Time-resolved photoluminescence studies of the energy transfer from excitons confined in Si nanocrystals to oxygen molecules

The formation of singlet oxygen due to the energy transfer from excitons confined in silicon nanocrystals to oxygen molecules is studied using time-resolved photoluminescence spectroscopy. The process of the excitation of oxygen molecules from the ground triplet state to the second excited singlet state is studied at low temperatures, where oxygen molecules are physisorbed on the surface of silicon nanocrystals and at room temperature in gaseous oxygen ambient and in oxygen-saturated water. The low temperature measurements reveal that the energy transfer time is the shortest for the resonant energy transfer. The involvement of one energy-conserving transversal optical phonon results in about 40% increase of the energy transfer time. The excitation rate of oxygen dimers is found to be similar to that measured for oxygen molecules. At room temperature, the time of the energy transfer to oxygen molecules is about $17\mu \mathrm{s}$. The photosensitizing efficiency of silicon nanocrystals at room temperature is found to be as high as 80% for gaseous oxygen ambient and for oxygen-saturated water.

Figure 1

(a) CW PL spectra of a $P\mathrm{Si}$ layer in vacuum (dashed line) and a layer having physisorbed oxygen molecules on the surface (solid line) at 5 K. Electron-spin configurations and spectroscopic labeling of molecular oxygen states are shown in the inset. (b) Time-resolved PL spectra at 5 K (dashed line, in vacuum; solid line, with physisorbed oxygen molecules). The measurement gate width is 100 ns. The data for the measurement delay time with respect to the excitation pulse of 0.08, 0.18, 0.28, 0.48, 0.78, 1.08, 1.48, 2.08, $3.08\mu \mathrm{s}$ are shown.

Figure 2

Time-resolved PL spectra of a $P\mathrm{Si}$ layer having oxygen molecules physisorbed on the surface divided by those of $P\mathrm{Si}$ in vacuum [$I_{\mathit{\text{Oxygen}}}(\omega ,t)∕I_{\mathit{Vac}}(\omega ,t)$]. The measurement gate width is 100 ns. The data for the measurement delay time with respect to the excitation pulse of 0.08, 0.18, 0.28, 0.48, 0.78, 1.08, 1.48, 2.08, $3.08\mu \mathrm{s}$ are shown. The energy positions of the ${}^3\Sigma -{}^1\Sigma$ transition of ${\mathrm{O}}_2$ and that of the $2{}^3\Sigma -2{}^1\Delta$ transition of oxygen dimers are indicated by dashed lines.

Figure 3

Sketch of energy-level diagram of molecular oxygen and Si nanocrystals having the band-gap energies of 1.63 eV and 1.57 eV. Both nanocrystals luminesce at 1.57 eV, while only the first one can contribute to the energy transfer followed by the ${}^1\Sigma$ state excitation. $I_{\mathit{\text{Oxygen}}}(\omega ,t)∕I_{\mathit{Vac}}(\omega ,t)$ at 80 ns delay time is shown on the left side.

Figure 4

$I_{\mathit{\text{Oxygen}}}(\omega ,t)∕I_{\mathit{Vac}}(\omega ,t)$ as a function of time at detection energies of 1.44, 1.57, 1.63, 1.69, and 1.76 eV. The energy transfer time can be estimated from the slopes.

Figure 5

PL spectra measured in vacuum (dotted line) and in oxygen ambient at 1 bar (solid line). Inset: spectral dependence of the PL suppression level measured at 1 bar of ${\mathrm{O}}_2$ ambient. $T=300\mathrm{K},E_{\mathit{ex}.}=2.54\mathrm{eV}$. Energy of the ${}^3\Sigma -{}^1\Sigma$ transition of ${\mathrm{O}}_2$ is indicated by a dotted line.

Figure 6

Time-resolved PL spectra of $P\mathrm{Si}$ in vacuum (a) and in oxygen ambient at 1 bar (b). Different delay times of PL measurements with respect to the excitation pulse and measurement gate times have been used. From the top to the bottom: delay time 0, 10, 50, 100, 250, $500\mu \mathrm{s}$, gate time 1, 10, 20, 50, 100, $1000\mu \mathrm{s}$, respectively. $E_{\mathit{ex}.}=2.33\mathrm{eV}$. Energy of the ${}^3\Sigma -{}^1\Sigma$ transition of ${\mathrm{O}}_2$ is indicated by a dotted line. All PL spectra are normalized to 1.

Figure 7

Spectral dependencies of the PL lifetime measured in vacuum (squares) and at 1 bar of oxygen ambient (circles). Inset: spectral dependence of the energy transfer time in oxygen ambient at 1 bar. $T=300\mathrm{K},E_{\mathit{ex}.}=2.33\mathrm{eV}$. Energy of the ${}^3\Sigma -{}^1\Sigma$ transition of ${\mathrm{O}}_2$ is indicated by a dotted line.

Figure 8

PL spectra of $P\mathrm{Si}$ powder immersed in degassed water (dotted line) and in oxygen-saturated water (solid line). Inset: spectral dependence of the PL suppression level measured for $P\mathrm{Si}$ powder immersed in oxygen-saturated water. $T=300\mathrm{K},E_{\mathit{ex}.}=2.54\mathrm{eV}$. Energy of the ${}^3\Sigma -{}^1\Sigma$ transition of ${\mathrm{O}}_2$ is indicated by a dotted line.

Figure 9

Spectral dependencies of the PL lifetime measured in degassed water (squares) and in oxygen-saturated water (circles). Inset: spectral dependence of the energy transfer time in oxygen-saturated water. $T=300\mathrm{K},E_{\mathit{ex}.}=2.33\mathrm{eV}$. Energy of the ${}^3\Sigma -{}^1\Sigma$ transition of ${\mathrm{O}}_2$ is indicated by a dotted line.