22
2 Entering a Two-Dimensional Materials World
properties [6] are as important for applications as their optoelectronic features and
their various forms of polarisation-based excitations (polaritons, hybrid light–matter
modes
1 ) [111].
Recent studies revealed the high elastic modulus of 2D materials such as graphene,
MoS 2 or WS 2 [112], which open up new pathways to ultrathin mechanical membranes for different micro/nano- (opto/electro-) mechanical applications. Similarly,
the demonstration of strong piezoelectricity in single-layer MoS 2 suggested possible applications in nanoscale electromechanical devices for sensing and energy
harvesting [11]. Presumably, one of the most outstanding possible applications of
suspended graphene membranes in biology is the detection of DNA, when drilled
nanoholes in that graphene layer enable an indirect estimation of the molecule’s size
and conformation through DNA translocation from one side of the pin-holed sheet
to the other one [113]. Indeed, many of the features, which 2D semiconductors of
the TMDC class exhibit, had been also discussed for applications in the field of
biological systems [15].
2D Honeycomb Lattice Made of Carbon: Graphene
Graphene, a single sheet of carbon atoms arranged in a hexagonal 2D lattice and
highly conductive, is the most studied representative of the class of 2D materials.
One can think of a pencil’s graphite tip being thinned down to the minimum of one
single layer, which is about a quarter of a nanometer thick but, remarkably, exhibits a
universal broadband absorption of about πα = 2.3%.
2 Correspondingly, graphene’s
optical contrast on certain substrates is good enough to identify monolayer flakes
visually under the microscope. How the remarkable properties could be used for
optoelectronic applications is briefly highlighted in the following subsection.
Finding Gaped 2D-Material Systems
However, the lack of a band gap renders graphene unsuitable for many digital electronic and optoelectronic applications. Consequently, significant efforts have been
devoted to identifying alternative 2D materials of semiconducting type [91, 116].
In the past decade, several classes of non-carbonic compounds with layered structure have been synthesised, including hBN [117], TMDCs [10], oxides [118, 119],
hydroxides [120], and oxychlorides [121]. Remarkably, they address different optical
frequencies (indicated for instance in spectral diagrams in [9, 122, 123], see Fig. 2.1).
Moreover, they provide a unique pool of vdW materials that has even become attractive for vdW epitaxy (see for instance map of energy gap versus lattice constant in
[124], see Fig. 7.1 in Chap. 7).
1 For 2D materials, the literature discusses species such as plasmon–polaritons, phonon–polaritons,
exciton–polaritons, cavity–polaritons, Cooper-pair–polaritons, and magnon–polaritons.
2 Here, α corresponds to Sommerfeld’s (fine-structure) constant, a fundamental natural constant
related to the electro-magnetic vacuum field’s interaction with charged elementary particles (important for QED)—so-to-say the coupling constant for electromagnetism determining the strength of
interactions between charged particles. Thus, it drew the attention of scientists working on graphene,
such as K. Novoselov, that graphene’s universal absorption (around its Dirac cone/point) was
‘quite a good direct measure’ of that constant [8].
2 Entering a Two-Dimensional Materials World
properties [6] are as important for applications as their optoelectronic features and
their various forms of polarisation-based excitations (polaritons, hybrid light–matter
modes
1 ) [111].
Recent studies revealed the high elastic modulus of 2D materials such as graphene,
MoS 2 or WS 2 [112], which open up new pathways to ultrathin mechanical membranes for different micro/nano- (opto/electro-) mechanical applications. Similarly,
the demonstration of strong piezoelectricity in single-layer MoS 2 suggested possible applications in nanoscale electromechanical devices for sensing and energy
harvesting [11]. Presumably, one of the most outstanding possible applications of
suspended graphene membranes in biology is the detection of DNA, when drilled
nanoholes in that graphene layer enable an indirect estimation of the molecule’s size
and conformation through DNA translocation from one side of the pin-holed sheet
to the other one [113]. Indeed, many of the features, which 2D semiconductors of
the TMDC class exhibit, had been also discussed for applications in the field of
biological systems [15].
2D Honeycomb Lattice Made of Carbon: Graphene
Graphene, a single sheet of carbon atoms arranged in a hexagonal 2D lattice and
highly conductive, is the most studied representative of the class of 2D materials.
One can think of a pencil’s graphite tip being thinned down to the minimum of one
single layer, which is about a quarter of a nanometer thick but, remarkably, exhibits a
universal broadband absorption of about πα = 2.3%.
2 Correspondingly, graphene’s
optical contrast on certain substrates is good enough to identify monolayer flakes
visually under the microscope. How the remarkable properties could be used for
optoelectronic applications is briefly highlighted in the following subsection.
Finding Gaped 2D-Material Systems
However, the lack of a band gap renders graphene unsuitable for many digital electronic and optoelectronic applications. Consequently, significant efforts have been
devoted to identifying alternative 2D materials of semiconducting type [91, 116].
In the past decade, several classes of non-carbonic compounds with layered structure have been synthesised, including hBN [117], TMDCs [10], oxides [118, 119],
hydroxides [120], and oxychlorides [121]. Remarkably, they address different optical
frequencies (indicated for instance in spectral diagrams in [9, 122, 123], see Fig. 2.1).
Moreover, they provide a unique pool of vdW materials that has even become attractive for vdW epitaxy (see for instance map of energy gap versus lattice constant in
[124], see Fig. 7.1 in Chap. 7).
1 For 2D materials, the literature discusses species such as plasmon–polaritons, phonon–polaritons,
exciton–polaritons, cavity–polaritons, Cooper-pair–polaritons, and magnon–polaritons.
2 Here, α corresponds to Sommerfeld’s (fine-structure) constant, a fundamental natural constant
related to the electro-magnetic vacuum field’s interaction with charged elementary particles (important for QED)—so-to-say the coupling constant for electromagnetism determining the strength of
interactions between charged particles. Thus, it drew the attention of scientists working on graphene,
such as K. Novoselov, that graphene’s universal absorption (around its Dirac cone/point) was
‘quite a good direct measure’ of that constant [8].