Graphene Junctions Research Articles

Switchable valley injection into graphene

The generation and control of the valley-polarized current play the key roles in valleytronics applications, but still remain as tough challenges. We propose a general idea of switchable valley injection, and predict theoretically that a ferromagnet-covered graphene junction would take the role of a valley injector, whose on/off state can be easily switched only by changing the magnetic direction. This scheme can be extended to other two-dimensional crystals and may lead to an alternative path for valleytronic practical applications.

Thermoelectric and thermospin transport in a ballistic junction of graphene

We consider theoretically a wide graphene ribbon, that on both ends is attached to electronic reservoirs which generally have different temperatures. The graphene ribbon is assumed to be deposited on a substrate, that leads to a spin-orbit coupling of Rashba type. We calculate the thermally induced charge current in the ballistic transport regime as well as the thermoelectric voltage (Seebeck effect). Apart from this, we also consider thermally induced spin current and spin polarization of the graphene ribbon. The spin currents are shown to have generally two components; one parallel to the temperature gradient and the other one perpendicular to this gradient. The latter corresponds to the spin current due to the spin Nernst effect. Additionally, we also consider the heat current between the reservoirs due to transfer of electrons.

Excitonic Josephson effect in double-layer graphene junctions

We show that double-layer graphene (DLG), where an external potential induces an charge-imbalance between $n$- and $p$-type layers, is a promising candidate to realize an exciton condensate in equilibrium. To prove this phenomenon experimentally, we suggest to couple two DLG systems, separated by a thin insulating barrier, and measure the excitonic Josephson effect. For this purpose we calculate the ac and dc Josephson currents induced by tunneling excitons and show that the former only occurs when the gate potentials of the DLG systems differ, irrespective of the phase relationship of their excitonic order parameters. A dc Josephson current develops if a finite order-parameter phase difference exists between two coupled DLG systems with identical gate potentials.

Ballistic Josephson junctions in edge-contacted graphene.

Hybrid graphene-superconductor devices have attracted much attention since the early days of graphene research. So far, these studies have been limited to the case of diffusive transport through graphene with poorly defined and modest-quality graphene/superconductor interfaces, usually combined with small critical magnetic fields of the superconducting electrodes. Here, we report graphene-based Josephson junctions with one-dimensional edge contacts of molybdenum rhenium. The contacts exhibit a well-defined, transparent interface to the graphene, have a critical magnetic field of 8 T at 4 K, and the graphene has a high quality due to its encapsulation in hexagonal boron nitride. This allows us to study and exploit graphene Josephson junctions in a new regime, characterized by ballistic transport. We find that the critical current oscillates with the carrier density due to phase-coherent interference of the electrons and holes that carry the supercurrent caused by the formation of a Fabry-Pérot cavity. Furthermore, relatively large supercurrents are observed over unprecedented long distances of up to 1.5 μm. Finally, in the quantum Hall regime we observe broken symmetry states while the contacts remain superconducting. These achievements open up new avenues to exploit the Dirac nature of graphene in interaction with the superconducting state.

Strain filter with gate control in a gapped graphene junction

We study transport properties of massive Dirac electrons tunneling through a strained barrier in graphene junction. The interplay of strained barrier and gap opening in graphene is focused. As an interesting result, we find that the applied strain in the barrier would cause a maximum conductance for some strain values leading to “strain filtering effect”. Perfect strain filter is predicted at the Fermi level approaching the energy gap. The selected strain value is found to be tunable by the gate voltage at the barrier. The strain filtering effect does not occur when graphene is gapless. The gate-controlled strain filter is almost linearly dependent on gate voltage for large strain. Our work reveals the potential of graphene for application of strain sensor or filter device.

