## 3 Results and Discussion
### Photoluminescence Properties
The shift of the Si-O-Si stretching mode to a lower wavenumber indicates a phase change towards an amorphous (non-stoichiometric silicon oxide) and porous structure [33; 34; 35; 36; 37]. This could be related to the formation of a SiO\({}_{X}\) shell around silicon nanocrystals formed by a wide variety of Si-O bond defects. In a recent publication, [PERSON] et al. have corroborated the formation of a core shell structure with a crystalline core surrounded by an interface mainly composed by Si\({}_{3}\)O defects [43]. Figure 7: (Color Online) PL (Photoluminescence) spectra obtained at room temperature from colloidal solution (**a**); and from coated films with **red** and **blue** Si-NCs without annealing (**b**). The thermal annealing also appears to cause the loss of the H-passivated characteristics of the Si-NCs, modifying its configuration into an oxygen-passivated surface. This is corroborated by the FTIR results (Figure 5c). Therefore, it is likely that part of the emission remains due to recombination inside the nanocrystals, but oxygen defects also contribute to the broadening of the range of emission in both types of coated films. The broad PL spectra were decomposed in Gaussian curves centered in the position of reported Si-O defect emissions, as shown in Figure 8a,b. As a result, PL bands at about 3.02 eV, 2.73 eV, 2.37 eV,1.99 eV, and 1.55 eV, were identified, which are respectively related to: weak-oxygen bond (WOB) [44], Neutral Oxygen Vacancy (NOV) [45], E'\({}_{S}\) center [46], non-bridging oxygen hole center (NBOHC) [18; 47; 48] and dangling bonds (DB) [49]. It was found that NBOHC and E'\({}_{S}\) centers are the dominant radiative defects for each of the coated films. The band around 2.7 eV has also been related to a twofold coordinated Si [16; 17]. However, recently, the red emission around 1.9 eV has been related to Si\({}_{3}\)O, and so the main contribution can be also ascribed to this defect [43]. These results agree with a previous report in which the main emission mechanism was associated to Si-O defects [29]. Figure 9 shows the PL emission spectra of embedded films with red (a) and blue (b) Si-NCs before and after thermal annealing. It is worth mentioning that any PL emission is observed when a Si-NCs:SOG volume ratio is lower than 3:1. As can be seen, the PL spectra are different than those observed in coated Si-NCs. Even though coated and embedded films were fabricated using the same Si-NCs, their respective spectrum has noticeable alterations. Before thermal annealing, the wavelength range of PL emission of both coated and embedded Si-NCs films are similar. However, after annealing, the emission of the embedded films shows practically no variation, contrary to the coated films. The PL spectrum obtained from the embedded films with red Si-NCs presents a peak at \(\sim\)715 nm, and a more intense one at \(\sim\)410 nm, which is related to the SOG PL emission, as observed in Figure 9a. The embedded films with blue Si-NCs exhibit a maximum PL peak at \(\sim\)430 nm, and a less intense one at \(\sim\)850 nm, as shown in Figure 9b. The PL emission observed in embedded films with both red and blue Si-NCs before thermal annealing is similar to that observed in the colloidal solutions (Figure 7a). Since the SOG around the Si-NCs is not chemically bonded to them, the embedded film emission is ascribed to the quantum confinement mechanism, as proposed by other authors for similar films [25; 50]. Figure 8: (Color Online) PL emission of coated films with **red** (**a**) and **blue** (**b**) Si-NCs after annealing at 1100 \({}^{\circ}\)C in N\({}_{2}\). Compared to Figure 7, these spectra are wider, and both shift towards the **blue-green** region of the visible spectrum. When thermal annealing is applied to these embedded films, it is clear that the PL emission changes in intensity, but not in shape, as shown in Figure 9a. Because of this, the PL of the embedded films (with both red and blue Si-NCs) can be attributed to the quantum confinement effect before and after thermal annealing.