Figure 4. Electric field enhancement and surface melting of gold nanoparticles (AuNPs).

(A) Schematic of proposed mechanism for fragmentation under ps laser. The strong enhanced electric field combing with surface partial melting leads to a layer of material ejection (L_liq) and particle fragmentation. The final product is composed of particles with two different sizes (d_small and d_large). (B) The enhancement of electric field around a AuNP with the diameter of 15 nm. Scale bare is 10 nm. (C) The electric filed strength (|E|) and gold lattice temperature (T_l) at the surface of the 15 nm AuNP when expose to the fragmentation threshold fluence (2.3 mJ/cm²). The reported surface partial melting point of gold (377 K) and the whole particle melting point (1257 K for 15 nm AuNP) are marked by red dash lines. (D) The maximum gold lattice temperature at the surface of particles with different sizes at their fragmentation threshold fluences. For comparison, the surface melting temperature and particle melting temperature are shown. (E-G) The fitting for the distribution of particles after laser treatment for 5 nm (E), 15 nm (F), 100 nm (G) AuNPs by starting with the experimentally determined size distributions before laser irradiation and assuming 6~10% surface melting.