To elucidate these, we applied Tr-ESFG spectroscopy. A 515nm (2.4eV, 200 fs) pump pulse excites electrons to the CB of monolayer MoS 2 (E g = 1.9eV). The subsequent charge dynamics are then probed by the ESFG (Figure ). The ESFG probe utilizes a mid-IR and near-IR pulse to interact with the sample which creates a signal at the sum of the two incoming beam frequencies. If the energy of the emitted signal resonates with an electronic transition at interfaces, the emitted signal is enhanced. (Figure ). We scanned the IR frequency across a broad spectral range to cover the possible electronic transitions. The tr-ESFG spectra of pristine monolayer MoS 2 and Ru-MoS 2 showed distinct time-dependent features. At first glance, pristine TMD (see Figure ) had a negative pump-probe signal right after time zero and recovered back to zero with a lifetime of 16.4 ps and then turned into a positive amplitude with a long lifetime of 129.0 ps. The negative transient signal has a center located at 1.79 eV, indicating a bleach of trion. Interestingly, the positive signal redshifts to 1.76 eV. For the Ru-MoS 2 (Figure ), a negative signal persists over the entire scanning range and only decays to a smaller amplitude. We note the Ru(DPPZ) 2 L-Pro monolayer alone shows negelegible static and transient ESFG signal.