Effect of Nanostructured Domains in Self-Assembled Block Copolymer

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Effect of Nanostructured Domains in Self-Assembled Block Copolymer Films on Sequential Infiltration Synthesis Qing Peng, Yu-Chih Tseng, Yun Long, Anil U. Mane, Shane DiDona, Seth B Darling, and Jeffrey W. Elam Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02922 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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Effect of Nanostructured Domains in Self-Assembled Block Copolymer Films on Sequential Infiltration Synthesis Qing Peng,a§* Yu-Chih Tseng,b† Yun Long,c Anil U. Mane,a Shane DiDona,d Seth B. Darling,be* and Jeffrey W. Elama* a

Energy Systems Division and bCenter for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass. Ave., Argonne, IL, 60439, USA; cDepartment of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576; d Electrical and Computer Engineering Department, Duke University, Durham, NC, 27708, USA; e Institute for Molecular Engineering, The University of Chicago, Chicago, IL, 60637, USA. § Current Address: Chemical and Biological Engineering Department, the University of Alabama, Tuscaloosa, AL, 35487, USA. †Current Address: CanmetMATERIALS, Natural Resources of Canada, Hamilton, ON Canada L8P 0A5 *Corresponding Authors: [email protected]; [email protected]; [email protected]

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Abstract There are broad interests in selective and localized synthesis in nanodomains of self-assembled block copolymers (BCPs) for a variety of applications. Sequential infiltration synthesis (SIS) shows promise to selectively grow a controllable amount of materials in one type of nanodomains of a self-assembled BCP film. However, the effect of nanostructured domains in a BCP film and SIS cycles on materials growth behavior of SIS are rarely studied. In this work, we investigate material growth behavior of TiO2 SIS within self-assembled polystyrene-block-poly (methyl methacrylate) (PS-b-PMMA) films and the two corresponding pure homopolymer films (PS and PMMA) by using in situ quartz crystal microbalance (QCM). Reactant purge steps are essential to enable a high selectivity of SIS in PMMA nanodomains in a BCP film by eliminating undesired homogeneous reactions. Continuous PS nanodomain is found as the main channel in transporting reactants to PMMA nanodomains in a self-assembled PS-b-PMMA BCP film. Segregated nanoscale PMMA nanodomains a BCP film show dramatically different TiCl4 diffusion/reaction behavior than a continuous PMMA film. The mass gain per SIS cycle within PMMA nanodomains decreases quickly with increasing cycle number; After 7 TiO2 SIS cycles, TiO2 SIS can only take place at the interface between PS and PMMA nanodomains in the BCP film. TiO2 SIS process can uniformly modify PMMA nanodomains through a self-assembled PSb-PMMA film upto the diffusion depth owing to the unique nanostructure-enabled diffusion. SIS cycle number and chemistry of a BCP will strongly affect material growth behavior of a SIS chemistry on the BCP film, therefore the final morphology of resulting nanomaterial. Detailed studies are warranted for a SIS process on a self-assembled BCP film of different chemistry.


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Introduction Block copolymers (BCPs) can self-assemble into a variety of different nanostructures consisting of chemically distinct nanoscale domains with controllable morphologies, chemistries, sizes, and hierarchical structures owing to the phase separation of chemically dissimilar polymer blocks.1-4 These nanoscale domains have been widely explored as templates for patterning a range of inorganic materials, with applications in microelectronics, sensors, data storage, and optics.1-4 In a typical synthesis process, a phase in a self-assembled BCP thin film is selectively infiltrated with one reactant, such as a metal-containing precursor, and then exposed to a second reactive agent, such as H2O or H2, to generate metal oxides, metals, or other materials.1,4-11 Sequential infiltration synthesis (SIS) in self-assembled BCPs has promising applications in lithography,12-17 optoelectronics,18-21 magnetic materials,22 enhancing electron contrast,23,24 nanogenerators,25 super-hydrophobic surface,26 nanomaterials synthesis,27-30 SIS borrows its chemistries from atomic layer deposition,6,7,14,31 and consists of cyclic step-wise infiltration and reaction. In principle, SIS relies on cyclic sequential substrate-site-limited heterogeneous reactions between vapor-phase reactants and a BCP substrate to growth materials in selected nanodomains. Between reaction steps, a purge step is used to remove physically trapped reactants from a substrate by steps.7 Owing to sequential reaction steps, SIS in self-assembled BCPs is able to synthesize patterned nanoscale materials, e.g., Al2O3, TiO2, and SiO2, with tunable dimensions by simply adjusting SIS cycle number.6,7,10,32 As more SIS cycles modify the polymer film or the nanodomains in BCP film to a higher extent, it is expected that SIS cycle number and nanostructure chemistry of a self-assembled BCP film will affect the material growth behavior of SIS in a BCP film. However, few work focuses on understanding reaction process in vapor-phase processing on BCPs. Considering the rising technical importance of SIS3 ACS Paragon Plus Environment

