National Institute Of Materials Physics - Romania

As a new class of sustainable carbon material, "carbon dots" is an umbrella term covering many types of materials. Herein, a broad range of techniques was used to develop the understanding of hydrothermally synthesized carbon dots, and it is shown how fine-tuning the structural features by simple reduction/oxidation reactions can drastically affect their excited-state properties. Structural and spectroscopic studies found that photoluminescence originates from direct excitation of localized fluorophores involving oxygen functional groups, whereas excitation at graphene-like features leads to ultrafast phonon-assisted relaxation and largely quenches the fluorescent quantum yields. This is arguably the first study to identify the dynamics of photoluminescence including Stokes shift and allow the relaxation pathways in these carbon dots to be fully resolved. This comprehensive investigation sheds light on how understanding the excited-state relaxation processes in different carbon structures is crucial for tuning the optical properties for any potential commercial applications.

Electronic phase separation is crucial for the fascinating macroscopic properties of the LaAlO3/SrTiO3 (LAO/STO) paradigm oxide interface, including the coexistence of superconductivity and ferromagnetism. We investigate this phenomenon using angle-resolved photoelectron spectroscopy (ARPES) in the soft-x-ray energy range, where the enhanced probing depth combined with resonant photoexcitation allow us access to fundamental electronic structure characteristics - momentum-resolved spectral function, dispersions and ordering of energy bands, Fermi surface - of buried interfaces. Our experiment uses x-ray irradiation of the LAO/STO interface to tune its oxygen deficiency, building up a dichotomic system where mobile weakly correlated Ti t(2g) electrons coexist with localized strongly correlated Ti e(g) ones. The ARPES spectra dynamics under x-ray irradiation shows a gradual intensity increase under constant Luttinger count of the Fermi surface. This fact identifies electronic phase separation (EPS) where the mobile electrons accumulate in conducting puddles with fixed electronic structure embedded in an insulating host phase, and allows us to estimate the lateral fraction of these puddles. We discuss the physics of EPS invoking a theoretical picture of oxygen-vacancy clustering, promoted by the magnetism of the localized Ti e(g) electrons, and repelling of the mobile t(2g) electrons from these clusters. Our results on the irradiation-tuned EPS elucidate the intrinsic one taking place at the stoichiometric LAO/STO interfaces.

Angle-resolved photoelectron spectroscopy (ARPES) is the main experimental tool to explore electronic structure of solids resolved in the electron momentum k. Soft-X-ray ARPES (SX-ARPES), operating in a photon energy range around 1 keV, benefits from enhanced probing depth compared to the conventional VUV-range ARPES, and elemental/chemical state specificity achieved with resonant photoemission. These advantages make SX-ARPES ideally suited for buried heterostructure and impurity systems, which are at the heart of current and future electronics. These applications are illustrated here with a few pioneering results, including buried quantum-well states in semiconductor and oxide heterostructures, their bosonic coupling critically affecting electron transport, magnetic impurities in diluted magnetic semiconductors and topological materials, etc. High photon flux and detection efficiency are crucial for pushing the SX-ARPES experiment to these most photon-hungry cases.

Eu3+,Tb3+-doped Sr3Al2O6 powder phosphor was synthesized via a precursor route and subjected to a subsequent thermal treatment in reducing atmosphere. Photoluminescence and thermoluminescence properties of Sr3Al2O6:Eu3+/Eu2+,Tb3+ were investigated. The structure and morphology of oxides were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) was used for the nanocrystals surface composition analysis. X-ray diffraction patterns confirmed the formation of the cubic structure specific to Sr3Al2O6 with space group Pa3 and lattice parameter a = 15.8322 angstrom, while SEM investigations revealed equiaxial, polycrystalline particles, with sizes in the submicronic range, for both Sr3Al2O6:Eu3+,Tb3+ and Sr3Al2O6:Eu3+/Eu2+,Tb3+ samples. The photoluminescence spectra showed the typical f-f luminescence lines of the Tb(3+ )and Eu3+ - ions, accompanied by a broad Eu2+ luminescence band at 510 nm (after calcination in reducing atmosphere). The "after-glow" luminescence signal and the thermoluminescence were assigned to the recombination of close neighbor partners (electron and Eu2+ - hole centers) within the same complex of defects.

