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Top 20 Most Read Articles
July 2010
The 20 articles with the most full-text downloads during the month, in descending order.
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Electromechanical probing of ionic currents in energy storage materials Appl. Phys. Lett. 96, 222906 (2010); http://dx.doi.org/10.1063/1.3446838 (3 pages) Online Publication Date: 4 June 2010
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The electrochemical processes in energy storage materials are generally linked with changes of molar volume of the host compound. Here, the frequency dependent strain response of one-dimensional electrochemically active system to periodic electric bias is analyzed. The sensitivity and resolution of electrochemical strain measurements are compared to the current-based electrochemical impedance spectroscopy. The resolution and detection limits of interferometric and atomic force microscopy based systems for probing electrochemical reactions on the nanoscale are analyzed.
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High-power and high-speed organic three-dimensional transistors with submicrometer channels Appl. Phys. Lett. 97, 013301 (2010); http://dx.doi.org/10.1063/1.3458867 (3 pages) Online Publication Date: 6 July 2010
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Three-dimensional organic field-effect transistors with high current density and high switching speed are developed with multiple submicrometer channels arranged perpendicularly to substrates. The short channel length is defined by the height of a multicolumnar structure without an electron-beam-lithography process. For devices using dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene, extremely high current density exceeding 10 A/cm2 and fast switching within 0.2 μs are realized with an on-off ratio of 105. The unprecedented performance is beyond general requirements to control organic light-emitting diodes, so that even more extensive applications to higher-speed active-matrices and display-driving circuits can be realized with organic semiconductors.
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Nonuniformity of a planar polarizer for spin-transfer-induced vortex oscillations at zero field Appl. Phys. Lett. 96, 212507 (2010); http://dx.doi.org/10.1063/1.3441405 (3 pages) Online Publication Date: 28 May 2010
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We discuss a possible mechanism of the spin-transfer-induced oscillations of a vortex in the free layer of spin-valve nanostructures, in which the polarizer layer has a planar magnetization. We demonstrate that if such planar polarizer is essentially nonuniform, steady gyrotropic vortex motion with large amplitude can be excited. The best excitation efficiency is obtained for a circular magnetization distribution in the polarizer. In this configuration, the conditions for the onset of the oscillations depend on the vortex chirality but not on the direction of its core.
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Graphene field-effect transistors with self-aligned gates Appl. Phys. Lett. 97, 013103 (2010); http://dx.doi.org/10.1063/1.3459972 (3 pages) Online Publication Date: 7 July 2010
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We present a device fabrication process that produces graphene-based field-effect transistors with self-aligned gates. This process utilizes the inherent nucleation inhibition of atomic-layer-deposited films with the graphene surface to achieve electrical isolation of the gate electrode from the source/drain electrodes while maintaining electrical access to the graphene channel. Self-alignment produces access lengths of 15–20 nm, which allows for improved device stability, performance, and a minimal normalized contact resistance of 540 Ω μm.
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Appl. Phys. Lett. 97, 021101 (2010); http://dx.doi.org/10.1063/1.3462942 (3 pages) Online Publication Date: 12 July 2010
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We have generated single-transverse-mode optical pulses with 100 W peak power and 3 ps duration at 1 GHz repetition from a blue-violet GaInN mode-locked laser diode (MLLD) and a semiconductor optical amplifier (SOA) without the use of any pulse compression. The generation of clean optical pulses without subpulse components from the MLLD and the reduction in amplified spontaneous emission in the SOA by incorporating a flare waveguide structure resulted in effective amplification of optical pulses to produce over 100 W peak power.
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An infrared invisibility cloak composed of glass Appl. Phys. Lett. 96, 233503 (2010); http://dx.doi.org/10.1063/1.3447794 (3 pages) Online Publication Date: 7 June 2010
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We propose to implement a nonmetallic low-loss cloak for the infrared range from identical chalcogenide glass resonators. Based on transformation optics for cylindrical objects, our approach does not require metamaterial response to be homogeneous and accounts for the discrete nature of elementary responses governed by resonator shape, illumination angle, and inter-resonator coupling. Air fractions are employed to obtain the desired distribution of the cloak effective parameters. The effect of cloaking is verified by full-wave simulations of the true multiresonator structure. The feasibility of cloak fabrication is demonstrated by prototyping glass grating structures with the dimensions characteristic for the cloak resonators.
