Adsorption geometry and bonding state of thiophene on Ge(100)

Abstract

We have studied the adsorption geometry and the bonding state of the thiophene molecule on the Ge(100) surface using hybrid density functional theory (DFT) calculations, scanning tunneling microscopy (STM), and high resolution core-level photoemission spectroscopy (HRPES). STM images show that thiophene molecules preferentially form one-dimensional molecular chains on Ge(100) under 0.25 ML at room temperature. At high coverage (over 0.25 ML), the thermodynamically stable features are additionally observed between adjacent molecular chains. In HRPES study, we observe three bonding states; a weakly bound state, a chemisorption state, and a decomposition state, change systematically depending on the molecular coverage and the annealing temperature. From the findings, we demonstrate under 0.25 ML, thiophene molecules adsorb on Ge(100) by the Lewis acid-base reaction. At high coverage, over 0.25 ML, additional thiophene molecules are adsorbed through the [4+2] cycloaddition reaction. Temperature dependent behaviors of thiophene on Ge(100) suggest that the dative bondig thiophene desorb followed by the [4+2] cycloaddition reaction product as the molecular thiophene or decompose to form metallocyclic compounds and sulfur atoms.