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Exciton polarizability in semiconductor nanocrystals

Nature Materials volume 5, pages 861–864 (2006)Cite this article

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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Figure 1: THz electric-field waveform transmitted through an unexcited suspension of CdSe QDs and the photoinduced change in this waveform.
Figure 2: Spectral dependence of the change in the real (Δχs′) and imaginary part (Δχs′′) of the photoinduced sheet susceptibility of the sample.
Figure 3: Polarizability of quantum-confined excitons in photoexcited CdSe QDs as a function of the QD radius R.

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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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Authors and Affiliations

  1. Departments of Physics and Electrical Engineering, Columbia University, New York, New York, 10027, USA

    Feng Wang & Tony F. Heinz

  2. Department of Physics, Case Western Reserve University, Cleveland, Ohio, 44106, USA

    Jie Shan

  3. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, 10027, USA

    Mohammad A. Islam & Irving P. Herman

  4. FOM-Institute AMOLF, Kruislaan 407, 1098 SJ, Amsterdam, The Netherlands

    Mischa Bonn

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Correspondence to Tony F. Heinz.

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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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