First-principles quantum transport modeling of thermoelectricity in single-molecule nanojunctions with graphene nanoribbon electrodes

We overview the nonequilibrium Green function combined with density functional theory (NEGF-DFT) approach to modeling of independent electronic and phononic quantum transport in nanoscale thermoelectrics with examples focused on a new class of devices where a single organic molecule is attached to two metallic zigzag graphene nanoribbons (ZGNRs) via highly transparent contacts. Such contacts make possible injection of evanescent wavefunctions from the ZGNR electrodes, so that their overlap within the molecular region generates a peak in the electronic transmission around the Fermi energy of the device. Additionally, the spatial symmetry properties of the transverse propagating states in the semi-infinite ZGNR electrodes suppress hole-like contributions to the thermopower. Thus optimized thermopower, together with diminished phonon thermal conductance in a ZGNR|molecule|ZGNR inhomogeneous heterojunctions, yields the thermoelectric figure of merit ZT≃0.4 at room temperature with maximum ZT≃3 reached at very low temperatures T≃10 K (so that the latter feature could be exploited for thermoelectric cooling of, e.g., infrared sensors). The reliance on evanescent mode transport and symmetry of propagating states in the electrodes makes the electronic-transport-determined power factor in this class of devices largely insensitive to the type of sufficiently short organic molecule, which we demonstrate by showing that both 18-annulene and C10 molecule sandwiched by the two ZGNR electrodes yield similar thermopower. Thus, one can search for molecules that will further reduce the phonon thermal conductance (in the denominator of ZT) while keeping the electronic power factor (in the nominator of ZT) optimized. We also show how the often employed Brenner empirical interatomic potential for hydrocarbon systems fails to describe phonon transport in our single-molecule nanojunctions when contrasted with first-principles results obtained via NEGF-DFT methodology.     

Time-dependent transport in open systems based on quantum master equations

Electrons in the active region of a nanostructure constitute an open many-body quantum system, interacting with contacts, phonons, and photons. We review the basic premises of the open system theory, focusing on the common approximations that lead to Markovian and non-Markovian master equations for the reduced statistical operator. We highlight recent progress on the use of master equations in quantum transport, and discuss the limitations and potential new directions of this approach.     

石墨烯量子点制备方法研究进展

石墨烯量子点(graphene quantum dots,GQDs)是三维尺度限域的零维纳米材料,以其优异的电学、热学和光学性能在众多领域备受关注。介绍了石墨烯量子点的主要制备方法,自上而下法主要包括水热法、电化学法和化学剥离碳纤维法,自下而上法主要包括溶液化学法、微波辅助和超声法,对纳米刻蚀法和钌催化C60转化法也做了简要介绍。     

纳米石墨烯掺杂浓度对XLPE击穿性能的影响

制备了分散性良好的6种不同配比XLPE/纳米石墨烯聚合物,运用差式扫描量热法排除了结晶度对击穿性能的影响;对试样进行了工频击穿强度测量并通过拉伸强度测试评估了机械性能。实验表明,掺杂纳米石墨烯能使聚合物的工频击穿强度最大提高27. 66%,拉伸强度最大提高10. 64%,在工业实现上有积极意义。通过等温去极化电流计算聚合物陷阱密度及能级,并结合动态热机械分析从界面分子结合力探究不同含量石墨烯影响击穿性能的机理。研究表明掺杂少于0. 01 wt%的纳米石墨烯能增大陷阱能级,同时增大聚乙烯基体分子间作用力,进而提升聚合物击穿性能。     

Scattering resonances in 1D coherent transport through a correlated quantum dot: An application of the few-particle quantum transmitting boundary method

We present a method for the calculation of the scattering states of a N+1th-particle coherently interacting with N correlated particles confined in a nanostructure and placed within an open domain. The method is based on a generalization of the quantum transmitting boundary method [C. Lent and D. Kirkner, J. App. Phys. 67, 6353 (1990)]. The antisymmetry conditions of the N+1-identical particles current-carrying state results from a proper choice of the boundary conditions. As an example which is relevant to coherent electronics, we apply the method to compute the exact transmission functions and phases of an electron crossing a 1D quantum dot with zero, one or two bound electrons.     

