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Top 20 Most Read Articles
November 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Small divergence edge-emitting semiconductor lasers with two-dimensional plasmonic collimators Appl. Phys. Lett. 93, 181101 (2008); http://dx.doi.org/10.1063/1.3009599 (3 pages) Online Publication Date: 3 November 2008
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Using quantum cascade lasers with a two-dimensional metallic aperture-grating structure defined on the facet the authors demonstrate a collimated laser beam with small divergence angle perpendicular and parallel to the laser waveguide layers (2.7° and 3.7°, respectively). These values represent a reduction by a factor of ∼ 30 and ∼ 10, respectively, compared to those of the original 8.06-μm- wavelength laser without plasmonic collimation. The devices preserve good room temperature performance with output power as high as 53% of that of the original unpatterned lasers.
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Electrically pumped ultraviolet ZnO diode lasers on Si Appl. Phys. Lett. 93, 181106 (2008); http://dx.doi.org/10.1063/1.3012579 (3 pages) Online Publication Date: 4 November 2008
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Electrically pumped ZnO quantum well diode lasers are reported. Sb-doped p-type ZnO/Ga-doped n-type ZnO with an MgZnO/ZnO/MgZnO quantum well embedded in the junction was grown on Si by molecular beam epitaxy. The diodes emit lasing at room temperature with a very low threshold injection current density of 10 A/cm2. The lasing mechanism is exciton-related recombination and the feedback is provided by close-loop scattering from closely packed nanocolumnar ZnO grains formed on Si.
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Experimental observation of narrow surface plasmon resonances in gold nanoparticle arrays Appl. Phys. Lett. 93, 181108 (2008); http://dx.doi.org/10.1063/1.3012365 (3 pages) Online Publication Date: 5 November 2008
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We demonstrate that coupling between grating diffraction and localized surface plasmons in two-dimensional gold nanoparticle arrays in water leads to narrow near-infrared resonance peaks in measured far field extinction spectra. Good agreement is obtained between finite difference time domain (FDTD) calculations and experimental extinction spectra. The FDTD calculations predict that the gold nanoparticle arrays exhibit near-field electric field intensity (E2) enhancements approximately one order of magnitude greater than those of single isolated gold nanoparticles.
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Graphene: A perfect nanoballoon Appl. Phys. Lett. 93, 193107 (2008); http://dx.doi.org/10.1063/1.3021413 (3 pages) Online Publication Date: 12 November 2008
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We have performed a first-principles density functional theory investigation of the penetration of helium atoms through a graphene monolayer with defects. The relaxation of the graphene layer caused by the incoming helium atoms does not have a strong influence on the height of the energy barriers for penetration. For defective graphene layers, the penetration barriers decrease exponentially with the size of the defects but they are still sufficiently high that very large defects are needed to make the graphene sheet permeable for small atoms and molecules. This makes graphene a very promising material for the construction of nanocages and nanomembranes.
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Silicon photonic crystal nanostructures for refractive index sensing Appl. Phys. Lett. 93, 181103 (2008); http://dx.doi.org/10.1063/1.3009203 (3 pages) Online Publication Date: 3 November 2008
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The authors present the fabrication and optical investigation of silicon on insulator photonic crystal drop filters for use as refractive index sensors. Two types of defect nanocavities (L3 and H1−r) are embedded between two W1 photonic crystal waveguides to evanescently route light at the cavity mode frequency between input and output waveguides. Optical characterization of the structures in air and various liquids demonstrates detectivities in excess of Δn/n = 0.018 and Δn/n = 0.006 for the H1−r and L3 cavities, respectively. The measured cavity frequencies and detector refractive index responsivities are in good agreement with simulations, demonstrating that the method provides a background free transducer signal with frequency selective addressing of a specific area of the sensor chip.
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High-resolution characterization of defects in oxide thin films Appl. Phys. Lett. 93, 182101 (2008); http://dx.doi.org/10.1063/1.2982082 (3 pages) Online Publication Date: 3 November 2008
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Nanometer sized defects in thin HfOx films are detected by atomic force microscopy facilitated leakage current measurements. Differences in the electrical properties of individual defects were distinguished. The effects of two mechanisms that localize the tip-sample interaction and increase spatial resolution were calculated. The expected increase in tip-sample current due to stress induced phase transformations and band gap narrowing has been calculated, and a behavior diagram is presented that shows the pressure necessary to generate a detectable current increase as a function of tip radius.
