Block Copolymer Self-Assembly Directed Hierarchically Structured

Perspective Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Block Copolymer Self-Assembly Directed Hierarchically Structured Materials from Nonequilibrium Transient Laser Heating Kwan Wee Tan*,†,‡ and Ulrich Wiesner*,† †

Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, United States School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore

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ABSTRACT: We highlight two recent approaches operating far from equilibrium for the synthesis of hierarchical porous thin film materials by coupling block copolymer-directed selfassembly with transient laser heating. In first block copolymerinduced writing by transient heating experiments, or BWRITE, an all-organic block copolymer−resols hybrid film is heated by submillisecond carbon dioxide laser irradiation, directly generating 3D mesoporous continuous resin structures and shapes. Harnessing the highly unique resin materials properties under laser heating conditions, in the second approach block copolymer-directed resin templating is coupled with nanosecond pulsed excimer laser annealing to generate complementary crystalline silicon nanostructures. The underlying structure formation mechanisms for such laser-induced organic and inorganic nanostructured materials are discussed, emphasizing that the nonequilibrium nature of these transient laser annealing approaches opens up vast and new processing windows beyond traditional stability limits of organic polymer materials. Finally, we highlight opportunities and challenges for possible future research directions and applications of laserinduced block copolymer-directed hierarchical porous materials formation including structure control, materials diversification, scale-up, on-chip applications, and additive manufacturing, which may provide solutions in areas as diverse as catalysis, sensing, and energy storage and conversion.

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INTRODUCTION

BCP self-assembly under nonequilibrium conditions has been adapted for the scalable synthesis of hierarchically porous polymer materials.5,6,27−29 In a first example, Peinemann et al. combined BCP self-assembly with nonsolvent-induced phase separation (now referred to as SNIPS30) to form asymmetrical porous polymer membrane structures, consisting of an ∼100− 200 nm thick top surface layer with uniform vertically oriented cylindrical mesopores on top of a several tens of micrometers thick asymmetric meso- to macroporous substructure.27 While initial work focused on AB diblock copolymer-based SNIPS, as a result of improved mechanical properties subsequent studies employed ABC triblock terpolymers to generate asymmetric membranes.28 We developed an alternative approach to hierarchical structure formation combining spinodal decomposition and BCP phase separation by first mixing a BCP with a water-soluble additive in an organic solvent, subsequent solvent evaporation leading to hierarchical structure formation, and final washing out of the organic additive in water or alcohol to generate the hierarchical porosity.29 To impart new properties and functionality, both methods have been further optimized to generate BCP-derived hierarchical hybrid and fully inorganic materials of carbon, metals, and metal oxides.31−36 An example demonstrating the power of using concepts in polymer physics to generate inorganic materials

Porous and hierarchically structured materials are highly desirable for a plethora of applications, ranging from separation, sensing, and energy conversion and storage to tissue engineering.1−6 Their often multifunctional properties, including the combination of high and accessible surface area with fast mass transport, are a collective consequence of their multiscale architecture and individual component chemistry. Despite considerable research interest, the tunable synthesis or fabrication of hierarchically ordered materials with the desired properties from simple approaches remains challenging. Block copolymer (BCP) self-assembly provides a direct pathway to construct a variety of two- and three-dimensional (2D/3D) periodic morphologies with periodicities typically ranging from 5 to 50 nm.7,8 In particular, BCPs have been employed as structure-directing agents to self-assemble organic and inorganic additives or nanoparticles into novel organic− organic and organic−inorganic hybrid nanostructured materials with unique property profiles.9−18 Specific properties and functionalities can be incorporated into BCP-directed hybrid nanostructured materials by tuning macromolecular chemistry and morphology as well as by additive materials selection.19,20 While the growth of self-assembled BCP-directed hybrid nanostructures at or close to equilibrium conditions is now reasonably well-understood experimentally as well as theoretically,21−25 it is often time-consuming and limits accessible structures and functionality.26 © XXXX American Chemical Society

Received: September 21, 2018 Revised: December 18, 2018

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DOI: 10.1021/acs.macromol.8b01766 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Transient laser heating-induced hierarchical structured materials. (a) Direct laser writing in commercial resins to form a microbull model (left panel) and a photonic crystal (right panel). Adapted with permission from refs 49 and 50, respectively. (b) Laser-assisted nanoimprinting of silicon grating arrays. Adapted with permission from ref 40. (c) Infrared laser-induced transformation of polyimide (lower left panel) and reduction of graphite oxide (right panel) into graphene structures. Adapted with permission from refs 43 and 42, respectively. (d) Millisecond laser-induced alignment of periodic BCP thin films. Adapted with permission from ref 51. (e) Nanosecond laser annealing of BCP-directed single crystal epitaxial semiconductor and metal nanostructures. Adapted with permission from ref 41. (f) All-organic BCP templating of laser-induced 3D periodic silicon nanostructures. Adapted with permission from ref 44.

