Graphene Nanoribbon Junctions Research Articles

Molecular Graphene Nanoribbon Junctions.

One of the challenges for the realization of molecular electronics is the design of nanoscale molecular wires displaying long-range charge transport. Graphene nanoribbons are an attractive platform for the development of molecular wires with long-range conductance owing to their unique electrical properties. Despite their potential, the charge transport properties of single nanoribbons remain underexplored. Herein, we report a synthetic approach to prepare N-doped pyrene-pyrazinoquinoxaline molecular graphene nanoribbons terminated with diamino anchoring groups at each end. These terminal groups allow for the formation of stable molecular graphene nanoribbon junctions between two metal electrodes that were investigated by scanning tunneling microscope-based break-junction measurements. The experimental and computational results provide evidence of long-range tunneling charge transport in these systems characterized by a shallow conductance length dependence and electron tunneling through >6 nm molecular backbone.

Gate Voltage-Modulated Conductance in Zigzag Graphene Nanoribbon Junctions

Using the Green’s function method, we study the modulation of the conductance in zigzag graphene nanoribbon (ZGNR) junctions by the gate voltages. As long as the difference between the gate voltages applied on the left and right ZGNRs (ΔV) remains unchanged, the conductance profiles for different cases are exactly the same, except to a displacement along EF-axis. It is found that the transmission of electrons from the upper/lower edge state of the left ZGNR to the lower/upper edge state of the right ZGNR is forbidden, therefore, the width of the conductance gap increases first and then decreases as |ΔV| increases. The upper/lower edge states and conduction/valence subbands of ZGNR under higher/lower gate voltage (VH/VL) determine step positions of the conductance when EF >VH/EF < VL. But when VL ≤ EF ≤ VH, the conductance profile is mainly determined by the upper and lower edge states, a few lowest conduction subbands/topmost valence subbands of ZGNR under lower/higher gate voltage. These results are helpful to the exploration and application of a new kind of field effect transistor based on ZGNR junctions.

Realizing Pure Spin Current With Photogalvanic Effect in Zigzag Graphene Nanoribbon Junctions: The Roles of Tetragon and Hexagon Connectors

We construct zigzag graphene nanoribbon junctions for generating pure spin current through the photogalvanic effect (PGE) by first-principles calculations. Two devices with spatial inversion symmetry are designed, both consisting of three zigzag graphene nanoribbons (ZGNRs) that introduce either two carbon tetragons (“2-C4”) or two carbon hexagons (“2-C6”) as connectors. The results show that robust pure spin current without charge current can be generated in both structures. The mechanism originates from the spatial inversion antisymmetric spin density as well as the spatial inversion symmetry of the geometrical structures. Furthermore, when more connectors are involved, we generalize that for structures with carbon tetragons as connectors, an even number of connectors are required to generate pure spin currents, while for structures with carbon hexagon connectors, any number of connectors can form pure spin currents. In addition, we find that larger pure spin currents can be obtained in the case of using hexagons as connectors. These findings provide a scheme for obtaining pure spin currents based on graphene nanoribbons, which has important reference implications for the design of spintronic devices with composite nanostructures.

The effect of light-irradiated area on the spin dependent photocurrent in zigzag graphene nanoribbon junctions.

In this work, we study the photogalvanic effect of a zigzag graphene nanoribbon junction with a centro-symmetrical structure which consists of 8 zigzag chains by density functional calculations. Specifically, we focus on the cases where the irradiated region is just part of the central region and located at different positions, with an aim to see how the spin dependent photocurrents will change and whether pure spin current can be obtained. It is found that the magnitude of the spin-dependent photocurrents increases with a gradual increase of the irradiated region and pure spin current is achieved when and only when the entire central region is irradiated. In addition, we studied the additive effect in this device to see that if we divide the central region into two parts, whether the sum of the spin current generated by irradiating the two parts individually is equal to that produced when the entire central region is irradiated. It is found that the sum of the spin currents produced by irradiating the two parts individually is smaller than that obtained by irradiating the whole central region, which means that the rule of "1 + 2 = 3" does not hold and the coupling effect between the two parts is important in photocurrent generation.

Nucleobase-Bonded Graphene Nanoribbon Junctions: Electron Transport from First Principles.

