Self assembly of two dimensional colloidal alloys for metamaterials applications

基于纳米尺度结构的材料和器件的系统设计和构建是21世纪材料科学研究的关键挑战。特别是，该领域的一个主要挑战是在纳米尺度（低于光波长）上设计金属/介电复合结构，因为这导致了一类新的材料，统称为超材料，其表现出不寻常的光学特性，如负渗透率和折射率。这些独特的光学特性使我们能够以前所未有的程度操纵光，开辟新的研究领域，如完美透镜，纳米光子器件，集成光学电路，高效太阳能电池，生物传感器等。然而，超材料研究的一个严重瓶颈是，用于超材料应用的金属/介电纳米结构传统上是使用“自上而下”的方法制造的，例如电子束光刻或聚焦离子束光刻，这些方法昂贵、缓慢且在最小特征方面受到限制。近年来，自组装已经成为一种“自下而上”的方法来制造微纳米结构材料，这种方法用途广泛，快速且廉价。当自组装涉及两个或多个具有各种光学、电子和磁性的组件时，这种方法变得特别强大。在最近的一项突破性研究中，我们表明，通过在油/水界面上自组装疏水和亲水球形二氧化硅颗粒的混合物，可以获得丰富多样的二维二元晶体结构。该项目的目的是扩展我们的自组装方法，用不同形状（如球形和棒状）和尺寸（微米到纳米）的金属颗粒代替亲水二氧化硅颗粒，以创建用于超材料应用的介电/金属复合结构。为了实现这一目标，我们采用多学科的方法，结合理论和实验来研究混合胶体单层的自组装和光学性质。具体来说，我们将首先研究液体界面上不同类型胶体之间的相互作用，以建立颗粒性质（例如，材料，润湿性，形状和大小）与颗粒相互作用之间的关系。这将允许我们通过改变粒子属性来调整粒子间的相互作用。接下来，我们将研究这些相互作用如何控制混合单层的自组装，以获得获得特定复合结构的设计规则。然后，我们将分析这种混合单层的光学响应，以确定最有希望的超材料应用结构。最后，在确定并创造了所需的微纳米尺度的超材料之后，作为一个特定的应用，我们将在这些超材料上沉积活性材料，如共轭聚合物或胶体量子点，以研究超材料如何改变活性材料的发射强度和方向性。这将使我们能够创造出混合等离子体结构，这将成为下一代纳米光子器件的基石。

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.