Showing papers by "Xiang Zhang published in 2004"

Terahertz magnetic response from artificial materials.

Abstract: We show that magnetic response at terahertz frequencies can be achieved in a planar structure composed of nonmagnetic conductive resonant elements. The effect is realized over a large bandwidth and can be tuned throughout the terahertz frequency regime by scaling the dimensions of the structure. We suggest that artificial magnetic structures, or hybrid structures that combine natural and artificial magnetic materials, can play a key role in terahertz devices.

Computational definition of sequence motifs governing constitutive exon splicing

Abstract: We have searched for sequence motifs that contribute to the recognition of human pre-mRNA splice sites by comparing the frequency of 8-mers in internal noncoding exons versus unspliced pseudo exons and 5' untranslated regions (5' untranslated regions [UTRs]) of transcripts of intronless genes. This type of comparison avoids the isolation of sequences that are distinguished by their protein-coding information. We classified sequence families comprising 2069 putative exonic enhancers and 974 putative exonic silencers. Representatives of each class functioned as enhancers or silencers when inserted into a test exon and assayed in transfected mammalian cells. As a class, the enhancer sequencers were more prevalent and the silencer elements less prevalent in all exons compared with introns. A survey of 58 reported exonic splicing mutations showed good agreement between the splicing phenotype and the effect of the mutation on the motifs defined here. The large number of effective sequences implied by these results suggests that sequences that influence splicing may be very abundant in pre-mRNA.

Measurement of the flux of ultrahigh energy cosmic rays from monocular observations by the High Resolution Fly's Eye experiment.

Abstract: We have measured the cosmic ray spectrum above 10(17.2) eV using the two air-fluorescence detectors of the High Resolution Fly's Eye observatory operating in monocular mode. We describe the detector, phototube, and atmospheric calibrations, as well as the analysis techniques for the two detectors. We fit the spectrum to a model consisting of galactic and extragalactic sources.

Plasmon Resonance of Finite One-Dimensional Au Nanoparticle Chains

Abstract: We report experimental and theoretical studies on the plasmon resonances of finite one-dimensional chains of Au nanoparticles excited by evanescent light waves with polarization parallel to the chains. The experimental results show that the plasmon resonance peak wavelengths of these finite 1D chains are significantly red-shifted in comparison to that of single Au nanoparticle. Contrary to previous findings, the peak wavelengths are observed to be a nonmonotonic function of particle numbers in the chain. This phenomenon is reproduced in the theoretical results obtained by using the transfer-matrix method and is shown to occur only for larger particles where phase retardation effects are important in plasmon coupling.

Large positive and negative lateral optical beam displacements due to surface plasmon resonance

Abstract: We report abnormally large positive and negative lateral optical beam shifts at a metal–air interface when the surface plasmon resonance of the metal is excited. The optimal thickness for minimal resonant reflection is identified as the critical thickness above which a negative beam displacement is observed. Experimental results show good agreement with theoretical predictions and the large observed bidirectional beam displacements also indicate the existence of forward and backward surface propagating waves at the surface plasmon resonance of the metal.

The Metastability of an Electrochemically Controlled Nanoscale Machine on Gold Surfaces