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BCP method, it is timely to investigate the effect of nanostructures of BCP thin films on mass gain behavior during SIS. In situ quartz crystal microbalance (QCM) has been used to probe mass gain behavior during vapor exposure on various homopolymer coatings.33-37 These studies have provided great insights into the effect of state variables such as temperature and partial pressure of reactants, and reaction cycles on the behavior of mass gain during alternating exposure of reactants to pure polymer substrates. In this work, we studied chemical reaction dynamics in TiO2 SIS (TiCl4/H2O) in polystyrene-block-poly(methylmethacrylate) copolymers (PS-b-PMMA) with in situ QCM. The experimental results confirm step-wise substrate-site-limited heterogeneous reaction mode of SIS within a PS-b-PMMA BCP film. A purge step is essential to enabling substrate-site-limited heterogeneous reaction by removing physically-bonded reactants. The localized generated TiO2 in PMMA film and nanodomains through SIS process significantly reduced the amount of mass gain per SIS cycle. The nanostructure in a self-assembled BCP film has three interesting effects: (i) The continuous PS phase acts as the main channel to deliver TiCl4 precursor even before PMMA nanodomains are significantly modified; (ii) The interface between PS and PMMA provides dense reactive sites for SIS reaction even after PMMA becomes impermeable; (iii) Nanoscale dimension of PMMA nanodomains increases diffusion rate of TiCl4 than a PMMA film. Experimental Chemicals. PS-b-PMMA (Mw = 50,500/20,900) block copolymer (BCP) was purchased from Polymer Source Inc. and purified through Soxhlet extraction to remove excess PS homopolymer. The eluent for Soxhlet extraction is toluene. PS was purchased from Polymer Source Inc. (99%, molecular weight: 17,500). PMMA (electronic grade, molecular weight: 950,000) was purchased 4 ACS Paragon Plus Environment

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from Microchem Inc. Titanium tetrachloride (TiCl4, 99.9%) was purchased from Sigma-Aldrich and used as received. Deionized H2O was generated from an on-site purifier. Ultrahigh purity N2 (99.999%) was used as purge gas and carrier gas and further purified with an inert gas filter (Aeronex Gatekeeper®) before entering the reactor. Synthesis of polymer thin films. BCP films were prepared by spin coating from a toluene solution (13 mg/mL) onto a QCM crystal (Polished, Au-coated, 6 MHz, Colorado Crystal Corp.). The deposited BCP films were annealed at 250˚C for 2h in a vacuum oven, then slowly cooled down in vacuum to room temperature over night to obtain self-assembled BCP films. We use 250°C because a higher temperature enables a much faster self-assembly process. Only 2 h were needed to phase segregate the block copolymer. As the process took place in a vacuum oven, no degradation was observed. In-plane PMMA cylinders were 30 ± 3 nm in diameter, and center-tocenter lateral distance was 60 ± 5 nm as described previously.6 PS and PMMA films were prepared on QCM crystals by spin coating at 3000 rpm from a 2.5 wt.% toluene solution and 4 wt.% in anisole respectively, then dried on a hot plate at 120 °C for 20 min. TiO2 ALD and SIS and in situ QCM. TiO2 ALD was performed at 135 °C with TiCl4 and H2O as reactants, which were introduced into the reactor by injecting into N2 carrier gas. Transition pressure increases during is ~ 20 milltorr (mtorr) and 150 mtorr respectively for pulses of TiCl4 and H2O. TiO2 ALD consists of TiCl4 pulse (1 s), purge (10 s), H2O pulse (1 s), and purge (10 s). It is noteworthy that backside of QCM crystals was continuously purged with N238 so that only the front surface of QCM is exposed to ALD reactants. TiO2 SIS was performed at 135 °C with TiCl4 and H2O as following:6 Before experiments, a sample was sitting in the reactor for ~ 2h at 135 ºC, 300 sccm N2 flow at 1 Torr to achieve


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temperature equilibrium (i.e., QCM signal becomes stable). Then the reactor was evacuated to < 20 mTorr before commencing SIS. All precursors were at room temperature before introduced into the reactor. Next, 5 Torr of the first reactant was admitted into the reactor for a predetermined exposure period (exposure). After the exposure, excess reactants and byproducts were removed from the reactor by evacuating the reactor to < 20 mTorr (evacuation). The chamber was then purged with ultra-high purity N2 gas (300 sccm at 1 Torr) for a predetermined period (purge). After this purge step, the chamber was evacuated again < 20 mTorr (evacuation). The exposure step followed by the evacuation-purge-evacuation (EPE) step was used for both reactants and repeated cyclically. The timing sequence of TiO2 SIS is given as: a/b/c/d where a and c are the exposure times to TiCl4 and H2O, respectively, and b and d are corresponding times for purge. All times are given in seconds (s). The sequence of a/b/c/d was 1200/1200/600/1200 s. In TiO2 SIS, the adsorption behavior of reactants in BCPs and homopolymers was analyzed by in situ QCM. In these experiments, a QCM crystal coated with a polymer film was sealed into the QCM fixture using conductive epoxy (Epotek P1011).38 Mass changes were sensed by a QCM crystal and recorded by Maxtek TM-400 with a custom-designed LABVIEW program. Both top and bottom surfaces of a QCM crystal were exposed to reactants during SIS. Sauerbrey equation is used here to calculate mass change from shift of resonance frequency of a QCM crystal.39,40 In this study, thin polymer films (< 300 nm) and small mass gain from SIS process induce a small ∆௙

frequency change ( ௙బ ≪ 0.02 , ∆݂ is frequency change, ݂ ଴ fundamental frequency 6 MHz), which will not cause a significant deviation from the Sauerbrey equation.36,38 In addition, thin polymer films before and after SIS oxide are rigid enough to maintain the validity of Sauerbrey equation.36


