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Top 20 Most Read Articles
September 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 92, 013125 (2008); http://dx.doi.org/10.1063/1.2828990 (3 pages) Online Publication Date: 9 January 2008
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The ability to detect individual impurity atoms has been greatly enhanced by the development of aberration-corrected electron microscopes. The reduced depth of focus potentially enables three-dimensional reconstructions of impurity atoms from through-focal series. We test the robustness of this depth-sectioning method for detecting impurity atoms in gate oxides using multislice simulations. For amorphous materials, dopants can be reliably imaged, and are accurately described by a simpler three-dimensional linear imaging model. For crystalline materials, however, channeling artifacts can render the signal uninterpretable. These artifacts can be eliminated by orienting the crystal slightly off the zone axis, which still preserves atomic resolution.
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Appl. Phys. Lett. 93, 091901 (2008); http://dx.doi.org/10.1063/1.2976330 (3 pages) Online Publication Date: 2 September 2008
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Three-dimensional-confined structures with triangular-lattice air-hole photonic crystal patterns were fabricated to enhance the light output from silicon rich oxide/SiO2 multilayer stack. The intensity and profile of spontaneous emission were found to be efficiently modulated by controlling the optical modes of the periodic arrays via varying their structural parameters. With lattice constant/radius of 700 nm/280 nm, the photoluminescence intensity was found to be enhanced by nearly nine times in the vertical direction. The mechanisms for different enhancement features have been theoretically analyzed based on coherent scattering and quantum electrodynamic effects, well supporting the experimental observation.
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Different curvatures of tunable liquid microlens via the control of laminar flow rate Appl. Phys. Lett. 93, 084101 (2008); http://dx.doi.org/10.1063/1.2976210 (3 pages) Online Publication Date: 26 August 2008
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This letter reports the tunable liquid microlens using three laminar flows injected into an expansion chamber. Different lens shapes and curvatures can be achieved and tuned through the control of three flow rates. The expansion chamber is designed to improve the fluidic stability and maintain the ideal lens shape for precise microscale optical measurement. The optical aberration is also eliminated by minimizing the diffusive broadening at the interfaces. The collimation and focusing capabilities of three liquid microlenses are demonstrated. The tunable liquid microlens is promising as a tool to realize different optical components that can be integrated onto a microchip.
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Nanoparticle-induced light scattering for improved performance of quantum-well solar cells Appl. Phys. Lett. 93, 091107 (2008); http://dx.doi.org/10.1063/1.2973988 (3 pages) Online Publication Date: 3 September 2008
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We report on the improved performance of InP/InGaAsP quantum-well waveguide solar cells via light scattering from deposited dielectric or metal nanoparticles. The integration of metal or dielectric nanoparticles above the quantum-well solar cell device is shown to couple normally incident light into lateral optical propagation paths, with optical confinement provided by the refractive index contrast between the quantum-well layers and surrounding material. With minimal optimization, short-circuit current density increases of 12.9% and 7.3% and power conversion efficiency increases of 17% and 1% are observed for silica and Au nanoparticles, respectively.
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Terahertz emission from black silicon Appl. Phys. Lett. 93, 091106 (2008); http://dx.doi.org/10.1063/1.2978096 (3 pages) Online Publication Date: 3 September 2008
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We report on a terahertz emitter made out of black silicon. The black surface structure absorbs the whole optical pump power in the very surface. In contrast to expectations for indirect semiconductors, the black structure shows an emission in the terahertz range. The emitted radiation of the black silicon crystal is characterized for different parameters using terahertz time-domain spectroscopy.
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Organic solar cells with solution-processed graphene transparent electrodes Appl. Phys. Lett. 92, 263302 (2008); http://dx.doi.org/10.1063/1.2924771 (3 pages) Online Publication Date: 1 July 2008
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We demonstrate that solution-processed graphene thin films can serve as transparent conductive anodes for organic photovoltaic cells. The graphene electrodes were deposited on quartz substrates by spin coating of an aqueous dispersion of functionalized graphene, followed by a reduction process to reduce the sheet resistance. Small molecular weight organic solar cells can be directly deposited on such graphene anodes. The short-circuit current and fill factor of these devices on graphene are lower than those of control device on indium tin oxide due to the higher sheet resistance of the graphene films. We anticipate that further optimization of the reduction conditions will improve the performance of these graphene anodes.
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Efficient multilayer organic solar cells using the optical interference peak Appl. Phys. Lett. 93, 043307 (2008); http://dx.doi.org/10.1063/1.2962986 (3 pages) Online Publication Date: 1 August 2008
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A multilayer structure of copper phthalocyanine/poly(3-hexylthiophene-2,5-diyl): [6,6]-phenyl-C61-butyric acid methyl ester (CuPc/P3HT:PCBM) is used to extend the light absorption spectrum covering almost the entire visible spectrum. To maximize the light absorption, the total number of excitons created in the multilayer structure as a function of layer thickness of both CuPc and P3HT:PCBM is simulated by using the optical transfer matrix formalism. The solar cells with a device structure of ITO/PEDOT:PSS/CuPc/P3HT:PCBM/Al are fabricated with different layers thicknesses. The optimized solar cell with a high short circuit current density of 12.54 mA/cm2 and power conversion efficiency as high as 4.13% is achieved, owing to the utilization of the second optical interference peak in the multilayer structure for the enhanced light absorption.
