The effect of surface ligands on the surface chemical states and photoluminescence characteristics in cesium lead bromide perovskite nanocrystals

This paper presents the results of our study on the relationship between the surface chemical states, which are influenced by ligands, and photoluminescence (PL) characteristics in cesium lead halide perovskite nanocrystals (NCs). NCs were synthesized <i>via</i> the Ligand-Assisted Reprecipitation (LARP) and Ultrasonic-Assisted (URSOA) methods, which were able to produce NCs with and without ligands. Although both synthesis methods used similar precursor composition and processing steps, the resulting crystal structures of NCs are different. The LARP method yielded orthorhombic CsPbBr₃ NCs, while the URSOA method yielded a mixture of hexagonal Cs₄PbBr₆ NCs and orthorhombic CsPbBr₃ NCs with an approximate weight ratio of ∼10 : 1. The X-ray diffraction data indicated that both NCs with and without ligands have the same crystal structure. However, photoelectron spectroscopy (XPS and HAXPES) analysis showed that chemical states in NCs without ligands differ between the inner side and the surface, which could be associated with surface defect species from the accumulation of Cs⁺ atoms, Pb atoms with zero oxidation state (Pb⁰), unbonded Br atoms, and Br vacancies at the surface of the NCs. The difference appears to be correlated with the observed PL characteristics. Although photoelectron spectroscopy measures the core level orbitals, the measured chemical states may indicate electronic structure alteration in valence orbitals, which are involved in photoexcitation and exciton relaxation processes. The PL of LARP NCs (orthorhombic CsPbBr₃) shows two components of PL decay, which are largely suppressed in NCs with purification or NCs without ligands. However, for URSOA NCs (predominantly Cs₄PbBr₆ NCs), the PL decays are almost similar for both with and without ligands. The present experimental results clearly show that the variations in PL characteristics, besides the crystal structure that determines the intrinsic properties of the formed excitons, may also come from surface states or surface defect species influenced by surface ligands. In addition, the results can also explain the much higher degree of defect tolerance properties in URSOA NCs compared to LARP NCs. The insights gained from this work may be useful not only for further development of passivation molecules in a general context but also for designing buffer layer molecules in perovskite heterojunction devices.