National Institute Of Materials Physics - Romania

The manipulation of magnetic anisotropy represents the fundamental prerequisite for the application of magnetic materials. Here we present the vectorial magnetic properties of nanostructured systems and thin films of Fe and FeCo prepared on linearly trenched Mo templates with thermally controlled periodicity. The magnetic properties of the nanosystems are engineered by tuning the shape, size, thickness, and composition parameters of the thin films. Thus, we control coercivity, magnetization, orientation of the easy axis of magnetization, and the long-range magnetic order of the system in the function of the temperature. We distinguish magnetic components that emerge from the complex morpho-structural features of the undulating Fe or FeCo nanostructured films on trenched Mo templates: (i) assembly of magnetic nanowires and (ii) assembly of magnetic islands/clusters. Uniaxial anisotropy at room temperature was proven, characterized, and explained in the case of all systems. Our work contributes to the understanding of magnetic properties necessary for possible further applications of linear systems and undulated thin films.

Antiferroelectric random access memory (AFERAM) is one of the newest alternative non-volatile memory technologies to emerge in recent years. ZrO2-based antiferroelectric films are exceptionally well-suited for memory applications with very high cycling endurance (>10(10)) and low operating voltages (< 2 V). Lightly alloying ZrO2 with HfO2 is performed to assess AFERAM device performance with back-end-of-line compatible thin film Zr1-xHfxO2 (x <= 0.13) capacitors. The transition fields associated with antiferroelectric behavior are reduced with more Hf incorporation, yielding a larger magnitude switching polarization and memory window. Cycling endurance beyond 10(10) cycles is conducted on thin film capacitors where wake-up in AFERAM first leads to an increase, then a decrease in the memory window at a cumulative cycle number found to be dependent on the amount of Hf-incorporation. Hf-incorporation into ZrO2 is demonstrated to be a feasible way to improve the memory window in ZrO2-based AFERAM.

Bi-phasic calcium phosphates (BCPs) are considered prominent candidate materials for the fabrication of bone graft substitutes. Currently, supplemental cation-doping is suggested as a powerful path to boost biofunctionality, however, there is still a lack of knowledge on the structural role of such substituents in BCPs, which in turn, could influence the intensity and extent of the biological effects. In this work, pure and Mg- and Sr-doped BCP scaffolds were fabricated by robocasting from hydrothermally synthesized powders, and then preliminarily tested in vitro and thoroughly investigated physically and chemically. Collectively, the osteoblast cell culture assays indicated that all types of BCP scaffolds (pure, Sr- or Sr-Mg-doped) delivered in vitro performances similar to the biological control, with emphasis on the Sr-Mg-doped ones. An important result was that double Mg-Sr doping obtained the ceramic with the highest beta-tricalcium phosphate (beta-TCP)/hydroxyapatite mass concentration ratio of similar to 1.8. Remarkably, Mg and Sr were found to be predominantly incorporated in the beta-TCP lattice. These findings could be important for the future development of BCP-based bone graft substitutes since the higher dissolution rate of beta-TCP enables an easier release of the therapeutic ions. This may pave the road toward medical devices with more predictable in vivo performance.

An innovative research has been conducted focused on demonstrating the ability of novel dual-emissive glutathione-stabilized gold nanoclusters (GSH-AuNCs) to perform bright near-infrared (NIR)-emitting contrast agents inside tissue-mimicking agarose-phantoms via two complementary confocal fluorescence imaging techniques. First, using a new and fast microwave-assisted approach, we synthesized photostable dual-emitting GSH-AuNCs with an average size of 3.2 +/- 0.4 nm and NIR emission quantum yield of 9.9%. Steady-state fluorescence measurements coupled with fluorescence lifetime imaging microscopy (FLIM) assays performed on lyophilized GSH-AuNCs revealed that the obtained GSH-AuNCs exhibit PL emissions at 610 nm (red PL) and, respectively, 800 nm (NIR PL) in both solution and powder solid-state. Time-resolved fluorescence measurements showed that the two PL components are characterized by average lifetimes of 407 ns (red PL) and 1821 ns (NIR PL), respectively. Additionally, due to a partial overlap between the red PL and the absorption of the NIR PL, an energy transfer between the two coexisting emissive centers was discovered and confirmed via steady-state and time-resolved fluorescence measurements. Furthermore, the FLIM analysis performed on powder GSH-AuNCs under 640 nm, an excitation more suitable for bioimaging applications, revealed a homogeneous and photostable NIR PL signal from GSH-AuNCs. Finally, the ability of GSH-AuNCs to operate as reliable NIR-emitting contrast agents inside tissue-mimicking agarose-phantoms was demonstrated here for the first time via complementary FLIM and re-scan confocal fluorescence imaging techniques. In consequence, GSH-AuNCs show great promise for future in vivo imaging applications via confocal fluorescence microscopy.

