are arbitrarily scaled. Full lines plot the best fit curves of Eq. (2.78).
Table 2.1 Best‐fitting parameters obtained by Eq. (2.78) for the decay kinetics of the two PL sub‐bands measured in the sample containing the surface‐NBOHC (
| E exc (eV) | E em (eV) | τ (μs) | γ |
|---|---|---|---|
| 2.07 | 1.92 | 35.2 ± 0.5 | 0.79 ± 0.02 |
| 1.99 | 29.9 ± 0.5 | 0.75 ± 0.02 | |
| 4.77 | 1.92 | 44.3 ± 0.5 | 0.78 ± 0.02 |
| 1.99 | 41.2 ± 0.5 | 0.76 ± 0.02 |
1
The associated errors derive from the best‐fitting procedures.
2.3.2 Zero‐Phonon Line Probed by Site‐Selective Luminescence
The results reported in the previous section have evidenced that surface‐NBOHC (≡Si – O–)3Si – O• is characterized by a small Stokes shift between its excitation and emission transitions peaked around 2 eV. This implies the possibility to detect, under site‐selective excitation, the ZPL and the vibrational structures with which the electronic transition is coupled. The main purposes of this study are: (i) the measure of the stretching frequency of the Si─O• bond in the ground and in the excited electronic state; (ii) the measure of the phonon coupling parameters; (iii) the measure of the inhomogeneous distribution of the ZPL.
Vibrational properties: Figure 2.9 shows the effects of temperature on time‐resolved PL spectra measured with E exc = 1.997 eV. At T = 290 K, the emission is characterized by two sub‐bands peaked at 1.92 ± 0.01 and 1.99 ± 0.01 eV, and it extends over the anti‐Stokes region. On lowering temperature, the PL amplitude increases, the anti‐Stokes part vanishes and, below 150 K, the ZPL resonant with the excitation is increasingly evident together with a vibrational structure at 920 cm−1 apart from it. The origin of the 920 cm−1 line will be clarified in the following.
The temperature dependence of the ratio between the intensities of ZPL and the whole band, I ZPL/I TOT, namely the Debye–Waller factor α(T), is shown in Figure 2.10; it allows to quantify the thermal deactivation of the environment vibrational modes the PL transition is coupled to. Panel (a) illustrates the measure of I TOT (shaded area) and I 0L (shaded area in the inset) in the spectrum detected at T = 8 K. Panel (b) evidences that α(T) decreases from 0.11 to 0.005 on increasing temperature from 8 to 137 K. As reported in the previous section, the expression of α(T) is derived under the straightforward approximation (homogeneous system of defects characterized by an electronic transition linearly coupled to a single mode of mean effective frequency ϖ), so that all defects are selectively excited, namely ZPL is in resonance with laser light:
Figure 2.9 Time‐resolved PL spectra of surface‐NBOHC (
Figure 2.10 Panel (a): Time‐resolved PL spectrum of surface‐NBOHC (
where
The most significant features of the local vibrations, coupled to the electronic transition around 2 eV, are derived by the emission spectra reported in the upper and lower side of Figure 2.11. The emission excited at 1.997 eV shows the ZPL, whose FWHM is ≈1.4 meV (11 cm−1), coincident with the laser line; that is, the ZPL originates from those centers located within the laser spectral linewidth in a much larger inhomogeneous distribution. At lower energies one observes two phonon sidebands centered at 923 ± 3 and 1840 ± 10 cm−1 apart from the ZPL.
