Atomic-scale imaging of carbon nanofibre growth

The synthesis of carbon nanotubes with predefined structure and functionality plays a central role in the field of nanotechnology, whereas the inhibition of carbon growth is needed to prevent a breakdown of industrial catalysts for hydrogen and synthesis gas production. The growth of carbon nanotubes and nanofibres has therefore been widely studied. Recent advances in in situ techniques now open up the possibility of studying gas-solid interactions at the atomic level. Here we present time-resolved, high-resolution in situ transmission electron microscope observations of the formation of carbon nanofibres from methane decomposition over supported nickel nanocrystals. Carbon nanofibres are observed to develop through a reaction-induced reshaping of the nickel nanocrystals. Specifically, the nucleation and growth of graphene layers are found to be assisted by a dynamic formation and restructuring of mono-atomic step edges at the nickel surface. Density-functional theory calculations indicate that the observations are consistent with a growth mechanism involving surface diffusion of carbon and nickel atoms. The finding that metallic step edges act as spatiotemporal dynamic growth sites may be important for understanding other types of catalytic reactions and nanomaterial syntheses.     

混合价态双核钌配合物合成及其光谱电化学性质

合成了一种新的以吡啶和酚为配体的混合价态双核金属钌的配合物Ru2BMPB,并对其光谱电化学性能进行了研究.该配合物具有可逆的单电子氧化还原过程(E1/2=0.19 V,-0.50 V),很好的稳定性(平衡常数K = 1.46×1012).可见与近红外吸收光谱显示,该配合物在410、665和1 580 nm 处均出现较强的吸收.在可控制的电位下,中心离子Ru的电子转移引起Ru2BMPB的可见与近红外吸收光谱的变化,具有可控制的开关性能.     

Atomic-scale imaging of DNA using scanning tunnelling microscopy

The scanning tunnelling microscope (STM) has been used to visualize DNA under water, under oil and in air. Images of single-stranded DNA have shown that submolecular resolution is possible. Here we describe atomic-resolution imaging of duplex DNA. Topographic STM images of uncoated duplex DNA on a graphite substrate obtained in ultra-high vacuum are presented that show double-helical structure, base pairs, and atomic-scale substructure. Experimental STM profiles show excellent correlation with atomic contours of the van der Waals surface of A-form DNA derived from X-ray crystallography. A comparison of variations in the barrier to quantum mechanical tunnelling (barrier-height) with atomic-scale topography shows correlation over the phosphate-sugar backbone but anticorrelation over the base pairs. This relationship may be due to the different chemical characteristics of parts of the molecule. Further investigation of this phenomenon should lead to a better understanding of the physics of imaging adsorbates with the STM and may prove useful in sequencing DNA. The improved resolution compared with previously published STM images of DNA may be attributable to ultra-high vacuum, high data-pixel density, slow scan rate, a fortuitously clean and sharp tip and/or a relatively dilute and extremely clean sample solution. This work demonstrates the potential of the STM for characterization of large biomolecular structures, but additional development will be required to make such high resolution imaging of DNA and other large molecules routine.     

Atomic-scale imaging of nanoengineered oxygen vacancy profiles in SrTiO3

At the heart of modern oxide chemistry lies the recognition that beneficial (as well as deleterious) materials properties can be obtained by deliberate deviations of oxygen atom occupancy from the ideal stoichiometry. Conversely, the capability to control and confine oxygen vacancies will be important to realize the full potential of perovskite ferroelectric materials, varistors and field-effect devices. In transition metal oxides, oxygen vacancies are generally electron donors, and in strontium titanate (SrTiO3) thin films, oxygen vacancies (unlike impurity dopants) are particularly important because they tend to retain high carrier mobilities, even at high carrier densities. Here we report the successful fabrication, using a pulsed laser deposition technique, of SrTiO3 superlattice films with oxygen doping profiles that exhibit subnanometre abruptness. We profile the vacancy concentrations on an atomic scale using annular-dark-field electron microscopy and core-level spectroscopy, and demonstrate absolute detection sensitivities of one to four oxygen vacancies. Our findings open a pathway to the microscopic study of individual vacancies and their clustering, not only in oxides, but in crystalline materials more generally.     

