Conductance of p-n-p graphene structures with "air-bridge" top gates

We have fabricated graphene devices with a top gate separated from the graphene layer by an air gap-a design which does not decrease the mobility of charge carriers under the gate. This gate is used to realize p-n-p structures where the conducting properties of chiral carriers are studied. The band profile of the structures is calculated taking into account the specifics of the graphene density of states and is used to find the resistance of the p-n junctions expected for chiral carriers. We show that ballistic p-n junctions have larger resistance than diffusive ones. This is caused by suppressed transmission of chiral carriers at angles away from the normal to the junction.     

Spin-polarized quantum pumping in bilayer graphene

We study adiabatic quantum pumping in bilayer graphene where two-barrier potentials are weakly modulated as pumping parameters. Comparing the results with those for a normal quantum pump of non-chiral quasiparticles, we find that the chirality of quasiparticles in bilayer graphene heavily affects the pumped current through chiral tunnelling. When an exchange splitting induced by the proximity of a ferromagnetic insulator is introduced, the pumped current becomes spin-polarized. It is interesting that an almost 100% polarized charge current and a pure spin current with vanishing charge current can all be achieved under suitable conditions. The experimental feasibility and the interlayer asymmetric effect in bilayer graphene caused by the gate and the ferromagnet structures are also discussed. The results are useful for spintronics applications based on graphene.     

Reconfigurable Diodes Based on Vertical WSe2 Transistors with van der Waals Bonded Contacts

New device concepts can increase the functionality of scaled electronic devices, with reconfigurable diodes allowing the design of more compact logic gates being one of the examples. In recent years, there has been significant interest in creating reconfigurable diodes based on ultrathin transition metal dichalcogenide crystals due to their unique combination of gate‐tunable charge carriers, high mobility, and sizeable band gap. Thanks to their large surface areas, these devices are constructed under planar geometry and the device characteristics are controlled by electrostatic gating through rather complex two independent local gates or ionic‐liquid gating. In this work, similar reconfigurable diode action is demonstrated in a WSe₂ transistor by only utilizing van der Waals bonded graphene and Co/h‐BN contacts. Toward this, first the charge injection efficiencies into WSe₂ by graphene and Co/h‐BN contacts are characterized. While Co/h‐BN contact results in nearly Schottky‐barrier‐free charge injection, graphene/WSe₂ interface has an average barrier height of ≈80 meV. By taking the advantage of the electrostatic transparency of graphene and the different work‐function values of graphene and Co/h‐BN, vertical devices are constructed where different gate‐tunable diode actions are demonstrated. This architecture reveals the opportunities for exploring new device concepts.     

Approaching ballistic transport in suspended graphene

The discovery of graphene raises the prospect of a new class of nanoelectronic devices based on the extraordinary physical properties of this one-atom-thick layer of carbon. Unlike two-dimensional electron layers in semiconductors, where the charge carriers become immobile at low densities, the carrier mobility in graphene can remain high, even when their density vanishes at the Dirac point. However, when the graphene sample is supported on an insulating substrate, potential fluctuations induce charge puddles that obscure the Dirac point physics. Here we show that the fluctuations are significantly reduced in suspended graphene samples and we report low-temperature mobility approaching 200,000 cm2 V-1 s-1 for carrier densities below 5 x 109 cm-2. Such values cannot be attained in semiconductors or non-suspended graphene. Moreover, unlike graphene samples supported by a substrate, the conductivity of suspended graphene at the Dirac point is strongly dependent on temperature and approaches ballistic values at liquid helium temperatures. At higher temperatures, above 100 K, we observe the onset of thermally induced long-range scattering.     

New Frontiers in Electron Beam–Driven Chemistry in and around Graphene

Modern aberration corrected transmission electron microscopes offer the potential for electron beam sensitive materials, such as graphene, to be examined with low energy electrons to minimize, and even avoid, damage while still affording atomic resolution, and thus providing excellent characterization. Here in this review, the exploits in which the electron beam interactions, which are often considered negative, are explored to usefully drive a wealth of chemistry in and around graphene, importantly, with no other external stimuli. After introducing the technique, this review covers carbon phase reactions between amorphous carbon, graphene, fullerenes, carbon chains, and carbon nanotubes. It then explores different studies with clusters and nanoparticles, followed by coverage of single atom and molecule interactions with graphene, and finally concludes and highlights the anticipated exciting future for electron beam driving chemistry in and around graphene.     

