National Institute Of Materials Physics - Romania

Despite the promise of high-k dielectrics, inherent limitations persist in transistor scaling and enhancing energy efficiency, including a fundamental threshold of 60 mV/dec for increasing drain current by an order of magnitude. Proposed solutions involve negative capacitance at the gate oxide to overcome this barrier using ferroelectric structures. Efforts to understand and regulate the switching dynamics and intricate electrostatic configurations of ferroelectric structures towards achieving negative capacitance regimes have intensified. While standalone ferroelectric capacitors cannot stabilize negative capacitance without external fields, multilayered thin films offer a promising solution. Typically, ferroelectric layers are paired with dielectrics/insulator, demonstrating steady-state negative capacitance, often at nanoscale or specific temperature domains. This study aims to stabilize negative capacitance in ferroelectric structures by inducing internal electric fields, aligning the system near coercivity, particularly in bilayer structures formed by two ferroelectric layers with slight differences in polarization values, such as p-n heterojunctions using Pb (Zr,Ti)O3 PZT) with different doping as Fe, Nb, Bi. Most of these structures exhibit evident amplification of capacitance compared to the equivalent series-connected capacitance, across a large temperature domain. The complex capacitance-frequency characteristic of these structures indicates a complex equivalent circuit. Analysis of these complex circuits compared with simple component layers concludes that at least one of the FE layers in these bilayer structures is in a negative capacitance (NC) state.

Cerium dioxide (CeO2) thin films with pyramidal nanostructures exhibiting a fractal aspect are suitable for various applications that require a large surface area. An accurate model of these films is valuable not only for optimizing the properties for specific applications but also for predicting and understanding the growth mechanism. In this paper, we present the foundation of a simulation for the growth of CeO2-x nanostructured thin films. We propose a 3-dimensional model of the surface nanostructures and link the morphology with crystallographic orientations and growth modes. To validate our model, we fabricated CeO2_ x thin films with different thicknesses using pulsed laser deposition (PLD) and characterized their morphological and structural properties. The evolution of our films shows the representative features of the Stranski-Krastanov model. The morphology changes from compact and smooth to dendritic with pyramidal nanostructures. The texture of our film also changes with thickness, and the preferential orientations are (111) and (220). Additionally, we characterize the CeO2_ x thin films from the chemical and optical points of view. The stoichiometry of CeO2 is not fully achieved, our thin films present the Ce3+ oxidation state at the surface. The formation of C-type Ce2O3 with fluorite structure can be associated with a small refractive index and small band gap.

The precise control of the magnetic compensation temperature (theta c) in ferrimagnetic garnets is essential for the development of cutting-edge ultrafast customizable spintronic devices. In this work we demonstrate how fine variation in stoichiometry and cation distribution in iron gadolinium garnets significanty influences theta c. Two samples of Gd3Fe5O112 garnets synthesized via a new hydrothermal method and a conventional solid-state reaction, respectively, were considered. The complex study was carried out using a complex approach combining X-ray diffraction, magnetometry, and M & ouml;ssbauer spectroscopy. Atomic-scale analysis revealed with unprecedent accuracy a cationic inversion between Fe3+ ang Gd3+ at octahedral and dodecahedral sites in both samples, and their chemical compositions were determined as Gd2.70Fe4.76O11.9 and Gd2.96Fe4.68O11.5, respectively. These local rearrangements have been shown to have a consistent influence on theta c (290 K and 317 K, respectively) around room temperature, emphasizing the high sensitivity of exchange interactions to internal atomic order. Results clearly illustrate the strong correlation between the processing, atomic configuration and macroscopic magnetic behavior, establishing a new paradigm for the design of garnet-based materials with tunable theta c. The strategy for the accurate determination of cation inversion illustrated in this work exhibits great potential in guiding material innovations for next-generation spintronics.

This study explores the fabrication and optimization of superstrate-structured antimony sulfide (Sb2S3) thin-film solar cells using RF magnetron sputtering of antimony (Sb) followed by sulfurization. The study systematically investigates the effects of varying absorber and buffer layer thicknesses on the photovoltaic performance of FTO/ CdS/Sb2S3/Spiro-OMeTAD/Au solar cell devices. Analytical techniques confirmed the structural and chemical properties of the Sb2S3 films obtained after Sb post-sulfurization, demonstrating improved crystallinity and a composition consistent with a primarily Sb2S3 phase. Optimizing the Sb2S3 absorber thickness to 100 nm resulted in a maximum power conversion efficiency of the champion device of 2.76%, with enhanced short-circuit current density (J(sc)) up to 14 mA/cm(2) and open-circuit voltage (V-oc) of up to 650 mV. The device exhibited semi-transparency up to 20% in the wavelength range of 380-740 nm, making it suitable for indoor and building-integrated photovoltaic applications. The results underscore the potential of magnetron-sputtered Sb2S3 for emerging transparent thin-film photovoltaics while highlighting the importance of thickness control and interface engineering for efficiency improvements.

