National Institute Of Materials Physics - Romania

Matrix-assisted pulsed laser evaporation (MAPLE) was used to deposit hybrid nanocomposite thin films based on cobalt phthalocyanine (CoPc), C60 fullerene and ZnO nanoparticles. The inorganic nanoparticles, with a size of about 20 nm, having the structural and optical properties characteristic of ZnO, were chemically synthesized by a simple precipitation method. Furthermore, ZnO nanoparticles were dispersed in a dimethyl sulfoxide solution in which CoPc and C60 had been dissolved, ready for the freezing MAPLE target. The effect of the concentration of ZnO nanoparticles on the structural, morphological, optical and electrical properties of the CoPc:C60:ZnO hybrid nanocomposite layers deposited by MAPLE was evaluated. The infrared spectra of the hybrid nanocomposite films confirm that the CoPc and C60 preserve their chemical structure during the laser deposition process. The CoPc optical signature is recognized in the ultraviolet-visible (UV-Vis) spectra of the obtained layers, these being dominated by the absorption bands associated to this organic compound while the ZnO optical fingerprint is identified in the photoluminescence spectra of the prepared layers, these disclosing the emission bands linked to this inorganic semiconductor. The hybrid nanocomposite layers exhibit globular morphology, which is typical for the thin films deposited by MAPLE. Current-voltage (J-V) characteristics of the structures developed on CoPc:C60:ZnO layers reveal that the addition of an appropriate amount of ZnO nanoparticles in the CoPc:C60 mixture leads to a more efficient charge transfer between the organic and inorganic components. Due to their photovoltaic effect, structures featuring such hybrid nanocomposite thin films deposited by MAPLE can have potential applications in the field of photovoltaic devices.

Our study focused on the long-term degradation under simulated conditions of coatings based on different compositions of polycaprolactone-polyethylene glycol blends (PCL-blend-PEG), fabricated for titanium implants by a dip-coating technique. The degradation behavior of polymeric coatings was evaluated by polymer mass loss measurements of the PCL-blend-PEG during immersion in SBF up to 16 weeks and correlated with those yielded from electrochemical experiments. The results are thoroughly supported by extensive compositional and surface analyses (FTIR, GIXRD, SEM, and wettability investigations). We found that the degradation behavior of PCL-blend-PEG coatings is governed by the properties of the main polymer constituents: the PEG solubilizes fast, immediately after the immersion, while the PCL degrades slowly over the whole period of time. Furthermore, the results evidence that the alteration of blend coatings is strongly enhanced by the increase in PEG content. The biological assessment unveiled the beneficial influence of PCL-blend-PEG coatings for the adhesion and spreading of both human-derived mesenchymal stem cells and endothelial cells.

Single-phase Ba1-xSrxTiO3 (BST) perovskite ceramics with 0.3 <= x <= 0.4 were prepared from powders synthesized via sol-gel route. The compositions have the ferroelectric-paraelectric phase transition close to room temperature. At 20 degrees C the BST ceramics are ferroelectric for 0.3 <= x <= 0.35 and paraelectric for x = 0.375 and x = 0.40. The study follows the relation between the structural changes produced when increasing the Sr content and the dielectric properties at low intensity electric fields. It is found that the grain size and tetragonality decreases as the Sr content increases. Analyses of complex permittivity and impedance spectroscopy reveal the temperature and frequency dependencies of the dielectric properties. The phase transitions seem to be of first order for all compositions, with a thermal hysteresis that decreases with increasing the Sr content, fact attributed to the corresponding increase of the grain boundaries weight allowing a more efficient stress relaxation in the structure during the change of the symmetry from cubic to tetragonal. The diffusiveness degree during the phase transition is increasing with Sr content, suggesting some relaxor-type contribution attributed to smaller grain size. The ac conductivity follows the universal Jonscher law, with an ac component having the power parameter s independent of Sr content, and a dc component that it is thermally activated with an activation energy of about 0.7-0.77 eV attributed to oxygen vacancies acting as donor-like defects. The fit of impedance spectra at different temperatures and frequencies is obtained by using an equivalent circuit accounting the grains, grain boundaries, electrode interfaces and the local contributions produced by reorientation of defect dipoles or defect clusters. All the component circuits have significant variations around phase transitions. These are discussed in relation to structural changes occurring during transition and considering the changes in the distribution of various charges when polarization vanishes.

Organometallic compounds embedded in thin films are widely used for Organic Light-Emitting Diodes (OLED), but their functionalities are strongly correlated with the intrinsic properties of those films. Controlling the concentration of the organometallics in the active layers influences the OLED performances through the aggregation processes. These aggregations could lead to crystallization processes that significantly modify the efficiency of light emission in the case of electroluminescent devices. For functional devices with organometallic-based thin films, some improvements, such as the optimization of the charge injection, are needed to increase the light output. One dual emitter IrQ(ppy)2 organometallic compound was chosen for the aggregation correlations from a multitude of macromolecular organometallics that exist on the market for OLED applications. The choice of additional layers like conductive polymers or small molecules as host for the active layer may significantly influence the performances of the OLED based on the IrQ(ppy)2 organometallic compound. The use of the CBP small molecule layer may lead to an increase in the electroluminescence versus the applied voltage.

