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Top 20 Most Read Articles
May 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Electronic structure and optical gain of wurtzite ZnO nanowires Appl. Phys. Lett. 92, 181101 (2008); http://dx.doi.org/10.1063/1.2918445 (3 pages) Online Publication Date: 5 May 2008
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The electronic structure and optical gain of wurtzite ZnO nanowires are investigated in the framework of effective-mass envelope-function theory. We found that as the elliptical aspect ratio e increases to be larger than a critical value, the hole ground states may change from optically dark to optically bright. The optical gain of ZnO nanowires increases as the hole density increases. For elliptical wire with large e, the y-polarized mode gain can be several thousand cm−1, while the x-polarized mode gain may be 26 times smaller than the former, so they can be used as ultraviolet linearly polarized lasers.
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Self-assembled organic donor/acceptor nanojunction arrays Appl. Phys. Lett. 92, 193301 (2008); http://dx.doi.org/10.1063/1.2920199 (3 pages) Online Publication Date: 12 May 2008
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We have fabricated well-ordered organic donor/acceptor nanojunction arrays comprising p-sexiphenyl (6P) and C60 via self-assembly of C60 on the molecular nanotemplate of 6P nanostripes on Ag(111). Our scanning tunneling microscopy measurements show that the preferential trapping of C60 triplets atop 6P single molecule leads to the formation of the C60/6P vertical nanojunctions with well-defined two-dimensional arrangement. Annealing this binary molecular superstructure at 380 K results in the insertion of C60 linear chains between the neighboring 6P nanostripes and thereby the appearance of the periodic C60/6P lateral nanojunction arrays.
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Silicon nanowire array photoelectrochemical solar cells Appl. Phys. Lett. 92, 163103 (2008); http://dx.doi.org/10.1063/1.2909555 (3 pages) Online Publication Date: 22 April 2008
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Silicon nanowires (SiNWs) arrays prepared by electroless etching show excellent optical antireflectivity over a wide spectral bandwidth from 300 to 1000 nm and surface defect-induced electrical conductivity. Both characteristics make the SiNWs a promising material for photovoltaic cell applications. Photoelectrochemical (PEC) measurements showed the electroless etching SiNWs are remarkably photoactive and effective in enhancing photovoltaic properties including photocurrent and photovoltage. Since electroless etching can enable simple, wafer-scale fabrication of SiNWs without the need of doping. SiNWs array thus prepared show great promise as low-cost and scalable photovoltaic-type PEC materials.
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Advanced light trapping management by diffractive interlayer for thin-film silicon solar cells Appl. Phys. Lett. 92, 181102 (2008); http://dx.doi.org/10.1063/1.2919727 (3 pages) Online Publication Date: 5 May 2008
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Thin-film silicon solar cells made of amorphous and microcrystalline silicon in tandem cell configuration enable high efficiency and low-cost production. Precise control of the absorption in each diode by a wavelength-selective and diffractive interlayer provides optimized current matching. For this purpose, intermediate reflectors with periodically textured interfaces are investigated. The propagation of electromagnetic waves is simulated using a three dimensional Maxwell solver which considers both near field and far field optics. Design rules for intermediate reflectors and textured interfaces are presented.
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Appl. Phys. Lett. 92, 201101 (2008); http://dx.doi.org/10.1063/1.2919051 (3 pages) Online Publication Date: 19 May 2008
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We report on our progress in the development of a terahertz quantum cascade laser source based on intracavity terahertz difference-frequency mixing in a dual-wavelength mid-infrared quantum cascade laser with the active region engineered to possess giant second-order nonlinear susceptibility. In this letter, we demonstrate devices that operate in mid-infrared at λ1 = 8.9 μm and λ2 = 10.5 μm and produce terahertz output at λ ≈ 60 μm via difference-frequency generation with 7 μW output power at 80 K, 1 μW output at 250 K, and still approximately 300 nW output at 300 K.
