Graphene Junctions Research Articles

Carbazole‐Based Tetrapodal Anchor Groups for Gold Surfaces: Synthesis and Conductance Properties

AbstractAs the field of molecular‐scale electronics matures and the prospect of devices incorporating molecular wires becomes more feasible, it is necessary to progress from the simple anchor groups used in fundamental conductance studies to more elaborate anchors designed with device stability in mind. This study presents a series of oligo(phenylene‐ethynylene) wires with one tetrapodal anchor and a phenyl or pyridyl head group. The new anchors are designed to bind strongly to gold surfaces without disrupting the conductance pathway of the wires. Conductive probe atomic force microscopy (cAFM) was used to determine the conductance of self‐assembled monolayers (SAMs) of the wires in Au–SAM–Pt and Au–SAM–graphene junctions, from which the conductance per molecule was derived. For tolane‐type wires, mean conductances per molecule of up to 10−4.37 G0 (Pt) and 10−3.78 G0 (graphene) were measured, despite limited electronic coupling to the Au electrode, demonstrating the potential of this approach. Computational studies of the surface binding geometry and transport properties rationalise and support the experimental results.

Covalently bonded 3D rebar graphene foam for ultrahigh-areal-capacity lithium-metal anodes by in-situ loose powder metallurgy synthesis

A free-standing covalently bonded CNTs/graphene foam (LPM-3D rebar GF) is developed by using an in-situ loose powder metallurgy templates method for high-performance dendrite-free Li metal anodes. The LPM-3D rebar GF possesses a macroporous structure with interconnected porous pipelines. The homogeneously dispersed CNTs can induce an in-situ growth for covalently bridging graphene sheets, resulting in the increase of electrical conductivity, structural stability, and active Li nucleation sites. With the guidance of CNTs, Li metal deposit preferentially on the junctions of CNTs and graphene to achieve a uniform deposition and avoid the formation of Li dendrites effectively, which is verified by both experiments and first-principle calculations. As the anode of Li metal battery, the LPM-3D rebar GF shows outstanding cycling stability even at 20 mA cm−2 for 20 mA h cm−2. Notably, this anode can still operate 90 cycles at 30 mA h cm−2 with a Coulombic efficiency (CE) of >98%. These properties indicate promising prospects of LPM-3D rebar GF as a new and dendrite-free anode with large areal capacity for high-performance Li metal batteries, and in other fields requiring 3D carbon monoliths including fuel cells and sodium-ion batteries.

Topological and transport properties of graphene-based nanojunctions subjected to a magnetic field

We study the topological properties of finite-size S-shaped graphene junctions with distinctive edge features subjected to the perpendicular magnetic field, using the tight-binding model. The quantum confinement and edge effects induced by the specific junction give rise to significant modifications in the Hofstadter spectra of the bent flakes, when compared to those of their perfect forms. Moreover, the results show that in absence of a magnetic field, the sharpest zigzag-edged corners support the edge states rather than the others, but the magnetic field leads to the localization of the edge states along the whole perimeter of the flakes. Furthermore, based on the Green’s function method, we investigate the electron transport through our proposed junctions. We show that, under magnetic flux, one can effectively control the energy gap and the conductance around the Fermi energy. Moreover, the transitions between metallic, semimetallic, and semiconducting phases are possible by the magnetic flux in the S-shaped junctions.

Thermal Transport Behavior of Carbon Nanotube–Graphene Junction under Deformation

We employ molecular dynamics simulations to explore the effect of tensile strain on the thermal conductivity of carbon nanotube (CNT)–graphene junction structures. Two types of CNT–graphene junctions are simulated; a seamless junction between CNT and graphene with pure [Formula: see text] covalent bonds, and a junction with mixed [Formula: see text] covalent bonds are studied. The most interesting observation is that the thermal conductivity of a CNT–graphene junction structure increases with an increase in mechanical strain. For the case of a (6,6) CNT–graphene junction structure with an inter-pillar distance (the length of graphene floor between two CNT–graphene junctions) of 15[Formula: see text]nm, the thermal conductivity is improved by 22.4% with 0.1 tensile strain. The thermal conductivity improvement by mechanical strain is enhanced when a larger graphene floor is placed between junctions since a larger graphene floor allows larger deformation (larger tensile strain) without breaking bonds in the junction structure. However, the thermal conductivity is found to more strongly depend on the C–C bond hybridization at the intramolecular junctions with pure [Formula: see text] hybridization showing a higher thermal conductivity when compared to mixed [Formula: see text] bonding regardless of the amount of tensile strain. The obtained results will contribute to the development of flexible electronics by providing a theoretical background on the thermal transport of three-dimensional carbon nanostructures under deformation.

