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Top 20 Most Read Articles
May 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 98, 183101 (2011); http://dx.doi.org/10.1063/1.3584006 (3 pages) Online Publication Date: 2 May 2011
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Here we report chemical vapor deposition of graphene on gold surface at ambient pressure. We studied effects of the growth temperature, pressure, and cooling process on the grown graphene layers. The Raman spectroscopy of the samples reveals the essential properties of the graphene grown on gold surface. In order to characterize the electrical properties of the grown graphene layers, we have transferred them on insulating substrates and fabricated field effect transistors. Owing to distinctive properties of gold, the ability to grow graphene layers on gold surface could open new applications of graphene in electrochemistry and spectroscopy.
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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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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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Nanostructured three-dimensional thin film silicon solar cells with very high efficiency potential Appl. Phys. Lett. 98, 163503 (2011); http://dx.doi.org/10.1063/1.3583377 (3 pages) Online Publication Date: 22 April 2011
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We report on the experimental realization of amorphous/microcrystalline silicon tandem solar cells (Micromorph) based on our three-dimensional design. An enhancement is reached in the short-circuit current by 40%, with an excellent open-circuit voltage of 1.41V and a fill factor of 72%. We have used nanoholes or microholes dry etched into the ZnO front contact layer. Monte Carlo optical modeling shows that stable efficiency of amorphous silicon p-i-n solar cells in over 12% range is possible. For the Micromorph cells, efficiency over 15% with the thickness of amorphous Si below 200 nm and of microcrystalline Si around 500 nm is possible.
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Appl. Phys. Lett. 98, 151904 (2011); http://dx.doi.org/10.1063/1.3576906 (3 pages) Online Publication Date: 11 April 2011
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We report a mechanically and optically robust folding structure to realize a foldable active matrix organic-light-emitting-diode (AMOLED) display without a visible crease at the junction. A nonlinear stress analysis, based on a finite element method, provided an optimized design. The folding-unfolding test on the structure exhibited negligible deterioration of the relative brightness at the junction of the individual panels up to 105 cycles at a folding radius of 1 mm, indicating highly reliable mechanical and optical tolerances. These results demonstrate the feasibility of seamless foldable AMOLED displays, with potentially important technical implications on fabricating large size flexible displays.
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Diffractive optical elements based on plasmonic metamaterials Appl. Phys. Lett. 98, 191101 (2011); http://dx.doi.org/10.1063/1.3587622 (3 pages) Online Publication Date: 9 May 2011
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The dispersive properties of plasmonic metamaterials and the ability to tailor their complex transmission strongly suggest their use in versatile optical elements. Here we introduce the idea of such an application in diffractive elements and describe, as a proof-of-principle, two numerical implementations of computer-generated holograms at visible wavelengths that are based on fishnet metamaterials. These holograms consist of large arrays of metamaterial unit cells which have locally varying geometrical parameters into which the desired far-field optical response is encoded. We describe the entire design process for such holograms, discuss their efficiency and critically assess their limitations.
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Resistive switching behaviors of NiO films with controlled number of conducting filaments Appl. Phys. Lett. 98, 192104 (2011); http://dx.doi.org/10.1063/1.3589825 (3 pages) Online Publication Date: 10 May 2011
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We deposited NiO films by using reactive dc sputtering on Pt/Ti/SiO2/Si substrates. Before depositing top Pt electrodes, conducting filaments are preformed using conductive atomic force microscope. Pt/NiO/Pt capacitor structures with controlled number of conducting filaments show unipolar resistive switching behaviors strongly dependent on the number of conducting filaments. Especially, set switching voltage distribution increases with the number of conducting filament.
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Origin of the efficiency enhancement in ferroelectric functionalized organic solar cells Appl. Phys. Lett. 98, 183301 (2011); http://dx.doi.org/10.1063/1.3587630 (3 pages) Online Publication Date: 3 May 2011
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We have investigated the efficiency enhancement of organic solar cells upon incorporation of a thin ferroelectric polymer layer. For non-Ohmic contacts the enhancement is due to an increased open circuit voltage, which is, however, independent of the ferroelectric polarization direction. Ferroelectricity cannot play a role due to depolarization and charge compensation. The same improvement is also obtained with a non-ferroelectric fluorinated polymer. The data unambiguously show that the improved efficiency is solely due to improvement of the cathode. The fluorinated polymer layer plays the role of LiF in an optimized solar cell.
