Exciton states in monolayer MoSe2: impact on interband transitions

TL;DR

This study combines optical spectroscopy and ab initio calculations to analyze exciton states in monolayer MoSe2, revealing detailed excitonic energy levels, resonances, and bandstructure features that influence interband transitions.

Contribution

It provides new insights into the excitonic states and bandstructure of MoSe2 monolayers, highlighting differences from similar materials like MoS2 and WSe2.

Findings

Measured A- and B-exciton energy separation of 220 meV.

Detected 2p exciton states 180 meV above 1s states.

Observed significant SHG enhancement at excitonic resonances.

Abstract

We combine linear and non-linear optical spectroscopy at 4K with ab initio calculations to study the electronic bandstructure of MoSe2 monolayers. In 1-photon photoluminescence excitation (PLE) and reflectivity we measure a separation between the A- and B-exciton emission of 220 meV. In 2-photon PLE we detect for the A- and B-exciton the 2p state 180meV above the respective 1s state. In second harmonic generation (SHG) spectroscopy we record an enhancement by more than 2 orders of magnitude of the SHG signal at resonances of the charged exciton and the 1s and 2p neutral A- and B-exciton. Our post-Density Functional Theory calculations show in the conduction band along the $K-\Gamma$ direction a local minimum that is energetically and in k-space close to the global minimum at the K-point. This has a potentially strong impact on the polarization and energy of the excitonic states that govern the interband transitions and marks an important difference to MoS2 and WSe2 monolayers.