Department of Chemistry, University of California, Berkeley, California 94720, United States.
Nano Lett. 2013 Apr 10;13(4):1367-72. doi: 10.1021/nl3007165. Epub 2012 Jul 23.
The kinetics and thermodynamics of structural transformations under pressure depend strongly on particle size due to the influence of surface free energy. By suitable design of surface structure, composition, and passivation it is possible, in principle, to prepare nanocrystals in structures inaccessible to bulk materials. However, few realizations of such extreme size-dependent behavior exist. Here, we show with molecular dynamics computer simulation that in a model of CdSe/ZnS core/shell nanocrystals the core high-pressure structure can be made metastable under ambient conditions by tuning the thickness of the shell. In nanocrystals with thick shells, we furthermore observe a wurtzite to NiAs transformation, which does not occur in the pure bulk materials. These phenomena are linked to a fundamental change in the atomistic transformation mechanism from heterogeneous nucleation at the surface to homogeneous nucleation in the crystal core.
由于表面自由能的影响，压力下结构相变的动力学和热力学强烈依赖于颗粒尺寸。通过对表面结构、组成和钝化的适当设计，原则上可以制备出大块材料无法获得的纳米晶体。然而，这种极端的尺寸依赖行为很少得到实现。在这里，我们通过分子动力学计算机模拟表明，在 CdSe/ZnS 核/壳纳米晶体的模型中，可以通过调节壳的厚度使核在高压下的结构在环境条件下处于亚稳状态。在壳较厚的纳米晶体中，我们还观察到了纤锌矿到 NiAs 的转变，而这种转变在纯体材料中不会发生。这些现象与原子转变机制从表面的非均匀成核到晶体核心的均匀成核的根本变化有关。
