The Analysis of Periodic Order in Monolayers of Colloidal Superballs

Daniël N Ten Napel,Andrei V Petukhov,Janne-Mieke Meijer

doi:10.3390/app11115117

Daniël N Ten Napel, Andrei V Petukhov + Show 1 more

Open Access

https://doi.org/10.3390/app11115117

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Abstract

The characterization of periodic order in assemblies of colloidal particles can be complicated by the coincidence of Bragg diffraction peaks of the structure and minima in the form factor of the particles. Here, we demonstrate a general strategy to overcome this problem that is applicable to all low-dimensional structures. This approach is demonstrated in the application of small-angle X-ray scattering (SAXS) for the characterization of monolayers of colloidal silica superballs prepared using the unidirectional rubbing method. In this method, the ordering of the colloidal superballs is achieved by mechanically rubbing them onto a polydimethylsiloxane (PDMS)-coated surface. Using three differently shaped superballs, ranging from spherical to almost cubic, we show that certain Bragg peaks may not appear in the diffraction patterns due to the presence of minima in the form factor. We show that these missing Bragg peaks can be visualized by imaging the colloidal monolayers at various orientations. Moreover, we argue that the same strategy can be applied to other techniques, such as neutron scattering.

Highlights

The average shape parameter m and diameter D of the synthesized superball colloids were determined from transmission electron microscopy (TEM) images using a customwritten analysis script written in Python
To elucidate the role of the particles’ form factor on the structural analysis in (X-ray) diffraction patterns, monolayers of superball colloids with three different shapes, namely spherical, semi-cubic and cube-like, were prepared using the unidirectional rubbing method and investigated with small-angle X-ray scattering (SAXS). These colloidal monolayers were rotated around the rubbing direction (x-axis)
We focus on 2D SAXS patterns and their analysis for a monolayer obtained from spherical colloids that can be described as a superball with a shape parameter of m = 2

Summary

Introduction

Colloidal monolayers are used in a wide variety of applications, such as anti-reflective coatings, photonic materials, photovoltaics and biosensors [1,2]. The assembly of colloids into monolayers is often used as a step in colloidal syntheses in order to locally modify the particles in order, for example, to produce Janus colloids [3,4,5]. Various methods are known for the preparation of colloidal monolayers, such as drying methods, spin coating, the assembly on liquid interfaces and the horizontal and vertical deposition methods [2]. Each of these preparation methods have their own strengths and weaknesses

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