Widely Tunable Morphologies in Block Copolymer Thin Films Through Solvent Vapor Annealing Using Mixtures of Selective Solvents

Chemically Nanostructured Organogel Monoliths from Cross-Linked Block Copolymers for Selective Infusion Templating

Soft gels with spatially defined mesoscale distributions of chemical activity that guide and accelerate reactions by chemical nanoconfinement are found ubiquitously in nature but are rare in artificial systems. In this study, we introduce chemically nanostructured bulk organogels with periodically ordered morphologies from self-assembled block copolymer monoliths with a single selectively cross-linked block (xBCP). Ordered bulk organogels are fabricated with various distinct morphologies including hexagonally packed cylinders and two gyroidal three-dimensionally periodic network structures that exhibit macroscopic and nanoscopic structural integrity upon swelling. Small-angle X-ray scattering and transmission electron microscopy confirm that the periodic arrangement of the chemically distinct blocks in the self-assembled xBCP is retained at polymer fractions as low as 15 vol %. Our results reveal that the swelling equilibrium is not exclusively determined by the cross-linked block despite its structural role but is strongly influenced by the weighted interactions between solvent and the individual nanophases, including the non-cross-linked blocks. Therefore, substantial swelling can be obtained even for solvents that the cross-linked block itself has unfavorable interactions with. Since these ordered organogels present a class of solvent-laden bulk materials that exhibit chemically distinct nanoenvironments on a periodic mesoscale lattice, we demonstrate their use for selective infusion templating (SIT) in a proof-of-concept nanoconfined synthesis of poly(acrylonitrile) from which a monolithic ordered gyroidal mesoporous carbon is obtained. Going forward, we envision using xBCP gels and SIT to enable the fabrication of traditionally hard-to-template materials as periodically nanostructured monoliths due to the extensive tunability in their physicochemical parameter space.

The Promise of Soft-Matter-Enabled Quantum Materials

The field of quantum materials has experienced rapid growth over the past decade, driven by exciting new discoveries with immense transformative potential. Traditional synthetic methods to quantum materials have, however, limited the exploration of architectural control beyond the atomic scale. By contrast, soft matter self-assembly can be used to tailor material structure over a large range of length scales, with a vast array of possible form factors, promising emerging quantum material properties at the mesoscale. This review explores opportunities for soft matter science to impact the synthesis of quantum materials with advanced properties. Existing work at the interface of these two fields is highlighted, and perspectives are provided on possible future directions by discussing the potential benefits and challenges which can arise from their bridging.

Processive Pathways to Metastability in Block Copolymer Thin Films

Block copolymers (BCPs) self-assemble into intricate nanostructures that enhance a multitude of advanced applications in semiconductor processing, membrane science, nanopatterned coatings, nanocomposites, and battery research. Kinetics and thermodynamics of self-assembly are crucial considerations in controlling the nanostructure of BCP thin films. The equilibrium structure is governed by a molecular architecture and the chemistry of its repeat units. An enormous library of materials has been synthesized and they naturally produce a rich equilibrium phase diagram. Non-equilibrium phases could potentially broaden the structural diversity of BCPs and relax the synthetic burden of creating new molecules. Furthermore, the reliance on synthesis could be complicated by the scalability and the materials compatibility. Non-equilibrium phases in BCPs, however, are less explored, likely due to the challenges in stabilizing the metastable structures. Over the past few decades, a variety of processing techniques were introduced that influence the phase transformation of BCPs to achieve a wide range of morphologies. Nonetheless, there is a knowledge gap on how different processive pathways can induce and control the non-equilibrium phases in BCP thin films. In this review, we focus on different solvent-induced and thermally induced processive pathways, and their potential to control the non-equilibrium phases with regards to their unique aspects and advantages. Furthermore, we elucidate the limitations of these pathways and discuss the potential avenues for future investigations.

