Oscar Grizzi

Self-assembly of alkanedithiols on Au(111) from solution: effect of chain length and self-assembly conditions.

A comparative study on the adsorption of buthanedithiol (BDT), hexanedithiol (HDT), and nonanedithiol (NDT) on Au(111) from ethanolic and n-hexane solutions and two different preparation procedures is presented. SAM characterization is based on reflection-absorption infrared spectroscopy, electrochemistry, X-ray photoelectron spectroscopy, and time of flight direct recoil spectroscopy. Results indicate that one can obtain a standing-up phase of dithiols and that the amount of the precursor lying-down phase decreases from BDT to NDT, irrespective of the solvent and self-assembly conditions. A good ordering of the hydrocarbon chains in the standing-up configuration is observed for HDT and NDT when the system is prepared in degassed n-hexane with all operations carried out in the dark. Disulfide bridges at the free SH terminal groups are formed for HDT and to a lesser extent for NDT prepared in ethanol in the presence of oxygen, but we found no evidence of ordered multilayer formation in our experiments. No disulfides were observed for BDT that only forms the lying-down phase. Our results demonstrate the key role of the chain length and the procedure (solvent nature and oxygen presence) in controlling the surface structure and chemistry of SAMs dithiols on Au(111).

Positive ion neutralisation on chlorine covered silver surfaces

We report a study of the effect of chlorine adsorption on the interaction of positive ions with the silver surfaces. Neutralisation rates of He+ were measured and an ion scattering spectroscopy (ISS) study of Cl covered Ag(1 1 1) was performed. The Cl ISS peak remains vanishingly small during the Cl chemisorption submonolayer phase and then rapidly increases above 1.5 L. The neutralisation rate of He+ is found to increase during the first phase and then decrease above 1.5 L, an effect that we attribute to AgCl island formation. This points out a chemical environment dependance of the Cl detection with ISS due to changes in particular quasi-resonant neutralisation rates involving Cl(3s) core levels. In the case of H+, Ne+ and Ar+ neutralisation on Ag(1 1 1) (1 1 0) and (1 0 0) a striking oscillation of the neutralisation rate is found for the three surfaces at low coverages. A first minimum is reached around 0.5 to 2/3 of the full coverage and a maximum is then attained at full coverage. Angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and metastable deexcitation spectroscopy (MDS) measurements allow one to describe these features in terms of the modifications in Auger neutralisation (AN) rates due to changes of the density of states as adsorption changes from smaller to higher packing of adsorbed Cl. Beyond the saturation limit the neutralisation rate is found to decrease again on Ag(1 1 1) but remains constant on Ag(1 1 0) and Ag(1 0 0). This is ascribed to the fact that, in the exposure range reported, AgCl formation occurs only on the (1 1 1) face. On the two other faces an ordered chemisorption phase remains stable until higher coverages.

Assembly and thermal stability of thin EP-PTCDI films on Ag(111)

We report a multi-technique study of the adsorption kinetics and self-assembly characteristics of EP-PTCDI grown on Ag(111) at UHV conditions and room temperature. Changes in the valence band characteristics for the mono and the multilayer film and the stability as a function of the annealing temperature are discussed. The results show that the molecules start to adsorb on the step edges forming ordered islands that grow to fully cover a monolayer. Further exposure results in the stacking of similarly ordered islands of several layers. The desorption experiment shows that the film is stable up to 150 degrees C where a rapid desorption of the multilayer takes place, followed by a decomposition of the molecules for temperatures higher than 170 degrees C.