Collaborative Research: Ultrafast Laser-Driven Phase Transitions in Nanoparticles near their Melting

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Non-Technical AbstractAlthough the melting of solids is the most ubiquitous of phase transitions, its atomistic mechanism is still not well understood. Experimental observations show that melting nucleates at surfaces and extended defects. Besides their technological applications, nanoparticles provide interesting opportunities to study the melting phase transition since their properties can be dominated by their surfaces. The study of melting is complicated by the ultrafast nature of the transition. In this project, ultrafast electron diffraction is used to resolve the atomistic dynamics for nanoparticles heated with a short laser pulse. The electron diffraction maps the structural evolution of the nanoparticles while they melt. Modeling using an accurate description of atomic bonding simulates the electron diffraction results. Improved understanding of nanoparticle response to the laser excitation is likely to make a strong impact on the development of biomedical, nano-electronics, and sensing applications. A post-doc, two Ph.D. students, and undergraduates are actively involved in this research and contribute to outreach activities to high school students and the general public.Technical AbstractThe main objective of this project is to understand the mechanisms of the rapid phase transitions in metal nanoparticles driven up to their melting by femtosecond laser irradiation. Ultrafast electron diffraction (UED) is used to map the structural dynamics of the laser-irradiated nanoparticles. Molecular dynamics (MD) simulations with a realistic description of the laser energy coupling and partitioning in the nanoparticles complements the UED studies and provide atomic-level insights into the laser-induced phase transitions. Since melting and diffusionless solid-solid phase transitions can occur on a picosecond time scale, ultrafast measurements are needed to probe transient states along the transition pathways. Ultrafast laser melting of well-characterized, size-selected metal nanoparticles (In, Pb, and Bi) will be studied. The low vapor pressure of these nanoparticles enables structural studies near their melting point without affecting their size. The experiments are conducted on nanoparticles fabricated in an ultrahigh vacuum on substrates with weak surface van der Waals forces. The experiments are designed to study the structural pathways through which the phase transitions occur and their dependence on heating rate; electronic excitation effects; electron-phonon coupling; limits of superheating and supercooling; and interface and substrate effects on melting and solid-state transitions. MD simulations include the effect of the thermal pressure from the excited electrons, which is parameterized based on the predictions of ab initio calculations and the UED results.