Strong Negative Differential Resistance Research Articles

Enhancing Spin-Transport Characteristics, Spin-Filtering Efficiency, and Negative Differential Resistance in Exchange-Coupled Dinuclear Co(II) Complexes for Molecular Spintronics Applications.

Single-molecule spintronics, where electron transport occurs via a paramagnetic molecule, has gained wide attention due to its potential applications in the area of memory devices to switches. While numerous organic and some inorganic complexes have been employed over the years, there are only a few attempts to employ exchange coupled dinuclear complexes at the interface, and the advantage of fabricating such a molecular spintronics device in the observation of switchable Kondo resonance was demonstrated recently in the dinuclear [Co2(L)(hfac)4] (1) complex (Wagner et al., Nat. Nanotechnol. 2013, 8, 575-579). In this work, employing an array of theoretical tools such as density functional theory (DFT), the ab initio CASSCF/NEVPT2 method, and DFT combined with nonequilibrium Green Function (NEGF) formalism, we studied in detail the role of magnetic coupling, ligand field, and magnetic anisotropy in the transport characteristics of complex 1. Particularly, our calculations not only reproduce the current-voltage (I-V) characteristics observed in experiments but also unequivocally establish that these arise from an exchange-coupled singlet state that arises due to antiferromagnetic coupling between two high-spin Co(II) centers. Further, the estimated spin Hamiltonian parameters such as J, g values, and D and E/D values are only marginally altered for the molecule at the interface. Further, the exchange-coupled state was found to have very similar transport responses, despite possessing significantly different geometries. Our transport calculations unveil a new feature of the negative differential resistance (NDR) effect on 1 at the bias voltage of 0.9 V, which agrees with the experimental I-V characteristics reported. The spin-filtering efficiency (SFE) computed for the spin-coupled states was found to be only marginal (∼25%); however, if the ligand field is fine-tuned to obtain a low-spin Co(II) center, a substantial SFE of 44% was noted. This spin-coupled state also yields a very strong NDR with a peak-to-valley ratio (PVR) of ∼56 - a record number that has not been witnessed so far in this class of compounds. Additionally, we have established further magnetostructural-transport correlations, providing valuable insights into how microscopic spin Hamiltonian parameters can be associated with SFE. Several design clues to improve the spin-transport characteristics, SFE and NDR in this class of molecule, are offered.

Spin filtering and negative differential resistance in PAQR-ZGNR junctions

The spin transport of a Polyacene quinone radical (PAQR) molecule sandwiched between two ferromagnetic ZGNR electrodes with an even width number of 6 is studied employing the nonequilibrium Green's function method combined with the density functional theory (NEGF-DFT). Lateral symmetry mismatch of wave functions between PAQR and ZGNR may suppress all the transport channels except only one spin-down channel near the chemical potential in the linear regime. A half-metallic behavior with perfect spin filtering efficiency and strong negative differential resistance is expected. Under high bias voltage, the spin filtering efficiency changes from −100% to 90% when other transport channel is involved. Applying an external gate voltage, we can modulate greatly the conductance of the junction. These unique features are insensitive to the length of PAQR chain and suggest great application prospect for PAQR-ZGNR molecule devices.

TPDH-graphene: A new two dimensional metallic carbon with NDR behaviour of its one dimensional derivatives

Abstract A new two dimensional carbon allotrope TPDH-graphene (Tetra-Penta-Deca-Hexagonal-graphene) belonging to the tetragonal-pentagonal carbon ring family is proposed in this work using density-functional method. The allotrope satisfies all the conditions of structural stability. This allotrope has lower cohesive energy than many existing planar carbon allotropes. It can withstand temperature as high as 1000 K without loosing its structural integrity. However, its electronic structure reflects metallic character due to delocalised pz orbital near the Fermi level. Further, this mechanically stable elastically anisotropic structure shows directional variation of In-plane Young's modulus and Poisson's ratio. It is stronger than graphene in a particular direction. Moreover, The material can be identified by four characteristic peaks in the electron energy loss spectra within 10 eV energy. It has optical reflectance peaks in the visible range at ∼600 nm yellow colour. Interestingly, some nanoribbons of this material show semimetallic, semiconducting and metallic behaviour. Non-equilibrium Green's function method along with density-functional theory is employed to study the nanodevices made of these nanoribbons. Strong current regulation property and robust negative differential resistance effect with a peak-to-valley ratio 3.3 are observed in two nanodevices making TPDH-graphene an attractive material for use in nanoelectronics.

