Au-MgB2-Au纳米结点的负微分电阻效应

运用密度泛函理论结合非平衡格林函数的方法对MgB2直线原子链与两半无限Au（100）电极构成纳米结点的电子输运特性进行了第一性原理计算．在模拟Au-MgB2-Au纳米结点的拉伸过程中,计算了结点在不同距离下的结合能与电导．结果发现结点的Au-B键长为1．90A,B-Mg键长为2．22A时,结合能最大,结构最稳定,此时结点平衡电导为0．51G0（G0=2e^0／h）．通过计算投影态密度发现电子通过结点时主要是通过B、Mg原子的px和py电子轨道形成的π键进行传输的．在-1．5～1．5V的电压范围内,结点的电流-电压近似为线性关系,表现出类似金属的导电性质,而当正负电压高于15V时,电流对称性逐渐减小,即存在负微分电阻效应,从不同电压下透射谱的变化对负微分电阻现象进行了分析与讨论．     

Zn(NCN)单轴的负热膨胀性及机理研究

负热膨胀(NTE)是一种反常的物理现象, 已在合金和框架结构化合物等材料中被观察到, 但NTE材料的种类仍然有限. 本文合成了一种单轴NTE材料Zn(NCN), 该材料在c轴方向及在100~475 K下的热膨胀系数为-3.35×10^?6 K^?1, 而a轴和b轴方向则呈低热膨胀性, 体积具有低的热膨胀系数[6.13×10^?6 K^?1(100~475 K)]. 通过同步辐射X射线衍射、 扩展X射线吸收精细结构和拉曼光谱等方法, 研究了Zn(NCN)的NTE机理. 结果表明, Zn—N键具有明显的横向振动, 一些低频振动模Grüneisen参数为负值. 直接的实验证据表明, N=C=N的横向振动以及准刚性ZnN₄四面体的耦合旋转和扭摆导致了c轴方向的NTE.     

具有增强电荷转移驱动的磺酸基功能化g-C3N4光催化剂的设计合成及应用

随着全球环境问题日益严重以及能源需求的不断增长,人们对高效环境修复与能源转换技术的需求日益增强.以半导体材料为光催化剂,可将可再生的太阳能转化为化学能,有望成为解决人类面临的能源和环境问题的有效途径.其中,开发高效稳定的光催化剂是该技术得以实际应用的关键.近几十年,研究人员开发出多种半导体材料并应用于光催化研究.其中,具有可见光响应的有机非金属光催化剂石墨相氮化碳(g-C₃N₄)因其稳定的分子结构,较小的禁带宽度(~2.7 e V)以及合适的能带结构而备受关注.然而,与大多数半导体光催化剂相似,由于传统g-C₃N₄上的光生电子和空穴极易复合,表面催化活性位点较少,可见光响应范围较窄,使得其催化效率不高.基于g-C₃N₄独特的有机分子结构,通过引入功能化的特定基团以优化g-C₃N₄的电子能带结构,促进载流子传输,拓展可见光响应范围,是提高其光催化效率的有效途径.已有研究表明,在各种功能化官能团中,具有强电负性的含氧基团对g-C₃N₄的Melon单元优化是非常有效的.因此,本文通过g-C₃N₄与氨基磺酸间的简单固相热反应成功合成了磺酸基功能化的g-C₃N₄纳米片(SACN),并实现了同步增强的相互作用.根据固体强酸特性,氨基磺酸可以在热处理的辅助下对g-C₃N₄进行酸刻蚀,从而增加其比表面积以及表面催化活性位点.更重要的是,理论计算与实验表征结果表明,磺酸基团的吸电子诱导效应所产生的电荷驱动力可极大改善g-C₃N₄的电荷转移动力学,有效抑制了它们的再结合.此外,吸电子诱导效应还可促进g-C₃N₄的局域电子再分布,进而降低g-C₃N₄的导带电位,增强光诱导电子的还原能力.光催化性能测试结果表明,SACN-400样品(前驱体中氨基磺酸加入量为400 mg)在光催化分解水制备氢气以及光降解传统污染物领域展现出较好的性能,其在入射光波长为420±15 nm时的产氢表观量子效率为11.03%.综上,本文为设计合成具有较高产氢性能以及污染物降解效率的石墨相氮化碳基光催化剂提供了一种简便有效的策略.     

