Equilibrating Nanoparticle Monolayers UsingWetting Films

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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).


Fig. 1. (a) Illustration of the nanoparticles used. (b) Gold core size distribution histogram from TEM with double Gaussian fit (line). (c) Bright field TEM (contrast enhanced) and (d) AFM images of preannealing nanoparticle monolayers (scale bars 300 A˚ ).

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.

Experimental Method

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).

Fig. 2. Pictorial summary of various system conditions. Top to bottom: Initial dry sample, thickest wetting liquid, annealed sample in thin liquid, redried sample after annealing.

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 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 condition of the systems in such process.

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.