National Institute Of Materials Physics - Romania

Structural and electronic transport features of high quality epitaxial metastable fcc W and beta-W2N thin films compatible with spintronic multilayer nanosystems are reported in a parallel approach. The epitaxial layers have been successfully prepared in a broad range of thicknesses from a few to tens of nanometers. The scalable method of auxiliary plasma assisted substrate magnetron sputtering was used. The epitaxy relationship between the thin films and MgO (100) single-crystal substrate is highlighted by high resolution XRD investigations of the reciprocal space. Electron transport properties are discussed with respect to the striking behavior observed via resistivity over temperature cycles. The chemical stability as active buffer layers of both fcc W and beta-W2N thin films is shown in spintronic configurations, where a thin film of Fe is interposed between W or W2N layers. The reported results show that the obtained high quality W based epitaxial layers are superior template-matched buffers for epitaxial Fe layers, enabling a new class of functional spintronic heterostructures.

Using solid-state interactions, composites based on molybdenum disulfide (MoS2), poly(ortho-toluidine) (POT), and reduced graphene oxide (RGO) were prepared. Raman scattering and FTIR spectroscopic studies have demonstrated that the interaction of the RGO and MoS2 sheets with the POT-emeraldine base state (EB) leads to composites composed of RGO sheets non-covalently functionalized with pseudo-protonic POT doped with MoS2 and RGO covalently functionalized with POT-leucoemeraldine salt (LS). An alternative way to prepare these composites is the solid-state interaction of the RGO and MoS2 sheets with the POT-emeraldine salt (ES), resulting in composites composed of RGO non-covalently functionalized with POT-salt having pseudo-protonic acidic entities doped with MoS2 and RGO covalently functionalized with POT-leucoemeraldine base (LB). Using UV-Vis spectroscopy, we demonstrate that the exposure of these composites to UV light in protic polar solvents leads to the generation of partially de-doped POT-LB and POT-ES. The features of the composites after light exposure, as well as their transformation to the salt state, are highlighted by surface-enhanced Raman scattering. This study contributes to better understanding of the properties of the composites based on MoS2, POT, and RGO, highlighting the need to avoid the exposure of composites dissolved in polar protic solvents to UV light.

Strain engineering is an effective strategy to tune the electronic structure and catalytic performance of functional materials. Here, we demonstrate a single-step continuous hydrothermal flow synthesis (CHFS) that produces multiphase titanate-TiO2-reduced graphene oxide (rGO) nanocomposites capable of both photocatalytic and mechanocatalytic activity. In this approach, in situ KOH-mediated reduction of graphene oxide introduces interfacial lattice strain and directs the co-crystallisation of anatase TiO2, layered K2Ti4O9, and metastable TiO2(B). The resulting multiphase composites feature strain gradients and nanoscale heterojunctions that enhance visible-light absorption, charge separation, and mechanically assisted catalytic response. Density functional theory (DFT) simulations confirm strain-induced bandgap narrowing and charge redistribution at oxide-2D interfaces. Under simultaneous visible-light irradiation and mechanical stirring, these hybrids achieved near-complete degradation of concentrated aqueous methylene blue (0.04 mM) within 60 min, with an apparent dual-mode rate constant of 0.1046 min-1, outperforming conventional photocatalysts. This single-step, scalable platform provides a general route to design multifunctional catalysts for environmental remediation and solar-to-chemical energy conversion.

We present experimental results on VO2, which is the most utilized Mott material, grown by atomic layer deposition on single-crystalline Si (100) (VO2/Si) wafers and followed by thermal treatment at 500 degrees C for different times: 30, 60 and 90 min. All these structures have shown in-plane ferroelectricity, the best results being obtained for VO2 treated thermically at 500 degrees C during 60 min. The remanent polarization in this case is 100 mu C cm-2 at room temperature at coercive voltages +/- 10 V. Current-voltage and polarization-voltage dependences at various temperatures have shown that VO2 has a reversible insulator-metal transition at 66 degrees C.

Efficient recycling of polypropylene (PP) from complex waste streams remains a significant challenge, particularly for multilayer non-woven materials. Here, we report a rapid dissolution-reprecipitation process for polypropylene recovery using a tailored ionic liquid formed from tris(2-ethylhexyl)amine and 3,3-dimethylbutyric acid. The process operates at 160 degrees C under ambient pressure, achieving up to 99% polymer recovery, with complete dissolution occurring in less than one minute. An integrated hot filtration step simultaneously removes pigments and additives, eliminating the need for additional purification stages. The recovered polymer exhibits thermal, structural, and mechanical properties comparable to virgin polypropylene, confirming preservation of polymer integrity. Quasi-elastic neutron scattering (QENS) provides molecular-level insight into the dissolution mechanism, revealing enhanced polymer mobility and plasticisation induced by the ionic liquid. Complementary spectroscopic and chromatographic analyses confirm the chemical stability of the solvent across the investigated temperature range, supporting its potential for reuse. This work establishes a mechanistically informed, solvent-based strategy for polypropylene recycling, offering a scalable pathway for recovering polyolefins from complex waste streams.

SrVO3 has long been considered a prototype for studying electronic correlations due to its simple electronic structure and coherent t2g quasiparticle band. However, the origin of the incoherent satellite at 1.5 eV binding energy seen in photoemission remains enigmatic, having been attributed to Hubbard physics, oxygen vacancies, or plasmonic coupling. Here, we investigate the t2g band and satellite feature in epitaxially strained thin films of SrVO3 using angle-resolved photoemission spectroscopy. We observe an increase in the quasiparticle effective mass due to tensile strain as a result of the narrowing of the corresponding bandwidth. While a low-energy kink at 60 meV indicates additional electron-phonon interactions, no strain-dependent behavior is detected. The increased effective mass coincides with a decrease in the plasma frequency from reported ellipsometry measurements and theoretical calculations. From this, we associate the plasma frequency with the quasiparticle-satellite energy separation that follows the same strain-induced changes. This consistency affirms the identification of the satellite feature as a replica band due to coherent coupling between quasiparticles and low-energy plasmons and emphasizes the role of dynamical screening as a key organizing principle in the electronic structure of correlated oxides.