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Atomic force microscope local oxidation nanolithography of graphene Appl. Phys. Lett. 93, 093107 (2008); http://dx.doi.org/10.1063/1.2976429 (3 pages) Online Publication Date: 3 September 2008
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We demonstrate the local oxidation nanopatterning of graphene films by an atomic force microscope. The technique provides a method to form insulating trenches in graphene flakes and to fabricate nanodevices with sub-nanometer precision. We demonstrate fabrication of a 25-nm-wide nanoribbon and submicron size nanoring from a graphene flake. We also found that we can write either trenches or bumps on the graphene surface depending on the lithography conditions. We attribute the bumps to partial oxidation of the surface and incorporation of oxygen into the graphene lattice.
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Current efficiency in organic light-emitting diodes with a hole-injection layer Appl. Phys. Lett. 93, 093306 (2008); http://dx.doi.org/10.1063/1.2978349 (3 pages) Online Publication Date: 5 September 2008
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We have systematically investigated the effect of layer structures on the current efficiency of prototypical hole-injection layer (HIL)/hole-transport layer (HTL)/electron-transport layer (ETL) organic light-emitting diodes based on 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (MTDATA) as the HIL, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) as the HTL, and tris(8-quinolinolato)aluminum (Alq) as the ETL. With bilayer devices, the current efficiency is limited by exciplex emissions in the case of MTDATA/Alq and quenching of Alq emissions by NPB+ radical cations in NPB/Alq. The improved current efficiency in trilayer MTDATA/NPB/Alq devices can be attributed to a reduction in NPB+ radical cations at the NPB/Alq interface and a strong electric field in the NPB layer.
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An amplifier concept for spintronics Appl. Phys. Lett. 93, 102513 (2008); http://dx.doi.org/10.1063/1.2977964 (3 pages) Online Publication Date: 12 September 2008
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Typical spin-dependent devices proposed for information processing lack one of the most important features provided by charge based logic: they do not provide gain. In this letter we show the basic concept of a spin amplifier and propose ways to amplify a spin current at room temperature.
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Appl. Phys. Lett. 93, 103502 (2008); http://dx.doi.org/10.1063/1.2978068 (3 pages) Online Publication Date: 8 September 2008
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Limitations in extraction efficiency of gallium nitride (GaN) photonic crystal (PhC) light emitting diodes (LEDs) are addressed by implementing an LED design using both two-dimensional PhCs in-plane and index guiding layers (IGLs) in the vertical direction. The effects of PhCs on light extraction and emission directionality from GaN LEDs are studied experimentally. Angular-resolved electroluminescence clearly shows the combined effect of controlling the vertical mode profile with the IGLs and tailoring the emission profile with the periodicity of the PhC lattice. Increases in directional emission as high as 3.5 times are achieved by taking advantage of this directionality and guided mode control.
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Appl. Phys. Lett. 93, 103301 (2008); http://dx.doi.org/10.1063/1.2973342 (3 pages) Online Publication Date: 8 September 2008
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A low-cost lithographic technique to pattern poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) films with 10 nm deep features of 700 nm periodicity is demonstrated. The use of these patterned films in poly(3-hexylthiophene):[6,6]-phenylC61-butyric acid methyl ester organic photovoltaic devices leads to an increase in short circuit current (Jsc), fill factor, and power conversion efficiency (PCE) with only a slight reduction in open circuit voltage. Patterning the PEDOT:PSS at 150 °C increases Jsc from 2.44 to 3.03 mA/cm2 improving the PCE from 0.63% to 0.81% with similar increases due to patterning also being obtained at other temperatures.
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Room temperature midinfrared electroluminescence from InSb/InAs quantum dot light emitting diodes Appl. Phys. Lett. 93, 091101 (2008); http://dx.doi.org/10.1063/1.2976551 (3 pages) Online Publication Date: 2 September 2008
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Self-assembled InSb submonolayer quantum dots (QDs) in an InAs matrix have been grown by molecular beam epitaxy using Sb2 and As2 fluxes. The structures exhibit bright midinfrared photoluminescence up to room temperature. Intense room temperature electroluminescence with a peak at wavelength near 3.8 μm was observed from p-i-n light emitting diode structures containing ten InSb submonolayer QD sheets inserted within the InAs active region.