This study aims to obtain uniform and homogeneous bismuth germanate oxides thin films by spin coating and using the sol-gel technique with different precursors, followed by low-temperature annealing at 560 degrees C. By using Bi(NO3)(3) precursors, we have obtained transparent, yellowish thin films with a 200 nm thickness. The structural analysis of the initial sol-gel powder has shown the presence of two crystalline structures, the cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5 crystallites, which evolves towards the BGO structure after annealing. The elemental analysis confirmed the composition of the desired compound Bi4Ge3O12 with 60 wt % GeO2 and 40 wt % Bi2O5. On the other hand, by changing the precursor to (Bi(CH3COO)(2), the film thickness increased to 500 nm thicker due to the high viscosity of the sol, and a dominant monoclinic Bi2GeO5 crystalline structure appeared. The elemental analysis revealed a nonstoichiometric composition with 38 wt % GeO2 and 62 wt % Bi2O3. Due to the low GeO2 phase content that reacted with metastable Bi2GeO5, we obtained cubic Bi4Ge3O12 as a secondary phase, with Bi2GeO5 as a dominant crystalline phase. The redshifts of both absorptions and emissions spectra peaks confirmed a different disorder structure as an interplay between the cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5 phases.

Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NC s composition and size. In this study, SiGe-SiO2 amorphous films were deposited by magnetron sputtering on Si substrate followed by rapid thermal annealing at 700, 800 and 1000 degrees C. We investigated films with Si:Ge:SiO2 compositions of 25:25:50 vol.% and 5:45:50 vol.%. TEM investigations reveal the major changes in films morphology (SiGe NCs with different sizes and densities) produced by Si:Ge ratio and annealing temperature. XPS also show that the film depth profile of SiGe content is dependent on the annealing temperature. These changes strongly influence electrical and photoconduction properties. Depending on annealing temperature and Si:Ge ratio, photocurrents can be 10(3) times higher than dark currents. The photocurrent cutoff wavelength obtained on samples with 25:25 vol% SiGe ratio decreases with annealing temperature increase from 1260 nm in SWIR for 700 degrees C annealed films to 1210 nm for those at 1000 degrees C. By increasing Ge content in SiGe (5:45 vol%) the cutoff wavelength significantly shifts to 1345 nm (800 degrees C annealing). By performing measurements at 100 K, the cutoff wavelength extends in SWIR to 1630 nm having high photoresponsivity of 9.35 AW(-1).

The time evolution of a quantum dot exciton in Coulomb interaction with wetting layer carriers is treated using an approach similar to the independent boson model. The role of the polaronic unitary transform is played by the scattering matrix, for which a diagrammatic, linked cluster expansion is available Similarities and differences to the independent boson model are discussed. A numerical example is presented.

We propose a concept of hybrid nanoelectronic-magnetic device made of magnetic Fe-C core-shell nanoparticles deposited onto prepatterned Si (111) substrate with basic circuitry made of metallic conductive lines. The synthesis of magnetic material and the creation of nanoelectronic prepatterned interdigitated die are reported and to prove the effectiveness in devices, their magnetotransport properties are investigated. Magnetic Fe/FeC nanoparticles, 11 nm diameter, with a core-shell structure have been prepared by laser pyrolysis. Two different layouts of prepatterned interdigitated die, have been conceived using e-beam lithography, with various geometries. A range of microscopy techniques, transmission electron, scanning and optical, were employed for morphological characterization of the as-obtained structures. Magnetic and magnetotransport characterization using SQUID magnetometry has been performed onto both the core-shell nanoparticles and onto the hybrid device obtained by depositing centrifugated and dispersed core-shell nanoparticles from liquid carrier solutions. From magnetotransport measurements, it has been revealed that the hybrid device made of Fe/FeC nanosized materials on prepatterned interdigitated die exhibit a large giant magnetoresistive (GMR) effect of about 8% at 300 K. This result is promising in view of the use of such devices as arrays of nanosensors and in spintronic applications.

Hybrid organic-inorganic thin films based on zinc phthalocyanine (ZnPc) and ZnO nanoparticles were deposited by Matrix Assisted Pulsed Laser Evaporation (MAPLE). Synthesized by a simple wet chemical precipitation method, the ZnO nanoparticles were featured by a hexagonal wurtzite structure, a band-gap value of similar to 3.3 eV and emission bands typical for this semiconductor. The hybrid films containing ZnPc and various amounts of ZnO nanoparticles were evaluated from morphological, compositional, structural, optical and electrical point of view. No chemical decomposition of the organic compound was observed in the FTIR spectra of the deposited layers. The transmittance and photoluminescence spectra recorded on hybrid films disclose the optical signature of both organic (ZnPc) and inorganic (ZnO) components. The electrical measurements carried out under illumination emphasized the importance of the quantity of the inorganic component on the performance parameters of the structures prepared with the hybrid films. Our study provides new insight in the MAPLE deposition of the organic-inorganic hybrid films with potential applications in the photovoltaic cells area.

We show that a simple ethanol (EtOH) refluxing treatment at mild temperature (120 degrees C) allows producing blue-colored and reduced titanium dioxide (TiO2-x) exhibiting improved visible-light (VIS) photocatalytic properties. The treatment causes an increase in the density of Ti(III) species and the appearance of two optical absorption features: a broad absorption band-responsible for the blue coloration- extending from the green region (similar to 2.3 eV) up to the near-infrared and a subgap absorption tail close to the band gap energy. The experimental results combined with a computation of the density of states via hybrid Hartree-Fock density functional support the hypothesis that the EtOH reflux treatment leads to formation of surface and subsurface oxygen (O) vacancies. We also show that the excitation-resolved photoluminescence technique allows a high-contrast detection of a subgap optical excitation band peaked at about 430 nm (similar to 2.9 eV), associated with anatase photoluminescence, whose intensity increases after the EtOH reflux treatment. This result gives a very direct support to the debated hypothesis identifying O vacancy states as the energy levels involved in the radiative transition of anatase TiO2. Improved photocatalytic degradation by the processed TiO2 under VIS illumination is demonstrated, and the possible mechanism involved in the formation of surface O vacancies is discussed. The method outlines a very simple, low-cost, and fast procedure to target the formation of O vacancies in the TiO2 surface region.