Heat transport in bulk/nanoporous/bulk silicon devices

We study heat transport in bulk/nanoporous/bulk silicon devices; we show that, despite bulk/nanoporous devices may act as thermal rectifiers, the non-linear aspects of their joint thermal conductance are not strong enough to lead to a negative differential thermal resistance, necessary to allow bulk/nanoporous/bulk Si devices to act as thermal transistors. Furthermore, we explicitly study the effective thermal conductivity of the mentioned devices for several temperatures, geometries, porosities, and pore size.     

The p-type ZnO thin films obtained by a reversed substitution doping method of thermal oxidation of Zn_3N_2 precursors

P-type ZnO is crucial for the realization of ZnO-based homojunction ultraviolet optoelectronic devices. The problem associated with the preparation of stable p-type ZnO with high hole density still hinders device applications. In this paper,we introduce an alternative route to stabilizing N in the oxidation process, the thermal stability of p-ZnO is significantly improved. Finally, we discuss the limitations of the alternative doping method and provide some prospective outlook of the method.     

Barrier asymmetry and the mm-wave performance of resonant tunnel diodes

The effects of barrier asymmetry in a resonant tunnel diode (RTD) on the frequency response of the negative dynamical resistance are described for (i) DC biasing in the positive differential resistance (PDR) region and (ii) DC biasing in the negative differential resistance (NDR) region. Collector barriers more transparent than emitter barriers enhance performance for NDR DC-biased devices. Asymmetry has no apparent effect for PDR DC-biased devices operating in transit time mode.     

以石墨烯为电极的有机噻吩分子整流器的设计及电输运特性研究

传统硅基半导体器件受到了量子尺寸效应的限制,发展分子电子学器件有可能解决这一难题.本文提出了由石墨烯电极和有机噻吩分子相结合构造分子器件的思想,建构了"石墨烯-噻吩分子-石墨烯"结构的分子器件,并运用非平衡态格林函数结合密度泛函理论的方法研究了其电输运特性.系统地分析了电子给体"氨基"和电子受体"硝基"两种取代基的位置对有机噻吩分子电输运的影响.计算表明,有机噻吩二聚物被"氨基"和"硝基"取代后会产生明显的负微分电阻效应和整流效应.进一步对产生这些效应的物理机制进行分析,发现氨基的位置可以调整负微分电阻的强弱,硝基的位置可以改变整流的方向.     

The electronic transport properties in C60 molecular devices with different contact distances

By applying non-equilibrium Green?s functions in combination with the density-functional theory, we investigate the transport behavior of molecular devices composed by metal electrode-C₆₀ molecule-metal electrode. Our results show that the electronic transport properties are affected obviously by the different contact distances between the electrodes, and the tunneling current decreases approximately exponentially at a certain bias with the increasing of contact distances. The negative differential resistance is observed and the peak-to-valley ratio can be tuned by different contact distances. The mechanisms of the contact distance effect and the negative differential resistance behavior are proposed.     

Managing Quantum Heat Transfer in a Nonequilibrium Qubit-Phonon Hybrid System with Coherent Phonon States

We investigate quantum heat transfer in a nonequilibrium qubit-phonon hybrid open system,dissipated by external bosonic thermal reservoirs.By applying coherent phonon states embedded in the dressed quantum master equation,we are capable of dealing with arbitrary qubit-phonon coupling strength.It is counterintuitively found that the effect of negative differential thermal conductance is absent at strong qubit-phonon hybridization,but becomes profound at weak qubit-phonon coupling regime.The underlying mechanism of decreasing heat flux by increasing the temperature bias relies on the unidirectional transitions from the up-spin displaced coherent phonon states to the down-spin counterparts,which seriously freezes the qubit and prevents the system from completing a thermodynamic cycle.Finally,the effects of perfect thermal rectification and giant heat amplification are unraveled,thanks to the effect of negative differential thermal conductance.These results of the nonequilibrium qubit-phonon open system would have potential implications in smart energy control and functional design of phononic hybrid quantum devices.     

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.     

Tuning spin-filtering,rectifying, and negative differential resistance by hydrogenation on topological edge defects of zigzag silicene nanoribbons

By first-principles calculations, we propose three heterojunction nanodevices based on zigzag silicene nanoribbons with different edge-hydrogenated topological line defects. The devices all present excellent spin-filtering properties with 100% spin polarization as well as remarkable rectifying effect (with rectification ratio around 10²) and negative differential resistance behaviors. Our findings shed new light on the design of silicon-based nanodevices with intriguing spintronic applications.     

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.     

B/N掺杂类直三角石墨烯纳米带器件引起的整流效应

基于密度泛函理论结合非平衡格林函数的方法, 研究了硼(氮)非对称掺杂类直三角石墨烯纳米带器件的电子输运性能. 计算结果表明: 单个硼或氮原子取代类直三角石墨烯纳米带顶点的碳原子后, 增强了体系的电导能力, 并且出现了新颖的整流效应. 分析表明: 这是由于硼氮掺杂类直三角石墨烯纳米带器件在正负偏压下分子能级的移动方向和前线分子轨道空间分布的不对称而产生的. 最重要的是, 当左右类直三角石墨烯纳米带的顶端原子同时被硼和氮掺杂后, 体系的整流效应显著增强, 而且出现负微分电阻效应.     

