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Self-assembled block copolymer templates for atomic layer deposition: The effect of processing solvent
Accepted Manuscript Self-assembled block copolymer templates for atomic layer deposition: The effect of processing solvent Moshe Moshonov, Yaron Kauffmann, Gitti L. Frey PII:
S0032-3861(16)30949-1
DOI:
10.1016/j.polymer.2016.10.036
Reference:
JPOL 19136
To appear in:
Polymer
Received Date: 22 August 2016 Revised Date:
13 October 2016
Accepted Date: 15 October 2016
Please cite this article as: Moshonov M, Kauffmann Y, Frey GL, Self-assembled block copolymer templates for atomic layer deposition: The effect of processing solvent, Polymer (2016), doi: 10.1016/ j.polymer.2016.10.036. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Self-assembled block copolymer templates for
solvent
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atomic layer deposition: the effect of processing Moshe Moshonov, Yaron Kauffmann and Gitti L. Frey*
Haifa, 32000 Israel.
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ABSTRACT
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Department of Materials Science and Engineering, Technion, Israel Institute of Technology,
In this study thin films of a rod-coil block copolymer (BCP) are deposited from different solvents and used as templates for atomic layer deposition (ALD) of ZnO. The BCP, P3HT-bPEO, is known to self-organize into fiber-like phase-separated nanostructures, with central crystalline P3HT domains and outer amorphous PEO domains. The P3HT block in this study is
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short so that the size of the P3HT domains in the films is solvent insensitive. However, processing from poor PEO solvents results in thin PEO domains and good PEO solvents allow extended PEO chains and wide domains. The crystallinity of the P3HT-block and the affinity of the PEO-block to the ALD ZnO precursors lead to selective deposition of ZnO in the PEO
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domains. Therefore, the solvent-controlled morphology of the PEO domains in the BCP films can be translated to the size, distribution and morphology of the deposited ZnO. We show that
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films with swollen PEO domains have higher ZnO uptake compared to films with collapsed PEO chains. Furthermore, oriented PEO domains template oriented ZnO domains and wormlike morphologies direct wormlike ZnO nanostructures. Therefore, solvent selection is a toll to manipulate the BCP self-assembly and hence the ALD template and the morphology of the hybrid organic/inorganic film.
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INTRODUCTION A versatile approach for the synthesis of nanostructured hybrid organic/inorganic thin films is by depositing the inorganic phase into pre-formed, self-assembled block copolymer (BCP) templates.[1] [2] [3] [4] The BCP template morphologies, i.e domain size, shape, periodicity, and
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arrangement, are tunable through the BCP molecular weight and chemical composition, and the selection of processing solvents and temperature. Generally, the BCP possess a sacrificial block that is removed after the self-assembly, followed by deposition of the inorganic phase into the template voids.[5] [6] [7] [8] [9]. A useful technique to introduce the inorganic phase is low
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temperature atomic layer deposition (ALD).[10, 11] In this case, the gaseous ALD precursors diffuse into the porous organic template and deposit on the void’s surfaces. The hybrid film
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could be further manipulated by burning out the organic template and back filling the (now) inorganic porous nanostructure with organic or inorganic materials. This procedure was recently demonstrated for fabrication of conjugated polymer/ZnO continuous organic/inorganic networks for hybrid photovoltaic devices. [7]
BCPs with no sacrificial block are also suitable as ALD templates when one polymer block
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allows the diffusion of the ALD precursors into it and the second block resists it. In this case, the inorganic phase is not deposited on void surfaces, but rather inside one of the BCP blocks.[6] The diffusion and retention of the ALD precursors in a polymer block strongly depend on its chemical structure, the film morphology and the deposition conditions.[6, 8, 12] Polymer blocks
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with affinity to the precursors will allow their diffusion into the film, while blocks with no affinity to the precursors will resist the diffusion. Similarly, amorphous domains support precursor diffusion, while crystalline domains hinder it.[13] [14, 15] Accordingly, in a previous
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study we used films of poly(3-hexylthiophene)23-b-poly(ethylene oxide)23, P3HT-b-PEO BCP, as templates for ALD of ZnO. This P3HT-b-PEO phase separates and self organizes into fibril-like structures of separated P3HT and PEO domains. We showed that the affinity of the PEO-block to the ZnO precursors, in combination with the crystallinity of the P3HT block, lead to the exclusive growth of ZnO in the PEO domains.[16] Here, we show that the processing solvents can be used to tune the size and distribution of the PEO blocks in the film and directly manipulate the amount and location of ZnO in the film. Using optical absorption, Electron Microscopy and X-ray techniques, we show that this BCP self organizes into continues films of
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fibrils regardless of the type of processing solvent. However, poor PEO solvents result in fibrils with thin PEO shells, while good PEO solvents allow extended PEO chains and wide domains. All BCP films can be used as ALD templates with selective deposition of the inorganic phase in the PEO domains. Films with swollen PEO domains show higher ZnO uptake compared to films
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with collapsed PEO chains; oriented PEO domains template oriented ZnO domains and wormlike morphologies direct wormlike ZnO nanostructures. Therefore, the solvent-controlled morphology of the PEO domains is translated to the size, distribution and morphology of the
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ZnO deposited by ALD.
EXPERIMENTAL
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Materials
Methanol, Acetone, Chloroform and Isopropanol, purchased from AR BioLab Israel, were used as received. Toluene anhydrous 99.8%; THF inhibitor free anhydrous 99.9%; Methanol anhydrous 99.8%; hexane anhydrous 95%; Poly(ethylene glycol) methyl ether azide with PEG average Mn 1000 gr/mole; 2,5-dibromo-3-hexylthiophene (DBHT) 97%; tert-Butylmagnesium chloride solution 2.0 M in diethyl ether; 1,3 Bis(diphenylphosphino)propane]dichloronickel(II)
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[Ni(dppp)Cl2] and Ethynylmagnesium bromide solution 0.5 M in THF, were all purchased from Sigma-Aldrich and used as received. Copper(I) Iodide anhydrous, 99.995% trace metals basis, purchased from Sigma-Aldrich, was purified by the dissolution–precipitation process followed by drying under vacuum, providing a colorless sample.
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Synthesis and film processing
All chemical reactions were conducted under inert conditions. The polymers were synthesized
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according to the literature[17] with these synthesis procedures and characterization presented in the electronic supporting information section: ethynyl-terminated regioregular poly(3hexylthiophene), P3HT GRIM polymerization procedure, and the poly(3-hexylthiophene)23-bpoly(ethylene oxide)23 (P3HT23-b-PEO23) click reaction procedure, (Figure S1 and S2 - 1H NMR spectra; Figure S3 - DSC thermogram; Figure S4 – GPC characterization). Films were prepared by dissolving the synthesized P3HT-b-PEO (20 mg/ml) in a dry solvent. Three solvent systems were separately used; toluene, THF and a solvent mixture of THF/MeOH at an 80/20 % volume ratio. The solutions were stirred for ~10 h at 60°C under N2 atmosphere. Films, 100 nm thick, were spun onto glass, silicon or quarts substrates under N2 atmosphere
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(glove-box conditions). The spun films were then left in a covered glass petri dish for 1 hour. It is important to note that the spinning conditions from the different solvents were optimized based on the absorption spectra to obtain the same thickness for all films. ALD of ZnO into the organic films was performed using an MVD100E Applied MST system
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with an integrated oxygen plasma module. Deposition temperature was set to 60°C and alternating 1 torr pulses of diethy zinc (DEZ) and water were applied. The precursor reaction time was limited to 1 sec for both DEZ and water in every cycle. The reaction chamber was purged with nitrogen between different precursor pulse injections. ALD processes were
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performed with 20, 40, 60, or 80 cycles. Film Characterization
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The absorption spectra of the films were measured on quartz substrates using a Varian Cary 100 Scan UV-vis spectrophotometer in the 250-800 nm range.
