Surface nanopatterning of functional polymers by parallel scanning probe lithography

Abstract

Engineering surface with patterned functional polymers is a crucial issue in a wide range of research fields. The recently developed parallel scanning probe arrays have emerged as promising candidates for desktop nanofabrication with high resolution and high throughput. Yet, there are only a few literature reports on the large-area patterning of functional polymer nanostructures via parallel SPL, with lack of fundamental understandings and practical applications. Moreover, it still remains some inherent limitations in current parallel SPL techniques regarding the resolution, uniformity and versatility. This thesis describes several approaches to tackle the challenges in the two aspects. The statement of challenges and objectives of this research is presented in the beginning, followed by a comprehensive literature review on nanopatterning techniques and a general introduction of methodologies. Next, Chapter 4 introduces the studies on ink transport in direct writing of polymers and applications of liquid polymer matrix. Chapter 5 states a molecular printing approach towards high-throughput fabrication of 3D polymer brushes. Chapter 6 describes the development of an apertureless polymer pen array to deliver both optical energy and materials. Chapter 7 addresses the uniformity challenge in polymer pen lithography through engineering the tip structure and new materials. Finally, Chapter 8 provides conclusions and future outlooks. First, direct printing of polymer ink materials was studied by dip-pen nanolithography (DPN) and polymer pen lithography (PPL). Two typical polymer inks were investigated, including polyelectrolytes and hygroscopic polymers. For polyelectrolytes, the ink transport was found to be tuned from molecular diffusion to liquid deposition mechanism by adding additives or using polymer pens. Moreover, a catalytic printing was developed for generating metal structures on flexible substrates by employing the hygroscopic polymer ink as delivery matrix and surface-grafted polymer as receiving matrix for catalysts. Second, by combining PPL with surface-initiated polymerization, for the first time we demonstrated the high-throughput generation of 2D and 3D complex polymer brush nanostructures over square-centimeter areas. The control on the 3D structures of polymer brush patterns was achieved by rational design of PPL parameters. Furthermore, the large-area patterned polymer brushes were explored as etching resists for micro/nanofabrication, as well as robust and versatile platforms for bioimmobilization. In addition to material printing, an apertureless polymer pen array was developed to enable serial writing with light at the sub-wavelength scale. This was achieved by combining the self-light-focusing polymer pyramids with an opaque coating on the flat backing regions. The elastomeric pens also show the capability of force-tuned illumination and simultaneous delivery of materials and optical energy. Furthermore, a novel dual-elastomer tip array was designed and fabricated to improve the resolution and the large-area uniformity in PPL. The ultrasharp tip with hard-apex, soft-base layered structure was verified to reduce tip deformation upon contact with the substrate by both experimental and mechanical simulation results, leading to decreased feature size and minimized size variation over large areas. In conclusions, large-area, high-throughput nanopatterning of functional polymers by parallel SPL is achieved through approaches of direct printing, surface-initiated growth and photochemical modification. Meanwhile, the development of apertureless cantilever-free pen array and dual-elastomer tip array will extend the patterning capability of SPL and lower the barrier for desktop nanofabrication. Given the broad applications of functional polymers and lithographic techniques, the work presented here is expected to have great impact on physical, chemical, material, biological and engineering fields.