Nanoscale Control of Layer Thickness for EL Devices by Mass-controlled Layer-by-layer Sequential Adsorption Process
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using this process, an
EL device was fabricated by depositing the thin film of micelle-wrapping ruthenium complex monomer on ITO and metal electrode on top of the film. Light emission was observed by applying voltage to this device.
1.
INTRODUCTION
There are two major processes for the fabrication of ultra-thin organic films: dry process and wet process. Vacuum evaporation and molecular beam epitaxy are the two popular dry processes. These two dry processes require high temperatures and high vacuums for thin film deposition. Spin coating and dip coating are the two popular wet processes. These two methods can be carried out room temperature and normal pressure. However, they lack accuracy for controlling the film thickness. The Langmuir-Blodgett technique can be used to control film thickness with • order accuracy.
However, the film formed by this method usually has poor
durability. In 1990, Decher reported a new fabrication process for ultra thin organic films. The process is called layer-by-layer sequential adsorption1). This process makes it possible to fabricate thin organic films simply through the sequential dipping of a substrate into dilute polycation and polyanion solutions with rinsing between each deposition step. It utilizes the self-adsorption and self- organization of the oppositely charge polyelectrolytes.
By using this process, it was also
demonstrated that the layer thickness of the thin film can be controlled with • order simply by controlling the solution pH2). Layer-by-layer sequential adsorption is expected to be a promising film fabrication process.
High performance devices such as electro luminescence (EL) devices3),
smoke sensors4) and air cleaning filters5) have been produced using this process. However, this method has been applied only for polyelectrolytes. If this process can be applied not only to polymers but monomers, the variety of the materials used for this process will be
greatly enlarged. In this study, hydrophobic Ru complex monomer, tris(bipyridyl) ruthenium(II) hexafluorophosphate (abbreviate as Ru(bpy)3(PF6)2
) 6) was wrapped with anionic surfactant and the
fabrication of thin organic films by the layer-by-layer sequential adsorption process was carried out. The change in material deposition with the change in solution pH was also investigated. Finally, the electro luminescence (EL) was observed from a device with the structure of metal/ (micelle wrapped Ru complex) / ITO. 2. EXPERIMENTAL The experimental setup consisting of an automatic dipping machine with in-situ QCM for the fabrication of layer-by-layer self-assembled films is shown in Fig.1.
As shown in this figure,
A0 and B0 are the dipping baths for the polycation and polyanion, respectively.
Polyelectrolyte
-2
dipping solutions of 10 M (based on the repeat unit molecular weight) were made from 18M ohm purified water. The purified water was used for rinsing. A quartz crystal substrate is attached near the dipping arm and was used to monitor the mass of the polyelectrolytes adsorbed onto the substrate.
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