Film thickness was measured using a Daktak XT Entry profilometer. It is important to note that the spinning conditions from the different solvents were optimized based on the absorption spectra to obtain the same thickness for all films.
Energy dispersive spectrometer (EDS) was used to analyze the composition of the sample.
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Measurements were conducted at a 5 KeV operating voltage and at a working distances of ~2mm. The Zn/S wt% ratio was evaluated from the EDS Kα signal of sulfur and 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. Furthermore, all studied films showed
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identical optical density confirming that the film thickness and amount of P3HT is identical in all films. Under such conditions, the total mass of sulfur is constant in all films and the measured
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Zn/S ratio is proportional to the total mass of ZnO in each film. At least two samples of each type were measured and averaged on four different locations. High-resolution scanning electron microscopy (HRSEM) cross section images of films on silicon substrates were obtained using the Zeiss Ultra plus high resolution scanning electron microscope (HRSEM), equipped with a Schottky field emission source, operating at 2 KeV. The images were acquired using both secondary electrons (in-lens detector) and backscattered electrons (in-lens energy selected detector), at a relatively low accelerating voltage of 1.5-2 kV at working distances of ~2mm.
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High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) surface and cross section measurements were performed on a FEI Titan 80-300 KeV S/TEM operating at 200 keV. Samples for the cross section measurements were thinned by a focused ion beam (FIB) integrated in the Strata 400 STEM DualBeam system of a field-emission scanning
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electron microscope (FE-SEM). At least two samples of each type of film were prepared and measured.
Grazing Incident Small Angle X-ray Scattering (GISAXS) measurements were performed using a Pilatus 300K detector and a Xenocs GeniX Low Divergence Cu Kα radiation, (Dectris,
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Baden, Switzerland) with scatterless slits. For this set-up, the sample−detector distance was 2.48 m with the wavelength tuned to 1.54 Å, yielding a measurement q range of 0.006−0.15 Å−1.[18]
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Each spectra was constructed of consecutive multiple frames acquired over 1 at an incident angle of about 0.16 degree, which is above the critical angle of the hybrid thin film and below the critical angle of the substrate. These conditions ensure that the X-ray beam penetrates the entire film thickness while minimizing the scattering from the Si substrate. Furthermore, a bare Si substrate was measured under identical conditions for reference. The 2D scattering patterns were radially averaged to produce one-dimensional scattering profiles using the SAXSGUI data
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reduction program.
RESULTS AND DISCUSSION
Self-organized P3HT-b-PEO BCP films were recently used as templates for ALD with
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selective deposition of the metal oxide, ZnO, in the PEO domains.[16] P3HT was selected because it tends to crystallize and because it serves as the active component in a variety of
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organic-electronic devices. PEO, on the other hand, is not opto-electronically active, but has an affinity to the ALD precursors of ZnO, diethyl zinc (DEZ) and water. The amphiphilic character of P3HT-b-PEO and the π-π stacking in the P3HT blocks drive the self-assembly into fibril-like morphologies with a central crystalline P3HT stack surrounded by flexible hydrophilic PEO segments.[19] [20] [21] [22] [23] [24] [25] [26] The type of solvent used during the BCP selfassembly determines the dimensions of the fibrils, i.e. the dimeter and length of the structures.[19] [25] [26, 27] For example, solvents good for PEO swell the PEO domains by extending the chains; while poor PEO solvents quench the PEO domains by shrinking the chains.[19] The P3HT stacks are less sensitive to the type of solvent due to the strong π-π
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interactions.[28] Here, we harness this solvent effect to tune the BCP template used for ALD and thereby manipulate the morphology of the final hybrid organic/inorganic film. To do so, we selected three solvents that are known to direct P3HT-b-PEO fibrils of different dimensions: Toluene - a good solvent for P3HT but a poor solvent for PEO; THF - a common solvent for
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both blocks; and a mixture of THF and methanol- where methanol is an anti-solvent for P3HT but a good solvent for PEO.
The solubility of the P3HT block in the different solvents is apparent from the optical
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absorption spectra of the BCP solutions, as shown Figure 1. The BCP toluene solution is translucent yellow with a broad single transition at 392.5 nm confirming full solubility of the P3HT block with no aggregation. The BCP THF solution is transparent with a brownish hue also
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showing the broad transition only, but very slightly red shifted compared to the toluene solution, possibly indicating some aggregation. The THF:MeOH solution is dark purple and the spectrum is significantly red shifted, peak position at 404 nm, with a noticeable vibronic manifold indicative of strong aggregation due to π-π stacking.
The three solutions were spun into BCP thin films and the absorption spectra of the films
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measured a also presented in Figure 1. In contrast to the solutions, all three pristine BCP films displayed a similar spectrum. The spectrum shows intra-molecular vibronic transitions at 518 and 552 nm, and a shoulder at 605 nm, indicating ordered semi-crystalline P3HT domains.[29] These results confirm that, as previously mentioned, the solvent has a negligible effect on the
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crystalline morphology of the P3HT block in the spun BCP films.[28] Two important points are noteworthy: 1) the BCP absorption spectra reflect the morphology of the P3HT block, so we can
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only speculate on the optically-silent PEO block; and 2) the optical density (OD) and spectra of the three BCP films are basically identical confirming the same thickness and amount of P3HT in all three films. This point is critical because the identical film thicknesses (obtained by optimizing the spinning conditions) allows quantitative analysis of ZnO uptake and film swelling.
The BCP films, deposited from the three different solvents, were introduced to the ALD system and exposed to sequences of DEZ and water. Figure 1 shows the optical absorption spectra of the films after 20, 40, 60 and 80 ALD cycles. The spectra of all films after the ALD
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processes show that the P3HT features, and hence the P3HT morphology, are maintained in the all ALD processes regardless of the processing solvent and number of cycles. The persistency of the P3HT crystalline morphology is also confirmed by complimentary GIXRD measurements (Figure S5). The GIXRD patterns of the BCP films before the ALD process show three peaks
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associated with the (100), (200) and (300) reflections of the crystalline P3HT block with the typical lamellar distance of 1.6 nm. [30] The positons of the (100) peak, extracted from Figure S5, are 5.41°, 5.41° and 5.40° for the toluene , THF and THF/MeOH processed films,
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persistency of the P3HT-block crystallinity in all films.
