Raluca Savu

Self-Assembled and One-Step Synthesis of Interconnected 3D Network of Fe3O4/Reduced Graphene Oxide Nanosheets Hybrid for High-Performance Supercapacitor Electrode

In the present work, we have synthesized three-dimensional (3D) reduced graphene oxide nanosheets (rGO NSs) containing iron oxide nanoparticles (Fe3O4 NPs) hybrids (3D Fe3O4/rGO) by one-pot microwave approach. Structural and morphological studies reveal that the as-synthesized Fe3O4/rGO hybrids were composed of faceted Fe3O4 NPs induced into the interconnected network of rGO NSs. The morphologies and structures of the 3D hybrids have been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectrometer (XPS). The electrochemical studies were analyzed by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy, which demonstrate superior electrochemical performance as supercapacitors electrode application. The specific capacitances of 3D hybrid materials was 455 F g-1 at the scan rate of 8 mV s-1, which is superior to that of bare Fe3O4 NPs. Additionally, the 3D hybrid shows good cycling stability with a retention ratio of 91.4 after starting from ∼190 cycles up to 9600 cycles. These attractive results suggest that this 3D Fe3O4/rGO hybrid shows better performance as an electrode material for high-performance supercapacitors.

P-type semiconducting gas sensing behavior of nanoporous rf sputtered CaCu3Ti4O12 thin films

The fabrication of nanoporous sputtered CaCu3Ti4O12 thin films with high gas sensitivity is reported in this work. The porous microstructure and the nanocrystalline nature of the material promoted the diffusion of the atmosphere into the film, shortening the response time of the samples. Behaving as p-type semiconductor, the material presents enhanced sensitivity even at low working temperatures. Impedance spectroscopy measurements were performed in order to investigate the mechanisms responsible for the performance of the devices.

Fabrication of interdigitated micro-supercapacitor devices by direct laser writing onto ultra-thin, flexible and free-standing graphite oxide films

In this work we present graphene-based in-plane flexible interdigitated micro-supercapacitor devices fabricated through direct laser writing onto ultra-thin graphite oxide (GO) films. The fabrication route is simple, fast, additive-free, mask-free and cost effective. This involves direct micro-writing of reduced graphene oxide (rGO) by a pulsed UV laser on a very small area (1.14 cm2). The fabricated micro-supercapacitor contains nineteen pairs of rGO electrodes separated by the unreduced portion of the GO film. The single laser patterned rGO electrode presents low resistivity, while the unpatterned portion is non-conducting. Under the optimized laser parameters the 2.2 μm ultra-thin GO films were completely and uniformly reduced. The electrochemical measurements showed that the micro-supercapacitor, packed in a glass cavity, and in the presence of a liquid electrolyte have a capacitance nearly 288% higher (288.7 mF cm−3) compared to the as-fabricated device (0.36 mF cm−3). The as-fabricated micro-supercapacitor without electrolyte also shows some capacitance due to the presence of free ions in the unreduced portion of GO which plays a crucial role. Furthermore, the cycling stability of the as-fabricated micro-supercapacitor is robust, with not much performance degradation for more that 5000 cycles.

Resistive-Switching Behavior in Polycrystalline CaCu3Ti4O12 Nanorods

Highly aligned CaCu(3)Ti(4)O(12) nanorod arrays were grown on Si/SiO(2)/Ti/Pt substrates by radio-frequency sputtering at a low deposition temperature of 300 °C and room temperature. Structural and morphological studies have shown that the nanostructures have a polycrystalline nature and are oriented perpendicular to the substrate. The high density of grain boundaries in the nanorods is responsible for the nonlinear current behavior observed in these arrays. The current-voltage (I-V) characteristics observed in nanorods were attributed to the resistive memory phenomenon. The electrical resistance of microcapacitors composed of CaCu(3)Ti(4)O(12) nanorods could be reversibly switched between two stable resistance states by varying the applied electric field. In order to explain this switching mechanism, a model based on the increase/decrease of electrical conduction controlled by grain boundary polarization has been proposed.

