Resist Free Patterning of Nonpreferential Buffer Layers for Block Copolymer Lithography

We report the design of a direct electron beam patternable buffer layer to spatially control the orientation of the microdomains in an overlaying polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) block copolymer (BCP) film. The buffer layer consists of a surface anchored low molecular weight PS-b-PMMA, with the PMMA segment anchored to the surface and a short PS block at the buffer layer/BCP interface. The block architecture of the buffer layer combines the essential features of “bottom up” and “top down” approaches as it functions as a nonpreferential layer to dictate perpendicular orientation of BCP domains from the substrate interface and as an e-beam resist to allow top-down lithographic process to spatially define the buffer layer on the substrate. The composition of the buffer layer can be tuned by changing the relative block lengths to create a nonpreferential surface which effectively induces perpendicular orientation of domains in an overlying BCP film. The grafted block copolymer can be locally shaved by e-beam lithography resulting in spatial control of domain orientation in the BCP film. The direct patterning approach reduces the number of steps involved in forming chemical patterns by conventional lithography

Synthesis of High Etch Contrast Poly(3-hydroxystyrene)-Based Triblock Copolymers and Self-Assembly of Sub‑5 nm Features

We demonstrate the successful synthesis of a series of poly­(3-hydroxystyrene)-block-poly­(dimethylsiloxane)-block-poly­(3-hydroxystyrene) (P3HS-b-PDMS-b-P3HS) triblock copolymers by atom transfer radical polymerization (ATRP). This system is a promising candidate for pattern transfer of single-digit nanometer features, due to its intrinsic high etch contrast, etch versatility, and the triblock architecture. Hydroxy-terminated PDMS polymers were directly functionalized to initiate the ATRP of an acetal-protected 3-hydroxystyrene monomer. The resulting triblocks have dispersity ranging from 1.10 to 1.26, and the synthesis provides robust control over molecular weights and volume fractions. The large chemical incompatibility between the blocks enabled the formation of ordered structures at low molecular weights, which yielded lamellar periodicities as small as 9.3 nm. This triblock hence permits access to sub-5 nm feature sizes. The phase diagram is asymmetric, with lamellar morphologies observed at lower volume fractions of the P3HS block, relative to the diblocks. This asymmetry is ascribed to the dispersity of the middle block relative to the end blocks

A Single-Component Inimer Containing Cross-Linkable Ultrathin Polymer Coating for Dense Polymer Brush Growth

We have developed a highly versatile universal approach to grow polymer brushes from a variety of substrates with high grafting density by using a single-component system. We describe a random copolymer which consists of an inimer, <i>p</i>-(2-bromoisobutyloylmethyl)­styrene (BiBMS), copolymerized with glycidyl methacrylate (GMA) synthesized by reversible addition–fragmentation chain-transfer (RAFT) polymerization. Thermal cross-linking created a mat that was stable during long exposure in organic solvent even with sonication or during Soxhlet extraction. The absolute bromine density was determined via X-ray photoelectron spectroscopy (XPS) to be 1.86 ± 0.12 Br atoms/nm³. The ratio of experimental density to calculated absolute initiator density suggests that ∼25% of the bromine is lost during cross-linking. Surface-initiated ATRP (SI-ATRP) was used to grow PMMA brushes on the substrate with sacrificial initiator in solution. The brushes were characterized by ellipsometry, XPS, and atomic force microscopy (AFM) to determine thickness, composition, and homogeneity. By correlating the molecular weight of polymer grown in solution with the brush layer thickness, a high grafting density of 0.80 ± 0.06 chains/nm² was calculated. By synthesizing the copolymer before cross-linking on the substrate, this single-component approach avoids any issues with blend miscibility as might be present for a multicomponent curable mixture, while resulting in high chain density on a range of substrates

Dose-Controlled, Floating Evaporative Self-assembly and Alignment of Semiconducting Carbon Nanotubes from Organic Solvents

Arrays of aligned semiconducting single-walled carbon nanotubes (s-SWCNTs) with exceptional electronic-type purity were deposited at high deposition velocity of 5 mm min^–1 by a novel “dose-controlled, floating evaporative self-assembly” process with excellent control over the placement of stripes and quantity of s-SWCNTs deposited. This approach uses the diffusion of organic solvent on the water–air interface to deposit aligned s-SWCNT (99.9%) tubes on a partially submerged hydrophobic substrate, which is withdrawn vertically from the surface of water. By decoupling the s-SWCNT stripe formation from the evaporation of the bulk solution and by iteratively applying the s-SWCNTs in controlled “doses”, we show through polarized Raman studies that the s-SWCNTs are aligned within ±14°, are packed at a density of ∼50 s-SWCNTs μm^–1, and constitute primarily a well-ordered monodispersed layer. The resulting field-effect transistor devices show high performance with a mobility of 38 cm² V^–1 s^–1 and on/off ratio of 2.2 × 10⁶ at 9 μm channel length

Emergent Dispersion and Sorting Behaviors of Carbon Nanotubes When Combining Short Conjugated Oligomers with Nonconjugated Coil Segments

