We have developed a colloidal synthesis of nearly monodisperse nanocrystals of pure Cs₄PbX₆ (X = Cl, Br, I) and their mixed halide compositions with sizes ranging from 9 to 37 nm. The optical absorption spectra of these nanocrystals display a sharp, high energy peak due to transitions between states localized in individual PbX₆^4– octahedra. These spectral features are insensitive to the size of the particles and in agreement with the features of the corresponding bulk materials. Samples with mixed halide composition exhibit absorption bands that are intermediate in spectral position between those of the pure halide compounds. Furthermore, the absorption bands of intermediate compositions broaden due to the different possible combinations of halide coordination around the Pb²⁺ ions. Both observations are supportive of the fact that the [PbX₆]^4– octahedra are electronically decoupled in these systems. Because of the large band gap of Cs₄PbX₆ (>3.2 eV), no excitonic emission in the visible range was observed. The Cs₄PbBr₆ nanocrystals can be converted into green fluorescent CsPbBr₃ nanocrystals by their reaction with an excess of PbBr₂ with preservation of size and size distributions. The insertion of PbX₂ into Cs₄PbX₆ provides a means of accessing CsPbX₃ nanocrystals in a wide variety of sizes, shapes, and compositions, an important aspect for the development of precisely tuned perovskite nanocrystal inks.

Identifying the composition of the solvated iodoplumbate complexes that are involved in the synthesis of perovskites in different solution environments is of great relevance in order to link the type and quantity of precursors to the final optoelectronic properties of the material. In this paper we clarify the nature of these species and the involved solution equilibria by combining experimental analysis and high-level theoretical calculations, focusing in particular on the DMSO and DMF solvents, largely employed in the perovskites synthesis. The specific molecular interactions between the iodoplumbate complexes, [PbIm]2-m, and the solvent molecules, X, were analyzed by identifying the most thermodynamically stable structures in various solvent solutions and characterizing their optical properties trough DFT and TD-DFT calculations. A comparison with the experimental UV-Vis absorption spectra allows for the first time the definition of the number of iodide and solvent ligands bonded to the Pb2+ ion and of the complex formation constants of the involved species.

We report a quantum mechanical investigation on the nature of electronic trap states in realistic models of individual and sintered anatase TiO₂ nanocrystals (NCs) of ca. 3 nm diameter. We find unoccupied electronic states of lowest energy to be localized within the central part of the NCs, and to originate from under-coordinated surface Ti atoms lying mainly at the edges between the (100) and (101) facets. These localized states are found at about 0.3–0.4 eV below the fully delocalized conduction band states, in good agreement with both electrochemical and spectro-electrochemical results. The overall Density-Of-States (DOS) below the conduction band (CB) can be accurately fitted to an exponential distribution of states, in agreement with capacitance data. Water molecules adsorbed on the NC surface raise the energy and reduce the number of localized states, thus modifying the DOS. As a possible origin of additional trap states, we further investigated the oriented attachment of two TiO₂ NCs at various possible interfaces. For the considered models, we found only minor differences between the DOS of two interacting NCs and those of the individual constituent NCs. Our results point at the presence of inherent trap states even in perfectly stoichiometric and crystalline TiO₂ NCs due to the unavoidable presence of under-coordinated surface Ti(IV) ions at the (100) facets.