Observation of Long-Lived Interlayer Excitons in Monolayer MoSe2-WSe2 Heterostructures (1403.4985v1)

Abstract: Two-dimensional (2D) materials, such as graphene1, boron nitride2, and transition metal dichalcogenides (TMDs)3-5, have sparked wide interest in both device physics and technological applications at the atomic monolayer limit. These 2D monolayers can be stacked together with precise control to form novel van der Waals heterostructures for new functionalities2,6-9. One highly coveted but yet to be realized heterostructure is that of differing monolayer TMDs with type II band alignment10-12. Their application potential hinges on the fabrication, understanding, and control of bonded monolayers, with bound electrons and holes localized in individual monolayers, i.e. interlayer excitons. Here, we report the first observation of interlayer excitons in monolayer MoSe2-WSe2 heterostructures by both photoluminescence and photoluminescence excitation spectroscopy. The energy and luminescence intensity of interlayer excitons are highly tunable by an applied vertical gate voltage, implying electrical control of the heterojunction band-alignment. Using time resolved photoluminescence, we find that the interlayer exciton is long-lived with a lifetime of about 1.8 ns, an order of magnitude longer than intralayer excitons13-16. Our work demonstrates the ability to optically pump interlayer electric polarization and provokes the immediate exploration of interlayer excitons for condensation phenomena, as well as new applications in 2D light-emitting diodes, lasers, and photovoltaic devices.

• The paper demonstrates that stacking monolayer MoSe2 and WSe2 creates interlayer excitons with a measured lifetime of about 1.8 ns.
• The study employs photoluminescence spectroscopy to identify exciton behavior and confirm type II band alignment in the heterostructure.
• The research highlights tunability via gate voltage and temperature, underscoring promising applications in next-generation optoelectronic devices.

Observation of Long-Lived Interlayer Excitons in Monolayer MoSe-WSe$_2$ Heterostructures

The paper "Observation of Long-Lived Interlayer Excitons in Monolayer MoSe-WSe$_2$ Heterostructures" presents a pivotal exploration of two-dimensional (2D) material heterostructures, particularly focusing on transition metal dichalcogenides (TMDs). Through precise stacking of monolayer MoSe$_2$ and WSe$_2$, the authors observe the formation of long-lived interlayer excitons, a phenomenon with promising implications for optoelectronic applications.

At the core of the paper is the capability to manipulate excitonic properties in TMDs, which dramatically influence the optical response due to strong Coulomb interactions. The authors address the challenge of generating interlayer excitons with a unique method that incorporates mechanical exfoliation and subsequent transfer to highly controlled substrate environments. Photoluminescence (PL) and PL excitation spectroscopy are utilized to effectively identify and monitor these excitons within the heterostructure.

A significant experimental result from this research is the identification and characterization of interlayer excitons with a measured lifetime of approximately 1.8 ns. This lifetime is notably an order of magnitude longer than that of intralayer excitons, implying significant potential for these states in future device technologies. The tunability of the excitons via external gate voltage, which alters luminescence intensity and spectral position, highlights the control over interlayer interactions in the heterojunction.

This paper does not just make strong observational claims but also explores theoretical predictions aligned with these findings. The demonstration of type II band alignment within the MoSe$_2$-WSe$_2$ system underpins potential developments in light-emitting diodes, lasers, and photovoltaic systems. The spatial separation of electrons and holes in different material layers, together with their strong binding within the heterostructure, opens avenues for further investigation into Bose-Einstein condensation phenomena, a theoretical landscape previously dominated by studies in GaAs systems.

Additionally, the research investigates the relationship between temperature and photoluminescence, showing that at lower temperatures, interlayer excitons are predominantly formed and recombine radiatively, while higher temperatures lead to non-radiative relaxation channels. The implications of these temperature-dependent characteristics of TMD heterostructures on device performance could prove vital for fine-tuning optoelectronic components.

From a practical standpoint, the paper’s innovation in fabricating and characterizing these heterostructures lays a strong foundation for advancing 2D material electronics. By providing direct evidence of long-lived interlayer excitons, this research underscores the technological potential for low-power, high-efficiency light-related applications.

In conclusion, this paper contributes significantly to the understanding of exciton dynamics in TMD heterostructures, presenting robust experimental evidence and theoretical interpretation. Future research may focus on the exploration of new TMD combinations, further enhancement of excitonic lifetimes, and integration into scalable electronic systems, potentially elevating the capacity of 2D materials in commercial applications. The findings serve as a crucial step towards leveraging the exotic quantum possibilities within van der Waals heterostructures.