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Comment on “Guided modes in graphene waveguides” [ Appl. Phys. Lett. 94, 212105 (2009) ] Appl. Phys. Lett. 96, 186101 (2010); http://dx.doi.org/10.1063/1.3425716 (2 pages) Online Publication Date: 5 May 2010
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Appl. Phys. Lett. 97, 013501 (2010); http://dx.doi.org/10.1063/1.3459139 (3 pages) Online Publication Date: 6 July 2010
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We have grown nitrogen-doped MgxZn1−xO:N films on Zn-polar ZnO single crystal substrates by molecular beam epitaxy. As N-sources, we employed NO-plasma or NH3 gas itself. As x increased, optimum growth temperature window for smooth film morphology shifted to higher temperatures, while maintaining high N-concentration ( ∼ 1×1019 cm−3). The heterosructures of MgxZn1−xO:N (0.1 ≤ x ≤ 0.4)/ZnO were fabricated into light emitting diodes of 500-μm-diameter. We observed ultraviolet near-band-edge emission (λ ∼ 382 nm) with an output power of 0.1 μW for a NO-plasma-doped LED and 70 μW for a NH3-doped one at a bias current of 30 mA.
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Enhancement of electroluminescence in GaN-based light-emitting diodes by metallic nanoparticles Appl. Phys. Lett. 96, 261105 (2010); http://dx.doi.org/10.1063/1.3457349 (3 pages) Online Publication Date: 1 July 2010
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The enhanced electroluminescence of GaN-based light-emitting diodes (LEDs) with noble metallic nanoparticles (MNPs) is demonstrated. The sample with well-designed Ag MNPs has shown the best performance enhancement of 126% in electroluminescent intensity compared with a conventional LED sample, even though the MNPs are placed at least 200 nm away from the quantum-well active layer. The MNPs provide enhanced photon scattering and coupling between localized surface plasmon resonance (LSPR) modes and photon modes internally trapped in a device. To investigate this effect, the peculiarities of the LSPR and the corresponding structural properties of the MNPs are discussed through the effective medium approach.
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InGaN/GaN self-organized quantum dot green light emitting diodes with reduced efficiency droop Appl. Phys. Lett. 97, 011103 (2010); http://dx.doi.org/10.1063/1.3460921 (3 pages) Online Publication Date: 7 July 2010
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High density (2–5×1010 cm−2) self-organized InGaN/GaN quantum dots were grown by plasma-assisted molecular beam epitaxy. Room temperature photoluminescence shows that the quantum dots have strong emission ranging from 430 to 524 nm. The internal quantum efficiency of dots emitting at 500 nm was determined to be 32% by temperature dependent photoluminescence measurements. A recombination lifetime of 0.57 ns is derived from time resolved photoluminescence measurements. These superior optical properties are attributed to a small piezoelectric field in the quantum dots. Light emitting diodes fabricated with the InGaN/GaN quantum dots and emitting at λ = 524 nm demonstrate a small blueshift with current injection and reduced efficiency droop.
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Appl. Phys. Lett. 97, 011101 (2010); http://dx.doi.org/10.1063/1.3460281 (3 pages) Online Publication Date: 6 July 2010
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Dodecagonal ZnO microrods were fabricated by vapor transport method. A dodecagonal ZnO microrod with diameter of 6.35 μm was employed as a whispering gallery mode microcavity, the low threshold exciton lasing was stimulated. In another smaller microrod, the electron-hole plasma lasing was observed under intense excitation as the carrier density was larger than the Mott transition density. The interference condition in the dodecagonal cavity was deduced, and the mode structures were investigated. Compared to the hexagonal microcavity with same diameter, the dodecagonal microcavity has higher lasing performance.
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Reduction in the efficiency droop effect of a light-emitting diode through surface plasmon coupling Appl. Phys. Lett. 96, 261104 (2010); http://dx.doi.org/10.1063/1.3459151 (3 pages) Online Publication Date: 30 June 2010
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The reduction in the external quantum efficiency (EQE) droop effect of an InGaN/GaN quantum-well (QW) light-emitting diode (LED) through the mechanism of surface plasmon (SP) coupling with QW is demonstrated. With a current spreading grid pattern on the mesa surface, a smaller grid period leads to more effective carrier transport into the QW regions of Ag deposition for stronger SP–QW coupling such that the droop effect is more significantly reduced, as indicated by the increase in injection current density of maximum EQE and the decrease in drooping slope. The claim of the SP–QW coupling effect in the samples of thin p-GaN is supported by the different droop behaviors of the LED samples fabricated with another epitaxial structure of thick p-GaN, in which the SP–QW coupling effect is expected to be weak.
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Appl. Phys. Lett. 96, 186102 (2010); http://dx.doi.org/10.1063/1.3425717 (2 pages) Online Publication Date: 5 May 2010
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Appl. Phys. Lett. 96, 261109 (2010); http://dx.doi.org/10.1063/1.3460288 (3 pages) Online Publication Date: 1 July 2010
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Significant photocurrent enhancement has been achieved for evaporated solid-phase-crystallized polycrystalline silicon thin-film solar cells on glass, due to light trapping provided by Ag nanoparticles located on the rear silicon surface of the cells. This configuration takes advantage of the high scattering cross-section and coupling efficiency of rear-located particles formed directly on the optically dense silicon layer. We report short-circuit current enhancement of 29% due to Ag nanoparticles, increasing to 38% when combined with a detached back surface reflector. Compared to conventional light trapping schemes for these cells, this method achieves 1/3 higher short-circuit current.
