Platinum diselenide (PtSe₂) is one of the most studied materials of the emerging group-10 transition-metal dichalcogenides, with interesting chemical and physical properties as the semimetal-to-semiconductor transition when approaching the monolayer thickness.

In this work, we investigate the electrical conduction and the photoconduction in ultrathin films of PtSe₂ synthesized by direct selenization of platinum deposited onto SiO₂/Si substrates. The PtSe₂ ultrathin films are exploited as the channel of back-gated field-effect transistors (FETs) and their electric conductance is investigated at different temperatures and pressures as well as under the irradiation of a super-continuous white light source. The increasing conductance with raising temperature confirms the semiconducting nature of the PtSe₂ film, while the gate modulation reveals p-type conduction with hole field-effect mobility up to 40 cm²/(Vs). The PtSe₂ conductivity is higher in air than in high vacuum due to the p-doping effect of oxygen. Moreover, electrical conduction measured along different directions shows isotropic transport ascribed to the polycrystalline structure of the film.

A reduction of the PtSe₂ conductance (negative photoconductivity) is observed under exposure to light in air, while positive or negative photoconductivity is observed in vacuum depending on the intensity of the light and the pressure. Such a behavior can be explained by the combination of the photogating effect caused by photo-charge accumulation in the SiO₂ dielectric and the adsorption/desorption of adsorbates.

Nowadays, nanoparticles (NPs) are used to make safe and more effective biomedical technologies for applications in highly targeted therapeutics and drug-delivery vehicles. This helps avoid low cellular penetration and accumulation of the drug in endosomal intracellular compartments that are not of interest for a particular therapy. A way to enhance the therapeutic efficiency is through nanoparticle loading systems, for which the aim of this study is to develop both low molecular (LMW) and high molecular weight (HMW) chitosan, as well as type B gelatin NPs. To enhance cell-penetration, the NPs were interfaced with the translocating peptide Buforin II. The obtained nanobioconjugates were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), confocal microscopy and transmission electron microscopy (TEM). Their size and surface zeta potential were estimated via DLS (Zetasizer Nano). Furthermore, to visualize their endosomal escape, the NPs were marked with the fluorophore Rhodamine B and imaged with the aid of the confocal microscope. The FTIR results showed bands corresponding to the polymers and to Buforin II after conjugation. The average NPs diameters were of about 250nm. The zeta potential of the chitosan NPs approached neutrality, which may be problematic due to low colloidal stability, while gelatin zeta potential of -7mV was closer to the value required for colloidal stability, i.e., ±10 mV. SEM microscopy of LMW and HMW chitosan NPs showed a round-shape and oval morphology, respectively, while the gelatin NPs had a filamentous morphology. SEM also shows agglomerates of the NPs. TEM microscopy results confirmed the LMW chitosan NPs morphology and showed that their nominal size was 5-10 nm.

Endothelialization is a key phenomenon required to maintain the patency in tissue engineered vascular grafts (TEVGs). Different approaches have been used to functionalize the surface of the lumen of TEVGs to induce Endothelial cells (ECs) adhesion and spreading. A promising strategy to promote integrin mediated signal transduction is to functionalize the TEGVs surface with peptide motifs. This approach is known to induce ECs adhesion, spreading, and tubular formation. However, surface functionalization is challenging given the availability of functional groups and the peptide - cell affinity under hemodynamic flow conditions. For that reason, herein we propose a chemo-mechanical model to study the optimal ligand distribution to improve cell adhesion and spreading under laminar flow. A computational multiphysics model was performed aided by the software COMSOL Multiphysics 5.5. The coupling of the Chemistry and Surface Reaction modules was carried out in order to predict the interaction between EC Integrins and Surface Functionalized peptides on the TEVG intima layer. The attachment and spreading of a single unit EC submitted to laminar flow and its influence on EC matrix interface interaction was evaluated with a Fluid-Structure Interaction model. A positive correlation between the concentration of Surface Functionalized peptides and ECs attachment and spreading time was found until equilibrium. Maximum cell spreading occurred under high binding integrin-surface affinity and physiological baseline low flow velocities. The proposed model elucidated the role of binding forces and flow velocities over cell spreading and detachment depending on ligand concentration. This model can contribute in optimizing surface functionalization of TEVGs for promoting successful endothelialization.

