For the first time, scientists have observed a single copper sulfide (Cu2S) nanocrystal, an important semiconductor expected to play a big role in future energy technologies as it undergoes structural transformation. The work could lead to the design of groundbreaking materials that serve next-generation devices for solar-energy harvesting and energy-storage batteries. Direct observation of structural transformations within Cu2S nanocrystals had never before been performed. Paul Alivisatos and his team at the US Department of Energy’s Lawrence Berkeley National Laboratory used TEAM 0.5, one of the world’s most powerful transmission electron microscopes (TEMs), to observe structural fluctuations in the material as it transitioned between the low- and high-chalcocite solid-state phases. This could help reveal how ion transport occurs within electrodes during the charging and discharging cycle of a battery. “Cu2S nanocrystals have been studied for solar cell applications. An understanding of the nature of phase transition is important to control the metastability of phases and the stability of the solar cells,” said Haimei Zheng, a Berkeley Lab chemist. “More generally, an understanding of first-order phase transition is critical for the understanding or controlling of a wide range of processes. The study of structural transformation dynamics in a Cu2S nanorod at [the] atomic level using advanced TEM may reveal [the] nature of phase transition that has not been reached before.” Phase transformation of Cu2S nanorod. (a) Atomic-resolution TEM image of low-chalcocite phase (left) and high-chalcocite phase (right), which are enlarged sections from a Cu2S nanorod. (b) TEM image of a transient structure during the transition. The low-chalcocite phase is indicated in green, and the high-chalcocite appears in red. A defect – i.e., a stacking fault – blocks the phase propagation during the transition. (c) Trajectories of the fluctuations between two structures during transition, which are from the two domains shown in b. Courtesy of Haimei Zheng, Lawrence Berkeley National Lab. TEAM 0.5 – a transmission electron aberration-corrected microscope that produces images with half-angstrom resolution – delivers rapid sample imaging with single-atom sensitivity across the elements of the periodic table with greater collection efficiency, Alivisatos said. With these capabilities, the scientists can study structural transformation dynamics in situ with atomic resolution. He also explained that the dynamics of the change itself are strongly influenced by the amount and type of defects that exist in the nanorod crystal. Based on the data collected, the team is confident that it is possible to assist or suppress the transformations to create new materials. Materials that could not be developed before now may be possible using the new and controlled phases. “Such knowledge will aid in the future design of materials with new and controlled phases,” Zheng said. “In addition, since Cu2S phase transition is associated with ion transport within a crystal lattice, our observation is highly relevant to questions on how ion transport occurs within electrodes during charge and discharge of batteries and how material changes at the electrode/electrolyte interface.” The team would like to study the structural transformation of materials in their working environments, Zheng added. Some examples include structural changes of nanoparticles during catalysis, material changes at the electrode/electrolyte interface, and ion transport within battery electrodes. “Our development of dynamic TEM imaging through liquids or gases as well as at the applied electric biasing will be a powerful tool for these studies,” Zheng said. “We expect such fundamental research will contribute to the design of novel materials for the next generation of energy conversion and storage devices.” The group’s observations were published in the July 8 issue of Science (doi: 10.1126/science.1204713).