Wang Feng, Shan Jie, Islam Mohammad A, Herman Irving P, Bonn Mischa, Heinz Tony F
Department of Physics, Columbia University, New York, New York 10027, USA.
Nat Mater. 2006 Nov;5(11):861-4. doi: 10.1038/nmat1739. Epub 2006 Oct 8.
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 nanocrystals 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 pulses. The electronic response is found to be instantaneous even for THz frequencies, in contrast to the behaviour reported in related measurements for larger nanocrystals and nanocrystal assemblies. The measured polarizability of an electron-hole pair (exciton) amounts to approximately 10(4) A(3) 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.
电荷对外加电场的响应是任何材料系统的一项重要基本特性，也是许多应用中的关键特性。在此，我们研究完全局限于纳米长度尺度上的电荷的行为和动力学。这是通过使用半径可控（1 - 2.5纳米）的CdSe纳米晶体作为原型量子系统来实现的。在室温下，通过光激发在这些结构中产生单个电子 - 空穴对，并通过太赫兹（THz）电磁脉冲进行探测。与较大纳米晶体和纳米晶体组件的相关测量中所报道的行为相反，即使对于太赫兹频率，电子响应也被发现是即时的。测量得到的电子 - 空穴对（激子）的极化率约为10(4) A(3)，并且大致与纳米晶体半径的四次方成正比。这种尺寸依赖性和即时响应反映了由强量子限制效应在系统中诱导产生的分立电子能级的存在。
