National Institute Of Materials Physics - Romania

We identify new lasing regimes of high-beta nanolasers with extended gain media. There, coherent emission can occur after the typical threshold non-linearity in the I-O-curve due to increased spontaneous emission. As an example for this device-regime, we present room temperature lasing in III-nitride nanobeam cavities.

The effect of room temperature hydrogen plasma treatment on the photoconductive properties of the SiO2 matrix containing SiGe nanoparticles is investigated. A considerable increase in photocurrent intensity is observed after plasma treatment. The increase is partly attributed to neutralization of dangling bonds around the nanoparticles and partly to passivation of non-radiative centers and defects in the matrix and at the nanoparticles-matrix interfaces.

Ferromagnetic shape memory alloys (FSMA) have attained strong interest over the last years and Fe-Pd alloys seems to be more suited for engineering and medical applications, due to their improved ductility and biocompatibility compared to the well-known Ni2MnGa alloy. The shape memory effect in disordered Fe-Pd (30 at.% Pd) is associated with FCC-FCT thermoelastic martensitic transformation. The melt-spinning technique enables to get ribbons with the FCC meta-stable structure preventing the precipitation of undesirable BCT irreversible phase and subsequent proper thermal treatments could improve the characteristic parameters of the martensitic transformation. The present work reports the effect of the rapid solidification (via melt-spinning technique) and the different thermal treatments on the microstructure, martensitic transformation and magnetic properties of the Fe69.4Pd30.6 ribbons. The samples were investigated by calorimetry, X-ray diffractometry, scanning electron microscopy and magnetometry. Two different structures induced by the distinct thermal treatments and responsible for the characteristic behavior of the martensitic transformation, were noticed and discussed in details. The high temperature treatment for short time stabilized the FCC phase and slightly decrease the martensitic transformation temperature, while the annealing at low temperature for longer time promotes the reduction of the amount of transforming FCC phase by its partial decomposition in the stable phases, causing the fall of the heat of transformation.

Trilayer MOS capacitors gate HfO2 / floating gate of Ge nanocrystals in HfO2 / tunnel HfO2 / Si substrate were prepared in the aim to be used for the detection of ionizing radiation. Magnetron sputtering and rapid thermal annealing were used for their fabrication. Capacitance-voltage measurements showed that Ge nanocrystals are the most important charge storage centres in our structure. The possibility to use these trilayer MOS capacitors as dosimeters was investigated, and the sensitivity to alpha particle irradiation was extracted.

We report unusual effect of channeled magnetic flux motion in YBa2Cu3O7-delta/PrBa2Cu3O7-delta superlattices grown by pulsed laser deposition. Magneto-optical imaging reveals that flux moves along a set of parallel and perpendicular lines, while optical microscopy does not show any features on the surface that may be linked to this effect. In contrast, scanning electron microscopy registers sub-micron fractures in the superlattices corresponding to these flux lines, but the magnetic flux channels are much wider than the width of these fractures. To further clarify the origin of flux channels, electrical transport measurements on the superlattices have been performed. Their current-voltage characteristics reveal the presence of distinctive branches related to the flux motion along the selective channels, following which magnetic flux crosses sample in a shortest and least resistive way. The application of very large current overheated the superlattice along these channels, evaporating superconducting material and exposing, wider than in the superconductor, fractures in the substrate. It is concluded that motion of flux in the channels is controlled not only by the presence of nano-fractures in YBa2Cu3O7-delta/PrBa2Cu3O7_delta, but also by stress developed in the superconducting material due to the fracturing of the substrate.