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respectively. All reflections are preserved after all ALD process, Figure S5, confirming the
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Figure 1: Absorbance spectra of P3HT-b-PEO BCP solutions (dashed lines) and films spun from toluene (black), THF (red) and THF/MeOH mixture (blue) before (continuous lines) and after exposure to 20 (full squares), 40 (empty squares), 60 (full circles) and 80 (empty circles) ALD
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cycles of DEZ and water. The green arrow indicates increasing the number of ALD cycles. The GIXRD patterns of the BCP films after the ALD processes also show three new reflections associated with the (001), (002) and (101) wurtzite ZnO, and confirm the formation of crystalline ZnO. The optical absorption spectra confirm the deposition of ZnO in all three films
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exposed to ALD, as evident from a new optical transition at ∼300 nm associated with the absorption of ZnO (Figure 1). Generally, the intensity of the ZnO peak increases in all films with the number of ALD cycles indicating that the amount of ZnO deposited depends on the number
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of ALD cycles. However, the rate of ZnO uptake, revealed by the evolution of the absorption peak with the number of ALD cycles, is not the same for all three films. Particularly, the THF:MeOH processed film shows the fastest and highest ZnO uptake, followed by the film processed from THF; and the lowest ZnO uptake is observed for the toluene-processed film. To study the correlation between ZnO mass accumulation and the solvent used for the film
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processing, we performed SEM EDS measurements. Importantly, the EDS measurement conditions guaranteed probing the full film thickness of each measurement (and not surface only). Therefore, the quantitative EDS analysis in not limited to the ZnO accumulation on the films surfaces, but rather determines the ZnO accumulation through the entire film thickness
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(See experimental section). As mentioned above, the absorption measurements confirm that the amount of P3HT is identical in all films. Hence, the Zn/S % weight ratio, evaluated from the
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EDS, represents the Zn mass within each film. The Zn/S % weight ratio as a function of solvent and number of ALD cycles is presented in Figure 2. The EDS analysis clearly shows that increasing the number of ALD cycles increases the amount of ZnO accumulation in all three films. Moreover, the EDS analysis shows that the ZnO accumulation depends on the solvent used for the BCP processing. The highest ZnO accumulation is in the BCP film processed form THF:MeOH (blue columns in Figure 2), followed by that processed from THF (red columns in Figure 2), and the least amount of ZnO is found in the BCP film processed from toluene (black columns in Figure 2).
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Figure 2: Relative Zn/S mass calculated by EDS measurements as a function of processing
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solvent and number of ALD cycles; toluene in black, THF in red and THF/MeOH mixture in blue. (The error bars reflect the statistical experimental error of each measurement and the standard deviation of different measurements on each film)
To follow the location, size, and distribution of the ALD ZnO domains within the three P3HTb-PEO films as a function of the processing solvent, we performed cross-section HRSEM imaging of the BCP films after 80 ALD cycles. In the BSE images, the dark regions are the
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organic phase, while the bright domains are ZnO. The cross section images of all three films, Figure 3 a-c, confirm that the ZnO is deposited within the film and not strictly on the film surfaces. Hence, although ALD is conventionally used for conformal coatings, here, the ZnO is
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deposited within the BCP films. Moreover, in all films the ZnO structures are nearly evenly distributed through the BCP films down to the substrates, and show spherical morphologies a few nm in diameter. Most of the ZnO particles are organized in pearl-necklace-like structures
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varying in length and in random orientations. Notably, despite the similar original BCP film thicknesses (100 nm), the thickness of the films after the ALD process strictly depends on the processing solvent. Namely, the ALD process causes film swelling, and the extent of swelling depends on the solvent used for processing the original BCP film. Figure 3a shows that the film processed from toluene is only slightly affected by the ALD, while the films processed from THF and THF:MeOH (Figure 3b and 3c, respectively) swell dramatically. Combining the optical absorption, EDS and HRSEM results indicates that the degree of swelling correlates with the
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amount of ZnO accumulated in the film. In other words, the ZnO accumulation is the reason for
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the film swelling.
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Figure 3: Cross-section BSE HRSEM micrographs of P3HT-b-PEO films processed from selected solvents after 80 ALD cycles of DEZ and water; (a) from toluene, (b) from THF, and (c)
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from THF/MeOH mixture (the scale bar is the same for all images).
The HESEM results are in agreement with the results showing that the absorption and crystallinity of the P3HT-block were not modified by the ZnO accumulation (Figure 1 and Figure S5), and confirm that the ZnO is accumulated exclusively within the PEO domains throughout the thickness of the film. Therefore, we suggest that the film swelling reflects the PEO volume available for ZnO uptake, directed by the processing solvent. More specifically,
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when the BCP film is processed from a good PEO solvent, the PEO chains are extended in the fibril “shell” and provide low density and expandable volume for ZnO uptake. Accumulation of ZnO in these domains results significant swelling of the film. In contrast, when a poor PEO
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solvent is used, the PEO chains are collapsed and entangled exposing a small and dense volume for ZnO uptake. Under such conditions, the uptake of ZnO is low and the films maintain their original thickness. Accordingly, when exposing films processed from THF or THF:MeOH to the
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ALD precursors, the precursors diffuse and deposit in the spacious extended PEO domain. This process continues during the ALD cycles accumulating ZnO, expanding the PEO domains, and considerably swelling the film. In contrast, the precursors’ diffusion and deposition in the PEO domains of films processed from toluene is limited because the PEO chains are collapsed and densely entangled. As a result, the ZnO uptake and film swelling are significantly lower. These results demonstrate tuning the template features by the processing solvent. To further study the effect of the selected solvent on the uptake of the ZnO phase, we performed HAADF STEM that provides high contrast and high spatial resolution, as shown in
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Figure 4. Notably, in the HAADF-STEM mode, the dark and bright regions represent the organic and ZnO domains, respectively. The HAADF-STEM micrographs show networks of fiber-like ZnO particles a few nm in thickness and extending over tens to hundreds of nanometers. This fiber-like morphology of the ZnO phase is in good agreement with the reported fibril
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morphology of P3HT-b-PEO films deposited from poor P3HT solvents.[25] Importantly, the low Mw of the P3HT block used in this study causes the π-π interactions to prevail over the phase separation and the fibril morphology is present in all films regardless of the solvent type. The HAADF STEM high-resolution and contrast imaging allow quantitative analysis of the
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dimensions of the organic-stack and inorganic “shell” of the fibrils (red and blue domains, respectively, in the schematic insets of Figure 4). Note that, as illustrated in Figure 4, the PEO
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blocks phase-separate to form amorphous domains on both sides of the central P3HT π-stack. When two fibers are packed close to each other, the PEO chains can overlap and the corresponding ZnO phase will represent the intermixed PEO domains.[31] It is also important to note that the fiber-like structure are also present inside the films (and not only on the top surface), evident from the cross-section HRSEM images (Figure 3) showing the ZnO particle assemblies that occupy the PEO domains in the film. The ZnO pearl-necklace-like assemblies
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form non-aligned worm-like structures inside the film, while those on the surface show some preferential alignment parallel to the substrate.
The TEM images were used to evaluate the width of the central P3HT stack and the surrounding PEO:ZnO domains for over 100 fibrils for each sample (for analysis details see the
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electronic supporting information). Figure 4 shows histograms representing the width of the organic (Figures 4d-f) and inorganic (Figure 4g-I) domains, and schematic illustrations of the
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corresponding nanostructures. The average width of the organic domains, corresponding to the dark regions in the HAADF-STEM micrographs, is ~8.5 ± 1.0 nm in all films regardless of the processing solvent (Figure 4d-f). This value is in good agreement with the length of the P3HT block, n=23, measured by NMR and GPC.[16] Assuming a unit length of 0.385 nm,[32] the calculated P3HT stack width is 8.85 nm. This result suggests that in all films the central block of the fibrils is composed of extended P3HT chains. Indeed, this result confirms the absorption and GIXRD results showing that the P3HT stack is crystalline in all films regardless of the type of solvent used for BCP film processing and remains crystalline regardless of the ALD cycle number.
