Bryant Garnett W, Jaskolski W
Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8423, USA.
J Phys Chem B. 2005 Oct 27;109(42):19650-6. doi: 10.1021/jp0535543.
Surface effects significantly influence the functionality of semiconductor nanocrystals. A theoretical understanding of these effects requires an atomic-scale description of the surface. We present an atomistic tight-binding theory of the electronic and optical properties of passivated and unpassivated CdS nanocrystals and CdS/ZnS core/shell nanocrystals. Fully passivated dots, with all dangling bonds saturated, have no surface states in the fundamental band gap, and all near-band-edge states are quantum-confined internal states. When surface anion dangling bonds are unpassivated, an anion-derived, narrow (bandwidth 0.05 eV), surface-state band lies 0.5 eV above the valence band edge, and a broader (0.2 eV) band of back-bonded surface states exists in the gap just above the valence band edge. When surface cation dangling bonds are unpassivated, a broad band of mixed surface/internal states exists above the conduction band edge. Partial passivation can push internal levels above the internal levels of a fully passivated dot or into the band gap. Because of this sensitivity to passivation, explicit models for surface effects are needed to describe accurately internal states. Capping the CdS dot with ZnS reduces the effect of the surface on the internal electronic states and optical properties. Six monolayers of ZnS are needed to eliminate the influence of any surface states on the internal states.
表面效应显著影响半导体纳米晶体的功能。对这些效应的理论理解需要对表面进行原子尺度的描述。我们提出了一种关于钝化和未钝化的CdS纳米晶体以及CdS/ZnS核壳纳米晶体的电子和光学性质的原子级紧束缚理论。完全钝化的量子点，所有悬空键均被饱和，在基本带隙中没有表面态，并且所有近带边态都是量子限制的内部态。当表面阴离子悬空键未被钝化时，一个由阴离子衍生的、窄的（带宽0.05 eV）表面态带位于价带边缘上方0.5 eV处，并且在价带边缘上方的能隙中存在一个更宽的（0.2 eV）反键表面态带。当表面阳离子悬空键未被钝化时，在导带边缘上方存在一个混合表面/内部态的宽带。部分钝化可以将内部能级推至完全钝化量子点的内部能级之上或推入带隙中。由于对钝化的这种敏感性，需要明确的表面效应模型来准确描述内部态。用ZnS包覆CdS量子点可降低表面对内部电子态和光学性质的影响。需要六个ZnS单分子层来消除任何表面态对内部态的影响。
