Discrimination of chiral adsorption configurations: styrene on germanium(100)

We investigated the adsorption geometries and desorption temperatures of styrene on a Ge surface using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and ab initio density functional theory (DFT) calculations. Our STM results show that styrene molecules attach via the vinyl group in two adsorption configurations: (i) on the top of single Ge dimers by forming di-σ bonds (OT) and (ii) in a paired end-bridge between two adjacent Ge dimers within the same dimer row (PEB). Moreover, the PEB configurations are divided into two types of adsorption configurations depending on the arrangement of the phenyl rings of the two styrene molecules: either the axis running through the two phenyl rings is diagonal to the dimer row direction (trans-PEB) or perpendicular to the dimer row direction (cis-PEB). STM images can discriminate the OT configurations with (S) and (R) chiralities, as well as the PEB configurations with diastereomeric (R,S) and enantiomeric (R,R) and (S,S) chiralities at the molecular level. The TPD measurements and DFT calculations show that the PEB configurations are thermodynamically more stable than the OT configurations because the chemisorbed styrene molecules desorb from Ge(100) at 320 K (OT) and at 365 K (PEB), respectively. Among the PEB configurations, STM results show that the trans-PEB configuration is more prevalent than the cis-PEB configuration at the initial coverage because of adsorbate-induced surface reconstruction. The cis- and trans-PEB configurations can induce phases with short-range order along the dimer row direction at high coverage of styrene.