Here we focus on the PL peak position. Clearly, in Figure 3, we can see that BIBW2992 concentration due to heating, PL spectra of Si NPs move towards smaller emission energy. Figure 4 describes this evolution of the temperature-dependent PL peak position of Si NPs in squalane and in octadecene. Both are compared to the band gap variation of the bulk Si in the same temperature range obtained from the Varshni model [22]. From our measurements, significant linear red shifts were extracted with a slope equal to −0.63 meV/K (0.28 nm/K) and −0.91 meV/K (0.39 nm/K) in octadecene and squalane, respectively. As evidenced from Figure 4,
the temperature dependence of our NP fluorescence energy is much more important than the bulk material band gap variation (three times for Si NPs in octadecene and four times for NPs in squalane). Several experiments have reported on the temperature dependence of PL matrix-embedded (ME) Si NPs [23, 24]. They concluded that the blueshift of the PL peak position with decreasing temperature behaves similarly to that of bulk silicon, i.e., the PL blueshift decreases by about 50 meV when the temperature drops from 300 down to 3 K. Near 300 K, the variation is almost linear with a maximal slope below 0.3 meV/K. ACY-1215 molecular weight As reported by Chao et al. [25], upon vacuum find more ultraviolet excitation of alkylated Si nanocrystallites, intense blue and orange
emission bands were found simultaneously. Both peak positions are shifted to longer wavelengths as the temperature increases from 8 K to room temperature: the orange peak position shifts from 600 ± 2 to 630 ± 2 nm. They suggest that this results Lumacaftor nmr from the population of localized tail states formed by the disordered potential at the surface [26] due to the surface roughness and variations in surface stoichiometry. A recent
study by Kůsová et al. [27] on free-standing (FS) Si nanocrystals obtained from electrochemical anodization has shown a considerably higher blueshift of the emission: 200 meV from 300 down to 4 K with a variation at 300 K of around −1 meV/K which is close to our results for Si NPs in NPLs. Kůsová et al. [27] explained the difference in the shift between FS and ME NPs by the presence of compressive strain in ME NPs which is absent in the case of FS NPs. This explanation is supported by the consideration of a strongly enhanced thermal expansion coefficient for Si NPs (9.10−6 K−1 instead of 2.10−6 K−1 for the bulk material). Nevertheless, in another recent work, size-purified plasma-synthesized Si NPs have been studied in the form of pure nanocrystal films and in the form of nanocomposite of Si NPs embedded in polydimethylsiloxane (PDMS) [28]. Strong compressive strain by an oxide matrix cannot be considered in this case. The quantitative deviation of the PL energy E with temperature (dE/dT) for both Si NP samples was found to be the same. A small deviation in comparison with the bulk material is shown in this work with a maximal variation at 300 K of −0.4 meV/K for the smallest NPs (3.