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

Bulk and Thin Film Morphological Behavior of Broad Dispersity Poly(styrene-<i>b-</i>methyl methacrylate) Diblock Copolymers

We describe the morphological implications of broad molecular weight dispersity on the bulk and thin film self-assembly behavior of seven model poly­(styrene-<i>block</i>-methyl methacrylate) (SM) diblock copolymers. Derived from sequential nitroxide-mediated polymerizations, these unimodal diblock copolymers are comprised of narrow dispersity S blocks (<i>Đ</i> ≤ 1.14) and broad dispersity M blocks (<i>Đ</i> ∼ 1.7) with total molecular weights <i>M</i>_n,total = 29.2–42.9 kg/mol and M volume fractions <i>f</i>_M = 0.35–0.63. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) analyses demonstrate that these diblock copolymers microphase separate into lamellar and cylindrical morphologies with substantially larger microdomain spacings at lower overall molecular weights as compared to their narrow dispersity analogues. The observed microphase-separated melt stabilization is also accompanied by a substantial shift in the lamellar phase composition window to higher values of <i>f</i>_M. In thin films, these polydisperse copolymers form perpendicularly oriented morphologies with modest degrees of lateral order on substrates functionalized with P­(S-<i>ran</i>-MMA) neutral polymer brush layers

A Dual Functional Layer for Block Copolymer Self-Assembly and the Growth of Nanopatterned Polymer Brushes

We present a versatile method for fabricating nanopatterned polymer brushes using a cross-linked thin film made from a random copolymer consisting of an inimer (<i>p</i>-(2-bromoisobutyloylmethyl)­styrene), styrene, and glycidyl methacrylate (GMA). The amount of inimer was held constant at 20 or 30% while the relative amount of styrene to GMA was varied to induce perpendicular domain orientation in an overlying P­(S-<i>b</i>-MMA) block copolymer (BCP) film for lamellar and cylindrical morphologies. A cylinder forming BCP blend with PMMA homopolymer was assembled to create a perpendicular hexagonal array of cylinders, which allowed access to a nanoporous template without the loss of initiator functionality. Surface-initiated ATRP of 2-hydroxyethyl methacrylate was conducted through the pores to generate a dense array of nanopatterned brushes. Alternatively, gold was deposited into the nanopores, and brushes were grown around the dots after removal of the template. This is the first example of combining the chemistry of nonpreferential surfaces with surface-initiated growth of polymer chains

Chemical Patterns for Directed Self-Assembly of Lamellae-Forming Block Copolymers with Density Multiplication of Features

Lamellae-forming polystyrene-<i>block</i>-poly­(methyl methacrylate) (PS-<i>b</i>-PMMA) films, with bulk period <i>L</i>₀, were directed to assemble on lithographically nanopatterned surfaces. The chemical pattern was comprised of “guiding” stripes of cross-linked polystyrene (X-PS) or poly­(methyl methacrylate) (X-PMMA) mats, with width <i>W</i>, and interspatial “background” regions of a random copolymer brush of styrene and methyl methacrylate (P­(S-<i>r</i>-MMA)). The fraction of styrene (<i>f</i>) in the brush was varied to control the chemistry of the background regions. The period of the pattern was <i>L</i>_s. After assembly, the density of the features (domains) in the block copolymer film was an integer multiple (<i>n</i>) of the density of features of the chemical pattern, where <i>n</i> = <i>L</i>_s/<i>L</i>₀. The quality of the assembled PS-<i>b</i>-PMMA films into patterns of dense lines as a function of <i>n</i>, <i>W</i>/<i>L</i>₀, and <i>f</i> was analyzed with top-down scanning electron microscopy. The most effective background chemistry for directed assembly with density multiplication corresponded to a brush chemistry (<i>f</i>) that minimized the interfacial energy between the background regions and the composition of the film overlying the background regions. The three-dimensional structure of the domains within the film was investigated using cross-sectional SEM and Monte Carlo simulations of a coarse-grained model and was found most closely to resemble perpendicularly oriented lamellae when <i>W</i>/<i>L</i>₀ ∼ 0.5–0.6. Directed self-assembly with density multiplication (<i>n</i> = 4) and <i>W</i>/<i>L</i>₀ = 1 or 1.5 yields pattern of high quality, parallel linear structures on the top surface of the assembled films, but complex, three-dimensional structures within the film