Orienting block copolymer microdomains with block copolymer brushes

Interface Manipulations Using Cross-Linked Underlayers and Surface-Active Diblock Copolymers to Extend Morphological Diversity in High-χ Diblock Copolymer Thin Films

Top and bottom interfaces of high-χ cylinder-forming polystyrene-block-maltoheptaose (PS-b-MH) diblock copolymer (BCP) thin films are manipulated using cross-linked copolymer underlayers and a fluorinated phase-preferential surface-active polymer (SAP) additive to direct the self-assembly (both morphology and orientation) of BCP microdomains into sub-10 nm patterns. A series of four photo-cross-linkable statistical copolymers with various contents of styrene, a 4-vinylbenzyl azide cross-linker, and a carbohydrate-based acrylamide are processed into 15 nm-thick cross-linked passivation layers on silicon substrates. A partially fluorinated analogue of the PS-b-MH phase-preferential SAP additive is designed to tune the surface energy of the top interface. The self-assembly of PS-b-MH thin films on top of different cross-linked underlayers and including 0-20 wt % of SAP additive is investigated by atomic force microscopy and synchrotron grazing incidence small-angle X-ray scattering analysis. The precise manipulation of the interfaces of ca. 30 nm thick PS-b-MH films not only allows the control of the in-plane/out-of-plane orientation of hexagonally packed (HEX) cylinders but also promotes epitaxial order-order transitions from HEX cylinders to either face-centered orthorhombic or body-centered cubic spheres without modifying the volume fraction of both blocks. This general approach paves the way for the controlled self-assembly of other high-χ BCP systems.

Direct Visualization of the Self-Alignment Process for Nanostructured Block Copolymer Thin Films by Transmission Electron Microscopy

Herein, this work aims to directly visualize the morphological evolution of the controlled self-assembly of star-block polystyrene-block-polydimethylsiloxane (PS-b-PDMS) thin films via in situ transmission electron microscopy (TEM) observations. With an environmental chip, possessing a built-in metal wire-based microheater fabricated by the microelectromechanical system (MEMS) technique, in situ TEM observations can be conducted under low-dose conditions to investigate the development of film-spanning perpendicular cylinders in the block copolymer (BCP) thin films via a self-alignment process. Owing to the free-standing condition, a symmetric condition of the BCP thin films can be formed for thermal annealing under vacuum with neutral air surface, whereas an asymmetric condition can be formed by an air plasma treatment on one side of the thin film that creates an end-capped neutral layer. A systematic comparison of the time-resolved self-alignment process in the symmetric and asymmetric conditions can be carried out, giving comprehensive insights for the self-alignment process via the nucleation and growth mechanism.

Emergence of layered nanoscale mesh networks through intrinsic molecular confinement self-assembly

Block copolymer self-assembly is a powerful tool for two-dimensional nanofabrication; however, the extension of this self-assembly concept to complex three-dimensional network structures is limited. Here we report a simple method to experimentally generate three-dimensional layered mesh morphologies through intrinsic molecular confinement self-assembly. We designed triblock bottlebrush polymers with two Janus domains: one perpendicular and one parallel to the polymer backbone. The former enforces a lamellar superstructure that intrinsically confines the intralayer self-assembly of the latter, giving rise to a mesh-like monoclinic (54°) M¹⁵ network substructure with excellent long-range order, as well as a tetragonal (90°) T¹³¹ mesh. Numerical simulations show that the spatial constraints exerted on the polymer backbone drive the assembly of M¹⁵ and yield T¹³¹ in the strong segregation regime. This work demonstrates that intrinsic molecular confinement is a viable path to bottom-up assembly of new geometrical phases of soft matter, extending the capabilities of block copolymer nanofabrication.

