National Institute Of Materials Physics - Romania

Hafnium oxide (HfO2) thin films were deposited on n-type gallium arsenide (GaAs) substrates by Oxide-Molecular Beam Epitaxy (Oxide-MBE) method using Hafnium (Hf) metallic flow in an oxidizing atmosphere of 10-6 mbar molecular oxygen. The Hf metallic flow was provided by an e-beam evaporator and a deposition rate 10 nm/h was established. Semiconductor surface preparation was done prior to deposition, beginning with chemical wet etching and aggressively adjusted by in treatments until a desired stoichiometry was reached. Heterojunctions with HfO2 thin layers of 1 nm, 3 nm, 10 nm and 20 nm were fabricated. X-Ray Photoelectron Spectroscopy (XPS) and ARXPS(Angle Resolved XPS) in-situ analyses provided a clear picture of the structure of the interfaces, the chemical bonds and composition. The interfaces are chemically stable and abrupt. A small amount of Ga2O3 provides a passivating effect of the semiconductor surface. The electrical properties of the heterostructures were determined using the Kraut method and Reflection Electron Energy Loss Spectroscopy (REELS) technique. Band offsets Delta EC=1.75 eV and Delta EV=2.62 eV confirm a high application potential. Additionally, data on the morphology and continuity of the layers were obtained by Atomic Force Microscopy (AFM) technique while the amorphous growth was monitored by XRD(X-ray Diffraction), GIXRD (Grazing Incidence XRD) and XRR(X-ray Reflectivity) measurements. The dielectric layers showed values of the constant k in the range of 19-22, established by electrical measurements on MOS capacitors.

Two series of photocatalysts (TYAu, TYCeAu) were obtained. Ti was incorporated by direct synthesis with zeolite Y, while Ce and Au were immobilized by double incipient wetness impregnation method. The experimental weight percents (XRF analysis) were for titanium (0.7 %, 1.9 %, 3.5 %), Ce (1 %), and Au (0.3 %, 0.1 %). The typical crystalline structure of zeolite Y was preserved in all samples except those with 3.5 % Ti, where XRD revealed reduced pattern intensity. SEM and TEM analyses showed morphological changes at higher Ti contents. CO2-TPD confirmed a decrease in basicity with increasing Ti, consistent with the diminished zeolite contribution. XPS analysis indicated the presence of varying Au0/Au+ and Ce3+/Ce4+ ratios on the surface, depending on the Ti content. The intra- and extraframework TiO2 as amorphous or anatase phases were Raman confirmed in ceriumcontaining samples. For materials with high Ti content, the dominant effect was from the Ce and Ti species, accentuated by gold. The surface plasmon resonance effect of Au nanoparticles and decreasing in band gap energy after Ce immobilization was evidenced by UV-Vis spectroscopy. The photocatalytic properties of the synthesized materials were evaluated in CO2 reduction with water and H2 production via water splitting under visible light (525 nm). Higher Ti content enhanced CO2 conversion and reduced CH4 selectivity, favoring the production of CH3OH and CH2O. A greater amount of hydrogen was produced by the samples with the lowest Ti concentration while the reaction was favored by the presence of cerium in the rich titanium samples.

The development of titania-based hybrid nanostructures with enhanced visible-light photocatalytic activity has been a key research area recently. The present study addresses current limitations of the TiO2 based composite photocatalyst by newly-experimental designing of flower-like multifunctional hybrid nanostructure with visible light capability through Ferrihydrite (Fh) surface TiO2 functionalization. Here, we present a versatile nanocompartimentalization process of which, core anatase TiO2 nanoparticles are emebeded into Fh lamellar shell. Physico-chemical properties related to the chemical structure and morphology of the prepared nanomaterials were comprehensively analysed using complementary analytical techniques, such as powder X-Ray Diffraction (XRD), Field-Emission Scanning Electron Microscopy (FE-SEM), Ultra-High Resolution Transmission Electron Microscopy (UHR-TEM), X-Ray Photoelectron Spectroscopy (XPS) and soft X-Ray Absorption Spectroscopy (XAS). The conducted visible-light-driven photocatalytic water splitting tests highlights significant enhancement in the Oxygen Evolution Reaction (OER) performance for TiO2-Fh core-shell nanoheterostructure of 25.6 mu mol/L of molecular oxygen after 60 min of visible light irradiation (AM1.5G), and a photocatalytic water oxidation activity rate of 341.3 mu mol L-1 g-1h-1. The biocompatibility assessment of the developed core-shell structures combined with their enhanced photocatalytic water oxidation activity under visible light illumination suits them as excellent candidates for the development of sustainable environmental remediation technologies.

