We have analysed scanning tunnelling microscopc (STM) images of self-assembled didodecylbenzene (DDB) molecules physisorbed on graphite from a DDB solution using octylbenzene as solvent. The DDB images were obtained alternating with images of the graphite substrate using two different bias voltages. The well-known lattice constant of the graphite substrate was used for an accurate determination of the calibration factors and drift of the STM. The DDB unit cells were detected and measured in the Fourier domain and corrected by the calibration data. The commensurability between the graphite lattice and the DDB lattice was analysed. Although the graphite lattice and the DDB lattice planes are parallel, incommensurability was observed as superstructures in the Fourier pattern. The molecules dioctadecyldiselenide and dioctadecyldisulphide 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 substrate. Theoretical and experimental observations show that S and Se atoms yield markedly different contrasts in the STM images. When imaging mixed surface layers, the dynamic adsorption and dissolution of individual molecules can be followed. A time scale of around 1 s was found for this process. The ratio between fractions of adsorbed molecules of different species was found to be very similar to the ratio in solution, indicating a small difference in bonding energy.
Citation: Surface and Coatings Technology
Pub Type: Journals