Perovskite solar cells, in which decaphenylcyclopentasilane (DPPS) layers were formed on the surface of the CH₃NH₃PbI₃-bsaed perovskite layer, were developed. The photovoltaic properties were improved by controlling the annealing temperature of the perovskite layer. For perovskite layers annealed at high temperatures in the range of 170~220 °C, the perovskite crystals were densely formed and the surface coverage of the perovskite layer was improved. The DPPS-laminated devices suppressed the formation of PbI₂ crystals and the stability was improved by the DPPS layer. Furthermore, the conversion efficiencies were improved over extended periods of time.

Effects of co-addition of CuBr₂ and NaCl to CH₃NH₃PbI₃(Cl) perovskite solar cells were investigated on the photovoltaic properties and microstructures. Short-circuit current densities and conversion efficiencies were improved by the simultaneous addition of CuBr₂ and NaCl. In addition, the efficiencies of the devices were maintained even after 10 weeks. The perovskite structure changed from a tetragonal to a cubic system by the addition of Na, which resulted in the improvement in the stabilization of the perovskite crystal.

Horizontally shifted hysteresis loops and asymmetric hysteresis loops are commonly related with exchange-biased samples, consisting of a ferromagnet exchange-coupled to an antiferromagnet or a ferrimagnet. In pure ferromagnetic samples, such effects may experimentally occur erroneously due to undetected minor loops or additional anisotropies, while in simulations they may occur due to thermal effects. However, performing simulations of ferromagnetic nanostructures at zero temperature with large enough saturation fields should not result in such asymmetries. Here we report on micromagnetic simulations using the Object Oriented MicroMagnetic Framework (OOMMF) at zero temperature, performed on symmetric sputtered nanoparticles with different shapes and holes or slits inside. Our results show that not only the small deviations of the systems under investigation due to random anisotropy orientations in the different grains may result in strong deviations of the magnetization reversal processes and hysteresis loops, but that there may also occur clearly asymmetric, horizontally shifted hysteresis loops in a purely ferromagnetic nanoparticle.

The search for high-energy density batteries especially stimulates the design of electrode materials with enhanced electrochemical storage properties. Downsizing the material to extend its electrochemically active surface as well as creating vacancies to create more available insertion sites are common approaches to improve the performance of electrode materials (1,3). In this work, our strategy is to maximize the cationic vacancies into nano-sized spinel Fe₂O₃ through substituting iron by molybdenum, with the final objective of extending the electrochemical insertion domain and accessing higher specific capacities. Our electrode materials were prepared by a simple solvothermal route which allows a carefully tuning of the cationic precursors.[2] The stabilization of molybdenum cations inside the spinel structure, and consequently the creation of cationic vacancies, were characterized by a wide range of complementary techniques, including pair distribution function analysis, X-ray absorption spectroscopy and ⁵⁷Fe Mössbauer spectroscopy. Interestingly, it is possible to tune both size and crystallinity of such nanomaterials by modifying the iron precursors and the synthesis conditions.

The positive influence of this nanoscale engineering was firstly verified by evaluating the synthesized materials as positive electrodes in lithium batteries, with a significant enhancement of the initial specific capacity (from 40 to 100 mAh/g) for Li insertion. As magnesium-ion batteries are emerging electrochemical storage systems that are still facing lack of positive electrode materials, we are also currently evaluating the magnesium insertion inside the Mo-substituted nanosized Fe₂O₃.

