10 Hot Carrier Transfer and Carrier Manipulation of Semiconductor …
183
10.6 Control of Multiexciton Dynamics Using Plasmonic
Nanostructures
Colloidal semiconductor nanocrystal QDs are one of the most attractive fluorophores
because of the size-dependent emission wavelengths, narrow emission line width,
and high photodurability. A crucial property of the QDs is the simultaneous existence of multiple excitons (MX) in a single QD. By utilizing the MX, the efficiency of
the optoelectronic devices can be considerably increased. Furthermore, multiphoton
emission from triexciton (TX) and biexciton (BX) states can behave as correlated
photon pairs, i.e., quantum entangled photons, to realize quantum information technologies. However, when MX are produced in a single QD, nonradiative Auger
recombination (AR) efficiently occurs, i.e., the MX decay to a single exciton (SX)
by AR, and thus, the excitons are wastefully consumed by AR. AR also caused the
emission blinking behavior which is called Auger ionization. Therefore, AR suppression has been extensively studied for effective use of the excitons. On the other hand,
AR promotes single-photon emission from a single QD, i.e., photon antibunching in
the QD emission, because the SX as a result of AR can emit a single photon even
when MX are generated in a single QD. The single-photon emission is also important
emission property of a single QD for quantum information technologies. Therefore,
it is crucial to control the MX dynamics and subsequent emission photon statistics
for the applications employing QDs.
We previously reported that the emission photon statistics, i.e., multiphoton emission and a single-photon emission from a single QD, can be modified by interactions of the QD with plasmonic nanostructures, i.e., metallic nanostructures (MNSs)
[29–36]. After that, similar enhancements of multiphoton emission were reported
using several QD-MNS systems [37–54]. Two possibilities have been discussed as
the mechanism for the enhancement of the BX emission. One mechanism is the
enhancement of the BX emission rate by the MNS, which is same as our previous
results. Another is the quenching of SX emission by the MNS, i.e., a decrease in
the quantum yield of SX emission rather than an actual increase in the quantum
yield of BX emission. Thus, the interaction of QD with MNS, particularly the influence of the MNS on the MX dynamics, is unrevealed. To elucidate the influence, a
single QD-MNS system in which both the spectral overlap and the distance are fully
controlled would be used because the interaction between the QD and MNS strongly
depends on the spectral overlap and the distance. However, the investigation using
such systems has not been reported. Additionally, one would have to directly observe
changes in the emission behavior of a single QD that accompany the interaction with
an MNS. To achieve the direct observation, we used two atomic force microscopy
(AFM) techniques.
183
10.6 Control of Multiexciton Dynamics Using Plasmonic
Nanostructures
Colloidal semiconductor nanocrystal QDs are one of the most attractive fluorophores
because of the size-dependent emission wavelengths, narrow emission line width,
and high photodurability. A crucial property of the QDs is the simultaneous existence of multiple excitons (MX) in a single QD. By utilizing the MX, the efficiency of
the optoelectronic devices can be considerably increased. Furthermore, multiphoton
emission from triexciton (TX) and biexciton (BX) states can behave as correlated
photon pairs, i.e., quantum entangled photons, to realize quantum information technologies. However, when MX are produced in a single QD, nonradiative Auger
recombination (AR) efficiently occurs, i.e., the MX decay to a single exciton (SX)
by AR, and thus, the excitons are wastefully consumed by AR. AR also caused the
emission blinking behavior which is called Auger ionization. Therefore, AR suppression has been extensively studied for effective use of the excitons. On the other hand,
AR promotes single-photon emission from a single QD, i.e., photon antibunching in
the QD emission, because the SX as a result of AR can emit a single photon even
when MX are generated in a single QD. The single-photon emission is also important
emission property of a single QD for quantum information technologies. Therefore,
it is crucial to control the MX dynamics and subsequent emission photon statistics
for the applications employing QDs.
We previously reported that the emission photon statistics, i.e., multiphoton emission and a single-photon emission from a single QD, can be modified by interactions of the QD with plasmonic nanostructures, i.e., metallic nanostructures (MNSs)
[29–36]. After that, similar enhancements of multiphoton emission were reported
using several QD-MNS systems [37–54]. Two possibilities have been discussed as
the mechanism for the enhancement of the BX emission. One mechanism is the
enhancement of the BX emission rate by the MNS, which is same as our previous
results. Another is the quenching of SX emission by the MNS, i.e., a decrease in
the quantum yield of SX emission rather than an actual increase in the quantum
yield of BX emission. Thus, the interaction of QD with MNS, particularly the influence of the MNS on the MX dynamics, is unrevealed. To elucidate the influence, a
single QD-MNS system in which both the spectral overlap and the distance are fully
controlled would be used because the interaction between the QD and MNS strongly
depends on the spectral overlap and the distance. However, the investigation using
such systems has not been reported. Additionally, one would have to directly observe
changes in the emission behavior of a single QD that accompany the interaction with
an MNS. To achieve the direct observation, we used two atomic force microscopy
(AFM) techniques.