Strongly-bound excitons and trions in anisotropic 2D semiconductors

TL;DR

This paper reports the experimental observation of strongly-bound excitons and trions in bilayer phosphorene, demonstrating highly anisotropic optical properties and large exciton binding energies, protected from oxidation via hBN encapsulation.

Contribution

It provides the first experimental evidence of anisotropic excitons and trions in bilayer phosphorene, with detailed optical characterization and insights into their binding energies and anisotropic behavior.

Findings

Excitons and trions exhibit highly anisotropic optical spectra.

Neutral exciton binding energy exceeds 100 meV.

Electrostatic doping enables observation of positive trions.

Abstract

Monolayer and few-layer phosphorene are anisotropic quasi-two-dimensional (quasi-2D) van der Waals (vdW) semiconductors with a linear-dichroic light-matter interaction and a widely-tunable direct-band gap in the infrared frequency range. Despite recent theoretical predictions of strongly-bound excitons with unique properties, it remains experimentally challenging to probe the excitonic quasiparticles due to the severe oxidation during device fabrication. In this study, we report observation of strongly-bound excitons and trions with highly-anisotropic optical properties in intrinsic bilayer phosphorene, which are protected from oxidation by encapsulation with hexagonal boron nitride (hBN), in a field-effect transistor (FET) geometry. Reflection contrast and photoluminescence spectroscopy clearly reveal the linear-dichroic optical spectra from anisotropic excitons and trions in the hBN-encapsulated bilayer phosphorene. The optical resonances from the exciton Rydberg series indicate that the neutral exciton binding energy is over 100 meV even with the dielectric screening from hBN. The electrostatic injection of free holes enables an additional optical resonance from a positive trion (charged exciton) ~ 30 meV below the optical bandgap of the charge-neutral system. Our work shows exciting possibilities for monolayer and few-layer phosphorene as a platform to explore many-body physics and novel photonics and optoelectronics based on strongly-bound excitons with two-fold anisotropy.