Self‐Assembly of Colloidal Spheres toward Fabrication of Hierarchical and Periodic Nanostructures for Technological Applications

Abstract

DOI: 10.1002/admt.201800541 to potentially fabricate numerous hierarchical and periodic micro/nanostructure arrays in a large scale.[1–5] Importantly, these ordered micro/nanostructure arrays are essential active components for many technological applications, ranging from data storage,[6] solar cells,[7–9] plasmonics to functional coatings, and many others.[10–15] Combined with the recent advance in colloidal science, the colloidal spheres with uniform morphology and excellent disperse stability can be further realized by a number of methods, which include the suspension,[16,17] dispersion polymerization,[18,19] emulsion,[20,21] and Stöber techniques.[22] The diameter of colloidal spheres can also be precisely controlled in a wide range, spanning from several micrometers all the way to down to tens of nanometers. Uniquely, these colloidal spheres can be self-assembled into 2D monolayers and 3D periodic multilayers, where they are subsequently utilized as the versatile masks (e.g., optical lens) to achieve fabrication templates by simple surface patterning onto underlying substrates, facilitating the construction of micro/nanostructure arrays with excellent tunability.[23–25] Since then, these 2D monolayer colloidal crystals (MCCs) and 3D multilayers have attracted extensive attention for many of their further utilizations. For instance, by using the oxygen plasma and reactive ion etching (RIE) methods, the periodicity, the size, and the shape of individual spheres of MCCs are accurately controlled.[26,27] With further modification, etching, and decoration, the MCCs can be exploited as either the masks for metal catalyst deposition to generate hierarchical and periodic nanostructures on underlying substrates, or the templates for the growth of patterned nanostructures.[28,29] Furthermore, these MCCs can also be utilized as optical lens to construct the periodic nanostructures with photosensitive materials, broadening the preparing routes of hierarchical and periodic nanostructures.[30] In order to prepare the periodic nanostructures, the fabrication process, including the self-assembly of MCCs, the morphological modification of MCCs, and the functionalized decoration, is commonly referred as the nanosphere lithography (NSL).[31–33] As compared with conventional lithographic techniques, such as photolithography,[34,35] electron beam lithography,[36] and focused ion beam lithography,[37] NSL can distinctively reduce the manufacturing cost and complexity, The current research status on the self-assembly of colloidal spheres for the fabrication of various hierarchical and periodic nanostructures is summarized, in which these structures exhibit unique properties for different technological applications in plasmonics, surface-enhanced Raman scattering, solar cells, and others. The fundamentals of colloidal self-assembly are first introduced. After which, the functions of the obtained monolayer of colloidal spheres (e.g., nanosphere monolayer) to act as the patterned mask for the subsequent pattern transfer onto underlying substrates are thoroughly discussed. In employing these nanosphere lithography techniques and fabrication templates, the focus switches to the successive construction of numerous nanostructures and their promising utilizations in a wide range of application domains. Finally, the current challenges and future opportunities of colloidal self-assembly are presented. This review not only aims to provide further insights for researchers to enable the ingenious designs of nanostructures, but also to facilitate the exciting research and development utilizing colloidal self-assembly for future manufacturing technologies.