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Top 20 Most Read Articles
August 2009
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 95, 013107 (2009); http://dx.doi.org/10.1063/1.3167775 (3 pages) Online Publication Date: 7 July 2009
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A direct transfer method for fabricating flexible electronics without the assistance of an adhesive layer and stamp is reported in this paper. This rapid and simple method provides an approach for the application of vertically aligned carbon nanotubes (VA-CNTs) on plastic substrates. After transfer, the VA-CNTs maintained their initial orientation in the designed pattern and showed sufficient adhesion to the substrate under extreme bending conditions. The flexible device performed an emission on the transparent substrate and showed a low turn-on of 1.13 V/μm. This VA-CNT-based flexible device, which exhibits electrical resistance sensitive to bending, is also described herein.
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Organic light-emitting diode with liquid emitting layer Appl. Phys. Lett. 95, 053304 (2009); http://dx.doi.org/10.1063/1.3200947 (3 pages) Online Publication Date: 6 August 2009
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We demonstrate an original organic light-emitting diode (OLED) having a neat liquid host of 9-(2-ethylhexyl)carbazole (EHCz) doped with a guest emitter of 5,6,11,12-tetraphenylnapthacene (rubrene). The device structure is composed of indium tin-oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate)/EHCz:rubrene/Cs2CO3/ITO. We demonstrate that the liquid organic semiconducting layer surely transports charge carriers, leading to electroluminescence from rubrene with the highest external quantum efficiency of ηext = 0.03% at a current density of 0.26 mA/cm2. Our demonstration of the liquid-OLEDs will open another possibility of organic semiconductors and light-emitting applications.
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Enhanced open-circuit voltage in polymer solar cells Appl. Phys. Lett. 95, 043301 (2009); http://dx.doi.org/10.1063/1.3157278 (3 pages) Online Publication Date: 27 July 2009
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We reported that the open-circuit voltage (VOC) of polymer solar cells (PSCs) can be enhanced by 0.3 V via inserting a thin layer of alcohol/water-soluble polyelectrolyte between the active layer and metal electrode. The origin of the increase in VOC can be attributed to the superposition of the built-in field with the interfacial dipole of the interlayer, and the existence of the interlayer as a dielectric was confirmed by ac impedance spectroscopy. The significant enhancement in VOC and the simplicity of incorporation of interlayer by solution processing make this method a good candidate for further enhancing the performance of PSCs.
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Demonstration of nonpolar a-plane InGaN/GaN light emitting diode on r-plane sapphire substrate Appl. Phys. Lett. 95, 071101 (2009); http://dx.doi.org/10.1063/1.3206666 (3 pages) Online Publication Date: 17 August 2009
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High crystalline a-plane (11
 0) GaN epitaxial layers with smooth surface morphology were grown on r-plane (1 02) sapphire substrate by metalorganic chemical vapor deposition. The full width at half maximum of x-ray rocking curve was measured as 407 arc sec along c-axis direction, and the root mean square roughness was 1.23 nm. Nonpolar a-plane InGaN/GaN light emitting diodes were subsequently grown on a-plane GaN template, and the optical output power of 0.72 mW was obtained at drive current of 20 mA (3.36 V) and 2.84 mW at 100 mA (4.62 V) with the peak emission wavelength of 477 nm. |
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Self-assembled bioinspired quantum dots: Optical properties Appl. Phys. Lett. 94, 261907 (2009); http://dx.doi.org/10.1063/1.3167354 (3 pages) Online Publication Date: 2 July 2009
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Until now, the wide research field of quantum dots (QDs) focused on inorganic structures. In the present study, we report on quantum confinement phenomena found in peptide nanocrystalline regions formed within self-assembly peptide nanospheres. These bioinspired nanostructures exhibit the optical absorption characteristics of QDs with pronounced luminescence of excitons whose origin is at the UV region. Theoretical estimations based on experimental data show that the radius of the self assembled peptide QDs is 1.3 nm.
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Photoluminescence study of single ZnO nanostructures: Size effect Appl. Phys. Lett. 95, 053113 (2009); http://dx.doi.org/10.1063/1.3200232 (3 pages) Online Publication Date: 6 August 2009
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Spatially resolved photoluminescence (PL) investigations were carried out on ZnO single nanowires, tetrapods, and nanocrystals. The fractional intensity for bound exciton (BX) transitions was shown to be correlated with the size in all these ZnO nanostructures. This size dependency is attributed to the inhomogeneous density distribution of the defects as binding sites for BX in the ZnO nanostructures, in good agreement with a simple model calculation.
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Highly efficient polycarbazole-based organic photovoltaic devices Appl. Phys. Lett. 95, 063304 (2009); http://dx.doi.org/10.1063/1.3182797 (3 pages) Online Publication Date: 14 August 2009
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We combined experimental and computational approaches to tune the thickness of the films in poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT)-based organic solar cells to maximize the solar absorption by the active layer. High power-conversion efficiencies of 5.2% and 5.7% were obtained on PCDTBT-based solar cells when using [6,6]-phenyl C61-butyric acid methyl ester (PC60BM) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) as the electron acceptor, respectively. The cells are designed to have an active area of 1.0 cm2, which is among the largest organic solar cells in the literature, while maintaining a low series resistance of 5 Ω cm2.
