Vibronic excitations in molecules are key to the fundamental understanding of the interaction between vibrational and electronic degrees of freedom. In order to probe the genuine vibronic properties of a molecule even after its adsorption on a surface appropriate buffer layers are of paramount importance. Here, vibrational progression in both molecular frontier orbitals is observed with submolecular resolution on a graphene-covered metal surface using scanning tunnelling spectroscopy. Accompanying calculations demonstrate that the vibrational modes that cause the orbital replica in the progression share the same symmetry as the electronic states they couple to. In addition, the vibrational progression is more pronounced for separated molecules than for molecules embedded in molecular assemblies. The entire vibronic spectra of these molecular species are moreover rigidly shifted with respect to each other. This work unravels intramolecular changes in the vibronic and electronic structure owing to the efficient reduction of the molecule-metal hybridization by graphene.

Hexagonal boron nitride (h-BN) was grown on Pt(111) and Ru(0001) to serve as a buffer layer for molecular adsorbates. Hydrocarbon lander molecule C64H36 does not exhibit preference for specific h-BN adsorption sites on Pt(111), while on Ru(0001), wire and pore sites of the two-dimensional mesh are favored. The spectroscopic signatures of C64H36 frontier orbitals show a strong dependence on the adsorption site. For h-BN on Pt(111), C64H36 frontier orbital energies exhibit a common shift that leaves the gap between the orbitals invariant and reflects local work function changes of the h-BN lattice the molecule is weakly coupled to. In contrast, h-BN on Ru(0001) leads to a nonuniform behavior of the frontier orbital energies, which is tentatively attributed to additional charge transfer processes between the molecule and the surface.

The efficiency of hexagonal boron nitride and graphene to separate the hydrocarbon molecule C64H36 from Ru(0001) and Pt(111) surfaces is explored in low-temperature scanning tunneling microscopy and spectroscopy experiments. Both 2D materials enable the observation of the Franck–Condon effect in both frontier orbitals. On hexagonal boron nitride, vibronic progression with two vibrational energies gives rise to sharp orbital sidebands that are clearly visible up to the second order of the vibrational quantum number with different Huang–Rhys factors. In contrast, on graphene, orbital and vibronic spectroscopic signatures exhibit broad line shapes, with the second-order progression being hardly discriminable. Only a single vibrational quantum energy leaves its fingerprint in the Franck–Condon spectrum.

The molecular donor tetraphenyldibenzoperiflanthene (DBP) forms coverage-dependent superstructures on Au(111). At submonolayer coverage, the molecules align parallel to each other. They arrange in row-like structures, which exhibit a nearly rectangular primitive unit cell. By contrast, the molecular monolayer is characterized by a herringbone-type DBP arrangement spanned by an almost square unit cell containing two molecules. Both superstructures occur simultaneously in a narrow coverage range close to completion of the molecular monolayer. The adsorbate-substrate interaction is similar to other physisorbed molecular films on Au(111), but differs for the two adsorption phases as inferred from the different modification of the Au(111) surface reconstruction. Structural properties were consistently probed in real and reciprocal space by scanning tunneling microscopy and low-energy electron diffraction, respectively.

A cheap and flexible method is introduced that enables the epitaxial growth of bilayer graphene on Pt(111) by sequential chemical vapour deposition. Extended regions of two stacked graphene sheets are obtained by, first, the thermal decomposition of ethylene and the subsequent formation of graphene. In the second step, a sufficiently thick Pt film buries the first graphene layer and acts as a platform for the fabrication of the second graphene layer in the third step. A final annealing process then leads to the diffusion of the first graphene sheet to the surface until the bilayer stacking with the second sheet is accomplished. Scanning tunnelling microscopy unravels the successful growth of bilayer graphene and elucidates the origin of moiré patterns. arXiv:1812.04540v1 [cond-mat.mtrl-sci]