National Institute Of Materials Physics - Romania

New approaches for the synthesis of magnetite (Fe3O4) nanoparticles (NPs) are of considerable interest due to their potential applications in various fields, such as biomedicine, industry, environmental remediation, and catalysis. This study presents a novel approach for synthesizing Fe3O4 NPs using high-energy electron beam (EB) irradiation starting from organic (acetylacetonates) iron precursors. EB irradiation is a challenging nanoparticle synthesis method, being at the same time efficient and rapid. The synthesis is carried out at room temperature and is based on the water radiolysis process. This eliminates the need for chemical-reducing agents and may provide precise control over particle formation. Using high-energy EB irradiation of an organic Fe precursor, we demonstrate the successful synthesis of well-dispersed Fe3O4 NPs with controlled size, morphology and magnetic properties, as proven by morpho-structural, M & ouml;ssbauer spectroscopy and magnetic investigations. In particular, using organic iron precursors, such as iron acetylacetonates, NPs with distinct surface characteristics and improved thermal stability compared to those synthesized from inorganic precursors were obtained. These findings suggest that integrating organic precursors in EB-assisted synthesis can enhance the functional properties of Fe3O4 NPs, making them more suitable for specific applications. The versatility of this method opens up new avenues for the targeted design of nanomaterials with specific functionalities, paving the way for advanced applications in various technological fields. The current study is also motivated by the lack of literature data on the synthesis of metallic iron or iron oxide NPs mediated by EB radiolysis.

In this work, Cu2Zn1-xNixSnS4 (CZ1-xNixTS) thin films were successfully synthesized via the solution-based spin-coating technique, and the effects of partial substitution of Zn by Ni (x = 0.2, 0.4, and 0.6) on their structural, morphological, optical, and photovoltaic properties were systematically investigated. Structural analyses demonstrated that Ni substitution in CZTS leads to lattice contraction and partial structural distortion due to the smaller ionic radius of Ni2+. Increasing Ni content enhances these effects until a solubility limit is reached at x approximate to 0.6, beyond which secondary phases such as NiS and Cu2SnS3 emerge. Raman spectroscopy further supports these findings, indicating compositional inhomogeneity and phase segregation at high Ni concentrations. Scanning Electron Microscope (SEM) micrographs further show that moderate Ni incorporation (x = 0.2) promotes improved surface compactness and grain growth, while excessive substitution results in grain fragmentation and increased surface roughness. Optical characterization reveals a tunable band gap, decreasing from 1.42 eV (x = 0) to 1.08 eV (x = 0.2), followed by a slight increase at higher doping levels, consistent with the formation of secondary phases. SCAPS-1D simulations of Mo/C2Z1-xNixSnS4/CdS/ZnO:Al/Al solar cell structures indicate improved photo-generated current and optical absorption at x = 0.2, whereas higher Ni substitution reduces the open circuit voltage (Voc) and the fill factor FF due to increased carrier recombination. Overall, moderate Ni incorporation enhances light absorption and carrier transport, whereas excessive Ni incorporation induces structural disorder, degrading device performance. These findings contribute to understanding the structural and compositional stability of Ni-doped CZTS thin films and their optimization for solar energy applications.

We report a detailed investigation of flux dynamics in YBa2Cu3O7-x (YBCO) superconducting thin films with strong synergetic pinning centers, using complex AC susceptibility measurements over a wide range of frequencies and DC magnetic fields. A new method is proposed to determine the characteristic pinning energy Uc from the electric field-current density (E-J) characteristics extracted at short time scales from AC susceptibility data. This method overcomes the limitations imposed by the use of standard Arrhenius plots, which rely on assuming a linear temperature dependence (p = 1) of the effective energy barrier UeffT,Hdc, J=Uc0 K,HdclnJc(T,Hdc)/J1-T/Tcp . Our results provide a more accurate approach for evaluating the magnetic field dependence of Uc, yielding a power-law dependence on the magnetic field induction Uc proportional to Bdc-0.32 , consistent with strong pinning behavior.

