Author Correction: Apparent Power Law Scaling of Variable Range Hopping Conduction in Carbonized Polymer Nanofibers.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Tuning the electronic structure of single-walled carbon nanotube by high-pressure H2 exposure.

We report on an electronic structure change of single-walled carbon nanotube (SWNT) on hexagonal boron nitride due to electron doping via high-pressure H2 exposure. The fractional coverage of hydrogenated carbon atom is estimated to be at least θ = 0.163 from the in situ I ds-V g measurements of the release process. Raman spectroscopy and x-ray photoelectron spectroscopy were carried out to support the in situ electrical measurements. In particular, we used the dissociative Langmuir-type model to yield the desorption coefficient k des by fitting it to the in situ electrical data. Finally, we applied this hydrogenation method to the SWNT network on the commercial Si/SiO2 substrate to open the possibility of the scalable n-type semiconducting SWNT FETs.

Uniform doping of graphene close to the Dirac point by polymer-assisted assembly of molecular dopants.

Tuning the charge carrier density of two-dimensional (2D) materials by incorporating dopants into the crystal lattice is a challenging task. An attractive alternative is the surface transfer doping by adsorption of molecules on 2D crystals, which can lead to ordered molecular arrays. However, such systems, demonstrated in ultra-high vacuum conditions (UHV), are often unstable in ambient conditions. Here we show that air-stable doping of epitaxial graphene on SiC-achieved by spin-coating deposition of 2,3,5,6-tetrafluoro-tetracyano-quino-dimethane (F4TCNQ) incorporated in poly(methyl-methacrylate)-proceeds via the spontaneous accumulation of dopants at the graphene-polymer interface and by the formation of a charge-transfer complex that yields low-disorder, charge-neutral, large-area graphene with carrier mobilities ~70 000 cm2 V-1 s-1 at cryogenic temperatures. The assembly of dopants on 2D materials assisted by a polymer matrix, demonstrated by spin-coating wafer-scale substrates in ambient conditions, opens up a scalable technological route toward expanding the functionality of 2D materials.

Probing variable range hopping lengths by magneto conductance in carbonized polymer nanofibers.

Using magneto transport, we probe hopping length scales in the variable range hopping conduction of carbonized polyacetylene and polyaniline nanofibers. In contrast to pristine polyacetylene nanofibers that show vanishing magneto conductance at large electric fields, carbonized polymer nanofibers display a negative magneto conductance that decreases in magnitude but remains finite with respect to the electric field. We show that this behavior of magneto conductance is an indicator of the electric field and temperature dependence of hopping length in the gradual transition from the thermally activated to the activation-less electric field driven variable range hopping transport. This reveals magneto transport as a useful tool to probe hopping lengths in the non-linear hopping regime.

Large-area synthesis of high-quality monolayer 1T'-WTe2 flakes.

Large-area growth of monolayer films of the transition metal dichalcogenides is of the utmost importance in this rapidly advancing research area. The mechanical exfoliation method offers high quality monolayer material but it is a problematic approach when applied to materials that are not air stable. One important example is 1T'-WTe2, which in multilayer form is reported to possess a large non saturating magnetoresistance, pressure induced superconductivity, and a weak antilocalization effect, but electrical data for the monolayer is yet to be reported due to its rapid degradation in air. Here we report a reliable and reproducible large-area growth process for obtaining many monolayer 1T'-WTe2 flakes. We confirmed the composition and structure of monolayer 1T'-WTe2 flakes using x-ray photoelectron spectroscopy, energy-dispersive x-ray spectroscopy, atomic force microscopy, Raman spectroscopy and aberration corrected transmission electron microscopy. We studied the time dependent degradation of monolayer 1T'-WTe2 under ambient conditions, and we used first-principles calculations to identify reaction with oxygen as the degradation mechanism. Finally we investigated the electrical properties of monolayer 1T'-WTe2 and found metallic conduction at low temperature along with a weak antilocalization effect that is evidence for strong spin-orbit coupling.

Apparent Power Law Scaling of Variable Range Hopping Conduction in Carbonized Polymer Nanofibers.