Nb2C MXene, obtained from Nb2AlC by Al3+ etching and exfoliation, was characterized using XRD, HRTEM and AFM, with the data confirming the crystallinity of the sample and the 2D morphology of the sheets with an average layer thickness of 1.5 nm. Surface analysis using XPS revealed the presence of structural defects, and NH3- and CO2-TPD profiles confirmed the low density of acid and basic sites in the range of tens of mu mol gcatalyst-1 of weak and moderate strengths. The combination of acid and basic sites in close proximity on the solid surface was responsible for the remarkable catalytic activity of Nb2C MXene in promoting aldolic condensation with high turnover frequencies of up to 855 h-1, which was comparable to the values of benchmark catalysts, such as MgO or HZSM-5. Nb2C MXene also catalyzed the aerobic oxidative aniline coupling to azo- and azoxy-benzene and hydrogenation of azoxybenzene to azobenzene.

A synthetic rational design of core-shell magnetic nanomaterials has garnered significant attention for their potential in photocatalysis and adsorption applications. This study presents the synthesis and characterization of a TiO2-based core-shell photocatalyst functionalized with FexOy co-catalyst for the efficient adsorption and degradation of synthetic methylene blue dye. The chemical synthesis technique involved a three-step process consisting in the preparation of TiO2 nanoparticles followed by surface nanocompartmentalization with a ferrihydrite layer exhibiting a flower-like morphology and lastly the calcination of the resulting composite at 400 degrees C to produce a magnetic core-shell nanomaterial. Comprehensive physicochemical characterization was performed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultra-high-resolution transmission electron microscopy (UHR-TEM) to elucidate the structural and morphological properties of the synthesized materials. Photodegradation experiments were conducted under both UV and Visible light irradiation using methylene blue as a model contaminant. The results revealed remarkable photocatalytic performance, with nearly instantaneous adsorption of the dye onto the catalyst surface, followed by efficient photodegradation. Detailed investigations confirmed that the adsorption process occurred at an exceptionally rapid rate, which was attributed to the unique surface functionalization and nanocompartmentalized structure of the core-shell material.

Electrochemical water splitting is regarded as a viable solution to future energy demands. Considering this, an innovative method to produce efficient oxygen evolution electrodes based on Co and Ni was proposed and successfully developed, where the metal atoms are intimately mixed before the calcination treatment. Electrochemical measurements demonstrated the high oxygen evolution activity and stability of the thus synthesized electrodes, EDX, and XPS revealing that the surface exhibits a remarkable oxidation resistance, allowing the active phase to better maintain its state when subjected to the aggressive positive potential required for oxygen evolution. Moreover, low electrical resistivity was recorded as a result of reduced thickness of the catalytic layer, further increasing the efficiency. These findings provide new insights into the design of durable and high-performance OER electrodes.

We report large-scale synthesis of monolayer WS2 films obtained by sulfurization of oxidized magnetron sputtered monolayer W precursors. Literature routes typically require similar to 800 degrees C, well above the 400 degrees C limit imposed by back-end-of-line (BEOL) integration. Here, using an enhanced chemical vapor deposition (CVD) approach, the magnetron sputtered ultrathin W precursor (a W monolayer film, 0.27 nm thick, which in ambient air becomes a WOx monolayer) is sulfurized at the lowest possible temperature (450 degrees C) within a microreactor, which consists of a sandwich-like structure formed by the precursor and a clean Si substrate. The obtained WS2 material has a good crystallinity and uniform morphology across the entire growth substrate, as confirmed by detailed characterization. These results highlight the versatility of the method combining magnetron sputtering and microreactor-CVD, facilitating its applications to wafer-scale synthesis of monolayer WS2, heterogeneously integrated into electronic circuits (a major objective for next-generation electronics and optoelectronics). Additionally, we investigate in detail the properties of WS2 films synthesized from a bilayer W precursor (0.43 nm thick), under the same conditions, and we calculated the frequencies of the second-order Raman scattering modes. For electrical measurements, we fabricated WS2/few-layer-graphene heterostructures, whose atomically clean interface yields reliable, low-resistance contacts. These devices exhibit resistive switching behavior, likely governed by vacancy migration, making it a promising candidate for memristive applications. Our results demonstrate that electronics-grade monolayer WS2 can be synthesized at 450 degrees C, approaching the BEOL requirement of 400 degrees C.

