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Top 20 Most Read Articles
March 2009
The 20 articles with the most full-text downloads during the month, in descending order.
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Practical enhancement of photoluminescence by metal nanoparticles Appl. Phys. Lett. 94, 101103 (2009); http://dx.doi.org/10.1063/1.3097025 (3 pages) Online Publication Date: 9 March 2009
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We develop a simple yet rigorous theory of the photoluminescence (PL) enhancement in the vicinity of metal nanoparticles. The enhancement takes place during both optical excitation and emission. The strong dependence on the nanoparticle size enables optimization for maximum PL efficiency. Using the example of InGaN quantum dots (QDs) positioned near Ag nanospheres embedded in GaN, we show that strong enhancement can be obtained only for those QDs, atoms, or molecules that are originally inefficient in absorbing as well as in emitting optical energy. We then discuss practical implications for sensor technology.
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Appl. Phys. Lett. 94, 111101 (2009); http://dx.doi.org/10.1063/1.3097278 (3 pages) Online Publication Date: 16 March 2009
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As an attempt to collect more in-plane emission power out of wavelength size two-dimensional photonic crystal defect lasers, edge-emitting photonic crystal double-heterostructure quantum well membrane lasers were fabricated by shortening the number of cladding periods on one side. 120 μW peak output power was collected from the facet of the single mode laser at room temperature. Laser efficiencies were analyzed and agree very well with three-dimensional finite-difference time-domain modeling.
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Ultrahigh spontaneous emission extraction efficiency induced by evanescent wave coupling Appl. Phys. Lett. 94, 091102 (2009); http://dx.doi.org/10.1063/1.3086887 (3 pages) Online Publication Date: 3 March 2009
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A spontaneous emission extraction efficiency greater than 50% is observed in a GaAs/AlGaAs quantum well structure grown on the subwavelength-sized ridge top facet of a V-grooved substrate by means of photoluminescence study. This is an extraction efficiency about 20 times higher than that of a similar structure grown on a flat substrate. It is demonstrated both experimentally and theoretically that the high extraction efficiency is the result of the efficient conversion of evanescent waves into propagating waves in air through constructive coupling of evanescent waves generated on the two sidewall facets of the V-grooved substrate by total internal reflection.
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All-inorganic light emitting device based on ZnO nanoparticles Appl. Phys. Lett. 94, 091115 (2009); http://dx.doi.org/10.1063/1.3093675 (3 pages) Online Publication Date: 5 March 2009
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We report on room-temperature electroluminescence from an all-inorganic light emitting device based on spin-coated ZnO nanoparticles. A tight submicron thick layer was fabricated on a fluorine doped tin oxide glass as a substrate using commercially available ZnO nanoparticles from the gas phase. After the evaporation of the top Al electrode, a diodelike I-V characteristic was obtained. An emission peak at around 390 nm and a broad defect-related electroluminescence in the visible range were observed at voltages below 10 V and ambient air conditions.
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Infrared transparent carbon nanotube thin films Appl. Phys. Lett. 94, 081103 (2009); http://dx.doi.org/10.1063/1.3075067 (3 pages) Online Publication Date: 24 February 2009
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We have measured the infrared properties of optically transparent and electrically conductive single walled carbon nanotube thin films. We found that nanotube films with sheet resistance values of 200 Ω/sq show outstanding transmittance in the infrared range up to at least 22 μm, with an average transmittance greater than 90% over this range. The infrared properties of various materials were compared and we found that transparent nanotube electrodes and transparent graphene electrodes outperform the others in several key categories. This study opens another important application area for conductive nanotube thin films.
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Appl. Phys. Lett. 94, 113302 (2009); http://dx.doi.org/10.1063/1.3099947 (3 pages) Online Publication Date: 17 March 2009
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The short-circuit current density of inverted organic solar cells comprising a solution-processed titania electron transport layer increases with continuous illumination in air and saturates after 10 min. On extended exposure (>2 days), the open-circuit voltage of the devices increases also. The improvement in device characteristics over short time scales is attributed to the filling of shallow electron traps in titania. With an increase in photoconductivity of titania, the short-circuit current increases accordingly. The increase in open-circuit voltage on extended exposure to air is attributed to an increase in the electrostatic field across the diodes when polythiophene is doped by oxygen.
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Role of interface roughness in the transport and lasing characteristics of quantum-cascade lasers Appl. Phys. Lett. 94, 091101 (2009); http://dx.doi.org/10.1063/1.3093819 (3 pages) Online Publication Date: 3 March 2009
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A density-matrix based theory of transport and lasing in quantum-cascade lasers reveals that large disparity between luminescent linewidth and broadening of the tunneling transition changes the design guidelines to favor strong coupling between injector and upper laser level. This conclusion is supported by the experimental evidence.
