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Single-slit laser diffractography was used to image the growth of carbon nanotubes. A silicon dioxide slit with a minimum width of 150 μm was prepared and positioned inside a chemical vapor deposition (CVD) reactor in alignment with a laser source. Carbon nanotubes were grown inside the slit width, producing corresponding changes in the diffraction pattern due to the optical opacity of these structures and to their high density and alignment. Changes in the diffraction pattern were recorded and used for the direct measurement of nanotube growth. The results show an exponential increase of length vs time for 45 min experiments, best fit with a double exponential function, which is interpreted in terms of the concurrence of base-growth and tip-growth modes for successive catalyst particles. Scanning electron microscopy confirms the diffractographic data at a high level of precision. The innovation brought by this in situ method to the kinetic study of nanotube synthesis is discussed and compared to a posteriori studies based solely on microscopy for a range of different nanotube lengths. [ABSTRACT FROM AUTHOR]

Vertical integration of van der Waals (vdW) materials with atomic precision is an intriguing possibility brought forward by these two-dimensional (2D) materials. Essential to the design and analysis of these structures is a fundamental understanding of the vertical transport of charge carriers into and across vdW materials, yet little has been done in this area. In this report, we explore the important roles of single layer graphene in the vertical tunneling process as a tunneling barrier. Although a semimetal in the lateral lattice plane, graphene together with the vdW gap act as a tunneling barrier that is nearly transparent to the vertically tunneling electrons due to its atomic thickness and the transverse momenta mismatch between the injected electrons and the graphene band structure. This is accentuated using electron tunneling spectroscopy (ETS) showing a lack of features corresponding to the Dirac cone band structure. Meanwhile, the graphene acts as a lateral conductor through which the potential and charge distribution across the tunneling barrier can be tuned. These unique properties make graphene an excellent 2D atomic grid, transparent to charge carriers, and yet can control the carrier flux via the electrical potential. A new model on the quantum capacitance's effect on vertical tunneling is developed to further elucidate the role of graphene in modulating the tunneling process. This work may serve as a general guideline for the design and analysis of vdW vertical tunneling devices and heterostructures, as well as the study of electron/spin injection through and into vdW materials. [ABSTRACT FROM AUTHOR]

Despite its importance in the large-scale synthesis of transition metal dichalcogenides (TMDC) molecular layers, the generic quantum effects on electrical transport across individual grain boundaries (GBs) in TMDC monolayers remain unclear. Here we demonstrate that strong carrier localization due to the increased density of defects determines both temperature dependence of electrical transport and low-frequency noise at the GBs of chemical vapor deposition (CVD)-grown MoS2 layers. Using field effect devices designed to explore transport across individual GBs, we show that the localization length of electrons in the GB region is -30-70% lower than that within the grain, even though the room temperature conductance across the GB, oriented perpendicular to the overall flow of current, may be lower or higher than the intragrain region. Remarkably, we find that the stronger localization is accompanied by nearly 5 orders of magnitude enhancement in the low-frequency noise at the GB region, which increases exponentially when the temperature is reduced. The microscopic framework of electrical transport and noise developed in this paper may be readily extended to other strongly localized two-dimensional systems, including other members of the TMDC family. [ABSTRACT FROM AUTHOR]

Heterostructures of compositionally and electronically variant two-dimensional (2D) atomic layers are viable building blocks for ultrathin optoelectronic devices. We show that the composition of interfacial transition region between semiconducting WSe2 atomic layer channels and metallic NbSe2 contact layers can be engineered through interfacial doping with Nb atoms. WxNb1-xSe2 interfacial regions considerably lower the potential barrier height of the junction, significantly improving the performance of the corresponding WSe2-based field-effect transistor devices. The creation of such alloyed 2D junctions between dissimilar atomic layer domains could be the most important factor in controlling the electronic properties of 2D junctions and the design and fabrication of 2D atomic layer devices. [ABSTRACT FROM AUTHOR]