石墨烯量子点材料及在电源中的应用

介绍石墨烯量子点(GQD)材料的几种合成方法:电化学法、酸氧化法、水热/溶剂热法、微波/超声波法和溶液化学法等。综述GQD材料在燃料电池、超级电容器、有机太阳电池和染料敏化太阳电池等电源中的应用,展望GQD材料在电源中的应用前景。     

大型汽轮发电机并联环和主引线水路故障分析和预防

大型水氢氢汽轮发电机并联环过热引发的电气事故多次发生，其根源在于水路故障和缺少有效的监测措施。分析和试验表明并联环水路设计是合理的，安装错误和运行不当是引起故障的主要原因，分析引起气堵的各种因素和发展为汽堵的机理，提出预防和改进措施，类似事故是完全可以避免的。     

Application of the R-matrix method in quantum transport simulations

The paper gives an overview of recently developed method for effective quantum transport simulations in nanoscale electronic devices. In the present formulation the device is treated as a set of independent close subsystems with appropriate low-dimensional basis representations. In continuous transport models, the local R-matrix basis makes it possible to avoid discretization of the device area and achieve a much higher numerical accuracy with a lower computational burden compared to common grid schemes. Furthermore, the local basis representation provides a suitable framework for studying ionized impurity scattering by adjusting the shape of the device elements and their internal coordinate representation. Non-equilibrium current carrying electronic states are constructed by a recursive propagation scheme such that the major portion of the computation time scales linearly with the device volume. As an illustration, we apply the method to study ionized impurity scattering in a short Si channel.     

Application of the R-matrix method in quantum transport simulations

The R-matrix method based on a continuous model is generalized as to become applicable to the atomistic transport simulations with a tight-binding device Hamiltonian. The device elements are introduced as arbitrary atomic clusters in the device area and freedom in the device fragmentation can be used to reduce computations. In the ballistic regime, the best computer performance is achieved by taking individual atoms as the device elements. Non-equilibrium current carrying electronic states are constructed by the forward-backward R-matrix propagation scheme which does not require large computer operations and mass storage. The method is applied to quantum transport in p-Si nanowire with random vacancies and surface roughness.     

Partially coherent electron transport in terahertz quantum cascade lasers based on a Markovian master equation for the density matrix

We derive a Markovian master equation for the single-electron density matrix, applicable to quantum cascade lasers (QCLs). The equation conserves the positivity of the density matrix, includes off-diagonal elements (coherences) as well as in-plane dynamics, and accounts for electron scattering with phonons and impurities. We use the model to simulate a terahertz-frequency QCL, and compare the results with both experiment and simulation via nonequilibrium Green’s functions (NEGF). We obtain very good agreement with both experiment and NEGF when the QCL is biased for optimal lasing. For the considered device, we show that the magnitude of coherences can be a significant fraction of the diagonal matrix elements, which demonstrates their importance when describing THz QCLs. We show that the in-plane energy distribution can deviate far from a heated Maxwellian distribution, which suggests that the assumption of thermalized subbands in simplified density-matrix models is inadequate. We also show that the current density and subband occupations relax toward their steady-state values on very different time scales.     

