Graphene Nanoribbon Electrodes Research Articles

Tunable pure spin current realized by photogalvanic effect in triangulene dimer based nano-devices

The unique magnetic properties of triangulene and its homologues have generated a great deal of interest in recent years. Triangulene dimers, with their magnetic ground states and the detection of singlet-triplet spin excitations, have been synthesized on-surface by Mishra, and shown to be suitable for spintronics applications. In our study, we use first-principles calculations to investigate the photogalvanic effect (PGE) of triangulene dimers based nano-devices. The devices are constructed by two graphene nanoribbon electrodes and a triangulene dimer molecule connected by carbon chains, which has a spatial inversion symmetry. We find that when light irradiation is applied to the device, there are both spin-up and spin-down current induced with the same magnitude and opposite flowing directions. In this case, there is no accompanying charge current, but a finite pure spin current arises. Furthermore, we find that the currents generated by PGE can be tuned by impurity states. When nitrogen atoms are doped into the triangulene dimer molecule, we observe the emergence of impurity states, which supply other electron transition modes and result in a photo-current under different photon energy. Our findings provide a useful insight into the real application of tunable photoelectric carbon-based nano-devices. The PGE properties of triangulene dimers can be applied in the development of efficient spintronics devices.

Tuning exponential decay factor in oligophenylene molecular junctions with graphene nanoribbon electrodes

We explore the transport properties of oligophenylene molecular junctions, where the center molecule containing 1, 2, or 3 phenyls is sand-wiched between two graphene nanoribbons (GNR) with different edge shapes. According to the obtained results of the first-principles calculations combined with non-equilibrium Green’s function method, we find that the molecular length-dependent resistance of all examined oligophenylene molecular junctions follows well the exponential decay law with different slopes, and the exponential decay factor is sensitive to the edge shape of GNRs and the molecule-electrode connecting configuration. These observations indicate that the current through the oligophenylene molecular junction can be effectively tuned by changing the edge shape of GNRs, the molecular length, and the molecular contacting configuration. These findings provide theoretical insight into the design of molecular devices using GNRs as electrodes.

Rotating single molecule-based devices: Single-spin switching, negative differential electrical and thermoelectric resistance

Single molecule-based devices have been one of the most candidates to realize the miniaturization of traditional electronic devices. Here we investigate the spin-polarized transport properties of an iron-complex molecule sandwiched between two ferromagnetic zigzag-edged graphene nanoribbon electrodes based on the non-equilibrium Green’s function formalism combined with the density functional theory. When the iron-complex molecule rotates, a single-spin switching effect is observed at the Fermi level, and meanwhile a perfect spin filtering effect appears in the molecular device. As the electric bias increases, an obvious negative differential electrical resistance is observed. In addition, we also find a negative differential thermoelectric resistance in the absence of the electric bias under a small molecular rotation, and the sign and magnitude of the thermally driven current can be tuned by the temperature. These theoretical findings imply that the iron-complex molecular devices have potential applications in the next-generation spin electric and thermoelectric devices.

Effect of DNA bases on the thermoelectric performance in graphene nanoribbons

The thermoelectric transport properties of four kinds of DNA bases sandwiched between two zigzag-edge graphene nanoribbon electrodes are investigated by first-principles calculation. The modulation of quantum interference, molecular length, and molecular torsion on thermoelectric transport properties are studied. It is found that the thermopower and efficiency can be obviously enhanced in the guanine and electrode coupling geometry that manifests a quantum interference effect. A decrease of thermopower with a number of DNA bases has been found, while the efficiencies have a different trend with the increase of the number of different molecules. In addition, the thermopower and efficiency also can be considerably enhanced by the twisting angle between the molecule and graphene electrodes.

