High resistivity measurement of SiC wafers using different techniques

To establish fast, nondestructive, and inexpensive methods for resistivity measurements of SiC wafers, different resistivity-measurement techniques were tested for characterization of semi-insulating SiC wafers, namely, the four-point probe method with removable graphite contacts, the van der Pauw method with annealed metal and diffused contacts, the current-voltage (I-V) technique, and the contactless resistivity-measurement method. Comparison of different techniques is presented. The resistivity values of the semi-insulating SiC wafer measured using different techniques agree fairly well. As a result, application of removable graphite contacts is proposed for fast and nondestructive resistivity measurement of SiC wafers using the four-point probe method. High-temperature van der Pauw and room-temperature Hall characterization for the tested semi-insulating SiC wafer was also obtained and reported in this work.     

Many-Body Terms in van der Waals Cohesion Energy of Nanotubes

We have developed a model for the calculation of van der Waals force for layered systems with axial symmetry. Our result can be applied to compute the cohesion of a carbon nanotube to a substrate, the cohesion between nanotubes, and between shells of multiwall nanotubes. We have obtained unusal power laws for the distance dependence of the many-body van der Waals potential.     

Crystalline complex between poly(γ-methyl l-glutamate) and dimethyl phthalate

The crystalline complex between poly(γ-methyl l-glutamate) (PMLG) and dimethyl phthalate (DMP) has been formed in films cast from a solution in dichloroethane. It has the stoichiometry of 1 mol of DMP to three or four residues of PMLG and shows two definite characteristics in X-ray diffraction patterns; one is the large hexagonal unit cell with the edge of around 28 Å and another the ‘extra’ 5.07 Å meridional reflection which can not be interpreted by a PMLG α-helical conformation. The structural examination for the films with various DMP contents is carried out by X-ray, viscoelastic, and d.s.c. measurements and the following structure is proposed for the crystalline complex. Four PMLG are associated to form a group which is hexagonally packed and DMP molecules, located in the gaps between groups, form a specific favourable helical structure along PMLG chains in which the van der Waals stacking of benzene rings of DMP is significant.     

试论建筑理论研究的目的与方法——以范·德·拉恩的比例理论为例

以范·德·拉恩的比例理论为例,对建筑理论研究的目的和方法进行了探讨,结论是：建筑理论研究的根本目的,是从自然规律中提取若干可以指导实践的基本原则或者＂法则＂;为了获得这些法则,人们通常采用的是隐喻和分类,类比和还原等基本策略;在最指实的地方,往往可以得到一种禅宗所说的＂顿悟＂,从这个意义上说,任何真正的建筑理论都不是以神秘主义为直接目标的.     

InAs基范德华异质结界面电荷转移特性第一性原理计算的研究进展

由低维InAs材料和其他二维层状材料堆叠而成的垂直范德华异质结构在纳米电子、光电子和量子信息等新兴领域中应用广泛。探索跨结界面的电荷转移机制对于全面理解该类器件的非凡特性至关重要。第一性原理计算在揭示界面电荷转移特性与各种能量稳定型InAs基范德华异质结的电、光、磁等原理物理特性和器件性能变化之间的内在关系方面发挥着不可比拟的作用。文中梳理、总结和探讨了近年来InAs基范德华异质结间界面电荷转移特性的理论研究工作与潜在的功能应用,提出在理论方法和计算精度方面大力发展第一性原理计算的几个途径,为更好地开展InAs基范德华异质结的基础科学研究和应用器件设计提供可借鉴的量化研究基础。     

Tunable SnSe2/WSe2 Heterostructure Tunneling Field Effect Transistor

The burgeoning 2D semiconductors can maintain excellent device electrostatics with an ultranarrow channel length and can realize tunneling by electrostatic gating to avoid deprivation of band‐edge sharpness resulting from chemical doping, which make them perfect candidates for tunneling field effect transistors. Here this study presents SnSe₂/WSe₂ van der Waals heterostructures with SnSe₂ as the p‐layer and WSe₂ as the n‐layer. The energy band alignment changes from a staggered gap band offset (type‐II) to a broken gap (type‐III) when changing the negative back‐gate voltage to positive, resulting in the device operating as a rectifier diode (rectification ratio ~10⁴) or an n‐type tunneling field effect transistor, respectively. A steep average subthreshold swing of 80 mV dec^?1 for exceeding two decades of drain current with a minimum of 37 mV dec^?1 at room temperature is observed, and an evident trend toward negative differential resistance is also accomplished for the tunneling field effect transistor due to the high gate efficiency of 0.36 for single gate devices. The I _ON/I _OFF ratio of the transfer characteristics is >10⁶, accompanying a high ON current >10^?5 A. This work presents original phenomena of multilayer 2D van der Waals heterostructures which can be applied to low‐power consumption devices.     

电动物流车远程监控系统

设计了一种电动物流车远程监控系统．该系统综合运用全球卫星定位系统（GPS）、公共移动通讯网（GPRS）、互联网（Intemet）和地理信息系统（GIS）等技术,对运行车辆状态和位置进行实时监控．系统的主控制器采用XC2267单片机,对GPS定位模块接收的车辆位置信息和CAN总线采集的车辆实时运行数据进行处理,然后通过GPRS网络将数据发送到中心服务器上,客户端或者充电站可向数据中心请求发送数据并在界面上显示．     

On the SPH tensile instability in forming viscous liquid drops

Smoothed Particle Hydrodynamics (SPH) simulations of elastic solids and viscous fluids may suffer from unphysical clustering of particles due to the tensile instability. Recent work has shown that in simulations of elastic or brittle solids the instability can be removed by an artificial stress whose form is derived from a linear perturbation analysis of the full set of governing SPH equations. While a linear analysis cannot be used to derive the corresponding form of the artificial stress for a viscous fluid, here we show that the same construction which applies to elastic solids may also work for viscous fluids provided that the constant parameter ? entering in the definition of the artificial stress is properly chosen. As a suitable test case, we model the formation of a circular van der Waals liquid drop and show that the tensile instability is removed when an artificial viscous force and energy generation term are added to the standard SPH equations of motion and energy, respectively. The optimal value of the constant ? is constrained by the ability of the model simulation to reproduce both a sufficiently smoothed density profile and the van der Waals phase diagram.