Quasiparticle dynamics and optical properties of alkali metal doped few-layered transition metal dichalcogenides

• Applicant: Professor Dr. Alexander Grüneis
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2016 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 278161773

The goal of the present project is to unravel and engineer the electronic and optical properties, the electronic spin and related many-body effects in novel two-dimensional transition metal dichalcogenides (TMDs), such as MoSe2. The suggested work comprises of in-situ sample synthesis and functionalization and a multispectroscopic approach: on the one hand, spin- and angle dependent photoemission spectroscopy will be employed in order to investigate the electron energy band structure and renormalizations; on the other hand Raman spectroscopy will be employed to investigate the vibrational properties and electron-phonon coupling. The control parameters by which we can engineer the physical properties are: (1) layer number, (2) stacking, (3) alloying (4) alkali metal doping and (5) lateral quantum confinement into nanoribbons. It is believed that these parameters allow for a wide tuning of the physical properties including a fully spin polarized electron energy band structure, optical bandgap, charge-density waves and superconducting regimes. In particular, the layer number of TMDs is expected to control the spin polarization since odd-numbered layers do not have an inversion center and hence have fully spin polarized bands. Since the electronic structure of TMDs such as MoSe2 consists of two valleys with opposite spin, the carriers these materials are often envisioned as building blocks for valleytronics. Stacking of the different TMDs provides a new type of heterostructure with extraordinary optoelectronic properties. Alloying (for example replacing Mo by W or replacing Se by S) is expected to yield full control over the bandgap. Alkali-metal doping turns semiconducting TMDs into metals and the charge carrier concentration (which can be controlled by alkali(ne) metal type) dictates whether charge density wave or superconducting regimes dominate. If it is possible to grow ribbons of TMDs on stepped surfaces, a very rich excitation spectrum consisting of valley dependent selection rules and zonefolded bands could be engineered. Due to the presence of van-Hove singularities in the density of states, the energy of the maximum absorption can be engineered by the ribbon width. The novel aspects of the suggested proposal are related to how the spin-polarized nature of the Fermi surface will affect the renormalization and scattering rates, exploring the full breadth of dopants (alkali and alkaline earth) and on the functionalization and quantum confinement aspects. The proposed work's feasibility is firmly grounded on the proposers' angle-resolved photoemission work on doped bulk transition metal compounds and graphene and the recent stormy developments in high-quality sample synthesis by molecular beam epitaxy. The suggested work will provide a deep understanding, not only of the physics of TMDs but of two-dimensional materials in general.

• DFG Programme: Research Grants