Invalidity of Band-Gap Engineering Concept for Bi3+ Heterovalent Doping in CsPbBr3 Halide Perovskite.

Heterovalent CsPbBr3 doping with Bi results in a significant red shift of the optical absorption of both single-crystal and powdered samples. The results of low-temperature (3.6 K) photoluminescence studies of perovskite single crystals indicate that the position of the excitonic luminescence peak remains unaffected by Bi doping that, in turn, infers that the band gap of Bi-doped perovskite is not changed as well. The position and state density distribution of the valence band and Fermi level of single-crystal perovskites were determined by another direct method of ultraviolet photoelectron spectroscopy. The obtained results show that Bi3+ doping causes no changes in the valence band structure but an increase in the Fermi level by 0.6 eV. The summary of the obtained results directly demonstrates that the concept of the band-gap engineering in Bi3+-doped CsPbBr3 halide perovskite is not valid.

Low Inhomogeneous Broadening of Excitonic Resonance in MAPbBr3 Single Crystals.

We present an optical study of MAPbBr3 single crystal grown from solution. The crystal Pm3m symmetry was confirmed by electron backscatter diffraction. Our major attention was focused on optical effects related to the excitonic states in MAPbBr3. Photoluminescence temperature dependence of narrow exciton resonance showed encouragingly low inhomogeneous broadening Γ ≈ 0.5 meV that allows one to distinguish the signals from free excitons and those arising from recombination of excitons localized on defects. Excitonic origin of the resonance was proved by its superlinear pump intensity dependence, in contrast to the linear behavior of the defect-assisted recombination bands. For the first time, the phonon replicas originating from free exciton recombination accompanied by partial energy transfer to the phonons were observed in high-resolution PL spectra and confirmed by independent low-temperature Raman scattering experiments. In turn, low-temperature, low-frequency Raman scattering studies let us resolve the structure of the low-frequency phonon spectrum.

Photoinduced hydrophilic conversion of hydrated ZnO surfaces.

Here we focused on the study of the effect of the zinc oxide nano-coatings surface acidity on the surface hydrophilicity and photoinduced hydrophilic conversion. A three-step procedure was used to have the initial state of the ZnO surface free of organics and to control the conditions during the water wetting and the irradiation steps. The kinetics of photoinduced hydrophilic conversions for ZnO films were obtained and demonstrated a dependence on the initial hydrophilic state. No conversion into the superhydrophilic state was observed for ZnO nano-films independently of the surface acidity. Irradiation in the ultraviolet and visible spectral regions showed the existence of "slow" and "fast" processes in the photoinduced hydrophilic conversion of the ZnO surface. A multi-layered model of hydroxyl-hydrated coverage of the surface is proposed to explain the experimental results.