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Top 20 Most Read Articles
April 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 98, 113305 (2011); http://dx.doi.org/10.1063/1.3567516 (3 pages) Online Publication Date: 17 March 2011
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We fabricate near-infrared absorbing organic photovoltaics that are highly transparent to visible light. By optimizing near-infrared optical-interference, we demonstrate power efficiencies of 1.3±0.1% with simultaneous average visible transmission of >65%. Subsequent incorporation of near-infrared distributed-Bragg-reflector mirrors leads to an increase in the efficiency to 1.7±0.1%, approaching the 2.4±0.2% efficiency of the opaque cell, while maintaining high visible-transparency of >55%. Finally, we demonstrate that a series-integrated array of these transparent cells is capable of powering electronic devices under near-ambient lighting. This architecture suggests strategies for high-efficiency power-generating windows and highlights an application uniquely benefiting from excitonic electronics.
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Defect-reduced green GaInN/GaN light-emitting diode on nanopatterned sapphire Appl. Phys. Lett. 98, 151102 (2011); http://dx.doi.org/10.1063/1.3579255 (3 pages) Online Publication Date: 11 April 2011
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Green GaInN/GaN quantum well light-emitting diode (LED) wafers were grown on nanopatterned c-plane sapphire substrate by metal-organic vapor phase epitaxy. Without roughening the chip surface, such LEDs show triple the light output over structures on planar sapphire. By quantitative analysis the enhancement was attributed to both, enhanced generation efficiency and extraction. The spectral interference and emission patterns reveal a 58% enhanced light extraction while photoluminescence reveals a doubling of the internal quantum efficiency. The latter was attributed to a 44% lower threading dislocation density as observed in transmission electron microscopy. The partial light output power measured from the sapphire side of the unencapsulated nanopatterned substrate LED die reaches 5.2 mW at 525 nm at 100 mA compared to 1.8 mW in the reference LED.
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Soft generators using dielectric elastomers Appl. Phys. Lett. 98, 142903 (2011); http://dx.doi.org/10.1063/1.3572338 (3 pages) Online Publication Date: 5 April 2011
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The potential to produce light-weight, low-cost, wearable dielectric elastomer generators has been limited by the requirement for bulky rigid, and expensive external circuitry. In this letter, we present a soft dielectric elastomer generator whose stretchable circuit elements are integrated within the membrane. The soft generator achieved an energy density of 10 mJ/g at an efficiency of 12% and simply consisted of low-cost acrylic membranes and carbon grease mounted in a frame.
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Effect of cavity mode volume on photoluminescence from silicon photonic crystal nanocavities Appl. Phys. Lett. 98, 171102 (2011); http://dx.doi.org/10.1063/1.3582035 (3 pages) Online Publication Date: 25 April 2011
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We experimentally investigated photoluminescence (PL) from silicon photonic crystal nanocavities with different mode volumes at room temperature. The integrated cavity mode intensity, which was estimated from the observed PL signal by considering extraction and collection efficiencies for each cavity mode, increased as the cavity mode volume decreased. This result suggests that smaller cavities have larger mode emission efficiency per volume than that for larger cavities at room temperature.
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Adsorption geometry of individual fullerene on Si surface at room-temperature Appl. Phys. Lett. 97, 253106 (2010); http://dx.doi.org/10.1063/1.3529446 (3 pages) Online Publication Date: 20 December 2010
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The adsorption properties of individual fullerene molecules at room-temperature have been investigated by scanning tunneling microscopy. Statistical analysis of the images demonstrates that fullerene molecules prefer to adsorb on five specific sites: corner hole, the middle of the faulted and unfaulted parts, and the top of the rest atoms within the faulted and unfaulted parts. High-resolution images of individual fullerene molecules on each of the adsorption sites show distinct intramolecular structures. By combining these internal fine structures with theoretical simulations, the molecular orientations at each of the adsorption sites can be unambiguously determined.
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Appl. Phys. Lett. 97, 251908 (2010); http://dx.doi.org/10.1063/1.3526754 (3 pages) Online Publication Date: 21 December 2010
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It is shown that the introduction of zero-point energy and thermal effects to density functional theory with an empirical van der Waals correction results in a significant improvement in the prediction of equilibrium volumes and isothermal equations of state for hydrostatic compressions of energetic materials at nonzero temperatures. This method can be used to predict the thermophysical properties of these materials for a wide range of pressures and temperatures.
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Ultracompact, broadband slot waveguide polarization splitter Appl. Phys. Lett. 98, 151101 (2011); http://dx.doi.org/10.1063/1.3579243 (3 pages) Online Publication Date: 11 April 2011
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In this letter, we demonstrate an ultracompact polarization splitter design leveraging the giant birefringence of silicon-on-insulator slot waveguides. The fabricated splitter device has a coupling length of only 13.6 μm, and shows average polarization extinction ratios of 21 dB and 17 dB for the TE and TM polarizations, respectively, over the entire C-band.