Superoxide plays a significant role in maintaining physiological states of living systems, with major roles in eradicating invading microorganisms and in cell signaling. It is regulated intricately by the enzyme superoxide dismutase (SOD), and when not properly regulated it can lead to cascade biological pathways with severe and irreversible damage to biofilms, tissue, and organs, being linked with many neurodegenerative diseases, atherosclerotic and cardiovascular diseases. Therefore, superoxide anion (O center dot-2 ) detection has a tremendous potential in clinical diagnostics to assess oxidative stress in living cells. This comprehensive review aims to explore, discuss, and analyze recent trends in the electrochemical detection of O center dot-2 in living systems, focusing not only on the recognition mechanism for in vitro assays (living cell cultures/tissues) but also on the importance of the electrode design and operational parameters for in vivo measurements (implantable sensors). By analyzing current in vitro/in vivo electrochemical strategies we gather information that is helpful to overcome existing limitations in the dynamic monitoring of O center dot-2 , and further improve electrochemical strategies that can be adopted and applied to prevent its negative effect, with an insight into the pathophysiology of neurodegenerative disorders and even cellular malignancies that derive from its accumulation in living systems.

In this study, thermal analysis (including differential thermal analysis and thermogravimetric analysis coupled with mass spectrometry) was used to design the sintering process of alumina as well as Al2O3-Mo and Al2O3-Ni green bodies obtained by digital light processing (DLP) 3D printing. The measurements were performed for selected organic additives, which are commonly used in the DLP technique, such as photoinitiators, dispersing agents, and organic monomers. Additionally, metallic powders (Ni, Mo), as well as ceramic and composite green bodies, have been subjected to thermal analysis. The obtained results allowed us to determine proper sintering conditions for a two-step sintering program. Firstly, the organic phase was burnt out at 400 degrees C in the air. At this temperature, metallic powders have not yet started to oxidize, and most of the organic additives have already been eliminated from the sample. The second step was performed using spark plasma sintering at 1150 degrees C with a pressure equaling 60 MPa in an argon atmosphere to prevent the oxidation of metals. The samples were gradually cooled down to 800 degrees C at a cooling rate of 35 degrees C min-1 and then furnace-cooled to room temperature, preventing the formation of intrinsic defects (microcracks) in a multicomponent ceramic-metal composite. The XRD and SEM-EDS analysis allowed us to conclude that the obtained composites are well densified, no other phases apart from alumina and metals are present in the samples, and that the alumina grain growth is smaller than for conventional sintering. An increase in fracture toughness for the composite samples was observed compared to pure alumina.

Chromium-doped hydroxyapatite (7CrHAp) and chromium-doped hydroxyapatite in chitosan matrix (7CrHAp-CH) coatings were synthesized in order to address the need for biomaterials with improved physico-chemical and biological properties for biomedical applications. Both chromium-doped hydroxyapatite (7CrHAp) and chromium-doped hydroxyapatite in chitosan matrix (7CrHAp-CH) coatings could represent promising materials for biomedical applications due to their superior properties. This study aims to evaluate the physico-chemical and in vitro biological properties of 7CrHAp and 7CrHAp-CH coatings to determine the impact of chitosan incorporation on the physico-chemical and biological features. The results reported in this study indicate that addition of chitosan improves surface uniformity and biological properties, highlighting their potential for uses in biomedical applications. In this study, coatings of chromium-doped hydroxyapatite (7CrHAp, with xCr = 0.07) and its composite variant embedded in a chitosan matrix (7CrHAp-CH) were systematically analyzed using a suite of characterization techniques: X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and metallographic microscopy (MM). The results of the XRD analysis revealed that the average crystal size was 19.63 nm for 7CrHAp and 16.29 nm for 7CrHAp-CH, indicating a decrease in crystallite size upon CH incorporation. The films were synthesized via the dip coating method using stable suspensions, whose stability was assessed through ultrasonic measurements (double-distilled water serving as the reference medium). The values obtained for the stability parameter were 2.5910-6 s-1 for 7CrHAp, 8.6410-7 s-1 for 7CrHAp-CH, and 3.1410-7 s-1 for chitosan (CH). These data underline that all samples are stable: CH is extremely stable, followed by 7CrHAp-CH (very stable) and 7CrHAp (stable). The in vitro biocompatibility of the 7CrHAp and 7CrHAp-CH coatings was evaluated with the aid of the MG63 cell line. The cytotoxic potential of these coatings towards MG63 cells was quantified using the MTT assay after 24 and 48 h of incubation. Our results highlight that both 7CrHAp and 7CrHAp-CH coatings exhibit high biocompatibility with MG63 cells, maintaining cell viability above 90% at both incubation times, thus supporting osteoblast-like cell proliferation. Furthermore, the antimicrobial efficacy of both 7CrHAp and 7CrHAp-CH samples was evaluated in vitro against the Pseudomonas aeruginosa 27853 ATCC (P. aeruginosa) reference strain. The in vitro antibacterial activity of the 7CrHAp and 7CrHAp-CH coatings was further evaluated against Pseudomonas aeruginosa 27853 ATCC (P. aeruginosa), Escherichia coli ATCC 25922 (E. coli) and Staphylococcus aureus ATCC 25923 (S. aureus) reference strains. In addition, atomic force microscopy (AFM) analysis was also used to investigate the ability of P. aeruginosa, E. coli and S. aureus cells to adhere and to develop colonies on the surfaces of the 7CrHAp and 7CrHAp-CH coatings. The results from the biological assays indicate that both coatings exhibit promising antibacterial properties, highlighting their potential for being used in biomedical applications, particularly in the development of novel antimicrobial devices.

