介观系统中的电导量子化

对介观系统电子输运性质的研究是当前凝聚态物理研究中的活跃领域。介观尺度下,由于量子力学效应,出现了一些特殊的物理现象,如电导量子化。本文从理论和实验两方面对介观输运领域的研究情况作了简要回顾,介绍了一些较新的研究成果,并讨论了电导量子化在下一代集成电路中可能的应用方向。     

Dynamic Stability of Organic Conducting Polymers and Its Replication in Electrical Conduction and Degradation Mechanisms

The evolving usefulness of organic conducting polymers, of metallic or semiconducting type, is primarily dependent on their mechanisms of electrical conduction and degradation. Understanding these mechanisms is crucial for improving the efficiency and lifetime of technologies derived from this class of polymers. There is demand for a model that provides a vivid and more precise evaluation of the electrical conduction mechanism in these polymers – especially when they act as hosts to guest species, such as acid dopant ions and nanoparticles. If, for example, the motional behavior of a host–guest organic conducting polymer structure, as related to dynamic stability, is either asynchronous or synchronous, is this reflected in the mechanism of electrical conduction and does it account for the pace of material's degradation? Here, we demonstrate that the answer is affirmative: asynchronous structural motions arising due to loosely bound or free guest species within the host polymer lead to anomalous electrical conduction mechanisms, increased fragility and short lifetime, at odds with the synchronous behavior.     

Tuning Electrical Conductance of Serpentine Single‐Walled Carbon Nanotubes

Changes in resistivity of serpentine single‐walled carbon nanotubes are presented as a function of bending radius, r_b, in the range of 100–2000 nm. Resistivity (ρ) is observed to increase with curvature (1/r_b), which is consistent with theoretical speculation on strain‐induced bandgap increment. Furthermore, a sharp bend (r_b < 50nm) in the nanotubes results in a drastic change in the field‐effect behavior, i.e., from ambipolar to p type across the bend. Local Raman spectra show that the G‐band Raman frequencies shift along the curvature, which may be attributed to local deformation and broken cylindrical symmetry in the nanotubes. The results suggest the possibility to tune the electrical properties by bending nanotubes and to build an all‐nanotube device by modulating the structure of the same tube.     

Open‐Shell Donor–Acceptor Conjugated Polymers with High Electrical Conductivity

Conductive polymers largely derive their electronic functionality from chemical doping, processes by which redox and charge‐transfer reactions form mobile carriers. While decades of research have demonstrated fundamentally new technologies that merge the unique functionality of these materials with the chemical versatility of macromolecules, doping and the resultant material properties are not ideal for many applications. Here, it is demonstrated that open‐shell conjugated polymers comprised of alternating cyclopentadithiophene and thiadiazoloquinoxaline units can achieve high electrical conductivities in their native “undoped” form. Spectroscopic, electrochemical, electron paramagnetic resonance, and magnetic susceptibility measurements demonstrate that this donor–acceptor architecture promotes very narrow bandgaps, strong electronic correlations, high‐spin ground states, and long‐range π‐delocalization. A comparative study of structural variants and processing methodologies demonstrates that the conductivity can be tuned up to 8.18 S cm^?1. This exceeds other neutral narrow bandgap conjugated polymers, many doped polymers, radical conductors, and is comparable to commercial grades of poly(styrene‐sulfonate)‐doped poly(3,4‐ethylenedioxythiophene). X‐ray and morphological studies trace the high conductivity to rigid backbone conformations emanating from strong π‐interactions and long‐range ordered structures formed through self‐organization that lead to a network of delocalized open‐shell sites in electronic communication. The results offer a new platform for the transport of charge in molecular systems.     