Cobalt-free LiNiO2 (LNO) cathodes promise high-capacity lithium-ion batteries, but they suffer from rapid surface degradation and Nickel (Ni) dissolution, which significantly limit cycle life and rate performance. In this study, we demonstrate that ultrathin (similar to 10 nm) TiOx coatings deposited via high-power impulse magnetron sputtering (HiPIMS) effectively stabilize LNO electrodes. This low-temperature process yields dense, conformal coatings that penetrate porous structures, improving interfacial integrity without compromising bulk conductivity. In half-cells, coated LNO retains 213 mAh.g(-1) after 30 cycles at 0.1C (versus 180 mAh.g(-1) for pristine), with 79% higher capacity at 5C. In graphite based full cells, the coated electrodes retain 77% of their initial capacity after 100 cycles at 0.5C (compared to 63% for the uncoated electrodes). This improvement is attributed to suppressed Ni migration (27% reduction confirmed by GD-OES) and reduced impedance growth. High-resolution TEM and density functional theory (DFT) calculations confirm structural stability at the LNO/TiOx interphase. These findings demonstrate a promising strategy for improving the stability of cobalt-free cathodes for next-generation energy storage.

Hafnia (hafnium oxide) nanostructures, both unmodified and silica-modified with minor and major silica content, were synthesized using an adapted sol-gel method with D-L tartaric acid as an internal template. After thermal treatment, structural non-stoichiometry and light absorptive properties were identified in the resulting hafnium-based nanostructures, indicating their potential for various applications, including photocatalysis. The ability of these materials to photogenerate reactive oxygen species (ROS), namely superoxide anion radicals (center dot O2-) under simulated solar light (AM 1.5) and singlet oxygen (O-1(2)) under visible light (lambda > 390 nm), was evaluated and monitored by UV-Vis and photoluminescence spectroscopy. Functionalization of hafnium-based oxides with protoporphyrin IX was employed to enhance singlet oxygen photogeneration. The reactivity of the generated (O-1(2)) was assessed by quenching of DL alpha-tocopherol photoluminescence under visible light irradiation. Photocatalytic experiments conducted under anaerobic conditions demonstrated the ability of the hafnia-based nanostructures to reduce 1,4-benzoquinone (BQ) to 1,4-hydroquinone (H(2)Q). Furthermore, embedding the hafnia-based powders into polyvinyl alcohol hydrogels enabled the obtainment of photoactive coatings on glass substrates, for which their mechanical properties were evaluated using force-distance spectroscopy measurements. Morphological and structural characterization of the materials was performed using scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), atomic force microscopy (AFM), X-ray diffraction and fluorescence (XRD, XRF), X-ray photoelectron spectroscopy (XPS), N-2 adsorption-desorption measurements, UV-Vis spectroscopy, photoluminescence (PL) spectroscopy, and zeta potential measurements. These investigations revealed that adding silica induces significant modifications in the morphology, texture, and structure of the hafnia, thereby enhancing the functional properties of the resulting materials.

Three methods were used to prepare poly(ortho-toluidine)/reduced graphene oxide (POT/RGO) composites: (i) the interaction of the two constituents in the solid state, (ii) the chemical polymerization of ortho-toluidine (OT) in the presence of the RGO sheets, and (iii) the electrochemical polymerization of OT in the presence of the RGO sheets. Combining the results of Raman scattering, FTIR spectroscopy, and X-ray photoelectron spectroscopy, we demonstrated the following: (i) the interaction of POT with RGO in the solid state leads to the non-covalent functionalization of RGO with POT; (ii) the chemical polymerization of OT in the presence of RGO results in the covalent functionalization of RGO with POT in emeraldine base (EB) and leucoemeraldine salt (LS) and (iii) the electrochemical polymerization of OT assisted by RGO sheets leads to the covalent functionalization of RGO with POT-LS and POT-emeraldine salt (ES). The cyclic voltammetry (CV) studies indicated that the values for the voltammetric output currents of the POT/RGO composites are superior to those reported for POT. Electrochemical studies demonstrated a (a) battery-type behavior in the case of the electrodes based on POT-ES and POT/RGO composites prepared by chemical and electrochemical polymerizations and a (b) pseudocapacitive behavior for the electrodes based on POT-EB and composites prepared by the interaction of POT-EB with RGO in the solid state. High capacitance values of up to 1197.23 and 1524.62 mF cm-2 were obtained for the symmetrical supercapacitors based on the electrodes containing the POT/RGO composites prepared by chemical and electrochemical polymerization, respectively, of OT in the presence of RGO sheets.

The glass/FTO/TiO2/Sb2S3-x/P3HT/Au solar cells were investigated, with a focus on identifying the origin of performance degradation after exposure to high relative humidity and to mild heat stress. Analyzing the photovoltaic output and Deep-Level Transient Spectroscopy (DLTS) results, we probed the evolution of defect states after thermal exposure and after long-term storage of Sb2S3-x-based devices. The DLTS analysis revealed that shallow hole traps govern the stability of the Sb2S3-x cells. Significantly, it was shown that this shallow defect is not related to chlorine in the absorber, indicating an intrinsic origin. In the broader context of Sb2S3-x photovoltaics, this study underlines the paramount importance of defect engineering towards achieving more stable and efficient Sb2S3-x solar cells.