National Institute Of Materials Physics - Romania

We model a change of the spin configuration in a two-dimensional square array, or a lateral superlattice, of quantum rings in an external perpendicular homogeneous magnetic field. The electron system is placed in a circular cylindrical far-infrared photon cavity with a single circularly symmetric photon mode. Our numerical results reveal that the spin ordering of the two-dimensional electron gas in each quantum ring can be influenced or controlled by the electron-photon coupling strength and the energy of the photons. The Coulomb interaction between the electrons is described by a spin-density functional approach, but the para- and diamagnetic electron-photon interactions are modeled via a configuration interaction formalism in a truncated many-body Fock-space, which is updated in each iteration step of the density functional approach. In the absence of external electromagnetic pulses this reordering of the spin configuration is replicated in the orbital magnetization of the rings. The change in the spin configuration can be suppressed by a strong electron-photon interaction. In addition, fluctuations in the spin configuration are found in dynamical calculations, where the system is excited by a time-dependent coupling scheme to a cylindrical cavity mode for emphasizing the diamagnetic electron-photon interaction not leading to simple electrical dipole oscillations. The diamagnetic interaction is enhanced by the rotational electric field of the particular cavity mode.

Herein, alpha-MoO3 micro- and nanoparticles were synthesized by a modified Pechini method, and the impact of the crystal structure and crystal growth orientation on the formation of ionic defects and, consequently, on the catalytic performance of the materials in the ethylic transesterification reaction for biodiesel production was investigated. Structural refinements from X-ray diffraction data and Raman spectra revealed the formation of alpha-MoO3 in a Pbnm orthorhombic phase, with nanoplate-like morphology at 500 degrees C (thickness between 100 and 260 nm) or ribbon-like morphology at 700 degrees C (thickness between 400 and 900 nm). An anisotropic crystal orientation along the [010] direction was observed with an increase of the calcination temperature. We emphasize the dependence of the orientation change with the elimination of ionic-type defects (oxygen vacancies and reduced Mo5+ centers) by the temperature using complementary techniques such as X-ray photoelectron and electron paramagnetic resonance spectroscopies. The catalytic activity of the samples depends on the orientation process and the presence of defects that act as acid-active sites on the catalyst surface and therefore play an important role in biodiesel production. This effect was confirmed by surface stability and reactivity simulated by density functional theory calculations, suggesting that the Mo and O surface terminals greatly impacted the interface catalytic reaction. The highest catalytic performance toward the biodiesel conversion (89% of conversion at 150 degrees C for 2 h) was achieved for the polycrystalline catalyst calcined at 500 degrees C, which was correlated with random crystal orientation and the presence of reduced Mo5+ and oxygen vacancy centers on the different facets exposed on the surface. The biodiesel production was confirmed by H-1 and C-13 NMR spectroscopy and gas chromatography analysis.

Most photocatalytic processes involve physicochemical phenomena occurring at the semiconductor-water interface. The interfacial charge transfer strongly depends on the charge carrier self-trapping or defect-based trapping mechanisms active in the crystal lattice of the photocatalyst. Thus, the crystal lattice distortion is expected to influence the photocatalytic efficiency during polymorphic phase transformations (PPT). A simple synthesis method involving the ultrasound-assisted excess hydrolysis of titanium tetra-isopropoxide (TTIP) (hydrolysis ratio (number of moles of water/number of moles of TTIP) r = 245) was used to obtain multiphase titanium dioxide (TiO2) nanomaterials with complex defect structures. Electron paramagnetic resonance (EPR) spectroscopy was employed to characterize the paramagnetic centers in the synthesized TiO2 and their behavior during incipient PPT. The calcined samples showed a complex defect structure comprising three types of paramagnetic centers: F+-centers (an electron trapped in an oxygen vacancy (Ov)), V-centers (oxygen ions with trapped holes) and paramagnetic centers involving Ti3+ such as Ti3+- Ov. The sample obtained at 600 degrees C, temperature marking the onset of a massive mixed transformation of anatase into rutile and brookite, composed of approximately 81 % anatase, 10 % brookite, and 9 % rutile, exhibited an intense and broadened EPR signal and enhanced photocatalytic activity for hydroxyl radical generation and hydrogen production by water splitting, despite its rather low specific surface area of 34 m2/g. The results revealed the synergistic effects of charge carrier trapping mechanisms in the early stages of PPT, boosting the photocatalytic performance of TiO2. The present study supports the design of facile synthesis methods for better TiO2 photocatalysts and promotes the development of further studies regarding lattice defect engineering during phase transformations in nanomaterials.

