Al4SiC4 w\"urtzite crystal: structural, optoelectronic, elastic and piezoelectric properties

New experimental results supported by theoretical analyses are proposed for aluminum silicon carbide (Al4SiC4). A state of the art implementation of the Density Functional Theory is used to analyze the experimental crystal structure, the Born charges, the elastic and piezoelectric properties. The Born charge tensor is correlated to the local bonding environment for each atom. The electronic band structure is computed including self-consistent many-body corrections. Al4SiC4 material properties are compared to other wide band gap W\"urtzite materials. From a comparison between an ellipsometry study of the optical properties and theoretical results, we conclude that the Al4SiC4 material has indirect and direct band gap energies of about 2.5eV and 3.2 eV respectively.Comment: 10 pages, 4 figure

Vibrational properties of SrCu2O2 studied via Density Functional Theory calculations and compared to Raman and infrared spectroscopy measurements

International audienceThe SrCu2O2 material is a p-type transparent conductive oxide. A theoretical study of the SrCu2O2 crystal is performed with a state of the art implementation of the Density Functional Theory. The simulated crystal structure is compared with available X-ray diffraction data and previous theoretical modeling. Density Functional Perturbation Theory is used to study the vibrational properties of the SrCu2O2 crystal. A symmetry analysis of the optical phonon eigenvectors at the Brillouin zone center is proposed. The Raman spectra simulated using the derivatives of the dielectric susceptibility, show a good agreement with Raman scattering experimental results

Isotope Exchange Raman Spectroscopy (IERS): a novel technique to probe physicochemical processes inin situsitu

We have developed a novel in situ methodology for the direct study of mass transport properties in oxides with spatial and unprecedented time resolution, based on Raman spectroscopy coupled to isothermal isotope exchanges. Changes in the isotope concentration, resulting in a Raman frequency shift, can be followed in real time, not accessible by conventional methods, enabling complementary insights for the study of ion transport properties of electrode and electrolyte materials for advanced solid-state electrochemical devices. The proof of concept and strengths of isotope exchange Raman spectroscopy (IERS) are demonstrated by studying the oxygen isotope back-exchange in gadolinium-doped ceria (CGO) thin films. Resulting oxygen self-diffusion and surface exchange coefficients are compared to conventional time-of-flight secondary ion mass spectrometry (ToF-SIMS) characterisation and literature values, showing good agreement, while at the same time providing additional insight, challenging established assumptions. IERS captivates through its rapidity, simple setup, non-destructive nature, cost effectiveness and versatile fields of application and thus can readily be integrated as new standard tool for in situ and operando characterization in many laboratories worldwide. The applicability of this method is expected to consolidate our understanding of elementary physicochemical processes and impact various emerging fields including solid oxide cells, battery research and beyond

Anisotropy of thermal expansion in YAlO3 and NdGaO3

3 pagesInternational audienceYAlO3 and NdGaO3 thermal expansion coefficients were measured using in-situ powder X-ray diffraction in the temperature range 28-650°C. They exhibit a clear anisotropy: the expansion, quite similar along the [100] and [001] directions, is much lower along the [010] direction. The formation of cracks observed in YBa2Cu3O7-d thin films deposited on YAlO3 and NdGaO3 substrates is likely related to the anisotropy. Stress value calculations have been performed in both systems. They indicate the intrinsic components are specifically high in YAlO3

TiO2-Ag° plasmonic sensor for the SERS detection of organic molecules

National audienc

Annealing of thin Zr films on Si1-xGex (0≤x≤1): X-ray diffraction and Raman studies