双量子环的隧穿电导和隧穿磁电阻

研究了双量子环的隧穿电导和隧穿磁电阻,研究结果表明:隧穿电导和隧穿磁电阻都随半导体环增大做周期性等幅振荡。δ势垒、Rashba自旋轨道耦合、AB磁通对隧穿电导或隧穿磁电阻的影响各不相同。隧穿磁电阻随AB磁通增强发生周期性等幅振荡,并随φ0角的减小而增大。两电极磁矩方向反平行时,隧穿磁电阻恒为零。     

Control of Switching Modes and Conductance Quantization in Oxygen Engineered HfOx based Memristive Devices

Hafnium oxide (HfO_x)‐based memristive devices have tremendous potential as nonvolatile resistive random access memory (RRAM) and in neuromorphic electronics. Despite its seemingly simple two‐terminal structure, a myriad of RRAM devices reported in the rapidly growing literature exhibit rather complex resistive switching behaviors. Using Pt/HfO_x/TiN‐based metal–insulator–metal structures as model systems, it is shown that a well‐controlled oxygen stoichiometry governs the filament formation and the occurrence of multiple switching modes. The oxygen vacancy concentration is found to be the key factor in manipulating the balance between electric field and Joule heating during formation, rupture (reset), and reformation (set) of the conductive filaments in the dielectric. In addition, the engineering of oxygen vacancies stabilizes atomic size filament constrictions exhibiting integer and half‐integer conductance quantization at room temperature during set and reset. Identifying the materials conditions of different switching modes and conductance quantization contributes to a unified switching model correlating structural and functional properties of RRAM materials. The possibility to engineer the oxygen stoichiometry in HfO_x will allow creating quantum point contacts with multiple conductance quanta as a first step toward multilevel memristive quantum devices.     

Nanomorphology and Charge Generation in Bulk Heterojunctions Based on Low‐Bandgap Dithiophene Polymers with Different Bridging Atoms

Carbon bridged (C‐PCPDTBT) and silicon‐bridged (Si‐PCPDTBT) dithiophene donor–acceptor copolymers belong to a promising class of low bandgap materials. Their higher field‐effect mobility, as high as 10^?2 cm² V^?1 s^?1 in pristine films, and their more balanced charge transport in blends with fullerenes make silicon‐bridged materials better candidates for use in photovoltaic devices. Striking morphological changes are observed in polymer:fullerene bulk heterojunctions upon the substitution of the bridging atom. XRD investigation indicates increased π–π stacking in Si‐PCPDTBT compared to the carbon‐bridged analogue. The fluorescence of this polymer and that of its counterpart C‐PCPDTBT indicates that the higher photogeneration achieved in Si‐PCPDTBT:fullerene films (with either [C60]PCBM or [C70]PCBM) can be correlated to the inactivation of a charge‐transfer complex and to a favorable length of the donor–acceptor phase separation. TEM studies of Si‐PCPDTBT:fullerene blended films suggest the formation of an interpenetrating network whose phase distribution is comparable to the one achieved in C‐PCPDTBT:fullerene using 1,8‐octanedithiol as an additive. In order to achieve a balanced hole and electron transport, Si‐PCPDTBT requires a lower fullerene content (between 50 to 60 wt%) than C‐PCPDTBT (more than 70 wt%). The Si‐PCPDTBT:[C70]PCBM OBHJ solar cells deliver power conversion efficiencies of over 5%.     

基于电荷传输层优化的量子点发光二极管

采用溶液法旋涂薄膜、真空蒸镀铝电极,制备了ITO/PEDOT∶PSS/空穴传输材料/量子点/纳米氧化锌(ZnO Nanoparticles)/Al结构的量子点发光二极管(QLED)器件。对比了不同纳米氧化锌分散剂对器件性能的影响。当用乙醇和乙醇胺分散氧化锌时,对量子点层破坏较小,器件的亮度最高达22 940cd/m2,电流效率达28.9cd/A。研究了在聚乙烯咔唑(PVK)中掺杂不同比例4,4′-环己基二[N,N-二(4-甲基苯基)苯胺](TAPC)器件的发光特性。在PVK中掺杂TAPC材料能够促进器件空穴传输以及电子空穴注入平衡,当PVK∶TAPC=3∶1时,器件的空穴传输层形貌较为平整,亮度较高;当PVK∶TAPC=1∶1时,器件的开启电压最低。通过对器件膜层表面形貌以及电学、光学性能的对比,分析了电荷传输层优化对器件特性改善的原因。     

