Sangho Lee

Ashwanth Subramanian

Nikhil Tiwale

Kim Kisslinger

Muhammad Mumtaz

Ling-Ying Shi

Karim Aissou

Chang-Yong Nam

Caroline A Ross
email: caross@mit.edu

Resolving Triblock Terpolymer Morphologies by Vapor-Phase Infiltration

Keywords: three-dimensional self-assembly, triblock terpolymer, infiltration synthesis, metal oxide, three-color nanostructure

The spontaneous formation of well-organized three-dimensional (3D) nanostructures from self-assembled block copolymers (BCPs) holds promise for nanofabrication and lithography. The addition of a third block to BCPs provides access to a plethora of 3D geometries, but it remains difficult to resolve the geometry of such "three-color" structures when there is low contrast between the polymeric components at length scales of a few nm. Here, we apply vapor-phase infiltration synthesis to a silicon-containing triblock terpolymer, poly(1,1dimethyl silacyclobutane-b-styrene-b-lactide) (PDMSB-b-PS-b-PLA or DSL) to distinguish the 3D microdomain morphologies of the two organic blocks, PLA and PS. Selective infiltration of ZnO within the PLA microdomains reveals morphologies consisting of three-color lamellae or lamellae combined with vertically aligned core-shell cylinders, depending on the volume fractions of each block. The infiltration produces ZnO nanoparticles throughout the 260 nm thickness of the DSL film, generating 3D nanocomposites containing ZnO and SiOx. These results provide a strategy for synthesizing multicomponent 3D nanostructures as well as visualizing the phase behavior of multi-block copolymers.

INTRODUCTION

Three-dimensional (3D) nanofabrication is essential for producing a host of useful materials and structures for applications such as electronic devices, nanoporous membranes and functionalized surfaces. [START_REF] Shulaker | Three-Dimensional Integration of Nanotechnologies for Computing and Data Storage on a Single Chip[END_REF][START_REF] Javey | Layer-by-Layer Assembly of Nanowires for Three-Dimensional, Multifunctional Electronics[END_REF][START_REF] Li | Three-Dimensional Self-Assembly of Core/Shell-Like Nanostructures for High-Performance Nanocomposite Permanent Magnets[END_REF][START_REF] Fernández-Pacheco | Three-Dimensional Nanomagnetism[END_REF][START_REF] Von Freymann | Three-Dimensional Nanostructures for Photonics[END_REF][START_REF] Castles | Blue-Phase Templated Fabrication of Three-Dimensional Nanostructures for Photonic Applications[END_REF] Self-assembly of block copolymer (BCP) thin films in particular offers an efficient route to produce a variety of 3D nanostructures, taking advantage of its simple sequence of processing steps. [START_REF] Ross | Three-Dimensional Nanofabrication by Block Copolymer Self-Assembly[END_REF][START_REF] Tavakkoli | Templating Three-Dimensional Self-Assembled Structures in Bilayer Block Copolymer Films[END_REF][START_REF] Oh | Universal Perpendicular Orientation of Block Copolymer Microdomains Using a Filtered Plasma[END_REF][START_REF] Rahman | Non-Native Three-Dimensional Block Copolymer Morphologies[END_REF][START_REF] Cheng | Imparting Superhydrophobicity with a Hierarchical Block Copolymer Coating[END_REF][START_REF] Chuang | Three-Dimensional Self-Assembly of Spherical Block Copolymer Domains into V-Shaped Grooves[END_REF][START_REF] Lee | Double-Layer Morphologies from a Silicon-Containing ABA Triblock Copolymer[END_REF][START_REF] Ren | Synthesis of Orthogonally Assembled 3D Cross-Stacked Metal Oxide Semiconducting Nanowires[END_REF] Compared with diblock copolymers, triblock and multiblock polymers can produce nanostructures with greater structural diversity and additional chemical functionalities, [START_REF] Choi | Hierarchical Templating of a BiFeO3-CoFe2O4 Multiferroic Nanocomposite by a Triblock Terpolymer Film[END_REF][START_REF] Chuang | Templated Self-Assembly of Square Symmetry Arrays from an ABC Triblock Terpolymer[END_REF][START_REF] Son | Highly Ordered Square Arrays from a Templated ABC Triblock Terpolymer[END_REF][START_REF] Robbins | Block Copolymer Self-Assembly-Directed Synthesis of Mesoporous Gyroidal Superconductors[END_REF][START_REF] Docampo | Triblock-Terpolymer-Directed Self-Assembly of Mesoporous TiO2: High-Performance Photoanodes for Solid-State Dye-Sensitized Solar Cells[END_REF][START_REF] Vignolini | A 3D Optical Metamaterial Made by Self-Assembly[END_REF] and there has been an increasing emphasis on understanding their selfassembly and 3D morphologies. [START_REF] Löbling | Rational Design of ABC Triblock Terpolymer Solution Nanostructures with Controlled Patch Morphology[END_REF][START_REF] Löbling | Bulk Morphologies of Polystyrene-block-Polybutadiene-block-Poly(tertbutyl methacrylate) Triblock Terpolymers[END_REF][START_REF] Lee | In Situ Study of ABC Triblock Terpolymer Self-Assembly under Solvent Vapor Annealing[END_REF][START_REF] Nunns | Synthesis and Bulk Self-Assembly of ABC Star Terpolymers with a Polyferrocenylsilane Metalloblock[END_REF][START_REF] Aissou | Bicontinuous Network Nanostructure with Tunable Thickness Formed on Asymmetric Triblock Terpolymer Thick Films[END_REF] However, it is challenging to resolve the morphologies of 3D BCPs comprising more than two polymeric components unless there is good contrast between the microdomains, for example a contrast in density, chemical composition, or reactivity. Block-selective etching followed by planview or cross-sectional imaging using scanning electron microscopy (SEM) is commonly used to characterize BCP morphologies, but the organic blocks are often difficult to distinguish. [START_REF] Lee | In Situ Study of ABC Triblock Terpolymer Self-Assembly under Solvent Vapor Annealing[END_REF][START_REF] Gu | Pattern Transfer Using Block Copolymers[END_REF] X-ray scattering is used for 3D structural analysis, but it requires a scattering contrast between the blocks, for example the presence of metal or Si in one block, [START_REF] Lee | Double-Layer Morphologies from a Silicon-Containing ABA Triblock Copolymer[END_REF][START_REF] Lee | In Situ Study of ABC Triblock Terpolymer Self-Assembly under Solvent Vapor Annealing[END_REF] and analysis of triblock morphologies (beyond measuring the periodicity) requires modeling the structure factor.

