Controlling Pattern Formation in Nanoparticle Assemblies via Directed Solvent Dewetting

We have achieved highly localized control of pattern formation in two-dimensional nanoparticle assemblies by direct modification of solvent dewetting dynamics. A striking dependence of nanoparticle organization on the size of atomic force microscope-generated surface heterogeneities is observed and reproduced in numerical simulations. Nanoscale features induce a rupture of the solvent-nanoparticle film, causing the local flow of solvent to carry nanoparticles into confinement. Microscale heterogeneities instead slow the evaporation of the solvent, producing a remarkably abrupt interface between different nanoparticle patterns.

Figure 1

(a) An AFM image of a two-level cellular structure formed by spin-casting gold nanoparticles in toluene onto a silicon substrate, (b) a simulated structure formed by linking the chemical potential ($\mu$) to the solvent density [in the form $\mu (v)={\mu }_0+(0.1{\mu }_0\times v^{0.7})$, where $v$ is the fraction of solvent that has become vapor, and ${\mu }_0$ is the value of $\mu$ at the beginning of the simulation], (c) an AFM image of gold nanoparticles spun onto silicon from chloroform, and (d) a simulated image with a much stronger coupling between $\mu$ and solvent coverage [in the form $\mu (v)={\mu }_0+(0.5{\mu }_0\times v^{0.7})$].

Figure 2

(a) An AFM image of gold nanoparticles spun from toluene onto $\mathrm{H}:\mathrm{Si}(111)$ with an AFM-patterned $4\mu \mathrm{m}\times 4\mu \mathrm{m}$ square of oxide in the center and (b) the result of a simulation with $|\mu |$ 6% lower on the oxide area.

Figure 3

(a) An AFM image showing the effect of $1\mu \mathrm{m}$ diameter rings of oxide on nanoparticle pattern formation. The oxide features in the image are almost entirely free of Au nanoparticles and a denuded zone extends round each ring for a further 100 nm. In the regions between the rings, the Au nanoparticle layer forms a network arising from nucleated dewetting of the solvent. (b) A simulation in which sections of solvent and particles have been removed (indicated in white) to approximate the effect of dewetting from the oxide regions before the realm of the simulation.

Figure 4

(a) A three-dimensional rendering of an AFM image of raised oxide areas on $\mathrm{H}:\mathrm{Si}(111)$ and (b) the same areas covered with trapped nanoparticles after slow evaporation of a gold nanoparticle solution.