4D Structures of Colloidal Nanoparticles using Graphene Liquid Cell TEM and Self-Supervised Machine Learning Analysis

Colloidal nanocrystals containing a few tens to hundreds of atoms have applications in a range of areas, from electronics to catalysis and biological sensors. This versatility stems from the high sensitivity of nanocrystal properties to size, chemical composition, and shape, which are largely determined by the synthesis route by which they are produced. However, structure characterization based on bulk-originating methods is usually not enough because of the unique structural characteristics emerging at the nanoscale. Exposed surfaces, defects, dislocations, and quantum effects are dominant in nanocrystals of finite size. In addition, the ensemble of nanocrystals produced from conventional colloidal synthesis displays a large degree of heterogeneity in the atomic structures and has the effects of organic ligands and solvent molecules. Thus, understanding the structures of nanocrystals at a level where fundamental structure-property relationships can be linked requires new analysis methods that allow precise and reproducible determination of the positions of the constituent atoms of single nanocrystals directly from the solution phase. We developed a “one-particle Brownian 3D reconstruction method” for the analysis of the three-dimensional (3D) structures of colloidal nanocrystals based on graphene liquid cell electron microscopy. “One-particle Brownian 3D reconstruction” can be applied to not only uni-component metallic nanoparticles but also multi-component nanoparticles such as CdSe, PbSe, and FePt. We further extend 3D structure characterization approach to time-resolved domain such that colloidal nanoparticles undergoing structural transition in solution can be captured in time-resolved 3D resolution. To enable 3D and 4D reconstruction of colloidal nanoparticles directly from liquid phase TEM imaging, a sereis of large image data processing, a part of which is potentially enhanced by machine-learning based data analysis, is developed.