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Appl. Phys. Lett. 94, 113110 (2009); http://dx.doi.org/10.1063/1.3103208 (3 pages) Online Publication Date: 18 March 2009
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Nanostructured metamaterials, especially arrays of metallic nanoparticles, which sustain the excitation of localized plasmon polaritons, provide excellent opportunities to mold the flow of light in the linear regime. We suggest a metamaterial structure whose properties are determined not only by its inner geometry but also by its entire shape. We call this structure a metamaterial nanotip. We evaluate the potential of this nanotip to control the size and the location of the field enhancement. Two-dimensional implementations of this metamaterial nanotip were comprehensively numerically simulated and confirm the expected, physically distinct regimes of operation.
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Current-driven vortex formation in a magnetic multilayer ring Appl. Phys. Lett. 94, 082501 (2009); http://dx.doi.org/10.1063/1.3085971 (3 pages) Online Publication Date: 23 February 2009
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Current-driven domain wall (DW) motion has been studied in the NiFe layer of a Co/Cu/NiFe thin film ring using giant-magnetoresistance measurements in a four-point contact geometry. The NiFe layer is initially in an onion state configuration with two 180° DWs. An electric current drives the walls around the ring so that they annihilate and the NiFe layer forms a DW-free vortex state. The direction of motion of the two DWs is determined by the current polarity, enabling the vortex chirality to be selected.
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Appl. Phys. Lett. 94, 103303 (2009); http://dx.doi.org/10.1063/1.3098472 (3 pages) Online Publication Date: 11 March 2009
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Effects of different cathode metals, such as aluminum, calcium, and silver, and difference in their reflectivity on the photocurrent generation in pentacene-C60 heterojunction solar cell are presented. Using optical transfer matrix calculations, we find that metal reflectivity has a profound impact on the electrical field confinement within the multilayer device structures. Silver as cathode offers better optical-field confinement close to the pentacene-C60 interface over generally preferred aluminum cathode. External quantum efficiency measurements confirm higher exciton dissociation efficiency and high photocurrent generation ability of silver over aluminum cathode making the choice of cathode metal an important parameter in device optimization.
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Appl. Phys. Lett. 94, 111109 (2009); http://dx.doi.org/10.1063/1.3100773 (3 pages) Online Publication Date: 17 March 2009
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Room-temperature photoluminescence (PL) measurements are performed on GaInN/GaN multiple-quantum-well heterostructures grown on GaN-on-sapphire templates with different threading-dislocation densities. The selective optical excitation of quantum wells and the dependence of integrated PL intensity on excitation power allow us to determine the internal quantum efficiency (IQE) as a function of carrier concentration. The measured IQE of the sample with the lowest dislocation density (5.3×108 cm−2) is as high as 64%. The measured nonradiative coefficient A varies from 6×107 to 2×108 s−1 as the dislocation density increases from 5.3×108 to 5.7×109 cm−2, respectively.
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Appl. Phys. Lett. 94, 101105 (2009); http://dx.doi.org/10.1063/1.3095495 (3 pages) Online Publication Date: 10 March 2009
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In order to obtain efficient surface emission, we propose and demonstrate a GaN light-emitting diode (LED) structure. A two-dimensional photonic crystal (PC) pattern is integrated to the sapphire substrate before the epigrowth by employing laser holography. In addition, angled sidewall deflectors (ASDs) are formed on the mesa sidewalls. Both the PC and ASDs redirect guided photons into the surface-normal direction. When compared to a conventional LED structure, we could obtain a twofold increase in the total surface emission and the surface-normal emission intensity enhanced by a factor of 2.5.
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Appl. Phys. Lett. 94, 103301 (2009); http://dx.doi.org/10.1063/1.3086897 (3 pages) Online Publication Date: 9 March 2009
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Bulk heterojunction solar cells are fabricated from blends of oligothiophene with a dialkylated diketopyrrolopyrrole chromophore:[6,6]-phenyl C71 butyric acid methyl ester. Absorption and photocurrent of the films extend to 800 nm. A power conversion efficiency (PCE) of 3.0% is obtained under simulated 100 mW/cm2 AM1.5 illumination with a 9.2 mA/cm2 short-circuit current density and an open-circuit voltage of 0.75 V. The hole and electron mobilities in the 50:50 blend are fairly balanced, 1.0×10−4 and 4.8×10−4 cm2/V s, respectively. This is the highest PCE reported to date for solar cells using solution processable small molecules.
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Metal mirror assisting light extraction from patterned AlGaInP light-emitting diodes Appl. Phys. Lett. 94, 101102 (2009); http://dx.doi.org/10.1063/1.3097017 (3 pages) Online Publication Date: 9 March 2009
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We demonstrate light extraction from metal reflector-based AlGaInP photonic crystal (PhC) light-emitting diodes (LEDs). The photons reflected by a high-reflectivity, small-absorption, bottom Ag mirror steadily interact with the PhC, and thus enhanced light extraction is achieved. The square lattice PhC patterns are fabricated on an upper n-doped AlGaInP surface with a depth of 500 nm. An optical power measurement using an integration sphere shows that the extraction efficiency of the PhC LED is ∼ 1.8 times larger than that of the nonpatterned LED. A three-dimensional finite difference time domain simulation is performed to understand the output enhancement extracted by the PhC and the effect of internal absorption.