Controlled Self-Assembly of Polystyrene-block-Polydimethylsiloxane for Fabrication of Nanonetwork Silica Monoliths

Herein, this work aims to carry out controlled self-assembly of single-composition block copolymer for the fabrication of various nanonetwork silica monoliths. With the use of lamellae-forming polystyrene-block-polydimethylsiloxane (PS-b-PDMS), nanonetwork-structured films could be fabricated by solvent annealing using a PS-selective solvent (chloroform). By simply tuning the flow rate of nitrogen purge to the PS-selective solvent for the controlled self-assembly of the PS-b-PDMS, gyroid- and diamond-structured monoliths can be formed due to the difference in the effective volume of PS in the PS-b-PDMS during solvent annealing. As a result, well-ordered nanonetwork SiO₂ (silica) monoliths can be fabricated by templated sol-gel reaction using hydrofluoric acid etched PS-b-PDMS film as a template followed by the removal of the PS. This bottom-up approach for the fabrication of nanonetwork materials through templated synthesis is appealing to create nanonetwork materials for various applications.

Well-Defined Nanostructures by Block Copolymers and Mass Transport Applications in Energy Conversion

With the speedy progress in the research of nanomaterials, self-assembly technology has captured the high-profile interest of researchers because of its simplicity and ease of spontaneous formation of a stable ordered aggregation system. The self-assembly of block copolymers can be precisely regulated at the nanoscale to overcome the physical limits of conventional processing techniques. This bottom-up assembly strategy is simple, easy to control, and associated with high density and high order, which is of great significance for mass transportation through membrane materials. In this review, to investigate the regulation of block copolymer self-assembly structures, we systematically explored the factors that affect the self-assembly nanostructure. After discussing the formation of nanostructures of diverse block copolymers, this review highlights block copolymer-based mass transport membranes, which play the role of “energy enhancers” in concentration cells, fuel cells, and rechargeable batteries. We firmly believe that the introduction of block copolymers can facilitate the novel energy conversion to an entirely new plateau, and the research can inform a new generation of block copolymers for more promotion and improvement in new energy applications.

Solvent-assisted self-assembly of block copolymer thin films

Solvent-assisted block copolymer self-assembly is a compelling method for processing and advancing practical applications of these materials due to the exceptional level of the control of BCP morphology and significant acceleration of ordering kinetics. Despite substantial experimental and theoretical efforts devoted to understanding of solvent-assisted BCP film ordering, the development of a universal BCP patterning protocol remains elusive; possibly due to a multitude of factors which dictate the self-assembly scenario. The aim of this review is to aggregate both seminal reports and the latest progress in solvent-assisted directed self-assembly and to provide the reader with theoretical background, including the outline of BCP ordering thermodynamics and kinetics phenomena. We also indicate significant BCP research areas and emerging high-tech applications where solvent-assisted processing might play a dominant role.

Thin film block copolymer self-assembly for nanophotonics

The nanophotonic engineering of light-matter interactions has profoundly changed research behind the design and fabrication of optical materials and devices. Metasurfaces-arrays of subwavelength nanostructures that interact resonantly with electromagnetic radiation-have emerged as an integral nanophotonic platform for a new generation of ultrathin lenses, displays, polarizers and other devices. Their success hinges on advances in lithography and nanofabrication in recent decades. While existing nanolithography techniques are suitable for basic research and prototyping, issues of cost, throughput, scalability, and substrate compatibility may preclude their use for many metasurface applications. Patterning via spontaneous self-assembly of block copolymer thin films offers an enticing alternative for nanophotonic manufacturing that is rapid, inexpensive, and applicable to large areas and diverse substrates. This review discusses the advantages and disadvantages of block copolymer-based nanopatterning and highlights recent progress in their use for broadband antireflection, surface enhanced Raman spectroscopy, and other nanophotonic applications. Recent advances in diversification of self-assembled block copolymer nanopatterns and improved processes for enhanced scalability of self-assembled nanopatterning using block copolymers are also discussed, with a spotlight on directions for future research that would enable a wider array of nanophotonic applications.