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Appl. Phys. Lett. 93, 193301 (2008); http://dx.doi.org/10.1063/1.3021022 (3 pages) Online Publication Date: 10 November 2008
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Spray deposition is emerging as an attractive low-cost and high throughput method for organic solar cell (OSC) fabrication. In this letter, we demonstrate that a highly conductive formulation of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) can be spray deposited to form the top electrode for an OSC. An inverted solar cell fabricated in this way with a blend of poly(3-hexylthiophene) and [6,6]-phenyl-C61 butyric acid methyl ester as the active layer achieved a power conversion efficiency of 2.0% under AM 1.5 100 mW/cm2 illumination.
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Appl. Phys. Lett. 93, 192501 (2008); http://dx.doi.org/10.1063/1.3013857 (3 pages) Online Publication Date: 10 November 2008
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An attractive approach for bit patterned media fabrication is the blanket deposition of magnetic material onto prepatterned substrates with elevated pillars and recessed trench areas. One issue with this method is the residual magnetic material in the trenches that causes disturbing stray fields during writing and readback. Here we present a technique to suppress the magnetic moment in the trenches with an additional annealing step by using prepatterned substrates consisting of SiN pillars on a Si wafer. The annealing triggers an interdiffusion process between the magnetic media and the Si in the trenches that results in the formation of a nonmagnetic silicide, while the magnetic moment on top of the SiN pillars remains substantially unaltered.
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Origin of the open-circuit voltage in multilayer heterojunction organic solar cells Appl. Phys. Lett. 93, 193308 (2008); http://dx.doi.org/10.1063/1.3027061 (3 pages) Online Publication Date: 14 November 2008
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From temperature dependent studies of pentacene/C60 solar cells in the dark, the reverse saturation current is found to be thermally activated with a barrier height that corresponds to the difference in energy between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor corrected for vacuum level misalignments and the presence of charge-transfer states. From the reverse saturation current in the dark and the short-circuit current under illumination, the open-circuit voltage can be predicted. Examination of several donor materials supports the relationship between reverse saturation current, this barrier height, and open-circuit voltage.
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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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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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Converging and wave guiding of Gaussian beam by two-layer dielectric rods Appl. Phys. Lett. 93, 201101 (2008); http://dx.doi.org/10.1063/1.3025818 (3 pages) Online Publication Date: 17 November 2008
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We have shown that a two-layer dielectric structure can give excellent beaming and enhanced transmission simultaneously of a Gaussian source. The front surface of the layer of dielectric rods supports surface states and the rear grading layer couples the surface states to radiation modes. By repeating periodically this two-layer structure, one can obtain excellent beaming and enhanced transmission for very long distances.
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Inverted small molecule organic photovoltaic cells on reflective substrates Appl. Phys. Lett. 93, 173304 (2008); http://dx.doi.org/10.1063/1.3005173 (3 pages) Online Publication Date: 29 October 2008
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We demonstrate top-illuminated, inverted, small molecule photovoltaic cells grown on reflective substrates employing copper phthalocyanine as the donor and 3,4,9,10-perylenetetracarboxylic bis-benzimidazole as the acceptor, with a sputter-deposited transparent indium tin oxide top cathode and a metal anode, thereby reversing the conventional charge extraction properties of these contacts. The best device achieved a peak power conversion efficiency of 0.74±0.03%, reasonably consistent with the optical simulations under 1 sun AM1.5G illumination giving 0.83±0.02%. This work suggests that inverted organic solar cells grown on reflective substrates have potential uses such as for power-generating coatings on opaque surfaces.
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The effect of electrode heat sink in organic-electronic devices Appl. Phys. Lett. 93, 183301 (2008); http://dx.doi.org/10.1063/1.3021071 (3 pages) Online Publication Date: 4 November 2008
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Most of organic devices showed poor thermal stability and short lifetime due to Joule heating by current injection during operation. To increase the lifetime of the devices, thermal management must be considered. We demonstrated the polymer light-emitting diodes with thermally conductive substrate and Al/Cu double cathode to enhance the thermal stability of the device. Also, we proposed the correlation between lifetime (Δt) and device heat sink (ΔT). The heat sink of all organic devices is required to enhance device durability.
