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27 Aug 2007

Volume 91, Issue 9, Articles (09xxxx)

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Appl. Phys. Lett. 91, 093110 (2007); http://dx.doi.org/10.1063/1.2775801 (3 pages)

M. Schmidbauer, Zh. M. Wang, Yu. I. Mazur, P. M. Lytvyn, G. J. Salamo, D. Grigoriev, P. Schäfer, R. Köhler, and M. Hanke
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Localized, low-voltage electro-osmotic pumping across nanoporous membranes

A. Gupta, H. Denver, A. H. Hirsa, J. A. Stenken, and D.-A. Borca-Tasciuc

Appl. Phys. Lett. 91, 094101 (2007); http://dx.doi.org/10.1063/1.2775836 (3 pages) | Cited 2 times

Online Publication Date: 28 August 2007

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This work demonstrates the principle of localized, low-voltage, continuous current electro-osmotic pumping across thin membranes. Localization of the pumping effect is obtained by employing ring electrodes, microfabricated on each side of nanoporous alumina membranes. Low voltage actuation is achieved by having the separation of the two electrodes in the microscale range—the thickness of the membrane. The measured electro-osmotic velocity increases linearly from 40 μm/s at 5 V to 200 μm/s at 20 V. Experimental results compare well with predictions of an electro-osmotic flow model considering the nonuniform distribution of the electric field between the electrodes.
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61.46.-w Structure of nanoscale materials
82.45.-h Electrochemistry and electrophoresis

Capacitive micromachined ultrasonic transducers for chemical detection in nitrogen

K. K. Park, H. J. Lee, G. G. Yaralioglu, A. S. Ergun, Ö. Oralkan, M. Kupnik, C. F. Quate, B. T. Khuri-Yakub, T. Braun, J.-P. Ramseyer, H. P. Lang, M. Hegner, Ch. Gerber, and J. K. Gimzewski

Appl. Phys. Lett. 91, 094102 (2007); http://dx.doi.org/10.1063/1.2776348 (3 pages) | Cited 14 times

Online Publication Date: 28 August 2007

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The authors present the prototype of a chemical sensor using a capacitive micromachined ultrasonic transducer array. Each element in the array consists of a large number of resonating membranes connected in parallel. A five-channel oscillator circuit operates at the resonant frequency around 6 MHz in this prototype. The surface of the elements in the array is coated by polymers such as polyallylamine hydrochloride, polyethylene glycol, and polyvinyl alcohol to detect different chemicals. By measuring shift in oscillation frequencies due to the mass-loading effect, analytes, e.g., water and isopropanol, with concentrations around 20 ppbv (parts per 109 by volume) range can be detected.
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07.10.Cm Micromechanical devices and systems
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
43.35.Yb Ultrasonic instrumentation and measurement techniques
43.38.Hz Transducer arrays, acoustic interaction effects in arrays

Spatial-frequency multiplication via absorbance modulation

Hsin-Yu Tsai, Gregory M. Wallraff, and Rajesh Menon

Appl. Phys. Lett. 91, 094103 (2007); http://dx.doi.org/10.1063/1.2775092 (3 pages) | Cited 6 times

Online Publication Date: 28 August 2007

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The absorbance of a thin film of photochromic material can be reversibly modified by exposure to two different wavelengths, λ1 and λ2. When such a film is illuminated by both wavelengths simultaneously, and the longer wavelength λ2 possesses a node in its intensity distribution, then the absorbance of the layer can be made high except at an arbitrarily small region near the node. By exploiting the large nonlinearity introduced by this mechanism, combined with the reversibility of the absorbance of the photochromic layer, the authors demonstrate that spatial frequencies larger than those present in incident intensity distributions may be generated. They show photoresist exposures to demonstrate this technique.
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78.66.-w Optical properties of specific thin films
42.65.-k Nonlinear optics
61.80.-x Physical radiation effects, radiation damage

Giant Kerr nonlinearities and solitons in a crystal of molecular magnets

Ying Wu and Xiaoxue Yang

Appl. Phys. Lett. 91, 094104 (2007); http://dx.doi.org/10.1063/1.2775094 (3 pages) | Cited 42 times

Online Publication Date: 28 August 2007

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The authors show the formation of microwave solitons in a crystal of molecular magnets via an electromagnetically induced transparency and have the giant cross-phase modulation phase shifts with the advantages of low pump powers, low absorptions, high sensitivities, and certain frequency tunability.
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75.50.Xx Molecular magnets
78.20.Ls Magneto-optical effects
42.65.Tg Optical solitons; nonlinear guided waves
42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption

Radiation detector resolution over a continuous energy range

Yanwen Zhang, Brian D. Milbrath, William J. Weber, Mikael Elfman, and Harry J. Whitlow