Optical control of edge chirality in graphene

We performed optical annealing experiments at the edges of nanopatterned graphene to study the resultant edge reconstruction. The lithographic patterning direction was orthogonal to a zigzag edge. μ-Raman spectroscopy shows an increase in the polarization contrast of the G band as a function of annealing time. Furthermore, transport measurements reveal a 50% increase of the GNR energy gap after optical exposure, consistent with an increased percentage of armchair segments. These results suggest that edge chirality of graphene devices can be optically purified post electron beam lithography, thereby enabling the realization of chiral graphene nanoribbons and heterostructures.     

Electron beam induced structural transformations of SWNTs and DWNTs grown on Si3N4/Si substrates

Electron beam induced structural transformations are investigated in single-wall carbon nanotubes (SWNTs), double-wall carbon nanotubes (DWNTs) and crossed nanotube junctions. The nanotubes studied here are synthesized by the chemical vapor deposition method. The response of the nanotubes to an electron beam is found to be influenced by the presence of coatings of amorphous carbon, graphene fragments and structural defects on the tube surface. The dependence of structural modifications on electron beam irradiation dose is measured. While nanotubes with amorphous carbon, graphene fragment coverage and/or defects undergo rapid transformation leading to structure disintegration, those without such coverage or defects are more resistant to beam damage. In addition, it is shown that the amorphous carbon coverage on the double-wall nanotubes can be transformed into graphene layers during electron beam irradiation of coated nanotubes. Finally, the relative stability of nanotube side-wall and end-walls are investigated through sub-threshold energy and above threshold energy irradiation of a model system, C60-filled nanotubes (Peapods). The data indicates that electron beams could be used to join nanotubes end-to-end without damaging the side-walls.     

Hydrothermal synthesis of graphene–CdS composites with improved photoelectric characteristics

Natural flake graphite was used as raw materials to prepare graphite oxide and further treated with ultrasonic oscillation to get graphene oxide. Graphene–CdS composites were prepared by one step hydrothermal synthesis, using cadmium acetate as Cd precursors, sulfourea as S precursors and graphene oxide as support. Graphene–CdS composites were characterized by X-ray diffraction and scanning electron microscopy for the structure and morphology of the composites, and further investigated by transient photocurrent response and cyclic voltammetry. Improved photoelectric characteristics can be obtained over graphene–CdS composites than that of pure CdS nanoparticles due to electron capture and transfer ability of graphene resulting in a more efficient separation of the photoexcited charge carriers from graphene–CdS composites.     

Multioperation-Mode Light-Emitting Field-Effect Transistors Based on van der Waals Heterostructure

2D semiconductors have shown great potential for application to electrically tunable optoelectronics. Despite the strong excitonic photoluminescence (PL) of monolayer transition metal dichalcogenides (TMDs), their efficient electroluminescence (EL) has not been achieved due to the low efficiency of charge injection and electron–hole recombination. Here, multioperation-mode light-emitting field-effect transistors (LEFETs) consisting of a monolayer WSe₂ channel and graphene contacts coupled with two top gates for selective and balanced injection of charge carriers are demonstrated. Visibly observable EL is achieved with the high external quantum efficiency of ≈6% at room temperature due to efficient recombination of injected electrons and holes in a confined 2D channel. Further, electrical tunability of both the channel and contacts enables multioperation modes, such as antiambipolar, depletion,and unipolar regions, which can be utilized for polarity-tunable field-effect transistors and photodetectors. The work exhibits great potential for use in 2D semiconductor LEFETs for novel optoelectronics capable of high efficiency, multifunctions, and heterointegration.     

Observation of the fractional quantum Hall effect in an oxide

The quantum Hall effect arises from the cyclotron motion of charge carriers in two-dimensional systems. However, the ground states related to the integer and fractional quantum Hall effect, respectively, are of entirely different origin. The former can be explained within a single-particle picture; the latter arises from electron correlation effects governed by Coulomb interaction. The prerequisite for the observation of these effects is extremely smooth interfaces of the thin film layers to which the charge carriers are confined. So far, experimental observations of such quantum transport phenomena have been limited to a few material systems based on silicon, III-V compounds and graphene. In ionic materials, the correlation between electrons is expected to be more pronounced than in the conventional heterostructures, owing to a large effective mass of charge carriers. Here we report the observation of the fractional quantum Hall effect in MgZnO/ZnO heterostructures grown by molecular-beam epitaxy, in which the electron mobility exceeds 180,000 cm(2) V(-1) s(-1). Fractional states such as ν = 4/3, 5/3 and 8/3 clearly emerge, and the appearance of the ν = 2/5 state is indicated. The present study represents a technological advance in oxide electronics that provides opportunities to explore strongly correlated phenomena in quantum transport of dilute carriers.     

