National Institute Of Materials Physics - Romania

The present paper concerns a formalism aimed to describe the behavior and trajectories of complex scalar waves with space-varying amplitude in media characterized by variable refractive index. The classical wave equation is transformed into an eikonal-type equation, without imposing the geometrical optics limit, satisfied by an effective refractive index. The effective index combines the refractive properties of the medium with self-refraction phenomena generated by the space dependence of the wave amplitude. It also induces wave packet dispersion, leading to relations between the group and phase velocities that are particularly relevant in the context of de Broglie waves. The equations of motion are provided, and it is shown that the self-refraction term bends the trajectory of the wave and changes its velocity. The dynamics is dictated by the curvature of the trajectory expressed in terms of the gradient of the effective refractive index. The uncovered findings point toward a possible generalization of geometrical optics with relevant implications for its relation to quantum mechanics, general relativity and transformation optics.

Integrating two-dimensional transition-metal dichalcogenides with graphene is attractive for low-power memory and neuromorphic hardware, yet sequential wet transfer leaves polymer residues and high contact resistance. We demonstrate a complementary metal-oxide-semiconductor (CMOS)-compatible, transfer-free route in which an atomically thin amorphous MoS2 precursor is RF-sputtered directly onto chemical vapor-deposited few-layer graphene and crystallized by confined-space sulfurization at 800 degrees C. Grazing-incidence X-ray reflectivity, Raman spectroscopy, and X-ray photoelectron spectroscopy confirm the formation of residue-free, three-to-four-layer 2H-MoS2 (roughness: 0.8-0.9 nm) over 1.5 cm x 2 cm coupons. Lateral MoS2/graphene devices exhibit reproducible non-volatile resistive switching with a set transition (SET) near +6 V and an analogue ON/OFF approximate to 2.1, attributable to vacancy-induced Schottky-barrier modulation. The single-furnace magnetron sputtering + sulfurization sequence avoids toxic H2S, polymer transfer steps, and high-resistance contacts, offering a cost-effective pathway toward wafer-scale 2D memristors compatible with back-end CMOS temperatures.

Pnictogen chalcohalide semiconductors are emerging materials with broad potential in energy-related applications, including solar cells, photocatalysis, photodetectors, batteries, supercapacitors, thermoelectric and piezoelectric generators. Their compositional flexibility allows fine tuning of structural and optoelectronic properties. In this study, microcrystalline powders of Sb1-xBixSeI (x = 0-1) were synthesized from binary precursors by a solid-state method in evacuated quartz ampoules. Energy dispersive spectroscopy confirmed the successful substitution of Sb with Bi in Sb1-xBixSeI. The formation of solid solutions was also supported by Raman spectroscopy and X-ray diffraction (XRD). All materials exhibited needle-shaped crystal morphologies and orthorhombic crystal structure (Pnma), regardless of the Bi/Sb ratio. XRD patterns shifted toward smaller angles with increasing Bi content, indicating lattice expansion. Calculated lattice parameters (b and c) increased linearly with Bi incorporation, while the lattice parameter (a) remained constant. Raman spectra exhibited characteristic peaks at 182 cm- 1 for Bi-Se vibration and 209 cm- 1 for Sb-Se vibration, with intensity ratios reflecting Bi content. UV-Vis-NIR diffuse reflectance spectroscopy revealed a direct band gap that decreased from 1.7 eV (SbSeI) to 1.29 eV (BiSeI). Room-temperature photoluminescence measurements exhibited a single emission band, shifting from 1.75 eV to 1.41 eV with increasing Bi content. Ultraviolet photoelectron spectroscopy indicated a shift in the valence band maximum from 0.44 eV (SbSeI) to 1.1 eV (BiSeI). These findings highlight the tunability of Sb1-xBixSeI compounds, offering pathways for optimizing their properties for specific optoelectronic applications.

