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Excitonic properties of semiconducting monolayer and bilayer MoTe2

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.