A scanning tunneling microscopy study of distyrylbenzene on Ag/Ge(111)-(sqr rt of 3 x sqr rt of 3)R30 degrees

The adsorption and self-organized monolayers of trans,trans-distyrylbenzene (tt-DSB) and cis,cis-distyrylbenzene (cc-DSB) on Ag/Ge(111)-(sqr rt of 3 x sqr rt of 3)R30 degrees (Ag/Ge(111)-sqr rt of 3) were studied by low-temperature scanning tunneling microscopy (STM) in ultrahigh vacuum. tt-DSB and cc-DSB overlayers were prepared by vapor deposition at a substrate temperature of 200 K and imaged after the samples were cooled to 100 K. High-resolution images allow identification of the internal structure of individual tt-DSB molecules with three phenyl rings and their molecular arrangements on the Ag/Ge(111)-sqr rt of 3 surface. It is found that the intermolecular distance between two terminal phenyl rings in tt-DSB is about twice the lattice constant of Ag/Ge(111)-sqr rt of 3. Such a lattice match makes Ag/Ge(111)-sqr rt of 3 an ideal substrate for tt-DSB self-organization and the formation of a (3 x 1) overlayer unit cell. The structural model and the molecule registry corresponding to STM images for the adlayers of tt-DSB on Ag/Ge(111)-sqr rt of 3 are proposed and discussed. For cc-DSB adsorption on Ag/Ge(111)-sqr rt of 3, uniform molecular overlayers with two discernible molecular images corresponding to two major types of cc-DSB conformers were observed. The coexistence of multiple conformers and the mismatch of molecular dimension of cc-DSB with the substrate unit cell length limit the growth of large cc-DSB domains.