2.4 Layered Systems Based on Monolayer Semiconductors
33
Fig. 2.9 Manual heterostructuring enables rich variations in the stacking sequence as well as interlayer alignment of single-layer and few-layer vdWs materials. a–f Example of a stacking variation
involving primarily monolayers of WSe 2 . Red arrows point at the heterostructure regions. The most
common heterostructures are indeed achieved by hBN buffering (d), capping (e) or encapsulation
(b), having a noticeable influence on the properties of ultrathin 2D TMDCs, else often directly
deposited on substrates (a) due to simplicity (common in the early phase of 2D-materials research).
Property improvements and modifications are nowadays widely targeted with the help of sophisticated stackings, ideally to achieve “band-gap engineering”. Monolayer–monolayer heterostructures
such as in (c) and (f) have become particularly promising due to the achievable band offsets that
give rise to charge-transfer states across type-II heterojunctions. While (a, b, d, e) clearly address
changes of the dielectric environment including in both half-spaces surrounding the monolayer, c,
f show even more effects, using manually incorporated MoSe 2 above (c) and below (f) the WSe 2
specimen, as studied in [40] (published 2017) and many more works of the same time. Remarkably, it can be deduced that the capping of an hBN-supported WSe 2 monolayer by even one single
monolayer TMDC (c) is enough to achieve a similarly strong impact on the linewidth and energy of
excitonic features as the capping with multilayer hBN (b). This indicates that the dominant screening effect takes place within the first surrounding layers, while it should be noted that MoSe 2 has
not only a larger dielectric constant, but can also drain charge carriers (transfer in a unipolar fashion
under phase-matching conditions, see [212]) from or feed energy (excitation resonantly transferred,
see [220]) to the WSe 2 [40]. Left insets: Sketches of the over-simplified band alignments (CB/VB
energy versus z coordinate). Shaded areas between “CB minimum” and “VB maximum” levels
indicate the energy gap. Right insets: Stacking configurations for these examples. Colour code:
grey, light grey, blue and red correspond to substrate, hBN, MoSe 2 and WSe 2 , respectively. a–f
Adapted with permission. [40] Copyright 2017 Elsevier B.V
Alignment of Bands in Heterostructures
Fundamental properties of resulting heterostructures depend crucially on the alignment of electronic bands at the interface (cf. insets in Fig. 2.9), thereby affecting the
electron and hole wave-functions. In type-I (type-II) heterostructures, wave-functions
are confined in a common (different) spatial region. Type-II heterostructures [210,
221–223] (cf. Fig.2.9c, f), which can induce spatial charge separation [224, 225],
lead to the desired indirect-exciton species [222, 226], as well as interlayer coupling
strength and charge-transfer rate, which can be tuned by introducing barriers between
the two layers of interest [221].
33
Fig. 2.9 Manual heterostructuring enables rich variations in the stacking sequence as well as interlayer alignment of single-layer and few-layer vdWs materials. a–f Example of a stacking variation
involving primarily monolayers of WSe 2 . Red arrows point at the heterostructure regions. The most
common heterostructures are indeed achieved by hBN buffering (d), capping (e) or encapsulation
(b), having a noticeable influence on the properties of ultrathin 2D TMDCs, else often directly
deposited on substrates (a) due to simplicity (common in the early phase of 2D-materials research).
Property improvements and modifications are nowadays widely targeted with the help of sophisticated stackings, ideally to achieve “band-gap engineering”. Monolayer–monolayer heterostructures
such as in (c) and (f) have become particularly promising due to the achievable band offsets that
give rise to charge-transfer states across type-II heterojunctions. While (a, b, d, e) clearly address
changes of the dielectric environment including in both half-spaces surrounding the monolayer, c,
f show even more effects, using manually incorporated MoSe 2 above (c) and below (f) the WSe 2
specimen, as studied in [40] (published 2017) and many more works of the same time. Remarkably, it can be deduced that the capping of an hBN-supported WSe 2 monolayer by even one single
monolayer TMDC (c) is enough to achieve a similarly strong impact on the linewidth and energy of
excitonic features as the capping with multilayer hBN (b). This indicates that the dominant screening effect takes place within the first surrounding layers, while it should be noted that MoSe 2 has
not only a larger dielectric constant, but can also drain charge carriers (transfer in a unipolar fashion
under phase-matching conditions, see [212]) from or feed energy (excitation resonantly transferred,
see [220]) to the WSe 2 [40]. Left insets: Sketches of the over-simplified band alignments (CB/VB
energy versus z coordinate). Shaded areas between “CB minimum” and “VB maximum” levels
indicate the energy gap. Right insets: Stacking configurations for these examples. Colour code:
grey, light grey, blue and red correspond to substrate, hBN, MoSe 2 and WSe 2 , respectively. a–f
Adapted with permission. [40] Copyright 2017 Elsevier B.V
Alignment of Bands in Heterostructures
Fundamental properties of resulting heterostructures depend crucially on the alignment of electronic bands at the interface (cf. insets in Fig. 2.9), thereby affecting the
electron and hole wave-functions. In type-I (type-II) heterostructures, wave-functions
are confined in a common (different) spatial region. Type-II heterostructures [210,
221–223] (cf. Fig.2.9c, f), which can induce spatial charge separation [224, 225],
lead to the desired indirect-exciton species [222, 226], as well as interlayer coupling
strength and charge-transfer rate, which can be tuned by introducing barriers between
the two layers of interest [221].