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Top 20 Most Read Articles
December 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Photonic integration in k-space: Enhancing the performance of photonic crystal dye lasers Appl. Phys. Lett. 93, 231101 (2008); http://dx.doi.org/10.1063/1.3040686 (3 pages) Online Publication Date: 8 December 2008
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We demonstrate how two optical functionalities can be implemented in a single photonic crystal structure by carefully engineering dispersion in several different bands at several different wavelengths. We use the concept for optically pumped dye doped hybrid polymer band edge lasers and show how a rectangular photonic crystal lattice imprinted into the surface can provide both feedback for in-plane band edge lasing and couple pump light into the device plane, thus increasing the emitted intensity and lowering the lasing threshold by more than an order of magnitude.
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Appl. Phys. Lett. 93, 221105 (2008); http://dx.doi.org/10.1063/1.3039787 (3 pages) Online Publication Date: 1 December 2008
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Herein the authors report the experimental application of a powerful light trapping scheme, the textured photonic crystal (TPC) backside reflector, to thin film Si solar cells. TPC combines a one-dimensional photonic crystal as a distributed Bragg reflector with a diffraction grating. Light absorption is strongly enhanced by high reflectivity and large angle diffraction, as designed with scattering matrix analysis. 5 μm thick monocrystalline thin film Si solar cells integrated with TPC were fabricated through an active layer transfer technique. Measured short circuit current density Jsc was increased by 19%, compared to a theoretical prediction of 28%.
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Plasmonic photonic crystal with a complete band gap for surface plasmon polariton waves Appl. Phys. Lett. 93, 231105 (2008); http://dx.doi.org/10.1063/1.3043581 (3 pages) Online Publication Date: 9 December 2008
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A dielectric plasmonic photonic crystal for manipulating surface plasmon polariton (SPP) fields has been designed, fabricated, and tested. The band structure of SPP fields inside the plasmonic photonic crystal has been calculated using the plane wave expansion method and validated by full wave numerical simulations. The fabricated device was characterized using our far-field SPP imaging technique. The transmittance of incident SPP waves is about 5% at 1520 nm (a designed band gap frequency), confirming the designed band structure. The results show, both experimentally and theoretically, a complete two-dimensional band gap for in-plane SPP waves. The SPP fields at frequencies within this specified 0.14ωa/2πc-wide band gap frequency range are forbidden to propagate through the plasmonic photonic crystal.
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Appl. Phys. Lett. 93, 223505 (2008); http://dx.doi.org/10.1063/1.3041643 (3 pages) Online Publication Date: 3 December 2008
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This report covers the fabrication of a fully transparent resistive random access memory (TRRAM) device based on an ITO (indium tin oxide)/ZnO/ITO capacitor structure and its resistive switching characteristics. The fabricated TRRAM has a transmittance of 81% (including the substrate) in the visible region and an excellent switching behavior under 3 V. The retention measurement suggests that the memory property of the TRRAM device could be maintained for more than 10 years. We believe that the TRRAM device presented in this work could be a milestone of future see-through electronic devices.
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GaN nanostructured p-i-n photodiodes Appl. Phys. Lett. 93, 221104 (2008); http://dx.doi.org/10.1063/1.3041641 (3 pages) Online Publication Date: 1 December 2008
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We report the fabrication of nanostructured p-i-n photodiodes based on GaN. Each device comprises arrays of ∼ 200 nm diameter and 520 nm tall nanopillars on a 1 μm period, fabricated by e-beam lithography. Strong rectifying behavior was obtained with an average reverse current per nanopillar of 5 fA at −5 V. In contrast to conventional GaN diodes, nanostructured devices reproducibly show ideality factors lower than 2. Enhanced tunneling through sidewall surface states is proposed as the responsible mechanism for this behavior. Under backillumination, the quantum efficiency in nanostructured devices is partly limited by the collection efficiency of holes into the nanopillars.
