Atomically-thin molybdenum disulfide is emerging as a new nanomaterial with potential applications in the fields of electronic and photonics. Charge carrier dynamics plays an essential role in determining its electronic and optical properties. We report spatially and temporally resolved pump-probe studies of charge carriers in atomically-thin molybdenum disulfide samples fabricated by mechanical exfoliation. Carriers are injected by interband absorption of a 390-nm pump pulse and detected by measuring differential reflection of a time-delayed and spatially-scanned probe pulse that is tuned to an exciton transition. Several parameters on charge carrier dynamics are deduced, including carrier lifetime, diffusion coefficient, diffusion length, and mobility.

Carrier and phonon dynamics in Bi 2 Se 3 crystals are studied by a spatially resolved ultrafast pump-probe technique. Pronounced oscillations in differential reflection are observed with two distinct frequencies and are attributed to coherent optical and acoustic phonons, respectively. The rising time of the signal indicates that the thermalization and energy relaxation of hot carriers are both sub-ps in this material. We found that the thermalization and relaxation time decreases with the carrier density. The expansion of the differential reflection profile allows us to estimate an ambipolar carrier diffusion coefficient on the order of 500 cm 2 /s. A long-term slow expansion of the profile shows a thermal diffusion coefficient of 1.2 cm 2 /s.

We report an optical study of charge transport in graphene. Diffusion of hot carriers in epitaxial graphene and reduced graphene oxide samples are studied using an ultrafast pump-probe technique with a high spatial resolution. Spatiotemporal dynamics of hot carriers after a point-like excitation are monitored. Carrier diffusion coefficients of 11,000 and 5,500 squared centimeters per second are measured in epitaxial graphene and reduced graphene oxide samples, respectively, with a carrier temperature on the order of 3,600 K. The demonstrated optical techniques can be used for non-contact and non-invasive in-situ detection of transport properties of graphene.Comment: 5 pages, 3 figure

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There is a growing interest in the earth abundant and nontoxic iron disulfide (FeS(2)) photovoltaic materials. Here, we report the synthesis of FeS(2) nanoplates with different spectral features which we have associated with thicknesses and crystallization. The structure and crystalline order of ultrathin FeS(2) nanoplates have a strong influence on the carrier lifetime, electronic and optical properties. We demonstrate that two-dimensional FeS(2) nanoplates show great promise for fabrication of hybrid bulk heterojunction solar cells. This opens up a host of applications of these materials as inexpensive solar cells and photocatalysts.