Two‐Step Anti‐Stokes Photoluminescence of CsPbX₃ Nanocrystals

Abstract

Cesium lead halide (CsPbX3, X = Cl, Br, and I) perovskite nanocrystals (NCs) exhibit efficient anti‐Stokes photoluminescence (ASPL) upon sub‐gap single‐photon excitation, which has great potential for all‐solid‐state laser cooling. However, the mechanism responsible for the ASPL still remains elusive. Here, ASPL properties of CsPb(BrI)3 NCs and CsPbI3 NCs in inorganic solid matrix are investigated. Excitation wavelength‐ and power‐dependent ASPL demonstrates that it is a phonon‐assisted single‐photon process. Femtosecond transient absorption spectra show that ASPL is mediated by the Urbach tail states, where localized excitons are generated upon sub‐gap excitation. Through electron–phonon coupling, these localized excitons change into free exciton to generate the exciton bleaching, and then thermally dissociate into free carriers accompanied by band edge bleaching, leading to the efficient ASPL. Such a two‐step up‐conversion process is further confirmed by the carrier temperature extracted from the band edge bleaching band. This work deepens the understanding of ASPL in CsPbX3 NCs, and is valuable for the development of materials for solid‐state laser cooling.

Anti-Stokes Photoluminescence Regulated by Lattice Distortion of CsPbX3 Nanocrystals in Glasses

“…These values are comparable to those previously reported. , Notably, the electron–phonon interaction strength ( A EP ) of CsPb­(BrI) 3 NCs in the WG specimen is found to be 126.1 meV, much larger than that obtained from the NG specimen (84.4 meV) (Figure S3d), confirming that electron–phonon interaction in CsPb­(BrI) 3 NCs is enhanced due to the water quenching-induced distortion of [PbX 6 ] octahedra. It has been reported that the distortion of the [PbX 6 ] octahedra can enhance the tail states, leading to the localization of photogenerated charge carriers via the interaction with phonons . Feldmann et al reported that the incorporation of Mn 2+ ions into the CsPb­(ClBr) 3 NCs breaks the lattice periodicity, leading to charge carrier localization and enhancing the intrinsic radiative recombination .…”

“…Upon SGE, the maximal upconversion energies (difference between the ASPL peak energy and excitation energy) are 163.7 meV for the NG specimen and 174.3 meV for the WG specimen. These features of ASPL from the NG and WG specimens are consistent with our previous research and other literature, , confirming that the ASPL of the CsPb­(BrI) 3 NCs in the NG and WG specimens is closely related to the Urbach tail states.…”

“…These XB bands are converted to PB bands within 2 ps (Figures and S8). The energy differences between the XB bands and PB bands are found to be ∼21 and ∼18 meV for the NG and WG specimens, respectively, corresponding to the exciton binding energy reported previously. , It is also noted that the spectral positions of these XB bands are independent of the SGE pump wavelength, and as a result, the XB bands are induced by the free exciton, instead of the localized exciton. The direct conversion of these localized excitons into free carriers is unlikely to occur since much more energy and involvement of multiple phonons are required to convert these distorted [PbBr 6 ] octahedron- trapped excitons into free carriers .…”

“…The band tail states of CsPbX 3 NCs in glasses also affect their PL properties. In our previous work, ASPL of CsPbX 3 NCs in glasses was realized, and the ASPL was considered to be realized through the excitation into the band tail states composed of distorted [PbX 6 ] octahedra on the surface layer of the CsPbX 3 NCs and subsequent thermalization to the band edge of CsPbX 3 NCs . Based on this mechanism, introducing a distorted octahedron to adjust the tail state distribution is expected to improve the ASPL of CsPbX 3 NCs.…”

“…In our previous work, ASPL of CsPbX 3 NCs in glasses was realized, and the ASPL was considered to be realized through the excitation into the band tail states composed of distorted [PbX 6 ] octahedra on the surface layer of the CsPbX 3 NCs and subsequent thermalization to the band edge of CsPbX 3 NCs. 37 Based on this mechanism, introducing a distorted octahedron to adjust the tail state distribution is expected to improve the ASPL of CsPbX 3 NCs. Since CsPbX 3 NC-embedded glasses are mainly prepared via high-temperature heat treatment, it is possible to control the crystalline quality (distortion of the [PbX 6 ] octahedra and band tail states) of these CsPbX 3 NCs in glasses and adjust the PL properties of CsPbX 3 NCs by modulating the thermal processing conditions.…”