Progress in functional 2D ordered arrays based on monolayer colloidal crystals

Abstract

Two-dimensionally (2D) ordered micro- and nanostructure arrays have attracted considerable attention because of their novel and/or enhanced properties arising from the patterned structures. Monolayer colloidal crystals (MCCs) assembled from colloidal spheres have been widely employed for the controllable fabrication of a variety of 2D ordered micro- and nanostructure arrays because this strategy has been demonstrated to be facile, inexpensive, efficient, and versatile. This review paper summarizes the recent progress in the functional 2D ordered arrays based on MCCs, including their applications in 2D photonic crystal sensors, plasmonic nanostructure arrays, solar energy conversion devices, optic and optoelectronic devices, surface wetting and adhesion, biomedical materials, etc. Particular attention is paid to our recent advances in the controlled fabrication and applications of ZnS nanobowl arrays, heterostructured Ag2S-Ag nanobowl arrays, heterostructured TiO2 nanorod@nanobowl arrays, and single crystal calcite microlens arrays. Firstly, large-area ZnS nanobowl arrays with a monolayer inverse opal structure were synthesized facilely by nanosphere lithography at solution surface through direct solution deposition. Owing to the 2D periodic structure of the hexagonally ordered nanobowl arrays and the high refractive index of ZnS, the obtained ZnS nanobowl arrays showed intensive structural colors and tunable photonic band-gap properties. The as-prepared ZnS nanobowl arrays can be used as a sensitive 2D photonic crystal sensor for fast detection of organic solvents and for visual oil sensing. After surface functionalization, the ZnS nanobowl arrays can be used as an effective 2D photonic crystal biosensor for sensitive detection of avidin molecules. Secondly, unique Ag2S-Ag heterostructured nanobowl arrays consisting of Ag2S nanonets lying on Ag nanobowl arrays were fabricated by two-step nanosphere lithography at the gas-liquid interface. The heterostructured nanobowl arrays exhibited effective resistance switching behaviors, thus showing the potential application in electronic switches and memory devices. Meanwhile, the heterostructured nanobowl arrays exhibited enhanced photocatalytic activity owing to their unique structure when they were used as a photoanode for photoelectrochemical (PEC) water splitting. This result may open up new possibilities for facile and rational synthesis of large-area ordered arrays of semiconductor-noble metal hybrid nanostructures with multifunctions. Thirdly, heterostructured TiO2 nanorod@ nanobowl (NR@NB) arrays consisting of rutile TiO2 nanorods grown on the inner surface of arrayed anatase TiO2 nanobowls were designed and fabricated as a new type of photoanodes for PEC water splitting. The unique heterostructures with a hierarchical architecture were readily fabricated by interfacial nanosphere lithography followed by hydrothermal growth. The heterostructured TiO2 NR@NB arrays on the FTO substrate showed significantly enhanced PEC properties for water splitting because they possessed multiple scattering centers, increased electrolyte accessible surface area, and beneficial anatase-rutile heterojunction. This work may open new avenues towards building complex semiconductor nanostructures with desirable architectures for efficient harvesting and utilization of solar energy. Lastly, unprecedented concave microlens arrays (MLAs) made of calcite single crystals with tunable crystal orientations were readily fabricated by MCC-assisted epitaxial growth in solution without additives under ambient conditions. Calcite single crystal MLAs with two different crystallographic orientations, namely, calcite (104) microlens arrays and calcite (001) microlens arrays, were produced using selected calcite substrates, respectively. While the non-birefringent calcite (001) microlens array showed excellent imaging performance like brittlestar’s microlens arrays, the birefringent calcite (104) microlens array exhibited remarkable polarization-dependent optical properties. In general, this work may open new pathways towards the fabrication of micropatterned single crystals for various optical applications and shed light on fundamental mechanisms in biomineralization. After a brief summary of the recent progress in this research field, the challenges and perspectives for future research are addressed.