DMSE Doctoral Thesis Defense

Designing and Fabricating 3D Nanostructures through Directed Self-Assembly of Block Copolymers

Hejin Huang

Friday, August 06, 2021

9:00am – 10:00am EDT

Contact dmse-gradoffice@mit.edu for Zoom link.

Intricate designs of 3D nanostructures are needed in different research areas. Our current techniques for fabrication of 3D structures are limited by either length scale or efficiency. Compared to the conventional methods such as ‘top-down’ lithography, self-assembly of block copolymer (BCP) provides a promising avenue to fabricate 3D nanostructures in a ‘bottom-up’ way. By applying an external field, the BCP self-assembly process will be biased, and single-crystalline nanostructures with well-controlled orientation are fabricated upon annealing. Over the past two decades, researchers have successfully created arbitrary 2D complex structures through directed self-assembly of block copolymers.

Despite the research efforts in directed self-assembly of BCP, most key achievements reported so far are on novel 2D nanostructures. How to create uniform and complex 3D nanostructures through BCP self-assembly in a controlled way remains unsolved. In this Thesis, a novel route to create complex 3D nanostructures has been discovered by simulation: through layer-by-layer stacking, defects and aperiodicity introduced in the base layer will propagate to subsequent layers to generate a multilayer aperiodic structure.

In the first stage, dissipative particle dynamic (DPD) has been reparametrized to give accurate predictions of self-assembled structures of BCP thin films. DPD is a particle-based simulation, which gives intuitive representations of the various experiment conditions compared to field-based simulation such as self-consistent field theory (SCFT). The previous parametrization of DPD simulation manages to reproduce the BCP phase structures in bulk, but fails in most of the thin film structures. Our reparametrized model reproduced all the experiment conditions in BCP thin films. Furthermore, effects of important parametrizations in the DPD simulation are studied to understand the theoretical background. Then this reparametrized DPD simulation serves as the tool to investigate how to fabricate novel 3D nanostructures. A novel approach to fabricate 3D nanostructure, named self-directed self-assembly (SDSA) of block copolymer, is proposed. The feasibility of this method is tested by DPD simulation. Through layer-by-layer stacking of two block copolymers, AB and AC, structural information of the base layer will propagate to subsequent layers to generate uniform 3D nanostructures. Different uniform 3D nanostructures, such as parallel cylinders, sphere aligned with cylinders and bilayer nanomeshes have been fabricated by SDSA. Finally, inverse design of 2D and 3D nanostructures have been achieved by combining the evolutionary algorithm with DPD simulation. The methods encompass rapid algorithms for characterizing the internal structures of BCP morphologies from the atom coordinates generated by MD simulations to substrate optimizing algorithms for developing effective routes to propagate information from the substrate into the BCP film. Four design rules of 3D nanostructures with connections have been discovered. These findings helps us to fabricate 3D nanostructures with selective connections between the layers effectively and efficiently.

Thesis Supervisor
Alfredo Alexander-Katz, Professor, Materials Science and Engineering, Massachusetts Institute of Technology

Thesis Committee
Caroline A. Ross, Toyota Professor, Materials Science and Engineering, Massachusetts Institute of Technology
Niels Holten-Andersen, Lecturer, Materials Science and Engineering, Massachusetts Institute of Technology