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Appl. Phys. Lett. 93, 221101 (2008); http://dx.doi.org/10.1063/1.3036234 (3 pages) Online Publication Date: 1 December 2008
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The authors have demonstrated the enhancement of linearly polarized lasing emission intensity using a structure made by a simple fabrication process. The enhanced lasing is achieved using a nanoimprinted distributed feedback structure together with spin-coated polymeric liquid crystals. The backward linearly TE-polarized lasing emission is transformed to left-handed circularly polarized light (L-CPL) by employing a dye-doped polymeric nematic liquid crystal (PNLC) film as a (−1/4)λ[ = (3/4)λ] plate. The L-CPL is effectively reflected by a L-polymeric cholesteric liquid crystal film as a reflector and transformed back to TE-polarized light by the PNLC film; as a result one-directional emission intensity is enhanced.
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Transparent and conductive electrodes based on unpatterned, thin metal films Appl. Phys. Lett. 93, 223304 (2008); http://dx.doi.org/10.1063/1.3028046 (3 pages) Online Publication Date: 4 December 2008
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Transparent electrodes composed of ultrathin, unpatterned metal films are investigated in planar heterojunction (PHJ) and bulk heterojunction organic photovoltaic (OPV) cells. Optimal electrode composition and thickness are deduced from electrical and optical models and experiments, enabling a PHJ-OPV cell to be realized using a silver anode, achieving power conversion efficiency parity with an analogous cell that uses an indium tin oxide anode. Beneficial aspects of smooth, unpatterned metal films as transparent electrodes in OPV cells are also discussed in the text.
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Appl. Phys. Lett. 93, 223302 (2008); http://dx.doi.org/10.1063/1.3041633 (3 pages) Online Publication Date: 1 December 2008
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The existence of a ground-state charge-transfer (CT) complex in a conjugated polymer:metal oxide nanoparticle bulk heterojunction photovoltaic cell is demonstrated by Fourier-transform photocurrent spectroscopy (FTPS). The CT complex between poly(3-hexylthiophene) (P3HT) and titanium dioxide (TiO2) is characterized by a weak additional photocurrent band (onset 1 eV) in the FTPS spectra, situated below the conjugated polymer bandgap of 2 eV. The presence of CT interaction between P3HT and TiO2 in relation to frontier orbital alignment is discussed, as well as the contribution of a sub-bandgap interfacial CT state to the electron transfer process in P3HT:TiO2 solar cells.
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Appl. Phys. Lett. 93, 233304 (2008); http://dx.doi.org/10.1063/1.3028094 (3 pages) Online Publication Date: 11 December 2008
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The performance of inverted bulk-heterojunction solar cells with zinc oxide nanoparticles as the electron selective contact is compared to those modified with a fullerene self-assembled monolayer (C60-SAM). The devices modified with a C60-SAM show very significant improvement in conversion efficiencies compared to unmodified devices leading to efficiencies as high as 4.9%. This is due to enhanced electronic coupling of the inorganic/organic interface from the C60-SAM leading to improved fill factor and photocurrent. Furthermore, devices fabricated in an inert environment were compared to those fabricated in ambient showing comparable device performance.
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On the resistive switching mechanisms of Cu/ZrO2:Cu/Pt Appl. Phys. Lett. 93, 223506 (2008); http://dx.doi.org/10.1063/1.3039079 (3 pages) Online Publication Date: 5 December 2008
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We use convincing experimental evidences to demonstrate that the nonpolar resistive switching phenomenon observed in Cu/ZrO2:Cu/Pt memory devices conforms to a filament formation and annihilation mechanism. Temperature-dependent switching characteristics show that a metallic filamentary channel is responsible for the low resistance state (ON state). Further analysis reveals that the physical origin of this metallic filament is the nanoscale Cu conductive bridge. On this basis, we propose that the set process (switching from OFF state to ON state) and the reset process (switching from ON to OFF state) stem from the electrochemical reactions in the filament, in which a thermal effect is greatly involved.