Tuning transport properties of deformed carbon nanocages by electric field, electrode material, and type of coupling

In this paper, we study the structural, electronic, and quantum transport properties in the smallest fullerene, C20, and the most famous fullerene, C60, and their corresponding fulleranes, C20H20 and C60H60. The molecules are attached to cumulene and graphene nanoribbon (GNR) electrodes. We examine the factors affecting electron transport in carbon molecular bridges, such as the application of an electric field, and changes in the type of and position electrodes. We first perform structural and electronic calculations with density functional theory (DFT), then we introduce the tight-binding parameters of the system for studying the transport properties of it, with the help of the non-equilibrium green's function (NEGF) method. Our findings show that fullerane molecular bridges are always electrically insulator, but fullerene systems can be metal, semiconductor, or even insulator, depending on the position of connection points to the electrodes. Larger fullerene molecular bridges, such as C60, are more chemically stable and can be used as molecular nanoelectronic devices. Besides, by applying an electric field to the C20H20 cage, we observe a phase transition from the semiconductor to the metal.

Regulation and mechanism of graphene electrode bending on negative differential resistance of 2-phenylpyridine molecular devices

Combining non-equilibrium Green’s function with density functional theory, we study the electronic transport properties of the molecular devices comprised of 2-phenylpyridine and zigzag graphene nanoribbon (ZGNR) electrodes. The <i>I-V</i> characteristics and transmission coefficients under external voltage biases are analyzed, and the results show that the negative differential resistance (NDR) is effectively adjusted by the bending of ZGNR electrode, which reduces the peak voltage (<i>V</i><sub>p</sub>) and increases the peak-valley ratio (PVR) of the device. When the electrode bending angle is 15°, the PVR of device M2 is a maximum value of 12.84 and <i>V</i><sub>p</sub> is 0.1 V, which is low enough for practical applications. The transmission spectra, the density of states and the real-space scattering state distribution at <i>E</i><sub>f </sub> of device under zero bias explain that the weaker coupling between the molecules and the electrodes is caused by the bending of the ZGNR electrode, which might be responsible for the adjustability of NDR. The analysis shows that the bending of the electrode changes the electronic structure between the 2-phenylpyridine molecule and the ZGNR electrode, and then changes the wave functions overlap between them, the coupling between the molecule and the electrodes gets weaker. An external bias can induce the level to shift. The transmission coefficient for the weaker coupling between the molecules. The electrodes can fluctuate wildly from level to level, and large NDR effect under very low bias is obtained with the variation of external bias. Therefore, for highly symmetric molecular devices, the electronic transport properties can be effectively adjusted by changing the coupling between the central molecule and the electrodes. Our investigations indicate that the 2-phenylpyridine molecular device with ZGNR electrodes may have potential applications in the field of low-power dissipation molecules device.

High Switching Performance and Spin Filtering of Photoswitchable Dimethyldihydropyrene/Cyclophanediene Single-Molecule Switch with Zigzag Graphene Nanoribbon Electrodes

High Switching Performance and Spin Filtering of Photoswitchable Dimethyldihydropyrene/Cyclophanediene Single-Molecule Switch with Zigzag Graphene Nanoribbon Electrodes

Tunnel magnetoresistance of trilayer graphene-based spin valve

Using the non-equilibrium Green’s function method and within the framework of the tight-binding Hamiltonian model in Landauer–Büttiker formalism, the spin-dependent transport and tunnel magnetoresistance ( T M R ) of AAA- and ABC-stacked trilayer zigzag graphene nanoribbon (TLG) connected to two ferromagnetic ( F M ) semi-infinite single-layer zigzag graphene nanoribbons (FM/TLG/FM system) have been theoretically investigated. Results show that the T M R for both AAA- and ABC-stacked cases can be increased about 100% because of the band-selective rule, and the magnitude of this 100% T M R region, enhances with the increase of the magnetization strength in FM graphene nanoribbon electrodes . Also, the TLG dimension has a prominent effect on the T M R ratio of the system. It is shown that the large values of the TMR ratio can be achieved very nicely by the variation of the width and length of the FM/TLG/FM system. The present theoretical results may be useful for designing few-layer graphene-based spin valves . • The TMR of trilayer graphene-based spin valve have been investigated. • The TMR for both AAA- and ABC-stacked cases can be increased about 100%. • The magnitude of 100% TMR region enhances with the magnetization strength. • The effects of width and length on the TMR ratio have been investigated.

