Chemical analysis of ligand-free silicon nanocrystal surfaces by surface   enhanced Raman spectroscopy
  Surface enhanced Raman spectroscopy (SERS) was used to probe the surface chemistry of chlorine-terminated silicon nanocrystal (Si-NC) surfaces in an air-free environment. SERS effect was observed from the thin films of Ag$_x$O using 514 nm laser wavelength. When a monolayer of Si-NCs were spin-coated on Ag$_x$O SERS substrates, a very clear signal of surface states, including Si-Cl$_x$, and Si-H$_x$ were observed. Upon air-exposure, we observed the temporal reduction of Si-Cl$_x$ peak intensity, and a development of oxidation-related peak intensities, like Si-O$_x$ and Si-O-H$_x$. In addition, first, second and third order transverse optical (TO) modes of Si-NCs were also observed at 519, 1000 and 1600 cm$^{-1}$, respectively. As a comparison, Raman analysis of a thick film (> 200 nm) of Si-NCs deposited on ordinary glass substrates were performed. This analysis only demonstrated the first TO mode of Si-NCs, and the all the other features originated from SERS enhancement did not appear in the spectrum. These results conclude that, SERS is not only capable of single-molecule detection, but also a powerful technique for monitoring the surface chemistry of nanoparticles. 
\.Ilker Do\u{g}an, Ryan Gresback, Tomohiro Nozaki and Mauritius C.M.   van de Sanden
cond-mat.mtrl-sci