Periodically Spaced CaF2_2 Semi-Insulating Thin Ribbons Growth Study on the Si(100) Surface

The use and the study of semi-insulating layers on metals and semiconductors surfaces have found continuous interest in the past decades. So far, the control of the sizes and growth location of the insulating islands on the substrate is either ill-defined or usually constrained to the use of evaporation masks which size can easily exceed tenth of nanometers. Here, we show that it is possible to grow self-organized periodically spaced thin ribbons of semi-insulating stripes on the bare Si(100) surface. The epitaxial growth of these structures is obtained by the evaporation of CaF$_2$ molecules on the silicon surface with a coverage of 1.2 monolayers. They are investigated via scanning tunneling techniques at low temperature (9K). The obtained ribbons exhibit a surface bandgap of ~3.2 eV as well as a resonant state at the central part of the ribbons at ~2.0 eV below the Fermi level energy. The use of the density functional theory allows suggesting a model structure of the observed ribbons and reproducing the experimental STM topographies. The formation of the thin ribbons is discussed and we point out the influence of the mechanical forces inside and between the structures that may influence their periodicity

Polarization properties of coherent VUV light at 125 nm generated by sum frequency four wave mixing in mercury

International audienceThe polarization of the VUV light generated by four-wave sum frequency mixing process w4 = 2w1 +w2 in mercury vapor at room temperature is analyzed in detail. Due to the specific two-photon transition used to enhance the nonlinear process, the polarization of the VUV wave is shown to be identical to the polarization of the wave at the frequency w2. In particular, circularly polarized VUV is observed with degree of circular polarization exceeding 0.99

Atomic-scale study of the adsorption of calcium fluoride on Si(100) at low-coverage regime

We investigate, experimentally and theoretically, the initial stage of the formation of Ca/Si and Si/F structures that occurs during the adsorption of CaF2 molecules onto a bare Si(100) surface heated to 1000 K in a low-coverage regime (0.3 monolayer). A low-temperature (5 K) scanning tunneling microscope (STM) is used to observe the topographies and the electronic properties of the exposed silicon surfaces. Our atomic-scale study reveals that several chemical reactions arise during CaF2 deposition, such as dissociation of the CaF2 molecules and etching of the surface silicon dimers. The experimental and calculated STM topographies are compared using the density functional theory, and this comparison enables us to identify two types of reacted structures on the Si(100) surface. The first type of observed complex surface structure consists of large islands formed with a semiperiodic sequence of 3 × 2 unit cells. The second one is made of isolated Ca adatoms adsorbed at specific sites on the Si(100)-2 × 1 surface.Peer reviewe

The PP2A inhibitor I2PP2A is essential for sister chromatid segregation in oocyte meiosis II.

Haploid gametes are generated through two consecutive meiotic divisions, with the segregation of chromosome pairs in meiosis I and sister chromatids in meiosis II. Separase-mediated stepwise removal of cohesion, first from chromosome arms and later from the centromere region, is a prerequisite for maintaining sister chromatids together until their separation in meiosis II [1]. In all model organisms, centromeric cohesin is protected from separase-dependent removal in meiosis I through the activity of PP2A-B56 phosphatase, which is recruited to centromeres by shugoshin/MEI-S332 (Sgo) [2-5]. How this protection of centromeric cohesin is removed in meiosis II is not entirely clear; we find that all the PP2A subunits remain colocalized with the cohesin subunit Rec8 at the centromere of metaphase II chromosomes. Here, we show that sister chromatid separation in oocytes depends on a PP2A inhibitor, namely I2PP2A. I2PP2A colocalizes with the PP2A enzyme at centromeres at metaphase II, independently of bipolar attachment. When I2PP2A is depleted, sister chromatids fail to segregate during meiosis II. Our findings demonstrate that in oocytes I2PP2A is essential for faithful sister chromatid segregation by mediating deprotection of centromeric cohesin in meiosis II

Manipulations électroniquement induites de molécules individuelles à la surface de semiconducteurs (vers les dispositifs bi-moléculaires)

