34
2 Entering a Two-Dimensional Materials World
Fig. 2.10 Two schematic hexagonal lattices on top of each other with two different twist angles
showing moiré patterns, representing typical scenarios for arbitrarily stacked TMDC heterobilayers.
These angles were visually approximated in [228] for a manually-stacked monolayer–monolayer
heterostructure in a study of intra- and interlayer excitons in such artificial TMDC heterobilayer
with expected moiré landscape. Large superlattices are generally expected for small twists around
0 (aligned) and 60 ◦ (anti-aligned for systems with three-fold symmetry). The sketch indicates the
translation unit vectors (a 1 and a 2 ) of the formed supercell and labels high-symmetry points of the
heterosystem’s supercell according to the nomenclature in [101]
Charge and Energy Transfer
The preservation of spin- and valley-degrees of freedom after charge transfer [210,
224] even renders these heterostructures applicable in the field of valleytronics. In
fact, charge transfer in a type-II 2D heterostructure can be accompanied by a Försterlike energy transfer which prevails even with thin interlayer-hBN spacers [220]. In
addition, electrical control of the exciton flux in vdWs heterostructures was recently
demonstrated [227]. Hereby, existing studies clearly show the attractiveness and
potential of heterostructuring for science and applications.
Stacking Sequence and Orientations
It is the choice of 2D materials and their stacking sequence that make 2D heterostructures unique in terms of optoelectronic properties and their use, e.g. for
high-performance photodetectors (see e.g. [229, 230]) or efficient photovoltaics (e.g.
[98, 231, 232] and references therein). It shall be noted that many of these applications can also benefit from in-plane as well as out-of-plane homojunctions of
differently doped monolayers. Furthermore, owing to the emergence of interesting
physics in twisted heterobilayers (sketched in Fig. 2.10), which introduce periodic
potential landscapes that give rise to moiré minibands [99, 100], new concepts such
as twistable electronics with dynamically rotatable heterostructures were recently
envisaged [233].
hBN-Based Heterostructures
In addition to conventional monolayer–monolayer heterostructures, heterostructures
of monolayers with hBN attracted considerable attention due to their impact on the
excitonic signatures [57–59], as well as dynamics and phonon coupling [38, 40]. On
the one hand, the emission linewidth considerably narrows when monolayer TMDCs
are encapsulated by hBN. On the other hand, the lifetime, exciton–exciton annihilation behaviour and the exciton/trion coupling to phonons is altered, as reported
among others in [40]. Moreover, the conditions obtained through hBN encapsulation
2 Entering a Two-Dimensional Materials World
Fig. 2.10 Two schematic hexagonal lattices on top of each other with two different twist angles
showing moiré patterns, representing typical scenarios for arbitrarily stacked TMDC heterobilayers.
These angles were visually approximated in [228] for a manually-stacked monolayer–monolayer
heterostructure in a study of intra- and interlayer excitons in such artificial TMDC heterobilayer
with expected moiré landscape. Large superlattices are generally expected for small twists around
0 (aligned) and 60 ◦ (anti-aligned for systems with three-fold symmetry). The sketch indicates the
translation unit vectors (a 1 and a 2 ) of the formed supercell and labels high-symmetry points of the
heterosystem’s supercell according to the nomenclature in [101]
Charge and Energy Transfer
The preservation of spin- and valley-degrees of freedom after charge transfer [210,
224] even renders these heterostructures applicable in the field of valleytronics. In
fact, charge transfer in a type-II 2D heterostructure can be accompanied by a Försterlike energy transfer which prevails even with thin interlayer-hBN spacers [220]. In
addition, electrical control of the exciton flux in vdWs heterostructures was recently
demonstrated [227]. Hereby, existing studies clearly show the attractiveness and
potential of heterostructuring for science and applications.
Stacking Sequence and Orientations
It is the choice of 2D materials and their stacking sequence that make 2D heterostructures unique in terms of optoelectronic properties and their use, e.g. for
high-performance photodetectors (see e.g. [229, 230]) or efficient photovoltaics (e.g.
[98, 231, 232] and references therein). It shall be noted that many of these applications can also benefit from in-plane as well as out-of-plane homojunctions of
differently doped monolayers. Furthermore, owing to the emergence of interesting
physics in twisted heterobilayers (sketched in Fig. 2.10), which introduce periodic
potential landscapes that give rise to moiré minibands [99, 100], new concepts such
as twistable electronics with dynamically rotatable heterostructures were recently
envisaged [233].
hBN-Based Heterostructures
In addition to conventional monolayer–monolayer heterostructures, heterostructures
of monolayers with hBN attracted considerable attention due to their impact on the
excitonic signatures [57–59], as well as dynamics and phonon coupling [38, 40]. On
the one hand, the emission linewidth considerably narrows when monolayer TMDCs
are encapsulated by hBN. On the other hand, the lifetime, exciton–exciton annihilation behaviour and the exciton/trion coupling to phonons is altered, as reported
among others in [40]. Moreover, the conditions obtained through hBN encapsulation