Graphene Junctions Research Articles

Giant Thermal Rectification from Single-Carbon Nanotube-Graphene Junction.

We describe the influence of the geometry parameters on the thermal rectification of single-carbon nanotube-graphene junction. The two-dimensional (2D) distribution of the thermal rectification with respect to the tube length and the side length of the graphene nanosheet are calculated and visualized. The maximum thermal rectification ratios of the designed single-carbon nanotube-graphene junction can reach 1244.1% and 1681.6% at average temperatures of 300 and 200 K, respectively. These values are much higher than those reported for single-material nanostructure-based thermal rectifiers. The thermal rectification ratios of the nanotube-graphene junction are fairly sensitive to geometry size and are almost entirely dominated by the degree of overlap of the power spectra under negative thermal bias. These findings could offer useful guidelines for the design and performance improvement of thermal diodes.

Strain control of real- and lattice-spin currents in a silicene junction

We investigate real- and lattice-spin currents controlled by strain in a silicene-based junction, where chemical potential, perpendicular electric field and circularly polarized light are applied into the strained barrier. We find that the junction yields strain filtering effect with perfect strain control of real- (or lattice-)spin currents. (i) By applying electric field without circularly polarized light we show that total current is carried by pure lattice-spin up (or down) electrons tunable by strain. (ii) When circularly polarized light is irradiated onto silicene sheet without applying electric field, total current is carried by pure real-spin up (or down) electrons tunable by strain. High conductance peaks associated with pure real- (or lattice-)spin currents in case ii (or i) occur at specific magnitude of strain, yielding strain filtering effect. Magnitudes of filtered strain due to pure real- (or lattice-) spin currents may be tunable by varying chemical potential. Sensitivity may be enhanced by increasing thickness of strained barrier. Significantly, (iii) when both perpendicular electric field and circularly polarized light are applied, the total current is carried by three species of electron groups tunable by strain. This may lead to controllable numbers of electron species to transport. This result shows that strain filtering effect in a silicene-based junction is quite different from that in graphene junction. Our work reveals potential of silicene as a nano-electro-mechanical device and spin-valleytronic applications.

Andreev Reflection in a Bilayer Graphene Junction: Role of Spatial Variation of the Charge Neutrality Point

A graphene sheet partially covered with a bulk superconductor serves as a normal conductor--superconductor (NS) junction, in which electron transport is mainly governed by Andreev reflection (AR). As excess carriers induced over the covered region penetrate into the uncovered region over a screening length, the charge neutrality point (CNP) in the uncovered region shifts only near the NS interface. We theoretically study the electron transport in a bilayer graphene junction taking account of such spatial variation of the CNP in the electron-doped case. When the Fermi level is close to the CNP away from the NS interface, the AR takes place in a specular manner owing to the diffraction of a reflected hole occurring at a $pn$ junction, which is naturally formed in the uncovered region. It is shown that the differential conductance shows an unusual asymmetric behavior as a function of bias voltage under the influence of the $pn$ junction. It is also shown that, if the Fermi level is located below the CNP, the $pn$ junction gives rise to quasi-bound states near the NS interface, leading to the appearance of resonant peaks in the differential conductance.

Influence of graphene growth temperature by chemical vapour deposition on the hydrogen response of palladium–graphene junction

The hydrogen sensing by palladium-graphene junction was dependent on the atmospheric pressure chemical vapour deposition growth temperature of the graphene films. The growth temperature window adopted in this study was 900–1000 °C, and the hydrogen sensor performance of the palladium–graphene junction (0.5–2.0% H2 in air) was studied in the temperature range 30–150 °C. Raman spectroscopy study with the as grown graphene films revealed the multilayer nature and the Pd–graphene planar structure showed a temperature dependent n- to p-type conductivity change in presence of hydrogen. Such a conductivity transition in presence of a reducing gas like hydrogen was experimentally studied in detail, and the hydrogen sensor results were correlated with the multilayer character of the graphene thin film, which induces hydrogen intercalation between the graphene layers.

Scanning gate microscopy of magnetic focusing in graphene devices: quantum versus classical simulation

We compare classical versus quantum electron transport in recently investigated magnetic focusing devices (Bhandari et al 2016 Nano Lett. 16 1690) exposed to the perturbing potential of a scanning gate microscope (SGM). Using the Landauer–Büttiker formalism for a multi-terminal device, we calculate resistance maps that are obtained as the SGM tip is scanned over the sample. There are three unique regimes in which the scanning tip can operate (focusing, repelling, and mixed regime) which are investigated. Tip interacts mostly with electrons with cyclotron trajectories passing directly underneath it, leaving a trail of modified current density behind it. Other (indirect) trajectories become relevant when the tip is placed near the edges of the sample, and current is scattered between the tip and the edge. We point out that, in contrast to SGM experiments on gapped semiconductors, the STM tip can induce a pn junction in graphene, which improves contrast and resolution in SGM. We also discuss possible explanations for spatial asymmetry of experimentally measured resistance maps, and connect it with specific configurations of the measuring probes.