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Experimental verification of extraordinary transmission without surface plasmons Appl. Phys. Lett. 95, 071102 (2009); http://dx.doi.org/10.1063/1.3206738 (3 pages) Online Publication Date: 17 August 2009
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This letter provides an experimental demonstration of extraordinary transmission in a closed waveguide system loaded with an electrically small diaphragm. This is a situation where the standard surface plasmon polariton (SPP) theory does not apply. The theoretical explanation is then based on the concept of impedance matching. This concept has previously been applied by some of the authors to account for enhanced transmission in situations where surface plasmon theory can be used: periodic arrays of small holes or slits in flat metal screens. The experiment in this letter supports the impedance matching model, valid for when SPPs are present or not.
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InGaN laser diodes with 50 mW output power emitting at 515 nm Appl. Phys. Lett. 95, 071103 (2009); http://dx.doi.org/10.1063/1.3206739 (3 pages) Online Publication Date: 17 August 2009
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We demonstrate direct green laser operation from InGaN based diodes at wavelengths as long as 515.9 nm with 50 mW output power in pulse operation. A factor of ∼ 10 defect reduction for the In-rich InGaN quantum wells based on improvements of the epitaxial growth process and design of the active layers on c-plane GaN-substrates makes it possible to demonstrate laser operation at room temperature. Micrometer-scale photoluminescence mappings and electro-optical measurements confirm the reduction of nonradiative defects in the emitting layers. The 11 μm broad-area gain-guided laser structures were driven in pulse operation to minimize thermal effects and to accurately measure the laser temperature dependence. The threshold current density was ∼ 9 kA/cm2 and the fitted slope efficiency had a value of ∼ 130 mW/A for an optical output up to 50 mW.
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Designing periodic arrays of metal nanoparticles for light-trapping applications in solar cells Appl. Phys. Lett. 95, 053115 (2009); http://dx.doi.org/10.1063/1.3200948 (3 pages) Online Publication Date: 7 August 2009
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We present criteria for optimizing the light-trapping efficiency of periodic arrays of metal nanoparticles for Si solar cell applications. The scattering cross section of the nanoparticles and the diffraction efficiency of the grating should be maximized in the long wavelength range. The grating pitch should be chosen to allow higher order diffraction modes for long wavelengths while maintaining the highest possible fill factor. These conditions place strong constraints on the optimal parameters (particle size of ∼ 200 nm and pitch of ∼ 400 nm) for periodic arrays of metal nanoparticles, in contrast to dielectric gratings, where a relatively wide range of periods and feature sizes can be used for efficient light trapping.
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Fabry–Pérot nanocavities in multilayered plasmonic crystals for enhanced biosensing Appl. Phys. Lett. 95, 051105 (2009); http://dx.doi.org/10.1063/1.3202391 (3 pages) Online Publication Date: 6 August 2009
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We have demonstrated extraordinary light transmission effect through Fabry–Pérot cavities in multilayered plasmonic crystals formed by coupling two physically separated metallic nanohole and nanodisk array layers. Superior field-medium overlap is observed with Fabry–Pérot resonances as a result of stronger electromagnetic field confinement in the dielectric region far from the metallic surfaces. We show that these cavity resonances are highly sensitive to refractive index changes. The large field-material overlap combined with simple fabrication scheme used here makes these structures an ideal candidate for biosensing applications.
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Size dependent thermoelectric properties of silicon nanowires Appl. Phys. Lett. 95, 063102 (2009); http://dx.doi.org/10.1063/1.3204005 (3 pages) Online Publication Date: 10 August 2009
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By using first-principles tight-binding electronic structure calculation and Boltzmann transport equation, we investigate the size dependence of thermoelectric properties of silicon nanowires (SiNWs). With cross section area increasing, the electrical conductivity increases slowly, while the Seebeck coefficient reduces remarkably. This leads to a quick reduction of cooling power factor with diameter. Moreover, the figure of merit also decreases with transverse size. Our results demonstrate that in thermoelectric application, NW with small diameter is preferred. We also predict that isotopic doping can increase the value of ZT significantly. With 50% 29Si doping (28Si0.5 29Si0.5 NW), the ZT can be increased by 31%.
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Semitransparent inverted polymer solar cells with MoO3/Ag/MoO3 as transparent electrode Appl. Phys. Lett. 95, 053303 (2009); http://dx.doi.org/10.1063/1.3196763 (3 pages) Online Publication Date: 4 August 2009
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Semitransparent inverted polymer solar cells were developed using thermally evaporable MoO3/Ag/MoO3 as transparent anode. The ultrathin inner MoO3 layer was introduced as a buffer layer to improve hole collection, while the outer MoO3 layer served as a light coupling layer to enhance optical transmittance of the device. The dependence of the device performances on the thickness of the outer MoO3 layer was investigated. The results showed that the addition of the outer MoO3 layer improves the transmittance of the anode compared to MoO3/Ag anode and the performances of the semitransparent devices with the outer MoO3 layer are improved due to the reduced series resistance.
