Mechanism of Metal Oxide Deposition from Atomic Layer Deposition

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The mechanism of metal oxide deposition from ALD inside non-reactive polymer matrices: Effects of polymer crystallinity and temperature Stas Obuchovsky, Hadar Frankenstein, Jane Vinokur, Anna K Hailey, Yueh-Lin Loo, and Gitti L. Frey Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00159 • Publication Date (Web): 23 Mar 2016 Downloaded from http://pubs.acs.org on March 26, 2016

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Chemistry of Materials

The mechanism of metal oxide deposition from ALD inside nonreactive polymer matrices: Effects of polymer crystallinity and temperature Stas Obuchovsky1, Hadar Frankenstein1, Jane Vinokur1, Anna K. Hailey2, Yueh-Lin Loo2 and Gitti L. Frey1* (1) (2)

Department of Materials Science and Engineering, Technion, Israel Institute of Technology, Haifa, 32000 Israel. Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, USA.

ABSTRACT: 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 sub-surface deposition. Hence, ALD into polymer films could be used for the preparation of inorganic-in-organic nano-composite materials. However, harnessing this approach requires deep understanding of the mechanisms that govern the infiltration, nucleation and in-situ 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 non-reactive 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 non-reactive 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 (HRSEM), we show that in the case of non-reactive 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 non-reactive polymer matrices offers new approaches for nanoscale engineering of hybrid materials with an eye towards creating inorganic-organic heterostructures for organic electronic device applications.

INTRODUCTION Atomic layer deposition (ALD) is a widely used technique for the deposition of thin inorganic coatings by selflimiting surface reactions through sequential exposure to a vapor-phase organometallic precursor and a suitable coreactant. Among its many merits are fine control over thickness, film conformity and uniformity on large substrates, and relatively low deposition temperatures.1, 2 Although it is mostly known for coating applications, ALD was also shown as a promising route for the fabrication of hybrid organicinorganic nanostructured materials.3 When ALD is applied onto organic films with no surface functionalization, the precursors can infiltrate the sample leading to sub-surface deposition.4 This phenomenon introduced several ALDderived infiltration techniques, such as sequential vapor infiltration (SVI)5, 6 multiple pulse infiltration (MPI)7 and sequential infiltration synthesis (SIS)8, 9 which, in contrast to conventional ALD, use extended precursor exposure times to ensure precursor infiltration and conversion. The process of ALD into organic matrices to form inorganic-in-organic nanocomposites is utilized in a wide range of fabrication procedures for example, to modify the surface energy of the

matrix 10 or enhance its mechanical11, 12 or optical13, 14 properties. This process was also used to indirectly characterize the complex morphology of organic blends 15-18 and as a processing tool for three dimensional inorganic structures where the polymer matrix is used as a sacrificial layer.6 As in classic metal oxide ALD, the vapor-based subsurface deposition inside organic films is performed by a series of alternating organometallic precursor/water injection pulses, separated by a purge of an inert gas. Recently, extensive work on the ALD of ZnO and Al2O3 from water and diethyl zinc (DEZ) or trimethylaluminum (TMA), respectively, under different deposition conditions and into a wide variety of polymer samples, such as polymethyl methacrylate,19 poly alcohols,12, 20 polyamides,5, 20, 21 polyesters,5, 6 polyolefins22-24 and various block copolymers,9, 17, 18 has shed light on the deposition mechanism and the parameters that affect the hybrid organic/inorganic morphology. In general, when exposing the polymer film to the organometallic precursor, the precursor can diffuse into the polymer film, followed by its partial retention inside. Similarly, the subsequent introduction of water results in its

