Carrier scattering and impact ionization in bilayer graphene

Transport properties of carriers in bilayer graphene (BLG) were studied. Several analytical models were developed for drift velocity, scattering rate and ionization coefficient of BLG for the first time. Then, the joint effect of temperature and potential difference of layers were addressed on the modeled parameters. The accuracy of the proposed models for drift velocity and scattering rate was verified by the simulation results of published works. In addition, the analytical results of ionization coefficient of BLG were compared with those of silicon.     

Short-range potential scattering and its effect on graphene mobility

Over the past few years, the amazing properties of graphene have led to predictions for its use in a variety of areas, not the least of which is in semiconductor devices. However, it appears that graphene is dominated by short-range potential scattering which can arise from intrinsic defects which limit the mobility to relatively low values, well below those predicted based upon its intrinsic band structure. Here, we examine the mobility in graphene on BN, SiC, and SiO₂ when it is dominated by these defects.     

First principles study of structural and electronic properties of BNNTs

In this article the electronic and structural properties of single-walled boron nitride (BN) nanotubes with a diameter range of 4–22 Å have been investigated using the generalized gradient approximation with both Perdew, Burke and Ernzerhof (PBE) and Becke–Lee–Yang–Parr (BLYP) functionals based on density functional theory. The variation of radial distribution, bond length, lattice constant, density of states, buckling separation, total energies and the electronic band gap of BN nanotubes have been studied in terms of the diameter of the nanotube. Our results revealed a correlation between the buckling and band gap: the higher the buckling, the lower the band gap, and by decreasing the tube diameter, the buckling separation increases. It is revealed that for both armchair and zigzag boron nitride nanotubes (BNNT) the value of the band gap increases by increasing the nanotube diameter. Moreover, it is concluded that for small BN nanotubes by reducing the radius, the band gap of the armchair nanotube remains almost constant, while the band gap of the zigzag nanotubes has a rapidly decreasing trend. The value of band gap obtained by the BLYP hybrid functional is more accurate than the PBE functional. The PDOS calculations revealed that the VBM comes from the N 2p orbitals, while the CBM is ruled by the B 2p orbitals. According to the obtained results, BNNTs are suggested as good materials for applications in the nanoscale optoelectronic devices.     

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.     

Multiscale modeling of screening effects on conductivity of graphene in weakly bonded graphene-dielectric heterostructures

Graphene is often surrounded by different dielectric materials when integrated into realistic devices. The absence of dangling bonds allows graphene to bond weakly via the van der Waals interaction with the adjacent material surfaces and to retain its peculiar linear band structure. In such weakly bonded systems, however, the electronic properties of graphene are affected by the dielectric screening due to the long-range Coulomb interaction with the surrounding materials. Including the surrounding materials in the first principles density functional theory (DFT) calculations is computationally very demanding due to the large supercell size required to model heterogeneous interfaces. Here, we employ a multiscale approach combining DFT and the classical image-potential model to investigate the effects of screening from the surrounding materials (hBN, SiC, SiO₂, Al₂O₃, and HfO₂) on the dielectric function and charged impurity scattering limited conductivity of graphene. In this approach, the graphene layer is modeled using DFT and the screening from the surrounding materials is incorporated by introducing an effective dielectric function. The dielectric function and conductivity of graphene calculated using the simplified two-band Dirac model are compared with DFT calculations. The two-band Dirac model is found to significantly overestimate the dielectric screening and charged impurity scattering limited conductivity of graphene. The multiscale approach presented here can also be used to study screening effects in weakly bonded heterostructures of other emerging two-dimensional materials such as metal dichalcogenides.     

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。     

High‐frequency modeling of Cu‐graphene heterogeneous interconnects

One novel interconnect scheme consisting of both Cu and graphene sheet is proposed in this paper, with the advantages of both materials exploited greatly. It is shown that the introduction of graphene layers in such heterogeneous interconnect scheme can reduce its effective resistance and thereby improve its transmission performance. On the other hand, it is also demonstrated that both coated and double‐coated structures possess better electrical performance than that of the sandwich one at high frequencies, because the graphene is placed at the interconnect surface where current is crowded. With the help of Partial Element Equivalent Circuit method, together with equivalent circuit technique, the transmission characteristics of some Cu‐graphene interconnects are captured and compared with that of Cu wire, and the advantages of such heterogeneous interconnects can be enlarged with the advanced technology. Copyright © 2015 John Wiley & Sons, Ltd.     

