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.

The composition of materials in a micro-/nano-devices plays a key role in determining their mechanical, physical, and chemical properties. Especially, for devices with a compositional change on nanoscale which can often be achieved by point-by-point direct writing technology using a focused ion beam (FIB), electron beam (EB), or laser beam (LB), but so far, nanoscale composition analysis of a large-area micro/nano structures with a variation composition remains a big challenge in cost, simpleness, and flexibility. Here we present a feasible route to realize large-area composition analysis with nanoscale spatial resolution by using Raman spectroscopy. We experimentally verified the capability of this method by analyzing a complex Sn-SnO_x system of a microscale grayscale mask with nanoscale spatial resolution of composition. Further analyses using Auger electron spectroscopy, transmission electron microscopy, and atomic force microscopy indicated the effectiveness and practicality of our method. This work opens up a way to analyze the composition of a large-area complex system at a nanoscale spatial resolution, and the method can be extended to many other material systems.