Carbon and hydrogen bonding constitute the backbone of life; in the form of graphene, possibly functionalized by DNA nucleobases, these hold promise for the programmable assembly of graphene-based nanoelectronic devices. It is still unknown how hydrogen-bonded junctions inherent in such devices will perform as electron transport media. Here, we design nucleobase-bonded graphene nanoribbons and quantify their quantum transport characteristics using first-principles calculations. Pronounced rectifying behavior and negative differential resistance are found, as well as high conductance of certain structures, with the guanine-cytosine junction in general being superior to the adenine-thymine junction. The identified sensitivity of the conductance to atomic details of the interfaces offers initial hints and guidance for experimental realization. The dependence of current on electrostatic gate doping, with an on/off ratio of ∼102, shows the potential of the junction as a field effect transistor.

Thermodefect voltage in graphene nanoribbon junctions

Thermoelectric junctions are often made of components of different materials characterized by distinct transport properties. Single material junctions, with the same type of charge carriers, have also been considered to investigate various classical and quantum effects on the thermoelectric properties of nanostructured materials. We here introduce the concept of defect-induced thermoelectric voltage, namely, thermodefect voltage, in graphene nanoribbon (GNR) junctions under a temperature gradient. Our thermodefect junction is formed by two GNRs with identical properties except the existence of defects in one of the nanoribbons. At room temperature the thermodefect voltage is highly sensitive to the types of defects, their locations, as well as the width and edge configurations of the GNRs. We computationally demonstrate that the thermodefect voltage can be as high as 1.7 mV K−1 for 555–777 defects in semiconducting armchair GNRs. We further investigate the Seebeck coefficient, electrical conductance, and electronic thermal conductance, and also the power factor of the individual junction components to explain the thermodefect effect. Taken together, our study presents a new pathway to enhance the thermoelectric properties of nanomaterials.

Single-layered assembly of vanadium pentoxide nanowires on graphene for nanowire-based lithography technique

Graphene nanoribbon (GNR)-based materials are a promising device material because of their potential high carrier mobility and atomically thin structure. Various approaches have been reported for preparing the GNR-based materials, from bottom-up chemical synthetic procedures to top-down fabrication techniques using lithography of graphene. However, it is still difficult to prepare a large-scale GNR-based material. Here, we develop a procedure to prepare a large-scale GNR network using networked single-layer inorganic nanowires. Vanadium pentoxide (V2O5) nanowires were assembled on graphene with an interfacial layer of a cationic polymer via electrostatic interaction. A large-scale nanowire network can be prepared on graphene and is stable enough for applying an oxygen plasma. Using plasma etching, a networked graphene structure can be generated. Removing the nanowires results in a networked flat structure whose both surface morphology and Raman spectrum indicate a GNR networked structure. The field-effect device indicates the semiconducting character of the GNR networked structure. This work would be useful for fabricating a large-scale GNR-based material as a platform for GNR junctions for physics and electronic circuits.

Valley-resolved transport in zigzag graphene nanoribbon junctions

Applying the transfer matrix and Green’s function methods, we study the valley-resolved transport properties of zigzag graphene nanoribbon (ZGNR) junctions. The width of the left and right ZGNRs are N L and N R , and N L ≥ N R . The step/dip positions of the conductance G, the intravalley transmission coefficients (T KK and ), and the valley polarization efficiency correspond to the subband edges of the right/left ZGNR that are controlled by N R /N L . The intervalley transmission coefficients ( and ) exhibit peaks at most of the subband edge of the left and right ZGNRs. In the bulk gap of the right ZGNR, =0, and = ±1, the valley polarization is well preserved. As N R increases, the energy region for = ±1 decreases. When N L is fixed and N R decreases, G, T KK , and exhibit more and more dips, and the peaks of () become more and more high, especially when (N L − N R )/2 is odd. These characters are quite useful for manipulating the valley dependent transport properties of carriers in ZGNR junctions by modulating N L or N R , and our results are helpful to the design of valleytronics based on ZGNR junctions.

Molecular Dynamics Study on Graphene-Nanoflake Sensor Sandwiched Between Crossed Graphene-Nanoribbon Junctions.