with exquisite control over hierarchical structures is recent work in which the segregation of one or two homopolymers in the presence of block copolymers directs the assembly of various inorganic materials into mesoporous spheres and mesoporous hollow spheres.37 An alternative and emerging approach operating far from equilibrium is the transient laser heating of polymeric structures on nano- to millisecond time scales to generate advanced hierarchical functional materials. Well established and routinely deployed in multiple industries, laser thermal processing has many advantages, including (1) spatially controlled heating and pattern transfer of nanoscale morphologies and shapes on solid and flexible substrates,38−44 (2) ultrafast materials phase transformations,40−47 and (3) enhancement of materials properties over conventional temperature limits enabling expansion of processing windows.48 Figure 1 summarizes recent examples for laser-induced organic and inorganic hierarchical and functional structure formation. In particular, the combination of bottom-up BCP selfassembly with top-down transient laser heating is expected to open new opportunities for the fabrication of novel functional

nano- and mesostructured materials and their applications. After setting the stage by discussing related work on the thermal behavior of polymer thin films under transient laser heating, this Perspective focuses on recent methods developed by the Wiesner group at Cornell University, coupling BCP selfassembly with transient laser heating to generate laser-induced BCP-directed organic and inorganic hierarchical porous structures. In addition to reviewing results from past and ongoing studies, we finally speculate about possible future research and development directions of this hybrid approach, which ultimately may lead to both improved fundamental understanding of the underlying physical and chemical processes and identification of new application fields for the resulting materials.

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THERMAL BEHAVIOR OF POLYMER THIN FILMS UNDER TRANSIENT LASER HEATING Laser thermal processing is routinely deployed in many different applications, in particular in the microelectronics industry,52 including laser lithography patterning of silicon grating arrays for thin film transistor fabrication40 (Figure 1b), excimer laser annealing of polycrystalline silicon for flat display B


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Macromolecules panel applications,53 and laser spike annealing for dopant activation in silicon.54 To generate more complex patterns, femtosecond pulsed lasers have been employed for direct laser writing of submicrometer 3D animal models and photonic crystals in commercial resins (Figure 1a).49,50 To achieve finer feature sizes, however, processing cost and time increase tremendously as more sophisticated instrumentation, optics, and exclusive designer photoresists are required.55−58 To overcome some of these limitations, self-assembly of BCPs has been coupled with various nonequilibrium processing techniques enabling access to rapid and scalable fabrication of complex periodic nanoscale structures. Organic BCP thin film materials have been widely adapted for a number of applications, including microelectronics,59,60 hybrid photovoltaics,61,62 and as functional templates.59,63 BCP thin films prepared by spin coating are typically kinetically trapped in a disordered state characterized by short-range order due to rapid drying conditions. Nonequilibrium postpreparation processing methods such as transient laser heating,51,64−69 microwave annealing,70,71 thermal flash annealing,72,73 and cold zone annealing74,75 have been explored to obtain well-defined periodic BCP morphologies and pattern alignment over large areas on very short time scales. It should be noted that while the above-mentioned large-area (nonlaser) heating techniques (e.g., thermal flash annealing) can enable rapid BCP self-assembly within millisecond time frames, they also generate large thermal stresses in BCP films and cause warpage of substrates due to the buildup of enormous temperature gradients.76 Tang and Tsai published an early report on laser heating of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) thin films with a pulsed carbon dioxide (CO2) laser to rapidly improve morphology reordering kinetics.64 During laser scanning, the locally heated areas experience high temperatures and thermal gradients, enhancing mobility of BCP chains and self-assembling kinetics by several orders of magnitude. However, the laser-heated PS-b-PMMA films were significantly damaged after application of multiple laser scans at high laser powers.64 To improve film thermal stability and ordering kinetics, Singer et al. combined solvent vapor annealing with laser heating of polystyrene-block-poly(dimethylsiloxane) (PSb-PDMS) films to form well-defined cylindrical microdomains and pattern alignment with respect to the laser scan direction.65 Jin and co-workers demonstrated laser-induced reordering and alignment of lamellar-forming PS-b-PMMA films (with thickness up to ∼300 nm) on high curvature glass and flexible polyimide substrates with chemically modified graphene (CMG) as the laser-absorbing overlayer (Figure 2a).66 Majewski and Yager exploited the combined effects of millisecond laser heating with shear fields to generate highly impressive uniform alignment of polystyrene-block-poly(2vinylpyridine) (PS-b-P2VP) cylinders (Figure 1d)51 as well as periodic 2D arbitrary superlattice structures grown layer by layer.67 Selective infiltration of metal oxide and metal precursors into the P2VP block, followed by a separate etching process to remove the PS block, yielded inorganic 2D nanoscale mesh structures (Figure 2b).67 While still relatively unexplored, transient laser heating of BCP-directed hybrid systems is expected to further expand in terms of process and system space to obtain multifunctional mesoporous structures and shapes. To that end, Jung et al.48 investigated the thermal behavior of a model copolymer photoresist under conventional hot plate

Figure 2. (a) Schematic of area-selective BCP self-assembly on a CMG/substrate under laser irradiation (first row, left panel) and corresponding SEM micrographs of BCP films formed on a flexible CMG/polyimide substrate with the lamellar morphology (first and second rows). Adapted with permission from ref 66. (b) SEM micrographs of various two-layered (third row) and three-layered (fourth row) platinum nanomeshes after metallization and BCP removal. Adapted with permission from ref 67.

bake and transient CO2 laser heating for heating dwells of 60 s−500 μs. Figure 3 shows that the copolymer photoresist exhibited essentially identical thermal behavior for all heating durations up to temperatures of 800 °C. Even more surprising was that polymer thermal stability increased by ∼400 °C as heating dwell was reduced by about 5 orders of magnitude (from 60 s to 500 μs). Moreover, by defining the lateral temperature gradients across the width of a single laser scan (i.e., units of °C/μm), transient laser heating can provide greater spatial and temporal controls to develop a deeper fundamental understanding of thermal and kinetic BCP selfassembly behaviors in otherwise inaccessible temperature processing windows.48,68,77 An interesting question arising from the work of Jung et al. was whether a polymer film could tolerate even higher temperature ranges (e.g., >1000 °C) by further reducing heating dwells into nanosecond time scales, thereby potentially opening an entirely new field of study.

