Asymmetric Nanostructures by Confinement of Block Copolymers in Anodized Aluminum Oxide Template for Plasmonic Metamaterials

Abstract

Metamaterials are artificial composite structures exhibiting optical properties that do not exist in nature and can be applied to negative refractive index, hyperbolic dispersion, and perfect absorption. Because metals have only a dielectric constant, complex three-dimensional (3D) metal nanostructures, such as split-ring resonators or vertically stacked rods, are required to show artificial magnetic permeability. Though many studies based on top-down approaches, for instance, direct laser writing and multiple e-beam lithography, have been performed to obtain metamaterial properties, the fabrication of complex 3D metal nanostructure in a large area is very difficult. Therefore, there is a need for novel fabrication that can create geometrically complicated plasmonic nanostructures over a large area. Block copolymers (BCPs) have been intensively studied due to their self-assembly such as spheres, cylinders, lamellae, and gyroids depending on the volume fraction of the constituent blocks (fA), the degree of polymerization (N), the Flory–Huggins interaction parameter (χ), and the chain architecture. Because nanopatterns are easily fabricated in a large area, they have been employed for next-generation lithography, high-density data storage media, solar cells, membranes, and sensors. However, the conventional microdomain morphologies could not be used for advanced optics such as metamaterials. Thus, different shapes of microdomains not attained from BCP in bulk or thin films could be obtained by utilizing the confinement of BCP microdomains in cavities. In this thesis, I prepared complex and unique nanostructures derived from BCP self-assembly via confinement in nanopores of anodized aluminum oxide (AAO) template for plasmonic metamaterials. In chapter 2, I investigated the morphology of lamellae-forming polystyrene-block-poly(methyl methacrylate) copolymer (PS-b-PMMA) confined in asymmetric hemisphere nanocavities which were prepared by oblique angle deposition of gold with various thicknesses. When the thickness of the deposited gold layer (tAu) was 0.5L0 of PS-b-PMMA (L0 is the lamellar domain spacing of PS-b-PMMA in bulk), concentric lamellar patterns were formed on the top surface of the nanocavities. Interestingly, at tAu = 1L0, WiFi-like nanopatterns were observed on the top surface. This is because of the reduction of dislocations of PS and PMMA lamellar microdomains near the center of the nanocavity. The experimentally observed morphologies are consistent with the prediction by self-consistent field theory. In addition, the inner wall of the hemispherical nanocavity was modified by grafting three different polymer brushes (PS, PMMA, and PS-r-PMMA) to change the affinity to each block. When the nanocavity was grafted by PMMA, WiFi-like nanopatterns were observed. On the other hand, laterally stacked U-shaped nanopatterns were formed in a nanocavity grafted by PS-r-PMMA with neutral affinity to PS and PMMA. I also fabricated the array of silver WiFi-like nanopatterns composed of laterally stacked split-ring resonators after selective silver deposition only on the PS microdomains. They showed unique plasmonic resonances depending on the polarization angle of incident light in near infra-red (NIR) wavelengths. The nanopatterns with broken symmetry obtained in this study can be used in advanced optical devices for structural coloration and optical anti-counterfeiting. In chapter 3, I fabricated vertically stacked split-ring resonators (SSRRs) arrays exhibiting polarization-sensitive dichroic responses in both visible and near-infrared wavelengths in a large area (~cm2). The SSRRs arrays were derived from pagoda-like nanorods fabricated from the confinement of PS-b-PMMA in cylindrical pores of AAO template. Along the nanorod direction, PS and PMMA nanodomains were alternately stacked with the same distance. Silver crescents and semi-hemispherical covers, which are essential for SSRRs with polarization sensitivity, were obliquely deposited on the single side of the nanorod after removing the AAO template and reactive-ion etching treatment. These sophisticated nanoscale architectures made by bottom-up fabrication can be applied to structural color, optical anti-counterfeiting, and commercial optical components in a large area.