Direct Laser Writing of Functional 3D Micro/nanostructures by Two-photon Polymerization for Enhanced Mechanical and Electrical Properties

Precise fabrication of three-dimensional (3D) micro/nanostructures with multiple functionalities is now a key component in a broad range of research and industry fields. Two-photon-polymerization (TPP) is of increasing interest due to its unique combination of true 3D fabrication capability and ultrahigh spatial resolution. However, the stringent requirements of nonlinear resins seriously limit the functionality of 3D micro/nanostructures fabricated by TPP. To unleash the full protentional of TPP, the research efforts described in this dissertation focused on the following points. 1) TPP is being used to deterministically print low-density, low-atomic-number (CHO) polymer foams at millimeter scale with nanoscale accuracy and precision, serving as small custom experimental packages (“targets”) to support research in areas of high-energy-density (HED) plasma physics. The design and printing process, as well as the assembly and handling of these delicate, brittle samples, demonstrates the flexibility, versatility, and efficiency of TPP for the fabrication of low-density targets. 2) A comprehensive method was developed to analyze the deformation and shrinkage during development and drying of the foam structures. The extent of polymerization was quantified by infrared and Raman spectroscopy, the mechanical strength was examined, and the magnitude of the shrinkage was quantified and simulated using finite element analysis. 3) Enhancement of the 3D micro/nanofabrication properties by TPP was investigated using carbon nanotubes (CNTs) as filling materials in acrylate polymer resins. Complex 3D micro/nanoscale conductive structures were successfully fabricated using composite photoresists. The CNT-polymer composites showed significantly enhanced electrical conductivity (up to 46.8 S/m) and mechanical strength with strong anisotropic properties. Precise assembly of multiwalled carbon nanotubes (MWNTs) of ~100 nm spatial resolution was achieved by selective thermal evaporation. 4) A novel method was developed to realize metallic 3D micro/nanostructures with silver nanowire (AgNW)-polymer composites via TPP followed by femtosecond laser nanojoining. The loading of AgNWs and joining of junctions further enhanced the electrical conductivity (up to 92.9 S/m at room temperature). Moreover, a reversible switching to a higher conductivity was observed, up to ~105 S/m at 523 K, for the first time. The temperature-dependent conductivity of the AgNW-polymer composite was analyzed following the variable range hopping and thermal activation models