Twist angle dependent interlayer transfer of valley polarization from excitons to free charge carriers in WSe2/MoSe2 heterobilayers

Abstract

Transition metal dichalcogenides (TMDs) have attracted much attention in the fields of valley- and spintronics due to their property of forming valley-polarized excitons when illuminated by circularly polarized light. In TMD-heterostructures it was shown that these electron-hole pairs can scatter into valley-polarized interlayer exciton states, which exhibit long lifetimes and a twist-angle dependence. However, the question how to create a valley polarization of free charge carriers in these heterostructures after a valley selective optical excitation is unexplored, despite its relevance for opto-electronic devices. Here, we identify an interlayer transfer mechanism in twisted WSe2/MoSe2 heterobilayers that transfers the valley polarization from excitons in WSe2 to free charge carriers in MoSe2 with valley lifetimes of up to 12 ns. This mechanism is most efficient at large twist angles, whereas the valley lifetimes of free charge carriers are surprisingly short for small twist angles, despite the occurrence of interlayer excitons.

Anomalous redshift in interlayer exciton emission with increasing twist angle in WSe2/MoSe2 heterostructures

“…We attribute the observed IX emission to K-K transition in the mentioned samples (also illustrated in figure 1(c)). Considering the stacking order of the investigated samples, MoSe 2 sustains similar strain with changing twist angle which leads to the Q valley being energetically above the K valley in CB, as MoSe 2 is in contact with substrate [21,35]. Thus, IX formation and relaxation in the studied samples mainly happen through the momentum direct K-K transition.…”

“…Nevertheless, the band offset can be manipulated by altering the Fermi level of the individual materials layers at thermal equilibrium by, for instance, varying the local density of carriers (e.g. electrostatic doping) [35,[44][45][46][47]. In the TMDC material family, further aspects must be considered when studying a heterojunction formed by few-or mono-layers.…”

“…In the TMDC material family, further aspects must be considered when studying a heterojunction formed by few-or mono-layers. Interestingly, only few theoretical and experimental reports have discussed the band alignment of different monolayer materials [35,38], and its importance in the properties of TMDC HSs. However, it has not yet been established how the band offset of these systems is affected not only by the choice of materials but also by the angle between the layers that form the junction.…”

Computational exploration of two-dimensional vacancy-free boridene sheet and its derivatives: high stabilities and the promise for hydrogen evolution reaction

“…Figure 5 presents the calculated band structures for six different M 6 B 6 T 6 sheets. The band structures revealed that all sheets exhibit metallic characteristics, similar to borophene [62] and some MXenes such as V 2 C and Nb 2 C [63] . The band dispersions of M 6 B 6 T 6 were strongly affected by the surface terminals and different metal elements.…”

Advance in twisted transition metal dichalcogenides: synthesis, characterization, and properties

“…During the stacking, the crystal orientation is a key factor. This is because the properties of t-TMDCs are closely related to the relative orientation between the two layers [25]. Thus, it is essential to understand and control the crystal orientation of the involved monolayer before stacking.…”