Ahmed Charaï

Transmission electron microscopy investigation of tin sub‐oxide nucleation upon SnO2 deposition on silicon

Transmission electron microscopy analysis of tin dioxide films grown by aerosol‐assisted chemical vapor deposition onto oxidized or etched silicon displayed the formation of a sub‐oxide phase that was identified as Sn2O3. Such a phase is observed to disappear upon heat treatment, and is believed to be one of the factors responsible for the instability of tin dioxide films used as gas sensing layers.

Heart rhythm characterization through induced physiological variables

Atrial fibrillation remains a major cause of morbi-mortality, making mass screening desirable and leading industry to actively develop devices devoted to automatic AF detection. Because there is a tendency toward mobile devices, there is a need for an accurate, rapid method for studying short inter-beat interval time series for real-time automatic medical monitoring. We report a new methodology to efficiently select highly discriminative variables between physiological states, here a normal sinus rhythm or atrial fibrillation. We generate induced variables using the first ten time derivatives of an RR interval time series and formally express a new multivariate metric quantifying their discriminative power to drive state variable selection. When combined with a simple classifier, this new methodology results in 99.9% classification accuracy for 1-min RR interval time series (n = 7,400), with heart rate accelerations and jerks being the most discriminant variables. We show that the RR interval time series can be drastically reduced from 60 s to 3 s, with a classification accuracy of 95.0%. We show that heart rhythm characterization is facilitated by induced variables using time derivatives, which is a generic methodology that is particularly suitable to real-time medical monitoring.

Origin of the superstructure elucidated by atomic resolution HAADF-STEM and HREM in the Ce10W22O81 lanthanide tungstate phase

The present paper provides new information on the attribution of the cationic sites of the orthorhombic Ce10W22O81 crystal phase prepared in the CeO2–Ce2O3–WO3 ternary system. Atomic resolution HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy) and HREM (high-resolution electron microscopy) investigations have highlighted the presence of two mixed columns of Ce and W cations along the a axis that were previously assigned to pure W cations in the asymmetric unit. This discovery explains the presence of a commensurate superstructure doubling the orthorhombic unit-cell length ao.

Toward a nanoimprinted nanoantenna to perform optical rectification through molecular diodes

This work presents investigations about the realization and modelization of rectenna solar cells. Rectennas are antennas coupled with a rectifier to convert the alternative current originating from the antenna into direct current that can be harvested and stored. By reducing the size of the antennas to the nanoscale, interactions with visible and near-infrared light become possible. If techniques such as nanoimprint lithography make possible the fabrication of sufficiently small plasmonic structures to act as optical antennas, the concept of rectenna still faces several challenges. One of the most critical point is to achieve rectification at optical frequencies. To address this matter, we propose to use molecular diodes (ferrocenyl-alkanethiol) that can be self-assembled on metallic surfaces such as gold or silver. In this paper, we present a basic rectenna theory as well as finite-difference time-domain (FDTD) optical simulations of plasmonic structures and experimental results of both nanoimprint fabrication of samples and characterizations by electron microscopy, Raman spectroscopy, and cyclic voltammetry techniques.

Molecular Diodes for Optical Rectennas

The photo conversion efficiencies of the 1st and 2nd generat ion photovoltaic solar cells are limited by the physical phenomena involved during the photo-conversion processes. An upper limit around 30% has been predicted for a monojunction silicon solar cell. In this work, we study 3rd generation solar cells named rectenna which could direct ly convert visible and infrared light into DC current. The rectenna technology is at odds with the actual photovoltaic technologies, since it is not based on the use of semi-conducting materials. We study a rectenna architecture consist ing of plasmonic nano-antennas associated with rectifying self assembled molecular diodes. We first opt imized the geometry of plasmonic nano-antennas using an FDTD method. The optimal antennas are then realized using a nano-imprint process and associated with self assembled molecular diodes in 11- ferrocenyl-undecanethiol. Finally, The I(V) characterist ics in darkness of the rectennas has been carried out using an STM. The molecular diodes exhibit averaged rect ification ratios of 5.

Superstructures In The Scheelite-Type Rare Earth Doped Tungstate Phases

The aim of the present work deals with the TEM study of scheelite related AWO4 compounds [1] showing modulated microstructures not revealed with synchrotron XRD investigations. These materials have potential applications in many fields such as photoluminescence, microwave, scintillator materials, humidity sensors and catalysis. Rare earth (RE) tungstate RE2(WO4)3 crystal phases are based on a cation-deficient superstructure of CaWO4. Cation substitution of RE are investigated in order to correlate the microstructure to physical properties especially for new potential applications in white-LEDs and lasers. In our study, a two-phase powder containing the monoclinic Ce2(WO4)3 and orthorhombic Ce10W22O81 structures [2], was obtained by complexing method using EDTA and citrate ions. In the latter phase, PED and HRTEM investigations showed a C2/c superspace group (SSG) doubling the smallest a cell parameter [3]. Additionally, another cerium tungstate structure substituted with strontium cations was also investigated revealing a (3+2)D incommensurately modulated structure.

Low kV High Resolution Scanning Electron Microscopy Study of Si Nanowire Surfaces

The characterization of small objects like nanometric gold nanocrystals on nanowire (NW) surface allowed us to highlight the improvements of new imaging detectors in SEM. We have employed different detectors for both conventional imaging signals (SE and BSE), coupled with analytical techniques. In previous work [3], saw-tooth faceting and non-homogeneous gold repartition have been characterized by coupling Scanning Transmission Electron Microscopy High Angle Annular Dark Field (STEM-HAADF), electron tomography and Energy-dispersive X-ray spectroscopy (EDS).