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Radially polarized optical vortex converter created by femtosecond laser nanostructuring of glass Appl. Phys. Lett. 98, 201101 (2011); http://dx.doi.org/10.1063/1.3590716 (3 pages) Online Publication Date: 16 May 2011
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We demonstrate the generation of optical vortices with radial or azimuthal polarization using a space variant polarization converter, fabricated by femtosecond laser writing of self-assembled nanostructures in silica glass. Manipulation of the induced form birefringence is achieved by controlling writing parameters, in particular, the polarization azimuth of the writing beam. The fabricated converter allows switching from radial to azimuthal polarization by controlling the handedness of incident circular polarization.
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Thermal transport in nanoclusters Appl. Phys. Lett. 98, 193107 (2011); http://dx.doi.org/10.1063/1.3590265 (3 pages) Online Publication Date: 11 May 2011
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Nonequilibrium and equilibrium molecular dynamics simulations are employed to study the thermal transport in sintered silicon nanoclusters made of 15 nm diameter nanoparticles arranged on a simple cubic lattice. Both simulation techniques indicate a reduction in the thermal conductivity from ∼ 120 W/m K (bulk) to 1.5 W/m K (nanoclusters) at 500 K. This dramatic reduction is attributed to the reduced thermal conductivity of nanoparticle (15 W/m K) and most prominently to the nanosized constriction resistance due to necking between the two nanoparticles. Comparison with the existing models, radial distribution function and vibrational analysis show that the phonon transport in the nanosized neck region is ballistic rather than diffusive.
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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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InGaN/GaN disk-in-nanowire white light emitting diodes on (001) silicon Appl. Phys. Lett. 98, 193102 (2011); http://dx.doi.org/10.1063/1.3588201 (3 pages) Online Publication Date: 9 May 2011
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High density ( ∼ 1011 cm−2) GaN nanowires and InGaN/GaN disk-in-nanowire heterostructures have been grown on (001) silicon substrates by plasma-assisted molecular beam epitaxy. The nanowires exhibit excellent uniformity in length and diameter and a broad emission is obtained by incorporating InGaN disks of varying composition along the length of the nanowires. Monolithic lighting emitting diodes were fabricated with appropriate n- and p-doping of contact layers. White light emission with chromaticity coordinates of x = 0.29 and y = 0.37 and a correlated color temperature of 5500–6500 K at an injection current of 50 A/cm2 is measured. The measured external quantum efficiency of the devices do not exhibit any rollover (droop) up to an injection current density of 400 A/cm2.
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Room-temperature spin-dependent tunneling through molecules Appl. Phys. Lett. 98, 172501 (2011); http://dx.doi.org/10.1063/1.3583585 (3 pages) Online Publication Date: 25 April 2011
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We have fabricated assemblies of molecular junctions comprised of superparamagnetic Fe3O4 nanoparticles self-assembled with alkane molecules of different lengths as the spacer. The electrical resistance increases exponentially over nearly two decades as the molecular length varies from 0.7 to 2.5 nm, indicating that electrons tunnel through the molecules that are chemically bonded with Fe3O4 nanoparticles. Up to ∼ 21% room-temperature magnetoresistance is observed. Remarkably, the tunneling magnetoresistance ratio stays nearly independent of molecular length, which entails room-temperature spin-conserving transport in organic molecules.
