Emergence of multiple fluorophores in individual cesium lead bromide nanocrystals

Abstract

Cesium-based perovskite nanocrystals (PNCs) possess alluring optical and electronic properties via compositional and structural versatility, tunable bandgap, high photoluminescence quantum yield and facile chemical synthesis. Despite the recent progress, origins of the photoluminescence emission in various types of PNCs remains unclear. Here, we study the photon emission from individual three-dimensional and zero-dimensional cesium lead bromide PNCs. Using photon antibunching and lifetime measurements, we demonstrate that emission statistics of both type of PNCs are akin to individual molecular fluorophores, rather than traditional semiconductor quantum dots. Aided by density functional modelling, we provide compelling evidence that green emission in zero-dimensional PNCs stems from exciton recombination at bromide vacancy centres within lead-halide octahedra, unrelated to external confinement. These findings provide key information about the nature of defect formation and the origin of emission in cesium lead halide perovskite materials, which foster their utilization in the emerging optoelectronic applications.

Fig. 1

Representative blinking behavior of a molecular emitter and a colloidal nanocrystal quantum dot. a–c A single Alexa 594 dye molecule and d–f individual CdSe/CdS core/shell nanocrystal quantum dot. PL lifetimes extracted from different intensity levels are color-coded to highlighted regions in blinking traces. Molecular blinking is characterized by burst-like emission with nearly uniform distribution of lifetimes across the intensity range. NQD’s blinking shows well-defined emission intensity levels with discrete lifetime values due to radiative recombination in various excitonic complexes. X⁰—neutral exciton, X⁻ —negatively charged trion, X⁺—positively charged trion, XX*—multiply charged excitons or charged biexcitons. Bin size is 20 ms

Fig. 2

Structural and optical comparison between 3D CsPbBr₃ NCs and 0D Cs₄PbBr₆ NCs. a XRD patterns of perovskite nanocrystals, with inset schematics showing a corner-sharing octahedra network and a discrete octahedra lattice, respectively. Room-temperature absorption (Abs, solid line), photoluminescence excitation (PLE, emission monitored at 520 nm, dotted line), and photoluminescence (PL, excitation at 350 nm, green line) spectra of b colloidal CsPbBr₃ NCs and c Cs₄PbBr₆ NCs, respectively. d Blinking trace recorded for an individual CsPbBr₃ NC at the excitation level of <N> = 1. e Extracted PL lifetimes from short, sub-500 ms bursts in a blinking trace and color-coded with highlighted regions in d. f Antibunching trace

Fig. 3

Blinking statistics of an individual 0D PNC at low a–i and high j–m excitation fluxes, respectively. a Wide-field illumination image of isolated 0D PNCs, excitation wavelength 400 nm. b PL spectrum of an individual 0D PNC. c Antibunching functions at different excitation levels, both show single emitter behavior (g²(0) <0.5). d, e Blinking trace and intensity distribution histogram at <N> = 0.2 with ‘ON’ and ‘GRAY’ states. f, g Expanded view of the blinking trace and PL lifetime extracted from the sub-second ‘ON’ intensity level shown by shaded bar. h, i Same for sub-second ‘GRAY’ level. j–m Excitation level of <N> = 1.3. j Blinking trace exhibiting burst-like behavior. k–m PL lifetimes extracted from different sub-second intensity bursts and color-coded to shaded bars in (j). Bin size is 20 ms

Fig. 4

Blinking in two individual 0D PNCs (a, b) and (c–h) with the emergence of multiple emitters. a Blinking trace exhibiting large bursts from multiple emitters. Inset—antibunching trace with large g²(0) value. b PL lifetimes extracted from different sub-second intensity bursts and color-coded to shaded boxes or bars in (a). Excitation level <N> = 1. c PL intensity histogram and d blinking trace for another dot at low excitation level of <N> = 0.16. Inset—antibunching trace indicative of single emitter. e Intensity histogram for the same dot at <N> = 0.5. Black trace—three Gaussian fit, colored traces—decomposition to individual intensity levels. f Blinking trace with several emitter levels highlighted by colored bars. g PL intensity of each emission level as function of the excitation <N>. Solid dots correspond to the increase of excitation to <N> = 1, triangles—decrease to <N> = 0.08. Dotted lines—linear fits. Error bars correspond to the widths of the Gaussian distributions in intensity histograms. h PL lifetimes at several intensity levels and color-coded to shaded bars in the corresponding blinking panels. Bin size is 20 ms

Fig. 5

Experimental and theoretical explanation of the Br-vacancy origin of PL in 0D Cs₄PbBr₆ NC. a HRTEM image and corresponding FFT pattern of a single Cs₄PbBr₆ PNC along the [021] zone axis, showing clear lattice fringes without any impurity phase. b DFT calculation of V_Br-induced mid-gap state between conduction band minima (CBM) and valence band maxima (VBM) of Cs₄PbBr₆ crystal, c DFT calculation of molecular energy levels of a single Cs₄PbBr₆ unit, insets show a highly localized density of states of LUMO level. Note that both DFTs were performed at HSE + SOC level. d Schematics of the activated/inactivated Br vacancies at different excitation powers