Isoemissive Photoluminescence from a Quaternary System of Valley-Polarized, Defect-Bound Excitons and Trions in Two-Dimensional Transition Metal Dichalcogenides

Abstract

Interest in the use of local band extrema, known as valleys, as a possible information carrier has in recent years been advanced by the isolation of 2D transition metal dichalcogenides (TMDCs). In some monolayer TMDCs, the structural inversion asymmetry leads to spin-valley locked states that present potential advantages for information storage and manipulation beyond existing charge- and spin-based semiconductor technologies. However, understanding of the role of defects on exciton recombination and valley scattering remains lacking and limits progress toward applications. Here, we report gate voltage-dependent polarized photoluminescence measurements on CVD-grown monolayer WS2 and MoS2 field-effect transistors and observe a quaternary system of valley polarized excitons and trions. Intriguingly, we find the spectra to exhibit isoemissive points, which were previously seen only as evidence of a binary system. We propose a rate model to explain the existence of isoemission in a quaternary system and find that the presence of isoemissive points imply a kinetic relationship between recombination and valley scattering. This relationship sheds light on the inverse relation seen between the polarization degree and photoluminescence intensity. Further consideration of the phenomenon also led to the identification of a possible new mechanism for charge carrier detrapping using the energy released from trion recombination.

Liquid-Metal-Printed Ultrathin Oxides for Atomically Smooth 2D Material Heterostructures

“…We investigated spin-valley polarization through CDPL mapping of CVD grown monolayer WS 2 crystals with and without Ga 2 O 3 cover (Figure a–f) at T = 3.6 K. Notably, the degree of circular polarization (DoCP) in 2D WS 2 remained homogeneous with no significant changes between regions covered and uncovered by Ga 2 O 3 (Figure c,f), indicating preservation of spin-valley coupling, similar to or better than reports of hBN-encapsulated TMDCs. − These measurements suggest LM-printed Ga 2 O 3 to be an excellent dielectric for 2D TMDC-based applications in optoelectronics and valleytronics. CDPL reflects a complex interplay between recombination and valley scattering rates of exciton species. − Our technique for creating Ga 2 O 3 /WS 2 heterostructures allows further investigations and insights into the physics and sensitivity of intervalley scattering in 2D WS 2 to its immediate environment.…”

Dielectrics for Two-Dimensional Transition-Metal Dichalcogenide Applications

“…This limitation can be circumvented in 2D TMD semiconductors. , 2D TMDs are useful for optoelectronic applications as they can address a wavelength range (∼1–2.5 eV) including the visible, have higher absorption (from ≈10% in monolayer to ≈90% in multilayer), − and have excellent gate tunability. Optical properties of 2D TMDs in visible range are dominated by sharp excitonic resonances which are extremely sensitive to the dielectric environment, − especially in the monolayer form. Therefore, the dielectric can influence the performance of optoelectronic devices.…”

Quantum Technologies for Engineering: the materials challenge

“…This technique was used to rapidly screen our TMDC materials (exfoliated or grown) before using them, after lithographic patterning, and after capping with thin dielectric layers. In addition, it was also useful for elucidating the role of defects (within the material, or at contact interfaces) on exciton recombination, and the population exchange between excitons and trions in such 2D TMDCs under the influence of an external electric field, such as those typically experienced in devices under the application of a gate voltage [54].…”

“…Amongst the first capabilities built in Goh's labs were two characterisation techniques that specifically allowed us to access the degree of valley selectivity in 2D TMDC materials-circular dichroic photoluminescence, and spin-and angle-resolved photoemission (ARPES) [38,53,54]. The presence of such valley selectivity assures the spin-valley coupling/locking remains a feature we can exploit in the material.…”