The purpose of this study was to investigate the effect of the addition of a third cation, M (M = Ti, Sn, Sb and Ta) on the physicochemical properties and catalytic performance of the Nb-containing NiO catalyst (Ni0.85Nb0.15O) during ethane oxidative dehydrogenation to ethylene. Hydrothermal and solvent evaporation methods were used for the preparation of tricationic oxides of type Ni0.765Nb0.135M0.1O, which were then calcined at 450 degrees C. The catalysts were characterized by XRD, SEM-EDX, H2-TPR, MicroRaman, and XPS, and evaluated via in situ electrical conductivity measurements under varying conditions. The findings indicate that the addition of a third cation substantially changes the structural, electronic and redox properties of the NiNbO system, with considerable effects on its catalytic activity in the oxidative dehydrogenation of ethane. Among all the catalysts tested, the Ta(5)-NiNbO-8 sample-a 5 at % Ta-doped NiNbO catalyst prepared under alkaline conditions (pH 8)-exhibited the best performance: 40% ethane conversion and 75% ODH selectivity at 350 degrees C. It outperforms the undoped NiNbO system in terms of both ethane conversion and ethylene selectivity over the whole temperature range studied. Its superior behavior is attributed to an optimal balance between redox ability and surface composition, particularly a reduced density of nonselective active species and enhanced lattice oxygen exchangeability under reaction conditions. However, none of the catalysts, including Ta(5)-NiNbO-8, demonstrated sustained stability at 400 degrees C. Progressive deactivation was linked to a gradual loss of p-type conductivity and diminished reoxidation capacity, consistent with a decrease in the density of active lattice O- species. Surface compositional changes together with structural changes were also associated with catalyst deactivation. These results demonstrate how important the type of dopant is in adjusting the physicochemical characteristics and catalytic activity of Nb-promoted NiO systems for ethane ODH. Future efforts will focus on exploring additional high-valence dopants and surface modifications to improve long-term stability and ODH selectivity.

Mg2Si1-xSnx solid solutions are promising thermoelectric materials due to the inexpensive and abundant nature of their components, their environmental friendliness and their flexibility in terms of optimising their thermoelectric properties. In this study, we report on the thermoelectric properties enhancement of n-type Mg2Si0.3Sn0.7 solid solutions doped with aliovalent elements, donor (Bi) and acceptor (Ag), respectively. Samples were synthesized via conventional melting followed by spark plasma sintering. Co-doping led to carrier concentration and mobility tuning, resulting in a significantly increased Seebeck coefficient while maintaining high electrical conductivity. Consequently, the power factor reached 44.67 x 10(-3) Wm(-)(1) K-2 at 650 K for Mg1.98Ag0.02Si0.29Sn0.69Bi0.02. Furthermore, lattice thermal conductivity was reduced via enhanced phonon scattering caused by point defects and suppressed bipolar conductivity attributed to a widened band gap and increased carrier density. The combined effects yielded improved ZT values, peaking at 1.12 at 674 K and 1.13 at 721 K for Mg1.99Ag0.01Si0.29Sn0.69Bi0.02 and Mg1.98Ag0.02Si0.29Sn0.69Bi0.02, respectively. These results highlight the potential of Ag-Bi co-doping as a robust strategy to enhance the thermoelectric efficiency of Mg2Si1-xSnx based materials.

Zeolitic imidazolate frameworks (ZIFs) have recently emerged as promising precursors for the synthesis of heteroatom-doped nanocarbon materials. The chemical and structural features of these frameworks are influenced by the synthesis methodology, which directly affects their catalytic efficiency and stability. This study aims to investigate such frameworks by exploring a Co-ZIF structure doped with iron. Part of the FexCoy-ZIF (x = 0.05-0.15; y = 0.95-0.85) precursors is directly pyrolyzed to form FexCoy-NPC (NPC-nanoporous carbon), while another part is coated with a silica shell, followed by the pyrolysis of the FexCoy-ZIF@SiO2 intermediates to produce FexCoy-NCF (NCF-nanocarbon framework). To elucidate their chemical, structural, and catalytic properties, the synthesized materials are comprehensively characterized and finally investigate in the base-free oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA). The optimal catalyst (Fe0.15Co0.85-NCF) demonstrates complete conversion of HMF (>99.9%) to FDCA with a pretty high selectivity (82.4%) after 6 h reaction at 80 degrees C. The correlation of the catalytic features with the efficiency of the catalysts provides insight into the catalytic characteristics responsible for the highest HMF conversion and selectivity to FDCA. The stability and recyclability of the catalysts are also examined.