In this study, we investigate the electrochemical performance and stability of silicon thin films, deposited by magnetron sputtering, as anodes for lithium-ion batteries (LIBs). The conventional direct current magnetron sputtering and the highly ionized technique high power impulse magnetron sputtering HiPIMS, known to produce high-density thin films, were used for the purpose. Silicon films with thicknesses of 50, 75, 124, 600 and 1000 nm were prepared and the morphology, density, and electrochemical performances of the films produced using both techniques were compared. Characterization of the prepared electrodes showed that the DCMSdeposited films exhibited lower density and large grain sizes, while the HiPIMS-deposited films exhibited higher density and relatively smaller grain sizes. The electrochemical tests revealed that the DCMS-deposited electrodes exhibited discharge capacities of 800 mAh g- 1 and 406 mAh g- 1 at a current density of 5 A g- 1 for electrode thicknesses of 600 nm and 1000 nm , respectively, demonstrating their high potential for fast charging applications. Additionally, these electrodes showed remarkable initial coulombic efficiency (ICE), with the 1000 nm thick electrode achieving an ICE of 96.08 %, outperforming traditional graphite anodes, which was attributed to a larger particle size and a lower SEI resistance. Furthermore, DCMS-deposited Si electrodes showed higher resistance to plastic deformation, which enhanced capacity retention and mitigated volume expansion, demonstrating their potential for high-performance anodes.

Ag-decorated TiO2 aerogels were synthesized using an acid-catalyzed sol-gel method, followed by drying under supercritical CO2 and annealing within the temperature range of 350-500 degrees C, with 50 degrees C increments. This study explores the preparation-structure-performance relationships of Ag-TiO2 aerogels influenced by the annealing process, focusing on their morphological, (micro)structural, optical, and textural properties and surface defects concerning photocatalytic activity. X-ray diffraction (XRD) and Raman spectroscopy confirmed that all aerogels exhibited a single anatase phase of TiO2, while electron microscopy (SEM/TEM) and XPS analysis demonstrated the presence of components. Increasing the annealing temperature resulted in particle size and pore structure changes, reducing the aerogel's overall surface area and porosity, as observed by SEM and nitrogen (N2) sorption analysis. Additionally, according to the Williamson-Hall (W-H) analysis based on the X-ray peak profile, the lattice microstrain value decreased while the crystallite size increased with rising annealing temperature. Optical investigation showed a strong UV light absorption characteristic of TiO2 and a visible light absorption band attributed to the plasmonic effect of silver nanoparticles. Moreover, a gradual photoluminescence (PL) quenching trend was observed with decreasing annealing temperature, indicating a reduction in the recombination rate of photo-induced electrons and holes in Ag-TiO2, alongside the formation of oxygen vacancies and structural defects, consistent with electron paramagnetic resonance (EPR) measurements. The Ag-decorated TiO2 aerogels demonstrated enhanced visible-light photocatalytic activity for methylene blue (MB) degradation, with the Ag-TiO2 aerogel annealed at 500 degrees C exhibiting the highest photocatalytic performance. This improvement can be attributed to the synergistic effects of chemical composition, plasmonic enhancement, morphological properties, and light absorption characteristics.

This study investigates the development of electrochemical genosensors using gold-coated electrospun polymeric fibers electrodes, Au/PMMA/PET and immobilized phosphorothioated oligonucleotides. Scanning electron microscopy (SEM) with energy-dispersive X-rays spectroscopy (EDS) revealed a uniform distribution of oligonucleotides on the fibers, contrary to planar gold electrodes Au/Ti/SiO2/Si, where network-like films were observed. X-ray photoelectron spectroscopy (XPS) confirmed the successful immobilization of the phosphorothioated oligonucleotides via strong covalent gold-sulfur bonds, while surface plasmon resonance (SPR) indicated superior binding affinity, with significantly lower equilibrium dissociation constants, when compared to unmodified probes. The detection of BCR/ABL fusion gene of chronic myeloid leukemia using differential pulse voltammetry and methylene blue as electroactive indicator, showed that the Au/PMMA/PET electrodes achieved a sensitivity of 379 +/- 12 mu A cm(-)(2) pM(-)(1) and a limit of detection of similar to 5.00 +/- 0.01 fM, outperforming the Au/Ti/SiO2/Si planar electrodes. Reduced non-specific adsorption was observed on the Au/PMMA/PET electrodes and attributed to the inherent charges introduced during the electrospinning process, which created localized electrostatic fields that repelled weakly adsorbing molecules. These findings demonstrate the potential of Au/PMMA/PET electrodes as a robust platform for further development of high-performance clinical diagnostic devices.