L objectif de cette thèse est d explorer le contrôle de processus électroniquement induits dans diverses molécules fonctionnalisées adsorbées sur la surface du Si(100). Ce travail s inscrit dans le contexte des nanosciences moléculaires et a été réalisé à l aide d un microscope à effet tunnel (STM) à basse température (9K). Nous avons utilisé une approche combinant étude statistique et modélisation théorique afin de pouvoir explorer la physique des divers processus observés. Cette thèse débute par l étude de la molécule d hexaphényle benzène (HPB) dont les phényles latéraux permettent un découplage électronique entre la molécule et la surface du silicium. Grâce à cet effet, nous avons pu contrôler la diffusion directive et réversible de la molécule d HPB physisorbée le long des marches de type SA à la surface du Si(100) 2.1 à travers un processus combinant l action des électrons tunnels et celle du champ électrostatique induit par la pointe du STM. Ces premiers résultats ont permis d envisager l étude d un couple de molécules de tétraphényles porphyrines métalliques adsorbées à la surface du Si(100) 2.1. Il s agit de NiTPP et de CuTPP qui, comme pour l HPB, possèdent des cycles phényles latéraux. Plusieurs conformations d adsorption de ces deux molécules ont été caractérisées et leurs réponses à des excitations électroniques étudiées. Ceci nous a permis, pour la molécule de NiTPP, d aboutir au contrôle de l activation réversible d un bistable intra-moléculaire en dépit de la chimisorption partielle de la molécule sur le silicium. L étude de la molécule de CuTPP, quant à elle, montre des courbes de conductance I(V) en forme d hystérésis associées à des changements réversibles de conformations réalisant ainsi une fonction mémoire. Dès lors, nous avons pu étudier la co-adsorption des molécules de NiTPP et de CuTPP sur le Si(100) afin de réaliser un binôme moléculaire. Divers couples de molécules ont pu être étudiés. Sur l un d entre eux, nous avons pu activer des processus d excitations inter-moléculaires en excitant électroniquement l une des molécules afin d observer un changement de conformation de la seconde molécule du binôme. Ce résultat réalise ainsi le contrôle électronique d un dispositif bi-moléculaire en s affranchissant des processus électroniques induits via le substrat. Enfin, à titre de perspective, ce travail de thèse présente un procédé novateur permettant le contrôle local de l hydrogénation de la surface de Si(100). Ceci est réalisé grâce à la passivation de la pointe du STM par l hydrogène moléculaire à 9K. Les électrons tunnels sont ensuite utilisés pour induire la dissociation intra-dimer des molécules d H2 sur la surface du Si(100). Cette technique peut être envisagée pour la passivation du Si(100) ou pour agir localement sur des circuits moléculaires.The objective of this thesis is to explore the control of electronically induced processes in various functionalized molecules adsorbed on the surface of silicon (100). In the context of molecular nanoscience, this work has been carried out using a scanning tunneling microscope operating at low temperature (9K). We used an approach combining statistical study and theoretical modelling in order to explore the physics of the various observed processes. This thesis begins with the study of the Hexaphenylbenzene (HPB) molecule for which the lateral phenyl rings enable the molecule-silicon surface electronic decoupling. Thanks to this effect, we could achieve a directive and reversible diffusion control of physisorbed HPB molecules along the SA silicon step edge through a process combining the joint actions of tunnel electrons and the local STM tip induced electrostatic field. These first results allowed considering the study of a couple of metaltetraphenyl porphyrin molecules adsorbed on the Si(100)-2x1 surface. Similarly to the HPB molecules, the two chosen metalloporphyrins: NiTPP and CuTPP, have lateral phenyl rings. Several adsorption conformations for these molecules were characterized and their response to electronic excitation has been studied. In the case of NiTPP, this led to the control of the reversible activation of an intra-molecular bistable despite the partial chemisorption of the molecule on the silicon surface. As for CuTPP molecule, our study revealed hysteresis behavior on the I(V) conduction curves associated with reversible conformation changes which represents the realization of a memory function. Following the study of each molecule apart, we performed the co-adsorption of the two molecules on the Si(100) surface to study molecular pairs. Various pairs of molecules have been studied. On one of them, we were able to activate an inter-molecular excitation transfer process by locally exciting one molecule and observing a conformation change of the second molecule of the pair. This result thus shows the electronic control of a bi-molecular device getting rid of substrate mediated electronic process. Finally, as a perspective, this thesis presents a novel technique allowing the controlled local hydrogenation of the Si(100) surface. This is achieved thanks to the passivation of the STM tip by molecular hydrogen at 9K. The tunnel electrons are then used to induce the intra-dimer dissociative adsorption of H2 molecules on the Si(100) surface. This technique could be considered for the passivation of Si(100) or to locally modify molecular circuits.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Periodically Spaced CaF2 Semi-Insulating Thin Ribbons Growth Study on the Si(100) Surface

International audienceThe use and the study of semi-insulating layers on metals and semiconductors surfaces have found continuous interest in the past decades. So far, the control of the sizes and growth location of the insulating islands on the substrate is either ill-defined or usually constrained to the use of evaporation masks which size can easily exceed tenth of nanometers. Here, we show that it is possible to grow selforganized periodically spaced thin ribbons of semi-insulating stripes on the bare Si(100) surface. The epitaxial growth of these structures is obtained by the evaporation of CaF2 molecules on the silicon surface with a coverage of 1.2 monolayers. They are investigated via scanning tunneling techniques at low temperature (9K). The obtained ribbons exhibit a surface bandgap of ~3.2 eV as well as a resonant state at the central part of the ribbons at ~2.0 eV below the Fermi level energy. The use of the density functional theory allows suggesting a model structure of the observed ribbons and reproducing the experimental STM topographies. The formation of the thin ribbons is discussed and we point out the influence of the mechanical forces inside and between the structures that may influence their periodicity