Correlation between the luminescence and Raman peaks in quantum-confined systems

Two of the main models for the explanation of strong visible-light emission in silicon, namely physical quantum confinement of electrons in nanometer-size "wires" or spherical crystallites, and chemical quantum confinement to subnanometer-size silicon particles due to the isolating effect of oxygen atoms, are considered with regard to a possible correlation between Raman shift and photoluminescence (PL) peak position. The physical confinement model predicts opposite shifts with changing size of the confinement. In the chemical confinement model, the shift of the gap is not a size effect; it occurs owing to chemical substitution of the bond terminators of the silicon atoms. This is accompanied by a parallel shift in the Raman frequency. The calculation of the vibration spectra of small size wires and spheres allows the correct assignment of experimentally observed Raman peaks. With the help of this assignment, the analysis of the observed spectra shows a parallel shift of Raman and PL peaks. The calculated frequencies for siloxene derivatives (a known manifestation of chemical quantum confinement) are lower than those observed in most porous silicon samples: still the parallel shift favors the idea of chemical as against physical quantum confinement.