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diffusion into the film and a possible sub-surface reaction with the organometallic precursor that remained in the film. The precursor’s diffusion and retention processes are strongly affected by the chemical composition of the organic sample, its morphology and the deposition conditions.4 Importantly, when reactive groups are present in the polymer host, they serve as reaction and nucleation sites for the organometallic precursors. However, information and understanding about the factors that control the growth mechanism, i.e. the precursor retention and initial nucleation and growth mechanism in polymer hosts with no reactive groups, is scarce. In this work we study the sub-surface ALD of a metal oxide into a polymer film with no reactive moieties by controlling the deposition temperature and the crystallinity of the polymer matrix. The model system is ZnO from DEZ and water deposited into a film of the conjugated polymer poly(3hexylthiophene-2,5-diyl) (P3HT). P3HT was selected because it does not interact with the ALD precursors,25 and its crystallinity is easily controlled by the location of the hexyl chain on the thiophene ring. Namely, films of regioregular (RRe) P3HT are semicrystalline (up to 60%),26 while films of regiorandom (RRa) P3HT are amorphous.27 Blending the two isomers allows intermediate crystallinity.28 Furthermore, the optical properties of P3HT are strongly correlated with its structure and conformation. 29 Hence, its absorption spectrum could be used to monitor changes in the film morphology. Characterization of the morphology is also possible by HRSEM due to P3HT’s relatively high carrier mobility.30 Finally, P3HT is the most commonly used hole-conducting polymer in a wide spectrum of organic electronic applications,31, 32 and its coupling with metal oxide ALD has recently been demonstrated for photovoltaic15, 33 and organic transistor applications.34, 35 Here, we expose films of P3HT to DEZ/water ALD sequences to study the effects of P3HT crystallinity and deposition temperature on the metal oxide nucleation and growth in a non-reactive polymer matrix. Using high-resolution electron microscopy, Energy-dispersive X-ray spectroscopy (EDS) elemental analysis and optical measurements, we find that the retention of the ALD precursors inside the inert organic matrix is the limiting factor of sub-surface deposition. The retention is promoted by matrix crystallinity and is hindered at elevated temperatures. By recognizing the considerations governing the sub-surface deposition, we are able to direct the resulting hybrid morphology. EXPERIMENTAL SECTION Materials: Diethylzinc (DEZ, cylinder packaged for use in ALD deposition systems) was purchased from Sigma Aldrich. Two Poly(3-hexylthiophene-2,5- diyl) (P3HT) isomers were purchased from Rieke Metals, Inc. U.S.A: 4002-E Regioregular (RRe >90%) and 4007 Regiorandom (RRa); all were used as-received. Sample preparation: Substrates used in this study include quartz and silicon. All substrates were cleaned thoroughly by sonication in acetone, methanol and isopropanol for 15 minutes each. Three types of 180 nm P3HT films were spin coated from chloroform: (i) RRa, (ii) RRe, and (iii) 1:2 RRe:RRa blend. RRe P3HT films were spun from a 20 mg/ml solution at 3000 RPM for 30 seconds, while the RRa P3HT and blend films were spun from 25 mg/ml solutions at 2000 RPM for 30 seconds. Bilayer films were prepared by

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transferring an approximately 110 nm RRa-P3HT film on top of a 180 nm RRe-P3HT film using soft contact lamination.36, 37 The RRe-P3HT films were prepared using the procedure described above for the single layer films. The 110 nm RRaP3HT films were prepared by spin coating a 15 mg/ml solution of RRa-P3HT in chloroform at 1500 RPM for 60 seconds onto pre-cleaned SiO2 substrates. A polydimethylsiloxane (PDMS) stamp, prepared by casting and curing a Dow Corning Sylgard 184 formulation, was laminated against the as-spun thin film of RRa-P3HT. The entire assembly was soaked in water, which selectively wicks to the film-substrate interface and thus transfers the RRaP3HT film onto the PDMS stamp. The RRa-P3HT film was then transferred from the PDMS stamp onto the RRe-P3HT film. The single-layer films were thermally treated at 170 oC under low vacuum for 60 min for solvent outgassing and residual water removal, which could affect the sub-surface nucleation. The bilayer RRe-P3HT/RRa-P3HT samples were dried for 8 hours under 10-6 torr vacuum with no temperature treatment to avoid layer intermixing. ALD growth of ZnO was conducted in an Applied Microstructures MVD100E system. Deposition temperature varied between 45-90 oC, and alternating pulses of DEZ and water were applied. The precursor reaction time was limited to 1 second for both DEZ and water in each cycle. Chamber purging consisted a series of 10 torr N2 dozes, 5 after a DEZ pulse and 10 after a water pulse, each followed by chamber evacuation. This multistep, harsh cleaning process is equivalent to 90 and 180 second purges after DEZ and water, respectively. Characterization: Optical absorbance measurements of films on quartz were performed using a Varian Cary 100 Scan UVvis spectrophotometer. Grazing-incidence X-ray diffraction (GIXRD) measurements (ω=3o) of films on silicon substrates were conducted on a D/MAX-2500 series, RIGAKU system with Cu Kα radiation (λ=1.5418 Å). High-resolution scanning electron microscopy (HRSEM) micrographs of films on silicon were acquired using a Zeiss Ultra-Plus FEG-SEM operating at 2 keV and the EDS measurements were conducted at a 5 keV operating voltage. EDS measurements include Kα signals of sulfur, carbon and oxygen as well as the Lα signal of zinc. The Si (substrate) signal was observed in all EDS measurements to ensure that the interaction volume was larger than the film thickness. RESULTS AND DISCUSSION To study the effect of host crystallinity on the growth of metal oxides by ALD inside non-reactive polymers, we prepared three sets of P3HT samples with different extents of crystallinity. Solutions containing RRe, RRa and a 1:2 RRe:RRa weight ratio of P3HT were spun from chloroform to form thin films. Absorbance spectra, shown in Figure 1A, indicate that the optical density of all films studied is comparable and relates to a film thickness of approximately 180 nm. Further examination of the data shows that introducing RRe-P3HT to RRa-P3HT induces a significant bathochromic shift in the absorption spectrum of the polymer, with λmax moving from 441 nm in the absorption spectrum for RRa-P3HT films to 468 nm and 518 nm in the absorption spectra for the RRe:RRa blend and RRe-P3HT samples, respectively. This shift corresponds to an extension of conjugation length in the samples that contain RRe-P3HT, indicating increased order and better polymer packing with the


