The Science: A team of scientists has developed four-dimensional (the three dimensions of space plus the fourth dimension of time) atomic electron tomography. Tomography is a technique for creating images of cross sections of an object using X-rays or ultrasound. The technique directly images the dynamics of structural changes at the atomic scale during nucleation. Nucleation is the creation of structure in a vapor, solution, or liquid. The scientists found that the nuclei came in a broad range of shapes and sizes and possess a diffuse interface surrounding a stable core. Their observations challenge the long-held classical nucleation theory that posits nucleation begins with the formation of perfectly spherical nuclei that grow after they reach a certain critical size.
The Impact: Everyday transitions from one state of matter to another – such as freezing, melting, or evaporation—start with a process called “nucleation.” Nucleation is the process by which tiny clusters of atoms or molecules (called “nuclei”) begin to coalesce. Nucleation plays a critical role in phenomena as diverse as the formation of clouds and the onset of neurodegenerative disease. By capturing nucleation, growth, dissolution, and merger of grains at the atomic scale, the technique showed limitations in current nucleation theories. The results of this research provide experimental data of nucleation on the atomic scale. This data will be important to the creation of new theories and models of nucleation.
Summary: Scientists have gained a never-before-seen view of nucleation, capturing how the atoms rearrange at 4D atomic resolution (that is, in three dimensions of space and across time). The findings differ from predictions based on the classical theory of nucleation that has long appeared in textbooks. The scientists measured the atomic positions in the nanoparticle at the core of a nucleation process after different annealing (heat treatment) times. The observations, confirmed by experiment and data analysis, reveal that nucleation primarily occurs at the heterogeneous surfaces of the nanoparticle, not in the homogeneous interior. The scientists also found that the nuclei form a variety of shapes and possess diffuse interfaces. These findings differ from the spherical shape and smooth interface that current theory predicts are required for successful nucleation.
The scientists conducted the research using the TEAM 0.5 (Transmission Electron Aberration-corrected Microscope) at the National Center for Electron Microscopy at the Molecular Foundry at Lawrence Berkeley Laboratory. In much the same way a CAT scan generates a 3D X-ray of the human body, atomic electron tomography uses electrons to create 3D images of atoms within a material as the sample is rotated under the microscope.
Funding: This research was supported by the National Science Foundation (3D tomographic reconstructions, data analysis and modeling) and the U.S. Department of Energy Office of Science, Basic Energy Sciences, Materials Sciences and Engineering (sample preparation and data acquisition). The ADF-STEM imaging with TEAM 0.5 was performed at the Molecular Foundry, which is a scientific user facility supported by the Office of Science, Office of Basic Energy Sciences of the U.S. DOE.
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