Dark and Bright Excitons in Halide Perovskite Nanoplatelets

Abstract

Semiconductor nanoplatelets (NPLs), with their large exciton binding energy, narrow photoluminescence (PL), and absence of dielectric screening for photons emitted normal to the NPL surface, could be expected to become the fastest luminophores amongst all colloidal nanostructures. However, super-fast emission is suppressed by a dark (optically passive) exciton ground state, substantially split from a higher-lying bright (optically active) state. Here, the exciton fine structure in 2-8 monolayer (ML) thick Cs n − 1 Pb n Br 3n + 1 NPLs is revealed by merging temperature-resolved PL spectra and time-resolved PL decay with an effective mass model taking quantum confinement and dielectric confinement anisotropy into account. This approach exposes a thickness-dependent bright-dark exciton splitting reaching 32.3 meV for the 2 ML NPLs. The model also reveals a 5-16 meV splitting of the bright exciton states with transition dipoles polarized parallel and perpendicular to the NPL surfaces, the order of which is reversed for the thinnest NPLs, as confirmed by TR-PL measurements. Accordingly, the individual bright states must be taken into account, while the dark exciton state strongly affects the optical properties of the thinnest NPLs even at room temperature. Significantly, the derived model can be generalized for any isotropically or anisotropically confined nanostructure.

How Exciton–Phonon Coupling Impacts Photoluminescence in Halide Perovskite Nanoplatelets

“…Below 50 K, a strong discontinuity is seen for the 2 and 3 ML samples, with the PL emission jumping to lower energies with decreasing temperature by 33 and 16 meV, respectively. This low-temperature shift can be explained by the excitonic fine structure of the NPLs, as we recently showed . For the thinnest NPLs, the splitting between the lower lying “dark” excitonic level and the lowest “bright” level increases significantly, and a thermal transition of an exciton from the dark into a bright level is not possible at low temperatures.…”

“…The in-plane exciton is the energetically favorable bright state for all thicknesses but the 2 ML NPLs. For these, the lowest bright exciton possesses the out-of-plane polarization, resulting in a more significant fraction of the overall exciton population polarized out-of-plane . As an LO phonon in a polar crystal leads to a macroscopic oscillating electric field with which the excitons interact, the polarization of an exciton could strongly affect this interaction.…”

Energy Transfer in Stability-Optimized Perovskite Nanocrystals

“…1,2 Scientifically, their flexible compositions and precise size control allow for studies on exciton fine structure, level inversion, polaron formation, carrier dynamics, and phonon interaction. [3][4][5][6][7][8][9] With emission wavelengths tunable throughout the visible range, quantum yields approaching unity, cheap and facile syntheses, and abundant precursor materials, potential applications range from light emission (light-emitting diodes (LEDs), lasers, and displays) to solar cells, photodetectors, field-effect transistors, and even photocatalysis. [10][11][12][13][14] Despite sounding like the perfect material, halide perovskites also (currently) exhibit limitations that have been impeding their widespread commercialization.…”

Effects of Electronic Coupling on Bright and Dark Excitons in a 2D Array of Strongly Confined CsPbBr3 Quantum Dots

“…30,42,46 ΔE BD was also found to decrease as the thickness of the nanoplatelets increased. 47 A direct comparison of the effect of the size increase in uncoupled QDs and the delocalization of excitons limited to the lateral direction in a 2D array of the coupled QDs will not be quantitatively valid. However, comparing these data the sizedependent ΔE BD values reported in ref 30 indicates that the It is interesting to note that τ fast at 5 K is smaller in NaBrpassivated QDs than OLAB-passivated QDs, which contrasts with the bright exciton relaxation on a nanosecond time scale at 300 K in NaBr-passivated QDs (τ = 8.8 ns) being slower than that in OLAB-passivated QDs (τ = 4.2 ns), as shown in Figure 3.…”