We present a design of a nanoscale inertial measurement unit or a data archive using a graphene-nanoflake (GNF) sandwiched between crossed graphene-nanoribbon (GNR) junctions. When an external force applied is below the retracting force, the inertial force exerted on the movable GNF can telescope it. Then, the self-restoring force increases as the attractive van der Waals force between the GNF and the GNRs, which enables the GNF to automatically and fully retract back into the sandwich position immediately after the externally applied force is released. When the external force exceeds the retracting force, the GNF escapes from the crossed GNR junctions, which enables the device to be used as non-volatile memory. The heterostructure of GNR/h-BN/GNR can be considered as an advanced structure in the proposed scheme.

Spatial spin flipping and spin switching phenomena on a Y-shaped graphene nanoribbon ferromagnetic junction with Rashba spin orbit coupling and strain

We theoretically study a controllable spatial spin converter and spin valve by using a Y-shaped even-chain zigzag graphene nanoribbon (ZGR) junction with Rashba spin orbit coupling (SOC) and strain. By calculating the spin-dependent conductance spectra of different tunneling terminals through the multi-terminal Landauer formula, we find that the spin up electron can flip its spin orientation, and convert to spin down one on the left electrode, the same spin converting phenomena can be inhibited on the right terminal. The above spin flipping and its valve phenomena are derived from the interactions of the Rashba SOC, strain and magnetization layout. When the strain’s orientation is along (perpendicular) to the zigzag chain’s direction, the valley valve effect of the even-chain ZGR is remained, the Rashba SOC takes little effect on the spatial spin switching, one can get 100% polarized spin up (down) electrons at different terminals of the Y-shaped ZGR due to the valley valve effect. When the strain is changed to other direction, the valley valve effect is partly destroyed, Rashba SOC can enhance the spin flipping conductance and break the ON/OFF transport states of spin valve. Our investigations might be useful in designing a multi-parameter controllable spin valve and spin detector based on a multi-terminal graphene nanoribbon junction.

Rectification, transport properties of doped defective graphene nanoribbon junctions

The transport properties and rectification behavior of junctions which contain armchair graphene nanoribbons (AGNRs) with double vacancy defects or nitrogen-doped in three different sizes of 9, 10 and 12 atoms are studied. The non-equilibrium Green function method and density functional based tight-binding approach are used for different computations. The double vacancy (DV) defects are along the direction of current pathways of graphene devices. We calculated transmission probability, density of states, the current–voltage curves, rectification ratio, and electrodes band structures. We found that I–V graph has nonlinear characteristic and displays rectification behavior. Devices which posses the size of 9 atoms show significant sign of rectification in contrast to other cases (10, 12 atoms). But the current value is more important for the device of 12 atoms size. Moreover, it is shown that extra energy bands are created by the DV defects and nitrogen (N) doped atoms. These bands of DV defects and N-doped cause the Fermi level to shift upwards and can change the behavior (n-type semiconductor, or metal-like) of devices of 9, 10 and 12 AGNRs. Also, various orbital distributions of MPSH (molecularly projected self-consistent Hamiltonian) states in the DV-9AGNR device are investigated.

Doping-induced large spin-filter behavior and rectification behavior in zigzag graphene nano-ribbon junction

Due to the edge states of zigzag graphene nanoribbon (ZGNR), heterojunctions of p- and n-type doped ZGNRs with either high rectification ratios or high spin-filter efficiencies are constructed. Four ZGNR-based heterojunctions are designed theoretically by replacing the edge carbon atoms of two ZGNR electrodes with boron atoms and nitrogen atoms, respectively. The electron-transport properties and the spin-transport properties are studied by applying non-equilibrium Green's function method combined with first principles calculations. The numerical results show that, such heterojunctions show 100% spin-filter efficiencies in the lower negative bias regime. This excellent spin-filter behavior originates from the localized spin-up sub-bands that lie on the Fermi level of boron-doped electrode and lead to a destructive interference of electrons in the scattering region, thereby preventing the spin-up electrons from entering the electrode and leading to a near-zero spin-up current in lower bias regime. When the dopant boron and nitrogen atoms are hydrogenated, the 100% spin-filter efficiency disappears, but the rectification ratio up to 106 is obtained. This amazing rectification ratio results from the vanishing overlap of conduction bands of the two electrodes for both spin-up and spin-down sub-bands when the absolute value of the negative bias is greater than 0.4 V.

