Speaker
Description
Femtosecond electron diffraction (FED) allows direct observation of a crystal lattice’s response to laser excitation. It is ideally suited to study the ultrafast energy flow from electrons to phonons as well as other photo-induced changes of the lattice, such as structural phase transitions, coherent phonons, and lattice distortions.
We have employed FED to probe lattice dynamics in 2d transition metal dichalcogenides (TMDCs), a family of layered van der Waals bonded materials. We have focused primarily on semiconducting monolayer systems with strong excitonic effects, (e.g. WSe2 and ReS2 presented herein), as well as heterostructures of the such monolayers (namely of WSe2 and MoSe2). We have also begun studying WTe2, a polar metal.
The results presented include collective structural variations on sub picosecond timescales in monolayer ReS2 and in WTe2, as well as element resolved atomic vibrations in monolayer WSe2. The latter is the first reported example of such a real-space picture on ultrafast timescales, and allows identifying stages within the relaxation of the phonon system. We also present a “stamping” setup for synthesizing monolayer-monolayer heterostructure samples, with the first examples being WSe2-MoSe2 heterostructures. “stamping” one monolayer onto another is done controllably such that the angle between the two can be defined. A particular aspect of the setup is the need to center a third layer for FED measurements: a Platinum pinhole is placed with micron precision, which ensures that the femtosecond electron pulses only probe the desired sample area.
