Yongfeng Lu, University of Nebraska-Lincoln, USA

Title

3D nanoprinting of functional structures

Abstract

Nanoscale 3D printing by two-photon polymerization (TPP) has been established as one of the most promising methods for achieving 3D fabrication in micro/nanoscales, due to its ability to produce arbitrary and complex 3D structures with subwavelength resolution. However, the lack of TPP-compatible and functional materials represents a significant barrier to realize functionalities of the fabricated structures devices, such as high electrical conductivity, high environmental sensitivity, high mechanical strength, and fast writing speed. To address these barriers, we have investigated the TPP 3D nanofabrication based on blended precursors. To unleash the full potential of TPP, it is essential to realize sufficient structural stability, high fabrication throughput, and fine linewidth. We introduced thiol-acrylate chemistry into TPP to improve all three of the factors simultaneously. Micro/nanofabrication by TPP was investigated using thiol-acrylic resins containing different amounts of tetrafunctional acrylic monomers and tetrafunctional thiol molecules. Compared with the pure acrylic resin, the thiol-acrylate resin (30 wt% of thiol concentration) can achieve higher writing speed, higher mechanical strength, finer linewidth, and lower percent of shrinkage in TPP fabrication. We also investigated a thiol grafting method in functionalizing multiwalled carbon nanotubes (MWNTs) to develop TPP-compatible MWNT-thiol-acrylate (MTA) composite resins. Significantly enhanced electrical and mechanical properties of the 3D micro/nanostructures were achieved. Microelectronic devices made of the MTA composite polymer were demonstrated. Similarly, we also realized metallic 3D micro/nanostructures with silver-thiol-acrylate composites via TPP followed by femtosecond laser nanojoining. Complex 3D micro/nanoscale conductive structures have been successfully fabricated with ~200 nm resolution. The loading of silver nanowires (AgNWs) and joining of junctions successfully enhance the electrical conductivity of the composites from insulating to 92.9 S m^-1 at room temperature. The nanomaterial assembly and joining method demonstrated in this study paved a way toward a wide range of device applications, including 3D electronics, sensors, memristors, micro/nanoelectromechanical systems (MEMS/NEMS), biomedical devices, and fuel targets for inertial confinement fusion. 

Biography

Dr. Yongfeng Lu is currently the Lott Distinguished Professor of Engineering at the University of Nebraska-Lincoln (UNL). He received his bachelor degree from Tsinghua University (China) in 1984 and M.Sc. and Ph.D. degrees from Osaka University (Japan) in 1988 and 1991, all in electrical engineering. From 1991 to 2002, he was a faculty in the Department of Electrical and Computer Engineering at National University of Singapore. He joined the Department of Electrical and Computer Engineering at UNL in 2002. He has more than 25 years of experience in processing and characterization of micro/nanostructured materials. His group has research projects funded by NSF, AFOSR, ONR, DTRA, DOE, DOT, NCESR, NRI, private companies, and foundations, with research expenditures over $27 million in the past a few years. His research has led to a number of commercialization and product developments. Dr. Lu has authored or co-authored over 480 journal papers and 440 conference papers. He is currently the president of International Academy of Photonics and Laser Engineering (IAPLE, UK). He served as the President of the Laser Institute of America (LIA, USA) in 2014. He has been elected to SPIE fellow, LIA fellow, OSA fellow, and IAPLE fellow. He has also served as chair and general chair for major international conferences in the field including the general congress chair for the International Congress of Applications of Lasers and Electro-Optics in 2007 and 2008, and general co-chair for LASE symposia in Photonics West 2014-2017. He is also the recipient of a number of prestigious award, including the Schawlow Award of LIA in 2016.