Abstract
The response of charge to externally applied electric fields is an important basic property of any material system, as well as one critical for many applications. Here, we examine the behaviour and dynamics of charges fully confined on the nanometre length scale. This is accomplished using CdSe nanocrystals1,2,3 of controlled radius (1–2.5 nm) as prototype quantum systems. Individual electron–hole pairs are created at room temperature within these structures by photoexcitation and are probed by terahertz (THz) electromagnetic pulses4. The electronic response is found to be instantaneous even for THz frequencies, in contrast to the behaviour reported in related measurements for larger nanocrystals5 and nanocrystal assemblies6,7. The measured polarizability of an electron–hole pair (exciton) amounts to ∼104 Å3 and scales approximately as the fourth power of the nanocrystal radius. This size dependence and the instantaneous response reflect the presence of well-separated electronic energy levels induced in the system by strong quantum-confinement effects.
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Acknowledgements
Research at Columbia University was supported primarily by the MRSEC Program of the National Science Foundation under award number DMR-0213574 and by the New York State Office of Science, Technology and Academic Research (NYSTAR), with additional support from US Department of Energy, Office of Basic Energy Sciences, through the Catalysis Science Program. Work at Case Western Reserve University was supported by NSF grant DMR-0349201.
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Wang, F., Shan, J., Islam, M. et al. Exciton polarizability in semiconductor nanocrystals. Nature Mater 5, 861–864 (2006). https://doi.org/10.1038/nmat1739
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DOI: https://doi.org/10.1038/nmat1739
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