The dark exciton ground state promotes photon-pair emission in individual perovskite nanocrystals

Abstract

Cesium lead halide perovskites exhibit outstanding optical and electronic properties for a wide range of applications in optoelectronics and for light-emitting devices. Yet, the physics of the band-edge exciton, whose recombination is at the origin of the photoluminescence, is not elucidated. Here, we unveil the exciton fine structure of individual cesium lead iodide perovskite nanocrystals and demonstrate that it is governed by the electron-hole exchange interaction and nanocrystal shape anisotropy. The lowest-energy exciton state is a long-lived dark singlet state, which promotes the creation of biexcitons at low temperatures and thus correlated photon pairs. These bright quantum emitters in the near-infrared have a photon statistics that can readily be tuned from bunching to antibunching, using magnetic or thermal coupling between dark and bright exciton sublevels.

Revealing the Exciton Fine Structure in Lead Halide Perovskite Nanocrystals

“…Early investigations of the PL decay from ensembles of CsPbBr 3 perovskite NCs have unveiled the onset of a long-time component under high magnetic fields (above 10 T), pointing to magnetic brightening of a long-lived state attributed to the dark exciton [ 50 , 51 ]. The direct spectroscopic signature of the dark exciton emission under magnetic fields has recently been discovered on hybrid organic–inorganic FAPbBr 3 and fully inorganic CsPbI 3 single NCs [ 40 , 42 ], showing evidence for both perovskite species that the ground exciton state is the dark singlet state lying a few meV below the bright triplet in these perovskite NCs ( Figure 2 a–c). The identification of the bright state that is coupled with the dark state is readily deduced from the polarization of the emission lines, as shown in Figure 2 d for a FAPbBr 3 NC.…”

“…At temperatures of a few Kelvins, the PL intensity of single lead halide perovskite NCs is usually stable, with shot-noise limited variations over minutes of integration time ( Figure 1 d). Since thermal dephasing is significantly reduced, single NCs of various compositions such as CsPbBr 3 [ 4 , 15 , 30 , 33 , 37 , 38 , 39 ], CsPbI 3 [ 32 , 40 , 41 ], FAPbBr 3 [ 42 , 43 ], FAPbI 3 [ 44 ], MAPbBr 3 [ 45 ] and MAPbI 3 [ 46 ] show extremely sharp lines in their PL spectrum, with line widths often limited by the resolution of the spectrograph (few hundreds of μeV), as shown in Figure 1 e,f. These peaks are assigned to zero-phonon radiative recombination lines (ZPLs) of the band-edge neutral exciton.…”

“…The triplet spectral structures observed in the single-NC PL spectra offer the opportunity to investigate their differences from one NC to the other. The observation of one, two or three ZPLs is associated with different degrees of degeneracy of the bright triplet, which results from both crystalline structure and NC shape anisotropies ( Figure 1 c) [ 29 , 31 , 40 ]. For example, assuming a cubic crystal structure (resp.…”

“…A theoretical model based on a variational approach has been developed to evaluate the exchange interaction in cuboid shaped NCs. Assuming a cubic crystal structure [ 40 ], it takes into account the effects of quantum confinement and dielectric confinement in NCs of various sizes and aspect ratios. It reproduces well the set of singlet–triplet splittings experimentally observed on CsPbI 3 NCs [ 40 ] ( Figure 2 e), without resorting to the hypothetical Rashba effect.…”

“…Assuming a cubic crystal structure [ 40 ], it takes into account the effects of quantum confinement and dielectric confinement in NCs of various sizes and aspect ratios. It reproduces well the set of singlet–triplet splittings experimentally observed on CsPbI 3 NCs [ 40 ] ( Figure 2 e), without resorting to the hypothetical Rashba effect. The calculations also show that among all common lead halide perovskite species, CsPbBr 3 perovskites have the largest bulk electron-hole exchange interaction, which could explain the failed attempts to observe brightening of the optically inactive state in single CsPbBr 3 NCs with magnetic fields of less than 7 T [ 30 ].…”