Vapor-phase infiltration (VPI) refers to variations of ex situ material hybridization techniques based on atomic layer deposition (ALD) including multi-pulse infiltration, [START_REF] Lee | Greatly Increased Toughness of Infiltrated Spider Silk[END_REF] sequential vapor infiltration (SVI), [START_REF] Gong | Sequential Vapor Infiltration of Metal Oxides into Sacrificial Polyester Fibers: Shape Replication and Controlled Porosity of Microporous/Mesoporous Oxide Monoliths[END_REF] and sequential infiltration synthesis (SIS), [START_REF] Peng | Nanoscopic Patterned Materials with Tunable Dimensions via Atomic Layer Deposition on Block Copolymers[END_REF] where vapor-phase inorganic precursors perfuse the polymer matrix to generate unique organic-inorganic hybrids with improved material properties. When applied to a self-assembled BCP, it can greatly enhance the scattering contrast and the etch resistance between blocks by selectively incorporating inorganic material such as ZnO, TiO2, or Al2O3 into specific microdomains. [START_REF] Lee | Greatly Increased Toughness of Infiltrated Spider Silk[END_REF][START_REF] Gong | Sequential Vapor Infiltration of Metal Oxides into Sacrificial Polyester Fibers: Shape Replication and Controlled Porosity of Microporous/Mesoporous Oxide Monoliths[END_REF][START_REF] Peng | Nanoscopic Patterned Materials with Tunable Dimensions via Atomic Layer Deposition on Block Copolymers[END_REF][START_REF] Peng | A Route to Nanoscopic Materials via Sequential Infiltration Synthesis on Block Copolymer Templates[END_REF][START_REF] Segal-Peretz | Characterizing the Three-Dimensional Structure of Block Copolymers via Sequential Infiltration Synthesis and Scanning Transmission Electron Tomography[END_REF][START_REF] Cummins | Strategies for Inorganic Incorporation using Neat Block Copolymer Thin Films for Etch Mask Function and Nanotechnological Application[END_REF][START_REF] Chai | Using Cylindrical Domains of Block Copolymers To Self-Assemble and Align Metallic Nanowires[END_REF][START_REF] Subramanian | Three-Dimensional Electroactive ZnO Nanomesh Directly Derived from Hierarchically Self-Assembled Block Copolymer Thin Films[END_REF][START_REF] Subramanian | Review of Recent Advances in Applications of Vapor-Phase Material Infiltration Based on Atomic Layer Deposition[END_REF][START_REF] Subramanian | Enhanced Hybridization and Nanopatterning via Heated Liquid-Phase Infiltration into Self-Assembled Block Copolymer Thin Films[END_REF][START_REF] Aissou | Highly Ordered Nanoring Arrays Formed by Templated Si-Containing Triblock Terpolymer Thin Films[END_REF] The infiltration process proceeds via sorption (diffusion) and block-selective reaction of vapor-phase organometallic precursors into the polymer blocks with suitable reactive moieties to generate infused inorganic materials within the target BCP microdomains. [START_REF] Waldman | The Chemical Physics of Sequential Infiltration Synthesis-A Thermodynamic and Kinetic Perspective[END_REF][START_REF] Peng | Effect of Nanostructured Domains in Self-Assembled Block Copolymer Films on Sequential Infiltration Synthesis[END_REF] The hybrid nanocomposite thus formed can also be converted to an inorganic nanostructure inheriting the original structure of the polymer microdomains, [START_REF] Subramanian | Three-Dimensional Electroactive ZnO Nanomesh Directly Derived from Hierarchically Self-Assembled Block Copolymer Thin Films[END_REF][START_REF] Subramanian | Enhanced Hybridization and Nanopatterning via Heated Liquid-Phase Infiltration into Self-Assembled Block Copolymer Thin Films[END_REF] not only producing various functional nanostructures in a simple process but also enabling all-organic BCP morphologies to be fully resolved. [START_REF] Segal-Peretz | Characterizing