Sébastien Gauthier

STM study of the nucleation and annealing of ion bombardment induced defects on Cu(100)

The morphology of the (100) face of copper after sputtering with 600 eV Ar+ ions has been investigated by scanning tunneling microscopy (STM) as a function of ion beam flux and fluence. This process generates vacancy and adatom islands bounded by monatomic steps. These islands exhibit their equilibrium shape, which can be described as a square with rounded corners. At low flux, vacancy as well as adatom islands coexist and the removal mode is three-dimensional, whereas at high flux, adatom islands disappear and the removal mode becomes two-dimensional, that is layer-by-layer. Moreover, the observation of distinct vacancy or adatom islands depleted zones in the vicinity of pre-existing monatomic steps demonstrates that the kinetics of adsorption of vacancies or adatoms on a step depends on the side by which they reach the edge. The energy barriers which control these processes also affect the interlayer mass transport. These observations can be related in a consistent way if the height of these barriers is assumed to decrease for steps bounding small-sized islands.

Self-Assembly of Fivefold-Symmetric Molecules on a Threefold-Symmetric Surface†

Buckybowls: The adsorption of penta-tert-butylcorannulene, a molecule with fivefold symmetry, on Cu(111), a surface with threefold symmetry, is investigated by scanning tunneling microscopy complemented by structure calculations. The symmetry mismatch is resolved by the formation of threefold-symmetric subunits consisting of three molecules, which combine with single molecules to form a nearly perfect filling of the plane (see picture).

An STM study of the adsorption of Pb on Cu(100): formation of an ordered surface alloy

Abstract The adsorption of Pb on Cu(100) from 0 to 1 ML was investigated by UHV scanning tunneling microscopy. We obtained atomic resolution images of the different superstructures which appear at 300 K with increasing coverage (c(4 × 4), c(2 × 2) and c( √2)R45°). We confirm recent results and propose, partly on the basis of low temperature studies, new arguments in favour of an incorporation of lead atoms in the surface layer of copper for low coverage. We demonstrate that the c(4 × 4) superstructure corresponds to an ordered surface alloy of Pb 3 Cu 4 composition, by investigating separately the alloying and de-alloying transitions. De-alloying occurs during the first-order transition between the c(4 × 4) and c(2 × 2) superstructures.

An NC-AFM and KPFM study of the adsorption of a triphenylene derivative on KBr(001)

Summary The adsorption on KBr(001) of a specially designed molecule, consisting of a flat aromatic triphenylene core equipped with six flexible propyl chains ending with polar cyano groups, is investigated by using atomic force microscopy in the noncontact mode (NC-AFM) coupled to Kelvin probe force microscopy (KPFM) in ultrahigh vacuum at room temperature. Two types of monolayers are identified, one in which the molecules lie flat on the surface (MLh) and another in which they stand approximately upright (MLv). The Kelvin voltage on these two structures is negatively shifted relative to that of the clean KBr surface, revealing the presence of surface dipoles with a component pointing along the normal to the surface. These findings are interpreted with the help of numerical simulations. It is shown that the surface–molecule interaction is dominated by the electrostatic interaction of the cyano groups with the K+ ions of the substrate. The molecule is strongly adsorbed in the MLh structure with an adsorption energy of 1.8 eV. In the MLv layer, the molecules form π-stacked rows aligned along the polar directions of the KBr surface. In these rows, the molecules are less strongly bound to the substrate, but the structure is stabilized by the strong intermolecular interaction due to π-stacking.

Co‐regulation of yeast purine and phosphate pathways in response to adenylic nucleotide variations

Adenylate kinase (Adk1p) is a pivotal enzyme in both energetic and adenylic nucleotide metabolisms. In this paper, using a transcriptomic analysis, we show that the lack of Adk1p strongly induced expression of the PHO and ADE genes involved in phosphate utilization and AMP de novo biosynthesis respectively. Isolation and characterization of adk1 point mutants affecting PHO5 expression revealed that all these mutations also severely affected Adk1p catalytic activity, as well as PHO84 and ADE1 transcription. Furthermore, overexpression of distantly related enzymes such as human adenylate kinase or yeast UMP kinase was sufficient to restore regulation. These results demonstrate that adenylate kinase catalytic activity is critical for proper regulation of the PHO and ADE pathways. We also establish that adk1 deletion and purine limitation have similar effects on both adenylic nucleotide pool and PHO84 or ADE17 expression. Finally, we show that, in the adk1 mutant, upregulation of ADE1 depends on synthesis of the previously described effector(s) (S)AICAR ((N‐succinyl)‐5‐aminoimidazol‐4‐carboxamide ribotide), while upregulation of PHO84 necessitates the Spl2p positive regulator. This work reveals that adenylic nucleotide availability is a key signal used by yeast to co‐ordinate phosphate utilization and purine synthesis.

A virtual dynamic atomic force microscope for image calculations

Calculations of frequency modulation-atomic force microscopy (FM-AFM) images are presented. A virtual FM-AFM, which realistically simulates the experiment by including the control system of the microscope, is implemented in order to go beyond the usual static approximation. It is shown that the results obtained within the static approach can be recovered in the limit of small scanning speed, while images at realistic scanning speed are distorted. The influence of the experimental noise on the images is investigated, allowing us to evaluate the sensitivity of the instrument.

Nanostenciling for fabrication and interconnection of nanopatterns and microelectrodes

Stencil lithography is used for patterning and connecting nanostructures with metallic microelectrodes in ultrahigh vacuum. Microelectrodes are fabricated by static stencil deposition through a thin silicon nitride membrane. Arbitrary nanoscale patterns are then deposited at a predefined position relative to the microelectrodes, using as a movable stencil mask an atomic force microscopy (AFM) cantilever in which apertures have been drilled by focused ion beam. Large scale AFM imaging, combined with the use of a high precision positioning table, allows inspecting the microelectrodes and positioning the nanoscale pattern with accuracy better than 100nm.

Dynamics of high index step equilibrium fluctuations as observed by scanning tunneling microscopy

We present experimental evidence for a dramatic influence on the aspect of tunneling images of the orientation relative to a [1110] direction of the steps on vicinal copper surfaces. A model is proposed which relates these observations to the mobility of geometrical kinks of the step edges. This model allows one to deduce from the experimental data the mean emission time of an adatom from a kink on a step oriented near the [1110] direction on Cu(100).

Step-step interactions on copper vicinal surfaces

Abstract STM images of clean vicinal copper surfaces are presented. The order of magnitude of the diffusion coefficient deduced from these images show that they reflect the structure of the surface in equilibrium at room temperature, on a length-scale of tens of nm. The correlation function along step edges and the terrace-width distribution are evaluated and used to estimate the strength of step-step interactions.

Synthesis, photovoltaic performances and TD-DFT modeling of push–pull diacetylide platinum complexes in TiO2 based dye-sensitized solar cells

In this joint experimental-theoretical work, we present the synthesis and optical and electrochemical characterization of five new bis-acetylide platinum complex dyes end capped with diphenylpyranylidene moieties, as well as their performances in dye-sensitized solar cells (DSCs). Theoretical calculations relying on Time-Dependent Density Functional Theory (TD-DFT) and a range-separated hybrid show a very good match with experimental data and allow us to quantify the charge-transfer character of each compound. The photoconversion efficiency obtained reaches 4.7% for 8e (see TOC Graphic) with the tri-thiophene segment, which is among the highest efficiencies reported for platinum complexes in DSCs.