Morphological transformation of nanocrystals

Abstract

The structure of organic/molecular crystals plays an important role in their properties and applications. While myriads of organic compounds have been synthesized under the macroscopic scale, the shape control of organic crystals remains at a primitive level at the micro- or nanoscale. (Chapter 1). Thus, it is urgent to develop new synthetic handles for designing organic crystals. A simple and effective solution-phase strategy has been developed to achieve hexagonal C60 mesh networks made of aligned nanorods, which were obtained by crystal transformation from C60 microplates (Chapter 2). The crystal transformation was caused by removing and embedding the co-solvent molecules. The epitaxial growth of rod on plate by the lattice matching leads to the ordered nanorod arrays. The unconventional role of solvents provides a good opportunity for exploring the morphologies of organic/molecular crystals. Thus, we propose solvent exchange as a new synthetic handle for creating complex micro-or nano- structures of organic crystals. Understanding the mechanism of solvent exchange and the effect of solvent are basis for the generality of the synthetic method (Chapter 3). Here, we systematically explored the role of solvents and their competitive relationship during the destabilization and on-site crystallization of the crystals. Therefore, with mitigated driving force in solution phase, solvent loss or exchange in C60 plates does not lead to structural collapse, but leads to crystal transformation to mesh networks. The generality of crystal transformation has been explored. The C60 nanorods also can transform to orthogonal plates with an overall rod shape. In addition, the C60/ferrocene mesh networks were obtained by crystal transformation of the fcc C60/ferrocene plates. Besides the hexagonal C60 plates, the rhombic C60·trichlorobenzene plates transformed to rhombic nano-meshes by solvent exchange. Intrestingly, C70 meshes were converted from C70 cubes by incubation in the transformative solution. However, this method is not only applied to molecular crystals (C60 or C70), but also to most organic crystals with solvent incorporation. The TPE-4Br organic crystal has been demonstrated to form different shapes (plates, prism-like structures or wires) by tuning the embedded solvent molecules (Chapter 4). Furthermore, the p-xylene-rich TPE-4Br plate can undergo crystal transformation to microrod arrays by solvent exchange. In addition, the crystal transformation was accompanied by the luminescence changes. Considering the wide variety of solvate crystals in the literature, our new synthetic handle and the understanding of mechanism opens a window for designing sophisticated crystal morphologies.