This paper presents the synthesis of the organic–inorganic hybrid perovskite, CH3NH3PbI3, doped in the Pb2+ position with Sn2+, Sr2+, Cd2+ and Ca2+. The incorporation of the dopants into the crystalline structure was analysed, observing how the characteristics of the dopant affected properties such as the crystalline phase, emission and optical properties. XRD showed how doping with Sn2+, Sr2+ and Cd2+ did not modify the normal tetragonal phase. When doping with Ca2+, the cubic phase was obtained. Moreover, DR-UV-Vis spectroscopy showed how the band gap decreased with the dopants, the values following the trend Sr2+ < Cd2+ < Ca2+ < CH3NH3PbI3 ≈ Sn2+. The biggest decrease was generated by Sr2+, which reduced the CH3NH3PbI3 value by 4.5%. In turn, cathodoluminescence (CL) measurements confirmed the band gap obtained. Periodic-DFT calculations were performed to understand the experimental structures. The DOS analysis confirmed the experimental results obtained using UV-Vis spectroscopy, with the values calculated following the trend Sn2+ ≈ Pb2+ > Cd2+ > Sr2+ for the tetragonal structure and Pb2+ > Ca2+ for the cubic phase. The electron localization function (ELF) analysis showed similar electron localizations for undoped and Sn2+-doped tetragonal structures, which were different from those doped with Sr2+ and Cd2+. Furthermore, when Cd2+ was incorporated, the Cd–I interaction was strengthened. For Ca2+ doping, the Ca–I interaction had a greater ionic nature than Cd–I. Finally, an analysis based on the non-covalent interaction (NCI) index is presented to determine the weak-type interactions of the CH3NH3 groups with the dopant and I atoms. To our knowledge, this kind of analysis with these hybrid systems has not been performed previously.

A good correlation was obtained between the electronic properties of Cu-doped anatase TiO₂ by virtue of both physical chemistry characterization and theoretical calculations. Pure and Cu-doped TiO₂ were synthesized. The composition, structural and electronic properties, and the band gap energy were obtained using several techniques. The method of synthesis used produces Cu-doped anatase TiO₂, and XRD, XPS and Raman spectroscopy indicate that Cu atoms are incorporated in the structure by substitution of Ti atoms, generating a distortion of the structure and oxygen vacancies. In turn, the band gap energy of the synthesized samples decrease drastically with the Cu doping. Moreover, periodic density functional theory (DFT-periodic) calculations were carried out both to model the experimentally observed doped structures and to understand theoretically the experimental structures obtained, the formation of oxygen vacancies and the values of the band gap energy. From the analysis of density of states (DOS), projected density of states (PDOS) and the electron localization function (ELF) a decrease in the band gap is predicted upon increasing the Cu doping. Thus, the inclusion of Cu in the anatase structure implies a covalent character in the Cu–O interaction, which involves the appearance of new states in the valence band maximum with a narrowing in the band gap.