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Characterizations. The thickness of a polymer film on a QCM crystal was measured using a Ellipsometer (Alpha-SE, J. A. Woollam Co. Inc.), whose data were fitted with a stack film model composed of a Cauchy layer model (for the polymer film) and a semi-infinite gold film (for the substrate). A field emission scanning electron microscope (SEM, Hitachi S-4700-II) was used to examine the morphology of resulting materials after removing polymers by heating in air at 500˚C for 6 h. Surface morphology of samples was also analyzed by atomic force microscopy (AFM, Digital Instrument Dimension 3100) in tapping mode. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Kratos Analytical Axis Ultra instrument with a monochromatic Al Kα source operated at 15 kV. For all samples, C 1s peak (-C-H2) was normalized to 284.6 eV. In survey scans, 160 eV was used for the pass energy. Before collecting each spectrum, sample surfaces were cleaned in situ for 5 min with Ar sputtering. Results and Discussion We performed a control experiment of ALD TiO2 on an impermeable bare QCM crystal in order to establish a baseline prior to exploring TiO2 SIS in polymer films. As shown in Figure 1, during a TiCl4 pulse (1 s), there is a saturated mass gain of ~38 ng/cm2 from chemical sorption of TiCl4 on the surface. Physisorbed TiCl4 on the surface is negligible as evidenced by a small mass drop during the following purge step (10 s). In the following H2O pulsing step (1 s), there is a loss of mass (~ -19 ng/cm2) owing to desorption of HCl, a byproduct from hydrolysis reaction between surface bonded Ti-Cl ligands and H2O. Mass loss in the subsequent purge step (10 s) is negligible. It is important to note that in TiO2 ALD experiment, TiO2 only grows on the front surface of the crystal since the backside of crystal is protected by a N2 purge. The steady-state mass gain is ~ 19 ng/cm2 per cycle, which is consistent with reported value of TiO2 ALD.41 The


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QCM data demonstrate that TiO2 ALD growth is a substrate-site-limited process and our reactor and measurement system work well. In probing reaction dynamics of SIS steps in polymer films, front surface of an Au-coated QCM crystal was covered by a thin layer of PS, PMMA, or self-assembled PS-b-PMMA BCP. Figure 2a presents mass gains recorded at 135 °C by QCM during TiO2 SIS in a pure PS thin film (~180 nm thick). When the fresh PS film is exposed to H2O vapor (5 Torr), mass increases. However, mass then decreases during the following EPE process. This H2O half cycle generates negligible net mass change, which indicates that H2O interacts with the PS film through reversible physical adsorption owing to inert chemistry of PS toward H2O. This result suggests that if the preloaded reactants are not firmly bonded to the substrate, they will diffuse out from the polymer film when there is a gradient of chemical potential (e.g., concentration gradient here). If these out-diffusing reactants are not fully removed from a polymer substrate, they will react with the incoming reactant homogeneously. Exposure to TiCl4 vapor (5 Torr) yields a net mass gain of ~180 ng/cm2. During the 1200 s exposure, mass gain approaches saturation as evidenced by a rapid initial mass increase followed by a reduced rate of mass uptake in the later stage. In the following EPE step, an abrupt mass loss with an initial rate of 3 ng/cm2/s was observed due to rapid desorption of physisorbed TiCl4 in PS. The first TiCl4 SIS half cycle generates a stable net mass gain of ~ 150 ng/cm2, which is ascribed to irreversible chemical reactions. This irreversible mass gain indicates that the whole TiCl4 exposure (1200 s) on PS is not a simple Fickian process. After exposed to H2O vapor, this film occurs an abrupt mass loss owing to removal of HCl from the film. HCl is produced from hydrolysis between Ti-Cl ligands and H2O, which also generates OH groups. Condensation between adjacent Ti-OH groups can release H2O and contribute to


