| Summary: | Defect-carrier interaction in transition metal dichalcogenides (TMDs) plays important
roles in carrier relaxation dynamics and carrier transport, which determines the
performance of electronic devices. With femtosecond laser time-resolved spectroscopy, we
investigated the effect of grain boundary/edge defects on the ultrafast dynamics of
photoexcited carrier in molecular beam epitaxy (MBE)-grown MoTe2 and
MoSe2. We found that, comparing with exfoliated samples, the carrier
recombination rate in MBE-grown samples accelerates by about 50 times. We attribute this
striking difference to the existence of abundant grain boundary/edge defects in MBE-grown
samples, which can serve as effective recombination centers for the photoexcited carriers.
We also observed coherent acoustic phonons in both exfoliated and MBE-grown
MoTe2, indicating strong electron-phonon coupling in this materials. Our
measured sound velocity agrees well with the previously reported result of theoretical
calculation. Our findings provide a useful reference for the fundamental parameters:
carrier lifetime and sound velocity and reveal the undiscovered carrier recombination
effect of grain boundary/edge defects, both of which will facilitate the defect
engineering in TMD materials for high speed opto-electronics.
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