Copper-zinc-tin-sulfide (Cu2ZnSnS4, CZTS) kesterites a promising earth-abundant and non-toxic absorber for next-generation thin-film photovoltaics. However, sulfur-based CZTS solar cells remain limited in performance, largely due to intrinsic defects and interfacial recombination losses. Here, we systematically investigate the impact of solvent chemistry and extrinsic doping and compositional engineering on the quality of solution-processed CZTS absorbers. A comparative study of three solvents, 2-methoxyethanol (MOE), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) reveals that all yield phase-pure, and compact films due to a robust Cu+-Sn4+ molecular precursor system, with MOE producing the most uniform morphology and superior optoelectronic properties. Rather than focusing on pure CZTS, the solvent screening is conducted on Cd-alloyed CZTS (CZCTS), as Cd incorporation substantially alters precursor coordination and crystallization behavior. Building on the optimized processing route, a synergistic compositional strategy combining silver (Ag) - cadmium (Cd) co-alloying with lithium (Li) doping is introduced to suppress cation disorder, passivate grain-boundary defects, and enhance carrier transport. As a result, the optimized he optimized absorber delivers a champion device with a power conversion efficiency of 10.6%, placing it among the highest efficiencies reported for solution-processed, selenium-free CZTS solar cells. These results highlight the critical role of solvent engineering coupled with targeted extrinsic doping in overcoming the long-standing limitations of CZTS photovoltaics and provide a scalable pathway toward environmentally benign thin-film solar technologies.

Alloys (A) of crystalline Ge50Te50 and partially amorphous Ge20Te80 were added to MgB2 superconductor. Samples with composition (MgB2)A0.01 were ex situ spark plasma sintered (SPS) and characterized. Results are compared with pristine and Te ((MgB2)Te0.01) or Ge, ((MgB2)Ge0.01) added samples. Samples with Ge-Te alloys show the fastest densification rate of 3.4 to 5.2 times larger than for the other samples. Although there is a strong influence of Te, alloys synergistically contribute structural and microstructural development and superconducting properties as determined by magnetometry. The proposed methodology of densification curves analysis allows prediction of the final bulk density, MgO and MgB4 amounts, the amount of carbon substituting boron and critical temperature. The alloys addition promotes MgB4 formation and a slightly lower critical current density Jc for Ge50Te50. For Ge20Te80, Jc is approximately similar to pristine sample, while the highest Jc is for Te. Samples with Ge-Te have lower maximum pinning force values.

Magnetic superconductor EuRbFe4As4 is a quite unique system in which macroscopic superconductivity and magnetic ordering coexist, with interesting interactions between Abrikosov vortices and Eu2+ spins that were investigated mostly by static (DC) magnetization measurements. Our aim is to study the dynamic interactions between the two sub-systems using AC susceptibility measurements in a wide range of temperatures and superimposed DC fields. In low DC fields, the magnetic transition at 15 K is clearly visible. We have observed very little difference between the AC susceptibility in different cooling regimes, but large difference for different field orientation. For field perpendicular to the superconducting planes, we have observed an anomalous dependence just below the critical temperature, which is absent in the parallel field orientation. We explained the anomaly by the interplay between the sample dimensions and the temperature dependence of the London penetration depth which may allow the paramagnetic Meissner-like response to be detected in the temperature dependence of the AC susceptibility. We stress that the newly reported phenomenon reflects an AC-susceptibility manifestation of a field-stabilized critical state rather than a thermodynamic phase. In addition, we have observed a paramagnetic AC response in the normal phase, in both field orientations, indicative of interactions between Eu2+ spins and flux lines.

Lithium borohydride (LiBH4) is a promising hydrogen storage material releasing 13.8 wt% H2 upon decomposition in lithium hydride and boron, significantly surpassing other complex hydrides. However, sluggish dehydrogenation kinetics still hinder the use for practical applications. The infiltration of LiBH4 into carbon nanoscaffolds has proved to be effective in improving the hydrogen absorption/desorption (a/d) kinetics. Further improvement of storage kinetics can be achieved by modification of the nanocarbon with dopant elements. The present work compares nanoporous carbon (C-MSU-H) and C-MSU-H doped either with 1 at% N or 1 at% Ni as the matrix for infiltration of LiBH4. The catalytic effect of nitrogen proved to be superior to that of nickel (keeping the same doping level) for improving the hydrogen a/d kinetics of LiBH4 infiltrated in doped C-MSU-H. X-ray photoelectron spectroscopy was used to detect the amount and chemical proximity of nitrogen in nanoporous carbon following the thermal treatment in ammonia flow. The morphology and porosity of doped C-MSU-H were investigated by X-ray diffraction, FTIR, TEM, and BET. Hydrogen a/d kinetics of LiBH4@C-MSU-H nanocomposites was investigated by a volumetric method. The desorption peak temperatures (measured at 2 degrees C min-1 rate) are 339 degrees C for the undoped LiBH4@C-MSU-H, 328 degrees C for the LiBH4@C-MSU-H doped with 1 at% Ni and 318 degrees C for the LiBH4@C-MSU-H doped with 1 at% N nanocomposites. The activation energies of hydrogen desorption for the investigated nanocomposites were obtained from Kissinger plots: 142.7 kJ mol-1 for undoped LiBH4@C-MSU-H, 123.8 kJ mol-1 for LiBH4@C-MSU-H 1 at% Ni and 119.5 kJ mol-1 for LiBH4@C-MSU-H 1 at% N nanocomposites. The catalytic effect on LiBH4 dehydrogenation due to N-doping of nanocarbons is discussed.

