| AVS 63rd International Symposium & Exhibition | |
| 2D Materials Focus Topic | Monday Sessions |
| Session 2D+MI+SA-MoM |
| Session: | 2D Materials Characterization including Microscopy and Spectroscopy |
| Presenter: | Aubrey Hanbicki, Naval Research Laboratory |
| Authors: | A.T. Hanbicki, Naval Research Laboratory K.M. McCreary, Naval Research Laboratory M. Currie, Naval Research Laboratory G. Kioseoglou, University of Crete C.S. Hellberg, Naval Research Laboratory A.L. Friedman, Naval Research Laboratory B.T. Jonker, Naval Research Laboratory |
| Correspondent: | Click to Email |
Monolayer transition metal dichalcogenides (TMDs) such as MoS2 or WS2 are semiconductors with degenerate, yet inequivalent k-points labeled K and K’ that define the direct bandgap. The valence band maximum in each valley has only one spin state in which the spins are opposite for K and K’. Consequently, one can selectively populate each valley independently with circularly polarized light and determine the valley populations via the polarization of emitted light. Monitoring changes in emitted polarization, therefore provide insights into the fundamental processes of intervalley scattering. We prepare single-layer WS2 films such that the photoluminescence is from either the neutral exciton or the negatively charged trion [1,2]. In most TMDs, the optical polarization is small at room temperature, and we find that the neutral exciton emission indeed has zero polarization at room temperature. However, we observe a room temperature optical polarization in excess of 40% for the trion. The trion polarization always exceeds that of the exciton and exhibits a pronounced, non-monotonic temperature dependence – the polarization nearly doubles as the temperature increases from 125 K to 175 K. The observed increase in optical polarization directly correlates with a decrease in emission intensity between 125-175 K indicating that this effect is a consequence of the onset of nonradiative processes. Because this dependence involves trion systems, one can use gate voltages to modulate the polarization or intensity emitted from TMD structures. Using an applied gate voltage, we can modulate the electron density and subsequently the polarization of WS2 trions continuously from 20-40%. Both the polarization and the emission energy monotonically track the gate voltage with the emission energy increasing by 45 meV. We discuss the role electron capture of the trion has on suppressing the intervalley scattering process. This work was supported by core programs at NRL and the NRL Nanoscience Institute, and by the Air Force Office of Scientific Research #AOARD 14IOA018-134141.
[1] M. Currie, A.T. Hanbicki, G. Kioseoglou, and B.T. Jonker, Appl. Phys. Lett. 106, 201907 (2015).
[2] A.T. Hanbicki, G. Kioseoglou, M. Currie, C.S. Hellberg, K.M. McCreary, A.L. Friedman, and B.T. Jonker, Sci. Rep. 6, 18885 (2016).