the Three-Dimensional Structure of Block Copolymers via Sequential Infiltration Synthesis and Scanning Transmission Electron Tomography[END_REF] ALD of metal oxide precursors into cylinder-and gyroidforming BCPs has been used to generate 1D and 3D nanostructures for a broad variety of nanotechnologies such as gas sensors, solar cells, photocatalysts, and pattern transfer masks. [START_REF] Ren | Synthesis of Orthogonally Assembled 3D Cross-Stacked Metal Oxide Semiconducting Nanowires[END_REF][START_REF] Cummins | Strategies for Inorganic Incorporation using Neat Block Copolymer Thin Films for Etch Mask Function and Nanotechnological Application[END_REF][START_REF] Subramanian | Review of Recent Advances in Applications of Vapor-Phase Material Infiltration Based on Atomic Layer Deposition[END_REF][START_REF] Kim | Gyroid-Structured 3D ZnO Networks Made by Atomic Layer Deposition[END_REF][START_REF] Rahman | Sub-50nm Self-Assembled Nanotextures for Enhanced Broadband Antireflection in Silicon Solar Cells[END_REF][START_REF] Akyildiz | Photoluminescence Mechanism and Photocatalytic Activity of Organic-Inorganic Hybrid Materials Formed by Sequential Vapor Infiltration[END_REF] While the majority of the reports exploiting infiltration synthesis of BCPs are focused on diblock copolymers, the process is also applicable to BCPs containing three or more blocks such as triblock terpolymers (TBTPs). [START_REF] Aissou | Highly Ordered Nanoring Arrays Formed by Templated Si-Containing Triblock Terpolymer Thin Films[END_REF] In this work we demonstrate the infiltration of inorganic species into poly(1,1-dimethyl silacyclobutane-block-styrene-block-lactide) (PDMSB-b-PS-b-PLA or DSL), a silicon-containing TBTP with two organic blocks, PS and PLA. Our previous study of one composition of DSL reported the formation of microdomains consisting of core-shell cylinders perpendicular to perforated lamellae, based on SEM and grazing-incidence small-angle X-ray scattering (GISAXS) characterization. [START_REF] Lee | In Situ Study of ABC Triblock Terpolymer Self-Assembly under Solvent Vapor Annealing[END_REF] The PDMSB was clearly resolved but it was difficult to differentiate between the PLA and the PS blocks. In this work, we demonstrate the effects of ZnO infiltration into a series of DSL compositions that includes both lamellar and lamellar plus hexagonal cylinder morphologies. We first investigate the phase behavior of four DSLs with different volume fractions of each block and then show that infiltration synthesis introduces ZnO into the PLA microdomains. These results not only reveal the microdomain morphologies of the DSL, but demonstrate the formation of nanocomposites with ZnO, silica and organic components. DSL (Figure 1a) was synthesized using anionic polymerization combined with ring opening polymerization as described in the Methods section and Ref (23). We previously described the syntheses of PDMSB-b-PS-b-poly(methyl methacrylate) (DSM) [START_REF] Lee | In Situ Study of ABC Triblock Terpolymer Self-Assembly under Solvent Vapor Annealing[END_REF][START_REF] Aissou | Highly Ordered Nanoring Arrays Formed by Templated Si-Containing Triblock Terpolymer Thin Films[END_REF][START_REF] Aissou | Periodic Bicontinuous Structures Formed on the Top Surface of Asymmetric Triblock Terpolymer Thick Films[END_REF] and PDMSB-b-PS-b-poly(-2-vinylpyridine) (DSV). [START_REF] Aissou | Bicontinuous Network Nanostructure with Tunable Thickness Formed on Asymmetric Triblock Terpolymer Thick Films[END_REF] The PDMSB (not to be confused with PDMS, polydimethylsiloxane) monomeric unit is composed of butane with one Si atom in the backbone. DSL films of 180-260 nm thicknesses were spin cast onto Si substrates and annealed in a solvent vapor flow of CHCl3.

