National Institute Of Materials Physics - Romania

In this paper we report a set of experiments at the wafer level regarding field-effect transistors with a graphene monolayer channel transferred on the ferroelectric HfO2/Ge-HfO2/HfO(2)three-layer structure. This kind of transistor has a switching ratio of 10(3)between on and off states due to the bandgap in graphene induced by the ferroelectric structure. Both top and back gates effectively control the carriers' charge flow in graphene. The transistor acts as a three-terminal memristor, termed a memtransistor, with applications in neuromorphic computation.

Biopolymers provide versatile platforms for designing naturally-derived wound care dressings through eco-friendly pathways. Eggshell membrane (ESM), a widely available, biocompatible biopolymer based structure features a unique 3D porous interwoven fibrous protein network. The ESM was functionalized with inorganic compounds (Ag, ZnO, CuO used either separately or combined) using a straightforward deposition technique namely radio frequency magnetron sputtering. The functionalized ESMs were characterized from morphological, structural, compositional, surface chemistry, optical, cytotoxicity and antibacterial point of view. It was emphasized that functionalization with a combination of metal oxides and exposure to visible light results in a highly efficient antibacterial activity against Escherichia coli when compared to the activity of individual metal oxide components. It is assumed that this is possible due to the fact that an axial p-n junction is created by joining the two metal oxides. This structure separates into components the charge carrier pairs promoted by visible light irradiation that further can influence the generation of reactive oxygen species which ultimately are responsible for the bactericide effect. This study proves that, by employing inexpensive and environmentally friendly materials (ESM and metal oxides) and fabrication techniques (radio frequency magnetron sputtering), affordable antibacterial materials can be developed for potential applications in chronic wound healing device area.

We report the facile and low-cost preparation as well as detailed characterization of dense arrays of passivated ferromagnetic nickel (Ni) nanotubes (NTs) vertically-supported onto solid Au-coated Si substrates. The proposed fabrication method relies on electrochemical synthesis within the nanopores of a supported anodic aluminum oxide (AAO) template and allows for fine tuning of the NTs ferromagnetic walls just by changing the cathodic reduction potential during the nanostructures' electrochemical growth. Subsequently, the experimental platform allowed further passivation of the Ni NTs with the formation of ultra-thin antiferromagnetic layers of nickel oxide (NiO). Using adequately adapted magnetic measurements, we afterwards demonstrated that the thickness of the NT walls and of the thin antiferromagneticNiO layer, strongly influences the magnetic behavior of the dense array of exchange-coupled Ni/NiO NTs. The specific magnetic properties of these hybrid ferromagnetic/antiferromagnetic nanosystems were then correlated with the morpho-structural and geometrical parameters of the NTs, as well as ultimately strengthened by additionally-implemented micromagnetic simulations. The effect of the unidirectional anisotropy strongly amplified by the cylindrical geometry of the ferromagnetic/antiferromagnetic interfaces has been investigated with the magnetic field applied both parallel and perpendicular to the NTs axis.

Nanocomposite thin films, sensitive to methane at the room temperature (25-30 degrees C), have been prepared, starting from SnSe2 powder and Zn(II)-5,10,15,20-tetrakis-(4-aminophenyl)- -porphyrin (ZnTAPP) powder, that were fully characterized by XRD, UV-VIS, FT-IR, Nuclear Magnetic Resonance (H-1-NMR and C-13-NMR), Atomic Force Microscopy (AFM), SEM and Electron Paramagnetic Resonance (EPR) techniques. Film deposition was made by drop casting from a suitable solvent for the two starting materials, after mixing them in an ultrasonic bath. The thickness of these films were estimated from SEM images, and found to be around 1.3 mu m. These thin films proved to be sensitive to a threshold methane (CH4) concentration as low as 1000 ppm, at a room temperature of about 25 degrees C, without the need for heating the sensing element. The nanocomposite material has a prompt and reproducible response to methane in the case of air, with 50% relative humidity (RH) as well. A comparison of the methane sensing performances of our new nanocomposite film with that of other recently reported methane sensitive materials is provided. It is suitable for signaling gas presence before reaching the critical lower explosion limit concentration of methane at 50,000 ppm.

ZnO nanostructures with intrinsic and extended defects were prepared by rapid decomposition of zinc-propio-nate and annealing at 400 degrees C-970 degrees C. The correlation between the structure/morphology, type of native defects (photoluminescence (PL), EPR) and ferromagnetism was investigated, together with ammonia adsorption capacity. All the samples show room temperature ferromagnetism (RTFM). Crystallite size increases while the unit cell volume, c-axis constant, microstrain and saturation magnetization relax with increasing temperature; morphology varies from aggregated nanoparticles to frameworks of well-welded crystals. 400 degrees C-800 degrees C annealed samples show a broad visible and/or a prominent violet-blue PL emission and, two narrow g = 2.0065 and g = 1.9632 EPR signals. 800 degrees C-970 degrees C annealed samples exhibit very intense green-yellow photoluminescence. The intrinsic defects in conjunction with a deformed lattice and/or pinned by grain-boundaries appear responsible for RTFM and Curie temperature exceeding 700 degrees C. Tuning the morphology, PL intensity and ferromagnetic signal by choice of annealing temperature can find applications in (gas) sensing, photonic/optoelectronic and spintronic devices.