Abstract: The advent of supramolecular chemistry has provided chemists with the wherewithal to construct molecule-level machines 3] in an efficient manner using the protocol of template-direction. Synthetically accessible, linear motor molecules come in the guise of bistable [2]rotaxanes in which the ring component can be induced to move relative to the dumbbell-shaped one by altering the redox characteristics of the molecules. Such precisely controllable nanoscale molecular machines and switches have attracted a lot of attention 3] because of their potential to meet the expectations of a visionary and to act as some of the smallest components for the engineering of nanoelectromechanical systems (NEMs) and the fabrication of nanoelectronic devices. Although the redox-switching properties of numerous bistable [2]rotaxanes have been demonstrated in solution, the lack of coherence of the switches in this phase makes it difficult to harness the potential envisaged by Feynman. It is essential that we establish how to self-assemble these tiny switches in an orderly manner at surfaces and to investigate their switching properties in conjunction with their introduction into solid-state devices that have been shown to function as two-dimensional molecular electronic circuits. The fabrication of such devices required the design and synthesis of bistable [2]rotaxanes that are amphiphilic, so that they can be transferred 14±16] as molecular monolayers using the Langmuir ± Blodgett (LB) technique into a device setting. A molecular switch tunnel junction (MSTJ) has been fabricated by sandwiching such self-organized LB monolayers between a bottom Si electrode and a top Ti/Al electrode in a crossbar device architecture. The switch-on (high conductance) and switch-off (low conductance) states of each junction can be addressed respectively upon applying a 2 V or a 2 V bias. The proposed electromechanical switching mechanism (Figure 1) suggests that the ground state, where the cyclobis(paraquat-p-phenylene) (CBPQT , blue) ring initially encircles the tetrathiafulvalene (TTF, green) unit, represents the switch-off state. When a 2 V bias is applied, the CBPQT ring moves mechanically to the 1,5-dioxynaphthalene (DNP, red) ring system as a result of oxidation of the TTF unit to its radical cation. Although, when the bias is removed, neutrality is restored to the TTF unit, the CBPQT ring continues to reside on the DNP ring system, forming the metastable state. The observation of a switch-on state can be attributed to this slow-decaying metastable state that can be erased by applying a 2 V bias for a fleeting moment during the switching cycle. Since the mechanical motion associated with this decay is an activated process, these devices exhibit a hysteretic current ± voltage response. Herein, we describe how we have utilized a custom-designed variable temperature (VT) electrochemical apparatus to investigate the redox-switching behavior of an Au surface-confined linear motor-molecule, that is, a disulfide-tethered bistable [2]rotaxane SSR ¥ 4PF6, together with the corresponding dumbbell-shaped control compound SSD. In both cases, the appended disulfide function is used to immobilize the redox-active [2]rotaxane and dumbbell control onto gold surfaces as selfassembled monolayers (SAMs). The [2]rotaxane SSR ¥ 4PF6 was obtained (Figure 2) by a template-directed protocol wherein a CBPQT ring was clipped around the TTF unit of the dumbbellshaped precursor SSD. Here, we report i) the results of a semiquantitative electrochemical investigation carried out on the surface-confined SSR and the control (SSD) at room temperature in MeCN, leading to the identification of a [28] W. L. Jorgensen, J. Tirado-Rives, J. Am. Chem. Soc. 1988, 110, 1657. [29] Gaussian 98 (Revision A.11.3), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian, Inc. , Pittsburgh, PA, 2002. [30] C. Breneman, K. Wiberg, J. Comput. Chem. 1990, 11, 361. [31] S. J. Weiner, P. A. Kollman, D. A. Case, U. C. Singh, C. Ghio, G. Alagona, S. Profeta, P. Weiner, J. Am. Chem. Soc. 1984, 106, 765. [32] D. Fincham, D. Heyes, Adv. Chem. Phys. 1985, 63, 493. [33] M. Parrinello, A. Rahman, J. App. Phys. 1981, 52, 7182. [34] T. Darden, D. York, L. Pedersen, J. Chem. Phys. 1993, 98, 10089. [35] C. Zannoni, in The Molecular Physics of Liquid Crystals (Eds. : G. R. Luckhurst, G. W. Gray), Academic Press, London 1979, pp. 51 ± 83. [36] I. Haller, Prog. Solid State Chem. 1975, 10, 103. [37] K. Toyne, in Thermotropic Liquid Crystals (Ed. : G. W. Gray), Wiley, London, 28 ±63. [38] M. Hird, in Physical Properties of Liquid Crystals, Vol. 1: Nematics, (Eds. : D. A. Dunmur, A. Fukuda, G. R. Luckhurst), EMIS, IEE, London 2000, pp. 3 ± 16. [39] A. Ferrarini, P. L. Nordio, J. Chem. Soc. Perkin Trans. 1998, 2, 456. [40] M. E. Tuckerman, B. Berne, G. Martyna, J. Chem. Phys. 1992, 97, 1990.

