Observing Multiexciton Correlations in Colloidal Semiconductor Quantum Dots via Multiple-Quantum Two-Dimensional Fluorescence Spectroscopy

Abstract

Correlations between excitons, that is, electron–hole pairs, have a great impact on the optoelectronic properties of semiconductor quantum dots and thus are relevant for applications such as lasers and photovoltaics. Upon multiphoton excitation, these correlations lead to the formation of multiexciton states. It is challenging to observe these states spectroscopically, especially higher multiexciton states, because of their short lifetimes and nonradiative decay. Moreover, solvent contributions in experiments with coherent signal detection may complicate the analysis. Here we employ multiple-quantum two-dimensional (2D) fluorescence spectroscopy on colloidal CdSe1–x S x /ZnS alloyed core/shell quantum dots. We selectively map the electronic structure of multiexcitons and their correlations by using two- and three-quantum 2D spectroscopy, conducted in a simultaneous measurement. Our experiments reveal the characteristics of biexcitons and triexcitons such as transition dipole moments, binding energies, and correlated transition energy fluctuations. We determine the binding energies of the first six biexciton states by simulating the two-quantum 2D spectrum. By analyzing the line shape of the three-quantum 2D spectrum, we find strong correlations between biexciton and triexciton states. Our method contributes to a more comprehensive understanding of multiexcitonic species in quantum dots and other semiconductor nanostructures.

Fluorescence‐Detected Pump–Probe Spectroscopy

“…In these QDs there are several spectrally overlapping excitonic states. [ 45 , 46 , 47 ] In PP, the higher‐energy exciton transitions are, however, completely overlapped by the ESA into multi‐excitonic states. [48] As these recombine with a timescale on the order of 100 ps,[ 49 , 50 ] their FL yield is the same as that of the one‐exciton states.…”

“…As before, we measured both PP (Figure 4 A) and F‐PP (Figure 4 B) under the same conditions. In these QDs there are several spectrally overlapping excitonic states [45–47] . In PP, the higher‐energy exciton transitions are, however, completely overlapped by the ESA into multi‐excitonic states [48] .…”

“…The energy transfer can be seen on the lowest-energy peak trace ( % 1.845 eV,red in Figure 4A,B), whose absolute magnitude slightly increases with T. These dynamics are relatively slow compared to the pulse duration, and thus aselective narrow- band excitation of aparticular excitonic state would make the dynamics much more pronounced, both in PP and F-PP.Y et another alternative would be a( F-)2DES measurement. [47,52] Finally,t here are coherent oscillations discernible in the waiting-time dynamics.InFigure S7 in Section S7 of the SI we compare the spectrum of the QD early waiting-time oscillations in the spectral regions marked in Figures 4A,B. Between 70 and 150 fs,both PP and F-PP display oscillations at the frequency of the low-energy optical phonons.F -PP, being free of the coherent artifact, allows analysis of earlier waiting times than PP.S tarting from T = 30 fs,w ef ind additional, relatively short-lived higher-energy oscillations at energies ranging from 180 meV to 350 meV.Oscillations of such energies correspond to the inter-excitonic spacing, and lifetimes on the order of 80 fs have been observed in CdSe QDs and were attributed to inter-excitonic coherences.…”

“…As before,wemeasured both PP (Figure 4A)a nd F-PP (Figure 4B)u nder the same conditions.I nt hese QDs there are several spectrally over-lapping excitonic states. [45][46][47] In PP,the higher-energy exciton transitions are,h owever,c ompletely overlapped by the ESA into multi-excitonic states. [48] As these recombine with atimescale on the order of 100 ps, [49,50] their FL yield is the same as that of the one-exciton states.A saresult, similar to the situation in the squaraine dimers,ESA is not present in F-PP of these QDs.W ec an thus directly observe the transitions from the ground state to the higher excitonic states.A part from the exciton line shapes,the F-PP spectrum in Figure 4B is modulated by the probe spectrum (Figure S2 in the SI).…”

Fluoreszenz‐detektierte Pump‐Probe‐Spektroskopie

“…Wie zuvor haben wir sowohl PP (Abbildung 4 A) als auch F‐PP (Abbildung 4 B) unter den gleichen Bedingungen gemessen. In diesen QDs gibt es mehrere spektral überlappende exzitonische Zustände [45–47] . Bei PP werden die höherenergetischen Exzitonenübergänge jedoch vollständig durch die ESA in Mehrexzitonenzustände überlagert [48] .…”