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Selective organization of solution-processed organic field-effect transistors Appl. Phys. Lett. 92, 173301 (2008); http://dx.doi.org/10.1063/1.2912822 (3 pages) Online Publication Date: 28 April 2008
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Semiconductor channels of organic field-effect transistors have been directly self-organized from a solution phase. The alkyl-modified surface was locally patterned by using a phenyl self-assembled monolayer (SAM) for the channels. Drop-cast small organic molecules were selectively crystallized on the phenyl SAM region. The self-organized process allows the simultaneous formation of polycrystalline transistor arrays from the patterned channels. The phenyl SAM under the channel is critical for the improvement of device stability. Further optimization of the deposition process realized direct growth of a single crystal channel from solution between prefabricated electrodes, and the single-crystal transistors exhibited excellent performance.
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High performance light emitting transistors Appl. Phys. Lett. 92, 183304 (2008); http://dx.doi.org/10.1063/1.2920436 (3 pages) Online Publication Date: 9 May 2008
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Solution processed light emitting field-effect transistors (LEFETs) with peak brightness exceeding 2500 cd/m2 and external quantum efficiency of 0.15% are demonstrated. The devices utilized a bilayer film comprising a hole transporting polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b] thiophene) and a light emitting polymer, Super Yellow, a polyphenylenevinylene derivative. The LEFETs were fabricated in the bottom gate architecture with top-contact Ca/Ag as source/drain electrodes. Light emission was controlled by the gate voltage which controls the hole current. These results indicate that high brightness LEFETs can be made by using the bilayer film (hole transporting layer and a light emitting polymer).
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Lithographic alignment to site-controlled quantum dots for device integration Appl. Phys. Lett. 92, 183101 (2008); http://dx.doi.org/10.1063/1.2920189 (3 pages) Online Publication Date: 5 May 2008
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We report on a scalable fabrication technology for devices based on single quantum dots (QDs) which combines site-controlled growth of QDs with an accurate alignment procedure. Placement of individual QDs and corresponding device structures with a standard deviation of around 50 nm from the target position was achieved. The potential of the technology is demonstrated by fabricating arrays of mesas, each containing one QD at a defined position. The presence of single, optically active QDs in the mesas was probed by scanning microphotoluminescence of the mesa arrays.
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Tunable wettability in surface-modified ZnO-based hierarchical nanostructures Appl. Phys. Lett. 92, 173104 (2008); http://dx.doi.org/10.1063/1.2918447 (3 pages) Online Publication Date: 29 April 2008
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Designing geometrical structures and making chemical modifications are two effective routes to tailor wettability. ZnO-based hierarchical nanostructures, in particular, vertically aligned nanoneedles and nanonails were employed as a platform to study the effect of surface morphology. The hydrophobicity and the variation of contact angle in the as-grown samples were attributed to the combined effects of surface roughness and partial water-solid contact. Subsequent chemical modifications with stearic acid led to superhydrophobic ZnO surfaces, while annealing in air resulted into superhydrophilicity. Under the alternations of coating and removal of stearic acid, reversible transitions between superhydrophilicity and hydrophobicity were realized.
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Engineering the randomness for enhanced absorption in solar cells Appl. Phys. Lett. 92, 171114 (2008); http://dx.doi.org/10.1063/1.2919094 (3 pages) Online Publication Date: 2 May 2008
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Photon management by means of random textured surfaces is known to be a promising route to increase the light absorption in a solar cell. To date this randomness was only a posteriori assessed and related to the absorption. Here, we will outline a meaningful strategy for a priori and purposely tailoring the randomness. By defining appropriate angular scattering functions and optimizing the surface profiles, it is shown that the number of absorbed photons can be enhanced by 55% compared to flat-surface solar cells.