The influence of dipolar species on charge carrier transport in alinear polysilicon

Effect of polar substituents and polar dopants on the transport of charge carriers in poly[methyl(phenyl)silanediyl] (PMPSI) was studied. The transport in undoped PMPSI can be explained within the theory of disordered polarons which postulates that the temperature and field dependence of the charge carrier mobility contain contributions both from the dynamic disorder, i.e. the polaron barrier, and from the environment-dependent static disorder. Polar moieties, chemically attached to the main polymer chain, give rise to a decrease of the on-chain mobility, due to the formation of local electronic polarization states. On the macroscopic level, these moieties and polar dopants give rise to a decrease of the drift (hopping) mobility due to a broadening of the distribution of the density of transport states as a consequence of the electrostatic interaction of the charge carrier with the dipole     

NEMO-3D based atomistic simulation of a double quantum dot structure for spin-blockaded transport

This work combines an atomistic electronic structure calculation with many-body rate equations to simulate the current-voltage (I–V) characteristics of a weakly-coupled Double Quantum Dot (DQD) system in the spin-blockade regime. Here we performed a NEMO-3D based, atomistic simulation of the geometry of the DQD to obtain its single electron eigen-states, hopping parameters, and Coulomb integrals followed by the evaluation of I–V characteristics with the many-electron spectrum of the DQD system, derived from this single-electron parameter set. The many-electron spectra and wave-functions are evaluated by exact-diagonalization of the many-electron system. The Hamiltonian is constructed from single electron eigen-states, hopping parameters and Coulomb integrals derived from atomistic NEMO 3-D simulations. Calculated I–V characteristics exhibit multiple regions of prominent Negative Differential Resistances (NDRs) that resemble the experimental trends. Unlike resonant tunnelling devices, however, level crossings in DQDs are negligible, and the NDRs result from a delicate interplay of delocalization, orbital offset and Coulomb interaction. B. Muralidharan, H. Ryu and Z. Huang contributed equally to this work.     

基于石墨烯薄膜的静电扬声器理论建模与仿真分析

针对基于石墨烯薄膜的新型静电扬声器设计缺乏理论指导的问题,对石墨烯静电扬声器进行了理论建模和仿真分析研究。在静电扬声器原理的基础上,采用质量-弹簧-阻尼系统建模分析了石墨烯薄膜在工作时的振动特性,研究了薄膜半径,厚度和应力对其振动特性的影响,利用COMSOL软件建立了石墨烯静电扬声器的有限元模型,对比分析了不同薄膜半径、厚度和应力对应的石墨烯静电扬声器的频率响应特性,验证了理论模型的准确性。结果表明,石墨烯薄膜半径越大,厚度越薄,应力越小,相应的石墨烯静电扬声器频率响应特性越好。     

基于量子技术的交流电压幅值测量准确度研究

在基于量子技术的交流信号测量中,采样是最常使用的方法,而在此方法中,差分电压的测量精度是决定交流电压幅值测量准确度的主要因素。为此,分析了交流量子电压台阶数、采样率和窗函数对测量准确度的影响。在交流量子电压不存在过渡过程的情况下,当采样率为常数时,通过改变交流量子电压的台阶个数,发现补偿后的相对误差随着台阶个数增加而减小;当交流量子电压的台阶个数为常数时,通过改变采样率,发现补偿后的相对误差随着采样率的提高而减小。在过渡过程和噪声干扰均存在的情况下,采用增加窗函数处理后,有效地提高了测量准确度,可使测量准确度至少提高两个数量级。     

Self-consistent quantum transport theory: Applications and assessment of approximate models

We have implemented a fully self-consistent non-equilibrium Green’s function approach for vertical quantum transport in open quantum devices with contacts and study theoretically quantum well heterostructures, resonant tunneling diodes and quantum cascade laser structures in this formalism. We systematically investigate the role and consequences of several widely used approximations such as decoupling the equations for the scattering states and their occupation, neglect of inelastic scattering, and neglect of nonlocal scattering self-energies.     

Analysis of the influence of band non-parabolicity on the subband structure of a Si quantum wire

In this work we present an investigation on the subband structure of a Si quantum wire, and how it is influenced by the non-parabolicity of the bulk bands. We carry out a study on the curvature of the subbands around their minima, and discuss the suitability of using parabolic fittings.