Improvement in Mechanical Properties of 3D-Printed PEEK Structure by Nonsolvent Vapor Annealing

The broad applications of 3D-printed poly-ether-ether-ketone (3D-PEEK) structures are largely hampered by their inadequate mechanical properties which can be improved by post treatments. At present, thermal annealing is generally used to improve the mechanical properties of 3D-PEEK. However, it cannot simultaneously improve the strength and ductility. Here, a cost-effective post-processing method is developed to improve the mechanical properties of 3D-PEEK, based on annealing in nonsolvent vapor at room temperature. The annealing in nonsolvent vapor at room temperature simultaneously improves the strength, ductility, and fracture energy of as-printed 3D-PEEK by 22.6%, 151.3% and 109.1%, respectively. The improved mechanical properties are attributed to enhanced interfacial bonding, increased crystallinity, decreased pinhole defects, and stress relaxation in the 3D-PEEK. Moreover, the annealing in both polar solvents (such as acetone and chloroform) and nonpolar solvents (such as n-hexane) are demonstrated to be effective for improving the mechanical properties of 3D-PEEK. The nonsolvent vapor-annealed 3D-PEEK can thus have potential applications in the fields of medical implants, automotive, aerospace, and more. This article is protected by copyright. All rights reserved.

Optical Reflection from Unforbidden Diffraction of Block Copolymer Templated Gyroid Films

We present material substitutions and optical characterization of block copolymer (BCP)-templated gyroid structures that are obtained from a volume-asymmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA). In addition to the structural analyses reported earlier, we elucidate the optical responses to the nonaffine gyroid planes, in which the PMMA channels are complexed with Al₂O₃ by sequential infiltration synthesis and the organic components are further eliminated to produce an inorganic air-Al₂O₃ gyroid film. Grazing-incidence small-angle X-ray scattering measurements show that three-dimensional gyroid lattices are retained in both in-plane and out-of-plane directions through these material substitution processes. Our BCP-templated gyroid films respond to the middle UV wavelength from 200 to 300 nm, and peculiar optical reflectance peaks correlate with the unforbidden {110} diffraction spots. Together with the red- and blue-shifts of the reflectance peaks by the component substitutions, the air-Al₂O₃ gyroid structure reveals the high-amplitude spectrum due to the large refractive-index difference between channel and matrix.

Micro-and-nanometer topological gradient of block copolymer fibrous scaffolds towards region-specific cell regulation

Regulating cell behavior and function by surface topography has drawn significant attention in tissue engineering. Herein, a gradient fibrous scaffold comprising anisotropic aligned fibers and isotropic annealed fibers was developed to provide a controllable direction of cell migration, adhesion, and spreading. The electrospun aligned fibers were engraved to create surface gradients with micro-and-nanometer roughness through block copolymer (BCP) self-assembly induced by selective solvent vapor annealing (SVA). The distinct manipulation of cell behavior by annealed fibrous scaffolds with tailored self-assembled nanostructure and welded fibrous microstructure has been illustrated by in situ/ex situ small angle X-ray scattering (SAXS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and in vitro cell culture. Further insights into the effect of integrated gradient fibrous scaffold were gained at the level of protein expression. From the perspective of gradient topology, this region-specific scaffold based on BCP fibers shows the prospect of guiding cell migration, adhesion and spreading and provides a generic method for designing biomaterials for tissue-engineering.

Developing Anisotropy in Self-Assembled Block Copolymers: Methods, Properties, and Applications

Block copolymers (BCPs) self-assembly has continually attracted interest as a means to provide bottom-up control over nanostructures. While various methods have been demonstrated for efficiently ordering BCP nanodomains, most of them do not generically afford control of nanostructural orientation. For many applications of BCPs, such as energy storage, microelectronics, and separation membranes, alignment of nanodomains is a key requirement for enabling their practical use or enhancing materials performance. This review focuses on summarizing research progress on the development of anisotropy in BCP systems, covering a variety of topics from established aligning techniques, resultant material properties, and the associated applications. Specifically, the significance of aligning nanostructures and the anisotropic properties of BCPs is discussed and highlighted by demonstrating a few promising applications. Finally, the challenges and outlook are presented to further implement aligned BCPs into practical nanotechnological applications, where exciting opportunities exist.