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Appl. Phys. Lett. 97, 252107 (2010); http://dx.doi.org/10.1063/1.3528211 (3 pages) Online Publication Date: 21 December 2010
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The role of interface between electrode and NiO on switching characteristics has been investigated for unipolar NiO-based resistive switching. The ten time reduction of reset current could be achieved relative to a few milliamperes in many literatures by inserting a thin nickel interfacial layer between cathodic electrode and NiO. A model describing the reduction of reset current mechanism was derived from the combination effect of oxygen vacancy formation/migration and the interfacial oxide layer at cathodic electrode.
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Aluminum plasmonic nanostructures for improved absorption in organic photovoltaic devices Appl. Phys. Lett. 98, 133305 (2011); http://dx.doi.org/10.1063/1.3574091 (3 pages) Online Publication Date: 31 March 2011
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We model the absorption enhancement in organic photovoltaic devices induced by incorporating Al, Ag, and Au nanoparticles in the active layer. We find that Al nanoparticles should yield significantly greater enhancement than Ag or Au. This is because the much higher plasma frequency of Al ensures a better overlap between plasmon resonance and absorption band of organic semiconductors. Our predictions are verified experimentally by demonstrating enhanced absorbance in a poly(3-hexylthiophene-2,5-diyl): [6,6]-phenyl C61 butyric acid methyl ester layer with embedded functionalized Al nanoparticles.
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Enhancement of the spin pumping efficiency by spin wave mode selection Appl. Phys. Lett. 97, 252504 (2010); http://dx.doi.org/10.1063/1.3528207 (3 pages) Online Publication Date: 21 December 2010
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The spin pumping efficiency of standing spin wave modes in a rectangular Y3Fe5O12/Pt sample has been investigated by means of inverse spin-Hall effect (ISHE). Standing spin waves drive spin pumping, the generation of spin currents from magnetization precession, into the Pt layer which is converted into a detectable voltage due to the ISHE. We discovered that the spin pumping efficiency is significantly higher for standing surface spin waves, hybridized with thickness modes, rather than for volume spin wave modes. The results suggest that the use of higher-mode surface spin waves allows for the fabrication of an efficient spin-current injector.
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Indirect Auger recombination as a cause of efficiency droop in nitride light-emitting diodes Appl. Phys. Lett. 98, 161107 (2011); http://dx.doi.org/10.1063/1.3570656 (3 pages) Online Publication Date: 19 April 2011
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InGaN-based light-emitting diodes (LEDs) exhibit a significant efficiency loss (droop) when operating at high injected carrier densities, the origin of which remains an open issue. Using atomistic first-principles calculations, we show that this efficiency droop is caused by indirect Auger recombination, mediated by electron-phonon coupling and alloy scattering. By identifying the origin of the droop, our results provide a guide to addressing the efficiency issues in nitride LEDs and the development of efficient solid-state lighting.
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Growth temperature dependent graphene alignment on Ir(111) Appl. Phys. Lett. 98, 141903 (2011); http://dx.doi.org/10.1063/1.3548546 (3 pages) Online Publication Date: 4 April 2011
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The morphology of graphene monolayers on Ir(111) prepared by thermal decomposition of ethylene between 1000 and 1530 K was studied with high resolution low energy electron diffraction. In addition to a well-oriented epitaxial phase, randomly oriented domains are observed for growth temperatures between 1255 and 1460 K. For rotational angles of ±3° around 30° these domains lock-in in a 30° oriented epitaxial phase. Below 1200 K the graphene layer exhibits high disorder and structural disintegrity. Above 1500 K the clear moiré spots reflect graphene in a single orientation epitaxial incommensurate phase.
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Photonic crystal microdisk lasers Appl. Phys. Lett. 98, 131109 (2011); http://dx.doi.org/10.1063/1.3567944 (3 pages) Online Publication Date: 30 March 2011
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A photonic crystal (PC) microdisk laser cavity was introduced and demonstrated. The microlaser utilizes lossless surface modes within the PC forbidden band for vertical confinement and whispering gallery modes for lateral confinement. Analysis showed that this type of cavity mode has a smaller mode volume and a larger confinement factor than other resonant modes in the microdisk stacks. Initial experiments demonstrated lasing of optically pumped wavelength-size microdisks with four period GaAs/AlGaAs PCs and InGaAs quantum dots as gain media.
