Formation of crownlike and related nanostructures on thin supported gold films irradiated by single diffraction-limited nanosecond laser pulses

A type of laser-induced surface relief nanostructure—the nanocrown—on thin metallic films was studied both experimentally and theoretically. The nanocrowns, representing a thin corrugated rim of resolidified melt and resembling well-known impact-induced water-crown splashes, were produced by single diffraction-limited nanosecond laser pulses on thin gold films of variable thickness on low-melting copper and high-melting tungsten substrates, providing different transient melting and adhesion conditions for these films. The proposed model of the nanocrown formation, based on a hydrodynamical (thermocapillary Marangoni) surface instability and described by a Kuramoto-Sivashinsky equation, envisions key steps of the nanocrown appearance and gives qualitative predictions of the acquired nanocrown parameters.

Figure 1

Experimental setup for surface nanostructuring.

Figure 2

(a)–(f) AFM images of the laser-induced surface structures in the 50-nm-thick Au/Pd film on the Cu substrate at laser energies $E$ ≈ 1.9 (a), 2.15 (b), 2.3 (c), 2.7 (d), 4.9 (e), and 5.5 nJ (f).

Figure 3

Squared radii of $x$ and $y$ semiaxes of nanocrowns versus $\ln E$ on the 25-nm-thick (blue symbols) and 50-nm-thick (red symbols) gold films on the Cu substrates. The straight lines fit the experimental data using the functions $R_{x,y}^2={\Delta }_{x,y}^2\ln \left(E/E_c\right)$, where ${\Delta }_{x,y}$ are the characteristic lateral scales of laser energy deposition.

Figure 4

AFM and SEM images of the laser-induced surface structures in the 25-nm-thick Au/Pd film on the Cu substrate at laser energies $E$ ≈ 0.5 (a), 0.87 (b), 1.55 (c), and 2.4 nJ (d).

Figure 5

(a)–(c) Pairs of AFM and SEM images of the laser-induced surface structures (nanorims and nanocraters) in the 50-nm-thick Au/Pd film on a tungsten substrate at the laser energies $E$ ≈ 2.2 (a), 2.9 (b), and 4.6 nJ (c). (d) Squared radii of $x$ and $y$ semiaxes of the rims versus $\ln E$.

Figure 6

(a) Sketch of a molten pool with the rim of the radius $R_m$ and the periodically (azimuthally) modulated height $h=h_m+h_1$ (the nanocrown). (b) Angular fragment of the nanocrown shown in (a), which illustrates the height modulation of the molten rim with the small amplitude $h_1$ ($h_1\gg h_m$) and the wavelength Λ.