Laser-Induced Photo-Thermal Reduction—Mechanism & Applications

Assembly of materials into microstructures under laser guidance is attracting wide attention. The ability to pattern various materials and to form 2D and 3D structures with micron/sub-micron resolution and less energy and material waste compared with standard top-down methods make laser-based printing promising for many applications in sensing, medical devices, and microelectronics. Assembly from liquids provides smaller feature size than powders and has advantages over other states of matter in terms of the relatively simple setup, easy handling, and recycling. However, the simplicity of the setup conceals a variety of underlying mechanisms, which cannot be identified simply according to the starting or resulting materials.<br/>Here, we shed light on one of the mechanisms through systematic analysis of photo-thermal 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. We found that focusing the laser inside the solution leads to exclusive formation of crystalline products, while focusing on 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. Our 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. Finally, we demonstrate how microfluidics could be utilized to enhance the capabilities of multi-layered laser micro-printing by quickly switching between precursors. The concepts presented here could provide new opportunities for fabrication of multi-layered micro-electronic devices and sensors.