Nanochannel Arrays for Molecular Sieving and Electrochemical Analysis by Nanosphere Lithography Templated Graphoepitaxy of Block Copolymers

The ability to design, fabricate, and manipulate materials at the nanoscale is fundamental to the quest to develop technologies to assemble nanometer-scale pieces into larger-scale components and materials, thereby transferring unique nanometer-scale properties to macroscopic objects. In this work, we describe a new approach to the fabrication of highly ordered, ultrahigh density nanochannel arrays that employs nanosphere lithography to template the graphoepitaxy of polystyrene-polydimethylsiloxane, diblock copolymers. By optimizing the well-controlled solvent vapor annealing, overcoating conditions, and the subsequent reactive ion etching processes, silica nanochannel (SNC) arrays with areal densities, ρ_A, approaching 1000 elements μm^-2, are obtained over macroscopic scales. The integrity and functionality of the SNC arrays was tested by using them as permselective ion barriers to nanopore-confined disk electrodes. The nanochannels allow cations to pass to the disk electrode but reject anions, as demonstrated by cyclic voltammetry. This ion gating behavior can be reversed from cation-permselective to anion-permselective by chemically inverting the surface charge from negative to positive. Furthermore, the conformal SNC array structures obtained could easily be lifted, detached, and transferred to another substrate, preserving the hierarchical organization while transferring the nanostructure-derived properties to a different substrate. These results demonstrate how nanoscale behavior can be replicated over macroscale distances, using electrochemical analysis as a model.

Morphology, directed self-assembly and pattern transfer from a high molecular weight polystyrene-block-poly(dimethylsiloxane) block copolymer film

The self-assembly of block copolymers with large feature sizes is inherently challenging as the large kinetic barrier arising from chain entanglement of high molecular weight (MW) polymers limits the extent over which long-range ordered microdomains can be achieved. Here, we illustrate the evolution of thin film morphology from a diblock copolymer of polystyrene-block-poly(dimethylsiloxane) exhibiting total number average MW of 123 kg mol^-1, and demonstrate the formation of layers of well-ordered cylindrical microdomains under appropriate conditions of binary solvent mix ratio, commensurate film thickness, and solvent vapor annealing time. Directed self-assembly of the block copolymer within lithographically patterned trenches occurs with alignment of cylinders parallel to the sidewalls. Fabrication of ordered cobalt nanowire arrays by pattern transfer was also implemented, and their magnetic properties and domain wall behavior were characterized.

Protein nanorings organized by poly(styrene-block-ethylene oxide) self-assembled thin films

This study explores the use of block copolymer self-assembly to organize Lsmα, a protein which forms stable doughnut-shaped heptameric structures. Here, we have explored the idea that 2-D crystalline arrays of protein filaments can be prepared by stacking doughnut shaped Lsmα protein into the poly(ethylene oxide) blocks of a hexagonal microphase-separated polystyrene-b-polyethylene oxide (PS-b-PEO) block copolymer. We were able to demonstrate the coordinated assembly of such a complex hierarchical nanostructure. The key to success was the choice of solvent systems and protein functionalization that achieved sufficient compatibility whilst still promoting assembly. Unambiguous characterisation of these structures is difficult; however AFM and TEM measurements confirmed that the protein was sequestered into the PEO blocks. The use of a protein that assembles into stackable doughnuts offers the possibility of assembling nanoscale optical, magnetic and electronic structures.

Morphological Evolution of Gyroid-Forming Block Copolymer Thin Films with Varying Solvent Evaporation Rate

In this study, we aim to examine the morphological evolution of block copolymer (BCP) nanostructured thin films through solvent evaporation at different rates for solvent swollen polystyrene-block-poly(l-lactide) (PS-PLLA). Interesting phase transitions from disorder to perpendicular cylinder and then gyroid can be found while using a partially selective solvent for PS to swell PS-PLLA thin film followed by solvent evaporation. During the transitions, gyroid-forming BCP thin film with characteristic crystallographic planes of (111)G, (110)G, and (211)G parallel to air surface can be observed, and will gradually transform into coexisting (110)G and (211)G planes, and finally transforms to (211)G plane due to the preferential segregation of constituted block to the surface (i.e., the thermodynamic origin for self-assembly) that affects the relative amount of each component at the air surface. With the decrease on the evaporation rate, the disorder phase will transform to parallel cylinder and then directly to (211)G without transition to perpendicular cylinder phase. Most importantly, the morphological evolution of PS-PLLA thin films is strongly dependent upon the solvent removal rate only in the initial stage of the evaporation process due to the anisotropy of cylinder structure. Once the morphology is transformed back to the isotropic gyroid structure after long evaporation, the morphological evolution will only relate to the variation of the surface composition. Similar phase transitions at the substrate can also be obtained by controlling the ratio of PLLA-OH to PS-OH homopolymers to functionalize the substrate. As a result, the fabrication of well-defined nanostructured thin films with controlled orientation can be achieved by simple swelling and deswelling with controlled evaporation rate.

