PhD defense of Yimin GUAN

Molecules are versatile building blocks that can generate remarkably diverse macromolecular systems. They are nowadays often used in ‘bottom-up’ nano-fabrication approaches to create various molecular architectures on surfaces through molecular assembly. This approach is of major scientific and technological interest, particularly in the fields of organic electronics, photonics, photovoltaic cells and advanced materials.

The aim of this work is to study in detail the formation of C₆₀/Perylene composite molecular layers on a silver substrate. Understanding how the C₆₀ and perylene molecules organize themselves at the interface into molecular layers, their structure and their electronic states is essential for controlling the properties of the composite material.

C₆₀ (fullerene) is an excellent electron acceptor, while perylene (and its derivatives such as PTCDA) is an organic semiconductor known for its hole transport properties. By studying their interface, it is possible to optimize charge separation and transport, which is crucial for organic solar cells (OPVs) and organic light-emitting diodes (OLEDs).

In addition, one of the avenues for research into improving solar cells is to use the process of “triplet annihilation upconversion” (TTA-UC) of neighbouring molecules, which enables two low-energy photons to produce one high-energy photon. In some applications, triplet organic photo-sensitisers are used for TTA-UC. In particular, it has already been shown that our two molecules of interest, perylene and C60 fullerene, can be involved in efficient TTA-UC processes.

Another interesting aspect is that the formation and properties of perylene layers on the Ag(110) surface, which have already been addressed by the team through STM studies, have shown unique structural characteristics, both in monolayers and multilayers. In the latter case, we obtain stacks of molecules in π–π bonds that self-assemble into a three-dimensional epitaxial layer. These “anchored” or “floating” columns self-assemble by hydrogen bonding. This gives the overlayer the remarkable ability to adapt to the morphology of the underlying substrate and to extend over the surface terraces as a single structure. We have dubbed this unique property ‘complete epitaxy’.

Our experimental study involves the use of diverse but complementary surface science techniques. The structural information is probed by “Scanning Tunneling Microscope” (STM), electronic characteristics by “Photoelectron Spectroscopy” (UPS, XPS) and certain optical properties by “Surface Differential Reflectivity Spectroscopy” (SDRS). The study was threefold. Firstly, we characterized the electronic and optical properties of the perylene multilayer. Then we studied the deposition of C60 directly on the silver surface, to serve as a reference system. Then we carried out all these characterization measurements on C₆₀/Perylene composite layers, formed by depositing C₆₀ on perylene layers previously formed on the metal substrate.