Prediction of very large values of magnetoresistance in a graphene nanoribbon device

Graphene has emerged as a versatile material with outstanding electronic properties that could prove useful in many device applications. Recently, the demonstration of spin injection into graphene and the observation of long spin relaxation times and lengths have suggested that graphene could play a role in 'spintronic' devices that manipulate electron spin rather than charge. In particular it has been found that zigzag graphene nanoribbons have magnetic (or spin) states at their edges, and that these states can be either antiparallel or parallel. Here we report the results of first-principles simulations that predict that spin-valve devices based on graphene nanoribbons will exhibit magnetoresistance values that are thousands of times higher than previously reported experimental values. These remarkable values can be linked to the unique symmetry of the band structure in the nanoribbons. We also show that it is possible to manipulate the band structure of the nanoribbons to generate highly spin-polarized currents.     

Exploring minimal scenarios to produce transversely bright electron beams using the eigen-emittance concept

Next generation hard X-ray free electron lasers require electron beams with low transverse emittance. One proposal to achieve these low emittances is to exploit the eigen-emittance values of the beam. The eigen-emittances are invariant under linear beam transport and equivalent to the emittances in an uncorrelated beam. If a correlated beam with two small eigen-emittances can be produced, removal of the correlations via appropriate optics will lead to two small emittance values, provided non-linear effects are not too large. We study how such a beam may be produced using minimal linear correlations. We find it is theoretically possible to produce such a beam, however, it may be more difficult to realize in practice. We identify linear correlations that may lead to physically realizable emittance schemes and discuss promising future avenues.     

Graphene–Ruthenium Complex Hybrid Photodetectors with Ultrahigh Photoresponsivity

The maximum responsivity of a pure monolayer graphene‐based photodetector is currently less than 10 mA W^?1 because of small optical absorption and short recombination lifetime. Here, a graphene hybrid photodetector functionalized with a photoactive ruthenium complex that shows an ultrahigh responsivity of ≈1 × 10⁵ A W^?1 and a photoconductive gain of ≈3 × 10⁶ under incident optical intensity of the order of sub‐milliwatts is reported. This responsivity is two orders of magnitude higher than the precedent best performance of graphene‐based photodetectors under a similar incident light intensity. Upon functionalization with a 4‐nm‐thick ruthenium complex, monolayer graphene‐based photodetectors exhibit pronounced n‐type doping effect due to electron transfer via the metal?ligand charge transfer (MLCT) from the ruthenium complex to graphene. The ultrahigh responsivity is attributed to the long lifetime and high mobility of the photoexcited charge carriers. This approach is highly promising for improving the responsivity of graphene‐based photodetectors.     

Nanopatterning of fluorinated graphene by electron beam irradiation

We demonstrate the possibility to selectively reduce insulating fluorinated graphene to conducting and semiconducting graphene by electron beam irradiation. Electron-irradiated fluorinated graphene microstructures show 7 orders of magnitude decrease in resistivity (from 1 TΩ to 100 kΩ), whereas nanostructures show a transport gap in the source-drain bias voltage. In this transport gap, electrons are localized, and charge transport is dominated by variable range hopping. Our findings demonstrate a step forward to all-graphene transparent and flexible electronics.     

Low Temperature Direct of Graphene onto Metal Nano‐Spindt Tip with Applications in Electron Emission

Graphene has been known for its superior electronic properties ever since its discovery in 2004. The high aspect ratio and ballistic transport properties exhibited by this one‐dimensional material are especially useful for electron emission applications. However, they are typically grown horizontally and excess efforts, such as the use of transfer techniques, is required to orientate them before effective electron emission from the graphene edges can occur. These transfer techniques have been shown to lead to additional defects to the as‐grown graphene structure, thereby degrading its properties. Here, we present an approach to directly fabricate graphene onto metal nano‐sized spindt tips (or nanocones) using the solid‐state transformation of carbon deposited from a pulsed laser system at low temperature. Besides providing a layer of chemical and mechanical protection for the metal nanocones, the graphene‐on‐metal nanocones gave enhanced emission properties compared to bare metal nanocones. This was due to the reduction of effective field emission tunneling barrier, which was a result of graphene‐metal charge transfer interactions. Controlling the metal nanocones density was also an important factor in determining the field emission performance, as electron screening from neighboring cones should be minimized.     

