Letter abstract
Nature Materials 5, 861 - 864 (2006)
Published online: 8 October 2006 | doi:10.1038/nmat1739
Subject Categories: Semiconductors | Optical, photonic and optoelectronic materials | Nanoscale materials
Exciton polarizability in semiconductor nanocrystals
Feng Wang1, Jie Shan2, Mohammad A. Islam3, Irving P. Herman3, Mischa Bonn4 & Tony F. Heinz1
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.
- Departments of Physics and Electrical Engineering, Columbia University, New York, New York 10027, USA
- Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, USA
- Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA
- FOM-Institute AMOLF, Kruislaan 407, 1098 SJ, Amsterdam, The Netherlands
Correspondence to: Tony F. Heinz1 e-mail: tony.heinz@columbia.edu
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