Probing the different stages in contacting a single molecular wire

A crucial problem in molecular electronics is the control of the electronic contact between a molecule and its electrodes. As a model system, we investigated the contact between the molecular wire group of a C90H98 (Lander) molecule and the edge of a Cu(111) monatomic step. The reproducible contact and decontact of the wire was obtained by manipulating the Lander with a low temperature scanning tunneling microscope. The electronic standing wave patterns on the Cu(111) surface serve to monitor the local electronic perturbation caused by the interaction of the wire end with the step edge, giving information on the quality of the contact.     

Structure,transport and field-emission properties of compound nanotubes: CN<Subscript>x</Subscript> vs. BNC<Subscript>x</Subscript> (<Emphasis Type="Italic">x</Emphasis><0.1)

Transport and field-emission properties of as-synthesized CN_x and BNC_x (x<0.1) multi-walled nanotubes were compared in detail. Individual ropes made of these nanotubes and macrofilms of those were tested. Before measurements, the nanotubes were thoroughly characterized using high-resolution and energy-filtered electron microscopy, electron diffraction and electron-energy-loss spectroscopy. Individual ropes composed of dozens of CN_x nanotubes displayed well-defined metallic behavior and low resistivities of ∼10–100 kΩ or less at room temperature, whereas those made of BNC_x nanotubes exhibited semiconducting properties and high resistivities of ∼50–300 MΩ. Both types of ropes revealed good field-emission properties with emitting currents per rope reaching ∼4 μA(CN_x) and ∼2 μA (BNC_x), albeit the latter ropes se- verely deteriorated during the field emission. Macrofilms made of randomly oriented CN_x or BNC_x nanotubes displayed low and similar turn-on fields of ∼2–3 V/μm. 3 mA/cm² (BNC_x) and 5.5 mA/cm² (CN_x) current densities were reached at 5.5 V/μm macroscopic fields. At a current density of 0.2–0.4 mA/cm² both types of compound nanotubes exhibited equally good emission stability over tens of minutes; by contrast, on increasing the current density to 0.2–0.4 A/cm², only CN_x films continued to emit steadily, while the field emission from BNC_x nanotube films was prone to fast degradation within several tens of seconds, likely due to arcing and/or resistive heating. Received: 29 October 2002 / Accepted: 1 November 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/51-6280, E-mail: golberg.dmitri@nims.go.jp     

信噪比对偏振耦合测试影响分析

以白光麦克尔逊干涉仪对双折射保偏光纤的偏振耦合进行测试,并基于仪器结构建立了测试扫描结果的数学模型.分析及仿真了信噪比对耦合强度及耦合位置检测的影响大小及变化趋势.信噪比大于32dB,可满足偏振耦合测试的要求.     

Benzene thermal conversion to nanocrystalline indium nitride from sulfide at low temperature

A benzene thermal conversion route has been successfully developed to prepare nanocrystalline indium nitride at 180-200 degrees C by choosing NaNH(2) and In(2)S(3) as novel nitrogen and indium sources. This route has been also extended to the synthesis of other group III nitrides. The product InN was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution TEM, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and infrared spectroscopy (IR). The optical properties of nanocrystalline InN were also recorded by means of UV-vis absorption spectroscopy and photoluminescence (PL) spectroscopy, indicating that the as-prepared sample was within the quantum confinement regime. Finally, the formation mechanism was also investigated.     

Dynamic NMR line-shape analysis demonstrates that the villin headpiece subdomain folds on the microsecond time scale

There is considerable interest in small proteins that fold very rapidly. These proteins have become attractive targets for both theoretical and computational studies. The independently folded 36-residue villin headpiece subdomain has been the subject of a number of such studies and is predicted to fold quickly. We demonstrate using dynamic NMR line-shape analysis that the protein folds on the time scale of 10 mus. Folding rates were directly estimated between 56 and 78 degrees C using resolved protein resonances from three different residues at both 500 and 700 MHz. The rates estimated using different residues and different field strengths agree well with each other. The estimated folding rate lies between 0.5 and 2.0 x 105 s-1 over this temperature range. The folding rate depends only weakly on temperature.     

Multicolor luminescence patterning by photoactivation of semiconductor nanoparticle films

Indiscriminate adsorption of nanoparticles (NPs) significantly complicates the preparation of mesoscale NP patterns considered as enabling technology for many devices and processes. Instead of selected chemical functionalization of the substrate surface prior to the assembly of nanocolloids, the required optical properties - in our case, high quantum yield luminescence - are imparted to the layer-by-layer assembled films by spatially selected photoactivation. The films are made by sequential adsorption of a positively charged polyelectrolyte and a negatively charged CdSe/CdS aqueous dispersion with an initial quantum yield of 0.5-2%. The photoactivation process takes place in the presence of oxygen and may be accompanied by photoetching. A 50-500-fold increase in the luminescence intensity of CdSe/CdS citrate-stabilized particles (quantum yield 25-45%) after visible light illumination provides excellent pattern contrast. Micron scale luminescence patterns were produced from NPs of various CdSe core diameters with red, yellow, and green emission. It was also demonstrated that different emission colors such as orange and green can be combined in one image by taking advantage of spatially selective photoetching. The presented optical patterning technique significantly simplifies the preparation of luminescence patterns as compared to conventional methods. The high signal-to-noise ratio associated with it is essential for optical devices, information processing, and biophotonics. The most immediate use of this approach is expected in cryptography and cell monitoring.     

Asymmetric hydrogenation of itaconic acid and enol acetate derivatives with the Rh-TangPhos catalyst

[reaction: see text] The Rh-TangPhos catalyst has been used for asymmetric hydrogenation of itaconic acid and enol acetate derivatives. A variety of chiral 2-substituted succinic acids and chiral acetates have been obtained in excellent ee values (up to 99% ee).     

Oscillations of nonlinear partial difference systems

This paper studies the following two-dimensional nonlinear partial difference systems

     

Symmetric and Rees algebras of Koszul cycles and their Gröbner bases

In this paper we study the symmetric algebra S(E _i ) and Rees algebra R(E _i ) of the modules E _i of i-cycles of the Koszul complex associated with the sequence of indeterminates of a polynomial ring . For i=2 and i=n–2 we show that is a d-sequence on S(E _i ) and R(E _i ) and we determine Gröbner bases and Sagbi bases related to these algebras. Mathematics Subject Classification (2000):13A30, 13D02, 13H10, 13P10The second author is grateful to the National Natural Science Foundation of China for support.Part of this work was made while the third author was visiting the Fachbereich Mathematik und Informatik der Universität Essen, to which he would like to thank for its hospitality.