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Adsorption cycles

Figure 10. (a) SEM image of cross-section of (45-nm silica/PDDA)4 multilayer on silver electrode (b) QCM-monitored, 45-nm silica growth. Reprinted with permission from [203], F. Hua et al., Nanotechnology 14, 453 (2003). © 2003, Institute of Physics.

into account component densities (p = 1.43, pSiO2 = 2.2, and pPDDA = 1.1 g/cm3), the volume ratio is obtained as Kpdda/Vbiiayer = 0.1. From the equation PpddaVpdda + Psio2 Vsio2 + Pair Vdr = pV, where the air-term is very small, VSiO2/V = 0.7. This is very close to the theoretical dense-packing coefficient for spheres (0.63), and corresponds to details in the SEM micrographs. SiO2/PDDA film volume composition is: 70% SiO2 + 10% polycation + 20% air-filled pores. These pores are formed by closely packed 45-nm SiO2and have a typical dimension of 15 nm. Therefore, the dielectric constant of our silica/PDDA multilayer is different from silica due to about 30% of inclusions, such as air, polyion layers, etc. In the analysis of the MOS devices, it is found that the dielectric constant was slightly higher than the one for thermal silica. In our group, it is possible to produce ultra-thin multilayers of silica nanoparticles in the thickness ranging from 100 nm to hundreds of nm with a precision of about 10 nm. These films have a porous structure related to the close packing of silica spheres in the layer.

As shown in Figure 11, clear patterns of the capacitor arrays with sharp borders were created on a silicon wafer. The arrays consist of round and square capacitors with various sizes. All 45 nm SiO2 spheres were closely packed to form a dense structure. The surface roughness of the capacitor was 6.5 nm measured by RST. The growth step can be easily estimated by measuring the frequency shift of the quartz crystal microbalance resonator, and the monolayer thickness can be calculated accordingly by the Sauerbrey equation. Figure 10(b) gives the QCM monitoring of alternate PDDA and SiO2 adsorption, where the thickness was calculated from frequency shifts with formula 2. As recorded by QCM, at every assembly step, the component

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