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Fabrication of Ordered 2D Colloidal Crystals on Flat and Patterned Substrates by Spin Coating

Spin coating is a simple and rapid method for fabricating ordered monolayer colloidal crystals on flat as well as patterned substrates. In this article, we show how a combination of factors, particularly concentration of the dispensed colloidal solution (Cn) and spin-coating speed, influences the ordering process. We have performed systematic experiments on different types of substrates with two types of colloidal particles (polystyrene and silica). We also show that even when perfect ordering is achieved at some locations, there might be a significant spatial variation in the deposit morphology over different areas of the sample. Our experiments reveal that higher Cn is required for obtaining perfect arrays, as the diameter of the colloids (dD) increases. Interestingly, a combination of higher Cn and rotational speed (expressed as revolutions per minute) is required to achieve perfect ordering on a topographically patterned substrate, as compared to that on a flat surface, because of loss of inertia of the particles during outward flow because of impact on the substrate features. Finally, we also identify the relation between the particle diameter and the height of the pattern features to achieve topography-mediated particle ordering.


S1.0 Surface Charge of the colloids:
The PS particles and synthesized silica particles show a net negative surface charge, because of which an anionic surfactant (SDS) was added to the colloidal dispersion. Apart from SDS, a non-ionic surfactant (Triton-X) was also used for stabilizing the particles. However, the colloidal dispersion was rendered unstable on addition of a cationic surfactant (HTAB). This is attributed to coulombic binding of the positively charged head group of HTAB onto PS and silica particles; both of which have negative surface charge, as verified by measurement of zeta potential.

Figure S1. (A) The surface charge of PS particles was found to be -38.9 mV (B) Surface of silica
particles was found to be -18.6 mV.

S2.0 HCP array formation by PS and Silica colloids on different surfaces:
Large-area HCP formation by colloidal particles depends on various factors; spin coating condition and dispersion concentration being the dominant ones. These factors have been optimized for different type (PS, silica) and size of particles (300, 600, 800 nm). Also, the nature of the surface governs the quality of the array formed and the dispersion concentration and RPM has to be modulated for obtaining large-area HCP ordering. The FESEM images for HCP formation by PS and silica colloids on different surfaces is shown in Figure S2, S3, S4 and S5.

S3.0 Morphology of Patterned Substrates
The AFM morphology of the PS, PMMA and PDMS films, patterned using the same stamps, is shown below in Figure S6. The grating patterns obtained with a DVD foil had a periodicity λ P ≈ 750 nm, groove width l P ≈ 350 nm and groove depth h P ≈ 100 nm. The films patterned with a CD foil had a periodicity of λ P ≈ 1.5 µm, groove width l P ≈ 750 nm and groove depth h P ≈ 250 nm. The patterns on all the three type of films was identical.

S4.0 Synthesis of silica colloids:
During the silica synthesis process two main reactions are involved: (i) Formation of silanol group by hydrolysis

Si-(OR) 4 + H 2 O ↔ Si-(OH) 4 + 4R-OH (ii) Formation of siloxane bridges by condensation polymerization reactions 2Si-(OH) 4 → 2(Si-O-Si) + 4H 2 O
Ethanol was first taken in a beaker and kept in a sonication bath, to which a pre-calculated amount of TEOS was added. After 20 minutes of sonication, 28% ammonium hydroxide was added as a catalyst to promote the condensation reaction. Sonication was continued for another 60 min to get a white turbid suspension. All the steps were conducted at room temperature. The silica suspension was allowed to stand for an hour and then dried and calcined to obtain silica powder. The average size of the silica particles was found to be d D ≈ 350 nm. The silica powder was re-dispersed in methanol.

S5.0 Fractional Coverage of Surface with Colloids (F s )
Fractional coverage (F s ) at a particular location is defined as the area occupied by the ordered colloidal domains relative to the total area of the region. Conceptually, this is similar to the term occupation factor introduced by M.  F s was determined using Pico Image Basic (Version 5.1), an integrated AFM and SPM imaging and analysis software package. The "Slice" feature of the software was used to render and further analyse the rendered AFM images. Here as an example we demonstrate three cases for fractional coverage measurement from 10μm x10μm area 2D AFM image S6 using the software. The first case is of a perfect monolayer HCP array where F s is equal to 1.
The second case is of a partially filled monolayer, where F s is less than 1. And the third case is of a multilayer array. Here the F s obtained from the software is added to 1 and hence the net F s is greater than 1.
When a uniform monolayer HCP array is obtained over the entire surface, F s is equal to1 ( Figure S8). When a scattered array is obtained on the substrate, F s is less than 1 ( Figure S9). When a multilayer is obtained, the bottom layer is considered as a uniform layer and the area covered by the top layer is calculated from the software. This is then added to 1 to get the fractional coverage ( Figure S10).