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Top 20 Most Read Articles
June 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Organic photovoltaic cells based on an acceptor of soluble graphene Appl. Phys. Lett. 92, 223303 (2008); http://dx.doi.org/10.1063/1.2938865 (3 pages) Online Publication Date: 2 June 2008
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In this paper, an organic photovoltaic device based on an acceptor of solution-processable functionalized graphene was designed. A short circuit current density (Jsc) of 4.0 mA cm−2, open circuit voltage (Voc) of 0.72 V, and a solar power conversion efficiency of 1.1% were obtained for the device of indium tin oxide/poly(ethylene dioxythiophene) doped with polystyrene sulfonic acid (40 nm)/poly(3-hexylthiophene-1, 3-diyl):graphene (graphene 10 wt %, 100 nm)/LiF (1 nm)/Al (70 nm) after an annealing treatment under simulated AM1.5G 100 mW illumination in air. Because of the low price, ease of preparation, and inertness against ambient conditions, soluble graphene will be a promising candidate used for acceptor materials in the photovoltaic applications.
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Terahertz near-field imaging of metallic subwavelength holes and hole arrays Appl. Phys. Lett. 92, 231101 (2008); http://dx.doi.org/10.1063/1.2936303 (3 pages) Online Publication Date: 9 June 2008
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Metallic microstructures are investigated by time-resolved terahertz near-field imaging. By our approach, we can directly follow field diffraction from subwavelength structures as well as coupling to the surface. Near-field images of the spectral amplitude and phase of the electric field show the formation, propagation, and attenuation of surface waves and allow us to distinguish between propagating and stationary modes. Our results show that the field enhancement in an individual hole, together with the formation of standing waves on the metal surface between the holes, are key mechanisms for the extraordinary transmission phenomenon through periodic hole arrays.
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Tuning field-induced energy gap of bilayer graphene via interlayer spacing Appl. Phys. Lett. 92, 243101 (2008); http://dx.doi.org/10.1063/1.2943414 (3 pages) Online Publication Date: 16 June 2008
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Our first-principles calculations reveal surprisingly high sensitivity of the field-induced energy gap of bilayer graphene to changes in its interlayer spacing. Small adjustments in the interlayer spacing near its equilibrium value produce large modulations in the gap over a wide range of field strength. We elucidate the mechanism for the extremely effective gap tuning by examining the interlayer charge redistribution driven by the coupled electric field and nanomechanical effect.
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Surface-plasmon enhanced transparent electrodes in organic photovoltaics Appl. Phys. Lett. 92, 243304 (2008); http://dx.doi.org/10.1063/1.2938089 (3 pages) Online Publication Date: 17 June 2008
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Random silver nanohole films were created through colloidal lithography techniques and metal vapor deposition. The transparent electrodes were characterized by uv-visible spectroscopy and incorporated into an organic solar cell. The test cells were evaluated for solar power-conversion efficiency and incident photon-to-current conversion efficiency. The incident photon-to-current conversion efficiency spectra displayed evidence that a nanohole film with 92 nm diameter holes induces surface-plasmon-enhanced photoconversion. The nanohole silver films demonstrate a promising route to removing the indium tin oxide transparent electrode that is ubiquitous in organic optoelectronics.
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Polarization-dependent confocal Raman microscopy of an individual ZnO nanorod Appl. Phys. Lett. 92, 223102 (2008); http://dx.doi.org/10.1063/1.2938701 (3 pages) Online Publication Date: 2 June 2008
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In this study, polarized-Raman scattering measurements of an individual ZnO nanorod were carried out by using a confocal microscope together with a high-resolution piezoelectric stage. A predominant A1 (TO) mode at 378 cm−1 in the parallel polarization (E∥) configuration and a predominant E2 (high) mode at 438 cm−1 in the perpendicular polarization (E⊥) configuration demonstrate the strong polarization dependent Raman scattering signals of an individual ZnO nanorod. The Raman intensity images of the individual ZnO nanorods with different orientation configurations can also be obtained, which reflect the interplay between the local phonon behavior and geometric anisotropy.
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Whispering gallery mode lasing in zinc oxide microwires Appl. Phys. Lett. 92, 241102 (2008); http://dx.doi.org/10.1063/1.2946660 (3 pages) Online Publication Date: 17 June 2008
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Lasing on whispering gallery modes was excited by optical pumping in single zinc oxide microwires fabricated by a simple carbothermal evaporation process. The experimentally observed laser modes agree precisely with the predicted energetic positions obtained from a plane wave model. Systematic diameter dependent measurements have been carried out for diameters of the microwires between 3 and 12 μm. The investigated microlasers are found to have a lasing threshold of about 170 kW/cm2 at 10 K.
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Appl. Phys. Lett. 92, 223301 (2008); http://dx.doi.org/10.1063/1.2937403 (3 pages) Online Publication Date: 2 June 2008
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We report a significant improvement in the efficiency of deep-blue phosphorescent organic light-emitting devices based on the electrophosphorescent dye bis(4′,6′-difluorophenylpyridinato)tetrakis (1-pyrazolyl) borate (FIr6). Using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as the hole transport layer (HTL), we achieved a maximum external quantum efficiency of ηEQE = (18±1)%, which is approximately 50% higher than ηEQE = 12% in a previously reported device with bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl as the HTL. The maximum luminous power efficiency was also improved from (14±1) lm/W to (18±1) lm/W. We attribute this efficiency improvement to the enhanced electron and exciton confinement provided by TAPC.
