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Top 20 Most Read Articles
July 2009
The 20 articles with the most full-text downloads during the month, in descending order.
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A piezomagnetoelastic structure for broadband vibration energy harvesting Appl. Phys. Lett. 94, 254102 (2009); http://dx.doi.org/10.1063/1.3159815 (3 pages) Online Publication Date: 25 June 2009
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This letter introduces a piezomagnetoelastic device for substantial enhancement of piezoelectric power generation in vibration energy harvesting. Electromechanical equations describing the nonlinear system are given along with theoretical simulations. Experimental performance of the piezomagnetoelastic generator exhibits qualitative agreement with the theory, yielding large-amplitude periodic oscillations for excitations over a frequency range. Comparisons are presented against the conventional case without magnetic buckling and superiority of the piezomagnetoelastic structure as a broadband electric generator is proven. The piezomagnetoelastic generator results in a 200% increase in the open-circuit voltage amplitude (hence promising an 800% increase in the power amplitude).
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Appl. Phys. Lett. 95, 021101 (2009); http://dx.doi.org/10.1063/1.3176435 (3 pages) Online Publication Date: 13 July 2009
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The opening of the light extraction cone of a high index substrate (GaP) is demonstrated using plasmonic gratings. We show that the excitation of surface plasmons on the metal grating leads to the extraction of light otherwise undergoing total internal reflection in the substrate with high efficiency. This effect has an immediate application in the context of light emitting diodes, where resonant metal gratings are promising for the design of dual purpose electric contact/extraction structures.
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Carbon nanotube photovoltaic device with asymmetrical contacts Appl. Phys. Lett. 94, 263501 (2009); http://dx.doi.org/10.1063/1.3136905 (3 pages) Online Publication Date: 29 June 2009
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A photovoltaic (PV) device based on “high-work-function metal/single-walled carbon nanotube/low-work-function metal” hybrid junction has been studied theoretically by the self-consistent nonequilibrium Green’s function approach. The PV effect and power conversion efficiency (η) of the device under light illumination are simulated, with a monochromatic η of higher than 40% for incident photon energies near the nanotube band-gap energy predicted. It is shown that the gate voltage and gate oxide thickness have an important influence on the device η.
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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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Sunlight-style color-temperature tunable organic light-emitting diode Appl. Phys. Lett. 95, 013307 (2009); http://dx.doi.org/10.1063/1.3176217 (3 pages) Online Publication Date: 10 July 2009
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We demonstrate a man-made lighting device of organic light-emitting diode (OLED) capable of yielding a sunlight-style illumination with various daylight chromaticities, whose color temperature ranges between 2300 and 8200 K, fully covering those of the entire daylight at different times and regions. The OLED employs a device architecture capable of simultaneously generating all the emissions required to form a series of daylight chromaticities. The wide color-temperature span may be attributed to that the recombination core therein can easily be shifted along the different emissive zones simply by varying the applied voltage via the use of a thin carrier-modulating layer.
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Appl. Phys. Lett. 94, 261103 (2009); http://dx.doi.org/10.1063/1.3159629 (3 pages) Online Publication Date: 29 June 2009
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The temperature influence on the luminescence characteristics of light-emitting diodes (LEDs) is investigated to reveal the connection of photoluminescence (PL) with photon absorption and electroluminescence (EL) through current injection. By inspecting the PL and EL spectra at identical injection intensities, it has been found that the normalized spectra in PL and EL exhibit obvious similarities in shape and apparent differences in spectral characteristic values. Furthermore, the differences are found to originate from the junction temperatures in diverse injection modes. The observations are conducted on AlGaInP red LED chips.
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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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Characterization of blue-green m-plane InGaN light emitting diodes Appl. Phys. Lett. 94, 261108 (2009); http://dx.doi.org/10.1063/1.3167824 (3 pages) Online Publication Date: 1 July 2009
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High indium content blue-green (460–520 nm) m-plane InGaN light emitting diodes (LEDs) were grown on low defect-density m-plane GaN substrates. Systematic studies were performed on packaged blue-green LED lamps by using a range of well and barrier thicknesses. Photoluminance and electroluminance peak wavelengths increased while the well width was increased from 2 to 4 nm. The highest output power was achieved for well width of 2.5 nm. The output power improved significantly with the increase in barrier thickness. Nearly blueshift-free emission was observed in all LEDs from 1–400 A/cm2 current density under pulsed operation.
