Mechanism for negative differential resistance in molecular electronic devices: local orbital symmetry matching

A new mechanism for negative differential resistance (NDR) originating from local orbital symmetry matching between an electrode and a molecule in a single molecular electronic device is proposed and demonstrated by a joint experimental and theoretical scanning tunneling microscope study of a cobalt phthalocyanines (CoPc) molecule on a gold substrate. For two different metal tips used, Ni and W, NDR occurs only with Ni tips and shows no dependence on the geometrical shape of the tip. Calculations reveal that such a behavior is a result of local orbital symmetry matching between the Ni tip and Co atom.     

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.     

金属-绝缘体-金属隧道发光结的电子隧穿和负阻现象

薄膜Au-Al₂O₃-Al隧道结(MIMTJ)在产生可见光发射的同时表现出了明显的负阻现象.这种负阻现象的物理机制是由于结中产生了作为发光中介作用的表面等离极化激元(SPP)对隧穿电子的阻挡作用.通过MIMTJ的电子输运的电路模拟和I-V特性的数值计算，揭示了SPP在I-V特性曲线中的负阻、隧道结发光中所起的关键作用. 关键词：     

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.     

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.     

Investigation of an AlInAs/GaInAs superlattice-emitter resonant tunneling bipolar transistor (SE-RTBT)

A new bipolar transistor with a 20-period i-AlInAs/n⁺-InGaAs superlattice, prepared by metal organic chemical vapour deposition, has been fabricated and demonstrated. This superlattice is used as a confinement barrier for holes and a resonant tunneling (RT) route for electrons. Due to the RT effect within the 20-period superlattice near emitter–base p–n junction region, the N-shaped negative-differential-resistance (NDR) phenomena are observed under normal operation. A transistor action with a common-emitter current gain of 13 and a small offset voltage (90 mV) is achieved at room temperature.     

Controlling single-molecule negative differential resistance in a double-barrier tunnel junction

We observed the transition from negative differential resistance (NDR) to the absence of NDR in the differential conductance (dI/dV) spectra of single copper-phthalocyanine (CuPc) molecules adsorbed on one, two, and three atomic layers of NaBr grown on a NiAl(110) substrate. Through numerical simulation, this transition is attributed to two phenomena in the double-barrier tunnel junction: (i) the opposite bias dependence of the vacuum and NaBr barrier heights, and (ii) the changing barrier widths for CuPc molecules adsorbed on different layers of NaBr.     

Negative differential resistance in ZnO coated peptide nanotube

We investigate the room temperature electronic transport properties of a zinc oxide (ZnO) coated peptide nanotube contacted with Au electrodes. Current–voltage (I–V) characteristics show asymmetric negative differential resistance (NDR) behavior along with current rectification. The NDR phenomenon is observed in both negative and positive voltage sweep scans, and found to be dependent on the scan rate and humidity. Our results suggest that the NDR is due to protonic conduction arising from water molecule redox reaction on the surface of ZnO coated peptide nanotubes rather than the conventional resonant tunneling mechanism.     

The operation of THz quantum cascade laser in the region of negative differential resistance

We investigate the light-current-voltage characteristics and emission spectra of 2.3 THz quantum cascade laser operating in the negative differential resistance (NDR) region. It was shown that the formation of electric field domains (EFDs) leads to a large number of discontinuities on the current-voltage and the total optical power on current characteristics. Measurements of emission spectra at different current (before the NDR region and in the NDR region) shows that the formation of EFDs results in decrease of the output intensity, but does not influence on Fabry-Perot multi-mode structure of THz QCL. The developed theoretical model predicts the formation of EFDs in the NDR region and qualitatively explain the experimental results.     

