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Top 20 Most Read Articles
December 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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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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An effective light trapping configuration for thin-film solar cells Appl. Phys. Lett. 91, 243501 (2007); http://dx.doi.org/10.1063/1.2789677 (3 pages) Online Publication Date: 10 December 2007
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Many thin-film solar cells make a compromise between achieving complete optical absorption using films that are thicker than the optical absorption length and achieving efficient conversion of the absorbed photons into photocurrent which is favored in thinner structures. We evaluate the performance of a V-shaped light trapping configuration that substantially increases the photocurrent generation efficiency for all angles of incidence and that is applicable to a broad class of low-cost thin-film solar cells. We experimentally demonstrate its effectiveness for small molecular weight and polymer organic solar cells. A 52% efficiency enhancement is obtained for a 170-nm-thick polymer cell.
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A few-electron vertical In0.56Ga0.44As quantum dot with an insulating gate Appl. Phys. Lett. 91, 232101 (2007); http://dx.doi.org/10.1063/1.2818712 (3 pages) Online Publication Date: 3 December 2007
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Using an Al2O3 gate insulator by atomic layer deposition and air-bridge drain electrode, we fabricated a quantum dot with few electrons based on an In0.56Ga0.44As resonant tunneling diode structure. Artificial atomic properties manifested themselves in magnetotransport, enabling the determination of effective electron g factors. Results show that the insulating gate structure used here is effective for realizing quantum dots made of narrow-gap semiconductors for studying spin-related phenomena.
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A highly efficient wide-band-gap host material for blue electrophosphorescent light-emitting devices Appl. Phys. Lett. 91, 233501 (2007); http://dx.doi.org/10.1063/1.2821116 (3 pages) Online Publication Date: 3 December 2007
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We report on an efficient wide-band-gap host material for blue electrophosphorescence devices, namely, 1,2-trans-di-9-carbazolylcyclobutane (DCz). Photophysical studies show that lower-energy excimer formation between the carbazole units can be efficiently suppressed in a DCz film, thus maintaining its high triplet-state energy and inducing an exothermic energy transfer from DCz to iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic). Electrophosphorescent devices comprising a FIrpic:DCz emitting layer exhibit a superior performance with a maximum external quantum efficiency of 9.8%, a maximum luminance efficiency of 21.5 cd/A, and a maximum power efficiency of 15.0 lm/W at 0.01 mA/cm2.
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Widely tunable single-mode quantum cascade laser source for mid-infrared spectroscopy Appl. Phys. Lett. 91, 231101 (2007); http://dx.doi.org/10.1063/1.2816909 (3 pages) Online Publication Date: 3 December 2007
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We demonstrate a compact, single-mode quantum cascade laser source continuously tunable between 8.7 and 9.4 μm. The source consists of an array of single-mode distributed feedback quantum cascade lasers with closely spaced emission wavelengths fabricated monolithically on a single chip and driven by a microelectronic controller. Our source is suitable for a variety of chemical sensing applications. Here, we use it to perform absorption spectroscopy of fluids.
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Appl. Phys. Lett. 91, 231901 (2007); http://dx.doi.org/10.1063/1.2820385 (3 pages) Online Publication Date: 3 December 2007
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A multidimensional ZnO light-emitting diode (LED) structure comprising film/nanorods/substrate was fabricated on a p-type Si substrate using metal organic chemical vapor deposition at relatively low growth temperature. The filmlike top layer used for the metal contact was continuously formed on the ZnO nanorods by varying the growth conditions and the resulting structure allowed us to utilize the nanorods with intense emission as an active layer. We investigated the performance of the resulting multidimensional LED. An extremely high breakdown voltage and low reverse leakage current as well as typical rectification behavior were observed in the I-V characteristics.
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Coherent control of molecular modulation Appl. Phys. Lett. 91, 241101 (2007); http://dx.doi.org/10.1063/1.2821382 (3 pages) Online Publication Date: 10 December 2007
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We demonstrate coherent control of a molecular modulation process using an incident set of seven optical sidebands spanning two octaves of bandwidth. We utilize a genetic algorithm to optimize the relative phases of the incident sidebands to generate additional UV sidebands with nearly 1% efficiency, change the ratio of energy between sidebands by more than a factor of 50, and efficiently alter individual sideband energies by millijoules.
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Appl. Phys. Lett. 91, 251101 (2007); http://dx.doi.org/10.1063/1.2822844 (3 pages) Online Publication Date: 17 December 2007
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We report the fabrication of both orthorhombic and tetragonal photonic crystal templates using a phase mask technique. Three-dimensional photonic crystal structures were formed by a double exposure of SU8 to three-beam interference patterns generated by a phase mask. Lattice structures and photonic band gap can be controlled by rotational angles of the phase mask between two exposures. Band gap computation predicts that photonic crystal structures with the optimized band gap can be realized when the rotational angle is set between 50° and 70°. A photonic crystal template with 60° phase mask rotation was fabricated, showing improved lattice structures required for the band gap opening.
