One of distinguishing features of metal halide perovskites is their long (up to microseconds) photoluminescence (PL) lifetimes, which are regularly observed for this class of semiconductors in spite of their direct gap origin. It is difficult to explain this contradiction in the framework of usual two-level Jablonski photophysical diagram because the absorption coefficient (i.e., the oscillator strength of the direct optical transition) is too high in perovskites to hold such long PL lifetimes. In this paper, we describe practical steps how the PL decay kinetics of perovskites in the forms of (1) passivated nanocrystals and (2) thin multicrystalline films can be described. In case of nanocrystals, the three-level delayed luminescence model is described by including shallow non-quenching traps providing multiple trapping and de-trapping of carriers and thus essentially lengthening the observed PL lifetime. In the case of perovskite thin films limited by interfacial recombination, the PL decay kinetics is usually determined by the non-radiative recombination on the film surfaces and can be satisfactorily described in terms of one-dimensional diffusion equation. The limits of applicability of such approaches are discussed.

A novel sensing system was designed for pH measurements based on the enhanced and quenched photoluminescence (PL) and UV-Vis absorption of the diluted water solutions of F-, O-, and N-containing carbon nanoparticles (FON-CNPs). These FON-CNPs were solvothermally synthesized, dissolved, ultra-iltrated, and separated by thin-layer chromatography. The total luorine content in them was found to be 1.2-1.5 mmol per gram. Their TGA showed a total weight loss of 52.7% because of the thermal decomposition and detachment of the surface groups and the partial burning of the functionalized shell on the carbon core at temperatures below 1200 °C. TEM and Raman data conirmed the presence of graphitic structures in the carbon core. From the results of ATR FTIR and UV-Vis spectroscopies, we showed that a carbon shell incorporates diferent functional groups covering the carbon core. The surface groups of the carbon shell include carboxyl, phenolic, and carbonyl groups. Heterocyclic N-containing and amino groups and triluoromethyl groups supporting the hydrophobicity were also found. We suggested the possible reasons for the pH responses obtained with the sensing system considering them dependent on the de-protonation of functional groups with pH change.

Anti-Stokes photoluminescence (ASPL), which is an up-conversion phonon-assisted process of the radiative recombination of photoexcited charge carriers, was investigated in methylammonium lead bromide (MALB) perovskite nanocrystals (NCs) with mean sizes that varied from about 6 to 120 nm. The structure properties of the MALB NCs were investigated by means of the scanning and transmission electron microscopy, X-ray diffraction and Raman spectroscopy. ASPL spectra of MALB NCs were measured under near-resonant laser excitation with a photon energy of 2.33 eV and they were compared with the results of the photoluminescence (PL) measurements under non-resonant excitation at 3.06 eV to reveal a contribution of phonon-assisted processes in ASPL. MALB NCs with a mean size of about 6 nm were found to demonstrate the most efficient ASPL, which is explained by an enhanced contribution of the phonon absorption process during the photoexcitation of small NCs. The obtained results can be useful for the application of nanocrystalline organometal perovskites in optoelectronic and all-optical solid-state cooling devices.

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