Physikalische Chemie - Direktor: Prof. Dr. Martin Wolf
Department Online Seminar
Chair: Sebastian Mährlein

Thursday, August 19, 2021, 10:00 am
Lawson T. Lloyd
University of Chicago
Resolving Femtosecond Exciton Dynamics and Couplings in Atomically Thin Semiconductors with Multidimensional Spectroscopy
Atomically thin transition metal dichalcogenides (TMDs) have emerged as promising candidates for next-generation optoelectronic applications. In the monolayer limit, TMDs possess a direct optical bandgap, robust exciton complexes, and an optically addressable ‘valley’ degree of freedom. In particular, the non-equivalent band extrema, or valleys, at the K/K’ points of the hexagonal Brillouin zone can be used to store and readout information by exploiting the valley-dependent chiral optical selection rules. However, intervalley coupling after optical excitation leads to a loss of the valley polarization on ultrafast timescales. Identifying the microscopic mechanisms driving valley depolarization is therefore critical to advancing useful applications.
Here, I will describe how we have leveraged multidimensional electronic spectroscopy to track the exciton dynamics and couplings in large-area monolayer MoS2 with femtosecond and valley resolution. In particular, we show that intervalley coupling between the K/K’ valley exciton states occurs both on the timescale of excitation (~10 fs) and with minimal excitation fluence and temperature dependence. This coupling is distinct from previously described dynamic coupling or scattering mechanisms and is robust to sample properties such as grain size, posing a challenge for large-area exciton-based “valleytronics.” I will also describe recent work revealing the dominance of carrier-induced bandgap renormalization over biexciton formation on the ~100 fs timescale at cryogenic temperatures. These results shed light on the strong many-body interactions governing the femtosecond exciton dynamics in TMDs and the factors limiting the realization of novel technologies built around the valley degree of freedom.

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