First-principles study of the electronic transport properties of a C131 -based molecular junction

Using first-principles density functional theory and the non-equilibrium Green’s function formalism, we have studied the electronic transport properties of the dumbbell-like fullerene dimer C₁₃₁-based molecular junction. Our results show that the current-voltage curve displays an obvious negative differential resistance phenomenon in a certain bias voltage range. The negative differential resistance behavior can be understood in terms of the evolution of the transmission spectrum and the projected density of states with applied bias voltage. The present findings could be helpful for the application of the C₁₃₁ molecule in the field of single molecular devices or nanometer electronics.     

First-principle study of the electronic transport properties of a new dumbbell-like carbon nanocomposite

Using a first-principle density functional theory and non-equilibrium Green's function formalism for quantum transport calculation, we have investigated the electronic transport properties of a new dumbbell-like carbon nanocomposite, in which one carbon nanotube segment is capped with two C₆₀ fullerenes. Our results show that the current–voltage curve reveals a highly nonlinear feature. A negative differential resistance (NDR) behavior is obtained at a very low bias, which is expected to be helpful for the development of low bias NDR-based molecular devices. Moreover, the carbon nanotube length and fullerene type can affect the NDR behavior strongly. The electronic transport is analyzed from the transmission spectra and the molecular projected self-consistent Hamiltonian states under different applied biases.     

Electronic transport properties of zigzag carbon- and boron-nitride-nanotube heterostructures

Using first-principles density functional theory and non-equilibrium Green's function formalism for quantum transport calculation, we have investigated the electronic transport properties of heteronanotubes by joining a zigzag (6,0) carbon nanotube and a zigzag (6,0) boron nitride nanotube with different atomic compositions and joint configurations. Our results show that the atomic composition and joint configuration affect strongly the electronic transport properties. Obvious negative differential resistance behavior and large rectifying behavior are obtained in the heterostructure with certain composition and joint configuration. Moreover, tube length and tube radius can affect strongly the observed NDR and rectifying behaviors. The observed negative differential resistance and rectifying behaviors are explained in terms of the evolution of the transmission spectrum with applied bias combined with molecular projected self-consistent Hamiltonian states analysis.     

Transport properties and mechanism of C60 coupled to carbon nanotube electrode

By applying non-equilibrium Green's functions in combination with density-functional theory, we investigate electronic transport properties of C₆₀ coupled to carbon nanotubes and Li electrodes. The results show that electronic transport properties of CNT-C₆₀-CNT and Li-C₆₀-Li systems are completely different. Nonlinear I-V characteristic, varistor-type behavior and negative differential resistance (NDR) phenomenon are observed when electrodes are carbon nanotubes. We discuss the mechanism of I-V characteristics of CNT-C₆₀-CNT systems in details. Our results suggest conductance, energy level of Frontier molecular orbitals, energy gap between HOMO and LUMO, the coupling between molecular orbitals and electrodes are all playing critical roles in electronic transport properties.     

3D electrothermal simulations of organic LEDs showing negative differential resistance

Organic semiconductor devices show a pronounced interplay between temperature-activated conductivity and self-heating which in particular causes inhomogeneities in the brightness of large-area OLEDs at high power. We consider a 3D thermistor model based on partial differential equations for the electrothermal behavior of organic devices and introduce an extension to multiple layers with nonlinear conductivity laws, which also take the diode-like behavior in recombination zones into account. We present a numerical simulation study for a red OLED using a finite-volume approximation of this model. The appearance of S-shaped current–voltage characteristics with regions of negative differential resistance in a measured device can be quantitatively reproduced. Furthermore, this simulation study reveals a propagation of spatial zones of negative differential resistance in the electron and hole transport layers toward the contact.     

Effect of the encapsulation of Li atom on the electronic transport properties of C20F20 cage

Carrying out theoretical calculations using a self-consistent ab initio approach that combines the non-equilibrium Green′s function formalism with density functional theory, we investigate the effect of the center encapsulation of Li atom on the electronic transport properties of C₂₀F₂₀ cage sandwiched between two bulk gold electrodes. The results show that the electrical conductivity of the endohedral complex Li@C₂₀F₂₀ becomes better than that of the empty C₂₀F₂₀ in the bias voltages ranging from 0 to 1.2 V. The novel negative differential resistance behavior in the I-V characteristic curves can be observed by inserting Li atom into C₂₀F₂₀ cage. The mechanism for the negative differential resistance behavior of Li@C₂₀F₂₀ is suggested.     

