Colloidal crystals find uses across many sectors, from photonic products to fabricating electrode materials for batteries. These crystals can be manufactured through self-assembly from colloidal particles. The spread of particle size (polydispersity) is critical to this process, with high polydispersity limiting crystal growth and resulting in poorer quality crystals. Developing a detailed understanding of how polydispersity links to crystal quality is becoming increasingly important as companies look to use more environmentally friendly methods to synthesis colloidal particles. These greener techniques are often associated with broader particle size distributions.
It is known that crystal quality drops with increasing polydispersity. However, there has been no experimental work that explores this relationship systematically over a range of polydispersities to identify where the critical values lie. The team at Edinburgh undertook such a study and investigated the structure of thin films of colloidal crystals fabricated via vertical drying using silica particles with polydispersities ranging from 6% to 15%. To examine the resulting 2D structures in the dried films, the top layer was imaged using Scanning Electron Microscopy (SEM). The 3D stacking structure of the films was also explored via transmission spectroscopy and via taking cross sections using the Focused Ion Beam of the University’s Cryo-FIB-SEM facility. These cross sections were again imaged using SEM.
As expected, the silica particles in samples with low polydispersity were arranged relatively uniformly in hexagonal packed structures, with this order being lost as polydispersity increased. This loss in structure was seen in both the 2D and 3D SEM images of the films. In particular, the team found significant drops in structural order at 8% and 12% polydispersity. These values provide insight into where the limits lie for manufacturing colloidal crystals from particle systems with higher polydispersity, such as those created using more sustainable synthesis methods. These practical thresholds can help inform process controls and in-line metrology in industrial settings.
Cracks in the films were also seen to increase in shape and size, from relatively narrow cracks at low polydispersity to smooth and wide fractures observed at high polydispersity. This intriguing observation will be investigated further by the team in a future study.
The study also provided insights into how the crystals grow as the films dry. For low polydispersity samples, the local order of the crystalline layers was found to increase with the distance from the substrate that the films grew on. The team hypothesised that this could result from the first layer immediately adhering to the solid substrate and therefore keeping the order it had at this time, whilst the second (and subsequent) layers are able to rearrange a little after deposition (via Brownian motion), leading to increased crystal quality. This supports an overall crystal growth picture of 2D layers forming first and then stacking into the 3D assemblies, as previously suggested in the literature for vertical drying.
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