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Multifunctional van der Waals Materials (MWM) - Research
Group leader: Dr. Eugenio Zallo (Chair of Prof. Dr. Jonathan Finley)
Epitaxial growth of
nanostructures
We study the epitaxial growth of nanostructures by molecular
beam epitaxy (MBE) with a special focus on the van der Waals epitaxy of 2D materials (group-III-monochalcogenides/nitrides).
MBE is well known for its high purity material and scalability, superior
control of both thickness and doping profile and sharp interfaces. By
correlating in situ reflection high energy electron diffraction and
line-of-sight quadrupole mass spectrometry we obtain fundamental information on
the growth processes, which are governed by thermodynamics (energetics) and
kinetics (diffusion and nucleation). In addition, it allows to realize more
complex structures where different 2D materials are stacked on top of one
another or in the same plane, namely vertical
and lateral heterostructures,
respectively. The last ones are very intriguing and disclose a realm of new
nanomaterials. Finally, we aim at understanding the critical role of the
substrate surface on the lattice dynamics and the interplay between the
symmetries of the substrate and the epitaxial layer. These topics are explored in close
collaboration with Prof. Gregor Koblmüller from SNQS and
the Chair led by Prof. Ian D. Sharp.
Relevant
publications:
In-situ spectroscopy of van der Waals materials
We
are interested in the optical and electronic properties of few layers 2D
materials “beyond graphene”. When heterostructures are formed by stacking 2D
materials, unique physical features emerge such as interlayer excitons, where the electron and hole are located in
different layers, or dipolar excitons
as strongly interacting many-body states, opening up the study of a new
exciting field named Moiré flat band
physics. For these purposes, the control of the surface and interface
quality, grain orientation, layer thickness, substrate screening and band alignment are essential.
This is finally possible with our all-UHV 2D-MBE-analytical
cluster. Ultrapure few layer 2D materials and heterostructures with
different alignment and hybrid configurations can be realized by MBE and the
optical, electronic and excitonic properties are investigated by in-situ Raman
and photoluminescence spectroscopies. Importantly, pristine information of
air-reactive materials (for example, group-III-monochalcogenides) can be now
unveiled, above all, the symmetry, chemical and excitonic structures,
electronic gap, carrier recombination and lifetime. These topics are explored in close collaboration with Dr. Elena Blundo, Dr. Gregor Koblmüller, Dr. Andreas Stier and Dr. Nathan Wilson from SNQS.
Relevant
publications:
Defect states in van der Waals materials
Defects
or traps play a crucial role in device engineering for determining the
suitability of a specific material in terms of performance and reliability. Our
goal is to shed light on the main trap characteristics by performing standard deep level transient spectroscopy (DLTS)
and optical DLTS (ODLTS and DLOS) of the MBE grown layered materials. DLTS is an
electrical technique for monitoring deep levels that exist in the depletion
region of single junctions or semiconductor devices by performing a temperature
dependence capacitive transient. Interestingly, the capacitance in 2D materials,
where defects are abundant and of different type, is very large due to quantum
confinement and screening effects. Thus, we are able to obtain several
fundamental parameters such as nature of majority/minority carrier trap,
emission rates and activation energy, capture cross-sections, amount and
concentration profile. Information about carrier dynamics will then be given
via ODLTS/DLOS by exciting the auto-trap states with optical pulses and will open
up the study of new functionalities by means of defect
engineering. These topics are explored in close collaboration with Prof. Hubert Ebert from LMU within the e-conversion Cluster of Excellence.
Van der Waals materials for optoelectronics
and energy conversion
Van
der Waals materials have proven to be an attractive playground for applied
materials, owing to their unique properties in flexible and transparent
electronics and optoelectronics that go beyond the capability of conventional
thin films and their applications in sustainable energy conversion. In our
group, we investigate several means for widely tuning the 2D materials bandgaps
such as control of layer numbers, alloying, substrate engineering, formation of
heterostructures. In addition, due to the large amount of strain they can
endure and the flat band structures, we employ ex-situ strain and electrical
field engineering to enhance the material functionalities and correlation
effects. We are especially interested in layered group-III monochalcogenides
since they are efficient photoabsorbers
and show high photoresponsivity with fast response time. Strikingly, they can
be used for photocatalytic solar water
splitting due to their band gaps in the visible, the vicinity of the band
edges to the water redox potentials, large carrier mobilities and small exciton
binding energies. Both these applications can be pursued by the exploitation of
the high-quality material produced in our 2D-MBE-Analytical cluster. In addition, we focus on atomically thin group-III 2D-nitrides as an emerging class of 2D-materials for light generation and light-induced energy harvesting applications.The final
goal is to optimize devices with multiple functions at the material scale in
order to create superior systems.
Relevant
publications:
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