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Top 20 Most Read Articles
June 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Shrinking an arbitrary object as one desires using metamaterials Appl. Phys. Lett. 98, 204101 (2011); http://dx.doi.org/10.1063/1.3590203 (3 pages) Online Publication Date: 16 May 2011
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Based on transformation optics, we present a shrinking device, which can transform an arbitrary object virtually into a small-size object with different material parameters as one desires. Such an illusion device will confuse the detectors or the viewers, and hence the real size and material parameters of the enclosed object cannot be perceived. We fabricated and measured a shrinking device by using metamaterials, which works at the nonresonant frequency and has low loss. The device has been validated by both numerical simulations and experiments on circular and square objects. Good shrinking performance has been demonstrated.
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Tuning laser-induced band gaps in graphene Appl. Phys. Lett. 98, 232103 (2011); http://dx.doi.org/10.1063/1.3597412 (3 pages) Online Publication Date: 7 June 2011
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Could a laser field lead to the much sought-after tunable band gaps in graphene? By using Floquet theory combined with Green's functions techniques, we predict that a laser field in the mid-infrared range can produce observable band gaps in the electronic structure of graphene. Furthermore, we show how they can be tuned by using the laser polarization. Our results could serve as a guidance to design optoelectronic nanodevices.
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Appl. Phys. Lett. 98, 212902 (2011); http://dx.doi.org/10.1063/1.3595484 (3 pages) Online Publication Date: 26 May 2011
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We demonstrate a low voltage nonvolatile memory field effect transistor comprising thermal SiO2 tunneling and HfO2 blocking layers as the gate dielectric stack and Au nanocrystals as charge storage nodes. The structure exhibits a memory window of ∼ 2 V at an applied sweeping voltage of ±3 V which increases to 12.6 at ±12 V. Retention tests show an extrapolated loss of 16% after ten years in the hysteresis width of the threshold voltage. Dynamic program/erase operation reveal an approximately pulse width independent memory for pulse durations of 1 μs to 10 ms; longer pulses increase the memory window while for pulses shorter than 1 μs, the memory windows vanishes. The effective oxide thickness is below 10 nm with very low gate and drain leakage currents.
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Appl. Phys. Lett. 98, 212102 (2011); http://dx.doi.org/10.1063/1.3593096 (3 pages) Online Publication Date: 23 May 2011
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Transparent nanofloating gate memory devices based on top-gate zinc oxide thin-film transistors were developed. The proposed devices contained a facile and dry-synthesized palladium nanocluster array as a charge-trapping layer. The good programmable memory characteristics were exhibited due to the thin tunneling oxide, caused by the top-gate structure. The good endurance, data retention capability, and environmental stability demonstrated by the proposed device made it suitable for nonvolatile memory applications. As the whole processes were carried at room temperature, this letter has a potential use in fabricating high-performance and high-reliability nonvolatile memory devices on flexible substrates.
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Single-electron shuttle based on a silicon quantum dot Appl. Phys. Lett. 98, 212103 (2011); http://dx.doi.org/10.1063/1.3593491 (3 pages) Online Publication Date: 23 May 2011
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We report on single-electron shuttling experiments with a silicon metal-oxide-semiconductor quantum dot at 300 mK. Our system consists of an accumulated electron layer at the Si/SiO2 interface below an aluminum top gate with two additional barrier gates used to deplete the electron gas locally and to define a quantum dot. Directional single-electron shuttling from the source to the drain lead is achieved by applying a dc source-drain bias while driving the barrier gates with an ac voltage of frequency fp. Current plateaus at integer levels of efp are observed up to fp = 240 MHz operation frequencies. The observed results are explained by a sequential tunneling model, which suggests that the electron gas may be heated substantially by the ac driving voltage.
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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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Single InGaN nanodisk light emitting diodes as full-color subwavelength light sources Appl. Phys. Lett. 98, 233101 (2011); http://dx.doi.org/10.1063/1.3597211 (3 pages) Online Publication Date: 6 June 2011
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Subwavelength electroluminescent sources with spatial, spectral, and polarization controlling capabilities are critical elements for optical imaging and lithography beyond the diffraction limit. Here, we show that the electroluminescence from single, strain-free InGaN nanodisks embedded in self-assembled GaN p-n nanorods can span the entire visible spectrum with a large linear polarization ratio ( ∼ 0.85). Furthermore, this unique nanodisk-in-nanorod geometry enables the realization of the ultrasmall footprint light-emitting diodes (LEDs) to be used as subwavelength light sources. Using these nano-LEDs, we are able to demonstrate near-field, subwavelength photolithography by controlling the exposure time and light intensity from single InGaN nanodisks at chosen wavelengths.