Magnetic nanoparticles (MNPs) have evolved tremendously during recent years, in part due to the rapid expansion of nanotechnology and to their active magnetic core with a high surface-to-volume ratio, while their surface functionalization opened the door to a plethora of drug, gene and bioactive molecule immobilization. Taming the high reactivity of the magnetic core was achieved by various functionalization techniques, producing MNPs tailored for the diagnosis and treatment of cardiovascular or neurological disease, tumors and cancer. Superparamagnetic iron oxide nanoparticles (SPIONs) are established at the core of drug-delivery systems and could act as efficient agents for MFH (magnetic fluid hyperthermia). Depending on the functionalization molecule and intrinsic morphological features, MNPs now cover a broad scope which the current review aims to overview. Considering the exponential expansion of the field, the current review will be limited to roughly the past three years.

Monocrystalline (100) semi-insulating gallium arsenide (GaAs) samples were irradiated with a 250-keV Xe+ ion beam at room temperature. To investigate the effect of ion irradiation on GaAs samples, the ion fluence was varied from 1 x 1012 to 3 x 1016 ions/cm2. The spectroscopic ellipsometry (SE) method and the Rutherford backscattering spectrometry (RBS) with nuclear reaction (NR) method were used to determine the properties of the virgin and implanted samples. The SE approach reveals that the pseudo-dielectric function of implanted GaAs samples varies with ion fluence. The RBS approach, which exposes the depth-profile of As, Ga, and Xe in the implanted samples, allows us to correlate changes in the pseudo-dielectric function with structural modifications caused by the ion irradiation. The NR approach points out the existence of an oxygen-enriched layer on the surface of implanted GaAs samples. Lastly, the optical study involving the Cauchy and Urbach dispersion model measures the refractive index n and extinction coefficient k of the native oxide layer on the surface of GaAs samples.

A deeper understanding of the coupling at the interface of multiferroics heterostructures is being achieved by the use of synchrotron radiation techniques. Here, the authors use k-resolved soft X-ray photoemission spectroscopy and first principles calculations to investigate the band structure of several multiferroic heterostructures, isolating the distinct signature of the interface. Physics of the multiferroic interfaces is currently understood mostly within a phenomenological framework based on screening of the polarization field and depolarizing charges. Additional effects still unexplored are the band dependence of the interfacial charge modulation and the associated changes of the electron-phonon interaction, coupling the charge and lattice degrees of freedom. Here, multiferroic heterostructures of the colossal-magnetoresistance manganite La1-xSrxMnO3 buried under ferroelectric BaTiO3 and PbZrxTi1-xO3 are investigated using soft-X-ray angle-resolved photoemission. The experimental band dispersions from the buried La1-xSrxMnO3 identify coexisting two-dimensional hole and three-dimensional electron charge carriers. The ferroelectric polarization modulates their charge density, affecting the coupling of the 2D holes and 3D electrons with the lattice which forms large Frohlich polarons inherently reducing mobility of the charge carriers. Our k-resolved results on the orbital occupancy, band filling and electron-lattice interaction in multiferroic oxide heterostructures modulated by the ferroelectric polarization disclose most fundamental physics of these systems needed for further progress of beyond-CMOS ferro-functional electronics.

Carbon quantum dots (CQDs) derived from biomass, a suggested green approach for nanomaterial synthesis, often possess poor optical properties and have low photoluminescence quantum yield (PLQY). This study employed an environmentally friendly, cost-effective, continuous hydrothermal flow synthesis (CHFS) process to synthesise efficient nitrogen-doped carbon quantum dots (N-CQDs) from biomass precursors (glucose in the presence of ammonia). The concentrations of ammonia, as nitrogen dopant precursor, were varied to optimise the optical properties of CQDs. Optimised N-CQDs showed significant enhancement in fluorescence emission properties with a PLQY of 9.6% compared to pure glucose derived-CQDs (g-CQDs) without nitrogen doping which have PLQY of less than 1%. With stability over a pH range of pH 2 to pH 11, the N-CQDs showed excellent sensitivity as a nano-sensor for the highly toxic highly-pollutant chromium (VI), where efficient photoluminescence (PL) quenching was observed. The optimised nitrogen-doping process demonstrated effective and efficient tuning of the overall electronic structure of the N-CQDs resulting in enhanced optical properties and performance as a nano-sensor.

