Session A43: Focus Session: Thin Film Block Copolymers I

Study of Complex Morphology of ABC Triblock Copolymer with Resonant Soft X-ray Scattering

Combining the spectroscopy sensitivity with scattering, resonant soft x-ray scattering (RSoXS) is an ideal tool for characterizing morphology of multi-component soft materials thin films. Both elemental and chemical sensitivity can be achieved by changing incident photon energy close to the absorption edge of the constituent atoms. In this work, the morphologies in thin film and bulk of A-B-C triblock copolymer, Poly(1,4-isoprene)-block-polystyrene-block-poly(2-vinlypyridine), were investigated with RSoXS together with scanning force microscopy, small angle x-ray scattering and transmission electron microscopy. In thin film, hexagonal array of nanostructures were observed by all the techniques, however, RSoXS revealed the core-shell nanostructures. In bulk, hexagonally packed cylindrical microdomains was observed. By selectively staining different polymer block, TEM tomography results suggested spatial arrangement of two different cylindrical microdomains consisted of poly(1,4-isoprene) and poly(2-vinlypyridine) in polystyrene matrix. Using soft x-ray at selected photon energies to isolate the scattering contribution from two different polymer blocks, RSoXS unambiguously revealed the two different hexagonally packed lattice.   

STED Microscopy as a Characterization Tool for Three Dimensionally Nanostructured Block Copolymer Thin Films

STED microscopy is an emerging method in the characterization of block copolymer film morphologies. We demonstrate a complete experimental platform that allows us to obtain in situ Three Dimensional images with microdomain specificity. IsoSTED, a variant of STED microscopy that coherently combines two opposing lenses, yields the necessary resolution, while the necessary fluorescence contrast is achieved by deterministically confining fluorophores to the targeted microdomain. These capabilities are combined to provide unambiguous images of various nano-structured block copolymer morphologies.   

Interfaces between Block Copolymer Domains

Block copolymers naturally form nanometer scale structures which repeat their geometry on a larger scale. Such a small scale periodic pattern can be used for various applications such as storage media, nano-circuits and optical filters. However, perfect alignment of block copolymer domains in the macroscopic scale is still a distant dream. The nanostructure formation usually occurs with spontaneously broken symmetry; hence it is easily infected by topological defects which sneak in due to entropic fluctuation and incomplete annealing. Careful annealing can gradually reduce the number of defects, but once kinetically trapped, it is extremely difficult to remove all the defects. One of the main reasons is that the defect finds a locally metastable morphology whose potential depth is large enough to prohibit further morphology evolution. In this work, the domain boundaries between differently oriented lamellar structures in thin film are studied. For the first time, it became possible to quantitatively study the block copolymer morphology in the transitional region, and it was shown that the twisted grain boundary is energetically favorable compared to the T-junction grain boundary. [Nano Letters, {\bf 9}, 2300 (2010)]. This theoretical method successfully explained the experimental results.   

Patterning of Multiple Block Copolymers per Layer with Orthogonal Processing

In this work we demonstrate the concept of orthogonal processing of block copolymers. By using a semi-flourinated photoresist/solvent system, we are able to selectively pattern or lift-off and then recover the block copolymer film intact. This approach can enable removable templating of self- assembly and also multiple block copolymers, morphologies or domain sizes on the same layer which can open the door to self-assembly of a wider range of geometries than possible before. We highlight the interplay between the various parameters for a successful additive and subtractive patterning and directions for further theoretical investigation as well as the limitations of this technique.   

Highly specific placement of Au Nanoparticles on chemical brush patterns prepared by combination of top-down and block copolymer lithography

Metallic nanoparticles (NPs) with interesting optical, electronic and reactivity properties show great promise for a range of scientific fields and technological applications, such as plasmonics, catalysis, and nanowire growth. However taking advantage of these properties, particularly for device fabrication often requires immobilization of pre-synthesized particles with high specificity and precise control of density and spacing of NPs at sub 100 nm scale. Here we show that lithographic patterning of a cross-linkable polystyrene brush and subsequent filling with poly(2-vinyl pyridine) (P2VP) leads to high contrast chemical patterns leading to site-specific placement of pre-synthesized Au NPs (13 nm) with the precise control of number from single to tens of particles per spot. Moreover we show that this approach is extendible to large area patterning of NPs with costs and process times suitable for technological applications using block-copolymer (BCP) lithography.   

Synthesis and graphoepitaxial placement control of block copolymer mediated silver nanoparticles

The strong interactions of plasmons in metal nanoparticle assemblies can render many possible applications ranging from sensors to imaging and information technology. To realize such applications, synthesis of well defined metal nanoparticles and precise control over assembly are critical. In this paper, we report a synthetic scheme of silver nanoparticles and their combination with dielectrics and/or gain media and their assembly on substrates. Silver nanoparticles are synthesized using a block copolymer of polystyrene and poly(4-vinyl pyridine) (PS-b-P4VP). Well defined nanoparticles were assembled on substrates using a graphoepitaxial approach with topographic patterns prepared by E-beam lithography. The effect of shapes and scales of topographic patterns on the nanoparticle assembly was investigated. Careful optical characterization and potential applications will be discussed.   

