Spin Coating of Silica Nanocolloids on Mica: Self-Assembly of Two-Dimensional Colloid Crystal Structures and Thin Films

The viability of spin-coating methods for the self-assembly of 150 nm diameter silica nanocolloids into large crystal structures on mica was investigated using different colloidal concentrations, accelerations, and rotational speeds. The samples were imaged by atomic force microscopy (AFM) in intermittent contact mode. Low colloidal concentration led to a size-dependent ordering configuration. The largest nanocolloidal particles formed crystalline close-packed structures that were surrounded by increasingly smaller nanocolloids configured into more polycrystalline or amorphous formations. This phenomenon became increasingly suppressed by increasing colloidal concentration. Two dimensional-fast Fourier transform (2D-FFT) radially averaged profiles of the topography images revealed increasing interparticle spacing with increasing rotational acceleration, from close-packed structuring at low accelerations to increasingly spaced packing at high acceleration (>800 rpm/s). This behaviour is attributed to rapid liquid shedding from the increased acceleration. Analysis with radial distribution functions quantified the extent of ordering and revealed an optimum spin speed that caused the formation of large, highly crystalline structures. This optimum spin speed is governed by the relationship between the rotational speed and the liquid film thickness that affect the uniformity of the film and the magnitude of the capillary forces generated.