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In contrast to the P3HT domains, the width of the ZnO (bright) domains strongly depends on the solvent used for the BCP film processing and are 14.5 ± 2.0 nm, 16.5 ± 4.0 nm, and 18.5 ± 4.5 nm for the toluene, THF and THF/MeOH films, respectively (Figures 4g-i). Due to the selective deposition of ZnO exclusively in the PEO domains, the ZnO domain width corresponds
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directly to the morphology of the PEO chains in the BCP film prior to the ALD process. This analysis corroborates our hypothesis that the solubility of the PEO block in the processing solvent can direct the ZnO accumulation in the film. Namely, when processed from a good PEO solvent, the PEO chains are extended allowing high ZnO accumulation and swelling of the PEO
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domains. In contrast, poor PEO solvents collapse the PEO chains limiting the accumulation and swelling of the PEO domains.
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In addition to the fiber’s organic and inorganic dimensions, the HAADF-STEM micrographs also reveal that the processing solvent has an effect on the orientation and order of the fibers. Figure 4a shows that processing from toluene, a poor PEO but good P3HT solvent, leads to µmlong fibrils packed in elongated and locally parallel bundles. Figure 4b, on the other hand, shows that processing from THF, a good solvent for both blocks, leads to curved, entangled worm-like fibrillar morphologies. Finally, adding methanol, a poor solvent for P3HT but good solvent for
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PEO, to THF, leads to growth of some extended (tens of µm long) non-ordered or aligned fibrils, as presented in Figure 4c. The correlation between the processing solvent, the fiber dimensions and film morphology is confirmed by 2D GISAXS scattering measurements and extracted 1D profiles (electronic supporting information section Figures S6 and S7, respectively). The
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GISAXS pattern and profile are associated with the long-range order in the film, and not only the surface. The 2D GISAXS pattern and 1D scattering profile of the toluene-processed hybrid film
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(Figure S6a and Figure S7a, respectively) show a noticeable sharp peak evident of long-range periodicity throughout the film. The peak position at q~0.042 [1/Å], d=14.9 [nm], is in good agreement with the HAADF-STEM calculated ZnO domain size histogram of ~15 nm indicating presence of packed fibrils through the film. The signals are significantly weaker in the patterns and profiles of the hybrid film processed from THF (Figures S6b and S7b), and barely noticed in those of THF:MeOH (Figures S6c and S7c). The gradual decrease in peak intensity with the increase of PEO solubility is also in good agreement with the HAADF-STEM images showing a more dispersed fibrillar structure in the film processed from THF, and disorganized fibrillar array THF:MeOH.
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These results confirm that the solvent choice clearly has an effect on the BCP fibrillar morphology that is then translated to the hybrid film morphology. Because the solvent does not affect the P3HT packing, the morphology, like the fiber dimensions, reflects the solvent effect on the PEO blocks. For example, the poor solubility of PEO in toluene leads to collapsed thin PEO
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domains that induce parallel alignment (Figure 4a). A similar alignment-effect was demonstrated for self-assembled fibrils of P3HT with other coil blocks.[23] The study showed that BCPs with non-bulky coil blocks induce densely packed, locally parallel fibril morphology; while fibrils from P3HT BCPs with bulky coil blocks lack the long-range parallel ordering. THF, on the other
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hand, is a good solvent for both blocks. It was recently reported that solvent casting P3HT-bPEO from chloroform, also a good solvent for both blocks, lead to a disordered wormlike pattern
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typical of two-phase separated nanostructures.[26] Indeed, the film processed in this study from THF also resulted in disordered, worm-like fibrillary morphology (Figure 4b). The very long fibrils obtained from the THF:MeOH solvent mixture, Figure 4c, are in agreement with the reported long P3HT-b-P2VP fiber-like micelles formed in THF:alcohol mixtures.[33] [34] It was suggested there that the slow removal of the common solvent, THF, induces the long fibril formation. We speculate that in the THF:MeOH mixture used in our study, the P3HT block
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aggregate due to the MeOH, (evident form the absorption spectrum) and act as seed micelles for the growth of the long fibrils during the THF evaporation. However, since THF is in excess, 80%, some of the fibers still attain the THF-directed worm-like fibrillar morphology seen in Figure 4b. The worm-like fibers fill up the space between the elongated seeded-growth fibers, to
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obtain the morphology observed in Figure 4c.
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Figure 4: HAADF-STEM images and schematic illustrations (a-c), and dimension histograms (inorganic domains d-f; and organic domains g-i) in P3HT-b-PEO films processed from the different solvents after 80 ALD cycles of DEZ and water: (a, d, g) from toluene, (b, e, h) from THF, and (c, f, i) from THF/MeOH mixture.
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CONCLUSIONS
In summary, in this study we demonstrated that the morphology of functional hybrid films can be tuned by solvent-controlling the morphology of BCP films that serve as templates for metal
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oxide ALD. We used selected solvents to tune the morphology of an amphiphilic conjugated rodcoil BCP, P3HT-b-PEO, that is known to self-assemble into fibril morphologies. The central P3HT block was short so that the π-π interactions prevail over the block-solvent interactions so that the dimensions and morphology of the fibril P3HT-domains are identical in all fibrils
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regardless of processing solvents. The PEO block, on the other hand, and hence the fibril “shell”, is highly sensitive to the selected solvents. Therefore, processing the BCP films from poor PEO
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solvents resulted in fibrils with thin PEO shells; while good PEO solvents allowed extended PEO chains and wider domains. The self-assembled BCP films were used as templates for ALD of ZnO. The crystallinity of the P3HT block and the affinity of the PEO block to the ZnO precursors lead to selective deposition of ZnO in the PEO domains. Hence, the solventcontrolled morphology of the PEO domains is translated to the size, distribution and morphology of the deposited ZnO. Namely, the ZnO follows the PEO domain template. For example, films
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with swollen PEO domains show higher ZnO uptake compared to films with collapsed PEO chains; oriented PEO domains template oriented ZnO domains and wormlike morphologies direct wormlike ZnO nanostructures. Therefore, judicious selection of the solvent is a toll to manipulate the BCP self-assembly and hence the ALD template. This approach could be useful
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for nanoscale engineering of hybrid materials, with an eye towards creating new inorganic-
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organic heterostructures for various applications.
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Acknowledgements
This research was partially supported by the Israeli Nanotechnology Focal Technology Area project on “Nanophotonics and Detection.”
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Electronic Supporting Information
Data supporting this study are provided as supplementary information accompanying this paper including: Instrumentation used for the polymers characterization; NMR, DSC and GPC. Synthesis procedures. Characterization of BCP by DSC and GPC. GIXRD patterns of P3HT-b-
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PEO films before and after ALD cycles. HAADF-STEM analysis clarification. 2D GISAXS
Author Information
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scattering patterns and extracted 1D line-cut profiles of the hybrid films.
Moshe Moshonov. Email: [email protected] Yaron Kauffmann. Email: [email protected] Gitti L. Frey. Email: [email protected]
4. 5. 6. 7. 8. 9. 10. 11.
