Control of the orbital character of indirect excitons in MoS2/WS2 heterobilayers

Abstract

Valley selective hybridization and residual coupling of electronic states in commensurate van der Waals heterobilayers enable the control of the orbital character of interlayer excitons. We demonstrate electric field control of layer index, orbital character, lifetime and emission energy of indirect excitons in MoS2/WS2 heterobilayers embedded in an vdW field effect structure. Different excitonic dipoles normal to the layers are found to stem from bound electrons and holes located in different valleys of MoS2/WS2 with a valley selective degree of hybridization. For the energetically lowest emission lines, coupling of electronic states causes a field-dependent level anticrossing that goes along with a change of the IX lifetime from 400 ns to 100 ns. In the hybridized regime the exiton is delocalized between the two constituent layers, whereas for large positive or negative electric fields, the layer index of the bound hole is field-dependent. Our results demonstrate the design of novel van der Waals solids with the possibility to in-situ control their physical properties via external stimuli such as electric fields.

Interlayer exciton formation, relaxation, and transport in TMD van der Waals heterostructures

“…Since electrons and holes are confined in opposite layers of a type-II vdW heterostructure, interlayer excitons have a static electric dipole along the out-of-plane direction ( p = e · d , where e is the charge quantity and d is the charge separation distance), which allows their energy to be tuned (Δ ε ) by an external electric field ( E ) along the dipole axis (i.e., the Stark effect, Δ ε = − p · E ) 22 , 95 , 117 . Strong and linear tuning of the interlayer exciton PL energy with an applied electric field was indeed observed in several studies with a tuning range of ~80–138 meV 22 , 26 , 30 , 86 , 95 , 117 , reflecting that the linear Stark effect mainly contributed to the energy shift (Fig. 3d ).…”

“…Such a stacking mode or twist angle dependence was also revealed in the MoS 2 /WSe 2 heterostructure, with lifetimes ranging from ~50 ps to ~3 ns 56 . Surprisingly, the interlayer exciton lifetime in the hBN-capsulated MoS 2 /WS 2 heterostructure was found to be significantly longer (~100 ns) 26 than that of the above uncapsulated MoS 2 /WS 2 heterostructure 42 , the reason for which is obscure and may be related to complicated factors such as the different material structures, fabrication methods, temperatures, twist angles between layers, and detection techniques.…”

Light–matter interaction in van der Waals hetero-structures

“…In photoluminescence experiments, multiplet emission lines from IX recombination are reported from different combinations of TMDC hetero-structures, pointing towards a complex interplay between multi-valley physics and hybridization of electronic states [54][55][56][96][97][98][99]. Doublet structures have been observed in the photoluminescence from MoSe 2 /WSe 2 hetero-bilayer by several groups [35,54,96,97].…”

“…(d) Room temperature IX photoluminescence spectrum of a hBN encapsulated MoS2/WS2 hetero-bilayer clearly indicating a triplet structure. The individual contributions I1, I2, I3 are included from Voigt fits to the spectrum[95] (similar spectra shown in[55]). (e) DFT band structure of MoS2/WS2 hetero-bilayer with projections of bands onto individual layers are shown with a color gradient showing high degree of hybridization at the Γ and Q/Σ point[95].…”

Light-field and spin-orbit-driven currents in van der Waals materials

“…Due to their weak interlayer coupling, van der Waals materials can be combined (or stacked) into atomically sharp heterostructures and superlattices without the lattice matching constraints imposed by traditional thin film epitaxy [7]. This ability to create vertical heterostructures has been used, for example, in semiconducting TMDs to study photogenerated Coulomb-bound electron hole pairs, socalled interlayer excitons, featuring prolonged radiative lifetimes [8][9][10]. Furthermore, adjusting the twist angle between two adjacent layers opens up the possibility to engineer the lateral band structure towards collective electronic excitations and altered optical properties via the formation of in-plane moiré superlattices at will [11][12][13].…”