Superelastic electron scattering by metastable strontium atoms

Superelastic electron scattering by metastable strontium atoms is experimentally studied using intersecting electron and atomic beams and an electron spectrometer. The energy dependence of the effective cross section of superelastic electron scattering in the energy range 0.15–2.0 eV has been obtained for the first time.     

Laser modification of silver nanoclusters in SiO2 thin films

Thin films of silica containing silver nanoclusters have been deposited by magnetron co-sputtering followed by thermal annealing. Laser modification of the mean cluster size was performed using the fourth harmonic of a Nd:YAG laser with energies of between 35 and 125 mJ/cm². The mean size of the clusters was estimated from the shape of the plasmon resonance band in the optical absorption spectra with the help of a computer simulation based on the Mie theory in static approximation. It was found that laser treatment with fluences above a certain threshold leads to a reduction of the mean size of the clusters and this reduction is greater for greater fluences. After a long treatment with the same fluence the effect saturates. The final mean size of the clusters after saturation depends only on the laser fluence and not on the initial mean cluster size. When lower laser fluences were used it was possible after laser annealing to return the mean cluster size to its initial value by thermal annealing. In this way by using a combination of laser treatment and thermal annealing a predetermined mean cluster size could be achieved. The mechanism of laser-induced cluster-size modification is discussed. PACS 81.07.-b; 42.62.-b; 36.40.Qv     

Flexible Polymer–Organic Solar Cells Based on P3HT:PCBM Bulk Heterojunction Active Layer Constructed under Environmental Conditions

In this study, some crucial parameters were determined of flexible polymer–organic solar cells prepared from an active layer blend of poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) mixed in 1:1 mass ratio and deposited from chlorobenzene solution by spin-coating on poly(ethylene terephthalate) (PET)/ITO substrates. Additionally, the positive effect of an electron transport layer (ETL) prepared from zinc oxide nanoparticles (ZnO np) on flexible photovoltaic elements’ performance and stability was investigated. Test devices with above normal architecture and silver back electrodes deposed by magnetron sputtering were constructed under environmental conditions. They were characterized by current-voltage (I–V) measurements, quantum efficiency, impedance spectroscopy, surface morphology, and time–degradation experiments. The control over morphology of active layer thin film was achieved by post-deposition thermal treatment at temperatures of 110–120 °C, which led to optimization of device morphology and electrical parameters. The impedance spectroscopy results of flexible photovoltaic elements were fitted using two R||CPE circuits in series. Polymer–organic solar cells prepared on plastic substrates showed comparable current–voltage characteristics and structural properties but need further device stability improvement according to traditionally constructed cells on glass substrates.     

Spectroscopic study of the decay of negative ions formed through electron interaction with metastable magnesium atoms

The formation of negative magnesium atoms through electron interaction with metastable ions in the energy region below 2 eV was recorded. The radiative mechanism of their decay is discussed. The energy dependences of the cross sections for formation and decay of negative magnesium ions are presented.     

Optical extinction resonance of Au and Ag clusters formed by ion irradiation in SiO2 and Al2O3

Three methods based on ion-beam irradiation were used to fabricate Ag and Au colloids in silica and alumina. Their surface-plasmon resonance in the visible was characterised by transmittance measurements and interpreted on the basis of transmission electron microscope observations. Despite their bimodal size distribution, particles formed by ion-beam mixing of sandwich layers exhibit much narrower resonances than those obtained by ion implantation. This unusual effect of an inhomogeneity in cluster size is ascribed to the spatial organisation of these clusters. Irradiation of supersaturated solid solutions at much lower ion fluences produces colloids with more uniform size and spatial distributions, and equally strong resonances. Received: 17 March 2001 / Accepted: 31 July 2001 / Published online: 11 February 2002     

Depth dependence of photoluminescence and chemical bonding in porous silicon

Porous silicon (PS) is studied by stepwise peeling of the surface layer to clarify the non-uniformity in the photoluminescence (PL) and correlate it with the in-depth chemical bonding and structure of the 30 μm thick layer. The PL intensity grows by an order of magnitude after the peeling off of the first 10 μm and decreases five times in the next 5 μm while the peak maximum position shifts from 730 to 800 nm. X-ray photoelectron spectroscopy (XPS) measurements show that Si–Si and Si–O bonds are present both on the surface and below, and the preferential oxidation state of silicon changes from 3+ and 4+ on the surface to 1+ and 2+ below 10 μm. Using Raman spectroscopy silicon nanocrystals are shown to exist. Their mean size can be estimated at about 3 nm. These results show that the strongest PL comes from a region in the PS layer where silicon nanocrystallites are surrounded by oxides with a low level of oxidation and not from the strongly oxidized surface layer.     

Ion Beam Induced Conversion of Si-Based Polymers and Gels Layers into Ceramics Coatings

The transformations induced by ion irradiation in films of polycarbosilane and gels derived from tetraethoxysilane, methyltriethoxysilane and triethoxysilane have been studied. Hydrogen is released selectively under the effect of electronic excitations at a rate depending on the probability of H radicals to combine for forming molecules inside the film. The radiolysis efficiency of C—H bonds is the lowest and that of Si—H bonds the highest. Beside the conversion of the polymeric structure into an amorphous ceramic, ion irradiation induces the precipitation of C or Si clusters, according to the precursor, leading to a noticeable luminescence in the visible. This precipitation has been put into evidence for the first time by energy filtered electron microscopy.     

Optical,film surface and photovoltaic properties of PTB7-Fx-based polymer-organic solar cells prepared in ambient conditions

Bulk heterojunction organic solar cells based on active layer of PTB7-Fx (x?=?0–100), conjugated block copolymer incorporating benzo[1,2-b:4,5-b′]dithiophene and partly fluorinated thieno[3,4-b]thiophene units, and fullerene derivatives as phenyl C₆₁ and phenyl C₇₁ butyric acid methyl esters sandwiched between a transparent anode of indium tin oxide, a hole conducting layer of either poly(ethylene dioxythiophene) : polystyrene sulfonate or magnetron-sputtered molybdenum oxide and evaporated aluminum cathodes were fabricated. Polymer-organic thin films were prepared at 1:1.5 mass ratio of the donor:acceptor mixture and deposited from dichlorobenzene solution of different concentrations by spin coating in ambient conditions. To control the active layer morphology, the films were subjected either to post-deposition treatments (annealing) at different temperatures or incorporation of an additive such as diiodooctane. Optical transmission, film surface topography and thickness measurements were used to characterize the active layer. Test devices with the architecture described above were prepared and their current–voltage characteristics, quantum efficiency and impedance spectra were measured and used to compare the different active layers, architectures and deposition sequences.