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Enhanced optical transmission: Role of the localized surface plasmon Appl. Phys. Lett. 93, 101113 (2008); http://dx.doi.org/10.1063/1.2977488 (3 pages) Online Publication Date: 12 September 2008
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We report the observation of enhanced optical transmission through the metal subwavelength hole arrays with nanoparticle inside the hole, and demonstrate the transmission enhancement and the suppression due to the excitation of the localized surface plasmon (LSP). We interpret this phenomenon with the Fano theory considering the real and imaginary parts of the polarizability of the LSP.
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Appl. Phys. Lett. 93, 103311 (2008); http://dx.doi.org/10.1063/1.2981525 (3 pages) Online Publication Date: 11 September 2008
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We show that ultrathin metal layers (Ag or Al/Ag) are feasible as transparent top contacts for zinc phthalocyanine: C60 bulk-heterojunction inverted organic solar cells thermally evaporated on glass substrates. Furthermore, it is demonstrated that the introduction of an organic capping layer drastically increases light incoupling and photon harvesting, in accordance with optical simulations. Proof of principle tin-doped indium oxide (ITO)-free solar cells employing a transparent metal contact and a capping layer reach efficiencies of 1.06%, compared to 0.69% without addition of the capping layer.
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The impact of water vapor transmission rate on the lifetime of flexible polymer solar cells Appl. Phys. Lett. 93, 103306 (2008); http://dx.doi.org/10.1063/1.2975185 (3 pages) Online Publication Date: 9 September 2008
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In this paper we perform accelerated lifetime testing on high efficiency flexible poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) solar cells encapsulated with food package quality barrier films with a water vapor transmission rate of 0.2 g/(m2 day) at 65 °C/85% relative humidity. We show that lifetimes exceeding 1250 h, even at high temperature/high humidity conditions, may be reached, proving that organic solar cells are significantly less sensitive against the environmental effects of water and oxygen than previously expected.
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Plasmon enhanced performance of organic solar cells using electrodeposited Ag nanoparticles Appl. Phys. Lett. 93, 073307 (2008); http://dx.doi.org/10.1063/1.2967471 (3 pages) Online Publication Date: 22 August 2008
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To enhance solar harvesting in organic solar cells, uniform-sized metal nanoparticles of ∼ 13 nm were incorporated to the device via pulse-current electrodeposition, which is a kind of simple and quick solution process that can control the density and size of metal nanoparticles. By incorporating plasmonic Ag nanoparticles on surface modified transparent electrodes, overall power conversion efficiency was increased from 3.05% to 3.69%, mainly resulting from the improved photocurrent density as a result of enhanced absorption of the photoactive conjugate polymer due to the high electromagnetic field strength in the vicinity of the excited surface plasmons.
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Synthesis and optical properties of ZnO and carbon nanotube based coaxial heterostructures Appl. Phys. Lett. 93, 103108 (2008); http://dx.doi.org/10.1063/1.2952487 (3 pages) Online Publication Date: 10 September 2008
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Carbon nanotubes and ZnO based functional coaxial heterostructured nanotubes have been fabricated by using atomic layer deposition. An irregular structured shell composed of ZnO nanocrystals was deposited on pristine nanotubes, while a highly defined ZnO shell was deposited on the tubes after its functionalization with Al2O3. Photoluminescence measurements of the ZnO shell on Al2O3/nanotube show a broad green band emission, whereas the shell grown on the bare nanotube shows a band shifted to the orange spectral range.
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A hybrid green light-emitting diode comprised of n-ZnO/(InGaN/GaN) multi-quantum-wells/p-GaN Appl. Phys. Lett. 93, 081111 (2008); http://dx.doi.org/10.1063/1.2975165 (3 pages) Online Publication Date: 27 August 2008
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Hybrid green light-emitting diodes (LEDs) comprised of n-ZnO/(InGaN/GaN) multi-quantum-wells/p-GaN were grown on semi-insulating AlN/sapphire using pulsed laser deposition for the n-ZnO and metal organic chemical vapor deposition for the other layers. X-ray diffraction revealed that high crystallographic quality was preserved after the n-ZnO growth. LEDs showed a turn-on voltage of 2.5 V and a room temperature electroluminescence (EL) centered at 510 nm. A blueshift and narrowing of the EL peak with increasing current was attributed to bandgap renormalization. The results indicate that hybrid LED structures could hold the prospect for the development of green LEDs with superior performance.
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Appl. Phys. Lett. 93, 101112 (2008); http://dx.doi.org/10.1063/1.2981640 (3 pages) Online Publication Date: 12 September 2008
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The effect of the last quantum barrier (LQB) on the internal quantum efficiency of GaN-light emitting diode (LED) was systematically investigated using a dual-wavelength GaN-LED design. Compared with a conventional GaN-LQB, a high indium contained In0.03Ga0.97N-LQB efficiently reduced the unintentional Mg impurity in the last quantum well and improved its photoluminescence and electroluminescence intensity up to 72% and 15%, respectively.
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