Electronic band gap reduction and intense luminescence in Co and Mn   ion-implanted SiO$_2$
  Cobalt and manganese ions are implanted into SiO$_2$ over a wide range of concentrations. For low concentrations, the Co atoms occupy interstitial locations, coordinated with oxygen, while metallic Co clusters form at higher implantation concentrations. For all concentrations studied here, Mn ions remain in interstitial locations and do not cluster. Using resonant x-ray emission spectroscopy and Anderson impurity model calculations, we determine the strength of the covalent interaction between the interstitial ions and the SiO$_2$ valence band, finding it comparable to Mn and Co monoxides. Further, we find an increasing reduction in the SiO$_2$ electronic band gap for increasing implantation concentration, due primarily to the introduction of Mn- and Co-derived conduction band states. We also observe a strong increase in a band of x-ray stimulated luminescence at 2.75 eV after implantation, attributed to oxygen deficient centers formed during implantation. 
R. J. Green, D. A. Zatsepin, D. J. St. Onge, E. Z. Kurmaev, N. V.   Gavrilov, A. F. Zatsepin, A. Moewes
cond-mat.mtrl-sci