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Top 20 Most Read Articles
October 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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Auger recombination in InGaN measured by photoluminescence Appl. Phys. Lett. 91, 141101 (2007); http://dx.doi.org/10.1063/1.2785135 (3 pages) Online Publication Date: 1 October 2007
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The Auger recombination coefficient in quasi-bulk InxGa1−xN (x ∼ 9%–15%) layers grown on GaN (0001) is measured by a photoluminescence technique. The samples vary in InN composition, thickness, and threading dislocation density. Throughout this sample set, the measured Auger coefficient ranges from 1.4×10−30 to 2.0×10−30 cm6 s−1. The authors argue that an Auger coefficient of this magnitude, combined with the high carrier densities reached in blue and green InGaN/GaN (0001) quantum well light-emitting diodes (LEDs), is the reason why the maximum external quantum efficiency in these devices is observed at very low current densities. Thus, Auger recombination is the primary nonradiative path for carriers at typical LED operating currents and is the reason behind the drop in efficiency with increasing current even under room-temperature (short-pulsed, low-duty-factor) injection conditions.
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Two-dimensional polymer photonic crystal band-edge lasers fabricated by nanoimprint lithography Appl. Phys. Lett. 91, 151101 (2007); http://dx.doi.org/10.1063/1.2798250 (3 pages) Online Publication Date: 8 October 2007
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We report on the fabrication and characterization of two-dimensional polymer photonic crystal band-edge lasers operating in the visible range. The components have been fabricated in a dye chromophore-loaded polymer matrix by nanoimprint lithography. High-symmetry band-edge modes are used to generate laser emission. The experimental lasing frequencies are in good agreement with those calculated using a two-dimensional plane wave algorithm. These results demonstrate the potential of nanoimprint lithography for the fabrication of two-dimensional planar photonic crystal structures in an active medium in a one-step process.
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Plasmonic quantum cascade laser antenna Appl. Phys. Lett. 91, 173113 (2007); http://dx.doi.org/10.1063/1.2801551 (3 pages) Online Publication Date: 23 October 2007
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We report a plasmonic quantum cascade laser antenna that confines coherent midinfrared radiation well below the diffraction limit. The antenna was fabricated on the facet of a midinfrared quantum cascade laser and consists of a pair of gold nanorods separated by a gap. The antenna near field was characterized by an apertureless near-field scanning optical microscope; field confinement of about 100 and 70 nm, limited by the gap size, was demonstrated at wavelengths of 7.0 and 5.3 μm, respectively. This device may find important applications in midinfrared subwavelength chemical and biological imaging and spectroscopy.
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Energy pathways in nanoscale thermal radiation Appl. Phys. Lett. 91, 153101 (2007); http://dx.doi.org/10.1063/1.2793688 (3 pages) Online Publication Date: 8 October 2007
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We show in this letter that when nanoscale radiation between two parallel plates is considered, the Poynting vector is decoupled for each parallel wavevector component (β) due to the nature of thermal emission, as manifested by the fluctuation-dissipation theorem. The streamlines calculated by tracing the Poynting vector vividly demonstrate that the spectral radiative energy travels in infinite directions along curved lines. Depending on the β value, the energy pathway may exhibit considerable lateral shift. This letter elucidates the fundamental characteristics of nanoscale thermal radiation that is important for applications, such as near-field optical sensors and energy conversion devices.
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Transport of light in amorphous photonic materials Appl. Phys. Lett. 91, 171904 (2007); http://dx.doi.org/10.1063/1.2800372 (3 pages) Online Publication Date: 23 October 2007
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Amorphous photonic materials based on dense assemblies of high refractive index spherical particles are presented. Light transmission through these photonic glasses shows a nontrivial wavelength dependence. The transmission spectra can be quantitatively reproduced by modeling the optical properties starting from their building blocks. Our results emphasize the relevance of including short range order correlations and an appropriate effective refractive index in the analysis of light transmission through amorphous photonic materials.