Evolution of homopolymer thin-film instability on surface-anchored diblock copolymers varying in composition

The stability of molecularly thin polymer films deposited on various material substrates is of critical importance to many contemporary nanotechnologies involving functional coatings and nano/micropatterned surfaces, in which case the causes responsible for film destabilization must be fully understood. Previous experimental studies report that factors such as film thickness and polymer molecular weight play significant roles in governing the rate, as well as mechanism, of destabilization. Complementary theoretical predictions reveal that surface heterogeneities can likewise induce (and regulate the process of) destabilization. In this study, we investigate the destabilization rate and mechanism of homopolystyrene (PS) films differing in thickness on top of poly(styrene-b-methyl methacrylate) (SM) diblock copolymer monolayers varying in chemical composition anchored to flat silica-like substrates to examine the effect of surface constitution on PS stability. Copolymers with a long M block consistently promote PS dewetting by nucleation and growth, wherein the linear dewetting rate decreases monotonically with increasing PS molecular weight, film thickness, and S fraction in the SM copolymer. In analogous studies involving a copolymer with a relatively short M block, however, PS dewetting proceeds instead by spinodal dewetting that evolves gradually into nucleation and growth as the film thickness is increased.

Directed self-assembly of block copolymers: a tutorial review of strategies for enabling nanotechnology with soft matter

Self-assembly of soft materials is broadly considered an attractive means of generating nanoscale structures and patterns over large areas. However, the spontaneous formation of equilibrium nanostructures in response to temperature and concentration changes, for example, must be guided to yield the long-range order and orientation required for utility in a given scenario. In this review we examine directed self-assembly (DSA) of block copolymers (BCPs) as canonical examples of nanostructured soft matter systems which are additionally compelling for creating functional materials and devices. We survey well established and newly emerging DSA methods from a tutorial perspective. Special emphasis is given to exploring underlying physical phenomena, identifying prototypical BCPs that are compatible with different DSA techniques, describing experimental methods and highlighting the attractive functional properties of block copolymers overall. Finally we offer a brief perspective on some unresolved issues and future opportunities in this field.

Hybrid bulk heterojunction solar cells based on the cooperative interaction of liquid crystals within quantum dots and diblock copolymers

In this article, the conjugated rod-rod polythiophene diblock copolymers comprising a regioregular poly(3-hexylthiophene) (P3HT) segment and a side-chain liquid-crystalline polythiophene segment bearing cyanobiphenyl mesogenic pendants (PTcbp), polythiophene-b-poly{3-[10-(4'-cyanobiphenyloxy)decyl]thiophene} (P3HT-b-PTcbp), were rationally designed and synthesized. It was observed that the diblock copolymers could self-assemble into high crystalline and oriented nanofibrils upon 1,2-dichlorobenzene solvent vapor annealing, originating from the crystallization of two segments and the orientation of cyanobiphenyl side-chain mesogens. Hybrid bulk heterojunction (BHJ) solar cells were then fabricated using P3HT-b-PTcbp as electron donors and ZnO and CdS quantum dots (QDs) modified by 4'-hydroxy-[1,1'-biphenyl]-4-carbonitrile (cbp) liquid-crystalline ligands (cbp@ZnO and cbp@CdS) as electron acceptors. The interaction between the cbp ligands on the surface of ZnO and CdS QDs and cyanobiphenyl side-chain mesogens of diblock copolymers promoted the cooperative self-assembly and controllable well-dispersion of QDs in the polymer matrix and, as a consequence, yielded an intimately contacted polymer-QD nanocomposites. The power conversion efficiency (PCE) of the device based on P3HT-b-PTcbp/cbp@ZnO hybrids was improved by 2.6 times compared with that of P3HT/ZnO hybrids from 0.58 to 0.97. In addition, an overall PCE of a homologous device based on the P3HT-b-PTcbp/cbp@CdS hybrid active layer reached 2.3%. The research paved the way for the further development of high-efficiency hybrid BHJ solar cells by introducing block copolymer nanofibrils with favored crystalline domain orientations and liquid-crystalline organization properties.

