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

TL;DR

This study demonstrates the electrical control and observation of charged excitons in monolayer MoSe2, revealing large trion binding energies and nearly identical electron and hole effective masses, advancing understanding of 2D semiconductors.

Contribution

It provides the first clear demonstration of electrostatic tunability of excitons and trions in monolayer MoSe2 with detailed spectroscopic analysis.

Findings

Large trion charging energy of 30 meV

Narrow linewidth of 5 meV below 55 K

Nearly identical charging energies for X+ and X-

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.