Interlayer transfer of valley polarization from excitons to free charge carriers in WSe2/MoSe2 heterobilayers

Scientists from RWTH Aachen, AMO GmbH, Forschungszentrum Julich and the University of Regensburg have shown that in twisted heterobilayers of WSe2 and MoSe2 there is a transfer of valley polarization from excitons in WSe2 to free carriers in MoSe2. This mechanism, which is strongly dependent on the twist angle, may allow the realization of opto-valleytronic devices where the valley polarization is optically excited but extracted and measured by electrical means. The results are reported in npj 2D Materials and Applications.

Cover image on 2D materials and applications. Image credits: Sebastian Staacks.

The scientists investigated five WSe2/MoSe2-heterobilayer devices with different twist angles, using a number of different experimental techniques, including photo-luminescence, time resolved Kerr rotation (TKRR), and angle resolved photo emission spectroscopy (ARPES). The measurements were performed as a function of gate voltage, which allowed to vary the Fermi level into the system. The measurements indicate that the twist angle has a significant impact on the dynamics of valley-polarized free charge carriers. For large twist angles, the excitation of valley-polarized excitons in the WSe2 layer results in a pronounced valley polarization of free charge carriers in the MoSe2 layer. This transfer is most efficient at the onsets of either valence or conduction band and indicates the possibility of extracting and utilizing this valley polarization by electrical means.  Vice versa, for small twist angles the polarization of the  polarization of the free charge carriers in MoSe2 is ultimately suppressed by scattering mechanisms.  

Schematic device structure of the heterobilayer devices. Out of the five investigated devices, only three presented a graphite gate. The other were gated via the SiO2/Si++ subrate.

This research was led by Bernd Beschoten and is the result of collaboration between the groups of Christoph Stampfer (RWTH), Claus M. Schneider and Lukasz Plucinski (FZ J├╝lich), and Jaroslav Fabian (Uni Regensburg).

Bibliographic Informarion:
Twist angle dependent interlayer transfer of valley polarization from excitons to free charge carriers in WSe2/MoSe2 heterobilayers
F. Volmer, M. Ersfeld, P.E. Faria Junior, L. Waldecker, B. Parashar, L. Rathmann, S. Dubey, I. Cojocariu, V. Feyer, K. Watanabe, T. Taniguchi, C.M. Schneider, L. Plucinski, C. Stampfer, J. Fabian, and B. Beschoten, npj 2D Mater. Appl. 7, 58 (2023)