Optical and scintillation properties of hybrid manganese(II) bromides with formamidinium and acetamidinium cations.

In recent years, hybrid manganese(II) halides (HMHs) have attracted wide attention due to their impressive optical properties, low toxicity, and facile synthetic processibility. Being effective reabsorption-free phosphors, these compounds demonstrate the potential to be used as low-cost solution-processable scintillators. However, most of the HMHs studied to date contain bulk organic cations and, as a result, are characterized by low density and low X-ray stopping power. For this reason, we studied manganese(II) bromides with compact organic cations such as formamidinium (FA+) and acetamidinium (AcA+). In particular, we synthesized four new phases, two of which are characterized by octahedral coordination of manganese ions ((FA)MnBr3 and (AcA)MnBr3) and red emission, whereas the other two have tetrahedrally coordinated Mn2+ ions ((FA)3MnBr5 and (AcA)2MnBr4) and green emission. Photoluminescence (PL) and radioluminescence measurements demonstrated high PL quantum yields and reasonable scintillation light yields of acetamidinium-based compounds. In addition, unlike most known HMH-based scintillators, the discovered materials have a relatively high density due to the small fraction of the volume occupied by organic cations, so their X-ray attenuation coefficients are comparable to the well-known oxide scintillators.

Exceptional structural diversity of hybrid halocuprates(I) with methylammonium and formamidinium cations.

Hybrid halocuprates(I) are nowadays the subject of intensive studies as promising materials for various optoelectronic applications. This class of materials is characterized by wide structural diversity enabled by a great variety in the size and shape of organic cations. Therefore, the study of composition-structure-property relationships is a key step for the rational design of new halocuprate materials with desired properties. In this paper, we comprehensively studied MABr/CuBr and FABr/CuBr systems (MA+ = methylammonium and FA+ = formamidinium) and established the existence of five novel phases (namely, MACu2Br3, FA2[Cu4Br6], MACuBr2, FACuBr2, and FA3CuBr4) related to four different structural types and three distinct A+ : Cu+ stoichiometries (A+ = MA+/FA+). The optical properties of the discovered phases are studied by absorption and low-temperature photoluminescence spectroscopy. Based on a crystal-chemical analysis, we explained a unique structural diversity of the MA- and FA-based bromocuprates, as well as revealed new structure-property relationships.

SERS uncovers the link between conformation of cytochrome c heme and mitochondrial membrane potential.

The balance between the mitochondrial respiratory chain activity and the cell's needs in ATP ensures optimal cellular function. Cytochrome c is an essential component of the electron transport chain (ETC), which regulates ETC activity, oxygen consumption, ATP synthesis and can initiate apoptosis. The impact of conformational changes in cytochrome c on its function is not understood for the lack of access to these changes in intact mitochondria. We have developed a novel sensor that uses unique properties of label-free surface-enhanced Raman spectroscopy (SERS) to identify conformational changes in heme of cytochrome c and to elucidate their role in functioning mitochondria. We have verified that molecule bond vibrations assessed by SERS are a reliable indicator of the heme conformation during changes in the inner mitochondrial membrane potential and ETC activity. We have demonstrated that cytochrome c heme reversibly switches between planar and ruffled conformations in response to the inner mitochondrial membrane potential (ΔΨ) and H+ concentration in the intermembrane space. This regulates the efficiency of the mitochondrial respiratory chain, thus, adjusting the mitochondrial respiration to the cell's consumption of ATP and the overall activity. We have found that under hypertensive conditions cytochrome c heme loses its sensitivity to ΔΨ that can affect the regulation of ETC activity. The ability of the proposed SERS-based sensor to track mitochondrial function opens broad perspectives in cell bioenergetics.

Crystallization Pathways of FABr-PbBr2-DMF and FABr-PbBr2-DMSO Systems: The Comprehensive Picture of Formamidinium-Based Low-Dimensional Perovskite-Related Phases and Intermediate Solvates.

In this study, we systematically investigated the phase diversity and crystallization pathways of the FABr excessive regions of two ternary systems of FABr-PbBr2-DMF and FABr-PbBr2-DMSO (where FA+-formamidinium cations, DMF-dimethylformamide and DMSO-dimethyl sulfoxide solvents). In these systems, a new FA3PbBr5 phase with a structure containing chains of vertex-connected PbBr6 octahedra is discovered, and its crystal structure is refined. We experimentally assess fundamental information on differences in the mechanisms of crystallization process in FABr-PbBr2-DMF and FABr-PbBr2-DMSO systems and determine possible pathways of crystallization of hybrid perovskites. We show that intermediate solvate phases are not observed in the system with DMF solvent, while a number of crystalline solvates tend to form in the system with DMSO at various amounts of FABr excess.