Structural transformation of graphite by passage of electric current

The passage of an electric current through graphite, or few-layer graphene, at high temperatures can produce dramatic structural transformations. These involve the formation of carbon structures with highly irregular morphologies, displaying many unusual features, including nanotube–graphene junctions. A number of studies of the transformed carbon have been carried out, but many aspects of its structure and formation mechanism are still not understood. This paper summarises the studies that have been made on this phenomenon, drawing mainly on the author’s own work. It is shown, using high-tilt transmission electron microscopy imaging, that the transformed carbon is largely flat, and not three-dimensional as suggested in previous papers. The possible mechanism of the transformations is discussed.

Structures formed by passage of current through graphite: flat or three-dimensional?

The passage of an electric current, or the application of an electrostatic charge through graphite at high temperatures can produce dramatic structural transformations. These involve the formation of carbon structures made up of few-layer graphene with highly irregular morphologies, displaying many unusual features, including nanotube–graphene junctions. A number of studies of the transformed carbon have been carried out, but many aspects of its structure are still not understood. Here, transmission electron microscopy is used to image regions of the transformed carbon at a series of tilt angles, in order to determine their overall shapes. It is found that both flat regions and three-dimensional structures are present. Along the edges of the flat regions, hollow strips are observed, due to the separation of graphene layers. These sometimes contain encapsulated nanotubes or nanoparticles. As well as describing the experimental results, I discuss the possible mechanisms whereby these interesting structures form.

Spin-orbit interaction and snake states in a graphene p-n junction

We study a model of a $p$-$n$ junction in single-layer graphene in the presence of a perpendicular magnetic field and spin-orbit interactions. By solving the relevant quantum-mechanical problem for a potential step, we determine the exact spectrum of spin-resolved dispersive Landau levels. Close to zero energy, we find a pair of linearly dispersing zero modes, which possess a wave-vector-dependent spin polarization and can be regarded as quantum analogous of spinful snake states. We show that the Rashba spin-orbit interaction, in particular, produces a wave vector shift between the dispersions of these modes with observable interference effects. These effects can in principle provide a way to detect the presence of Rashba spin-orbit interaction and measure its strength. Our results suggest that a graphene $p$-$n$ junction in the presence of strong spin-orbit interaction could be used as a building block in a spin field-effect transistor.

Magnetic field effect on strained graphene junctions

We investigate the spin-dependent transport properties of a ferromagnetic/strained/normal graphene junctions with central region subjected to a magnetic field $B$. An analytical approach, based on Dirac equation, is implemented to obtain the eigenstates and eigenvalues of the charge carrier in three regions. Using the transfer matrix method, we determine the spin-dependent transmission in the presence of an applied strain along the armchair and zigzag directions of the graphene sample. We find that the strain remarkably modifies the Landau levels (LLs) originating from the applied $B$. It is shown that the spin up/down energy bands, in the first region, are shifted by the exchange $H_{ex}$ and left the whole spectrum linear as in the case of pristine graphene. In the central region, the position of the Dirac point changes due to the uniaxial strain and $B$. It is also found that the uniaxial strain in graphene induces a contraction of the LLs spectra. Moreover, the strain and $B$ modify the shape and position of some peaks in the transmission probabilities.

