Fabrication of floating colloidal crystal monolayers by convective deposition

Hypothesis: Well-defined two-dimensional colloidal crystal monolayers (CCM) have numerous applications, such as photonic crystal, sensors, and masks for colloidal lithography. Therefore, significant effort was devoted to the preparation of preparing CCM. However, the fabrication of CCM that can float in the continuous phase and readily transfer to other substrate remains an elusive challenge. Experiments: In this article a facile approach to prepare floating CCM from polymeric colloids as building blocks is reported. The key to obtain floating CCM is the selection of an appropriate solvent to release the formed CCM from the substrate. There are two steps involved in the preparation of floating CCM: formation and peeling off. Findings: First, colloids are dispersed in a solvent. Evaporation of this solvent results in the formation of a meniscus structure of the air-liquid interface between the colloids that are on the substrate. The deformation of the meniscus gives rise to capillary attraction, driving the colloids together in a dense mono layer. Once a crystallization nucleus is formed, a convective flow containing additional colloids sets in, resulting in the formation of CCM on the substrate. Second, the remaining bulk dispersion is replaced by an extracting solvent that wets the substrate and peels the formed CCM off. The influence of the several solvents, the substrate materials, and the types of colloids on the CCM formation are investigated systematically. The robustness of the approach facilitates the preparation of CCM. Furthermore, the floating feature of the CCM in principle makes transfer of the CCM to other substrates possible, which broadens its applications. (c) 2020 Elsevier Inc. All rights reserved.