Large Negative Differential Resistance Research Articles

Effects of symmetry and spin configuration on spin-dependent transport properties of iron-phthalocyanine-based devices

Spin-dependent transport properties of nanodevices constructed by iron-phthalocyanine (FePc) molecule sandwiched between two zigzag graphene nanoribbon electrodes are studied using first-principles quantum transport calculations. The effects of the symmetry and spin configuration of electrodes have been taken into account. It is found that large magnetoresistance, large spin polarization, dual spin-filtering, and negative differential resistance (NDR) can coexist in these devices. Our results show that 5Z-FePc system presents well conductive ability in both parallel (P) and anti-parallel (AP) configurations. For 6Z-FePc-P system, spin filtering effect and large spin polarization can be found. A dual spin filtering and NDR can also be shown in 6Z-FePc-AP. Our studies indicate that the dual spin filtering effect depends on the orbitals symmetry of the energy bands and spin mismatching of the electrodes. And all the effects would open up possibilities for their applications in spin-valve, spin-filter as well as effective spin diode devices.

Spin transport properties of partially edge-hydrogenated MoS2 nanoribbon heterostructure

We report ab initio calculations of electronic transport properties of heterostructure based on MoS2 nanoribbons. The heterostructure consists of edge hydrogen-passivated and non-passivated zigzag MoS2 nanoribbons (ZMoS2NR-H/ZMoS2NR). Our calculations show that the heterostructure has half-metallic behavior which is independent of the nanoribbon width. The opening of spin channels of the heterostructure depends on the matching of particular electronic orbitals in the Mo-dominated edges of ZMoS2NR-H and ZMoS2NR. Perfect spin filter effect appears at small bias voltages, and large negative differential resistance and rectifying effects are also observed in the heterostructure.

Large negative differential resistance and rectifying performance modulated by contact sites in fused thiophene trimmer-based molecular devices

Abstract By applying density functional theory with non-equilibrium Greenʼs function formalism, we have carried out a theoretical study of the electron transport in fused thiophene trimmer-based molecular devices with ethylene connections at three different sites. The simulation results indicate that the electronic transport properties strongly depend on the contact sites. Negative differential resistance and rectifying behaviors occur simultaneously in the current–voltage curves when ethylene connects the fused thiophene trimer at one second-nearest site and one third-nearest site. A larger negative differential resistance occurs only when ethylene connects the fused thiophene trimer at two second-nearest sites.

Large negative differential resistance and rectifying behaviors in isolated thiophene nanowire devices

We design isolated molecular nanowires composed of thiophene oligomers sandwiched between two one-dimensional gold electrodes. Electronic transport through the molecular junctions with two interface geometries is studied by performing the first principles calculations based on density functional theory and nonequilibrium Green's function. The current-voltage (I-V) curves of the molecular wires display an unexpected negative differential resistance and rectifying behaviors along with the oscillation effects, different from other theoretical and experimental studies about the analogous thiophene devices. The significant difference is attributed to the design of the one-dimensional gold electrodes with large enough vacuum layer in transverse direction in order to suppress the interaction between wires. Such transport behaviors indicate that the thiophene molecular device would be an important candidate in future molecular electronics.

Intrinsic region length scaling of heavily doped carbon nanotube p–i–n junctions

We investigated the dependence of the transport properties of heavily doped intratube single-walled carbon nanotube (SWCNT) p-i-n junctions on the length of the intrinsic region by using empirical self-consistent quantum transport simulations. When the length of the intrinsic region is scaled from a few angstroms to over 10 nanometers, the SWCNT p-i-n junction evolves from a tunneling diode with a large negative rectification and large negative differential resistance to one with a large positive rectification (like a conventional positive rectifying diode). The critical length of the intrinsic length is about 8.0 nm. Therefore, one can obtain nanoscale diodes of different performance types by changing the intrinsic region length.

First principles calculations of transport properties in Si nanowires: The role of crystal orientation

The electron transport properties of Si nanowires along four different orientations (〈110〉, 〈111〉, 〈100〉,〈112〉) are investigated by density functional theory and non-equilibrium Green’s function methods. It is found that electron transport property depends sensitively on the crystal orientation. The transmission probability of 〈110〉-oriented nanowires displays a smaller conductance gap than that of other directions. Orientation induced orbital overlap reflected from PDOS indicates that 〈110〉 wires have smaller band gap than those along other directions due to larger “electrode-molecule” overlap. The current–voltage characteristic confirms that 〈〉〈110〉-oriented nanowires present larger conductance at low bias than that of others. Moreover, a large negative differential resistance appears in 〈110〉 nanowires, which shows potential advantage as Si based nano-functional electronic device.