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Titanium suboxide as an optical spacer in polymer solar cells Appl. Phys. Lett. 95, 013302 (2009); http://dx.doi.org/10.1063/1.3159622 (3 pages) Online Publication Date: 7 July 2009
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The use of a layer of titanium suboxide (TiOx) between the bulk heterojunction (BHJ) thin film and the metal contact in “plastic” solar cell has been shown to increase both the short-circuit current and the fill factor. Using a combination of optical modeling and device experiments, we clarify the role of the TiOx layer as an optical spacer. For an appropriate choice of the BHJ thickness and composition, the TiOx layer can enhance the absorption within the active layer, thereby increasing the photocurrent and the power conversion efficiency.
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Appl. Phys. Lett. 95, 013301 (2009); http://dx.doi.org/10.1063/1.3163056 (3 pages) Online Publication Date: 6 July 2009
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Photoelectron spectroscopy was used to investigate poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), and their blends on various conductive substrates. The study shows a P3HT-rich layer at the top of the P3HT:PCBM blend films. The energy level alignment of the top P3HT changes with the work function of the substrate and the PCBM concentration at the bottom surface of the blend film. The results can be explained using the integer charge transfer model.
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High efficiency GaN-based light-emitting diodes fabricated on dielectric mask-embedded structures Appl. Phys. Lett. 95, 011108 (2009); http://dx.doi.org/10.1063/1.3166868 (3 pages) Online Publication Date: 7 July 2009
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We report on the enhanced quantum efficiency of GaN-based light-emitting diodes (LEDs) fabricated on inverted hexagonal pyramid dielectric mask (IHPDM)-embedded structure. The ray-tracing calculation showed that the extraction efficiency of LEDs fabricated on IHPDM-embedded structure could be enhanced up to 56%. Compared to the reference, the n-GaN template grown on IHPDM-embedded structure also showed a reduction in the dislocation density by 57%, leading to an increase in photoluminescence intensity by 82%. The LED fabricated on IHPDM-embedded structure exhibited a reduction in the forward leakage current by one order of magnitude (<1.5 V) and an enhancement in the output power by 41%.
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Improved output power of GaN-based light-emitting diodes grown on a nanopatterned sapphire substrate Appl. Phys. Lett. 95, 011110 (2009); http://dx.doi.org/10.1063/1.3173817 (3 pages) Online Publication Date: 8 July 2009
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This letter describes the improved output power of GaN-based light-emitting diodes (LEDs) formed on a nanopatterned sapphire substrate (NPSS) prepared through etching with a self-assembled monolayer of 750-nm-diameter SiO2 nanospheres used as the mask. The output power of NPSS LEDs was 76% greater than that of LEDs on a flat sapphire substrate. Three-dimensional finite-difference time-domain calculation predicted a 40% enhancement in light extraction efficiency of NPSS LEDs. In addition, the reduction of full widths at half maximum in the ω-scan rocking curves for the (0 0 2) and (1 0 2) planes of GaN on NPSS suggested improved crystal quality.
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Appl. Phys. Lett. 95, 013305 (2009); http://dx.doi.org/10.1063/1.3174914 (3 pages) Online Publication Date: 8 July 2009
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We have explored the effect of gold nanoparticle (Au NP)-induced surface plasmons on the performance of organic photovoltaic devices (OPVs). The power conversion efficiency of these OPVs was improved after blending the Au NPs into the anodic buffer layer. The addition of Au NPs increased the rate of exciton generation and the probability of exciton dissociation, thereby enhancing the short-circuit current density and the fill factor. We attribute the improvement in device performance to the local enhancement in the electromagnetic field originating from the excitation of the localized surface plasmon resonance.
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Nonlinear optical infrared microscopy with chemical specificity Appl. Phys. Lett. 94, 261101 (2009); http://dx.doi.org/10.1063/1.3167286 (3 pages) Online Publication Date: 29 June 2009
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We demonstrate a realization of nonlinear optical infrared microscopy that is capable of chemically specific imaging with high spatial resolution. It exploits the nonlinear refraction of the target material that is thermally induced by resonantly absorbed mid-infrared radiation and probed with a short wavelength near-infrared laser beam. The O–H and C–H2 vibrational modes in a water-oil mixture sample are selectively imaged at two different midinfrared wavelengths and the spatial resolution is measured to be 1.1 μm in the condition of numerical aperture 0.5, which is not attainable with conventional infrared microscopes.