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mass loss. The slow mass gain during the later portion of H2O exposure is attributed to physisorption of H2O, as evidenced by mass loss during the following EPE step. Net mass gain from the 1st SIS cycle in PS is ~120 ng/cm2, which is ~6× of the mass gain produced by one TiO2 ALD cycle on a smooth bare impermeable QCM substrate (~ 19 ng/cm2). There are two reasons for this difference. First, during SIS, both the front and back surfaces of QCM crystal are exposed to reactants, while in ALD TiO2 only the front surface of the QCM crystal is exposed to reactants. Second, although benzene rings and alkyl ligands in polystyrene are inert to both TiCl4 and H2O, impurities such as polymerization initiators, contamination from solvent, trapped H2O, and defective sites (e.g., chain ends of PS) may all provide reactive sites for TiCl4. During each step of subsequent TiO2 SIS cycles, the pattern of QCM response is similar to the first SIS cycle. However, net mass gain decreases to ~69 ng/cm2 during the 2nd cycle, and then stabilizes at ~45 ng/cm2 for the following cycles. The reduced mass gain indicates reduced number of reactive sites in the substrate, which may be caused by double reactions42 and steric hindrance.43 TiO2 SIS in a PMMA film (~240 nm thick) is also monitored by in situ QCM. As shown in Figure 2b, when the fresh PMMA-coated QCM crystal is exposed to TiCl4 (5 Torr), there is a large initial mass gain followed by a much slower rate of mass uptake. During the following EPE process (~1200 s), mass drops at a constant rate of ~ 0.15 ng/cm2/s throughout the entire period. Rate of desorption of TiCl4 in the EPE step is much smaller than adsorption rate of TiCl4. This small constant mass decreasing rate suggests desorption is probably controlled by the reverse reaction (i.e., conversion of TiCl4 that is coordinated to O=C to free TiCl4). The 1st TiCl4 half cycle generates a net mass gain of ~ 1800 ng/cm2, which is 7.5 wt.% of the PMMA film by assuming the density of PMMA is 1 g/cm2. This mass gain results mainly from chemisorption of 9 ACS Paragon Plus Environment

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TiCl4 inside of the PMMA film since nearly 80% of this mass remains after the EPE step. Although mass gain of 1800 ng/cm2 is significant, the reaction consumes only ~ 10% of carbonyl groups in the PMMA film by assuming each TiCl4 coordinates with average two carbonyl groups as Ti4+ can coordinate with six bonds.44-46 After the TiCl4 half cycle, the PMMA-coated QCM substrate is exposed to H2O vapor at 5 Torr for 600 s. Mass dropped immediately upon H2O vapor exposure, and then stabilizes within 100 s. This short time suggests a fast kinetics of the reaction between H2O and TiCl4 modified PMMA. This mass loss is due to release of HCl byproduct through hydrolysis of Ti-Cl within PMMA matrix, and possibly also from condensation between Ti-OH groups to release H2O. In the following EPE step, mass drops slightly owing to desorption of weakly bonded free H2O. In the 2nd TiO2 SIS cycle, net mass gain per cycle decreases significantly to ~1200 ng/cm2. Given that a significant portion (> 80%) of carbonyl groups in the PMMA film had not yet reacted with TiCl4, such a dramatic decrease of net mass gain must have resulted from reduced permeability of TiCl4 in the TiO2 SIS treated PMMA. Interestingly, in the TiCl4 EPE step of the 2nd cycle, mass loss is negligible, indicating absence of physisorbed TiCl4. In the 3rd SIS cycle and beyond, the mass gain from TiCl4 exposure has a plateau, indicating TiCl4 adsorption saturates within 1200 s exposure. There is an unexpected positive mass gain from the 2nd H2O half cycle and negligible mass loss in the 3rd H2O half cycle, as HCl from H2O pulse will desorb from the film and reduce the mass. These phenomena require further experiments to ascertain their origin. Overall, net mass gain per TiO2 SIS cycle decreases significantly with increasing cycles. Figure 2c displays in situ QCM data during TiO2 SIS in a self-assembled PS-b-PMMA BCP film (~280 nm). In a thick PS-b-PMMA film, microphase-separated PMMA nanodomains are expected to be randomly oriented. This type BCP film is chosen to demonstrate the generality of 10 ACS Paragon Plus Environment

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SIS for selective domain functionalization in a self-assembled BCP film. When the fresh PS-bPMMA film was exposed to TiCl4 vapor (5 Torr), mass gain approaches to saturation within 1200 s. Surprisingly, all of free or physisorbed TiCl4 desorbs within the first 100 s of the EPE step as demonstrated by rapid mass loss and subsequent flat QCM signal in the step (Figure 2c). This result suggests that TiCl4 desorbs from the self-assembled BCP film at a much faster rate than from the pure PMMA film (Figure 2b). The desorption behavior indicates that nanostructured PMMA domains have a higher rate for desorption than the PMMA film. In fact, on the BCP film, desorption behavior of TiCl4 (Figure 2c) is qualitatively similar to that from the pure PS film in Figure 2a. In spite of rapid TiCl4 desorption from the BCP film, net mass gain from the 1st TiO2 half-cycle is 1290 ng/cm2, ~15 wt % of PMMA nanodomains in the BCP film, and is higher than 7.5 wt.% in the PMMA film even though the BCP film contains about half of the amount of PMMA. This finding implies that TiCl4 reacts with PMMA nanodomains to a higher degree than a PMMA film under same exposure conditions. This enhanced mass gain rate is most likely a product of the PS percolation pathway and nanoscale PMMA nanodomains inside the BCP film. During the 1st H2O half cycle, the mass loss is negligible, suggesting that the mass of OH and O added to the film equals to the mass of Cl lost from the film. In subsequent SIS cycles, mass is accumulated during TiCl4 half cycles and mass is reduced during H2O half cycles, as expected. Overall, the net mass gain per cycle decreases with increasing number of SIS cycles. Figure 2d summarizes net mass gains vs. cycle number extracted from Figures 2a-2c. During the 1st TiO2 SIS cycle, net mass gain is much smaller in the PS film (120 ng/cm2) than in PMMA (1380 ng/cm2) and PS-b-PMMA films (1290 ng/cm2). This is due to selective coordination reaction between the carbonyl group of PMMA and TiCl4.45 However, mass gain per SIS cycle in 11 ACS Paragon Plus Environment

