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Top 20 Most Read Articles
August 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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Enhanced outcoupling from organic light-emitting diodes using aperiodic dielectric mirrors Appl. Phys. Lett. 90, 241112 (2007); http://dx.doi.org/10.1063/1.2748859 (3 pages) Online Publication Date: 14 June 2007
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Aperiodic dielectric stacks between the substrate and transparent anode in organic light-emitting diodes are used to improve the optical outcoupling efficiency. The authors demonstrate that a nine-layer SiO2/SiNx aperiodic dielectric stack improves the brightness by 80% within a 60° viewing cone for a red-emitting organic light-emitting diode, while maintaining a Lambertian emission pattern. As the refractive index contrast between the two materials used in a two-component multilayer dielectric stack is increased, a brightness improvement of 170% in a 60° viewing cone is achievable while maintaining a Lambertian emission profile.
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Directed assembly of gold nanoparticle nanowires and networks for nanodevices Appl. Phys. Lett. 91, 063101 (2007); http://dx.doi.org/10.1063/1.2763967 (3 pages) Online Publication Date: 6 August 2007
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Alternating electric field is used to assemble gold nanoparticle nanowires from liquid suspensions. The effects of electrode geometry and the dielectrophoresis force on the chaining and branching of nanowire formation are investigated. The nanowire assembly processes are modeled using finite element calculations, and the particle trajectories under the combined influence of dielectrophoresis force and viscous drag are simulated. Nanoparticle nanowires with 10 nm resolution are fabricated. The wires can be further oriented along an externally introduced flow. This work provides an approach towards rapid assembly and organization of ultrasmall nanoparticle networks.
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Experimental realization of a silicon spin field-effect transistor Appl. Phys. Lett. 91, 072501 (2007); http://dx.doi.org/10.1063/1.2770656 (3 pages) Online Publication Date: 13 August 2007
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A longitudinal electric field is used to control the transit time (through an undoped silicon vertical channel) of spin-polarized electrons precessing in a perpendicular magnetic field. Since an applied voltage determines the final spin direction at the spin detector and hence the output collector current, this comprises a spin field-effect transistor. An improved hot-electron spin injector providing ≈ 115% magnetocurrent, corresponding to at least ≈ 37% electron current spin polarization after transport through 10 μm undoped single-crystal silicon, is used for maximum current modulation.
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Appl. Phys. Lett. 91, 063124 (2007); http://dx.doi.org/10.1063/1.2768624 (3 pages) Online Publication Date: 10 August 2007
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Microfabrication of graphene devices used in many experimental studies currently relies on the fact that graphene crystallites can be visualized using optical microscopy if prepared on top of Si wafers with a certain thickness of SiO2. The authors study graphene’s visibility and show that it depends strongly on both thickness of SiO2 and light wavelength. They have found that by using monochromatic illumination, graphene can be isolated for any SiO2 thickness, albeit 300 nm (the current standard) and, especially, ≈ 100 nm are most suitable for its visual detection. By using a Fresnel-law-based model, they quantitatively describe the experimental data.
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Enhancement of polycrystalline silicon solar cells using ultrathin films of silicon nanoparticle Appl. Phys. Lett. 91, 063107 (2007); http://dx.doi.org/10.1063/1.2766958 (3 pages) Online Publication Date: 6 August 2007
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Ultrathin films of highly monodispersed luminescent Si nanoparticles are directly integrated on polycrystalline Si solar cells. The authors monitor the open-circuit voltage and the short circuit current. The results demonstrate that films of 1 nm blue luminescent or 2.85 nm red luminescent Si nanoparticles produce large voltage enhancements with improved power performance of 60% in the UV/blue range. In the visible, the enhancements are ∼ 10% for the red and ∼ 3% for the blue particles. The results point to a significant role for charge resonant transport across the nanofilm and Schottky-like rectification at nanoparticle-metal interface.