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Selective growth of catalyst-free ZnO nanowire arrays on Al:ZnO for device application Appl. Phys. Lett. 91, 233112 (2007); http://dx.doi.org/10.1063/1.2811717 (3 pages) Online Publication Date: 6 December 2007
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Vertically aligned ZnO nanowire (NW) arrays have been synthesized selectively on patterned aluminum-doped zinc oxide (AZO) layer deposited on silicon substrates without using any metal catalysts. The growth region was defined by conventional photolithography with an insulating template. Careful control of the types of template materials and growth conditions allows good alignment and growth selectivity for ZnO NW arrays. Sharp ultraviolet band-edge peak observed in the photoluminescence spectra of the patterned ZnO NW arrays reveals good optical qualities. The current-voltage characteristics of ZnO NWs/AZO/p-Si device suggest that patterned and aligned ZnO NW arrays on AZO may be used in optoelectronic devices.
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Appl. Phys. Lett. 91, 243502 (2007); http://dx.doi.org/10.1063/1.2817930 (3 pages) Online Publication Date: 10 December 2007
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This letter analyzes the influence of the energy of the highest-energy occupied molecular orbital (HOMO) and the single occupied molecular orbital (SOMO) (i.e., the low-energy polaronic level) of different electron donors (D) on the open circuit voltage (VOC) value delivered by bilayer and bulk-heterojunction solar cells containing fullerenes as acceptor (A). The analysis further confirms that the VOC of these devices scales with the energy difference between the HOMO of the D and the lowest-energy unoccupied molecular orbital of the A. As observed, a better relationship is obtained if the SOMO energy of the D—extracted from photoinduced absorption spectra—is considered.
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Electrical detection of kidney injury molecule-1 with AlGaN/GaN high electron mobility transistors Appl. Phys. Lett. 91, 222101 (2007); http://dx.doi.org/10.1063/1.2815931 (3 pages) Online Publication Date: 26 November 2007
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AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect kidney injury molecule-1 (KIM-1), an important biomarker for early kidney injury detection. The gate region consisted of 5 nm gold deposited onto the AlGaN surface. The gold was conjugated to highly specific KIM-1 antibodies through a self-assembled monolayer of thioglycolic acid. The HEMT source-drain current showed a clear dependence on the KIM-1 concentration in phosphate-buffered saline solution. The limit of detection was 1 ng/ml using a 20×50 μm2 gate sensing area. This approach shows potential for both preclinical and clinical kidney injury diagnosis with accurate, rapid, noninvasive, and high throughput capabilities.
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Size dependency of the elastic modulus of ZnO nanowires: Surface stress effect Appl. Phys. Lett. 91, 231912 (2007); http://dx.doi.org/10.1063/1.2821118 (3 pages) Online Publication Date: 5 December 2007
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Relation between the elastic modulus and the diameter (D) of ZnO nanowires was elucidated using a model with the calculated ZnO surface stresses as input. We predict for ZnO nanowires due to surface stress effect: (1) when D>20 nm, the elastic modulus would be lower than the bulk modulus and decrease with the decreasing diameter, (2) when 20 nm>D>2 nm, the nanowires with a longer length and a wurtzite crystal structure could be mechanically unstable, and (3) when D<2 nm, the elastic modulus would be higher than that of the bulk value and increase with a decrease in nanowire diameter.
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Appl. Phys. Lett. 91, 243506 (2007); http://dx.doi.org/10.1063/1.2807272 (3 pages) Online Publication Date: 12 December 2007
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Auger recombination is determined to be the limiting factor for quantum efficiency for InGaN–GaN (0001) light-emitting diodes (LEDs) at high current density. High-power double-heterostructure (DH) LEDs are grown by metal-organic chemical vapor deposition. By increasing the active layer thickness, DH LEDs can reach a maximum in quantum efficiency at current densities above 200 A/cm2. Encapsulated thin-film flip-chip DH LEDs with peak wavelength of 432 nm have an external quantum efficiency of 40% and output power of 2.3 W at 2 A.
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Semiconducting chains of gold and silver Appl. Phys. Lett. 91, 223115 (2007); http://dx.doi.org/10.1063/1.2820450 (3 pages) Online Publication Date: 30 November 2007
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The authors introduce a geometry for ultrathin Au and Ag wires that ab initio calculations indicate to be more stable than previously considered planar geometries for these systems by about 0.1 eV per atom. This structure is insulating for both metals and for related Ag0.5Au0.5 alloys with gaps of 1.3 eV for Au, 0.8 eV for Ag, and varying between 0.1 and 1.9 eV for the alloys. The insulating nature of the geometry is not a result of Peierls instabilities and is analyzed in terms of an interplay between geometric and electronic structure effects.