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Highlights: •
P3HT-b-PEO block copolymer (BCP) films are used as templates for atomic layer deposition (ALD) of ZnO The ZnO is deposited selectively in the PEO domains
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The type of solvent used for processing the BCP film directs the size and
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orientation of the PEO domains and hence the morphology of the hybrid film.
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S0032-3861(16)30949-1
DOI:
10.1016/j.polymer.2016.10.036
Reference:
JPOL 19136
To appear in:
Polymer
Received Date: 22 August 2016 Revised Date:
13 October 2016
Accepted Date: 15 October 2016
Please cite this article as: Moshonov M, Kauffmann Y, Frey GL, Self-assembled block copolymer templates for atomic layer deposition: The effect of processing solvent, Polymer (2016), doi: 10.1016/ j.polymer.2016.10.036. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Self-assembled block copolymer templates for
solvent
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atomic layer deposition: the effect of processing Moshe Moshonov, Yaron Kauffmann and Gitti L. Frey*
Haifa, 32000 Israel.
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ABSTRACT
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Department of Materials Science and Engineering, Technion, Israel Institute of Technology,
In this study thin films of a rod-coil block copolymer (BCP) are deposited from different solvents and used as templates for atomic layer deposition (ALD) of ZnO. The BCP, P3HT-bPEO, is known to self-organize into fiber-like phase-separated nanostructures, with central crystalline P3HT domains and outer amorphous PEO domains. The P3HT block in this study is
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short so that the size of the P3HT domains in the films is solvent insensitive. However, processing from poor PEO solvents results in thin PEO domains and good PEO solvents allow extended PEO chains and wide domains. The crystallinity of the P3HT-block and the affinity of the PEO-block to the ALD ZnO precursors lead to selective deposition of ZnO in the PEO
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domains. Therefore, the solvent-controlled morphology of the PEO domains in the BCP films can be translated to the size, distribution and morphology of the deposited ZnO. We show that
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films with swollen PEO domains have higher ZnO uptake compared to films with collapsed PEO chains. Furthermore, oriented PEO domains template oriented ZnO domains and wormlike morphologies direct wormlike ZnO nanostructures. Therefore, solvent selection is a toll to manipulate the BCP self-assembly and hence the ALD template and the morphology of the hybrid organic/inorganic film.
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INTRODUCTION A versatile approach for the synthesis of nanostructured hybrid organic/inorganic thin films is by depositing the inorganic phase into pre-formed, self-assembled block copolymer (BCP) templates.[1] [2] [3] [4] The BCP template morphologies, i.e domain size, shape, periodicity, and
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arrangement, are tunable through the BCP molecular weight and chemical composition, and the selection of processing solvents and temperature. Generally, the BCP possess a sacrificial block that is removed after the self-assembly, followed by deposition of the inorganic phase into the template voids.[5] [6] [7] [8] [9]. A useful technique to introduce the inorganic phase is low
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temperature atomic layer deposition (ALD).[10, 11] In this case, the gaseous ALD precursors diffuse into the porous organic template and deposit on the void’s surfaces. The hybrid film
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could be further manipulated by burning out the organic template and back filling the (now) inorganic porous nanostructure with organic or inorganic materials. This procedure was recently demonstrated for fabrication of conjugated polymer/ZnO continuous organic/inorganic networks for hybrid photovoltaic devices. [7]
BCPs with no sacrificial block are also suitable as ALD templates when one polymer block
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allows the diffusion of the ALD precursors into it and the second block resists it. In this case, the inorganic phase is not deposited on void surfaces, but rather inside one of the BCP blocks.[6] The diffusion and retention of the ALD precursors in a polymer block strongly depend on its chemical structure, the film morphology and the deposition conditions.[6, 8, 12] Polymer blocks
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with affinity to the precursors will allow their diffusion into the film, while blocks with no affinity to the precursors will resist the diffusion. Similarly, amorphous domains support precursor diffusion, while crystalline domains hinder it.[13] [14, 15] Accordingly, in a previous
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study we used films of poly(3-hexylthiophene)23-b-poly(ethylene oxide)23, P3HT-b-PEO BCP, as templates for ALD of ZnO. This P3HT-b-PEO phase separates and self organizes into fibril-like structures of separated P3HT and PEO domains. We showed that the affinity of the PEO-block to the ZnO precursors, in combination with the crystallinity of the P3HT block, lead to the exclusive growth of ZnO in the PEO domains.[16] Here, we show that the processing solvents can be used to tune the size and distribution of the PEO blocks in the film and directly manipulate the amount and location of ZnO in the film. Using optical absorption, Electron Microscopy and X-ray techniques, we show that this BCP self organizes into continues films of
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fibrils regardless of the type of processing solvent. However, poor PEO solvents result in fibrils with thin PEO shells, while good PEO solvents allow extended PEO chains and wide domains. All BCP films can be used as ALD templates with selective deposition of the inorganic phase in the PEO domains. Films with swollen PEO domains show higher ZnO uptake compared to films
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with collapsed PEO chains; oriented PEO domains template oriented ZnO domains and wormlike morphologies direct wormlike ZnO nanostructures. Therefore, the solvent-controlled morphology of the PEO domains is translated to the size, distribution and morphology of the
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ZnO deposited by ALD.
EXPERIMENTAL
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Materials
Methanol, Acetone, Chloroform and Isopropanol, purchased from AR BioLab Israel, were used as received. Toluene anhydrous 99.8%; THF inhibitor free anhydrous 99.9%; Methanol anhydrous 99.8%; hexane anhydrous 95%; Poly(ethylene glycol) methyl ether azide with PEG average Mn 1000 gr/mole; 2,5-dibromo-3-hexylthiophene (DBHT) 97%; tert-Butylmagnesium chloride solution 2.0 M in diethyl ether; 1,3 Bis(diphenylphosphino)propane]dichloronickel(II)
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[Ni(dppp)Cl2] and Ethynylmagnesium bromide solution 0.5 M in THF, were all purchased from Sigma-Aldrich and used as received. Copper(I) Iodide anhydrous, 99.995% trace metals basis, purchased from Sigma-Aldrich, was purified by the dissolution–precipitation process followed by drying under vacuum, providing a colorless sample.
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Synthesis and film processing
All chemical reactions were conducted under inert conditions. The polymers were synthesized
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according to the literature[17] with these synthesis procedures and characterization presented in the electronic supporting information section: ethynyl-terminated regioregular poly(3hexylthiophene), P3HT GRIM polymerization procedure, and the poly(3-hexylthiophene)23-bpoly(ethylene oxide)23 (P3HT23-b-PEO23) click reaction procedure, (Figure S1 and S2 - 1H NMR spectra; Figure S3 - DSC thermogram; Figure S4 – GPC characterization). Films were prepared by dissolving the synthesized P3HT-b-PEO (20 mg/ml) in a dry solvent. Three solvent systems were separately used; toluene, THF and a solvent mixture of THF/MeOH at an 80/20 % volume ratio. The solutions were stirred for ~10 h at 60°C under N2 atmosphere. Films, 100 nm thick, were spun onto glass, silicon or quarts substrates under N2 atmosphere
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(glove-box conditions). The spun films were then left in a covered glass petri dish for 1 hour. It is important to note that the spinning conditions from the different solvents were optimized based on the absorption spectra to obtain the same thickness for all films. ALD of ZnO into the organic films was performed using an MVD100E Applied MST system
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with an integrated oxygen plasma module. Deposition temperature was set to 60°C and alternating 1 torr pulses of diethy zinc (DEZ) and water were applied. The precursor reaction time was limited to 1 sec for both DEZ and water in every cycle. The reaction chamber was purged with nitrogen between different precursor pulse injections. ALD processes were
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performed with 20, 40, 60, or 80 cycles. Film Characterization
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The absorption spectra of the films were measured on quartz substrates using a Varian Cary 100 Scan UV-vis spectrophotometer in the 250-800 nm range.