This study explores the optimization of multi-walled carbon nanotube (MWCNT) concentration in nylon-based nanocomposites to enhance the performance of triboelectric nanogenerators (TENGs). Nylon/MWCNT nanocomposite films were fabricated using the spin-coating method, and their electrical output was systematically evaluated as a function of MWCNT concentration. Results show that the open-circuit voltage (Voc) and short-circuit current (Isc) increase with MWCNT loading up to 0.05 wt%, reaching a peak of 29.7 V and 3.0 mu A, respectively, compared to 17.5 V and 1.8 mu A for pristine nylon-based TENGs. However, a decline in output was observed at 0.1 wt% due to MWCNT agglomeration, which disrupts charge transfer and introduces charge leakage. The enhancement at optimal concentration is attributed to improved charge trapping and increased dielectric constant, while excessive CNT loading reduces the effective contact area and limits triboelectric charge generation. These findings underscore the crucial role of nanomaterial dispersion in optimizing TENG performance and offer valuable insights for the development of high-efficiency triboelectric energy harvesting systems.

A facile and versatile strategy to obtain NPs@ZnAlO nanocomposite materials, comprising controlled-size nanoparticles (NPs) within a ZnAlO matrix is reported. The mono-(Au, Ni) and bimetallic (Ni-Au) NPs serving as an active phase were prepared by the polyol-alkaline method, while the ZnAlO support was obtained via the thermal decomposition of its corresponding layered double hydroxide (LDH) precursors. X-ray diffraction (XRD) patterns confirmed the successful fabrication of the nanocomposites, including the synthesis of the metallic NPs, the formation of LDH-like structure, and the subsequent transformation to ZnO phase upon LDH calcination. The obtained nanostructures confirmed the nanoplate-like morphology inherited from the original LDH precursors, which tended to aggregate after the addition of gold NPs. According to the UV-Vis spectroscopy, loading NPs onto the ZnAlO support enhanced the light absorption and reduced the band gap energy. ATR-DRIFT spectroscopy, H2-TPR measurements, and XPS analysis provided information about the functional groups, surface composition, and reducibility of the materials. The catalytic performance of the developed nanostructures was evaluated by the photodegradation of bisphenol A (BPA), under simulated solar irradiation. The conversion of BPA over the bimetallic Ni-Au@ZnAlO reached up to 95% after 180 min of irradiation, exceeding the monometallic Ni@ZnAlO and Au@ZnAlO catalysts. Its enhanced activity was correlated with good dispersion of the bimetals, narrower band gap, and efficient charge carrier separation of the photo-induced e-/h+ pairs.

This study investigates the influence of different aromatic side groups on the 2D-benzodithiophene (BDT) unit in donor-acceptor conjugated polymers for organic solar cell (OSC) applications. Three new polymers, P1, P2, and P3, featuring phenyl, thienyl, and thienothienyl side chains on the 2D-BDT backbone, respectively, were synthesized using the Stille cross-coupling reaction. The benzotriazole (BTz) unit served as the electron acceptor with a selenophene it-bridge to enhance electronic interactions. The optical band gaps were determined to be 1.79 eV, 1.74 eV, and 1.73 eV for P1, P2, and P3, respectively. OSCs fabricated using these polymers and PC71BM as the acceptor showed the best performance for the thienyl-substituted polymer (P2), achieving a PCE of 4.34 % with a JSC of 10.08 mA/cm2, an VOC of 0.67 V, and a FF of 64 %. Compared to P1 and P3, the P2-based blend exhibited a more defined interpenetrating network with PC71BM, enhancing charge transport and promoting exciton dissociation due to its thinner active layer and optimized morphology. These findings highlight the importance of side-chain engineering in improving the optoelectronic properties, morphology, and photovoltaic performance of OSCs. This study highlights the critical role of side-chain engineering in tuning the optoelectronic properties, morphology, and performance of OSCs. The findings emphasize that thienyl side chains in P2 facilitate better it-it stacking and molecular organization, resulting in superior device performance compared to phenyl and thienothienyl-substituted counterparts.

