  Hybrid quantum dot (QD) / transition metal dichalcogenide (TMD)
heterostructures are attractive components of next generation optoelectronic
devices, which take advantage of the spectral tunability of QDs and the charge
and exciton transport properties of TMDs. Here, we demonstrate tunable
electronic coupling between CdSe QDs and monolayer WS$_2$ using variable length
alkanethiol ligands on the QD surface. Using femtosecond time-resolved second
harmonic generation (SHG) microscopy, we show that electron transfer from
photoexcited CdSe QDs to single-layer WS$_2$ occurs on ultrafast (50 fs - 1 ps)
timescales. Moreover, in the samples exhibiting the fastest charge transfer
rates ($\leq$ 50 fs) we observed oscillations in the time-domain signal
corresponding to an acoustic phonon mode of the donor QD, which coherently
modulates the SHG response of the underlying WS$_2$ layer. These results reveal
surprisingly strong electronic coupling at the QD/TMD interface and demonstrate
the usefulness of time-resolved SHG for exploring ultrafast
electronic-vibrational dynamics in TMD heterostructures.
