Electrical Control of Two-Dimensional Neutral and Charged Excitons in a Monolayer Semiconductor

Abstract: Monolayer group VI transition metal dichalcogenides have recently emerged as semiconducting alternatives to graphene in which the true two-dimensionality (2D) is expected to illuminate new semiconducting physics. Here we investigate excitons and trions (their singly charged counterparts) which have thus far been challenging to generate and control in the ultimate 2D limit. Utilizing high quality monolayer molybdenum diselenide (MoSe2), we report the unambiguous observation and electrostatic tunability of charging effects in positively charged (X+), neutral (Xo), and negatively charged (X-) excitons in field effect transistors via photoluminescence. The trion charging energy is large (30 meV), enhanced by strong confinement and heavy effective masses, while the linewidth is narrow (5 meV) at temperatures below 55 K. This is greater spectral contrast than in any known quasi-2D system. We also find the charging energies for X+ and X- to be nearly identical implying the same effective mass for electrons and holes.

• The paper demonstrates a method to electrostatically tune excitonic states in a MoSe2 monolayer using FET configurations.
• It reports a significant trion binding energy of approximately 30 meV and narrow 5 meV linewidths at sub-55 K temperatures.
• The study highlights reversible conversion between neutral excitons and trions, paving the way for tunable optoelectronic devices.

Electrical Control of Neutral and Charged Excitons in a Monolayer Semiconductor: An Analysis

This paper presents a significant experimental investigation into excitonic phenomena within a two-dimensional (2D) context, using monolayer molybdenum diselenide (MoSe$_2$) as the material of choice. The authors provide a substantive contribution to the study of exciton physics in the ultimate 2D limit, demonstrating how monolayer group VI transition metal dichalcogenides (TMDs) can be leveraged to explore novel semiconducting behaviors beyond those traditionally observed in bulk 3D materials or quasi-2D quantum wells.

Key Findings and Methodologies

Utilizing photoluminescence within field-effect transistor (FET) configurations, the authors report on the unambiguous observation and electrostatic tunability of excitonic states, notably neutral excitons (X$^0$) and their charged counterparts, positive (X$^{+}$) and negative trions (X${}^{-}$). The experimental results show a significant trion binding energy of approximately 30 meV and narrow emission linewidths of 5 meV at sub-55 K temperatures. These findings indicate higher spectral contrast compared to known quasi-2D systems and underscore the potential for optical and electronic tunability that such 2D materials offer.

The study employs back-gated FET devices on high-quality MoSe$_2$ samples, allowing for the reversible conversion between different excitonic species through electrostatic doping. The large energy similarity between X$^{+}$ and X$^{-}$ also implies equivalence in effective masses for both electrons and holes in MoSe$_2$.

Implications for Theory and Application

The findings elucidate new facets in excitonic physics that can be explored due to the inherent 2D nature and strong Coulomb interactions in TMD monolayers. This research contributes to the growing understanding of excitonic behavior in true 2D systems and paves the way for potential applications in optoelectronic devices such as LEDs and excitonic circuits. The ability to control exciton and trion populations electrostatically suggests innovative pathways for developing devices with tunable properties and enhanced performance metrics. Additionally, the observed stability and distinct spectral properties open up opportunities for further exploration of phenomena like exciton condensation and the Fermi-edge singularity.

Future Directions in 2D Materials Research

The study proposes monolayer MoSe$_2$ as a promising platform for advanced investigations into spin-valley coupling and charge dynamics in TMDs, complementing the already significant research into valleytronics. Future work could extend to exploring superlattice structures or heterostructures incorporating different TMD layers to achieve further control over excitonic interactions and charge carrier dynamics.

As material quality and device fabrication techniques continue to progress, researchers may focus on reducing contact resistance and enhancing electron mobility, thus mitigating current limitations on carrier concentration equilibrium at low temperatures. This paper underscores the transformative potential of 2D materials in the continued evolution of semiconductor research and technology, warranting deeper theoretical and experimental explorations of their unique properties.