The effects of spin-filter and negative differential resistance on Fe-substituted zigzag graphene nanoribbons

Using the first-principle calculations, we investigate the spin-dependent transport properties of Fe-substituted zigzag graphene nanoribbons (ZGNRs). The substituted ZGNRs with single or double Fe atoms, distributing symmetrically or asymmetrically on both edges, are considered. Our results show Fe-substitution can significantly change electronic transport of ZGNRs, and the spin-filter effect and negative differential resistance (NDR) can be observed. We propose that the distribution of the electronic spin-states of ZGNRs can be modulated by the substituted Fe and results in the spin-polarization, and meanwhile the change of the delocalization of the frontier molecular orbitals at different bias may be responsible for the NDR behavior.     

Photonic-sensitive InAlGaAs/InP negative-differential-resistance heterojunction bipolar transistor

A novel photonic-sensitive InAlGaAs/InP negative-differential-resistance heterojunction bipolar transistor (NDR-HBT) is fabricated and demonstrated. Due to the appropriately designed narrow base width and the employment of a δ -doped sheet at the emitter–base (E–B ) heterojunction, the base resistance effect results in the significant NDR phenomenon. In addition, the experimental results show that the device studied is very sensitive to the applied light source. The N-shaped NDR phenomena are clearly observed under illumination. This phenomenon is attributed to the base resistance and barrier lowering effect resulting from holes induced by the applied light source.     

Negative differential resistance behaviour in N-doped crossed graphene nanoribbons

By using first-principles calculations and nonequilibrium Green’s function technique, we study elastic transport properties of crossed graphene nanoribbons. The results show that the electronic transport properties of molecular junctions can be modulated by doped atoms. Negative differential resistance (NDR) behaviour can be observed in a certain bias region, when crossed graphene nanoribbons are doped with nitrogen atoms at the shoulder, but it cannot be observed for pristine crossed graphene nanoribbons at low biases. A mechanism for the negative differential resistance behaviour is suggested.     

Effect of spin–orbit coupling on spin transport at graphene/transition metal interface

In spite of large spin coherence length in graphene due to small spin–orbit coupling, the created potential barrier and antiferromagnetic coupling at graphene/transition metal (TM) contacts strongly reduce the spin transport behavior in graphene. Keeping these critical issues in mind in the present work, ferromagnetic (Co, Ni) nanosheets are grown on graphene surface to elucidate the nature of interaction at the graphene/ferromagnetic interface to improve the spin transistor characteristics. Temperature dependent magnetoconductance shows unusual behavior exhibiting giant enhancement in magnetoconductance with increasing temperature. A model based on spin–orbit coupling operated at the graphene/TM interface is proposed to explain this anomalous result. We believe that the device performance can be improved remarkably tuning the spin–orbit coupling at the interface of graphene based spin transistor. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

栅极电势对强光场下石墨烯场效应管中电子隧穿的影响

利用时域有限差分方法研究了强光场下石墨烯场效应管中栅极电势对电子隧穿的影响. 在强光场下由于光学stark效应,石墨烯场效应管的完美手征透射被抑制.这种抑制除了 可以利用光场来调控外,也可以通过改变栅极电势的宽度、势垒高度等来调控. 研究了非方势垒中电子的隧穿. 研究发现,当电势的倾斜较小时,电子隧穿概率变化不大.而当电势倾斜很大时,电子隧穿概率急剧改变.     

Contact position and width effect of graphene electrode on the electronic transport properties of dehydrobenzoannulenne molecule under bias

By applying nonequilibrium Green?s function formalism in combination with density functional theory, we have investigated the electronic transport properties of dehydrobenzoannulenne molecule attached to different positions of the zigzag graphene nanoribbons (ZGNRs) electrode. The different contact positions are found to drastically turn the transport properties of these systems. The negative differential resistance (NDR) effect can be found when the ZGNRs electrodes are mirror symmetry under the xz midplane, and the mechanism of NDR has been explained. Moreover, parity limitation tunneling effect can be found in a certain symmetry two-probe system and it can completely destroy electron tunneling process. The present findings might be useful for the application of ZGNRs-based molecular devices.     