We induce dramatic changes in the structure of conducting polymer nanofibers by carbonization at 800 °C and compare charge transport properties between carbonized and pristine nanofibers. Despite the profound structural differences, both types of systems display power law dependence of current with voltage and temperature, and all measurements can be scaled into a single universal curve. We analyze our experimental data in the framework of variable range hopping and argue that this mechanism can explain transport properties of pristine polymer nanofibers as well.

Scalable Production of Sensor Arrays Based on High-Mobility Hybrid Graphene Field Effect Transistors.

We have developed a scalable fabrication process for the production of DNA biosensors based on gold nanoparticle-decorated graphene field effect transistors (AuNP-Gr-FETs), where monodisperse AuNPs are created through physical vapor deposition followed by thermal annealing. The FETs are created in a four-probe configuration, using an optimized bilayer photolithography process that yields chemically clean devices, as confirmed by XPS and AFM, with high carrier mobility (3590 ± 710 cm2/V·s) and low unintended doping (Dirac voltages of 9.4 ± 2.7 V). The AuNP-Gr-FETs were readily functionalized with thiolated probe DNA to yield DNA biosensors with a detection limit of 1 nM and high specificity against noncomplementary DNA. Our work provides a pathway toward the scalable fabrication of high-performance AuNP-Gr-FET devices for label-free nucleic acid testing in a realistic clinical setting.

Citrus-Peel-Derived, Nanoporous Carbon Nanosheets Containing Redox-Active Heteroatoms for Sodium-Ion Storage.

Advanced design of nanostructured functional carbon materials for use in sustainable energy storage systems suffers from complex fabrication procedures and the use of special methods and/or expensive precursors, limiting their practical applications. In this study, nanoporous carbon nanosheets (NP-CNSs) containing numerous redox-active heteroatoms (C/O and C/N ratios of 5.5 and 34.3, respectively) were fabricated from citrus peels by simply heating the peels in the presence of potassium ions. The NP-CNSs had a 2D-like morphology with a high aspect ratio of >100, high specific surface area of 1167 m(2) g(-1), and a large amount of nanopores between 1 and 5 nm. The NP-CNSs also had an electrical conductivity of 2.6 × 10(1) s cm(-1), which is approximately 50 times higher than that of reduced graphene oxide. These unique material properties resulted in superior electrochemical performance with a high specific capacity of 140 mAh g(-1) in the cathodic potential range. In addition, symmetric full-cell devices based on the NP-CNSs showed excellent cyclic performance over 100,000 repetitive cycles.

Sodium-Ion Storage in Pyroprotein-Based Carbon Nanoplates.

Pyroprotein-based carbon nanoplates are fabricated from self-assembled silk proteins as a versatile platform to examine sodium-ion storage characteristics in various carbon environments. It is found that, depending on the local carbon structure, sodium ions are stored via chemi-/physisorption, insertion, or nanoclustering of metallic sodium.

Ultralong Ordered Nanowires from the Concerted Self-Assembly of Discotic Liquid Crystal and Solvent Molecules.

The realization of long, aligned molecular wires is a great challenge, and a variety of approaches have been proposed. Interestingly, hexapentyloxytriphenylene (HAT5) discotic liquid crystal molecules, a model system of molecules with flat and aromatic cores, can spontaneously form well-aligned, micrometer long, yet only tens of nanometers thick, nanowires on solid surfaces. We have investigated the formation mechanism of these wires using different solvents with selected characteristics, including chemical structure, boiling point, vapor pressure, and surface tension. When casting from toluene and benzene solutions, atomic force microscopy reveals that the discotics spontaneously form very long and thin wires, self-aligning along a common orientation. If instead dodecane or heptane are used, different and in general thicker structures are obtained. The chemical structure of the solvent appears to have a key role, coupling to the liquid crystal self-assembly by allowing solvent molecules to enter the ordered structure if their design matches the core of HAT5 molecules, thereby guiding the assembly. However, other aspects are also relevant in the assembly, including the nature of the substrate and the rate of solvent evaporation, and these can favor or interfere with the self-assembly into long structures. The use of solvents with aromatic structure is advantageous not only because it affects the geometry of the assembly, promoting long wire formation, but it is also compatible with good quality of the intermolecular order, as suggested by a high anisotropy of the Raman spectra of the nanowires formed from these solvents. Finally, the electrical properties of ordered systems show a clearly higher electrical conductivity compared to the disorganized aggregates.