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Direct modulation of excited state quantum dot lasers Appl. Phys. Lett. 95, 061101 (2009); http://dx.doi.org/10.1063/1.3193664 (3 pages) Online Publication Date: 10 August 2009
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The use of the excited state quantum dot lasers for high speed direct modulation is proposed and demonstrated. A direct comparison of lasers utilizing the ground state and excited state from the same laser material reveals a factor of two increase in the K-factor limited bandwidth. This is attributed to an increase in the saturated gain and reduced carrier scattering time of the excited state compared to the ground state.
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Photovoltaic effects in BiFeO3 Appl. Phys. Lett. 95, 062909 (2009); http://dx.doi.org/10.1063/1.3204695 (3 pages) Online Publication Date: 14 August 2009
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We report a photovoltaic effect in ferroelectric BiFeO3 thin films. The all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages ∼ 0.8–0.9 V and external quantum efficiencies up to ∼ 10% when illuminated with the appropriate light. Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric-based devices. The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO3/tin doped indium oxide interface.
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High efficiency light emitting diode with anisotropically etched GaN-sapphire interface Appl. Phys. Lett. 95, 041109 (2009); http://dx.doi.org/10.1063/1.3190504 (3 pages) Online Publication Date: 28 July 2009
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We report the fabrication and study of high efficiency ultraviolet light emitting diodes with inverted micropyramid structures at GaN-sapphire interface. The micropyramid structures were created by anisotropic chemical wet etching. The pyramid structures have significantly enhanced the light output efficiency and at the same time also improved the crystal quality by partially relieving the strain and reducing the dislocation defects in GaN. The electroluminescent output power at normal direction was enhanced by 120% at 20 mA injection current and the output power integrated over all directions was enhanced by 85% compared to a reference sample.
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Ultrafast pulse characterization using cross phase modulation in silicon Appl. Phys. Lett. 95, 051101 (2009); http://dx.doi.org/10.1063/1.3193538 (3 pages) Online Publication Date: 3 August 2009
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Based on the high nonlinearity of the chip-scale silicon waveguide with small dispersion, a compact frequency-resolved optical gating system has been demonstrated using cross phase modulation for ultrafast pulse characterization. The principal component generalized projections algorithm is used to retrieve the amplitude and phase from the spectrogram. Amplitude and phase of a 540 fs pulse have been measured. The measured amplitude result is confirmed by the autocorrelation measurement.
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Facile construction of nanofibrous ZnO photoelectrode for dye-sensitized solar cell applications Appl. Phys. Lett. 95, 043304 (2009); http://dx.doi.org/10.1063/1.3193661 (3 pages) Online Publication Date: 31 July 2009
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A facile method to prepare nanofibrous ZnO photoelectrodes with tunable thicknesses by electrospinning is reported. A “self-relaxation layer” is formed spontaneously between ZnO nanofibers and fluorine-doped SnO2 (FTO) substrate, which facilitates the release of interfacial tensile stress during calcination, resulting in good adhesion of ZnO film to FTO substrate. Dye-sensitized solar cells (DSSCs) based on the nanofibrous ZnO photoelectrodes are fabricated and an energy conversion efficiency of 3.02% is achieved under irradiation of AM 1.5 simulated sunlight with a power density of 100 mW cm−2, which shows good promise of electrospun nanofibrous ZnO as the photoelectrode in DSSCs.
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Energy-level alignment and charge injection at metal/C60/organic interfaces Appl. Phys. Lett. 95, 043302 (2009); http://dx.doi.org/10.1063/1.3189176 (3 pages) Online Publication Date: 27 July 2009
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The energy-level alignment and charge injection at metal/C60/organic interfaces have been studied by ultraviolet photoelectron spectroscopy and temperature dependent current-voltage (IV) measurements. It is found that the Fermi level at the interface is pinned to ∼ 4.7 eV by adsorbed C60 molecules on the metal surface, resulting in more favorable energy level alignment for charge injection. The findings are in excellent agreement with interface dipole theory derived from traditional semiconductor physics.
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Appl. Phys. Lett. 95, 061104 (2009); http://dx.doi.org/10.1063/1.3204446 (3 pages) Online Publication Date: 12 August 2009
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Three-layer staggered InGaN quantum wells (QWs) light-emitting diodes (LEDs) emitting at 520–525 nm were grown by metal-organic chemical vapor deposition by employing graded growth-temperature profile. The use of staggered InGaN QW, with improved electron-hole wave functions overlap design, leads to an enhancement of its radiative recombination rate. Both cathodoluminescence and electroluminescence measurements of three-layer staggered InGaN QW LED exhibited enhancements by 1.8–2.8 and 2.0–3.5 times, respectively, over those of conventional InGaN QW LED.
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