Anomalous refinement and uniformization of grains in metallic thin films

When a laser beam writes on a metallic film, it usually coarsens and deuniformizes grains because of Ostwald ripening, similar to the case of annealing. Here we show an anomalous refinement effect of metal grains: A metallic silver film with large grains melts and breaks into uniform, close-packed, and ultrafine (~ 10 nm) grains by laser direct writing with a nanoscale laser spot size and nanosecond pulse that causes localized heating and adaptive shock-cooling. This method exhibits high controllability in both grain size and uniformity, which lies in a linear relationship between the film thickness (h) and grain size (D), D ∝ h. The linear relationship is significantly different from the classical spinodal dewetting theory obeying a nonlinear relationship (D ∝ h^5/3) in common laser heating. We also demonstrate the application of such a silver film with a grain size of ~ 10.9 nm as a surface-enhanced Raman scattering chip, exhibiting superhigh spatial-uniformity and low detection limit down to 10⁻¹⁵ M. This anomalous refinement effect is general and can be extended to many other metallic films.