Failure of Perturbative DFT-Derived STM Images of Organic Molecules on Semiconductor Surfaces

摘要

Tersoff-Hamann and Bardeen perturbation theory derived empty-state STM images of the [4 + 2] intradimer adduct of 1,3-cyclohexadiene (CHD) adsorbed on Si(100)-2 x 1 using a density functional description of the electronic structure appear to capture the π~* C=C orbital observed experimentally but ultimately produce images dominated by the CH2 groups. Neither averages over finite temperature structures nor use of W(110) tips with O or Si adsorbed on the apex yield the correct image. Strong tip-CHD interactions substantially lower the energy of the π~* orbital relative to the Fermi energy and change the CHD geometry. In addition to perturbing the C=C bond, it is found that the tip pushes aside the CH2 groups. The former electronic effect enhances the prominence of the π~* orbital, while the latter geometric effect suppresses contributions from the remainder of the CHD.