Fluorescent Self-Assembled Molecular Monolayer on Graphene

International audienceWe report the first fluorescent molecular self-assembly on graphene. The quenching of the fluores-cence of the adsorbed dye by the adjacent graphene is hindered at the molecular scale based on a spacer approach , through a specifically designed dual-functionalized self-assembling building block. This 3D tecton presents two faces, one forming a non-covalent graphene-binding pedestal and the other carrying a dye group linked by a spacer to the pedestal. The spontaneous ordering of the adsorbed layer is investigated by scanning tunneling microscopy whereas the resulting optical properties of the whole graphene-dye hybrid system are characterized by absorption and fluorescence spectroscopies. Graphene has focused intensive research in the past ten years due to its unusually high electron mobility, atomic thickness, broadband optical absorption and unique flexibility. 1,2,3,4 CVD-grown graphene being easily transferable onto arbitrary substrates while preserving high electronic mobilities 5 it soon appeared as a tantalizing candidate for various applications in photonics 6 , such as solar cells 7 , high-speed photodetectors, 8 light sources, 9 ultrafast lasers 10 or metamaterials 11. However, as a zero-bandgap semimetal, pristine graphene has a narrow range of roles, merely limited to transparent electrodes. Advanced applications require graphene to be synergistically combined with nanomaterials providing complementary properties. Recently, non-covalent functionalization of graphene with organic molecular building blocks has appeared as a promising way to modulate its properties in view of functional applications. Actually, graphene provides an atomic scale crystallographic lattice acting as a template guiding supramolecular self-assembly. 12 This bottom-up elaboration process, which implies the physisorption onto graphene of mostly planar molecules (tectons), is now well mastered in view of electronic applications. For example , supramolecular self-assembly on CVD grown gra-phene was used to dope graphene and back-gated gra-phene field-effect transistor (G-FET) devices were obtained. 13 Surprisingly, by comparison with electronics, the non-covalent functionalization of graphene is still in its infancy as concerns applications in photonics. Yet, organic dyes offer a high flexibility in the design of innovative photonic devices. Actually, because of their high oscillator strengths, they can play the role of light harvesters, pho-ton sources, exciton funnels, etc. and as such should provide opportunities to enhance or extend the properties of graphene towards light-based applications. Very recently, the distinctive optical absorption signature of self-assembled dye-graphene architectures, the structure of which were confirmed by scanning tunneling microscopy, have been analyzed by absorption micro-spectroscopy 14. However, the main challenge remains the emission of light by such systems, since graphene is known as a strong quencher of electronic excited states through ultrafast non-radiative electron-tunneling exchanges (the so-called Dexter excitation transfer) between the dyes and the graphene 15,16,17. Hence, the realization of light emitting devices based on dye-graphene assemblies requires to overwhelm Dexter transfers through introduction of an accurately controlled dye-to-graphene electron barrier, for which planar flat-lying molecules are inefficient. Self-assembling building blocks of increasing three-dimensional (3D) complexity emerged recently, in particular following the so-called Janus tecton paradigm 18,19. These systems have been proposed as a platform to attach functional molecular moieties at a controlled distance from graphene 20. Here, we demonstrate the first fluorescent supramolec-ular self-assembly on graphene with a specifically designed 3D Janus tecton. We investigate its two-dimensional ordering by scanning tunneling microscopy (STM) and its surface optical properties by absorption and fluorescence spectroscopies