220
7 Structuring Possibilities
7.2.3 Stacking of van-der-Waals Materials
Mechanical exfoliation has provided an incredibly easy and hands-on technique
to produce monolayered samples by oneself on the lab scale, as K. Novoselov has
highlighted in his Nobel lecture [45]. This triggered the vast field of graphene research
and beyond, in an incredible fashion that it caused the plethora of studies on graphenerelated materials and 2D materials in the wider sense, such as monolayer TMDCs,
and their vertical heterostructures [46–48]. As was outlined in a news feature in the
journal Nature, the natural question which arises is, “2D, or not 2D” [49].
In the sense of entering the 3D world with 2D materials for different purposes, heterostructuring of van-der-Waals materials has grown drastically in significance—for
instance for charge-transfer- or tunneling-current-based electronic or optoelectronic
devices, which may need schemes for electronic coupling between layers or charge
separation at interfaces.
Moreover, it has been understood that the optical quality of monolayer semiconductors and the energetics of their resonances [51–55] are strongly altered by the
environment, which in the most fundamental case is encapsulation by insulating 2D
material hexagonal boron nitride (hBN, also known as α-BN, labelled graphitic BN).
But even buffer layers of hBN, which provide an atomically-smooth, chemicallyinert and undoped interface as (hydrophobic) vdW substrate material, are known
to improve optoelectronic properties such as the luminescence yield from common
TMDCs, as investigated in [53] (see Fig. 2.9 for optical micrographs of various manually stacked vdW heterostructures using primarily hBN and WSe 2 ). Moreover, they
can practically act as an insulator, as needed for instance in [50] to suppress the
(unexpected) quenching of PL when TMDCs are deposited directly on transparent
GaP. To achieve the goal of PL enhancement by employing a specially designed and
processed photonic nanostructure in GaP, different mechanical exfoliation, transfer
and stacking steps were needed, the overview of which is shown as a microscope
image series in Fig. 7.6.
Recent experiments with high-quality tungsten-diselenide monolayers sandwiched
between hBN flakes (Fig. 5.17a) even showed a considerable energy–momentum
dispersion of the excitonic resonances in optical measurements [55]. Such measurements also demonstrated considerable helicity variations for emerging exciton
complexes. The radiation patterns of these exciton complexes were separately characterised in a study of in-plane and out-of-plane exciton species in WSe 2 [56], and
probed as a function of the excitation detuning as well as detected with a momentumdependent valley-pseudo-spin texture [57]. Those experiments rely on high-quality
WSe 2 bulk single crystals, which were grown in an excess selenium flux with a defect
density of 5 × 10
10 cm
−2 (see [19]).
Encapsulated samples comprising monolayer WSe 2 and hBN with very high quality are produced for instance in the author’s partner groups of J. Hone and K. Barmak at the Columbia University, with the help of S. Esdaille, D. A. Rhodes, and
provided for joint experimental work. Therefor, 2D-materials flakes are initially
exfoliated from bulk single crystals onto SiO 2 before stacking them sequentially
7 Structuring Possibilities
7.2.3 Stacking of van-der-Waals Materials
Mechanical exfoliation has provided an incredibly easy and hands-on technique
to produce monolayered samples by oneself on the lab scale, as K. Novoselov has
highlighted in his Nobel lecture [45]. This triggered the vast field of graphene research
and beyond, in an incredible fashion that it caused the plethora of studies on graphenerelated materials and 2D materials in the wider sense, such as monolayer TMDCs,
and their vertical heterostructures [46–48]. As was outlined in a news feature in the
journal Nature, the natural question which arises is, “2D, or not 2D” [49].
In the sense of entering the 3D world with 2D materials for different purposes, heterostructuring of van-der-Waals materials has grown drastically in significance—for
instance for charge-transfer- or tunneling-current-based electronic or optoelectronic
devices, which may need schemes for electronic coupling between layers or charge
separation at interfaces.
Moreover, it has been understood that the optical quality of monolayer semiconductors and the energetics of their resonances [51–55] are strongly altered by the
environment, which in the most fundamental case is encapsulation by insulating 2D
material hexagonal boron nitride (hBN, also known as α-BN, labelled graphitic BN).
But even buffer layers of hBN, which provide an atomically-smooth, chemicallyinert and undoped interface as (hydrophobic) vdW substrate material, are known
to improve optoelectronic properties such as the luminescence yield from common
TMDCs, as investigated in [53] (see Fig. 2.9 for optical micrographs of various manually stacked vdW heterostructures using primarily hBN and WSe 2 ). Moreover, they
can practically act as an insulator, as needed for instance in [50] to suppress the
(unexpected) quenching of PL when TMDCs are deposited directly on transparent
GaP. To achieve the goal of PL enhancement by employing a specially designed and
processed photonic nanostructure in GaP, different mechanical exfoliation, transfer
and stacking steps were needed, the overview of which is shown as a microscope
image series in Fig. 7.6.
Recent experiments with high-quality tungsten-diselenide monolayers sandwiched
between hBN flakes (Fig. 5.17a) even showed a considerable energy–momentum
dispersion of the excitonic resonances in optical measurements [55]. Such measurements also demonstrated considerable helicity variations for emerging exciton
complexes. The radiation patterns of these exciton complexes were separately characterised in a study of in-plane and out-of-plane exciton species in WSe 2 [56], and
probed as a function of the excitation detuning as well as detected with a momentumdependent valley-pseudo-spin texture [57]. Those experiments rely on high-quality
WSe 2 bulk single crystals, which were grown in an excess selenium flux with a defect
density of 5 × 10
10 cm
−2 (see [19]).
Encapsulated samples comprising monolayer WSe 2 and hBN with very high quality are produced for instance in the author’s partner groups of J. Hone and K. Barmak at the Columbia University, with the help of S. Esdaille, D. A. Rhodes, and
provided for joint experimental work. Therefor, 2D-materials flakes are initially
exfoliated from bulk single crystals onto SiO 2 before stacking them sequentially