Film thickness was measured using a Daktak XT Entry profilometer. It is important to note that the spinning conditions from the different solvents were optimized based on the absorption spectra to obtain the same thickness for all films.
Energy dispersive spectrometer (EDS) was used to analyze the composition of the sample.
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Measurements were conducted at a 5 KeV operating voltage and at a working distances of ~2mm. The Zn/S wt% ratio was evaluated from the EDS Kα signal of sulfur and 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. Furthermore, all studied films showed
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identical optical density confirming that the film thickness and amount of P3HT is identical in all films. Under such conditions, the total mass of sulfur is constant in all films and the measured
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Zn/S ratio is proportional to the total mass of ZnO in each film. At least two samples of each type were measured and averaged on four different locations. High-resolution scanning electron microscopy (HRSEM) cross section images of films on silicon substrates were obtained using the Zeiss Ultra plus high resolution scanning electron microscope (HRSEM), equipped with a Schottky field emission source, operating at 2 KeV. The images were acquired using both secondary electrons (in-lens detector) and backscattered electrons (in-lens energy selected detector), at a relatively low accelerating voltage of 1.5-2 kV at working distances of ~2mm.
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High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) surface and cross section measurements were performed on a FEI Titan 80-300 KeV S/TEM operating at 200 keV. Samples for the cross section measurements were thinned by a focused ion beam (FIB) integrated in the Strata 400 STEM DualBeam system of a field-emission scanning
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electron microscope (FE-SEM). At least two samples of each type of film were prepared and measured.
Grazing Incident Small Angle X-ray Scattering (GISAXS) measurements were performed using a Pilatus 300K detector and a Xenocs GeniX Low Divergence Cu Kα radiation, (Dectris,
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Baden, Switzerland) with scatterless slits. For this set-up, the sample−detector distance was 2.48 m with the wavelength tuned to 1.54 Å, yielding a measurement q range of 0.006−0.15 Å−1.[18]
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Each spectra was constructed of consecutive multiple frames acquired over 1 at an incident angle of about 0.16 degree, which is above the critical angle of the hybrid thin film and below the critical angle of the substrate. These conditions ensure that the X-ray beam penetrates the entire film thickness while minimizing the scattering from the Si substrate. Furthermore, a bare Si substrate was measured under identical conditions for reference. The 2D scattering patterns were radially averaged to produce one-dimensional scattering profiles using the SAXSGUI data
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reduction program.
RESULTS AND DISCUSSION
Self-organized P3HT-b-PEO BCP films were recently used as templates for ALD with
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selective deposition of the metal oxide, ZnO, in the PEO domains.[16] P3HT was selected because it tends to crystallize and because it serves as the active component in a variety of
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organic-electronic devices. PEO, on the other hand, is not opto-electronically active, but has an affinity to the ALD precursors of ZnO, diethyl zinc (DEZ) and water. The amphiphilic character of P3HT-b-PEO and the π-π stacking in the P3HT blocks drive the self-assembly into fibril-like morphologies with a central crystalline P3HT stack surrounded by flexible hydrophilic PEO segments.[19] [20] [21] [22] [23] [24] [25] [26] The type of solvent used during the BCP selfassembly determines the dimensions of the fibrils, i.e. the dimeter and length of the structures.[19] [25] [26, 27] For example, solvents good for PEO swell the PEO domains by extending the chains; while poor PEO solvents quench the PEO domains by shrinking the chains.[19] The P3HT stacks are less sensitive to the type of solvent due to the strong π-π
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interactions.[28] Here, we harness this solvent effect to tune the BCP template used for ALD and thereby manipulate the morphology of the final hybrid organic/inorganic film. To do so, we selected three solvents that are known to direct P3HT-b-PEO fibrils of different dimensions: Toluene - a good solvent for P3HT but a poor solvent for PEO; THF - a common solvent for
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both blocks; and a mixture of THF and methanol- where methanol is an anti-solvent for P3HT but a good solvent for PEO.
The solubility of the P3HT block in the different solvents is apparent from the optical
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absorption spectra of the BCP solutions, as shown Figure 1. The BCP toluene solution is translucent yellow with a broad single transition at 392.5 nm confirming full solubility of the P3HT block with no aggregation. The BCP THF solution is transparent with a brownish hue also
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showing the broad transition only, but very slightly red shifted compared to the toluene solution, possibly indicating some aggregation. The THF:MeOH solution is dark purple and the spectrum is significantly red shifted, peak position at 404 nm, with a noticeable vibronic manifold indicative of strong aggregation due to π-π stacking.
The three solutions were spun into BCP thin films and the absorption spectra of the films
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measured a also presented in Figure 1. In contrast to the solutions, all three pristine BCP films displayed a similar spectrum. The spectrum shows intra-molecular vibronic transitions at 518 and 552 nm, and a shoulder at 605 nm, indicating ordered semi-crystalline P3HT domains.[29] These results confirm that, as previously mentioned, the solvent has a negligible effect on the
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crystalline morphology of the P3HT block in the spun BCP films.[28] Two important points are noteworthy: 1) the BCP absorption spectra reflect the morphology of the P3HT block, so we can
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only speculate on the optically-silent PEO block; and 2) the optical density (OD) and spectra of the three BCP films are basically identical confirming the same thickness and amount of P3HT in all three films. This point is critical because the identical film thicknesses (obtained by optimizing the spinning conditions) allows quantitative analysis of ZnO uptake and film swelling.
The BCP films, deposited from the three different solvents, were introduced to the ALD system and exposed to sequences of DEZ and water. Figure 1 shows the optical absorption spectra of the films after 20, 40, 60 and 80 ALD cycles. The spectra of all films after the ALD
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processes show that the P3HT features, and hence the P3HT morphology, are maintained in the all ALD processes regardless of the processing solvent and number of cycles. The persistency of the P3HT crystalline morphology is also confirmed by complimentary GIXRD measurements (Figure S5). The GIXRD patterns of the BCP films before the ALD process show three peaks
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associated with the (100), (200) and (300) reflections of the crystalline P3HT block with the typical lamellar distance of 1.6 nm. [30] The positons of the (100) peak, extracted from Figure S5, are 5.41°, 5.41° and 5.40° for the toluene , THF and THF/MeOH processed films,
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persistency of the P3HT-block crystallinity in all films.