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Intermediate-band photovoltaic solar cell based on ZnTe:O Appl. Phys. Lett. 95, 011103 (2009); http://dx.doi.org/10.1063/1.3166863 (3 pages) Online Publication Date: 6 July 2009
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Oxygen doping in ZnTe is applied to a junction diode in the aim of utilizing the associated electron states 0.5 eV below the bandedge as an intermediate band for photovoltaic solar cells. The ZnTe:O diodes confirm extended spectral response below the bandedge relative to undoped ZnTe diodes, and demonstrate a 100% increase in short circuit current, 15% decrease in open circuit voltage, and overall 50% increase in power conversion efficiency. Subbandgap excitation at 650 and 1550 nm confirms the response via a two-photon process and illustrates the proposed energy conversion mechanism for an intermediate band solar cell.
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Hybrid core-multishell nanowire forests for electrical connector applications Appl. Phys. Lett. 94, 263110 (2009); http://dx.doi.org/10.1063/1.3148365 (3 pages) Online Publication Date: 1 July 2009
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Electrical connectors based on hybrid core-multishell nanowire forests that require low engagement forces are demonstrated. The physical binding and electrical connectivity of the nanowire electrical connectors arise from the van der Waals interactions between the conductive metallic shells of the engaged nanowire forests. Specifically, the nanofibrillar structure of the connectors causes an amplification of the contact area between the interpenetrating nanowire arrays, resulting in strong adhesion with relatively low interfacial resistance. The nanowire electrical connectors may enable the exploration of a wide range of applications involving reversible assembly of micro- and macroscale components with built-in electrical interfacing.
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Advantages of blue InGaN multiple-quantum well light-emitting diodes with InGaN barriers Appl. Phys. Lett. 95, 011116 (2009); http://dx.doi.org/10.1063/1.3176406 (3 pages) Online Publication Date: 10 July 2009
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The advantages of blue InGaN light-emitting diodes (LEDs) with InGaN barriers are studied. The L-I curves, carrier concentrations in the quantum wells, energy band diagrams, and internal quantum efficiency are investigated. The simulation results show that the InGaN/InGaN LED has better performance over its conventional InGaN/GaN counterpart due to the enhancement of electron confinement, the reduced polarization effect between the barrier and well, and the lower potential barrier height for the holes to transport in the active region. The simulation results also suggest that the efficiency droop is markedly improved when the traditional GaN barriers are replaced by InGaN barriers.
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Appl. Phys. Lett. 94, 263303 (2009); http://dx.doi.org/10.1063/1.3168523 (3 pages) Online Publication Date: 2 July 2009
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High-performance pentacene (μsat = 6.3 cm2/V s) and poly(3-hexylthiophene) (μsat = 0.43 cm2/V s) field effect transistors have been realized on flexible substrate with low operating voltage (<−5 V), utilizing a trilayer sol-gel silica gate dielectric. Furthermore, the permittivity of the dielectric was tuned from ∼ 7 to ∼ 10 by varying plasma treatments, allowing the study of charge carrier mobility variation with k. A 65% reduction in the saturation mobility of the devices was observed when k increases, suggesting that the energetic disorder at the interface between the active layer and the dielectric can be modulated by the high polarizability of the bulk dielectric.
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A theoretical study on thermoelectric properties of graphene nanoribbons Appl. Phys. Lett. 94, 263107 (2009); http://dx.doi.org/10.1063/1.3171933 (3 pages) Online Publication Date: 30 June 2009
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We investigate the thermoelectric properties of graphene nanoribbons (GNRs) by solving atomistic electron and phonon transport equations in the nonequilibrium Green’s function formalism. The dependence of thermopower on temperature and chemical potential is compared to that of graphene, which shows the important role of quasi-one-dimensional geometry in determining the thermoelectric properties of a GNR. The edge roughness and lattice vacancy are found to increase the thermopower but decrease the thermoelectric ZT factor because the decrease in the electronic conductance outweighs the decrease in the thermal conductance and the increase in the thermopower.
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Measuring the profile of the emission zone in polymeric organic light-emitting diodes Appl. Phys. Lett. 94, 263301 (2009); http://dx.doi.org/10.1063/1.3159836 (3 pages) Online Publication Date: 30 June 2009
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The profile of the emission zone (PEZ) in the emissive layer (EML) of polymeric organic light-emitting diodes (OLEDs) is determined by fitting the measured electroluminescence spectrum of the device with a series of simulated spectra that correspond to different emitter locations. The studied OLEDs are based on two different blue-emitting conjugated polymers. In one case, the PEZ is confined to a 10 nm thin sheet at the anode indicating electron dominated current in the EML. In OLEDs based on the other emitter, the PEZ spreads over the entire EML.
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