition with varying Si deposition time are investigated using Raman
spectroscopy/mapping for studying the variation of Si phonon frequency observed
in these NCps. The lower frequency (LF) phonons (~ 495 - 510 cm-1) and higher
frequency (HF) phonons (~ 515 - 519 cm-1) observed in Raman mapping data (Fig.
1A) in all samples studied are attributed to have originated from surface
(Si-SiO2 interface) and core of Si nanocrystals, respectively. The consistent
picture of this understanding is developed using Raman spectroscopy monitored
laser heating/annealing and cooling (LHC) experiment at the site of a desired
frequency chosen with the help of Raman mapping, which brings out clear
difference between core and surface (interface) phonons of Si nanocrystals. In
order to further support our attribution of LF being surface (interface)
phonons, Raman spectra calculations for Si41 cluster with oxygen termination
are performed which shows strong Si phonon frequency at 512 cm-1 corresponding
to the surface Si atoms. This can be considered analogous to the observed
phonon frequencies in the range 495 - 510 cm-1 originating at the Si-SiO2
interface (extended). These results along with XPS data show that nature of
interface (oxygen bonding) in turn depends on the size of nanocrystals and thus
LF phonons originate at the surface of smaller Si nanocrystals. The
understanding developed can be extended to explain large variation observed in
Si phonon frequencies of Si-SiO2 nanocomposites reported in the literature,
especially lower frequencies.
