Jean-Claude Vial

The role of fluctuations and stress on the effective viscosity of cell aggregates

Cell aggregates are a tool for in vitro studies of morphogenesis, cancer invasion, and tissue engineering. They respond to mechanical forces as a complex rather than simple liquid. To change an aggregates shape, cells have to overcome energy barriers. If cell shape fluctuations are active enough, the aggregate spontaneously relaxes stresses (“fluctuation-induced flow”). If not, changing the aggregates shape requires a sufficiently large applied stress (“stress-induced flow”). To capture this distinction, we develop a mechanical model of aggregates based on their cellular structure. At stress lower than a characteristic stress τ*, the aggregate as a whole flows with an apparent viscosity η*, and at higher stress it is a shear-thinning fluid. An increasing cell–cell tension results in a higher η* (and thus a slower stress relaxation time tc). Our constitutive equation fits experiments of aggregate shape relaxation after compression or decompression in which irreversibility can be measured; we find tc of the order of 5 h for F9 cell lines. Predictions also match numerical simulations of cell geometry and fluctuations. We discuss the deviations from liquid behavior, the possible overestimation of surface tension in parallel-plate compression measurements, and the role of measurement duration.

Fourier transform IR monitoring of porous silicon passivation during post-treatments such as anodic oxidation and contact with organic solvents

Abstract The surface passivation of porous silicon is a determining factor in the emission efficiency of the material. The hydrogen surface coverage has been shown to provide very efficient passivation. In this work, we have performed Fourier transform IR (FTIR) measurements to monitor the Si-H surface coverage, which is readily obtained after the layer formation in HF, during different post-treatments (anodic oxidation and contact with organic solvent) and to relate it to the emission efficiency. FTIR studies, performed at different steps of the electrochemical oxidation, indicate that, during the anodic treatment, the hydrogen surface coverage is preserved and that the oxidation takes place on the back bonds of the surface silicon atoms. The importance of the hydrogen coverage is also shown by the analysis of porous layers treated in boiling CCl 4 after formation. This treatment provokes the desorption of the hydrogen atoms and results in a drastic decrease in the photoluminescence. When samples are immersed in boiling methanol after formation, FTIR analyses show that there is also a partial loss of the hydrogen coverage, but accompanied with an oxidation of the material, so that no significant changes in the emission efficiency can be observed.

Blue light electroluminescent devices based on a copolymer derived from fluorene and carbazole

Abstract Thin layers of poly(dihexylfluorene-co-N-hexylcarbazole) (poly(DHF-co-NHK) sandwiched between indium tin oxide (ITO) and aluminium electrodes show stable photoluminescence (PL) and electroluminescence (EL) at room temperature and ambient atmosphere. I(V) and EL vs. voltage curves present a much lower threshold than device made up of poly(DHF) homopolymer. The behaviour of both materials has been quantitatively investigated through a Fowler–Nordheim analysis allowing the determination of the energy offsets between the Fermi level of the ITO electrode and the HOMO of polymers. When using the copolymer, the barrier height for hole injection is reduced to a value close to 0.17 eV compared to 0.83 eV for the homopolymer. This difference in offsets has been confirmed by electrochemical investigations.

Fabrication and optical properties of Si/CaF2(111) multi‐quantum wells

We have synthesized, by molecular beam epitaxy, Si/CaF2(111) multi‐quantum wells which are photoluminescent at room temperature after ageing in air. In this article, we report on the structural properties and on a detailed optical study of these heterostructures. The photoluminescence spectra for various confinements and the temperature dependence of the lifetimes as a function of emission wavelength are described in comparison with the corresponding characteristics of porous silicon and hydrogenated amorphous silicon. A model based on quantum confinement is proposed to explain the experimental data.

Tunneling dynamics of delocalized protons in benzoic acid dimers: A study of the temperature dependence by time and frequency domain optical spectroscopy

Abstract The translational tunneling of the acid protons in benzoic acid dimers coupled to a dye molecule is investigated in single crystals and at liquid helium temperatures. The optical transitions of the dye are used to monitor the dynamics of protons delocalized over the two wells of the double-minimum potential, which governs their motion. Using transient grating as well as optical hole-burning techniques in protonated and deuterated crystals the population relaxation rate and its temperature dependence is determined between 1.35 and 4.2 K. It is found that this rate is immeasurably small at the lowest temperatures (⩽ 107 s −1 ) As the temperature is raised the rate increases very rapidly and attains at 4.2 K a value of 2.5 × 10 9 s −1 . This value contrasts with the rates 2 × 10 −8 −4 × 10 8 s −1 , measured previously in systems where the protons of the benzoic acid dimers are localized by the larger environment induced asymmetry of the double-well potential. These results demonstrate the importance of multi-(two-)photon processes in the relaxation of delocalized protons which dominate the one-phonon processes at higher temperatures.

