Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.050
Publication date: 18th July 2019
We studied the photogeneration, mobility and decay dynamics of charge carriers and excitons in colloidally synthesized or liquid exfoliated 2D materials. Examples include metal chalcogenide nanosheets, superlattices of connected PbSe quantum dots with square or honeycomb geometry, and black phosphorus. The studies were performed using ultrafast pump-probe laser spectroscopy with optical or terahertz conductivity detection.
The composition and nanogeometry of the material were found to have pronounced effects on the relative yield of free mobile charges and neutral excitons. The relative yield of excitons was found to increase with excitation density. This effect could be described on the basis of the Saha equation, which accounts for more charge recombination at higher photoexcitation density. Surprisingly, excitons in CdSe nanosheets are stable even at high densities were they start to exhibit spatial overlap. This counter intuitive result can be understood theoretically from the fact that the Coulomb screening length, and thus the exciton binding energy, remain non-zero even at high density. This is a particular result for 2D materials and does not hold for 3D semiconductors.
The mobility of charge carriers depends strongly on the nanogeometry and material composition. In PbSe honeycomb superlattices mobilities are of the order of 1 cm2/Vs, while in square superlattices and PbS nanosheets values as high as a few hundred cm2/Vs were found. The frequency dependence of the mobility could be described theoretically by the Drude-Smith model, which includes effects of charge scattering on phonons as well as static defects.
The Netherlands Organisation of Scientific Research (NWO) and Toyota Motor Europe are acknowledged for financial support.
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