Strain effect on spin polarization in a graphene junction

We investigate the strain effect on the spin-dependent transport in a graphene junction with spin–orbit coupling and a gate voltage. We find that uniaxial strain along the armchair direction breaks the transmission probability symmetry with respect to the incident angle, reduces the spin-flipped transmission probability, and extends the transmission gap as regard to the Fermi energy, while strain along the zig-zag direction has very little effect on transmission probabilities. We analyze the spin polarization as a function of the strain magnitude, direction, voltage, and area width. Selecting the proper strain direction and magnitude for both the Klein tunneling and classical cases, the direction of the spin-polarization vector can be controlled and its magnitude is dramatically enhanced. Strain will expand the non-zero range of the magnitude of the spin-polarization vector with respect to voltage. Increasing the strain area width over a threshold, keeps the magnitude of the spin-polarization vector stable.

Pure valley and spin polarization current in ferromagnetic graphene junction

We investigate the band structure and spin- and valley-dependent transport in ferromagnetic graphene double junctions using the transfer matrix method. The ferromagnetic double junctions include a lateral layout of normal/magnetic vector potential/normal/effective exchange field/normal graphene regions. We find that the strain combined with magnetic vector potentials breaks the valley degeneracy, and the strain combined with the effective exchange field breaks the spin degeneracy, so that there exists a spin- and valley-dependent gap in the ferromagnetic graphene double junctions, which allows only one spin species with special valley characteristics to be transported. Thus, this type of junction can achieve simultaneously pure spin- and pure valley-polarized currents and function as a perfect valley filter and a perfect spin filter. The exact nature of the valley filtering and spin filtering can be tuned by the strain, magnetic barrier, and effective exchange field strength.

In situ mechanical investigation of carbon nanotube-graphene junction in three-dimensional carbon nanostructures.

Hierarchically organized three-dimensional (3D) carbon nanotubes/graphene (CNTs/graphene) hybrid nanostructures hold great promises in composite and battery applications. Understanding the junction strength between CNTs and graphene is crucial for utilizing such special nanostructures. Here, in situ pulling an individual CNT bundle out of graphene is carried out for the first time using a nanomechanical tester developed in-house, and the measured junction strength of CNTs/graphene is 2.23 ± 0.56 GPa. The post transmission electron microscopy (TEM) analysis of remained graphene after peeling off CNT forest confirms that the failure during pull-out test occurs at the CNT-graphene junction. Such a carefully designed study makes it possible to better understand the interfacial interactions between CNTs and graphene in the 3D CNTs/graphene nanostructures. The coupled experimental and computational effort suggests that the junction between the CNTs and the graphene layer is likely to be chemically bonded, or at least consisting of a mixture of chemical bonding and van der Waals interactions.

Signatures of Phonon and Defect-Assisted Tunneling in Planar Metal-Hexagonal Boron Nitride-Graphene Junctions.

Electron tunneling spectroscopy measurements on van der Waals heterostructures consisting of metal and graphene (or graphite) electrodes separated by atomically thin hexagonal boron nitride tunnel barriers are reported. The tunneling conductance, dI/dV, at low voltages is relatively weak, with a strong enhancement reproducibly observed to occur at around |V| ≈ 50 mV. While the weak tunneling at low energies is attributed to the absence of substantial overlap, in momentum space, of the metal and graphene Fermi surfaces, the enhancement at higher energies signals the onset of inelastic processes in which phonons in the heterostructure provide the momentum necessary to link the Fermi surfaces. Pronounced peaks in the second derivative of the tunnel current, d2I/dV2, are observed at voltages where known phonon modes in the tunnel junction have a high density of states. In addition, features in the tunneling conductance attributed to single electron charging of nanometer-scale defects in the boron nitride are also observed in these devices. The small electronic density of states of graphene allows the charging spectra of these defect states to be electrostatically tuned, leading to "Coulomb diamonds" in the tunneling conductance.

THz-circuits driven by photo-thermoelectric, gate-tunable graphene-junctions.