Polarons and confinement of electronic motion to two dimensions in a layered manganite

A remarkable feature of layered transition--metal oxides-most famously, the high-temperature superconductors--is that they can display hugely anisotropic electrical and optical properties (for example, seeming to be insulating perpendicular to the layers and metallic within them), even when prepared as bulk three-dimensional single crystals. This is the phenomenon of 'confinement', a concept at odds with the conventional theory of solids, and recognized as due to magnetic and electron-lattice interactions within the layers that must be overcome at a substantial energy cost if electrons are to be transferred between layers. The associated energy gap, or 'pseudogap', is particularly obvious in experiments where charge is moved perpendicular to the planes, most notably scanning tunnelling microscopy and polarized infrared spectroscopy. Here, using the same experimental tools, we show that there is a second family of transition-metal oxides--the layered manganites La(2-2x)Sr(1+2x)Mn2O7--with even more extreme confinement and pseudogap effects. The data demonstrate quantitatively that because the charge carriers are attached to polarons (lattice- and spin-textures within the planes), it is as difficult to remove them from the planes through vacuum-tunnelling into a conventional metallic tip, as it is for them to move between Mn-rich layers within the material itself.     

高温超导薄膜的原子尺度制备和表征

高温超导体因其丰富的物理性质和潜在的应用价值已成为30余年来凝聚态物理学备受关注的前沿方向之一.探索和发现新型高温超导体并以此建立非常规高温超导电性的物理机制是超导物理学家们长期追求的目标.本文从高温超导体的研究现状和瓶颈出发,介绍基于异质外延薄膜高温超导电性的原子尺度研究,阐述近年来分子束外延技术在高温超导薄膜制备和量子调控方面取得的研究进展.在外延薄膜超导特性表征方面,着重介绍基于原位扫描隧道显微镜对超导层的原子尺度研究.在新型高温超导体探索方面,主要介绍基于异质外延薄膜界面超导体系的构筑.本文侧重于展示实验设计思路、研究方法以及对高温超导电性微观机制的理解,力图以此启发相关领域的研究人员.     

Control of the electronic phase of a manganite by mode-selective vibrational excitation

Controlling a phase of matter by coherently manipulating specific vibrational modes has long been an attractive (yet elusive) goal for ultrafast science. Solids with strongly correlated electrons, in which even subtle crystallographic distortions can result in colossal changes of the electronic and magnetic properties, could be directed between competing phases by such selective vibrational excitation. In this way, the dynamics of the electronic ground state of the system become accessible, and new insight into the underlying physics might be gained. Here we report the ultrafast switching of the electronic phase of a magnetoresistive manganite via direct excitation of a phonon mode at 71 meV (17 THz). A prompt, five-order-of-magnitude drop in resistivity is observed, associated with a non-equilibrium transition from the stable insulating phase to a metastable metallic phase. In contrast with light-induced and current-driven phase transitions, the vibrationally driven bandgap collapse observed here is not related to hot-carrier injection and is uniquely attributed to a large-amplitude Mn-O distortion. This corresponds to a perturbation of the perovskite-structure tolerance factor, which in turn controls the electronic bandwidth via inter-site orbital overlap. Phase control by coherent manipulation of selected metal-oxygen phonons should find extensive application in other complex solids--notably in copper oxide superconductors, in which the role of Cu-O vibrations on the electronic properties is currently controversial.     

