a. Photo-physical properties of 2D materials and their heterostructures

 b. Van der Waal materials and hybrid stacks for energy conversion and (photo-)catalysis

 c. Exciton ensembles in van der Waals heterostructures and GaAs quantum well bilayers

 d. Interaction physics, phase transitions and collective phenomena

 e. Toolbox

a.  Photo-physical properties of emerging 2D materials and their heterostructures

We are interested in the photo-physical properties of novel two-dimensional materials such as semiconducting transition metal dichalcogenides (e.g. MoS2 or WSe2) and related hetero- and hybrid-structures. We are working towards a better understanding of the optical and electronic properties of pristine devices, in dependence of the environment and embedded in field-effect structures and circuits. We recently demonstrated that the dielectric environment impacts the intrinsic doping level, the single particle electronic bands, the exciton binding energy, the contact morphology and also the light in- and out-coupling efficiencies.
Moreover, we explore the properties of deterministically positioned point defects in those materials that are induced with a high lateral precision by a focused helium ion beam. We investigate novel two-dimensional materials and their hetero- and nanostructures with the aim to develop an in-depth understanding of the underlying physical properties and to tailor their properties on-demand. 
To achieve those ambitious goals, we established a comprehensive toolbox for the preparation, nano-fabrication and (in-situ) investigation of 2D materials.

Recent papers:

Present collaboration partners:

Alexander Holleitner, Jonathan Finley (WSI-TU München), Peter Thiesen, Sebastian Funke (Accurion GmbH), Michael Lorke, Alexander Steinhoff, Frank Jahnke (Uni Bremen), Abhay Pasupathy (Columbia University).

b. Van der Waals hetero- and hybrid stacks for energy conversion via (photo-)catalysis

We study the in-situ and in-operando properties of the photo-physical system and material properties of individual micro-and nanoscale (photo-)catalytic materials, particular micromechanically exfoliated 2D materials. Our aim is the generation of a microscopic understanding of the underlying processes contributing to the photo-catalytic reaction including charge carrier generation, separation, transport and the role of catalytic active sites. We developed a photo-electrochemical microcell for (photo-) electrochemical investigation and in-situ and in-operando µ-optical characterization. We recently demonstrated the photocatalytic stability of MoS2 flakes und we measured the layer dependent (photo-)catalytic activity for the hydrogen evolution reaction (HER) and demonstrated the importance of defect sites in the basal plane for the overall activity. On the other hand side, we study the catalytic activity of helium ion beam induced point defects in semiconducting transition metal dichalcogenides.

Recent papers:

Present collaboration partners:
Alexander Holleitner (WSI-TUM), Aliaksandr Bandarenka (TUM), Joel W. Ager III (LBNL and UCB Berkeley)

c. Excitons ensembles in van der Waals heterostructures and GaAs bilayers

Excitons are electron-hole pairs that are coupled by Coulomb interaction. As composite bosons, exciton ensembles obey a fascinating interaction-driven quantum phase diagram with classical and quantum phases including degenerate quantum gases, quantum liquids and quantum solids. The most intriguing state is the manybody ground state with the system condensed into a macroscopic ground state wave-function forming a Bose-Einstein condensate. We study dense ensembles of indirect or interlayer excitons (IX) hosted either in GaAs double quantum well or in hetero-bilayers of van der Waals materials.

Recent papers:

Present collaboration partners:

Alexander Holleitner, Jörg Kotthaus (LMU), Michael Lorke, Alexander Steinhoff, Frank Jahnke (Uni Bremen), Aron Pinczuk (Columbia University), Michael J. Manfra (Purdue University).

d.  Interaction physics and collective phenomena

Many properties of solid state materials are governed by manifold interaction physics. Minor changes in the interactions can cause completely different behavior of the material, e.g. superconductivity, magnetism, metal-insulator transitions or fractional quantum Hall effect states.
We are interested in fundamental interaction driven phenomena caused by electron-electron, electron-phonon, exciton-phonon, spin and valley interaction, and  (quantum) phase transitions. Those phases can be identified by their unique low-energy excitation spectra. Our approach to these intriguing states and phase transitions is a set of complementary experiments including resonant inelastic and resonant Rayleigh light scattering, absorbance, photoluminescence and transport measurements. We explored the low energy excitation spectra unique to exotic and even non-Abelian FQHE states, we demonstrated that a hole plasmon excitation of a two-dimensional hole system couples coherently to a coexisting exciton ensemble. Moreover, we study doping induced coupling mechanism in semiconducting 2D materials such as exciton-phonon and electron-electron coupling towards superconductivity.

Recent papers:

Present collaboration partners:

Alexander Holleitner (WSI-TUM), Jörg Kotthaus (LMU), Aron Pinczuk (Columbia University), Jainendra Jain (Penn State University), Michael J. Manfra (Purdue University), Werner Wegscheider (ETH Zurich).


We thank the Alexander Humboldt Foundation (AvH) and the Deutsche Forschungsgemeinschaft (DFG) for funding in several projects. In particular, we are grateful for the support of the Nanosystems Initiative Munich (NIM).

e. Methodology

Our experimental “tool-box” comprise advanced van der Waals assembly, nanofabrication and -analysis together with various spectroscopy, light scattering and electrical measurement techniques. We have different cryostats available to perform those measurements in the full temperature range from above room temperature down to cryogenic temperatures using liquid nitrogen and liquid helium gas.  For optical and optoelectronic measurements various pulsed and cw laser sources, (home-built) microscopes/objectives, spectrometers and detectors are available. Tunable cw lasers enable resonant inelastic light scattering and resonant Rayleigh scattering experiments.

Van der Waals heterostructures are prepared by micromechanical exfoliation from bulk crystals and a dry transfer process. We have designed and built a micro-mechanical transfer and vdW stacking tool with micrometer-manipulation in x, y, z and a rotational degree of freedom for the preparation of vdW HS. This tool is implemented in a fully motorized Leica microscope system suitable for reflectance and transmission measurements. Moreover, we modified this commercial microscope such that in-situ µ-PL, µ-Raman and differential reflectance and second harmonic generation measurements are possible. The hybrid tool can be operated in inert-gas atmosphere.