All-carbon Devices Research Articles

Theoretical investigation on current–voltage characteristics in all-carbon molecular device with different contact geometries

Abstract Applying nonequilibrium Green's functions in combination with the first-principles density-functional theory, we investigate electronic transport properties of an all-carbon molecular device consisting of one phenalenyl molecule and two zigzag graphene nanoribbons. The results show that the electronic transport properties are strongly dependent on the contact geometry and device's currents can drop obviously when the connect sites change from second-nearest sites from the central atom of the molecule (S site) to third-nearest sites from the central atom of the molecule (T site). More importantly, the negative differential resistance behavior is only observed on the negative bias region when the molecule connects the graphene nanoribbons through two T sites.

Visualizing Electrical Breakdown and ON/OFF States in Electrically Switchable Suspended Graphene Break Junctions

Narrow gaps are formed in suspended single- to few-layer graphene devices using a pulsed electrical breakdown technique. The conductance of the resulting devices can be programmed by the application of voltage pulses, with voltages of 2.5 to ~4.5 V, corresponding to an ON pulse, and ~8 V, corresponding to an OFF pulse. Electron microscope imaging of the devices shows that the graphene sheets typically remain suspended and that the device conductance tends to zero when the observed gap is large. The switching rate is strongly temperature dependent, which rules out a purely electromechanical switching mechanism. This observed switching in suspended graphene devices strongly suggests a switching mechanism via atomic movement and/or chemical rearrangement and underscores the potential of all-carbon devices for integration with graphene electronics.

A simple scheme of molecular electronic devices with multiwalled carbon nanotubes as the top electrodes

A simple fabrication scheme for molecular electronic junctions is presented with multiwalled carbon nanotubes (MWCNTs) as the top electrodes. Results indicate that our approach retains the molecular character of the chosen molecules [a self-assembled monolayer of octadecanethiol on gold bottom electrodes] and opens the door for studying a wide variety of organothiol candidates for molecular electronics. The fabrication scheme is designed in a way that it can be modified into all-carbon devices in the future by using graphitic carbon bottom electrodes functionalized with nitrozoabenzene, for example, and MWCNTs or graphene as the top electrodes. Alternatively, the scheme is applicable for all-gold devices with gold bottom electrodes and gold nanowire top electrodes.

Patterning and Electronic Tuning of Laser Scribed Graphene for Flexible All-Carbon Devices

Engineering a low-cost graphene-based electronic device has proven difficult to accomplish via a single-step fabrication process. Here we introduce a facile, inexpensive, solid-state method for generating, patterning, and electronic tuning of graphene-based materials. Laser scribed graphene (LSG) is shown to be successfully produced and selectively patterned from the direct laser irradiation of graphite oxide films under ambient conditions. Circuits and complex designs are directly patterned onto various flexible substrates without masks, templates, post-processing, transferring techniques, or metal catalysts. In addition, by varying the laser intensity and laser irradiation treatments, the electrical properties of LSG can be precisely tuned over 5 orders of magnitude of conductivity, a feature that has proven difficult with other methods. This inexpensive method for generating LSG on thin flexible substrates provides a mode for fabricating a low-cost graphene-based NO(2) gas sensor and enables its use as a heterogeneous scaffold for the selective growth of Pt nanoparticles. The LSG also shows exceptional electrochemical activity that surpasses other carbon-based electrodes in electron charge transfer rate as demonstrated using a ferro-/ferricyanide redox couple.

Synthesis of large-scale undoped and nitrogen-doped amorphous graphene on MgO substrate by chemical vapor deposition

Despite amorphous graphene (a-graphene) domains having been created and observed within crystalline graphene, large-area synthesis is still in its infancy. Here, using MgO single crystals as a catalytic substrate, large-area (10 × 5 mm) undoped and nitrogen-doped a-graphene were synthesized by chemical vapor deposition with benzene and pyridine as precursors, respectively. The dependence of the number of layers and the crystallinity of a-graphene on the growth temperature was observed by transmission electron microscopy, selected area electron diffraction and Raman spectroscopy. Few-layer (2–5 layers) a-graphene films are demonstrated to have high transmittance (>85% at 550 nm) and low conductivity (<1 × 10−3 S cm−1). When doped with nitrogen (N/C ratio = ∼0.09), the conductivity of the a-graphene film could be increased. The preparation of large-area a-graphene films not only extends to two-dimensional carbon materials but also might provide building blocks for future all-carbon devices.

Integrated modelling of a JET type-I ELMy H-mode pulse and predictions for ITER-like wall scenarios

A baseline type-I ELMy H-mode JET tokamak discharge in low triangularity has been analysed using the JINTRAC integrated code suite to obtain a self-consistent description of the plasma edge dynamics and core plasma confinement. The inter- and intra-ELM transport model of JINTRAC has been adapted as such to match the experimental pre- and post-ELM plasma pedestal profiles and at the same time to recover the observed ELM dynamics in terms of ELM frequency, ELM energy loss, target ELM wetted area and target heat flows. The presented and validated modelling results for the JET all-carbon device reference case are utilized to predict a type-I ELMy H-mode for the JET ITER-like wall (ILW) assuming a full-tungsten divertor and beryllium main-chamber wall using JINTRAC. By keeping all relevant transport parameters fixed for the inter- and intra-ELM phase as in the all-carbon reference case it is observed that a moderate amount of seeded neon impurity is necessary to compensate for a similar level of radiation when carbon is absent in the system. The results of the ILW model setup are finally taken to estimate the total amount of tungsten particles eroded per ELM from the target plates. A rough estimate of the core radiative fraction due to W accumulation is given, predicting that no substantial impact on typical a JET ILW type-I ELMy H-mode discharge core plasma performance is expected.