Nuclear fusion is a promising energy source. The International Thermonuclear Experimental Reactor aims to study the feasibility of tokamak-type reactors and test technologies and materials for commercial use. One major challenge is developing materials for the reactor's divertor, which supports high thermal flux. Tungsten was chosen as the plasma-facing material, while a CuCrZr alloy will be used in the cooling pipes. However, the gradient between the working temperatures of these materials requires the use of a thermal barrier interlayer between them. To this end, refractory high-entropy (CrFeTiTa)70W30 and VFeTiTaW alloys were prepared by mechanical alloying and sintering, and their thermal and irradiation resistance was evaluated. Both alloys showed phase growth after annealing at 1100 degrees C for 8 days, being more pronounced for higher temperatures (1300 degrees C and 1500 degrees C). The VFeTiTaW alloy presented greater phase growth, suggesting lower microstructural stability, however, no new phases were formed. Both (as-sintered) alloys were irradiated with Ar+ (150 keV) with a fluence of 2.4 x 1020 at/m2, as well as He+ (10 keV) and D+ (5 keV) both with a fluence of 5 x 1021 at/m2. The morphology of the surface of both samples was analyzed before and after irradiation showing no severe morphologic changes, indicating high irradiation resistance. Additionally, the VFeTiTaW alloy presented a lower deuterium retention (8.58%) when compared to (CrFeTiTa)70W30 alloy (14.41%).

Magnesium and its alloys, pure or doped MgO and its solid solutions show an increasing interest for future applications in the field of biomaterials. In this paper, MgO nanoparticles were prepared by sol-gel route starting from magnesium acetylacetonate and 2-methoxyethanol. A complex thermal analysis was used to investigate in situ the decomposition reactions, the gaseous products evolved from the gel precursor of MgO during the thermal treatment procedure, in the temperature range of 293.2-1123.2 K (20-850 degrees C), and crystallization of cubic MgO phase. We found 5 steps for the pyrolysis of the precursor gel and, as gaseous products removed: water, carbon dioxide and fragments of acetylacetone and 2-methoxyethanol. The structure and morphology of the resulting MgO powder were also investigated by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The results of these analyses demonstrated that as-prepared MgO powder show a microstructure consisting of particles with rounded shape and size ranging from 7 to 12 nm and, a structure of cubic phase by heating the gel precursor at 773.2 K (500 degrees C), 1 h in 5%H2/Ar.

In this work, we investigated the influence of reduced graphene oxide (RGO) on the electrical performances of ZnO/Ag multilayer transparent conductive electrodes. RGO flakes were successfully laser transferred by matrix assisted pulsed laser evaporation (MAPLE) using frozen targets obtained from suspensions of commercially chemically RGO powder dispersed in dimethyl sulfoxide (DMSO). The main deposition parameters such as RGO concentration, laser fluence, number of the laser pulses and deposition pressure were varied to identify the optimal morphological and optical characteristics. The laser transfer conditions for the RGO structures with the best transmittance were further employed in preparation of RGO/Ag/ZnO multilayer transparent conductive electrodes (MTCE). Thus, the MAPLE deposited RGO structures were covered with metal (Ag) by vacuum thermal evaporation (VTE) and subsequently with metal oxide (ZnO) by radio frequency magnetron sputtering (RF-MS). In comparison to the optical and electrical properties of a ZnO/Ag/ZnO reference structure, the results emphasize that the RGO/Ag/ZnO are featured by a similar transmittance (similar to 82-85 %) and improved sheet resistance (similar to 10.6 Omega/square, meaning up to 2-fold smaller).

Oxygen in the excited state is essential for organic synthesis and medical treatment. Herein, a novel phenomenon is reported in which the magnetic ground state of molecular oxygen undergoes a transition at room temperature from S = 1 to S = 1/2, corresponding to the transition of O2 from a triplet to a doublet state after stable physical adsorption on the defect-free surface of bulk VSe2. This density functional theory (DFT) calculations demonstrate the stable physical adsorption of O2 on both 1T- and 2H-VSe2 surfaces without further decomposition. Electron spin resonance (ESR) measurements confirm the spin state transition. Theoretical simulations reveal the charge transfer from entangled V-3d and Se-4p bands to oxygen as the leading cause of the spin state transition. This mechanism has not been previously proposed and offers multiple potential applications, from organic synthesis to medicine. Moreover, this approach can be extended to reveal new aspects of known catalytic materials and to design novel catalysts.