Large negative differential resistance behavior in arsenene nanoribbons induced by vacant defects

We have studied the electronic structures of arsenene nanoribbons with different edge passivations by employing first-principle calculations. Furthermore, the effects of the defect in different positions on the transport properties of arsenene nanoribbons are also investigated. We find that the band structures of arsenene nanoribbons are sensitive to the edge passivation. The current-voltage characteristics of unpassivated and O-passivated zigzag arsenene nanoribbons exhibit a negative differential resistance behavior, while such a peculiar phenomenon has not emerged in the unpassivated and O-passivated armchair arsenene nanoribbons. The vacant defects on both top and bottom edges in unpassivated armchair arsenene nanoribbon can make its current-voltage characteristic also present a negative differential resistance behavior. After expanding the areas of the top and bottom defects in unpassivated armchair arsenene nanoribbon, the peak-to-valley ratio of the negative differential resistance behavior can be enlarged obviously, which opens another way for the application of arsenene-based devices with a high switching ratio.     

Theoretical investigation on current–voltage characteristics in all-carbon molecular device with different contact geometries

Applying nonequilibrium Green's functions in combination with the first-principles density-functional theory, we investigate electronic transport properties of an all-carbon molecular device consisting of one phenalenyl molecule and two zigzag graphene nanoribbons. The results show that the electronic transport properties are strongly dependent on the contact geometry and device's currents can drop obviously when the connect sites change from second-nearest sites from the central atom of the molecule (S site) to third-nearest sites from the central atom of the molecule (T site). More importantly, the negative differential resistance behavior is only observed on the negative bias region when the molecule connects the graphene nanoribbons through two T sites.     

Low-bias negative differential resistance in combined nanostructure of two zigzag-edged trigonal graphenes

Using the density functional theory combined with the non-equilibrium Green's function method, we have investigated the electron transport properties of combined nanostructures of two zigzag-edged trigonal graphenes linked by their vertex carbon atoms bridged between two gold electrodes. The results show that obvious negative differential resistance behavior can be obtained at low bias (0.3 V) in such combined systems. The observed low-bias negative differential resistance behavior is analyzed by the bias-dependent transmission spectra, projected density of states, and voltage drop.     

First-principles study of transport properties of endohedral Li@C20 metallofullerene

The transport properties of the endohedral Li@C₂₀ metallofullerene are studied using density functional non-equilibrium Green’s function method. The equilibrium conductance of Li@C₂₀ metallofullerene becomes larger than that of the empty C₂₀ fullerene molecule. The I–V curve under low-bias voltage shows the characteristic of metallic behavior; another, the novel negative differential resistance behavior is also observed. It is found that the doping effect of Li atom significantly changes the transport properties of C₂₀ fullerene.     

Negative differential resistance in the unsymmetrical C121-based molecular junction

Using first-principles density functional theory and non-equilibrium Green?s function formalism for quantum transport calculation, we have investigated the electronic transport properties of the unsymmetrical C₁₂₁-based molecular junction. Our results show that the current-voltage curve displays a negative differential resistance phenomenon in a certain bias voltage range. The mechanism for the negative differential resistance phenomenon is suggested. The present findings could be helpful for the application of the C₁₂₁ molecule in the field of single molecular devices or nanometer electronics.     

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

By applying density functional theory with non-equilibrium Green?s 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.     

Contact transparency inducing negative differential resistance in nanotube–molecule–nanotube junction predicted by first-principles study

We construct a molecular junction where propyl contacts two armchair carbon nanotubes through five-member ring and perform the first-principles calculations of its transport properties. The negative differential resistance effect with peak-to-valley ratio of 700% is present. Our investigations indicate that contact transparency can induce negative differential resistance in nanotube–molecule–nanotube junction, which may promise the potential application in nano-electronics devices in the future.     