A series of five Cu(x)CeMgAlO mixed oxides with different copper contents (x) ranging from 6 to 18 at. % with respect to cations, but with fixed 10 at. % Ce and Mg/Al atomic ratio of 3, were prepared by thermal decomposition of layered double hydroxide (LDH) precursors at 750 degrees C. The solid containing 15 at. % Cu, i.e. Cu (15)CeMgAlO, was also calcined at 550 and 650 degrees C. Powder XRD was used to characterize the crystalline structure and SEM-EDX was used to monitor the morphology and chemical composition of both as prepared and calcined materials. Additionally, the textural properties and the reducibility of the mixed oxide catalysts were studied by nitrogen adsorption/desorption and temperature programmed reduction with hydrogen (H-2-TPR) techniques, respectively. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical state of the elements on the catalyst surface and the diffuse reflectance UV-vis spectroscopy, to obtain information about the stereochemistry and aggregation of copper in the Cu-containing mixed oxides. Their catalytic properties in the total oxidation of methane, used as a volatile organic compound (VOC) model molecule, were evaluated and compared with those of an industrial Pd/Al2O3 catalyst. Their catalytic behavior was explained in correlation with their physicochemical properties. Cu(15)CeMgAlO mixed oxide was shown to be the most active catalyst in this series, with a T-50 (temperature corresponding to 50% methane conversion) value of only ca. 45 degrees C higher than that of a commercial Pd/Al2O3 catalyst. This difference becomes as low as ca. 25 degrees C for the Cu(15)CeMgAlO system calcined at 550 degrees C. The influences of the contact time and of the methane concentration in the feed gas on the catalytic performances of the Cu(15)CeMgAlO catalyst have been investigated and its good stability on stream was evidenced.

Mixtures of nitrogen-doped titanium dioxide (TiO2:N) with different concentrations of Ag and/or SiO2 particles (0.5, 1 and 2 wt.%) were prepared in solid state by mechanico-chemical interactions. Using UV-VIS spectroscopy, Raman scattering, photoluminescence (PL) and photoluminescence excitation (PLE), the influence of the particles on the host material is evaluated. UV-VIS spectroscopy studies indicate a TiO2:N band gap shift to the UV range with increasing concentrations of SiO2 and Ag particles. PL intensities decrease with increasing concentrations of Ag and/or SiO2 particles in the TiO2:N host matrix, which in turn could effectively restrict the electron and hole recombination. To explain these processes, the different de-excitation ways will be advanced, taking into account the energy levels diagram of TiO2:N/Ag, TiO2:N/SiO2 and TiO2:N/Ag/SiO2 systems. PLE spectra show a gradual decrease in their relative intensities after 165 min of continuous irradiation due to photosensitivity of TiO2:N. The plasmonic effect of Ag particles in the TiO2:N/Ag system is highlighted for the first time by PLE studies.

Control of structural coupling at complex-oxide interfaces is a powerful platform for creating ultrathin layers with electronic and magnetic properties unattainable in the bulk. However, with the capability to design and control the electronic structure of such buried layers and interfaces at a unit-cell level, a new challenge emerges to be able to probe these engineered emergent phenomena with depth-dependent atomic resolution as well as element- and orbital selectivity. Here, we utilize a combination of core-level and valence-band soft x-ray standing-wave photoemission spectroscopy, in conjunction with scanning transmission electron microscopy, to probe the depth-dependent and single-unit-cell resolved electronic structure of an isovalent manganite superlattice [Eu0.7Sr0.3MnO3/La0.7Sr0.3MnO3] x 15 wherein the electronic-structural properties are intentionally modulated with depth via engineered oxygen octahedra rotations/tilts and A-site displacements. Our unit-cell resolved measurements reveal significant transformations in the local chemical and electronic valence-band states, which are consistent with the layer-resolved first-principles theoretical calculations, thus opening the door for future depth-resolved studies of a wide variety of heteroengineered material systems.