Trans-polyacetylene based organic spin valve for a multifunctional spin-based device: A first principle analysis

Carbon-based materials are the promising candidates for the design of multifunctional spintronic devices, owing to their fascinating inherent attributes such as the ballistic transport nature, long spin coherence time, etc. Here, we report the spin-dependent electron transport properties of a trans-polyacetylene channel sandwiched between two semi-infinite hydrogen-passivated zigzag graphene nanoribbon (ZGNR) electrodes within the framework of spin-polarized density functional theory (DFT) and non-equilibrium Green's function (NEGF) formalisms. The proposed device exhibits an excellent spin-dependent transport behavior within the bias window of [-0.5 V, 0.5 V]. In parallel configuration (PC), the device offers an ideal spin filtration efficiency of almost 100% within the bias range of [-0.4 V, 0.4 V], and in antiparallel configuration (APC), it exhibits negative differential resistance (NDR), dual-spin rectification, and dual-spin filtration effects. Also, a high tunnel magnetoresistance (TMR) of 1.2× 10 5 % is achieved. To validate the aforementioned effects, the transmission spectra along with the area enclosed by the curve, eigenvalues, eigenstates, and quantum conductance have also been analyzed. In addition, the impact of hybridization and torsion defects on the spin-dependent transport phenomenon through trans-polyacetylene has been investigated. Furthermore, the modelled device is observed to exhibit the spin Seebeck effect in PC under the influence of temperature gradient. The proposed device may stand as a good contender for various spintronic and spin caloritronic applications. • First principle based analysis of a trans-polyacetylene based multifunctional spin-based device. • The device displays a perfect spin filtration efficiency and dual rectification effects. • A high tunnel magnetoresistance (TMR) of 1.2× 10 5 % is achieved. • Thermal Spin Seebeck and spin filtering effects are observed in the proposed model. • A potential candidate in the field of spintronic and spin caloritronic applications.

Thermal spin molecular logic gates modulated by an electric field

Logic gates are fundamental structural components in all modern digital electronic devices. Here, nonequilibrium Green’s functions are incorporated with the density functional theory to verify the thermal spin transport features of the single-molecule spintronic devices constructed by a single molecule in series or parallel connected with graphene nanoribbons electrodes. Our calculations demonstrate that the electric field can manipulate the spin-polarized current. Then, a complete set of thermal spin molecular logic gates are proposed, including AND, OR, and NOT gates. The mentioned logic gates enable different designs of complex thermal spin molecular logic functions and facilitate the electric field control of thermal spin molecular devices.

The thermal spin molecular logic gates modulated by light

Digital electronics and spintronics are based on logic operations. Here, nonequilibrium Green’s functions are incorporated with the density functional theory to evaluate the thermal spin transportation features of the photochromic molecule spintronic devices constructed by dihydropyrene/cyclophanediene (DDP/CPD) molecule in alone, series, or parallel, connected with graphene nanoribbons electrodes. Based on calculations, the spin-polarized currents can be manipulated by light. Then, a complete set of thermal spin molecular logic gates is proposed, including AND, OR, and NOT gates. The above logic gates can be employed to perform complex thermal spin molecular logic operations and facilitate all-optical control of thermal spin molecular devices.

Electronic transport properties of compressed and stretched helicene-graphene nanostructures, a theoretical study

We present our ab initio simulations of strained helicene molecular junction. First, we present ab-initio models of [n]helicenes up to [100]helicene done by two different recent reactive force field molecular geometry optimization methods (ReaxFF CHOCsKNaClIFLi, 2019 and CHONSMgPNaCuCl, 2018). We used the [4]helicene and [10]helicene models for Density Functional Theory (DFT) electronic transport calculations of various graphene-helicene-graphene molecular junction configurations on Local Density Approximation (LDA) or Generalized Gradient Approach (GGA) level of exchange and correlation energy with Perdew-Zinger (PZ) or Perdew–Burke–Ernzerhof (PBE) functionals and Grimme DFT-D3 van der Waals correction. The electronic transport properties are studied by using of the Green's functions formalism. The electron transmissions and densities of states of [4]helicene (and [10]helicene) attached as a molecular junction in three different configurations to zigzag graphene nanoribbon (ZGNR) electrodes of three different widths were obtained. The structures are analyzed while relaxed, compressed or stretched. The current-voltage and current-strain characteristics are presented and results are compared and discussed. The work is supported by a short review summarizing some of the recent studies related to graphene, helicene, and molecular junctions under strain.

