Epitaxial growth and properties of lead-free ferroelectric Na0.5Bi0.5TiO3 thin films grown by pulsed laser deposition on various single crystal substrates

International audienceThe epitaxial growth of lead-free ferroelectric Na0.5Bi0.5TiO3 (NBT) thin films on various single crystal substrates was successfully achieved, using the pulsed laser deposition technique (PLD). The present work is divided in two parts, focused on: (i) the growth of NBT layers on c- and r-sapphire (Al2O3) substrates, with and without introducing a CeO2 buffer layer, and (ii) the growth of NBT layers on bare (001)SrTiO3 substrates, with and without introducing a LaNiO3 layer, that could be used as a bottom electrode. In the first part, it was shown that the introduction of a CeO2 buffer layer completely modifies the out-of-plane growth orientation of the NBT films, as well as their microstructure. Indeed, (001)NBT films epitaxially grow only on r-Al2O3 substrates buffered with epitaxial (001)CeO2 layers, while, growing simply NBT on top of bare c or r-Al2O3 substrates, or on top of CeO2/c-Al2O3 heterostructures leads to polycrystalline or textured films. In the second part, we demonstrate that (001)-oriented NBT layers deposited on either bare (001)SrTiO3 or (001)SrTiO3 substrates (STO) covered with (001)LaNiO3 (LNO) are systematically epitaxially grown. Furthermore, the microstructure of the samples is strongly affected by the introduction of the LaNiO3 layer

Electronic and physico-chemical properties of nanmetric boron delta-doped diamond structures

Heavily boron doped diamond epilayers with thicknesses ranging from 40 to less than 2 nm and buried between nominally undoped thicker layers have been grown in two different reactors. Two types of [100]-oriented single crystal diamond substrates were used after being characterized by X-ray white beam topography. The chemical composition and thickness of these so-called deltadoped structures have been studied by secondary ion mass spectrometry, transmission electron microscopy, and spectroscopic ellipsometry. Temperature-dependent Hall effect and four probe resistivity measurements have been performed on mesa-patterned Hall bars. The temperature dependence of the hole sheet carrier density and mobility has been investigated over a broad temperature range (6K<T<450 K). Depending on the sample, metallic or non-metallic behavior was observed. A hopping conduction mechanism with an anomalous hopping exponent was detected in the non-metallic samples. All metallic delta-doped layers exhibited the same mobility value, around 3.660.8 cm2/Vs, independently of the layer thickness and the substrate type. Comparison with previously published data and theoretical calculations showed that scattering by ionized impurities explained only partially this low common value. None of the delta-layers showed any sign of confinement-induced mobility enhancement, even for thicknesses lower than 2 nm.14 page

Positioning Europe for the EPITRANSCRIPTOMICS challenge

The genetic alphabet consists of the four letters: C, A, G, and T in DNA and C,A,G, and U in RNA. Triplets of these four letters jointly encode 20 different amino acids out of which proteins of all organisms are built. This system is universal and is found in all kingdoms of life. However, bases in DNA and RNA can be chemically modified. In DNA, around 10 different modifications are known, and those have been studied intensively over the past 20 years. Scientific studies on DNA modifications and proteins that recognize them gave rise to the large field of epigenetic and epigenomic research. The outcome of this intense research field is the discovery that development, ageing, and stem-cell dependent regeneration but also several diseases including cancer are largely controlled by the epigenetic state of cells. Consequently, this research has already led to the first FDA approved drugs that exploit the gained knowledge to combat disease. In recent years, the ~150 modifications found in RNA have come to the focus of intense research. Here we provide a perspective on necessary and expected developments in the fast expanding area of RNA modifications, termed epitranscriptomics.SCOPUS: no.jinfo:eu-repo/semantics/publishe

Synthesis, Resolution, and Absolute Configuration of Chiral Tris(2-pyridylmethyl)amine-Based Hemicryptophane Molecular Cages

International audienceThe synthesis, characterization, and chiroptical properties of a new class of hemicryptophane cages combining a cyclotriveratrylene unit and a tris(2-pyridylmethyl)amine (TPA) moiety are reported. Changing the linkers between these two units allows for the modification of the size and shape of the cavity. The synthesis is straightforward and efficient, providing gram-scale of cage compounds. The racemic mixture of each hemicryptophane host can be readily resolved by chiral HPLC, giving an easy access to the enantiopure molecular cages of which absolute configurations have been assigned by ECD spectroscopy. These new hemicryptophanes are available chemical platforms ready to use for various purposes due to the versatile metal complexation properties of the TPA unit. A Zn(II)@hemicryptophane complex has been obtained and used as a heteroditopic host for the selective recognition of zwitterionic guests