For future on-chip communication schemes, it is essential to integrate nanoscale materials with an ultrafast optoelectronic functionality into high-frequency circuits. The atomically thin graphene has been widely demonstrated to be suitable for photovoltaic and optoelectronic devices because of its broadband optical absorption and its high electron mobility. Moreover, the ultrafast relaxation of photogenerated charge carriers has been verified in graphene. Here, we show that dual-gated graphene junctions can be functional parts of THz-circuits. As the underlying optoelectronic process, we exploit ultrafast photo-thermoelectric currents. We describe an immediate photo-thermoelectric current of the unbiased device following a femtosecond laser excitation. For a picosecond time-scale after the optical excitation, an additional photo-thermoelectric contribution shows up, which exhibits the fingerprint of a spatially inverted temperature profile. The latter can be understood by the different time-constants and thermal coupling mechanisms of the electron and phonon baths within graphene to the substrate and the metal contacts. The interplay of the processes gives rise to ultrafast electromagnetic transients in high-frequency circuits, and it is equally important for a fundamental understanding of graphene-based ultrafast photodetectors and switches.

Photoresponse in graphene induced by defect engineering

We present a photoresponse study on a lateral defect/pristine graphene junction device fabricated by a simple plasma irradiation method. The junction between pristine graphene and plasma-modified graphene was created by controlling the location of Ar+ plasma treatment. We found that a distinct photocurrent was generated at the junction by photocurrent line scanning measurements, and further analysis reveals that the photo-thermoelectric (PTE) effect, instead of the photovoltaic (PV) effect, dominates the photocurrent generation at the interface. Additionally, the obtained results suggest that tuning the defect density could be effective in modulating the optoelectronic performance of junctions in our device.

Electron optics with p-n junctions in ballistic graphene.

Electrons transmitted across a ballistic semiconductor junction are expected to undergo refraction, analogous to light rays across an optical boundary. In graphene, the linear dispersion and zero-gap band structure admit highly transparent p-n junctions by simple electrostatic gating. Here, we employ transverse magnetic focusing to probe the propagation of carriers across an electrostatically defined graphene junction. We find agreement with the predicted Snell's law for electrons, including the observation of both positive and negative refraction. Resonant transmission across the p-n junction provides a direct measurement of the angle-dependent transmission coefficient. Comparing experimental data with simulations reveals the crucial role played by the effective junction width, providing guidance for future device design. Our results pave the way for realizing electron optics based on graphene p-n junctions.

Tunable anomalous Andreev reflection and triplet pairings in spin-orbit-coupled graphene

We theoretically study scattering process and superconducting triplet correlations in a graphene junction comprised of ferromagnet-RSO-superconductor in which RSO stands for a region with Rashba spin orbit interaction. Our results reveal spin-polarized subgap transport through the system due to an anomalous equal-spin Andreev reflection in addition to conventional back scatterings. We calculate equal- and opposite-spin pair correlations near the F-RSO interface and demonstrate direct link of the anomalous Andreev reflection and equal-spin pairings arised due to the proximity effect in the presence of RSO interaction. Moreover, we show that the amplitude of anomalous Andreev reflection, and thus the triplet pairings, are experimentally controllable when incorporating the influences of both tunable strain and Fermi level in the nonsuperconducting region. Our findings can be confirmed by a conductance spectroscopy experiment and provide better insights into the proximity-induced RSO coupling in graphene layers reported in recent experiments.

Theory of ballistic quantum transport in the presence of localized defects

We present an efficient numerical approach for treating ballistic quantum transport across devices described by tight binding (TB) Hamiltonians designated to systems with localized potential defects. The method is based on the wave function matching approach, Lippmann-Schwinger equation (LEQ) and the scattering matrix formalism. We show that the number of matrix elements of the Green's function to be evaluated for the unperturbed system can be essentially reduced by projection of the time reversed scattering wave functions on LEQ which radically improves the speed and lowers the memory consumption of the calculations. Our approach can be applied to quantum devices of an arbitrary geometry and any number of degrees of freedom or leads attached. We provide a couple of examples of possible applications of the theory, including current equilibration at the p-n junction in graphene and scanning gate microscopy mapping of electron trajectories in the magnetic focusing experiment on a graphene ribbon. Additionally, we provide a simple toy example of electron transport through 1D wire with added onsite perturbation and obtain a simple formula for conductance showing that Green's function of the device can be obtained from the conductance versus impurity strength characteristics.

Spin and charge thermopower effects in the ferromagnetic graphene junction

Using wave function matching approach and employing the Landauer-Buttiker formula, a ferromagnetic graphene junction with temperature gradient across the system is studied. We calculate the thermally induced charge and spin current as well as the thermoelectric voltage (Seebeck effect) in the linear and nonlinear regimes. Our calculation revealed that due to the electron-hole symmetry, the charge Seebeck coefficient is, for an undoped magnetic graphene, an odd function of chemical potential while the spin Seebeck coefficient is an even function regardless of the temperature gradient and junction length. We have also found with an accurate tuning external parameter, namely, the exchange filed and gate voltage, the temperature gradient across the junction drives a pure spin current without accompanying the charge current. Another important characteristic of thermoelectric transport, thermally induced current in the nonlinear regime, is examined. It would be our main finding that with increasing thermal gradient applied to the junction the spin and charge thermovoltages decrease and even become zero for non zero temperature bias.