Graphene oxide film is made of stacked graphene layers with chemical functionalities, and we report that plasticity in the film can be engineered by strain rate tuning. The deformation behavior and plasticity of such functionalized layered systems is dominated by shear slip between individual layers and interaction between functional groups. Stress-strain behavior and theoretical models suggest that the deformation is strongly strain rate dependent and undergoes brittle to ductile transition with decreasing strain rate. [ABSTRACT FROM AUTHOR]

MoS2 is a promising and low-cost material for electrochemical hydrogen production due to its high activity and stability during the reaction. However, the efficiency of hydrogen production is limited by the amount of active sites, for example, edges, in MoS2. Here, we demonstrate that oxygen plasma exposure and hydrogen treatment on pristine monolayer MoS2 could introduce more active sites via the formation of defects within the monolayer, leading to a high density of exposed edges and a significant improvement of the hydrogen evolution activity. These as-fabricated defects are characterized at the scale from macroscopic continuum to discrete atoms. Our work represents a facile method to increase the hydrogen production in electrochemical reaction of MoS2 via defect engineering, and helps to understand the catalytic properties of MoS2. [ABSTRACT FROM AUTHOR]

The practical recycling of carbon dioxide (CO2) by the electrochemical reduction route requires an active, stable, and affordable catalyst system. Although noble metals such as gold and silver have been demonstrated to reduce CO2 into carbon monoxide (CO) efficiently, they suffer from poor durability and scarcity. Here we report three-dimensional (3D) graphene foam incorporated with nitrogen defects as a metal-free catalyst for CO2 reduction. The nitrogen-doped 3D graphene foam requires negligible onset overpotential (-0.19 V) for CO formation, and it exhibits superior activity over Au and Ag, achieving similar maximum Faradaic efficiency for CO production (~85%) at a lower overpotential (-0.47 V) and better stability for at least 5 h. The dependence of catalytic activity on N-defect structures is unraveled by systematic experimental investigations. Indeed, the density functional theory calculations confirm pyridinic N as the most active site for CO2 reduction, consistent with experimental results. [ABSTRACT FROM AUTHOR]

Here we report a unique method to locally determine the mechanical response of individual covalent junctions between carbon nanotubes (CNTs), in various configurations such as "X", "Y", and "Λ"-like. The setup is based on in situ indentation using a picoindenter integrated within a scanning electron microscope. This allows for precise mapping between junction geometry and mechanical behavior and uncovers geometry-regulated junction stiffening. Molecular dynamics simulations reveal that the dominant contribution to the nanoindentation response is due to the CNT walls stretching at the junction. Targeted synthesis of desired junction geometries can therefore provide a "structural alphabet" for construction of macroscopic CNT networks with tunable mechanical response. [ABSTRACT FROM AUTHOR]

An atomically thin optoelectronicmemory array for image sensingis demonstrated with layered CuIn7Se11and extendedto InSe and MoS2atomic layers. Photogenerated charge carriersare trapped and subsequently retrieved from the potential well formedby gating a 2D material with Schottky barriers. The atomically thinlayered optoelectronic memory can accumulate photon-generated chargesduring light exposure, and the charges can be read out later for dataprocessing and permanent storage. An array of atomically thin imagememory pixels was built to illustrate the potential of fabricatinglarge-scale 2D material-based image sensors for image capture andstorage. [ABSTRACT FROM AUTHOR]

Thegate-controllability of the Fermi-edge onset of interband absorptionin graphene can be utilized to modulate near-infrared radiation inthe telecommunication band. However, a high modulation efficiencyhas not been demonstrated to date, because of the small amount oflight absorption in graphene. Here, we demonstrate a ∼40% amplitudemodulation of 1.55 μm radiation with gated single-layer graphenethat is coupled with a silicon microring resonator. Both the qualityfactor and resonance wavelength of the silicon microring resonatorwere strongly modulated through gate tuning of the Fermi level ingraphene. These results promise an efficient electro-optic modulator,ideal for applications in large-scale on-chip optical interconnectsthat are compatible with complementary metal-oxide-semiconductor technology. [ABSTRACT FROM AUTHOR]