Bilayer metallic electrodes of magnesium and zinc-aluminum alloys Mg/Zn:Al were prepared using combined laser-thermionic vacuum arc methods. Plasma diagnosis suggests laser-plasma interaction by increasing the discharge voltage when the laser is on, which is converted by thermal annealing of plasma ions. A magnesium thin film with a lower work function, covered with a Zn:Al (4:1) layer was successfully used for efficient charge injection into electroluminescent diodes. The zinc-aluminum combination preserved the electrical conductivity of the thin magnesium film and protected it against oxidation. The capacitance-voltage measurements confirmed the integrity of the magnesium layer, whereas the work function was not affected. The zinc-aluminum alloy obtained through a thermionic vacuum arc exhibits good conductivity compared to pure zinc layers and better stability against degradation.

Synchrotron-based small- and wide-angle X-ray scattering is used to elucidate the structure of low-dimensional lepidocrocite-titanate-based nanofilaments. In the colloidal state, they consist of quantum-confined 1D NFs, loosely associated into nanoribbons, one lepidocrocite sheet thick (about 4 & Aring;), 30-40 & Aring; wide (5-8 NFs), and more than 300 & Aring; long. In the dry state, they reach a final state of extended sheets, stacked three to about twenty high, whose crystallinity increases with stack height, in parallel with a decrease in photocatalytic activity. These findings suggest a kinetic pathway for the self-assembly of initially 1D titanate nanoribbons into 2D and ultimately 3D structures, providing context for a recent body of work on these low-dimensional materials.

In this study we present the effect of the irradiation of sintered MgB2 with protons of intermediate energies, from 8.6 to 15.07 MeV, at a constant fluence of 2.86 x 10(16) p/cm(2). We found clear evidence that the disorder generated by irradiation leads to a weak suppression of the critical temperature T-c and an increase of the critical current density J(c) with increasing proton energy, as expected. However, it was found that thermomagnetic instabilities and vortex creep phenomena strongly depend on the ratio between proton range and the sample thickness. The number of the macroscopic flux jumps (MFJ) is highest and manifest up to 28 K in the sample with the proton range shorter than sample thickness (P-1) and decrease as the proton energy increases. Similarly, the relaxation rate of the irreversible magnetization is the lowest in the sample P-1 and increases with proton energy. We tentatively attribute this effect to the protons that stop within MgB2 and interact with the local structure/atomic composition.

This study aims to investigate the impact of clays on the quality of the Nubia reservoir in the Saqqara field, Gulf of Suez, Egypt. The research will contribute to a broader knowledge of reservoir characterization, offering valuable insights for similar geological settings in other regions, thereby aiding in the optimization of resource management in global petroleum industries. The shale evaluation procedure involves three primary steps: estimating shale volume, identifying clay minerals, and assessing shale distribution. The neutron-density (N-D) method was employed to estimate the shale volume in the Nubia reservoir, yielding an average of 0.6% across four wells, with a maximum recorded value of 2.2% in well GS323-3. These values represent that the Nubia reservoir contains a negligible amount of shale, indicating that the porosity and permeability are high. Clay mineral analysis, based on a Potassium-Thorium (K-Th) and Potassium-PEF cross-plots, identified the presence of chlorite, illite, montmorillonite, and heavy thorium-bearing minerals, where chlorite and illite enhance the mechanical stability of the reservoir, while montmorillonite may cause issues with swelling and pressure. Thorium-bearing heavy minerals are typically associated with reduced permeability due to their influence on chemical interactions. The shale distribution analysis, conducted using the Thomas and Stieber model, confirms the overall cleanliness of the Nubia reservoir. Most formation data points align with the 0% shale line, indicating high total porosity, while only a few points fall along the dispersed shale line. In conclusion, the findings indicate that the Nubia reservoir exhibits minimal shale content, predominantly clean lithology, and favorable porosity and permeability characteristics. Consequently, the reservoir is classified as high-quality, making it suitable for efficient hydrocarbon production.