242
8 Conclusion and Outlook
for instance [97, 98, 105, 106]) that form a superlattice of artificial quantum dots
and are particularly attractive for light absorbing devices, such as detectors or solar
cells. Preliminary results by the author’s team showed strong absorption in the visible
range for interlayer moiré features of a WSe 2 /WS 2 bilayer with small twist angle
[110]. Collaborative research aims at the exploration of moiré bilayers with good
charge-carrier mobility with regard to THz emission from optically or electrically
excited carriers drifting in the spatially oscillating potential. Such work may provide
fundamental insights into charge transport in these materials and pave the way for
the achievement of conceptually new THz emission schemes.
GaTe-Based Sensing Devices
One promising post-TMDC material for optoelectronic applications is GaTe, which
has remarkable properties as a layered material with 2D crystal lattice that is slightly
different than that of the more famous TMDCs. Previous collaborative work by the
research group of H.Z. Wu in Hangzhou with the author [120] showed that GaTe
multilayers decorated by gold nanoparticles act as very sensitive probe systems for
certain aromatic molecules. This renders them attractive for surface-enhanced Raman
spectroscopy, which is explored by partners from Hangzhou. With this “GoldenGaTe” towards ultra-sensitive sensing in sight, currently, further research is going on
cooperatively to understand charge-carrier dynamics and temperature dependencies
of Au-GaTe in comparison to pristine and annealed flakes of GaTe. Moreover, the
influence of gold decoration on the optical properties is studied.
Mid-Infrared and THz Detectors
High-speed mid-infrared photodetectors with a two-dimensional electron gas (2DEG)
in CdTe/PbTe heterojunctions are a topic of further investigation within the cooperation group FNMS-COOP. Such 2DEG is formed at the heterojunction’s interface. With this platform of a novel 2DEG established in the group of H.Z. Wu in
Hangzhou, the cooperation partners aim at the development of a high-speed and
room-temperature operating mid-IR photodetector. Making use of the conductionband and valence-band offsets of the heterojunction, the photoresponse of the detectors originating from the intrinsic response of PbTe shall open the path to broadband
detection schemes spanning the IR and THz range (studied jointly via ultrafast spectroscopy). Moreover, the development of suitable optoelectronic structures is pursued
and other concepts involving the thermoelectric effect in graphene may be explored
with partners. The underlying work by the group of H. G. Roskos in Frankfurt builds
on a careful evaluation of the performance of THz detectors based on graphene
field-effect transistors [121], which has led to the development of a circuit simulator,
which takes rectification by both resistive mixing and the hot-carrier thermoelectric
effect in graphene into account [122].
2D–3D Interface and Nanoparticle-Enhanced Perovskite-Based Structures
The optical characterisation of perovskite solar cell materials was targeted with the
work carried out by Kong et al. involving the current cooperation-group coordinators
from Hangzhou and Marburg and continued by W. Kong and co-workers in a new
research environment at the Southern University of Science and Technology, Shenzhen, and at Hebei University afterwards [123–125]. The cooperation partners’ next
8 Conclusion and Outlook
for instance [97, 98, 105, 106]) that form a superlattice of artificial quantum dots
and are particularly attractive for light absorbing devices, such as detectors or solar
cells. Preliminary results by the author’s team showed strong absorption in the visible
range for interlayer moiré features of a WSe 2 /WS 2 bilayer with small twist angle
[110]. Collaborative research aims at the exploration of moiré bilayers with good
charge-carrier mobility with regard to THz emission from optically or electrically
excited carriers drifting in the spatially oscillating potential. Such work may provide
fundamental insights into charge transport in these materials and pave the way for
the achievement of conceptually new THz emission schemes.
GaTe-Based Sensing Devices
One promising post-TMDC material for optoelectronic applications is GaTe, which
has remarkable properties as a layered material with 2D crystal lattice that is slightly
different than that of the more famous TMDCs. Previous collaborative work by the
research group of H.Z. Wu in Hangzhou with the author [120] showed that GaTe
multilayers decorated by gold nanoparticles act as very sensitive probe systems for
certain aromatic molecules. This renders them attractive for surface-enhanced Raman
spectroscopy, which is explored by partners from Hangzhou. With this “GoldenGaTe” towards ultra-sensitive sensing in sight, currently, further research is going on
cooperatively to understand charge-carrier dynamics and temperature dependencies
of Au-GaTe in comparison to pristine and annealed flakes of GaTe. Moreover, the
influence of gold decoration on the optical properties is studied.
Mid-Infrared and THz Detectors
High-speed mid-infrared photodetectors with a two-dimensional electron gas (2DEG)
in CdTe/PbTe heterojunctions are a topic of further investigation within the cooperation group FNMS-COOP. Such 2DEG is formed at the heterojunction’s interface. With this platform of a novel 2DEG established in the group of H.Z. Wu in
Hangzhou, the cooperation partners aim at the development of a high-speed and
room-temperature operating mid-IR photodetector. Making use of the conductionband and valence-band offsets of the heterojunction, the photoresponse of the detectors originating from the intrinsic response of PbTe shall open the path to broadband
detection schemes spanning the IR and THz range (studied jointly via ultrafast spectroscopy). Moreover, the development of suitable optoelectronic structures is pursued
and other concepts involving the thermoelectric effect in graphene may be explored
with partners. The underlying work by the group of H. G. Roskos in Frankfurt builds
on a careful evaluation of the performance of THz detectors based on graphene
field-effect transistors [121], which has led to the development of a circuit simulator,
which takes rectification by both resistive mixing and the hot-carrier thermoelectric
effect in graphene into account [122].
2D–3D Interface and Nanoparticle-Enhanced Perovskite-Based Structures
The optical characterisation of perovskite solar cell materials was targeted with the
work carried out by Kong et al. involving the current cooperation-group coordinators
from Hangzhou and Marburg and continued by W. Kong and co-workers in a new
research environment at the Southern University of Science and Technology, Shenzhen, and at Hebei University afterwards [123–125]. The cooperation partners’ next