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Appl. Phys. Lett. 98, 071102 (2011); http://dx.doi.org/10.1063/1.3554426 (3 pages) Online Publication Date: 14 February 2011
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We report on a self-organized nanoscale patterning method by using oblique angle deposition to enhance the light extraction in a GaInN light-emitting diode (LED). The method offers one-step processing with good controllability of the feature size and density of the nanopatterns by varying the deposition angle during oblique angle deposition, eliminating the need for photolithography and annealing. A 5-nm-thick silver (Ag) film, when deposited by using oblique angle deposition, spontaneously forms a nanoscale island-like morphology on the substrate. This method is used to texture p-type GaN with nanoscale features, which results in increased light extraction from a GaInN LED. At 100 mA, the nanotextured LED shows a 46% higher light output than a standard LED with unpatterned (planar) p-type GaN.
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Appl. Phys. Lett. 98, 133301 (2011); http://dx.doi.org/10.1063/1.3566979 (3 pages) Online Publication Date: 28 March 2011
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The open circuit voltage Voc and the corresponding charge carrier density were measured in dependence of temperature and illumination intensity by current–voltage and charge extraction measurements for poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) and P3HT:bisPCBM solar cells. At lower temperatures a saturation of Voc was observed which can be explained by energetic barriers at the contacts (metal–insulator–metal model). Such injection barriers can also influence Voc at room temperature and limit the performance of the working solar cell, as was assured by macroscopic device simulations on temperature dependent IV characteristics. However, under most conditions—room temperature and low barriers—Voc is given by the effective band gap.
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Continuous roll-to-roll growth of graphene films by chemical vapor deposition Appl. Phys. Lett. 98, 133106 (2011); http://dx.doi.org/10.1063/1.3573866 (3 pages) Online Publication Date: 30 March 2011
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Few-layer graphene is obtained in atmospheric chemical vapor deposition on polycrystalline copper in a roll-to-roll process. Raman and x-ray photoelectron spectroscopy were employed to confirm the few-layer nature of the graphene film, to map the inhomogeneities, and to study and optimize the growth process. This continuous growth process can be easily scaled up and enables the low-cost fabrication of graphene films for industrial applications.
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Two-dimensional broadband distributed-feedback quantum cascade laser arrays Appl. Phys. Lett. 98, 141101 (2011); http://dx.doi.org/10.1063/1.3574555 (3 pages) Online Publication Date: 4 April 2011
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We present two-dimensional broadband quantum cascade laser arrays based on distributed-feedback (DFB) ring cavity surface emitting lasers. The 16-element arrays exhibit a linear tuning range of 180 cm−1 centered at a wavelength of 8.2 μm when operated in pulsed mode at room temperature. The devices show single-mode emission with a side mode suppression ratio of 30 dB. Given by the facetless nature of the single emitters, the spectral dependent threshold current densities and optical power reflect the gain profile of the incorporated material and are not impaired by the diversity of underlying DFB designs.
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Single-layer graphene cathodes for organic photovoltaics Appl. Phys. Lett. 98, 123303 (2011); http://dx.doi.org/10.1063/1.3569601 (3 pages) Online Publication Date: 23 March 2011
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A laminated single-layer graphene is demonstrated as a cathode for organic photovoltaic devices. The measured properties indicate that graphene offers two potential advantages over conventional photovoltaic electrode materials; work function matching via contact doping, and increased power conversion efficiency due to transparency. These findings indicate that flexible, light-weight all carbon solar cells can be constructed using graphene as the cathode material.
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Appl. Phys. Lett. 98, 162108 (2011); http://dx.doi.org/10.1063/1.3582925 (3 pages) Online Publication Date: 22 April 2011
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Nonvolatile memory (NVM) that is based on gate-all-around (GAA) and polycrystalline silicon (poly-Si) nanowires structure with silicon nanocrystals (NCs) as the storage nodes is demonstrated. The GAA poly-Si–SiO2–Si3N4–SiO2–poly-Si (SONOS) NVMs are also fabricated and compared. The GAA NCs NVMs have a 4.2 V of threshold voltage shift at 18 V for 1 ms, and are faster than the GAA SONOS NVMs do. In reliability studies, this NVM shows superior endurance after 104 program/erase (P/E) cycles, and loses only 14% of its charges lose after ten years at 85 °C.
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Low-resistance Ni/Al Ohmic contacts applied to a nonpolar a-plane n-type GaN Appl. Phys. Lett. 98, 161101 (2011); http://dx.doi.org/10.1063/1.3579252 (3 pages) Online Publication Date: 18 April 2011
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The authors report upon a low-resistance Ni/Al Ohmic contact to a nonpolar n-type a-plane GaN with respect to the annealing temperature. The Schottky behavior of the Ni/Al contact changes to a linear Ohmic behavior at a 700 °C annealing, at which the specific contact resistivity of the Ni/Al contact became as low as 5.8×10−5 whereas that of a typical Ti/Al contact was 1.6×10−3 Ω cm2. This improvement is attributed to a lowering of the Schottky barrier height via a Ni–Al interdiffused layer, formed at the interface between the metal and the nonpolar a-plane n-type GaN during the annealing process.
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