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

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.

One step growth of PVP spheres embedded with nearly monodispersive CdS nanocrystals using chemical bath deposition

We have used simple chemical bath deposition technique to grow nearly monodispersive CdS nanocrystals in PVP matrix. Systematic study of variation of growth parameters has revealed that optimized growth of CdS nanocrystals in PVP matrix depends on relative concentration of Cd acetate/Thiourea to polyvinyl pyrrolidone in the bath. It is also observed that higher concentration (1M) of Cd acetate/Thiourea gives rise to smaller NCs compared to lower concentration (0.5M), however density of particles is large in thin film grown using 1M concentration. Scanning electron microscopic studies show that it is a nanoparticulate film of spheres of size around 100-200nm. Further, absorption, energy dispersive spectroscopy and transmission electron microscopic investigations reveal that nearly monodispersive CdS nanocrystals are embedded in 100-200 nm PVP spheres for the range 0.5 M, 1M Cd acetate/Thiourea concentration (figure 1). The effect of varying PVP, Cd acetate/Thiourea concentration, sequence and addition of ingredients and heating/cooling cycles have been studied and results are corroborated with existing theory.