Scientists in the Center for Nanoscale Materials’ Theory & Modeling and EMMD Groups, working with Argonne APS and MSD researchers, revealed previously unobserved behaviors that show how details of the transfer of heat at the nanoscale cause nanoparticles to change shape in ensembles. The new findings depict three distinct stages of evolution in groups of gold nanorods, from the initial rod shape to the intermediate shape to a sphere-shaped nanoparticle. The research suggests new rules for the behavior of nanorod ensembles, providing insights into how to increase heat transfer efficiency in a nanoscale system.
Many potential industrial, medical, and environmental applications of metal nanorods rely on the physics and resultant kinetics and dynamics of the interaction of these particles with light. The team discovered a surprising kinetics transition in the global melting of femtosecond laser-driven gold nanorod aqueous colloidal suspension. At low laser intensity, the melting exhibits a stretched exponential kinetics, which abruptly transforms into a compressed exponential kinetics when the laser intensity is raised. The relative formation and reduction rate of intermediate shapes play a key role in the transition. Supported by both molecular dynamics simulations and a kinetic model, the behavior is traced back to the persistent heterogeneous nature of the shape dependence of the energy uptake, dissipation and melting of individual nanoparticles.
Synchrotron X-rays were used to analyze the changing shapes via small angle X-ray scattering at the Advanced Photon Source. Molecular dynamics was performed using the 10-petaflop Mira supercomputer at the Argonne Leadership Computing Facility. Using factors such as the shape, temperature and rate of change, the researchers built simulations of the nanorod in many different scenarios to see how the structure changes over time.
Little is known about how nanorods behave in ensembles of millions. Understanding how the individual behavior of each nanorod, including how its orientation and rate of transition differ from those around it, impacts the collective kinetics of the ensemble and is critical to using nanorods in future technologies. At the nanoscale, individual gold nanorods have unique electronic, thermal and optical properties. Understanding these properties and managing how collections of these elongated nanoparticles absorb and release this energy as heat will help drive new research towards next-generation technologies such as water purification systems, battery materials and cancer research.
Y. Li et al., “Femtosecond Laser Pulse Driven Melting in Gold Nanorod Aqueous Colloidal Suspension: Identification of a Transition from Stretched to Exponential Kinetics,” Nature Scientific Reports, 5, 8146 (2015).