Influence of Line Defects on the Electrical Properties of Single Crystal TiO2

One‐dimensional defects are created in [001] and [110] oriented TiO₂ single crystals by uniaxial pressure. Transmission electron microscopy (TEM) characterization shows them to preferably lie on {110} planes. Electrical properties studied as a function of oxygen partial pressure reveal their influence on ionic and electronic charge carriers. At high oxygen partial pressures (1 bar–10^?5 bar) the conductivity due to positive charge carriers is strongly enhanced, e.g., the ionic conductivity is increased by more than two orders of magnitude, when the electrical measurement axis lies on the slip plane. In contrary, no changes are observed when the measurement axis does not lie on the slip planes. At low oxygen partial pressures (<10^?15 bar), irrespective of orientation and presence of dislocation, there is no change in the n‐type conductivity. The observed phenomena can be well explained within the space charge model, assuming the dislocation cores to exhibit an excess negative charge (increased titanium vacancy concentration). The present study gives a clear correlation between line defects and point defect concentrations in such an oxide for the first time.     

关于阀控密封铅蓄电池容量与电导的相关性

阀控密封铅蓄电池荷电态在50％以上时，其内阻(或电导)是看不出什么变化的。只是荷电态在50％以下时，铅蓄电池的内阻才逐渐增大。若将荷电态在0至100％的全部铅蓄电池都统计进去，其容量与电导才有线性相关关系。然而对荷电态大于80％的在线使用的铅蓄电池而言，却不存在上述关系。因而不能用铅蓄电池的电导值去推断在线使用的铅电池的荷电态或健康状态。     

Improved conductive atomic force microscopy measurements on organic photovoltaic materials via mitigation of contact area uncertainty

Physical processes that lead to conversion of light into electrical energy inside photovoltaic devices happen at the nanoscale. Therefore, understanding of electrical properties of photovoltaic materials at this length scale is of paramount importance for improvement of device performance. In this paper, we describe and validate a new framework for high‐resolution quantitative measurements of electrical and mechanical properties of compliant materials with sub‐100‐nm resolution. Previous approaches have generally suffered from uncertainty in the quantitative level of contact between the probe and the material being measured; the methodology presented here overcomes this obstacle. We use the broadly studied ITO/PEDOT:PSS/P3HT:PC₆₁BM system as an example to illustrate variability of chemical composition and electrical properties of the active layer at hundred‐nanometers and micrometer length scales. Copyright © 2012 John Wiley & Sons, Ltd.     

Hybrid Spintronic Materials from Conducting Polymers with Molecular Quantum Bits

Hybrid materials consisting of organic semiconductors and molecular quantum bits promise to provide a novel platform for quantum spintronic applications. However, investigations of such materials, elucidating both the electrical and quantum dynamical properties of the same material have never been reported. Here the preparation of hybrid materials consisting of conducting polymers and molecular quantum bits is reported. Organic field-effect transistor measurements demonstrate that the favorable electrical properties are preserved in the presence of the qubits. Chemical doping introduces charge carriers into the material, and variable-temperature charge transport measurements reveal the existence of mobile charge carriers at temperatures as low as 15 K. Importantly, quantum coherence of the qubit is shown to be preserved up to temperatures of at least 30 K, that is, in the presence of mobile charge carriers. These results pave the way for employing such hybrid materials in novel molecular quantum spintronic architectures.     

Atom‐by‐Atom Fabrication of Single and Few Dopant Quantum Devices

Atomically precise fabrication has an important role to play in developing atom‐based electronic devices for use in quantum information processing, quantum materials research, and quantum sensing. Atom‐by‐atom fabrication has the potential to enable precise control over tunnel coupling, exchange coupling, on‐site charging energies, and other key properties of basic devices needed for solid‐state quantum computing and analog quantum simulation. Using hydrogen‐based scanning probe lithography, individual dopant atoms are deterministically placed relative to atomically aligned contacts and gates to build single electron transistors, single atom transistors, and gate‐controlled quantum sensing devices. The key steps required to fabricate and demonstrate the essential building blocks needed for spin selective initialization/readout and coherent quantum manipulation are described.     

Temperature‐Dependent Electrical Transport in Polymer‐Sorted Semiconducting Carbon Nanotube Networks

The temperature dependence of the electrical characteristics of field‐effect transistors (FETs) based on polymer‐sorted, large‐diameter semiconducting carbon nanotube networks is investigated. The temperature dependences of both the carrier mobility and the source‐drain current in the range of 78 K to 293 K indicate thermally activated, but non‐Arrhenius, charge transport. The hysteresis in the transfer characteristics of FETs shows a simultaneous reduction with decreasing temperature. The hysteresis appears to stem from screening of charges that are transferred from the carbon nanotubes to traps at the surface of the gate dielectric. The temperature dependence of sheet resistance of the carbon nanotube networks, extracted from FET characteristics at constant carrier concentration, specifies fluctuation‐induced tunneling as the mechanism responsible for charge transport, with an activation energy that is dependent on film thickness. Our study indicates inter‐tube tunneling to be the bottleneck and implicates the role of the polymer coating in influencing charge transport in polymer‐sorted carbon nanotube networks.     