掺杂六角形石墨烯电子输运特性的研究

锯齿型和扶手椅型六角形石墨烯分别跨接在两Au电极上, 构成分子纳器件, 同时考虑对六角形石墨烯分别进行B, N和BN局部规则掺杂. 利用第一性原理方法, 系统地研究了这些器件的电子输运特性. 计算结果表明: B及BN掺杂到扶手椅型六角形石墨烯, 对其电流有较好的调控效应, 同时发现本征及掺杂后的锯齿型六角形石墨烯均表现为半导体性质, 且N及BN掺杂时, 表现出明显的负微分电阻现象, 特别是N掺杂的情况, 能呈现显著的负微分电阻效应, 这也许对于发展分子开关有重要应用. 通过其透射特性及掺杂诱发的六角形石墨烯电子结构的变化, 对这些结果的内在原因进行了说明.     

Resonant tunneling and enhanced Goos–Hänchen shift in a graphene double velocity barrier structure

Resonant transmission and Goos–Hänchen (GH) shift for Dirac fermion beams tunneling through graphene double velocity barrier structures (DVBs) are investigated theoretically. Analytical and numerical results demonstrate that strong resonant tunneling effect occurs in this structure and is highly dependent on the incident angle and the structure of velocity barriers. The resonant tunneling in graphene DVBs belongs to the Fabry–Pérot resonance and leads to oscillated conduction at wide energy range. It is also found that GH shifts in this structure can be enhanced by the resonant tunneling and multi-GH shift peaks with giant magnitudes can occur at these resonant energy positions. These special properties of GH shifts in graphene DVBs may have good application in lateral manipulation of electron beams and valley or spin beam splitter.     

Negative differential resistance and rectification effects in zigzag graphene nanoribbon heterojunctions: Induced by edge oxidation and symmetry concept

By applying non-equilibrium Green's functions (NEGF) in combination with tight-binding (TB) model, we investigate and compare the electronic transport properties of H-terminated zigzag graphene nanoribbon (H/ZGNR) and O-terminated ZGNR/H-terminated ZGNR (O/ZGNR–H/ZGNR) heterostructure under finite bias. Moreover, the effect of width and symmetry on the electronic transport properties of both models is also considered. The results reveal that asymmetric H/ZGNRs have linear I–V characteristics in whole bias range, but symmetric H-ZGNRs show negative differential resistance (NDR) behavior which is inversely proportional to the width of the H/ZGNR. It is also shown that the I–V characteristic of O/ZGNR–H/ZGNR heterostructure shows a rectification effect, whether the geometrical structure is symmetric or asymmetric. The fewer the number of zigzag chains, the bigger the rectification ratio. It should be mentioned that, the rectification ratios of symmetric heterostructures are much bigger than asymmetric one. Transmission spectrum, density of states (DOS), molecular projected self-consistent Hamiltonian (MPSH) and molecular eigenstates are analyzed subsequently to understand the electronic transport properties of these ZGNR devices. Our findings could be used in developing nanoscale rectifiers and NDR devices.     

Multifunctionality for the nanojunction of a rotating p-phenylene vinylene molecule between graphene leads

With the multi-functional molecular device based on graphene nanoribbon being deeply studied in experiment, the zigzag-edged graphene device is still worth to investigate. Employing the ab-initio method, the spin transport properties have been studied for the nanojunctions consisting of a p-phenylene vinylene (PPV) molecule sandwiched between two-probe leads of zigzag-edged graphene nanoribbons (ZGNRs). A series of obvious electromagnetic transmission functionalities, including spin switching, negative differential resistance (NDR), dual spin-filtering, magnetoresistance and spin-diode behaviors, are numerically referred in the proposed molecular junction within spin parallel or antiparallel configurations. The performance of switching and double spin filtering can be explained by the transport spectra or total transmission pathways. Besides, the rectification effect is due to the asymmetry spatial distribution of the local density of states as well as the corresponding coupling between the PPV molecule and leads. It is expected that the designed models can be ideal candidate for future spintronic device.     