A single molecule switch based on two Pd nanocrystals linked by a conjugated dithiol

Tunneling spectroscopy measurements have been carried out on a single molecule device formed by two Pd nanocrystals (dia. ∼5 nm) electronically coupled by a conducting molecule, dimercaptodiphenylacetylene. TheI-V data, obtained by positioning the tip over a nanocrystal electrode, exhibit negative differential resistance (NDR) on a background M-I-M characteristics. The NDR feature occurs at ∼0.67 V at 300 K and shifts to a higher bias of 1.93 V at 90 K. When the tip is held in the middle region of the device, a Coulomb blockade region is observed (±∼0.3 V)     

Fabrication of heterostructure-emitter bipolar transistor with a doping-superlattice collector

Grown by molecular beam epitaxy, a sawtooth-doping-superlattice (SDS) structure has been employed in the collector region of a heterostructure-emitter bipolar transistor. For the studied structure, conventional transistor behavior and controllable S-shaped negative-differential-resistance (NDR) performance were simultaneously achieved. First, due to the avalanche multiplications within SDS periods or emitter-base p-n junction depletion region, a bi-directional switching phenomenon was exhibited under the two-terminal operation. In addition, when a base current or bias was applied, transistor properties and a controllable S-shaped NDR family in the large current regime were obtained. The best common-emitter current gain was 25.     

Large negative differential resistance in graphene nanoribbon superlattices

A graphene nanoribbon superlattice with a large negative differential resistance (NDR) is proposed. Our results show that the peak-to-valley ratio (PVR) of the graphene superlattices can reach 21 at room temperature with bias voltages between 90–220 mV, which is quite large compared with the one of traditional graphene-based devices. It is found that the NDR is strongly influenced by the thicknesses of the potential barrier. Therefore, the NDR effect can be optimized by designing a proper barrier thickness. The large NDR effect can be attributed to the splitting of the gap in transmission spectrum (segment of Wannier–Stark ladder) with larger thicknesses of barrier when the applied voltage increases.     

Study on NDR property of nitro-Benzene SAMs by using UHV-STM

Recently, research on conducting molecules containing thiol functional groups, such as benzenethiol has been progressing. This conducting molecule is applicable to the study of NDR and switching properties of logic devices. The 4,4′-di(ethynylphenyl)-2′-nitro-1-(thioacetyl)benzene molecule contains a thiol functional group as in benzenethiol. Thus, we measured the property of NDR by using the self-assembly method in STM. Au substrate was exposed to a 1 mM solution of 1-dodecanethiol in ethanol for 24 h to form a monolayer. After thoroughly rinsing the sample, it was exposed to a 0.1 mM solution of 4,4′-di(ethynylphenyl)-2′-nitro-1-(thioacetyl)benzene in dimethylformamide (DMF) for 30 min and kept in the dark during immersion to avoid photo-oxidation. After assembly, we measured the electrical properties of self-assembly monolayers (SAMs) by using ultra-high-vacuum scanning tunneling microscopy (UHV-STM) and scanning tunneling spectroscopy (STS) [L.A. Bumm, J.J. Arnold, M.T. Cygan, T.D. Dunbar, T.P. Burgin, L. Jones ll, D.L. Allara, J.M. Tour, P.S. Weiss, Science 271 (1996) 1705]. As a result, we confirmed the property of NDR in a positive region between negative regions. The energy gap (E_g) obtained by using differential conductance (dI/dV–V) was verified through UV/visible. This molecule is applicable to the fabrication of molecular junctions.     

Negative differential resistance of a metal–insulator–metal device with gold nanoparticles embedded in polydimethylsiloxane

Metal–insulator–metal (MIM) devices play an important role in information storage cells. In this research, a MIM with an insulator made from polydimethylsiloxane blended with gold nanoparticles has been investigated. The current–voltage characteristic demonstrates a negative differential resistance (NDR) and memory effect. This article attempts to explain the NDR and memory effect, using the charge trapping and releasing mechanisms of the gold nanoparticles and also electron tunneling mechanisms.     

描述光伏效应的新解析模型

 对陆启生等人提出的描述光伏效应的解析模型涉及的边界条件进行了讨论，提出了一个适用性更宽的解析模型。通过对新模型、陆的模型以及另一个解析模型的比较，对前两个模型能够描述光伏型光电探测器在强光辐照时的信号饱和效应的原因进行了解释。     

Multiple spin-resolved negative differential resistance and electrically controlled spin-polarization in transition metal-doped [6]cycloparaphenylenes

In structure, a [n]cycloparaphenylene ([n]CPP) molecule is constructed by fully conjugated bent benzenes, i.e., hexangular rings. Based on first-principles calculations, the spin-dependent electronic transport of transition metal-doped CPP, X@[6]cycloparaphenylene (X@[6]CPP) (X?=?Fe, Co and Ni), contacted with Au electrodes is investigated. (Multiple) negative differential resistance (NDR) is observed for all the doping cases, suggesting it is the intrinsic feature of such systems. Due to the spin dependence of the NDR, electrical switch of the direction of spin polarization for a current is realized. Further analysis shows that it is the suppression of the transmission peaks around the Fermi level as the bias increases that results in the NDR. The suppression is caused by the decay of the local density of states on the scattering region. As electrically controlled spin polarization is a promising area in nanoelectronics, we believe our results would be quite beneficial to the development of spintronic devices.     

