National Institute Of Materials Physics - Romania

The effect of potential tetragonal and triclinic distortions of the energetic favorable cubic crystalline structure in CoFeZrSi Heusler compound, was investigated, using semiclassic Boltzmann theory on the thermoelectric functionalities. Chemical potential dependence of the conductivity integral sigma/tau for relaxed cubic and tetragonal structures confirms p-type thermoelectric characteristics. When triclinic deformation is investigated, the electrical conductivity response indicates that the material's ability to conduct electric current decreases. The calculated Seebeck coefficients exhibit positive values for the crystalline structures whose angles are equal to 90 degrees (cubic and tetragonal), over the 300-1200K temperature range. The figures of merit ZT, for relaxed cubic and tetragonal structures, at optimum unit cell volume or higher, present around room temperature, promising features as a potential thermoelectric material (i.e. ZT = 0.94 for 350K in optimum cubic structure). (C) 2019 Elsevier B.V. All rights reserved.

The influence of temperature factor to crystal structure as well as magnetic and electric properties of strontium hexaferrite partially substituted with diamagnetic indium ions has been investigated. Ferroelectric properties have been found out in SrFe11.9In0.1O19 compound that contradicts to conventional opinion, which describe its crystal structure within the framework of centrosymmetric space group P6(3)/mmc (No. 194). For determination features of the crystal structure, which are responsible for ferroelectric properties of strontium hexaferrite, have been carried out neutron diffraction measurements with high resolution in temperature range from 1.5 to 740 K. The analysis of hexaferrite structure has been executed within the framework both centrosymmetric and non-centrosymmetric space group. Values of coefficients of magneto crystalline anisotropy at 5 and 300 K and influence of ambient temperature to linear size of magnetic regions have been determined from magnetic measurements. (C) 2019 Elsevier B.V. All rights reserved.

In this study, the synergistic behavior of Ni species and bimodal mesoporous undoped SnO2 is investigated in oxygen evolution reactions (OERs) under alkaline conditions without any other modification of the compositional phases or using noble metals. An efficient and environmentally friendly hydrothermal method to prepare bimodal mesoporous undoped SnO2 with a very high surface area (>130 m(2) g(-1)) and a general deposition-precipitation method for the synthesis of well-dispersed Ni species on undoped SnO2 are reported. The powders were characterized by adsorption-desorption isotherms, TG-DTA, XRD, SEM, TEM, Raman, TPRH2, and XPS. The best NiSn composite generates, under certain experimental conditions, a very high TOF value of 1.14 s(-1) and a mass activity higher than 370 A g(-1), which are remarkable results considering the low amount of Ni deposited on the electrode (3.78 ng). Moreover, in 1 M NaOH electrolyte, this material produces more than 24 mA cm(-2) at an overpotential value of approximately +0.33 V, with only 5 wt % Ni species. This performance stems from the dual role of undoped SnO2, on the one hand, as a support for active and well-dispersed Ni species and on the other hand as an active player through the oxygen vacancies generated upon Ni deposition.

This paper presents some studies on the organic heterostructures realized by Matrix Assisted Pulsed Laser Evaporation in both bi-layer and mixed layer configurations on glass substrates covered by flat or nano-patterned ITO. The donor, a star-shaped arylenevinylene compound, 4,4',4 ''-tris[(4'-diphenylamino) styryl] triphenylamine, and acceptor, a non-fullerene compound, N,N'-bis-(1-dodecyl)perylene-3,4,9,10 tetracarboxylic diimide, were blended in three weight ratios: 1:2, 1:3 and 1:4. A grating of cylindrical pillars with a periodicity of 1.1 mu m has been developed by UV-Nanoimprint Lithography in a polymer layer. The shape of the nanostructures changed to cone trunk by the Pulsed Laser Deposition of ITO on this nanostructured surface. The effect of the nanostructures and composition on the optical and electrical properties of the heterostructures was analyzed. The nano-patterning affected both the UV-Vis transmission and photoluminescence through the multiple reflections inside the cavities and at interfaces and the particularities of the molecular arrangement. The patterning was preserved independently of composition, but the roughness increased with increasing acceptor amount. The I-V characteristics drawn at room temperature in dark revealed an ohmic contact behavior for all heterostructures. The nano-patterning had a similar effect on the current in the heterostructures with mixed layer (1:2) and stacked bi-layer.

Carbon nanotube - highly reduced graphene oxide - transition metal oxide (ZnO) nanohybrid layers were synthesized using a one-step laser technique. Commercial multiwall carbon nanotubes (MWCNTs), graphene oxide (GO) platelets and ZnO nanoparticles were used as starting materials. We discuss the influence of carbon/metal oxide ratio on the physico-chemical properties of the nanohybrid layers, geometrical characteristics, shape and dimensions of constituent nanoentities, chemical composition and chemical bonding states, optical properties, UV-visible absorption, band gap values, as well as charge transfer properties. In the followings the relation between these properties and functional characteristics, removal of water contaminants, antibiotic molecules, and charge storage performances of the ternary, MWCNTs/reduced GO/ZnO layers are presented, identifying the optimum relative concentrations of the constituting nanomaterials. The high photocatalytic efficiencies both under UV and visible light irradiations, even after several consecutive degradation cycles, were attributed to effective separation of photogenerated charge carriers by carbon nanomaterials as well as formation of oxygen deficient ZnO(x-1 )nanocrystals. The enhanced charge storage capacity of ternary nanohybrid electrodes is based on combined electrochemical double layer capacitance and pseudocapacitance implying redox reactions on the surface and subsurface of the layers in contact with the electrolyte. Both functional properties are strongly influenced by the relative concentrations of the nanomaterials constituting the ternary layers.