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Design principles for particle plasmon enhanced solar cells Appl. Phys. Lett. 93, 191113 (2008); http://dx.doi.org/10.1063/1.3021072 (3 pages) Online Publication Date: 14 November 2008
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We develop fundamental design principles for increasing the efficiency of solar cells using light trapping by scattering from metal nanoparticles. We show that cylindrical and hemispherical particles lead to much higher path length enhancements than spherical particles, due to enhanced near-field coupling, and that the path length enhancement for an electric point dipole is even higher than the Lambertian value. Silver particles give much higher path length enhancements than gold particles. The scattering cross section of the particles is very sensitive to the thickness of a spacer layer at the substrate, which provides additional tunability in the design of particle arrays.
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Light-emitting polymer space-charge-limited transistor Appl. Phys. Lett. 93, 223301 (2008); http://dx.doi.org/10.1063/1.3027057 (3 pages) Online Publication Date: 1 December 2008
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Polymer light-emitting transistor is realized by vertically stacking a top-emitting polymer light-emitting diode on a polymer space-charge-limited transistor. The transistor modulates the current flow of the light-emitting diode by the metal-grid base voltage. The active semiconductor of the transistor is poly(3-hexylthiophene). Yellow poly(para-phenylene vinylene) derivative is used as the yellow emitting material. As the cathode is fixed at −12 V and the grid base voltage varies from 0.9 to −0.9 V the light emission is turned on and off with on luminance up to 1208 cd/m2. The current efficiency of the light-emitting transistor is 10 cd/A.
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Ultrahigh-brightness 850 nm GaAs/AlGaAs photonic crystal laser diodes Appl. Phys. Lett. 93, 221102 (2008); http://dx.doi.org/10.1063/1.3040322 (3 pages) Online Publication Date: 1 December 2008
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One-dimensional photonic crystal lasers emitting in the 850 nm range show high internal quantum efficiencies of 93% and very narrow vertical beam divergence of 7.1° (full width at half maximum). 50 μm broad area lasers with unpassivated facets exhibit a high total output power of nearly 20 W in pulsed mode with a divergence of 9.5°×11.3° leading to a record brightness of 3×108 W cm−2 sr−1, being presently the best value ever reported for a single broad area laser diode. 100 μm broad devices with unpassivated facets show continuous wave operation with an output power of 1.9 W.
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Wave propagation and tunneling through periodic structures Appl. Phys. Lett. 93, 231110 (2008); http://dx.doi.org/10.1063/1.3033826 (3 pages) Online Publication Date: 10 December 2008
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The phenomenon of tunneling manifests itself in nearly every field of physics. The ability to distinguish a wave tunneling through a barrier from one propagating is important for a number of applications. Here we explore the properties of the wave traveling through the band gap created by a lattice, either as a consequence of tunneling through the barrier or due to the presence of a pass band inside the gap. To observe the pass band for studying tunneling and propagating waves simultaneously, a localized lattice defect was introduced. The differences between the two phenomena are highlighted via waves’ dispersion characteristics.
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Carrier mobility, structural order, and solar cell efficiency of organic heterojunction devices Appl. Phys. Lett. 93, 133308 (2008); http://dx.doi.org/10.1063/1.2996029 (3 pages) Online Publication Date: 2 October 2008
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The loss mechanisms limiting solar cell efficiency in organic heterojunction photodiodes are discussed. We show that the energy loss when the exciton is split, and absence of Langevin recombination at the interface, are related to the magnitude of the electron-phonon interaction and the wavefunction extent of the electron and hole. In contrast to several recent publications, it is argued that well-ordered, high mobility organic materials will provide higher efficiency cells.