We have shown in previous investigations that the low temperature collective magnetism observed in mesoporous cubic ZnS:Mn nanocrystalline powders prepared by colloidal synthesis, with nominal doping concentrations above 0.2 at.%, is due to the formation of Mn2+ clusters with distributed antifer romagnetic coupling localized in an amorphous phase found between the cubic ZnS:Mn nanocrystals. Here we investigate the composition, origin and thermal annealing behavior of this amorphous phase in such a mesoporous ZnS:Mn sample doped with 5 at.% Mn nominal concentration. Correlated analytical transmission electron microscopy, multifrequency electron paramagnetic resonance and Fourier transform infrared spectroscopy data show that the amorphous nanomaterial consists of unreacted precursor hydrated zinc and manganese acetates trapped inside the pores and on the surface of the cubic ZnS nanocrystals. The decomposition of the acetates under isochronal annealing up to 270 degrees C, where the mesoporous structure is still preserved, lead to changes in the nature and strength of the magnetic inter actions between the aggregated Mn2+ ions. These results strongly suggest the possibility to modulate the magnetic properties of such transition metal ions doped II-VI mesoporous structures by varying the synthesis conditions and/or by post-synthesis thermochemical treatments. (C) 2017 Elsevier B.V. All rights reserved.

Here we report a ferroelectric capacitor structure obtained by alternating ferroelectric and insulator thin-film layers which allows an increase of up to 2(n) polarization states, with n the number of ferroelectric layers. Four and up to eight distinct, stable and independently addressed polarization states are experimentally demonstrated in this work. The experimental findings are supported by a theoretical model based on the Landau-Ginzburg-Devonshire theory. The key parameter is the change in the strain conditions of ferroelectric layers induced by the insulating separator. Notably, the 2(n) increase in the storage capacity can be achieved without major changes in the present technology used for FeRAM devices. The test structures demonstrate very good memory characteristics such as retention and fatigue, opening the way towards the design of high density ferroelectric memories.

Upon reduction of the film thickness we observe a metal-insulator transition in epitaxially stabilized, spin-orbit-coupled SrIrO3 ultrathin films. By comparison of the experimental electronic dispersions with density functional theory at various levels of complexity we identify the leading microscopic mechanisms, i.e., a dimensionality-induced readjustment of octahedral rotations, magnetism, and electronic correlations. The astonishing resemblance of the band structure in the two-dimensional limit to that of bulk Sr2IrO4 opens new avenues to unconventional superconductivity by "clean" electron doping through electric field gating.

The present study shows the results obtained in anodic electrodeposition of hematite precursor films in agarose gel template and photoelectrochemical characterization of the prepared hematite films. The peak current in cyclic voltammetry increases after each scan of the potential between -0.1 and 1.3 V for FTO electrode covered with a film of agarose gel in 0.1 M iron sulfate solution. The hematite samples with different thicknesses were prepared using template electrodeposition and annealing in air. The obtained nanostructured films were characterized by field emission scanning electron microscopy, X-ray diffraction, UV-vis spectroscopy, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The results show that a synergistic improvement of photocurrent density (1.14 mAcm(-2) at 1.23 V vs RHE) is induced by Sn-doping and slight surface modification of hematite with thin and discontinuous NiO film. The results indicate that thicker hematite films and hematite films strongly modified with NiO nanostructures show a reduced photoactivity. (C) 2017 Elsevier Ltd. All rights reserved.

Un-doped and doped strontium aluminates (Sr3Al2O6, Sr3Al2O6:Tb3+, Sr3Al2O6:Tb3+,Eu3+ and Sr3Al2O6:Tb3+,Eu3+/Eu2+) were synthesized through a soft chemical method - the tartarate precursor route. The tartarate precursors were characterized by infrared spectroscopy (IR) and thermal analysis. The strontium aluminates were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared spectroscopy (IR). The XRD patterns confirmed the formation of single-phase cubic structure Sr3Al2O6 with average crystallite size of 18 nm. SEM micrographs showed porous polycrystalline powder microstructures and EDX analysis confirmed the incorporation of the dopant in the Sr3Al2O6 powders. The photoluminescence spectrum recorded on Sr3Al2O6: Tb3+,Eu3+ (obtained from tartarate precursor calcined in air) showed the typical f-f luminescences of the RE-ions. Subsequent calcination of this sample in reducing atmosphere is accompanied by new photoluminescence and thermoluminescence features associated to the Eu2+ ions.