F. Sellam

Flat Lying Pin-Stripe Phase of Decanethiol Self-Assembled Monolayers on Au(111)

Self-assembled monolayers with low molecular surface density of decanethiol on Au(111) have been prepared by annealing of the densely packed (√3 × √3)R30° structure. High-resolution scanning tunneling microscope (STM) imaging reveals the formation of a 11.5 × √3 stripe structure where the molecules are aligned along the substrate surface in double lamellae. We can deduce from the images that the molecules bind to specific sites at the Au(111) surface rather than adsorb as didecane disulfides. For the first time, a lattice structure for an alkanethiol stripe phase can be derived from the obtained real-space data.

Direct observation of organic–organic heteroepitaxy: perylene-tetracarboxylic-dianhydride on hexa-peri-benzocoronene on highly ordered pyrolytic graphite

Abstract We investigate the heteroepitaxial growth of two films of large planar molecules on an inorganic substrate: for the particular system [perylene-tetracarboxylic-dianhydride (PTCDA) on hexa-peri-benzocoronene (HBC) on highly ordered pyrolytic graphite (HOPG)], large domains of several hundred of nanaometers in size with interfaces that are flat and almost defect-free on the molecular scale are observed using scanning tunneling microscopy (STM). The images obtained represent, to the best of our knowledge, the first STM observation of the ordered growth of large planar molecules in a heteroepitaxially stacked film. The mutual lattice alignment and epitaxial relations between the two organic layers are determined from the pronounced moire pattern in the STM images. Beside these findings, the intramolecular contrast of HBC on HOPG is used to determine the molecular orientation of the molecules within the two-dimensional unit cell by STM.

Annealed decanethiol monolayers on Au(111) – intermediate phases between structures with high and low molecular surface density

Abstract Self-assembled monolayers of decanethiol on Au(111) have been investigated using high resolution scanning tunneling microscopy (STM). After the annealing of the as-adsorbed densely packed c(2 3 ×4 3 )R30° structure and before the formation of a previously described low density (11.5× 3 ) stripe phase three different phases with intermediate molecular surface density have been observed. We present highly resolved and distortion-corrected STM images for both the (7.5× 3 ) stripe phase and the meshlike structure. We describe a mixed (7.5/11.5× 3 ) stripe phase that shows a switching of the stripe arrangement.

Investigation of sensor characteristics of chemical semiconductor sensors with impedance spectroscopy

Abstract Impedance spectroscopy has been performed with EICS ( e lectrolyte/ i on c onductor/ s emiconductor) sensor structures using the fluoride-sensitive Si/LaF 3 system. LaF 3 bulk and interface properties have been determined. The blocked interface between the silicon substrate and the lanthanum fluoride thin film governs the electrical impedance behavior of the whole sensor structure in the high frequency range. This is important for capacitance-voltage measurements because the C - V curve depends in size and steepness on the quality of the Si/LaF 3 interface. Contamination free Si surfaces have been obtained using the MBE process. Epitaxial LaF 3 films of good quality were fabricated but they failed in fluoride-ion sensitivity.