Long-Lived Dark Exciton Emission in Mn-Doped CsPbCl₃ Perovskite Nanocrystals

Abstract

The unusual temperature dependence of exciton emission decay in CsPbX3 perovskite nanocrystals (NCs) attracts considerable attention. Upon cooling, extremely short (sub-ns) lifetimes were observed and were explained by an inverted bright–dark state splitting. Here, we report temperature-dependent exciton lifetimes for CsPbCl3 NCs doped with 0–41% Mn2+. The exciton emission lifetime increases upon cooling from 300 to 75 K. Upon further cooling, a strong and fast sub-ns decay component develops. However, the decay is strongly biexponential and also a weak, slow decay component is observed with a ∼40–50 ns lifetime below 20 K. The slow component has a ∼5–10 times stronger relative intensity in Mn-doped NCs compared to that in undoped CsPbCl3 NCs. The temperature dependence of the slow component resembles that of CdSe and PbSe quantum dots with an activation energy of ∼19 meV for the dark–bright state splitting. Based on our observations, we propose an alternative explanation for the short, sub-ns exciton decay time in CsPbX3 NCs. Slow bright–dark state relaxation at cryogenic temperatures gives rise to almost exclusively bright state emission. Incorporation of Mn2+ or high magnetic fields enhances the bright–dark state relaxation and allows for the observation of the long-lived dark state emission at cryogenic temperatures.

Magnetic-brightening and control of dark exciton in CsPbBr3 perovskite

“…And the electron-hole exchange interaction leads to the formation of an optically active bright exciton state (J=1, spin-allowed) and an optically inactive dark exciton state (J=0, spin-forbidden), respectively [31,39]. Based on the above discussion, a three-level model including a spinallowed bright state level, a spin-forbidden dark state level and a ground state level, has been proposed to understand the recombination mechanism of exciton [34,40]. As shown in Fig.…”

“…2b, in the absence of external magnetic field, the PL only originates from the recombination of bright exciton. However, when an external magnetic field is applied, the dark state is brightened by coupling with one of the bright triplet states combined with an extremely slow bright-dark state relaxation [30,34,40]. Thus, the external magnetic field opens a new radiative recombination channel for the dark excitons via mixing the dark state with the bright state (in Fig.…”

“…2c). In zero field, the bright exciton PL decay is fast; when a magnetic field is applied, a slow dark exciton radiative recombination component appears, and the long-time component drastically shortens with increasing magnetic field [30,34,40], thus the PL intensity of the dark excitons increases with increasing magnetic field. The signature of the magnetic brightened dark state was further explored by raising the magnetic field from 0 to 7 T. The emission spectra versus magnetic field at 10 K are plotted in Fig.…”

“…Magnetic-photoluminescence (PL) spectroscopy is a powerful tool to address exciton properties and disclose the fine-structure of exciton [33]. Recently, magnetic-dependent PL dynamics spectrum has been studied to validate the existence of dark exciton in CsPbX 3 NCs; a fast-decay and a slow-decay component can be observed, which are assigned to the bright and dark exciton, respectively [34,35]. The abnormal inversion of the dark-bright splitting was also observed in CsPbX 3 perovskite arising from the Rashba effect together with strong spin-orbital coupling (SOC) [36].…”

Reversible Thermochromism and Strong Ferromagnetism in Two‐Dimensional Hybrid Perovskites

“…Thet emperature-dependent conductivity change suggests athermally activated charge transport mechanism. Fitting the conducting data to the Arrhenius equation s = s 0 exp(ÀE a /k B T) [42,43] can give the activation energy (E a )o ft he materials for charge transport, where s is the electrical conductivity, s 0 is aprefactor, k B is the Boltzmann constant, and T is the absolute temperature.W eobtained an E a of 299 meV for the (PED)CuCl 4 ( Figure S14). In comparison, the E a of (BED) 2 CuCl 6 is estimated to be 960 meV.In2D perovskites,t he organic cations are considered as insulating groups between the conductive inorganic layers.…”