We report the synthesis of colloidal CsPbX3–Pb4S3Br2 (X = Cl, Br, I) nanocrystal heterostructures, providing an example of a sharp and atomically resolved epitaxial interface between a metal halide perovskite and a non-perovskite lattice. The CsPbBr3–Pb4S3Br2 nanocrystals are prepared by a two-step direct synthesis using preformed subnanometer CsPbBr3 clusters. Density functional theory calculations indicate the creation of a quasi-type II alignment at the heterointerface as well as the formation of localized trap states, promoting ultrafast separation of photogenerated excitons and carrier trapping, as confirmed by spectroscopic experiments. Postsynthesis reaction with either Cl– or I– ions delivers the corresponding CsPbCl3–Pb4S3Br2 and CsPbI3–Pb4S3Br2 heterostructures, thus enabling anion exchange only in the perovskite domain. An increased structural rigidity is conferred to the perovskite lattice when it is interfaced with the chalcohalide lattice. This is attested by the improved stability of the metastable γ phase (or “black” phase) of CsPbI3 in the CsPbI3–Pb4S3Br2 heterostructure.

Colloidal lead sulfide nanosheets have attracted broad interest for a wide variety of device applications, including field-effect transistors, solar cells, and spintronic devices. Whereas confinement effects in PbS quantum dots are well studied, they are still unclear in 2-dimensional ultrathin PbS nanosheets, especially in the 1 nm thickness range. In this work, we report a synthesis of monodisperse, rectangular-shaped PbS nanosheets with a thickness of 1.2 nm, using Pb­(thiocyanate)2 as a single source precursor. These nanosheets have an orthorhombic crystal structure, a direct bandgap, and weak optical absorption properties. This is evident from the lack of both excitonic absorption features and photoluminescence, and was corroborated by density functional theory calculations. Although these properties make the PbS nanosheets unsuitable for emission based applications, the nanosheets are highly photoconductive in films, with a responsivity up to 0.1 A W–1 and a detectivity of 1.3 × 109 Jones. We detected higher photoconductivity of these films under bending stress compared to that of films of PbS quantum dots.

The formation of cesium lead bromide (CsPbBr3) nanocrystal superlattices (NC SLs) is accompanied by a red shift in the NC photoluminescence (PL). The values of the PL red shift reported in the literature range from none to ∼100 meV without unifying explanation of the differences. Using a combination of confocal PL microcopy and steady-state optical spectroscopies we found that an overall PL red shift of ∼96 meV measured from a macroscopic sample of CsPbBr3 NC SLs has several contributions: ∼ 10–15 meV from a red shift in isolated and clean SLs, ∼ 30 meV from SLs with impurities of bulklike CsPbBr3 crystals on their surface, and up to 50 meV or more of the red shift coming from a photon propagation effect, specifically self-absorption. In addition, a self-assembly technique for growing micron-sized NC SLs on the surface of perfluorodecalin, an inert perfluorinated liquid and an antisolvent for NCs, is described.

Films made of colloidal CsPbBr 3 nanocrystals packed in isolated or densely-packed superlattices display a remarkably high degree of structural coherence. The structural coherence is revealed by the presence of satellite peaks accompanying Bragg reflections in wide-angle X-ray diffraction experiments in parallel-beam reflection geometry. The satellite peaks, also called “superlattice reflections”, arise from the interference of X-rays diffracted by the atomic planes of the orthorhombic perovskite lattice. The interference is due to the precise spatial periodicity of the nanocrystals separated by organic ligands in the superlattice. The presence of satellite peaks is a fingerprint of the high crystallinity and long-range order of nanocrystals, comparable to those of multilayer superlattices prepared by physical methods. The angular separation between satellite peaks is highly sensitive to changes in the superlattice periodicity. These characteristics of the satellite peaks are exploited to track the superlattice compression under vacuum, as well as to observe the superlattice growth in situ from colloidal solutions by slow solvent evaporation.