纳米颗粒团簇中的自发结构转变研究

Nanoparticles and atoms exhibit much similarity when they aggregate to form larger structures, in despite of the huge difference in their size. A typical example is that nanoparticles mediated by hydrophobic interaction can form clusters with icosahedral structure. This type structure, which is highly symmetrical but incompatible with the long-range translational order of normal crystals, is well-known in atom clusters. However, the instability of atom clusters makes it difficult to determine the spontaneous transition process to normal crystals during their growth. Here, we will take advantage of the high stability of nanoparticle clusters to study the process of the spontaneous structure transition in nanoparticle clusters in detail. Especially, we will try to experimentally determine when and how the spontaneous structure transition happens, and clarify the exact form of the size effect predicted by theory. Towards this, we will optimize the current method of nanoparticle synthesis, and explore the strategy for achieving spontaneous structure transition from icosahedral clusters to normal crystals in nanoparticle clusters during their growth. This study will not only promote our understanding about the corresponding process in atom clusters, but benefit the control of the nanoparticle cluster structure, and hence benefit the tuning of their structure-dependent collective properties.

纳米颗粒和原子尽管在尺寸上存在很大的差异，但它们在聚集形成更大尺度的结构时却表现出很高的相似性。一个典型的例子是纳米颗粒在疏水作用下会形成二十面体结构的团簇。这种高度对称却又和普通晶体的长程平移序不相容的结构在原子团簇中早已为人们所熟知。然而由于其结构的不稳定性，长期以来人们很难在实验上确定原子团簇在生长的过程中自发转变为普通晶体的具体过程。本课题将利用纳米颗粒团簇结构的高度稳定性，详细研究纳米颗粒团簇中自发结构转变的具体过程，尤其是在实验上确定该转变何时以何种方式发生，并弄清理论预言的尺寸效应在实际体系中的表现形式。为此，我们会优化现有纳米颗粒的制备方法，并探索实现纳米颗粒团簇在生长的过程中自发从二十面体结构转变为超晶体结构的途径。该研究有望增进我们对发生在原子团簇中对应过程的认识，并能使我们更好地控制纳米颗粒团簇的结构，从而有益于我们调控其依赖于结构的整体性能。

本项目主要研究了纳米颗粒团簇中自发结构转变现象。首先，优化了现有的金纳米颗粒的合成方法，制备出了尺寸可调的单分散的金纳米颗粒。接着用硫醇封端的有着不同聚合度的聚苯乙烯修饰金纳米颗粒，制备出了壳层厚度可调的Au@PS核壳纳米颗粒。以这些高质量的Au@PS核壳纳米颗粒为构筑基元，通过自组装的方法制备了纳米颗粒团簇。通过优化实验条件，使纳米颗粒团簇在生长的过程中自发地完成从非晶到超晶体的转变。还研究了纳米颗粒团簇中自发结构转变的具体方式以及影响因素。成功地观察到了理论预言的尺寸效应，发现团簇尺寸的增大是结构转变的直接诱因。上述研究为人们理解原子团簇中的对应过程提供了参照，也为人们制备具有不同性能的纳米颗粒团簇提供了可行的方法。