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Air-voids embedded high efficiency InGaN-light emitting diode Appl. Phys. Lett. 93, 191103 (2008); http://dx.doi.org/10.1063/1.2998596 (3 pages) Online Publication Date: 11 November 2008
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To improve the light extraction efficiency of InGaN-light emitting diode (LED), inverted hexagonal cone shaped air voids with {10–11} GaN crystal planes were formed between a patterned sapphire substrate and GaN epitaxial layer using a H3PO4-based hot chemical etching method. The air-voids embedded LED showed 12% and 210% higher optical power than a patterned substrate LED and a flat substrate LED, respectively. A ray tracing simulation revealed that the light extraction through the top face of the air-voids embedded LED was dramatically increased due to a strong light reflection and redirection by the air voids.
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Enhanced absorption by nanostructured silicon Appl. Phys. Lett. 93, 193103 (2008); http://dx.doi.org/10.1063/1.3021480 (3 pages) Online Publication Date: 10 November 2008
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Some applications such as ultrafast detectors or high efficiency photovoltaics require absorption by thin films. However, close to the bandgap, silicon absorbs very poorly. In this letter, we show that the absorption of a 100 nm slab can be as high as 50% in the range of wavelengths 700–830 nm when using a periodic structure properly designed.
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Why so strong for the lotus leaf? Appl. Phys. Lett. 93, 201909 (2008); http://dx.doi.org/10.1063/1.3036535 (3 pages) Online Publication Date: 21 November 2008
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The authors discussed the potential reasons why the lotus leaf is so strong by means of scanning electron microscopy (SEM). The results showed that the good mechanical properties of lotus leaf should be attributed to its architecture, such as paralleled microtubes structure, umbrellalike structure, and hierarchically layered hexagon structure. The important observation from this work is that the surface of the rear face of the lotus leaf seems to be constituted by the layers of hexagons whose hierarchical pilling up of size decreases as we go deeper from surface. This is a typical fractal-like phenomenon.
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Band alignment at metal/organic and metal/oxide/organic interfaces Appl. Phys. Lett. 93, 193310 (2008); http://dx.doi.org/10.1063/1.3030979 (3 pages) Online Publication Date: 14 November 2008
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Charge injection at metal/organic interfaces dictates the performance, lifetime, and stability of organic electronic devices. We demonstrate that interface dipole theory, originally developed to describe Schottky contacts at metal/semiconductor interfaces, can also accurately describe the injection barriers in real organic electronic devices. It is found that theoretically predicted hole injection barriers for various archetype metal/organic and metal/oxide/organic structures are in excellent agreement with values extracted from experimental transport measurements. Injection barriers at metal/organic and metal/oxide/organic interfaces can therefore be accurately predicted based on the knowledge of only a few fundamental material properties of the oxide and organic layers.
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Organic cesium salt as an efficient electron injection material for organic light-emitting diodes Appl. Phys. Lett. 93, 183302 (2008); http://dx.doi.org/10.1063/1.3012377 (3 pages) Online Publication Date: 4 November 2008
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Cesium quinoline-8-oxide (Csq), as an organic salt, was synthesized and applied as an electron injection material for organic light-emitting diodes. A typical bilayer structure of N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine/tris-(8-hydroxy-quinolinolato) aluminum (III) (Alq3) was used to evaluate the electron injection of Csq. The results showed Csq/Al had better electron injection than LiF/Al cathode, and a maximum current efficiency of 4.13 cd/A (3.33 cd/A for LiF/Al) was obtained when the Csq layer is 2.0 nm. It is proposed that the enhanced electron injection by Csq is related to the interfacial chemical reaction between Csq and Al and the self-doping of Csq by librated Cs.
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Appl. Phys. Lett. 93, 183501 (2008); http://dx.doi.org/10.1063/1.3006316 (3 pages) Online Publication Date: 3 November 2008
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We demonstrate the use of a simple charge extraction measurement to determine the charge carrier densities n in annealed poly(3-hexylthiophene):methanofullerene solar cells under operating conditions. By applying charge extraction to the device under forward bias in the dark (Jdark), we find Jdark∝n2.6. This dependence on charge density is the same as that we find for bimolecular recombination losses observed in such devices under irradiation at open circuit, suggesting that the dark current originates from bimolecular recombination at the polymer/fullerene interface.
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