Electron transport in graphene/graphene side-contact junction by plane-wave multiple-scattering method

Electron transport in graphene is along the sheet but junction devices are often made by stacking different sheets together in a ``side-contact'' geometry which causes the current to flow perpendicular to the sheets within the device. Such geometry presents a challenge to first-principles transport methods. We solve this problem by implementing a plane-wave-based multiple-scattering theory for electron transport. This implementation improves the computational efficiency over the existing plane-wave transport code, scales better for parallelization over large number of nodes, and does not require the current direction to be along a lattice axis. As a first application, we calculate the tunneling current through a side-contact graphene junction formed by two separate graphene sheets with the edges overlapping each other. We find that transport properties of this junction depend strongly on the AA or AB stacking within the overlapping region as well as the vacuum gap between two graphene sheets. Such transport behaviors are explained in terms of carbon orbital orientation, hybridization, and delocalization as the geometry is varied.

Spin transport in a Zigzag normal/ferromagnetic graphene junction

We study magnetic proximity effect induced low-energy spin transport in the normal/ferromagnetic junction of a semi-infinite zigzag graphene nanoribbon. Due to the absence of a spin flip in a single interface, the spin transfer in this model can be described by the “two-spin channel” model. We identify each spin channel as either a perfect conducting or a non-conducting channel. This feature leads to spin filter in symmetric zigzag graphene nanoribbon and spin precession in antisymmetric zigzag graphene nanoribbon, and helps to directly determine the exchange-splitting intensity directly, even without an external auxiliary bias.

Tunable quantum temperature oscillations in graphene nanostructures

We investigate the local electron temperature distribution in graphene nanoribbon and graphene junctions subject to an applied thermal gradient. Using a realistic model of a scanning thermal microscope, we predict quantum temperature oscillations whose wavelength is related to that of Friedel oscillations. Experimentally this wavelength can be tuned over several orders of magnitude by gating or doping, bringing quantum temperature oscillations within reach of the spatial resolution of existing measurement techniques.

Modeling the Charge Transport in Graphene Nano Ribbon Interfaces for Nano Scale Electronic Devices

In this research work we have modeled, simulated and compared the electronic charge transport for Metal-Semiconductor-Metal interfaces of Graphene Nano Ribbons (GNR) with different geometries using First-Principle calculations and Non-Equilibrium Green's Function (NEGF) method. We modeled junctions of Armchair GNR strip sandwiched between two Zigzag strips with (Z-A-Z) and Zigzag GNR strip sandwiched between two Armchair strips with (A-Z-A) using semi-empirical Extended Huckle Theory (EHT) within the framework of Non-Equilibrium Green Function (NEGF). I-V characteristics of the interfaces were visualized for various transport parameters. The distinct changes in conductance and I-V curves reported as the Width across layers, Channel length (Central part) was varied at different bias voltages from -1V to 1 V with steps of 0.25 V. From the simulated results we observed that the conductance through A-Z-A graphene junction is in the range of 10-13 Siemens whereas the conductance through Z-A-Z graphene junction is in the range of 10-5 Siemens. These suggested conductance controlled mechanisms for the charge transport in the graphene interfaces with different geometries is important for the design of graphene based nano scale electronic devices like Graphene FETs, Sensors.

Conductance enlargement in picoscale electroburnt graphene nanojunctions

Provided the electrical properties of electroburnt graphene junctions can be understood and controlled, they have the potential to underpin the development of a wide range of future sub-10-nm electrical devices. We examine both theoretically and experimentally the electrical conductance of electroburnt graphene junctions at the last stages of nanogap formation. We account for the appearance of a counterintuitive increase in electrical conductance just before the gap forms. This is a manifestation of room-temperature quantum interference and arises from a combination of the semimetallic band structure of graphene and a cross-over from electrodes with multiple-path connectivity to single-path connectivity just before breaking. Therefore, our results suggest that conductance enlargement before junction rupture is a signal of the formation of electroburnt junctions, with a picoscale current path formed from a single sp(2) bond.

Conductance oscillations induced by ballistic snake states in a graphene heterojunction.