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A InGaN/GaN quantum dot green (λ = 524 nm) laser Appl. Phys. Lett. 98, 221104 (2011); http://dx.doi.org/10.1063/1.3596436 (3 pages) Online Publication Date: 1 June 2011
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The characteristics of self-organized InGaN/GaN quantum dot lasers are reported. The laser heterostructures were grown on c-plane GaN substrates by plasma-assisted molecular beam epitaxy and the laser facets were formed by focused ion beam etching with gallium. Emission above threshold is characterized by a peak at 524 nm (green) and linewidth of 0.7 nm. The lowest measured threshold current density is 1.2 kA/cm2 at 278 K. The slope and wall plug efficiencies are 0.74 W/A and ∼ 1.1%, respectively, at 1.3 kA/cm2. The value of T0 = 233 K in the temperature range of 260–300 K.
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Boron nitride substrates for high mobility chemical vapor deposited graphene Appl. Phys. Lett. 98, 242105 (2011); http://dx.doi.org/10.1063/1.3599708 (3 pages) Online Publication Date: 13 June 2011
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Chemical vapor deposited (CVD) graphene is often presented as a scalable solution to graphene device fabrication, but to date such graphene has exhibited lower mobility than that produced by exfoliation. Using a boron nitride underlayer, we achieve mobilities as high as 37 000 cm2/V s, an order of magnitude higher than commonly reported for CVD graphene and better than most exfoliated graphene. This result demonstrates that the barrier to scalable, high mobility CVD graphene is not the growth technique but rather the choice of a substrate that minimizes carrier scattering.
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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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Multilayer graphene under vertical electric field Appl. Phys. Lett. 98, 222101 (2011); http://dx.doi.org/10.1063/1.3595335 (3 pages) Online Publication Date: 31 May 2011
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We study the effect of vertical electric field (E-field) on the electronic properties of multilayer graphene. We show that the effective mass, electron velocity, and density-of-state of a bilayer graphene are modified under the E-field. We also study the transformation of the band structure of multilayer graphenes. E-field induces finite (zero) band gap in the even (odd)-layer ABA-stacking graphene. On the other hand, finite band gap is induced in all ABC-stacking graphene. We also identify the optimum E-field to obtain the maximum band gap in the multilayer graphenes. Finally, we compare our results with the experimental results of a field-effect-transistor.
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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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Appl. Phys. Lett. 98, 243308 (2011); http://dx.doi.org/10.1063/1.3600665 (3 pages) Online Publication Date: 16 June 2011
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Photovoltaic performance of the polymer solar cell (PSC) based on poly(3-hexylthiophene) (P3HT) as donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor was improved by using the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) modification layer treated by ethanol or 2-propanol. Power conversion efficiency (PCE) of the PSC based on P3HT:PCBM (1:1, w/w) with the 2-propanol-treated PEDOT:PSS modification layer reached 4.74%, in comparison with a PCE of 3.39% for the PSC with the PEDOT:PSS layer without the organic solvent treatment. The enhanced performance of the PSCs is attributed to higher conductivity and optimized surface morphology of the PEDOT:PSS layers treated by the organic solvent.
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Charge-carrier dynamics in hybrid plasmonic organic solar cells with Ag nanoparticles Appl. Phys. Lett. 98, 253302 (2011); http://dx.doi.org/10.1063/1.3601742 (3 pages) Online Publication Date: 20 June 2011
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To understand the effects of Ag nanoparticles (NPs) on the performance of organic solar cells, we examined the properties of hybrid poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric-acid-methyl-ester:Ag NP solar cells using photoinduced charge extraction with a linearly increasing voltage. We find that the addition of Ag NPs into the active layer significantly enhances carrier mobility but decreases the total extracted carrier. Atomic force microscopy shows that the Ag NPs tend to phase segregate from the organic material at high concentrations. This suggests that the enhanced mobility results from carriers traversing Ag NP subnetworks, and that the reduced carrier density results from increased recombination from carriers trapped on the Ag particles.