12 pagesInternational audienceX-ray diffraction experiments have been combined to Raman scattering and transmission electron microscopy data to analyze the result of rapid thermal annealing applied to Zr films, 16 or 80 nm thick, sputtered on Si1-xGex epilayers (0≤x≤1). The C49 Zr(Si1-xGex)2 is the unique phase obtained after complete reaction. ZrSi1-xGex is formed as intermediate phase. The C49 formation temperature Tf is lowered by the addition of Ge in the structure. Above a critical Ge composition close to x=0.33, a film microstructure change was observed. Films annealed at temperatures close to Tf are continuous and relaxed. Annealing at T>Tf leads to discontinuous films: surface roughening resulting from SiGe diffusion at film grain boundaries occurred. Grains are ultimately partially embedded in a SiGe matrix. A reduction in the lattice parameters as well as a shift of Raman lines are observed as T exceeds Tf. Both Ge non-stoichiometry and residual stress have been considered as possible origins of these changes. However, as Ge segregation has never been detected, even by using very efficient techniques, it is thought that the changes originate merely from residual stress. The C49 grains are expected to be strained under the SiGe matrix effect and the shift of the Raman lines would indicate the stress is compressive. Some simple evaluations of the stress values indicate that it varies between -0.3 and -3.5 GPa for 0≤x≤1 which corresponds to a strain in the range [-0.11, -1.15 %]. X-ray and Raman determinations are in good agreement

Effects of Polyethylenimine and Its Molecular Weight on the Chemical Bath Deposition of ZnO Nanowires

The addition of polyethylenimine (PEI) in the standard chemical bath deposition (CBD) of ZnO nanowires has received an increasing interest for monitoring their aspect ratio, but the physicochemical processes at work are still under debate. To address this issue, the effects of PEI are disentangled from the effects of ammonia and investigated over a broad range of molecular weight (i.e., chain length) and concentration, varying from 1300 to 750 000 and from 1.5 to 10 mM, respectively. It is shown that the addition of PEI strongly favors the elongation of ZnO nanowires by suppressing the homogeneous growth at the benefit of the heterogeneous growth as well as by changing the supersaturation level through a pH modification. PEI is further found to inhibit the development of the sidewalls of ZnO nanowires by adsorbing on their nonpolar m-planes, as supported by Raman scattering analysis. The inhibition proceeds even in the low pH range, which somehow rules out the present involvement of electrostatic interactions as the dominant mechanism for the adsorption. Furthermore, it is revealed that PEI drastically affects the nucleation process of ZnO nanowires on the polycrystalline ZnO seed layer by presumably adsorbing on the nanoparticles oriented with the m-planes parallel to the surface, reducing in turn their nucleation rate as well as inducing a significant vertical misalignment. These findings, specifically showing the effects of the PEI molecular weight and concentration, cast light onto its multiple roles in the CBD of ZnO nanowires

Thermal expansion and stress development in the first stages of silicidation in Ti/Si thin films

8 pagesInternational audienceThe structural evolution in fiber-textured Ti/Si thin films has been investigated by in-situ X-ray diffraction, in a temperature range preceding the formation of silicide compounds. At low temperature, thermoelastic behavior of the metallic film was observed. Abnormal thermal behavior of both 002 and 101 diffraction profiles was observed at 360°C and 450°C, which could be understood in terms of Si diffusion, first in Ti grain boundaries, then, into the grains. By combining ex-situ strain studies with analysis of the Si local environment in the whole Ti silicide family from Ti5Si3 to TiSi2, it was possible to determine the stress-free lattice parameters of annealed films. They are significantly higher than the bulk parameters, and indicate that nearly 4.5 at. % Si is present in the Ti grains

Thermal annealing of amorphous Ti-Si-O thin films

International audienceTi-Si-O thin films were deposited using an aerosol chemical vapor deposition process at atmospheric pressure. The film structure and microstructure were analysed using several techniques before and after thermal annealing. Diffraction results indicate that the films remain X-ray amorphous after annealing whereas Fourier transform infrared spectroscopy gives evidence of a phase segregation between amorphous SiO2 and well crystallized anatase TiO2. Crystallization of anatase TiO2 is also clearly shown in the Raman spectra. Transmission electron microscopy analysis indicates that anatase TiO2 nanograins are embedded in a SiO2 matrix in an alternated SiO2/TiO2 multilayer structure

Growth and characterization of Sr-doped Cu 2 O thin films deposited by metalorganic chemical vapor deposition

International audienc