Nanofountain Pen for Writing Hybrid Plasmonic Architectures

Self-assembled colloidal nanostructures enable various interesting fabrication alternatives to current top-down lithographic approaches for supporting localized surface plasmon resonance (LSPR). Recently, there have been numerous efforts to manipulate the LSPR in structural or compositional engineering using clustered nanoparticles (NPs). However, creating heterogeneous colloidal clusters comprising two or more types of NPs requires a complicated chemical synthesis of the linker molecules on the NP surfaces. Moreover, the traditional assembly methods pose several challenges to 3D manufacturing. Herein, a fountain pen-inspired open-microfluidic approach demonstrating an exquisite balance between evaporation and capillary action is reported. This approach enables the direct writing of binary NP clusters. Microcapillaries are designed to precisely control the evaporation and capillary action of the binary NP suspension ink at the tip. This approach effectively guides the growth of NP clusters along the out-of-plane direction to fabricate a freeform microarchitecture. The growth characteristics are theoretically explained using a simplified balancing model. In addition, the optical properties can be tuned precisely using multicolloidal NP mixing. A Janus pillar comprising plasmonic NPs and biomolecules is showcased as a microactuator operating under chemical stimuli. It is expected that the proposed method paves the way for manufacturing numerous interesting structures in nanophotonics.