Anastasia Vassilakopoulou

Nanocrystalline/microcrystalline materials based on lead-halide units

The preparation, photoluminescence (PL) spectra and related properties of particulate (nanocrystalline/microcrystalline) materials based on Pb(BrxCl1−x)3, Pb(BrxI1−x)3, and Pb(ClxI1−x)3 (x = 0–1) units, in several macroscopic forms, are reported. The materials in the form of suspensions in toluene or in toluene containing polymethyl methacrylate (PMMA) were obtained from the corresponding n-layer materials (or their precursors), mainly, by a titration-like method. This is the injection of solutions of the corresponding compounds with the Pbn(BrxCl1−x)3n+1, Pbn(BrxI1−x)3n+1, and Pbn(ClxI1−x)3n+1 (n > 2) units, respectively, into neat toluene or toluene–PMMA, with parallel observation of the PL-spectra. With this method, the mass of the lead-halide materials is controlled and the position of the low frequency PL band could be tuned in a wide spectral range, i.e., from ca. 400 to ca. 700 nm. From the suspensions in toluene the corresponding materials in the form of precipitates (powders) were obtained by centrifugation and decantation. From suspensions in toluene–PMMA, the corresponding materials in the form of composite thin films (of PMMA-matrix) were obtained by drop-casting, deep-coating and/or spin-coating techniques. So, blue, green, yellow and red narrow PL-band (of 20–35 nm) emitters (in forms of suspensions, powders and films) were obtained and their PL and optical absorption spectra were investigated further. The PL-intensity varies as in the case of CsPbBr3 particles in a CsBr matrix. Also, some similarities with the spectra of metal chalcogenide quantum dots were found. They were suggested as candidate materials for several optical and optoelectronic applications.

Encapsulation and protection of carbon dots within MCM-41 material

The effective encapsulation of Carbon Dots (CDs) within spherical-like MCM-41 nanoparticles is reported in this work, using a simple, low cost synthetic procedure, here focusing on blue luminescent CDs. CDs of 2 nm average size, with strong photoluminescence, are embedded into MCM-41 nanoparticles during the sol-gel procedure, using tetrathylorthosilicate as the precursor for the glass structure, whereas a cationic surfactant is used as the structure directing agent under basic conditions. The white powder of the final hybrid material showed long term, stable photoluminescence due to the presence of the CDs, indicating the protective character of the silica matrix. Most importantly, it is reported that the photoluminescence of the final hybrid material is not significantly affected by a thermal treatment at 550 °C. The transparent nature of the MCM-41, allows loading this type of mesoporous materials with CDs, thus, providing some new key composite photoluminescent materials for energy and other related applications.Graphical Abstract

Energy transfer yellow light emitting diodes based on blends of quasi-2D perovskites

The new class of hybrid organic-inorganic semiconductor (HOIS) materials, based on halide perovskites, is constantly being pursued for applications such as Light Emitting Diodes (LEDs) and solar cells, due to their momentous optoelectronic properties. In this work, we present a single layer LED that operates due to energy transfer effects as well as a simple, instant and low cost method for its fabrication. A LED device based on a mixture of zero dimensional (OD) (CH 3 NH 3 ) 4 PbI 6, two dimensional (2D) (F- C 6 H 4 CH 2 CH 2 NH 2 ) 2 PbI 4 and three dimensional (3D) (CH 3 NH 3 )PbI 3 HOIS, is presented for the first time. The final composite material manifests simple, yet unique energy transfer optical effects, while its electroluminescence exhibits excitonic recombination bright yellow light, peaked at 592 nm. LED device fabricated under ambient air, readily functions at room temperature and low voltages. As for the active layer, it exhibited substantial film continuity in any form of deposition. Finally, with appropriate mixtures, it is possible to create films containing phase changes that exhibit dual color emission, here presented as yellow-green.

A chemical sensor for CBr4 based on quasi-2D and 3D hybrid organic–inorganic perovskites immobilized on TiO2 films

The effective immobilization of hybrid organic–inorganic semiconductors (HOIS) or their blends over standard mesoporous, transparent, semiconducting TiO2 films in order to study their electrochemical behavior as sensors through a cyclic voltammetry technique (CV) has been reported for the first time. The CV technique is used in order to acquire a comprehensive understanding of the electron transport dynamics within the TiO2 film, loaded with perovskites, while the structural and optical properties can be easily extracted from X-ray diffraction, optical absorption and photoluminescence measurements. The electrochemical behavior of the adsorbed perovskites was studied as a function of the potential applied to the semiconductive film, which induces transition from an insulating to a conductive state. The localized traps in the bandgap of the composite semiconductive film predominantly govern the electron transfer process. The HOIS employed are either 3D or mixtures of quasi-2D perovskites, for which there is strong evidence that the lead bromide and chloride based HOIS exhibit cathodic peaks at positive voltages. In addition, exploiting the capability of ions to migrate and interact with the HOIS lattice, a perovskite-based electrode was successfully used as an (electro-)chemical sensor for CBr4. All electrochemical processes concurring on the composite hybrid film vary with respect to the CBr4 concentration in the tested solution, leading to a CBr4 detectability range of the order of 20 ppb mol mol−1.