524
M. Vacha and S. Hirata
Spectral diffusion can originate from multiple defect levels of different energies within one particle. Activation and deactivation of such defect states would
lead to spectral changes and blinking. To selectively excite the possible different
defects, we used alternately two lasers, one (375 nm) with energy above the bandgap
E g , the other (532 nm) slightly below E g to measure PL spectra on the same single NC
(x = 0.3). The spectra that appear upon both excitations have maxima below 2.06 eV,
whereas the spectra that appear only upon the 375 nm excitation have maxima around
2.10 eV. Such excitation-dependent differences in PL energy are shown for statistical ensemble of single NCs in distributions NC spectral positions in Fig. 30.4b.
The difference in the PL distributions points to the coexistence of two distinct defect
states in individual NCs whose recombination energy differs on average by 85 meV.
Such multiple defect states within single NCs can be a significant factor contributing
to the large homogeneous width in the NCs.
Apart from the differences in PL spectra, excitation at different energies also
leads to differences in blinking behavior. To study the blinking phenomenon, we
used sequential excitation with 532 nm laser only (10 s), 532 nm and 375 nm lasers
simultaneously (10 s), and 375 nm laser only (10 s) and measured PL intensity traces.
An example of such traces shown in Fig. 30.4c confirms that the ON times are most
frequent during the simultaneous irradiation with both lasers. Statistical analysis of
PL intensity obtained with the different excitations on the same NCs shows that
the simultaneous irradiation leads on average to 1.85 times more emitted photons
compared to each of the individual irradiations.
The difference between the ON and OFF time response to the different irradiation
modes points to the involvement of multiple defect states in the emission process.
Considering the two coexisting defect states mentioned above, one such state (lower
in energy) can be temporarily in the form of a long-lived non-radiative trap that
quenches the excitons. Filling of such trap state with the lower-energy laser causes
the conduction band exciton to relax into the higher energy defect state from where
the emission occurs. This process enables optical modulation of PL blinking and
leads to an almost twofold increase in brightness in these semiconductor QDs.
M. Vacha and S. Hirata
Spectral diffusion can originate from multiple defect levels of different energies within one particle. Activation and deactivation of such defect states would
lead to spectral changes and blinking. To selectively excite the possible different
defects, we used alternately two lasers, one (375 nm) with energy above the bandgap
E g , the other (532 nm) slightly below E g to measure PL spectra on the same single NC
(x = 0.3). The spectra that appear upon both excitations have maxima below 2.06 eV,
whereas the spectra that appear only upon the 375 nm excitation have maxima around
2.10 eV. Such excitation-dependent differences in PL energy are shown for statistical ensemble of single NCs in distributions NC spectral positions in Fig. 30.4b.
The difference in the PL distributions points to the coexistence of two distinct defect
states in individual NCs whose recombination energy differs on average by 85 meV.
Such multiple defect states within single NCs can be a significant factor contributing
to the large homogeneous width in the NCs.
Apart from the differences in PL spectra, excitation at different energies also
leads to differences in blinking behavior. To study the blinking phenomenon, we
used sequential excitation with 532 nm laser only (10 s), 532 nm and 375 nm lasers
simultaneously (10 s), and 375 nm laser only (10 s) and measured PL intensity traces.
An example of such traces shown in Fig. 30.4c confirms that the ON times are most
frequent during the simultaneous irradiation with both lasers. Statistical analysis of
PL intensity obtained with the different excitations on the same NCs shows that
the simultaneous irradiation leads on average to 1.85 times more emitted photons
compared to each of the individual irradiations.
The difference between the ON and OFF time response to the different irradiation
modes points to the involvement of multiple defect states in the emission process.
Considering the two coexisting defect states mentioned above, one such state (lower
in energy) can be temporarily in the form of a long-lived non-radiative trap that
quenches the excitons. Filling of such trap state with the lower-energy laser causes
the conduction band exciton to relax into the higher energy defect state from where
the emission occurs. This process enables optical modulation of PL blinking and
leads to an almost twofold increase in brightness in these semiconductor QDs.