Self-assembly of diblock polythiophenes with discotic liquid crystals on side chains for the formation of a highly ordered nanowire morphology

Diblock copolymers bearing a triphenylene (TP) discotic liquid crystals moiety, poly(3-hexylthiophene)-block-poly[3-(10-(2,3,6,7,10-pentakis(hexyloxy)triphenylen)-decyloxy)thiophene] (P3HT-b-P3TPT), was successfully synthesized by Grignard metathesis polymerization. The self-assembled nanowire structures of these diblock copolymers have been investigated by atomic force microscopy and transmission electron microscopy. The domain size and crystallinity of the nanostructures can be easily controlled by tuning the P3HT/P3TPT block ratio and by employing different annealing processes such as thermal and solvent annealing. The results of X-ray diffraction indicate that both intermolecular interactions and mesogen packing are essential for the formation of nanostructures in the diblock copolymers. Although the block ratio of P3HT and P3TPT comes to 9:1 and the copolymer undergoes solvent annealing followed by thermal treatment, an optimal crystalline nanowire with a size of 16.9 nm is formed. In addition, solar cells based on these copolymers as electron donors in combination with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or N,N'-di(2-ethylhexyl)perylene-3,4,9,10-tetracarboxylbisimide (PDI) as electron acceptors have been constructed, and the effect of the nanomorphology on device performance has been investigated.

Perpendicular orientation of sub-10 nm channels in polystyrene-b-poly(4-hydroxyl styrene)/PEG oligomer blend thin films

Vertically oriented cylinders via the directional coalescence of the spheres embedded in the blend thin films from polystyrene-b-poly(4-hydroxyl styrene) (PS-b-PHS) and PEG induced by solvent annealing were achieved. Removal of PEG water led to the formation of nanochannels throughout the films. The diameter of these channels could be as small as 9 nm which might enhance the applicability of the nano-porous films as size-selective membranes and controllable drug delivery systems for the objects less than 10 nm.

Solvent-assisted directed self-assembly of spherical microdomain block copolymers to high areal density arrays

The fabrication process for 5 Tb/in(2) bit patterns using solvent-assisted directed self-assembly is investigated. The N-methyl-2-pyrrolidone solvent vapor-annealing method was used to achieve good long-range lateral ordering of low-molecular-weight polystyrene-block-polydimethylsiloxane with a lattice spacing of 11 nm on flat Si substrates, PS modified substrates and lithographically patterned substrates, respectively.

Long-range ordering of block copolymer cylinders driven by combining thermal annealing and substrate functionalization

This work presents a new method for forming well-defined nanostructured thin films from self-assembled polystyrene-block-poly(l-lactide) (PS-PLLA) on Si wafers with a functionalized SiO2 surface. Large, well-ordered, perpendicular PLLA cylinders in PS-PLLA thin films can be formed using the functionalized substrate. In contrast to random copolymers, a neutral substrate for the PS and PLLA blocks is formed by functionalizing a substrate with hydroxyl-terminated PS (PS-OH) followed by hydroxyl-terminated PLLA (PLLA-OH). The heterogeneous grafting of PS-OH and PLLA-OH can be substantially alleviated using this two-step functionalization. Accordingly, the surface properties can be fine-tuned by controlling the ratio of grafted PS-OH to PLLA-OH to control the orientation of the PLLA cylinders on the functionalized SiO2. Nevertheless, the orientation that is driven by the neutral substrate is surprisingly limited in that the effective length of orienting cylinders is less than twice the interdomain spacing. Thermal annealing at high temperature can yield a neutral air surface, rendering perpendicular PLLA cylinders that stand sub-micrometers from the air surface. Consequently, the neutral substrate can be used to enable truly film-spanning perpendicular cylinders in films to be fabricated using the high-temperature thermal treatment. In addition, the perpendicular cylinders can be laterally ordered by further increasing the annealing temperature. The ability to create these film-spanning perpendicular cylinders in films with a well-ordered texture and sub-micrometer thickness opens up possible applications in nanotechnology.