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Silicon nanowire tunneling field-effect transistors Appl. Phys. Lett. 92, 193504 (2008); http://dx.doi.org/10.1063/1.2928227 (3 pages) Online Publication Date: 13 May 2008
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We demonstrate the implementation of tunneling field-effect transistors (TFETs) based on silicon nanowires (NWs) that were grown using the vapor-liquid-solid growth method. The Si NWs contain p-i-n+ segments that were achieved by in situ doping using phosphine and diborane as the n- and p-type dopant source, respectively. Electrical measurements of the TFETs show a band-to-band tunneling branch in the transfer characteristics. Furthermore, an increase in the on-state current and a decrease in the inverse subthreshold slope upon reducing the gate oxide thickness are measured. This matches theoretical calculations using a Wenzel Kramer Brillouin approximation with nanowire diameter and oxide thickness as input parameters.
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An easy mechanical way to create ferromagnetic defective ZnO Appl. Phys. Lett. 92, 182504 (2008); http://dx.doi.org/10.1063/1.2921782 (3 pages) Online Publication Date: 7 May 2008
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We present an experiment for the creation of unconventional ferromagnetism in pure ZnO powder by application of mechanical force. The ferromagnetism is related to flakelike structures in planar compressed pieces of the powder with easy axis in the plane. It is associated with defect creation and disappears upon annealing in oxygen. Besides cluster formation, such defects might be one of the nonintrinsic origins of ferromagnetism in transition metal doped ZnO.
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Stable ZnO thin film transistors by fast open air atomic layer deposition Appl. Phys. Lett. 92, 192101 (2008); http://dx.doi.org/10.1063/1.2924768 (3 pages) Online Publication Date: 12 May 2008
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We report stable, high performance zinc oxide thin film transistors grown by an atmospheric pressure atomic layer deposition system. With all deposition and processing steps kept at or below 200 °C, the alumina gate dielectric shows low leakage (below 10−8 A/cm2) and high breakdown fields. Zinc oxide thin film transistors in a bottom gate geometry yield on/off ratios above 108, near zero turn-on voltage, little or no hysteresis, and mobility greater than 10 cm2/V s. With alumina passivation, shifts in threshold voltage under gate bias stress compare favorably to those reported in the literature.
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Appl. Phys. Lett. 92, 143101 (2008); http://dx.doi.org/10.1063/1.2905271 (3 pages) Online Publication Date: 7 April 2008
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We report a high-bendability flexible dye-sensitized solar cell (DSSC) based on a ZnO-nanowire photoelectrode, which was fabricated on polyethylene terephtalate/indium tin oxide substrate by low-temperature hydrothermal growth. Nanowire morphology shows preferable in crack resistance due to its efficient release of bending stress. The ZnO-nanowire film can be bended to an extreme radius of 2 mm with no crack observed. Flexible DSSCs based on this kind of ZnO-nanowire photoelectrodes showed good bending stability. With a ZnO-nanoparticle modification on the nanowires, the flexible DSSC fabricated showed a much improved power conversion efficiency. Meanwhile, the good bendablility of this nanoparticle-modified nanowire electrode is maintained. The results demonstrate that high quality ZnO nanowires fabricated by the low-temperature method is promising for efficient and flexible plastic solar cells.
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Strain effects and thickness dependence of ferroelectric properties in epitaxial BiFeO3 thin films Appl. Phys. Lett. 92, 182902 (2008); http://dx.doi.org/10.1063/1.2920192 (3 pages) Online Publication Date: 6 May 2008
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A “misfit strain-temperature” phase diagram for BiFeO3 thin film was constructed based on the Landau–Devonshire theory with the mechanical substrate effect. It is found that the polarization instabilities result in the formation of the monoclinic phases in BiFeO3 thin films. The effective substrate lattice parameter has been introduced to calculate the film thickness dependence of the polarization and the dielectric constants. The theoretical results are in agreement with the experimental data for the thickness dependence of ferroelectric properties of the BiFeO3 epitaxial thin films on SrTiO3 and Si substrates.
