Interplay of Spinodal and Diffusion-Limited Aggregation in Formation of Nanocrystal Assembled 2D Islands

Abstract

Although self-organization of semiconductor nanocrystals (or quantum dots) into 3D superlattices and exploration of their collective optical, magnetic, and transport properties have been demonstrated, little is known about the underlying physics of self-assembly, growth mechanisms, and interdot-coupling-induced collective properties. Here we report a facile way of preparing nanocrystal-assembled 2D islands by drop-casting nanocrystal suspension on a hot substrate. Growth mechanisms such as scaling function, spinodal decomposition phase separation, and diffusion-limited aggregation are investigated based on the observation of quasi-monolayer coverage. After a curve fitting to several theoretical growth models, the pair bond (interaction) energy, critical nucleus size, and the phase of growth patterns were determined. Moreover, by heating the substrate and controlling the concentration of nanocrystal suspension, the spinodal decomposition and diffusion-limited aggregation can be tuned to modulate growth patterns of 2D nanocrystal islands. The interplay of these two mechanisms results in a variation of wavelength in spinodal growth patterns and of fractal pattern dimensions. By using this experimental approach, various sizes and shapes of nanocrystal-assembled 2D islands can be deposited on a flat surface of either graphite or gold.