Geometry and chiral symmetry breaking of ripple junctions in 2D materials

In bulk crystals dislocation junctions underlie the physics of strain hardening. In two-dimensional (2D) crystals, dislocations take the form of surface ripples owing to the ease of bending and weak vdW adhesion of the atomic layers. Here we report that a ripple junction in 2D crystals features distinct morphologies and functions from their bulk counterparts. Our atomistic simulations show that a ripple junction in monolayer graphene exhibits four-fold symmetry. Upon biaxial compression the ripple junction undergoes helical instability, forming a helix with a random orientation. Differently, in-plane shear separates the junction into two individual ripples. We further demonstrate that the helicity of the junction can be controlled by a shear-compression loading sequence or the adsorption of a single chiral molecule at the junction. Shear-controlled helicity forms the basis for mechanical transduction and electro-opto-mechanical coupling, while adsorbate-driven helicity – which imparts the chirality of a single molecule onto a helical distortion pattern of the host 2D layer – has ramifications for sensing and chiral control.

Spin-dependent quantum beating of the conductance oscillations in asymmetric velocity modulated ferromagnetic graphene junction

Abstract The mismatch effect of a double Fermi-velocity barrier structures NG/FG 1 /NG/FG 2 /NG junction on the electrically driven SdH-like oscillations of spin-dependent conductance is investigated, where NG is normal graphene and FG 1,2 is the ferromagnetic-gate with Fermi-velocity modulation ξ 1,2 where ξ 1,2 is the Fermi velocity ratio between NG region and FG 1,2 region. The results show that the effect of asymmetry of the double Fermi-velocity barrier structures, the difference value between ξ 1 and ξ 2 cause of the oscillations the spin-dependent quantum beating pattern. Which the beating patterns of the spin transport appeared as a result of the superposition of the two oscillatory components in the double Fermi velocity barriers. In addition, we found that the shifting of the spin-dependent quantum beats of the G ↑ and G ↓ is due to the presence of the exchange field, H, that is, the G ↑ and G ↓ are shifted in the opposite direction. By the difference value of the phase shift can be estimated to be 2H . In case of U + η σ H > > E F which electrons are collimated, the oscillations behaviors show the quantum beating with holding constant amplitude and the period of the beating amplitude depend on the Fermi velocity ratio mismatch in FG regions. These interesting characteristic of spin-dependent quantum transport may identify the effect of Femi-velocity asymmetry on double proximity-induced ferromagnetic graphene structure for making spintronic devices.

Anomalous zero bias conductance peak in a ferromagnetic graphene junction with d-wave anisotropic superconducting pair symmetry

We theoretically present an anomalous zero bias conductance peak (ZBCP) in graphene junctions with proximity-induced ferromagnetism and d-wave anisotropic superconducting pair symmetry (ASPS) herein. It is revealed that in the thin insulator limit, the ZBCP can be periodically recovered by adjusting the sandwiched insulating barrier strength χ regardless of exchange field h and the phase of the periodical behaviours for h larger than its Fermi energy EF (h > EF) is exactly opposite to that for h < EF. Most interestingly, in the context of h > EF, the periodic oscillation of the nonzero bias conductance located in the ZBCP versus χ, is accompanied by an explicit splitting peak. Moreover, under the situation of the insulator with finite width, the conductance exhibits a stronger damping oscillation with bias voltage eV for any h, which is also accompanied by a splitting ZBCP at h > EF. These singular features originate from ferromagnetic-modulated midgap states characteristic by the relativistic nodal fermions, which confirms the spin polarization and ASPS of the graphene, and thus will be of great interest in the designing and fabrication of graphene superconducting spintronic devices.

Ultrawideband graphene three-port circulator for THz region

The suggested circulator is formed by a concave pattern graphene junction and three waveguides symmetrically connected to it. The graphene is supported by SiO2/Si layers. The circulation behavior is based on the nonsymmetry of the graphene conductivity tensor which appears due to magnetization by a DC magnetic field applied normally to the graphene plane. The symmetrical mode propagating in the nonmagnetized graphene waveguide, is transformed in magnetized region to an edge-guided one providing the propagation from one port to another port and isolating the third port. The device characteristics depend on the physical parameters of the graphene junction, its dimensions and parameters of the substrate. We discuss a choice of these parameters to maximize the frequency band and isolation level and to minimize the losses and the applied DC magnetic field. The theoretical arguments are confirmed by full-wave computations. In an example, we demonstrate that the circulator can have the frequency band of 42% (from 2.75THz to 4.2THz), with the isolation higher than 17 dB and the insertion losses better that 2dB, provided by the biasing DC magnetic field 1.5T and the chemical potential of graphene 0.15eV.

