Resonant Electronic Energy Transfer from Excitons Confined in Silicon Nanocrystals to Oxygen Molecules

We demonstrate efficient resonant energy transfer from excitons confined in silicon nanocrystals to molecular oxygen (MO). Quenching of photoluminescence (PL) of silicon nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with triplet-singlet splitting energy of oxygen molecules. The dependence of PL quenching efficiency on nanocrystal surface termination is consistent with short-range resonant electron exchange mechanism of energy transfer. A highly developed surface of silicon nanocrystal assemblies and a long radiative lifetime of excitons are favorable for achieving a high efficiency of this process.

Figure 1

PL spectra from the as-prepared PSi layer. Excitation energy is 2.41 eV. Dashed curve: $T=50\mathrm{K}$; layer is in vacuum. Blue curve: $T=50\mathrm{K}$; MO pressure is ${10}^{-2}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$. Green curve: $T=5\mathrm{K}$; MO pressure is ${10}^{-4}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$. Red line is emission due to the ${}^1\Delta \mathrm{\text{−}}{}^3\Sigma$ transition. Broad weak background of the Si dangling bonds PL band is subtracted for clarity. Energies of ${}^1\Sigma$ and ${}^1\Delta$ states are shown by vertical dotted lines. Arrows indicate spectral features discussed in the text. PL scaling factors are shown. Inset: sketch of the energy levels of molecular oxygen depending on electron spin configurations. Labeling and energies of transitions are indicated.

Figure 2

PL spectrum from as-prepared PSi (solid curve) and spectral dispersion of the PL decay time (circles) at $T=5\mathrm{K}$; MO pressure is ${10}^{-4}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$. Measurements have been performed at a weak level of PL quenching to keep the signal-to-noise ratio on a reliable level for time-resolved measurements.

Figure 3

The spectral dependencies of the PL quenching strength for hydrogen-terminated porous Si layers. (a) $T=293\mathrm{K}$, MO pressure is 1 bar. (b) $T=50\mathrm{K}$, MO pressure is ${10}^{-2}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$. (c) $T=5\mathrm{K}$, MO pressure is ${10}^{-4}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$. TO-phonon-related spectral signatures are indicated by arrows.