RESULTS AND DISCUSSION

The organic blocks were etched by oxygen plasma reactive ion etching (RIE) and both top and side views of the structure were characterized by SEM. Further analysis was carried out using transmission and scanning transmission electron microscopy (TEM/STEM) and elemental mapping using energy dispersive spectroscopy (EDS). The bulk morphology of vacuum annealed samples was characterized using small angle x-ray scattering (SAXS). correspond to the out-of-plane lamellar spacing and the in-plane pore spacing respectively.

Four DSL compositions were used in this study (Table 1). DSL-L1 and DSL-L2 had symmetric PDMSB and PS volume fractions, while DSL-H1 and DSL-H2 had a volume fraction of PS that was ~ 1.5 times that of PDMSB. DSL-H2 is the same polymer composition reported in Ref (23).

The The Lam structure agrees with the bulk morphology of the DSL-L1, but the DSL-L2 was Hex in bulk, and the Lam thin film structure may be a result of the preferential swelling of PDMSB domains by CHCl3 during the solvent annealing leading to an effective increase in volume fraction.

Note that differences between the bulk and thin film morphologies have also been observed for both DSV [START_REF] Aissou | Bicontinuous Network Nanostructure with Tunable Thickness Formed on Asymmetric Triblock Terpolymer Thick Films[END_REF] and DSM [START_REF] Aissou | Periodic Bicontinuous Structures Formed on the Top Surface of Asymmetric Triblock Terpolymer Thick Films[END_REF] samples.

For the DSL-H1 and H2 films, the top view SEM images (Figure 1b and S2) clearly exhibit a hexagonal pore array attributed to out-of-plane cylindrical morphology, Hex⊥. The pores show a core-shell morphology assumed to consist of a PS shell and a PLA core, evident from a difference in their etching rate in the O2 plasma. To examine the structure at different depths, we introduced an angled plasma RIE followed by SEM imaging from the top surface (see Methods). This reveals terraces showing vertical alignment of the core-shell cylinders throughout the film, in agreement with the DSL-H2 results in Ref (23). Cross-section imaging of both DSL-H1 and H2 show inplane Lam// with vertical features corresponding to the cylinders passing through the perforations in the lamellae. We therefore describe the structure as Lam//-Hex⊥. Our earlier work [START_REF] Lee | In Situ Study of ABC Triblock Terpolymer Self-Assembly under Solvent Vapor Annealing[END_REF] on DSL-H2 also showed about 20 % of the film had a morphology attributed to alternating in-plane cylindrical microdomains (Alt//); a similar result was found in the present work for DSL-H2 but no in-plane cylinders were found in DSL-H1.