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SUBMILLISECOND TRANSIENT LASER HEATING OF ALL-ORGANIC BCP-DIRECTED NANOCOMPOSITES We have developed a simple and rapid method enabling direct generation of meso- and macroscopic continuous porous C


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(Figure 4b). Infrared photons are absorbed by the Si substrate, thereby increasing the local film temperature on submillisecond time frames, followed by cooling at similar rates via thermal conduction into the substrate.48 During the submillisecond CO2 laser scanning process, the (negative-tone) resols additive thermopolymerizes into a continuous resin framework, while the (positive-tone) BCP simultaneously decomposes forming a pore network, yielding well-defined mesoporous resin structures (Figure 4b). Finally, non-laser-heated regions are removed in a solvent rinse, leaving 3D mesoporous continuous resin relief structures on the substrate (Figure 4c). The first systems studied using B-WRITE were hybrid mixtures of polyisoprene-block-polystyrene-block-poly(ethylene oxide) (PI-b-PS-b-PEO, ISO) triblock terpolymers with molar masses of 38 and 69 kg/mol (ISO-38/69) and resorcinol− formaldehyde resols (R) as the additive. The ISO-38/69-R hybrid samples were thermally cured and then scanned by sequential CO2 laser irradiations for 0.5 ms dwell to induce formation of B-WRITE mesoporous structures and shapes. Figure 4d shows macroscopic B-WRITE resin patterns in an ISO-38-R film sample after laser heating. Each ISO-38-R based trench was ∼2 cm long and 160 μm wide, containing ∼200 nm thick continuous network resin nanostructures (see scanning electron microscopy (SEM) images in Figure 4e,f), and notably only required ∼4.5 min to fabricate in air. Even more striking is the cross-section SEM micrograph of the laserheated ISO-69-R sample shown in Figure 4g, clearly demonstrating highly uniform mesoporous resin structure formation and coverage over large areas via B-WRITE. Tuning the BCP−resols composition and BCP molar mass provided control over pore size and pore size distribution. Figure 5 shows that the pore diameter and pore size

Figure 3. Thickness plot of copolymer photoresist film remaining as a function of temperature heated by CO2 laser heating (for dwells of 0.5, 1, and 5 ms) as compared to hot plate bake (60 s). Adapted with permission from ref 48.

structures and shapes by combining BCP-directed resols selfassembly with transient laser heating.44 BCP-induced writing by transient heating experiments (B-WRITE) is schematically illustrated in Figure 4. A mixture of structure-directing BCP and phenol/resorcinol−formaldehyde resols as the additive is dissolved in an organic solvent and spin-coated as an allorganic hybrid thin film on a silicon (Si) substrate (Figure 4a). The hybrid film is then scanned at predefined locations by a line-focused continuous wave CO2 laser (wavelength λ = 10.6 μm) for submillisecond time dwells under ambient conditions

Figure 4. (a−c) Schematic illustration of the B-WRITE process. Optical images of macroscopic linear B-WRITE patterns in (d) sample ISO-38-R film and (h) laser-heated ISO-69-R sample after solvent rinsing. SEM micrographs of (e) plan view and (f) cross section of laser-heated ISO-38-R structure depicted in (d) as well as (g, i) representative cross sections of laser-heated ISO-69-R structures. Inset in (g) shows the highlighted area at higher magnification. Adapted with permission from ref 44. D


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Figure 5. Plan view and cross section (insets) SEM micrographs of B-WRITE mesoporous resin structures with tailored pore size and pore size distribution. Adapted with permission from ref 44.

function of laser power (and corresponding temperature78). First, all the organic films remained thermally stable up to about 550 °C (≤35 W). Interestingly at 670 °C (40 W), the ISO-69 films experienced a rapid thickness loss attributed to laser heating-induced thermal decomposition. In contrast, under the same conditions the cross-linked resols films retained >50% of the original thickness. Notably a mixed behavior was observed in the ISO-69-R hybrid films that contained both ISO-69 and resols components. Specifically, a laser-induced temperature of 670 °C (40 W) was sufficient to induce simultaneous ISO-69 thermal decomposition, resulting in pore network formation, and resols thermopolymerization, resulting in resin framework formation. These results provided insights into the optimal transient laser heating conditions for rapid but controlled generation of mesoporous resin structures and were in stark contrast to the behavior of 1 h furnace heat treated hybrid samples, which decomposed at temperatures ∼600 °C lower as compared to the short times of transient laser treatments; compare the black dotted and solid curves in Figure 6 (all in air). Next, sequential laser irradiations of increasing laser powers (from 10 to 40 W for 0.5 ms dwell) in B-WRITE to heat the BCP−resols hybrid films yielded uniform 3D mesoporous continuous resin structure formation and film coverage (see SEM images in Figures 4 and 5). On the basis of these results, we hypothesized that B-WRITE promotes (1) controlled laserinduced BCP decomposition to enable gradual release of gaseous decomposition products and to reduce laser-induced thermal stresses as well as (2) increased cross-linking of resols, leading to enhanced mechanical and thermal properties. With the “soft” but “tough” organic materials properties, the resultant 3D mesoporous continuous resin structures were able to accommodate the laser-induced thermal and mechanical stresses and retain the structural integrity and mesoscopic morphology. B-WRITE provides a simple and rapid pathway to the generation of 3D mesoporous structures and macroscopic patterns that is compatible with standard semiconductor manufacturing processes. Figure 7 further demonstrates the versatility of the B-WRITE process. ISO-69-R hybrid films were spin-coated onto lithographically patterned Si substrates and converted into laser-heated mesoporous resin structures via B-WRITE. Cross-sectional SEM micrographs in Figure 7a− c shows that despite the complex surface topography, BWRITE mesoporous resin structure formation was highly homogeneous across and within Si holes and trenches. Further heat treatment of the B-WRITE resin structures at 800 °C under nitrogen yielded electrically conductive disordered carbon structures (Figure 7d), with the 3D continuous