Green Nanofabrication Opportunities in the Semiconductor Industry: A Life Cycle Perspective

The turn of the 21st century heralded in the semiconductor age alongside the Anthropocene epoch, characterised by the ever-increasing human impact on the environment. The ecological consequences of semiconductor chip manufacturing are the most predominant within the electronics industry. This is due to current reliance upon large amounts of solvents, acids and gases that have numerous toxicological impacts. Management and assessment of hazardous chemicals is complicated by trade secrets and continual rapid change in the electronic manufacturing process. Of the many subprocesses involved in chip manufacturing, lithographic processes are of particular concern. Current developments in bottom-up lithography, such as directed self-assembly (DSA) of block copolymers (BCPs), are being considered as a next-generation technology for semiconductor chip production. These nanofabrication techniques present a novel opportunity for improving the sustainability of lithography by reducing the number of processing steps, energy and chemical waste products involved. At present, to the extent of our knowledge, there is no published life cycle assessment (LCA) evaluating the environmental impact of new bottom-up lithography versus conventional lithographic techniques. Quantification of this impact is central to verifying whether these new nanofabrication routes can replace conventional deposition techniques in industry as a more environmentally friendly option.

Bending Behavior and Directed Self-Assembly of Rod-Coil Block Copolymers

The formation of zigzags, chevrons, Y-junctions, and line segments is demonstrated in thin films formed from cylindrical morphology silicon-containing conformationally asymmetric rod-coil diblock copolymers and triblock terpolymers under solvent annealing. Directed self-assembly of the block copolymers within trenches yields well-ordered cylindrical microdomains oriented either parallel or transverse to the sidewalls depending on the chemical functionalization of the sidewalls, and the location and structure of concentric bends in the cylinders is determined by the shape of the trenches. The innate etching contrast, the spontaneous sharp bends and junctions, and the range of demonstrated periodicity and line/space ratios make these conformationally asymmetric rod-coil polymers attractive for nanoscale pattern generation.

Vertical Lamellae Formed by Two-Step Annealing of a Rod-Coil Liquid Crystalline Block Copolymer Thin Film

Silicon-containing block copolymer thin films with high interaction parameter and etch contrast are ideal candidates to generate robust nanotemplates for advanced nanofabrication, but they typically form in-plane oriented microdomains as a result of the dissimilar surface energies of the blocks. Here, we describe a two-step annealing method to produce vertically aligned lamellar structures in thin film of a silicon-containing rod-coil thermotropic liquid crystalline block copolymer. The rod-coil block copolymer with the volume fraction of the Si-containing block of 0.22 presents an asymmetrical lamellar structure in which the rod block forms a hexatic columnar nematic liquid crystalline phase. A solvent vapor annealing step first produces well-ordered in-plane cylinders of the Si-containing block, then a subsequent thermal annealing promotes the phase transition from in-plane cylinders to vertical lamellae. The pathways of the order-order transition were examined by microscopy and in situ using grazing incidence small-angle X-ray scattering and wide-angle X-ray scattering.

Self-Assembly in ultrahigh molecular weight sphere-forming diblock copolymer thin films under strong confinement

Ultrahigh molecular weight (UHMW) diblock copolymers (DBCs) have emerged as a promising template for fabricating large-sized nanostructures. Therefore, it is of high significance to systematically study the influence of film thickness and solvent vapor annealing (SVA) on the structure evolution of UHMW DBC thin films. In this work, spin coating of an asymmetric linear UHMW polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) DBC is used to fabricate thin films, which are spherically structured with an inter-domain distance larger than 150 nm. To enhance the polymer chain mobility and facilitate approaching equilibrium nanostructures, SVA is utilized as a post-treatment of the spin coated films. With increasing film thickness, a local hexagonal packing of PMMA half-spheres on the surface can be obtained, and the order is improved at larger thickness, as determined by grazing incidence small angle X-ray scattering (GISAXS). Additionally, the films with locally hexagonal packed half-spherical morphology show a poor order-order-poor order transition upon SVA, indicating the realization of ordered structure using suitable SVA parameters.

