Nanoscale self-assembly and nonequilibrium transformation of colloidal nanoparticles visualized by liquid-phase TEM

Liquid-phase transmission electron microscopy (TEM) has been widely used for probing solution-phase nanoscale dynamics, such as nanoparticle growth/corrosion, electrochemical processes, and aggregation of soft materials (e.g., micelles, proteins). Among them, one particularly interesting direction is to resolve and understand the self-assembly pathways of nanosized building blocks, namely individual one of them diffusing and interacting with each other to form into functional superstructures. Complicated, nonclassical pathways occur as originated from the nonadditivity of nanoscale interactions and the resultant complex free-energy landscapes for nanosized entities. During my Ph. D., my research has provided insights in charting the energy diagram, fundamental interactions, formation of prenucleation precursors and interfacial energy of crystals using in-situ liquid-phase TEM. Such scientific understanding is enabled by a statistical mechanics based conceptual framework to extract spatiotemporal information from the liquid-phase TEM movies, which can be generalizable to a broad range of phase behavior studies for materials at the nanoscale. We used a model system of nanoparticles to study their crystallization pathways into well-shaped supracrystals. We first quantitatively studied the electron induced beam effect in aqueous environment and developed a protocol to control the self-assembly dynamics of nanoparticles inside liquid-phase TEM. Then, the full transition of crystallization was mapped from single nanoparticle towards a large-scale supracrystal, and the density and structure took place separately in time, following a nonclassical two-step crystallization. I elucidated the origin of this two-step crystallization by experimentally measuring the free-energy barrier for nucleation based on the statistical distribution of transient clusters at initial stage. During the growth process, the crystal surface fluctuates due to thermally excited capillary waves, based on which we were able to measure the interfacial energy of such supracrystals, again enabled by our in-situ liquid-phase TEM imaging. The supracrystal surface has a roughness driven by the thermal capillary waves, which we validated for the first time at the nanoscale. In summary, we have demonstrated the capability of utilizing liquid-phase TEM to investigate various spatiotemporal fluctuating phenomena at the nanoscale. Fast diffusion of nanoparticles enables investigation over various collective behaviors, such as glass transition, grain-boundary migration and aggregate coalescence, which have been challenging to probe due to the extremely long relaxation time. Apart from inorganic nanoparticles, the protocol can be extended to study other spatially heterogenous phenomena, where nanoscale dynamics play a critical role, such as the penetration of nanoparticles through a porous membrane and degradation dynamics of polymeric materials into micro/nanoparticles. Real-space investigations will offer insight into such physical processes, improving coating recipe engineering and efficiency of drinking water filtration. Such methodologies can also be applied to optical microscopy and impact the understanding of other materials systems, including a synthesis–imaging–analysis platform I developed to visualize the morphology transformation of metal–organic framework crystals during chemical etching.