Atomic-scale imaging of insulating diamond through resonant electron injection

The electronic properties of insulators such as diamond are of interest not only for their passive dielectric capabilities for use in electronic devices, but also for their strong electron confinement on atomic scales. However, the inherent lack of electrical conductivity in insulators usually prevents the investigation of their surfaces by atomic-scale characterization techniques such as scanning tunnelling microscopy (STM). And although atomic force microscopy could in principle be used, imaging diamond surfaces has not yet been possible. Here, we demonstrate that STM can be used in an unconventional resonant electron injection mode to image insulating diamond surfaces and to probe their electronic properties at the atomic scale. Our results reveal striking electronic features in high-purity diamond single crystals, such as the existence of one-dimensional fully delocalized electronic states and a very long diffusion length for conduction-band electrons. We expect that our method can be applied to investigate the electronic properties of other insulating materials and so help in the design of atomic-scale electronic devices.     

Impedance effect of manganite thin film-based photodetectors

We report on the photodetector structures based on perovskite manganite La0.67Ca0.33MnO3 thin films on tilted SrTiO3 （001） substrates. The photovoltaic effect has been observed in response to excitation by 308 nm ultraviolet laser pulse irradiation in duration of 20 ns at room temperature. The outputs obtained required no amplification. To reduce the deformation of the signal detected, a series of testing measurements were made to investigate the impedance effect. When the impedance at the oscilloscope end matched to the co-axis cable, the signal trace was almost triangular and symmetrical, with response time equal to the excitation laser. In addation, the response linearly depends on the irradiated area for low on-sample energy. The devices work well under unbiased conditions and so are simple to configure for practical applications.     

Atomic-scale imaging of individual dopant atoms and clusters in highly n-type bulk Si

As silicon-based transistors in integrated circuits grow smaller, the concentration of charge carriers generated by the introduction of impurity dopant atoms must steadily increase. Current technology, however, is rapidly approaching the limit at which introducing additional dopant atoms ceases to generate additional charge carriers because the dopants form electrically inactive clusters. Using annular dark-field scanning transmission electron microscopy, we report the direct, atomic-resolution observation of individual antimony (Sb) dopant atoms in crystalline Si, and identify the Sb clusters responsible for the saturation of charge carriers. The size, structure, and distribution of these clusters are determined with a Sb-atom detection efficiency of almost 100%. Although single heavy atoms on surfaces or supporting films have been visualized previously, our technique permits the imaging of individual dopants and clusters as they exist within actual devices.     

混合价态双核磺化酞菁钴在非水溶剂中的电化学性质研究

混合价态的双核磺化酞菁钴bi-CoPc在非水溶剂二甲基亚砜（DM－SO）中的循环伏安曲线分别呈现四准可逆和一非可逆电对的电流峰，其中Co^Ⅱ／Co^Ⅰ还原过程的峰出现了裂分。比较了不同支持电解质对bi-CoPc的循环伏安曲线的影响。根据裂分能计算了bi-CoPc在DMSO体系中生成Co^ⅡPc（－2）－Co^ⅠPc（－2）化合物的反应常数Kc。     

Colloidal ordering from phase separation in a liquid-crystalline continuous phase

Some binary mixtures exist as a single phase at high temperatures and as two phases at lower temperatures; rapid cooling therefore induces phase separation that proceeds through the initial formation of small particles and subsequent growth and coarsening. In solid and liquid media, this process leads to growing particles with a range of sizes, which eventually separate to form a macroscopically distinct phase. Such behaviour is of particular interest in systems composed of an isotropic fluid and a liquid crystal, where the random distribution of liquid-crystal droplets in an isotropic polymer matrix may give rise to interesting electro-optical properties. Here we report that a binary mixture consisting of an isotropic fluid and a liquid crystal forming the continuous phase does not fully separate into two phases, but self-organizes into highly ordered arrays of monodisperse colloidal droplet chains. We find that the size and spatial organization of the droplets are controlled by the orientational elasticity of the liquid-crystal phase and the defects caused by droplets exceeding a critical size. We expect that our approach to forming monodisperse, spatially ordered droplets in liquid crystals will allow the controlled design of ordered composites that may have useful rheological and optical properties.     