Automated solvent vapor annealing with nanometer scale control of film swelling for block copolymer thin films

Molecular self-assembly of block copolymers has been pursued as a next generation high-resolution, low-cost lithography technique. Solvent vapor annealing is a promising way of achieving self-assembled patterns from polymers with high interaction parameters, χ, or high molecular weights. Compared to thermal annealing, the assembly in a solvated state can be much faster, but the film swelling process is typically challenging to control and reproduce. We report the design and implementation of an automated solvent annealing system that addresses these issues. In this system the film swelling is controlled via local heating or cooling, which enables exceptionally fast and precise modulation of the swelling. The swelling of the polymer films follows preprogrammed annealing profiles with the help of a feedback loop that compares and tunes the film thickness with respect to the set point. The system therefore enables complex annealing profiles such as rapid cyclic swelling and deswelling. We show that the orientation of the pattern morphology and the amount of lattice defects are influenced by the used annealing profile. We demonstrate that optimized profiles significantly shorten the annealing time (<15 min) of high-χ and high-molecular weight poly(styrene-b-2-vinylpyridine).

Self-assembled full nanowire P(VDF-TrFE) films with both anisotropic and high bidirectional piezoelectricity

With the explosive growth of flexible electronics, the prototype piezoelectric polymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] has gained tremendous attention due to potential applications in flexible sensors, energy harvesters, and new smart devices. However, full realization of these applications is still challenging due to the lack of high quality films with strong piezoelectricity, which requires tailored molecular organization. Here we report unique 'full nanowire' P(VDF-TrFE) films with substantially enhanced bidirectional performance by a simple self-assembly via selective vapor annealing. Structural analysis showed that the solvent molecules significantly enhanced the copolymer chain mobility, giving highly ordered nanowires, whose quantity increased with time and finally formed a full flat-on lamellar nanowire array with backbones highly aligned along the film plane, leading to high lateral piezoelectricity as revealed by vector piezoresponse force microscopy and confirmed by electrical measurements. Surprisingly, the nanowire films also showed a much higher vertical piezoelectric coefficient (-35.2 pC N-1 directly measured by using a Berlincourt meter) than that of usually crystallized films owing to simultaneously enhanced molecular order and dipole switching ability. The scalability of the new method might boost industrial applications, and the findings may provide hints on new routes to nanostructured polymers with novel functionalities and deepen our understanding of the self-assembly of random copolymers.

Temperature-Controlled Solvent Vapor Annealing of Thin Block Copolymer Films

Solvent vapor annealing is as an effective and versatile alternative to thermal annealing to equilibrate and control the assembly of polymer chains in thin films. Here, we present scientific and practical aspects of the solvent vapor annealing method, including the discussion of such factors as non-equilibrium conformational states and chain dynamics in thin films in the presence of solvent. Homopolymer and block copolymer films have been used in model studies to evaluate the robustness and the reproducibility of the solvent vapor processing, as well as to assess polymer-solvent interactions under confinement. Advantages of utilizing a well-controlled solvent vapor environment, including practically interesting regimes of weakly saturated vapor leading to poorly swollen states, are discussed. Special focus is given to dual temperature control over the set-up instrumentation and to the potential of solvo-thermal annealing. The evaluated insights into annealing dynamics derived from the studies on block copolymer films can be applied to improve the processing of thin films of crystalline and conjugated polymers as well as polymer composite in confined geometries.

Core-Shell and Zigzag Nanostructures from a Thin Film Silicon-Containing Conformationally Asymmetric Triblock Terpolymer

The self-assembly of multiblock copolymers generates diverse hierarchical nanostructures and greatly extends the range of microdomain geometries beyond those produced by diblock copolymers. We report the synthesis of a conformationally asymmetric ABC triblock terpolymer in which the end blocks are a mesogen-jacketed liquid crystalline polymer and poly(dimethylsiloxane), respectively, and its self-assembly under mixed solvent vapor annealing forms a range of sphere, cylinder, and perforated lamellar core-shell morphologies, as well as stacked multilevel structures. Sub-7 nm diameter SiO_x nanopatterns were generated by selective plasma etching of the small volume fraction Si-containing core block giving a line/space ratio of ∼1:4. Moreover, the conformational asymmetry of this terpolymer leads to zigzag cylinders on a flat substrate and stable cylinder alignment transverse to template sidewalls within lithographically patterned trenches.