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Deterministic nanoassembly of a coupled quantum emitter–photonic crystal cavity system Appl. Phys. Lett. 98, 193103 (2011); http://dx.doi.org/10.1063/1.3571437 (3 pages) Online Publication Date: 9 May 2011
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Controlling the interaction of a single quantum emitter with its environment is a key challenge in quantum optics. Here, we demonstrate deterministic coupling of single nitrogen-vacancy (NV) centers to high-quality photonic crystal cavities. We preselect single NV centers and position their 50-nm-sized host nanocrystals into the mode maximum of photonic crystal S1 cavities with few-nanometer accuracy. The coupling results in a strong enhancement of NV center emission at the cavity wavelength.
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Detailed balance limit of the efficiency of multilevel intermediate band solar cells Appl. Phys. Lett. 98, 171108 (2011); http://dx.doi.org/10.1063/1.3583587 (3 pages) Online Publication Date: 27 April 2011
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Intermediate-band solar cells (IBSCs) promise ultrahigh solar-electricity energy conversion. We have calculated the detailed balance limit of the efficiency for IBSCs with multiple intermediate bands by optimizing IB’s energy levels. The results indicate that thermodynamic limit of IBSCs with 4 IBs is 74.6% which far exceeds 63% calculated in a previous study for the single IB case. By further increasing the total number IBs, the thermodynamic limit of IBSCs can ultimately approach nearly 80%.
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Metallic and insulating adsorbates on graphene Appl. Phys. Lett. 98, 192101 (2011); http://dx.doi.org/10.1063/1.3588035 (3 pages) Online Publication Date: 9 May 2011
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We directly compare the effect of metallic titanium (Ti) and insulating titanium dioxide (TiO2) on the transport properties of single layer graphene. The deposition of Ti results in substantial n-type doping and a reduction in graphene mobility by charged impurity scattering. Subsequent exposure to oxygen largely reduces the doping and scattering by converting Ti into TiO2. In addition, we observe evidence for short-range scattering by TiO2 impurities. These results illustrate the contrasting scattering mechanisms for identical spatial distributions of metallic and insulating adsorbates.
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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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Appl. Phys. Lett. 98, 183304 (2011); http://dx.doi.org/10.1063/1.3580629 (3 pages) Online Publication Date: 5 May 2011
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Organic solar cells are commonly susceptible to degradation in air. We present that insertion of a thin layer of thermally evaporated copper oxide (CuOx) between the organic active layer and the Al cathode can greatly extend the lifetime of P3HT:PCBM based bulk heterojunction solar cells. The performance can be further improved by applying an interfacial bilayer of CuOx/LiF. Our results suggest that the CuOx functions not only as a charge transport layer but also as a protection layer, which prevents formation of thick organic-Al interdiffusion area. This leads to a more air-resistive cathode/organic interface.
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Ultimate photovoltage in perovskite oxide heterostructures with critical film thickness Appl. Phys. Lett. 98, 181101 (2011); http://dx.doi.org/10.1063/1.3586250 (3 pages) Online Publication Date: 2 May 2011
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One order larger photovoltage is obtained with critical thicknesses of La0.9Sr0.1MnO3 films in both kinds of heterostructures of La0.9Sr0.1MnO3/SrTiO3 (0.8 wt % Nb-doped) and La0.9Sr0.1MnO3/Si fabricated at various oxygen pressures. Our self-consistent calculation reveals that the critical thickness of the La0.9Sr0.1MnO3 film with the ultimate value of photovoltage is just the thickness of the depletion layer of La0.9Sr0.1MnO3 in both heterojunctions, respectively.
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Band gap formation in graphene by in-situ doping Appl. Phys. Lett. 98, 203102 (2011); http://dx.doi.org/10.1063/1.3589364 (3 pages) Online Publication Date: 17 May 2011
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We report the formation of band gaps in as-grown stacks of epitaxial graphene with opposite doping. Control of in-situ doping during carbon source molecular beam epitaxy growth on SiC was achieved by using different carbon sources. Doping heterostructures were grown by stacking n-type material from a C60 source on p-type material from a graphite filament source. Activation energies for the resistivity and carrier concentration indicated band gaps up to 200 meV. A photoconductivity threshold was observed in the range of the electrical activation energies. Band gap formation is attributed to electric fields induced by spatially separated ionized dopants of opposite charge.
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