Real-Time Electron Microscopy of Nanocrystal Synthesis, Transformations, and Self-Assembly in Solution

Abstract

Solution-phase processes such as colloidal synthesis and transformations have enabled the formation of nanocrystals with exquisite control over size, shape, and composition. Self-assembly, in solution or at phase boundaries, can arrange such nanocrystal building blocks into ordered superlattices and dynamically reconfigurable "smart" materials. Ultimately, continued improvements in our ability to direct nanocrystal matter depend on progress in understanding colloidal chemistry and self-assembly in solution. The traditional approach for investigating the underlying, inherently dynamic processes involves sampling at different stages combined with ex situ characterization, for example, using electron microscopy. In situ studies have been restricted to a few methods capable of measuring in bulk liquids, either in reciprocal space by diffraction or scattering or using spatially averaging (e.g., optical) measurements. These strategies face clear limitations in obtaining mechanistic information, and they are unable to address heterogeneous systems that may harbor rich sets of configurations with different local properties. The development of microfabricated cells that hermetically encapsulate bulk solutions between ultrathin (electron transparent) membranes has paved the way for studying processes in liquids in real time by electron microscopy at resolution down to the atomic scale. Electrons interact much more strongly with matter than other probes, for example, X-rays. In ordinary inorganic samples, the main effects are atom displacements and defect formation via knock-on and ionization damage. In liquid-cell electron microscopy, the interaction of the beam with both the suspended nanostructures and the solution creates more diverse effects, so the straightforward scenario of imaging unperturbed nanocrystal chemistry in solution is rarely realized.In this Account, we discuss applications of real-time electron microscopy to the analysis of nanocrystal synthesis, transformations, and self-assembly in solution. While in the simplest case the effects of the electron beam are negligible, the interaction with high-energy electrons often provides excitation or stimulus for solution-phase processes or opens up competing chemical pathways. Real-time observations of self-assembly demonstrate particularly clearly the power of in situ microscopy in identifying key nucleation and growth mechanisms and providing information about preferred structural motifs that can be analyzed to quantify the balance of forces and the role of entropy in stabilizing ordered assemblies. Modifications of the solution by the electron beam can provide stimuli for on-demand self-assembly, for example, via an acid spike due to water radiolysis that locally lowers the pH in the imaged area. While in this and other cases (e.g., colloidal synthesis), beam-induced radicals become part of the experimental design, in imaging redox reactions such as galvanic transformations of nanocrystal templates, radicals need to be managed and if possible eliminated by suitable scavengers. Finally, excitation by the imaging electron beam can transfer energy to individual nanocrystals in solution, thus driving nonthermal (e.g., plasmon-mediated) synthesis or other chemistry while following the reaction progress with high resolution. Overall, with validation by ex situ control experiments, the unique ability of observing processes in solution at the nanometer scale should make liquid-cell electron microscopy an integral part of the toolkit for designing novel inorganic nanocrystal architectures.