The paradox of an insulating contact between a chemisorbed molecule and a wide band gap semiconductor surface

Abstract

Controlling the intrinsic optical and electronic properties of a single molecule adsorbed on a surface requires electronic decoupling of some molecular orbitals from the surface states. Scanning tunneling microscopy experiments and density functional theory calculations are used to study a perylene molecule derivative (DHH-PTCDI), adsorbed on the clean 3 × 3 reconstructed wide band gap silicon carbide surface (SiC(0001)-3 × 3). We find that the LUMO of the adsorbed molecule is invisible in I(V) spectra due to the absence of any surface or bulk states and that the HOMO has a very low saturation current in I(z) spectra. These results present a paradox that the molecular orbitals are electronically isolated from the surface of the wide band gap semiconductor even though strong chemical bonds are formed.

Non‐Local Atomic Manipulation on Semiconductor Surfaces in the STM: The Case of Chlorobenzene on Si(111)‐7×7

“…These features are assumed to be the LUMO and HOMO, respectively, of the chlorobenzene/Si(111)‐7×7 system. In view of previous experiments, we might expect the dangling bond states to disappear because of the covalent bond between adatom and molecule, but this seems not to be the case. This remaining adatom state might be due to the rehybridisation of the dangling bond state with the HOMO and LUMO of the molecule.…”

STM imagery and density functional calculations of C60fullerene adsorption on the 6H-SiC(0001)-3×3 surface

“…Charge transfer from the surface to the molecule is also observed for C 60 on a metal surface, 25 on Si(111), 34 and for other organic molecules on SiC. 36,37 In Fig. 5(a), our calculated density of states (DOS) of the adsorbed C 60 molecule in configuration (3) (orange spectrum) shows the existence of a peak at the Fermi level (arrow) compared to the free molecule (blue spectrum).…”

“…The (3) configuration is now the most stable ( − 1.57 eV) and the on-top (1) configuration the least ( − 1.22 eV). The adsorption energies for configurations (2) and (3) differ by 0.15 eV, which is small but not negligible, 36,37 because the differences between the sites (2) − (1) and (3) − (1) are only 0.21 and 0.36 eV, respectively. Using Grimme's approach, [46][47][48] the calculated vdW energy contribution is not small.…”

“…No experimental scanning tunneling spectroscopy is shown here, because I (V ) spectroscopy of molecules on the SiC surface is very difficult to perform and analyze, especially at low voltage close to the Fermi level due to the wide band gap of SiC substrate. 36 Inverse photoemission spectroscopy (IPES) experiments 52 reveal additional information confirming the chemisorption of C 60 on the SiC surface. Spectra taken as a function of temperature show that C 60 molecules desorb intact from the 3 × 3 surface in stark contrast to the molecular cage opening observed on the √ 3 × √ 3 SiC surface, or indeed other substrates such as Si(111) and Si(100).…”

“…Silicon carbide is a promising candidate for achieving electronic decoupling; with a wide indirect band gap of 3 eV, the SiC(0001)-3 × 3 surface reconstruction has no bulk electronic states over a broad energy range. Recent studies of the conjugated polyaromatic molecules H 2 Pc 35 and PTCDI 36,37 show that these molecules bind to the SiC(0001)-3 × 3 surface via a Diels-Alder cycloaddition reaction through the formation of two Si-N and Si-O bonds, respectively. However, the SiC(0001)-3 × 3 surface has three intrinsic surface states located within the bulk band gap.…”