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PMMA decreases dramatically with TiO2 SIS cycles. For example, by the 7th cycle, the net mass gain had decreased to ~40 ng/cm2, which is close to the corresponding mass gain on PS (~45 ng/cm2), and around twice the mass gain on a bare, planar QCM surface (19 ng/cm2, Figure 1). In the PS-b-PMMA film, the mass gain per TiO2 SIS cycle also decreases with increasing cycles. For example, mass gain drops from 1290 ng/cm2 (1st cycle) to ~350 ng/cm2 (7th cycle). However, in the PS-b-PMMA BCP film, the mass gain of ~350 ng/cm2 is ~18× of the corresponding rate in PMMA (40 ng/cm2). This difference is ascribed to the nanostructure in BCP film as discussed below. After 8 cycles of TiO2 SIS, polymer films were then removed by calcination in air at 500 °C. Figure 3a and 3b show SEM images of the resulting metal oxide structures templated from the BCP film. The discrete, randomly oriented nanowires have average diameter of 18.1 ± 2.4 nm, which suggests selective TiO2 growth within the PMMA nanodomains and negligible growth within the PS domain. The size of TiO2 nanowires is smaller than original PMMA nanodomains due to consolidation of a low density of titanium oxide from SIS in BCPs.7 These randomly oriented nanowires are stacked in layers as shown in Figure 3b. The resulting feature is consistent with the large mass shown in Figure 2. The nanowires are continuous with nanometer pores randomly distributed inside the nanowires, as shown by the TEM images (Figure S1). For the material that is templated by the PMMA film, SEM image (Figure 3c) reveals a relatively featureless surface compared to that from the BCP film (Figures 3a and 3b). It is difficult to resolve fine features of the coating due to electron charging effect in SEM. AFM imaging of the sample (Figure 3d) reveals a continuous coating with surface roughness of around 5 nm. According to XPS analysis (Figure 4a), the material in Figures 3a - 3b consists of Ti, O and minor adventitious carbon contamination. The coating in Figure 3c also consists mainly of Ti and 12 ACS Paragon Plus Environment

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O according to XPS analysis (Figure 4b). These results are consistent with the mass accumulation by TiO2 SIS on PMMA and PS-b-PMMA BCP films as shown in Figure 2 and verified that chemical composition of materials shown in Figure 3. Discussion Role of purge steps in localized synthesis in self-assembled BCP film As shown in Figures 2a - 2c, a mass loss is consistently observed during EPE step of TiCl4 and H2O half cycles. These mass losses are from desorption of free reactants and byproducts. If EPE steps were not implemented, unreacted reactants would likely react with the coming reactant. Through this homogeneous gas phase reaction, materials will be deposited into in inert undesired nanodomains of self-assembled BCP films. Therefore, SIS will lose its selectivity in desired nanodomains. In most current methods, purge steps have not been used in selective material growth in self-assembled of BCPs.1,4 According to our results, if a purging step was implemented in these methods, it is possible to achieve smoother edges and finer replication of nanoscale features of a BCP template.1,4 Deviation of mass gain behavior from Fickian model As is apparent from the QCM data, TiCl4 vapor generates irreversible mass gain in all polymer films (with a far greater gain in polymers containing chemical moieties that chemically bind precursors). The process, therefore, is not a simple diffusion-only process. Instead, the kinetics could be dominated by diffusion, reaction, or coupled processes in various stages of the reaction. Fickian diffusion model cannot perfectly fit the overall process. In addition, the infiltrated species may swell polymer films and cause further deviation from the Fickian model. Indeed, the standard plane Fickian diffusion model (Eq. 1) fits whole TiCl4 exposure steps poorly (Figure S2). 13 ACS Paragon Plus Environment

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Mt 8 =1 − 2 M∞ π

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D(2n + 1) 2 π 2 t 1 exp(− ) ∑ 2 l2 n =0 (2n + 1) ∞

(1)

wherein Mt is mass gain at time t, M∞ is mass gain at infinite time, D is diffusion coefficient, and l is thickness of a polymer film.47 Although a numerical model that can capture all physics behind a whole TiCl4 exposure step is not possible at this moment, QCM data is still valuable in understanding TiCl4 exposure steps. The shape of Mt / M∞ vs. t1/2 can provide characteristics of the infiltration process. If Mt / M∞ vs. t1/2 is approximately linear as far as Mt / M∞ = 50%, the initial stage is probably dominated by diffusion. The average diffusion constant can be estimated from the slope through Eq. 2. If otherwise, the initial process is probably not a diffusion-dominated process. ெ೟