Direct laser writing of micro-supercapacitors on thick graphite oxide films and their electrochemical properties in different liquid inorganic electrolytes

In this article we demonstrate a simple approach to fabricate interdigitated in-plane electrodes for flexible micro-supercapacitors (MSCs). A nanosecond ultraviolet laser treatment is used to reduce and pattern the electrodes on thick graphite oxide (GO) freestanding films. These laser-treated regions obtained by direct writing provide the conducting channels for electrons in the capacitors. The electrochemical performance of the MSCs was evaluated in the presence of two different electrolytes and they exhibit characteristics of nearly electrical double layer capacitors. The MSCs have areal capacitances as 2.40, 2.23 and 1.62μF/cm2 for NaOH, Na2SO4 and KCl electrolytes respectively, for measurements performed at the scan rate of 50mV/s. They retain ∼93.1% of their initial capacitances after 3500 cycles (scan rate=80mV/s) in NaOH electrolyte. The proposed laser treatment approach enables facile and fast fabrication of flexible MSCs without the need for tedious processing methods such as photolithographic micro-patterning and deposition of porous carbon or metallic current collectors.

Grain size effect on the electrical response of SnO2 thin and thick film gas sensors

Porous nano and micro crystalline tin oxide films were deposited by RF Magnetron Sputtering and doctor blade techniques, respectively. Electrical resistance and impedance spectroscopy measurements, as a function of temperature and atmosphere, were performed in order to determine the influence of the microstructure and working conditions over the electrical response of the sensors. The conductivity of all samples increases with the temperature and decreases in oxygen, as expected for an n-type semiconducting material. The impedance plots indicated the existence of two time constants related to the grains and the grain boundaries. The Nyquist diagrams at low frequencies revealed the changes that took place in the grain boundary region, with the contribution of the grains being indicated by the formation of a second semicircle at high frequencies. The better sensing performance of the doctor bladed samples can be explained by their lower initial resistance values, bigger grain sizes and higher porosity.

A wearable, highly stable, strain and bending sensor based on high aspect ratio graphite nanobelts.

A simple and scalable method was developed for the fabrication of wearable strain and bending sensors, based on high aspect ratio (length/thickness ∼10(3)) graphite nanobelt thin films deposited by a modified Langmuir-Blodgett technique onto flexible polymer substrates. The sensing mechanism is based on the changes in contact resistance between individual nanobelts upon substrate deformation. Very high sensor response stability for more than 5000 strain-release cycles and a device power consumption as low as 1 nW were achieved. The device maximum stretchability is limited by the metal electrodes and the polymer substrate; the maximum strain that could be applied to the polymer used in this work was 40%. Bending tests carried out for various radii of curvature demonstrated distinct sensor responses for positive and negative curvatures. The graphite nanobelt thin flexible films were successfully tested for acoustic vibration and heartbeat sensing.

Interference of apoptosis in the pathophysiology of subarachnoid hemorrhage

Programmed cell death is crucial for the correct development of the organism and the clearance of harmful cells like tumor cells or autoreactive immune cells. Apoptosis is initiated by the activation of cell death receptors and in most cases it is associated with the activation of the cysteine proteases, which lead to apoptotic cell death. Cells shrink, chromatin clumps and forms a large, sharply demarcated, crescent-shaped or round mass; the nucleus condenses, apoptotic bodies are formed and eventually dead cells are engulfed by a neighboring cell or cleared by phagocytosis. The authors have summarized the most important data concerning apoptosis in subarachnoid hemorrhage that have been issued in the medical literature in the last 20 years.

Correlation between mechanical and surface properties of SLS parts

Purpose – The purpose of this paper is to compare the results from mechanical testing with measurements of surface-dependent properties performed on polyamide parts made by selective laser sintering (SLS) to assess a possible correlation between them. Design/methodology/approach – Fabrication of Nylon 12 (Duraform PA®) samples using different laser power levels and their characterization by tensile testing, roughness and Raman scattering measurements. Findings – Among the surface methods investigated, the results from Raman spectroscopy are the best ones, but methods dependent on surface analysis are not really suitable as indicators of the mechanical properties. The correlation coefficients for linear fitting obtained when the normalized results of mechanical properties are plotted against the surface properties are too low. Furthermore, the ambiguity between surface and mechanical data makes it impossible to use these surface properties for prediction purposes in the industrial environment. Originality/...

Study of the thermal contact between carbon nanotubes and a metal electrode

The thermal resistance of the contact between a metal and carbon nanotubes (CNTs) and between CNT layers is estimated by the local heating of CNT arrays (films) of different thicknesses using a focused laser beam and measuring their local temperature via Raman spectroscopy. It is demonstrated that thermal contacts between nanotubes, and also between CNTs and an electrode, can be formed by means of laser annealing.