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introduction of RRe-P3HT.29 Moreover, introducing RReP3HT to RRa-P3HT also yields two new vibronic shoulders in the spectra, at 552 and 605 nm; both are associated with the optical transitions of intermolecular π-π interactions and thus are also indicative of ordered P3HT domains.38 Complimentary study of the P3HT film morphology was conducted by GIXRD, and is presented in Figure 1B. (The signal at 6°, which is present in all patterns, is induced by reflection from the sample surface and is not characteristic for the samples studied). Films cast from RRa-P3HT show no distinctive diffractions peaks. The broad signal at low angles is indicative of amorphous P3HT. In contrast, the GIXRD pattern of RRe-P3HT shows three peaks corresponding to the (100), (200), (300) reflections associated with the 1.6 nm lamellar distance typical of crystalline P3HT.30, 39 Finally, the GIXRD pattern of the RRe:RRa 1:2 blend film displays a combination of both the broad amorphous halo of RRa-P3HT and the diffraction peaks of RRe-P3HT, consistent with an intermediate degree of film crystallinity. Thus, both optical and GIXRD data confirm that increasing the regioregularity of P3HT induces a higher degree of film order and crystallinity, in agreement with previous reports of completely amorphous RRa-P3HT films and ~50-60% crystallinity in the RRe-P3HT sample.26

Figure 2. UV-VIS absorbance spectra of P3HT films of different regioregularity after exposure to cycles of the ALD DEZ/water process: (A) RRa, and absorbance of ZnO deposited on a bare substrate (B) 1:2 RRe:RRa mix (C) RRe.

Figure 1. (A) UV-VIS absorbance and (B) GIXRD pattern of thin P3HT films of different regioregularity.

The sub-surface growth of ZnO in the P3HT films was examined by applying a series of ALD sequences with varying number of cycles at 60 ºC. As previously reported,15, 16 exposure of P3HT film to an ALD sequence of DEZ and water results in diffusion of the precursors into the P3HT film and sub-surface ZnO deposition. We would like to emphasize that this study focuses on the effect of host crystallinity and ALD process temperature on the sub-surface deposition of metal oxides in the non-reactive organic film. Thus, we selected exposure and purge conditions that do not limit the sub surface mass accumulation. After confirming the conditions, these parameters were kept constant throughout the study to allow us to study the effects matrix crystallinity and temperature on the deposition mechanism.



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Figure 3. Cross section BSE HRSEM micrographs of P3HT films as a function of film crystallinity and number of deposition cycles. The bright domains represent the higher density ZnO dispersed in the darker P3HT matrix.

