In Situ Study On Novel Nucleation-Growth Mechanisms By Transmission Electron Microscopy

In this era of nanoscience,the synthesis of nanomaterials with designed structures and novel properties is in hot pursuit.With the increasing complexity of synthetic pathways,a thorough understanding of nucleation and growth mechanism for various environments has a great significance.In this regard,numerous experimental evidences challenge the traditional concepts of how crystalline phase nucleate and grow in synthetic and natural fields.In contrast to the monomer-by-monomer addition in Classical Nucleation Theory,the introduction of metastable intermediates with diverse structures before the emergence of stable nuclei(i.e.Non-classical Nucleation Theory)is now recognized as a common phenomenon.In consideration of the differences in components,structures,and stability for different intermediates,the non-classical trajectories present diverse dynamic behaviors,which is governed by the combination of both thermodynamic energy landscapes and reaction/interaction kinetics.In particular,the small-size nuclei show unstable structures and transient dynamics,which are really difficult to be visualized and traced in experimental observations.Therefore,due to the complexity in such issue regarding the nucleation-growth process and the limitations of common experimental probes,details concerning structural transformation and evolution pathway in crystallization is far from well-understood.Recent advances of in situ TEM(transmission electron microsopy)technique provide an unprecedent opportunity to trace the dynamic behaviors for physicochemical process in a high temporal-spatial resolution.Herein,focusing on the structural changes in crystal formation,especially the evolution dynamics in the early stage of crystallization,we performed a series in situ TEM observations to reveal the diverse nucleation and growth mechanisms in solid-state systems.The main research contents are listed as following:In chapter 2,the three-step bubbling process confined in two-dimensional(2D)materials is suggested by in situ TEM under beam irradiation.Firstly,decomposition of solid matrix induces the formation of numerous voids;later,the as-formed irregular voids change their shape to spherical with the continuous production of gas species,i.e.bubble nucleation;finally,further growth of bubbles is achieved by the coalescence of neighboring bubbles via formation of metastable or unstable neck structures,which is remarkably controlled by the confinement of 2D environment.In chapter 3,the crystallization of Ni O nanocrystals induced by decomposition of solid precursor shows a distinct multistep nucleation mechanism.Firstly,spinodal decomposition of matrix results the formation of interpenetrating Ni-rich/Ni-poor regions.Secondly,Ni-rich regions gradually transformed to spherical nuclei by continues mass transport.Thereafter,the spherical nuclei undergo reversible structure oscillations between crystalline and amorphous state before the formation of final stable crystal.In chapter 4,templated crystallization over spherical bubbles is found to be determined by curvature-induced stress.Multiple pre-crystallized domains are formed on the surface before nucleation.For an individual domain,heterogenous nucleation occurs along its boundary and leads to a single-crystalline grain.The incompatibility between the curved surface and crystalline structures causes diverse pathways for defects generation and propagation.And consequently,the final spherical crystal shows polycrystalline nature with abundant defects.In chapter 5,the topochemical transformation for a nanoribbon is finished via three stages.Firstly,considerable defects generate in the crystalline matrix.The subsequent nucleation of new phase initiates at the edge part of nanoribbon.More importantly,the crystalline orientation between nuclei and matrix has its dependence.After that,the growth of new phase relies on the migration of the interfaces via a latticematched manner.The overall structural evolution induces a single-crystal-to-singlecrystal phase transformation.In summary,the results in this thesis demonstrated novel nucleation-growth mechanisms in several solid-state crystallizations,which present new insights into the pathway for crystallization.