Strukturuntersuchung organischer Moleküle auf Au(111)-Oberflächen

The present work covers two topics namely the coadsorption of formic acid and water on Au(111) and the structure of biphenylalkanthiole SAMs on Au(111) surfaces. The coadsorption of formic acid and water on Au(111) surfaces has been investigated by means of vibrational and photoelectron spectroscopy (HREELS, XPS). Formic acid adsorbs at 90 K molecularly with vibrational modes characteristic for flat lying zig-zag chains in the mono- and multilayer regime, like in solid formic acid. The structure of the flat lying formic acid chains was determined by low energy electron diffraction (LEED) as a (2r3 x r19) unit cell. Annealing results in a complete desorption at 190 K. Sequential adsorption of formic acid and water at 90 K shows no significant chemical interaction. Upon annealing the coadsorbed layer to 140 K a hydrogenbonded cyclic complex of formic acid with one water molecule could be identified using isotopically labelled adsorbates. Upon further annealing this complex decomposes leaving molecularly adsorbed formic acid on the surface at 160 K, accompanied by a proton exchange between formic acid and water. The influence of the alkane spacer chain length on the structure of biphenylalkanethiols on Au(111) surfaces was investigated as well. A systematic study was done on BPn-SAMs deposited from the gas phase. For every chain length a structure was found by LEED. Furthermore the influence of temperature on the structure was investigated in the range from room temperature up to about 400 K. To obviate influences from different preparation methods BP3 and BP4 was deposited from gas phase and from solution. No LEED spots were observed on BP4 SAMs deposited from solution. For BP3 an influence of the preparation could be excluded. For all BPn-SAMs a good agreement between LEED and STM data’s was found. Nevertheless different unit cells were determined by LEED and STM consistent structures could be suggested considering the unit cell size given by LEED and the molecular packing given by STM. With these informations the area per molecule could be calculated. In addition complementary information’s given by the different contrast mechanism from LEED and STM could be worked out and included into the given structure models. Based on the assumption that the STM contrast is governed by the orientation of the terminal methyl group and the LEED structure is given by equivalent adsorbed molecules showing the same orientation of the biphenyl rings structures were given taking all the information’s into account. Comparing the occupied area per molecule it was found that BPn-SAMs with an odd number of methyl groups in the alkane chain make more densly packed structures as expected from the odd even effect. Only BP3 and BP5 showed the same structure. For the even numbered BPn SAMs the observed structures varied with the alkane chain length. From the big unit cell observed for BP2 SAMs it was concluded that the stress caused by the misfit of the preferred adsorption sites from the thiole on the Au surface and the required space from the biphenyl rings was compensated in the alkane chain. Only for BP4 SAMs different phases were found. The different packing densities predicted by the structure models for the different BP4 phases could be proved by XPS measurements