Electronic transport of recrystallized freestanding graphene nanoribbons.

The use of graphene and other two-dimensional materials in next-generation electronics is hampered by the significant damage caused by conventional lithographic processing techniques employed in device fabrication. To reduce the density of defects and increase mobility, Joule heating is often used since it facilitates lattice reconstruction and promotes self-repair. Despite its importance, an atomistic understanding of the structural and electronic enhancements in graphene devices enabled by current annealing is still lacking. To provide a deeper understanding of these mechanisms, atomic recrystallization and electronic transport in graphene nanoribbon (GNR) devices are investigated using a combination of experimental and theoretical methods. GNR devices with widths below 10 nm are defined and electrically measured in situ within the sample chamber of an aberration-corrected transmission electron microscope. Immediately after patterning, we observe few-layer polycrystalline GNRs with irregular sp(2)-bonded edges. Continued structural recrystallization toward a sharp, faceted edge is promoted by increasing application of Joule heat. Monte Carlo-based annealing simulations reveal that this is a result of concentrated local currents at lattice defects, which in turn promotes restructuring of unfavorable edge structures toward an atomically sharp state. We establish that intrinsic conductance doubles to 2.7 e(2)/h during the recrystallization process following an almost 3-fold reduction in device width, which is attributed to improved device crystallinity. In addition to the observation of consistent edge bonding in patterned GNRs, we further motivate the use of bonded bilayer GNRs for future nanoelectronic components by demonstrating how electronic structure can be tailored by an appropriate modification of the relative twist angle of the bonded bilayer.

Hierarchically porous carbon nanosheets from waste coffee grounds for supercapacitors.

The nanostructure design of porous carbon-based electrode materials is key to improving the electrochemical performance of supercapacitors. In this study, hierarchically porous carbon nanosheets (HP-CNSs) were fabricated using waste coffee grounds by in situ carbonization and activation processes using KOH. Despite the simple synthesis process, the HP-CNSs had a high aspect ratio nanostructure (∼20 nm thickness to several micrometers in lateral size), a high specific surface area of 1945.7 m(2) g(-1), numerous heteroatoms, and good electrical transport properties, as well as hierarchically porous characteristics (0.5-10 nm in size). HP-CNS-based supercapacitors showed a specific energy of 35.4 Wh kg(-1) at 11250 W kg(-1) and of 23 Wh kg(-1) for a 3 s charge/discharge current rate corresponding to a specific power of 30000 W kg(-1). Additionally, the HP-CNS supercapacitors demonstrated good cyclic performance over 5000 cycles.

Electronic transport in heterostructures of chemical vapor deposited graphene and hexagonal boron nitride.

CVD graphene devices on stacked CVD hexagonal boron nitride (hBN) are demonstrated using a novel low-contamination transfer method, and their electrical performance is systematically compared to devices on SiO(2). An order of magnitude improvement in mobility, sheet resistivity, current density, and sustained power is reported when the oxide substrate is covered with five-layer CVD hBN.

Correlating atomic structure and transport in suspended graphene nanoribbons.

Graphene nanoribbons (GNRs) are promising candidates for next generation integrated circuit (IC) components; this fact motivates exploration of the relationship between crystallographic structure and transport of graphene patterned at IC-relevant length scales (<10 nm). We report on the controlled fabrication of pristine, freestanding GNRs with widths as small as 0.7 nm, paired with simultaneous lattice-resolution imaging and electrical transport characterization, all conducted within an aberration-corrected transmission electron microscope. Few-layer GNRs very frequently formed bonded-bilayers and were remarkably robust, sustaining currents in excess of 1.5 µA per carbon bond across a 5 atom-wide ribbon. We found that the intrinsic conductance of a sub-10 nm bonded bilayer GNR scaled with width as GBL(w) ≈ 3/4(e(2)/h)w, where w is the width in nanometers, while a monolayer GNR was roughly five times less conductive. Nanosculpted, crystalline monolayer GNRs exhibited armchair-terminated edges after current annealing, presenting a pathway for the controlled fabrication of semiconducting GNRs with known edge geometry. Finally, we report on simulations of quantum transport in GNRs that are in qualitative agreement with the observations.