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500 nm electrically driven InGaN based laser diodes Appl. Phys. Lett. 94, 081119 (2009); http://dx.doi.org/10.1063/1.3089573 (3 pages) Online Publication Date: 27 February 2009
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Based on recent improvements of growth of In-rich InGaN quantum wells with low defect density, we demonstrate current driven InGaN laser diodes at wavelengths as long as 500 nm. The laser structures are grown on c-plane GaN substrate and are processed as broad oxide-insulated stripe laser diodes. We discuss the impact of the piezoelectric field on the emission energy of long wavelength laser diodes for this growth orientation. The combination of low threshold current density of 8.2 kA/cm2 with high slope efficiency of 650 mW/A enables high output powers up to several tens of milliwatts.
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Appl. Phys. Lett. 94, 103305 (2009); http://dx.doi.org/10.1063/1.3093831 (3 pages) Online Publication Date: 12 March 2009
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A significant increase in open circuit voltage (VOC) is obtained in the polymer-fullerene bulk heterojunction solar cell by using the e-beam deposited Al cathode. Compared with the device with the thermal evaporated Al cathode, an obvious enhancement of VOC from 596 to 664 mV is obtained, which makes the overall device power conversion efficiency improved by 12.4% (from 3.79% to 4.26%). Electrical characterizations suggest that the energetic particles in the e-beam deposition induce deep interface hole traps in the poly(3-hexylthiophene-2,5-diyl) (P3HT), while leaving the fullerene unaffected. The deep trapped holes near the P3HT/cathode interface can induce the image negative charges in the cathode and thus form “dipoles.” These dipoles lead to the lowering of the Al effective work function and cause the enhancement of VOC.
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Output power enhancement of GaN light emitting diodes with p-type ZnO hole injection layer Appl. Phys. Lett. 94, 103506 (2009); http://dx.doi.org/10.1063/1.3097243 (3 pages) Online Publication Date: 10 March 2009
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We report an enhancement of the optical output power of GaN light emitting diodes (LEDs) by addition of a p-type ZnO layer located in close proximity to the active layer (ZnO/GaN LEDs). Arsenic (As)-doped p-ZnO was used as a hole-injecting layer to overcome the drop in external quantum efficiency of GaN LEDs at high drive currents—the so-called “efficiency droop.” The output power in ZnO/GaN LEDs was improved up to 40%. This result is useful for development of highly efficient GaN LEDs operating at high current densities that will play a critical role in replacement of incandescent lamps by high efficiency solid-state light bulbs.
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Appl. Phys. Lett. 94, 071118 (2009); http://dx.doi.org/10.1063/1.3077606 (3 pages) Online Publication Date: 20 February 2009
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We report a dramatic increase in the light extraction efficiency of GaN-based blue light-emitting diodes (LEDs) by ZnO nanorod arrays on a planar indium tin oxide (ITO) transparent electrode. ZnO nanorods were grown into aqueous solution at the low temperature of 90 °C. With 20 mA current injection, the light output efficiency of the LED with ZnO nanorod arrays on ITO was increased by about 57% with no increase in a forward voltage over the conventional LEDs with planar ITO. The increased light extraction by the ZnO nanorod arrays is due to the formation of sidewalls and a rough surface, resulting in a multiple photon scattering at the LED surface.
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Appl. Phys. Lett. 94, 082106 (2009); http://dx.doi.org/10.1063/1.3088861 (3 pages) Online Publication Date: 25 February 2009
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We show that transport gap of semiconducting nanoparticles can be changed without disturbing the optical gap. This is achieved through inorganic-organic hybrid core-shell approach. Different inorganic nanoparticles with a bandgap in the UV to NIR range are used as the core; as a shell to the nanoparticles, a monolayer of different organic molecules is used. With the inclusion of the shell layer, optical gap of the nanoparticles does not change. Transport gap, as obtained from current-voltage characteristics of a single nanoparticle with scanning tunneling microscope tip, changes to that of the shell-material irrespective of the bandgap of core nanoparticles.
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Appl. Phys. Lett. 94, 103501 (2009); http://dx.doi.org/10.1063/1.3095436 (3 pages) Online Publication Date: 9 March 2009
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By adopting the coordinate transformation theory and the corresponding numerical approach, we investigated the method of designing open cloaks that have one or more windows to exchange information and matter with the outer environment. Compared with close cloaks, open cloaks have the quasiperfect cloaking performance and are applied to any objects especially to the moving ones that must communicate and exchange matter with the outer region, which thus opens up possibilities for practical applications of cloak technologies developed recently.
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