Substantialdifferences in charge storage mechanisms exist betweendielectric capacitors (DCs) and electrochemical capacitors (ECs),resulting in orders of magnitude difference of stored charge densityin them. However, if ionic diffusion, the major charge transport mechanismin ECs, is confined within nanoscale dimensions, the Helmholtz layersand diffusion layers will overlap, resulting in dismissible ionicdiffusion. An interesting contradiction between appreciable energydensity and unrecognizable ionic diffusion is observed in solid-statecapacitors made from reduced graphene oxide films that challenge thefundamental charge storage mechanisms proposed in such devices. Anew capacitive model is proposed, which combines the two distinctcharge storage mechanisms of DCs and ECs, to explain the contradiction,of high storage capacity yet undetectable ionic diffusion, seen ingraphene oxide based supercapacitors. [ABSTRACT FROM AUTHOR]

Conventionalwisdom suggests that decreasing dimensions of dielectricmaterials (e.g., thickness of a film) should yield increasing capacitance.However, the quantum capacitance and the so-called “dead-layer”effect often conspire to decrease the capacitance of extremely smallnanostructures, which is in sharp contrast to what is expected fromclassical electrostatics. Very recently, first-principles studieshave predicted that a nanocapacitor made of graphene and hexagonalboron nitride (h-BN) films can achieve superior capacitor properties.In this work, we fabricate the thinnest possible nanocapacitor system,essentially consisting of only monolayer materials: h-BN with grapheneelectrodes. We experimentally demonstrate an increase of the h-BNfilms’ permittivity in different stack structures combinedwith graphene. We find a significant increase in capacitance belowa thickness of ∼5 nm, more than 100% of what is predicted byclassical electrostatics. Detailed quantum mechanical calculationssuggest that this anomalous increase in capacitance is due to thenegative quantum capacitance that this particular materials systemexhibits. [ABSTRACT FROM AUTHOR]

Wedemonstrate how substrate interfacial chemistry can be utilizedto tailor the physical properties of single-crystalline molybdenumdisulfide (MoS2) atomic-layers. Semiconducting, two-dimensionalMoS2possesses unique properties that are promising forfuture optical and electrical applications for which the ability totune its physical properties is essential. We use self-assembled monolayerswith a variety of end termination chemistries to functionalize substratesand systematically study their influence on the physical propertiesof MoS2. Using electrical transport measurements, temperature-dependentphotoluminescence spectroscopy, and empirical and first-principlescalculations, we explore the possible mechanisms involved. Our datashows that combined interface-related effects of charge transfer,built-in molecular polarities, varied densities of defects, and remoteinterfacial phonons strongly modify the electrical and optical propertiesof MoS2. These findings can be used to effectively enhanceor modulate the conductivity, field-effect mobility, and photoluminescencein MoS2monolayers, illustrating an approach for localand universal property modulations in two-dimensional atomic-layers. [ABSTRACT FROM AUTHOR]

Gate-controllable transmission ofterahertz (THz) radiation makesgraphene a promising material for making high-speed THz wave modulators.However, to date, graphene-based THz modulators have exhibited onlysmall on/off ratios due to small THz absorption in single-layer graphene.Here we demonstrate a ∼50% amplitude modulation of THz waveswith gated single-layer graphene by the use of extraordinary transmissionthrough metallic ring apertures placed right above the graphene layer.The extraordinary transmission induced ∼7 times near-filedenhancement of THz absorption in graphene. These results promise complementarymetal–oxide–semiconductor compatible THz modulatorswith tailored operation frequencies, large on/off ratios, and highspeeds, ideal for applications in THz communications, imaging, andsensing. [ABSTRACT FROM AUTHOR]