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Spin field effect transistor with a graphene channel Appl. Phys. Lett. 91, 153105 (2007); http://dx.doi.org/10.1063/1.2798596 (3 pages) Online Publication Date: 9 October 2007
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A spin field effect transistor (FET) is proposed by utilizing a graphene layer as the channel. Similar to the conventional spin FETs, the device involves spin injection and spin detection by ferromagnetic source and drain. Due to the negligible spin-orbit coupling in the carbon based materials, spin manipulation in the channel is achieved via electrical control of the electron exchange interaction with a ferromagnetic gate dielectric. Numerical estimates indicate the feasibility of the concept if the bias can induce a change in the exchange interaction energy of the order of meV. When nanoribbons are used for a finite channel width, those with armchair-type edges can maintain the device stability against the thermal dispersion.
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Direct mechanical mixing in a nanoelectromechanical diode Appl. Phys. Lett. 91, 143101 (2007); http://dx.doi.org/10.1063/1.2789812 (3 pages) Online Publication Date: 1 October 2007
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We observe direct mechanical mixing in nanoelectromechanical transistors fabricated in semiconductor materials operating in the radio frequency band of 10–1000 MHz. The device is made of a mechanically flexible pillar with a length of 240 nm and a diameter of 50 nm placed between two electrodes in an impedance matched coplanar wave guide. We find a nonlinear I-V characteristic, which enables radio frequency mixing of two electromagnetic signals via the nanomechanical transistor. Potential applications for this mixer are ultrasensitive displacement detection or signal processing in communication electronic circuits requiring high-throughput insulation.
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Highly directional radiation pattern of microdisk cavities Appl. Phys. Lett. 91, 151103 (2007); http://dx.doi.org/10.1063/1.2789956 (3 pages) Online Publication Date: 8 October 2007
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The far field radiation pattern of microdisk cavities embedding a quantum well is measured through angle resolved microphotoluminescence. The radiation pattern presents two narrow lobes slightly off the disk plane, in apparent contradiction with previous theoretical predictions. The observed radiation pattern is shown to result from interferences with light reflected by the sample substrate. It can be fully reproduced given the microdisk precise geometry as well as the whispering gallery mode azimuthal number.
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Appl. Phys. Lett. 91, 152111 (2007); http://dx.doi.org/10.1063/1.2799257 (3 pages) Online Publication Date: 12 October 2007
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Vertically oriented TiO2 nanotube arrays formed by anodization offer a highly ordered material architecture for efficient charge generation and collection in photoelectrochemical devices. A blend of regioregular poly(3-hexylthiophene) and a methanofullerene (phenyl C71-butyric acid methyl ester) was infiltrated into transparent TiO2 nanotube films. The heterojunction poly(3-hexylthiophene) (P3HT)-([6,6]-phenyl-C71-butyric acid methyl ester) and P3HT-TiO2 interfaces both result in charge separation. The resulting solid state solar cells show a short-circuit current density of 12.4 mA/cm2, 641 mV open circuit potential, and a 0.51 fill factor, yielding power conversion efficiencies of 4.1% under AM 1.5 sun.
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Light localization at randomly textured surfaces for solar-cell applications Appl. Phys. Lett. 91, 171104 (2007); http://dx.doi.org/10.1063/1.2800374 (3 pages) Online Publication Date: 23 October 2007
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By using a rigorous diffraction theory, the localization of light near textured zinc oxide (ZnO) surfaces is theoretically investigated and compared with experimental data obtained from scanning-near-field-optical microscopy. Although random by nature, these surfaces show well-defined geometrical features, which cause the formation of localized light patterns near the surface. Particularly, photon jets are observed to emerge from conical surface structures. Because these structures are of primary importance for applications in photovoltaics, we analyze the “real” surface topography of textured ZnO used in silicon solar cells. With this work, valuable insight is provided into the mechanism of light coupling through randomly textured interfaces.