Preliminary investigation of ZnO growth inside the polymer matrix was conducted by absorbance measurements and is presented in Figure 2. All absorbance spectra show high reproducibility of the P3HT film thickness. Consequently, the organic matrix volume that is available for diffusion and nucleation of the precursors is similar in all samples. Exposure of P3HT to the ALD sequences does not alter the polymer absorbance spectra beyond 375 nm, as shown in Figure 2. The absence of spectral shifts, in particular the constant intensity of the optical transitions associated with the ordered domains (Figure 2 B, C), indicates that the morphology was not significantly modified during the ALD process. This observation is in good agreement with previous reports suggesting that the sub-surface ALD proceeds in the amorphous domains of the infiltrated samples.15, 16, 40 More importantly, the spectral stability confirms that there is no chemical reaction between P3HT and the ALD precursors because such reactions would have reduced the conjugation length and modified the optical properties.25 Exposing the P3HT samples to DEZ/water ALD sequences results in ZnO growth, as apparent from the appearance of a new optical transition below 375 nm in the absorbance spectra of all films, the intensity of the additional absorbance increases with the number of ALD cycles. This transition agrees with the absorbance of ZnO grown on bare substrates (presented in Figure 2 A) and corresponds to the optical band gap of ZnO.41 The effect of P3HT crystallinity on the deposition rate can be initially evaluated by the evolution of the ZnO absorbance signal in the different films as a function of the number of ALD cycles. A comparison of the relative change in the absorbance below 375 nm reveals that for RRa-P3HT, short deposition sequences of a few dozen ALD cycles don't induce observable spectral changes. Namely, the absorbance of the pristine films is identical to that of the same films after 38 pulses of DEZ and water (red and black curves in Figure 2A). In contrast, the absorbance of the RRe-P3HT films (Figure 2C), even after the short deposition sequences, shows the typical ZnO feature. Hence, the absorbance spectra give the first indication that the initial stages of the ZnO deposition are faster in the more crystalline film. However, in contrast to the initial ZnO growth stage, comparing the intensity of the spectra below 375 nm after a large number of ZnO cycles indicates that in the more advanced stages of the ZnO growth, the ZnO incorporation is more significant in the films with lower crystallinity.

Quantitative analysis of the ZnO mass accumulation in the films provides integrative information from the full thickness of the hybrid film. However, we would like to follow the changes in the deposition rate coupled with analysis of the location and dispersion of the ZnO grains. Therefore, we performed cross-section HRSEM analysis after the different ALD sequences, as presented in Figure 3. The images are obtained using backscattered electrons (BSE), so the bright domains represent the higher density ZnO dispersed in the darker P3HT matrix. Figure 3 shows a strong correlation between the number of ALD cycles and the ZnO mass inside the film, as we previously reported for pristine P3HT films.15 More specifically, in agreement with the absorbance measurements, the diffusion of DEZ and water through the available amorphous domains in all P3HT films produces the slow nucleation of the preliminary ZnO grains that are dispersed throughout the entire P3HT film. As the deposition progresses, ZnO domains continue to grow and promote swelling of the P3HT layer. A closer look at the ZnO morphology and spatial distribution reveals important dissimilarities between the P3HT films of different crystallinities. Interestingly, after 38 ALD cycles the RRa-P3HT films are yet to show any significant ZnO growth. However in the 1:2 RRe:RRa blend and even more in the RRe-P3HT film, 38 cycles are enough to yield first ZnO clusters dispersed inside the polymer layer. Furthermore, as can be seen from comparing the micrographs of the films after 50-75 ALD cycles, not only the amount of ZnO depends on the P3HT crystallinity, but so do grain size and distribution. Deposition into amorphous P3HT films generates smaller ZnO grains with a shorter grain-to-grain distance and more homogeneous dispersion inside the P3HT layer. Generally, Figure 3 shows that polymer films with higher extent of crystallinity display higher amounts of deposited ZnO in the first 75 ALD cycles. However, this trend is less observable after 87 cycles, and is completely inverted after 120 cycles. This inversion is also reflected in the thicknesses of the films after long deposition sequences. Figure 3 shows that after 120 ALD cycles the amorphous film (RRa-P3HT) swells substantially, by 40-50%, due to ZnO uptake, while the thickness of the RRe films increases by no more than 15%.


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Figure 4. Zn mass deposited inside P3HT films, calculated from EDS measurements, as a function of film crystallinity and number of deposition cycles. (The inset is an expansion of the low ALD cycles section).

