[Paper Review] Anomalous valley polarization in monolayer MoSe2

Anmin Zhang, Jiahe Fan+9 more|arXiv (Cornell University)|Mar 30, 2015

2D Materials and Applications2 references3 citations

TL;DR

This study investigates valley polarization in monolayer MoSe2 using polarized photoluminescence, revealing an anomalous 70% valley polarization for the B exciton—significantly higher than in MoS2—while the A exciton shows less than 3% polarization and a small negative polarization for the A trion. These results expose novel intra- and inter-valley scattering processes, advancing understanding of excitonic physics and valleytronic potential in transition-metal dichalcogenides.

ABSTRACT

Modern electronic devices heavily rely on the accurate control of charge and spin of electrons. The emergence of controllable valley degree of freedom brings new possibilities and presents a promising prospect towards valleytronics. Recently, valley excitation selected by chiral optical pumping has been observed in monolayer MoS2. In this work, we report polarized photoluminescence (PL) measurements for monolayer MoSe2, another member of the family of transition-metal-dichalcogenides (MX2), and observe drastic difference from the outcomes of MoS2. In particular, we identify a valley polarization (VP) up to 70% for B exciton, while that for A exciton is less than 3%. Besides, we also find a small but finite negative VP for A- trion. These results reveal several new intra- and inter-valley scattering processes which significantly affect valley polarization, hence provide new insights into exciton physics in monolayer MX2 and possible valleytronic applications.

Motivation & Objective

• To investigate valley polarization in monolayer MoSe2, a member of the transition-metal dichalcogenide (MX2) family.
• To compare valley polarization behavior in MoSe2 with that of MoS2, where chiral optical pumping has previously induced strong valley polarization.
• To identify and understand the underlying scattering processes responsible for the observed valley polarization anomalies in MoSe2.
• To explore the implications of these findings for excitonic physics and future valleytronic device applications.

Proposed method

• Polarized photoluminescence (PL) measurements were performed on exfoliated monolayer MoSe2 flakes to probe valley polarization.
• Circularly polarized light was used to selectively excite specific valleys, enabling the study of valley-selective emission.
• The degree of valley polarization was quantified by analyzing the intensity ratio of left- and right-circularly polarized photoluminescence components.
• The analysis focused on distinct excitonic states: A exciton, B exciton, and A trion, to compare their polarization behaviors.
• Supplementary information and control experiments were used to validate the observed polarization values and rule out artifacts.
• Theoretical interpretation of scattering processes was informed by the experimental results to explain the anomalous polarization trends.

Experimental results

Research questions

• RQ1 Why does monolayer MoSe2 exhibit significantly different valley polarization compared to MoS2 despite similar crystal structure and electronic properties?
• RQ2 What causes the strong valley polarization of 70% in the B exciton state of MoSe2, while the A exciton shows less than 3% polarization?
• RQ3 What are the roles of intra- and inter-valley scattering processes in determining the observed valley polarization in MoSe2?
• RQ4 Why does the A trion exhibit a small but finite negative valley polarization?
• RQ5 How do these polarization anomalies influence the potential for valleytronic applications in 2D transition-metal dichalcogenides?

Key findings

• The B exciton in monolayer MoSe2 exhibits a valley polarization of up to 70%, indicating strong valley selectivity under chiral optical excitation.
• In contrast, the A exciton shows a valley polarization of less than 3%, indicating minimal valley polarization under the same conditions.
• A small but finite negative valley polarization is observed for the A trion, suggesting the presence of non-trivial many-body effects or scattering pathways.
• The results reveal significant intra- and inter-valley scattering processes that strongly influence valley polarization, differing from the behavior in MoS2.
• These findings provide new insights into the complex excitonic many-body physics in monolayer MX2 materials.
• The anomalous polarization behavior in MoSe2 highlights its unique potential for valleytronic applications, particularly due to the high and selective B-exciton polarization.