ெಮ

=

ସ

஽௧ ଵ/ଶ

ቀ ቁ గబ.ఱ ௟ మ

(2)

Figure 5 summarizes Mt / M∞ vs t1/2 during the 1st, 2nd, and 7th TiCl4 exposure in PS (a-c), PMMA (d-f), and PS-b-PMMA (g-i). The data are extracted from Figures 2a - 2c. Right before exposure of TiCl4 in the ith SIS cycle, t and Mt are set to zero. Mt is the net mass gain at the new time t, M∞ is assigned as the total mass gain after 1200 s exposure of TiCl4 vapor. Note, the assigned M∞ is not the true M∞, but is a close approximate as the mass gain rate at 1200s is very slow. For the PS film, Mt / M∞ vs. t1/2 curves (Figures 5a – 5c) are linear until Mt / M∞ is ~ 0.6 for TiCl4 exposures in the 1st, 2nd, and 7th SIS cycles. After the initial linear portion, the slope of the curve decreases dramatically. The initial mass gain is probably controlled by TiCl4 diffusion in PS. Owing to the fact that initial slopes in all three figures are same, accumulated TiO2 from 7 cycles of SIS does not affect diffusion of TiCl4 in PS film. In TiCl4 exposure on a fresh PMMA film, the initial mass gain is linear until Mt / M∞ reaches 0.5 (Figure 5d). The figure then levels off. This 14 ACS Paragon Plus Environment

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shift suggests that TiCl4 diffusion dominates the initial stage (Mt / M∞ < 0.5). In the initial stage, diffusion rate of TiCl4 is much higher than its reaction rate with fresh PMMA film. Therefore, the mass gain mainly reflects the diffusion characteristics of TiCl4. The following stage of the exposure is probably controlled by coupled reaction and diffusion. During the 2nd TiCl4 exposure in PMMA film (Figure 5e), the initial linear portion is short and slope of the curve is reduced quickly. It is probable that the 1st SIS cycle produces a diffusion-resistant layer close to the surface of the PMMA film. In the 2nd SIS cycle, TiCl4 quickly saturates the top layer (a higher initial slope), then diffuses slowly through the layer into regions underneath (a smaller slope in the following stage). The curve from TiCl4 exposure in the 7th cycle is dramatically different from that in the 1st and 2nd cycles (Figure 5f). In summary, TiO2 SIS does not affect the diffusion of TiCl4 inside a PS film, however, it dramatically changes diffusion of TiCl4 inside a PMMA film. Mt / M∞ is also plotted against t1/2 for the 1st, 2nd, and 7th TiO2 SIS cycle in PS-b-PMMA film as shown in Figures 5g – 5i. In the 1st TiCl4 exposure on the fresh PS-b-PMMA film (Figure 5g), Mt / M∞ increases linearly with t1/2 until Mt / M∞ is ~ 0.6. The slope is then quickly reduced. The overshoot of the linear portion in Figure 5g is probably due to the measurement error. In the TiCl4 exposures of the 2nd and 7th SIS cycles (Figures 5h and 5i), Mt / M∞ increases linearly with t1/2 upto ~ 0.5. The slopes of the initial linear regions are the same as that of the 1st TiCl4 exposure (Figure 5g). This pattern is same as the plots for PS film in Figures 5a – 5c. This similarity suggests that the initial linear portion reflects the characteristics of TiCl4 diffusion in the continuous PS domain. Note, however, in the linear region, the net amount of TiCl4 (Mt) in the fresh PS-b-PMMA film is much higher than that in the PS film (Figures 2a – 2c). Therefore, a majority of TiCl4 molecules is inside PMMA region. It is interesting that the initial linear 15 ACS Paragon Plus Environment

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portion of Figure 5g did not show the effect of PMMA nanodomains on diffusion, even though PMMA nanodomains are able to transport and absorb significant amount of TiCl4 within 25 s exposure (Figure 5d). This result indicates that diffusion in PMMA nanodomains does not dominate the initial mass gain in PS-b-PMMA film. Otherwise, the initial portion of the curves would be changed in a way as that in the pure PMMA film. This fact is at least partially due to the smaller size of PMMA nanodomains (15 nm, radius of PMMA nanodomains) than PMMA film 240 nm becuase the time needed to infiltrate 15 nm is much smaller than 240 nm. In summary, the same slope of the initial linear regions (Figures 5g - 5i) suggests diffusion in PS domain controls the initial mass gain behavior in both fresh and TiO2 SIS modified PS-b-PMMA BCP films. After the initial linear region, the latter portion of Figures 5g – 5i changes with SIS cycles. The mass gain behavior in the latter stage is probably due to coupled diffusion and reaction in the self-assembled PS-b-PMMA BCP film. In the 7th TiCl4 exposure (Figure 5i), the latter portion of curve shows a behavior that is similar to the PMMA film (Figure 5f). The slope of the curve decreases, then increases, and then levels off. We did not try to simulate the latter portion of the curves (Figures 5g – 5i) because there are many factors could affect the curve, including nanostructures, changing polymer properties including swelling effect by the infiltrated chemicals and chemical reactions, coupled diffusion and reaction, and reaction kinetics, and we are not yet sure what are the major factors that contribute to the data. A comprehensive work is needed to understand these factors in order to setup a realistic model. By assuming that the initial stage is dominated by diffusion process for the three fresh films and diffusion coefficients are constant in the initial stage, we can estimate diffusion coefficients of TiCl4 in these films by using Eq. 2. Table 1 summarizes the results. The diffusion coefficient of 16 ACS Paragon Plus Environment