Mid/far-infrared photo-detectors based on graphene asymmetric quantum wells

We conducted a theoretical study on the electronic properties of a single-layer graphene asymmetric quantum well.Quantification of energy levels is limited by electron–hole conversion at the barrier interfaces and free-electron continuum.Electron–hole conversion at the barrier interfaces can be controlled by introducing an asymmetry between barriers and taking into account the effect of the interactions of the graphene sheet with the substrate.The interaction with the substrate induces an effective mass to carriers,allowing observation of Fabry–P′erot resonances under normal incidence and extinction of Klein tunneling.The asymmetry,between barriers creates a transmission gap between confined states and free-electron continuum,allowing the large graphene asymmetric quantum well to be exploited as a photo-detector operating at mid-and far-infrared frequency regimes.     

Tuning electronic properties of the S_2/graphene heterojunction by strains from density functional theory

Based on the density functional calculations, the structural and electronic properties of the WS_2/graphene heterojunction under different strains are investigated. The calculated results show that unlike the free mono-layer WS_2, the monolayer WS_2 in the equilibrium WS_2/graphene heterojunctionis characterized by indirect band gap due to the weak van der Waals interaction. The height of the schottky barrier for the WS_2/graphene heterojunction is 0.13 eV, which is lower than the conventional metal/MoS_2 contact. Moreover, the band properties and height of schottky barrier for WS_2/graphene heterojunction can be tuned by strain. It is found that the height of the schottky barrier can be tuned to be near zero under an in-plane compressive strain, and the band gap of the WS_2 in the heterojunction is turned into a direct band gap from the indirect band gap with the increasing schottky barrier height under an in-plane tensile strain. Our calculation results may provide a potential guidance for designing and fabricating the WS_(2~-)based field effect transistors.     

Gate control of lattice-pseudospin currents in graphene on SW2: Effect of sublattice symmetry breaking and spin–orbit interaction

Strong spin–orbit interaction (SOI) in graphene grown on tungsten disulfide (SW₂) has been recently observed, leading to energy gap opening by SOI. Energy gap in graphene may also be induced by sublattice symmetry breaking (SSB) where energy level in A-sublattice is not equal to that in B-sublattice. SSB-gap may be produced by growing graphene on hexagonal boron nitride or silicon carbide. In this work, we investigate transport property in a SOI/SSB/SOI gapped graphene junction, focusing the effect of interplay of SOI and SSB. We find that, lattice-pseudospin polarization (L-PSP) can be controlled perfectly from +100% to −100% by gate voltage. This is due to the fact that in graphene grown on SW₂, the carriers carry lattice-pseudospin degree of freedom “up and down”. The SSB-gapped graphene exhibits pseudo-ferromagnetism to play the role of lattice-pseudospin filtering barrier. It is also found that the SOI and SSB-gaps in graphene may be measured by characteristic of L-PSP in the junction. The proposed controllable-lattice-pseudospin currents may be applicable for graphene-based pseudospintronics.     

Strong n-type molecule as low bias negative differential resistance device predicted by first-principles study

A first-principles study of the transport properties of two thiolated pentacenes sandwiching ethyl is performed. The thiolated pentacene molecule shows strong n-type characteristics when contact Ag lead because of low work function about metal Ag. A strong negative differential resistance (NDR) effect with large peak-to-valley ratio of 758% is present under low bias. Our investigations indicate that strong n- or p-type molecules can be used as low bias molecular NDR devices and that the molecular NDR effect based on molecular-level leaving not on molecular-level crossing has no hysteresis.     

含氮-空位缺陷锯齿状石墨烯纳米条带中负微分电阻和自旋过滤效应

采用第一性原理和非平衡格林函数方法,系统研究了含氮空位缺陷锯齿状石墨烯纳米条带的自旋极化输运特性．理论计算结果表明边界非对称的这类石墨纳米条带的基态具有铁磁性,由其构建的分子结中负微分电阻效应具有鲁棒性,是电极局域的态密度及依赖偏压的散射区-电极耦合作用结果．此外,在特定偏压区域还观察到几乎完美的自旋过滤效应．     

Electronic transport for pristine and doped crossed graphene nanoribbon junctions with zigzag interfaces

Using the fully self-consistent non-equilibrium Green?s function (NEGF) method combined with density functional theory, we investigate numerically the electronic transport property for pristine and doped crossed graphene nanoribbon (GNR) junctions. It is demonstrated that in the case of zigzag interfaces, the I–V characteristics of the junction with or without doping always show semiconducting behavior, which is different from that in the case of armchair interfaces [Zhou, Liao, Zhou, Chen, Zhou, Eur. Phys. J. B 76 (2010) 421]. Interestingly, negative differential resistance (NDR) behavior can be clearly observed in a certain bias region for nitrogen-doped shoulder crossed junction. A mechanism for the NDR behavior is suggested.     

