Stabilized electronic state and its luminescence at the surface of oxygen-passivated porous silicon

Photoluminescence (PL) spectra of as-made porous Si samples were obtained in a wide peak-wavelength range. After exposure to air or coupling with ${\mathrm{C}}_{60}$ molecules, the PL peak shifts to a pinning wavelength within the range of 610–630 nm. This pinning wavelength is almost independent of the size of the original porous Si nanocrystallites and both redshifting and blueshifting can occur for different sizes. A self-consistent effective-mass calculation shows that the $\mathrm{Si}=\mathrm{O}$ binding states are responsible for the radiation of this pinning wavelength and the blueshift for the large nanocrystallites is due to the additional potential modulation within the Si nanocrystallite by the long-range Coulomb interaction of oxygen ions.