Monodisperse Long-Chain Sulfobetaine-Capped CsPbBr3 Nanocrystals and Their Superfluorescent Assemblies

Abstract

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.

Figure 1

(a) Scheme of size selection of CsPbBr₃ NCs. (b, c) The normalized absorption and PL spectra of crude supernatant (s) and the resulting fractions of NCs synthesized at 150 °C (7.3–4.7.nm; see also Table S3 and Figure S3). (d) Zwitterionic sulfobetaine ligand (ASC18) used in the synthesis.

Figure 2

(a) PL and absorption spectra for small (a = 4.5 nm, blue/black line, 1sf6) and large (a = 13 nm, red/black line, 5sf2) NCs isolated by size-selective precipitation; (b) their small-angle X-ray scattering traces fitted to an orthorhombic model revealing one NC dimension to be 20% shorter with respect to the other two. (c, d) Cryo-HAADF-STEM images of the NCs with edge length statistics and high resolution image. For small NCs, the lattice fringes could not be resolved even under cryo conditions.

Figure 3

(a) Normalized absorbance spectra of size-selected CsPbBr₃ NCs of different sizes. The p-p transition is marked with (∗). (b) Respective normalized PL spectra. In (c), the energy of the allowed optical transitions against the edge length is plotted. For the experimental values (black and red points), edge lengths were received from SAXS volume distributions, s-s and p-p transition energies from absorption spectra. The error bars indicate the absolute polydispersity and refer to the lower axis. The lines are the energies calculated using an effective mass/k·p model. Panel (d) shows the radiative lifetime as a function of the edge length. Exemplary traces are shown in the inset (red 5sf2 and blue 1sf6 remaining traces and fits; see Figure S9). The line indicates the theoretical expectation using a Boltzmann state mixing model, with (blue line) and without (black line) the inclusion of the electron-hole correlated motion.

Figure 4

Superlattices of OA/OLA-capped CsPbBr₃ NCs (left) and zwitterion-capped CsPbBr₃ NCs (right). (a, e) Fluorescence microscopy images of the obtained cuboid-shaped supercrystals (SEM image in Figure S21). (b) GISAXS patterns showing mosaicity (rings) in the OA/OLA-case and (f) sharp reflections indicating large domain sizes obtained with zwitterion-capped NCs. (c, g) The cross-sectional plots (see corresponding white lines in b, f) yield similar unit cell parameters but larger domain sizes for zwitterion-capped NCs. (d, h) Pictograms representing the crystallographic information found by GISAXS; the mosaicity (reported value is the average mosaicity) is mostly present within the a–b plane of the superlattices.

Figure 5

(a) A 6 K PL spectrum from a supercrystal comprising zwitterion-capped NCs (8.2 × 8.2 × 6.7 nm NCs). The experimental data have been fitted to a sum of two Lorentzian functions, depicted in gray and green. The inset shows a log–log plot of both peak heights with power-law fits (solid lines), yielding exponents of m = 0.92 and m = 0.98 for the gray and green peaks, respectively. (b) Color-coded streak camera image obtained with an excitation fluence of 114 μJ/cm². (c) Spectrally integrated time-resolved emission intensity traces for the several excitation fluences. The traces are shifted in time to account for a constant time offset (ca. 22 ps) present in the streak camera images. (d) Measured data (points) and the model curves of: (top) the decay time as a function of the excitation fluence, fitted according to the SF model; (middle) the SF burst peak height that increases superlinearly with excitation fluence, corresponding to a power-law dependence with an exponent m = 1.3; (bottom) the extracted build-up time decreases at high excitation fluence due to the increased interaction among the emitters, and can be well reproduced by the log(N)/N theoretical expected trend.