Controlling the structure of two-dimensional nanoparticle supracrystals from long-range order to anisotropy by tailoring ligand interactions

[摘要] Ligand-stabilized nanoparticles (NPs) assembled into long-range ordered arrays, also known as ;;nanoparticle supracrystals (NPSCs);;, are expected to provide a powerful general platform for designing new types of solids. In particular, the NPs are themselves self-assembled structures consisting of a core and a self-assembled monolayer of ligand molecules surrounding it. The self-assembled structure of the NPs themselves determines the structure of the self-assembled supracrystals. Ligands are of special interest in this respect, because it is an important component for the NP system which play a major role in the design of self-assembly of the complex matter and also provide a powerful entry into the supracrystal engineering. The increasing ability to control the way in which ligand molecules associate gives means for the designed generation of supraparticle architectures in the self-assembly. In spite of this, elucidation of how the ligands play a role in affecting the structural behavior of NPSCs remains largely unrevealed. In this thesis, the effect of ligands for the two dimensional (2D) self-assembled NPSCs structure was investigated. The key materials advancement that enables this work is that we have been able to synthesize monodisperse gold NPs of same core size but different ligand molecules. Additionally, a new method for monolayer film processing has been developed to prepare the 2D NPSCs, based on a Langmuir assembly through successive compression cycles. Importantly, as there is little effect exhibited by solvent interactions in the NPs structure obtained from this approach, the corresponding NPs structural variation in this work is truly driven by the different ligand interactions in NPSCs. Specifically, we show that such ligand interactions have direct consequences on the ordering and symmetry of the assembled NPSCs structures. Here, we report on a set of NPSC arrays in which small changes in either the NP ligand environment or the ligand configuration geometry induce significant variations in the order parameters of the crystal. First, we show that the packing organization of a 2D NPSC array of hydrophobic alkanethiol ligands varies with subtle chemical changes in the system, leading to a transition between long-range to short-range (almost glassy) ordered phases. The balance between long and short-range order is driven by small differences in intermolecular interpenetration of the ligand molecules, that can be related to ligand conformational and that can be rigorously the experimentally measured. Second, we show the first 2D NPSC structures to have unique anisotropic symmetry due to the interaction between amphiphilic NP ligand shells. It is understood that the ligand interactions on NPs through their unique molecular configuration of amphiphilic ligands may provide the anisotropic feature in the orientational alignment of NPSC symmetry.

[发布机构] Massachusetts Institute of Technology