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Power efficiency improvement in a tandem organic light-emitting diode Appl. Phys. Lett. 92, 223311 (2008); http://dx.doi.org/10.1063/1.2938269 (3 pages) Online Publication Date: 6 June 2008
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When a tandem light-emitting diode (OLED) utilizes unoptimized electroluminescent (EL) units, it is fairly easy to improve the power efficiency of such a device. However, when a tandem OLED utilizes optimized EL units, improved power efficiency can only be achieved if each intermediate connector has excellent carrier injection capabilities along with a negligible voltage drop across it. Four organic intermediate connectors were studied in this work, one of which consisting of a Li-doped 4,7-diphenyl-1,10-phenanthroline layer and a 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile layer, exhibits the best power efficiency improvement for tandem OLEDs.
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Appl. Phys. Lett. 92, 253301 (2008); http://dx.doi.org/10.1063/1.2945281 (3 pages) Online Publication Date: 23 June 2008
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The performance and stability of unencapsulated inverted bulk-heterojunction solar cells with zinc oxide (ZnO) made by different processes as the electron selective contact are compared to conventional bulk-heterojunction solar cells. The low temperature processed inverted devices using ZnO nanoparticles on indium tin oxide plastic substrates showed high power conversion efficiency of ∼ 3.3%. This inverted device structure possessed much better stability under ambient conditions retaining over 80% of its original conversion efficiency after 40 days while the conventional one showed negligible photovoltaic activity after 4 days. This is due to the improved stability at the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/Ag interface.
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Deep level defects in a nitrogen-implanted ZnO homogeneous p-n junction Appl. Phys. Lett. 92, 222109 (2008); http://dx.doi.org/10.1063/1.2940204 (3 pages) Online Publication Date: 5 June 2008
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Nitrogen ions were implanted into undoped melt grown ZnO single crystals. A light-emitting p-n junction was subsequently formed by postimplantation annealing in air. Deep level transient spectroscopy was used to investigate deep level defects induced by N+ implantation and the effect of air annealing. The N+ implantation enhanced the electron trap at EC−(0.31±0.01) eV (E3) and introduced another one at EC−(0.95±0.02) eV (D1), which were removed after annealing at 900 and 750 °C, respectively. Another trap D2 (Ea = 0.17±0.01 eV) was formed after the 750 °C annealing and persisted at 1200 °C.
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Modulated terahertz responses of split ring resonators by nanometer thick liquid layers Appl. Phys. Lett. 92, 221101 (2008); http://dx.doi.org/10.1063/1.2939215 (3 pages) Online Publication Date: 2 June 2008
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The terahertz responses of planar arrays of split ring resonators with 50 nm thin liquid layers introduced in the interspaces between the metal structures have been studied using the terahertz-time domain spectroscopy technique. In our experimental configuration, both the circular current driven and linear polarization induced resonances show redshifted and enhanced transmission. The dielectric functions extracted from an effective medium model indicate that the behavior of the surface and interface charge oscillations is highly sensitive to the interface environment changes. The results suggest that this kind of device may be used for sensing applications.
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Deformability of ultrahigh strength 5 nm Cu/Nb nanolayered composites Appl. Phys. Lett. 92, 231901 (2008); http://dx.doi.org/10.1063/1.2938921 (3 pages) Online Publication Date: 9 June 2008
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In this work, micropillar compression testing has been used to obtain stress-strain curves for sputter-deposited Cu–Nb nanolaminate composites with nominal bilayer thickness of 10 nm. In addition to the extremely high flow strength of 2.4 GPa, the 5 nm Cu/5 nm Nb nanolaminate exhibits significant ductility, in excess of 25% true strain.
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Optical properties of yellow light-emitting diodes grown on semipolar (11 Appl. Phys. Lett. 92, 221110 (2008); http://dx.doi.org/10.1063/1.2938062 (3 pages) Online Publication Date: 5 June 2008
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We demonstrate high power yellow InGaN single-quantum-well light-emitting diodes (LEDs) with a peak emission wavelength of 562.7 nm grown on low extended defect density semipolar (11
 2) bulk GaN substrates by metal organic chemical vapor deposition. The output power and external quantum efficiency at drive currents of 20 and 200 mA under pulsed operation (10% duty cycle) were 5.9 mW, 13.4% and 29.2 mW, 6.4%, respectively. It was observed that the temperature dependence of the output power of InGaN LEDs was significantly smaller than that of AlInGaP LEDs. |
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Suppressing series resistance in organic solar cells by oxygen plasma treatment Appl. Phys. Lett. 92, 233302 (2008); http://dx.doi.org/10.1063/1.2940236 (3 pages) Online Publication Date: 11 June 2008
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We demonstrate a low series resistance contact of gold (anode)/zinc-phthalocyznine in a reverse organic photovoltaic device aided by oxygen plasma treatment. The power conversion efficiency appreciated from 0.2% to 2.13%, post O2 plasma treatment, predominantly due to the reduction in the series resistance of the device. A depth profile of zinc from x-ray photoelectron spectroscopy study revealed a gold layer with graded zinc composition, instead of pure gold, to be an efficient hole-collector from the organic interface. The zinc alloying of the gold layer has clearly been promoted by the oxygen plasma treatment.
