In 2010, Haimei Zheng and colleagues observed the growth trajectories of single colloidal platinum nanocrystals using in situ transmission electron microscopy (TEM). They found that platinum nanocrystals can grow either by monomer attachment from solution onto existing particles or by coalescence between particles. Surprisingly, an initially broad size distribution of the nanocrystals can spontaneously narrow, suggesting that nanocrystals take different growth pathways based on their size- and morphology-dependent internal energies. These observations are expected to be highly relevant for other nanocrystal systems. The growth of colloidal nanocrystals has advanced significantly in the last decade, allowing for the synthesis of nanocrystals with narrow size distributions and high crystallinity. These nanocrystals can be shaped into various forms, including spheres, disks, rods, and more complex structures. The current state of nanocrystal synthesis has been largely achieved empirically, with some classical models serving as guides. In this paper, the authors demonstrate that it is possible to directly observe the growth trajectories of individual colloidal nanocrystals in solution using a newly designed liquid cell that operates inside a TEM. These trajectories reveal a diverse set of growth pathways more complex than previously envisioned. The study used a liquid cell reactor that can be placed in a special TEM sample holder to image the dynamic growth of Pt nanocrystals in solution. The liquid cell was designed to allow for high-resolution imaging in real time. The authors observed that platinum nanocrystals can grow through monomer addition or coalescence, and that coalescence events can lead to a narrowing of the size distribution. They also found that the growth of nanocrystals can be influenced by surfactants, which play a significant role in controlling particle size and shape. The study provides insights into the growth mechanisms of nanocrystals, showing that coalescence and punctuated growth can contribute to the focusing of the size distribution. The authors also found that the relaxation time after a coalescence event increases with the size of the coalesced particles, following a power law relationship. These findings suggest that the growth of nanocrystals is a complex process that involves multiple factors, including coalescence, monomer addition, and surfactant effects. The study highlights the importance of in situ TEM in understanding the growth mechanisms of nanocrystals, as it allows for the direct observation of individual nanoparticle growth trajectories. The results have implications for the synthesis of nanocrystals with more complex shapes and for the development of new nanomaterials. The study also demonstrates the potential of in situ TEM for addressing fundamental issues in materials science, chemistry, and other fields of science.In 2010, Haimei Zheng and colleagues observed the growth trajectories of single colloidal platinum nanocrystals using in situ transmission electron microscopy (TEM). They found that platinum nanocrystals can grow either by monomer attachment from solution onto existing particles or by coalescence between particles. Surprisingly, an initially broad size distribution of the nanocrystals can spontaneously narrow, suggesting that nanocrystals take different growth pathways based on their size- and morphology-dependent internal energies. These observations are expected to be highly relevant for other nanocrystal systems. The growth of colloidal nanocrystals has advanced significantly in the last decade, allowing for the synthesis of nanocrystals with narrow size distributions and high crystallinity. These nanocrystals can be shaped into various forms, including spheres, disks, rods, and more complex structures. The current state of nanocrystal synthesis has been largely achieved empirically, with some classical models serving as guides. In this paper, the authors demonstrate that it is possible to directly observe the growth trajectories of individual colloidal nanocrystals in solution using a newly designed liquid cell that operates inside a TEM. These trajectories reveal a diverse set of growth pathways more complex than previously envisioned. The study used a liquid cell reactor that can be placed in a special TEM sample holder to image the dynamic growth of Pt nanocrystals in solution. The liquid cell was designed to allow for high-resolution imaging in real time. The authors observed that platinum nanocrystals can grow through monomer addition or coalescence, and that coalescence events can lead to a narrowing of the size distribution. They also found that the growth of nanocrystals can be influenced by surfactants, which play a significant role in controlling particle size and shape. The study provides insights into the growth mechanisms of nanocrystals, showing that coalescence and punctuated growth can contribute to the focusing of the size distribution. The authors also found that the relaxation time after a coalescence event increases with the size of the coalesced particles, following a power law relationship. These findings suggest that the growth of nanocrystals is a complex process that involves multiple factors, including coalescence, monomer addition, and surfactant effects. The study highlights the importance of in situ TEM in understanding the growth mechanisms of nanocrystals, as it allows for the direct observation of individual nanoparticle growth trajectories. The results have implications for the synthesis of nanocrystals with more complex shapes and for the development of new nanomaterials. The study also demonstrates the potential of in situ TEM for addressing fundamental issues in materials science, chemistry, and other fields of science.