Mechanism of Metal Oxide Deposition from Atomic Layer Deposition inside Nonreactive Polymer Matrices: Effects of Polymer Crystallinity and Temperature

Atomic layer deposition (ALD) is conventionally used to deposit smooth and conformal coatings from the gas phase onto surfaces. ALD onto organic films, however, may lead to precursor infiltration into the sample and subsurface deposition. Hence, ALD into polymer films could be used for the preparation of inorganic-in-organic nanocomposite materials. However, harnessing this approach requires deep understanding of the mechanisms that govern the infiltration, nucleation, and <i>in situ</i> growth with respect to the processing and properties of the organic matrix. Here we investigate the effect of matrix crystallinity and growth temperature on the deposition into nonreactive polymer matrices (i.e., polymers that do not bear functional groups which interact with the ALD precursors). This is done by exposing films of a nonreactive polymer, poly­(3-hexylthiophene-2,5-diyl) (P3HT), with different extents of crystallinity, to ALD cycles of ZnO precursors at different deposition temperatures. In the case of polymer matrices that chemically react with the precursors, the amount of inorganic phase uptake is a result of the interplay between precursor diffusion and matrix reactivity. However, using absorption measurements and high-resolution scanning electron microscopy, we show that, in the case of nonreactive polymer matrices, the inorganic uptake is significantly affected by the rate of nucleation which is determined by the retention of the precursors in the matrix. Furthermore, we find that the retention in the film is facilitated by the presence of crystalline domains, probably due to physisorption of the precursor molecules. This retention-dependence mechanism is further supported by temperature dependence and deposition in amorphous/semicrystalline bilayers. We find that the precursors diffuse through the top amorphous layer but ZnO is deposited strictly in the bottom semicrystalline layer due to the preferred retention. Revealing the general growth mechanism in nonreactive polymer matrices offers new approaches for nanoscale engineering of hybrid materials with an eye toward creating inorganic–organic heterostructures for organic electronic device applications