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respectively. All reflections are preserved after all ALD process, Figure S5, confirming the
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Figure 1: Absorbance spectra of P3HT-b-PEO BCP solutions (dashed lines) and films spun from toluene (black), THF (red) and THF/MeOH mixture (blue) before (continuous lines) and after exposure to 20 (full squares), 40 (empty squares), 60 (full circles) and 80 (empty circles) ALD
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cycles of DEZ and water. The green arrow indicates increasing the number of ALD cycles. The GIXRD patterns of the BCP films after the ALD processes also show three new reflections associated with the (001), (002) and (101) wurtzite ZnO, and confirm the formation of crystalline ZnO. The optical absorption spectra confirm the deposition of ZnO in all three films
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exposed to ALD, as evident from a new optical transition at ∼300 nm associated with the absorption of ZnO (Figure 1). Generally, the intensity of the ZnO peak increases in all films with the number of ALD cycles indicating that the amount of ZnO deposited depends on the number
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of ALD cycles. However, the rate of ZnO uptake, revealed by the evolution of the absorption peak with the number of ALD cycles, is not the same for all three films. Particularly, the THF:MeOH processed film shows the fastest and highest ZnO uptake, followed by the film processed from THF; and the lowest ZnO uptake is observed for the toluene-processed film. To study the correlation between ZnO mass accumulation and the solvent used for the film
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processing, we performed SEM EDS measurements. Importantly, the EDS measurement conditions guaranteed probing the full film thickness of each measurement (and not surface only). Therefore, the quantitative EDS analysis in not limited to the ZnO accumulation on the films surfaces, but rather determines the ZnO accumulation through the entire film thickness
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(See experimental section). As mentioned above, the absorption measurements confirm that the amount of P3HT is identical in all films. Hence, the Zn/S % weight ratio, evaluated from the
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EDS, represents the Zn mass within each film. The Zn/S % weight ratio as a function of solvent and number of ALD cycles is presented in Figure 2. The EDS analysis clearly shows that increasing the number of ALD cycles increases the amount of ZnO accumulation in all three films. Moreover, the EDS analysis shows that the ZnO accumulation depends on the solvent used for the BCP processing. The highest ZnO accumulation is in the BCP film processed form THF:MeOH (blue columns in Figure 2), followed by that processed from THF (red columns in Figure 2), and the least amount of ZnO is found in the BCP film processed from toluene (black columns in Figure 2).
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Figure 2: Relative Zn/S mass calculated by EDS measurements as a function of processing
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solvent and number of ALD cycles; toluene in black, THF in red and THF/MeOH mixture in blue. (The error bars reflect the statistical experimental error of each measurement and the standard deviation of different measurements on each film)
To follow the location, size, and distribution of the ALD ZnO domains within the three P3HTb-PEO films as a function of the processing solvent, we performed cross-section HRSEM imaging of the BCP films after 80 ALD cycles. In the BSE images, the dark regions are the
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organic phase, while the bright domains are ZnO. The cross section images of all three films, Figure 3 a-c, confirm that the ZnO is deposited within the film and not strictly on the film surfaces. Hence, although ALD is conventionally used for conformal coatings, here, the ZnO is
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deposited within the BCP films. Moreover, in all films the ZnO structures are nearly evenly distributed through the BCP films down to the substrates, and show spherical morphologies a few nm in diameter. Most of the ZnO particles are organized in pearl-necklace-like structures
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varying in length and in random orientations. Notably, despite the similar original BCP film thicknesses (100 nm), the thickness of the films after the ALD process strictly depends on the processing solvent. Namely, the ALD process causes film swelling, and the extent of swelling depends on the solvent used for processing the original BCP film. Figure 3a shows that the film processed from toluene is only slightly affected by the ALD, while the films processed from THF and THF:MeOH (Figure 3b and 3c, respectively) swell dramatically. Combining the optical absorption, EDS and HRSEM results indicates that the degree of swelling correlates with the
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amount of ZnO accumulated in the film. In other words, the ZnO accumulation is the reason for
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the film swelling.
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Figure 3: Cross-section BSE HRSEM micrographs of P3HT-b-PEO films processed from selected solvents after 80 ALD cycles of DEZ and water; (a) from toluene, (b) from THF, and (c)
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from THF/MeOH mixture (the scale bar is the same for all images).
The HESEM results are in agreement with the results showing that the absorption and crystallinity of the P3HT-block were not modified by the ZnO accumulation (Figure 1 and Figure S5), and confirm that the ZnO is accumulated exclusively within the PEO domains throughout the thickness of the film. Therefore, we suggest that the film swelling reflects the PEO volume available for ZnO uptake, directed by the processing solvent. More specifically,
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when the BCP film is processed from a good PEO solvent, the PEO chains are extended in the fibril “shell” and provide low density and expandable volume for ZnO uptake. Accumulation of ZnO in these domains results significant swelling of the film. In contrast, when a poor PEO
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solvent is used, the PEO chains are collapsed and entangled exposing a small and dense volume for ZnO uptake. Under such conditions, the uptake of ZnO is low and the films maintain their original thickness. Accordingly, when exposing films processed from THF or THF:MeOH to the
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ALD precursors, the precursors diffuse and deposit in the spacious extended PEO domain. This process continues during the ALD cycles accumulating ZnO, expanding the PEO domains, and considerably swelling the film. In contrast, the precursors’ diffusion and deposition in the PEO domains of films processed from toluene is limited because the PEO chains are collapsed and densely entangled. As a result, the ZnO uptake and film swelling are significantly lower. These results demonstrate tuning the template features by the processing solvent. To further study the effect of the selected solvent on the uptake of the ZnO phase, we performed HAADF STEM that provides high contrast and high spatial resolution, as shown in
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Figure 4. Notably, in the HAADF-STEM mode, the dark and bright regions represent the organic and ZnO domains, respectively. The HAADF-STEM micrographs show networks of fiber-like ZnO particles a few nm in thickness and extending over tens to hundreds of nanometers. This fiber-like morphology of the ZnO phase is in good agreement with the reported fibril
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morphology of P3HT-b-PEO films deposited from poor P3HT solvents.[25] Importantly, the low Mw of the P3HT block used in this study causes the π-π interactions to prevail over the phase separation and the fibril morphology is present in all films regardless of the solvent type. The HAADF STEM high-resolution and contrast imaging allow quantitative analysis of the
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dimensions of the organic-stack and inorganic “shell” of the fibrils (red and blue domains, respectively, in the schematic insets of Figure 4). Note that, as illustrated in Figure 4, the PEO
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blocks phase-separate to form amorphous domains on both sides of the central P3HT π-stack. When two fibers are packed close to each other, the PEO chains can overlap and the corresponding ZnO phase will represent the intermixed PEO domains.[31] It is also important to note that the fiber-like structure are also present inside the films (and not only on the top surface), evident from the cross-section HRSEM images (Figure 3) showing the ZnO particle assemblies that occupy the PEO domains in the film. The ZnO pearl-necklace-like assemblies
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form non-aligned worm-like structures inside the film, while those on the surface show some preferential alignment parallel to the substrate.
The TEM images were used to evaluate the width of the central P3HT stack and the surrounding PEO:ZnO domains for over 100 fibrils for each sample (for analysis details see the
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electronic supporting information). Figure 4 shows histograms representing the width of the organic (Figures 4d-f) and inorganic (Figure 4g-I) domains, and schematic illustrations of the
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corresponding nanostructures. The average width of the organic domains, corresponding to the dark regions in the HAADF-STEM micrographs, is ~8.5 ± 1.0 nm in all films regardless of the processing solvent (Figure 4d-f). This value is in good agreement with the length of the P3HT block, n=23, measured by NMR and GPC.[16] Assuming a unit length of 0.385 nm,[32] the calculated P3HT stack width is 8.85 nm. This result suggests that in all films the central block of the fibrils is composed of extended P3HT chains. Indeed, this result confirms the absorption and GIXRD results showing that the P3HT stack is crystalline in all films regardless of the type of solvent used for BCP film processing and remains crystalline regardless of the ALD cycle number.
