55 research outputs found
Long Exciton Dephasing Time and Coherent Phonon Coupling in CsPbBrCl Perovskite Nanocrystals
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 timescale,
determining dephasing and density decay rates in these NCs. The experiments are
performed on CsPbBrCl NCs using transient resonant three-pulse four-wave
mixing (FWM) in heterodyne detection at temperatures ranging from 5 K to 50 K.
We found a low-temperature exciton dephasing time of 24.51.0 ps, inferred
from the decay of the photon-echo amplitude at 5 K, corresponding to a
homogeneous linewidth (FWHM) of 545 {\mu}eV. Furthermore, oscillations in
the photon-echo signal on a picosecond timescale are observed and attributed to
coherent coupling of the exciton to a quantized phonon mode with 3.45 meV
energy
Monodisperse Long-Chain Sulfobetaine-Capped CsPbBr₃ Nanocrystals and Their Superfluorescent Assemblies
Ligand-capped nanocrystals (NCs) of lead halide perovskites, foremost fully inorganic CsPbX₃ NCs, are the latest generation of colloidal semiconductor quantum dots. They offer a set of compelling characteristics—large absorption cross section, as well as narrow, fast, and efficient photoluminescence with long exciton coherence times—rendering them attractive for applications in light-emitting devices and quantum optics. Monodisperse and shape-uniform, broadly size-tunable, scalable, and robust NC samples are paramount for unveiling their basic photophysics, as well as for putting them into use. Thus far, no synthesis method fulfilling all these requirements has been reported. For instance, long-chain zwitterionic ligands impart the most durable surface coating, but at the expense of reduced size uniformity of the as-synthesized colloid. In this work, we demonstrate that size-selective precipitation of CsPbBr₃ NCs coated with a long-chain sulfobetaine ligand, namely, 3-(N,N-dimethyloctadecylammonio)-propanesulfonate, yields monodisperse and sizable fractions (>100 mg inorganic mass) with the mean NC size adjustable in the range between 3.5 and 16 nm and emission peak wavelength between 479 and 518 nm. We find that all NCs exhibit an oblate cuboidal shape with the aspect ratio of 1.2 × 1.2 × 1. We present a theoretical model (effective mass/k·p) that accounts for the anisotropic NC shape and describes the size dependence of the first and second excitonic transition in absorption spectra and explains room-temperature exciton lifetimes. We also show that uniform zwitterion-capped NCs readily form long-range ordered superlattices upon solvent evaporation. In comparison to more conventional ligand systems (oleic acid and oleylamine), supercrystals of zwitterion-capped NCs exhibit larger domain sizes and lower mosaicity. Both kinds of supercrystals exhibit superfluorescence at cryogenic temperatures—accelerated collective emission arising from the coherent coupling of the emitting dipoles
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