Optical properties of single CsPbBr3 perovskite quantum dots synthesized by a modified ligand-assisted reprecipitation method

Abstract

Colloidal perovskite quantum dots (pQDs) are promising quantum light emitters, and investigations at the single pQD scale have so far relied mostly on hot-injection synthesis, which requires precise temperature control and an inert atmosphere. While alternative synthesis routes under milder conditions are often associated with structural and surface defects that may have limited impact in ensemble measurements, demonstrating high optical quality at the level of individual pQDs constitutes the most stringent benchmark for a new synthesis protocol. Here, we demonstrate that a modified ligand-assisted reprecipitation (LARP) approach yields CsPbBr3 pQDs showing state-of-the-art optical properties at the scale of single emitters. By combining an amine-mediated post-synthetic size-trimming strategy with didodecyldimethylammonium bromide (DDAB) ligands for enhanced surface passivation and colloidal stability, we obtain isolated pQDs with stable emission and minimal spectral diffusion at cryogenic temperatures. Micro-photoluminescence experiments resolve the characteristic fine structure of the bright exciton, its low-energy optical phonon replicas, and the trion and biexciton states. Time-resolved and photon correlation measurements show a ~90 ps lifetime and high purity single photon emission, respectively. These results demonstrate that modified LARP synthesis constitutes an accessible alternative to hot injection, preserving the intrinsic excitonic and quantum optical properties of individual pQDs while offering greater flexibility for post-synthetic ligand engineering, as exemplified here by the use of DDAB for surface passivation.

Figures (11)

• Figure 1: (a) Schematic illustration of the optimized LARP-based synthesis, outlining the critical steps used to obtain a monodisperse solution of surface-passivated CsPbBr$_3$ pQDs. Step 1: LARP synthesis, leading to a turbid solution of polydisperse pQDs. Step 2: Addition of PPA amines for post-synthetic size-trimming, leading to a transparent solution of monodisperse pQDs. Step 3: Addition of DDAB ligands, leading to a stable colloidal solution of surface-passivated pQDs. (b) Room-temperature absorption (dashed black line) of the resulting pQDs solution and its corresponding PL spectrum (solid black line) centered at 512 nm with a FWHM of 19 nm. (c) High resolution transmission electron microscopy (HRTEM) image of an individual CsPbBr$_3$ pQD (left) and the corresponding FFT-derived diffraction pattern (right).
• Figure 2: (a) High-resolution PL map of an isolated single pQD (distinct from the pQD under study), obtained with a laser spot size of $\sim$1.2 µm and a scanning step of 0.2 µm. Lower panel: horizontal line cut through the center of the map, fitted with a Gaussian profile (FWHM = 1.5 µm). (b) PL spectrum of the excitonic emission from a single pQD at 4 K. The black dots are experimental data, and the solid red and blue lines are Lorentzian fits with linewidths of 1.0 meV and 0.7 meV, respectively, separated by 0.9 meV. (c) Emission polarization diagram, where the blue and red dots represent the intensity of each spectral component of the excitonic spectrum in (b). The faded points correspond to duplicated data used to extend the plot over the full 0-360° range. (d) Temporal PL trace recorded over 2 with a binning time of 200 ms.
• Figure 3: (a) Time-resolved PL of an individual pQD (blue dots). The black line is the Instrumental Response Function (IRF) measured at the pQD emission wavelength. The blue solid line is a bi-exponential fit of the PL decay, taking the IRF into account, with short- and long-time components $T_s\sim 87$ ps and $T_\ell\sim 751$ ps, respectively. (b) Intensity auto-correlation of the excitonic emission measured in a HBT setup under pulsed excitation, where $g^{(2)}(0)=0.12 \pm 0.06$ after background subtraction. (c) Same measurement as (b) under CW excitation, where the IRF of the HBT setup is shown as a grey shaded area and the experimental data (black line) are fitted by the theoretical $g_{\mathrm{fit}}^{(2)}$ function convoluted with the IRF (red line), giving the corrected value $g^{(2)}_{\mathrm{fit}}(0)=0.01 \pm 0.01$.
• Figure 4: (a) PL spectrum of a single pQD showing the exciton (X), trion (X*), and biexciton (XX) emission lines. (b) Integrated PL intensity of each line as a function of excitation power plotted on a log-log scale. Experimental data (circles) are fitted with power-law dependencies (solid lines), from which the exponent $\beta$ is extracted and indicated next to each fit. (c) PL spectra of three different pQDs, including the pQD analyzed throughout this work, plotted on a semi-logarithmic scale. Two optical phonon replicas of the excitonic emission are observed at $-3.5$ meV and $-6.3$ meV, highlighted by vertical dashed lines.
• Figure S1: (a,c) TEM images of the CsPbBr$_3$ pQDs synthesized by the optimized LARP synthesis protocol. (b,d) Corresponding particle size distribution histograms with average length of $10.3 \pm 1.7$ nm and $10.7 \pm 1.8$ nm, respectively. (e) Total size distribution obtained by summing histograms (b) and (d). (f) Aspect ratio extracted from the TEM images, with a mean value of $1.17 \pm 0.15$.