Proximity-Induced Spin Polarization of Graphene in Contact with Half-Metallic Manganite.

The role of proximity contact with magnetic oxides is of particular interest from the expectations of the induced spin polarization and weak interactions at the graphene/magnetic oxide interfaces, which would allow us to achieve efficient spin-polarized injection in graphene-based spintronic devices. A combined approach of topmost-surface-sensitive spectroscopy utilizing spin-polarized metastable He atoms and ab initio calculations provides us direct evidence for the magnetic proximity effect in the junctions of single-layer graphene and half-metallic manganite La0.7Sr0.3MnO3 (LSMO). It is successfully demonstrated that in the graphene/LSMO junctions a sizable spin polarization is induced at the Fermi level of graphene in parallel to the spin polarization direction of LSMO without giving rise to a significant modification in the π band structure.

Metal-graphene heterojunction modulation via H2 interaction

Combining experiment and theory, we investigate how a naturally created heterojunction (pn junction) at a graphene and metallic contact interface is modulated via interaction with molecular hydrogen (H2). Due to an electrostatic interaction, metallic electrodes induce pn junctions in graphene, leading to an asymmetrical resistance in electronic transport for electrons and holes. We report that the asymmetry in the resistance can be tuned in a reversible manner by exposing graphene devices to H2. The interaction between the H2 and graphene occurs solely at the graphene-contact pn junction and induces a modification on the electrostatic interaction between graphene and metallic contacts. We explain the experimental data with theory providing information concerning the length of the heterojunction and how it changes as a function of H2 adsorption. Our results are valuable for understanding the nature of the metal-graphene interfaces and have potential application for selective sensors of molecular hydrogen.

Inhibiting Klein Tunneling in a Graphene p-n Junction without an External Magnetic Field.

We study by first-principles calculations a densely packed island of organic molecules (F_{4}TCNQ) adsorbed on graphene. We find that with electron doping the island naturally forms a p-n junction in the graphene sheet. For example, a doping level of ∼3×10^{13} electrons per cm^{2} results in a p-n junction with an 800meV electrostatic potential barrier. Unlike in a conventional p-n junction in graphene, in the case of the junction formed by an adsorbed organic molecular island we expect that the Klein tunneling is inhibited, even without an applied external magnetic field. Here Klein tunneling is inhibited by the ferromagnetic order that spontaneously occurs in the molecular island upon doping. We estimate that the magnetic barrier in the graphene sheet is around 10mT.

Perfect spin filtering controlled by an electric field in a bilayer graphene junction: Effect of layer-dependent exchange energy**Project supported by the Kasetsart University Research and Development Institute (KURDI) and Thailand Research Fund (TRF) (Grant No. TRG5780274).

Magneto transport of carriers with a spin-dependent gap in a ferromagnetic-gated bilayer of graphene is investigated. We focus on the effect of an energy gap induced by the mismatch of the exchange fields in the top and bottom layers of an AB-stacked graphene bilayer. The interplay of the electric and exchange fields causes the electron to acquire a spin-dependent energy gap. We find that, only in the case of the anti-parallel configuration, the effect of a magnetic-induced gap will give rise to perfect spin filtering controlled by the electric field. The resolution of the spin filter may be enhanced by varying the bias voltage. Perfect switching of the spin polarization from + 100% to −100% by reversing the direction of electric field is predicted. Giant magnetoresistance is predicted to be easily realized when the applied electric field is smaller than the magnetic energy gap. It should be pointed out that the perfect spin filter is due to the layer-dependent exchange energy. This work points to the potential application of bilayer graphene in spintronics.

Single gate p-n junctions in graphene-ferroelectric devices

Graphene's linear dispersion relation and the attendant implications for bipolar electronics applications have motivated a range of experimental efforts aimed at producing p-n junctions in graphene. Here we report electrical transport measurements of graphene p-n junctions formed via simple modifications to a PbZr0.2Ti0.8O3 substrate, combined with a self-assembled layer of ambient environmental dopants. We show that the substrate configuration controls the local doping region, and that the p-n junction behavior can be controlled with a single gate. Finally, we show that the ferroelectric substrate induces a hysteresis in the environmental doping which can be utilized to activate and deactivate the doping, yielding an “on-demand” p-n junction in graphene controlled by a single, universal backgate.