High-efficiency switching effect and negative differential conductance in cyclo[18]carbon–graphene nanoribbon junction

The cyclo[18]carbon was synthesized experimentally for the first time in August 2019 [K. Kaiser et al., Science 365, 1299 (2019)]. The semiconductor property of the cyclo[18]carbon makes it possible to become an essential element of molecular electronic devices. In our work, the electron transport properties of zigzag graphene nanoribbon cyclo[18]carbon-zigzag graphene nanoribbon junction [(8-ZGNR)-C18-(8-ZGNR)] were studied, using the non-equilibrium Green’s function approach based on the density functional theory. The coplanar and perpendicular configuration constitutes the current ON and OFF state, respectively, with the high ON/OFF ratio approaching to 104. A significant negative differential conductance (NDC) with a peak-to-valley ratio of 1.5 in the bias range of 0.3 V–0.4 V was found. The NDC arises due to the resonance of the edge state of ZGNRs and the frontier orbitals of C18 within the bias window. Moreover, the high current ON/OFF ratio and NDC are robust against variations in the lead’s width, length of short-carbon chain, and the bridging way of C18 on leads. Specifically, the Ipeak/Ivalley of the coplanar configuration with the C5 spacer can increase to 5.7 in the bias range of 0.4 V–0.5 V. Our computational studies may guide design of multi-functional molecular devices based on C18.

Thermal transport and spin-dependent Seebeck effect in parallel step-like zigzag graphene nanoribbon junctions.

By using nonequilibrium molecular dynamics, thermal transport through a series of parallel step-like graphene nanoribbon (GNR) junctions is investigated. The theoretical results show that the thermal current flows preferentially from wide GNRs to narrow ones, displaying a pronounced thermal rectification effect. Moreover, several step-like GNR-based devices are designed, and the thermally driven spin-dependent currents are calculated by using density functional theory combined with the nonequilibrium Green's function approach. We find that thermal spin-dependent currents with opposite flow directions are generated when a temperature gradient is applied along the GNRs, indicating the occurrence of a spin-dependent Seebeck effect (SDSE). More interestingly, a negative differential SDSE occurs in the thermal spin currents, and the odd and even law appears in the spin-dependent currents, thermopowers and thermoelectric conversion efficiencies. Our theoretical results indicate that the parallel step-like GNRs are potential candidates to design spin caloritronics devices hosting thermal rectification and multiple thermal-spin transport functionalities.

Removing all periodic boundary conditions: Efficient nonequilibrium Green's function calculations

We describe a method and its implementation for calculating electronic structure and electron transport without approximating the structure using periodic super-cells. This effectively removes spurious periodic images and interference effects. Our method is based on already established methods readily available in the non-equilibrium Green function formalism and allows for non-equilibrium transport. We present examples of a N defect in graphene, finite voltage bias transport in a point-contact to graphene, and a graphene-nanoribbon junction. This method is less costly, in terms of CPU-hours, than the super-cell approximation.

Spin-Dependent Electronic and Thermoelectric Transport Properties for a Sawtoothlike Graphene Nanoribbon Coupled to Two Ferromagnetic Leads

The spin-dependent electronic transport and thermoelectric properties for a sawtoothlike graphene nanoribbon between two ferromagnetic leads with parallel or antiparallel magnetic moments are investigated theoretically. Both Christmas-tree graphene nanoribbon and tree-saw graphene nanoribbon have been considered to exemplify the effect of the chirality. By using ab initio calculations combined with a non-equilibrium Green’s function method, we obtain the important result that these sawtoothlike graphene nanoribbon junctions, regardless of their chirality, can exhibit giant charge and spin Seebeck coefficients as well as high thermoelectric efficiency at room temperature by putting ferromagnetic stripes or magnetic fields on the nanoribbons. Specifically, large $$|S_\mathrm{C}|\,(\sim 5.8~\hbox {mV/K})$$, $$|S_\mathrm{S}|(\sim 1.8~\hbox {mV/K})$$, charge figure of merit $$ZT_\mathrm{C}$$ and spin figure of merit $$ZT_\mathrm{S}(\sim 20)$$ were found in these sawtoothlike graphene nanoribbon junctions. In addition, negative differential resistance and spin filtering were also realized in these sawtoothlike graphene nanoribbon junctions. The maximum of spin filtering efficiency can be up to 100%. Our studies suggest that sawtoothlike graphene nanoribbon junctions are suitable for use as highly efficient spin caloritronics at room temperature.

