Department of Chemistry, University of Southern California , Los Angeles, California 90089, United States.
Bremen Center for Computational Materials Science, Universität Bremen , Otto-Hahn-Alle 1, 28359 Bremen, Germany.
Nano Lett. 2017 Apr 12;17(4):2389-2396. doi: 10.1021/acs.nanolett.6b05368. Epub 2017 Mar 2.
Recent experiments on CdSe nanoplatelets synthesized with precisely controlled thickness that eliminates ensemble disorder have allowed accurate measurement of quantum coherence at room temperature. Matching exactly the CdSe cores of the experimentally studied particles and considering several defects, we establish the atomistic origins of the loss of coherence between heavy and light hole excitations in two-dimensional CdSe and CdSe/CdZnS core/shell structures. The coherence times obtained using molecular dynamics based on tight-binding density functional theory are in excellent agreement with the measured values. We show that a long coherence time is a consequence of both small fluctuations in the energy gap between the excited state pair, which is much less than thermal energy, and a slow decay of correlation between the energies of the two states. Anionic defects at the core/shell interface have little effect on the coherence lifetime, while cationic defects strongly perturb the electronic structure, destroying the experimentally observed coherence. By coupling to the same phonon modes, the heavy and light holes synchronize their energy fluctuations, facilitating long-lived coherence. We further demonstrate that the electronic excitations are localized close to the surface of these narrow nanoscale systems, and therefore, they couple most strongly to surface acoustic phonons. The established features of electron-phonon coupling and the influence of defects, surfaces, and core/shell interfaces provide important insights into quantum coherence in nanoscale materials in general.
最近对通过精确控制厚度消除了集合无序的 CdSe 纳米板的实验进行了研究，这使得在室温下对量子相干性进行准确测量成为可能。我们精确匹配了实验研究粒子的 CdSe 核，并考虑了几种缺陷，从而确定了二维 CdSe 和 CdSe/CdZnS 核/壳结构中重空穴和轻空穴激发之间相干性损失的原子起源。基于紧束缚密度泛函理论的分子动力学得到的相干时间与测量值非常吻合。我们表明，长相干时间是激发态对之间能隙小波动的结果，这比热能小得多，并且两个态之间的能量相关性的缓慢衰减也是一个原因。核/壳界面上的阴离子缺陷对相干寿命几乎没有影响，而阳离子缺陷强烈地扰动电子结构，破坏了实验观察到的相干性。通过与相同的声子模式耦合，重空穴和轻空穴同步它们的能量波动，从而实现长寿命相干性。我们进一步证明，这些窄纳米尺度系统中的电子激发被局限在表面附近，因此它们与表面声子强烈耦合。电子-声子耦合的特征和缺陷、表面以及核/壳界面的影响为一般纳米材料中的量子相干性提供了重要的见解。