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In contrast to the P3HT domains, the width of the ZnO (bright) domains strongly depends on the solvent used for the BCP film processing and are 14.5 ± 2.0 nm, 16.5 ± 4.0 nm, and 18.5 ± 4.5 nm for the toluene, THF and THF/MeOH films, respectively (Figures 4g-i). Due to the selective deposition of ZnO exclusively in the PEO domains, the ZnO domain width corresponds
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directly to the morphology of the PEO chains in the BCP film prior to the ALD process. This analysis corroborates our hypothesis that the solubility of the PEO block in the processing solvent can direct the ZnO accumulation in the film. Namely, when processed from a good PEO solvent, the PEO chains are extended allowing high ZnO accumulation and swelling of the PEO
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domains. In contrast, poor PEO solvents collapse the PEO chains limiting the accumulation and swelling of the PEO domains.
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In addition to the fiber’s organic and inorganic dimensions, the HAADF-STEM micrographs also reveal that the processing solvent has an effect on the orientation and order of the fibers. Figure 4a shows that processing from toluene, a poor PEO but good P3HT solvent, leads to µmlong fibrils packed in elongated and locally parallel bundles. Figure 4b, on the other hand, shows that processing from THF, a good solvent for both blocks, leads to curved, entangled worm-like fibrillar morphologies. Finally, adding methanol, a poor solvent for P3HT but good solvent for
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PEO, to THF, leads to growth of some extended (tens of µm long) non-ordered or aligned fibrils, as presented in Figure 4c. The correlation between the processing solvent, the fiber dimensions and film morphology is confirmed by 2D GISAXS scattering measurements and extracted 1D profiles (electronic supporting information section Figures S6 and S7, respectively). The
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GISAXS pattern and profile are associated with the long-range order in the film, and not only the surface. The 2D GISAXS pattern and 1D scattering profile of the toluene-processed hybrid film
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(Figure S6a and Figure S7a, respectively) show a noticeable sharp peak evident of long-range periodicity throughout the film. The peak position at q~0.042 [1/Å], d=14.9 [nm], is in good agreement with the HAADF-STEM calculated ZnO domain size histogram of ~15 nm indicating presence of packed fibrils through the film. The signals are significantly weaker in the patterns and profiles of the hybrid film processed from THF (Figures S6b and S7b), and barely noticed in those of THF:MeOH (Figures S6c and S7c). The gradual decrease in peak intensity with the increase of PEO solubility is also in good agreement with the HAADF-STEM images showing a more dispersed fibrillar structure in the film processed from THF, and disorganized fibrillar array THF:MeOH.
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These results confirm that the solvent choice clearly has an effect on the BCP fibrillar morphology that is then translated to the hybrid film morphology. Because the solvent does not affect the P3HT packing, the morphology, like the fiber dimensions, reflects the solvent effect on the PEO blocks. For example, the poor solubility of PEO in toluene leads to collapsed thin PEO
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domains that induce parallel alignment (Figure 4a). A similar alignment-effect was demonstrated for self-assembled fibrils of P3HT with other coil blocks.[23] The study showed that BCPs with non-bulky coil blocks induce densely packed, locally parallel fibril morphology; while fibrils from P3HT BCPs with bulky coil blocks lack the long-range parallel ordering. THF, on the other
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hand, is a good solvent for both blocks. It was recently reported that solvent casting P3HT-bPEO from chloroform, also a good solvent for both blocks, lead to a disordered wormlike pattern
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typical of two-phase separated nanostructures.[26] Indeed, the film processed in this study from THF also resulted in disordered, worm-like fibrillary morphology (Figure 4b). The very long fibrils obtained from the THF:MeOH solvent mixture, Figure 4c, are in agreement with the reported long P3HT-b-P2VP fiber-like micelles formed in THF:alcohol mixtures.[33] [34] It was suggested there that the slow removal of the common solvent, THF, induces the long fibril formation. We speculate that in the THF:MeOH mixture used in our study, the P3HT block
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aggregate due to the MeOH, (evident form the absorption spectrum) and act as seed micelles for the growth of the long fibrils during the THF evaporation. However, since THF is in excess, 80%, some of the fibers still attain the THF-directed worm-like fibrillar morphology seen in Figure 4b. The worm-like fibers fill up the space between the elongated seeded-growth fibers, to
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obtain the morphology observed in Figure 4c.
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Figure 4: HAADF-STEM images and schematic illustrations (a-c), and dimension histograms (inorganic domains d-f; and organic domains g-i) in P3HT-b-PEO films processed from the different solvents after 80 ALD cycles of DEZ and water: (a, d, g) from toluene, (b, e, h) from THF, and (c, f, i) from THF/MeOH mixture.
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CONCLUSIONS
In summary, in this study we demonstrated that the morphology of functional hybrid films can be tuned by solvent-controlling the morphology of BCP films that serve as templates for metal
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oxide ALD. We used selected solvents to tune the morphology of an amphiphilic conjugated rodcoil BCP, P3HT-b-PEO, that is known to self-assemble into fibril morphologies. The central P3HT block was short so that the π-π interactions prevail over the block-solvent interactions so that the dimensions and morphology of the fibril P3HT-domains are identical in all fibrils
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regardless of processing solvents. The PEO block, on the other hand, and hence the fibril “shell”, is highly sensitive to the selected solvents. Therefore, processing the BCP films from poor PEO
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solvents resulted in fibrils with thin PEO shells; while good PEO solvents allowed extended PEO chains and wider domains. The self-assembled BCP films were used as templates for ALD of ZnO. The crystallinity of the P3HT block and the affinity of the PEO block to the ZnO precursors lead to selective deposition of ZnO in the PEO domains. Hence, the solventcontrolled morphology of the PEO domains is translated to the size, distribution and morphology of the deposited ZnO. Namely, the ZnO follows the PEO domain template. For example, films
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with swollen PEO domains show higher ZnO uptake compared to films with collapsed PEO chains; oriented PEO domains template oriented ZnO domains and wormlike morphologies direct wormlike ZnO nanostructures. Therefore, judicious selection of the solvent is a toll to manipulate the BCP self-assembly and hence the ALD template. This approach could be useful
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for nanoscale engineering of hybrid materials, with an eye towards creating new inorganic-
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organic heterostructures for various applications.
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Acknowledgements
This research was partially supported by the Israeli Nanotechnology Focal Technology Area project on “Nanophotonics and Detection.”
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Electronic Supporting Information
Data supporting this study are provided as supplementary information accompanying this paper including: Instrumentation used for the polymers characterization; NMR, DSC and GPC. Synthesis procedures. Characterization of BCP by DSC and GPC. GIXRD patterns of P3HT-b-
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PEO films before and after ALD cycles. HAADF-STEM analysis clarification. 2D GISAXS
Author Information
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scattering patterns and extracted 1D line-cut profiles of the hybrid films.
Moshe Moshonov. Email: [email protected] Yaron Kauffmann. Email: [email protected] Gitti L. Frey. Email: [email protected]
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Highlights: •
P3HT-b-PEO block copolymer (BCP) films are used as templates for atomic layer deposition (ALD) of ZnO The ZnO is deposited selectively in the PEO domains
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The type of solvent used for processing the BCP film directs the size and
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•
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orientation of the PEO domains and hence the morphology of the hybrid film.
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