Appl. Phys. Lett. 91, 094105 (2007); http://dx.doi.org/10.1063/1.2776978 (3 pages) | Cited 6 times

Online Publication Date: 29 August 2007

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An ion approach is demonstrated to determine energy resolution in both semiconductor detectors and scintillators over a continuous energy range. For semiconductors, the energy resolution of a silicon detector was measured as a function of helium ion energy, and the values from extrapolation to high energies are in good agreement with the literature data from alpha measurements. For scintillators, benchmark crystals subject to He+ irradiation were investigated, and the agreement of energy resolution between the ion and gamma measurements indicates that the ion approach can be used to predict the energy resolution of the candidate materials in thin-film form or small crystals when large crystals necessary for gamma-ray measurements are unavailable.
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29.40.Wk Solid-state detectors
29.40.Mc Scintillation detectors
07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
61.80.Jh Ion radiation effects

Nondestructive analysis of threading dislocations in GaN by electron channeling contrast imaging

Y. N. Picard, J. D. Caldwell, M. E. Twigg, C. R. Eddy, Jr., M. A. Mastro, R. L. Henry, R. T. Holm, P. G. Neudeck, A. J. Trunek, and J. A. Powell

Appl. Phys. Lett. 91, 094106 (2007); http://dx.doi.org/10.1063/1.2777151 (3 pages) | Cited 8 times

Online Publication Date: 29 August 2007

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Threading dislocations in metal-organic chemical-vapor grown GaN films were imaged nondestructively by the electron channeling contrast imaging (ECCI) technique. Comparisons between ECCI and cross-sectional transmission electron microscopy indicated that pure edge dislocations can be imaged in GaN by ECCI. Total threading dislocation densities were measured by ECCI for various GaN films on engineered 4H-SiC surfaces and ranged from 107 to 109 cm−2. A comparison between the ultraviolet electroluminescent output measured at 380 nm and the total dislocation density as measured by ECCI revealed an inverse logarithmic dependence.
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81.05.Ea III-V semiconductors
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.-y Defects and impurities in crystals; microstructure
61.72.Lk Linear defects: dislocations, disclinations
78.40.Fy Semiconductors
78.60.Fi Electroluminescence

Large impact of strontium substitution on photocatalytic water splitting activity of BaSnO3

Yupeng Yuan, Jun Lv, Xiaojun Jiang, Zhaosheng Li, Tao Yu, Zhigang Zou, and Jinhua Ye

Appl. Phys. Lett. 91, 094107 (2007); http://dx.doi.org/10.1063/1.2778631 (3 pages) | Cited 18 times

Online Publication Date: 30 August 2007

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The effects of Sr substitution for Ba on photocatalytic water splitting activity of BaSnO3 were investigated experimentally and theoretically. The chemical incorporation of Sr into BaSnO3 induced a great enhancement for H2 production. Density function theory calculations of Ba1−xSrxSnO3 (x = 0, 0.5, and 1.0) reveal that the bottom of the conduction band is gradually pushed up and the contribution of the Sr 5s orbitals to the bottom of the conduction band gradually becomes dominant with the increase of Sr concentration from x = 0 to x = 1.0. The participation of the normally inert Sr cation in the electronic structure of the SnO32− framework not only enhances the reduction ability of photoinduced electrons but also provides favorable opportunities for charge carrier transport, thus enhancing the photocatalytic activity of BaSnO3.
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82.50.-m Photochemistry
82.30.Vy Homogeneous catalysis in solution, polymers and zeolites
71.20.Nr Semiconductor compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

From superhydrophobic to superhydrophilic surfaces tuned by surfactant solutions

Feng-Ming Chang, Yu-Jane Sheng, Hui Chen, and Heng-Kwong Tsao

Appl. Phys. Lett. 91, 094108 (2007); http://dx.doi.org/10.1063/1.2779092 (3 pages) | Cited 9 times

Online Publication Date: 31 August 2007

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The wettability of hydrophobic surfaces is generally improved by surfactant solutions. The wetting behavior of superhydrophobic surfaces can be classified into two types, in terms of the variation of contact angle with surfactant concentration cs. Contact angle is controlled by surface tension for common linear surfactants and becomes independent of cs as cs>critical micelle concentration. Consequently, superhydrophobic surfaces remain in hydrophobic range, as reported. However, for branch-tailed surfactants such as sodium-bisethylhexylsulfosuccinate and didodecyldimethylammonium bromide, superhydrophobic surfaces can turn superhydrophilic by increasing cs owing to continuous reduction of solid-liquid interfacial tension. The superhydrophobicity is recoverable simply by water rinsing.
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68.08.Bc Wetting
68.03.Cd Surface tension and related phenomena
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