François Vonau

Covalent functionalization by cycloaddition reactions of pristine, defect-free graphene

Based on a low-temperature scanning tunneling microscopy study, we present a direct visualization of a cycloaddition reaction performed for some specific fluorinated maleimide molecules deposited on graphene. Up to now, it was widely admitted that such a cycloaddition reaction can not happen without pre-existing defects. However, our study shows that the cycloaddition reaction can be carried out on a defect-free basal graphene plane at room temperature. In the course of covalently grafting the molecules to graphene, the sp2 conjugation of carbon atoms was broken, and local sp3 bonds were created. The grafted molecules perturbed the graphene lattice, generating a standing-wave pattern with an anisotropy which was attributed to a (1,2) cycloaddition, as revealed by T-matrix approximation calculations. DFT calculations showed that while both (1,4) and (1,2) cycloadditions were possible on free-standing graphene, only the (1,2) cycloaddition could be obtained for graphene on SiC(0001). Globally averaging spectroscopic techniques, XPS and ARPES, were used to determine the modification in the elemental composition of the samples induced by the reaction, indicating an opening of an electronic gap in graphene.

Generating long supramolecular pathways with a continuous density of states by physically linking conjugated molecules via their end groups.

Self-assembly of conjugated 2,5-dialkoxy-phenylene-thienylene-based oligomers on epitaxial monolayer graphene was studied in ultrahigh vacuum by low-temperature scanning tunneling microscopy (STM). The formation of long one-dimensional (1D) supramolecular chain-like structures has been observed, associated to a physical linking of their ends which involved the rotation of the end thiophene rings in order to allow π-π stacking of these end-groups. dI/dV maps taken at an energy corresponding to the excited states showed a continuous electronic density of states, which tentatively suggests that within such molecular chains conjugation of electrons is preserved even across physically linked molecules. Thus, in a self-organization process conjugation may be extended by appropriately adapting conformations of neighboring molecules. Our STM results on such self-organized end-linked molecules potentially represent a direct visualization of J-aggregates.

A phenomenological approach of joint density of states for the determination of band structure in the case of a semi-metal studied by FT-STS

The authors show that an accurate determination of the band structure can be achieved by using Fourier transform scanning tunnelling microscopy (FT-STM) techniques in the case of a semi-metallic ErSi2 layer grown on a Si(111) substrate. This material provides an ideally confined 2D electron and hole gas that is reflected in a complex standing wave pattern at 77 K. The quasi-particles exist over a wide energy range from −800 to +300 meV without mixing with silicon bulk excitations. The Fourier transform of dI/dV maps have been successfully interpreted using the concept of the joint density of states (JDOS), which will be properly introduced. We present here an intuitive interpretation of the quasiparticle interference process based on a geometric construction which also allows us to clearly demonstrate that hole–hole and hole–electron quantum interferences dominate over electron–electron quantum interference.