Ultrafast charge dynamics in dispersions of MoS2 nanosheets in N-methyl-2-pyrrolidone are reported. Samples were prepared by the ultrasonication-assisted exfoliation of MoS2 powder, resulting in nanosheets that were predominantly monolayer and had an average sheet size of 32.4 ± 0.1 nm. These dispersions were characterized using absorption and photoluminescence spectroscopy, transient photoluminescence measurements, and atomic force microscopy before the ultrafast charge dynamics were studied via transient absorption spectroscopy. The transient absorption spectra exhibited bleach peaks and photoinduced absorption peaks in spectral regions corresponding to both the A and B excitons of MoS2. The growth and decay of the features in the B exciton region were determined largely by the dynamics of the exciton population, while the features in the A exciton region depend on the dynamics of both excitons and trions.

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The pH dependence of emission from graphene oxide is believed to be due to the protonation of surface functional groups. In this study we use transient absorption spectroscopy to study the sub-picosecond charge dynamics in graphene oxide over a range of pH values, observing dynamics consistent with an excited state protonation step for pH < 9.3. The timescale of this process is ~ 1.5 ps, and a corresponding change in recombination dynamics follows. A broad photo-induced absorption peak centred at 530 nm associated with excited state protonation is also observed.