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Silicon based organic semiconductor laser Appl. Phys. Lett. 91, 051124 (2007); http://dx.doi.org/10.1063/1.2764553 (3 pages) Online Publication Date: 2 August 2007
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The authors demonstrate silicon based visible lasers as potential optical interconnects by combining silicon processed resonators and solution processed light-emitting polymers. The high refractive index and absorption coefficient of silicon at these wavelengths were addressed by developing distributed Bragg reflector resonators on a silicon-on-insulator substrate. The performance of the hybrid structure was characterized and analyzed in comparison to an all-silica counterpart and mechanisms for controlling the number of longitudinal modes and for tuning the emission wavelength were explored.
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Fourier plasmonics: Diffractive focusing of in-plane surface plasmon polariton waves Appl. Phys. Lett. 91, 081101 (2007); http://dx.doi.org/10.1063/1.2772756 (3 pages) Online Publication Date: 20 August 2007
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An in-plane Fresnel zone plate (FZP) for focusing surface plasmon polariton (SPP) fields has been designed, fabricated, and tested. The fabricated device consists of 400 nm tall by 5 μm wide amorphous Si-based SPP FZP on an Al film integrated with a pair of two-dimensional nanohole arrays for excitation of the incident and detection of the diffracted SPP fields. Diffracted SPP fields from each Fresnel zone constructively interfere at the expected focal point to produce focusing with threefold intensity enhancement. Temporal and spatial characteristics of the focused SPP fields are studied with time-resolved spatial-heterodyne imaging technique. Good agreement with average power measurements is demonstrated. Diffractive focusing of SPP fields, based on Fourier plasmonics, represents an approach to SPP in-plane microscopy.
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Formation of one-dimensional nanoparticle chains Appl. Phys. Lett. 91, 063119 (2007); http://dx.doi.org/10.1063/1.2768891 (3 pages) Online Publication Date: 9 August 2007
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The authors report an approach to forming one-dimensional chains of nanoparticles. It is a purely physical process and thus is free from both chemical contamination and residue side products. The size of the dots as well as the interdot spacing can be manipulated by adjusting the conditions of a postannealing process, changing the initial wire width, or using different substrates. Taking advantage of the capability for precise positioning using e-beam lithograpy, this method is a relatively simple route to fabricating one-dimensional nanoparticle chains with precise geometries.
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p-type conduction in unintentional carbon-doped ZnO thin films Appl. Phys. Lett. 91, 072101 (2007); http://dx.doi.org/10.1063/1.2768917 (3 pages) Online Publication Date: 14 August 2007
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p-type conduction has been observed in unintentional carbon-doped ZnO thin films grown by metal organic chemical vapor deposition through postgrowth annealing treatment. The existence of carbon, which has been verified by secondary ion mass spectrometry and x-ray photoelectron spectroscopy, was predicted to immobilize the oxygen in the interstitial site in ZnO thin films after annealing. Using first principles calculations, the formation of carbon-oxygen cluster defect in ZnO was found to be favorable and acts as a shallow acceptor. The cryogenic photoluminescence of the p-type carbon-doped ZnO thin films shows an additional peak located at 3.3564 eV, which was attributed to the acceptor (carbon-oxygen cluster defect) bound exciton emission.
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Ink-jet printed ZnO nanowire field effect transistors Appl. Phys. Lett. 91, 043109 (2007); http://dx.doi.org/10.1063/1.2760041 (3 pages) Online Publication Date: 26 July 2007
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Semiconducting nanowires provide routes for realizing high-performance electronic devices, but for many applications of such devices low-cost manufacturing techniques are needed. The authors demonstrate here top-gated zinc oxide nanowire field effect transistors (NW-FETs) fabricated by ink-jet printing. High resolution submicrometer gold gaps between source and drain electrodes were defined by a self-aligned ink-jet printing technique, and the nanowires were deposited from solution onto these electrode arrays and gated from the top using a spin-coated poly(methyl methacrylate) gate dielectric. The typical NW-FETs exhibited a mobility of 2–4 cm2/V s, a current on/off ratio of 104, and a transconductance of 20.5 nS. The process provides a pathway for fabrication of NW-FETs by low-cost, large-area solution processing and direct printing techniques.
