A group of researchers recently published an article in the journal Advanced optical materials which demonstrated the effectiveness of a novel manufacturing method based on viral nano-building blocks to fabricate unconventional nano-metamaterials.
Study: Development of unconventional nano-metamaterials from viral nano-building blocks. Image Credit: GiroScience/Shutterstock.com
Importance of nano-metamaterials
Structured metamaterials are called periodically ordered nanostructures in which the dielectric constant can be modulated periodically on any length scale comparable to the operational wavelength, such as the wavelength of light. Interactions between light wave electric fields and sub-wavelength unit structures in metamaterials can generate unusual effects that are practically not achievable in natural materials.
Nanostructures with periodicities similar to the wavelength of light can trap certain colors of light. However, properly designed photonic metamaterials with periodically arranged sub-100 nanometer structures can effectively render an object invisible by guiding light waves around the object. Photonic bandgap (PBG) metamaterials, which are another form of dielectric nanostructures, can be modeled at the sub-wavelength or wavelength scale to create a photon bandgap that obstructs the propagation of light, causing unusual effects such as negative refraction.
To develop camouflage or stealth devices, selective control and trapping of light propagation must be achieved by engineering the basic properties of electromagnetic waves, such as frequency, wavelength, and speed. Although periodic three-dimensional (3D) nanostructures are suitable for developing stealth devices, fabricating these nanostructures with high atomic-level precision is extremely challenging. Thus, making a 3D object invisible remains a challenge.
Limits of existing nano-metamaterial fabrication techniques
Until now, most photonic nano-metamaterial fabrication approaches relied on traditional, time-consuming, expensive and difficult lithographic techniques, which required the identification of novel fabrication processes to fabricate highly ordered 3D structures with periodicities below a micrometer.
Submicron semiconductor arrays and solar cells have been successfully fabricated using a general modeling approach in which porous patterns are formed by selective degradation of self-assembled block copolymer (BCP) morphologies to separation of phases used to model inorganic materials. However, BCP morphologies are rare and their production is extremely difficult.
Importance of viral M13 nanostructures to efficiently fabricate nano-metamaterials
Self-assembled viral nanostructures based on bacteriophage M13 chassis offer greater versatility compared to BCP morphologies. Bacteriophage M13 can be obtained easily by bacterial fermentation and genetically and chemically modified to make structures with different chemical-physical characteristics and morphologies. Additionally, M13 is suitable for fabrication of nanostructures as it is resistant to solvents and heat.
Viral nanostructures can be assembled using multiple versions of functionalized M13 to interact with other materials through intermolecular interactions such as robust covalent bonds to design nanoscale architectures with complex structures and functions.
Although M13-based scaffolds have been evaluated to direct nanostructures for drug delivery and nanoparticle assembly at specific molecular binding sites, the potential of M13-based quasi-3D viral nanomodels for manufacturing of 3D nano-metamaterials has remained largely untapped.
A new way to fabricate 3D nano-metamaterials
In this study, the researchers proposed a new technique to create 3D nano-metamaterials using self-assembled M13-based viral building blocks as nanopatterns that were replicated in a quasi-3D metal nanostructure. The viral building blocks enabled the directed assembly of materials below 100 nanometers, which were used to create near-3D nanoarchitectures. M13 architectures have been used as nano-scaffolds to pattern metals to build a new range of nano-array structures with desired morphologies and create innovative optical metamaterials.
Initially, several nano- and micro-arrays with peak-to-groove (R/G) ratios ranging from 0.18 to 1.0 and concentric Fresnel lens-like nanomorphologies were fabricated by focused ion beam (FIB) lithography. . These were then used as guides to underlying topographic surfaces to fabricate different 3D nanoarchitectures from the viral nanostructures.
Subsequently, the nano-micro-structured surfaces were spin coated in a controlled manner or immersed in the M13 solution and a poly(methyl methacrylate) (PMMA)/epoxy matrix was embedded on top of the structures, followed by the removal of scaffolds to obtain M13 self-contained assembled 3D nanomorphologies.
Near-3D metallic viral nanostructures were fabricated by gold plating the nanomorphologies by sputtering and electrodeposition methods. These quasi-3D metallic viral nanostructures possessed an optical metamaterial architecture.
Atomic force microscopy, optical characterization and scanning electron microscopy were used to characterize the fabricated 3D nano-metamaterials.
Significance of the study
The viral nanostructures were spontaneously assembled into 3D periodic lattice structures. Several quasi-3D metallic nano-configurations have been fabricated by changing the underlying topologically structured substrates. These fabricated underlying substrates guided M13 self-assembly and acted as topological landmarks for fabricating chiral liquid crystal (LC) phase nanomorphologies, Fresnel-like nanoarchitectures, and horizontally oriented and vertically aligned nanoviral arrays. .
Several nanoviral structures displayed enhanced hydrophobicity as the specific orientation of the M13 filament on underlying substrates with a tightly packed phage nanostructure and varying length scales of roughness limited droplet propagation, leading to a rolling hydrophobicity and a larger contact angle.
The viral metallic/gold plasmonic network produced enhanced reflection at the substrate layer, with confined induced energy emerging from localized plasmons in the concentric gold-viral rings of the Fresnel-like nanostructures. Nanoscale ordered viral networks have further facilitated plasma shifting and frequency shifting, leading to the fabrication of nano-metamaterials with distinct optical properties that are different from bulk gold.
The fabricated viral gold nanoarray-like structures demonstrated high reflectance in the visible and from visible to near infrared. However, chiral LC nanostructures and Fresnel-like viral gold nanoarchitectures display average and lower reflectance at similar wavelengths, respectively.
In summary, the results of this study have demonstrated that the proposed fabrication method can serve as an efficient platform to fabricate and design new artificial nanostructures to create metamaterials, especially photonic metamaterials.
White, H., Mahajan, S., Schofield, Z. et al. Development of unconventional nano-metamaterials from viral nano-building blocks. Advanced optical materials 2022. https://doi.org/10.1002/adom.202102784