National Institute Of Materials Physics - Romania

ZnO is recognized as one of the most versatile semiconductor materials, due to its adjustable properties, making it ideal for various applications. Herein, C-doped ZnO nanoparticles were obtained by thermal decomposition and tested as photocatalysts for water depollution and electrode materials in a symmetric supercapacitor device. These nanoparticles exhibited polyhedral shapes with sizes ranging from 95 to 104 nm. The impact of the doping level on the evolution of defect centers was analyzed using EPR and PL spectroscopy. Under visible light, the photocatalytic performance was tested against two contaminants, Rhodamine B (RhB) dye and oxytetracycline (OTC) antibiotic. A 1% C doping level provided the optimal photocatalytic performance, achieving degradation rates of 80% for RhB and 50% for OTC. The photodegradation process was elucidated by examining ROS species generated, linked to the measured valence band position and defect levels. Additionally, the same sample showed superior electrochemical performance when tested in a symmetric supercapacitor device, achieving a maximum specific capacitance of 104.16 F/g at a 2 mV/s scan rate, with an energy density of 14.46 Wh/kg and excellent cyclic stability, retaining 99% capacity after 2000 cycles. These findings highlight the versatility of C-doped ZnO nanoparticles, making them effective in environmental remediation and energy storage applications.

Layers of HfO2 and (Hf,Zr)O2 crystalline nano-particles are synthesized via direct liquid injection atomic layer deposition, and a comprehensive set of structural, chemical, and electrical characterizations is employed to elucidate their phase composition and functional behavior. X-ray photoelectron spectroscopy revealed a compositional contrast between the films: (Hf,Zr)O2 layers contained up to 45% stoichiometric oxide, while pure HfO2 films are dominated by sub-oxides, especially under strongly reducing conditions, in which exclusively sub-oxide phases and p-type semiconducting behavior is revealed. Electrical measurements indicated room-temperature stabilization of polar phases and tetragonal-to-orthorhombic phase transition with a Curie temperature near 200 K. FTIR spectroscopy confirmed the presence of tetragonal and orthorhombic HfO2 phases, providing insight into minor features observed approximate to 30 degrees (2 theta) in X-ray diffraction patterns. Notably, devices incorporating an AlN interlayer demonstrated a significant enhancement in pyroelectric performance, suggesting this strategy to advance the pyroelectric performance of HfO2-based materials, supporting their development for lead-free sensor technologies.

Wide band gap semiconductors are essential for next-generation photovoltaics, especially indoor tandem applications, because they align well with both the solar spectrum and artificial light sources. Quaternary chalcogenides, such as Cu2ZnSnS4 (CZTS), offer tunable bandgaps, stability, and earth abundance. In this study, Ag-alloyed CZTS (ACZTS) thin films were synthesized via a controlled chemical solution process involving spin coating deposition process and sulfur annealing. Elemental composition and morphology analyses confirmed uniform grain distribution and precise control of the Ag/Cu ratio. Structural characterization via X-ray diffraction and Raman spectroscopy revealed a gradual transformation from the kesterite to the stannite phase as the Ag concentration increased. This transformation was accompanied by lattice expansion and a change in crystallite size. Optical measurements showed a clear widening of the bandgap from approximately 1.5 eV of pure CZTS to about 1.7 eV at high Ag levels, supporting its potential use as a top absorber in tandem solar cells. These findings demonstrate that alloying with Ag effectively tailors the properties of CZTS, making it a promising, non-toxic candidate for stable and efficient use in solar cells for indoor environments or high-efficiency tandem applications.

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.

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.

Cu2ZnSnS4 (CZTS) is an emerging material with significant potential as an absorber layer for solar cells. Precise control over the film preparation process is crucial for attaining optimal transport, electrical, and optical properties. This study investigates the effect of sulfurization duration on the properties of CZTS films deposited onto soda lime glass substrates via spray pyrolysis, followed by annealing at 550 degrees C in a sulfur-rich environment under argon flow. X-ray diffraction and Raman spectroscopy confirmed the formation of monophasic CZTS, with the highest phase purity observed for films sulfurized for 5 min. Scanning electron microscopy demonstrated notable morphological and microstructural enhancements due to the sulfurization process, while energydispersive spectroscopy confirmed near-ideal stoichiometric composition (Cu:Zn:Sn:S approximate to 2:1:1:4). Optical spectroscopy determined the band gap of the films to be between 1.40 and 1.50 eV. The electrical transport properties were investigated up to 130 degrees C, revealing p-type conductivity, with Seebeck coefficients ranging from 30 to 70 mu V K-2 and low electrical resistivity, displaying semiconductor-like behavior. The maximum power factor achieved was 0.36 mu W mK-2 at 130 degrees C for the sample sulfurized for 5 min. These findings suggest that a 5-min sulfurization time is optimal for producing single-phase CZTS films characterized by uniform morphology, accurate stoichiometric composition, and an ideal direct band gap. Given its favorable thermoelectric properties, CZTS shows significant promise as a material for thermoelectric applications, particularly in waste heat recovery systems. The results indicate that CZTS films could be further optimized for use in thermoelectric devices, and future studies could focus on enhancing their thermoelectric performance by adjusting sulfurization conditions and exploring material modifications.

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.

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.

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.

The morpho-structural and defect properties of SnO2 nanoparticles, obtained by hydrothermal synthesis at 120 degrees C, 140 degrees C and 160 degrees C, using a SnCl2 precursor, were comparatively investigated and correlated with their NO2 sensing performance for in-field conditions. The constructive contributions of the nanoparticle size, faceting and oxygen vacancy concentrations had a positive effect on the sensor performances for the two samples synthesized at lower temperatures. These samples had almost similar, smaller size and the proportion of the more active, higher-index facets over the {110} facets was significantly larger than for the sample prepared at 160 degrees C. The concentration of paramagnetic defects, associated to complexes of oxygen vacancies in the (101) planes at the SnO2 surface, increased with the synthesis temperature decrease. A sensor signal of 74 for the NO2 detection limit of 3 ppm, at the operating temperature of 100 degrees C, under dynamic air flow with in-field-like relative humidity of 50 %, was obtained for the sample grown at 120 degrees C. The sensor signal was about four times higher compared to the 140 degrees C sample with similar size and morphology and about nine times higher than in the case of the 160 degrees C sample. In addition to its high NO2 sensitivity, the 120 degrees C sample had a low sensor response for potential interfering gases as CH4 and CO2 and was relatively stable over a period of 20 months. Our results evidence the direct correlation between the sensing properties and the surface oxygen vacancy complexes and highlight the importance of an in-depth atomic-level investigation approach for the controlled synthesis of an application-oriented material.