Diblock Copolymer Thin Films Research Articles

Electrical bistable memory device based on a poly(styrene-b-4-vinylpyridine) nanostructured diblock copolymer thin film

This paper describes the performance of a nonvolatile memory device based on a solution-processed poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) diblock copolymer thin film. The Al/PS-b-P4VP/indium tin oxide memory device featuring metal-coordinated 30 nm P4VP cores exhibited an ON/OFF ratio of 2×105, an erase voltage of 0.75 V, a write voltage of −0.5 V, and a retention time of 104 s. The device exhibited a metallic behavior in the ON state, suggesting the formation of metallic filaments through the migration of Al atoms into the P4VP domain during writing. Such nanostructured diblock copolymer thin films open up avenues for fabricating organic memory devices using simple procedures.

A Transition from Cylindrical to Spherical Morphology in Diblock Copolymer Thin Films

Microphase-separated structures in poly(styrene-block-isoprene) (SI) block copolymer thin films were investigated by transmission electron microtomography (TEMT). The SI block copolymer showed cylindrical microdomains in the bulk state. Several block copolymer thin films with different thicknesses were prepared by spin-coating and were extensively annealed before the TEMT experiments. Intriguingly, although the cylindrical morphology orienting parallel to the substrate was observed in most of the cases, spherical microdomains were found at certain film thicknesses. The thickness dependence was investigated using a computer simulation based on the self-consistent-field theory, producing a morphological phase diagram based on minimizing free energy. We find that the distortion of the hexagonal lattice of the cylindrical microdomains caused the morphological transition to the spherical microdomains.

Directed Self-Assembly of Diblock Copolymer Thin Films on Chemically-Patterned Substrates for Defect-Free Nano-Patterning

We demonstrate that polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) can self-assemble in a well-aligned, long-range ordered nanopattern over arbitrarily large areas, commensurate with chemically prepatterned templates prepared by electron beam (EB) lithography. The epitaxially grown cylindrical microdomain formed a defect-free hexagonal lattice although the Chemically-Patterned substrate had some defects in its pattern and errors in pattern position. Furthermore, we demonstrated that the self-assembly process can interpolate points in between the EB generated pattern, thus multiplying the pattern density. The result suggests that the self-assembly of PS-b-PMMA can repair the defects of the patterned substrate, while the patterned substrate can align the domain structures of block copolymer into a long-range ordered single array. The developed process, which combines EB lithography and self-assembly of diblock copolymer provides a promising fabrication method for extension of top down-type lithogra...

Polymer Crystallization Influenced by Initial Orientation of Cylindrical Diblock Copolymers in Thin Films

The effect of the initial states (disordered perpendicular cylinder structure vs. parallel cylinder structure) on the crystallization of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) thin films during cyclohexane annealing was investigated. The cylindrical domains perpendicular or parallel to the surface were obtained by controlling the film thickness. During solvent annealing, for the film with the perpendicular cylinders, the ordering degree of cylinders was increased. The enthalpic increase is large enough for the forming of square-shaped crystals, and subsequently the square-shaped single crystals surrounded by the ordered hexagonally packed perpendicular cylinders evolved to the dendrite ones. For the film with the parallel cylinders, the parallel cylinders were translated to the perpendicular ones. The increased enthalpy was not large enough for the formation of square-shaped single crystals. Instead, the dendrite-like crystals started at the edge of terraces.

Order−Order and Order−Disorder Transitions in Thin Films of an Amphiphilic Liquid Crystalline Diblock Copolymer

In this study, we quantitatively investigated the temperature-dependent phase transition behaviors of thin films of an interesting amphiphilic diblock copolymer, poly(ethylene oxide)-b-poly(11-[4-(4-butylphenylazo)phenoxy]undecyl methacrylate) (p(EO)-b-p(MAAZ)) and the resulting morphological structures by using synchrotron grazing incidence X-ray scattering (GIXS) and differential scanning calorimetry. The quantitative GIXS analysis showed that the diblock copolymer in the homogeneous, isotropic melt state undergoes phase-separation near 190 degrees C and then forms a body-centered cubic (BCC) structure of spherical p(EO) domains in the p(MAAZ) matrix, at which point the p(EO) domains and the p(MAAZ) matrix are both in amorphous, liquid states. The BCC structure of spherical p(EO) domains is converted to a hexagonal cylinder structure near 120 degrees C, which is induced by the transformation of the isotropic phase of the p(MAAZ) matrix to the smectic A phase, which is composed of a laterally ordered structure of p(MAAZ) blocks with fully extended side groups. The resulting hexagonal cylinder structure is very stable below 120 degrees C. This microscopic hexagonal cylinder structure is retained as the smectic A phase of the p(MAAZ) matrix undergoes further transitions to smectic C near 104 degrees C and to a smectic X phase near 76 degrees C, while the amorphous, liquid phase of the p(EO) cylinders undergoes crystallization near -15 degrees C. These complicated temperature-dependent disorder-order and order-order phase transitions in the films were found to take place reversibly during the heating run. A face-centered orthorhombic structure of p(EO) domains was also found during the heating run and is an intermediate structure in the hexagonal cylinder structure to BCC structure transformation. We use these structural analysis results to propose molecular structure models at various temperatures for thin films of the diblock polymer.