Utilizing NDR effect to reduce switching threshold variations in memristive devices

Variations in the switching threshold voltage of memristive devices present significant challenges for their integration into large-scale circuits. In this paper, we propose to address this problem by adding a device exhibiting S-type (N-type) negative differential resistance (NDR) in series (parallel) with memristive devices. The main effect comes from the transition between low- and high-conductivity branches of the NDR device, which leads to a redistribution of the voltage drop inside the device stack, and, as a result, the effective lowering of variations in the switching threshold. The idea is checked experimentally using a TiO_2?x memristive device connected in parallel with a tunnel GaAs diode.     

Engineering of carbon-based superlight spin filter with negative differential resistance

Based on density functional theory and non-equilibrium Green's function, we investigate the edge hydrogenation and oxidation effects on the spin transport of devices consisting of a zigzag C₂N nanoribbon (ZC₂NNR) embedded in zigzag graphene nanoribbons in parallel (P) and antiparallel (AP) spin configurations. The results show that device with edge hydrogenation exhibits dual spin filtering effect in AP spin configuration and obvious negative differential resistance in both P and AP spin configuration. By substituting oxygen for hydrogen as passivation atoms of ZC₂NNR, the spin filtering efficiency is as high as 100% in the P spin configuration, and the negative differential resistance is largely enhanced with a peak to valley ratio in excess of 4×10³. Our theoretical studies suggest that zigzag C₂N nanoribbon modulated by edge substitution has great potential in the design of future multifunctional spin devices.     

Large magnetoresistance and perfect spin filter effect in Fe doped SnS2 half-metallic monolayers: A first principles study

In this work, the spin-resolved transport properties of the Fe doped SnS₂ monolayer devices have been investigated. The results show that the doping systems have the transport properties of negative differential resistance, large magnetoresistance effect and near 100% spin polarization effect when the magnetization directions of two electrodes are in parallel. Moreover, the mechanisms for the properties also have been discussed. The original reason of these results could be due to the half-metallic of the doping system. Our results imply that Fe doped SnS₂ monolayer is a promising candidate for the future spintronic devices.     

Self-assembled and electrochemically deposited mono/multilayers for molecular electronics applications

For the development of molecular electronics, it is desirable to investigate characteristics of organic molecules with electronic device functionalities. In near future, such molecular devices could be integrated with silicon to prepare hybrid nanoelectronic devices. In this paper, we review work done in our laboratory on study of characteristics of some functional molecules. For these studies molecular mono and multilayers have been deposited on silicon surface by self-assembly and electrochemical deposition techniques. Both commercially available and specially designed and synthesized molecules have been utilized for these investigations. We demonstrate dielectric layers, memory, switching, rectifier and negative differential resistance devices based on molecular mono and multilayers.     

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.     

Physical mechanism of negative differential conductance in substrateless metallic carbon nanotubes

The existence of pronounced negative differential conductance at room temperature in suspended metallic carbon nanotubes was recently proven. We investigate here the physical nature of this phenomenon, which is of considerable importance for high-frequency devices, such as oscillators working up to few hundreds of GHz. Besides previous explanations, we find a new physical mechanism that explains the negative differential conductivity at room temperature. The entire suspended metallic carbon nanotube behaves as a very large quantum well, the negative differential conductance occurring due to the depletion of carriers on high-energy resonant levels.     

Spin transport properties in Fe-doped graphene/hexagonal boron-nitride nanoribbons heterostructures

By combining non-equilibrium Green's function (NEGF) with density functional theory (DFT), we systematically study the spin-related transport properties of the heterostructures composed of graphene and hexagonal boron-nitride (h-BN) when the metal Fe is doped different positions of the heterostructures interface. The results show that the heterostructures exhibit obvious spin-filtering effect (SFE) and negative differential resistance (NDR) due to the different absorbing positions of the metal Fe. And the spin filtering ratio can reach more than 90% in a specific bias voltage range. Moreover, spin-rectifying behaviors are detected in the heterostructures. Whether it is for the design of multifunctional devices or the synthesis of spintronic devices, these findings will have some reference value.     

Switching behavior of current filaments inp-germanium connected in parallel

Impact ionization breakdown in extrinsic germanium at low temperatures as a phase transition of a non-conducting to a conducting state is associated with a region of negative differential resistance, typically resulting in a current-voltage characteristic of multiple hystereses and switching between different conducting states. As an experimental model, we investigate the parallel combination of two devices each with a perfectlyS-shaped characteristic. The behavior of this simple combination can serve for understanding the complexity of more general cases. Further, we observed an interesting switching behavior between both devices which cannot be understood as an instability of the individual devices or in terms of a local stability analysis of the coupled system.     

Conductivity of carbon-based molecular junctions from ab-initio methods

Carbon nanomaterials (CNMs) are prompting candidates for next generational electronics. In this review we provide a mini overview of recent results on the conductivity of carbon-based molecular junctions obtained from ab-initio methods. CNMs used as nanoelectrodes and molecular materials in molecular junctions are discussed. The functionalities that include the nanomechanically controlled molecular conductance switches, negative differential resistance devices, and electronic rectifiers realized by using CNMs have been demonstrated.     

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.