Laser prepared organic heterostructures based on star-shaped arylenevinylene compounds

This paper presents some studies about the preparation by matrix-assisted pulsed laser evaporation (MAPLE) technique of organic bulk heterojunctions made from the mixture of a star-shaped arylenevinylene compound, 4,4′,4″-tris[(4′-diphenylamino)styryl] triphenylamine as donor and fullerene derivative, [6, 6]-phenyl C61 butyric acid butyl ester, as acceptor, in the weight ratio 1:2. The mixed layer has been characterized by spectroscopic (UV–Vis, Fourier transform infrared) and microscopic (AFM) methods, and the effects of the deposition conditions (number of pulses) and of a buffer layer of poly(aniline-co-aniline propane sulfonic acid) or poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) have been analyzed. The study of the electrical properties has revealed a typical solar cell behavior for the heterostructure glass/ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/4,4′,4″-tris[(4′-diphenylamino)styryl] triphenylamine: [6, 6]-phenyl C61 butyric acid butyl ester/Al, confirming that MAPLE could be an adequate method for the preparation of active layer based on bulk heterojunction for solar cells.     

Charge density waves in nanocrystalline thin films of blue bronze K0.3MoO3

Thin granular films of charge density wave (CDW) system K_0.3MoO₃ were prepared by pulsed laser deposition and investigated by various standard characterization methods such as GI-XRD, electric transport, TOF-ERDA, AFM and UV–visible spectroscopy. While all these methods indicate that the thin films consist of nanometer grains of K_0.3MoO₃, it is only the non-destructive femtosecond time-resolved spectroscopy (fsTRS) that demonstrates the charge density wave nature of the ground state and therefore proves directly the presence of K_0.3MoO₃. Furthermore, the comparison of the fsTRS data obtained in thin films and in single crystals shows the reduction of the charge density wave transition temperature and of the photoinduced signal strength in granular thin films with respect to single crystals, which is attributed to the granularity and crystal growth morphology. Our results establish fsTRS technique as the essential tool for the detection and characterization of complex ground states in nano-sized systems.     

Ergodic properties for some non-expanding non-reversible systems

We study several properties of invariant measures obtained from preimages, for non-invertible maps on fractal sets which model non-reversible dynamical systems. We give two ways to describe the distribution of all preimages for endomorphisms which are not necessarily expanding on a basic set Λ. We give a topological dynamics condition which guarantees that the corresponding measures converge to a unique conformal ergodic borelian measure; this helps in estimating the unstable dimension a.e. with respect to this measure with the help of Lyapunov exponents. When there exist negative Lyapunov exponents of this limit measure, we study the conditional probabilities induced on the non-uniform local stable manifolds by the limit measure, and also its pointwise dimension on stable manifolds.     

Surface nitridation by multipulse excimer laser irradiation

A review is presented of the results obtained on the formation of nitride surface layers on semiconductor (Si) and metal (Ti) samples by multipulse (up to 2500) XeCl excimer laser (λ=308 nm) irradiation in N₂ and NH₃ atmosphere through a collaboration of Italian, Czechoslovak and Romanian laboratories. Different diagnostic techniques (optical and electron microscopy, Rutherford backscattering spectroscopy, nuclear reaction analysis, Auger and x-ray photoelectron spectroscopy) were used to positively identify the formed compounds. Silicon nitride formation was obtained only when laser irradiation was performed in ammonia atmosphere. In contrast, when titanium samples were irradiated the nitridation process resulted very efficient in both atmospheres. The characteristics of laser synthesized nitride layers are illustrated and discussed as a function of the kind of irradiated materials, the number of subsequent laser pulses and the nature of the ambient gas.     