distribution of transient laser-heated ISO-69-R structures were reduced from ∼30−200 nm (Figure 5a) to ∼20−50 nm (Figure 5b) by decreasing the ISO-69 to resols mass ratio. Using a small molar mass BCP such as poly(ethylene oxide)block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic F127) mixed with phenol−formaldehyde resols further reduced pore size and pore size distribution to ∼10−30 nm (Figure 5c) and demonstrated use of commercial block copolymer products in B-WRITE. To further enhance simplicity, B-WRITE was directly applied to as-deposited ISO−resols films, i.e., without pretreatment via thermal curing. Figure 4e shows B-WRITE ISO-69-R-based line patterns on a clean Si substrate after transient laser heating and solvent rinsing to remove the nonlaser-heated hybrid film sections (see schematic in Figure 4c and SEM in Figure 4i). Adjustment of the laser heating parameters such as laser power, dwell, and scan direction enabled selective removal of resin material to form more complex shapes, e.g., demonstrated via use of orthogonal scans in Figure 4h. Thermal behavior of all-organic hybrid thin films under transient laser heating in air could be understood by plotting remaining film thickness (Figure 6) of ISO-69 (blue solid curve), resols (orange solid curve), and ISO-69-R (black solid curve) films after a single CO2 laser irradiation for 0.5 ms as a

Figure 6. Thickness plot of ISO-69, resols, and ISO-69-R hybrid thin films heated by a single CO2 laser irradiation for 0.5 ms dwell and compared to furnace heated (1 h) hybrids. Adapted with permission from ref 44. E


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NANOSECOND EXCIMER LASER-BASED TRANSIENT ANNEALING INDUCED BCP-DIRECTED SINGLE-CRYSTAL STRUCTURES Transient laser heating at very short time scales can enable transient enhancement of materials properties such as thermal stability and structural properties and expand the process window to conventionally inaccessible temperatures. To that end, we demonstrated a novel approach coupling BCP-directed inorganic templating with nanosecond pulsed excimer transient laser annealing to synthesize complex single-crystal Si (semiconductor) and nickel silicide (metal) nanostructures with epitaxial relations to a single-crystal Si substrate.41 A schematic of these experiments together with the corresponding electron microscopy micrographs is shown in Figure 8. An ISO triblock terpolymer was employed as structure-directing agent for niobium oxide (Nb2O5) sol precursors and spin-coated as an organic−inorganic hybrid thin film on a single-crystal Si substrate. Mesoporous BCP-directed Nb2O5 network structures were obtained after removal of the organic components by oxygen plasma etching (Figure 8a,d). To achieve singlecrystal epitaxy, the native SiO2 layer on the substrate was subsequently removed by immersing in 0.5% hydrofluoric (HF) acid solution for ∼35 s, followed by filling the pore network with amorphous Si (a-Si) via chemical vapor deposition (CVD) (Figure 8b). The sample was then irradiated with 40 ns pulsed XeCl excimer laser irradiation (λ = 308 nm) to melt the a-Si precursor into the substrate, followed within ∼100 ns by solidification with the Si singlecrystal orientation directed epitaxially by the substrate. Finally, the inorganic metal oxide template was removed by concentrated HF, yielding the resultant ∼100 nm thick mesoporous single-crystal epitaxial Si network structures (Figure 8c,e). The congruence of lattice fringes in a singlecrystal Si strut and substrate displayed in the cross-sectional high-resolution transmission electron microscopy image in Figure 8f confirmed the laser-induced epitaxial relationship. A clean interface between the a-Si precursor and single-crystal Si substrate is the key for laser-induced melt solidification into a

Figure 7. (a−c) Cross-sectional SEM micrographs of B-WRITE hierarchical porous resin structures on lithographically patterned Si substrates. Inset in (c) shows the highlighted region at higher magnification. (d) Current/voltage plot of B-WRITE carbon mesostructures carbonized at 800 °C. Adapted with permission from ref 44.

network morphology remaining intact. Results suggested that B-WRITE could be suitable to generate porous structures with high surface area and connectivity for miniaturized device fabrication with applications ranging from sensing to energy storage applications (vide infra).

