Regular Article
Fabrication of floating colloidal crystal monolayers by convective deposition

https://doi.org/10.1016/j.jcis.2020.12.037Get rights and content

Abstract

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 monolayer. 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.

Introduction

Colloidal lithography is a powerful tool to prepare two-dimensional ordered nanostructures, which have potential applications in the areas of photonics, plasmonics, sensing, and solar cells [1], [2], [3], [4]. Colloidal crystal monolayers (CCM) are the templates for colloidal lithography [5], [6], [7], [8], [9]. Therefore, numerous effort has been devoted to assembling colloids into CCM. The methods developed up to now include convective deposition [10], drop casting [11], spin coating [12], and electrophoretic deposition [13]. However, the major drawbacks of the aforementioned methods are the requirement of (super-)hydrophilic substrates onto which colloids assemble, and the difficulty to transfer the formed colloidal crystals to other substrates. Therefore, a variety of approaches have been attempted to prepare floating CCM [14], [15], [16], [17], [18]. As the term suggests, floating CCM are not attached to a substrate but can freely float in the continuous phase. The most commonly employed approach to prepare floating CCM are the liquid interface mediated methods. Instead of using solid substrates like glass and mica, a liquid interface is used in these methods, such as gas–liquid interface and liquid–liquid interface [14], [15], [16]. For example, Kondo et al. [16] obtained floating CCM by first spreading monodisperse hydrophobic alkoxyl chains coated silica particles at the air-benzene interface and subsequent picking the formed monolayer up with a mica substrate. However, to increase the ordering and packing density in monolayer by the liquid interface mediated methods, the control of the hydrophobicity of colloids [16], the utility of Langmuir-Blodgett trough [19] or the addition of various polymers [20] or surfactants [21] are usually required. Besides the commonly employed liquid interface mediated methods, other methods have been reported. Ramos et al. uses a surfactant-mediated method to prepare floating CCM [17]. By mixing aqueous charge-stabilized polystyrene latex particles with a mixture of an oppositely charged and a neutral surfactant which self-assembled into vesicles, 2D colloidal crystal monolayers were formed on the vesicles. In that system, besides 2D colloidal crystal monolayers, there also were many free particles and random clusters present. Furthermore, the requirement of two types of surfactants as well as the formation of vesicles makes the system complicated and difficult to improve. Tang et al. reported the spontaneous formation of floating CCM of CdTe nanoparticles with tetrahedral shape. The authors ascribed the formation of floating CCM to a combination of electrostatic repulsion and anisotropic hydrophobic attraction [18]. However, the requirement of anisotropic shape limits the potential applications of this method. In addition, optical binding can be another tool to prepare floating CCM which required complicated optical experimental setup and was limited to very small numbers of colloids [22], [23].

As mentioned before, convective deposition is a widely employed method to prepare CCM. In a classical convective deposition, (super-)hydrophilic substrate is inserted into an aqueous colloidal dispersion and subsequent slowly withdrawn out of the dispersion. The colloids nucleate at the drying front via attractive capillary interactions. Once a crystallization nucleus is formed, a convective flow sets in that contains additional colloids, resulting in the formation of CCM [10]. In this article, a modified convective deposition method to prepare floating CCM is presented. As illustrated in Scheme 1, cross-linked polymeric colloids are first dispersed in a volatile dispersing solvent. Instead of withdrawing the substrate out of dispersion, we let the solvent evaporate for approximately 20 min in a fume hood. During evaporation, monolayers are formed onto the inner wall of the centrifugal tube. After removal of the remaining bulk dispersion, an extracting solvent is added. Subsequently, the formed monolayers are peeled off by manual shaking, and eventually dispersed in the extracting solvent. There are three crucial differences between the method reported here and earlier reported procedures that make use of convective deposition. The first difference is the dispersing solvent. While water is generally the solvent of choice, here volatile organic solvents are used which apparently accelerates the formation of CCM. Second, the hydrophobic inner wall of the centrifugal tube instead of (super-)hydrophilic glass is used as a substrate, hence the tedious pretreatment of the substrate is avoided. The last and most important difference is that by using the method present here, CCM can be easily peeled off, and freely float in the dispersion, which is ascribed to the use of appropriate extracting solvents.