The selectivity of poly(2-vinylpyridine-block-methyl methacrylate) copolymer films: an AFM study

In this study, the surface morphologies of poly(2-vinyl pyridine-block-methyl methacrylate), P(2VP-MMA), copolymer films were analyzed by atomic force microscopy. P(2VP-MMA) samples varying in total molar mass and individual block length were evaluated for variations in the surface morphologies of films cast from toluene on Si wafers. The incorporation of AuNPs into the polymer domain significantly influenced the surface morphology of the films. Variations in the surface morphology as a function of the polarity of the casting solvent were also examined. In this context toluene (a non-polar solvent), chloroform (of intermediate polarity) and ethyl acetate (a polar solvent) were employed as casting solvents. Toluene is a good solvent for PMMA compared to P2VP, chloroform has no preferential solvation, while ethyl acetate is a good solvent for P2VP compared to PMMA. The morphology of the films cast on substrates of distinctly different polarities, such as mica, Si wafers, and HOPG, were studied to appraise their selectivity. Finally, a detailed study of the effects of thermal annealing on the surface morphologies of P(2VP-MMA) and P(2VP-MMA)-AuNPs was conducted.

In this study, the surface morphologies of poly(2-vinyl pyridine-block-methyl methacrylate), P(2VP-MMA), copolymer films were analyzed by atomic force microscopy.

Orientation and Morphology Control of the Liquid Crystalline Block Copolymer Thin Film by Liquid Crystalline Solvent

The critical challenge to engineer the morphological structures in the strongly phase-segregated block copolymer thin film is to overcome the preferential wetting of the blocks at the interface and direct the self-assembly process. Herein, we utilize surface activity and selective solvation of a nematic (N) liquid crystalline (LC) solvent, 5CB, to facilely alter the LC anchoring and the orientation of the nanophase separated structures of the smectic-nematic (S-N) LC block copolymer thin film. For the neat S-N diblock copolymer thin film, the nanostructures are parallel aligned. In contrast, with continuous introduction of 5CB into the system, the orientations of the characteristic nanostructures and the morphologies of the LC thin film can be consequently changed, yielding the perpendicularly oriented lamellar or cylindrical structures with the feature size below 10 nm. The homeotropic alignment of the 5CB nematics near the air interface plays a critical role to induce this unique behavior in the S-N/5CB systems, which offers an opportunity to fine-tune the interfacial structures and the morphological patterning in the block copolymer thin film.

Controlling Self-Assembly in Gyroid Terpolymer Films By Solvent Vapor Annealing

The efficacy with which solvent vapor annealing (SVA) can control block copolymer self-assembly has so far been demonstrated primarily for the simplest class of copolymer, the linear diblock copolymer. Adding a third distinct block-thereby creating a triblock terpolymer-not only provides convenient access to complex continuous network morphologies, particularly the gyroid phases, but also opens up a route toward the fabrication of novel nanoscale devices such as optical metamaterials. Such applications, however, require the generation of well-ordered 3D continuous networks, which in turn requires a detailed understanding of the SVA process in terpolymer network morphologies. Here, in situ grazing-incidence small-angle X-ray scattering (GISAXS) is employed to study the self-assembly of a gyroid-forming triblock terpolymer during SVA, revealing the effects of several key SVA parameters on the morphology, lateral order, and, in particular, its preservation in the dried film. The robustness of the terpolymer gyroid morphology is a key requirement for successful SVA, allowing the exploration of annealing parameters which may enable the generation of films with long-range order, e.g., for optical metamaterial applications.

Fabrication and Thermal Insulation Properties of Bamboo-Shaped Polymer Fibers by Selective Solvent Vapor Annealing

Fibrillar materials have gained much attention recently because of their unique properties and potential applications. Although many methods have been developed to fabricate materials, it remains challenging to prepare fibrillar materials containing multicomponent materials or even with complex structures. Here, a facile strategy is developed to fabricate bamboo-shaped fibers by treating electrospun polymer core-shell fibers with solvent vapor annealing. Electrospun polystyrene (PS)/poly(methyl methacrylate) (PMMA) core-shell fibers are first prepared by electrospinning PS/PMMA blend solutions via a phase separation process. When the PS/PMMA core-shell fibers are annealed with the vapor of cyclohexane, which swells and delocalizes the PS domains selectively, the fibers transform into bamboo-shaped structures. The bamboo-shaped structures can be further examined by swelling and delocalizing the PMMA domains selectively, revealing the undulated PS structures. The thermal insulation properties of the fibers with bamboo-shaped structures are observed to be enhanced compared with the original polymer core-shell fibers.

