Interface structure of silicon nanocrystals embedded in an amorphous silica matrix

We performed molecular dynamics simulations of the atomic structure of silicon nanocrystals embedded in a stoichiometric amorphous silica matrix. The atom–atom interactions are described by a combination of well-assessed potentials for bulk silicon and SiO2, plus a mixing term to allow adjusting the charge transfer at the interface between Si and silica. For the free-standing Si nanocrystals, we find that the spherical structure is favoured with respect to the faceted one, up to at least a diameter of 6 nm. Correspondingly, the surface layer shows a higher diffusivity than the bulk. When embedded in the silica matrix, nanocrystals are under severe mechanical stress which is released by the combined formation of porosity at the interface and of bridging Si–O–Si bonds, whose density increases with the nanocrystal size. Vibrational frequencies specific to the interface bonding are identified and discussed.

We calculated the equilibrium structures of spherical Si nanoclusters embedded in an amorphous silica matrix by means of molecular dynamics simulations. High concentration of interface stress was observed, which leads to deformation of the smallest nanocrystals. As the nanocrystal size is increased, stress relaxation mechanisms such as the formation of Si–O–Si bridge bonds become more effective. However, the matrix-nanocrystal contact is found to be rather discontinuous, with dense contact regions separated by large areas of interfacial porosity. The vibrational spectra of the embedded nanocrystals display interesting features, such as the peak at 12.2 THz, that can be attributed to the formation of Si–O–Si interfacial bonds and to bonding defects at the interface. Speculations about the role of such vibrational states in the PL efficiency could motivate to proceed further with studies of electron–phonon coupling in these structures.Figure optionsDownload as PowerPoint slide