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Appl. Phys. Lett. 91, 171101 (2007); http://dx.doi.org/10.1063/1.2800802 (3 pages) Online Publication Date: 22 October 2007
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We propose an analytically tractable structure for controlling enhanced transmission through a metallic grating with subwavelength slits by filling the slits with an anisotropic material such as a liquid crystal. As the director rotates, the Fabry-Pérot condition is modified and the transmission peaks shift. Analytical results of a single-mode theory for the propagation constant, transmission efficiency, and the tunable range of peak wavelengths are given, taking the finite conductivity of metal into account. Analytical results are in good agreement with those calculated by the rigorous coupled-wave analysis, indicating that waveguide resonance plays a dominant role and the results are scalable.
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Bridged ZnO nanowires across trenched electrodes Appl. Phys. Lett. 91, 142108 (2007); http://dx.doi.org/10.1063/1.2794417 (3 pages) Online Publication Date: 3 October 2007
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Using a hydrothermal synthesis approach, large scale and laterally bridged nanowires have been grown across trenched Au/Si, Au/SiO2/Si, and ZnO/Si electrodes. This technique shows a low temperature (80 °C) approach for growing ZnO nanowires on a prepatterned substrate, showing its potential for integrating with silicon based technology. The I-V characteristics of the nanowires have been measured and their nonlinear behavior has been analyzed. It is suggested that the nonlinear behavior might be due to the dominant phonon scattering and the impurity involved grain boundaries in the multinanowire bridges. The bridged nanowire arrays could be useful for fabricating gas, chemical, or biochemical nanosensor arrays.
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Appl. Phys. Lett. 91, 173503 (2007); http://dx.doi.org/10.1063/1.2801624 (3 pages) Online Publication Date: 22 October 2007
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Fullerene single crystal nanorods (C60-nanorods) are synthesized by simple liquid/liquid interface precipitation method. Three dimensional bulk heterojunction donor-acceptor-type organic thick film solar cell is demonstrated having an active layer made from C60-nanorods and regioregular poly(3-octylthiophene). C60-nanorods seem to be promising material for organic solar cell applications. Our preliminary results opens an area of “organic thick film solar cells” which was almost thought to be impossible due to lower mobility of charge carriers and small exciton diffusion lengths in most of the organic materials, including small molecules and conjugated polymers.
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Fast single-charge sensing with a rf quantum point contact Appl. Phys. Lett. 91, 162101 (2007); http://dx.doi.org/10.1063/1.2794995 (3 pages) Online Publication Date: 15 October 2007
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We report high-bandwidth charge sensing measurements using a GaAs quantum point contact embedded in a radio frequency impedance matching circuit (rf-QPC). With the rf-QPC biased near pinch-off where it is most sensitive to charge, we demonstrate a conductance sensitivity of 5×10−6 e2/h Hz−1/2 with a bandwidth of 8 MHz. Single-shot readout of a proximal few-electron double quantum dot is investigated in a mode where the rf-QPC back action is rapidly switched.
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Large-area, ordered hexagonal arrays of nanoscale holes or dots from block copolymer templates Appl. Phys. Lett. 91, 143110 (2007); http://dx.doi.org/10.1063/1.2794010 (3 pages) Online Publication Date: 1 October 2007
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Hexagonal arrays of nanoscale holes or metal dots (25 nm in diameter and 39 nm in period), with orientational order extending over the entire square-centimeter array area, were fabricated on unpatterned silicon wafer substrates using a shear-aligned sphere-forming diblock copolymer template. Since two or more layers of spherical nanodomains are required to achieve alignment in the block copolymer film, but pattern transfer requires a single layer, a multistep etching process was developed, whereby the top layer of a shear-aligned bilayer was evenly removed, leaving the ordered bottom layer as the fabrication template for hole and dot arrays free from grain boundaries.
