Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target

TY - JOUR

T1 - Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target

AU - Wu, Chengping

AU - Christensen, Martin S.

AU - Savolainen, Juha-Matti

AU - Balling, Peter

AU - Zhigilei, Leonid V.

PY - 2015/1/12

Y1 - 2015/1/12

N2 - Structural transformations in a shallow surface region of a bulk Ag (001) target irradiated by a femtosecond laser pulse are investigated in large-scale atomistic simulations and experiments. The simulations reveal a complex interplay of fast laser melting, rapid resolidification, and dynamic relaxation of laser-induced stresses that leads to the formation of a subsurface porous region covered by a nanocrystalline surface layer. The generation of the porous region is consistent with the experimental observation of surface "swelling" occurring at laser fluences below the spallation/ablation threshold and may be related to the incubation effect in multipulse laser ablation of metals. The nanocrystalline layer is produced by massive nucleation of crystallites triggered by a deep undercooling of the melted surface region experiencing fast quenching at a rate on the order of 10(11) K/s. The predicted surface structure features random crystallographic orientation of nanograins and a high density of stacking faults, twins, and nanoscale twinned structural elements with fivefold symmetry, which suggests high hardness and possible enhancement of catalytic activity of the surface.

AB - Structural transformations in a shallow surface region of a bulk Ag (001) target irradiated by a femtosecond laser pulse are investigated in large-scale atomistic simulations and experiments. The simulations reveal a complex interplay of fast laser melting, rapid resolidification, and dynamic relaxation of laser-induced stresses that leads to the formation of a subsurface porous region covered by a nanocrystalline surface layer. The generation of the porous region is consistent with the experimental observation of surface "swelling" occurring at laser fluences below the spallation/ablation threshold and may be related to the incubation effect in multipulse laser ablation of metals. The nanocrystalline layer is produced by massive nucleation of crystallites triggered by a deep undercooling of the melted surface region experiencing fast quenching at a rate on the order of 10(11) K/s. The predicted surface structure features random crystallographic orientation of nanograins and a high density of stacking faults, twins, and nanoscale twinned structural elements with fivefold symmetry, which suggests high hardness and possible enhancement of catalytic activity of the surface.

KW - MOLECULAR-DYNAMICS SIMULATIONS

KW - EMBEDDED-ATOM-METHOD

KW - THIN METAL-FILMS

KW - PHASE EXPLOSION

KW - MICROSCOPIC MECHANISMS

KW - ELECTRON-DIFFRACTION

KW - ABLATION THRESHOLDS

KW - COPPER

KW - DEFORMATION

KW - PULSE

U2 - 10.1103/PhysRevB.91.035413

DO - 10.1103/PhysRevB.91.035413

M3 - Journal article

SN - 2469-9950

VL - 91

JO - Physical Review B

JF - Physical Review B

IS - 3

M1 - 035413

ER -