Self assembly of two dimensional colloidal alloys for metamaterials applications

The systematic design and construction of materials and devices based on structure at the nanometer scale is a key challenge for materials science research in the 21st century. In particular, a major challenge in this area is to engineer metal/dielectric composite structures on the nanometer scale (below the wavelength of light) as this leads to a new class of materials, collectively known as metamaterials, which exhibit unusual optical properties such as negative permeability and refractive index. These unique optical properties allow us to manipulate light to an unprecedented degree, opening new areas of research such as perfect lenses, nanophotonic devices, integrated optical circuits, high efficiency solar cells, bio-sensors etc. However a serious bottleneck in metamaterials research is that the metal/dielectric nanostructures used for metamaterials applications have traditionally been fabricated using 'top down' approaches such as electron-beam lithography or focused ion beam lithography which are expensive, slow and limited in terms of the smallest features that can be made. In recent years, self-assembly has emerged as an alternative 'bottom-up' method for making micro- and nano-structured materials which is versatile, fast and inexpensive. This approach becomes particularly powerful when the self-assembly involves two or more components with a variety of optical, electronic and magnetic properties.

In a recent ground-breaking study, we showed that it is possible to obtain a rich variety of 2D binary crystal structures through the self-assembly of mixtures of hydrophobic and hydrophilic spherical silica particles at an oil/water interface. The aim of the project is to extend our self-assembly method by replacing the hydrophilic silica particles with metallic particles of different shapes (e.g., spherical and rod shaped) and sizes (micron to nanometer) in order to create dielectric/metal composite structures for metamaterials applications. In order to achieve this aim, we use a multi-disciplinary approach that integrates both theory and experiment to study the self-assembly and optical properties of mixed colloidal monolayers.

Specifically we will first study the interactions between different types of colloids at a liquid interface in order to establish the relationship between particle properties (e.g., material, wettability, shape and size) and particle interactions. This will allow us to tune particle interactions by changing particle properties. Next, we will study how these interactions control the self assembly of mixed monolayers in order to obtain the design rules for obtaining specific composite structures. We will then analyse the optical response of such mixed monolayers in order to identify the most promising structures for metamaterials applications. Finally, having identified and created the desired micro and nano scale metamaterials, as a specific application, we will deposit active materials such as conjugated polymers or colloidal quantum dots on top of these metamaterials to investigate how the metamaterial modifies the emission intensity and directionality of the active material. This will allow us to create hybrid plasmonic structures that will form the building blocks for the next generation of nanophotonic devices.

The UK is internationally leading in the areas of colloid science and photonics. This proposal aims to combine these two world leading research areas to develop a novel colloidal self-assembly method that is versatile, fast and inexpensive in order to create electromagnetically inhomogeneous materials for advanced photonic applications. The proposal therefore makes a step change contribution to the EPSRC's vision to create a strong materials science base in the UK which is internationally leading in originality but also transformatory and applicable. The project also aligns with the EPSRC Grand Challenge themes of "Nanoscale Design of Functional Materials" and "Directed Assembly of Extended Structures" as well as the EPSRC research areas of "Photonic materials and metamaterials", "Light matter interaction and optical phenomenon" and "Biophysics and soft matter physics".

The ability to control structure on the nanoscale opens up new and exciting opportunities for the design of advanced functional materials which will benefit the UK economy in both the medium and long term. In particular, the ability to control the structure of electromagnetically inhomogeneous materials on the micro and nanoscales promises to revolutionise the way we transport and manipulate light from the IR to visible regime. This capability is particularly pressing given that current computing technology based on electron transport is reaching its speed and capacity limit. This limit is predominantly due to the thermal and signal delay associated with the electronic interconnection in integrated circuit technology. Using light to transport and process information can provide a solution to this problem as light based devices would be faster and more energy efficient than electron based devices with at least four orders of magnitude in bandwidth to power ratio. In addition, the development of metamaterials for visible light will make it possible to confine light below the optical diffraction limit, a critical step towards photonic miniaturization that will allow photonics to catch up with Moore's law for electronics and pave the way and new applications such as integrated optical circuits.

The project will train two PDRAs in conducting world class academic research within the context of an interdisciplinary team. Another key impact of the project is therefore to produce highly trained and versatile research personnel who will be able to contribute to the UK and global knowledge based economy. Finally, the project will provide striking examples of how self-assembly can be used to design new structures and materials. The project outcomes therefore naturally lend themselves to public engagement activities, illustrating both the usefulness and beauty of science.