Paper TF-ThP16
Porosity Analysis on Supported Thin Nanoporous Films

Thursday, October 18, 2007, 5:30 pm, Room 4C

Ordered nanoporous materials synthesized through surfactant templating exhibit channel structures of amorphous silicate or oxide framework, uniform apertures in the range of 2–30 nm and large surface area of ~1000 m2/g. Their intrachannel void domain provides the confined space to accommodate nanostructured molecules and clusters, and the hydroxyl groups on the intrachannel surface can be chemically modified through silynation. The applications of this type of nanoporous films are closely associated with their pore properties such as pore size and porosity as well as hydrophobicity of intrapore surface. The dielectric constant of nanoporous silica films was reported to decrease with porosity. Gas transport of these films was found to be somewhat affected by their pore size distribution. Furthermore, the uniform thickness and proper density of the nanoporous coating play key roles in its anti-reflection of light. The film texture of supported thin films was found to be affected by substrate. As those films and their substrates usually appear together in application, it is desirable to perform the characterization with on-substrate thin films instead of the films detached from their substrates. In this paper, three pore characterization techniques and their application on supported silica thin films are discussed. The porosity of nanoporous films on flat and dense substrates, including glass and silicon wafer, has been estimated from refractive index obtained by ellipsometric porosimetry and from film density and thickness obtained by specular X-ray reflectivity(XRR). After removal of organic template, such as nonionic P123 Pluronic block copolymer (EO20-PO70-EO20) and cationic surfactant CTAB (C16H33N(CH3)3Br, the intrachannel surface of nanoporous silica is rich in hydrophilic silanol groups, which can absorb water easily. The water absorbency makes the nanoporous silica difficult to maintain a constant density and low dielectric constant, especially at relative humidity > 25 %. Therefore, it is better to perform the XRR under low humidity using an in-situ sample cell and to use the film as a low dielectric layer after hydrophobic modification. To elucidate the relationship between porosity and pore size distribution, the physical adsorption of krypton on dehydrated samples has been measured, and this technique is performed in the absence of water uptake on the hydrophilic nanoporous materials.