To clarify the distribution of PS and PLA within the microphase-separated DSL films, we employed infiltration synthesis to infuse ZnO selectively within the PLA microdomains. ZnO infiltration consisted of cyclic exposures of the BCP film to diethylzinc (DEZ) and water, which are precursors respectively for Zn and O. In total 6 cycles were performed, assisted by an initial single AlOx infiltration cycle using trimethylaluminum (TMA) and water (referred to as AlOx priming). [START_REF] Peng | A Route to Nanoscopic Materials via Sequential Infiltration Synthesis on Block Copolymer Templates[END_REF][START_REF] Kamcev | Chemically Enhancing Block Copolymers for Block-Selective Synthesis of Self-Assembled Metal Oxide Nanostructures[END_REF][START_REF] Ocola | Infiltrated Zinc Oxide in Poly(methyl methacrylate): An Atomic Cycle Growth Study[END_REF] During an infiltration synthesis cycle, gaseous organometallic precursors, which are typically Lewis acids, diffuse into the polymer matrix and react with Lewis-basic functional groups within the polymer to form chemical bonds. A subsequent exposure of the film to an oxidizing agent, such as water, transforms the pre-infiltrated organometallic precursor into metal oxide molecules. In the DSL films, only the PLA block contains a Lewis-basic carbonyl group and, thus, metal oxide moieties are expected to be selectively infiltrated into the PLA domain. However, similar to the well-studied PMMA, the carbonyl group within PLA does not readily react with weakly Lewis-acidic DEZ. [START_REF] Peng | A Route to Nanoscopic Materials via Sequential Infiltration Synthesis on Block Copolymer Templates[END_REF][START_REF] Subramanian | Three-Dimensional Electroactive ZnO Nanomesh Directly Derived from Hierarchically Self-Assembled Block Copolymer Thin Films[END_REF][START_REF] Kamcev | Chemically Enhancing Block Copolymers for Block-Selective Synthesis of Self-Assembled Metal Oxide Nanostructures[END_REF][START_REF] Ocola | Infiltrated Zinc Oxide in Poly(methyl methacrylate): An Atomic Cycle Growth Study[END_REF] This was mitigated by the AlOx priming, because the more Lewis-acidic TMA can react better with the carbonyl group and generate -Al-OH species in the PLA domain once the priming cycle is completed with water. The available OH group then can readily react with DEZ, facilitating the subsequent infiltration cycles of ZnO. for DSM where the PMMA cores of core-shell cylinders were decorated with alumina. [START_REF] Aissou | Highly Ordered Nanoring Arrays Formed by Templated Si-Containing Triblock Terpolymer Thin Films[END_REF] DSL-L2 which includes a larger fraction of PLA (fPLA = 0.28) exhibits more homogeneous ZnO layers in between PS lamellae (Figure S3 and S4).

The ZnO-infiltrated DSL-L1 structure was analyzed by high-angle annular dark-field (HAADF) imaging and EDS elemental mapping (Figure 2c). HAADF images sensitive to Z-contrast reveal the three-color lamellar structure with ZnO present in the PLA throughout the film thickness. This demonstrates the potential of this approach to resolve the internal structure of even thick films.

Elemental mapping shows that as expected, Si is uniformly distributed in the PDMSB domain which exhibit sharp interfaces, but oxygen is also present. In fact, the oxidation appears to have occurred before the ZnO infiltration (Figure S5), likely due to ambient oxygen and water. The majority of Zn is selectively infiltrated into PLA but there are also ZnO particles present in the PDMSB, presumably due to its prior oxidation which could provide an increased reactivity of the PDMSB toward DEZ. Kinetic entrapment of precursors driven by weak physisorption within the polymer matrix might also contribute, as has been proposed to explain AlOx and ZnO infiltration into PDMS. [START_REF] Yu | Sequential Infiltration Synthesis of Doped Polymer Films with Tunable Electrical Properties for Efficient Triboelectric Nanogenerator Development[END_REF][START_REF] Gong | Hydrophilic Mechanical Buffer Layers and Stable Hydrophilic Finishes on Polydimethylsiloxane Using Combined Sequential Vapor Infiltration and Atomic/Molecular Layer Deposition[END_REF][START_REF] Leng | Vapor Phase Infiltration (VPI) for Transforming Polymers into Organic-Inorganic Hybrid Materials: A Critical Review of Current Progress and Future Challenges[END_REF] It is noted that the nominally unreactive blocks around the PLA are not expected to retard the infiltration of ZnO into the targeted PLA domain because the absence of strong interaction between infiltrating precursors and unreactive polymer domains renders their diffusion through the inert domains fast, as confirmed by a recent study by Waldman et al. examining the infiltration of weakly interacting organometallic precursors in homopolymer thin films. [START_REF] Waldman | Sequential Infiltration Synthesis of Electronic Materials: Group 13 Oxides via Metal Alkyl Precursors[END_REF] This is consistent with previous reports that have shown a high-fidelity block-selective infiltration of various inorganic materials into isolated PMMA domains surrounded by a majority of unreactive PS domains. [START_REF] Peng | A Route to Nanoscopic Materials via Sequential Infiltration Synthesis on Block Copolymer Templates[END_REF][START_REF] Segal-Peretz | Characterizing the Three-Dimensional Structure of Block Copolymers via Sequential Infiltration Synthesis and Scanning Transmission Electron Tomography[END_REF] though the difference exceeds that predicted from the ratio of (molecular weight (Mn)) 2/3 , (Mn,L1/Mn,L2) 2/3 = 1.07. For both structures, the PDMSB domains are 1.5 times wider than the organic domains although the volume fraction of PDMSB is 36-41 %. This is attributed to preferential swelling of the PDMSB during solvent annealing which is at least partly preserved after drying. ZnO was infiltrated into DSL-H films via the same process as described in Figure 4a. Unlike DSL-L, the DSL-H forms Lam// through which Hex⊥ of PLA-PS pass perpendicular to the substrate, i.e. the SEM indicates a morphology consisting of both in-plane and out-of-plane periodic features, a result supported by GISAXS [START_REF] Lee | In Situ Study of ABC Triblock Terpolymer Self-Assembly under Solvent Vapor Annealing[END_REF] and also described for thick films of DSV. 25 This is in agreement with the morphology of DSL-H2 resolved by employing hydrolysis to selectively degrade the PLA domains. [START_REF] Lee | In Situ Study of ABC Triblock Terpolymer Self-Assembly under Solvent Vapor Annealing[END_REF] However, unlike DSL-H1, thicker ZnO-infused regions corresponding to the cores of the core-shell cylinders are not evident in the image, even though the SEM plain view and cross-section (Figure 1b) as well as the bright-field cross-section of the un-infiltrated DSL-H2 (Figure S5b) demonstrate continuous vertical cylinders. We assume that the low volume fraction of the PLA limited infiltration of Zn into the cylinder cores which were not resolved in Figure S6. We note that self-assembled BCP films in general can be swelled when exposed to infiltrating precursors as organic vapors can do, [START_REF] She | Effect of the Micelle Opening in Self-assembled Amphiphilic Block Co-polymer Films on the Infiltration of Inorganic Precursors[END_REF] but our TEM data clearly indicates no disruption of the self-assembled BCP morphology by the precursor infiltration. This is most likely due to the purging step included in the infiltration protocol, which removes unreacted precursor out of the BCP film.

