DYNAMICS AND STRUCTURE OF SELF-ASSEMBLED ORGANIC-MOLECULES AT THE SOLID-LIQUID INTERFACE

We have analysed scanning tunnelling microscope (STM) images of self-a ssembled didodecylbenzene (DDB) molecules physisorbed on graphite from a DDB solution using octylbenzene as solvent. The DDB images were obt ained alternating with images of the graphite substrate using two diff erent bias voltages. The well-known lattice constant of the graphite s ubstrate was used for an accurate determination of the calibration fac tors and drift of the STM. The DDB unit cells were detected and measur ed in the Fourier domain and corrected by the calibration data. The co mmensurability between the graphite lattice and the DDB lattice was an alysed. Although the graphite lattice and the DDB lattice planes are p arallel, incommensurability was observed as superstructures in the Fou rier pattern. The molecules dioctadecyldiselenide and dioctadecyldisul phide also form well-ordered monolayers when adsorbed on graphite from solution. Dioctadecyldiselenide and dioctadecyldisulphide form ideal mixtures. It is therefore of interest to determine to what extent the molecules form mixed structures when physisorbed on the graphite subst rate. Theoretical and experimental observations show that S and Se ato ms yield markedly different contrasts in the STM images. When imaging mixed surface layers, the dynamic adsorption and dissolution of indivi dual molecules can be followed. A time scale of around 1 s was found f or this process. The ratio between fractions of adsorbed molecules of different species was found to be very similar to the ratio in solutio n, indicating a small difference in bonding energy.