Figure 8. (a−c) Schematic and (d−f) electron microscopy micrographs of complex BCP-directed single-crystal Si nanostructure formation. Adapted with permission from ref 41. F


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Figure 9. SEM micrographs of all-organic BCP-directed c-Si nanostructures by pulsed excimer laser annealing. (a−c) Cross sections and plan views (insets) of (a) mesoporous resin template by pyrolysis, (b) deposition of a-Si by CVD, and (c) inverse c-Si nanostructures after pulsed excimer laser annealing and template removal. Adapted with permission from ref 47. (d) Cross section of B-WRITE-derived c-Si nanostructures. (e, f) Higher resolution cross-section and plan view of region indicted in (d). (g−j) Plan views and cross sections of (g, h) mesoporous gyroidal resin template after pyrolysis as well as of (i, j) corresponding excimer laser-induced periodic c-Si nanostructures after template removal. Adapted with permission from ref 44.

challenges: (1) Well-defined, periodically ordered morphologies can be obtained in BCP−resols hybrid films after spin coating by postpreparation treatments such as thermal and solvent vapor annealing techniques.79−82 (2) Organic resin and carbon nanostructures exhibit high chemical resistance to common SiO2 etchants such as HF acid and aqueous potassium hydroxide solution.81−83 (3) B-WRITE can be employed to rapidly generate BCP-directed hierarchical resin templates for the laser-induced melt-crystallization process. As demonstrated in Figure 6, the mesoporous organic resin structures exhibited excellent thermal and structural stability at elevated temperatures during transient laser heating, i.e., up to 1000 °C for 0.5 ms.44 The question was whether the same resin structures would withstand temperatures above 1250 °C, i.e, the melting temperature of a-Si, necessary for crystalline Si nanostructure formation. Surprisingly, indeed that was the case. SEM images shown in Figure 9a−c illustrate the different stages of the corresponding synthesis process,47 which was similar to our earlier study41 (compare to schematic in Figure 8a−c). All-organic ISO-R hybrid films were spin-coated on Si substrates and pyrolyzed at 450 °C under nitrogen to form mesoporous resin network structures (Figure 9a). The resin template samples were immersed in 0.5% HF solution for ∼2 min to remove the native SiO2 layer on the Si substrate, followed by a-Si precursor backfilling into the pore network by CVD (Figure 9b).84 The a-Si/resin composite samples were then irradiated by a single excimer laser pulse to melt the a-Si

single-crystal nanostructure. For example, immersing the BCPdirected Nb2O5 template in 0.5% HF for only ∼20 s was insufficient to remove the native SiO2 layer, resulting in polycrystalline Si nanostructures. Laser irradiation performed through a mask led to the formation of hierarchically structured crystalline Si structures. In separate experiments, coupling colloidal crystal silica templating with pulsed excimer laser annealing yielded submicrometer 2D and 3D crystalline Si inverse colloidal structures with non-close-packed hexagonal symmetry.46 While these experiments successfully demonstrated first examples of synthesis of complex transient laser annealing induced 3D single-crystal and polycrystalline Si nanostructures with multiple length scale features, significant challenges remained originating from the use of BCP-directed inorganic (e.g., SiO2 and Nb2O5) templates. First, BCP-directed thin film metal oxide templates prepared by sol−gel chemistry are often limited to kinetically trapped disordered morphologies due to rapid reaction rates and fast drying conditions during spincoating. Second, oxide templates exhibit low selective etch resistance compared to the native SiO2 layer on Si substrates in the etching step necessary to enable single-crystal epitaxy, making it difficult to find optimized etching conditions. Third, using a combination of building blocks such as BCP and colloidal crystal templates to form hierarchical structures increases the overall processing complexity. The all-organic BCP−resols hybrid nanostructure system described in the first section met the aforementioned G


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Macromolecules precursor into the Si substrate, which subsequently solidified into crystalline Si (c-Si) phase after a melt duration of ∼50− 100 ns. Finally, the resin template was removed by wet acid solution treatments, leaving behind ∼400 nm thick complementing mesoporous c-Si nanostructures (Figure 9c). Despite immersing the mesoporous resin template in 0.5% HF solution before a-Si CVD, the excimer laser-induced Si network nanostructures were unexpectedly polycrystalline. This was attributed to regrowth of a new SiO2 layer on the Si substrate after the 0.5% HF treatment and before high vacuum was achieved in the static CVD chamber.84 Grazing incidence small-angle X-ray scattering (GISAXS) characterization of the mesoporous resin template (Figure 9a) and resulting inverse laser-induced c-Si nanostructure sample (Figure 9c) suggested effective pattern transfer from the resin template into laser-induced c-Si. Most notably, both local SEM (compare Figure 9a and Figure 9c) and millimeter scale GISAXS data (Figure 10a) supported that the organic resin template remained stable during the excimer laser-induced Si melt-crystallization process (at temperatures above 1250 °C for ∼50 ns), enabling 3D nano- and mesostructural pattern transfer.