Butanediammonium Salt Additives for Increasing Functional and Operando Stability of Light-Harvesting Materials in Perovskite Solar Cells.

Organic diammonium cations are a promising component of both layered (2D) and conventional (3D) hybrid halide perovskites in terms of increasing the stability of perovskite solar cells (PSCs). We investigated the crystallization ability of phase-pure 2D perovskites based on 1,4-butanediammonium iodide (BDAI2) with the layer thicknesses n = 1, 2, 3 and, for the first time, revealed the presence of a persistent barrier to obtain BDA-based layered compounds with n > 1. Secondly, we introduced BDAI2 salt into 3D lead−iodide perovskites with different cation compositions and discovered a threshold-like nonmonotonic dependence of the perovskite microstructure, optoelectronic properties, and device performance on the amount of diammonium additive. The value of the threshold amount of BDAI2 was found to be ≤1%, below which bulk passivation plays the positive effect on charge carrier lifetimes, fraction of radiative recombination, and PSCs power conversion efficiencies (PCE). In contrast, the presence of any amount of diammonium salt leads to the sufficient enhancement of the photothermal stability of perovskite materials and devices, compared to the reference samples. The performance of all the passivated devices remained within the range of 50 to 80% of the initial PCE after 400 h of continuous 1 sun irradiation with a stabilized temperature of 65 °C, while the performance of the control devices deteriorated after 170 h of the experiment.

Plasmonic features of free-standing chitosan nanocomposite film with silver and graphene oxide for SERS applications.

A scalable procedure of SERS substrates design was developed using a novel plasmonic structure based on a freestanding chitosan film, silver nanoparticles, and graphene oxide. Chitosan provides a uniform distribution of silver nanoparticles from a colloidal suspension and, therefore, a reproducible Raman signal from local areas of measurements of several tens of microns. The addition of graphene oxide (GO) to the colloidal solution of silver nanoparticles suppresses the tortuous background fluorescence signal from the analyte and leads to an increase in the signal-to-fluorescence background intensity ratio by up to 6 times as compared to structures without GO. The manufactured plasmonic polymer nanocomposite provides a detection limit of down to 100 pM for R6G using a laser wavelength of 532 nm through a portable ×10 objective. The high colloidal stability of GO in water and the use of an aqueous colloid of silver nanoparticles simplify the procedure for creating a substrate by applying the GO-silver composite on the surface of a chitosan film without a need to form a GO film. Therefore, our approach paves a promising avenue to provide more sensitive detection even for the fluorescent analytes with short-wavelength lasers (532, 633 nm) instead of IR (785, 1024 nm) and foster the practical application of the developed plasmonic composites on portable Raman spectrometers.

Molecular Immobilization and Resonant Raman Amplification by Complex-Loaded Enhancers (MIRRACLE) on copper (II)-chitosan-modified SERS-active metallic nanostructured substrates for multiplex determination of dopamine, norepinephrine, and epinephrine.

A unique approach based on Molecular Immobilization and Resonant Raman Amplification by Complex-Loaded Enhancers (MIRRACLE) on copper (II)-chitosan-modified SERS-active metallic nanostructured substrates is proposed for sensitive and rapid determination of the catecholamines (CA) dopamine, norepinephrine, and epinephrine. The ternary (CA)2Cu(4AAP)2 complexes were characterized by the appearance of new absorbance bands at 555, 600, and 500 nm for dopamine, norepinephrine, and epinephrine, respectively. The new absorbance band matched with a broad surface plasmon resonance band of utilized silver nanoparticles: 450-600 nm, and 633 excitation wavelength. We observed enhancement factors up to 3.6·106 due to the additional resonant enhancement. The multiplexing capabilities of quantitative spectral unmixing for Raman spectra of a group of CAs, which differ by only either hydroxy or methyl group, at the fingerprint region were successfully demonstrated with the direct classic least squares model. The achieved nM limits of detection with only 1.5 mW laser power and analysis of spiked human blood plasma samples proved the possibility of the multiplex determination of the catecholamines at the level of reference concentrations in the blood of healthy people as well as promise for the future facilitation in the precision diagnosis of neuroendocrine tumors and neurodegenerative diseases.

