Juliette Zito

Halide exchange in lead‐based halide perovskites has been studied extensively. While mixed Cl/Br or Br/I alloy compositions can be formed with no miscibility gaps, this is precluded for mixed Cl/I compositions, due to the large difference in Cl ⁻ and I ⁻ ionic radii. Here, perovskite‐chalcohalide CsPbCl ₃ –Pb ₄ S ₃ Cl ₂ nanocrystal heterostructures are exploited to study the Cl→I exchange and to isolate new types of intermediate structures. The epitaxial interface between the Pb ₄ S ₃ Cl ₂ chalcohalide and the CsPbCl ₃ perovskite significantly influences the intermediate stages of halide exchange in the perovskite domain, leading to coexisting CsPbCl ₃ and CsPbI ₃ domains, thereby delivering segmented CsPbI ₃ –CsPbCl ₃ –Pb ₄ S ₃ Cl ₂ , energetically favorable heterostructures, with partial I‐alloying of the CsPbCl ₃ domain and at the perovskite–chalcohalide interface. The I:CsPbCl ₃ domain between CsPbI ₃ and Pb ₄ S ₃ Cl ₂ enables a gradual lattice expansion across the heterostructure. This design accommodates interfacial strain, with a 5.6% mismatch at the CsPbCl ₃ –CsPbI ₃ interface and a 3.4% mismatch at the perovskite–chalcohalide interface. Full halide exchange leads to CsPbI ₃ –Pb ₄ S ₃ Cl ₂ heterostructures. Both in intermediate and fully exchanged heterostructures, the CsPbI ₃ domain is emissive. In the intermediate structures, the band alignment between the two perovskite domains is type‐I, with the carriers photogenerated in the CsPbCl ₃ domain quickly transferring to the CsPbI ₃ domain, where they can recombine radiatively.

We report the synthesis of nanocrystal heterostructures composed of CsPbCl₃ and PbS domains sharing an epitaxial interface. We were able to promote the growth of a PbS domain (in competition with the more commonly observed Pb₄S₃Cl₂ one) on top of the CsPbCl₃ domain by employing Mn²⁺ ions, the latter most likely acting as scavengers of Cl^- ions. Complete suppression of the Pb₄S₃Cl₂ domain growth was then achieved by additionally selecting an appropriate sulfur source (bis(trimethylsilyl)sulfide, which also acted as a scavenger of Cl^- ions) and reaction temperature. In the heterostructures, emission from the perovskite domain was quenched, while emission from the PbS domain was observed, pointing to a type-I band alignment, as confirmed by calculations. These heterostructures, in turn, could be exploited to prepare second-generation heterostructures through selective ion exchange on the individual domains (halide ion exchange on CsPbCl₃ and cation exchange on PbS). We demonstrate the cases of Cl^- → Br^- and Pb²⁺ → Cu⁺ exchanges, which deliver CsPbBr₃-PbS and CsPbCl₃-Cu_2-xS epitaxial heterostructures, respectively.

Current syntheses of CsPbBr₃ halide perovskite nanocrystals (NCs) rely on overstoichiometric amounts of Pb²⁺ precursors, resulting in unreacted lead ions at the end of the process. In our synthesis scheme of CsPbBr₃ NCs, we replaced excess Pb²⁺ with different exogenous metal cations (M) and investigated their effect on the synthesis products. These cations can be divided into two groups: group 1 delivers monodisperse CsPbBr₃ cubes capped with oleate species (as for the case when Pb²⁺ is used in excess) and with a photoluminescence quantum yield (PLQY) as high as 90% with some cations (for example with M = In³⁺); group 2 yields irregularly shaped CsPbBr₃ NCs with broad size distributions. In both cases, the addition of a tertiary ammonium cation (didodecylmethylammonium, DDMA⁺) during the synthesis, after the nucleation of the NCs, reshapes the NCs to monodisperse truncated cubes. Such NCs feature a mixed oleate/DDMA⁺ surface termination with PLQY values of up to 97%. For group 1 cations this happens only if the ammonium cation is directly added as a salt (DDMA-Br), while for group 2 cations this happens even if the corresponding tertiary amine (DDMA) is added, instead of DDMA-Br. This is attributed to the fact that only group 2 cations can facilitate the protonation of DDMA by the excess oleic acid present in the reaction environment. In all cases studied, the incorporation of M cations is marginal, and the reshaping of the NCs is only transient: if the reactions are run for a long time, the truncated cubes evolve to cubes.

We show how, in the synthesis of yellow-emissive Bi-doped Cs₂Ag_1-xNa_xInCl₆ double perovskite nanocrystals (NCs), preventing the transient formation of Ag⁰ particles increases the photoluminescence quantum yield (PLQY) of the NCs from ∼30% to ∼60%. Calculations indicate that the presence of even a single Ag⁰ species on the surface of a NC introduces deep trap states. The PL efficiency of these NCs is further increased to ∼70% by partial replacement of Na⁺ with K⁺ ions, up to a 7% K content, due to a lattice expansion that promotes a more favorable ligands packing on the NC surface, hence better surface passivation. A further increase in K⁺ lowers the PLQY, due to both the activation of nonradiative quenching channels and a lower oscillator strength of the BiCl₆→AgCl₆ transition (through which PL emission occurs). The work indicates how a deeper understanding of parameters influencing carrier trapping/relaxation can boost the PLQY of double perovskites NCs.