The realization of p-n junctions in graphene, combined with the gapless and chiral nature of its massless Dirac fermions has led to the observation of many intriguing phenomena such as the quantum Hall effect in the bipolar regime, Klein tunnelling and Fabry-Pérot interferences, all of which involve electronic transport across p-n junctions. Ballistic snake states propagating along the p-n junctions have been predicted to induce conductance oscillations, manifesting their twisting nature. However, transport studies along p-n junctions have so far only been performed in low mobility devices. Here, we report the observation of conductance oscillations due to ballistic snake states along a p-n interface in high-quality graphene encapsulated by hexagonal boron nitride. These snake states are exceptionally robust as they can propagate over 12 μm, limited only by the size of our sample, and survive up to at least 120 K. The ability to guide carriers over a long distance provide a crucial building block for graphene-based electron optics.

Carbon nanotube-graphene junctions studied by impedance spectra

Two kinds of carbon nanotube (CNT)-graphene structures, vertical CNT-graphene and paralleled CNT–graphene, were fabricated to investigate the geometrical effect on the transport properties of the CNT–graphene junctions by using AC impedance spectra. The results demonstrated that the geometrical structure showed obvious impact on the resistance rather than the capacity of the junction. It is proposed that the difference caused by the geometrical structure may be associated with the dangling bonds terminated by –OH or –COOH of the open-ended CNTs. The unsymmetrical chemical bonds will increase the dipole moment in CNTs, which enhance the interaction between vertical CNTs and graphene and reduce the contact resistance.

Transmission Through Graphene Junctions with Rashba Spin-Orbit Coupling

In this work we investigate electronic transport in a class of junctions based on graphene. The junctions consist of two parts: the first part is made of pristine graphene on a usual substrate (e.g. SiO2), while in the second part a strong Rashba spin-orbit coupling is additionally induced (e.g. by placing this part on a dierent substrate). Our main goal is to analyze the probabilities of transmission from the part with the Rashba spin-orbit coupling to the pristine one. We are especially interested in the spin polarization of the transmitted charge carriers. We find that the transmission through the junctions is spin-dependent. Therefore, an electric current flowing through the junction should be spin-polarized. This eect is robust against

Distinct photoresponse in graphene induced by laser irradiation

The graphene-based photodetector with tunable p-p+-p junctions was fabricated through a simple laser irradiation process. Distinct photoresponse was observed at the graphene (G)-laser irradiated graphene (LIG) junction by scanning photocurrent measurements, and its magnitude can be modulated as a result of a positive correlation between the photocurrent and doping concentration in LIG region. Detailed investigation suggests that the photo-thermoelectric effect, instead of the photovoltaic effect, dominates the photocurrent generation at the G-LIG junctions. Such a simple and low-cost technique offers an alternative way for the fabrication of graphene-based optoelectronic devices.

Observation of a giant two-dimensional band-piezoelectric effect on biaxial-strained graphene

Piezoelectric materials used in the development of nanoscale mechanical sensors, actuators and energy harvesters have received much attention. More recently, devices made of graphene are of particular interest because of graphene’s intriguing electronic and mechanical properties. Intrinsic graphene has long been considered devoid of the piezoelectric effect, although flexoelectricity has been exploited to demonstrate piezoelectricity in functionalized graphene and graphene nanoribbons. The perceived lack of this property has restricted graphene’s use in nanoelectromechanical systems (NEMS) for electromechanical coupling purposes. Here an unprecedented two-dimensional (2D) piezoelectric effect on a strained/unstrained graphene junction is reported. In stark contrast to the bulk piezoelectric effect that results from the occurrence of electric dipole moments in solids, the 2D piezoelectric effect arises from the charge transfer along a work function gradient introduced by the biaxial-strain-engineered band structure. The observed effect, termed the band-piezoelectric effect, exhibits an enormous magnitude due to the ultrathin structure of graphene. On the basis of the band-piezoelectric effect, a graphene nanogenerator and a pressure gauge were fabricated. The results not only provide a versatile NEMS platform for sensing, actuating and energy harvesting, but also pave the way for efficiently modulating graphene via strain engineering.

