(45b) Nucleation and Growth of Titanium Dioxide Thin Film on Polydimethylsiloxane By Atomic Layer Deposition (ALD)

Researchers, engineers and industry have enormous interest in polymers for their potential applications especially in biomaterials. Researchers have been trying to incorporate solid-state materials into polymers in order to attain additional desirable properties of the polymer material such as conductivity and antimicrobial activity. One promising technique for this is atomic layer deposition (ALD). By depositing of a very thin film of metal or metal oxide, one can take advantage of those material properties without losing the unique bulk properties of the polymer; however, deposition of such a material on polymers becomes complex due to the availability of both surface and subsurface film growth on such porous materials.

In this work, ALD of TiO₂ on polydimethylsiloxane (PDMS) is investigated at the early stages of nucleation and growth of TiO₂ on both O₂ plasma treated and non-plasma treated PDMS. Contact angle measurements showed that hydrophilicity of the PDMS surface increased significantly with plasma treatment resulting in decreasing water contact angle from 99° to 44°. This is believed to be the result of the substitution of nonpolar methyl groups (-CH₃) with hydrophilic silanol group (Si-OH). In this work pristine PDMS acted as a hydrophobic surface and oxygen plasma treated surface acted as a hydrophilic surface.

The ALD of TiO₂ on polydimethylsiloxane (PDMS) was done in a commercial ALD reactor, ALD-150LEâ„¢ from Kurt J. Lesker Comany®. Deposition pressure and temperature was 970 mtorr and 120 °C, respectively. The PDMS used in this study showed thermal stability up to 200 °C, well above the ALD temperature of 120 °C used. Each cycle of the ALD reaction consisted of 1.7/25 seconds of precursor pulse/purge, respectively, and 1.5/20 seconds of oxidizer pulse/purge, respectively.

X-ray photoelectron spectroscopy (XPS) was done to find the chemical composition of the PDMS surface before and after 25 and 50 deposition cycles of TiO₂. Based on these spectra, there appears to be no evidence of nitrogen (binding energy 402-397 eV) from potentially unreacted TDMAT precursor on either pristine or O₂ plasma-treated PDMS and thus reaction between TDMAT precursor and oxidizer (O₃) appears to have reached to completion. XPS spectra also showed that after 25 ALD cycles, TiO₂ appears mostly on the O₂ plasma treated PDMS and not on the pristine PDMS. Thus, the plasma-treated surface was chemically favorable for ALD, presumable due to the silanol groups formed from the plasma, whereas pristine PDMS lacking in silanol groups does not appear conducive to the ALD reaction.

To probe the formation of TiO₂ thin film on both surface and subsurface of the PDMS substrate, x-ray absorption near edge structure (XANES) was performed by scanning the Ti K-edge at 4966 eV and monitoring the relative intensity of x-ray fluorescence normalized to the incident x-ray beam flux. Contrary to our XPS results, at lower number of cycles (e.g. 25, 50) the edge-steps of the plasma treated samples were actually smaller than the non-treated PDMS indicating more Ti present in the non-plasma treated samples for the number of ALD cycles used. As the number of cycles increased, the edge step of both treated and non-treated PDMS increased. A plausible explanation of these results is that the exposure of the PDMS surface to O₂ plasma would functionalize the surface and graft groups which can act as surface reactive groups for the upcoming ALD reaction. Therefore, after pulsing/purging of TDMAT (precursor) and pulsing/purging of O₃ (oxidizer), nucleation took place on the functionalized surface and consequently the first atomic layer of TiO₂ formed. With increasing consecutive cycles, this TiO₂ thin film grew thicker on the PDMS surface. However, in the case of pristine PDMS substrates, the mechanism appears to be different and this difference most likely originates from the lack of reactive groups on the surface of PDMS.

Results indicate that an ALD-like surface growth of TiO₂ takes place on plasma-treated PDMS and it leads to a hydrophilic surface on the PDMS. For non-plasma treated, pristine PDMS substrates apparently favor in diffusion of TiO₂ into the subsurface of the polymer without noticeable surface deposition for at least the first 25 cycles of the ALD process.