Restricted meniscus convective self-assembly

doi:10.1016/j.jcis.2010.01.010

Journal of Colloid and Interface Science

Volume 344, Issue 2, 15 April 2010, Pages 315-320

https://doi.org/10.1016/j.jcis.2010.01.010 Get rights and content

Abstract

Convective (or evaporation-induced) self-assembly is a standard technique for depositing uniform, poly-crystalline coatings of nanospheres across multiple square centimeters on the timescale of minutes. In this paper, we present a variation of this technique, where the drying meniscus is restricted by a straight-edge located approximately 100 μm above the substrate adjacent to the drying zone. Surprisingly, we find this technique to yield films at roughly twice the growth rate compared to the standard technique. We attribute this to differing rates of diffusion of vapor from the drying crystal in the two cases. We also investigate the crystal growth rate dependence on ambient relative humidity and find, contrary to some previous reports, that the growth rate depends strongly on the humidity. We introduce a model which indicates that while the length of the drying zone may increase with humidity, this alone cannot compensate for the simultaneous reduction in evaporation rate, so a lower humidity must always lead to a higher growth speed. Comparing the model to our experimental results, we find that the length of the drying zone is constant and mostly independent of parameters such as humidity and surface tension.

Graphical abstract

A variant of convective self-assembly for thin colloidal crystal is introduced, with growth speeds twice those of the conventional technique. The scaling of growth speed with humidity is also investigated.

Introduction

Coatings of colloidal particles, consisting of single or multiple layers of densely packed nanoparticles, are of interest for applications such as antireflection coatings [1], photonic crystals [2], [3], [4], [5], optical filters [6], [7], sensors [8], [9], porous membranes [10], [11], surface enhanced Raman spectroscopy (SERS) [12], fabrication of “patchy” nanoparticles [13], and nanosphere lithography [14]. They can be fabricated through a number of different methods [15], [16], [17], including sedimentation [18], [19], [20], [21], slow evaporation [2], [22], spin or drop casting [23], [24], [25], [26], microfluidic packing [6], [27], electrostatic assembly [28], covalent attachment [29], or Langmuir–Blodgett methods [30]. In applications where the goal is to produce thin films, containing three layers of colloids or less, and where poly-crystalline ordering of the lattice is sufficient, convective assembly, also known as evaporation-induced self-assembly [31], [32], [33], [34], [35], [36], is probably the fastest and most convenient technique to implement.

In one standard version of this technique, illustrated in Fig. 1a, a plate is placed at an acute angle immediately above the substrate, and a small volume of nanoparticle suspension is placed in the corner formed by the plate and substrate. The plate is then withdrawn at a velocity v_w, dragging the suspension and a thin wetting film attached to the suspension with it. Evaporation from the film induces a flow J_w of solvent toward its edge. Particles are pulled along with the flow, which drives the growth of a thin colloidal crystal with one or more layers. Uniform films can be deposited on multiple square centimeters in a few minutes with this technique.

In our version of the technique, illustrated in Fig. 1b, the meniscus of the solvent is restricted by placing a straight-edge above the substrate just before the drying zone of the film. This can be accomplished simply by running the setup just described in reverse so that the upper contact line of the fluid meniscus from which the film grows is attached along the bottom edge of the angled plate. We will call this growth mode Restricted Meniscus Convective Self-Assembly (RMCSA) to distinguish it from the conventional configuration, where the upper contact line is free to attach anywhere along the flat side of the plate. Since both modes can be accommodated with the same apparatus run in opposite directions, we will use negative withdrawal speeds (v_w) to denote RMCSA and positive for conventional CSA.

Section snippets

Experimental

Au pellets (99.999% pure) and Ni pellets (99.98% pure) were obtained from Kurt J. Lesker (Clairton, PA). Surfactant free white carboxyl polystyrene latex nanosphere suspension (784 nm diameter, 4.2% w/v) was purchased from Invitrogen (Carlsbad, CA). Unless otherwise noted, the nanoparticles were concentrated by centrifugation to 21% w/v prior to use. Plain precleaned glass slides and all chemicals were purchased from Fisher Scientific (Pittsburgh, PA).

Following Prevo and Velev [35], the particle

Theory

As explained by Dimitrov and Nagayama [32], convective self-assembly occurs due to evaporation from a wetting film that extends into the colloidal crystal from the three-phase contact line between the suspension and the film. See Fig. 1c. We denote the total evaporative flow per unit width across the length of the film as J_E. In steady state, the evaporated water is replaced by a flow of water J_w₀ = J_E from the suspension. This flux carries a particle number flow $J_{N} \propto J_{W 0}$ along with it, which gets

Results and discussion

The results from a number of RMCSA and conventional CSA experiments all carried out with the same nanoparticles but at varying v_w and relative humidity are shown in Fig. 2. For each value of the humidity, there is a critical withdrawal speed $v_{c}^{(1)}$ above which only submonolayer colloidal films form, but below which the films will consist of one or more closepacked monolayers. For uniform monolayer films, free of inclusions of thicker or thinner films, v_w must lie within a few percentage of $v_{c}^{(1)}$

Conclusion

We have compared the crystal growth rates of two different forms of convective self-assembly, which differ only in the attachment of the upper contact line of the meniscus from which the crystal is grown. Contrary to expectations, we find that the growth rates of the two modes differ by as much as a factor of two. After excluding other explanations, we attribute this to different evaporation rates from the wetting films in the two cases, which in turn is due to the difference in shape between

Acknowledgments

This work was supported in part by a grant from the National Science Foundation under agreement CBET-0756693 and by a grant from the Thomas F. and Kate Miller Jeffress Memorial Trust.

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Restricted meniscus convective self-assembly

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Theory

Results and discussion

Conclusion

Acknowledgments

Chem. Phys. Lett.

Colloid Surf. A

J. Colloid Interf. Sci.

J. Mater. Chem.

Nature

Photonic Crystals

Phys. Rev. Lett.

Adv. Mater.

Langmuir

Appl. Spectroscopy

Pure Appl. Chem.

Nature

Nature

Science

J. Mater. Chem.

Langmuir

J. Phys. Chem. B

Chemphyschem

J. Mater. Chem.

Adv. Mater.