冰冻人血清中尿酸标准物质复制的定值EI北大核心CSCD

为了满足该标准物质的需求,进行了第二次尿酸血清标准物质的复制。针对复制的尿酸血清标准物质,基于单四极杆质谱的液相色谱-同位素稀释质谱法(LC-IDMS),用乙腈沉淀法去除蛋白质,BEH C18色谱柱和电喷雾离子源负离子模式(ESI^(-)),同位素稀释的单点校准法进行定值、均匀性检验、稳定性检验以及不确定度评定等研究。此定值方法经过CCQM-K109(血清中尿酸分析)国际关键比对进行验证。复制的2种不同浓度水平尿酸(肾病患者和正常人)血清标准物质的定值结果分别为(73.5±1.3)μg/g,(47.5±1.1)μg/g,其均匀性和稳定性评估结果良好。     

Negative refraction and imaging of acoustic waves in a two-dimensional square chiral lattice structure

The negative refraction behavior and imaging effect for acoustic waves in a kind of two-dimensional square chiral lattice structure are studied in this paper. The unit cell of the proposed structure consists of four zigzag arms connected through a thin circular ring at the central part. The relation of the symmetry of the unit cell and the negative refraction phenomenon is investigated. Using the finite element method, we calculate the band structures and the equi-frequency surfaces of the system, and confirm the frequency range where the negative refraction is present. Due to the rotational symmetry of the unit cell, a phase difference is induced to the waves propagating from a point source through the structure to the other side. The phase difference is related to the width of the structure and the frequency of the source, so we can get a tunable deviated imaging. This kind of phenomenon is also demonstrated by the numerical simulation of two Gaussian beams that are symmetrical about the interface normal with the same incident angle, and the different negative refractive indexes are presented. Based on this special performance, a double-functional mirror-symmetrical slab is proposed for realizing acoustic focusing and beam separation.     

A domain decomposition method for three species modeling of multi-electrode negative corona discharge – With applications to electrostatic precipitators

The negative corona discharge problem for multi-electrode geometries is modeled by a three species model. The equations are solved using domain decomposition, by recognizing that multiple species are only present in a small part of the domain, and hence only need to be incorporated locally in the model.The method is applied to an industrially relevant three wire electrostatic precipitator geometry. The calculated current density is in good agreement with experimental data. To further illustrate applications of the three species solution, it is used for coupled particle, fluid, and electrostatic simulations to analyze particle collection properties.     

A scheme for a topological insulator field effect transistor

We propose a scheme for a topological insulator field effect transistor. The idea is based on the gate voltage control of the Dirac fermions in a ferromagnetic topological insulator channel with perpendicular magnetization connecting to two metallic topological insulator leads. Our theoretical analysis shows that the proposed device displays a switching effect with high on/off current ratio and a negative differential conductance with a good peak to valley ratio.     

Modulating microstructure and optical properties of hydrogenated nanocrystalline silicon photovoltaic materials prepared under hydrogen diluted silane PECVD by various DC bias

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were fabricated by plasma enhanced chemical vapor deposition under the various negative substrate bias voltages with hydrogen as a diluent of silane. The microstructure and optical properties of nc-Si:H thin films were studied by Raman scattering spectroscopy, X-ray diffraction (XRD), transmission electron microscopy, and optical transmission spectroscopy. Raman spectra and XRD pattern reveal that applying negative bias voltages at the moderate level favors the enhancement of crystalline volume fraction, increase of crystallite sizes and decrease of residual stress. We also demonstrated that the negative direct current bias can be used to modulate the volume fraction of voids, refractive index, absorption coefficient, compactness and ordered degree of nc-Si:H films. It is found that the film deposited at −80 V shows not only high crystallinity, size of crystallite, and static index n₀ but also low residual stress and volume fraction of voids. Furthermore, the microstructural evolution mechanism of nc-Si:H thin films prepared at different bias voltages is tentatively explored.     

Observation of room temperature negative differential resistance in solution synthesized ZnO nanorod

We report the observation of negative differential resistance (NDR) in solution synthesized ZnO nanorod. The ZnO nanorod was fabricated as a two terminal planar device using lithographically patterned Au electrodes. The measured current–voltage response of the device has shown a negative differential resistance behavior. The peak-to-valley current ratio of the NDR is found to be greater than 4. The mechanism of this observed NDR effect has been explained based on charge trapping and de-trapping at the nanoscale contacts. It is the first observation of negative differential resistance effect in solution synthesized ZnO nanorod.     

Electro-optical properties of smectic liquid crystal-polymer composite with a negative dispersion of birefringence

Two-dimensionally self-organized smectic liquid crystal (LC)-polymer composite has been reported to show a negative dispersion of birefringence [H. Lee and J.-H. Lee, Opt. Lett. 39, 5146 (2014)]. In this paper, we separately measured the extraordinary and ordinary refractive indices n_e and n_o of the pure LC and LC-polymer composite and simulated the dependence of the transmittance and the color on the viewing angle for the display application. The LC-polymer composite with the negative dispersion property showed a wider viewing angle and a smaller color shift compared to the pure liquid crystal with a positive dispersion of birefringence.