Optical properties of an epitaxial Na0.5 Bi0.5 TiO3 thin film grown by laser ablation: Experimental approach and density functional theory calculations

International audienceThis study reports on the determination of the optical constants of a lead-free ferroelectric material, Na0.5Bi0.5TiO3. The optical transmission measurements were carried out in the 300–3000 nm wavelength range, on a 100-oriented Na0.5Bi0.5TiO3 thin film, epitaxially grown by laser ablation on a 100MgO single crystal substrate. Applying the “method of the envelopes,” developed by Manifacier et al. J. Phys. E 9, 1002 1976 and by Swanepoel J. Phys. E 16, 1214 1983, the analysis of the observed interference fringes allowed extracting some optical data for Na0.5Bi0.5TiO3, namely the linear refractive and extinction indices n and k, the absorption coefficient , and as well the nature direct or indirect transition and value of the optical band gap. It was found that n=2.321 and the transmission data suggested a 3.30 eV indirect transition. Our experimental results are thus in opposition to the few data reported so far for Na0.5Bi0.5TiO3, where a direct transition was evoked. Therefore, we have confronted the optical transmission data to supplementary diffuse reflectance and ellipsometry measurements, and finally, to first principles calculations. The diffuse reflectance data, collected for Na0.5Bi0.5TiO3 powders, indicate a 3.26 eV optical band gap. In addition, the ellipsometry measurements reveal a refractive index of 2.346 at 2.066 m, an energy gap of 3.20 eV, and also confirm the thickness of the layer. These additional data are then in very good agreement with the data derived from the optical transmission measurements. Finally, first principles calculations were carried out in the framework of the density functional theory for the three Na0.5Bi0.5TiO3 polymorphs i.e., rhombohedral, tetragonal, and cubic. For each polymorph, in order to consider the Na/Bi disorder on the perovskite A-site, three different Na/Bi distributions within the unit cell were investigated, leading to nine atomic configurations. The electronic structure of these nine configurations was then calculated after geometry optimization and only one of them did not converge. These calculations complete the above experimental data, as six electronic structures over eight presented an indirect band gap, with a band gap energy Eg falling in the 3.90–4.60 eV energy range. The latter value is higher than the experimental ones but is definitely more compatible with the energy band gap expected for ferroelectric oxide materials

Structural observation of piezoelectric inhomogeneity in a mixed-orientation Na0.5Bi0.5TiO3 perovskite thin film

International audienceThin films of the lead-free ferroelectric Na0.5Bi0.5TiO3 grown on thin-film Pt electrodes supported by SrTiO3 substrates have a complex microstructure consisting of crystalline grains with three distinct major crystallographic orientations. The piezoelectric response measured in spatially separated sub-micron grains using time-resolved synchrotron x-ray microdiffraction is highly inhomogeneous even among grains sharing the same major orientation. The piezoelectric coefficient d33 varies by nearly a factor of two in a series of areas sharing the 〈001〉 orientation. The piezoelectric inhomogeneity is linked to the peculiar microstructure of the film, arising from local variations in the stress imposed by surrounding grains with different crystallographic orientations and differing directions of the ferroelectric remnant polarization. A systematic nonlinearity of the piezoelectric strain is observed in applied electric fields with small magnitudes in all regions, consistent with the coexistence of domains of differing polarization direction at zero applied electric field

Improved hemicryptophane hosts for the stereoselective recognition of glucopyranosides

International audienceFour new enantiomerically and diastereomerically pure hemicryptophane hosts (M-SSS-2/P-SSS-2 and M-RRR-2/P-RRR-2 pairs) were designed for the recognition of sugar derivatives. Their absolute configuration was determined from the circular dichroism spectra and DFT calculations. The host molecules were then used for the stereoselective recognition of glucopyranosides. Binding constants were obtained from 1H NMR titration experiments showing an increase of affinity for this class of receptors, associated with an improved diastereo- and enantio-differentiation