Ultrafast energy transfer in transition metal dichalcogenide-graphene heterostructures revealed by pump-probe Raman spectroscopy


Intense light-matter interactions and unique structural and electrical properties make van der Waals heterostructures composed by graphene (Gr) and monolayer transition metal dichalcogenides (TMD) promising building blocks for tunneling transistors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics, and quantum light emitting devices (QLEDs). An efficient energy harvesting of photoexcited hot carriers is critical for the performances of such devices. In this respect, the way initially photogenerated excitons in the TMD are converted into an electric current in Gr is a highly controversial issue.

An experimental study, performed at the Femtoscopy labs of the Physics Department in collaboration with the Italian Institute of Technology and the University of Strasbourg, unveiled that the fundamental mechanism boosting the carrier injection in Gr from the TMD monolayer is an energy transfer occurring on the picosecond scale. “We exploited a picosecond pump pulse, resonant with the WS2 monolayer absorption, to inject photo-carriers in the TMD and then monitored the graphene response with a temporally delayed probe pulse.

By tracking the picosecond dynamics of the G and 2D Raman bands, we determined the electronic temperature profile of Gr in response to TMD photoexcitation” Tullio Scopigno, the research coordinator, says. “Our results indicate the existence of an additional conversion mechanism bridging such ultrafast energy transfer with the much slower charge transport involved in optoelectronics applications”. The research, recently published in the Proceedings of the National Academy of Science, provides an essential step for the microscopic understanding of ultrafast interlayer coupling in TMD-Gr heterostructures, showing the possibility to exploit photo-thermionic effects in graphene based photonic devices.

Carino Ferrante, Giorgio Di Battista, Luis E. Parra López, Giovanni Batignani, Etienne Lorchat, Alessandra Virga, Stéphane Berciaud, Tullio Scopigno 

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