Abstract
Fully inorganic cesium lead halide perovskite nanocrystals (NCs) have shown to exhibit outstanding optical properties such as wide spectral tunability, high quantum yield, high oscillator strength as well as blinking-free single photon emission, and low spectral diffusion. Here, we report measurements of the coherent and incoherent exciton dynamics on the 100 fs to 10 ns time scale, determining dephasing and density decay rates in these NCs. The experiments are performed on CsPbBr2Cl NCs using transient resonant three-pulse four-wave mixing (FWM) in heterodyne detection at temperatures ranging from 5 to 50 K. We found a low-temperature exciton dephasing time of 24.5 ± 1.0 ps, inferred from the decay of the photon-echo amplitude at 5 K, corresponding to a homogeneous line width (fwhm) of 54 ± 5 μeV. Furthermore, oscillations in the photon-echo signal on a picosecond time scale are observed and attributed to coherent coupling of the exciton to a quantized phonon mode with 3.45 meV energy.
Highlights
R ecently, a new type of colloidal nanocrystals (NCs) has emerged for optoelectronic applications which combines simplicity in synthesis with great spectral flexibility and exceptional optical properties
The experiments are performed on CsPbBr2Cl NCs using transient resonant three-pulse four-wave mixing (FWM) in heterodyne detection at temperatures ranging from 5 to 50 K
Almost blinking-free single photon emission and marginal spectral diffusion have been reported for CsPbX3 quantum dots at low temperature.[3,6]
Summary
Been applied on various materials such as ruby crystals,[11] atoms,[12] molecules,[13,14] and semiconductor nanostructures.[15−17] In general, the dephasing of excitons in semiconductor materials is caused by elastic and inelastic scattering processes with phonons and charge carriers, and by radiative population decay.[18]. It is attributed to the resonant excitation, selecting the states and nanocrystals in the ensemble with the highest oscillator strength, as observed in quantum wells with localized excitons.[27] in nonresonant PL, the dynamics can be influenced by the relaxation of the electron−hole pairs within the complex level structure by phonon-assisted processes This relaxation can have time scales similar or substantially longer than the radiative lifetimes, affecting the measured PL decay times, as observed for CdSe nanoplatelets.[21] Because the lowest state in cesium lead halide NCs is bright,[5] relaxation to the dark state is not relevant at low temperature. If we assume that the upper two fine-structure states are dephasing fast due to phonon-assisted transitions to the lowest state, there is an additional decay by a factor of 3 Taking all these corrections into account results in a ZPL weight of 0.91 at 5 K, as is shown in Figure 3d on the right vertical-axis.
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