In the present work, new luminescent lanthanide complexes with extended mesomorphic range were prepared by coordination to lanthanide ions (Eu3+, Sm3+ and Tb3+) of the new 4-pyridone based organic ligands (L) with 3,4-(7-n) and 3,5-di(alkyloxy)benzyl (8-n) mesogenic groups and variable length (n = 12 or 14 carbon atoms) onto the benzyl unit. The cumulative results of the elemental analyses as well as the H-1, C-13 NMR and IR spectroscopies support the structure of the organic derivatives and their lanthanide complexes [LnL(3)(NO3)(3)] (9-n/Ln and 10-n/Ln) described in this work. These complexes show characteristic lanthanide solid state emission, both at room and elevated temperatures corresponding to crystalline, glassy or liquid crystal states. The long range SmA phases displayed by all complexes were supported by a combination of characterization methods, including: differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and variable-temperature powder X-ray diffraction (XRD). This work shows that the number, substitution pattern and length of flexible alkoxy chains are important parameters to control the phase-transition characteristics of the lanthanide complexes. Complexes with 3,4-disubstituted pattern (9-n/Ln) show higher clearing temperatures (nearly 60 degrees C for Eu3+, 70 degrees C for Sm3+ and 85 degrees C for Tb3+ complexes) compared to their counterparts with 3,5-disubstituted pattern (10-n/Ln). Moreover, complexes 9-n/Ln crystallize when cooling their LC phase while complexes 10-n/Ln are stable in glassy state at room temperature as a consequence of the different close interdigitated molecular packing evidenced by XRD measurements. Dielectric spectroscopy was employed to detect the changes of order degree specific to each phases (crystalline, LC or isotropic). The variation of dielectric constant and the electrical conductivity versus temperature shows three transition intervals for selected complexes 10-14/Sm and 10-14/Tb, which delimit the main intervals: 45-60 degrees C, 90-110 degrees C; 140-160 degrees C corresponding to the Cr-1-Cr-2, Cr-2 - SmA and SmA-Iso transitions, and agree very well with the DSC results. The change of characteristic time, obtained by Havriliak-Negami fit function, with temperature also provides a very good correlation with the DSC and POM results. (C) 2022 Elsevier B.V. All rights reserved.

This paper presents the results concerning monotropic nematic liquid crystals 4-pentylphenyl 40-alkyl benzoate (5PnB) (n = 3 or 5 carbon atoms in the alkyl chain). Their mesophase properties were supported by images of the polarized optical microscopy. Molecular dynamics in the bulk samples or in the composites prepared with aerosil A380 was investigated by broadband dielectric spectroscopy in a large temperature range, appropriately chosen. Thermo gravimetric and infrared investigations were additionally performed. The data were compared with those of structurally related nematics like cyanophenyl pentyl benzoates, which have a cyan group instead of the pentyl chain. The dielectric spectra of the bulk 3P5B and 5P5B demonstrate a dielectric behavior with several relaxation processes as expected for nematic liquid crystals. The temperature dependence of the relaxation rates (and of the dielectric strength) seems to have two distinguished regimes. Thus, in the isotropic state, at higher temperatures the data obey the Vogel-Fulcher-Tammann law, whereas an Arrhenius law is fitted at lower temperature, in a close similarity to the behavior of a constrained dynamic glass transition. Samples with a high density of silica (larger than 7 g aerosil/1 g of 5PnB) were prepared to observe a thin layer adsorbed on the particle surface; it was estimated that almost each guest 5PnB molecule interacts with the aerosil surface. For the composites only one main relaxation process is observed at frequencies much lower than those for the corresponding bulk, which was assigned to the dynamics of the molecules in the surface layer. Infrared spectroscopy shows that these molecules interact with the surface by the ester carbonyl group leading to the monolayer self-assembly at liquid-solid interface. We note once more the importance of the functional unit(s) for the interaction with the hydroxyl groups on the aerosil surface. (c) 2022 Elsevier B.V. All rights reserved.