in an important and timely issue concerning a wide class of optical phenomena influenced by the size reduction of materials to nanoscale. Nanometer‐sized silica particles (nanosilica) are, indeed, characterized by a large specific surface area that favors a large concentration of defects that have a crucial role in determining the high emissivity, which is surprising if compared to the optical properties of their bulk counterpart, and are therefore the subject of huge attention in the specialized literature [15–20]. It also worth noting that in these nanosystems, the generation of defects is strongly conditioned by the accessibility of surface sites for molecules of the environment [21]. This is particularly advantageous from a fundamental viewpoint because the possibility of stabilizing specific defects, by controlled thermochemical processes, is crucial to better understand their structural and electronic properties.
At silica surfaces, the structure of NBOHC is in fact determined by its generation occurring by the chemical reaction of an axially symmetric surface‐E' center,
Figure 2.6 Structure of the NBOHC and p orbitals of the dangling oxygen in the (panel a) C 3v and (panel b) C S symmetries.
2.3.1 Emission Spectra and Lifetime Measurements
Several studies have evidenced that NBOHC at the silica surface emits a luminescence band around 2.0 eV with a composite excitation profile consisting of a peak at 2.0 eV, nearly overlapping with the emission, and an UV broadband with peaks at 4.8 and 6.0 eV [23–25]. Time‐resolved spectra have been performed in agreement with the experimental setup described in the previous section.
In Figure 2.7 are reported time‐resolved PL spectra acquired under pulsed laser excitation at 2.07 eV (panel a) and at 4.77 eV (panel b) with Δt = 4 μs and T D going from 1 to 248 μs, when the decay of the PL emission is almost completed.
To analyze the luminescence decay from the PL spectra reported in Figure 2.7, we have derived the decay kinetics of the two sub‐bands (E em = 1.91 eV and E em = 1.99 eV) under visible (E exc = 2.07 eV) and UV (E exc = 4.77 eV) excitation (Figure 2.8a). As evident from the semilogarithmic scale, the PL decay curves deviate from a single exponential law. This behavior is common to color centers embedded in amorphous network and is consistent with a multiexponential curve with decay constants inhomogeneously distributed. The quantitative analysis of the PL time decay, I(t), has been carried out by a stretched exponential decay function:
(2.78)
where τ is the lifetime and γ≤1 is a stretching parameter that measures the deviation from a single exponential decay. The results obtained from the fitting procedure are reported in Table 2.1.
To complete the lifetime study, in Figure 2.8b we show the temperature dependence of the decay kinetics of the PL band excited at E exc = 4.77 eV. Since the two sub‐bands have similar properties, we only display the curve related to one of them (E em = 1.99 eV). The deviation from a single exponential law is maintained in the investigated temperature range and the lifetime slightly increases from τ = 41.2 ± 0.5 μs, at T = 300 K, to τ = 52.0 ± 0.5 μs, at T = 10 K. Combining the luminescence lifetime at low temperature (τ≈50 μs) and the Einstein coefficient relation (Eq. 2.66), it is possible to estimate the oscillator strength of the 2.0 eV absorption band for the surface‐NBOHC: f ≈ 6 × 10−5.
Figure 2.7 Time‐resolved PL spectra acquired at different delays in the sample containing (
We observe that the spectral features of surface‐NBOHC can be accounted for by an energy‐level scheme, where two different pathways of excitation/emission can be distinguished. The first includes the cycle 2.0 eV (excitation)/1.9 eV (emission) occurring between two electronic states, whose small Stokes shift is consistent with a very weak electron–phonon coupling. The second cycle, UV and vacuum UV (excitation)/1.9 eV (emission), involves additional electronic states and the large Stokes shift is due to non‐radiative electronic relaxations. The excited state from which the PL takes place is commonly associated with a lone pair in both nonbonding 2p orbitals of the dangling oxygen [26–28]. In contrast, conflicting models have been put forward to account for the states originating the excitation bands; one of the most accepted hypotheses that the 2.0 eV band is associated with the charge transfer from the Si─O• bonding orbital to one of the nonbonding orbitals, while in the UV and vacuum‐UV bands, the charge transfer originates from the nonbonding 2p orbitals of the basal oxygen [21, 26, 28]. In this framework, the time‐resolved experiments help in the interpretation since the long lifetime (τ≈50 μs) points out the forbidden character of the PL transition due to the small overlap between the filled 2p orbitals of the nonbridging oxygen atom, which identify the excited state, and that where the charge transfer terminates.
Figure 2.8 Panel (a): Semilog plots of the PL decay in surface‐NBOHC (