A novel scheme for global scattering center modeling of radar targets

A novel scheme for extracting the global scattering center model of radar targets is proposed in this paper. The 2D/3D scattering center models can be reconstructed based on the wideband measurements at different viewing angles. The sphere spreading of the 1D scattering center projections is exploited. The 1D-2D/3D scatterer map (OTSM) is designed to manifest the high dimensional scattering characteristic of radar targets. The Hough transform and the least squares method are used to extract the stable scattering centers and their scattering coefficients. This modeling method does not need a high density of the spatial grid, which greatly cuts down the necessary original data. The model built in this way describes the stable point scattering mechanisms in a large spatial extent and can be extrapolated to other frequencies in the optical region. Examples verify the validity of both the model and the method. __________ Translated from Chinese Journal of Radio Science, 2007, 22(3): 436–441 [译自: 电波科学学报]     

First principles investigations of geometric, electronic and optical properties of 5-aminotetrazole derivatives

Density functional theory (DFT) is an important computational technique to study and predict the properties of isolated molecules. It is now a leading method for electronic structure calculations in chemistry and solid state physics. In this paper, we have investigated the geometric, electronic and optical properties of six 5-aminotetrazole derivatives employing DFT. The ground state geometries were optimized at B3LYP/6-311G^?? and B3LYP/6-31G^?? level of theories. The density of states, HOMOs and LUMOs and absorption spectra of all the compounds under study have been computed and discussed. The HOMOs are delocalized on aminotetrazole moiety in all the compounds. A comprehensible intra charge transfer has been observed from aminotetrazole moiety to entire compounds. In the absorption spectra, the wavelength of maximum absorption for triplets in all the systems is red shifted relative to their corresponding singlet wavelengths of absorption maximum. The B3LYP/6-311G^?? level of theory is found to give better results than B3LYP/6-31G^?? level of theory, to reproduce previously reported experimental data. In most of the cases B3LYP/6-31G^?? level of theory overestimate more the bond lengths.     

First-principles versus semi-empirical modeling of global and local electronic transport properties of graphene nanopore-based sensors for DNA sequencing

Using first-principles quantum transport simulations, based on the nonequilibrium Green function formalism combined with density functional theory (NEGF+DFT), we examine changes in the total and local electronic currents within the plane of graphene nanoribbon with zigzag edges (ZGNR) hosting a nanopore which are induced by inserting a DNA nucleobase into the pore. We find a sizable change of the zero-bias conductance of two-terminal ZGNR + nanopore device after the nucleobase is placed into the most probable position (according to molecular dynamics trajectories) inside the nanopore of a small diameter \(D=1.2\) nm. Although such effect decreases as the nanopore size is increased to \(D=1.7\) nm, the contrast between currents in ZGNR + nanopore and ZGNR + nanopore + nucleobase systems can be enhanced by applying a small bias voltage \(V_b \lesssim 0.1\) V. This is explained microscopically as being due to DNA nucleobase-induced modification of spatial profile of local current density around the edges of ZGNR. We repeat the same analysis using NEGF combined with self-consistent charge density functional tight-binding (NEGF+SCC-DFTB) or self-consistent extended Hückel (NEGF+SC-EH) semi-empirical methodologies. The large discrepancy we find between the results obtained from NEGF+DFT vs. those obtained from NEGF+SCC-DFTB or NEGF+SC-EH approaches could be of great importance when selecting proper computational algorithms for in silico design of optimal nanoelectronic sensors for rapid DNA sequencing.     

Conductance fluctuations in graphene nanoribbons

Over the past few years, the amazing properties of graphene have led to predictions for its use in a variety of areas, not the least of which is in semiconductor devices. But, the transport is an important aspect of any possible application. At low temperature, fluctuations are observed in the conductance through nanoribbons. These fluctuations arise from the presence of a random potential in the semiconductor, which arises from e.g. impurities present in the material structure. In this work, we examine the nature of these fluctuations in nanoribbons using an atomic basis quantum transport simulation.     

On pseudomagnetoresistance in graphene junctions

Using bilayer-graphene (BG), monolayer-graphene (MG), and hydrogenated-bilayer-graphene (hBG), we study the performance of three different pseudospinvalve (PSV) junctions: (1) BG-BG, (2) MG-BG, and (3) hBG-BG, modulated by vertical electric-field. Although pseudomagnetoresistance (PMR) of nearly 100 % at zero temperature could be obtained for the BG-BG structure, this PMR decreases rapidly as temperature increases, and the decrease is worsened with the increasing lateral distance between the required pairs of the electric gates. On the other hand, the decrease is suppressed by using a MG-BG structure, which is less sensitive to temperature and not affected by the electric-gate distance. Unlike our expectation, our results also show that the hBG-BG has the worst performance.     

