Electron microscopic studies of novel crystal morphologies and related growth mechanisms

Abstract

This project explores selected crystals with novel morphologies and corresponding growth mechanisms. Crystal samples from different growth stages are obtained by interrupting the reaction in order to study the growth mechanisms step by step. To achieve the characterisations of the crystals, electron microscopes and in-built electron spectroscopic techniques are the major implements. Together with electron microscopy, other techniques, such as X-ray diffraction and infrared spectroscopy, are also employed. Schematic visualisation is exploited to illustrate the formation mechanisms.

Copper pseudo-icosahedral crystals are prepared and studied. Polyvinylpyrrolidone-assisted growth route involving dual-step reduction and multiple intermediate phases is revealed. The attribute of pseudo-icosahedral is discussed morphologically. Using the same synthesis system but different reactant, Cu₂O spherulites are prepared. By examining the polarity of the spherulites and the feature of Cu₂O nanocrystallites in PVP matrix, the formation mechanism concerning intrinsic dipolar force is proposed.

Cs₄PbBr₆ rhombohedral crystals with embedded CsPbBr₃ nanocrystallites are produced via antisolvent preparation method. Despite the sensitivity to electron beam exposure, we manage to visualise the material using electron microscopy. With observed evidence of amorphous precursor, a formation mechanism associated with the precursor amorphous phase is proposed. Different antisolvents are tested on their effects of the morphology. Water triggered transformation of Cs₄PbBr₆ to CsPb₂Br₅ and CsPbBr₃ is realised.

Electron microscopic and spectroscopic information of Ti₃C₂Tₓ nanowire clusters synthesised at Beijing Normal University reveals the dimensional degradation mechanism. From the oxidation of layered Ti3C2Tₓ structure, interconnected Ti₃C₂O and Ti₃C₂ nanowires are produced.

We anticipate that this doctorate project can provide new perspectives in understanding non-classical crystal growth, set an example for the study of beam sensitive materials and inspire the morphology engineering of crystalline materials.