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Highly efficient inverted polymer solar cell by low temperature annealing of Cs2CO3 interlayer Appl. Phys. Lett. 92, 173303 (2008); http://dx.doi.org/10.1063/1.2918983 (3 pages) Online Publication Date: 30 April 2008
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We demonstrate a highly efficient inverted bulk heterojunction polymer solar cell based on regioregular poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester with a low temperature annealed interfacial buffer layer, cesium carbonate (Cs2CO3). This approach improves the power conversion efficiency of the inverted cell from 2.3% to 4.2%, with short-circuit current of 11.17 mA/cm2, open-circuit voltage of 0.59 V, and fill factor of 63% under AM1.5G 100 mW/cm2 irradiation. This result is comparable to the previous regular structure device on the same system. Ultraviolet photoelectron spectroscopy shows that the work function of annealed Cs2CO3 layer decreases from 3.45 to 3.06 eV. Further x-ray photoelectron spectroscopy results reveal that Cs2CO3 can decompose into low work function, doped cesium oxide Cs2O upon annealing, which is accountable for the work-function reduction and device efficiency improvement.
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Appl. Phys. Lett. 92, 183107 (2008); http://dx.doi.org/10.1063/1.2919800 (3 pages) Online Publication Date: 8 May 2008
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We report here a simple and easy method of fabricating arranged inorganic nanowire architecture via electrospinning method equipped with a devised collector and demonstrate hybrid photovoltaic cells that are fashioned of planar-aligned TiO2 nanowire architectures such as uniaxially aligned nanowires and multiple layers of cross-aligned nanowire arrays with poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene]. The power conversion efficiency can be significantly improved by at least 70% under 1 sun condition depending on the degree of aligning TiO2 nanowire arrays through enhancing charge collection and transport rate, as well as facilitating the polymer infiltration as compared to a randomly collected TiO2 nanowire electrode.
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Structural and optical properties of nonpolar GaN thin films Appl. Phys. Lett. 92, 171904 (2008); http://dx.doi.org/10.1063/1.2918834 (3 pages) Online Publication Date: 29 April 2008
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A correlation between the structural and optical properties of GaN thin films grown in the [11
 0] direction has been established using transmission electron microscopy and cathodoluminescence spectroscopy. The GaN films were grown on an r-plane sapphire substrate, and epitaxial lateral overgrowth was achieved using SiO2 masks. A comparison between the properties of GaN directly grown on sapphire and GaN laterally grown over the SiO2 mask is presented. The densities and dimensions of the stacking faults vary significantly with a high density of short faults in the window region and a much lower density of longer faults in the wing region. The low-temperature luminescence spectra consist of peaks at 3.465 and 3.41 eV, corresponding to emission from donor-bound excitons and basal-plane stacking faults, respectively. A correlation between the structural defects and the light emission characteristics is presented. |
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Hybrid optoelectronics: A polymer laser pumped by a nitride light-emitting diode Appl. Phys. Lett. 92, 163306 (2008); http://dx.doi.org/10.1063/1.2912433 (3 pages) Online Publication Date: 23 April 2008
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We demonstrate indirect electrically pumped lasing in a hybrid polymer laser. The lasers comprise a corrugated fluorene copolymer waveguide on an InGaN light-emitting diode and were driven under nanosecond pulsed operation. We observe the onset of distributed feedback lasing at 568 nm for peak drive currents above 144 A. Angle-resolved photoluminescence measurements identify the lasing mechanism as band edge feedback from a photonic stopband in the TE0 waveguide mode.
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Appl. Phys. Lett. 91, 233117 (2007); http://dx.doi.org/10.1063/1.2821113 (3 pages) Online Publication Date: 7 December 2007
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Silicon nanowire-based solar cells on metal foil are described. The key benefits of such devices are discussed, followed by optical reflectance, current-voltage, and external quantum efficiency data for a cell design employing a thin amorphous silicon layer deposited on the nanowire array to form the p-n junction. A promising current density of ∼ 1.6 mA/cm2 for 1.8 cm2 cells was obtained, and a broad external quantum efficiency was measured with a maximum value of ∼ 12% at 690 nm. The optical reflectance of the silicon nanowire solar cells is reduced by one to two orders of magnitude compared to planar cells.
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