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Top 20 Most Read Articles
November 2009
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 95, 183504 (2009); http://dx.doi.org/10.1063/1.3256223 (3 pages) Online Publication Date: 4 November 2009
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This letter reports the fabrication and electrical characterization of mechanically flexible and low operating voltage transistors and logic gates (NOT, NAND, and NOR gates) using printed silicon nanomembranes and self-assembled nanodielectrics on thin plastic substrates. The transistors exhibit effective linear mobilities of ∼ 680 cm2/V s, on/off ratios >107, gate leakage current densities <2.8×10−7 A/cm2, and subthreshold slopes ∼ 120 mV/decade. The inverters show voltage gains as high as 4.8. Simple digital logic gates (NAND and NOR gates) demonstrate the possible application of this materials combination in digital integrated circuits.
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Optical nanorod antennas as dispersive one-dimensional Fabry–Pérot resonators for surface plasmons Appl. Phys. Lett. 95, 201101 (2009); http://dx.doi.org/10.1063/1.3262947 (3 pages) Online Publication Date: 16 November 2009
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Resonant optical nanoantennas exhibit a different length scaling due to the surface plasmons compared to their radio frequency counterparts. In this letter, we address this difference by calculating the wavelength-dependent effective mode index neff for a cylindrical one-dimensional gold nanowire waveguide. Our results show that nanorod optical antennas act as dispersive and lossy Fabry–Pérot resonators for surface plasmons.
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Energetic disorder limits energy transfer in semiconductor nanocrystal–DNA–dye conjugates Appl. Phys. Lett. 95, 143101 (2009); http://dx.doi.org/10.1063/1.3242015 (3 pages) Online Publication Date: 5 October 2009
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We demonstrate the influence of spectral linewidths of individual donor-acceptor couples on energy transfer efficiency in semiconductor nanocrystal–DNA–organic dye conjugates. Temperature-dependent single molecule and ensemble spectroscopy data are analyzed using the Förster theory within the macroscopic and microscopic approaches. The results obtained evidence on the importance of the spectral overlap between emission of a single donor and absorption of a single acceptor in its close vicinity, which determines the microscopic resonance and transfer efficiency between individual neighbors. This realization poses important implications on the applicability of ensemble spectral overlap for the analysis of distance dependencies of nanoscopic objects.
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A pathway to p-type wide-band-gap semiconductors Appl. Phys. Lett. 95, 172109 (2009); http://dx.doi.org/10.1063/1.3247890 (3 pages) Online Publication Date: 30 October 2009
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Based on first-principles calculations we devise an alternative approach to p-type doping in AlN, ZnO, and ZnMgO. Instead of searching for acceptors on the left of the host atoms in the Periodic Table, we propose to search on the far right. We find that F placed at interstitial sites in AlN, ZnO, and ZnMgO acts as a shallow acceptor, leaving a hole in an effective-mass state near the valence-band maximum. We investigate the stability of F impurities and propose a procedure to selectively introduce F in the interstitial lattice sites of the above wide-band-gap semiconductors.
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Blue light-emitting diodes with a roughened backside fabricated by wet etching Appl. Phys. Lett. 95, 201102 (2009); http://dx.doi.org/10.1063/1.3262968 (3 pages) Online Publication Date: 16 November 2009
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The InGaN-based light-emitting diodes (LEDs) with a roughened patterned backside on the N-face GaN surface were fabricated through a crystallographic etching process to increase light-extraction efficiency. After laser decomposition, laser scribing, and a lateral crystallographic wet etching process at the GaN/Al2O3 interface, stable crystallographic etching planes were formed as the GaN {10
 } planes that included an angle with the top GaN (0001) plane measured at 58°. The GaN buffer layer acted as the sacrificial layer for the laser decomposition process and the lateral wet etching process with a 26 μm/min etching rate. The LED with the inverted pyramidal N-face GaN surface close to the GaN/Al2O3 interface has a larger light-scattering process than the conventional LED. The light-output power of the LED with the backside roughened surface had a 47% enhancement when measured in LED chip form. |
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Scalable arrays of rf Paul traps in degenerate Si Appl. Phys. Lett. 95, 173102 (2009); http://dx.doi.org/10.1063/1.3254188 (3 pages) Online Publication Date: 26 October 2009
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We report techniques for the fabrication of multizone linear radio frequency Paul traps that exploit the machinability and electrical conductivity of degenerate silicon. The approach was tested by trapping and laser cooling 24Mg+ ions in the two following trap geometries: a single-zone two-layer trap and a multizone surface-electrode trap. From the measured ion motional heating rate we determine an electric field spectral density at the ion’s position of approximately 1×10−10 (V/m)2⋅Hz−1 at ωz/2π = 1.125 MHz when the ion lies 40 μm above the trap surface. One application of these devices is controlled manipulation of atomic ion qubits, the basis of one form of quantum information processing.
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Appl. Phys. Lett. 95, 202101 (2009); http://dx.doi.org/10.1063/1.3263942 (3 pages) Online Publication Date: 16 November 2009
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This letter presents a simple method for transferring epitaxial sheets of graphene on silicon carbide to other substrates. The graphene was grown on the (0001) face of 6H-SiC by thermal annealing at 1550 °C in a hydrogen atmosphere. Transfer was accomplished using a peeling process with a bilayer film of gold/polyimide, to yield graphene with square millimeters of coverage on the target substrate. Raman spectroscopy provided evidence that the transferred material is single layer. Back gated field-effect transistors fabricated on oxidized silicon substrates with Cr/Au as source-drain electrodes exhibited ambipolar characteristics with hole mobilities of ∼ 100 cm2/V-s, and negligible influence of resistance at the contacts.