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Direct observation of conducting filaments on resistive switching of NiO thin films Appl. Phys. Lett. 92, 222106 (2008); http://dx.doi.org/10.1063/1.2931087 (3 pages) Online Publication Date: 2 June 2008
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The Hg/NiO/Pt capacitor with a Hg top electrode diameter of about 35 μm showed the typical bistable resistive switching characteristic. After the removal of the Hg top electrode, we directly observed the formation and removal of filaments for a high resistive state (Roff) and a low resistive state (Ron) by conducting atomic force microscope (CAFM). CAFM images for Roff and Ron states directly exhibit evidence of the formation and removal of filaments on the surface, which supports well the filament model as a switching mechanism of resistive random access memory.
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Method for full Bloch sphere control of a localized spin via a single electrical gate Appl. Phys. Lett. 92, 222502 (2008); http://dx.doi.org/10.1063/1.2937305 (3 pages) Online Publication Date: 3 June 2008
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We calculate the dependence on an applied electric field of the g tensor of a single electron in a self-assembled InAs/GaAs quantum dot. We identify dot sizes and shapes for which one in-plane component of the g tensor changes sign for realistic electric fields, and show that this should permit full Bloch sphere control of the electron spin in the quantum dot using only a static magnetic field and a single vertical electric gate.
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Investigation on polymer anode design for flexible polymer solar cells Appl. Phys. Lett. 92, 233308 (2008); http://dx.doi.org/10.1063/1.2945796 (3 pages) Online Publication Date: 13 June 2008
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Bilayer polymer anode composed of poly(3,4-ethylene-dioxythiophene): polystyrenesulfonate (PEDOT:PSS) (PH500) and PEDOT:PSS (Baytron P VP Al 4083) was used to construct flexible polymer solar cells on plastic substrates polyethylene terephthalate (PET) with a device structure of PET/polymer anode/APFO-3:PCBM/LiF/Al. The power conversion efficiency (PCE) of the indium tin oxide (ITO)-free solar cells achieved 2.2% under illumination of AM1.5 (100 mW cm−2), which is 80% of the PCE of the reference cells with ITO on glass. The simplicity of preparing bilayer polymer anode and the comparable performance achieved in the flexible solar cells made the bilayer polymer anode an alternative to ITO for flexible solar cells produced by printing technology.
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Sidewall corrugation lithography: Bulk fabrication of ordered nanowires, nanoribbons, and nanorings Appl. Phys. Lett. 92, 223109 (2008); http://dx.doi.org/10.1063/1.2939563 (3 pages) Online Publication Date: 4 June 2008
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A general technique for three-dimensional nanofabrication is demonstrated by employing photolithographic standing wave corrugations as nanoscale templates. The line-of-sight deposition of a variety of materials demonstrates vertical stacks of nanowires and nanoribbons that take the forms of straight lengths, circular rings, and multilayer structures. Contact photolithography is used to fabricate the micron-scale corrugated templates from which nanofeatures with widths ranging from 65 to ∼ 20 nm are derived.
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Photonic bandgap of gradient quasidiamond lattice photonic crystal Appl. Phys. Lett. 92, 231103 (2008); http://dx.doi.org/10.1063/1.2943278 (3 pages) Online Publication Date: 10 June 2008
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A three-dimensional (3D) photonic crystal (PhC) structure consisting of gradient quasidiamond lattices was fabricated using multiphoton photopolymerization nanofabrication technique. The photonic bandgap (PBG) of this 3D PhC was experimentally confirmed by reflection and transmission measurements and simulated with finite-difference time domain calculations. The results indicate that a 3D PhC with gradient lattices could effectively expand the width of the PBG and may be beneficial for developing complete-bandgap PhCs with low refractive index materials for applications in polymer based optoelectronic devices and integrated systems.
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On the effect of the electrical contact resistance in nanodevices Appl. Phys. Lett. 92, 243106 (2008); http://dx.doi.org/10.1063/1.2946663 (3 pages) Online Publication Date: 17 June 2008
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We have investigated the effect of the electric contact resistance in nano devices by manufacturing dedicated chips with four-point contacted platinum nanowires and individual, suspended, and four-point contacted carbon nanotubes. For both cases, we measured the intrinsic electrical resistance and the electrical contact resistance. Our results indicate that the electrical contact resistance of the platinum nanowires and the carbon nanotubes can be one to three orders of magnitude higher than the intrinsic electrical resistance of the sample itself. Subsequently, we tested the platinum nanowires as pressure sensors to quantify the impact of the electrical contact resistance on the performance of micro- and nanodevices.
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