Hexagonally Perforated Lamella-to-Cylinder Transition in a Diblock Copolymer Thin Film under an Electric Field

Using self-consistent-field simulation, a perforated lamella (PL) is investigated under an applied electric field. The perforated lamella is obtained by confining a cylinder-forming diblock copolymer melt in a thin film with a thickness of the spacing between cylinders in the bulk equilibrium state. Upon application of an electric field, the perforated lamella transforms to laying or standing cylinders depending on the direction of the electric field.

From Nanorings to Nanodots by Patterning with Block Copolymers

We demonstrate three different transfer patterns that can be achieved by use of a surface reconstructed block copolymer film where metal is evaporated onto the surface of the film, providing the contrast. Thin films of diblock copolymers having cylindrical microdomains oriented normal to the surface with long-range lateral order were used. Solvent reconstruction of the film, followed by a glancing angle metal evaporation and thermal annealing, led to three different decorations of the films with gold. These films were used as masks for pattern transfer of pores, columns, and rings to underlying substrate with high fidelity.

Phase-field modeling of the formation of lamellar nanostructures in diblock copolymer thin films under inplanar electric fields

Recent experiments show that external inplanar electric field can be employed to guide the molecular self-assembly in diblock copolymer (BCP) thin films to form lamellar nanostructures with potential applications in nanotechnology. We study this self-assembly process through a detailed coarse-grained phase-separation modeling. During the process, the free energy of the BCP films is modeled as the Ginzburg-Landau free energy with nonlocal interaction and electrostatic coupling. The resulting Cahn-Hilliard (CH) equation is solved using an efficient semi-implicit Fourier-spectral algorithm. Numerical results show that the morphology of order parameter formed in either symmetric or asymmetric BCP thin films is strongly influenced by the electric field. For symmetrical BCPs, highly ordered lamellar nanostructures evolved along the direction of the electric field. Phase nucleation and dislocation climbing in the BCP films predicted by the numerical simulation are in a good agreement with those observed in recent BCP electronanolithography. For asymmetrical BCPs, numerical simulation shows that nanodots are guided to align to the electric field. Furthermore, in the case of high electric field, nanodots formed in asymmetrical BCPs may further convert into highly ordered lamellar nanostructures (sphere-to-cylinder transition) parallel to the electric field. Effects of the magnitude of electric field, BCP asymmetry, and molecular interaction of BCPs on the self-assembly process are examined in detail using the numerical scheme developed in this study. The present study can be used for the prediction of the formation of nanostructures in BCP thin films and the quality control of BCP-based nanomanufacturing through optimizing the external electric fields.

Symmetric Diblock Copolymer Thin Films on Rough Substrates: Microdomain Periodicity in Pure and Blended Films

The lamellar dimension, D, for pure and blended block copolymer (BCPs) thin films of symmetric poly(styrene)-block-poly(methyl methacrylate) was measured using atomic force microscopy analysis of surface patterns of perpendicularly oriented lamellar structures. It was approximately verified, using SAXS and AFM analysis, that perpendicular structures in lamellar thin films bounded by a neutral and a roughened interface did not significantly alter the bulk block copolymer phase separation thermodynamics. In the case of blends, it was also verified that there was a uniform distribution of blend components throughout the thin film. These checks allowed a detailed comparison of D as a function of the number of statistical segments (N) and blend composition using existing bulk phase predictions and experiments. In pure BCPs we observed the two characteristic regimes; strong (D ∼ N0.66) and intermediate segregation regimes (D ∼ N0.85). In addition, we observed a more complicated, weakly segregated regime where D does not scale as N0.5 as expected for a random coil. In blended BCPs there was good agreement with existing theory when BCP components from the strongly segregated regime were used. In cases where intermediate or weakly segregating BCP components were used, an alternative semiempirical analytical function was developed. In cases where the ratio of N for the BCP blend components exceeds ∼5, macrophase separation was observed in the thin film with the smaller BCP component remaining at the air surface.

