Publications

We investigate a possible route for optimization of organic P3HT:ICBA photovoltaic cells. In order to ensure a more efficient charge separation and collection at the electrodes, two- and three-layer structures are produced, where additional P3HT and ICBA single layers are placed adjacent to the mixed layer. The J-V characteristics are modeled using Monte-Carlo simulations in a flexible computational framework, reproducing the typical morphologies of the active layers. We discuss the implications of the structural modifications, in particular the enhancement of the open circuit voltage. Qualitative features of the theoretical simulations are validated by experiment. The proposed fabrication technique of using solvents with different boiling points for successive deposition of the individual layers may constitute an accessible route for producing optimized solar cell structures. (C) 2017 Elsevier B.V. All rights reserved.

Thin films of chalcogenide, Cu(In, Ga) Se-2 have been obtained using a single quaternary target by radio frequency magnetron sputtering method, with thickness in the range 750 nm to 1200 nm. X-ray photoelectron spectroscopy investigations showed, that the composition of Cu(In, Ga) Se-2 thin films was very similar to that of the used target CuIn0.75Ga0.25Se2. Identification of the chemical composition of Cu(In, Ga) Se-2 thin films by XPS performed in high vacuum, emphasized that the samples exhibit surface features suitable to be integrated into the structure of solar cells. Atomic Force Microscopy and Scanning Electron Microscopy investigations showed that surface morphology was influenced by the increase in thickness of the Cu(In, Ga) Se-2 layer. From X-Ray Diffraction investigations it was found that all films were polycrystalline, having a tetragonal lattice with a preferential orientation along the (112) direction. The optical reflectance as a function of wavelength was measured for the studied samples. The increase in thickness of the Cu(In, Ga) Se-2 absorber determined a decrease of its optical bandgap value from 1.53 eV to 1.44 eV. The results presented in this paper showed an excellent alternative of obtaining Cu(In, Ga) Se-2 compound thin films from a single target. (C) 2016 Elsevier B.V. All rights reserved.

Using a fast and eco-friendly deposition method, ITO thin films with different thicknesses (0.5 mu m-0.7 mu m) were deposited on glass substrates by radio frequency magnetron sputtering technique. A comparative analysis of these oxide films was then carried out. AFM investigations showed that the deposited films were smooth, uniform and having a surface roughness smaller than 10 nm. X-ray diffraction investigations showed that all samples were polycrystalline and the grain sizes of the films, corresponding to (222) cubic reflection, were found to increase with the increasing film thickness. The optical properties, evaluated by UV-VIS-NIR (190-3000 nm) spectrophotometer, evidenced that the obtained thin films were highly transparent, with a transmission coefficient between 90 and 96%, depending on the film thickness. Various methods (Swanepoel and Drude) were employed to appreciate the optimal behaviour of transparent oxide films, in determining the dielectric optical parameters and refractive index dispersion for ITO films exhibiting interference patterns in the optical transmission spectra. The electrical conductivity also increased as the film thickness increased. (C) 2017 Elsevier B.V. All rights reserved.

We report the molecular beam epitaxy growth of Ni on a clean Ge(001) surface with an intermediate NiGe layer forming at the interface at room temperature. The crystallinity of the substrate is lost after the deposition of more than 2 Ni monolayers. The Schottky barrier formation is investigated by X-ray photoelectron spectroscopy. The method allows us to infer a 0.39-0.45 eV band bending at the interface between the compound and Ge(001). Magneto-optical Kerr effect measurements were conclusive in detecting the ferromagnetic ordering of Ni outermost layers. (C) 2017 Elsevier B.V. All rights reserved.

Green compacts of B4C or B4C added with 1 vol% graphite were infiltrated with molten Si and subsequently were subject of processing by floating zone partial re-melting (FZPR). In FZPR only the low temperature fusible component, in this case Si, is melted. A fully dense B4C-based ceramic is obtained. It contains free-Si, SiC and B4C. In the center of the FZPR ceramic without graphite addition, the amount of Si is decreased when compared to the infiltrated material. Some impurity elements such as Al, Fe, or Ti detected in the raw B4C powder are preferentially gathered at the edges of the sample. In the sample added with graphite, formation of a high amount of SiC in the infiltrated material hinders Si shift from the center to the edges. The pulling rate and the particle size of the B4C raw powders are also important. It is recognized that sintering of powders larger than 10-20 gm is usually difficult: our approach is demonstrated to be suitable for processing of B4C powders with a very different particle size, from 10 to 250 gm. The FZPR ceramic had a Vickers hardness of 9-38 GPa depending on location of the indentation imprint and on the sample. A tensile strength of 114-188 MPa that is up to about 2-3 times higher than for the infiltrated material was recorded. Work indicates that the proposed processing approach offers extended control possibilities towards fabrication of new composite materials not available by traditional technologies.

