Intermolecular interaction and arrangements of adsorbed 1,4-cyclohexadiene molecules on Si(1 0 0)(2×1)

The adsorbed states of 1,4-cyclohexadiene molecules on Si(1 0 0)(2 × 1) at 300 K have been investigated as a function of coverage in real space by means of scanning tunneling microscopy (STM). Two kinds of intermolecular interaction have been studied (i) between the neighboring molecules on the neighboring dimer rows and (ii) between the neighboring molecules on the same dimer row. By analyzing the observed arrangements of adsorbed molecules, we have concluded that the intermolecular interaction between the molecules on the neighboring dimer rows is very small. On the other hand, the intermolecular interaction between the neighboring molecules on the same dimer row becomes repulsive when the distance between two adsorbates is shorter than 2a (a=3.85 إ: the distance between the Si dimers on the same row). Various types of arrangements are discussed, based on the steric repulsion. The proposed models can explain the alternate height variations in consecutive molecular protrusions in STM images and the saturation coverage of 0.6 molecules per Si dimer at 300 K.