Diffusion-limited photopolymerization in scanning micro-stereolithography

Abstract: The trade-off between process speed and resolution in microstereolithography (μSL) roots on the diffusion-limited kinetics of photopolymerization. Using a numerical model, we have investigated the influence of diffusion dominant effect under high photon flux. Radical depletion turned out to limit the smallest feature achievable to the order of 10 μm under high process speed. A solution of pulsed laser curing is proposed in order to realize sub-micron resolution in high speed μSL process.

Sub-100 nm lithography using ultrashort wavelength of surface plasmons

Abstract: The development of a nanolithography technique utilizing ultrashort wavelength of surface plasmons (SPs) is presented in this article. The mask consists of silver thin film perforated with two-dimensional hole arrays exhibiting superior confinement due to SPs with a wavelength equal to 14 of that of the illuminating light (365 nm). This short wavelength of SPs can confine the field on an area much smaller compared to the excitation light wavelength, leading to the higher resolution lithography than conventional photolithography methods. Finite-difference time-domain simulations show significantly enhanced electric field and tight confinement of the near-field profile obtained from silver plasmonic masks, where features as small as 30 nm can be resolved. Furthermore, the lithography experiments have been performed with demonstration of sub-100 nm spatial resolution.

Temperature dependence of excitonic recombination in lateral epitaxially overgrown InGaN/GaN quantum wells studied with cathodoluminescence

Abstract: We have examined in detail the optical properties of InGaN quantum wells (QWs) grown on pyramidal GaN mesas prepared by lateral epitaxial overgrowth (LEO) in a metalorganic chemical vapor deposition system that resulted in QWs on {1-101} facets. The effects of In migration during growth on the resulting QW thickness and composition were examined with transmission electron microscopy (TEM) and various cathodoluminescence (CL) imaging techniques, including CL wavelength imaging and activation energy imaging. Spatial variations in the luminescence efficiency, QW interband transition energy, thermal activation energy, and exciton binding energy were probed at various temperatures. Cross-sectional TEM was used to examine thickness variations of the InGaN/GaN QW grown on a pyramidal mesa. CL imaging revealed a marked improvement in the homogeneity of CL emission of the LEO sample relative to a reference sample for a conventionally grown In0.15Ga0.85N/GaN QW. The characteristic phase separation that resulted in ...

Manufacturing at Nanoscale: Top-Down, Bottom-up and System Engineering

Abstract: The current nano-technology revolution is facing several major challenges: to manufacture nanodevices below 20 nm, to fabricate three-dimensional complex nano-structures, and to heterogeneously integrate multiple functionalities. To tackle these grand challenges, the Center for Scalable and Integrated NAno-Manufacturing (SINAM), a NSF Nanoscale Science and Engineering Center, set its goal to establish a new manufacturing paradigm that integrates an array of new nano-manufacturing technologies, including the plasmonic imaging lithography and ultramolding imprint lithography aiming toward critical resolution of 1–10 nm and the hybrid top-down and bottom-up technologies to achieve massively parallel integration of heterogeneous nanoscale components into higher-order structures and devices. Furthermore, SINAM will develop system engineering strategies to scale-up the nano-manufacturing technologies. SINAMs integrated research and education platform will shed light to a broad range of potential applications in computing, telecommunication, photonics, biotechnology, health care, and national security.

Suppression of porosity in beam weaving laser welding

Abstract: The present paper describes a beam weaving laser welding technique to suppress argon or nitrogen porosity, which may appear during laser welding of low carbon steel. Bead on plate welding was performed using a 3 kW CO2 laser. The weaving frequency was varied within 0–30 Hz and the weaving amplitude within 0–2 mm during welding. The experimental results show that under 2.4 kW laser power and 1.0 m min-1 welding speed, the nitrogen porosity decreases remarkably with increasing frequency, and it can be eliminated for a weaving frequency of 22 Hz with 0.5 mm weaving amplitude. Under 2.4 kW laser power and 1.5 m min-1 welding speed, beam weaving laser welding can also effectively reduce argon porosity at a weaving frequency of 22 Hz and amplitude of 1.0–1.5 mm.

