## 3 Results and Discussion
### Photoluminescence Properties  
Si-NCs obtained through porous silicon samples exhibit a typical strong red PL emission with a quasi-Gaussian curve [31], which is usually attributed to the quantum confinement effect [1; 6]. The PL spectra obtained from the Si-NCs-based colloidal solutions for this work are shown in Figure 7a. The PL spectrum of the red Si-NC colloidal solution has a main peak at around 639 nm (which is similar to that of pSi layers; not shown), and a lower intensity secondary PL peak at 400 nm. As observed in Figure 7a, the PL spectrum of the isopropanol solution without Si-NCs also exhibits peaks at about 415 nm, but its intensity is very low as compared to the PL from samples with them. The blue Si-NC colloidal solution exhibits an intense PL band at \(\sim\)400 nm and an additional PL band of lower intensity around 753 nm. The intense PL band centered at 400 nm is mainly associated with the blue Si-NCs, since the isopropanol luminescence is known to be too low. These PL emission bands have been reported before [8; 24; 25]. Figure 7b shows the PL spectra of the coated red and blue Si-NC films before thermal annealing. Coated red Si-NCs show a broad PL band with a maximum peak at \(\sim\)650 nm and a less intense band at \(\sim\)415 nm, while the coated blue Si-NC films emit an intense PL band with the main peak at \(\sim\)410 nm and a very small secondary peak at \(\sim\)800 nm. The PL emission of the red Si-NCs was related to their diameter distribution and their energy bandgap. Assuming a nearly spherical configuration of the nanocrystals, calculations of the energy gap were made using the mathematical expression [39]:  
Figure 6: (Color Online) FTIR spectra of coated and embedded films before and after annealing at 1100 \({}^{\circ}\)C for 3 h in N\({}_{2}\) atmosphere. (**a,b**) are the IR (infrared) spectra of coated films with **red** and **blue** Si-NCs, respectively; (**c,d**) are the IR spectra of embedded films with **red** and **blue** Si-NCs, respectively. \[E_{eh}=1.1 eV+h^{2}/(8*m_{e}R^{2})+h^{2}/(8*m_{h}R^{2}), \tag{1}\]  
where \(m_{e}\) and \(m_{h}\) are the effective masses of the electron and hole, respectively; \(R\) is the radius of the nanocrystals; and \(h\) is [PERSON]'s constant. According to Equation (1), considering an \(m_{e}=1.08m_{0}\), and a \(m_{h}=0.56m_{0}\), where \(m_{0}=9.11\times 10^{-31}\) kg is the free electron rest mass [40], the estimated diameter of the Si-NCs producing a PL emission at 650 nm is 2.3 nm. This is consistent with the diameter distribution obtained from HRTEM images for red Si-NCs, which is between 1.5 nm to 4.5 nm, with a mean of 2.5 nm. This supports the conclusion stating that the emission of red Si-NCs is mainly due to QCE. A similar result is obtained from blue colloidal solution and coated blue Si-NCs films before the thermal annealing. Using Equation (1) and the PL spectrum of the blue Si-NCs, the nanocrystals' diameter ranges from 1.3 nm to 1.9 nm with a mean of 1.4 nm. Unfortunately, the observation of Si-NCs of less than 1.5 nm is not possible in the available HRTEM. Nevertheless, the presence of the blue Si-NCs was confirmed by XRD. Moreover, the PL emission from the colloidal solution is similar to the one observed in the coated films. Therefore, the light emission mechanism in these coated films is likely related to QCE as well. As observed in Figure 8, after thermal annealing, the PL emission spectra from both red and blue Si-NCs coated films are broadened. It now presents a wavelength range from \(\sim\)400 nm to 850 nm. This behavior indicates that, regardless of having different Si-NCs, similar emission mechanisms take place in both samples after the thermal annealing, as discussed in a previous report [29]. The thermal annealing produces important effects in the response of the material. In particular, it results in an increase of the radiative surface states density at the Si-NCs/SiO\({}_{2}\) interface [41; 42], as discussed in the previous sections.