Rapid Shape Change\nand Spallation of Isolated Nanoscale\nAluminum Cubes, Rods, Pyramids, and Spheres: Implications for Plasmonic\nand Energetic Materials

Aluminum nanoparticles (Al NPs) are an attractive material\nfor\nplasmonic and energetic applications. Herein, we investigate the response\nof spherical and nonspherical (cubes, pyramids, and pentagonal rods)\nAl NPs to rapid laser heating. Rapid surface diffusion reconfigures\nthe shaped Al NPs toward a spherical geometry. Based on simulations,\nthe surface diffusion occurs below the bulk Al melting temperature.\nWhen exposed to sufficient laser flux, Al NPs of all geometries spall.\nThe critical laser flux to induce spallation depends on pulse duration\nand initial particle geometry. Thermal simulations indicate that complete\nAl melting occurs in less than 5 ns, with steady state occurring before\n20 ns. Experimentally, spallation was observed between 18 and 79 ns.\nThe laser conditions to induce spallation in an oxygen-deprived environment\n(<200 ppm oxygen) are identical to those in open air, suggesting\na thermomechanical spallation mechanism rather than a diffusion-reaction\nmechanism.