Electronic and transport properties of azobenzene monolayer junctions as molecular switches

We investigate from first principles the change in transport properties of a two-dimensional azobenzene monolayer sandwiched between two Au electrodes that undergoes molecular switching. We focus on transport differences between a chemisorbed and physisorbed top monolayer-electrode contact. The conductance of the monolayer junction with a chemisorbed top contact is higher in the trans configuration, in agreement with the previous theoretical predictions of one-dimensional single-molecule junctions. However, with a physisorbed top contact, the ON state, with larger conductance, is associated with the cis configuration due to a reduced effective tunneling pathway, which successfully explains recent experimental measurements on azobenzene monolayer junctions. A simple model is developed to explain electron transmission across subsystems in the molecular junction. We also discuss the effects of monolayer packing density, molecule tilt angle, and contact geometry on the calculated transmission functions. In particular, we find that a tip-like contact with chemisorption significantly affects the electric current through the cis monolayer, leading to highly asymmetric current-voltage characteristics as well as large negative differential resistance behavior.

Large Low Bias Negative Differential Resistance in an Endohedral Li@C60 Dimer Junction

By applying the nonequilibrium Green function formalism combined with density functional theory, we have investigated the electronic transport properties of the C60 dimer and its endohedral complex Li@C60 dimer. Our results show that the doping of Li atoms significantly changes the transport properties of the C60 dimer. Negative differential resistance is found in such systems. Especially, the doping of Li atoms can lead to a much larger negative differential resistance at much lower bias, and it is quite evident from the plot of differential conductance versus bias. The negative differential resistance behavior is understood in terms of the evolution of the transmission spectrum and projected density of states spectrum with applied bias combined with molecular projected self-consistent Hamiltonian states analyses.

A general formula for the transmission coefficient through a barrier and application to I–V characteristic

A general formula providing the transmission coefficient through a given barrier, sandwiched by semiconductor reservoirs under bias is presented in terms of the incoming carrier energy and the logarithmic wave function derivative at the start of the barrier. Furthermore, the formula involves the carrier effective masses in the barrier and reservoir regions. The procedure employed is based on solving an appropriate Riccati equation governing the logarithmic derivative along the barrier width at the end of which it is known in terms of the carrier energy and applied bias. On account of the facility provided for obtaining the transmission coefficient we obtained the I–V characteristic of a quantum dot carved barrier, which exhibits a region of quite a large negative differential resistance together with a high peak to valley ratio. Under the circumstances, the possibility of developing a nanostructure switch utilizing a small variation in the applied bias exists.

The peculiar transport properties in p-n junctions of doped graphene nanoribbons

Two kinds of junctions based on doped graphene nanoribbons (GNRs) are designed and studied in this article. One is the N-doped armchair GNR (AGNR) joined directly by B-doped AGNRs, and another is similar, but there is an undoped AGNR between them. The transport properties are calculated using the full self-consistent ab initio nonequilibrium Green’s function and density-functional theory methods under external bias. We find that the I-V curves for both junctions have a striking nonlinear feature and show large negative differential resistance properties, not only at the positive bias but also at the negative one. The results also indicate that the diode-like properties are kept and the rectification coefficient is very high within a wide bias region. Our calculations reveal that the formation of these peculiar transport behaviors is due to the great changes of the transmission spectra and the projected self-consistent Hamiltonian eigenvalues with the applied bias voltage. These findings suggest that the doped AGNRs may offer unique opportunities for the future development of nanoscale electronics.

Large negative differential resistance in a molecular device with asymmetric contact geometries: A first-principles study

We report a first-principles study of electronic transport properties and negative differential resistance (NDR) in a single molecular device consisting of a pyrene-based molecule sandwiched between two gold electrodes with different contact geometries. The results show that the electronic transport properties are strongly dependent on the contact geometry. The transmission coefficients and spatial distributions of molecular orbitals under various external biases voltage are analyzed, and it suggests that the asymmetry of the coupling between the molecule and the electrodes with external bias leads to NDR.

Transport properties of carbon atomic wire in the environment of H 2O molecules: An ab initio study

The transport properties of carbon atomic wire in the environment of H 2O molecules are studied by the non-equilibrium Green function method based on density functional theory. In particular, the carbon wire with seven atoms sandwiched between the Al(1 0 0) electrodes is considered. It is found that the transport properties are sensitive to the variation of the number and the position of the H 2O molecule adsorbed on the carbon wire. To our surprise, with different positions of a single H 2O molecule on the carbon wire, the equilibrium conductance shows an evident odd–even oscillatory behavior. For example, the equilibrium conductance of the carbon wire becomes bigger when the H 2O is adsorbed on the odd-numbered carbon atoms; an opposite conclusion is obtained for the H 2O adsorbed on the even-numbered carbon atoms. For the cases of two H 2O molecules, the equilibrium conductance varies largely and the contribution of the third eigenchannel becomes larger in some special configurations. The calculated current–voltage curves show different behavior with the variation of the positions of the H 2O molecules. In certain cases, large negative differential resistance (NDR) is shown, while in other cases, it only slightly deviates from the linear behavior. The above behavior is analyzed via the charge transfer and the density of states (DOS) and reasonable explanations are presented.

Effect of Gas Molecules on Resistance Switch Employing a Gold Nanogap Junction

We report the dependence of gas molecules on the resistance switch effect observed in a simple gold nanogap junction. A large negative differential resistance, which is a characteristic behavior of the current–voltage curve of the resistance switch in a vacuum, was observed even in argon and nitrogen gas environments. In contrast, the characteristic behavior was not observed in oxygen gas and air environments. These results indicate that oxygen and water molecules prevent the change in nanogap width and a nanogap device can be operated with inactive gas sealing.