Iodine Solution Treatment in Nonpolar Solvents as a Facile Approach to Improve the Morphology and Photostability of Perovskite Films.

We propose a new, simple, and easily implemented approach to improve the morphology of thin films of lead halide perovskites. A key feature of the approach is the controllable size increase of perovskite grains facilitated by polyiodides formed on the surface of the perovskite upon its treatment with iodine solutions in nonpolar solvents with the best results obtained for iodine solution in toluene saturated with MAI. Such a treatment demonstrated an increase in the average grain size of the films of up to 3.5 times in approximately 2 min followed by significantly enhanced photostability.

Detection of Hypertension-Induced Changes in Erythrocytes by SERS Nanosensors.

Surface-enhanced Raman spectroscopy (SERS) is a promising tool that can be used in the detection of molecular changes triggered by disease development. Cardiovascular diseases (CVDs) are caused by multiple pathologies originating at the cellular level. The identification of these deteriorations can provide a better understanding of CVD mechanisms, and the monitoring of the identified molecular changes can be employed in the development of novel biosensor tools for early diagnostics. We applied plasmonic SERS nanosensors to assess changes in the properties of erythrocytes under normotensive and hypertensive conditions in the animal model. We found that spontaneous hypertension in rats leads (i) to a decrease in the erythrocyte plasma membrane fluidity and (ii) to a decrease in the mobility of the heme of the membrane-bound hemoglobin. We identified SERS parameters that can be used to detect pathological changes in the plasma membrane and submembrane region of erythrocytes.

Nonmonotonic Photostability of BA2MAn-1PbnI3n+1 Homologous Layered Perovskites.

Layered lead halide perovskites (2D LHPs) are attracting considerable attention as a promising material for a new generation of solar cell devices. LHPs have been presented as a more stable alternative to the more widespread 3D bulk perovskite materials; however, a critical analysis of their photostability is still lacking. In this work, we perform a comparative study between BA2MAn-1PbnI3n+1 (BA─butylammonium and MA─methylammonium) 2D LHPs with different dimensionalities (n = 1-3) and MAPbI3 3D perovskites. We compare different stability testing protocols including photometrical determination of iodine-containing products in nonpolar solvents, X-ray diffraction, and photoluminescence (PL) spectroscopy. The resulting trends of the photostability in an inert atmosphere based on PL spectroscopy measurements demonstrate a nonmonotonic dependence of the degradation rate on the perovskite layer thickness n with a "stability island" at n ≥ 3, which is caused by a combination of antibate factors of electronic structures and chemical compositions in the family of 2D perovskites. We also identify a critical oxygen concentration in the surrounding environment that affects the mechanism and strongly enhances the rate of layered perovskite photodegradation.

Solubility of Hybrid Halide Perovskites in DMF and DMSO.

Solution methods remain the most popular means for the fabrication of hybrid halide perovskites. However, the solubility of hybrid perovskites has not yet been quantitively investigated. In this study, we present accurate solubility data for MAPbI3, FAPbI3, MAPbBr3 and FAPbBr3 in the two most widely used solvents, DMF and DMSO, and demonstrate huge differences in the solubility behavior depending on the solution compositions. By analyzing the donor numbers of the solvents and halide anions, we rationalize the differences in the solubility behavior of hybrid perovskites with various compositions, in order to take a step forward in the search for better processing conditions of hybrid perovskites for solar cells and optoelectronics.

Structural Disorder in Layered Hybrid Halide Perovskites: Types of Stacking Faults, Influence on Optical Properties and Their Suppression by Crystallization Engineering.

Layered hybrid halide perovskites (LHHPs) are an emerging type of semiconductor with a set of unique optoelectronic properties. However, the solution processing of high-quality LHHPs films with desired optical properties and phase composition is a challenging task, possibly due to the structural disorder in the LHHP phase. Nevertheless, there is still a lack of experimental evidence and understanding of the nature of the structural disorder in LHHPs and its influence on the optical properties of the material. In the current work, using 2D perovskites (C4H9NH3)2(CH3NH3)n-1PbnI3n+1 (further BA2MAn-1PbnI3n+1) with n = 1-4 as a model system, we demonstrate that deviations in LHHPs optical properties and X-ray diffraction occur due to the presence of continuous defects-Stacking Faults (SFs). Upon analyzing the experimental data and modeled XRD patterns of a possible set of stacking faults (SFs) in the BA2MAPb2I7 phase, we uncover the most plausible type of SFs, featured by the thickness variation within one perovskite slab. We also demonstrate the successful suppression of SFs formation by simple addition of BAI excess into BA2MAn-1PbnI3n+1 solutions.

