Correlation of size and oxygen bonding at the interface of Si nanocrystal in Si-SiO2 nanocomposite: A Raman mapping study

TL;DR

This study uses Raman spectroscopy and modeling to analyze how the size of silicon nanocrystals influences oxygen bonding at their interfaces in Si-SiO2 nanocomposites, revealing size-dependent surface phonon characteristics.

Contribution

It provides new insights into the correlation between nanocrystal size, interface oxygen bonding, and phonon behavior in Si-SiO2 nanocomposites using combined experimental and theoretical approaches.

Findings

LF phonons originate from surface/interface regions of Si nanocrystals.

Surface phonon frequency depends on nanocrystal size and oxygen bonding.

Raman modeling supports experimental attribution of phonon origins.

Abstract

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.