Selection of bright excitonic emission in monolayer TMDCs

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Shaul Katznelson




Since the discovery of graphene by Geim and Novoselov back in 2004, there has been a tremendous growth in research and development in 2D materials. A very promising group of Two-dimensional semiconductors are transition metal dichalcogenides (TMDCs), which possess immense potential for electronic and photonic applications. Their exceptional opto- electronic properties are a product of the TMDC’s direct band gap, tightly bound excitons, strong spin-orbit coupling and spin-valley degrees of freedom. The band splitting in the conduction and valance bands promotes two spin allowed (forbidden), nominally ‘bright’ (‘dark’), excitonic transitions. While the optically dominant bright excitons were profoundly explored, the study of dark excitons at room temperature is lacking, albeit exhibiting a fundamental key role in the optical properties of TMDCs. Owing to their out-of-plane dipole moment transition, dark excitons can be manipulated by the exciting electric fields polarization. Here we present a method for selective emission of the PL spectra between the Ab and Bb excitonic transitions, profoundly for MoX2 (where X=S, Se) monolayers, induced by relative excitations of in- and out-of-plane dipoles. we demonstrate how incorporation of out-of-plane excitation through the generation of Ad- exciton induce population in the upper conduction band (CB) resulting in a more dominant Bb- photoluminescence (PL) spectrum, whereas purely in-plane excitation results in a dominant Ab- spectrum. We further show that this modulation is thermally driven and relates to the CB splitting.

Supervisor: Asst. Prof. Elad Koren