硼氮原子取代掺杂对分子器件负微分电阻效应的影响

利用基于非平衡格林函数和密度泛函理论相结合的第一性原理计算方法,研究了硼氮原子取代掺杂对三并苯分子电子输运性质的影响.计算结果表明,三并苯分子器件的电流在特定偏压区间内随电压的增加而减小呈现出负微分电阻效应,电流的峰谷之比高达5.12.用硼原子或者氮原子取代分子的中心原子后,器件0.8V以内的电流明显增加,但是负微分电阻效应减弱,相应的电流峰谷比分别降至3.83和3.61.分析认为,输运系数在特定偏压下的移动是器件负微分电阻效应的主要成因.核外电子数的差异导致硼氮原子掺杂取代可以使器件轨道及其透射峰分别向高能方向或者低能方向移动从而有效地调控了器件的低偏压下的电子传输能力和负微分电阻效应.     

水分子对碳链的输运性质影响的第一性原理研究

用基于密度泛函理论的非平衡格林函数方法研究了水分子对7个碳原子组成的一维原子链的输运性质的影响.碳原子链放在具有有限截面的Al(100)电极中.研究发现，碳原子链上的水分子的数目和放置的位置的不同将对体系输运性质产生很大的影响.特别是，单个H₂O分子对碳链平衡电导的影响随其摆放位置的不同而出现奇偶振荡，例如，当位于奇数编号的碳原子上时，电导取极大值，当位于偶数编号的碳原子上时，取极小值.将两个H₂O分子置于不同的碳原子正上方时，在不同的位置平衡电导相差很大，在某些特殊的情况下原本受到抑止的第三个本征通道也有较大的贡献.此外，还研究了放置两个水分子时，体系的电流-电压(I-V)特性，随着水分子的数目和放置的位置不同，某些情况可能出现较大幅度的负微分电阻，而在另一些情况下却没有出现. 关键词： 平衡电导 透射谱 负微分电阻     

Transport properties of carbon atomic wire in the environment of H2O molecules: An ab initio study

The transport properties of carbon atomic wire in the environment of H₂O 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 H₂O molecule adsorbed on the carbon wire. To our surprise, with different positions of a single H₂O 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 H₂O is adsorbed on the odd-numbered carbon atoms; an opposite conclusion is obtained for the H₂O adsorbed on the even-numbered carbon atoms. For the cases of two H₂O 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 H₂O 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.     

Electronic transport through a C60– n X n (X=N and B) molecular bridge

By applying the non-equilibrium Green's function (NEGF) technique, the Landauer–Buttiker theory and the Fisher–Lee formula, we have investigated the transport behavior of a C_{60– n} X _n (X=N, B) molecule coupled to two semi-infinite SWCNT electrodes. In this study, the coupling through the carbon, boron and nitrogen atoms to the electrodes will be considered. We study the effects of different contact geometries, the electron–phonon interaction and the number of doped atoms on the current value and negative differential resistance (NDR) behavior of C_{60– n} X _n . Our results indicate that the transmission coefficient and the NDR behavior of C_{60– n} X _n vary on changing n and X. Moreover, NDR behavior is observed in C_{60– n} X _n with different contacts and in C₆₀ with C₅ and C₆ contacts. C_{60– n} X _n molecules are suggested for the operation of devices with a nanoscale current.     

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.     

Effects of the surface layer of La2/3Ca1/3MnO3 thin films

The surface layer effects on transport in epitaxial La_2/3Ca_1/3MnO₃ thin films are studied. It was found that the two-probe resistance is nonlinear which is enhanced with decreasing temperature. Similar to the resistance of intrinsic La_2/3Ca_1/3MnO₃ thin films reported in the literature, the apparent dynamic contact resistance behaves semiconducting at high temperatures, passes through a peak, and displays a metallic behavior. At lowest temperatures, the curve of the contact resistance versus temperature shows a little upturn. The temperature dependent work function difference between the surface layer and the thin film underneath, together with the tunneling process across either the resulting charge depleted layer or the semiconducting surface layer is used to explain our observations.     

Negative Differential Resistance in Atomic Carbon Chain-Graphene Junctions

We investigate the electronic transport properties of atomic carbon chain-graphene junctions by using the density-functional theory combining with the non-equilibrium Green's functions. The results show that the transport properties are sensitively dependent on the contact geometry of carbon chain. From the calculated I-V curve we find negative differential resistance (NDR) in the two types of junctions. The NDR can be considered as a result of molecular orbitals moving related to the bias window.