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Blue-emitting AlN:Eu2+ nitride phosphor for field emission displays Appl. Phys. Lett. 91, 061101 (2007); http://dx.doi.org/10.1063/1.2767182 (3 pages) Online Publication Date: 6 August 2007
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An Eu2+-activated AlN phosphor was synthesized by firing the powder mixture of AlN, α-Si3N4, and Eu2O3 at 2050 °C for 4 h under 1.0 MPa N2. This nitride phosphor emits a strong blue color with the chromaticity coordinates of x = 0.139 and y = 0.106 at an accelerating voltage of 3 kV. The cathodoluminescence properties of AlN:Eu2+ was evaluated by utilizing it in the Spindt-type field emission display panel. It shows that the nitride phosphor exhibits higher brightness, higher color purity, lower saturation, and longer lifetime than the currently used Y2SiO5:Ce3+, indicative of the suitability of the AlN:Eu2+ blue phosphor in field emission displays.
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Highly efficient organic tandem solar cells using an improved connecting architecture Appl. Phys. Lett. 91, 073519 (2007); http://dx.doi.org/10.1063/1.2772208 (3 pages) Online Publication Date: 16 August 2007
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Tandem solar cells based on the combination of a poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester and a copper phthalocyanine:fullerene subcell are reported. By using a highly transparent, high-work function WO3 layer as part of the interconnecting system for the two subcells, the authors demonstrate stacked devices with power conversion efficiencies as high as 4.6%. The efficiency of the stacked devices is close to the sum of the efficiencies of the individual subcells.
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Thin-film silicon solar cells with efficient periodic light trapping texture Appl. Phys. Lett. 91, 061116 (2007); http://dx.doi.org/10.1063/1.2768882 (3 pages) Online Publication Date: 8 August 2007
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For solar cells based on thin-film microcrystalline (μc-Si:H) or amorphous silicon (a-Si:H) with absorber layers in the micrometer range, highly effective light trapping and an optimal incoupling of the entire sun spectrum are essential. To investigate and optimize both effects the wave propagation in thin-film silicon solar cells is modeled in three dimensions (3D) solving the Maxwell equations rigorously. A periodic nanostructured texture is investigated as an alternative to the common randomly rough texture. Inverted 3D pyramids with a periodicity of 850 nm and structure height of 400 nm show promising high quantum efficiencies close to the Tiedje limit.
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AlN air-bridge photonic crystal nanocavities demonstrating high quality factor Appl. Phys. Lett. 91, 051106 (2007); http://dx.doi.org/10.1063/1.2757596 (3 pages) Online Publication Date: 31 July 2007
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The authors report an achievement of high quality AlN ultraviolet photonic crystal nanocavities. Convex AlN air-bridge structures with embedded GaN quantum dots have been formed by utilizing photoelectrochemical etching of 6H-SiC substrates. Room-temperature microscopic photoluminescence measurements reveal the high quality of the nanocavities. For the lowest-order cavity mode of a 150-nm-period nanocavity with seven missing holes, the highest Q factor (>2400) ever reported in nitride-based photonic crystals has been obtained.
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Appl. Phys. Lett. 91, 081102 (2007); http://dx.doi.org/10.1063/1.2772766 (3 pages) Online Publication Date: 20 August 2007
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The authors report on a spray deposition method as a cost-efficient technique for the fabrication of organic solar cells (OSCs). Active layers of OSCs were fabricated using conventional handheld airbrushes. Although the spray deposited film showed a relatively rougher surface than spin coated ones, pinhole-free and constant thickness films could be obtained. An optimized OSC showed 2.83% of power conversion efficiency and 52% of incident photon to current conversion efficiency even though the device was fabricated in air. The performance of sprayed OSCs was comparable to that of the spin coated devices fabricated in air.