Single-walled carbon nanotubes have unique electronic properties and the potential to advance the microelectronics industry. For these applications, semiconducting CNTs with high purity (>99.99%) and specific diameters or band gaps are required. High-molecular-weight conjugated polymers, especially polyfluorenes and their derivatives, have shown an exceptional ability to selectively wrap semiconducting over metallic CNTs while enriching a reduced number of diameters, whereas lower-molecular-weight analogues (<10,000 g/mol) form much less stable dispersions. Here, we report coil-conjugated-coil triblock copolymers (PS28-b-PFO16-b-PS28 and PS69-b-PFO16-b-PS69) that combine low-molecular-weight fluorene oligomers with polystyrene coils. Neither the short, conjugated oligomers nor the polystyrene coils alone have the ability to act as stable wrappers, but when combined, stable CNT dispersions are obtained with yields comparable to or exceeding those of high-molecular-weight PFO, with a semiconducting selectivity shifted to larger diameters. These results open the door to low-molecular-weight wrappers for CNT sorting and to solution-processing with wrappers with modified conformations that have the potential to alter the interactions of CNTs with their environment

Hierarchical Nanostructures of Organosilicate Nanosheets within Self-Organized Block Copolymer Films

Hierarchical Nanostructures of Organosilicate Nanosheets within Self-Organized Block Copolymer Film

Synthesis of Photoacid Generator-Containing Patternable Diblock Copolymers by Reversible Addition−Fragmentation Transfer Polymerization

Synthesis of Photoacid Generator-Containing Patternable Diblock Copolymers by Reversible Addition−Fragmentation Transfer Polymerizatio

Polymer-Coated Magnetic Microspheres Conjugated with Growth Factor Receptor Binding Peptides Enable Cell Sorting

The separation and sorting of human cells is an important step in the bioprocessing of cell-based therapeutics. Heterogeneous mixtures of cells must be sorted to isolate the desired cell type and purify the final product. This process is often achieved by antibody-based sorting techniques. In this work, we demonstrate that magnetic microspheres may be functionalized with peptides that selectively bind to cells on the basis of their relative concentration of specific surface proteins. Five-micrometer-magnetic microspheres were coated with the synthetic copolymer PVG (poly­(poly­(ethylene glycol)­methyl ether methacrylate-ran-vinyl dimethyl azlactone-ran-glycidyl methacrylate) and functionalized with the vascular endothelial growth factor receptor binding peptide (VRBP), which binds to the vascular endothelial growth factor receptor (VEGFR). These microspheres exhibited low cytotoxicity and bind to cells depending on their relative surface protein expression. Finally, coated, magnetic microspheres were used to separate heterogeneous populations of cells dependent on their VEGFR expression through magnetic-assisted cell sorting (MACS), demonstrating that peptide-based cell sorting mechanisms may be useful in the bioprocessing of human-cell-based products

Electronic Transport and Raman Scattering in Size-Controlled Nanoperforated Graphene

We demonstrate the fabrication and study of the structure–property relationships of large-area (>1 cm²) semiconducting nanoperforated (NP) graphene with tunable constriction width (<i>w</i> = 7.5–14 nm), derived from CVD graphene using block copolymer lithography. Size-tunable constrictions were created while minimizing unintentional doping by using a dual buffer layer pattern-transfer method. An easily removable polymeric layer was sandwiched between an overlying silicon oxide layer and the underlying graphene. Perforation-size was controlled by overetching holes in the oxide prior to pattern transfer into graphene while the polymer protected the graphene from harsh conditions during oxide etching and lift off. The processing materials were removed using relatively mild solvents yielding the clean isolation of NP graphene and thereby facilitating Raman and electrical characterization. We correlate the D to G ratio as a function of <i>w</i> and show three regimes depending on <i>w</i> relative to the characteristic Raman relaxation length. Edge phonon peaks were also observed at 1450 and 1530 cm^–1 in the spectra, without the use of enhancement methods, due to high density of nanoconstricted graphene in the probe area. The resulting NP graphene exhibited semiconducting behavior with increasing ON/OFF conductance modulation with decreasing <i>w</i> at room temperature. The charge transport mobility decreases with increasing top-down reactive ion etching. From these comprehensive studies, we show that both electronic transport and Raman characteristics change in a concerted manner as <i>w</i> shrinks

Side-Chain-Grafted Random Copolymer Brushes as Neutral Surfaces for Controlling the Orientation of Block Copolymer Microdomains in Thin Films

Random copolymers of P(S-r-MMA-r-HEMA)s with a distribution of surface reactive hydroxyl groups were synthesized to formulate neutral surface layers on a SiO2 substrate. The layers were designed to drive vertical orientation of lamellar microdomains in a top P(S-b-MMA) thin film. Copolymers with a styrene weight fraction (fSt) of 0.58 and a HEMA fraction (fHEMA) ranging from 0.01 to 0.03, with a corresponding MMA fraction (fMMA) ranging from 0.41 to 0.39, in the P(S-r-MMA-r-HEMA) copolymer showed neutral surface characteristics. The morphology of block copolymer thin films was studied by scanning electron microscopy (SEM). P(S-r-MMA-r-HEMA) copolymers prepared by both living and classical free-radical polymerizations were equally effective in demonstrating the neutrality of the surface. These side-chain-grafted random copolymer brushes showed faster grafting kinetics than the end-chain-grafted P(S-r-MMA) because of multipoint attachment to the surface. The modified surfaces had a very thin layer of random copolymer brush (5−7 nm), which is desirable for effective pattern transfer. Furthermore, neutral surfaces could be obtained even when the grafting time was reduced to 3 h. These results indicate that the composition of the random copolymer brush, rather than its PDI or molecular weights, is the most important factor in controlling the neutrality of the surface. These results also demonstrate the feasibility of using a third comonomer (C) in the random copolymer brush P(A-r-B-r-C) to alter the interfacial and surface energies of a diblock copolymer (A-b-B)