Novel Approaches to Biomolecular Sequence Indexing.

Abstract: In many biomolecular database applications involving string/sequence data, it is common to have similarity search in the form of near neighbor queries or nearest neighbor queries. The similarity between strings/sequences are typically measured in terms of the least costly set of allowed edit operations that transform one string/sequence to another. In this survey, we briefly describe some of the recent developments in biomolecular sequence indexing methods that allow efficient similarity search. Our focus here is on global similarity measures that compare sequences in full; such measures are important for comparing protein sequences and smaller biomolecules. Examples include character and block edit distances and their weighted variants. Two major approaches are summarized here: distance based indexing and embeddings of general sequence similarity measures to Hamming distance, for which efficient indexing methods are available.

Latent splice sites and stop codons revisited.

Abstract: The accuracy of the data we reported in an RNA Letter to the Editor earlier this year on the possible relationship between stop codons and splicing is questioned by Miriami et al. (this issue). We reply here that we see no inaccuracy in our data presentation and offer a possible explanation for their interpretation.

Tunable plasmonic wires at terahertz frequencies

Abstract: We employed micro-electro-mechanical system (MEMS) techniques to fabricate parallel sub-wavelength thin-wire structures of metals on elastomeric matrices. From the transmission measurement by Fourier Transform Infrared Spectroscopy, we observed the depressed plasma frequencies of these thin-wire structures at terahertz (THz) ranges. Furthermore, the behavior of depressed plasma frequencies is very sensitive to the polarization of the applied field. The reasons that these engineered materials exhibit unprecedented properties not observed in nature can be interpreted by two factors: the diluted electron densities and the enhancement of electron mass. In addition, the plasma frequencies are readily tunable over a broad frequency range by extending the elastomeric matrices to change their periodicity. These novel properties of tunable and polarization-dependant plasma frequencies at THz ranges promise abundant striking applications in THz optics.

Numerical study of the current conduction in single-layer organic light-emitting devices

Abstract: A numerical model for the current conduction in single-layer organic light-emitting devices is established under the basis of trapped charge-limited conduction with an exponential trap distribution. The dependences of the current density on the operation voltage, the thickness of the organic layer, and the trap properties are numerically studied. The current density decreases nearly exponentially with the thickness of the organic layer and the relative trap depth (l), and it is inversely proportional to the lth power of the total trap density. The results from simulations for the current–voltage characteristics agree very well with those from experiments.

Delamination/debond growth in Z-fibre reinforced composite T-joints: A finite elements simulation

Abstract: This paper addresses the damage tolerance capability of laminated T-joints having through-thickness reinforcement by Z-fibres. This kind of reinforcement, usually referred to as Z-pinning, has been developed by Foster-Miller and Aztex in the USA since 1992, to improve the damage tolerance of traditional laminated composites. A 3D FE model of a composite T-joint has been developed. The model represents delamination and debond in the web-to-base bonded flange by a proper combination of linear and nonlinear spring elements; the latter describes the bridging forces exerted by Z-pins during the delamination growth. The modelling strategy has been validated by means of experimental data regarding both unpinned and pinned composite T-joints.

Impact of Weak Localization on Wave Dynamics: Crossover from Quasi-1D to Slab Geometry

Abstract: Department of Physics, Queens College of the City University of New York, Flushing, New York 11367, USA(Dated: February 2, 2008)We study the dynamics of wave propagation in nominally diffusive samples by solving the Bethe-Salpeter equation with recurrent scattering included in a frequency-dependentvertex function, whichrenormalizes the mean free path of the system. We calculate the renormalized time-dependentdiffusion coefficient, D(t), following pulsed excitation of the system. For cylindrical samples withreflecting side walls and open ends, we observe a crossover in dynamics in the transformation froma quasi-1D to a slab geometry implemented by varying the ratio of the radius, R, to the length, L.Immediately after the peak of the transmitted pulse, D(t) falls linearly with a nonuniversal slopethat approaches an asymptotic value for R/L ≫ 1. The value of D(t) extrapolated to t = 0, dependsonly upon the dimensionless conductance g for R/L ≪ 1 and upon kl