Nanostructure and Mechanism of Metal Deposition by a Laser-Induced Photothermal Reaction

Laser-induced photothermal reaction of metal ions is a promising one-step method for production of conductive micropatterns for electronic devices and sensors. However, the mechanism that governs the process in which thermally activated materials are deposited on a substrate is not clear. Here, light is shed on the mechanism through systematic analysis of photothermal reaction products forming iron oxide and silver at different interfaces. Examination of the nanostructure of deposits on a substrate using high-resolution transmission electron microscopy and selected area diffraction pattern analysis reveals a combination of both amorphous and crystalline moieties. It is found that focusing the laser inside the solution leads to exclusive formation of crystalline products, while focusing at the liquid/air interface leads to formation of amorphous products due to kinetic considerations. Ring-shaped microstructures observed on the substrate indicate that microbubbles are involved in the deposition. The findings suggest that crystalline nanoparticles formed in solution are pinned to the base of the microbubbles. These stationary deposits absorb the laser light, resulting in extensive local heating, which leads to a fast thermal-reaction of the metal ions that are added as amorphous nanostructures. The presence of both crystalline and amorphous nanostructures therefore results from two different mechanisms.