Freely Suspended,van der Waals Bound Organic Nanometer‐Thin Functional Films: Mechanical and Electronic Characterization

Determining the electronic properties of nanoscopic, low‐dimensional materials free of external influences is key to the discovery and understanding of new physical phenomena. An example is the suspension of graphene, which has allowed access to their intrinsic charge transport properties. Furthermore, suspending thin films enables their application as membranes, sensors, or resonators, as has been explored extensively. While the suspension of covalently bound, electronically active thin films is well established, semiconducting thin films composed of functional molecules only held together by van der Waals interactions could only be studied supported by a substrate. In the present work, it is shown that by utilizing a surface‐crystallization method, electron conductive films with thicknesses of down to 6 nm and planar chiral optical activity can be freely suspended across several hundreds of nanometers. The suspended membranes exhibit a Young's modulus of 2–13 GPa and are electronically decoupled from the environment, as established by temperature‐dependent field‐effect transistor measurements.     

Atomically localized plasmon enhancement in monolayer graphene

Plasmons in graphene can be tuned by using electrostatic gating or chemical doping, and the ability to confine plasmons in very small regions could have applications in optoelectronics, plasmonics and transformation optics. However, little is known about how atomic-scale defects influence the plasmonic properties of graphene. Moreover, the smallest localized plasmon resonance observed in any material to date has been limited to around 10 nm. Here, we show that surface plasmon resonances in graphene can be enhanced locally at the atomic scale. Using electron energy-loss spectrum imaging in an aberration-corrected scanning transmission electron microscope, we find that a single point defect can act as an atomic antenna in the petahertz (10(15) Hz) frequency range, leading to surface plasmon resonances at the subnanometre scale.     

Electric field control of spin rotation in bilayer graphene

The manipulation of the electron spin degree of freedom is at the core of the spintronics paradigm, which offers the perspective of reduced power consumption, enabled by the decoupling of information processing from net charge transfer. Spintronics also offers the possibility of devising hybrid devices able to perform logic, communication, and storage operations. Graphene, with its potentially long spin-coherence length, is a promising material for spin-encoded information transport. However, the small spin-orbit interaction is also a limitation for the design of conventional devices based on the canonical Datta-Das spin field-effect transistors. An alternative solution can be found in magnetic doping of graphene or, as discussed in the present work, in exploiting the proximity effect between graphene and ferromagnetic oxides (FOs). Graphene in proximity to FO experiences an exchange proximity interaction, that acts as an effective Zeeman field for electrons in graphene, inducing a spin precession around the magnetization axis of the FO. Here we show that in an appropriately designed double-gate field-effect transistor, with a bilayer graphene channel and FO used as a gate dielectric, spin-precession of carriers can be turned ON and OFF with the application of a differential voltage to the gates. This feature is directly probed in the spin-resolved conductance of the bilayer.     

Ultrafast hot-carrier-dominated photocurrent in graphene

The combination of its high electron mobility, broadband absorption and ultrafast luminescence make graphene attractive for optoelectronic and photonic applications, including transparent electrodes, mode-locked lasers and high-speed optical modulators. Photo-excited carriers that have not cooled to the temperature of the graphene lattice are known as hot carriers, and may limit device speed and energy efficiency. However, their roles in charge and energy transport are not fully understood. Here, we use time-resolved scanning photocurrent microscopy to demonstrate that hot carriers, rather than phonons, dominate energy transport across a tunable graphene p-n junction excited by ultrafast laser pulses. The photocurrent response time varies from 1.5?ps at room temperature to 4?ps at 20?K, implying a fundamental bandwidth of ～500?GHz (refs?12, 13, 21). Gate-dependent pump-probe measurements demonstrate that both thermoelectric and built-in electric field effects contribute to the photocurrent, with the contribution from each depending on the junction configuration. The photocurrent produced by a single pulsed laser also displays multiple polarity reversals as a function of carrier density, which is a possible signature of impact ionization.     

Detection of individual gas molecules adsorbed on graphene

The ultimate aim of any detection method is to achieve such a level of sensitivity that individual quanta of a measured entity can be resolved. In the case of chemical sensors, the quantum is one atom or molecule. Such resolution has so far been beyond the reach of any detection technique, including solid-state gas sensors hailed for their exceptional sensitivity. The fundamental reason limiting the resolution of such sensors is fluctuations due to thermal motion of charges and defects, which lead to intrinsic noise exceeding the sought-after signal from individual molecules, usually by many orders of magnitude. Here, we show that micrometre-size sensors made from graphene are capable of detecting individual events when a gas molecule attaches to or detaches from graphene's surface. The adsorbed molecules change the local carrier concentration in graphene one by one electron, which leads to step-like changes in resistance. The achieved sensitivity is due to the fact that graphene is an exceptionally low-noise material electronically, which makes it a promising candidate not only for chemical detectors but also for other applications where local probes sensitive to external charge, magnetic field or mechanical strain are required.