Adsorption Configuration for Cysteine on Ge(100): Coverage-Dependent Surface Reorientation

Abstract

The adsorption structure and surface reorientation of cysteine molecules on a Ge(100) surface were studied using high-resolution core-level photoemission spectroscopy and low-energy electron diffraction (LEED) to track the variation in adsorption structure as a function of cysteine coverage. Analysis of the S 2p, C 1s, N 1s, and O 1s core-level spectra revealed quite different adsorption behaviors as a function of cysteine coverage. At 0.4 ML (below half a monolayer), a single S 2p peak and a single N 1s peak were observed, consistent with an adsorption structure that contained one type of thiol conformation and a neutrally charged amino moiety. At 0.60 ML (over half a monolayer), two S 2p peaks emerged with a binding energy difference of 0.91 eV, indicative of two types of thiols, and two N 1s peaks were observed, which were consistent with the presence of both neutrally charged NH2 and positively charged NH2+ moieties. The relative populations of the two thiols induced a structural change in the ordering, from a 2 x 1 to a l x I of reorientation of the cysteine molecule adsorbed on the Ge(100) surface. At a higher coverage (over 1.0 ML), the LEED pattern became diffuse, and a configurational change of the molecule resulted from protonation of the NH2 group. We systematically elucidated the evolution of cysteine adsorption structures on Ge(100) surfaces as a function of coverage density.