38
2 Entering a Two-Dimensional Materials World
Fig. 2.11 WS 2 in different highly-symmetric stacking configurations, the naturally occurring AA’stacked type of bilayer and an artificially-stacked AB type. Sketch of the valley-selective optical
transitions (circularly polarised light, σ ± , represented by grey/black vertical arrows) at the K
point between valence and conduction bands for the single-layer (a), AA’ (b) and AB (c) bilayer
configurations. Spin orientations up (red) and down (blue) are associated with each band. While here
only the behaviour at the K point is described, the spin situation and polarisation selectivity at the
K point behaves simply oppositely. The calculated band structures of the three systems (a–c) are
depicted in (d–f) in the same order. Arrows indicate direct/indirect gaps. Insets depict schematically
the stacking alignment with in-plane constant a and mean interlayer (tungsten–tungsten) distance
d WW . a–f and insets adapted with permission. [219] Copyright 2019 American Chemical Society
coupling [64–66] and promise the achievement of exciton and polariton condensation at relatively high temperatures (up to room temperature for polaritons) [207,
248, 249]. These systems can become ideal testbeds for many-body phenomena, e.g.
in optical microcavities, while others exploit 2D-material microcavities for secondharmonic generation [250, 251]. Moreover, the valley- and spin-dependent properties
of TMDCs are attractive for potential applications in information processing [16, 17,
246], promising novel concepts that enable optical “valleytronics”—using principles
of spintronics [252]—such as ‘spin-and-valley laser’ (making use of valley- and spin-
2 Entering a Two-Dimensional Materials World
Fig. 2.11 WS 2 in different highly-symmetric stacking configurations, the naturally occurring AA’stacked type of bilayer and an artificially-stacked AB type. Sketch of the valley-selective optical
transitions (circularly polarised light, σ ± , represented by grey/black vertical arrows) at the K
point between valence and conduction bands for the single-layer (a), AA’ (b) and AB (c) bilayer
configurations. Spin orientations up (red) and down (blue) are associated with each band. While here
only the behaviour at the K point is described, the spin situation and polarisation selectivity at the
K point behaves simply oppositely. The calculated band structures of the three systems (a–c) are
depicted in (d–f) in the same order. Arrows indicate direct/indirect gaps. Insets depict schematically
the stacking alignment with in-plane constant a and mean interlayer (tungsten–tungsten) distance
d WW . a–f and insets adapted with permission. [219] Copyright 2019 American Chemical Society
coupling [64–66] and promise the achievement of exciton and polariton condensation at relatively high temperatures (up to room temperature for polaritons) [207,
248, 249]. These systems can become ideal testbeds for many-body phenomena, e.g.
in optical microcavities, while others exploit 2D-material microcavities for secondharmonic generation [250, 251]. Moreover, the valley- and spin-dependent properties
of TMDCs are attractive for potential applications in information processing [16, 17,
246], promising novel concepts that enable optical “valleytronics”—using principles
of spintronics [252]—such as ‘spin-and-valley laser’ (making use of valley- and spin-