Unlike the HPL phase in DSV thick films where the perforations are stacked in an AB sequence, [START_REF] Aissou | Bicontinuous Network Nanostructure with Tunable Thickness Formed on Asymmetric Triblock Terpolymer Thick Films[END_REF] the DSL-H in this work shows an AA stacking sequence according to the SEM and STEM images in Figure 1 and4, respectively. The AA sequence refers to the perforations aligned above each other in adjacent lamellae, i.e. cylinders penetrating orthogonal to the lamellae. This is the same structure as we proposed in Ref (23) and it is also the structure observed in thick films of DSM blended with PDMSB-b-PS, which results in fPDMSB/fPS/fPMMA = 0.31/0.45/0.24. [START_REF] Aissou | Periodic Bicontinuous Structures Formed on the Top Surface of Asymmetric Triblock Terpolymer Thick Films[END_REF] Thus, the two DSL-H here and the DSM blend in Ref (43) have similar volume fractions and are found to exhibit AA stacked Lam//-Hex⊥ under solvent annealing in CHCl3, a morphology which has not been reported in other BCPs.

We note that the demonstrated use of block-selective infiltration in TBTPs achieves TEM contrast enhancement in a similar manner to traditional polymer staining methods based on OsO4 and RuO4 reagents, with the former known to oxidize polymers with olefinic functionalities and the latter to react primarily with aromatic and unsaturated polymers. [START_REF] Wang | Interfacial Staining of a Phase-Separated Block Copolymer with Ruthenium Tetroxide[END_REF][START_REF] Loo | Direct Imaging of Polyethylene Crystallites within Block Copolymer Microdomains[END_REF][START_REF] Trent | Ruthenium Tetraoxide Staining of Polymers for Electron Microscopy[END_REF] However, these methods are either very slow (e.g., even up to several days), [START_REF] Loo | Direct Imaging of Polyethylene Crystallites within Block Copolymer Microdomains[END_REF] or difficult to perform due to the high toxicity of these metal oxides, and it may be hard to predict the domains to be stained due to various other chemical and structural characteristics known to affect the preferential reactivity of blocks. [START_REF] Trent | Ruthenium Tetraoxide Staining of Polymers for Electron Microscopy[END_REF] On the other hand, infiltration synthesis, relying on a relatively straightforward Lewis acid-base interaction between infiltrating precursors and polymers, allows us to predict the domain to be preferentially infiltrated and is quick and safe to implement using a commercial ALD system. terpolymers with different volume fractions of each block in Table 1 were synthesized via the combination of anionic polymerization by sequential addition of monomers and ring opening polymerization using a strong organic base as catalyst (also called pseudo anionic polymerization).

