Growth of silicon thin film by LPE on porous silicon bilayers

The fabrication of solar cells based on the transfer of a thin silicon film on a foreign substrate is an attractive way to realise cheap and efficient photovoltaic devices. The aim of this work is to realise a thin mono-crystalline silicon film on a double porous silicon layer in order to detach and transfer it on mullite. The first step is the fabrication of a double porous silicon layer by electrochemical anodisation using two different current densities. The low current leads to a low porosity layer and during annealing, the recrystallisation of this layer allows epitaxial growth. The second current leads to a high porosity which permits the transfer on to a low cost substrate. Liquid Phase Epitaxy (LPE) performed with indium (or In+Ga) in the temperature range of 950–1050°C leads to almost homogeneous layers. Growth rate is about 0.35 μm min⁻¹. Crystallinity of the grown epilayer is similar on porous silicon and on single crystal silicon. In this paper, we focus on the realisation of porous silicon sacrificial layer and subsequent LPE growth. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ligand trans-effect: using an old concept as a novel approach to bis(dipolar) NLO-phores

In plane parallel arrangement and enhancement of NLO-activity are observed upon coordination of heteroditopic dipoles containing a phosphole ring on square-planar d8-palladium centre.     

Growth of anthracene thin films by matrix-assisted pulsed-laser evaporation

Thin films of anthracene (C₁₄H₁₀) have been grown by matrix-assisted pulsed-laser evaporation. Frozen solutions of 2 wt% anthracene (solute) in ortho-xylene or in chloroform were used as targets and were irradiated by a frequency-quadrupled Nd:YAG laser (266 nm). The deposited films were studied by the complementary use of infrared absorption and photoluminescence emission. The vibrational bands and fluorescence emission of bulk anthracene were clearly identified in the films. This means that the organic molecules were not modified by photochemical reactions during the laser irradiations, despite the strong absorption of anthracene at 266 nm. Among the growth parameters, the laser fluence was found to be critical in order to avoid the fragmentation of the anthracene molecules. The results show that matrix-assisted pulsed-laser evaporation is suitable for growing thin films of organic materials even if they absorb at the irradiating wavelength. PACS 81.15.Fg; 78.55.Kz; 33.20.Ea; 71.20.Rv     

Optimization of Experimental Parameters to Explore Small‐Ligand/Aptamer Interactions through Use of 1H NMR Spectroscopy and Molecular Modeling

Aptamers constitute an emerging class of molecules designed and selected to recognize any given target that ranges from small compounds to large biomolecules, and even cells. However, the underlying physicochemical principles that govern the ligand‐binding process still have to be clarified. A major issue when dealing with short oligonucleotides is their intrinsic flexibility that renders their active conformation highly sensitive to experimental conditions. To overcome this problem and determine the best experimental parameters, an approach based on the design‐of‐experiments methodology has been developed. Here, the focus is on DNA aptamers that possess high specificity and affinity for small molecules, L ‐tyrosinamide, and adenosine monophosphate. Factors such as buffer, pH value, ionic strength, Mg²⁺‐ion concentration, and ligand/aptamer ratio have been considered to find the optimal experimental conditions. It was then possible to gain new insight into the conformational features of the two ligands by using ligand‐observed NMR spectroscopic techniques and molecular mechanics.     

Photoluminescence Study of Functionalized Polysiloxane Gel

Photoluminescence of a polysiloxane gel film incorporating a partially conjugated copolymer polyether-polyphenylenevinylene (PEPPV) has been studied and compared with results obtained from bulk and vacuum evaporated films of the same starting material. The evaporation clearly affects the distribution of lengths of the conjugated parts, yet the incorporation in the gel matrix induces only slight changes in the spectroscopic properties of the polymer.     

Influence of LPE process technological conditions on Si ELO layers morphology

Epitaxial lateral overgrowth (ELO) is a suitable method which makes it possible to obtain thin layers for applications in the thin film solar cells technology. Due to the method it is possible to save the material which is used in production to make PV modules more economical. Moreover, an ELO layer is partly separated from the growth substrate by an SiO₂ cover which prevents propagation of defects into the ELO layer from the substrate. It means that even poor quality silicon substrates can be used to fabricate good quality solar cells. Growing a continuous thin silicon layer on a specially prepared growth silicon substrate is the first step to obtain photovoltaic (PV) modules. The morphology as well as the layer quality and the density of defects depend on various factors: growth temperature, cooling rate, growth time, atmosphere, substrate type, etc. This work presents an analysis of silicon ELO layers growth in different conditions using a horizontal LPE setup. The results can be used to determine the best conditions of growth in order to obtain optimal Si layers for PV applications.     

Synthesis and electronic properties of alternating alpha,alpha'-thiophene-phosphole oligomers

[structure: see text] Alternating thiophene-phosphole oligomers up to seven rings in length have been synthesized. A regular decrease of the HOMO-LUMO gap is observed upon increasing the length of the pi-conjugated system. The nature of the P-moieties dramatically influences the optical and electrochemical properties of these phosphole-based oligomers. Electro-oxidation of derivative 5a afforded an electroactive polymer exhibiting reversible p-doping.     

Connecting pi-chromophores by sigma-P-P bonds: new type of assemblies exhibiting sigma-pi-conjugation

To study the ability of sigma-P-P skeleton to mediate interaction between pi-chromophores, 1,1'-biphospholes bearing phenyl or thienyl substituents at the 2,2' and 5,5'-position have been prepared and studied. These air-stable derivatives are readily available via a "one-pot" synthesis starting from diynes. Theoretical studies and UV-vis data clearly establish that the two pi-systems interact via the sigma-P-P bridge. This through-bond interaction results in a lowering of the optical HOMO-LUMO gap of the assemblies. The nucleophilic sigma(3)-P centers of these 1,1'-biphospholes allow chemical modifications of the sigma-bridge. These modifications offer further tuning of the optical properties of the assembly. Electrooxidation of the thienyl-substituted 1,1'-biphosphole results in electroactive materials characterized by low optical band gap and reversible p-doping.     

Rational Design of a Redox‐Labeled Chiral Target for an Enantioselective Aptamer‐Based Electrochemical Binding Assay

A series of redox‐labeled L ‐tyrosinamide (L ‐Tym) derivatives was prepared and the nature of the functional group and the chain length of the spacer were systematically varied in a step‐by‐step affinity optimization process of the tracer for the L ‐Tym aptamer. The choice of the labeling position on L ‐Tym proved to be crucial for the molecular recognition event, which could be monitored by cyclic voltammetry and is based on the different diffusion rates of free and bound targets in solution. From this screening approach an efficient electroactive tracer emerged. Comparable dissociation constants K_d were obtained for the unlabeled and labeled targets in direct or competitive binding assays. The enantiomeric tracer was prepared and its enantioselective recognition by the corresponding anti‐D ‐Tym aptamer was demonstrated. The access to both enantiomeric tracer molecules opens the door for the development of one‐pot determination of the enantiomeric excess when using different labels with well‐separated redox potentials for each enantiomer.