Crystal structure of new formamidinium triiodide jointly refined by single-crystal XRD, Raman scattering spectroscopy and DFT assessment of hydrogen-bond network features.

A novel triiodide phase of the formamidinium cation, CH5N2 +·I3 -, crystallizes in the triclinic space group P at a temperature of 110 K. The structure consists of two independent isolated triiodide ions located on inversion centers. The centrosymmetric character of I3 - was additionally confirmed by the observed pronounced peaks of symmetrical oscillations of I3 - at 115-116 cm-1 in Raman scattering spectra. An additional structural feature is that each terminal iodine atom is connected with three neighboring planar formamidinium cations by N-H⋯I hydrogen bonding with the N-H⋯I bond length varying from 2.81 to 3.08 Å, forming a deformed two-dimensional framework of hydrogen bonds. A Mulliken population analysis showed that the calculated charges of hydrogen atoms correlate well with hydrogen-bond lengths. The crystal studied was refined as a three-component twin with domain ratios of 0.631 (1):0.211 (1):0.158 (1).

New Methylamine-Iodine-Mediated Solvent-Free Approach of Hybrid Perovskite Synthesis via the Redox Conversion of Metallic Lead Films.

The fast progress of lab-scale perovskite solar cells makes the problem of upscaling of perovskite thin-film deposition more and more acute; therefore, the development of new methods for perovskite deposition is highly desired. In this work, we proposed a new solution-free preparation approach for hybrid perovskite films based on the in situ generation of methylammonium iodide from methylamine and iodine vapors in the presence of an organic reducing agent conjugated with a redox process of metallic lead conversion with iodine vapor. At first, we demonstrated that either metallic lead or lead iodide powders can readily react with the solution of methylamine and iodine in the presence of isopropyl alcohol acting as a reducing agent, resulting in a phase-pure polycrystalline CH3NH3PbI3 perovskite. A possible mechanism of underlying chemical processes has been proposed. In order to convert the predeposited thin films of metallic lead to CH3NH3PbI3 perovskite, a protocol of sequential treatment by iodine and methylamine vapors was proposed. Finally, we revealed the optimal conditions of processing, which were proven to be facile and robust for the sake of the better control of perovskite grain morphology.

Dual-Purpose SERS Sensor for Selective Determination of Polycyclic Aromatic Compounds via Electron Donor-Acceptor Traps.

Toxic, carcinogenic, and mutagenic properties of polycyclic aromatic hydrocarbons (PAHs) and environmental pollution caused by polycyclic aromatic sulfur heterocycles (PASHs) postulate the importance of their selective and sensitive determination in environmental and oil fuel samples. Surface-enhanced Raman spectroscopy (SERS) opens up an avenue toward multiplex analysis of complex mixtures, however not every molecule gives high enhancement factors and, thus, cannot be reliably detected via SERS. However, the sensitivity can be drastically increased by additional resonant enhancement as a result of the analyte absorption band overlapping with the surface plasmon band of nanoparticles (NPs) and the laser excitation wavelength. Using this idea, we developed a dual-purpose SERS sensor based on trapping the target PAHs and PASHs into colored charge-transfer complexes (CTCs) with selected organic π-acceptor molecules on the surface of AgNPs. Studying, computing, and then comparing stability constants of the formed CTC served as a powerful explanation and prediction tool for a wise choice of π-acceptor indicator systems for the further silver surface modification. Moreover, we show that CTC formation can be effectively utilized for increasing both selectivity and sensitivity by simple liquid-liquid extraction prior to SERS measurements. For the first time, the dual-purpose SERS sensor allowed determination of two different classes of polycyclic aromatic fuel components down to 10 nM concentration, lower than that restricted by the ASTM regulation, and demonstrated multi-purpose capabilities of the developed approach.

New Acidic Precursor and Acetone-Based Solvent for Fast Perovskite Processing via Proton-Exchange Reaction with Methylamine.

A new solvent system for PbI2 based on HI solution in acetone with a low boiling point is proposed. High solubility of PbI2 is caused by the formation of iodoplumbate complexes, and reaches a concentration of 1.6 M. Upon its crystallization metastable solvate phases PbI2∙HI∙n{(CH3)2CO} are formed. The latter allows for their easy deposition on substrates in a form of smooth and uniform thin films by spin-coating. Through a fast acid-base reaction with a gaseous amine, the films of the intermediate phase can be completely converted to single-phase perovskite films. The developed method allows one to form smooth perovskite films with high crystallinity with a thickness up to 1 µm. Due to easy and fast processing, the developed method can be promising for perovskite technology upscaling.

