Strongly anisotropic diffusion of excitons in layered res2 observed with transient absorption microscopy
Nicolas Gauriot a, Jooyoung Sung a, Hope Bretscher a, Akshay Rao a
a Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK., J.J. Thomson Avenue, Cambridge, United Kingdom
nanoGe Fall Meeting
Proceedings of nanoGe Fall Meeting19 (NGFM19)
#Sol2D19. Two Dimensional Layered Semiconductors
Berlin, Germany, 2019 November 3rd - 8th
Organizers: Efrat Lifshitz, Cristiane Morais Smith and Doron Naveh
Poster, Nicolas Gauriot, 403
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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