CONCLUSION

A hydroxyl-terminated poly(1,1-dimethylsilacyclobutane-b-styrene) (PDMSB-b-PS-OH) was first prepared by the sequential anionic polymerization of DMSB and styrene monomers in various compositions using tetrahydrofuran/heptane (1:1) as a solvent at -48 °C accompanied by the addition of ethylene oxide. This reaction was terminated using degassed methanol in order to achieve the desired end-functionalized symmetric and asymmetric PDMSB-b-PS BCPs. The ring opening polymerization of D, L-lactide was then performed in toluene at 80 °C using PDMSB-b-PS-OH as macroinitiator in the presence of catalyst triazabicyclodecene (TBD). [START_REF] Simón | The Mechanism of TBD-Catalyzed Ring-Opening Polymerization of Cyclic Esters[END_REF] 180-260 nm thick DSL films were obtained by spin-coating 4 wt% DSL solutions in a mixture of tetrahydrofuran and propylene glycol monomethyl ether acetate (THF/PGMEA = 2:1) onto silicon substrates with native oxide layer. The as-cast film thickness was determined by the spin speed of 3,000 rpm and measured using a spectral reflectometer (FilMetrics F20-UV). All the solvents used in this study were purchased from Sigma Aldrich.

Solvent Vapor Annealing of DSL Films. Solvent vapor annealing was carried out using a continuous flow system where separate streams of nitrogen are bubbled through liquid solvents and the flow of each stream is regulated using a mass flow controller (MKS Inc., M100B). [START_REF] Gotrik | Morphology Control in Block Copolymer Films Using Mixed Solvent Vapors[END_REF] Ascast DSL films on substrates were placed inside the glass annealing chamber (volume ~ 80 cm 3 ) tightly sealed by a quartz plate along with a perfluoroelastomer O-ring (Markez Inc., Z1210), and were annealed in a vapor flow of CHCl3 and N2 for ~ 1 h at ambient temperature. Solvent vapors of CHCl3:N2 = 10 sccm:1 sccm were introduced to achieve an optimal swelling ratio of ~ 2.1 and ~ 1.9 for DSL-L and DSL-H, respectively which produced well-ordered DSL morphologies. The inflow tube was connected to a mixer fed by two flow channels: pure N2 gas (Airgas, Inc., 99.9997 % purity) for diluting the vapor pressure of CHCl3 and a solvent bubbler which contained CHCl3.

During the deswelling, swollen films were slowly dried over 10 min by stopping the vapor flow of CHCl3 and N2 followed by gently detaching the quartz lid from the chamber at room temperature.

ZnO Infiltration Synthesis in DSL Films. The infiltration synthesis of ZnO was carried out in a commercial ALD system (Cambridge Nanotech, Savannah S100) at 85 °C using DEZ and TMA as metal organic precursors along with water as an oxidant. The infiltration synthesis process consists of 1 AlOx-priming cycle followed by 6 ZnO infiltration cycles. The priming step consists of: exposure to TMA for 100 s under a static vacuum (~ 350 Torr); chamber purging using N2 (100 sccm) for 100 s; exposure to water vapor (~ 10 Torr) for 100 s; and chamber purging using N2 (100 sccm) for 100 s. The following ZnO infiltration cycle consists of exposure to DEZ for 6 min under a static vacuum (~ 1.7 Torr); chamber purging using N2 (100 sccm) for 2 min; exposure to water vapor (~ 10 Torr) for 6 min; and chamber purging using N2 (100 sccm) for 2 min.

Characterization of DSL Films.

As-annealed DSL films in Figure 1 were first etched by RIE (Plasma-Therm 790) to enhance the imaging contrast followed by Zeiss Merlin high-resolution SEM characterization at 3 kV from multiple angles. For the SEM images in plain view, the PDMSB surface layer and microdomains of organic blocks (PS and PLA) were selectively removed by RIE treatment with CF4 (5 s, 15 mTorr, 50 W) and O2 (30 s, 6 mTorr, 90 W), respectively. Terraced regions of PDMSB were obtained by covering the half of the sample,

Figure 1 .