The effects of a turnstile operation on the current-induced vibron dynamics in nanoelectromechanical systems (NEMS) are analyzed in the framework of the generalized master equation. In our simulations each turnstile cycle allows the pumping of up to two interacting electrons across a biased mesoscopic subsystem which is electrostatically coupled to the vibrational mode of a nanoresonator. The time-dependent mean vibron number is very sensitive to the turnstile driving, rapidly increasing/decreasing along the charging/discharging sequences. This sequence of heating and cooling cycles experienced by the nanoresonator is due to specific vibron-assisted sequential tunneling processes along a turnstile period. At the end of each charging/discharging cycle the nanoresonator is described by a linear combination of vibron-dressed states s(v). associated to an electronic configuration nu. If the turnstile operation leads to complete electronic depletion the nanoresonator returns to its equilibrium position, i.e., its displacement vanishes. It turns out that a suitable bias applied on the NEMS leads to a slow but complete cooling at the end of the turnstile cycle. Our calculations show that the quantum turnstile regime switches the dynamics of the NEMS between vibron-dressed subspaces with different electronic occupation numbers. We predict that the turnstile control of the electron-vibron interaction induces measurable changes on the input and output transient currents.

Among the prerequisites for the progress of single-molecule-based electronic devices are a better understanding of the electronic properties at the individual molecular level and the development of methods to tune the charge transport through molecular junctions. Scanning tunneling microscopy (STM) is an ideal tool not only for the characterization, but also for the manipulation of single atoms and molecules on surfaces. The conductance through a single molecule can be measured by contacting the molecule with atomic precision and forming a molecular bridge between the metallic STM tip electrode and the metallic surface electrode. The parameters affecting the conductance are mainly related to their electronic structure and to the coupling to the metallic electrodes. Here, the experimental and theoretical analyses are focused on single tetracenothiophene molecules and demonstrate that an in situ-induced direct desulfurization reaction of the thiophene moiety strongly improves the molecular anchoring by forming covalent bonds between molecular carbon and copper surface atoms. This bond formation leads to an increase of the conductance by about 50 % compared to the initial state.

The underlying mechanisms of the metal-insulator transition in correlated oxides are a rich source of interesting physics and a topic of long-standing investigation. Here, the authors use angle-resolved photoelectron spectroscopy to investigate changes in charge carrier properties and electron-phonon interactions as a function of Ce-doping across the metal-insulator transition in CaMnO3. Many transition metal oxides (TMOs) are Mott insulators due to strong Coulomb repulsion between electrons, and exhibit metal-insulator transitions (MITs) whose mechanisms are not always fully understood. Unlike most TMOs, minute doping in CaMnO3 induces a metallic state without any structural transformations. This material is thus an ideal platform to explore band formation through the MIT. Here, we use angle-resolved photoemission spectroscopy to visualize how electrons delocalize and couple to phonons in CaMnO3. We show the development of a Fermi surface where mobile electrons coexist with heavier carriers, strongly coupled polarons. The latter originate from a boost of the electron-phonon interaction (EPI). This finding brings to light the role that the EPI can play in MITs even caused by purely electronic mechanisms. Our discovery of the EPI-induced dichotomy of the charge carriers explains the transport response of Ce-doped CaMnO3 and suggests strategies to engineer quantum matter from TMOs.

The effect of UV light on the cationic photopolymerization of the SU8 negative photoresist is shown by photoluminescence (PL) studies. Our results demonstrate that the cationic photopolymerization reaction of the SU8 photoresist takes place predominantly under the influence of the UVA light. Using UVA light, the influence of carbon nanotubes [of the types single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs), multiwalled carbon nanotubes (MWNTs), and SWNTs functionalized with carboxyl groups (SWNTs-COOH)] on the cationic photopolymerization process of the SU8 photoresist is shown by PL studies. The cationic photopolymerization of the SU8 photoresist is monitored by the variations of the two emission bands with maxima at similar to 400-429 nm and 523-556 nm. The increase in the relative intensity of the PL band at similar to 523-556 nm is dependent on (i) the carbon nanotube concentration in the SU8 photoresist matrix; (ii) the type of carbon nanotubes, i.e., SWNTs, DWNTs, and MWNTs; and (iii) the nonfunctionalized and functionalized state of SWNTs. The results reported in this work demonstrate that PL can be used as a complementary method to Raman scattering and IR spectroscopy in the investigation of the cationic photopolymerization reaction of the SU8 negative photoresist. A decrease in the wrapping angle of carbon nanotubes with the SU8 photoresist is highlighted by anisotropic PL studies.

A one-step method assisted by hydrothermal treatment was approached to obtain nanocrystalline 1 and 10 mol. % In doped 2 mol.% Pd-SnO2 powders using a non-ionic surfactant Brij52 and Polyethylene glycol 6000 (PEG) as templates. Depending on In content, the samples were labeled as Pd1InSn and Pd10InSn. The obtained materials consist of nanosized crystallites packed into micrometric grains with a high porosity, as revealed by the morphological and structural investigations (SEM, TEM). A dependence of the grain size with respect to the In content has been revealed i.e. the sample Pd1InSn was showing an average grain size of around 10 nm, whilst for the sample Pd10InSn the average grain size was found to be around 5 nm. The XPS investigations highlighted the differences occurred in the surface chemistry in terms of surface hydroxylation as well as the chemical states of Pd. The sensing properties towards different CO concentrations have been examined under infield background conditions, at low operating temperature of 50 degrees C. The sensing mechanism model for CO was discussed in detail according to the possible interplay between oxygen and water related species based on the experimentally results acquired through simultaneous electrical resistance and work function measurements.