1,8-二巯基芘分子电学特性的理论研究

在第一性原理的基础上 ,对 1,8 二巯基芘分子的电学特性进行了理论研究 .采用了 3个Au原子构成的团簇来模拟Au表面 .首先利用密度泛函理论计算了 1,8 二巯基芘分子的电子结构及其和Au表面的相互作用 ,再利用前线轨道理论和微扰理论定量地确定了该分子和Au表面的相互作用能常数 .最后利用弹性散射格林函数法研究了该分子结的伏 安特性 .计算结果表明 ,分子中的硫原子和Au原子形成很强的共价键 .当外加偏压小于 1V时分子结存在电流禁区 ,随着偏压升高 ,分子结的电导出现平台结构 .分子结的电导特性和其电子结构密切相关 ,扩展分子轨道为电荷的迁移提供了通道 ,而局域轨道对电流贡献很小     

Ab initio calculation of transport properties in 1,3-diphenylpropynylidene based molecular device

Using an ab initio method based on non-equilibrium Green’s functions (NEGF) combined with density functional theory (DFT), a calculation of the transport properties of a single molecular junction based on 1,3-diphenylpropynylidene (PhC₃Ph) ‘radical-π-radical’ is performed. The obvious negative differential resistance (NDR), spin current polarisation (SCP) and dual-spin current rectification (SCR) effects in this device are obtained. The total current for magnetic parallel configuration (PC) is larger at first and then less than that for magnetic antiparallel configuration (APC) as the bias increases, which suggests the abnormal magnetoresistance (MR) effect and can be used as a molecular switch with two working voltages. The evolution of the spin-polarised transmission spectrums and the frontier molecular orbitals (MOs) with applied bias is used to explain the above interesting results. Our calculations may be helpful for designing multifunctional molecular spintronics devices in the future.     

DFT-NEGF study of transport properties and NDR behavior in fused furan and thiophene dimmers

The nonequilibrium Green's function approach in combination with density-functional theory is used to perform quantum mechanical calculations of the electron transport properties of furan and thiophene dimmers. Both the molecular systems have two S-linker and translated into the Gold junction with (1 1 1) surfaces. The studied molecular junctions at zero bias voltage are HOMO-based junctions and currents through these systems are driven by hole transport. The current–voltage characteristics of the both studied molecular junctions illustrate that negative differential resistance (NDR) feature is observed over the bias voltage of 2.0 V. Higher conductivity of fused furan dimmer and NDR character have been explained by the monitoring of the transmission resonance peak across the bias window against varying bias voltages.     

Effective interactions in molecular dynamics simulations of lysozyme solutions

On the basis of ab-initio calculations, we predict the effect of conformation and molecule-electrode distance on transport properties of asymmetric molecular junctions for different electrode materials M (M = Au, Ag, Cu, and Pt). The asymmetry in these junctions is created by connecting one end of the biphenyl molecule to conjugated double thiol (model A) and single thiol (model B) groups, while the other end to Cu atom. A variety of phenomena viz. rectification, negative differential resistance (NDR), switching has been observed that can be controlled by tailoring the interface state properties through molecular conformation and molecule-electrode distance for various M. These properties are further analyzed by calculating transmission spectra, molecular orbitals, and orbital energy. It is found that Cu electrode shows significantly enhanced rectifying performance with change in torsion angles, as well as with increase in molecule-electrode distances than Au and Ag electrodes. Moreover, Pt electrode manifests distinctive multifunctional behavior combining switch, diode, and NDR. Thus, the Pt electrode is suggested to be a good potential candidate for a novel multifunctional electronic device. Our findings are compared with available experimental and theoretical results.