Simulation Field Effect Transistor Bipolar Graphene Nano-ribbon

The purpose of this paper is to use software package ATK-SE package, in combination with virtual Nano-Lab (VNL), can be used to investigate a Nano-scale transistor. For the transistor structure we will use a graphene junction device, where ATK is used to investigate the properties of a similar system. The effect of various parameters on the structure of graphene Nano-ribbon checked. It consists of 3 regions and forms a metal-semiconductor-metal junction. By applying a gate potential to the central region, the system can function as a field effect transistor, which is able to calculate properties, Transmission spectrum, Temperature dependent conductance, Conductance and Current as function of gate potential and temperature. So in this paper, the device design and simulation parameters are associated with improved performance.

Tailoring 10 nm scale suspended graphene junctions and quantum dots.

The possibility to make 10 nm scale, and low-disorder, suspended graphene devices would open up many possibilities to study and make use of strongly coupled quantum electronics, quantum mechanics, and optics. We present a versatile method, based on the electromigration of gold-on-graphene bow-tie bridges, to fabricate low-disorder suspended graphene junctions and quantum dots with lengths ranging from 6 nm up to 55 nm. We control the length of the junctions, and shape of their gold contacts by adjusting the power at which the electromigration process is allowed to avalanche. Using carefully engineered gold contacts and a nonuniform downward electrostatic force, we can controllably tear the width of suspended graphene channels from over 100 nm down to 27 nm. We demonstrate that this lateral confinement creates high-quality suspended quantum dots. This fabrication method could be extended to other two-dimensional materials.

Giant magnetoresistance modulated by magnetic field in graphene p-n junction

We investigate the tunneling transport across a graphene p-n junction under the influence of a perpendicular magnetic field (B field). We observe a sideway deflection of the transmission profile, which can be quantitatively explained by invoking the classical Lorentz force. By considering the trajectory of the Dirac fermions along their cyclotron orbits, we analytically derive the incident angles for transmission across the graphene junction under a B field, as well as the critical magnetic field for full suppression of tunneling across the junction. These analytical predictions are consistent with the numerical results obtained via the non-equilibrium Green's function method. A stronger B-field conductance modulation is obtained for a p-n as opposed to an n-n or p-p type graphene junction. The magnetic field also induces a forbidden region of almost zero transmission for electron energy close to the Dirac point, which can be utilized to achieve a giant magnetoresistance effect. Based on our analysis, we devise an optimal magneto-electrical transport modulation, which can potentially realize a giant magnetoresistance effect in graphene p-n junction systems.

Strain-modulation of spin-dependent transport in graphene

We investigate strain modulation of the spin-dependent electron transport in a graphene junction using the transfer matrix method. As an analogy to optics, we define the modulation depth in the electron optics domain. Additionally, we discuss the transport properties and show that the modulation depth and the conductance depend on the spin-orbit coupling strength, the strain magnitude, the width of the strained area, and the energy of the incident electron. The conductances of the spin-down and spin-up electrons have opposite and symmetrical variations, which results in the analogous features of their modulation depths. The maximum conditions for both the modulation depth and the electron spin upset rate are also analyzed.

Conduction gap in graphene strain junctions: direction dependence

It has been shown in a recent study (Nguyen et al 2014 Nanotechnology 25 165201) that unstrained/strained graphene junctions are promising candidates to improve the performance of graphene transistors which is usually hindered by the gapless nature of graphene. Although the energy bandgap of strained graphene still remains zero, the shift of Dirac points in the k-space due to strain-induced deformation of graphene lattice can lead to the appearance of a finite conduction gap of several hundred meV in strained junctions with a strain of only a few per cent. However, since it depends essentially on the magnitude of the Dirac point shift, this conduction gap strongly depends on the direction of applied strain and the transport direction. In this work, a systematic study of conduction-gap properties with respect to these quantities is presented and the results are carefully analyzed. Our study provides useful information for further investigations to exploit graphene-strained junctions in electronic applications and strain sensors.