To quantitatively determine the ZnO uptake as a function of host crystallinity we performed SEM EDS measurements. All pristine P3HT films are of the same thickness and hence have the same total mass of sulfur. The total mass of Zn, on the other hand, increases with the number of cycles and represents the mass accumulation of ZnO. Under such conditions, the Zn/S wt. ratio, evaluated from the EDS Kα signal of sulfur and Lα signal of zinc, is directly proportional to the total mass of zinc accumulated in each film.15 Figure 4 displays the calculated ZnO mass as a function of the number of ALD cycles. All films, regardless of P3HT crystallinity, display a non-linearly increasing deposition rate. The initial mass accumulation/cycle is very low, however as the deposition proceeds, the rate consistently increases in all films. This behavior is in contrast to ideal classical ALD on impenetrable reactive substrates, where the constant surface area and conditions for the chemical reaction generate a fixed mass/cycle ratio.1 Previous reports on ALD into inert polymers, i.e. polypropylene,23 polyethylene22 and P3HT,15 showed that the absence of a chemical reaction between the polymer and the ALD precursors hinders initial metal oxide nucleation. Nevertheless, once the preliminary metal oxide clusters have been created, the deposition can propagate through their direct reaction with the diffusing ALD precursors, generating additional metal oxide growth that again increases the surface area available to subsequent ALD cycles. The differences in the rate of ZnO mass accumulation between the P3HT films with different extents of crystallinity are apparent in Figure 4. The EDS measurements show that from the initial stages and until the advanced stages of approximately 75 ALD cycles, the semicrystalline RRe-P3HT films (black squares) accumulate a larger mass of ZnO compared with the amorphous RRa-P3HT samples (red circles). The RRa:RRe blend films exhibit intermediate ZnO mass values (blue triangles). However, as the number of ALD cycles increases (>75), the ZnO mass accumulation in the amorphous RRa-P3HT increases faster than that in the semicrystalline film, and eventually the amount of ZnO inside the RRa-P3HT films prevails.

The combined absorbance, HRSEM and EDS observations allow us to gain insight on the sub-surface deposition mechanism and the effect of sample crystallinity on the nucleation and the growth of ZnO inside P3HT. A recent study by Jur et al. examined the deposition of Al2O3 into polyethylene terephthalate (PET) fibers.40 Unlike P3HT, PET is reactive towards the diffusing precursors. It was reported that an increase in the crystallinity of the polymer film hinders the sub-surface deposition because the crystalline domains are impenetrable to the ALD precursors. Under such conditions, the process limiting the initial nucleation of the metal oxide particles is the diffusion of the precursors into the film. Precursors that successfully diffused into the film are then anchored in the matrix by the chemical reaction with the polymer. Namely, when comparing the mass accumulation in reactive hosts with different extents of crystallinity, higher crystallinity limits the volume that is available for the diffusion of the ALD precursors and hence results in lower mass accumulation compared to corresponding amorphous films. However, in contrast to the above, for the P3HT films studied here, the initial ZnO accumulation is higher in the more crystalline films. Actually, after low numbers of ALD cycles there is basically no ZnO accumulation in the amorphous film (38th cycle image in Figure 3). Therefore, we can conclude that ALD into a non-reactive polymer matrix follows a completely different mechanism than that in a reactive polymer matrix. Generally, the nucleation of a new ZnO grain requires several consecutive processes: the diffusion of DEZ into the film, its retention inside the matrix during the nitrogen purge, followed by the successful diffusion of water and a fertile reaction of the two precursors inside the P3HT film. We speculate that for a non-reactive polymer matrix (i.e. no chemical reaction between DEZ/water and P3HT), the bottleneck of the deposition process is not the precursor diffusion into the film, but rather its retention inside it. With no chemical reaction between the precursors and the matrix, the retention (during nitrogen purging) will depend on the out diffusion process, which most likely mirrors the in-diffusion process, and the physisorption of the precursor in the film. We suggest that the presence of crystalline domains, although hinders the diffusion of the precursors into the film,40 also promotes their retention probably by facilitating their physisorption in the film. Therefore, in contrast to PET, where the crystalline domains only limit the presence of the precursors in the film and hence the metal oxide nucleation, the crystalline domains in P3HT actually enhance the precursor presence in the film and hence the probability for the preliminary nucleation. Considering this suggested mechanism, the presence, size and distribution of the crystalline domains in a non-reactive polymer play a determining role on the rate, spatial distributions and morphology of in-situ grown metal oxide particles.