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ZnO nanowire arrays: Optical scattering and sensitization to solar light Appl. Phys. Lett. 93, 233119 (2008); http://dx.doi.org/10.1063/1.3040054 (3 pages) Online Publication Date: 11 December 2008
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Arrays of ZnO nanowires with different lengths (0.5–2 μm) and diameters (100–330 nm) were electrodeposited to study the influence of the nanowire dimensions on light scattering. The nanowire length and diameter were found to be major parameters in modifying the intensity and the wavelength of the scattered radiation, respectively. A significant scattering for the whole visible wavelength range was attained in arrays of ZnO nanowires of ∼ 1.5 μm in length and ∼ 330 nm in diameter. ZnO nanowire arrays were sensitized to solar light with a conformally deposited thin CdSe layer. A clear correlation between light scattering before coating and absorption in the resulting ZnO/CdSe core-shell nanostructures was found. The enhancement in the scattering for wavelengths where CdSe exhibits a relatively low absorption coefficient resulted in an effective absorption, in the 400–725 nm range of the AM1.5 solar spectrum, as high as 88% with a CdSe shell thickness of ∼ 20 nm only.
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Appl. Phys. Lett. 93, 222104 (2008); http://dx.doi.org/10.1063/1.3031726 (3 pages) Online Publication Date: 3 December 2008
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This study examined a fundamental aspect of ZnO-based thin film transistors (TFTs): the connection between the deposition conditions and the microstructure of ZnO films, and the electrical performance of the TFTs. We characterized the microstructure of ZnO films deposited under various rf powers by using high resolution transmission electron microscopy and x-ray diffraction. In further investigating the effects of the microstructure on the device performance, we experimentally demonstrated that the electrical mobility of the devices was coupled to the grain size of the ZnO films in an exponential function.
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Appl. Phys. Lett. 93, 231109 (2008); http://dx.doi.org/10.1063/1.3046130 (3 pages) Online Publication Date: 10 December 2008
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We report directional light extraction from AlGaInP thin-film resonant cavity light emitting diodes (RCLEDs) with shallow photonic crystals (PhCs). Diffraction of guided modes into the light extraction cone enhances the light extraction by a factor of 2.6 compared to unstructured RCLEDs, where the farfields still show higher directionality than Lambertian emitters. The external quantum efficiency is 15.5% to air and 26% with encapsulation, respectively. The PhC-RCLEDs are also more stable to a temperature induced wavelength shift than unstructured RCLEDs.
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Appl. Phys. Lett. 93, 231111 (2008); http://dx.doi.org/10.1063/1.3046099 (3 pages) Online Publication Date: 11 December 2008
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The enhanced and partially polarized output of a green light-emitting diode (LED), in which its InGaN/GaN quantum well (QW) couples with surface plasmons (SPs) on a surface Ag grating structure, is demonstrated. Compared with a LED sample without (flat) Ag coating, the total output intensity of an LED of SP-QW coupling can be enhanced by ∼ 59% ( ∼ 200)% when the grating period and groove depth are 500 and 30 nm, respectively. Also, a bottom-emission polarization ratio of 1.7 can be obtained under the condition of 15 nm in groove depth.
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Nanocrystalline multiferroic BiFeO3 ultrafine fibers by sol-gel based electrospinning Appl. Phys. Lett. 93, 222904 (2008); http://dx.doi.org/10.1063/1.3040010 (3 pages) Online Publication Date: 3 December 2008
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Novel nanocrystalline BiFeO3 ultrafine fibers have been synthesized by sol-gel based electrospinning, with fiber diameter in the range of 100–300 nm and grain size of around 20 nm. Phase pure perovskite BiFeO3 can be obtained if the fibers are fired in Ar atmosphere, eliminating impurity phases often observed when fired in air or N2 atmosphere. Excellent piezoelectricity and clear ferroelectric domain structure of the ultrafine fibers are characterized by high voltage piezoresponse force microscopy. Enhanced weak ferromagnetism arising from the nanocrystalline structure of ultrafine fibers is also observed.
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