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Observation of phase shifts in a vertical cavity quantum dot switch Appl. Phys. Lett. 98, 231101 (2011); http://dx.doi.org/10.1063/1.3596704 (3 pages) Online Publication Date: 6 June 2011
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We have studied the possibility to utilize semiconductor quantum dots (QDs) as an optical phase shifter within a vertical geometry for ultrafast information processing. From theoretical analyses, an optical phase nonlinearity in QD structures has been predicted which can be enhanced through the use of an vertical optical cavity. Asymmetric cavity structures with 16/30 periods of GaAs/AlGaAs layers for the front/back mirrors have been fabricated to demonstrate a practical device with significant nonlinear characteristics for optical switching. A phase shift of 18° has been initially observed with a tilted pump scheme. This observation paves the way toward a Mach–Zehnder optical switch using QDs inside a vertical cavity.
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Bragg polariton luminescence from a GaN membrane embedded in all dielectric microcavity Appl. Phys. Lett. 98, 221101 (2011); http://dx.doi.org/10.1063/1.3595481 (3 pages) Online Publication Date: 31 May 2011
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We report on the development of a band gap-selective photochemical etching technique capable of producing 200 nm thick optical quality freestanding GaN membranes. The use of low electrolyte concentration combined with intense excitation by a laser source are shown to yield highly anisotropic etch profile with optical quality etched surfaces. Using this technique, high quality GaN microcavity is fabricated by embedding the GaN membrane inside an all-dielectric mirror cavity. In these structures, Bragg polariton photoluminescence is observed at room temperature.
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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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The oxygen dimer in Si: Its relationship to the light-induced degradation of Si solar cells? Appl. Phys. Lett. 98, 182101 (2011); http://dx.doi.org/10.1063/1.3584138 (3 pages) Online Publication Date: 2 May 2011
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It is widely believed that the light induced degradation of crystalline silicon solar cells is due to the formation of a BsO2i recombination center created by the optically excited migration of the oxygen dimer (charge-state-driven motion). In this letter the concentration dependence of the neutral state of O2i on [Oi] in p- and n-type Cz–Si has been determined using infrared absorption. A systematic search for the absorption signature of the dimer in the doubly positively charged state has been unsuccessful. These data strongly suggest that charge-state-driven motion (Bourgoin–Corbett mechanism) of the oxygen dimer cannot occur in typical solar silicon and hence bring into question the accepted degradation mechanism.
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Appl. Phys. Lett. 98, 223305 (2011); http://dx.doi.org/10.1063/1.3595679 (3 pages) Online Publication Date: 3 June 2011
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A short series of alkyl substituted perylenediimides (PDIs) with varying steric bulk are used to demonstrate the relationship between molecular structure, materials properties, and performance characteristics in organic photovoltaics. Devices were made with the structure indium tin oxide/copper phthalocyanine (200 Å)/PDI (200 Å)/bathocuproine (100 Å)/aluminum (1000 Å). We found that PDIs with larger substituents produced higher open circuit voltages (VOC’s) despite the donor acceptor interface gap (ΔEDA) remaining unchanged. Additionally, series resistance was increased simultaneously with VOC the effect of reducing short circuit current, making the addition of steric bulk a tradeoff that needs to be balanced to optimize power conversion efficiency.
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Organic photovoltaics incorporating electron conducting exciton blocking layers Appl. Phys. Lett. 98, 243307 (2011); http://dx.doi.org/10.1063/1.3598426 (3 pages) Online Publication Date: 15 June 2011
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We demonstrate that 3,4,9,10 perylenetetracarboxylic bisbenzimidazole (PTCBI) and 1,4,5,8-napthalene-tetracarboxylic-dianhydride (NTCDA) can function as electron conducting and exciton blocking layers when interposed between the acceptor layer and cathode. A low-resistance contact is provided by PTCBI, while NTCDA acts as an exciton blocking layer and optical spacer. Both materials serve as efficient electron conductors, leading to a fill factor as high as 0.70. By using an NTCDA/PTCBI compound blocking layer structure in a functionalized-squaraine/C60-based device, we obtain a spectrally corrected power conversion efficiency of 5.1±0.1% under 1 sun, AM 1.5G simulated solar illumination, an improvement of >25% compared to an analogous device using a conventional bathocuproine layer that has previously been shown to conduct electrons via damage-induced midgap states.
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