Design of 3D Complex Nanostructures Using Block Copolymer Self-Assembly

Abstract

The continuously pursuit of designing high quality complex nanostructures is required by the modern nanotechnology. However, the conventional 'top-down' lithography is limited by the rapidly increasing cost and difficulty when operating on length scales below 20 nm. As an alternative, block copolymer (BCP) self-assembly offers a 'bottom-up' approach for the fabrication of various nanopatterns. By implementing external guidance and modifying molecular architectures, long-range ordered BCP microdomains can be oriented and registered in specific locations during the annealing process. Over the past few decades, BCP self-assembly has become well-established for generating arbitrary complex 2D patterns with feature sizes of a few nanometers.

Despite the remarkable success in creating 2D patterns, efforts to achieve highly ordered 3D nanostructures through BCP self-assembly have continued. In this Thesis, a series of novel routes to fabricate oriented nanomeshes, the overlaid parallel lines, are explored and evaluated using both experimental and simulation methods. The discovered mechanisms, proposed modeling setups, and polymer architecture design strategies shed light on a path towards fabricating various 3D geometries and even multicomponent nanopatterns through BCP self-assembly.

The first stage involves introducing external guidance to traditional linear diblock copolymers (di-BCPs). A newly designed multi-mechanism directed self-assembly (MMDSA) approach is proposed to construct metallic nanomeshes without requiring pattern transfer or high-resolution lithographic templating. Three mechanisms, including trench wall guidance, edge nucleation, and underlayer guidance, are systematically evaluated through both experiments and dissipative particle dynamics (DPD) simulations. The DPD model, a particle-based method, is reparametrized to accurately reproduce all the experimental findings in the MMDSA experiments and provide an accurate description of the self-assembled phase structures in both bulk and thin film states.

The next motivation stems from the desire to expand the library of self-assembled structures. To this end, an unconventional "A-branch-B" diblock Janus bottlebrush copolymer (di-JBBCP) is synthesized and studied. Di-JBBCP consists of a rigid backbone grafted with alternating A and B sidechains, which allows for effective and efficient microphase separation of the two blocks parallel to the backbone. The phase diagram of di-JBBCP is fully investigated, revealing the bulk-stable perforated lamella, unconventional Frank-Kasper A15 spheres, and hexagonally close-packed spheres, along with cylinder, gyroid, and lamellar morphologies attainable through a simple annealing step.

Finally, taking a step beyond di-JBBCP, we introduce a third block to form a triblock JBBCP with two Janus domains: one perpendicular and one parallel to the backbone. The perpendicular Janus domain enforces a superstructure that intrinsically confines the intralayer self-assembly of the parallel Janus domain. This results in a phase-in-phase structure that gives rise to a mesh-like monoclinic network as well as a tetragonal counterpart with excellent long-range order. Furthermore, under the guidance of a topographical template with appropriate surface conditions, the phase-in-phase structure can be aligned with unique preferred orientations. These findings demonstrate a valuable pathway to bottom-up fabrication of original 3D nanostructures via soft matter, which extends the capabilities of BCP self-assembly.