Intervalley conversion in bilayer-monolayer graphene junctions

We studied the valley dependent transport properties of bilayer-monolayer graphene junctions consisting of a device region of bilayer zigzag ribbon connected to monolayer zigzag ribbon leads. We determined numerically using the python package Kwant the valley dependent conductance through the junctions in an energy range in which each valley has at most two subbands in a single layer ribbon. There is a Fermi energy in this range in which the K to K conductance drops significantly to below 0.2e2/h (spin degree of freedom is included), while the K to K′ conductance remains high. This means a high percentage of the incident K electrons are transferred to the K′ valley in the outgoing lead. A number of junctions can be designed with K to K′ conductance larger than 1.2e2/h, which have a conversion efficiency higher than 84% and there are also junctions with K to K′ conductance between 1.2e2/h and 0.6e2/h (the conversion efficiency > 66%). Changing the transverse dimension, the conductance features are shifted in energy without significant change to the qualitative behavior. When the longitudinal dimension of the junction is changed, the qualitative features are changed in a way, which resembles the relation between the band structure of armchair ribbons and the dimer numbers along the ribbon's transverse direction. For some junctions, this interesting intervalley scattering property can be explained by Fano resonance. Our results show that these bilayer-monolayer junctions can be used as efficient intervalley converters in valleytronic devices.

Tunneling conductance of the s-wave and d-wave pairing superconductive graphene–normal graphene junction

Within the framework of the Blonder–Tinkham–Klapwijk formalism we calculate and analyze the conductance of the normal graphene — s-wave and independently d-wave pairing superconductive graphene junction. The eigenfunctions, the Andreev and the normal reflection rates are obtained by solving the Dirac–Bogoliubov–de Gennes equations. The Fermi velocity is believed to be different in the normal and in the superconductive regions. We consider the options of gapless and gapped graphene for both cases: s-wave and independently d-wave pairing. It is demonstrated that the characteristics of the junction considered are sensitive to the ratio vFN/vFS where vFN, vFS are the Fermi velocities in the normal and the superconductive graphene respectively. This conclusion refers to the Andreev reflection as well as to the normal one. The first of them is shown to be the dominant process for the formation of the conductivity. These results are true for an arbitrary value of the orientational angle of the d-waves. Each of four cases considered: s-, d-wave pairing and gapless and gapped graphene displays its own specific features of the conductance. The dependence of the conductance on the external electrostatic potential as well as on the Fermi energy is also analyzed in every case. The obtained results may be useful for controlling the transport properties of the normal graphene–superconductive graphene junction.

Chemical vapor deposition-grown carbon nanotubes/graphene hybrids for electrochemical energy storage and conversion

Chemical vapor deposition (CVD)-grown carbon nanotubes (CNTs)/graphene hybrids produce a novel three-dimensional carbon composite structure with seamless C–C junctions of one-dimensional CNTs and two-dimensional graphene. The CNTs/graphene hybrids promote the advantages of their building blocks, exhibiting large surface area, high electrical conductivity, excellent mechanical and chemical stability. Hence they are becoming an ideal electrode material platform for electrochemical energy storage and conversion including supercapacitors, lithium batteries and fuel cells. In this review, we first summarize the CVD growth for the fabrication of three types of CNTs/graphene hybrids, including CNTs/graphene oxide, CNTs/planar graphene and CNTs/graphene foam. And then we discuss the applications of CVD-grown CNTs/graphene hybrids in supercapacitors, lithium batteries and fuel cells. Finally, we propose the challenges and opportunities in future development of the CNTs/graphene hybrids.