To enable pattern transfer from organic resin templates into excimer laser-induced crystalline silicon nanostructures, an a-Si overlayer on the resin template and enhanced physical and chemical properties of the resin template during transient laser heating are essential. To avoid excimer laser ablation of the phenolic resin template, a dense a-Si overlayer was deposited on the organic template (see Figure 9b) to selectively absorb the ultraviolet spectrum photons and induce melting and solidification of a-Si. To correlate the resin template stability at temperatures of liquid Si (i.e., >1250 °C for ∼50 ns), pyrolyzed mesoporous resin template samples were irradiated with a single CO2 laser irradiation in air for dwells of 0.05−2 ms. The remaining film thickness as a function of transient laser temperatures was then plotted (Figure 10b).47 From this data it was apparent that the resin template material became increasing thermally stable as CO2 laser heating dwells were reduced into the submillisecond time frames. In particular, no noticeable film thickness reduction was detected in resin template samples laser heated for 0.05 ms dwell up to ∼1320 °C. Raman spectroscopy confirmed that the resin template samples were carbonized during transient laser heating. From these results it is likely that upon excimer laser irradiation the dense a-Si overlayer absorbs most of the laser energy to induce the Si melting process, thereby “shielding” the resin template from interactions with the ultraviolet photons. As the Si melt front moves into the substrate, as a result of the short time scales involved the resin material does not have enough time to oxidize and acts as a “hard” template85−87 for conformal liquid Si infiltration. The melt subsequently solidifies within ∼50 ns. Complex 3D c-Si nanostructures could be generated using B-WRITE-derived mesoporous resin templates as shown in Figure 9d−f (compare with B-WRITE resin template in Figure 5a) via an all-laser-induced process, i.e., mesoporous resin template formation by B-WRITE, a-Si deposition, pulsed excimer laser annealing and template removal. Finally, it is worth emphasizing that a key advantage of BCP-directed selfassembly is straightforward access to 3D periodically ordered morphologies at the nanoscale and pattern transfer into other functional materials. To that end, first proof-of-principle experiments indicated it is possible to obtain 3D periodic, networked excimer laser-induced c-Si nanostructures (Figure 9i,j) from a highly ordered resin template with the bicontinuous alternating gyroidal morphology18 (Figure 9g,h). These results together demonstrated that soft matter self-assembly derived porous 3D structure formation in thin films in combination with hard matter backfilling and transient laser annealing paves the way for the formation of well-defined, highly ordered single-crystal epitaxial nano- and mesostructures of varying morphologies. Such nano- and mesostructures may become useful in answering fundamental questions about structure−property correlations in nano- and mesostructured materials. They may also find their way into diverse applications ranging from microelectronics and sensing to energy conversion and storage.

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OUTLOOK: OPPORTUNITIES AND CHALLENGES Laser processing technology is well-established in many manufacturing industries as it enables high speed and accurate processing (e.g., laser cutting, welding, engraving, and surface treatment) of a wide range of materials (such as glass, metals, plastics, and semiconductors). Coupling bottom-up selfassembly with top-down laser processing provides a highly accessible technique to directly generate 3D nanostructures

Figure 10. (a) Azimuthally integrated intensity plots of GISAXS patterns of mesoporous resin template and inverse laser-induced c-Si nanostructure after template removal. (b) Film thickness plots of pyrolyzed mesoporous resin template samples heated by a single CO2 laser irradiation for different submillisecond time dwells as indicated. Adapted with permission from ref 47. H


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(data not shown). We hypothesize that “hard” and “inflexible” BCP−metal oxide hybrid films are unable to accommodate the rapidly generated thermal stresses as well as mechanical distortions from the escaping BCP decomposition products at such short time scales, leading to film damage. A potential workaround solution is to blend BCP and inorganic precursors with oligomeric resols additives. Cooperative self-assembly of multicomponent BCP-directed hybrid mixtures are wellstudied with many examples, such as F127−resols−silicon dioxide,83 F127−resols−aluminum oxide,104 and polystyreneblock-poly(ethylene oxide)−resols−silicon dioxide,36 as well as other alternative BCP−organic−inorganic hybrid systems.105−108 The addition of “soft” resols is expected to improve flexibility and toughness of the tricomponent BCP− resols−metal oxide hybrid films to withstand the laser-induced mechanical and thermal stresses and retain mesoporous structure integrity. Removal of the resin component via oxidation would yield inorganic laser-induced metal oxide films.36,83 Alternatively, the resin material in the transient laser heating induced organic−inorganic hybrid material could be carbonized to form multifunctional mesoporous carbon/metal oxide104 or carbon/metal carbide108 nanocomposites. Scale-Up of Laser Processing of Self-Assembly Systems. B-WRITE enables expansion of process and systems space to obtain multifunctional mesoporous BCP structures and shapes. However, a current limitation for large-scale fabrication of transient laser heating induced hierarchical mesoporous materials is the serial mode of operation. To improve scalability and throughput, B-WRITE could be performed using a high-power continuous wave laser source with a line-focused beam profile coupled with beam splitting optics that would permit laser-induced heating of the BCPdirected hybrid films at several positions simultaneously. Furthermore, performing B-WRITE through an optical mask41,51 could generate more complex macroscopic patterns at higher resolutions and shorter processing duration. BWRITE could be further incorporated with roll-to-roll film processing74,109,110 to enable mass and volume production of laser-induced self-assembly directed hierarchical porous materials for a variety of applications. Transient Laser Heating Induced Hierarchical Mesoporous Materials for On-Chip Applications. Laser processing of polymeric materials is widely used in the fabrication of miniaturized on-chip structures for applications such as microfluidics,111,112 energy storage,42,43,113 and sensors.114 Coupling laser processing with BCP-directed selfassembly (B-WRITE) permits direct patterning of macroscopic shapes as well as control of pore size and pore size distribution at the nanoscale in the resultant 3D mesoporous resin structures. Furthermore, carbonization under an inert environment18,44 and direct laser heating47,115 yield electrically conductive hierarchical carbon structures (vide supra). As a proof of concept for microfluidic applications, we constructed high-surface-area microfluidic devices with B-WRITE hierarchical porous structures in the channel bottom to increase accessible surface area and mass flux potentially useful for microfluidic sensors and catalysis reactors (see Figure 11).44 BWRITE could also be used to write interdigitated patterns of 3D hierarchical carbon materials to fabricate microbatteries and supercapacitors with high energy and power densities.92,116−118 Surface functionalization of the hierarchical carbons119 and generation of alternative laser-induced multicomponent hybrid materials (vide supra) could enable

