Equilibrating Nanoparticle Monolayers UsingWetting Films
Entry: Chia Wei Hsu, AP 225, Fall 2010
D. Pontoni, K. Alvine, A. Checco, O. Gang, B. Ocko, and P. Pershan, Equilibrating Nanoparticle Monolayers Using Wetting Films, Phys. Rev. Lett. 102, 016101 (2009).
The authors study monolayers of gold nanoparticles (NP) formed on silicon substrate. The gold NPs consist of mostly small NPs and some large ones. They pre-assemble the NPs on the silicon substrate, and then use controlled under-saturated toluene solvent vapors to re-assemble the monolayer. The nanoscale packing structure closely resembles those observed in micron-sized binary hard-sphere systems.
Thiol-stabilized gold NPs are prepared in a solution of octane-thiol and mercaptopropionic acid. The coating thickness <math>t</math> is about 12 <math>\AA</math> (Fig 1a). The distribution of the particle size (<math>s</math>) is bimodal, with the two peaks at 17 <math>\AA</math> for small particles and 76 <math>\AA</math> for large particles (fig 1b). About 85% of the particles are small. These gold NPs are hydrophobic and disperse readily in toluene.
The authors first prepare a dry monolayer of particles on silicon surface using the Langmuir-Schaefer approach. The resulting structure shows correlated regions of the more abundant small particles surrounded by ribbon-like structure of larger particles (TEM image in fig 1c and AFM image in fig 1d).
Then, toluene vapor is used for the controlled wetting of the NP monolayer. They achieve so by accurate control of the chemical potential of the toluene. They first inject toluene vapor at <math>\Delta T=15K</math>, where <math>\Delta T=T_s-T_r</math> is the temperature difference between the substrate and the toluene vapor reservoir. This creates a thin wetting film of about 1 nm thick. They they slowly cool the substrate to decrease <math>\Delta T</math> to 15 mK. In such process the film thickness grows to 10 nm. Finally, they slowly heat up (anneal) the substrate back to <math>\Delta T</math> = 15K (the wetting film get back to 1 nm thin in the process). In the end they re-dry the sample. Fig 2 provides a pictorial summary of the particle conditions of the systems in such process, based on observations.
X-ray diffraction is used to study the structure of the sample through out the whole process, and microscopy is used to examine the initial dry monolayer and the final re-dried monolayer.
Within the 10 nm thick solvent, the small particles experience a nearly 3D environment, while the large particles are still confined to a 2D geometry. This is supported by scattering data. Such confinement imbalance facilitate the preferential diffusion of small particles into initially empty cracks. Indeed, in the re-assembled monolayer, small particles cover more substrate area then before (a factor of 1.24). This suggests that when the solvents are not dried yet, the particles are mobile and explore the entire area by diffusion. Another consequence is that the re-dried sample exhibit enhanced size-segregation of particles.
Before wetting, the system's correlation function is dominated by the small particles, similar to predictions for a model of binary hard-sphere mixtures with predominantly small hard-spheres. After solvent annealing, the small particles still dominate the system's correlation function.
Connection to Soft Matter
This study provides a method to systematically investigate size and solvent effects on the structure and dynamics of NP assembly, with the confinement varying continuously between purely 2D to nearly 3D. The wetting-induced restructuring provides possibility for controlled assembly with varying degrees of order. These might eventually lead to technological applications.