From Metallic Lead Films to Perovskite Solar Cells through Lead Conversion with Polyhalide Solutions.

Solutions of methylammonium and formamidinium polyhalides (AX1+n, A = MA, FA, X = I, Br) in isopropanol are introduced as novel versatile precursors for the fabrication of APbX3 hybrid perovskite thin films via oxidation of metallic Pb. The polyhalide solution with adjustable reactivity is distributed over a metallic Pb layer followed by iodine vapor postprocessing to tune the morphology and composition of the film using only the elements inherently present in the perovskite. This method is easily reproducible in any materials science laboratory with equipment commonly used for perovskite solar cell fabrication and resulted in power conversion efficiencies of 16.2 and 17.2% for planar solar cells using MAPbI3 and MA0.25FA0.75PbI2.75Br0.25 perovskites, respectively, as a proof of concept. Implementation of metallic lead thin films as the single Pb-containing precursors reduces a number of in-lab handling hazards compared to classical PbI2 powder and solutions and provides a variety of scalable deposition options.

New Features of Photochemical Decomposition of Hybrid Lead Halide Perovskites by Laser Irradiation.

We found that laser irradiation, being widely used in perovskite photovoltaics for both laser scribing and materials characterization, inevitably causes a cascade of complex photo- and thermochemical conjugated reactions, material melting, and ablation with deep morphological and composition changes of perovskite thin films over a much larger area compared to the initial laser spot. A crucial issue in the advancing or suppression of these degradation processes is related to the origin of the surrounding atmosphere. In particular, an effective approach utilizing an inert gas flow directed onto the exposed area is suggested for the first time to eliminate the negative consequences of perovskite laser scribing. This finding is naturally related to experimental observations of spreading the volatile decomposition products, including elemental iodine, over the pristine perovskite material, regardless of its composition, followed by laser-induced formation of liquid polyiodides. Suppression of decomposition product amount by proper selection of the gas atmosphere and power regime of the laser treatment is of interest to enhance the scribing procedure.

Silver-chitosan nanocomposite as a plasmonic platform for SERS sensing of polyaromatic sulfur heterocycles in oil fuel.

Herein, a silver-chitosan nanocomposite for application in surface enhanced Raman spectroscopy (SERS) sensing was proposed. It was shown that optically transparent chitosan coatings with 0.8 µm thickness allow penetration of target analytes to silver nanoparticles and the analysis in both polar and nonpolar solvents. Under the chosen conditions, chitosan formed continuously smooth films and coatings stabilizing rough nanostructured metallic surfaces and served as a suitable matrix for immobilization, uniform spreading, and preconcentration of the analytes. Polycyclic aromatic sulfur heterocycles were chosen as target analytes being one of the most important fuel quality markers, hazardous components, and the hardest-to-remove impurities. For the most effective immobilization and even distribution of the analytes onto a nanostructured metallic surface, an additional polymer layer of chitosan was found to be needed. The presence of thin films of chitosan resulted in higher reproducibility of SERS spectra as compared to bare nanostructured silver substrates. Additionally, the developed nanocomposite SERS sensors provided the rapid determination of dibenzothiophene and its derivatives in isooctane with the threshold of detection better than 0.1 µM. This approach was successfully applied in the analysis of real fuel samples and the results agreed well with independently measured FTIR and GC-MS data.

Methylammonium Polyiodides: Remarkable Phase Diversity of the Simplest and Low-Melting Alkylammonium Polyiodide System.

Newly discovered methylammonium polyiodides (MAIx) are unique precursors for innovative solvent-free technologies in perovskite photovoltaics because MAIx are liquids at room temperature and demonstrate high chemical reactivity. We investigated the features of an MAI-I2 system and built up a first phase diagram in wide temperature and composition ranges using data from differential scanning calorimetry, single-crystal X-ray diffraction, and visual thermal analysis. The phase diagram has been found to differ drastically from that of any related systems owing to the unique propensity of methylammonium toward forming a diversity of polyiodides with complicated crystal structures, namely, MAI2, MAI2.67, MAI4, and MAI5.5, found in this system for the first time. The performed density functional theory calculations revealed the crucial role of entropy contributing to the formation of higher methylammonium polyiodides, in good agreement with experimental data.