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Design and characterization of GaN/InGaN solar cells Appl. Phys. Lett. 91, 132117 (2007); http://dx.doi.org/10.1063/1.2793180 (3 pages) Online Publication Date: 28 September 2007
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We experimentally demonstrate the III-V nitrides as a high-performance photovoltaic material with open-circuit voltages up to 2.4 V and internal quantum efficiencies as high as 60%. GaN and high-band gap InGaN solar cells are designed by modifying PC1D software, grown by standard commercial metal-organic chemical vapor deposition, fabricated into devices of variable sizes and contact configurations, and characterized for material quality and performance. The material is primarily characterized by x-ray diffraction and photoluminescence to understand the implications of crystalline imperfections on photovoltaic performance. Two major challenges facing the III-V nitride photovoltaic technology are phase separation within the material and high-contact resistances.
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Surface plasmon coupling effect in an InGaN/GaN single-quantum-well light-emitting diode Appl. Phys. Lett. 91, 171103 (2007); http://dx.doi.org/10.1063/1.2802067 (3 pages) Online Publication Date: 23 October 2007
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The authors demonstrate the coupling effects between the quantum well (QW) and surface plasmon (SP) generated nearby on the p-type side in an InGaN/GaN single-QW light-emitting diode (LED). The QW-SP coupling leads to the enhancement of the electroluminescence (EL) intensity in the LED sample designed for QW-SP coupling and reduced SP energy leakage, when compared to a LED sample of weak QW-SP coupling or significant SP energy loss. In the LED samples of significant QW-SP coupling, the blueshifts of the photoluminescence and EL emission spectra are observed, indicating one of the important features of such a coupling process. The device performance can be improved by using the n-type side for SP generation such that the device resistance can be reduced and the QW-SP coupling effect can be enhanced (by further decreasing the distance between the QW and metal) because of the higher carrier concentration in the n-type layer.
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In situ health monitoring and repair in composites using carbon nanotube additives Appl. Phys. Lett. 91, 133102 (2007); http://dx.doi.org/10.1063/1.2783970 (3 pages) Online Publication Date: 24 September 2007
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The authors demonstrate a simple, effective and real-time diagnostic, and repair technique featuring electrically conductive carbon nanotube additives that are infiltrated into the host structure. They show that by monitoring volume and through-thickness resistance, one can determine the extent and propagation of fatigue-induced damage such as crack and delamination growth in the vicinity of stress concentrations. The conductive nanotube network also provides opportunities to repair damage by enabling fast heating of the crack interfaces; the authors show up to 70% recovery of the strength of the undamaged composite. These advances could result in enhanced safety, reliability, and service life for polymer composites that are used in engineering applications.
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Phototunable photonic crystals with reversible wavelength choice Appl. Phys. Lett. 91, 141114 (2007); http://dx.doi.org/10.1063/1.2793624 (3 pages) Online Publication Date: 3 October 2007
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We report a phototunable photonic crystal device whose reflection at the stop band position can be switched between valley and peak. The device is comprised of a colloidal crystal film, a photoresponsible polymer-dispersed liquid crystal film, and a holophote. The reflection spectrum of the device can be reversibly switched by ultraviolet and visible light irradiations. It was demonstrated that this device is not only used for the application of wavelength selection but is also useful for applications such as display.
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Gigahertz frequency flexible carbon nanotube transistors Appl. Phys. Lett. 91, 153111 (2007); http://dx.doi.org/10.1063/1.2798583 (3 pages) Online Publication Date: 10 October 2007
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We investigate the high frequency performances of flexible field-effect transistors based on carbon nanotubes. A large density of mostly aligned carbon nanotubes deposited on a flexible substrate by dielectrophoresis serves as the channel. The transistors display a constant transconductance up to at least 6 GHz and a current gain cutoff frequency (fT) as high as 1 GHz at VDS = −700 mV. Bending tests show that the devices can withstand a high degree of flexion characterized by a constant transconductance for radius of curvature as small as 3.3 mm.
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