The Development and Atomic Structure of Zinc Oxide Crystals Grown within Polymers from Vapor Phase Precursors

Sequential infiltration synthesis (SIS), also known as vapor phase infiltration (VPI), is a quickly expanding technique that allows growth of inorganic materials within polymers from vapor phase precursors. With an increasing materials library, which encompasses numerous organometallic precursors and polymer chemistries, and an expanding application space, the importance of understanding the mechanisms that govern SIS growth is ever increasing. In this work, we studied the growth of polycrystalline ZnO clusters and particles in three representative polymers: poly(methyl methacrylate), SU-8, and polymethacrolein using vapor phase diethyl zinc and water. Utilizing two atomic resolution methods, high-resolution scanning transmission electron microscopy and synchrotron X-ray absorption spectroscopy, we probed the evolution of ZnO nanocrystals size and crystallinity level inside the polymers with advancing cycles—from early nucleation and growth after a single cycle, through the formation of nanometric particles within the films, and to the coalescence of the particles upon polymer removal and thermal treatment. Through in situ Fourier transform infrared spectroscopy and microgravimetry, we highlight the important role of water molecules throughout the process and the polymers’ hygroscopic level that leads to the observed differences in growth patterns between the polymers, in terms of particle size, dispersity, and the evolution of crystalline order. These insights expand our understanding of crystalline materials growth within polymers and enable rational design of hybrid materials and polymer-templated inorganic nanostructures.

It is important to note that Figure b which shows atomic resolution imaging of PMCHO was obtained from a different sample type than that described in the methods section of the manuscript (SiNx windows).PMCHO has proven to be much more sensitive to the electron beam than the other two polymers.For that reason, we were unable to obtain atomic resolution imaging of particles grown within the polymer on SiNx.Instead, the polymer was spin-cast unto a water-soluble sacrificial layer on a Si wafer, which was then inserted into water.The PMCHO layer, which floated to the water surface, was then scooped onto a TEM grid, dried, and then inserted into the ALD chamber for the SIS process.The overall mass thickness of the sample (including substrate, polymer and ZnO) was lower than obtained on SiNx windows, thus allowing atomic resolution imaging of ZnOx crystals in PMCHO.Unfortunately, polymer film thickness was inconsistent when made in this manner, a fact that prompted our choice of SiNx windows for HR-TEM study of ZnO SIS in these polymers.Table S4.Structural parameters for the first and second coordination shells of Zn obtained from the fitting of the Zn K-edge EXAFS spectra.Here N is a coordination number (±0.4), R is an average interatomic distance (±0.02Å), MSRD is mean-square relative displacement (s 2 , ±0.002 Å 2 ), also known as the Debye-Waller factor, and C3 is a third cumulant (±0.0005Å 3 ) which accounts for a deviation of the radial distribution function from the Gaussian shape.The table compares data acquired after 5 SIS cycles, and after 5 cycles and 600°C thermal treatment.

Figure S10.
Representative QCM measurements of growth in different polymers.Experiments were done at 120°C.Each cycle was comprised of 900 s exposure to the organometallic precursor, followed by 1200 s N2 purge and then 900 s exposure to water, followed by 1200 s N2 purge.Exposures were done in static mode, during which the chamber is completely closed and there is no N2 flow.
The first cycle in the PMMA experiment (green plot) was done using trimethyl aluminum (TMA) as the organometallic precursor.To gain statistically significant data, each experiment was repeated a few times, and the averaged results are shown in the main text.

Water Absorption Experiments
In order to evaluate the water intake (or hygroscopy) of the polymers, we conducted QCM water exposure experiments (Figure 8).After preparation (described in the methods section), which included sample drying in a desiccator for at least 2 hours, each sample was kept in a N2 glove box.Prior to each experiment, the sample was placed in the ALD at 120°C for at least 4 hr at 50 sccm N2 flow, to eliminate water residues from the atmosphere.
Stabilization ended and the experiment began only when frequency variations were lower than 1 Hz during a 5 hr period.Experiments consisted of three water/purge half cycles.The pressure profile of this experiment, measured using capacitance manometer in a PMMA experiment together with details on the flow at each stage and the valve position, can be found below in Figure S11e.In each half cycle the sample was exposed to water vapors, during which time the chamber was sealed in a completely static mode for 300 s. Chamber valves were then opened, and the chamber was purged for 300 s with 20 sccm N2 flow.Prior to these three half cycles, a similar half cycle was done once without opening the water valve, as a control experiment meant to evaluate the effect of N2 pressure and valve opening and closing on the mass measurement (for example, at 5400 sec, N2 flow is decreased from 50 sccm, which is the instrument flow rate during the drying stage, to 5 sccm -the flow rate preceding each pulse).
A word of caution-these measurements are close to the sensitivity limit of the in-situ QCM and should be performed on a stable polymer film and a stable QCM to obtain repeatable results.Experiments were done at 120°C.Each cycle was comprised of 300 s exposure to water, followed by 300 s N2 purge.Before the first exposure to water, a single cycle was performed without opening the water valve, to help eliminate N2 and chamber valve procedure artifacts (gray area).Exposures were done in static mode, during which the chamber is completely closed and there is no N2 flow.(e) A typical pressure profile, valve position, and flow through the chamber during the water uptake experiments.

Figure S3 .
Figure S3.FTIR of PMMA: ex-situ (a) and in-situ (b, c) measurements.(a) shows a standard measurement of the pristine polymer, denoting relevant peaks.(b) and (c) show a measurement done during SIS, with one cycle of TMA/H2O exposure, followed by 5 DEZ/H2O exposures.TMA, DEZ and water are denoted by red, grey and teal, respectively.In (b) data is presented by following select peaks of interest as a function of time, while (c) shows the entire data in a height map form.

Figure S4 .
Figure S4.FTIR of SU-8: ex-situ (a) and in-situ (b, c) measurements.(a) shows a standard measurement of the pristine polymer, denoting relevant peaks.(b) and (c) show a measurement done during SIS, with 5 DEZ/H2O exposures.DEZ and water are denoted by grey and teal, respectively.In (b) data is presented by following select peaks of interest as a function of time, while (c) shows the entire data in a height map form.

Figure S5 .
Figure S5.FTIR of PMCHO: ex-situ (a) and in-situ (b, c) measurements.(a) shows a standard measurement of the pristine polymer, denoting relevant peaks.(b) and (c) show a measurement done during SIS, with 5 DEZ/H2O exposures.DEZ and water are denoted by grey and teal, respectively.In (b) data is presented by following select peaks of interest as a function of time, while (c) shows the entire data in a height map form.

Figure S6 .
Figure S6.Thermal treatment STEM FFT Analysis: STEM images (a-b) and FFT patterns (c-d) for SU-8 (a, c) and PMMA (b, d).Rings drawn in the FFT patterns illustrate the periodicities found to match those of ZnO wurtzite-type d-spacings.

Figure S11 .
Figure S11.QCM water uptake evaluation raw data for SU-8 (a), PMCHO (b), PMMA (c) and the PMMA/AlOx hybrid (d), represented on separate axes.Experiments were done at 120°C.Each cycle was comprised of 300 s