Electronic properties and conductance suppression of defected and doped zigzag graphene nanoribbons

By using the first-principles calculation based on density functional theory, we investigate the electronic structures and transport properties of the defected and doped zigzag graphene nanoribbons (ZGNRs). The effects of multivacancies defects and impurities have been considered. The results show that band structures of ZGNRs can be tuned strongly and currents drop drastically due to the defect and impurities. Moreover, the notable suppression of conductance can be found near the Fermi level, leading to the negative differential resistance (NDR) behavior under low bias. This effect presents a possibility in novel nanoelectronics devices application.     

Spin-dependent delay time and Hartman effect in asymmetrical graphene barrier under strain

We study the spin-dependent tunneling time, including group delay and dwell time, in a graphene based asymmetrical barrier with Rashba spin–orbit interaction in the presence of strain, sandwiched between two normal leads. We find that the spin-dependent tunneling time can be efficiently tuned by the barrier width, and the bias voltage. Moreover, for the zigzag direction strain although the oscillation period of the dwell time does not change, the oscillation amplitude increases by increasing the incident electron angle. It is found that for the armchair direction strain unlike the zigzag direction the group delay time at the normal incidence depends on the spin state of electrons and Hartman effect can be observed. In addition, for the armchair direction strain the spin polarization increases with increasing the RSOI strength and the bias voltage. The magnitude and sign of spin polarization can be manipulated by strain. In particular, by applying an external electric field the efficiency of the spin polarization is improved significantly in strained graphene, and a fully spin-polarized current is generated.     

Effect of electrical contact on performance of WSe2 field effect transistors

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as tungsten diselenide (WSe₂) have spead many interesting physical properties, which may become ideal candidates to develop new generation electronic and optoelectronic devices. In order to reveal essential features of 2D TMDCs, it is necessary to fabricate high-quality devices with reliable electrical contact. We systematically analyze the effect of graphene and metal contacts on performance of multi-layered WSe₂ field effect transistors (FETs). The temperature-dependent transport characteristics of both devices are tested. Only graphene-contacted WSe₂ FETs are observed with the metal-insulator transition phenomenon which mainly attributes to the ultra-clean contact interface and lowered contact barrier. Further characterization on contact barrier demonstrates that graphene contact enables lower contact barrier with WSe₂ than metal contact, since the Fermi level of graphene can be modulated by the gate bias to match the Fermi level of the channel material. We also analyze the carrier mobility of both devices under different temperatures, revealing that graphene contact can reduce the charge scattering of the device caused by ionized impurities and phonon vibrations in low and room temperature regions, respectively. This work is expected to provide reference for fabricating 2D material devices with decent performances.     

Effects of Stone–Wales defect upon adsorption of formaldehyde on graphene sheet with or without Al dopant: A first principle study

The adsorption mechanisms of formaldehyde (H₂CO) on modified graphene, including aluminum doping, Stone–Wales (SW) defects, and a combination of these two, were investigated via density functional theory (DFT). It was found that the graphene with SW defect is more sensitive than that of perfect graphene for detecting H₂CO molecules. Compared with Al-doped graphene/H₂CO complex, the binding energy for Al-doped SW defect complex can be enhanced by the introduction of a SW defect. The large values of binding energy and net charge transfer for this complex indicate a strong chemisorption and a larger affinity with H₂CO for the modified graphene. Furthermore, the density of states (DOS) of the complex shows that the effect of defect–dopant combination on adsorption mechanisms is due to the orbital hybridization between the Al atom and its adjacent C atoms. In addition, it can be expected that adsorption of H₂CO on the surface of Al-doped SW defect may occur easily, and the Al-doped SW graphene is more suitable for H₂CO gas detection.