Blue-green light-emitting diodes and electrochemical cells based on a copolymer derived from fluorene

Abstract A copolymer derived from fluorene has been synthesised using a fluorene monomer functionalised with poly(ethylene oxide) (PEO)-like segments as comonomer in a poly(dihexylfluorene) main chain. Efficient blue-green polymer light-emitting diodes (LEDs) based on this material and working under ambient atmosphere are reported. When using ITO as anode and aluminium as cathode, operating voltage at around 25 V are required to obtain emission intensity of 1 μW cm−2. By decreasing the film thickness from 250 to 110 nm and using calcium instead of aluminium, the corresponding operating voltage can be reduced by approximately 50% without significant loss of luminescence efficiency. Light-emitting electrochemical cells (LECs) are also demonstrated, leading to threshold operating voltages close to the electrochemical gap of poly(fluorene).

Carrier localization in porous silicon investigated by time‐resolved luminescence analysis

We analyzed the photoluminescence (PL) mechanisms of porous silicon, and in particular, the origin of the PL high quantum efficiency (QE) at room temperature. For this we used postformation treatments, anodic oxidation, and hydrofluoric acid (HF) etching (known for their strong QE enhancement effect) correlated with a PL time resolved analysis. A third parameter was the temperature which, for heating above room temperature, gave a reversible quenching of the PL. All three parameters give a similar evolution of the PL decay shape, which we consider to originate from the same evolution of the carrier dynamics. Porous silicon is described as an undulating wire. The high QE at room temperature is attributed to carrier localization inside minima of the fluctuating potential along the wire; these considerations are extended to another porous material: amorphous porous silicon. Anodic oxidation and HF dissolution diminish the wire size, giving a reduction of the localization length of the carriers and progressiv...

Energy transfer in dye impregnated porous silicon

Two kinds of porous silicon layers (fresh and oxidized) were impregnated with laser dyes solutions before drying. An original technique of diffuse reflectance allowed us to give an estimation of the concentration of the dye molecules in the pores, which appears to be higher for oxidized samples than for fresh ones. Selective excitation of the red photoluminescence of the silicon crystallites with a pulsed ultraviolet laser was performed and it showed a rapid luminescence of the laser dye. This emission is more important for fresh samples than for oxidized ones, although the dye concentration is lower in fresh samples. This indicates that the luminescence of the dye molecules does not come from direct excitation of the laser but that it is provided by an energy transfer from the porous silicon crystallites. In as far as the coupling between the crystallite and the dye molecule theoretically decreases when the distance between them increases, a lowering of the transfer on oxidized samples for whom the distance is larger is expected and is experimentally observed.

Photoluminescence and electroluminescence from electrochemically oxidized porous silicon layers

Abstract In this paper we review the luminescence properties of porous silicon layers formed on p-type silicon substrates and subsequently oxidized by anodic polarization in an aqueous electrolyte. The electrochemical oxidation of the porous material leads to a large increase in the photoluminescence intensity, accompanied by a blue shift of the emitted spectra. A bright visible electroluminescence is also observed during anodic treatment, with characteristics showing similar trends to that of the photoluminescence. The features of the emission are analyzed using a model that expresses the energy dependence of the emitted intensity. The model is developed on the hypothesis that the visible light emission originates in the confinement of charge carriers in the quantum-sized crystallites which form the material, and that its efficiency is determined by nonradiative processes, which involve the carrier escape from the confined zone where they are created (or injected) through a tunneling mechanism. This model is shown to be well supported by the experimental results, and allows an understanding of the spectral shifts and the intensity variations of both photoluminescence and electroluminescence during electrochemical oxidation of the porous layers.

Optical measurements of methyl group tunneling in molecular crystals: Temperature dependence of the nuclear spin conversion rate

The tunneling methyl groups in dimethyl‐s‐tetrazine (DMST) doped single crystals of durene were investigated by high resolution optical spectroscopy using spectral hole burning. The experiments probe the level structure as well as the relaxation dynamics of the tunneling methyl groups in different electronic states of DMST. The tunneling splitting differs by 1.24 GHz in the ground and the first excited singlet states of DMST. In the ground electronic state, relaxation (spin conversion) between the spin 3/2 (A) and 1/2 (E) tunneling levels was measured between 1.5 and 12 K. The spin conversion time is larger than 100 h at 1.5 K and decreases with Arrhenius‐type behavior above 3.5 K. The activation energy of 20 cm−1 also is observed as a phonon sideband in emission, and is, in agreement with theoretical predictions, tentatively assigned to a librational mode of the methyl group.