Ternarytwo-dimensional dichalcogenide alloys exhibit compositionallymodulated electronic structure, and hence, control of dopant concentrationwithin each individual layer of these compounds provides a powerfultool to efficiently modify their physical and chemical properties.The main challenge arises when quantifying and locating the dopantatoms within each layer in order to better understand and fine-tunethe desired properties. Here we report the synthesis of molybdenumdisulfide substitutionally doped with a broad range of selenium concentrations,resulting in over 10% optical band gap modulations in atomic layers.Chemical analysis using Z-contrast imaging provides direct maps ofthe dopant atom distribution in individual MoS2layersand hence a measure of the local optical band gaps. Furthermore, ina bilayer structure, the dopant distribution is imaged layer-by-layer.This work demonstrates that each layer in the bilayer system containssimilar local Se concentrations, randomly distributed, providing newinsights into the growth mechanism and alloying behavior in two-dimensionaldichalcogenide atomic layers. The results show that growth of uniform,ternary, two-dimensional dichalcogenide alloy films with tunable electronicproperties is feasible. [ABSTRACT FROM AUTHOR]

If not for its inherently weak opticalabsorption at visible andinfrared wavelengths, graphene would show exceptional promise foroptoelectronic applications. Here we show that by nanopatterning agraphene layer into an array of closely packed graphene nanodisks,its absorption efficiency can be increased from less than 3% to 30%in the infrared region of the spectrum. We examine the dependenceof the enhanced absorption on nanodisk size and interparticle spacing.By incorporating graphene nanodisk arrays into an active device, wedemonstrate that this enhanced absorption efficiency is voltage-tunable,indicating strong potential for nanopatterned graphene as an activemedium for infrared electro-optic devices. [ABSTRACT FROM AUTHOR]

Varioustwo-dimensional (2D) materials have recently attracted great attentionowing to their unique properties and wide application potential inelectronics, catalysis, energy storage, and conversion. However, large-scaleproduction of ultrathin sheets and functional nanosheets remains ascientific and engineering challenge. Here we demonstrate an efficientapproach for large-scale production of V2O5nanosheetshaving a thickness of 4 nm and utilization as building blocks forconstructing 3D architectures via a freeze-drying process. The resultinghighly flexible V2O5structures possess a surfacearea of 133 m2g–1, ultrathin walls,and multilevel pores. Such unique features are favorable for providingeasy access of the electrolyte to the structure when they are usedas a supercapacitor electrode, and they also provide a large electroactivesurface that advantageous in energy storage applications. As a consequence,a high specific capacitance of 451 F g–1is achievedin a neutral aqueous Na2SO4electrolyte as the3D architectures are utilized for energy storage. Remarkably, thecapacitance retention after 4000 cycles is more than 90%, and theenergy density is up to 107 W·h·kg–1ata high power density of 9.4 kW kg–1. [ABSTRACT FROM AUTHOR]

Wereport on the low-temperature electrical transport propertiesof large area boron and nitrogen codoped graphene layers (BNC). Thetemperature dependence of resistivity (5 K < T< 400 K) of BNC layers show semiconducting nature and displaya band gap which increases with B and N content, in sharp contrastto large area graphene layers, which shows metallic behavior. Ourinvestigations show that the amount of B dominates the semiconductingnature of the BNC layers. This experimental observations agree withthe density functional theory (DFT) calculations performed on BNCstructures similar in composition to the experimentally measured samples.In addition, the temperature dependence of the electrical conductivityof these samples displays two regimes: at higher temperatures, thedoped samples display an Arrhenius-like temperature dependence thusindicating a well-defined band gap. At the lowest temperatures, thetemperature dependence of the conductivity deviates from activatedbehavior and displays a conduction mechanism consistent with Mott’stwo-dimensional (2D) variable range hopping (2D-VRH). The abilityto tune the electronic properties of thin layers of BNC by simplyvarying the concentration of B and N will provide a tremendous boostfor obtaining materials with tunable electronic properties relevantto applications in solid state electronics. [ABSTRACT FROM AUTHOR]