References
[1] B. P. Hahn, J. W. Long, and D. R. Rolison, “Something from nothing: Enhancing electrochemical charge storage with cation vacancies,” Acc. Chem. Res., vol. 46, no. 5, pp. 1181–1191, 2013.
[2] B. P. Hahn, J. W. Long, A. N. Mansour, K. A. Pettigrew, M. S. Osofsky, and D. R. Rolison, “Electrochemical Li-ion storage in defect spinel iron oxides: The critical role of cation vacancies,” Energy Environ. Sci., vol. 4, no. 4, pp. 1495–1502, 2011.

Chemical vapor deposition synthesis of graphene on copper foil from methane is the most promising technology for industrial production. However, an important problem of the formation of the second and subsequent graphene layers during synthesis arises due to the strong roughness of the initial copper foil. Here we demonstrate the various approaches to prepare a smooth copper surface before graphene synthesis to reduce the formation of multi-layer graphene islands. Six methods of surface processing of copper foils are studied, and the decrease of the roughness from 250 to as low as 80 nm is achieved. The correlation between roughness and the formation of multi-layer graphene is demonstrated. Under optimized conditions of surface treatment, the content of the multi-layer graphene islands drops from 9 to 2.1%. The quality and the number of layers of synthesized graphene are analyzed by Raman spectroscopy, scanning electron microscopy, and measurements of charge mobility.

Pyrophosphate nanocomposite materials containing transition metal ions have unique physical, chemical, electrical and mechanical properties . Especially interesting characteristics show compounds containing cooper atoms, which have received a lot of attention in research and application due to its excellent conductivity as well as good biocompatibility and increased surface Raman scattering activity (SERS). In our research, we apply the density functional theory and lattice dynamics calculations to get a better insight into the structural and dynamic properties of the Cu₂P₂O₇ crystal. Copper pyrophosphate has a monoclinic crystal structure described by the C2/c (15) space group and lattice parameters: a=6.901 Å, b=8.108 Å, c=9.176 Å, β = 109.65^o. All calculations were performed with the Vienna Ab initio Simulation Package (VASP) within the general gradient approximation (GGA). The stability of the crystal structure was tested by determining the phonon dispersion relations and the phonon density of states (PDOS) using the PHONON program based on the direct method . In the optimized structure all non-equivalent atoms were shifted from equilibrium positions and Hellmann-Feynman (HF) forces acting on all atoms were calculated.

The obtained results were compared with the experimental measurements of nanocrystalline copper pyrophosphates. The comparison of theoretically determined Raman scattering spectra with the experimental data enables the identification of the crystalline phases of pyrophosphates.

Nanosized coatings of ZrO₂ were deposited on silicon substrates using sol-gel and spin-coating techniques. The precursor solutions were prepared from ZrOCl₂.8H₂O with the addition of different percentage (0.5-5%) of rare earth Gd³⁺ ions as dopant. The thin films were homogeneous, with average thickness of 115 nm and refractive index (n) of 1.83. The X-ray diffraction analysis (XRD) revealed the presence of a varying mixture of monoclinic and tetragonal ZrO₂ polycrystalline phases, depending on the dopant concentration, all of which with nanosized crystallites. Scanning electron microscopy (SEM) as well as atomic force microscopy (AFM) methods were deployed to investigate the surface morphology and roughness of the thin films, respectively. They revealed a smooth, well uniform and crack-free surface with average roughness of 0.5 nm. It was established that the dopant concentration affects the photoluminescence (PL) properties of the samples. The undoped films exhibited broad violet PL emission, while the addition of Gd³⁺ ions resulted in new narrow bands in both UV and visible light regions, characteristic of the rare earth metal. These exact emissions can find useful applications in medical lamps or as red phosphors.

Acknowledgement

Research equipment of distributed research infrastructure INFRAMAT (part of Bulgarian National roadmap for research infrastructures) supported by Bulgarian Ministry of Education and Science under contract D01-284/17.12.2019 was used in this investigation.