Thermoelectric performance of porphyrin molecular junction with graphene nanoribbon electrodes

Thermoelectric performance of porphyrin molecular junction is investigated by using first-principles calculation combined with nonequilibrium Green function (NEGF) method. It is found that porphyrin molecule rotation (90[Formula: see text] rotated) has no obvious effect on its thermoelectric conversion efficiency due to the conjugate structure of central molecule. Through the functions of energy and thermal conductivity, it is found that phonon thermal conductivity accounts for the main part, which provides a potential for further optimizing thermoelectric performance. Moreover, it can be seen from phonon transmission spectrum that porphyrin molecule has a certain filtering effect on high-frequency phonon mode. For power factor and figure of merit (ZT), wave functions for two cases highly coincide. The maximum ZT values of por1 and por2 are 0.38 and 0.34 near Fermi level (FL), respectively. Porphyrin molecular junction could have a good application prospect in nanoscale thermoelectric devices.

Significantly enhanced thermoelectric performance of Van der Waals interface coupling molecular junction with nitrogen-doped graphene nanoribbon electrodes

Rational design of model structure and effective strategy to overcome inherent mismatch between molecular orbital energy and Fermi level (FL) of electrodes are the key prerequisites to fully rationalize and optimize thermoelectric properties for a single molecular junction. A bis-phenylethynyl-anthrancene (BIS-molecule) with conjugated structure is contacted with armchair/zigzag graphene nanoribbon (AGNR/ZGNR) electrodes by van der Waals (vdW) interface coupling. The optimized distance between molecular and two leads is associated with interface stacking mode. Weak-coupling structure greatly suppresses phonon transport behavior due to interface resistance. Thermal conductance (κtol) has been greatly increased at FL with N-doping based on two different mechanisms. Mass matching effect plays a dominate role for κtol increase in N-doped AGNR_BIS_AGNR, meanwhile sharp and degenerated DOS is main contribution to κtol in N-doped ZGNR_BIS_ZGNR. Nitrogen-doping can adjust Fermi level of electrodes into molecular resonance region, resulting in the enhancement of electronic coupling between central molecular and leads, which is come from obvious charge transfer from graphene nanoribbon to BIS-molecule. Moreover, an excellent thermoelectric figure-of-merit (ZT) of 14.1/3.5 has been obtained near FL for nitrogen-doped molecular junction.

Perfect dual spin filtering effect and large magnetoresistance in all-carbon devices based on C18 cyclo molecule from first principles

The carbon-based nanomaterial would push silicon aside and create a post- complementary metal oxide semiconductor world. In this work, we proposed and theoretically investigated an all-carbon spintronic device composed of C18 cyclo molecule and two zigzag graphene nanoribbon (ZGNR) electrodes with two different sturdy connection modes. It is found that both structures have similar excellent spin-dependent properties. Adjusting the magnetization configuration of two electrodes to be anti-parallel, electrons in different spin states are filtered unidirectionally according to the sign of the applied bias voltage. The molecular junction exhibits a perfect dual spin filtering effect. Moreover, a huge magneto-resistance ratio was found in our propose junctions. The matching of symmetry type of the spin-polarized states in two ZGNR electrodes, as well as the states of C18 in central region plays an important role in the occurrence of these effects.

All-carbon multifunctional molecular spintronic device: A first-principles study

Based on the first-principles density functional theory combined with the nonequilibrium Green’s function methodology, we have designed and investigated the spin-polarized transport properties of an all-carbon molecular device, in which a C60 fullerene is sandwiched between two ferromagnetic zigzag-edge graphene nanoribbon electrodes. The results show that the spin-polarized properties are strongly dependent on the external magnetic field modulation and spin-filtering, giant magnetoresistance and spin-rectifying effects are realized in this device. The mechanisms are proposed for these interesting phenomena. These results demonstrate that the designed all-carbon device holds great potentials in the development of multifunctional molecular spintronic device.