We report the direct growth of large,atomically thin GaSe singlecrystals on insulating substrates by vapor phase mass transport. Acorrelation is identified between the number of layers and a Ramanshift and intensity change. We found obvious contrast of the resistanceof the material in the dark and when illuminated with visible light.In the photoconductivity measurement we observed a low dark current.The on–off ratio measured with a 405 nm at 0.5 mW/mm2light source is in the order of 103; the photoresponsivityis 17 mA/W, and the quantum efficiency is 5.2%, suggesting possibilityfor photodetector and sensor applications. The photocurrent spectrumof few-layer GaSe shows an intense blue shift of the excitation edgeand expanded band gap compared with bulk material. [ABSTRACT FROM AUTHOR]

Monolayermolybdenum disulfide (MoS2) with a directband gap of 1.8 eV is a promising two-dimensional material with apotential to surpass graphene in next generation nanoelectronic applications.In this Letter, we synthesize monolayer MoS2on Si/SiO2substrate via chemical vapor deposition (CVD) method andcomprehensively study the device performance based on dual-gated MoS2field-effect transistors. Over 100 devices are studied toobtain a statistical description of device performance in CVD MoS2. We examine and scale down the channel length of the transistorsto 100 nm and achieve record high drain current of 62.5 mA/mm in CVDmonolayer MoS2film ever reported. We further extract theintrinsic contact resistance of low work function metal Ti on monolayerCVD MoS2with an expectation value of 175 Ω·mm,which can be significantly decreased to 10 Ω·mm by appropriategating. Finally, field-effect mobilities (μFE) ofthe carriers at various channel lengths are obtained. By taking theimpact of contact resistance into account, an average and maximumintrinsic μFEis estimated to be 13.0 and 21.6 cm2/(V s) in monolayer CVD MoS2films, respectively. [ABSTRACT FROM AUTHOR]

Monolayer molybdenum disulfide (MoS2) is a two-dimensionaldirect band gap semiconductor with unique mechanical, electronic,optical, and chemical properties that can be utilized for novel nanoelectronicsand optoelectronics devices. The performance of these devices stronglydepends on the quality and defect morphology of the MoS2layers. Here we provide a systematic study of intrinsic structuraldefects in chemical vapor phase grown monolayer MoS2, includingpoint defects, dislocations, grain boundaries, and edges, via directatomic resolution imaging, and explore their energy landscape andelectronic properties using first-principles calculations. A richvariety of point defects and dislocation cores, distinct from thosepresent in graphene, were observed in MoS2. We discoverthat one-dimensional metallic wires can be created via two differenttypes of 60° grain boundaries consisting of distinct 4-fold ringchains. A new type of edge reconstruction, representing a transitionstate during growth, was also identified, providing insights intothe material growth mechanism. The atomic scale study of structuraldefects presented here brings new opportunities to tailor the propertiesof MoS2via controlled synthesis and defect engineering. [ABSTRACT FROM AUTHOR]

Although lithium ion batteries havegained commercial success owingto their high energy density, they lack suitable electrodes capableof rapid charging and discharging to enable a high power density criticalfor broad applications. Here, we demonstrate a simple bottom-up approachtoward single crystalline vanadium oxide (VO2) ribbonswith graphene layers. The unique structure of VO2-grapheneribbons thus provides the right combination of electrode propertiesand could enable the design of high-power lithium ion batteries. Asa consequence, a high reversible capacity and ultrafast charging anddischarging capability is achieved with these ribbons as cathodesfor lithium storage. A full charge or discharge is capable in 20 s.More remarkably, the resulting electrodes retain more than 90% ofthe initial capacity after cycling more than 1000 times at an ultrahighrate of 190C, providing the best reported rate performance for cathodesin lithium ion batteries to date. [ABSTRACT FROM AUTHOR]

Here, we design and develop high-power electric double-layercapacitors(EDLCs) using carbon-based three dimensional (3-D) hybrid nanostructuredelectrodes. 3-D hybrid nanostructured electrodes consisting of verticallyaligned carbon nanotubes (CNTs) on highly porous carbon nanocups (CNCs)were synthesized by a combination of anodization and chemical vapordeposition techniques. A 3-D electrode-based supercapacitor showedenhanced areal capacitance by accommodating more charges in a givenfootprint area than that of a conventional CNC-based device. [ABSTRACT FROM AUTHOR]