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Figure 5. Cross section BSE HRSEM micrographs of P3HT films after 62 ALD cycles as a function of film crystallinity and deposition temperature.

We can now analyze the absorbance, HRSEM and EDS results with respect to the mechanism suggested above. Retention of the precursor in the semicrystalline films results in the nucleation of ZnO in the RRe-P3HT even at low ALD cycles (Figure 3, top left image); while for the amorphous RRa-P3HT films the precursor was barely retained at low number of cycles and hence no ZnO is detected in the film (Figure 3, bottom left image). As the number of cycles increases, ~50 cycles, some precursor molecules are randomly retained in the amorphous film and they serve as nucleation centers for the ZnO particles. The absence of crystalline domains in RRa-P3HT films and hence the randomly dispersed nucleation centers induces not only slower nucleation, but also smaller size of the ZnO grains and a more uniform spatial distribution compared to that in the semicrystalline film where there are preferred adsorption and nucleation sites (50th cycle images in Figure 3). Once the initial ZnO grains are created and the mass accumulation is governed by their growth and not additional nucleation, P3HT behaves similarly to reactive polymer films and the sub-surface growth becomes limited by precursor diffusion. As a result, at the higher ALD cycles, films with higher amorphous P3HT content exhibit higher deposition rates compared to that in the semicrystalline films. The gap in mass accumulation between the semicrystalline and amorphous films narrows down and eventually, after approximately 100 ALD cycles the mass of ZnO is higher in the amorphous film. Correspondingly, as shown in Figure 3, the thickness of all films after 87 cycles are quite similar, but after 120 cycles, the thickness of the ZnO-containing RRaP3HTis significantly higher than that of the corresponding RRe-P3HT and RRe-RRa blend film due to the higher ZnO uptake. We identified that, in contrast to reactive polymers where the precursor diffusion and reaction with the polymer matrix are the main growth limiting factors, in non-reactive polymers it is the precursor retention in the host. Under such conditions, the temperature-dependence of the deposition in the non-reactive polymers should be completely different than that observed for reactive polymers. To study the effect of deposition temperature on the nucleation and growth mechanism in non-reactive polymers, with and without crystalline domains, we performed a series of ALD sequences at processing temperatures between 45 and 90 °C. For all temperatures, the P3HT films underwent 62 DEZ/water cycles. This intermediate deposition sequence was selected in

order to achieve substantial ZnO mass accumulation, while still remaining in the regime where nucleation plays a significant role in determining the mass accumulation and hybrid film morphology. The HRSEM images and EDS ZnO mass accumulation calculations for the RRe-P3HT and RRa-P3HT films after 62 ALD cycles at different deposition temperatures are presented in Figures 5 and 6, respectively. Essentially, both characterization methods display a similar trend: regardless of P3HT crystallinity, the initial increase in temperature (from 45 to 65 °C) enhances the amount of the deposited ZnO, while further increasing the temperature results in a reduction of the ZnO uptake. Finally, at 90 ºC there is basically no ZnO deposition observed in both the amorphous and semicrystalline films. Because the 62 cycles ensure that we are in the nucleation regime facilitated by the presence of crystalline domains (Figure 4), the ZnO uptake in the semicrystalline RRe-P3HT films (black squares Figure 6) is higher than that in the amorphous RRa-P3HT films (red circles in Figure 6) for all temperatures studied. However, the difference between the amount of ZnO accumulated in the semicrystalline and amorphous films strongly depends on the processing temperature. Namely, the RRe-P3HT samples reach the maximal ZnO mass accumulation at a 68 ºC, while for the RRa-P3HT films the maximum accumulation is reached at 60 ºC. Moreover, at 75 ºC, the RRe-P3HT samples still achieve considerable mass accumulation, while the equivalent deposition into the amorphous RRa-P3HT uptakes negligible amounts of ZnO. The temperature dependent ZnO mass accumulation trends are fully supported by the absorbance measurements as shown in the Supplementary Information section (see SI Fig S1). To understand the overall thermal dependence of the sub-surface deposition in non-reactive polymer matrices, one should consider the independent effect of temperature on the different processes which lead to nucleation and growth, specifically, precursor diffusion, reactivity and its retention in the matrix. Previous reports showed that the diffusion coefficients of precursors in all polymer matrices increase as the deposition temperature increases above the glass transition temperature (Tg) of the infiltrated polymer.19, 23 This is because above Tg the polymer chain-mobility increases and hence the free volume available for precursor infiltration increases with temperature. In addition, it was shown that in the case of reactive polymers, higher temperatures also promote higher reactivity between the precursors and polymer matrix.