Indirect band gaps in quantum dots made from direct-gap bulk materials

The conditions under which the band gaps of free standing and embedded semiconductor quantum dots are direct or indirect are discussed. Semiconductor quantum dots are classified into three categories; (i) free standing dots, (ii) dots embedded in a direct gap matrix, and (iii) dots embedded in an indirect gap matrix. For each category, qualitative predictions are first discussed, followed by the results of both recent experiments and state of the art pseudopotential calculations. We show that:

多注双间隙耦合腔电子电导计算与模拟

该文基于空间电荷波理论模型,推导出多注双间隙耦合腔电子电导的单模理论和多模理论计算公式。比较结果显示对于双间隙耦合腔,单模理论计算结果足够准确。由粒子模拟结果与计算结果的一致证明了计算结果的正确性。最后使用粒子模拟研究了电压调制系数和聚焦磁场对电子电导的影响。结果显示当电压调制系数小于0.1并且聚焦磁场大于1.5倍布里渊磁场时,小信号理论计算的电子电导是准确的。     

蓄电池容量与电导相关性的研究

通信电源系统中,蓄电池是直流供电系统的重要组成部分,是保证通信直流供电系统正常的最后一道防线,能否准确把握蓄电池容量现状,对保障通信具有重要指导意义。文中简要介绍蓄电池结构、模型及工作原理,阐述了蓄电池充放电机理,并通过对一组蓄电池电导、内阻进行长期跟踪测试,从大量实测数据中定量分析了蓄电池容量与电导之间的内在联系。最后利用福光蓄电池在线放电仪表FBI对目标蓄电池组进行容量测试,并得出蓄电池容量与电导具有相关性的结论。     

Functional Polymer Brushes on Diamond as a Platform for Immobilization and Electrical Wiring of Biomolecules

For the biofunctionalization of electronic devices, polymer brushes can provide a route which allows combining the advantages of other commonly used approaches, such as immobilization of functional biomolecules via self assembled monolayers or coated polymer matrices: high stability and loading capacity, efficient electron transport, and excellent biocompatibility. In the work presented here, poly(methacrylic acid) brushes are prepared by self‐initiated photografting and photopolymerization on diamond electrodes. In this straightforward process no prior grafting of initiators is required since the initiation of the polymerization can be conveniently controlled by the hydrogen or oxygen termination of the diamond surface. Boron doped nanocrystalline diamond as an electrode material provides extreme chemical inertness and stability, inherent biocompatibility, and superior electrochemical properties, such as the large accessible potential window and low background currents. As a proof of concept we demonstrate the amperometric detection of glucose by polymer brushes covalently modified with the redox enzyme glucose oxidase and aminomethyl ferrocene as electron mediator. Characterization by X‐ray photoelectron spectroscopy and atomic force microscopy both indicate a high loading of the ferrocene mediator. Consistently, electrochemical cyclic voltammetry shows a multilayer equivalent loading of ferrocene and highly efficient electron transfer throughout the polymer film. Overall, functionalized polymer brushes can provide a promising platform for the immobilization and electrical wiring of biomolecules for bioelectronic and biosensing applications.     

Anisotropic transport properties of quantum dot arrays fabricated by the edge-defined nanowires

The anisotropic transport properties of quantum dot arrays fabricated by the nanoimprinted multilayer quantum wires have been investigated. The vertical transport characteristics are estimated by the carrier capture time from the electron reservoir to the low dimensional structures, which turned out to be longer than the spontaneous emission lifetime. The lateral transport characteristics are modeled after the quantum conductance between quantum dot arrays which are readily available from the miniband structures. The resulting lateral transport revealed that lateral quantum conductance can be minimized when the Fermi level is kept at the center of the miniband gap when the gap is larger than 4 kT.