Observation of bright and dark exciton transitions in monolayer MoSe2 by photocurrent spectroscopy

TL;DR

This study uses photocurrent spectroscopy to identify bright and dark exciton transitions in monolayer MoSe2, revealing gate-induced activation of dark excitons and analyzing thickness-dependent optical properties.

Contribution

It demonstrates the direct observation of dark exciton activation via gate voltage and characterizes excitonic transitions in MoSe2 using photocurrent spectroscopy.

Findings

Bright A0 exciton peak observed at specific spectral profile.

Gate voltage induces a prominent peak at 30 meV above A0, indicating dark exciton activation.

Thickness-dependent optical bandgap characterized through absorption edge analysis.

Abstract

We investigate the excitonic transitions in single- and few-layer MoSe2 phototransistors by photocurrent spectroscopy. The measured spectral profiles show a well-defined peak at the optically active (bright) A0 exciton resonance. More interestingly, when a gate voltage is applied to the MoSe2 to bring its Fermi level near the bottom of the conduction band, another prominent peak emerges at an energy 30 meV above the A0 exciton. We attribute this second peak to a gate-induced activation of the spin-forbidden dark exciton transition, AD0. Additionally, we evaluate the thickness-dependent optical bandgap of the fabricated MoSe2 crystals by characterizing their absorption edge.