Materials capable of withstanding extreme environments open promising opportunities for nuclear fusion reactors. In this study, equiatomic CrFeTaVW and CrFeTiVW high-entropy alloys are investigated as interlayer materials between W and CuCrZr. Monte Carlo and Molecular Dynamics simulations predicted a bcc-type structure for both systems. Additionally, the Monte Carlo simulation predicts lower potential energy and a more stable structure for both systems than Molecular Dynamics. For CrFeTaVW, the chemical segregation values are lower in MC than in the MD simulation, whereas for CrFeTiVW, the opposite trend is observed, with MC indicating stronger segregation values. After simulation, the high-entropy alloys were prepared by planetary ball milling, consolidated by spark plasma sintering, and analyzed using X-ray diffraction, scanning electron microscopy, and thermal diffusivity. The experimental results for the milled powders confirmed the formation of a bcc structure in both alloys. The consolidated material revealed a bcc-type structure and an Fe2Ta Laves phase for the CrFeTaVW HEA, while the CrFeTiVW HEA exhibits two different bcc-type structures. The values of CrFeTaVW and CrFeTiVW thermal diffusivity are between 3.5 and 7 mm2/s, which is consistent with the expected values for high-entropy alloys. Overall, the findings indicate that these HEAs have promising properties that can be used in extreme environments.

Palladium diselenide (PdSe2) is a two-dimensional (2D) transition metal dichalcogenide van der Waals material that exhibits a unique wrinkled pentagonal structure that results in strong anisotropic layer-dependent properties. Owing to these unique properties the material has a great potential for applications in electronic, optoelectronic, photonic and thermoelectric devices. Here, we study the electronic properties of monolayer (ML) PdSe2 in a Schottky junction by contacting the material with a tungsten scanning tunneling microscope (STM) tip. The STM-induced lifting of the 2D layer leads to structural warping of the top layer, which causes significant changes in the electronic properties of PdSe2 as well as the electrostatic potential across the junction. We compare the STM-determined work function of the warped PdSe2 ML with the work function of bulk PdSe2 as obtained by Kelvin probe force microscopy and electronic structure obtained by synchrotron-based angle-resolved photoemission spectroscopy, and x-ray photoelectron spectroscopy.

Transition-metal phosphorus trichalcogenides are a class of van der Waals materials having the potential to transform a diverse range of fields, from electronics to energy storage. Among these, nickel phosphorous trisulphide (NiPS3) is particularly interesting with its strongly correlated zig-zag antiferromagnetic insulating state below the Neel temperature, which is, of about 150 K. This study delves into the electronic properties of NiPS3 above the transition temperature, using state-of-the-art synchrotron-based techniques, including x-ray absorption spectroscopy (XAS) combined with x-ray linear dichroism (XLD) and angle-resolved photoemission spectroscopy (ARPES), in order to probe the chemical state and the momentum-resolved band structure. The experimental results, corroborated by density functional theory (DFT) calculations, unveil orbital asymmetries linked to the crystallographic lattice and the orbital content of the electronic structure, which presents a nearly flat band close to the Fermi level. Moreover, the calculated dipole matrix elements reproduce well the polarization dependence observed in valence band ARPES. Our data reveal details on the electronic properties of NiPS3, which could lead to innovative solutions and applications in areas such as advanced computing, photo-electrochemical devices, and photodetectors.