20
2 Entering a Two-Dimensional Materials World
with various different structures [70–74], and particularly for valleytronic nanophotonics motivated by the circular dichroism [17, 75].
Numerous applications (also see sections and subsections below for further explanations and examples), which are expected to benefit considerably from the special
properties of 2D electronic quasi-particles in such layered crystals, require controlling, manipulating and first of all understanding the nature of the optical resonances that are attributed to exciton modes [52]. In fact, theory [76–79] and previous
experiments [34, 80, 81] have provided unique methods to the characterisation and
classification efforts regarding the band structure and optical modes in 2D materials,
whereas only recently the quasi-particles’ energy–momentum dispersion came under
exploration with different techniques in the bulk [51] and in the monolayer regime
[52], using electronic and optical mapping of the momentum-space, respectively.
Prospects of Van-der-Waals Materials
The properties of 2D materials in the few- and single-layer regime are widely tunable
by doping, strain, external fields and environmental effects [5, 10, 84, 85], owing to
the materials’ atomic thickness and the strong influence of the surroundings [40, 49,
58, 86–88]. Their size, features and sheet flexibility promises the development of
a new generation of nano/micro-devices, in which flexible ultrathin 2D membranes
can be integrated [9, 89, 90] and even be stacked layer by layer [56, 91]. Such
few- and single-layer-based heterostructures as well as devices benefit from the
pronounced vdW interaction between adjacent layers that give rise to high-quality
homo- or heterojunction interfaces with little constraints on lattice matching. This
encourages production schemes such as vdW epitaxy and offers compatibility with
existing semiconductor technologies. Accordingly, novel device concepts on the
nanoscale have been envisioned for electronic, photonic and quantum technological
applications (Fig. 2.3).
Heterostructuring Possibilities and Novel Twist-Related Phenomena
Leading back the 2D crystals into the 3D world, their out-of-plane (vertical) heterostructures gained unrivaled importance as a means of obtaining new functionality
(e.g. through hybridisation, environmental or proximity effects) by building (functionalised) sequences of layers or encapsulated materials in stacks [5, 56, 91, 92].
This was supported by theoretical considerations [93–96]. New design possibilities are furthermore enabled by lateral heterostructuring, offering in-plane interfaces
[94, 97], or monolayer preparation with different lateral doping, leading to in-plane
pn-junctions (examples in [43, 98]).
Recently, the whole research on vertical heterostructures even received a new
twist. This happened, on the one hand, with the emergence of in-plane superlattices of
2D-periodic potential islands and moiré minibands [99–101], as well as moiré states
[102–105]. And on the other hand, it happened with the discovery of the extraordinary
behaviours of twisted homobilayers. Among those extraordinary behaviours, which
were initially theoretically obtained [106] and later experimentally demonstrated,
are superconductivity [107, 108], Mott-insulation [109] or emerging ferromagnetism
[110] in graphene bilayers. Thus, this new degree of freedom with regard to crystal
orientations has been termed in the literature as ‘twistronics’ [99].
2 Entering a Two-Dimensional Materials World
with various different structures [70–74], and particularly for valleytronic nanophotonics motivated by the circular dichroism [17, 75].
Numerous applications (also see sections and subsections below for further explanations and examples), which are expected to benefit considerably from the special
properties of 2D electronic quasi-particles in such layered crystals, require controlling, manipulating and first of all understanding the nature of the optical resonances that are attributed to exciton modes [52]. In fact, theory [76–79] and previous
experiments [34, 80, 81] have provided unique methods to the characterisation and
classification efforts regarding the band structure and optical modes in 2D materials,
whereas only recently the quasi-particles’ energy–momentum dispersion came under
exploration with different techniques in the bulk [51] and in the monolayer regime
[52], using electronic and optical mapping of the momentum-space, respectively.
Prospects of Van-der-Waals Materials
The properties of 2D materials in the few- and single-layer regime are widely tunable
by doping, strain, external fields and environmental effects [5, 10, 84, 85], owing to
the materials’ atomic thickness and the strong influence of the surroundings [40, 49,
58, 86–88]. Their size, features and sheet flexibility promises the development of
a new generation of nano/micro-devices, in which flexible ultrathin 2D membranes
can be integrated [9, 89, 90] and even be stacked layer by layer [56, 91]. Such
few- and single-layer-based heterostructures as well as devices benefit from the
pronounced vdW interaction between adjacent layers that give rise to high-quality
homo- or heterojunction interfaces with little constraints on lattice matching. This
encourages production schemes such as vdW epitaxy and offers compatibility with
existing semiconductor technologies. Accordingly, novel device concepts on the
nanoscale have been envisioned for electronic, photonic and quantum technological
applications (Fig. 2.3).
Heterostructuring Possibilities and Novel Twist-Related Phenomena
Leading back the 2D crystals into the 3D world, their out-of-plane (vertical) heterostructures gained unrivaled importance as a means of obtaining new functionality
(e.g. through hybridisation, environmental or proximity effects) by building (functionalised) sequences of layers or encapsulated materials in stacks [5, 56, 91, 92].
This was supported by theoretical considerations [93–96]. New design possibilities are furthermore enabled by lateral heterostructuring, offering in-plane interfaces
[94, 97], or monolayer preparation with different lateral doping, leading to in-plane
pn-junctions (examples in [43, 98]).
Recently, the whole research on vertical heterostructures even received a new
twist. This happened, on the one hand, with the emergence of in-plane superlattices of
2D-periodic potential islands and moiré minibands [99–101], as well as moiré states
[102–105]. And on the other hand, it happened with the discovery of the extraordinary
behaviours of twisted homobilayers. Among those extraordinary behaviours, which
were initially theoretically obtained [106] and later experimentally demonstrated,
are superconductivity [107, 108], Mott-insulation [109] or emerging ferromagnetism
[110] in graphene bilayers. Thus, this new degree of freedom with regard to crystal
orientations has been termed in the literature as ‘twistronics’ [99].