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Appl. Phys. Lett. 97, 023301 (2010); http://dx.doi.org/10.1063/1.3462298 (3 pages) Online Publication Date: 12 July 2010
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We correlate carrier recombination via charge transfer excitons (CTEs) with the short circuit current, Jsc, in polymer/fullerene solar cells. Near infrared photoluminescence spectroscopy of CTE in three blends differing for the fullerene acceptor, gives unique insights into solar cell characteristics. The energetic position of the CTE is directly correlated with the open-circuit voltage, Voc, and more important Jsc decreases with increasing CTE emission intensity. CTE emission intensity is discussed from the perspective of blend morphology. The work points out the fundamental role of CTE recombination and how optical spectroscopy can be used to derive information on solar cell performances.
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Terahertz activated luminescence of trapped carriers in InGaAs/GaAs quantum dots Appl. Phys. Lett. 97, 031101 (2010); http://dx.doi.org/10.1063/1.3464163 (3 pages) Online Publication Date: 19 July 2010
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Optical properties and interdot transfer dynamics of trapped carriers in InGaAs quantum dots (QDs) are investigated. Time resolved photoluminescence (PL) was measured for time-delayed interband and intraband excitations. Terahertz activated luminescence (TAL) from trapped carriers having lifetimes of ∼ 250 ns at 8 K, was observed. Spectral shift of the TAL with respect to the PL showed the trionic nature of the PL in the n-doped QDs. With increasing terahertz excitation intensity, the TAL increased and reached saturation. The activation energy associated with the trapped carrier decay was quite close to the intersublevel transition energy ( ∼ 20 meV) indicating trapping in the QDs.
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Nanostructured interfaces in polymer solar cells Appl. Phys. Lett. 96, 263109 (2010); http://dx.doi.org/10.1063/1.3458809 (3 pages) Online Publication Date: 30 June 2010
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The morphology in organic photovoltaics plays a key role in determining the device efficiency. We propose a method to fabricate bilayer devices with controlled nanostructured interfaces by combining nanoimprinting and lamination techniques. This technique allows us to achieve a network structure of donor-acceptor material with a ∼ 80 nm periodicity and ∼ 40 nm width. These structures have an abrupt interface between the donor and acceptor materials and show an increased effective interfacial area and photovoltaic performance compared to bilayer solar cells. In contrast to blend films, they will allow an in depth analysis of the influence of morphology on interfacial physical processes.
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Organic tandem solar cells with complementary absorbing layers and a high open-circuit voltage Appl. Phys. Lett. 97, 033301 (2010); http://dx.doi.org/10.1063/1.3464169 (3 pages) Online Publication Date: 19 July 2010
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Tandem organic solar cells with peak conversion efficiencies (η) of 5.15% are demonstrated. This is achieved by stacking two different planar heterojunction devices, each with a high open-circuit voltage (Voc). The phthalocyanine based donor materials in the employed subcells possess complementary absorption, a quality of critical importance to optimize photocurrent in a series connected tandem cell. The tandem structure produces Voc values of nearly 2 V, while fill factor remains above 60%. The measured η corresponds to a 40% increase compared to η of the optimal single cells.
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Surface wave Cherenkov maser based on a periodic lattice Appl. Phys. Lett. 96, 261101 (2010); http://dx.doi.org/10.1063/1.3456618 (3 pages) Online Publication Date: 28 June 2010
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The excitation of a surface wave cavity based on a two-dimensional periodic structure by an energy source, such as relativistic electrons, results in stimulated, single frequency coherent emission. A high-Q cavity has been achieved via a resonant coupling between surface waves and volume waves. The concept of a Cherenkov maser based on the surface wave cavity is discussed and results of numerical studies presented. Links between the model described and the concept of a surface plasmon amplifier, which has been recently introduced are described.
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Appl. Phys. Lett. 97, 023701 (2010); http://dx.doi.org/10.1063/1.3462979 (3 pages) Online Publication Date: 12 July 2010
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The FluidFM technology uses microchanneled atomic force microscope cantilevers that are fixed to a drilled atomic force microscope cantilevers probeholder. A continuous fluidic circuit is thereby achieved extending from an external liquid reservoir, through the probeholder and the hollow cantilever to the tip aperture. In this way, both overpressure and an underpressure can be applied to the liquid reservoir and hence to the built-in fluidic circuit. We describe in this letter how standard atomic force microscopy in combination with regulated pressure differences inside the microchanneled cantilevers can be used to displace living organisms with micrometric precision in a nondestructive way. The protocol is applicable to both eukaryotic and prokaryotic cells (e.g., mammalian cells, yeasts, and bacteria) in physiological buffer. By means of this procedure, cells can also be transferred from one glass slide to another one or onto an agar medium.
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