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Final Report Summary - PHOTOSTM (Investigating Photo Catalytic Reactions at the Molecular Scale)

In this project we correlated the local electronic structure at the atomic level of semiconducting potential catalyst’s reaction spots to the local catalyst’s morphology. Understanding the mechanism of photo catalytic reactions is a key to systematically improve their efficiency and enhance substantially the impact of green chemistry. Photo catalytic reactions are determined by the electronic level alignment between a catalyst and the reactant. This energy level alignment depends strongly on the local morphology of the catalyst and the absorption site of the reactant, but is only poorly understood due to the difficulty to study photo catalytic reactions at the molecular length scale – the critical length scale for photo-catalytic processes.

Monolayer semiconducting transition metal dichalcogenides (TMDs) such as molybdenum diselenide (MoSe2) are direct-bandgap materials with an extraordinarily large exciton binding energy and high charge-carrier mobility. Their remarkable optical and electrical properties make them of interest for a wide variety of applications ranging from its combination with other 2D materials (graphene, boron nitride...) to fabricate novel optoelectronic devices to its use in photo catalysis or hydrogen production. Individual atomic-scale defects in 2D TMD semiconductors are particularly interesting because they are expected to modify charge transport or introduce ferromagnetism, whereas 1D defects such as grain boundaries and edges may alter electronic and optical properties and introduce magnetic or catalytic functionality.

In our work, by combining high-resolution non-contact atomic force microscopy (nc-AFM) and simultaneous measurement of the electronic structure by low temperature scanning tunneling spectroscopy (STS), we provide direct evidence for the existence of isolated, one-dimensional charge density waves at mirror twin boundaries (MTBs), intrinsic to single-layer semiconducting MoSe2. Our low-temperature scanning tunneling microscopy/spectroscopy measurements reveal a substantial band gap of 100 meV opening at the Fermi energy in the otherwise one-dimensional metallic structure. We find a periodic modulation in the density of states along the mirror twin boundary, with a wavelength of approximately three lattice constants. The modulations in the density of states above and below the Fermi level are spatially out of phase, consistent with charge density wave order. In addition to the electronic characterization, we determine the atomic structure and bonding configuration of the one-dimensional mirror twin boundary by means of high-resolution non-contact atomic force microscopy. Density functional theory calculations reproduce both the gap opening and the modulations of the density of states. This particular type of line defect could act as 1D type-I heterostructure, spatially localized within the 2D semiconductor.

This study highlights the importance of defects as a tool for tailoring new properties of single layer transition metal dichalcogenide semiconductors. Surface engineering through defects has an enormous impact on the development of new photo active materials not only from the point of fundamental understanding of their intrinsic properties, but also because it opens new possibilities of functionalization.

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