模糊随机需求条件下供应链定期库存订货策略研究

在提前期内需求为模糊随机变量且提前期为可变情形下,建立由制造商和购买商所组成的供应链优化决策模型,其中购买商订货周期、目标订货点和进货提前期、制造商目标库存点为决策变量。首先推导出模糊随机需求条件下购买商、制造商和供应链的成本函数,然后进一步考虑总需求为三角模糊数,推导出购买商、制造商和供应链的模糊成本函数。在此基础上从供应链成本最小角度对模型设计求解,并运用具体算例验证模型的有效性。     

一般加速带电带磁动态黑洞的非热辐射

在一般加速带电带磁的动态黑洞中,化简Klein-Gordon场方程,利用乌龟(Tortoise )坐标变换,得到在视界面附近的Hawking热谱和Hawking温度. 同时研究黑洞的非热辐射,计算出了荷电荷磁粒子的正负能级表达式和交错能级最大值.     

范畴中态射集的Baksalary-Hauke序

定义了范畴中态射集的Ｂａｋｓａｌａｒｙ－Ｈａｕｋｅ序,给出了它的等价刻划,并讨论了它与态射集星形序之间的关系     

Manipulation of elementary charge in a silicon charge-coupled device

The ultimate limit in the operation of an electronic device is the manipulation of a single charge. Such a limit has been achieved in single-electron tunnelling devices. However, these devices are based on multiple tunnel barriers and conductive islands, which are complex structures to fabricate. Here we demonstrate another type of device that can also manipulate elementary charge, but which is more suitable for large-scale integration. The device consists of two closely packed silicon wire-MOSFETs, which are commonly used building blocks of electronic circuits. We have developed a scheme to generate and store holes in the channels of either of these MOSFETs. Subsequently, holes can be transferred between the two MOSFETs at the level of an elementary charge, and their exact position can be monitored. This single-charge transfer device, which is operated at 25 K, is in effect a charge-coupled device. This is also the first realization of a silicon-based device that manipulates elementary charge.     

TPC探测器内空间电荷电场

为了评估时间投影室探测器内空间电荷对漂移电场的影响,根据Green函数法求解Poisson方程,推导和借鉴了数种适用于TPC的Green函数;给出了计算空间电荷电场的解析形式;计算了单元区域内电荷均匀分布的电势和电场强度;展示了求解TPC空间电场的具体步骤。同时,利用Maxwell软件对单元区域内电荷均匀分布的电场进行了仿真,与Green函数法计算的结果基本一致。结果表明:该文给出的Green函数法适合求解TPC内的空间电场,可以获取正确结果,更加简便和实用。     

非线性分析中的半序方法

系统论述了利用半序方法研究缺乏紧性和缺乏连续性的增算,减算以及混合单调算子最大不动点和最小不动点的存在性,不点动的存在唯一性,迭代序列的收敛性以及收敛速率估计等问题,并给出对于Ｂａｎａｃｈ空间非线性积分－微分方程的应用。文章还提出了若干提供一步研究的问题。     

Tuning the magnetic anisotropy in LSMO manganite films through non-magnetic nanoparticles

We report the observation of anomalous magnetic anisotropy driven by nonmagnetic ZrO2 nanoparticles in epitaxial La2/3Sr1/3MnO (LSMO) films grown on LaAlO3 (LAO) substrates. The compressive epitaxial strain imposed by the lattice mismatch of substrate and film is tuned by the density of ZrO2 nanoparticles embedded in the film matrix and affects the magnetic anisotropy as well as the magnetotransport properties. Epitaxial 54 nm thick LSMO thin films with different concentrations of ZrO2 nanoparticles demonstrate anisotropic hysteresis loops concomitant with anisotropic magnetotresistance behavior. The biaxial epitaxial strain, induced by the substrate/film lattice parameter mismatch is partially relaxed by increasing the density of precipitates and they serve as a tuning parameter for the strain state. We interpret our results by a strain-induced interplay of impurity scattering, weak localization and magnetic domain structure.