Pathways to Mesoporous Resin/Carbon Thin Films with Alternating Gyroid Morphology

Three-dimensional (3D) mesoporous thin films with sub-100 nm periodic lattices are of increasing interest as templates for a number of nanotechnology applications, yet are hard to achieve with conventional top-down fabrication methods. Block copolymer self-assembly derived mesoscale structures provide a toolbox for such 3D template formation. In this work, single (alternating) gyroidal and double gyroidal mesoporous thin-film structures are achieved via solvent vapor annealing assisted co-assembly of poly(isoprene-block-styrene-block-ethylene oxide) (PI-b-PS-b-PEO, ISO) and resorcinol/phenol formaldehyde resols. In particular, the alternating gyroid thin-film morphology is highly desirable for potential template backfilling processes as a result of the large pore volume fraction. In situ grazing-incidence small-angle X-ray scattering during solvent annealing is employed as a tool to elucidate and navigate the pathway complexity of the structure formation processes. The resulting network structures are resistant to high temperatures provided an inert atmosphere. The thin films have tunable hydrophilicity from pyrolysis at different temperatures, while pore sizes can be tailored by varying ISO molar mass. A transfer technique between substrates is demonstrated for alternating gyroidal mesoporous thin films, circumventing the need to re-optimize film formation protocols for different substrates. Increased conductivity after pyrolysis at high temperatures demonstrates that these gyroidal mesoporous resin/carbon thin films have potential as functional 3D templates for a number of nanomaterials applications.

Perpendicular Structure Formation of Block Copolymer Thin Films during Thermal Solvent Vapor Annealing: Solvent and Thickness Effects

Solvent vapor annealing of block copolymer (BCP) thin films can produce a range of interesting morphologies, especially when the perpendicular orientation of micro-domains with respect to the substrate plays a role. This, for instance, allows BCP thin films to serve as useful templates for nanolithography and hybrid materials preparation. However, precise control of the arising morphologies is essential, but in most cases difficult to achieve. In this work, we investigated the solvent and thickness effects on the morphology of poly(styrene-b-2 vinyl pyridine) (PS-b-P2VP) thin films with a film thickness range from 0.4 L₀ up to 0.8 L₀. Ordered perpendicular structures were achieved. One of the main merits of our work is that the phase behavior of the ultra-high molecular weight BCP thin films, which hold a 100-nm sized domain distance, can be easily monitored via current available techniques, such as scanning electron microscope (SEM), atomic force microscope (AFM), and transmission electron microscope (TEM). Systematic monitoring of the self-assembly behavior during solvent vapor annealing can thus provide an experimental guideline for the optimization of processing conditions of related BCP films systems.

Reduction of Line Edge Roughness of Polystyrene-block-Poly(methyl methacrylate) Copolymer Nanopatterns By Introducing Hydrogen Bonding at the Junction Point of Two Block Chains

To apply well-defined block copolymer nanopatterns to next-generation lithography or high-density storage devices, small line edge roughness (LER) of nanopatterns should be realized. Although polystyrene-block-poly(methyl methacrylate) copolymer (PS-b-PMMA) has been widely used to fabricate nanopatterns because of easy perpendicular orientation of the block copolymer nanodomains and effective removal of PMMA block by dry etching, the fabricated nanopatterns show poorer line edge roughness (LER) due to relatively small Flory-Huggins interaction parameter (χ) between PS and PMMA chains. Here, we synthesized PS-b-PMMA with urea (U) and N-(4-aminomethyl-benzyl)-4-hydroxymethyl-benzamide (BA) moieties at junction of PS and PMMA chains (PS-U-BA-PMMA) to improve the LER. The U-BA moieties serves as favorable interaction (hydrogen bonding) sites. The LER of PS line patterns obtained from PS-U-BA-PMMA was reduced ∼25% compared with that obtained from neat PS-b-PMMA without BA and U moieties. This is attributed to narrower interfacial width induced by hydrogen bonding between two blocks, which is confirmed by small-angle X-ray scattering. This result implies that the introduction of hydrogen bonding into block copolymer interfaces offers an opportunity to fabricate well-defined nanopatterns with improved LER by block copolymer self-assembly, which could be a promising alternative to next-generation extreme ultraviolet lithography.