The K alpha and K beta x-ray spectra of Se, Y, and Zr were studied experimentally and theoretically in order to obtain information on the K alpha(1) line asymmetry and the spin doublet in K beta(1,3) diagram lines. Using a high-resolution antiparallel double-crystal x-ray spectrometer, we obtained the line shapes, that is, asymmetry index and natural linewidths. We found that the corrected full width at half maximum of the K alpha(1) and K alpha(2) lines as a function of Z is in good agreement with the data in the literature. Furthermore, satellite lines arising from shake-off appear in the low-energy side of the K alpha(1) and K alpha(2) lines in Se but, in Y and Zr, it was very difficult to identify the contribution of the shake process to the overall lines. The K beta(1,3) natural linewidth of these elements was also corrected using the appropriate instrumental function for this type of x-ray spectrometer, and the spin doublet energies were obtained from the peak positions. The corrected full width at half maximum (FWHM) of the K alpha(1) x-ray lines increases linearly with Z, but this tendency was found to be, in general, not linear for K alpha(3) x-ray lines. This behavior may be due to the existence of satellite lines originated from shake processes. Simulated line profiles, obtained using the multiconfiguration Dirac-Fock formalism, accounting for radiative and radiationless transitions and shake-off processes, show a very good agreement with the high-resolution experimental spectra.

Continuous development of Si photonics requires ecological and cost-effective materials. In this work, SiGe nanocrystals (NCs) embedded in TiO2 are investigated as a photosensitive material for visible (VIS) to short-wave infrared (SWIR) broad-range detection. The TiO2 matrix has the advantage of a lower band gap than SiO2, facilitating transport of photogenerated carriers in NCs. The advantage of SiGe NCs over Ge NCs is emphasized by elucidating the mechanisms involved in rapid thermal annealing (RTA)-induced nanocrystallization. An efficiently increased NC stabilization is achieved by avoiding the detrimental fast Ge diffusion. For this, the structure, morphology, and composition were carefully characterized by high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy. Two types of structures were investigated, a film of SiGe-TiO2 alloy and a multilayer of a stack of six SiGe/TiO2 pairs. The layers have been deposited on Si wafers using magnetron sputtering of Si, Ge, and TiO2 followed by RTA in an inert atmosphere. The stabilization of SiGe NCs is achieved by the formation during RTA of protective SiO2 thin layers through Si oxidation at the SiGe NC surface, acting as a barrier for Ge diffusion. Thus, embedded Ge-rich SiGe NCs are obtained, resulting in the SWIR extension of the spectral photocurrent up to 1700 nm for films and 1600 nm for multilayers. This study has shown that in multilayers, the local anisotropy of crystallization is compensated by the stress field developed in the SiGe lattice, highly visible in the bottom part. Also, SiGe crystallizes faster than TiO2 in the rutile phase, and therefore, TiO2 remains mainly amorphous.

One of the key elements in assessing traffic safety on the roads is the detection of asphalt conditions. In this paper, we propose an optical sensor based on GeSi nanocrystals embedded in SiO2 matrix that discriminates between different slippery road conditions (wet and icy asphalt and asphalt covered with dirty ice) in respect to dry asphalt. The sensor is fabricated by magnetron sputtering deposition followed by rapid thermal annealing. The photodetector has spectral sensitivity in the 360-1350 nm range and the signal-noise ratio is 10(2)-10(3). The working principle of sensor setup for detection of road conditions is based on the photoresponse (photocurrent) of the sensor under illumination with the light reflected from the asphalt having different reflection coefficients for dry, wet, icy and dirty ice coatings. For this, the asphalt is illuminated sequentially with 980 and 1064 nm laser diodes. A database of these photocurrents is obtained for the different road conditions. We show that the use of both k-nearest neighbor and artificial neural networks classification algorithms enables a more accurate recognition of the class corresponding to a specific road state than in the case of using only one algorithm. This is achieved by comparing the new output sensor data with previously classified data for each algorithm and then by performing an intersection of the algorithms' results.

The influence of Ag and Au nanoparticles and reduced graphene oxide (RGO) sheets on the photodegradation of alpha-lipoic acid (ALA) was determined by UV-VIS spectroscopy. The ALA photodegradation was explained by considering the affinity of thiol groups for the metallic nanoparticles synthesized in the presence of trisodium citrate. The presence of excipients did not induce further changes when ALA interacts with Ag and Au nanoparticles with sizes of 5 and 10 nm by exposure to UV light. Compared to the Raman spectrum of ALA powder, changes in Raman lines' position and relative intensities when ALA has interacted with films obtained from Au nanoparticles with sizes between 5 and 50 nm were significant. These changes were explained by considering the chemical mechanism of surface-enhanced Raman scattering (SERS) spectroscopy. The photodegradation of ALA that had interacted with metallic nanoparticles was inhibited in the presence of RGO sheets.

Almost six decades ago, in Romania a small group of physicists begun to study chalcogenide compositions, motivated primarily by the desire to understand the phase-change phenomenon in these materials, discovered recently, at that time, by Stanford R. Ovshinsky. It took not too long for them to realize the challenges these materials set to the research. With newcomers to the field, the research was broadened. In some cases just for basic research, to model, and to understand the chalcogenide materials, whereas in other cases, the applicative potential was revealed and used. Herein, the evolution of the field of these somewhat exotic materials is followed, listing the main contributions done in Romania, both in basic and applied research.