Functional Nanomaterials based on Nanoporous Block Copolymer Templates

Nanoporous templates have been widely used for the development of new functional nanostructured materials suitable for electronics, optics, magnetism, and energy storage materials. We have prepared nanoporous templates by using thin films of mixtures of polystyrene-\textit{block}-poly(methyl methacrylate) (PS-$b$-PMMA) and PMMA homopolymers. These nanoporous films were found to be very effective for the separation of human Rhinovirus type 14, major pathogen of a common cold in humans. We found that when the pore size was effectively controlled down to 6 nm, a long-term constant \textit{in vitro} release of BSA and hGH was achieved without their denaturation up to 2 months. The long-term constant delivery based on this membrane for protein drugs within the therapeutic range can be highly appreciated for the patients with hormone-deficiency.\\[4pt] Work done in collaboration with Seung Yun Yang, Pohang University of Science and Technology.   

Effect of Boundary Conditions on the Directed Self-Assembly of Block Copolymer on Chemical Patterned Surfaces

Previously we determined the morphological phase behavior of lamellae-forming poly(styrene-block-methyl methacrylate) (PS-b-PMMA) density multiplication on chemically patterned surfaces in thin films. The stripe density of the chemical pattern was half of the block copolymer domain density. Parallel lamellae, vertical lamellae, mixed lamellae, PS-dots, and PMMA-dots were observed depending on the pattern stripe width and interaction strength between the polymer and the patterned surfaces. The density multiplied vertical lamellae exhibited three dimensional profiles, which was problematic in the subsequent pattern transfer process. Here we found that the block copolymer domain profiles could be improved by revising the boundary conditions. In the meanwhile, better line width and line edge roughness were obtained. The addition of homopolymers into the block copolymer and subsequent molecular transfer printing could offer a chemically patterned surface with improved boundary conditions for block copolymer directed assembly. The printed chemical patterns had a stripe density and a stripe width matching with block copolymer domains. The interfacial energies of the stripes were favorable to the block copolymer domains.   

Directed assembly of supramolecular copolymers in thin films

Using computer simulation of a coarse-grained model for supramolecular polymers we investigate the potential of quasi-block copolymers (QBCP) assembled on chemically patterned substrates for creating device-oriented nanostructures. QBCP are comprised of AB diblock copolymers and supramolecular B segments that can reversibly bond to any available B terminus, either on the copolymers or the B oligomers, creating a polydisperse blend of B homopolymers, AB and ABA copolymers. We focus on an AB incompatibility, $\chi$, and strength of supramolecular bonds where a lamellar morphology, a bicontinous structure and a macrophase-separated state have comparable free energy in the bulk. We consider substrate patterns with perpendicularly crossing, A-preferential lines and demonstrate their defect-free replication by QBCP. The same QBCP replicates simultaneously patterns differing by up to $50\%$ in their length scales, illustrating the high versatility of QBCP materials. We discuss the interplay between pattern geometry and distribution of molecular architectures and verify the key role of supramolecular associations for replicating patterns with different length scales.   

Self-Assembly of Block Copolymers on Well-modulated Sawtoothed Surface

The self-assembly of block copolymers on the faceted surfaces of sapphire and silicon substrates were investigated as a function of the amplitude and pitch of the sawtooth pattern. To generate these surfaces, sapphire substrates, cut along the M plane, were annealed at temperatures above 1000$^{\circ}$C. The pitch and amplitude of the sawtooth pattern was controlled by varying the annealing temperature. In addition, surfaces with a sawtooth pattern in silicon were generated by an anisotropic etching of the silicon. Block copolymers were spin-coated onto the pattered surfaces. To induce ordering of the BCP, the thin films were solvent-annealed in organic solvent vapors. Long-range lateral ordering of the BCP microdomains persisted across the entire surface with both types of substrates without further treatment. The ordering of the BCP is very sensitive to the geometry of sawtooth pattern.   

Templating non-hexagonal monolayers of block copolymer spheres in confined geometries

We investigate the ordering of poly(styrene-b-2vinylpyridine) [PS-PVP Mn = 56 kg/mol] sphere monolayers in wells of various shapes and sizes. Recent self-consistent field theory results on ordering of block copolymer (BCP) cylinders in square surroundings suggest that adding homopolymers of higher Mn can allow square arrays of BCP to form in small (4 to 5 cylinders across) square wells by relieving packing frustration. Experimentally, we adopt a similar strategy for ordering BCP spheres on silicon nitride membranes patterned by electron beam lithography to produce SiOx mesas, adding various volume fractions of PS homopolymer of different Mn. Scanning force microscopy, transmission electron microscopy and X-ray scattering has been used in a complementary manner to quantify the structures obtained after thermal annealing at 150 C. In the absence of PS additions, only defective hexagonal structures are observed even in wells containing 16 spheres. Adding 10{\%} PS of Mn = 112 kg/mol to the BCP results in a square packing of spheres in square wells containing as many as 81 spheres.   

Dynamics of interacting edge defects in copolymer lamellae

It is known that terraces at the interface of lamella forming diblock copolymers do not make discontinuous jumps in height. Rather, their profiles are smoothly varying. The width of the transition region between two lamellar heights is typically several hundreds of nanometres, resulting from a balance between surface tension, chain stretching penalties, and the enthalpy of mixing. What is less well known in these systems is what happens when two transition regions approach one another. In this study, we show that time dependent experimental data of interacting copolymer lamellar edges is consistent with a model that assumes a repulsion between adjacent edges. The range of the interaction between edge defects is consistent with the profile width of noninteracting diblock terraces.