Date of Award
Doctor of Philosophy
Chemical and Biological Engineering
Andrew C Hillier
Nanostructured architectures made with bioinspired motifs represent a new paradigm of functional materials. This dissertation focused on creating different optical nanostructures using deoxyribonucleic acid (DNA) as a template. Particular interest was to fabricate metamaterial building blocks with the directed metallization of DNA. Optical metamaterials are artificially engineered materials that exhibit extraordinary electromagnetic properties that are governed by the physical properties of constituent unit cells with dimensions smaller than the wavelength of light. Split ring resonators (SRR) are one of the basic forms of metamaterial unit cells. They are required to be approximately near one-tenth of the resonance frequency in size, a dimension which is very challenging to obtain with traditional top-down techniques for the metamaterials to function in the visible or near IR frequency. To address the problem, we proposed that DNA nanotechnology as a bottom-up technique can solve the current limitations to fabricate nanoscale metamaterials. In this report, we have developed a robust metallization method to create DNA-origami templated metal nanostructures and metamaterial unit cells that are ~130 nm in size. We applied different characterization techniques to investigate the optical properties of the fabricated nanomaterials and metamaterials building blocks.
This dissertation is divided in 5 Chapters that summarize the project in detail, from the conception to the fabrication and characterization of plasmonic nanomaterials and optical metamaterials. The first chapter introduces the fundamentals of the optical metamaterials and the promising electromagnetic and optical properties they exhibit. This chapter introduces the DNA nanotechnology as an alternative to the challenging top-down techniques to fabricate a variety of nano-objects with programmable design and manipulation. We have summarized some of the hybrid nanostructures we created with DNA origami technique that was metallized in the later sections to achieve plasmonic shapes. We have summarized the contemporary DNA metallization techniques and justified our three-step metallization scheme that involves i) Ag seeding of DNA, ii) electroless Au metallization of Ag seeded DNA origami, and iii) chemical annealing to obtain uniform metal structures, all to fabricate plasmon active nano-objects and metamaterials. In the second chapter, we have detailed our development of a novel method to coat the DNA origami with a thin layer of silver metals. In this method, we have selectively deposited ~4 nm Silver onto the DNA origami triangles with the repeated reduction of silver diamine using ~365 nm ultraviolet (UV) light. With multiple cycles of photoreduction, a self-termination of silver deposition was achieved where the overall shapes of the DNA were mostly un-altered, and the coatings were uniform. The silver layer on the DNA template was stable and conformal to the DNA molecules, which facilitated further growth of a gold film to realize plasmonic metamaterials that is described in the next chapter. The third chapter has been utilized to show the development of a secondary metallization of silver coated DNA to create ~20-40 nm thick metal nanostructures. We have developed an in-house electroless deposition (ELD) of gold that is based on the conception of seeded growth mechanism of nanoparticles to selectively deposit metal onto the Ag seeded DNA. We have designed a series of experiments varying the concentration of Au ELD bath (composed of chloroauric acid, silver nitrate, cetyl trimethyl ammonium bromide, and ascorbic acid), temperature, pH, and the deposition time to find the best set of process parameters to realize DNA origami templated nano-objects with a conformal and controllable metal growth, background nanoparticles free metal deposition with a maximum yield. Due to the metal growth on an average of 9-12 random nucleation sites on the DNA origami templates, the resulting DNA origami metallized nanostructures contained high surface roughness. We postulated two different approaches (chemical annealing method I and II) to anneal the as-fabricated rough nanostructures to result in highly uniform plasmonic nanostructures. During annealing, chloroauric acid in water or as in Au ELD bath acted as an oxidative etchant to remove Au atoms from the vertices and edges of the roughly coated metal objects and at the same time facilitated to grow more metal onto the existing metal layers to result in ultra-smooth meta-surfaces. In chapter 4, we have demonstrated numerical and experimental investigation of representative plasmonic nanoparticle and DNA-metallized nano-objects. The optimized metallization method from chapter 3 was successfully employed on the Ag coated DNA origami templates to realize hollow metal triangles and different shape of split ring resonators (SRR) such as V-shaped SRR (V-SRR), U-shaped (U-SRR), and triangular shaped SRR (T-SRR). In conjunction with the experimental optical characterization, Finite difference time domain (FDTD) numerical calculations with COMSOL Multiphysics was applied to estimate the optical resonance properties of gold nanorods, nanotriangles and different shapes of split ring resonators of their ideal forms as well as their DNA-metallized realistic shapes. The numerical simulation showed that the SRRs with a nominal size of ~125 nm and ~25 nm thickness exhibited two primary resonance modes, namely, electric dipole resonance and, magnetic resonance that appeared at wavelengths ranging from 500-700 nm, and 1000-1100 nm, respectively, depending on the shapes and the effective surface morphology of the SRRs. Single particle dark-field (DF) scattering spectroscopy analysis was performed on the nanorods, nanotriangle and origami templated metamaterial SRRs to capture the plasmonic resonances and compared the responses to the numerical findings. Due to the spectral limitations of our current DFM apparatus, only the electric dipole resonances were captured for the SRRs. Utilizing the RGB color intensities from the DFM images of the plasmonic nanostructures, we determined the orientations at which the electric mode and magnetic mode resonances of the SRRs are excited. A significant consistency was found between the experimental and numerical optical responses. Chapter 5 summarizes the accomplishments achieved in this dissertation, and in addition, we laid out some new studies based on the success of metallization and characterization of DNA origami templated nanostructures. Notably, we have demonstrated a method to use sonication to extract metallized DNA nanostructures from the Si surfaces into a suspension that could be used in 2-D or 3-D assembly of unique nanostructures into ordered meta-structures. In addition to the post metallized assembly of nanostructures, we provided a conception to assemble the DNA origami into a 2-D patterns. We also provided a facile method to create an array of DNA origami structures using Laser interference lithography (LIL) technique. In a third project, we have taken advantage of forming a hexagonal packed monolayer of DNA triangles on a flat surface to fabricate large order nanohole arrays (NHA), which promises numerous applications in chemical and biosensing. In addition to these, we have recommended a few of the novel ideas that can be realized using the metallization and characterization methods developed in this dissertation.
In summary, we have provided a novel metallization route to fabricate nanostructures using DNA origami as a template. We have created geometrically unique hollow gold nanotriangles and three different shapes of metamaterial unit cells with DNA-directed metallization. The metamaterial SRRs exhibited magnetic resonances in the near-IR wave frequency, which is critical in obtaining negative magnetic permeability of materials. The optical properties of the DNA-fabricated split rings from this report would lay foundation to create sub-100 nm DNA origami templates and metallize them to achieve metamaterials that would function in the near-IR or visible frequency.
Md Mir Hossen
Hossen, Md Mir, "DNA origami templated optical metamaterials" (2020). Graduate Theses and Dissertations. 18142.
Available for download on Saturday, September 04, 2021