2.4 Layered Systems Based on Monolayer Semiconductors
37
[216, 217, 244], enabling to obtain spin–layer or spin–valley locking scenarios for
the naturally (AA’) [219, 245] and artificially (AB)
9 stacked TMDCs, respectively
[219]. A recent study by the author’s team with the help of collaboration partners
showed among other spectral signatures that a noticeable difference of circular polarisation anisotropy is evidenced for selectively CVD-grown AA’ and AB configured
homobilayers (corresponding schematics and band structure calculations displayed
in Fig. 2.11), with the latter exhibiting a higher value [219]; for hBN-supported AA’
and AB flakes, for which dephasing induced by defects and doping is reduced (cf.
[7]), a more pronounced linear and circular polarisation anisotropy is obtained [219].
Furthermore, their theory predicted a lifted degeneracy between layers, observable
in reflection-contrast experiments. These distinct bilayer cases could be for instance
interesting for future ‘valleytronic’ or even ‘layertronic’ device schemes.
2.5 Photonics and Optoelectronics of 2D Semiconductor
TMDCs
Besides the recent progress in applications of 2D semiconductor TMDCs with an
emphasis on strong excitonic effects in monolayers and heterostructures [21, 205],
the mere electronic and optical properties of 2D materials, as well as the spin- and
valley-dependent properties, caught the attention of various research communities
worldwide (see for instance the activities described in [16, 17, 111, 246]). It is worth
noting that, while most of the 2D research takes place sailing the graphene ‘flagship’,
various accompanying 2D ‘catamarans’ have been recently explored and have shown
a strong potential for next generation optoelectronics and materials sciences (see for
instance [5, 7, 12, 18, 20, 56, 92, 98, 232, 239]).
Optoelectronic Application Potential
Optoelectronic applications are understood to gain significantly from the research on
2D materials in the future and have still remained largely untapped. The following
examples may emphasise the great potential they exhibit. Naturally, the high carrier
mobilities render 2D semiconductors ideal candidates for efficient photodetector
and transistor schemes. Furthermore, the direct gap nature, pronounced light–matter
interactions and the controllable optoelectronic properties clearly promote the use
in semiconductor photonic devices. In addition, complex excitonic species, such
as charged excitons (trions) and exciton molecules (e.g. biexcitons), can enrich the
electrical transport of photoexcitation [247].
Benefits for Photonic 2D Devices
Remarkably, the strong exciton binding energies and the correspondingly large exciton oscillator strength in TMDCs can enable room-temperature strong light–matter
9 Note that in the literature, the use and definition of bilayer abbreviations may not be consistent,
so that care has to be taken when comparing different works referring to AA, AA’, AB and other
stacking situations.
37
[216, 217, 244], enabling to obtain spin–layer or spin–valley locking scenarios for
the naturally (AA’) [219, 245] and artificially (AB)
9 stacked TMDCs, respectively
[219]. A recent study by the author’s team with the help of collaboration partners
showed among other spectral signatures that a noticeable difference of circular polarisation anisotropy is evidenced for selectively CVD-grown AA’ and AB configured
homobilayers (corresponding schematics and band structure calculations displayed
in Fig. 2.11), with the latter exhibiting a higher value [219]; for hBN-supported AA’
and AB flakes, for which dephasing induced by defects and doping is reduced (cf.
[7]), a more pronounced linear and circular polarisation anisotropy is obtained [219].
Furthermore, their theory predicted a lifted degeneracy between layers, observable
in reflection-contrast experiments. These distinct bilayer cases could be for instance
interesting for future ‘valleytronic’ or even ‘layertronic’ device schemes.
2.5 Photonics and Optoelectronics of 2D Semiconductor
TMDCs
Besides the recent progress in applications of 2D semiconductor TMDCs with an
emphasis on strong excitonic effects in monolayers and heterostructures [21, 205],
the mere electronic and optical properties of 2D materials, as well as the spin- and
valley-dependent properties, caught the attention of various research communities
worldwide (see for instance the activities described in [16, 17, 111, 246]). It is worth
noting that, while most of the 2D research takes place sailing the graphene ‘flagship’,
various accompanying 2D ‘catamarans’ have been recently explored and have shown
a strong potential for next generation optoelectronics and materials sciences (see for
instance [5, 7, 12, 18, 20, 56, 92, 98, 232, 239]).
Optoelectronic Application Potential
Optoelectronic applications are understood to gain significantly from the research on
2D materials in the future and have still remained largely untapped. The following
examples may emphasise the great potential they exhibit. Naturally, the high carrier
mobilities render 2D semiconductors ideal candidates for efficient photodetector
and transistor schemes. Furthermore, the direct gap nature, pronounced light–matter
interactions and the controllable optoelectronic properties clearly promote the use
in semiconductor photonic devices. In addition, complex excitonic species, such
as charged excitons (trions) and exciton molecules (e.g. biexcitons), can enrich the
electrical transport of photoexcitation [247].
Benefits for Photonic 2D Devices
Remarkably, the strong exciton binding energies and the correspondingly large exciton oscillator strength in TMDCs can enable room-temperature strong light–matter
9 Note that in the literature, the use and definition of bilayer abbreviations may not be consistent,
so that care has to be taken when comparing different works referring to AA, AA’, AB and other
stacking situations.