From photoinduced electron transfer to 3D metal microstructures via direct laser writing

We review the fundamental concepts of direct laser writing (DLW) of 3D metallic structures via photoreduction and give an overview over the state-of-the-art. On the one hand, metallic microstructures and nanostructures play an important role in photonic applications such as resonators, antennas, metamaterials, and polarizers. On the other hand, DLW offers a flexible and fast way to fabricate microstructures. Because the underlying mechanisms from the first photoreaction to the final 3D microstructure are quite complex and not yet well controlled, we believe that a review of the photochemistry and photophysics of the direct writing process of metal structures helps to promote development in this field. To this end, we first summarize the principles of electroplating and electroless plating as this helps understand the photoresist's components. Next, we describe the different photoreducing agents and photoreactions that lead to metal seeds and in consequence to nanoparticles. This is followed by insights into the physics of nanoparticle agglomeration to the desired microstructure. Finally, we give an overview over the state-of-the-art of DLW metallic 3D microstructures.