Nonclassical features of crystallization in solution have been recently identified both experimentally and theoretically. Especially, amorphous phase-mediated pathway is found in various systems of crystallization as an important route, different from classical nucleation and growth model. Here, we utilize high-resolution in-situ transmission electron microscopy with graphene liquid cells to study amorphous phase-mediated formation of Ni nanocrystals. An amorphous phase is precipitated in the initial stage of the reaction. Within the amorphous particles, crystalline domains nucleate and eventually form nanocrystals. In addition, unique crystallization behaviors, such as formation of multiple domains and dislocation relaxation, are observed in amorphous phasemediated crystallization. Theoretical calculations confirm that surface interactions can induce amorphous precipitation of metal precursors, which is analogous to the surface-induced amorphous-tocrystalline transformation occurring in biomineralization. Our results imply that an unexplored nonclassical growth mechanism is important for the formation of nanocrystals. Crystallization in liquid media is ubiquitous phenomena and is fundamental to understanding the formation of many materials including colloids, biominerals, and nanocrystals. 1 In the classical

Defining the redox activity of different surface facets of ceria nanocrystals is important for designing an efficient catalyst. Especially in liquid-phase reactions, where surface interactions are complicated, direct investigation in a native environment is required to understand the facet-dependent redox properties. Using liquid cell TEM, we herein observed the etching of ceria-based nanocrystals under the control of redox-governing factors. Direct nanoscale observation reveals facet-dependent etching kinetics, thus identifying the specific facet ({100} for reduction and {111} for oxidation) that governs the overall etching under different chemical conditions. Under each redox condition, the contribution of the predominant facet increases as the etching reactivity increases.

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers are unit-cell thick materials with tunable physical properties according to their size, morphology, and chemical composition. Their transition of lab-scale research to industrial-scale applications requires process development for the wafer-scale growth and scalable device fabrication. Herein, we report on a new type of atmospheric pressure chemical vapor deposition (APCVD) process that utilizes colloidal nanoparticles as process-scalable precursors for the wafer-scale production of TMD monolayers. Facile uniform distribution of nanoparticle precursors on the entire substrate leads to the wafer-scale uniform synthesis of TMD monolayers with the controlled size and morphology. Composition-controlled TMD alloy monolayers with tunable bandgaps can be produced by simply mixing dual nanoparticle precursor solutions in the desired ratio. We also demonstrate the fabrication of ultrathin field-effect transistors and flexible electronics with uniformly controlled performance by using TMD monolayers.