and shapes to facilitate novel nanotechnology-based applications. Here we provide our insights to highlight (1) challenges of nonequilibrium BCP self-assembly via transient laser heating, (2) new opportunities to advance our fundamental understanding, and (3) potential deployment of laser-induced hierarchical structures in integrated on-chip applications. Rapid Ordering of BCP Hybrid Nanostructures. Periodically ordered 3D BCP nanostructures, in particular the gyroid morphology, are of great interest for a number of scientific communities due to the highly accessible surface area and porosity, 3D interconnectivity, and exceptional mechanical stability.88 BCP gyroid structures usually consist of two interpenetrating gyroidal subvolumes that are fully 3D continuous, chiral, and related by an inversion operation. If these two subvolumes are constituted by the same material (e.g., in AB di-BCPs), the structure is referred to as a double gyroid (space group Ia3̅d).89 In the case that the two subvolumes are made up of different materials (e.g., in ABC triblock terpolymers) the structure is called the chiral single alternating gyroid (space group I4132).90 Based on structure− property correlations, 3D BCP gyroidal mesostructures (10− 100 nm) are expected to lead to a host of novel properties and applications, e.g., in energy conversion and storage,61,91,92 optical metamaterials,93−95 multifunctional catalysis,96−98 and more recently even in quantum materials in the form of superconductors and topologically protected Weyl materials.99,100 In order to obtain well-defined periodic morphologies in BCP−resols hybrid thin films, specifically the bicontinuous gyroidal nanostructure, we could couple B-WRITE with solvent vapor annealing65 to enhance mobility of the different components at lower laser-induced transient temperatures to prevent immobilization of the cross-linked resols. Solvent vapor annealing in mixed solvents could allow access to other periodic morphologies in the same laser-heated hybrid system.101 Fast removal of the solvent vapor to quench the desired morphology followed by B-WRITE would yield welldefined periodic mesoporous resin structures and shapes. Such ordered resin structures could serve as hard templates to shape other materials for novel functional properties and applications (e.g., see Figure 9g−j). Transient laser heating of BCP hybrid films under various controlled atmospheres (e.g., oxygen or fluorine-containing environments) could also provide new ways to further improve materials properties.102 To realize its full potential, it is vital to gain detailed understanding of nonequilibrium BCP self-assembly mechanisms at transient time scales, for example, by employing in situ GISAXS experiments to obtain real-time structure information.75,79,101,103 Finally, a recently developed technique enables transfer, for example, of solvent vapor annealed BCP−resols derived mesoporous thin films with self-assembled and nanostructured bicontinuous gyroidal morphology from silicon to other substrates,82 thereby providing expanded opportunities for epitaxial material solidification on substrates other than single-crystal silicon. Multicomponent BCP-Directed Hybrid Nanocomposites. Exploring transient laser heating of other BCP-directed hybrid systems, e.g., with metals, metal oxides, or quantum dots as additives, can introduce more functional properties and increase the accessible application space. However, in earlier BWRITE experiments submillisecond CO2 laser heating of ISOdirected metal oxide organic−inorganic hybrid films resulted in poorly developed, ruptured mesoporous inorganic thin films I


Perspective

Macromolecules

cracks (i.e., potential sites for crack initiation) in the 3D ceramic microlattice structures. We note that hierarchically porous structures comprised of well-ordered, 3D periodic and fully interconnected pore assemblies and solid constituents over the nano- to macroscopic length scales may also exhibit similar excellent mechanical and thermal properties.125,131−133 We envision an integrated approach combining nanoscale self-assembly with direct ink writing134,135 and transient laser heating to build well-ordered, multifunctional 3D hierarchical structures and devices with nanometer-level precision. Figure 12 shows a schematic of a possible hybrid approach.

Figure 11. (a) Schematic and (b) optical image of a B-WRITEderived microfluidic device on the probe station. (c) Bright-field and (d) fluorescence optical micrographs of a TRITC dye solution flowing through the high surface area microfluidic channel. Adapted with permission from ref 44. Figure 12. Additive manufacturing of self-assembly directed multifunctional, ordered hierarchical structures.

development of new materials with specific functions and properties. Based on the recent work by Jin et al.,66 B-WRITE could possibly be performed on modified polymers with a laser-absorbing overlayer on the substrate to generate BCPdirected hierarchical porous structures for potential flexible substrate applications. Integrating BCP Self-Assembly with Transient Laser Heating and Additive Manufacturing. Additive manufacturing (AM) enables design and direct fabrication of predefined complex 3D structures and shapes on the macroscopic length scale. For example, the 3D digital design can be read by a computer and divided into thin parallel “slices” which are then translated into the physical structure by assembling materials layer by layer.4,120,121 There are many variations of AM technologies, e.g., extrusion-based fused deposition modeling122 and direct ink writing,117,123 polymerization-based stereolithography,124−127 selective laser sintering,128,129 and selective laser melting,130 that are able to process different materials (e.g., polymers, ceramics, and metals) for use in aerospace, biomedical, and energy storage applications.121 Because the 3D objects and components are built using the same AM process, they exhibit high pattern fidelity (with respect to the design model) as well as exceptional structural properties. Eckel and co-workers recently reported construction of high strength 3D ceramics structures employing two different AM techniques of stereolithography and a self-propagating photopolymer waveguide (SPPW) method on specially designed photosensitive preceramic polymer resins.126 In particular, SPPW-derived silicon oxycarbide microlattice structures obtained after pyrolysis were completely dense, yielding compressive strength values that were ∼10 times higher than commercial ceramic foams of similar density. The high compressive strength property was attributed to the absence of noticeable porosity and surface

