Excitonic properties of semiconducting monolayer and bilayer MoTe2

TL;DR

This study investigates the electronic and optical properties of monolayer and bilayer MoTe2, revealing their excitonic states, band gaps, and recombination dynamics through simulations and various spectroscopic techniques.

Contribution

It provides a comprehensive analysis combining DFT-GW calculations and experimental spectroscopy to elucidate excitonic behavior in MoTe2 monolayers and bilayers, highlighting differences and similarities.

Findings

Monolayer MoTe2 has a direct band gap; bilayer has an indirect band gap.

Both monolayer and bilayer exhibit similar energies for direct excitonic states.

Reduced exciton-exciton annihilation in bilayer MoTe2.

Abstract

MoTe2 belongs to the semiconducting transition metal dichalcogenide family with some properties differing from the other well-studied members (Mo,W)(S,Se)2. The optical band gap is in the near infrared region and both monolayers and bilayers may have a direct optical band gap. We first simulate the band structure of both monolayer and bilayer MoTe2 with DFT-GW calculations. We find a direct (indirect) electronic band gap for the monolayer (bilayer). By solving the Bethe-Salpeter equation, we calculate similar energies for the direct excitonic states in monolayer and bilayer. We then study the optical properties by means of photoluminescence (PL) excitation, time-resolved PL and power dependent PL spectroscopy. We identify the same energy for the B exciton state in monolayer and bilayer. Following circularly polarized excitation, we do not find any exciton polarization for a large range of excitation energies. At low temperature (T=10 K), we measure similar PL decay times of the order of 4 ps for both monolayer and bilayer excitons with a slightly longer one for the bilayer. Finally, we observe a reduction of the exciton-exciton annihilation contribution to the non-radiative recombination in bilayer.