1 3
Top Curr Chem (Z) (2018) 376:10
Figure 15c, d, show the plot of the room-temperature 2DES map of the two samples
for a waiting time of T = 52 fs.
Diagonal and cross-peaks are clearly resolved at the electronic transitions involving different hole bands and labeled heavy-hole and light-hole excitons (HX and LX,
respectively). The presence of the cross-peaks is assigned to the strong coupling
between the two transitions expected because they share the same electronic state.
Monitoring the evolution of the amplitude of the peaks appearing in the 2DES
maps as a function of time allowed the study of coherent superpositions of exciton
states. Oscillations are evident and visible on both the diagonal and the lower crosspeaks in the CdSe NPL and in the CdSe/ZnS NPL heterostructure. The authors
reported the results for the oscillations observed at the lower cross-peak position in
three different samples (CdSe, heterostructure CdSe/ZnS and CdSe/ZnS with thinner
core) (Fig. 15e). The oscillations last for about 150 fs. The frequency of the oscillations in these two peaks matches the HX–LX frequency difference measured by the
absorption spectrum and they are assigned to electronic coherences. The HX–LX
electronic coherence dephases for the three different samples with different time
constants. From these results, it is evident that the pure homogeneous line broadening of the colloidal NPLs enables clear observation of the electronic coherence.
6 Photoreactivity
2DES provides a key advantage over methods such as pump-probe spectroscopy for
unraveling ultrafast kinetic processes because it decouples the frequency of excitation with the time resolution of the experiment. This renders possible since the excitation is resolved by an interferometric measurement between the two pump beams.
Thus the discrimination between a sequential and a parallel mechanism is achievable, even if the states are too entangled and/or their dynamics are too fast [7].
For a sequential mechanism (Fig. 16a), a loss of population (photobleaching) is
expected for the species A and a cross-peak will appear at the excitation frequency
of A and emission of B. The gain of population (photoproduct) will result in a signal with the opposite sign [78, 79]. In the case of an equilibrium between A and C
(Fig. 16b), two diagonal peaks will be detected as a photobleach for A and C and
two off-diagonal peaks will be of the opposite sign at the excitation of A and emission of C and excitation of C and emission of A. For instance, if A gives B and C
with A and C being in an equilibrium (Fig. 16c), two diagonal peaks are expected in
A and C, and three off-diagonal peaks are expected in excitation of A and emission
of B and C, and excitation of C and emission of A. Those cases are some extreme
cases to describe a 2DES map and will be useful to interpret the data.
The first example is the electron and hole relaxation dynamics in cadmium telluride (CdTe) nanorods (NRs) in toluene (Fig. 17a inset) [80]. Indeed, conventional
transient absorption spectroscopy cannot disentangle those phenomena because of
the overlap of the transitions and their very fast relaxation time scales. We saw in
the Heterogeneity paragraph that those systems are highly heterogeneous. Semiconductor nanocrystals are very interesting materials because the quantum confinement
effect leads to: i) discrete energy levels, ii) tunable energy bandgap depending on
49
Reprinted from the journal
Top Curr Chem (Z) (2018) 376:10
Figure 15c, d, show the plot of the room-temperature 2DES map of the two samples
for a waiting time of T = 52 fs.
Diagonal and cross-peaks are clearly resolved at the electronic transitions involving different hole bands and labeled heavy-hole and light-hole excitons (HX and LX,
respectively). The presence of the cross-peaks is assigned to the strong coupling
between the two transitions expected because they share the same electronic state.
Monitoring the evolution of the amplitude of the peaks appearing in the 2DES
maps as a function of time allowed the study of coherent superpositions of exciton
states. Oscillations are evident and visible on both the diagonal and the lower crosspeaks in the CdSe NPL and in the CdSe/ZnS NPL heterostructure. The authors
reported the results for the oscillations observed at the lower cross-peak position in
three different samples (CdSe, heterostructure CdSe/ZnS and CdSe/ZnS with thinner
core) (Fig. 15e). The oscillations last for about 150 fs. The frequency of the oscillations in these two peaks matches the HX–LX frequency difference measured by the
absorption spectrum and they are assigned to electronic coherences. The HX–LX
electronic coherence dephases for the three different samples with different time
constants. From these results, it is evident that the pure homogeneous line broadening of the colloidal NPLs enables clear observation of the electronic coherence.
6 Photoreactivity
2DES provides a key advantage over methods such as pump-probe spectroscopy for
unraveling ultrafast kinetic processes because it decouples the frequency of excitation with the time resolution of the experiment. This renders possible since the excitation is resolved by an interferometric measurement between the two pump beams.
Thus the discrimination between a sequential and a parallel mechanism is achievable, even if the states are too entangled and/or their dynamics are too fast [7].
For a sequential mechanism (Fig. 16a), a loss of population (photobleaching) is
expected for the species A and a cross-peak will appear at the excitation frequency
of A and emission of B. The gain of population (photoproduct) will result in a signal with the opposite sign [78, 79]. In the case of an equilibrium between A and C
(Fig. 16b), two diagonal peaks will be detected as a photobleach for A and C and
two off-diagonal peaks will be of the opposite sign at the excitation of A and emission of C and excitation of C and emission of A. For instance, if A gives B and C
with A and C being in an equilibrium (Fig. 16c), two diagonal peaks are expected in
A and C, and three off-diagonal peaks are expected in excitation of A and emission
of B and C, and excitation of C and emission of A. Those cases are some extreme
cases to describe a 2DES map and will be useful to interpret the data.
The first example is the electron and hole relaxation dynamics in cadmium telluride (CdTe) nanorods (NRs) in toluene (Fig. 17a inset) [80]. Indeed, conventional
transient absorption spectroscopy cannot disentangle those phenomena because of
the overlap of the transitions and their very fast relaxation time scales. We saw in
the Heterogeneity paragraph that those systems are highly heterogeneous. Semiconductor nanocrystals are very interesting materials because the quantum confinement
effect leads to: i) discrete energy levels, ii) tunable energy bandgap depending on
49
Reprinted from the journal