Colloidal Aziridinium Lead Bromide Quantum Dots

Abstract

The compositional engineering of lead-halide perovskite nanocrystals (NCs) via the A-site cation represents a lever to fine-tune their structural and electronic properties. However, the presently available chemical space remains minimal since, thus far, only three A-site cations have been reported to favor the formation of stable lead-halide perovskite NCs, i.e., Cs⁺, formamidinium (FA), and methylammonium (MA). Inspired by recent reports on bulk single crystals with aziridinium (AZ) as the A-site cation, we present a facile colloidal synthesis of AZPbBr₃ NCs with a narrow size distribution and size tunability down to 4 nm, producing quantum dots (QDs) in the regime of strong quantum confinement. NMR and Raman spectroscopies confirm the stabilization of the AZ cations in the locally distorted cubic structure. AZPbBr₃ QDs exhibit bright photoluminescence with quantum efficiencies of up to 80%. Stabilized with cationic and zwitterionic capping ligands, single AZPbBr₃ QDs exhibit stable single-photon emission, which is another essential attribute of QDs. In particular, didodecyldimethylammonium bromide and 2-octyldodecyl-phosphoethanolamine ligands afford AZPbBr₃ QDs with high spectral stability at both room and cryogenic temperatures, reduced blinking with a characteristic ON fraction larger than 85%, and high single-photon purity (g⁽²⁾(0) = 0.1), all comparable to the best-reported values for MAPbBr₃ and FAPbBr₃ QDs of the same size.

Keywords: aziridinium; ligands; nanocrystals; perovskite; photoluminescence; quantum dots.

Figure 1

(a) Calculated Goldschmidt tolerance factors for different cations (cesium, methylammonium, aziridinium, and formamidinium) in the APbBr₃ perovskite lattice. (b) Top panel: a reaction scheme; bottom panel: an overview of carboxylic and phosphonic acids as well as ligands used in the synthesis. (c) Optical absorption and (d) PL spectra of purified AZPbBr₃ NCs ranging from 4.5 to 14 nm in size (for visualization purposes, a cumulative vertical offset was applied to each subsequent spectra). (e) A high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image of purified 8 nm NCs with a high-resolution HAADF-STEM image of few single NC in the inset. (f) Size-dependent (absorption) band gap in AZPbBr₃ NCs for sizes obtained via TEM (green) and SAXS (gray); the experimental data sets (open squares, with error bars denoting the standard deviation) were fitted by a semiempirical sizing curve (solid line), with the bulk band gap (dashed lines) as one of the fit parameters.

Figure 2

(a) The fit of experimental SAXS data from an AZPbBr₃ NC colloidal suspension (gray line) via an analytical model (black line) yields cuboids with edge lengths of 8.27 ± 0.12, 8.28 ± 0.93, and 8.60 ± 0.48 nm, respectively. (b) Solvent-subtracted synchrotron WAXTS data (black line) and the best fit (green line) of AZPbBr₃ NCs using the split cubic model with locally tilted/disordered PbBr₆ octahedra (for a more extended discussion, see the Supporting Information); the inset shows a 2D map of the refined (number-based) log-normal size distribution function (D_ab: the diameter of the circle of area equivalent to the prisms basal plane and L_c, the height of the prismatic clusters). (c) The split cubic structural model of AZPbBr₃ NCs (the four Br of each PbBr₆ octahedra, shown in green, have 1/4 site occupancy factor each, i.e., only one out of four is stochastically present in each site) and the model of AZ cation formed by disordering in 12 symmetry-equivalent orientations (4 equivalent geometrical orientations × 3 “elemental” C–C/N dispositions, H atoms were omitted, similar to bulk AZPbBr₃). (d) The temperature dependence of the tetragonal unit cell parameters a′ = a/√2 and c for a weakly distorted (ca. 1%) cubic lattice, plotted in the entire 11–290 K range, showing an anomaly below 90 K. (e) The temperature dependence of the fwhm₀ parameter (the θ-dependence of the peak width is described according to the relation fwhm(θ) = fwhm₀/cos θ), which suggests additional peak broadening (hidden splitting) below 90 K.

Figure 3

(a) Solution ¹H NMR spectra of C₈C₁₂–PEA (black line) and DDAB-capped (blue line) AZPbBr₃ NCs in cyclohexane-d₁₂. (b) Solution ³¹P NMR spectra of C₈C₁₂–PEA-capped AZPbBr₃ NCs synthesized without (top) and with (middle) addition of alkylphosphonic acid (for instance, octylphosphonic acid-C₈PAc) during the synthesis and after their further digestion with DMSO-d₆ (bottom). (c) Solid-state ²⁰⁷Pb NMR spectrum of C₈C₁₂–PEA-capped AZPbBr₃ NCs.

Figure 4

Room-temperature PL of single AZPbBr₃ QDs with various surface capping ligands. (a) PL spectrum of a single C₈C₁₂–PEA-capped AZPbBr₃ QD displaying narrow-band emission with a fwhm of 82 meV. (b) Spectra series of a C₈C₁₂–PEA-capped QD exhibiting spectrally stable PL and small intensity variations. The highlighted time period (shaded area without PL spectral detection) corresponds to the acquisition of the second-order correlation function (g⁽²⁾(t), shown in panel (c)) and blinking trace by a Hanbury–Brown and Twiss experiment. (c) Second-order photon–photon correlation of a C₈C₁₂–PEA-capped AZPbBr₃ QD displaying high single-photon purity (g⁽²⁾(0) = 0.1). (d–g) Representative PL blinking traces (10 ms binning time) of a QD capped with branched C₈C₁₂–PEA ligands (panel (d)), lecithin (panel (e)), DDAB (panel (f)), and dicationic amine C₃–4C₁₂AB (panel (g)). (h) Histograms of the fraction of time that single QDs spend in their bright (ON) state. The numbers after the ligand name indicate the ensemble PL central wavelength of the respective sample.

Figure 5

Exciton fine-structure of single AZPbBr₃ QDs at 4 K. (a) Time series of a single QD (1 s integration time and 1800 g/mm grating); sub-meV spectral diffusion allows to resolve the exciton fine structure. This QD exhibits a doublet fine structure with peaks denoted as FS₁ and FS₂. The inset shows an associated histogram of the peak energies (bars), fitted with a Gaussian function (lines). (b) The PL spectra of one single QD measured at two different angles of a linear polarizer in the detection path: 45° (blue) and 135° (gray); this QD has a doublet exciton fine structure. The inset shows a polar plot with the respective PL intensities, as a function of the polarizer angle; both doublet sublevels exhibit highly linear polarization, oriented perpendicular to each other; (c, d) Fine-structure splitting energy (FSS) for all of the single QDs (capped with DDAB or C₈C₁₂–PEA ligands) exhibit doublet sublevels (panel (c)) or triplet sublevels (panel (d)). The insets in panels (c) and (d) show representative spectra for doublet and triplet exciton fine structures, respectively. (e) Second-order correlation function g⁽²⁾(τ) of a single AZPbBr₃ QD under 0.3 μJ/cm² before (dark gray line) and after (blue line) spectrally filtering out the biexciton emission by a tunable short-pass filter.