In photocatalysis, a common challenge is the rapid recombination of photogenerated electrons and holes, coupled with low surface reaction efficiency in aqueous environments. This study addresses these issues by improving the photocatalytic performance of Al3+-doped SrTiO3 perovskite through the strategic loading of CoOOH onto its surface. We successfully demonstrated that the combined approach of Al3+ doping and CoOOH co-catalyst functionalization significantly enhances the photocatalytic performance of SrTiO3. The Al3%:SrTiO3 was functionalized with CoOOH using a two-step process. This process involved the oxidation of Co2+ ions to Co3+ ions, followed by the precipitation of cobalt oxyhydroxide (CoOOH) onto the surface of Al3%:SrTiO3, resulting in the formation of an Al3%:SrTiO3@CoOOH composite heterostructure. The UV-Vis data shows an enhanced light absorption capabilities into the visible spectrum with a direct band gap of 1.73 eV in contrast to 3.24 eV for the pristine perovskite. XPS analysis confirms the surface functionalization with CoOOH co-catalyst and the determined 1:1 Sr:Ti stoichiometry, the reduced state of Al, and the absence of oxygen vacancies were identified as beneficial properties for photocatalytic applications, as shown in DFT calculations. The oxacillin photodegradation was tested at three different concentrations of Al3%:SrTiO3@CoOOH photocatalyst (0.25 g/L; 0.5 g/L and 1 g/L) and we observed that the removal efficiency significantly varies from 99 % for 1 g/L Al3%: SrTiO3@CoOOH to 78 % and 44 % for 0.5 g/L and, respectively 0.25 g/L. Additionally, under visible light irradiation, the Al3%:SrTiO3@CoOOH composite achieved an exceptional degradation rate of the (3-lactam antibiotic oxacillin of up to 99 %, with holes identified as key players in the photocatalytic process. This study highlights that effective surface modification using a well-chosen co-catalyst can substantially boost the photocatalytic efficiency of semiconductor-based materials, offering a promising strategy for developing advanced photocatalysts for environmental remediation applications.

Recently, spin asymmetry in O 2p related deep valence states was evidenced in SrTiO3(001) [Popescu et al., Phys. Scr. 99(10), 105925 (2024)]. In this work, we report the detection of a much higher (about four times) spin asymmetry in SrTiO3(011) by spin resolved photoelectron spectroscopy, with samples characterized also by core level x-ray photoelectron spectroscopy and low energy electron diffraction. The explanation of a so important spin asymmetry is related to the partial neutralization of O-2(4-) or SrTiO(4+) end layers. Missing electrons from O 2p states in the case of O-2 terminations enable robust atomic spins, according to Hund's rule. The parallel analysis of core level shifts for surface atoms and the amplitude of spin asymmetry suggests that 50% of the oxygens from the surface SrO layer of SrTiO3(001) have a 2p(5) configuration with an unpaired electron (the rest are in a 2p(6) configuration), while in the case of O-2 terminated SrTiO3(011), about 50% of surface oxygens have a 2p(5) configuration and 50% of surface oxygens are neutral (2p(4)), yielding a net charge per O-2 surface unit cell of (-1) instead of (-4). The magnetization is oriented along the rows formed by the (4 x 1) reconstruction in the 011 in-plane direction. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(https://creativecommons.org/licenses/by/4.0/).

We theoretically study the 2D Su-Schrieffer-Heeger model in the context of Floquet topological insulators (FTIs). FTIs are systems which undergo topological phase transitions, governed by Chern numbers, as a result of time reversal symmetry (TRS) breaking by a time periodic process. In our proposed model, the condition of TRS breaking is achieved by circularly polarized light irradiation. We analytically show that TRS breaking is forbidden in the absence of second order neighbors hopping. In the absence of light irradiation, we identify a symmetry-protected degeneracy and prove the appearance of a flat band along a specific direction in the momentum space. Furthermore, we employ a novel method to show that the four unit cell atoms, in the absence of irradiation, can be interpreted as conserved spin states. With the breaking of TRS via light irradiation, these spin states are no longer conserved, leading to the emergence of chiral edge states. We also show how the interplay between the TRS breaking and dimerization leads to some complex topological phase transitions. The validity of our findings is substantiated through Chern numbers, spectral properties, localization of chiral edge states and simulations of quantum Hall transport. Our model is suitable not only for condensed matter (materials), but also for cold gases trapped in optical lattices or topolectrical circuits.