Room temperature carrier transport in graphene

Graphene is a novel new material with an unusual zero-gap band structure, where electrons and holes are closely connected through a relativistic Dirac equation. It is of interest to study the various scattering mechanisms and the transport through device structures fabricated on this new material. Here, we use Rode’s method to study the transport through gated graphene devices. The results are compared with recent results obtained for both back-gates and electrochemical gates. The transport is dominated by the trapped charge at the graphene-SiO₂, but phonon scattering is shown to be important.     

Control principles of micro-source invertersused in microgrid

Since micro-sources are mostly interfaced to microgrid by power inverters, this paper gives an insight of the control methods of the micro-source inverters by reviewing some recent documents. Firstly, the basic principles of different inverter control methods are illustrated by analyzing the electrical circuits and control loops. Then, the main problems and some typical improved schemes of the ωU-droop grid-supporting inverter are presented. In results and discussion part, the comparison of different kinds of inverters is presented and some notable research points is discussed. It is concluded that the most promising control method should be the ωU-droop control, and it is meaningful to study the performance improvement methods under realistic operation conditions in the future work.     

Optical absorption in bilayer graphene superlattices

Journal of Computational Electronics - A periodic statistical potential applied in each layer of bilayer graphene can change it to a superlattice, enabling its application as a flexible...     

Competing topological phases in few-layer graphene

We investigate the effect of spin-orbit coupling on the band structure of graphene-based two-dimensional Dirac fermion gases in the quantum Hall regime. Taking monolayer graphene as our first candidate, we show that a quantum phase transition between two distinct topological states—the quantum Hall and the quantum spin Hall phases—can be driven by simply tuning the Fermi level with a gate voltage. This transition is characterized by the existence of a chiral spin-polarized edge state propagating along the interface separating the two topological phases. We then apply our analysis to the more difficult case of bilayer graphene. Unlike in monolayer graphene, spin-orbit coupling by itself has indeed been predicted to be unsuccessful in driving bilayer graphene into a topological phase, due to the existence of an even number of pairs of spin-polarized edge states. While we show that this remains the case in the quantum Hall regime, we point out that by additionally breaking the layer inversion symmetry, a non-trivial quantum spin Hall phase can re-emerge in bilayer graphene at low energy. We consider two different symmetry-breaking mechanisms: inducing spin-orbit coupling only in the upper layer, and applying a perpendicular electric field. In both cases, the presence at low energy of an odd number of pairs of edge states can be driven by an exchange field. The related situation in trilayer graphene is also discussed.     

Ultrafast terahertz responses in monolayer graphene

ABSTRACT

We theoretically investigate the ultrafast terahertz (THz) properties of monolayer graphene. The analytical formulations of the photon carrier, electric polarization and optical current are obtained by solving the Bloch-equations in present of the ultrafast THz Gaussian pulse. Graphene shows a large nonlinear and ultrafast optical response at THz frequencies due to the gapless and relativistic Dirac particles with nearly linear energy dispersion. It is found that the photon carrier density, electric polarization and optical current density increase with increasing the frequency of the THz pulse. These theoretical results are in agreement with recent experimental findings. This study confirms further that graphene exhibits important features and is relevant to the applications in the ultrafast THz fields.     

需求侧响应中的经济学原理

电力需求侧响应就是运用系统可靠性程序或基于市场的价格去影响需求的时间和水平。对需求侧响应理论研究和实践操作中密切相关的几个经济学原理进行了系统阐述,以对需求侧响应的经济学原理有一个全景理解,包据电力短期行业供应曲线的跃变特性,电力需求弹性的非线性和不对称性,以及电力需求侧响应的非纯公共物品性。指出应采用极短期电力需求弹性和长期需求弹性以及公共物品最优标准来进行需求侧响应的评估;     

Operating principles of pin diodes

Analytical models of the pin diode in a small‐current operation are not known yet. This article presents a simple analytical model of the pin diode operation with its confirmation by a numerical simulation. At the onset, carrier recombinations are not included for the sake of simplicity. The exact J_F?V_F characteristic could have been induced only by accounting for the Boltzmann distribution of each carrier across the junctions and the diffusion current of each minority carrier in a p‐anode or n‐cathode. Based on this new model, the modifications of hole–electron densities product (n_en_h) across junctions, a rough estimation of the large operational current, its carrier distributions, and the effect of carrier recombination on the carrier distribution are plainly estimated and are also compared with the simulation results. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(4): 47–56, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20844