Low-temperature synthesis of nanoscale silica multilayers – atomic layer deposition in a test tube

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Birgit Hausmann

Reference

B. Hatton et. al. "Low-temperature synthesis of nanoscale silica multilayers – atomic layer deposition in a test tube”, J. Mater. Chem., (20) 6009–6013 2010

Keywords

Atomic layer deposition, silica multilayer, colloidal crystal films

Overview

Tetramethoxysilane vapor is used alternately with ammonia vapor as a catalyst to grow uniform silica multilayers onto hydrophilic surfaces at ambient conditions.

Results and Discussion

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Alkoxysilane vapor (tetramethoxysilane, TMOS) has been used for the low temperature growth of silica multilayers, which can be useful when organic materials are to be coated which can't resist high temperature conventional ALD. Here, silica multilayers were deposited isotropically on polymer colloidal spheres and within a colloidal crystal (opal) structure (setup shown in Fig. 1), by alternating the exposure of the substrate samples to TMOS and <math> \mathrm{NH_3/H_2O}</math> vapors. The <math> \mathrm{NH_3}</math> vapor is used to catalyze the hydrolysis of remaining methoxy groups and aids the condensation polymerization of surface silanol groups. Fig. 2 shows TMOS-based <math> \mathrm{SiO_2}</math> growth at 80 C on <math> \mathrm{SiO_2}</math> colloidal crystal films for different TMOS exposure cycles from 0 to 100. The interstitial space got increasingly covered by multilayers. The changes in optical and mechanical properties were characterized as a function of silica deposition cycles: The infiltration growth within an opal is estimated from the position of the first stop gap in optical spectra for the 320 nm opal film as a function of the number of the TMOS exposure cycles (from 0 to 100) as shown in Fig. 3. A red-shift (i.e.; increase in the effective refractive index, neff) is observed as well as a reduced intensity of the first stop gap Bragg peak with increased silica deposition. the silica volume fraction (Vs) shown in Fig. 3b increased to approximately 0.93 and 0.97 for the 285 and 320 nm films, respectively (assuming nSiO2 ¼ 1.44). If the spheres were fully dense one would expect Vs to start at 0.74 for a close-packed structure (dotted line). Therefore, this simple model suggests there is densification of the silica spheres themselves, in addition to external layer growth between the spheres. The results indicate the average deposition rate is between 0.2 - 0.3 nm per TMOS exposure cycle, which is 3 - 4 times increased compared to 0.07 - 0.08 nm per cycle for NH3-catalyzed TEOS ALD at RT, measured by Ferguson et al.17 This growth rate depends on the relative humidity, and the thickness of the adsorbed hydration layer.30 While a more sophisticated, closed vacuum system is necessary to control these factors, our results demonstrate that the conditions of the ambient atmosphere are sufficiently suitable to achieve reasonable, nanoscale control over the layer thickness.

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