Polystyrene-b-polyisoprene thin films with hexagonally perforated layer structure: quantitative grazing-incidence X-ray scattering analysis

Grazing-incidence X-ray scattering (GIXS) formulas for hexagonally perforated layer (HPL) structures withABCandABstacking sequences were derived, and used in the quantitative analysis of the two-dimensional GIXS patterns of polystyrene-b-polyisoprene (PS-b-PI) diblock copolymer thin films supported on silicon substrates. This quantitative analysis provided detailed information (shape, size and size distribution, packing order, layer packing sequence, and orientation) about the HPL structure of the diblock copolymer films that cannot be easily obtained with conventional X-ray and neutron scattering techniques or with conventional microscopic methods.

Microdomain Ordering in Laterally Confined Block Copolymer Thin Films

We examine the effects of small-scale, hexagonal, lateral confinement on microdomain ordering in diblock copolymer thin films using self-consistent field theory simulations. Specifically, we examine a hexagonal confinement well with side length L approximately equal to five cylindrical microdomain lattice spacings. The commensurability constraints of the small-scale, lateral confinement, coupled with surface-induced effects allow the confining well to have a significant effect on the perfection of microdomain order. We identify commensurability windows in L that depend on the segment−wall interaction and the “temperature” annealing rate (modeled as a Flory χ ∼ 1/T annealing rate). The effect of added majority-block homopolymer is also explored.

Evaluation of Enzymatic Activity on Nanoscale Polystyrene-block-Polymethylmethacrylate Diblock Copolymer Domains

Understanding structural and functional changes of polymeric surface-bound proteins is extremely important as polymers play an increasingly significant role as arrays and substrates in proteomics applications. We carried out, for the first time, quantitative activity measurements of horseradish peroxidase (HRP) enzymes immobilized selectively on the polystyrene domains of microphase-separated polystyrene-block-polymethylmethacrylate ultrathin films. The specific enzymatic activity of HRP adsorbed on the diblock copolymer surface was evaluated and compared to that of HRP in free solution. We demonstrate that the polymeric surface-bound HRP molecules maintain approximately 85% of their activity in free solution. The unique advantages of diblock copolymer templates, involving nanoscale self-assembly and largely retained protein functionality, make the spontaneously constructed enzyme nanoarrays highly suitable as proteomics substrates. Our novel assembly method of providing functional enzymes on diblock copolymer thin films can be greatly beneficial for high-throughput and high-density protein assays.

Solvent-Induced Transition from Micelles in Solution to Cylindrical Microdomains in Diblock Copolymer Thin Films

It is well-known that block copolymer thin films can be easily prepared on different substrates by spin coating. Here, we show that the cylindrical microdomains of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) copolymers can be oriented normal to the surface over a large area by spin coating in toluene/THF solvent mixture. The structure and size of the PS-b-P4VP in solution were compared with those in thin films. It was found that the cylindrical microdomains, obtained by spin coating, oriented normal to the film surface depended strongly on the amount of THF in the casting solution. The study was extended to PS-b-P4VP copolymers with different molecular weights and compositions, which allowed the control of the size of the cylindrical domains and the areal density of the arrays.

Detailed analysis of gyroid structures in diblock copolymer thin films with synchrotron grazing-incidence X-ray scattering

In this study, a grazing-incidence X-ray scattering (GIXS) formula was derived for gyroid structures formed in thin films supported on substrates. Two-dimensional GIXS patterns were measured for gyroid structures formed in polystyrene-b-polyisoprene (PS-b-PI) diblock copolymer nanometre-scale thin films supported on silicon substrates, and a quantitative analysis of the obtained two-dimensional GIXS data was conducted with the scattering formula. This analysis provided details (lattice parameter, width of the PS phase, positional distortion factor, orientation and orientation distribution) of the gyroid structures developed in the diblock copolymer thin films that are not easily obtained using conventional techniques. Moreover, it was possible to simulate complete and detailed two-dimensional GIXS patterns with the determined structure parameters.

Enhanced Order of Block Copolymer Cylinders in Single‐Layer Films Using a Sweeping Solidification Front

Significant improvements of the microdomain order in cylinder-forming diblock copolymer thin film samples subjected to directional solidification are reported. Samples annealed in a sweeping solidification front have defect densities reduced by two orders of magnitude compared to regular thermal annealing, whereas adding an oscillatory component to the solidification front decreases defect densities by two additional orders of magnitude

A new process for fabricating nanodot arrays on selective regions with diblock copolymerthin film

A procedure for micropatterning a single layer of nanodotarrays in selective regions is demonstrated by using thin films ofpolystyrene-b-poly(t-butylacrylate) (PS-b-PtBA) diblock copolymer. The thin-film self-assembled into hexagonally arrangedPtBA nanodomains in a PS matrix on a substrate by solvent annealing with 1,4-dioxane. ThePtBA nanodomains were converted into poly(acrylic acid) (PAA) having carboxylic-acid-functionalizednanodomains by exposure to hydrochloric acid vapor, or were removed by ultraviolet (UV)irradiation to generate vacant sites without any functional groups due to the elimination ofPtBA domains. By sequential treatment with aqueous sodium bicarbonate and aqueous zinc acetatesolution, zinc cations were selectively loaded only on the carboxylic-acid-functionalizednanodomains prepared via hydrolysis. Macroscopic patterning through a photomask viaUV irradiation, hydrolysis, sequential zinc cation loading and calcination left a nanodotarray of zinc oxide on a selectively UV-shaded region.

