Proceedings of nanoGe Fall Meeting19 (NFM19)
Publication date: 18th July 2019
The development of new optoelectronics applications based on transition metal dichalcogenides will require a deep understanding of the temporal and spatial dynamics of the excitons and carriers they can host. While most of these materials are isotropic, a few exhibit anisotropic properties due to a reduced crystal symmetry. A degree of freedom that remains largely unexplored. They are also typically technology relevant only in the mono to few layers limit.
In that respect, ReS2 and ReSe2 stand out in the TMD family. Their disordered 1T phase make them anisotropic semiconductors. In addition, they host two linearly polarized excitons stable at room temperature from the monolayer to the bulk. The number of layers is therefore another degree of freedom that can be exploited to tune the materials properties.
Here, we study the anisotropic diffusion of excitons and free carriers in ReS2 with transient absorption microscopy for thicknesses from the monolayer to the bulk
A 10fs pump pulse focused to the diffraction limit generates excitons and carriers, their temporal and spatial dynamics is then probed by a 10 fs probe pulse with a larger focal spot (15 µm). Comparing the spatial profile of the transient absorption signal at time zero and later times allows to study the movement of excited species with 10 nm resolution.
Rotating the pump and probe linear polarisation we observe the strongly anisotropic diffusion of the two exciton populations We observe an increase in the exciton lifetime and diffusion coefficients with the number of layers that we attribute to a decrease of surface trap state as the sample get thicker.
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