Formule de Lewis | EPFL Graph Search

En chimie, une structure de Lewis est une représentation en deux dimensions de la structure électronique externe des atomes composant une molécule. Inventée par Gilbert Lewis, elle se base sur la topologie de la molécule (connexion entre les atomes par des liaisons covalentes). Elle concerne les atomes du groupe principal. Définition La structure de Lewis consiste à définir la localisation des électrons sur ou entre les atomes de la molécule. Seuls les électrons de valence sont considérés. On obtient ainsi une certaine vision de la structure électronique de la molécule par ses doublets libres, ses doublets liants (liaisons σ et π), ses lacunes et ses éventuels électrons célibataires (dans le cas des radicaux). Un code de représentation Dans cette représentation, les électrons célibataires sont notés par des points et les paires d'électrons par des traits (plus rarement par deux points). Les traits peuvent être localisés sur un atome (doublet libre ou non lia

The Cas A-Like Snr 1E 0102.2-7219 In The Small Magellanic Cloud: An Asymmetric Bipolar Explosion

Frédéric Vogt

We have used the Wide Field Spectrograph on the 2.3 m telescope at Siding Spring Observatory to map the [O III] lambda 5007 dynamics of the young oxygen-rich supernova remnant 1E 0102.2-7219 in the Small Magellanic Cloud. From the resultant data cube, we have been able to reconstruct the full three-dimensional structure of the system of [O III] filaments. From this we are able to deduce that the ejecta trace an asymmetric bipolar structure. The redshifted gas contains fainter clumps but with higher velocities than the blueshifted gas. The major axis of the explosion is inclined at similar to 40 degrees to the line of sight. This structure shows that the supernova explosion has been influenced both by the rotation of the progenitor star and, presumably, by a global rotational instability following core collapse.

2010

Lattice Energy as Emerging General Parameter Describing Ionic Organic Semiconducting Materials Properties

Donatas Gesevicius

Cyanine dyes represent a class of ionic functional materials used by a broad heterogeneous science community contain-ing biologists, chemists, physicists and materials scientists. Their diverse application opportunities are ranging from fluorescence markers, nonlinear optics, CD-R data storage, photography and organic electronics. Therefore the development of cyanine dyes and their applications in chemistry, engineering, medicine and pharmacology is growing continuously. A typical leitmotif of a cyanine dye contains two nitrogen heterocyclic rings joined by a conjugated chain of carbon atoms. Due to the synthetic approach, quaternary salt precursors are required resulting in ionic compounds. Such chromophores are positively charged and require a counter anion to maintain the overall charge neutrality. Several reports were presenting certain anions as efficiency enhancers in cyanine dye based organic electronic devices. Despite their long presence in science, there is a lack of knowledge behind the mechanisms causing these efficiency enhancements. Furthermore, in the field of organic electronics, the processability of cyanine dyes towards electronic devices was restricted to solution-based methods. In the present thesis work the three main challenges in cyanine dye research, namely: efficient ion exchange procedures, mechanisms behind organic electronics figures of merit enhancement as well as the introduction of physical vapour deposition method have been addressed. An efficient halide-for-anion exchange method with the potential for industry scale up was introduced. This allowed modifying a cationic cyanine chromophore with organic or inorganic anions yielding six salts. The following investigations of these six anions by either varying organic moieties or negative charge delocalisation paved the way towards a generalized concept describing the semiconducting properties of this cationic cyanine chromophore. The lattice energy was established as a universal parameter describing cyanine salt semiconducting properties on a universal energy scale. Such decoupling of an ionic structure from its Lewis formula allows future predictions of semiconducting properties which can be widened to all organic ionic functional materials. Furthermore weakly coordinating anion influence on the volatility of a cyanine chromophore established physical vapour deposition as thin film fabrication method. Using co-evaporation of the cyanine dye and the fullerene C60 a suitable bulk heterojunction morphology was obtained. This resulted in the fabrication of the first fully vacuum deposited cyanine dye bulk heterojunction device.

EPFL2018

A 3D printed beam splitter for polar neutral molecules

Sean Dennis Steven Gordon, Andreas Osterwalder

We describe a beam splitter for polar neutral molecules. An electrostatic hexapole initially confines and guides a supersonic expansion of ammonia, and it then smoothly transforms into two bent quadrupole guides, thus splitting the molecular beam in two correlated fractions. This paves the way towards molecular beam experiments wherein one beam is modified through interactions with, e.g. a laser beam or another molecular beam, while the other one remains unmodified and serves as a reference. Because both beams originate from the same parent beam, such differential experiments can dramatically enhance the sensitivity. The highly complex electrode structure required for the beam splitter would be very difficult to build by traditional means. Instead, we introduce a new method of production: 3D printing of a plastic piece, followed by electroplating. The 3D printed piece can take any desired shape and, since the entire structure can be printed as a single piece, provides inherently precise alignment. Electroplating produces atomically smooth metal surfaces that allow the application of electric potentials. The simple and powerful fabrication method introduced here opens a plethora of new avenues for research, far beyond molecular beams experiments, since 3D printing imposes practically no limitations on possible shapes, and the metal-plating produces chemically robust, conductive construction elements. It has the added advantage of dramatically reduced production cost and time: all components used in the present study were printed within less than 48 hours; electroplating is completed in one day, and the bottle neck of the entire process was the shipping to and from the plating company.

2017

Lattice Energy as Emerging General Parameter Describing Ionic Organic Semiconducting Materials Properties

Donatas Gesevicius

EPFL2018