Figure 1 Results of photoluminescence measurements. PL spectra of Si-NCs (VIS) doped with Er3+ (NIR) measured at 10 and 300 K at 488-nm excitation together with normalized PLE spectra detected at 0.81 eV for two Si concentrations: (a) 37 at.% and (b) 39 at.% of Si. The normalization was done for both spectra separately. Emission peak positions as function of temperature for two excitation wavelengths, 266 (squares) and 488 nm (circles), for two different Si concentrations, (c) 37 at.% and (d) 39 at.%, together with find more theoretically predicted Varshni formula.

For the Varshni formula, Si bandgap at 0 K has been set as 2.3 eV for better data presentation. The second band at 1.6 eV can be assigned to the recombination of excitons localized in the SRSO matrix. Moreover, from Figure 1a,

it can be seen that all VIS emission bands have a complex structure. This is due to interference effects caused by the refractive index contrast between SRSO and the Si substrate [35]. These interferences will modify the shape of the emission spectra in the entire VIS spectral range. However, Metformin mw Er3+ emission is not affected by this effect. Additionally, Figure 1a shows the PLE spectra measured for Er3+ at room temperature at 0.81 eV in a broad UV-VIS excitation band energy range. The obtained PLE spectra are also very similar to those obtained by us for undoped SRSO samples [36, 37]. The appearance of strong Er3+ emission at excitation wavelengths far from

resonance with erbium energy levels clearly indicates that we are dealing here with an efficient excitation transfer from the levels responsible for VIS emission (i.e., aSi-NCs, Si-NCs, or defects) to erbium ions. The main argument behind the conclusion that defect states can be excluded in this case is the Si-concentration-dependent position of the excitation spectra for Er3+ ions and VIS emission bands. It can be seen that when the Si content increases, the edge of excitation as well as emission bands shifts towards lower energies due to reduction of quantum confinement. This suggests that the observed VIS emission can be related either to aSi-NCs or to Si-NCs. Moreover, the position of these excitation bands at 4.3 and 3.4 eV for 37 and 39 at.% of Si, respectively, seems to be different than energies typically obtained for excitation bands Carnitine dehydrogenase of defects in SiO2 films: ‘non-bridging oxygen hole center’ at 4.8 and 5.8 eV [38], E’ center at 5.4 to 6.2 eV [39], or ‘oxygen-deficient center’ (ODC) at 7.6, 6.9, and 5.0 eV [40]. Another important conclusion from Figure 1a is that the emission band in the VIS spectral range cannot be assigned to Si-NCs or aSi-NCs only, but some contribution from defect states can also be clearly observed, especially for the sample with 39 at.% where weak emission bands at around 450 nm can be observed. These defect states are most probably due to ODC in the SiO2 matrix [41] or self-trapped excitons (STE) [42].