Scalable production of highly sensitive nanosensors based on graphene functionalized with a designed G protein-coupled receptor.

We have developed a novel, all-electronic biosensor for opioids that consists of an engineered µ-opioid receptor protein, with high binding affinity for opioids, chemically bonded to a graphene field-effect transistor to read out ligand binding. A variant of the receptor protein that provided chemical recognition was computationally redesigned to enhance its solubility and stability in an aqueous environment. A shadow mask process was developed to fabricate arrays of hundreds of graphene transistors with average mobility of ∼1500 cm(2) V(-1) s(-1) and yield exceeding 98%. The biosensor exhibits high sensitivity and selectivity for the target naltrexone, an opioid receptor antagonist, with a detection limit of 10 pg/mL.

Microporous carbon nanoplates from regenerated silk proteins for supercapacitors.

Novel carbon-based microporous nanoplates containing numerous heteroatoms (H-CMNs) are fabricated from regenerated silk fibroin by the carbonization and activation of KOH. The H-CMNs exhibit superior electrochemical performance, displaying a specific capacitance of 264 F/g in aqueous electrolytes, a specific energy of 133 Wh/kg, a specific power of 217 kW/kg, and a stable cycle life over 10000 cycles.

N-type graphene induced by dissociative H2 adsorption at room temperature.

Studies of the interaction between hydrogen and graphene have been increasingly required due to the indispensable modulation of the electronic structure of graphene for device applications and the possibility of using graphene as a hydrogen storage material. Here, we report on the behaviour of molecular hydrogen on graphene using the gate voltage-dependent resistance of single-, bi-, and multi-layer graphene sheets as a function of H2 gas pressure up to 24 bar from 300 K to 345 K. Upon H2 exposure, the charge neutrality point shifts toward the negative gate voltage region, indicating n-type doping, and distinct Raman signature changes, increases in the interlayer distance of multi-layer graphene, and a decrease in the d-spacing occur, as determined by TEM. These results demonstrate the occurrence of dissociative H2 adsorption due to the existence of vacancy defects on graphene.

Fibers of reduced graphene oxide nanoribbons.

Reduced graphene oxide nanoribbon fibers were fabricated by using an electrophoretic self-assembly method without the use of any polymer or surfactant. We report electrical and field emission properties of the fibers as a function of reduction degree. In particular, the thermally annealed fiber showed superior field emission performance with a low potential for field emission (0.7 V µm(-1)) and a giant field emission current density (400 A cm(-2)). Moreover, the fiber maintains a high current level of 300 A cm(-2) corresponding to 1 mA during long-term operation.

Enhancement of heating performance of carbon nanotube sheet with granular metal.

A strategy for enhancing the heating performance of freestanding carbon nanotube (CNT) sheet is presented that involves decorating the sheet with granular-type palladium (Pd) particles. When Pd is added to the sheet, the heating efficiency of CNT sheet is increased by a factor of 3.6 (99.9 °C cm(2)/W vs 27.3 °C cm(2)/W with no Pd). Suppression of convective heat transfer loss attributes to the enhanced heat generation efficiency. However, higher heating response of CNT/Pd sheet was observed compared to CNT sheet, hence suggesting that the electron-lattice energy exchange could be additional heating mechanism in the presence of granular-type particles of Pd having a diameter of 10 nm or less. CNT sheet/Pd is quite stable, retaining its initial characteristics even after 300 cycles of on-off voltage pulses and shows fast thermal responses of the heating and cooling rates being 154 and -248 °C/s, respectively.