Engineering Crossed Andreev Reflection in Double-Bilayer Graphene.

Crossed Andreev reflection (CAR) is a nonlocal process that converts an incoming electron (hole) from one normal electrode to an out-going hole (electron) in another normal electrode through a superconductor (SC). CAR corresponds to the inverse process of Cooper pair splitting, which generates a quantum-entangled electron pair with spatial separation. Here, we fabricated vertically stacked double bilayer graphene (BLG) connected via a superconducting electrode and achieved a spacing between BLG sheets of ∼14 nm, which is far shorter than the superconducting coherence length. We confirm the highly efficient CAR effect by observing strong negative differential resistance in a nonlocal configuration and demonstrate that the competing processes against the CAR can be effectively suppressed by separately tuning the chemical potential of each BLG. The dependence of nonlocal signals on bias voltage, temperature, and chemical potential is consistent with the predicted CAR process. Our results provide a new pathway to a novel SC-based quantum entangler with the in situ tunability of the correlated-pair-splitting efficiency.

Redox-mediated Negative Differential Resistance (NDR) Behavior in Perylenediimide Derivative: A Supramolecular Approach.

Deaggregated perylenediimide (PDI) derivatives exhibit exceptionally high quantum yields, photostability and appropriate molecular features for organic electronics. This work demonstrates a metal-dye-metal framework with a large and stable negative differential resistance (NDR) at ambient conditions, built using a supramolecular strategy. The deaggregation achieved through the encapsulation of the bay-substituted phenyl groups of aggregated (l/d)-Phe-PDI dyes by the β-CD macrocyclic host is validated through detailed spectroscopic and imaging techniques. The host-guest interaction resulted in a dramatic enhancement in the emission yield from 0.28 to 0.90. In the thin film deposits, the β-CD/(l/d)-Phe-PDI complex displayed well-connected sheet-like morphology, whereas the uncomplexed (l/d)-Phe-PDI dye remained as scattered lumps. The large and reversible I-V characteristics displaying strong NDR behavior is attributed to the oxidation/reduction processes involving the rigid π-rich PDI core and is stable at least for about six months at ambient conditions, a promising system for organic electronics applications.

Strong current-polarization and negative differential resistance in FeN3-embedded armchair graphene nanoribbons

Motivated by the recent advances of transition-metal-nitrogen-carbon (TM-N-C) materials in catalysis, we investigate the electronic structure and transport properties of FeN $_3$ -embedded armchair and zigzag graphene nanoribbons (FeN $_3$ @AGNRs, FeN $_3$ @ZGNRs) with different widths. The first-principles results indicate that the FeN $_3$ induces significant changes on the band structures of both ZGNRs and AGNRs, making the resultant systems quite different from the pristine ones and own room-temperature stable ferromagnetic (FM) ground states. While only FeN $_3$ @AGNRs possess a significant spin-dependent negative differential resistance (NDR) and a striking current polarization (nearly 100\%) behaviors, due to that FeN $_3$ introduces two isolated spin-down states, which contribute current with different performances when they couple with different frontier orbits. It is suggested that by embedding FeN $_3$ complexes, AGNRs can be used to build spin devices in spintronics.

Electron transport through phenylene sandwiched between zigzag graphene nanoribbons

We study systematically the electron transport through a phenylene rotor with an axis of atomic carbon chain (CPC) connected to twofold symmetric electrodes of nonmagnetic zigzag graphene nanoribbons. The density functional theory combined with the nonequilibrium Green’s function method is employed for the simulation. The CPC rotor is conductive with parabolic I–V characteristic when its ring is coplanar with the electrodes. Its rotation modulates the symmetry of its electron states and their matching to the states in the electrodes. The I–V curve then becomes characterized by sharp peaks with strong negative differential resistance (NDR) in a large range of the rotation angle. The corresponding shift of transport modes in energy with the rotation opens a way to efficient and accurate manipulation of NDR.