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Figure 6. Zn mass deposited inside P3HT films after 62 ALD cycles, calculated from EDS measurements, as a function of film crystallinity and deposition temperature.

Therefore, the overall temperature dependence of growth in reactive polymer matrices is a result of the precursor diffusion and reactivity. In the case of reactive polymer hosts, increasing the temperature should thus increase the mass accumulation. However, if the high reactivity generates dense metal oxide deposition close to the surface, subsequent diffusion into the bulk film is inhibited and under such conditions, increasing temperature could actually result in lower metal oxide uptake.42, 43 The general temperature dependence for the deposition of ZnO in P3HT (at deposition temperatures above the Tg),44-46 as illustrated in Figures 5 and 6, also show the same initial increase followed by decrease of deposition rate. However, in contrast to the trend in reactive polymers, the trend here cannot be attributed to enhanced and inhibited subsurface diffusion, because in non-reactive hosts there is no dense metal oxide layer formation at elevated temperatures that would stall diffusion. To explain the trend we observe, we return to our earlier demonstration showing that in the case of non-reactive polymer hosts, the rate of nucleation is determined by precursor retention. The temperature dependence of precursor retention should show a reduction with the increase of the temperature. Therefore, in the case of the non-reactive host, the deposition temperature dependence is an interplay between the availability of free volume for diffusion and precursor retention. The former increases with temperature because the temperature is above the Tg, while the latter decreases with temperature, resulting in the parabolic temperature dependency observed in Figure 6. Importantly, this diffusion/ retention interplay is also nicely reflected in the differences in ZnO accumulation between the amorphous and semicrystalline matrices at the different temperatures. In the amorphous films (red circles in Figure 6) the initial increase in temperature (to 60 °C) results in a higher deposition rate due to higher free volume for diffusion. However, at higher temperatures the lack of crystalline domains for physisorption results in a dramatic decrease in retention and hence a drop in ZnO accumulation. The drop is so significant that at 75 °C there is basically no ZnO deposition in the amorphous P3HT film even after 62 cycles. The trend is very similar for the semicrystalline film (black squares in Figure 6) with similar positive and negative slopes, but the turning point is at a higher temperature, at approximately 70 °C. The increase in

ZnO accumulation in the semicrystalline film above 60°C is due to the physisorption promoted by the crystalline domains. The dramatic drop in ZnO accumulation at temperatures above 70 °C, and the identical negative slopes observed for the amorphous and semicrystalline films, indicate that above 70 °C the preferred precursor retention, i.e. physisorption, no longer dominates. In other words, above 70 °C the kinetic energy of the precursors due to the temperature is higher than the physisorption energy. Therefore, we can use the observation of promoted precursor retention up to 70 °C to evaluate the sorption energy to be approximately 30 meV. This estimated value is well situated in the physisorption regime (10-100 meV), and supports our hypothesis that the presence of crystalline domains in non-reactive polymer matrices enhances nucleation by promoting physisorption of the precursors. The temperature dependence reported above, allows us to also experimentally corroborate the suggested growth mechanism in non-reactive polymer matrices. To do so, we prepared bilayers consisting of a bottom RRe-P3HT semicrystalline layer and a top RRa-P3HT amorphous layer, the top layer was transferred onto the spuncast bottom layer by the lamination-delamination method described in Kim et al.37 The bilayers were exposed to 62 ALD cycles at 60 °C. These ALD conditions, selected based on the results presented in Figures 3-6, ensure that the ZnO growth is still in the nucleation regime and that precursor retention is significant. HRSEM cross section micrographs of two different bilayer films after the ALD process are presented in Figure 7. The bilayer in Figure 7A was exposed to a deposition sequence without any preliminary thermal treatments, while that in Figure 7B was thermally treated at 170 °C for 60 min prior to the ALD process. The difference between the distribution of the ZnO particles inside the two bilayer films is quite striking. Figure 7A shows that the ZnO deposition in the non-heated bilayer is strictly in the bottom RRe-P3HT, despite the fact that the precursors are introduced from the top surface and that there is a top RRa-P3HT layer. Namely, the precursors diffused through the amorphous top RRa-P3HT layer and into the bottom RRe-P3HT layer (almost double the distance of diffusion in a single layer), with no ZnO deposition in the top amorphous RRa-P3HT layer. Therefore, the diffusion of the precursors is in both layers, but the retention and nucleation is selectively in the semicrystalline layer. In good agreement with the faster nucleation in the semicrystalline films compared to amorphous films (Figure 4), here too the nucleation depends on the precursor retention, which is preferred in the bottom semicrystalline film. While the amorphous layer is more permeable for the precursors, the crystalline domains in the semicrystalline layer enhance the retention of the precursors in the bottom film. The retained precursor molecules serve as nucleation sites in the subsequent ALD cycles, leading to growth of ZnO in the bottom semicrystalline layer only.