The present investigation deals with the development, characterization and application of nano-structured Pd doped Ni electrodes (Pd@Ni), which uses the electrochemical properties of Pd in synergy with the magnetic properties of Ni for biosensors development. The Pd@Ni electrodes have been characterized by X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. It has been shown that palladium presented spherical assemblies ranging 150-200 nm medium diameter size that covers large areas of the electrode surface while metallic nickel, which confers magnetic properties, showed a uniform granular structure with sizes between 20 and 50 nm. Cyclic voltammetry and electrochemical impedance spectroscopy were performed to understand the electrochemical process at the Pd@Ni electrodes in neutral media. The Pd@Ni electrodes were applied for the electrochemical detection of H2O2. Finally, Ni nanoparticles (NiNP) functionalized with the model enzyme glucose oxidase (GOx-NiNP) have been attached to the Pd@Ni electrode solely through magnetic interactions, and the obtained GOx-NiNP/Pd@Ni biosensor applied for glucose determination in aqueous solutions by fixed potential amperometry at -0.05 V (vs Ag/AgCl) with reduced interferences. (C) 2019 Elsevier Ltd. All rights reserved.

N-doped defective graphene [(N)G] obtained by pyrolysis at 900 degrees C of chitosan contains about 3.7% of residual N atoms, distributed as pyridinic, pyrrolic and graphitic N atoms. It has been found that (N)G acts as basic catalyst promoting two classical C-C bond forming nucleophilic additions in organic chemistry, such as the Michael and the Henry additions. Computational calculations at DFT level of models corresponding to the various N atoms leads to the conclusion that N atoms are more stable at the periphery of the graphene sheets and that H adsorption on these sites is a suitable descriptor to correlate with the catalytic activity of the various sites. According to these calculations the most active sites are pyridinic N atoms at zig-zag edges of the sheets. In addition, N as dopant changes the reactivity of the neigh. bour C atoms. Water was found a suitable solvent to achieve high conversions in both reactions. In this solvent the initial distribution of N atoms is affected due to the easy protonation of the N-py to N-pyH sites. As an effect, C edge sites adjacent at N-PyH with an appropriate reactivity towards the alpha-C-H bond breaking are formed. The present results show the general activity of N-doped graphene as base catalysts and illustrate the potential of carbocatalysis to promote reactions of general interest in organic synthesis. (C) 2019 Elsevier Inc. All rights reserved.

Ferroelectric capacitors based on aluminium (Al) doped hafnium oxide (HfO2) thin films grown on silicon substrates were fabricated by Atomic Layer Deposition (ALD), taking into account two methods. The first one involved the growth of a binary oxide, in a laminar way, by alternating the ALD cycles of HfO2 and Al2O3, and the second, the two precursors were sequentially mixed on the surface. The composition and structure of deposited aluminium doped hafnium oxide (Al: HfO2) thin films have been studied using X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray diffraction (GIXRD). XPS measurements show the formation of opposite ferroelectric polarization areas. Via GIXRD, it was found that the Al: HfO2 films deposited on Si have a structure with polycrystalline domains. Recording and investigation of ferroelectric domains were performed by Piezoresponse Force Microscopy (PFM), while the electrical performances of obtained devices were analysed by capacitance-voltage (C-V) and current-voltage (I-V) characteristics. The PFM measurements show there is a mechanical non-zero response even outside the written area and for an appropriate value of the electrical stress the difference in phase between successive areas is saturated to a value close to 180 degrees. The atomic force microscopy (AFM) analyses indicate a very low value of roughness average, for all grown thin films, similar to 0.2 nm, for a thickness of similar to 7 nm. From C-V characteristics, the memory window was extracted and the calculated values were found to be 0.8 V for the device obtained by the first ALD method, and 0.44 V for the second one, respectively. Moreover, in the case of the device based on the ferroelectric layer grown by the second ALD method, the memory window extends over a much wider applied voltage domain, in the range (+/- 4 V; +/- 8 V), at a signal of 100 kHz.