Periodically ordered 3D lattice structures of submillimeter resolution are fabricated via direct ink writing by extruding the ink through a nozzle/orifice to create the filamentary structure in a layer-wise sequence. The ink material could be a BCPdirected hybrid solution that could provide controlled formation of well-defined 3D periodic structures (e.g., bicontinuous gyroid morphology) with functional properties via self-assembly. For instance, the self-assembling ink could be prepared by mixing triblock terpolymers with resols,18 metals,24 and metal oxides99,136,137 or preceramic polymers138 to form mesoporous gyroidal structures with electrical conductivity as well as high thermal and mechanical stability. Selective transient laser heating of the self-assembling ink as it is extruded would provide precise, selective thermal curing/ sintering/crystallization of the additives and would result in simultaneous BCP decomposition, thereby avoiding superstructure collapse or substrate limitations in conventional hightemperature heat treatments. If successful, the integration of self-assembly with transient laser heating and direct writing could enable a new programmable synthetic platform for the creation of 3D hierarchical functional materials suitable for fundamental studies and potential applications such as photonic crystals, tissue engineering, energy storage, or sensing devices.

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CONCLUSIONS Coupling BCP-directed self-assembly with nonequilibrium processing in the form of transient laser heating provides a simple, deterministic, and potentially scalable strategy to design and direct complex hierarchically structured materials for a range of applications. In particular, the nonequilibrium nature of these transient laser annealing approaches opens up J


Perspective

Macromolecules vast and new processing windows beyond traditional stability limits of organic polymer materials. In the B-WRITE method, all-organic BCP−resols self-assembly is combined with submillisecond laser-induced heating permitting direct formation of 3D continuous BCP-directed polymer network nanostructures and shapes. Macroscopic areas showing hierarchical patterns of mesoporous materials are generated in minutes under ambient conditions. Nanoscale pore sizes with narrow pore size distributions are obtained by tuning the BCP molar mass and BCP/resols weight ratios and concentrations. The impressive physical and chemical properties exhibited by mesoporous BCP-directed resins under transient laser annealing enable their use as functional templates to generate, for example, laser-induced meltcrystallized Si nanostructures with high pattern transfer fidelity. The approach is not limited to Si, however, but should be applicable to any material with a reasonable melting temperature including other semiconductors or metals on substrates including, but not limited to silicon. The demonstration of transient laser annealing induced BCP-directed structure formation is quite promising and may pave the way to shape well-defined, 3D periodic single-crystal nanostructures into meso- or macroscopic thin film structures useful for fundamental studies as well as advanced applications. We hope this Perspective will generate interest to develop new fundamental understanding and concepts as well as exploit selfassembly mechanisms under such nonequilibrium conditions so as to accelerate transition of laboratory experiments to industrial manufacturing of advanced ordered functional nanomaterials and devices in the near future.

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Wiesner) and the Singapore−MIT Alliance for Research and Technology (with Prof. Carl V. Thompson and Prof. Yang ShaoHorn), he joined NTU as an assistant professor in the School of Materials Science and Engineering in 2017.

Ulrich (Uli) Wiesner studied chemistry at the University of Mainz, Germany, and UC Irvine, CA, gaining his PhD in 1991 with work under Prof. H. W. Spiess at the Max-Planck-Institute for Polymer Research, Mainz. After a two-year postdoc at the Ecole Superieure de Physique et de Chimie Industrielle de la ville de Paris (E.S.P.C.I.) with Prof. L. Monnerie, he returned to the group of Prof. H. W. Spiess in 1993 where he finished his Habilitation in 1998. He joined the Cornell University, NY, MS&E faculty in 1999 as a tenured Associate Professor, became a Full Professor in 2005, and since 2008 is the Spencer T. Olin Professor of Engineering.

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ACKNOWLEDGMENTS U.W. thanks the National Science Foundation (NSF) Single Investigator award (DMR-1707836) for financial support. K.W.T. acknowledges the Singapore Ministry of Education AcRF Tier 1 grant (2018-T1-001-084) and a startup grant from Nanyang Technological University, Singapore. The authors thank all contributors in the Wiesner group, at Cornell, and beyond for their efforts to develop this area of research. Particular thanks go to Prof. Michael O. Thompson for his continued interest in transient laser heating of soft polymer materials.

AUTHOR INFORMATION

Corresponding Authors

*(U.W.) E-mail: [email protected]. *(K.W.T.) E-mail: [email protected]. ORCID

Kwan Wee Tan: 0000-0002-8289-6503 Ulrich Wiesner: 0000-0001-6934-3755 Notes

The authors declare no competing financial interest.

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Biographies

REFERENCES

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