First-principles study of a vertical spin switch in atomic scale two-dimensional platform

Abstract High in-plane charge carrier mobility and long spin diffusion length makes graphene a unique material for spin-based devices. However, in a vertical graphene junction, the 2pz orbitals of carbon atoms in graphene can be tuned via suitable magnetic substrates; this would affect the spin injection into graphene. Here, a vertical spin switch has been designed by embedding a single layer of graphene as a tunnel layer between the Ni (1 1 1) substrate. Periodic density functional approach in conjunction with Julliere’s model is used to calculate the tunnel magnetoresistance (TMR). Further, single-layered hexagonal Boron Nitride (h-BN) is sandwiched between the graphene and Ni (1 1 1) substrate to understand the role of hybridization at the interface on TMR. Our calculation shows that in contrast to the graphene junction, a much higher TMR value is obtainable in the case of the graphene/h-BN multi-tunnel junction (MTJ). The TMR in graphene junction is found to decrease with the increase of an externally applied electric field, and drops to zero for a field greater than equal to 0.16 V/A. Similar phenomenon was observed in the case of h-BN/graphene MTJ, where TMR value remains unchanged for electric field up to 0.1 V/A beyond which it drops to zero. The change in hybridization and charge-carrier-population at the interface modifies the magnetic exchange interaction and magnetic anisotropy resulting in a spin flip at interface, leads to rapid drop in TMR after a threshold electric field. The high and tunable TMR value suggests h-BN assisted high performance graphene based vertical spin switch.

Klein tunnelling and Hartman effect in graphene junctions with proximity exchange field

Tunnelling of electrons in graphene-based junctions is studied theoretically. Graphene is assumed to be deposited either directly on a ferromagnetic insulator or on a few atomic layers of boron nitride which separate graphene from a metallic ferromagnetic substrate. Such junctions can be formed by appropriate external gating of the corresponding system. To describe low-energy electronic states near the Dirac points, certain effective Hamiltonians available in the relevant literature are used. These Hamiltonians include staggered potential and exchange interaction due to ferromagnetic substrates. Tunnelling in the systems under consideration is then spin-dependent. The main focus is on Klein tunnelling and also on the group delay and the associated Hartman effect. The impact of a gap induced in the spectrum at the Dirac points on tunnelling is analysed in detail.

Simulation analysis of graphene contacted perovskite solar cells using SCAPS-1D

To understand what cause the low performance in a perovskite cell with graphene back contact, we have performed simulations of device characteristics using SCAPs-1D simulation platform. The impact of increasing defect concentration at the interface of perovskite/graphene (RGO) and also graphene thickness on the current density, voltage, fill factor, and conversion efficiency of the cell was investigated. We converted the graphene nanostructure to a planar p-type layer at the top side of the cell as a hole transporting layer in order to enable to insert it into the SCAPS simulation platform. The simulation analysis were compared to the experimental data reported in literature. The voltage and fill factor of the simulation and experiments are almost the same but the current density is showing to be higher in simulation analysis which reminds a imperfect thickness and absorption by the graphene layer. Graphene was also compared to Spiro-MeOTAD showing to be a promising materials to act as both hole transporting layer and back contact. The experimental process could be improved by looking at our results as a method to fabricate a high performance cell with graphene electrode. It is shown that such a hybrid structure suffers from imperfect interface at graphene and perovskite junction where a high concentration of trap density impedes the carrier collection.

Краевое легирование в графеновых приборах на подложках SiO-=SUB=-2-=/SUB=-

AbstractThe conductivity of single layer and bilayer graphene ribbons 0.5–4 μm in width fabricated by oxygen plasma etching is studied experimentally. The dependence of the electron concentration in graphene on the ribbon width is revealed. This effect is explained by the edge doping of graphene due to defects arranged in SiO_2 near the ribbon edges. The method of the formation of abrupt p – n junctions in graphene and structures with a constant electron concentration gradient in the graphene plane with the help of edge doping is proposed.

Interface driven thermal rectification in a graphene–bilayer graphene junction from nonequilibrium molecular dynamics

We report on a series of controlled computational experiments based on nonequilibrium molecular dynamics and show that at the nanoscale, the thermal rectification is determined by the thermal boundary resistance, i.e., the thermal resistance of the interface, and cannot be explained without it. In the graphene–bilayer graphene system that we study, the sign of the thermal rectification is opposite to the value predicted from bulk-derived models, i.e., phonons preferentially flow in the opposite direction. This behavior derives from the temperature dependence of the thermal boundary resistance and from the fact that the latter, at the nanoscale, has large relative weight compared to the total thermal resistance. These results outline the importance of properly accounting for the active role of the interface.