2D materials

Two-dimensional materials such as transition-metal dichalcogenides are atomically thin crystalline materials with interesting optical properties.

For example, the optical properties of 2D transition-metal dichalcogenides (TMDs) such as MoSe₂ or WS₂, are dominated by excitons, Coulomb-bound particles comprised of an electron and a hole. Because 2D materials are so thin, any material directly adjacent to the TMD layer affects the properties of those excitons. We are interested in the following questions:

-        What are the fundamental properties of TMDs?

-        How does the dielectric / magnetic / topological environment of an atomically thin TMD affect the excitons?

-        Can we engineer the environment of TMDs in such a way as to enable interesting physical phases or to create interesting devices?

Present collaborators:

Richard Schmidt (MPQ), Christopher Gies, Matthias Florian, Alexander Steinhoff (U. Bremen).

Relevant publications:

1.      “Magnetooptics of exciton Rydberg states in a monolayer semiconductor”
Phys. Rev. Lett. 120, 057405 (2018), Editors Choice.

2.      ”Probing the influence of dielectric environment on excitons in monolayer WSe₂: Insight from high magnetic fields”
Nano Lett. 16, 7054 (2016)

3.      “Exciton diamagnetic shift and valley Zeeman splitting in monolayer WS₂ and MoS₂ to 65 Tesla”
Nature Comm. 7, 10643 (2016)

4.    "Controlling exciton many-body states by the electric field effect in monolayer MoS₂"

Quantum Sensing of 2D materials

In conventional spectroscopy, the state of a system is measured by investigating either the energy eigenstates or
the excitations out of the ground state. Using these pathways, we can attempt to understand the intricacies of
emergent quantum phases. However, especially in 2D systems, interactions are strongly enhanced and may
become so complicated that more information is needed in order to form a complete picture.

This is especially true for novel magnetic 2D materials, where spin, charge and valley degrees of freedom as well
as spontaneously broken symmetries at interfaces can come into play.

We are utilizing nitrogen vacancy centers in diamond for imaging of magnetic phases of novel 2D heterostructures. These highly sensitive sensors are essentially a quantum 2-level system that is influenced by the electric or magnetic fields of a proximized 2D material. Thus we can quantitatively image the quantum state of a specimen.         

Present collaborators:

Dominik Bucher (TUM), Zdenek Sofer (UCT Prague), Jaroslav Fabian (U. Regensburg).

Localized excitons in 2D materials

Localized excitons in 2D materials have shown to be sources for 
single photon emission. We follow two avenues for excitons localization in order to create single photon sources for quantum light applications exciton localization through dielectric engineering and strain-induced exciton localization. Our goal is to couple light from such localized emitters into photonic structures.

Present collaborators: Christopher Gies (U. Bremen)
                                       Katia Gallo (U. Stockholm)    
                                       Fei Ding (U. Hannover)
                                       R. Haug (U. Hannover)

This work is supported by the DFG through the SPP 2244.

Relevant publications:

1.      “Charged exciton kinetics in monolayer MoSe₂ near ferroelectric domain walls in periodically poled lithium niobate"
Nano Letters 21, 959 (2021)

2D perovskites

Similar to conventional III-V semiconductor superlattices, Ruddlesden-Popper halide perovskites are quantum well structures formed by two-dimensional layers of halide perovskites semiconductors, separated by bulky organic spacer layers. This layered structure can be tuned by varying the perovskite layer thickness and the composition of the organic layer.

Although efficient opto-electronic devices were made from such materials, fundamental questions concerning the nature of optical resonances remain. We strive to understand the basic optical properties of these fascinating materials.

     
Collaborators: J.C. Blancon (Rice University)

Relevant publications:

1.      “Scaling law for excitons in 2D perovskite quantum wells”
Nature Comm. 9, 2254 (2018)

2.    "Managanese doping for enhanced magnetic brightening and circular polarization control of dark excitons in parametric layered hybrid metal-halide perovskites"
Nature Comm. 12, 1 (2021)