Lattice dynamics of core-shell bimetallic nanocrystals during ultrafast laser excitation

K. Sasikumar, M. Cherukara, J.N. Clark, S.K.R.S. Sankaranarayanan, R. Harder
Argonne National Laboratory,
United States

Keywords: lattice dynamics, heat transfer, laser excitation, ultrafast heating

Summary:

Energy transport via lattice vibrations (phonons) play a crucial role in several applications such as heat dissipation in semiconductors, waste heat energy conversion via thermoelectric materials, and phase transitions and cavitation phenomena in intensely heated nanofluids. Investigation of the temporal behavior of externally stimulated materials, under severe thermal non-equilibrium conditions, can lead to crucial insights for energy research. Recently, experimental techniques have evolved to conduct time-dependent lattice dynamics measurements in nanomaterials. In particular, Bragg Coherent Diffraction Imaging (BCDI) in conjunction with optical pump-probe experiments via ultrafast x-ray lasers has been used to directly image the generation and subsequent evolution of coherent acoustic phonons within nanocrystals. In particular, experiments on bimetal (Au/Al) core-shell nanocrystals have revealed inhomogeneous effects in the lattice breathing upon heating with femtosecond x-ray lasers. Conventional theoretical models cannot be used to explain the physics of the phenomena in such non-equilibrium environments that involve extreme heat fluxes and temperature gradients. Atomistic molecular dynamics (MD) simulation is an appropriate technique to investigate lattice dynamics in such environments, particularly in core-shell structures where interfacial effects can play an important role in phonon scattering. With the convergence of time and length scales accessible by both experiments and simulations, we are now able to integrate experimental observations with multi-million atom MD simulations to enhance the fundamental understanding of materials behavior under extreme environments. In this talk, we focus on the MD simulations performed on core-shell bimetallic nanocrystals under the influence of extreme heat fluxes. We explore the vibrational modes that dominate lattice breathing and look into the effect the nanoparticle size and shape have on the same. In addition, we attempt to identify the origin of the inhomogeneous effects, observed experimentally, in the lattice breathing of core-shell structures. Finally we investigate the effects of lattice mismatch between the core and shell material, the shell-to-core size ratio and the interfacial structure to tune the lattice dynamics of core-shell nanocrystals.