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

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Birgit Hausmann

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

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 $\mathrm{NH_3/H_2O}$ vapors. The $\mathrm{NH_3}$ 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 $\mathrm{SiO_2}$ growth at 80 C on $\mathrm{SiO_2}$ 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. Fig. 3b shows the increased silica volume fraction to approximately 0.93 and 0.97 for the 285 and 320 nm films, respectively, which indicates an average deposition rate is between 0.2 - 0.3 nm per TMOS exposure cycle. To characterize the mechanical stabilty of the colloidal system the Young’s modulus (E) is shown in Fig. 3c as a function of TMOS exposure cycles. For the 285 nm and 320 nm films, E increases from practically zero to ~25 GPa and ~30 GPa, respectively. E appears to be a linear function of Vs as indicated in Fig. 3d. Silica growth at RT is demonstrated in Fig. 4 (5 cycles) and was found to uniformly coat the 700 nm PS spheres (Fig. 4a) with a SiO2 layer around 20 nm thick, as shown in the SEM (Fig. 4b), and TEM (Fig. 4c,d) images of the porous shells.