Competition of Lamellar Orientation in Thin Films of a Symmetric Poly(styrene)-b-poly(l-lactide) Diblock Copolymer in Melt State

We have studied the lamellar orientation in thin films of a model diblock copolymer, symmetric poly(styrene)-b-poly(l-lactide) (PS−PLLA), in the melt state on supported silicon wafer surface. In this system, while the PLLA block prefers to wet the polymer/substrate interface, the polymer/air as well as polymer/polymer interface is neutral for both blocks due to the similar surface energies of PS and PLLA in melt state. Our results demonstrate that the interplay of the interfaces during phase separation results in a series of structures before approaching the equilibrium state. Lamellar orientation of thin films with different initial film thicknesses at different annealing stages has been investigated using atomic force microscopy (AFM), transmission electronic microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). It is found that in the early stage (annealing time t < 10 min), the polymer/substrate interface dominates the structure evolution, leading to a parallel lamellar structure with holes or islands formed depending on the initial film thickness. Later on, the neutral air interface becomes important and leads to a transition of lamellar orientation from parallel to perpendicular. It is interesting to see that for films with thickness h > 2L, where L is the bulk lamellar period, the lamellar orientation transition can occur independently in different parallel lamellar domains due to the neutrality of polymer/polymer interface. This independent transition leads to a new intermediate stage; i.e., the perpendicular lamellae in two adjacent layers can be either parallel or cross to each other. Certainly, these interlaced structures are not the thermodynamic equilibrium forms; they emerge into the uniformed perpendicular structure upon further annealing.

Combinatorial Generation and Replication-Directed Assembly of Complex and Varied Geometries with Thin Films of Diblock Copolymers

Directed assembly of fine-scale, very complex patterns with a variety of features, including terminations, jogs, disclinations, acute and obtuse bends, and sharp radii of curvature, was achieved with a symmetric poly(styrene-block-methylmethacrylate) (PS-b-PMMA) copolymer. The complex pattern was generated spontaneously by spin coating and annealing a thin film of a lamellae-forming block copolymer on a chemically neutral surface. The resulting "fingerprint" pattern had a domain spacing of 47.5 nm. Oxygen plasma treatment of the block copolymer converted it into an insoluble chemical nanopattern that was quantified by XPS, goniometry, and the wetting behavior of the block copolymer. Spin coating a second thin film of the block copolymer and annealing resulted in directed assembly that replicated the fingerprint pattern, including the most complicated defect structures. A computer vision algorithm was developed and implemented to compare the patterns quantitatively, taking into account inherent differences in image contrast, scale, rotation, and translation.

Phase Behavior in Thin Films of Cylinder-Forming Diblock Copolymer: Deformation and Division of Heptacoordinated Microdomains

The microdomain areas in a 2D hexagonal array of vertically oriented PS-b-PMMA diblock copolymer cylinders have been studied, and we have found that their area increases as a function of their coordination number. For the 7-fold coordinated cylinders, the unit cell could adopt an anisotropic configuration due to the stress of the lattice. In this case, in order to minimize the free energy of the system, the PMMA domain deforms commensurately with the unit cell and adopts an elliptical or a lozenge-like cross section, in good agreement with strong segregation considerations. However, these strained configurations reveal to be metastable so that thermal fluctuations could induce a transition from a distorted column to two other circular columns. This phenomenon has been used to explain the motion of dislocations.

Tailored Ag nanoparticle array fabricated by block copolymer photolithography

Periodic arrays of metallic Ag nanoparticles have been fabricated on various solid substrates for the potential application in nano-electronics and nano-photonics by block copolymer photolithography. Spin-coated diblock copolymer thin film with well-ordered and vertically oriented cylindrical nanodomains was utilized as the template. Ag + was selectively doped in the cylindrical nanodomain by just placing a few drops of AgNO 3 aqueous solution on the film. Both photochemical reduction of Ag + to metallic Ag and concomitant photo-etching of the diblock copolymer template have been achieved under vacuum ultraviolet light irradiation, resulting in a direct transcription of the periodic pattern of diblock copolymer template to the metallic Ag nanoparticle array with the consistent periodicity.