Novel semiconductors represent a new frontier in electronics research and development. In this area, Monolayer Transition Metal Dichalcogenides (TMDs), including MoS2, have attracted substantial interest in recent years for their interesting electronic and optical properties. Excitonic effects give rise to many of the novel properties observed in these materials. These materials feature exciton binding energies estimated in the hundreds of meV, as well as charged excitons, or trions, with binding energies of around 20 meV. Electronic devices including strain-resistant FET’s, photodetectors and solar cells have already been made using TMD’s as the active element. However, ultrafast charge and energy transfer dynamics of the active element play an important role in device function and have yet to be fully explored for these materials. To better understand the role the number of layers plays in the ultrafast energy transport in TMDs, we have performed two-dimensional electronic spectroscopy (2DES) on monolayer and multilayer MoS2 samples. 2DES provides simultaneous temporal resolution on the five fs (10-15 s) timescale and .6 meV absorption energy resolution over a tunable 500 meV range from 1.37-2.48 eV. This combined fine temporal and excitation energy resolution makes 2DES a prime candidate for studies of nanomaterials, where energy transfer processes often proceed on the tens of femtoseconds timescale. We observe fast relaxation to the band edge on the sub-100fs timescale for both samples. Spectroscopic signatures of coupling between different excitons as well as oscillatory signals on the lower cross peak are observed in the multilayer sample. These results will be discussed with an eye toward how these novel excitonic properties can be applied to the development of a new generation of electronic devices.

Layer-number dependence of energy transfer and excitonic coupling observed in monolayer and multilayer MoS2 using ultrafast spectroscopy

256th National Meeting (2018)

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