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nucleation and deposition in higher crystallinity matrices could be used to direct and control the location of deposited metal oxides in non-reactive matrices. This approach provides a tool for the nanoscale engineering of hybrid organic/inorganic materials, or alternatively, this phenomenon could be exploited to map and study the spatial distribution of crystalline domains in thin organic films.

ASSOCIATED CONTENT Figure 7: Cross section BSE HRSEM micrographs of top RRaP3HT/bottom RRe-P3HT bilayer films, exposed to 62 ALD cycles at 60 ° (A). Film (B) was thermally annealed at 170 °C for 60 min before ALD.

Supporting Information Absorbance spectra of RRe/RRa P3HT as a function of temperature and XPS characterization of P3HT bilayer surface are available free of charge via the Internet at http://pubs.acs.org

The spatially selective nucleation in the crystalline domains was further confirmed by thermally annealing a bilayer film prior to the ALD deposition sequence. The annealing temperature, 170 °C, induces mixing of the bilayer. Figure 7B shows the distribution of the ZnO in the thermallytreated bilayer that was exposed to the same ALD sequence of the bilayer in Figure 7A. Noticeably, in contrast to the untreated bilayer, the distribution of the sub-surface ZnO grains is now more uniform, some ZnO grains are observed inside the top polymer layer and only a narrow portion of the top P3HT is completely depleted of ZnO. The annealing process provides sufficient thermal energy for inter-diffusion of the two P3HT stereoisomers. Following the intermixing of the layers, and the sample recrystallization during cooling, a decreasing crystallinity gradient is now formed towards the surface of the layer stack. The crystallinity gradient is translated to a similar ZnO gradient inside the film after the 62 ALD cycles. The distribution of the ZnO is not as homogenous as that observed for single layers of RRe-P3HT (Figure 3) because the thermal treatment applied here results in only partial intermixing of the polymers. Notably, in contrast to all single-layer P3HT films, the bilayer samples show patches of ZnO on the top surface. These patches are due to a reaction between the DEZ and residual PDMS present on the surface after the delamination from the PDMS stamp (for additional information, see SI Fig S2).

AUTHOR INFORMATION

CONCLUSIONS In this study we investigate the sub-surface infiltration and deposition of ZnO inside non-reactive P3HT matrices during a typical ALD process. We find that, in contrast to reactive polymer matrices, the rate-determining step of the deposition process is the retention of the diffusing precursors inside the polymer films. We find that although there is more free volume for precursor diffusion in completely amorphous films, the retention is significantly enhanced by polymer crystallinity. We show that the precursor retention is promoted by presence of crystalline domains probably due to physisorption. We speculate, at this stage, that the precursors are physisorbed at the amorphous/crystalline boundaries. By following the dependence of the ZnO mass uptake on the temperature in the semicrystalline films, we can calculate the physisorption energy to be approximately 30 meV. The promotion of precursor retention by crystalline domains is also demonstrated by diffusing the precursors into amorphous/semi crystalline bilayers. The precursors diffuse through the top amorphous layer, but ZnO deposition is strictly in the bottom semicrystalline layer due to the preferred retention. Therefore, harnessing the effect of preferred

Corresponding Author * Tel.: +972 4 [email protected]

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Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS We thank Miss Aya Osherov for preparation of the polymer films for the temperature dependence study. This research was partially supported by the Israeli Nanotechnology Focal Technology Area project on “Nanophotonics and Detection.” A.K.H. was supported by an NSF Graduate Research Fellowship.

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Mechanism of Metal Oxide Deposition from Atomic Layer Deposition

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