Hybridization of Organic Molecules with Metal Substrates in the Presence of Surface Passivation Layers

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Aromatic molecules adsorbed in a flat-lying manner on metal surfaces tend to couple electronically to the substrate, i.e., a hybridization of metal substrate and molecular states occurs. Controlling the degree of electronic coupling (or decoupling) is highly desirable from both a fundamental and an application-based point of view, as it governs important properties of the interface.Previous research in the literature mainly aims at measuring the bonding strength (feasible, e.g., with TDS), and to characterize the adsorption as (mostly) chemisorbed or (mostly) physisorbed. In contrast, the project proposed here strives for going beyond this existing binary categorization. In fact, we do not want to measure any integral bonding strength, but examine in detail the impact the hybridization has on the individual electronic states of molecular adsorbates, including both occupied and unoccupied states in the investigation. This will be accomplished by the selection of complementary research methods (EELS, IETS, STS ARUPS). While it has long been known theoretically that individual states interact differently with the substrate (see, e.g., the concept of charge back donation), detailed experimental studies are scarce. Of particular interest is the study of the influence of hybridization on electronic transitions, which should take place by means of Differential Reflectance Spectroscopy (DRS): diverse previous work of the applicant have shown that on various substrates differently broadened absorption spectra of the first monolayer of organic molecules are observed, ranging in shape from monomer-like (being similar to solvent spectra) to greatly broadened and even structureless, depending on the type of interaction. A direct comparison of the absorption spectra of one and the same molecule on different surfaces is feasible by the extraction of the complex dielectric function from the primary DRS data.By inclusion of selected model molecules we want to attempt to derive generalized rules for the hybridization of molecular states on metals. Experimentally, we will vary the degree of hybridization by employing different passivation layers. We will focus on graphene and hexagonal boron nitride (h-BN). We will further consider decoupling of aromatic molecular backbones from surfaces by using phenyl spacer groups. Here, we will concentrate on dibenzotetraphenylperiflanthene (DBP) since it has evolved as a promising material for applications in organic photovoltaic devices (OPVDs) and organic light emitting diodes (OLEDs).A further, even more ambitious project goal is to derive a quantitative measure (figure-of-merit) for the degree of hybridization based on our spectral information, especially on an analysis of the spectral shape and the spectral broadening of the optical spectra.