We study the validity of Hund's first rule for the spin multiplicity in circular molecules-made of real or artificial atoms such as quantum dots-by considering a perturbative approach in the Coulomb interaction in the extended Hubbard model with both on-site and long-range interactions. In this approximation, we show that an anti-Hund rule always defines the ground state in a molecule with 4N atoms at half-filling. In all other cases (i.e., number of atoms not a multiple of four, or a 4N molecule away from half-filling) both the singlet and the triplet outcomes are possible, as determined primarily by the total number of electrons in the system. In some instances, the Hund rule is always obeyed and the triplet ground state is realized mathematically for any values of the on-site and long-range interactions, while for other filling situations the singlet is also possible but only if the long-range interactions exceed a certain threshold, relatively to the on-site interaction.

A series of novel N-benzoyl-N'-aryl thiourea derivatives (BTU) bearing different number of alkoxy groups in terminal positions of benzoyl unit and a perfluorooctyl group on the other side have been designed and prepared. Their liquid crystalline properties were investigated by a combination of three techniques: polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and variable-temperature powder X-ray diffraction (XRD). Their thermal stability was studied by thermogravimetric analysis (TG). It was found that only the compounds which have only one alkoxy chain attached to benzoyl unit, 1a and 1b, show calamitic mesomorphic behavior, with smectic A and C phases being displayed. The type and stability of these mesophases are greatly influenced by the alkyl chain length as well as by the presence of perfluorooctyl group. The clearing and the mesophase ranges are significantly increased with the incorporation of perfluoroalkyl chains when compared to non-fluorinated analogues, with almost 40 degrees C. The attachment of additional alkoxy groups on the benzoyl moiety led to a significant decrease of the clearing points and suppression of the mesogenic character.

Magnetic nanoparticles embedded in the active layer of the Organic Light Emitting Diodes (OLEDs) significantly increases the electroluminescence and the charge transport without influencing the transparency of these devices. A brief comparison was done in order to identify which parameter influences these properties, by comparing the CoFe2O4 magnetic nanoparticles with CoFe2 metallic magnetic nanoparticles, the latter one being obtained by thermal reduction in hydrogen of cobalt ferrite nanoparticles. CoFe2 have shown a better efficiency of the metallic nanoparticles where probably the main advantage is the higher magnetization property instead of the coercive field. Concerning the charge transport across the OLEDs, these nanoparticles reduce the electron injection, acting as filling traps, which directly increases the electroluminescence and the current at the same voltage.

Morphology is a key element in the functionality of low dimensional structures including here electroactive polymers, especially when applications such as muscle like actuators are sought. The reason is that morphology in the context of a high specific surface object strongly influences specific parameters such as ionic diffusion, conductivity and consequently the actuation capability of the system. In the present work a new architecture for microtube-based actuating elements is presented. Free-standing fibrillar microtubes with diameter in the range of micrometers and with a core-shell polyaniline/gold structure are fabricated through a scalable approach. Aligned electrospun poly(methyl methacrylate) fibers are coated with gold and are further employed as microstructured electrodes for the electrochemical deposition of polyaniline. Further the poly(methyl methacrylate) core was dissolved, leading to a tubular structure. The polyaniline/gold microtubes show complex, rapid and reversible movement patterns, with great stability and consistency over repeated actuation cycles. Thus, when the potential is swept between -0.2 and 1 V at different rates, the microtubes move, this movement being associated with the morphological and structural characteristics of the deposited polyaniline layer, a mechanism based on the expansion/contraction and conformational changes of the polymer chains due to the insertion/expulsion of ions. The response time of these electroactive microstructures during one cycle is in the range of seconds, a consequence of their low dimensionality and specific structure. Moreover the actuation takes place in different electrolytes including simulated gastric fluid, which enables a wide range of applications. (C) 2017 Elsevier B.V. All rights reserved.

Some of the modern aspects of field emission based electron sources have been collated in a short and comprehensive review. The usually overlooked peculiar aspects in this research field have been particularly emphasized in order to increase the interest in further fundamental studies and technological applications. The vast material was roughly split in two main branches which occasionally overlap: the electron emission devices based on chemically homogeneous nanostructured surfaces and the more complex nanocomposite emitting surfaces.