 1 Figure 1. (a) Chemical structure of DSL and schematic illustrations of self-assembled DSL films.

  SAXS data for the bulk DSLs are shown in FigureS1. The SAXS contrast originates from the difference in scattering between the PDMSB and the organic blocks. DSL-L1 showed lamellar (Lam) and DSL-L2 showed hexagonally packed cylindrical (Hex) morphologies with period L0 = 23.2 nm and 31.4 nm respectively. DSL-H1 exhibited broad peaks, due to its larger molecular weight, and has been tentatively indexed to hexagonally-perforated lamellae (HPL) with an AB stacking (L100 = 39.2 nm), i.e. HPL with the perforations in one layer located above the junctions in the adjacent layers, while DSL-H2 showed peaks from a mixture of Hex (L0 = 19.0 nm) and Lam (L0 = 36.9 nm) structures. SEM images of DSL films after oxygen RIE are shown in Figure1b. In both DSL-L1 and L2, the plan view images show no periodic features but in-plane lamellae (Lam//) are observed in the cross-sectional images. This technique does not reveal any contrast between PS and PLA microdomains, showing only the PDMSB lamellae alternating with the non-Si-containing blocks.

Figure 2 .

 2 Figure 2. (a) Schematic illustrations depicting the solvent vapor annealing (SVA) followed by

Figure 2a shows schematics

  Figure2ashows schematics of the proposed structure of the DSL-L thin films after infiltration,

Figure 3 .

 3 Figure 3. EDS elemental mappings of both Si (red) and Zn (blue) for DSL structures: (a) DSL-L1

Figure 4 .

 4 Figure 4. (a) Schematic illustrations depicting SVA followed by infiltration synthesis to produce

Figure 4b shows

  Figure 4bshows cross-sectional HAADF STEM data comparing the DSL-H1 morphology before

  In summary, a vapor-phase infiltration synthesis was demonstrated to selectively deposit inorganic material (ZnO) into an organic block (PLA) of a triblock terpolymer film. The infiltration extends over film thicknesses of at least 260 nm and enables resolution of microdomain morphologies and volume fractions of the blocks. The triblock terpolymer, DSL, shows a transition from a three-color lamellar structure to perforated lamellae plus perpendicular core-shell cylinders on decreasing the volume fraction of the PDMSB. Environmental oxidation of the PDMSB domains formed SiOx, and infiltration of the PLA domains with ZnO results in hierarchical ZnO/polymer/SiOx nanostructures. Infiltration synthesis therefore provides access to multicomponent nanocomposites with both lateral and vertical periodicities as well as a process for investigating complex 3D BCP morphologies.EXPERIMENTAL SECTIONMaterials andPreparation of DSL Films. Four linear PDMSB-b-PS-b-PLA (DSL) triblock

Table 1 .

 1 DSL properties, morphologies, and periods in thin films.

		Molecular weight, Mn [kg/mol]	Dispersity, Mw/Mn	Composition, fPDMSB/fPS/fPLA	Thin film thickness [nm]	Thin film morphology	Period [nm] a)
	DSL-L1	28.4	1.15	0.41/0.40/0.19	180	Lam//	21
	DSL-L2	25.8	1.12	0.36/0.36/0.28	210	Lam//	16
	DSL-H1	55.5	1.09	0.25/0.40/0.35	240	Lam//-Hex⊥	21, 38
	DSL-H2	38.4	1.07	0.31/0.53/0.16	260	Lam//-Hex⊥	17, 35

Lam, alternating lamellae; Hex, hexagonally packed core-shell cylinders; //, lateral alignment; ⊥, vertical alignment; a) Estimated from SEM and TEM images; the two values for DSL-H1 and H2

ACKNOWLEDGMENTS

The authors acknowledge the support of the Semiconductor Research Corporation and Tokyo Electron. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

ASSOCIATED CONTENT

etching the other half with CF4 (60 s, 15 mTorr, 50 W) and O2 (30 s, 6 mTorr, 90 W), and imaging the boundary between these two regions. This process yields oxidized PDMSB patterns that exhibit the morphology of the PDMSB microdomains. The cross-sections were prepared by cracking the sample in liquid nitrogen followed by RIE with O2 (5 s, 6 mTorr, 90 W) and imaged after tilting the sample 70° to the SEM detector. The as-infiltrated organic-inorganic hybrids were characterized by TEM (JEOL 2100; 200 kV), and scanning TEM (FEI Talos F200X; 200 kV; equipped with the EDS elemental mapping capability). The cross-sectional TEM samples were prepared by the standard in situ lift-out procedure using Ga ion milling in a focused ion beam system (FEI Helios 600 Nanolab).

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Additional information on SAXS patterns for the bulk morphologies, top view SEM images

showing the vertical core-shell cylinders, and cross-sectional TEM/STEM images with EDS areal elemental mappings for DSL-L and DSL-H films

AUTHOR INFORMATION

Corresponding Author

*E-mail: caross@mit.edu (C.A.R.).

Notes

The authors declare no competing financial interest