The popular concept of phase transitions is that of a material, in response to temperature changes, undergoing a transformation from a solid to a liquid or gas, i.e., ice to water to steam. But some solid materials, especially at the nanoscale, when subjected to temperature changes can transition between two more different phases in their crystal structure. Copper sulfide, for example, can be transformed from a complex hexagonal structure known as the low-chalcocite phase, to a more simple hexagonal structure known as the high-chalcocite phase. Because such "first-order structural transformations" can alter the properties of a nanocrystal, they are of great interest to a broad range of scientific fields and hold important implications for numerous technologies.
"In nanoscale systems, the energetic barrier to a structural transformation scales with crystal size," says Alivisatos. "When the size of a nanocrystal is in a regime where thermal energy is comparable to the energy barrier for phase transformation, fluctuations between two stable structures occur at the transition point, and are relevant to many molecular and solid-state phenomena near equilibrium."
Alivisatos, the Larry and Diane Bock Professor of Nanotechnology at the University of California (UC) Berkeley, is a corresponding author of a paper in the journal Science titled "Observation of Transient Structural-Transformation Dynamics in a Cu2S Nanorod."
Co-authoring this paper were Haimei Zheng, Jessy Rivest, Timothy Miller, Bryce Sadtler, Aaron Lindenberg, Michael Toney, Lin-Wang Wang and Christian Kisielowski.
"During the phase transitions of copper sulfide between low-chalcocite and high-chalcocite structure, the sulfur ions remain in a rigid lattice frame while the copper ions move within the sulfur ion lattice," says Haimei Zheng, lead and co-corresponding author of the Science paper.
"We observed where the phase nucleates at
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DOE/Lawrence Berkeley National Laboratory