Low bias negative differential resistance in tour wires predicted by first-principles study

Tour wires are considered as potential block materials for molecular electronics in the future. This work provides a first-principles study of the transport properties of three Tour wires sandwiched between two Al leads by pyridine “alligator clip” connecting apex Al atom. We obtained strong negative differential resistance (NDR) effect under very low bias and such NDR effect can be modulated by the length of the Tour wire, which promise the potential applications in molecular devices in the future.

Overwhelming coexistence of negative differential resistance effect and RRAM.

An electronic cell that possesses synchronously multi-physical properties is of great importance in the applications of multifunctional electronic devices. In this study, an overwhelming coexistence of negative differential resistance (NDR) effect and resistive switching (RS) memory behavior at room temperature was observed based on Ag/Cu2ZnSnSe4 (CZTSe)/Mo devices. The long retention time of ∼104 s and high HRS/LRS resistance ratio of ∼215 can be achieved, indicating that our devices possess excellent resistance random access memory (RRAM). Moreover, strong NDR behavior was observed at room temperature, which provides a great potential application in advanced electronic devices. Finally, the combined physical model of conductive filament and Schottky barrier reinstallment is demonstrated to explain the coexistence phenomenon. Thus, in this, study we propose a new strategy for preparing a multifunctional electronic device with multiple physical attributes in the future.

Single-band negative differential resistance in metallic armchair MoS2 nanoribbons

Semiconductor armchair MoS2 nanoribbons can be converted into conductors by edge functionalization of H atoms or OH groups. Those metallic nanoribbons exhibit I–V characteristics of a single half-filled band with strong negative differential resistance (NDR) under a voltage bias less than 1 V. This originates from the spatial separation between electrons in the conduction and valence bands. The NDR becomes spin dependent if the H atoms or OH groups are not uniformly adsorbed on the edge. Furthermore, the spin polarization can be greatly enhanced in heterojunctions of H- and OH-passivated nanoribbons.

Pyridine “alligator-clip” as molecular negative differential resistor predicted by first-principles study

Based on non-equilibrium Green's function and density functional theory, a first-principles study of the transport properties of two benzene-pyridines sandwiching the σ barrier of ethyl is performed. Using symmetric leads, strong negative differential resistance (NDR) effects with high peak-to-valley ratios (PVRs) are present under low bias. When using asymmetric leads, the PVR can be modulated to higher value with the unchanging of voltage (Vpeak) corresponding current peak (Ipeak). Our investigations indicate that pyridine “alligator-clip” can be used as very good low bias molecular NDR devices.

Spin-polarized transport properties of 1,3-dimethylpropynylidene-based molecular devices

Based on non-equilibrium Green's function and density functional theory, a first-principles study of the spin-polarized transport properties and magnetism of 1,3-dimethylpropynylidene molecule sandwiched in two Ag electrodes is performed. Strong spin-polarized negative differential resistance and spin filtration effect are present. We also find that the net spin moments of molecular magnet abnormally decrease driven by electric field because the molecular orbitals around Fermi level have opposite spin direction and that non-spin-polarized electrical read out of spin states for single molecular magnets is possible.

Radical based molecular transport system as good molecular spintronics device predicted by first-principles study

Using a first-principles approach based on non-equilibrium Green’s functions combing density functional theory, the transport properties of “radical-σ-radical” molecule sandwiched between two gold leads are investigated. The strong negative differential resistance (DNR) effect and spin filtration are obtained. We also find that the bias can manipulate the spin current sign and the magnetization configuration, which indicates that such molecular transport system (TS) has strong magnetoelectric effect and promises potential applications in the field of molecular spintronics in the future.