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TiCl4 in the PMMA film falls between the common diffusion coefficient coefficients of similar size molecules (~10-12 – 10-14 cm2/s),48 but is smaller than the other reported value (~ 1.53×10-10 cm2/s).36 This discrepancy is likely due to larger molecular weight and lower reaction temperature in this study than in the previous reports.36,48 The PS-b-PMMA has a higher diffusion coefficient than the pure PS film. It is probably due to the lower molecular weight of PS in PS-bPMMA film and interface between PS and PMMA. As the reaction temperature (135 ºC) is higher than glass transition temperatures of PS and PMMA, molecular weight should not have a large effect on diffusion. However, further investigations are needed to ascertain these explanations. Table 1: Diffusion coefficients calculated from the 1st TiCl4 exposure on the fresh films.

Average thickness (࢒, nm) Slope (Mt / M∞ vs. t1/2) Diffusion constant (D, cm2/s)

PS

PMMA

PS-b-PMMA

180 nm

240 nm

280 nm

0.16

0.08

0.121

1.6×10-12

0.7×10-12

2.3×10-12

Mass gain vs. SIS cycle number in PMMA and PS-b-PMMA films Mass gain of each TiO2 SIS cycle decreases significantly with increasing the number of cycles in PMMA and PS-b-PMMA films, as shown in Figure 2d. It is unlikely that this reduced mass gain stems from a reduction in the number of reactive sites in polymer films since TiO2 SIS chemistry will naturally replenish reactive sites after each precursor exposure. Instead, we ascribe this behavior to a slower transport of TiCl4 through polymers, which is related to free volume of polymer films, the degree of cross-linking, and interactions between polymer substituents and vapor species.49 All of these parameters will likely be modified by TiO2 SIS and will affect


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transport rate of vapors. Accumulated TiO2 within PMMA nanodomains will reduce diffusion of reactants by cross-linking adjacent polymer chains and restricting movement of polymer chains. Therefore, permeability of TiCl4 in PMMA film or PMMA nanodomains decreases with increasing SIS cycles, and eventually TiO2 stop growing in a PMMA film or nanodomains. In comparison to the PMMA film, the PS-b-PMMA BCP film shows a different mass gain behavior, which is due to segregated PMMA nanodomains inside a continuous PS phase. Although vertically aligned PMMA nanodomains in Figure 6 do not accurately reflect randomly aligned PMMA nanodomains in the BCP film (Figures 3a and 3b), Figure 6 does catch the main feature of the BCP film: a continuous PS domain and segregated PMMA nanodomains. First, the continuous PS domain can channel SIS precursors. Owing to its chemical inertness, the PS domain is largely unaffected by TiO2 SIS (Figure 6b), so is the permeability to TiCl4 in it. In contrast, PMMA nanodomains are modified by TiO2 SIS and their permeability drops dramatically with increasing cycles as described above. Consequently, the PS domain functions as diffusion channels to transport reactants to reactive sites in PMMA nanodomains (carbonyl and Ti-OH) after PMMA nanodomains become much less permeable (Figures 5g – 5i). This mechanism holds no matter the segregated PMMA nanodomains are aligned vertically, laterally, or randomly. Second, segregated PMMA nanodomains in the BCP film show an enhanced kinetics of desorption of TiCl4 molecules than the PMMA film (Figures 2b and 2c). It is mainly due to the small diameter of nanoscale domains of PMMA and transportation of free TiCl4 through the continuous PS domain (Figure 6c). Third, in the 7th SIS cycle, the net mass gain is ~ 350 ng/cm2 in the PS-b-PMMA BCP film compared to ~ 40 ng/cm2 in the PMMA film (Figure 2d). This result is because interfaces between PS and PMMA nanodomains provide a large effective surface area for TiO2 growth in the BCP film. According to a simple calculation 18 ACS Paragon Plus Environment

