Article pubs.acs.org/Langmuir
Fabrication of Large Two-Dimensional Colloidal Crystals via SelfAssembly in an Attractive Force Gradient Xiaoyan Sun,† Yang Li,† Tian Hui Zhang,† Yu-qiang Ma,*,†,‡ and Zexin Zhang*,† †
Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, PR China National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, PR China
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ABSTRACT: Colloidal particles in a water−lutidine (WL) binary liquid mixture experience temperature-dependent attraction close to the mixture’s demixing temperature. This temperature-tunable interaction can be potentially harnessed to assemble colloids and grow colloidal crystals. In this article, for the first time a novel attractive force gradient method is presented to fabricate high-quality, single-domain colloidal crystals. The well-controlled attractive force gradient here arises from a temperature gradient in the WL mixture. The nucleation of colloidal crystals in such a WL mixture preferably occurs in the high-temperature region because of the stronger attraction there. Crystallization propagates from the high-temperature region to the low-temperature region in a well-controlled way. The growth of the colloidal crystal is characterized in detail by Voronoi construction, the pair correlation function, and the orientational order parameter. It is found that the number of crystal-like particles increases with time, and a single-domain 2D colloidal crystal can be produced. The mechanism of the defect-free crystallization process is discussed on the basis of an analogy to cluster beam deposition methods. This study demonstrates an efficient and robust way to prepare colloidal crystals with little to no defects, being suitable for applications such as colloidal lithography and the fabrication of perfect 3D colloidal crystals.
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INTRODUCTION Two-dimensional (2D) colloidal crystals have many important applications in various fields, including colloidal nanolithography,1−6 biosensors,7 and optical devices.8 Now there is renewed interest in using 2D colloidal crystals as templates for preparing graphene nanoribbons.9 For these applications, efficient and economical methods for preparing high-quality, large-area 2D colloidal crystals are critical. For example, in colloidal nanolithography, if a colloidal crystalline template has defects, then it is impossible for nanoparticles to form perfect patterns for further applications.3 For optical applications, defects and grain boundaries in colloidal crystals will greatly affect their optical properties. In previous attempts, a number of strategies have been developed to prepare 2D colloidal crystals, including Langmuir−Blodgett deposition,10 self-assembly under capillary forces,11−13 electrophoresis deposition,14,15 temperature gradients,16 convective assembly,17−20 spin coating,21,22 and other sophisticated assembly processes with special devices.23−25 Recent studies have shown that colloids can form aggregates in binary liquid mixtures near the mixture’s demixing temperature.26−28 For example, when colloids are dispersed in a water−lutidine (WL) mixture, an attractive force between colloids is generated when the system is heated to the demixing temperature of the WL mixture. Driven by the attractive force, colloids can aggregate and even crystallize. However, because the attractive force is spatially uniform, colloidal crystals can nucleate everywhere. As a result, polycrystals, instead of a single crystal, are typically formed. Herein, we demonstrate a novel method using a temperature gradient to induce an attractive © 2013 American Chemical Society
force gradient in colloidal suspensions. The attraction gradient controls the nucleation and growth of colloidal crystal. Thus, we successfully grow large, defect-free 2D colloidal crystals.
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EXPERIMENTAL SECTION
Monodisperse colloidal particles (polystyrene latex, nominal diameter σ = 1.16 μm, polydispersity
