Publications

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%).

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.

Biodegradable templates are sought for targeted antibiotic administration/delivery in bone infections to avoid detrimental reactions during bone regeneration. We investigated two key-aspects in this direction: 1) the optimal antibiotic delineation, with thermal stability equivalent to the temperature involved for templates preparation and 2) the antibacterial templates development by a two-step homogenization process. The selected antibiotic - ampicillin (AMP) was mechanically and then thermally mixed with the prime materials: poly(lactic acid) (PLA) - the polymeric matrix, biogenic bovine bone-derived hydroxyapatite (HA, particles <40 mu m), and graphene nanoplatelets (GnP, micrometric range). For the first time, all materials were used in simultaneous modulated ratios for the synthesis of PLA/AMP (1-5 wt.%)/HA (0-30 wt.%)/GnP (0-3 wt.%) composites. The influence of the concomitant materials modulation was surveyed through several assays. The FTIR-ATR spectroscopy depicted a three-level model of overlapping structures asserting the solid molecular cross-linking between all materials, without structural alterations induced to any material (XRD analysis). The addition of AMP to the PLA matrix caused a slight particle conglomeration, alleviated through GnP addition. The microporous HA particles supported the adhesion to the PLA matrix and promoted the AMP particles' entrapment/attachment. Linked to the enhanced wettability of the composite materials, the phosphate buffered saline solution (PBS) degradation profiles revealed a pronounced burst during the first 14-28 days of incubation and a long-term, low-level process hereafter. Thus, the formation of pores and cavities were signalled along with the HA particles' fragmentation. The released AMP (percentage) pictured an ascending trendline during the 24 h of analysis, at all targeted ratios. The preservation of AMP features at 200 degrees C was endorsed by the strong antibacterial activity of composite materials against Staphylococcus aureus (S. Aureus) growth and the medium response against Escherichia coli (E. Coli) - the higher the drug release, the higher the inhibitory effect on bacteria evolution.

When used as the active layers-either as a light absorber in photovoltaic devices or as an electroluminescent material in light-emitting devices-lead-free perovskites significantly impact the performance of optoelectronic devices. This study focuses on antimony-based perovskites, which are promising for lighting applications. These types of perovskites enable the formation of self-trapped excitons (STEs) with higher dissociation energy than lead-based perovskites, which generate excitons with lower dissociation energy. The (CH3NH3)3Sb2I9 crystals are synthesized using two methods, resulting in distinct spatial configurations - dimer and dimer/layered mixtures, each exhibiting unique structural and spectroscopic properties, as revealed by comprehensive multi-parametric complementary analyses. Their emissive properties underscore the efficiency of the STE photoluminescence, driven by electron-phonon interactions and influenced by Sb-Sb distances in (CH3NH3)3Sb2I9 powder, whether dispersed in polymethyl-methacrylate or solution. The phase transition from monoclinic to hexagonal (dimer) and trigonal (layered) structures enabled the tuning of the optical properties in direct correlation with the structural and electrical features. The photoluminescence behavior of the STEs, analyzed in conjunction with the Raman spectroscopy, elucidates the dynamic process of the electron-phonon coupling effects in the dimer (face-capping Sb-I octahedra) and layered (corner-sharing Sb-I octahedra) crystallographic structures.

In the last decade, it has been observed that the field of biomaterials has gained the attention of the researchers. This study presents the physicochemical and biological properties of coatings based on chromium-doped hydroxyapatite (CrHAp) and chromium-doped hydroxyapatite enriched with amoxicillin (CrHApAx). The coatings were obtained for the first time using the dip coating technique, beginning from dense suspensions of CrHAp and CrHApAx. The obtained layers were then analyzed by various methods in order to have a comprehensive overview of their physicochemical properties. Stability studies performed using ultrasound measurements showed that the CrHAp suspension has very good stability (S = 6.8610-6 s-1) compared to double-distilled water. The CrHApAx suspension (S = 0.00025 s-1) shows good but weaker stability compared to that of the CrHAp suspension. Following XRD studies, a single hydroxyapatite-specific phase was observed in the CrHAp sample, while in the case of the CrHApAx sample, an amoxicillin-specific peak was also observed. The AFM results showed that the CrHAp coatings had a surface topography of a homogenous and uniform layer, with no significant cracks and fissures, while the CrHApAx coatings exhibited a surface morphology of homogenous layers formed of particles conglomerates. The biocompatibility of CrHAp and CrHApAx coatings was assessed using the MG63 cell line. The cytotoxicity of the coatings was evaluated by measuring cell viability with the aid of an MTT assay after 24, 48, and 72 h of incubation with the CrHAp and CrHApAx coatings. The results demonstrated that both CrHAp and CrHApAx coatings exhibited good biocompatibility for all the tested time intervals. The in vitro antibacterial activity of the coatings was also assessed against Pseudomonas aeruginosa 27853 ATCC (P. aeruginosa) bacterial cells. The potential of P. aeruginosa bacterial cells to adhere and develop on the surfaces of CrHAp and CrHApAx coatings was also investigated using AFM analysis. The findings of the biological assays suggest that CrHAp and CrHApAx coatings could be considered as promising candidates for biomedical applications, including the development of novel antimicrobial materials.

In this paper, we present the development of magnesium-doped hydroxyapatite in chitosan matrix (MHA_Ch) powder by an adapted coprecipitation method. The MHA_Ch powder was then deposited as thin layers by radio frequency magnetron sputtering. The MHA_Ch layers were exposed to various irradiation doses and immersed in Dulbecco's Modified Eagle's Medium (DMEM) for various time intervals. We report, for the first time, the effects of DMEM on irradiated layers of magnesium-doped hydroxyapatite in a chitosan matrix. The surface morphology of the layers before and after irradiation and immersion in DMEM was evaluated by SEM, AFM, and MM studies. Additionally, data about the functional groups present in the layers and the changes induced by exposure of the layers to irradiation and DMEM were obtained by FTIR studies. In vitro biological assays were conducted using an MG63 cell line (ATCC CRL1427). Our results suggest that the magnesium-doped hydroxyapatite in chitosan matrix layers may be suitable candidates for applications in the biomedical domain.