Strong in-plane bonding and weak van der Waals interplanarinteractionscharacterize a large number of layered materials, as epitomized bygraphite. The advent of graphene (G), individual layers from graphite,and atomic layers isolated from a few other van der Waals bonded layeredcompounds has enabled the ability to pick, place, and stack atomiclayers of arbitrary compositions and build unique layered materials,which would be otherwise impossible to synthesize via other knowntechniques. Here we demonstrate this concept for solids consistingof randomly stacked layers of graphene and hexagonal boron nitride(h-BN). Dispersions of exfoliated h-BN layers and graphene have beenprepared by liquid phase exfoliation methods and mixed, in variousconcentrations, to create artificially stacked h-BN/G solids. Thesevan der Waals stacked hybrid solid materials show interesting electrical,mechanical, and optical properties distinctly different from theirstarting parent layers. From extensive first principle calculationswe identify (i) a novel approach to control the dipole at the h-BN/Ginterface by properly sandwiching or sliding layers of h-BN and graphene,and (ii) a way to inject carriers in graphene upon UV excitationsof the Frenkell-like excitons of the h-BN layer(s). Our combined approachcould be used to create artificial materials, made predominantly frominter planar van der Waals stacking of robust bond saturated atomiclayers of different solids with vastly different properties. [ABSTRACT FROM AUTHOR]

Nanoscale antennas sandwiched between two graphene monolayersyielda photodetector that efficiently converts visible and near-infraredphotons into electrons with an 800% enhancement of the photocurrentrelative to the antennaless graphene device. The antenna contributesto the photocurrent in two ways: by the transfer of hot electronsgenerated in the antenna structure upon plasmon decay, as well asby direct plasmon-enhanced excitation of intrinsic graphene electronsdue to the antenna near field. This results in a graphene-based photodetectorachieving up to 20% internal quantum efficiency in the visible andnear-infrared regions of the spectrum. This device can serve as amodel for merging the light-harvesting characteristics of opticalfrequency antennas with the highly attractive transport propertiesof graphene in new optoelectronic devices. [ABSTRACT FROM AUTHOR]

Highly conductive reduced graphene oxide (GO) polymer nanocomposites are synthesized by a well-organized in situ thermochemical synthesis technique. The surface functionalization of GO was carried out with aryl diazonium salt including 4-iodoaniline to form phenyl functionalized GO (I-Ph-GO). The thermochemically developed reduced GO (R-I-Ph-GO) has five times higher electrical conductivity (42 000 S/m) than typical reduced GO (R-GO). We also demonstrate a R-I-Ph-GO/polyimide (PI) composites having more than 104 times higher conductivity (∼1 S/m) compared to a R-GO/PI composites. The electrical resistances of PI composites with R-I-Ph-GO were dramatically dropped under ∼3% tensile strain. The R-I-Ph-GO/PI composites with electrically sensitive response caused by mechanical strain are expected to have broad implications for nanoelectromechanical systems. [ABSTRACT FROM AUTHOR]

Various three-dimensional (3D) battery architectures have been proposed to address effective power delivery in micro/nanoscale devices and for increasing the stored energy per electrode footprint area. One step toward obtaining 3D configurations in batteries is the formation of core−shell nanowires that combines electrode and electrolyte materials. One of the major challenges however in creating such architectures has been the coating of conformal thin nanolayers of polymer electrolytes around nanostructured electrodes. Here we show conformal coatings of 25−30 nm poly(methyl methacralate) electrolyte layers around individual Ni−Sn nanowires used as anodes for Li ion battery. This configuration shows high discharge capacity and excellent capacity retention even at high rates over extended cycling, allowing for scalable increase in areal capacity with electrode thickness. Our results demonstrate conformal nanoscale anode−electrolyte architectures for an efficient Li ion battery system. [ABSTRACT FROM AUTHOR]