Atomically resolved phonon and photon spectroscopies of 2D material stackings

• Applicant: Professor Dr. Jörg Kröger
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 467613242

The project addresses double-layers of graphene and stackings of graphene and hexagonal boron nitride (h-BN) on metal surfaces – Ru(0001), Rh(111) and Pt(111). The first aim is the preparation of the bilayer structures in a surface science approach in order to ensure the presence of domains with different twist angles between the stacked two-dimensional materials. The rotation angle between the 2D lattices will be inferred from the analysis of the resulting moiré patterns by electron diffraction and scanning tunnelling microscopy. The second target consists in exploring electronic properties, phonon and photon excitations of these stacked hybrid materials with scanning tunnelling spectroscopy. The focus is on the potential dependence of the aforementioned properties on the twist angle. The planned approach allows the direct comparison of local phonon and photon excitations with the electronic structure. In addition, owing to the presence of twist angle domains on one and the same sample, the experiments will identify the dependence of the quantum properties on the twist angle without the necessity to change the sample. The unique opportunities of the scanning tunnelling microscope shall be used to explore the importance of Van Hove singularities for phonons in bilayer graphene. A possible energy reduction of specific phonon modes due to an enhanced electron-phonon correlation in terms of a Kohn anomaly will be scrutinized. There is a debate on the role of Van Hove singularities in the photon emission of bilayer graphene, where ooptical transitions between the singularities and strongly bound excitons induced by the singularitues compete with each other. The local photon emission from the tunnelling barrier depending on the moiré lattice site and on the twist angle shall elucidate the debate. Single defects in the h-BN lattice on graphene will be prepared using argon ion bombardment in order to study their optical properties as colour centers. To this end, spectroscopy of the local electroluminescence induced by the injected tunnelling current will be performed.

• DFG Programme: Research Grants