Fascinating van der Waals structures: stacking, twisting & hybridization

Two-dimensional (2D) materials are atomically thin crystals characterized by strong in-plane chemical bonds and weak van der Waals (vdW) coupling between adjacent layers. This allows easy access to the crystals by exfoliation techniques such as the precedent “scotch-tape” tape methods. The weak coupling between the layers enables the combination of such 2D materials with other materials nearly without limitations e.g. towards wearables, flexible and bendable opto-/electronics and energy conversion applications. Atomically thin semiconducting transition metal dichalcogenides such as MoSe2 or WSe2 excel due to their strong exciton dominated light-matter interaction. Excitons are Coulomb-coupled electron-hole pairs and hence composite-bosons. The vdW nature allows the realization of precisely tailored 2D systems that can be engineered by stacking, precise twisting or by selectively inducing defects. In-situ control can be realized by external stimuli such as electric fields and charge doping in field effect structures.

Those VdW stacks can have properties individual layers or conventional 3D solids do not reveal: (i) twist-angle dependent moiré superstructure with periodic potential profiles and the formation of minibands [1] and (ii) momentum dependent hybridization of electronic state resulting in the competition between interlayer (IX) and intralayer (X) excitons [2,3]. In particular, we will discuss the low-temperature emission properties of dense interlayer exciton ensembles in MoSe2/WSe2 hetero-bilayers featuring several criticalities [4] and drastically reduced dipolar blueshift together with extended spatial coherence [5]. These findings are in agreement with the occurrence of a coherent many-body state of IX [5]. Moreover, we demonstrate that studying collective charge excitations between moiré-minibands in twisted WSe2 bilayers by means of resonant inelastic light scattering spectroscopy provide unique experimental access to the modulated bands in twisted bilayers structures [6].

[1] T. Deilmann et al., J. Phys.: Condens. Matter 32 333002 (2020).

[2] J. Kiemle et al., Phys. Rev. B 101, 121404(R) (2020).

[3] F. Sigger et al., Appl. Phys. Lett. 121, 071102 (2022).

[4] L. Sigl et al., Physical Review Research 2, 042044(R) (2020).

[5] M. Troue et al., Phys. Rev. Lett. 131, 036902 (2023).

[6] N. Saigal et al., in preparation.