In situ visualization of NaCl nanocrystal growth in aqueous environments using amorphous carbon encapsulated TEM

Crystal growth is the post-nucleation process in which molecules or clusters attach to a host crystal. Because of its diversity, complexity, and stochasticity, research at the sub-nano-level in aqueous environments presents a significant experimental challenge. In this study, amorphous carbon encapsulated transmission electron microscopy (ACE-TEM) is employed to observe the crystal growth processes of an electron-beam-sensitive nanocrystal within a custom-designed ACE liquid cell made from amorphous carbon films, enabling stable observation under an electron beam. NaCl nanocrystals are successfully encapsulated in an aqueous environment by deliberately evaporating a small amount of water before sealing the liquid cell. The crystal growth of the NaCl nanocrystals is observed through multiple simultaneous processes occurring in different planes. NaCl nanocrystal growth proceeds via stepwise elongation of lattice fringes, with step lengths consistent with the interatomic spacing of NaCl. In addition, growth involves oriented attachment of a rotating cluster-like feature consistent with a metastable cluster. These observations bridge classical and non‑classical growth pathways. The in situ ACE-TEM methodology introduced here paves the way for a more comprehensive understanding of nanoscale crystallization principles that are essential for progress across the materials science domain.