All-Carbon Molecular Tunnel Junctions

This Article explores the idea of using nonmetallic contacts for molecular electronics. Metal-free, all-carbon molecular electronic junctions were fabricated by orienting a layer of organic molecules between two carbon conductors with high yield (>90%) and good reproducibility (rsd of current density at 0.5 V <30%). These all-carbon devices exhibit current density-voltage (J-V) behavior similar to those with metallic Cu top contacts. However, the all-carbon devices display enhanced stability to bias extremes and greatly improved thermal stability. Completed carbon/nitroazobenzene(NAB)/carbon junctions can sustain temperatures up to 300 °C in vacuum for 30 min and can be scanned at ±1 V for at least 1.2 × 10(9) cycles in air at 100 °C without a significant change in J-V characteristics. Furthermore, these all-carbon devices can withstand much higher voltages and current densities than can Cu-containing junctions, which fail upon oxidation and/or electromigration of the copper. The advantages of carbon contacts stem mainly from the strong covalent bonding in the disordered carbon materials, which resists electromigration or penetration into the molecular layer, and provides enhanced stability. These results highlight the significance of nonmetallic contacts for molecular electronics and the potential for integration of all-carbon molecular junctions with conventional microelectronics.

The Possibility of Chemically Inert, Graphene-Based All-Carbon Electronic Devices with 0.8 eV Gap

Graphene is an interesting electronic material. However, flat monolayer graphene does not have significant gap in the electronic density of states, required for a large on-off ratio in logic applications. We propose here a novel device architecture, composed of self-folded carbon nanotube-graphene hybrids, which have been recently observed experimentally in Joule-heated graphene. These experiments demonstrated the feasibility of cutting, folding, and welding few-layer graphene in situ to form all-carbon nanostructures with complex topologies. The electronic gap of self-folded nanotubes can be combined with the semimetallicity of graphene electrodes to form a "metal-semiconductor-metal" junction. By ab initio calculations we demonstrate large energy gaps in the transmission spectra of such junctions, which preserve the intrinsic transport characteristics of the semiconducting nanotubes despite topologically necessary disinclinations at the flat graphene-curved nanotube interface. These all-carbon devices are proposed to be constructed by contact probe cutting and high-temperature annealing and, if produced, would be chemically stable at room temperature under normal gas environments.

Tunable Kondo effect in graphene with defects

Graphene is a model system for the study of electrons confined to a strictly two-dimensional layer1 and a large number of electronic phenomena have been demonstrated in graphene, from the fractional2, 3 quantum Hall effect to superconductivity4. However, the coupling of conduction electrons to local magnetic moments5, 6, a central problem of condensed matter physics, has not been realized in graphene, and, given carbon's lack of d or f electrons, magnetism in graphene would seem unlikely. Nonetheless, magnetism in graphitic carbon in the absence of transition-metal elements has been reported7-10, with explanations ranging from lattice defects11 to edge structures12, 13 to negative curvature regions of the graphene sheet14. Recent experiments suggest that correlated defects in highly-ordered pyrolytic graphite (HOPG) induced by proton irradiation9 or native to grain boundaries7, can give rise to ferromagnetism. Here we show that point defects (vacancies) in graphene15 are local moments which interact strongly with the conduction electrons through the Kondo effect6, 16-18 providing strong evidence that defects in graphene are indeed magnetic. The Kondo temperature TK is tunable with carrier density from 30-90 K; the high TK is a direct consequence of strong coupling of defects to conduction electrons in a Dirac material18. The results indicate that defect engineering in graphene could be used to generate and control carrier-mediated magnetism, and realize all-carbon spintronic devices. Furthermore, graphene should be an ideal system in which to probe Kondo physics in a widely tunable electron system.

Toward All-Carbon Electronics: Fabrication of Graphene-Based Flexible Electronic Circuits and Memory Cards Using Maskless Laser Direct Writing

Owing to its extraordinary electronic property, chemical stability, and unique two-dimensional nanostructure, graphene is being considered as an ideal material for the highly expected all-carbon-based micro/nanoscale electronics. Herein, we present a simple yet versatile approach to constructing all-carbon micro/nanoelectronics using solution-processing graphene films directly. From these graphene films, various graphene-based microcosmic patterns and structures have been fabricated using maskless computer-controlled laser cutting. Furthermore, a complete system involving a prototype of a flexible write-once-read-many-times memory card and a fast data-reading system has been demonstrated, with infinite data retention time and high reliability. These results indicate that graphene could be the ideal material for fabricating the highly demanded all-carbon and flexible devices and electronics using the simple and efficient roll-to-roll printing process when combined with maskless direct data writing.