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(supporting information), the total surface area of a 280 nm-thick PS-b-PMMA film, including sidewalls of PMMA nanocylinders, the top surface of a polymer film, and the bottom surface of the quartz crystal, is ~9.3 cm2. Given additional surface area from the roughness of the nanodomains and shorter discontinuous PMMA nanodomains (Figures 3a and 3b), this value matches reasonably well with 18× of mass gain rate in the BCP film relative to the planar and bare QCM surface (19 ng/cm2, Figure 1). Therefore, the apparent difference of mass gain behaviors in PMMA and PS-b-PMMA BCP is ascribed to the nanostructure inside the PS-bPMMA BCP film. Conclusions The result verifies substrate-site-limited reaction mode of TiO2 SIS within nanostructured PS-bPMMA, which is the key to a high selectivity of SIS in PMMA nanodomains. A purge step, which removes weakly bonded reactants before the introduction of the next reactant, is essential to maintain a high selectivity of SIS on BCPs. Permeability of TiCl4 is reduced significantly in PMMA by progressive infiltration of TiO2, and becomes zero after ~ 7 TiO2 SIS cycles. The continuous nanostructured PS domain, which is chemically inert, in a PS-b-PMMA BCP film functions as a channel to transport reactants to reactive sites located in PMMA nanodomains and at PS/PMMA interfaces. According to these results, SIS cycle number and chemistry of BCPs will strongly affect SIS on BCPs and the final structure of synthesized nanomaterials. For instance, our results predict that PMMA-b-PS, where PMMA is a continue phase, the mass gain behavior will be similar as a PMMA film. Although this study is about SIS process, the insights from this study can also be used to optimize other BCP templated synthesis. For instance, adding a purge step in other methods can improve selectivity of those chemistries in desired


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nanodomains. QCM can also be adapted to investigate reaction dynamics in other localized syntheses in BCPs. Supporting Information Mathematic model of pure Fickian diffusion in a plane polymer film (Eq. 1); TEM images of TiO2 nanowires by SIS process (Fig. S1); Fit of mass gain data during TiCl4 exposure on fresh PS, PMMA and BCP films (Fig. S2); Calculation of interfacial area of the BCP film; Acknowledgements: Q. P. thanks Prof. Jan Genzer and Prof. Maurice Balik from North Carolina State University for the helpful discussion related to diffusion and reaction processes in polymers. Q. P. also thanks UA seed funding in supporting part of his time in finishing this work. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.


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Figures

Figure 1: In situ QCM data of TiO2 ALD from TiCl4 and H2O at 135 °C. The net mass gain per cycle of TiO2 is ~ 19 ng/cm2. The red peak (red dash arrow) is H2O pulse (1 s) and the blue peak (blue solid arrow) is TiCl4 pulse (1 s). An Ar purge step of 10 s was used between the pulses of these two reactants.


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Figure 2: In situ QCM data of TiO2 SIS at 135 °C in thin films of (a) PS (~180 nm), (b) PMMA (~240 nm), and (c) PS-b-PMMA (~280 nm). The time sequence of SIS is TiCl4/Purge/H2O/Purge: 1200/1200/600/1200 s. (d) Summary of the net mass gains from each SIS cycle onto the three polymer thin films shown in (a-c). The wider and thinner rectangles in (a-c) represent the exposures to TiCl4 (5 Torr, 1200 s) and H2O vapor (5 Torr, 600 s), respectively. The time between the exposures is the evacuation process (to 20 mTorr) followed by a N2 purge (1 Torr, 1200 s, 300 sccm N2), and then evacuation (to 20 mTorr).


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Figure 3: (a-b) SEM images of TiO2 nanowires with a random alignment templated from selfassembled PS-b-PMMA BCP film on a QCM crystal after 8 SIS cycles at 135 ˚C; (c-d) SEM and AFM images of the TiO2 film obtained after 8 TiO2 SIS cycles in PMMA film on a QCM crystal at 135 ˚C. Polymer templates were removed by calcination at 500 ˚C for 10 h.


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Figure 4. (a) XPS spectrum of TiO2 nanowires shown in Figure 3a and 3b. Atomic concentrations of Ti, O, and C are 32.6, 65.4, and 2.0 at. % (atomic ratio). (b) XPS spectrum of the coating shown in Figures 3c and 3d. Atomic concentrations of Ti, O, and C are 25.1, 66.0, and 2.1 at. % (atomic ratio). Au (6.1 at. %) is from substrate and Zn 0.7 at. % may result from contamination. A brief Ar sputtering was used to clean the surface before the analysis.


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Figure 5. Adsorption behavior of TiCl4 in the 1st, 2nd, and 7th SIS cycle in a PS film (a, b, c), in a PMMA film (d, e, and f), and in a PS-b-PMMA film (g, h, and i). Right before exposure of TiCl4 in the ith SIS cycle, t and Mt are set to zero. Mt is the net mass gain at the new time t, M∞ is assigned as the total mass gain after 1200 s exposure of TiCl4 vapor. Note, the assigned M∞ is not the true M∞, but a close approximate. The solid lines are guides for the eye.


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Figure 6: Simplified schematic diagram of the reaction process during the TiCl4 half cycle for TiO2 SIS on the self-assembled PS-b-PMMA block copolymer film. (a) PS-b-PMMA thin film is exposed to TiCl4, which diffuses into the polymer film primarily through PS domain. It is not unexpected as fresh PMMA can also transport TiCl4. (b) TiCl4 molecules diffuse primarily through PS domain and encounter reactive sites (carbonyl group and Ti-OH) within PMMA domains. (c) Excess free TiCl4 molecules diffuse out the BCP film primarily through PS domain. The real situation is more complex than this, but the real PS-b-PMMA has continuous PS domain and segregated PMMA nanodomains. Note, PMMA nanodomains may be swollen by infiltrated reactants and TiO2.


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Table of Content:


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Effect of Nanostructured Domains in Self-Assembled Block Copolymer

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