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Appl. Phys. Lett. 91, 061903 (2007); http://dx.doi.org/10.1063/1.2767993 (3 pages) Online Publication Date: 6 August 2007
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Optical properties of p-type ZnO epilayers doped with different amounts of phosphorus by radio-frequency magnetron sputtering are investigated by x-ray diffraction, temperature dependent photoluminescence (PL), and time-resolved PL techniques. Bound exciton, free electrons-to-acceptors, donor-to-acceptor pair, and deep-level emissions are observed at about 3.356, 3.32, 3.24, and 2.4 eV at 10 K for p-type ZnO, respectively. The crystal quality and luminescence efficiency are improved with increasing phosphorus doping concentration. These results show that phosphorus doping plays an important role both in reducing native defects and in generating shallow acceptors in ZnO, leading to a good p-type conductivity in ZnO.
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Appl. Phys. Lett. 91, 061106 (2007); http://dx.doi.org/10.1063/1.2757134 (3 pages) Online Publication Date: 6 August 2007
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The authors investigate the modification of the spontaneous emission dynamics and the external quantum efficiency for self-assembled InGaAs quantum dots coupled to extended and localized photonic states in GaAs two-dimensional photonic crystals. The photonic band gap is shown to give rise to a five to ten times enhancement of the external quantum efficiency, while the spontaneous emission rate is simultaneously reduced by a comparable factor. The findings are quantitatively explained by a modal redistribution of spontaneous emission due to the modified local density of photonic states. The results suggest that quantum dots embedded within photonic crystals are suitable for practical single photon sources with high external efficiency.
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Highly efficient p-i-n-type organic light emitting diodes on ZnO:Al substrates Appl. Phys. Lett. 91, 063510 (2007); http://dx.doi.org/10.1063/1.2768865 (3 pages) Online Publication Date: 8 August 2007
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Aluminum doped zinc oxide (ZAO) is presented in this letter as an alternative transparent electrode: optimized ZAO films offer excellent parameters for organic light emitting diodes (OLEDs). The ZAO films are applied to various p-i-n-type OLEDs. By using green phosphorescent molecules in a double emitter structure, very high efficiencies were obtained, namely, 54.6 cd/A and 61.5 lm/W for 100 cd/m2 at 2.78 V. Additionally, white OLEDs on ZAO demonstrated pure white emission independent of the luminance and high efficiencies of 12.6 cd/A and 14.5 lm/W for 100 cd/m2 at 2.6 V, which is comparable to indium-tin-oxide based white OLEDs.
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Charge injection process in organic field-effect transistors Appl. Phys. Lett. 91, 053508 (2007); http://dx.doi.org/10.1063/1.2759987 (3 pages) Online Publication Date: 1 August 2007
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The charge injection process in top-contact organic field-effect transistors was energetically observed with displacement of the Fermi level as a result of scanning the gate voltage. Doping of charge-transfer molecules into the metal/organic interface resulted in low interface resistance, which unveiled the bulk transport of the injected charges from the contact metal into the channel. The authors found that the bulk transport clearly obeys the Meyer-Neldel rule, according to which the exponential density of states near the band edge limits the charge injection.
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High-gain pentacene-based inverter achieved through high and low energy ultraviolet treatments Appl. Phys. Lett. 91, 083504 (2007); http://dx.doi.org/10.1063/1.2772752 (3 pages) Online Publication Date: 21 August 2007
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The authors report on the fabrication of pentacene-based inverter with two p-channel thin-film transistors (TFTs) on polymer/AlOx bilayer dielectric, which has been patterned by high energy ultraviolet (UV) (254 nm) illumination. After pentacene channel growth on the dielectric, the inverter showed a high voltage gain of ∼ 10 under −6 V supply voltage (VDD) but at a transition voltage of −1 V which is too marginal to guarantee a desirable inverter operation between 0 and −6 V. When low energy UV (352 nm) was applied onto one of the two p TFTs, which plays as a load in the inverter circuit, the transition voltage shifted to an adequate value (−3 V) because the UV changes the threshold voltage of the load TFT to be lower. The UV-treated inverter demonstrated a high voltage gain of ∼ 150 under a VDD of −30 V.
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