Large Negative Differential Resistance in a Molecular Junction of Carbon Nanotube and Anthracene

We propose a novel molecular junction with single walled carbon nanotube (SWNT) as electrodes bridged by an anthracene molecule. It is found that when the coupling between the molecule and the SWNT is noncovalent, the current-voltage (I-V) curve shows a striking nonlinear feature and a large negative differential resistance (NDR) at small bias. While in covalent adsorption site, the I-V curve behaves nearly linear. Theoretical studies based on the nonequilibrium Green's function method demonstrate that mechanism of the NDR is due to the narrow features of the local density of states (LDOS) of the SWNT as well as the alignment between the peak of LDOS of the electrodes and the molecular energy levels.

Transport Properties of Binary Clusters

We present first-principles studies on the transport properties of small silicon and aluminium clusters: Al2, Si2, Al4 and AlSi sandwiched between two Al (100) electrodes. The variation of the equilibrium conductance as a function of contact distance for these two-probe systems is probed. Our results show that the transport properties are dependent on both the specific nanostructure and the separation distance between the central molecule and the electrodes. For equilibrium transport properties, the clusters with the similar structure show similar transmission spectra at large distances, the small difference can be explained by the electron filling. For current-voltage characteristics, all the clusters show the metallic behaviour at lower bias, however very different non-linear behaviour can be observed at higher bias. For AlSi and Al2, when the distance between the central cluster and the electrodes is 3.5 Å, large negative differential resistance (NDR) can be found in the bias range 0.8 V∼1.4 V.

First principles study on the effects of the H2O molecules on the transport properties of a carbon wire

We investigate the effects of H2O molecule environment on the transport properties of a seven-atom carbon wire coupled to two Al(100) electrodes based on a recently developed ab initio nonequilibrium Green function formalism. Our results show that the transport properties are sensitive to the variation of the number and the position of the H2O molecule on the carbon wire. Especially, the equilibrium conductance of the carbon wire with single H2O molecule exhibits an oscillatory behavior with the shift of positions of the H2O molecule. For the case of two H2O molecules, the contribution of the third eigenchannel becomes larger in some configurations. The calculated current-voltage curves show different behaviors with the variation of the positions of the H2O molecules. In certain cases, large negative differential resistance is found, but not in other cases.

Writing and erasing information in multilevel logic systems of a single molecule using scanning tunneling microscope

Large double negative differential resistance (NDR) has enabled a single quinone derivative to be stable in the neutral (0), singly reduced (1), typical neutral conformation generated by releasing the charge (2), and doubly reduced state (3) at 90K. The authors wrote and erased information by switching a single molecule among these four conducting states (0,1,2,3) showing random access memory and read only memory applications for terabit memory generation in future. Origin of NDR is investigated in typical functional groups of the molecule, in local environment, and at atomic junction with scanning-tunneling-microscope tip to conclude NDR as a collective phenomenon.

Negative differential resistance effect in organic devices based on an anthracene derivative

The authors observed a negative differential resistance (NDR) in organic devices consisting of 9,10-bis-(9,9-diphenyl-9H-fluoren-2-yl)-anthracene (DPFA) sandwiched between Ag and indium tin oxide electrodes. The large NDR shown in current-voltage characteristics is reproducible, resulting in that the organic devices can be electrically switched between a high conductance state (on state) and a low conductance state (off state). It can be found that the currents at both on to off states are space-charge limited and attributed to the electron traps at the Ag/DPFA interface. The large and reproducible NDR makes the devices of tremendous potential in low power memory and logic circuits.

Calculation of negative differential resistance in transport through benzene ring chains with NO 2 side-group

We describe a model, incorporating photon-assisted tunneling, certain localized states and bond-bending, to account for the observed transport characteristics between gold leads through a triple benzene molecule with NO 2 side-group (2′-amino-4-ethynylphenyl-5′-nitro-1-benzenethiolate). The model provides a natural explanation for the characteristic large negative differential resistance which emerges only when the NO 2 side-group is present.

Electron Transport Properties in a GaAs/AlGaAs Quantum Wire Grown on V-Grooved GaAs Substrate by Metalorganic Vapor Phase Epitaxy

A series of nonlinear conductance phenomena is investigated in the GaAs V-grooved quantum wire (QWR) field effect transistors (FETs) prepared by the flow-rate modulated epitaxy (FME) technique. These are attributed to Coulomb blockade at a subthreshold gate bias, universal conductance steps near the threshold, real space transfer under a forward gate bias and a large source-drain bias condition. Weak carrier coupling between sidewall quantum wells and QWR is responsible for the small step height of the measured universal conductance (Gm≪G0=2e2/h). Shubnikov de Haas oscillation measurements revealed that sidewall quantum wells in the V-groove quantum wire act as additional current paths and are switched or mixed with QWR depending on the gate bias conditions and device geometry. The gate-bias-dependent current path switching is found to be responsible for the large current steps and negative differential resistance (NDR) in the drain current (IDS)-gate bias (VGS) and IDS-drain bias (VDS) relationships, respectively.