Optical Imaging of Large Gyroid Grains in Block Copolymer Templates by Confined Crystallization

Block copolymer (BCP) self-assembly is a promising route to manufacture functional nanomaterials for applications from nanolithography to optical metamaterials. Self-assembled cubic morphologies cannot, however, be conveniently optically characterized in the lab due to their structural isotropy. Here, the aligned crystallization behavior of a semicrystalline-amorphous polyisoprene-b-polystyrene-b-poly(ethylene oxide) (ISO) triblock terpolymer was utilized to visualize the grain structure of the cubic microphase-separated morphology. Upon quenching from a solvent swollen state, ISO first self-assembles into an alternating gyroid morphology, in the confinement of which the PEO crystallizes preferentially along the least tortuous pathways of the single gyroid morphology with grain sizes of hundreds of micrometers. Strikingly, the resulting anisotropic alignment of PEO crystallites gives rise to a unique optical birefringence of the alternating gyroid domains, which allows imaging of the self-assembled grain structure by optical microscopy alone. This study provides insight into polymer crystallization within a tortuous three-dimensional network and establishes a useful method for the optical visualization of cubic BCP morphologies that serve as functional nanomaterial templates.

Templated Sub-100-nm-Thick Double-Gyroid Structure from Si-Containing Block Copolymer Thin Films

The directed self-assembly of diblock copolymer chains (poly(1,1-dimethyl silacyclobutane)-block-polystyrene, PDMSB-b-PS) into a thin film double gyroid structure is described. A decrease of the kinetics of a typical double-wave pattern formation is reported within the 3D-nanostructure when the film thickness on mesas is lower than the gyroid unit cell. However, optimization of the solvent-vapor annealing process results in very large grains (over 10 µm²) with specific orientation (i.e., parallel to the air substrate) and direction (i.e., along the groove direction) of the characteristic (211) plane, demonstrated by templating sub-100-nm-thick PDMSB-b-PS films.

Solvent-controlled reversible switching between adsorbed self-assembled nanoribbons and nanotubes

We have demonstrated that solutions of 3,5-bis-(5-hexylcarbamoylpentyloxy)-benzoic acid decyl ester (BHPB-10) can form metastable nanostructures on solid substrates and in the bulk. BHPB-10 is an achiral molecule involving several distinct, strongly interacting groups (SIGs), one aromatic-ester ring and two amide groups per molecule. Specific solvents affect the interactions between particular SIGs, thus promoting various nano-structures: lamellae, nanoribbons, helical ribbons, or nanotubes. In cyclohexane, a solvent allowing for both inter-amide hydrogen bonds and mutual attraction of rings, the formation of nanotubes with a diameter of 28 ± 5 nm was observed in the bulk and on surfaces. By contrast, in cyclohexanone, which suppresses inter-amide hydrogen bonds, flat nanoribbons with a specific width of 12 ± 4 nm were formed on solid substrates after drying. By annealing in cyclohexane vapor, we followed the process of switching surface structures from nanoribbons to nanotubes and observed helical ribbons as the precursor of nanotubes. We also turned nanotubes back into nanoribbons by adding cyclohexanone, thus demonstrating reversible switching along the route: tubes → lamellae → flat ribbons → helical ribbons → tubes. We propose models explaining the observed nanostructures and their transformations, including the origin of spontaneous chirality of the helical ribbons. Our findings on the self-assembly in the achiral BHPB-10 solutions provide insight into the influence of complementary inter-molecular specific SIG-based interactions and demonstrate an effective route for tailoring the shape and size of nanostructures derived from the same building unit.

Surface engineering to achieve organic ternary memory with a high device yield and improved performance

Organic memories fabricated on surface-engineered indium tin oxide show the highest ternary yield (82%) to date and better performance.

Squaraine molecules deposited on indium tin oxide (ITO) substrates modified with phosphonic acids crystalize more orderly than do those on untreated ITO. The as-fabricated electro-resistive memories show the highest ternary device yield observed to date (82%), a narrower switching voltage distribution, and better retention as well as resistance uniformity.