2.4 Layered Systems Based on Monolayer Semiconductors
35
have provided access to a measurable optical dispersion (in the meV range, corresponding to effective masses of < 10
−3 m 0 )
8 of TMDC monolayer resonances [52],
and indications of other effects, such as polaron formation (coupling of excitons to
the optical-phonon bath) at elevated temperatures above 100 K [52, 53]. Recently,
also the role of hBN encapsulation as a nonuniform environment configuration for
monolayers on the exciton properties was discussed, showing experimentally that
excitons in encapsulated TMDC monolayers exhibit a Rydberg-like behaviour in a
fractional dimension between 2D and 3D [60].
Multistacks of TMDCs
Towards vertical structures consisting of multiple 2D materials, double wells of
monolayers can be considered as an introduction into this domain, for instance to
achieve stronger coupling phenomena in microcavities [65]. Recently, spatially indirect excitons (in the sense of dipolar excitons with electron and hole in separate layers)
formed between two hBN-buffered MoS 2 monolayers were shown and compared to
their intralayer counterparts [234].
Direct Gap Materials for Heterostructuring
MoS 2 and WS 2 are the most studied materials in the TMDC family. They feature
a relatively bright PL spectrum due to their direct band gap in the monolayer limit.
On the other hand, MoSe 2 and WSe 2 monolayers have smaller band gaps and higher
electron mobility compared to their sulfide analogues [235], and exhibit a higher PL
intensity compared to MoS 2 monolayers due to a high non-radiative recombination
of electrons and holes in MoS 2 [231, 236]. Almost always, higher mobility leads to
better device performance rendering MoSe 2 and WSe 2 more promising for practical
applications, including transistors and optoelectronic detectors. However, an increasing number of layers for these TMDCs induces a transition from a direct to an indirect
band gap, thereby reducing the PL intensity significantly [133]. Thus, it is presumed
that their direct-gap character can get spoiled within hybridised heterobilayers (i.e.
for monolayer–monolayer heterosystem resembling a homobilayer-like situation).
In comparison, MoSe 2 and MoTe 2 retain their direct band gaps in a regime of two
layers (tellurides up to a few layers) [237, 238], leading to higher PL intensities
beyond the monolayer regime compared to MoS 2 and other TMDCs in monolayer
heterostructures. Accordingly, they could become interesting components for heterobilayers.
Telluride-Based Heterostructures
Early considerations revealed that the combination of MoSe 2 and MoTe 2 monolayers results in type-II heterostructures (see for instance the calculated alignments in
[93–96]), which find applications as photovoltaic devices and detectors. In contrast,
the combination of WSe 2 and MoTe 2 materials is expected to provide a type-I heterostructure (see aforementioned theoretical works) with direct excitons, which has
applications in low-threshold diode lasers. Moreover, owing to the aforementioned
direct-gap nature of mono-to-few-layer tellurides, they likely retain their direct gap
even in heterostructures of bilayer tellurides with other monolayer TMDCs.
8 Free electron mass m 0 , natural constant.
35
have provided access to a measurable optical dispersion (in the meV range, corresponding to effective masses of < 10
−3 m 0 )
8 of TMDC monolayer resonances [52],
and indications of other effects, such as polaron formation (coupling of excitons to
the optical-phonon bath) at elevated temperatures above 100 K [52, 53]. Recently,
also the role of hBN encapsulation as a nonuniform environment configuration for
monolayers on the exciton properties was discussed, showing experimentally that
excitons in encapsulated TMDC monolayers exhibit a Rydberg-like behaviour in a
fractional dimension between 2D and 3D [60].
Multistacks of TMDCs
Towards vertical structures consisting of multiple 2D materials, double wells of
monolayers can be considered as an introduction into this domain, for instance to
achieve stronger coupling phenomena in microcavities [65]. Recently, spatially indirect excitons (in the sense of dipolar excitons with electron and hole in separate layers)
formed between two hBN-buffered MoS 2 monolayers were shown and compared to
their intralayer counterparts [234].
Direct Gap Materials for Heterostructuring
MoS 2 and WS 2 are the most studied materials in the TMDC family. They feature
a relatively bright PL spectrum due to their direct band gap in the monolayer limit.
On the other hand, MoSe 2 and WSe 2 monolayers have smaller band gaps and higher
electron mobility compared to their sulfide analogues [235], and exhibit a higher PL
intensity compared to MoS 2 monolayers due to a high non-radiative recombination
of electrons and holes in MoS 2 [231, 236]. Almost always, higher mobility leads to
better device performance rendering MoSe 2 and WSe 2 more promising for practical
applications, including transistors and optoelectronic detectors. However, an increasing number of layers for these TMDCs induces a transition from a direct to an indirect
band gap, thereby reducing the PL intensity significantly [133]. Thus, it is presumed
that their direct-gap character can get spoiled within hybridised heterobilayers (i.e.
for monolayer–monolayer heterosystem resembling a homobilayer-like situation).
In comparison, MoSe 2 and MoTe 2 retain their direct band gaps in a regime of two
layers (tellurides up to a few layers) [237, 238], leading to higher PL intensities
beyond the monolayer regime compared to MoS 2 and other TMDCs in monolayer
heterostructures. Accordingly, they could become interesting components for heterobilayers.
Telluride-Based Heterostructures
Early considerations revealed that the combination of MoSe 2 and MoTe 2 monolayers results in type-II heterostructures (see for instance the calculated alignments in
[93–96]), which find applications as photovoltaic devices and detectors. In contrast,
the combination of WSe 2 and MoTe 2 materials is expected to provide a type-I heterostructure (see aforementioned theoretical works) with direct excitons, which has
applications in low-threshold diode lasers. Moreover, owing to the aforementioned
direct-gap nature of mono-to-few-layer tellurides, they likely retain their direct gap
even in heterostructures of bilayer tellurides with other monolayer TMDCs.
8 Free electron mass m 0 , natural constant.