Scanning tunneling microscopy and spectroscopy of rubrene on clean and graphene-covered metal surfaces.

Karl Rothe,Jörg Kröger,Nicolas Néel,Alexander Mehler

doi:10.3762/bjnano.11.100

Abstract

Rubrene (C42H28) was adsorbed with submonolayer coverage on Pt(111), Au(111), and graphene-covered Pt(111). Adsorption phases and vibronic properties of C42H28 consistently reflect the progressive reduction of the molecule–substrate hybridization. Separate C42H28 clusters are observed on Pt(111) as well as broad molecular resonances. On Au(111) and graphene-covered Pt(111) compact molecular islands with similar unit cells of the superstructure characterize the adsorption phase. The highest occupied molecular orbital of C42H28 on Au(111) exhibits weak vibronic progression while unoccupied molecular resonances appear with a broad line shape. In contrast, vibronic subbands are present for both frontier orbitals of C42H28 on graphene. They are due to different molecular vibrational quanta with distinct Huang–Rhys factors.

Highlights

Two-dimensional materials are emerging as monatomically thin buffer layers (BLs) on metal surfaces, i.e., as intermediate films that efficiently reduce the hybridization of an adsorbate with the metallic substrate
The molecular superstructure on graphene is similar to the assembly on Au(111), albeit with a lower molecule surface density, and dI/dV data exhibit vibronic progression in both frontier orbitals, which reflects the effective separation of C42H28 from the metal surface
The close-up view in Figure 2b shows that C42H28 molecules exhibit a submolecular structure consisting of two bright lobes separated by a central line with dim contrast

Summary

Introduction

Two-dimensional materials are emerging as monatomically thin buffer layers (BLs) on metal surfaces, i.e., as intermediate films that efficiently reduce the hybridization of an adsorbate with the metallic substrate. In these studies molecular orbitals, the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO), appear with spectroscopic fine structure in differential conductance (dI/dV, I: tunneling current, V: bias voltage) data, which is assigned to vibronic progression induced by a single group of molecular vibrations with similar quantum energies. On Au(111) the observations are compatible with a reduced hybridization since compact molecular islands with a regular superstructure form and vibronic progression of the HOMO dI/dV spectroscopic signature is visible.

Results

Conclusion