Monolayer Film of Gold Nanoparticles on a 3 inch or Larger Silicon Wafer
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Monolayer Film of Gold Nanoparticles on a 3 inch or Larger Silicon Wafer Matthew N. Martin and Sang-Kee Eah Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, U.S.A. ABSTRACT We chemically synthesized charged gold nanoparticles coated with hydrophobic organic molecules, which are stable in hexane but unstable in toluene. If a hexane droplet with charged gold nanoparticles is mixed with a larger toluene droplet, they immediately float to the airtoluene interface forming a monolayer of close-packed gold nanoparticles. The monolayer can be deposited to any substrate simply by the solvent molecules’ evaporation with no limit in size and without using any sophisticated instrument. As a demonstration, we fabricated a very large monolayer of close-packed gold nanoparticles covering the whole surface of a 3 inch silicon wafer. We further showed that excess organic ligand molecules do not affect the charged gold nanoparticles’ property of floating to and forming a monolayer at the air-toluene interface. However, these very slowly evaporating molecules remain on a substrate, affect the twodimensional ordering, and significantly reduce the contrast in scanning electron microscopy. INTRODUCTION Chemical synthesis of colloidal nanoparticles has been a very active research area with progress especially in the size and shape control, improvement in the size monodispersity, and three-dimensional (3D) self-assembly. Still, there are many challenges to two-dimensional (2D) self-assembly of colloidal nanoparticles for fabricating a monolayer or multilayer film of high quality in spatial uniformity and batch reproducibility, which is important for many applications. Most popular traditional 2D self-assembly techniques for colloidal nanoparticles in solution are spin-coating and the Langmuir–Blodgett (LB) methods, where various parameters, such as the compression pressure in the LB method [1], must be carefully controlled to obtain a monolayer of close-packed nanoparticles. Recently, new methods were reported for a higher quality monolayer film of nanoparticles by either spin-coating a mixture of polymers and nanoparticles [2], or excess surfactant molecules’ catching nanoparticles at the air-toluene interface [3], where a monolayer of nanoparticles sits above and/or below a thick layer of organic molecules, limiting the range of applications. In addition, all these 2D self-assembly methods have difficulties when applied to fabrication of a very large monolayer such as covering the whole surface of a silicon wafer. Here we present a very simple 2D self-assembly method for colloidal nanoparticles at the air-toluene interface of a toluene droplet in order to fabricate a monolayer of close-packed nanoparticles with high spatial uniformity and high batch reproducibility. The monolayer has no limit in size without using any sophisticated instruments. As a demonstration we fabricated a close-packed monolayer of 5 nm diameter gold nanoparticles covering the whole surface of a 3
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