238
8 Conclusion and Outlook
8.5 Towards Optoelectronic Devices and Microcavity
Experiments with 2D Materials
As light-based technologies have become indispensable in our modern world, a
demand for high-performing novel optical materials is growing and optimisation
of optical devices with respect to energy efficiency and cost effectiveness receives
much attention. Low-dimensional materials feature unique advantages over their bulk
counterparts and are expected to enable next-generation innovative technologies.
Particularly, the opportunity to vertically stack 2D materials offers vast possibilities
in terms of ‘heterostructuring’ and “band-gap engineering” [72, 73]. Nowadays,
2D materials are popular not only for conventional optoelectronics [74], but also for
valleytronics due to the optical and electronic properties related to the valley-pseudospin [75, 76].
Naturally, for the successful use in devices, the optoelectronic properties of, both,
individual monolayers (MLs) and stacked ML-based heterosystems (HSs) need to be
thoroughly explored. Previous works on monolayer materials have clearly shown how
the substrate [77–80] and the environment [81–85] alter the optical properties of 2D
semiconductors such as WS 2 , WSe 2 and so forth of the family of TMDCs. With regard
to high-quality monolayer-system studies, it has been shown that hBN encapsulation
has become an indispensable tool [83, 86–89]. In addition to the use of the surrounding materials’ dielectric properties (e.g. electronic screening) or interface-related
effects (e.g. electronic doping), optical properties can be further tailored through
light–matter interactions with the help of patterned substrate structures resembling
in-plane optical microcavities [80], or vertically constructed (monolithic or open,
tunable) Fabry-Pérot (FP) optical microcavities [90–95]. Last, but not least, homobilayers of different symmetry or heterostructures comprising different monolayers
with certain layer-to-layer orientations can open up new possibilities in the modification of material properties [97–100]. This can for instance happen via band structure
modifications [101–104], or the formation of moiré patterns and thereby in-plane
superlattices in the 2D potential landscape that can have a strong influence on the
spectral features [105–110], to name but a few.
In a current research project, a joint theoretical and experimental study of optical properties of the (yet little explored) MoSe 2 /MoTe 2 and WSe 2 /MoTe 2 ML–ML
heterostructures is conducted. Primarily, the understanding of this material class and
their possible applications is going to be demonstrated by investigating whether typeII heterostructures of MoSe 2 /MoTe 2 retain the direct band gap of their corresponding
monolayers and are promising candidates for photovoltaic devices and detectors due
to the presence of interlayer (i.e. spatially indirect) excitons—also known as chargetransfer excitons. Similarly, it can be clarified whether type-I heterostructures of
WSe 2 /MoTe 2 are ideal structures for utilisation in low-threshold 2D-materials diode
lasers due to the presence of intralayer (i.e. spatially direct) excitons. Moreover, it is
important to understand whether heterostructuring deteriorates the optical properties
of monolayers, and whether lattice mismatch and the substrate choice significantly
affect the optical properties of these heterostructures.
8 Conclusion and Outlook
8.5 Towards Optoelectronic Devices and Microcavity
Experiments with 2D Materials
As light-based technologies have become indispensable in our modern world, a
demand for high-performing novel optical materials is growing and optimisation
of optical devices with respect to energy efficiency and cost effectiveness receives
much attention. Low-dimensional materials feature unique advantages over their bulk
counterparts and are expected to enable next-generation innovative technologies.
Particularly, the opportunity to vertically stack 2D materials offers vast possibilities
in terms of ‘heterostructuring’ and “band-gap engineering” [72, 73]. Nowadays,
2D materials are popular not only for conventional optoelectronics [74], but also for
valleytronics due to the optical and electronic properties related to the valley-pseudospin [75, 76].
Naturally, for the successful use in devices, the optoelectronic properties of, both,
individual monolayers (MLs) and stacked ML-based heterosystems (HSs) need to be
thoroughly explored. Previous works on monolayer materials have clearly shown how
the substrate [77–80] and the environment [81–85] alter the optical properties of 2D
semiconductors such as WS 2 , WSe 2 and so forth of the family of TMDCs. With regard
to high-quality monolayer-system studies, it has been shown that hBN encapsulation
has become an indispensable tool [83, 86–89]. In addition to the use of the surrounding materials’ dielectric properties (e.g. electronic screening) or interface-related
effects (e.g. electronic doping), optical properties can be further tailored through
light–matter interactions with the help of patterned substrate structures resembling
in-plane optical microcavities [80], or vertically constructed (monolithic or open,
tunable) Fabry-Pérot (FP) optical microcavities [90–95]. Last, but not least, homobilayers of different symmetry or heterostructures comprising different monolayers
with certain layer-to-layer orientations can open up new possibilities in the modification of material properties [97–100]. This can for instance happen via band structure
modifications [101–104], or the formation of moiré patterns and thereby in-plane
superlattices in the 2D potential landscape that can have a strong influence on the
spectral features [105–110], to name but a few.
In a current research project, a joint theoretical and experimental study of optical properties of the (yet little explored) MoSe 2 /MoTe 2 and WSe 2 /MoTe 2 ML–ML
heterostructures is conducted. Primarily, the understanding of this material class and
their possible applications is going to be demonstrated by investigating whether typeII heterostructures of MoSe 2 /MoTe 2 retain the direct band gap of their corresponding
monolayers and are promising candidates for photovoltaic devices and detectors due
to the presence of interlayer (i.e. spatially indirect) excitons—also known as chargetransfer excitons. Similarly, it can be clarified whether type-I heterostructures of
WSe 2 /MoTe 2 are ideal structures for utilisation in low-threshold 2D-materials diode
lasers due to the presence of intralayer (i.e. spatially direct) excitons. Moreover, it is
important to understand whether heterostructuring deteriorates the optical properties
of monolayers, and whether lattice mismatch and the substrate choice significantly
affect the optical properties of these heterostructures.