Electronic transport via DTF-NEGF at bipyridine junctions with 1D organic electrodes

In this work, we study the properties of electronic transport in molecular junctions formed by bipyridine isomers as central region coupled at electrodes of carbyne wires. Through calculations of first principles , based on Density Functional Theory (DFT), combined with Non-Equilibrium Green’s Functions (NEGF), we obtain important properties such as electric current , differential conductance, transmission, and eigenchannels. The results showed that the presence of nitrogen atoms in the molecule–electrode interface strongly affects the coupling of the junction, providing better electronic conduction; this is corroborated by the transmission eigenchannels. The transport properties analyzed revealed that in bipyridine bridges, devices with carbyne electrodes, presented better performance when compared to other works that used metallic electrodes (Au, Ag, and Cu) or graphene nanoribbons electrodes. The devices proposed showed a Field Effect Transistor (FET) behavior when are formed by symmetric isomers, whereas for asymmetric systems we obtained characteristics of Molecular Diode (MD).

Tunable giant magnetoresistance ratio in bilayer CuPc molecular devices.

We investigate the influence of the distance between the buffer layer and the central molecule on the electrical transport, spin-filter transport, magnetoresistance effects and thermoelectric properties of a bilayer CuPc molecular device with V-shaped zigzag-edged graphene nanoribbon (VZGNR) electrodes by combining density functional theory and the non-equilibrium Green's function. The results show that the spin-dependent total conductance and spin filter efficiency of the bilayer CuPc molecular device reach a maximum with a parallel spin configuration (PC) when the carbon atom at the edge of the electrode is in the center of the carbon atom at the edge of the bilayer molecules due to the stronger coupling interaction between the double-layer molecules and the leads. Moreover, the spin polarization of the bilayer CuPc molecular device is reversed at certain distances; there is a minimum spin filter efficiency (SFE) of −99.93448% and a maximum SFE of 97.91% observed in the anti-parallel spin configuration (APC) of the device and there is a minimum SFE of −26.03175% and a maximum SFE of 99.99996% observed with the PC at zero bias. The SFE oscillates with increasing considered bias voltage in the PC and APC when the distances are d = 0 Å and d = −1.06 Å, and a negative differential resistance (NDR) effect was observed. For the PC and APC, there is a giant magnetoresistance (MR) effect and the MR ratio exceeds 5.21 × 107% (99.9996%), and the MR ratio oscillates with increasing considered bias voltage when d = 0 Å. The MR ratio could be reserved by applying a certain bias voltage. These transport behaviors can be well understood by analyzing the transmission spectra, projected density of states and scattering states. There are pure spin Seebeck coefficients and pure charge Seebeck coefficients at certain temperatures when the distances are certain values, which means that the corresponding temperature differences could produce pure spin current and pure charge current, respectively. Our results provide new ideas for designing ultrahigh-performance spintronic molecular devices.

Significantly Enhanced Thermoelectric Performance of Van Der Waals Interface Coupling Molecular Junction with Nitrogen-Doped Graphene Nanoribbon Electrodes

Significantly Enhanced Thermoelectric Performance of Van Der Waals Interface Coupling Molecular Junction with Nitrogen-Doped Graphene Nanoribbon Electrodes

Design of multifunctional spin logic gates based on manganese porphyrin molecules connected to graphene electrodes.

The spin-resolved transport properties of molecular logic devices composed of two Mn porphyrin molecules connected to each other via a six-carbon atomic chain were studied using the non-equilibrium Green's function combined with density functional theory. The molecules were symmetrically connected to armchair graphene nanoribbon electrodes through four-carbon atomic chains on the left- and right-hand sides. Our calculations revealed that the spin-resolved current-voltage curves depend on the initial spin setting of the transition metal Mn atoms and carbon atoms on the zigzag edges where the electrodes come in contact with the molecule. By simultaneously regulating the spin orientations of the intermediate functional molecules and the zigzag edges of the armchair graphene nanoribbon electrodes, seven spin polarization configurations were obtained. These configurations were examined in this study considering the spin-related symmetry of molecular junctions. By meticulously selecting different combinations according to the specific input and output signals, YES, NOT, OR, NOR, and XOR multifarious spin logic devices were created. The findings of this study are expected to contribute toward the extension of molecular junction functions in future spintronic integrated circuit design and further miniaturization.