We demonstrate the fabrication of nanoperforated graphene materials with sub-20-nm features using cylinder-forming diblock copolymer templates across >1 mm(2) areas. Hexagonal arrays of holes are etched into graphene membranes, and the remaining constrictions between holes interconnect forming a honeycomb structure. Quantum confinement, disorder, and localization effects modulate the electronic structure, opening an effective energy gap of 100 meV in the nanopatterned material. The field-effect conductivity can be modulated by 40x (200x) at room temperature (T = 105 K) as a result. A room temperature hole mobility of 1 cm(2) V(-1) s(-1) was measured in the fabricated nanoperforated graphene field effect transistors. This scalable strategy for modulating the electronic structure of graphene is expected to facilitate applications of graphene in electronics, optoelectronics, and sensing.

The ability of random copolymer brushes and cross-linked mats to induce the vertical orientation of domains in overlying films of lamellae-and cylinder-forming block copolymers was investigated as a function of the composition. The substrate-modifying layers consisted of styrene and methyl methacrylate random copolymers and contained either a terminal hydroxyl group or a third polar comonomer of 2-hydroxyethyl methacrylate (HEMA) for grafting brushes to silicon oxide surfaces or glycidyl methacrylate (GMA) for crosslinking the random copolymer into a mat. Polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) lamellaeand cylinder-forming block copolymers (both PS and PMMA minority block copolymers) were deposited and annealed on the modified surfaces. In all cases the vertical orientation of domains was observed for a range of random copolymer composition, but the ranges of composition were different for each combination of surface layer and block copolymer. The cylindrical domains of PS exhibited vertical structures for a very narrow range of compositions compared to cylindrical domains of PMMA or lamellae. As expected, the incorporation of polar HEMA or GMA monomers in the surface layers shifted the composition range for the perpendicular orientation of domains to higher fractions of styrene. The results are discussed in terms of the equilibration of the films in the presence of the chemically modified surfaces.

Nearly ballistic carbon nanotube array transistors are realized with current densities outmatching conventional semiconductors.

Lamellae-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films, with bulk period L 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 W, and interspatial “background” regions of a random copolymer brush of styrene and methyl methacrylate (P(S-r-MMA)). The fraction of styrene (f) in the brush was varied to control the chemistry of the background regions. The period of the pattern was L s. After assembly, the density of the features (domains) in the block copolymer film was an integer multiple (n) of the density of features of the chemical pattern, where n = L s/L 0. The quality of the assembled PS-b-PMMA films into patterns of dense lines as a function of n, W/L 0, and f was analyzed with top-down scanning electron microscopy. The most effective background chemistry for directed assembly with density multiplication corresponded to a brush chemistry (f) 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 W/L 0 ∼ 0.5–0.6. Directed self-assembly with density multiplication (n = 4) and W/L 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.

A major challenge to increasing bandwidth in optical telecommunications is to encode electronic signals onto a lightwave carrier by modulating the light up to very fast rates. Polymer electro-optic materials have the necessary properties to function in photonic devices beyond the 40-GHz bandwidth currently available. An appropriate choice of polymers is shown to effectively eliminate the factors contributing to an optical modulator's decay in the high-frequency response. The resulting device modulates light with a bandwidth of 150 to 200 GHz and produces detectable modulation signal at 1.6 THz. These rates are faster than anticipated bandwidth requirements for the foreseeable future.

We report the induction of perpendicularly oriented cylindrical domains in PS-b-PMMA block copolymer (BCP) films thicker than 100 nm by thermally annealing on a substrate modified with a random copolymer. The effects of annealing temperature, composition of the substrate-modifying random copolymer, and BCP film thickness on the morphology of PMMA cylinder forming PS-b-PMMA were studied. For BCP films thicker than 100 nm, the fabrication of perpendicular PMMA cylinders is highly dependent on both the substrate-modifying random copolymer and the annealing temperature as these two parameters control the interactions of the BCP with the substrate and the free surface, respectively. We found the best perpendicular structures to be created by using a random copolymer brush with a styrene fraction (F St) near 0.70 and an annealing temperature near 230 °C. Perpendicular cylinder structures were achieved in ∼300 nm thick films using these conditions. When the BCP film was thicker than 300 nm, nucleation and growth of the microdomains proceeded independently from each interface. We present scanning electron microscope (SEM) and cross-sectional transmission electron microscope (TEM) images of these perpendicular structures and explain the results on the basis of previous simulation reports.

We report a route to noncovalently latch dipolar molecules on graphene to create stable chromophore/graphene hybrids where molecular transformation can be used as an additional handle to reversibly modulate doping while retaining high mobilities. A light switchable azobenzene chromophore was tethered to the surface of graphene via π-π interactions, leading to p-doping of graphene with an hole concentration of ~5 × 10(12) cm(-2). As the molecules switch reversibly from trans to cis form the dipole moment changes, and hence the extent of doping, resulting in the modulation of hole concentration up to ~18% by alternative illumination of UV and white light. Light-driven conductance modulation and control experiments under vacuum clearly attribute the doping modulation to molecular transformations in the organic molecules. With improved sensitivities these "light-gated" transistors open up new ways to enable optical interconnects.

Chemically patterned surfaces comprised of polymer mats and brushes of well-defined chemistry were fabricated at the length scale of 10 nm. A key concept is the integration of new materials, cross-linked polymer mats, with traditional lithographic processing. Resist was patterned on top of cross-linked polystyrene mats. After etching, regions of the remaining mat with dimensions ranging from 10 to 35 nm were separated by interspatial openings to the underlying substrate. End-grafted polymer brushes, in this case hydroxyl-terminated poly(2-vinylpyridine) or polystyrene−poly(methyl methacrylate) random copolymer, were grafted into the exposed, interspatial regions from films spin-coated over the patterned mat. Both block copolymer wetting studies, with polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA), and near-edge X-ray fine structure spectroscopy showed that with sufficient cross-linking the polymer mat chemistry was unaffected by the subsequent grafting of the polymer brush. The precise definition of both the chemistry and the geometry was demostrated with two sensitive applications of nanoscale chemical patterns: the site-specific immobilization of Au nanoparticles and the directed assembly of overlying PS-b-PMMA films.