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InGaN/GaN light emitting diodes on nanoscale silicon on insulator Appl. Phys. Lett. 91, 231109 (2007); http://dx.doi.org/10.1063/1.2814062 (3 pages) Online Publication Date: 5 December 2007
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The authors report on the fabrication of InGaN/GaN-based light emitting diodes (LEDs) on nanoscale silicon-on-insulator (SOI) substrates. The LED structures are grown on (111)-oriented 45 nm thick SOI overlayer by metal organic chemical vapor deposition. Square-shaped mesa patterns are created by standard LED processing steps including multiple-mask photolithography, inductive coupled plasma etching, and contact metallization. Due to the high reflective Si/SiO2 beneath AlN buffer and high refractive contrasts at the interfaces, the authors observed multiple interference peaks from LEDs on SOI and such effect resulted in an increased integrated electroluminescence intensity when compared to LED structures fabricated on bulk Si(111).
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Effect of threading defects on InGaN/GaN multiple quantum well light emitting diodes Appl. Phys. Lett. 91, 231107 (2007); http://dx.doi.org/10.1063/1.2822395 (3 pages) Online Publication Date: 5 December 2007
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Photoelectrochemical etching was used to measure the threading defect (TD) density in InGaN multiple quantum well light-emitting diodes (LEDs) fabricated from commercial quality epitaxial wafers. The TD density was measured in the LED active region and then correlated with the previously measured characteristics of these LEDs. It was found that the reverse leakage current increased exponentially with TD density. The temperature dependence of this dislocation-related leakage current was consistent with a hopping mechanism at low reverse-bias voltage and Poole-Frenkel emission at higher reverse-bias voltage. The peak intensity and spectral width of the LED electroluminescence were found to be only weakly dependent on TD density for the measured TD range of 1×107–2×108 cm−2.
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Holographically fabricated photonic crystals with large reflectance Appl. Phys. Lett. 91, 241103 (2007); http://dx.doi.org/10.1063/1.2820449 (3 pages) Online Publication Date: 11 December 2007
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We report reflection and transmission spectra from three-dimensional polymer photonic crystals fabricated by holographic lithography. The measured peak reflectance matches that predicted by both a finite-difference time-domain method a simple transfer matrix theory and is ∼ 70%, significantly higher than previous reports of ∼ 30% reflectance.
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Air stable hybrid organic-inorganic light emitting diodes using ZnO as the cathode Appl. Phys. Lett. 91, 223501 (2007); http://dx.doi.org/10.1063/1.2809387 (3 pages) Online Publication Date: 26 November 2007
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An air stable hybrid organic-inorganic light emitting device is presented. This architecture makes use of metal oxides as charge injecting materials into the light emitting polymer, avoiding the use of air sensitive cathodes commonly employed in organic light emitting diode manufacturing. We report the application of zinc oxide as a cathode in an organic light emitting device. This electroluminescent device shows high brightness levels reaching 6500 cd/m2 at voltages as low as 8 V. Compared to a conventional device using low workfunction metal cathodes, our device shows a lower turn-on voltage and it can operate in air.
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Appl. Phys. Lett. 91, 212109 (2007); http://dx.doi.org/10.1063/1.2817747 (3 pages) Online Publication Date: 21 November 2007
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We present the electrical injection, detection, and magnetic field modulation of lateral diffusive spin transport through silicon using surface contacts. Fe/Al2O3 tunnel barrier contacts are used to create and analyze the flow of pure spin current in a silicon transport channel. Nonlocal detection techniques show that the spin current detected after transport through the silicon is sensitive to the relative orientation of the magnetization of the injecting and detecting contacts. Hanle effect measurements demonstrate that the spin current can be modulated by a perpendicular magnetic field, which causes the spin to precess and dephase in the transport channel.
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Thermodynamic limits of quantum photovoltaic cell efficiency Appl. Phys. Lett. 91, 223507 (2007); http://dx.doi.org/10.1063/1.2817753 (3 pages) Online Publication Date: 27 November 2007
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The intermediate band solar cell has been proposed as an ultrahigh efficiency source of energy due to the possibility of absorption of two sequential sub-band-gap photons to excite charge from a quantum confined (e.g., quantum dot or well) region into the large band gap barrier region [
A. Luque and A. Martí, Phys. Rev. Lett. 78, 5014 (1997)
]. Unfortunately, high efficiencies using this structure have not yet been realized. Here, we analyze the fundamental limits to power generation in quantum solar cells. When a difference in quasi-Fermi energies between the barrier and the quantum well regions exists due to the presence of photogenerated charge, an upper efficiency limit of 44.5% is achievable due to single photon absorption only. This efficiency is significantly higher than the Shockley-Queisser limit of ∼ 31% for homojunction cells, but remains below that predicted for two photon excitation (>63%) previously predicted for quantum cells.
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