Crown-like structure development on titanium exposed to multipulse Nd:YAG laser irradiation

We report micrometre-sized crown-like structure growth on a Ti surface by multipulse Nd:YAG (λ=1.064 μm,τ=170 ns) laser irradiation in air at atmospheric pressure. The irradiation was performed at 8×10⁷ W/cm² laser-pulse intensity, below the ablation threshold. A ring-shape structure develops in the centre of the irradiation spot after the action of five laser pulses. The further increase of the laser-pulse number leads gradually to a crown-like structure, which has, for 150 pulses, a height of 120–140 μm above the non-irradiated Ti surface. The forming crater’s depth does not exceed the height of the grown structure. In the neighbouring zone, after the action of 25 laser pulses, microcracks of the oxide surface layer develop. With the next pulses this leads to the formation of a surface microrelief. The crown-like-structure growth is originated by molten material movement attributed to the laser-induced plasma-recoil pressure. Received: 6 June 2001 / Accepted: 6 January 2002 / Published online: 26 March 2002     

Bioactive glass and hydroxyapatite thin films obtained by pulsed laser deposition

Bioactive glass (BG), calcium hydroxyapatite (HA), and ZrO₂ doped HA thin films were grown by pulsed laser deposition on Ti substrates. An UV KrF^* (λ = 248 nm, τ ≥ 7 ns) excimer laser was used for the multi-pulse irradiation of the targets. The substrates were kept at room temperature or heated during the film deposition at values within the (400-550 °C) range. The depositions were performed in oxygen and water vapor atmospheres, at pressure values in the range (5-40 Pa). The HA coatings were heat post-treated for 6 h in a flux of hot water vapors at the same temperature as applied during deposition. The surface morphology, chemical composition, and crystalline quality of the obtained thin films were studied by scanning electron microscopy, atomic force microscopy, and X-ray diffractometry. The films were seeded for in vitro tests with Hek293 (human embryonic kidney) cells that revealed a good adherence on the deposited layers. Biocompatibility tests showed that cell growth was better on HA than on BG thin films.     

Temperature field modeling during multi-modes CO2 laser irradiation of human enamel

We examine the temperature fields of human enamel [Yu D, Fox JL, Hsu J, Lynn Powell G, Higuchi WI. Computer simulation of surface temperature profiles during CO₂ laser irradiation of human enamel. Opt Eng 1993; 32(2)] during multi-modes CO₂ laser irradiation. For this we use the integral transform method as well as direct and inverse Laplace transform [Oane M, Sporea D. Temperature profiles modeling in IR optical components during high power laser irradiation. Infrared Phys Technol 2001; 42(1): 31–40; Oane M, Sporea D. Study of heat transfer in IR optical components during CO₂ laser irradiation. Proc SPIE 2001; 4430: 898–904; Oane M. Mathematical modeling of the thermal field distributions in solids under multiple laser irradiations. Proc SPIE 2003; 5227: 329–34; Oane M, Apostol I, Timcu A. Temperature field modeling in laser heated metals for laser cleaning of surfaces. Proc SPIE 2003; 5227: 323–8]. The enamel block is modeled as homogeneous cylinder in three dimensions. Results indicate that (i) the thermal field depends on multi-modes structure; (ii) heat transfer coefficient plays an important role in temperature distribution.     

Synthesis of functionally graded bioactive glass-apatite multistructures on Ti substrates by pulsed laser deposition

Functionally graded glass-apatite multistructures were synthesized by pulsed laser deposition on Ti substrates. We used sintered targets of hydroxyapatite Ca₁₀(PO₄)₆(OH)₂, or bioglasses in the system SiO₂-Na₂O-K₂O-CaO-MgO-P₂O₅ with SiO₂ content of either 57 wt.% (6P57) or 61 wt.% (6P61). A UV KrF* (λ = 248 nm, τ > 7 ns) excimer laser source was used for the multipulse laser ablation of the targets. The hydroxyapatite thin films were obtained in H₂O vapors, while the bioglass layers were deposited in O₂. Thin films of 6P61 were deposited in direct contact with Ti, because Ti and this glass have similar thermal expansion behaviors, which ensure good bioglass adhesion to the substrate. This glass, however, is not bioactive, so yet more depositions of 6P57 bioglass and/or hydroxyapatite thin films were performed. All structures with hydroxyapatite overcoating were post-treated in a flux of water vapors. The obtained multistructures were characterized by various techniques. X-ray investigations of the coatings found small amounts of crystalline hydroxyapatite in the outer layers. The scanning electron microscopy analyses revealed homogeneous coatings with good adhesion to the Ti substrate. Our studies showed that the multistructures we had obtained were compatible with further use in biomimetic metallic implants with glass-apatite coating applications.