Carbon atomic chains in a spin thermoelectric device

We investigated spin-dependent thermoelectric properties of a carbon atomic chain (CAC) between two zigzag-edged triangular graphene nanoflakes (GNFs), with two semi-infinite armchair graphene nanoribbons (AGNRs) as the electrodes. Using the non-equilibrium Green’s function method combined with the effective self-consistent mean-field Hubbard approximation, we found that the designed configuration shows large thermo-spin effects that could be tuned by the gate voltage, chemical potential as well as temperature. Our results show that the non-magnetic AGNRs with the edge magnetism of the GNFs and CAC located between them induces a notable spin Seebeck coefficient (SSC), comparable to or more than that in zigzag graphene nanoribbon junctions, for all sizes of CAC. Finally, applying a gate voltage and a temperature gradient across the chain-based caloritronic device could induce a pure spin current, a perfect spin filtering effect, and an unrivaled negative differential thermal resistance in a broad range of temperatures close to room temperature.

Thermosize voltage induced in a ballistic graphene nanoribbon junction

A thermoelectric voltage is induced in a junction, constituted of two dissimilar materials under a temperature gradient. Similarly, a thermosize voltage is expected to be induced in a junction made by the same material but having different sizes, so-called thermosize junction. This is a consequence of dissimilarity in Seebeck coefficients due to differences in classical and/or quantum size effects in the same materials with different sizes. The studies on thermosize effects in the literature are mainly based on semiclassical models under relaxation time approximation or even simpler local equilibrium ones where only very general ideas and results have been discussed without considering quantum transport approaches and specific materials. To make more realistic predictions for a possible experimental verification, here we consider ballistic thermosize junctions made by narrow and wide (n−w) pristine graphene nanoribbons with perfect armchair edges and calculate the electronic contribution to the thermosize voltage, at room temperature, by using the Landauer formalism. The results show that the maximum thermosize voltage can be achieved for semiconducting nanoribbons and it is about an order of magnitude larger than that of metallic nanoribbons. In the semiconducting case, the thermosize voltage forms a characteristic plateau for a finite range of gating conditions. We demonstrate, through numerical calculations, that the induced thermosize voltage per temperature difference can be in the scale of mV/K, which is high enough for experimental measurements. Owing to their high and persistent thermosize voltage values, graphene nanoribbons are expected to be good candidates for device applications of thermosize effects.

Synthesis of Regioisomeric Graphene Nanoribbon Junctions via Heteroprecursors

Graphene nanoribbons are one of the most promising materials for nanoscale electronics. While various structures have successfully been synthesized by on-surface reactions, curved connections remain less investigated, yet they could prove highly useful in the development of copolymerization technology. Here, we present the synthesis of three different regioisomeric junctions on Au (111) by using 10,10′-dibromo-9,9′-bianthryl and 1,3,6,8-tetrabromopyrene. The chemical structures and the electronic properties of junctions were studied with a combination of scanning tunneling microscopy with a CO terminated tip, scanning tunneling spectroscopy, and density functional theory calculations. We found that two identical nanoribbon-junction interface electronic states appear within the band gaps of connected graphene nanoribbons.

Spin-related thermoelectric transport in wedge-shaped graphene nanoribbon junctions

Spin-related thermoelectric effect in low-dimensional system has become a very important research filed in nanoelectronics and spintronics. In this work, we report a first principles calculation of spin-resolved Seebeck effect as well as transport properties in wedge-shaped graphene ribbon nanojunctions. A series of spin-resolved transmission dips are formed for the single channel transport due to the quasi-bound states induced by back scattering of electrons near the wedge-shape edges in the center of the junctions. Near the spin-resolved anti-resonant dips, large spin thermopower and spin figure of merits can be achieved. Moreover, by adjusting the chemical potential and temperatures of both leads, pure spin current without charge current can be obtained near the energies of transmission dips for each nanojunction. The proposed wedge-shaped graphene nanojunctions may have potential application as low-dimensional nanoscale thermoelectric devices.