In this work, we first demonstrate that using our new method, the formed assemblies are indeed monolayers and these monolayers can freely float in the dispersion. Subsequently, we systematically investigate the experimental parameters in terms of the dispersing solvent, the extracting solvent, the wall materials, and the colloid on the formation of the floating CCM. Eventually, a possible mechanism is proposed which includes CCM formation and peeling off: the CCM formation step involves the capillary attractions that originated from the deformation of a meniscus structure of the air–liquid interface between the colloids, while the CCM peeling off is caused by the penetration of the extracting solvent into the gap between the CCM and the substrate.

Section snippets

Materials

Styrene (St, 99%), divinylbenzene (DVB, 55% mixture of isomers, tech. grade), 1-(chloromethyl)-4-ethenylbenzene (VBC, ≥90%, tech. grade), acrylic acid (AA, 99%), methanol (anhydrous, 99.8%), N,N-dimethylformamide (DMF, ≥99%), acetone (AR, ≥99.5%), methyl acetate (anhydrous, 99.5%), acetonitrile (GC, ≥99.5%), 1,4-dioxane (DOX, ACS reagent, ≥99.0%), 1-methylpyrrolidin-2-one (NMP, anhydrous, 99.5%), 1-ethenylpyrrolidin-2-one (PVP, K30, Mw = 40 kg/mol), polyvinyl alcohol (PVA, Mw = 85–124 kg/mol,

Experimental proof of floating CCM

Chlorinated cross-linked colloids, further abbreviated as CPS-Cl, with a diameter of 418 ± 8 nm and a zeta potential of −38 ± 4 mV were first used as the building blocks. The colloids were synthesized by seeded emulsion polymerization and fluorescent labeling in combination with confocal microscopy indicates that the particles are chemically isotropic (S2, Supporting Information). THF was introduced as both the dispersing solvent and the extracting solvent for CPS-Cl, which were kept in a

Conclusion

We report a modified convective deposition method to prepare floating colloidal crystal monolayers (CCM) consisting of submicron polymeric building blocks. The method involves two major steps. In the first step, we make plausible that evaporation of the dispersing solvent results in the formation of a meniscus structure of the air–liquid interface between the colloids on the substrate. The deformation of the meniscus gives rise to capillary attraction, driving the colloids towards the crystal

CRediT authorship contribution statement

Yong Guo: Methodology, Investigation, Formal analysis, Writing - original draft, Writing - review & editing, Funding acquisition. Willem K. Kegel: Supervision, Project administration, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We would like to thank Kanvaly Lacina for taking the scanning electron microscopy images. Hans Meeldijk and Chris Schneijdenberg are thanked for helping with freeze-drying samples. Yong Guo is supported by a scholarship under State Scholarship Fund (File No. 201306200056) from the Chinese government.

References (39)

  • J.-T. Chen et al.

    Fabrication of hierarchical structures by wetting porous templates with polymer microspheres

    Langmuir

    (2009)
  • N. Vogel et al.

    From soft to hard: the generation of functional and complex colloidal monolayers for nanolithography

    Soft Matter

    (2012)
  • A.S. Dimitrov et al.

    Continuous convective assembling of fine particles into two-dimensional arrays on solid surfaces

    Langmuir

    (1996)
  • N. Denkov et al.

    Mechanism of formation of two-dimensional crystals from latex particles on substrates

    Langmuir

    (1992)
  • A. Mihi et al.

    Oriented colloidal-crystal thin films by spin-coating microspheres dispersed in volatile media

    Adv. Mater.

    (2006)
  • K.-Q. Zhang et al.

    In situ observation of colloidal monolayer nucleation driven by an alternating electric field

    Nature

    (2004)
  • F. Reincke et al.

    Spontaneous assembly of a monolayer of charged gold nanocrystals at the water/oil interface

    Angew. Chem. Int. Ed.

    (2004)
  • J.-T. Zhang et al.

    Fabrication of large-area two-dimensional colloidal crystals

    Angew. Chem. Int. Ed.

    (2012)
  • M. Kondo et al.

    Preparation of colloidal monolayers of alkoxylated silica particles at the air-liquid interface

    Langmuir

    (1995)
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