Negative differential resistance and switch behavior of T-BxNy (x, y = 5, 6, 11) molecular junctions

The electronic transport properties of T-BxNy (x, y=5, 6, 11) molecular junction are investigated based on first-principle density functional theory and non-equilibrium Green's function method. Strong negative differential resistance (NDR) behavior is observed for T-B5N6 molecule under negative and positive bias voltages, with an obvious switch effect for T-B6N5. However, only small NDR is shown for the complex of the two molecules. The projected device density of states, the spatial distribution of molecular orbitals, and the effect of transmission spectra under various bias voltages on the electronic transport properties are analyzed. The obvious effect of bias voltage on the changes in the electronic distribution of frontier molecular orbitals is responsible for the NDR or switch behavior. Therefore, different functional molecular devices can be obtained with different structures of T-BxNy.

Tuning the electronic and quantum transport properties of nitrogenated holey graphene nanoribbons

Strong negative differential resistance (NDR) behavior with a remarkable current peak-to-valley ratio for armchair C2N-hNRs and non-linear current–voltage characteristics for zigzag C2N-hNRs.

Tunable Esaki Effect in Catalyst-Free InAs/GaSb Core-Shell Nanowires.

We demonstrate tunable bistability and a strong negative differential resistance in InAs/GaSb core-shell nanowire devices embedding a radial broken-gap heterojunction. Nanostructures have been grown using a catalyst-free synthesis on a Si substrate. Current-voltage characteristics display a top peak-to-valley ratio of 4.8 at 4.2 K and 2.2 at room temperature. The Esaki effect can be modulated-or even completely quenched-by field effect, by controlling the band bending profile along the azimuthal angle of the radial heterostructure. Hysteretic behavior is also observed in the presence of a suitable resistive load. Our results indicate that high-quality broken-gap devices can be obtained using Au-free growth.

Tunnel Dielectric Field-Effect Transistors with High Peak-to-Valley Current Ratio

We present silicon-compatible tunnel dielectric field-effect transistors with strong negative differential resistance. On-state tunneling currents have been improved and fully suppressed ambipolarity with lowest subthreshold slope (SS) 10 mV/dec. In addition to the TFET mode, our device works as the negative transconductance characteristic that produces a high current peak-to-valley current ratio (PVR) (up to 107). Numerical simulations demonstrate the impact of tunnel dielectric layer thickness, gate oxide thickness and temperature on the PVR. With the significant improvement in SS, on-state current and high PVR, this tunnel dielectric transistor provides an effective technique for enhancing the drive current, and realizes its applications in logic and memory circuits.

Estimation of Joule heating and its role in nonlinear electrical response of Tb 0.5 Sr 0.5 MnO 3 single crystal

Abstract Highly non-linear I–V characteristics and apparent colossal electro-resistance were observed in non-charge ordered manganite Tb 0.5 Sr 0.5 MnO 3 single crystal in low temperature transport measurements. Significant changes were noticed in top surface temperature of the sample as compared to its base while passing current at low temperature. By analyzing these variations, we realize that the change in surface temperature ( Δ T sur ) is too small to have caused by the strong negative differential resistance. A more accurate estimation of change in the sample temperature was made by back-calculating the sample temperature from the temperature variation of resistance (R–T) data ( Δ T cal ) , which was found to be higher than Δ T sur . This result indicates that there are large thermal gradients across the sample. The experimentally derived Δ T cal is validated with the help of a simple theoretical model and estimation of Joule heating. Pulse measurements realize substantial reduction in Joule heating. With decrease in sample thickness, Joule heating effect is found to be reduced. Our studies reveal that Joule heating plays a major role in the nonlinear electrical response of Tb0.5Sr0.5MnO3. By careful management of the duty cycle and pulse current I–V measurements, Joule heating can be mitigated to a large extent.

Reverse rectification and negative differential resistance effects in doped armchair graphene ribbons device

In the present work, we perform first-principles calculations based on density functional theory and non-equilibrium Green’s function to study the electronic transport properties of the 10-armchair graphene ribbons devices doped by boron and phosphorus atoms. Two kinds of device show a strong inverse rectification and negative differential resistance (NDR) effect. The effect of doping position on rectifying phenomenon are analyzed by calculating the transmission spectra and the energy band structures of the related electrodes as well as the projected density of states for two devices at different bias. And the observed NDR effect is explained by the local density of states. The results indicate that the asymmetric doping of the impurity atom contributes to the electron transport of the device, being used to design a molecular rectifier with good performance.

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.