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Appl. Phys. Lett. 95, 183301 (2009); http://dx.doi.org/10.1063/1.3257361 (3 pages) Online Publication Date: 3 November 2009
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Oxidation of silver top electrodes in polymer solar cells induces the devices to exhibit a transition from conventional to inverted photovoltaic characteristics. As silver oxidizes, its work function increases from 4.3 to 5.0 eV. The silver top electrode thus reverts from collecting electrons to collecting holes during device operation. We have quantified the extent of this transformation and find it to be correlated with the rate of silver oxidation, which we can influence by altering the annealing sequence of the polymer photoactive layer, ultimately varying the grain size of silver in the top electrode.
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Design of graphene electronic devices using nanoribbons of different widths Appl. Phys. Lett. 95, 182104 (2009); http://dx.doi.org/10.1063/1.3257731 (3 pages) Online Publication Date: 5 November 2009
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We present a simple design of a field effect transistor based on graphene nanoribbons, taking advantage of the metallic and semiconductor nature of nanoribbons with different widths. Such device could be constructed by using lithography techniques. The conductance of the proposed device is obtained by using the Kubo formula, assuming a strong damping due to the substrate and imperfections of the lattice. By removing the control electrodes, the design could also be used as an electrical resistance.
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Improving optical performance of inverted organic solar cells by microcavity effect Appl. Phys. Lett. 95, 193301 (2009); http://dx.doi.org/10.1063/1.3262967 (3 pages) Online Publication Date: 11 November 2009
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Optical simulations have been performed to investigate the performance of inverted organic solar cells with metal-mirror microcavity structure formed by central active layer sandwiched between semitransparent silver (Ag) cathode and thick Ag anode. Compared to nearly optimized noncavity devices with indium tin oxide cathode, the total absorbed photons (TAPs) in a 70 nm-thick active layer for cavity devices can be improved by 16.3% due to microcavity effect. Furthermore, an improvement of over 10% in TAPs can be obtained when thin Ag layer is optimized to be 10–16 nm thick, indicating thin Ag layer is a good choice as transparent electrode material.
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A tunable few electron triple quantum dot Appl. Phys. Lett. 95, 193101 (2009); http://dx.doi.org/10.1063/1.3258663 (3 pages) Online Publication Date: 9 November 2009
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In this paper, we report on a tunable few electron lateral triple quantum dot design. The quantum dot potentials are arranged in series. The device is aimed at studies of triple quantum dot properties where knowing the exact number of electrons is important as well as quantum information applications involving electron spin qubits. We demonstrate tuning strategies for achieving required resonant conditions such as quadruple points where all three quantum dots are on resonance. We find that in such a device resonant conditions at specific configurations are accompanied by complex charge transfer behavior.
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Appl. Phys. Lett. 95, 193103 (2009); http://dx.doi.org/10.1063/1.3258490 (3 pages) Online Publication Date: 10 November 2009
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External quantum efficiency in solar cells based on junctions between PbSe quantum dots (QDs) and thin ZnO films is increased by replacing the ZnO films with a vertically oriented array of single-crystalline ZnO nanowires, and infiltrating this array with colloidal QDs. When illuminated with 100 mW/cm2 of simulated solar light, QD-nanowire solar cells exhibited power conversion efficiencies approaching 2%, approximately three times higher than that achieved with thin-film ZnO devices constructed with the same amount of QDs. Significant photocurrent and power conversion improvement with increasing nanowire length is consistent with higher exciton and charge collection efficiencies.
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Electroluminescence induced by photoluminescence excitation in GaInN/GaN light-emitting diodes Appl. Phys. Lett. 95, 191105 (2009); http://dx.doi.org/10.1063/1.3258488 (3 pages) Online Publication Date: 10 November 2009
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Optical emission resulting from 405 nm selective photoexcitation of carriers in the GaInN/GaN quantum well (QW) active region of a light-emitting diode reveals two recombination channels. The first recombination channel is the recombination of photoexcited carriers in the GaInN QWs. The second recombination channel is formed by carriers that leak out of the GaInN QW active region, self-bias the device in forward direction, induce a forward current, and subsequently recombine in the GaInN active region in a spatially distributed manner. The results indicate dynamic carrier transport involving active, confinement, and contact regions of the device.
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Electrically tunable terahertz quantum-cascade laser with a heterogeneous active region Appl. Phys. Lett. 95, 181101 (2009); http://dx.doi.org/10.1063/1.3253714 (3 pages) Online Publication Date: 2 November 2009
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We demonstrate experimentally a terahertz frequency quantum-cascade laser which can be tuned electronically, in a step-wise manner from typically 3.07 to 3.40 THz, by changing the applied electric field across the device. To achieve this, the gallium growth rate was successively reduced during growth of the active region from +6% to −4% of its nominal 1 μm/h operating value. This produced a heterogeneous active region, comprising 23 sections of different lasing units stacked together. With a single-metal surface-plasmon waveguide configuration, the devices operated in pulsed mode, up to a temperature of 81 K, and gave peak powers of ∼ 8 mW.
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Appl. Phys. Lett. 95, 103105 (2009); http://dx.doi.org/10.1063/1.3222901 (3 pages) Online Publication Date: 9 September 2009
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The authors report a direct examination of recorded marks in blue-ray recordable and erasable disks with scattering-type scanning near-field optical microscopy. The optical contrasts of the crystalline and amorphous regions of the phase-change layer match with the prediction based on their optical constants. The determined spatial optical variation in the recorded marks reflects the intensity profile of the recording laser beam. The identified nanometer-sized optical features are shown to correspond to 10 nm-sized crystalline domains within the amorphous recorded marks. The revealed near-field signatures show a potential influence on the carrier-to-noise ratio of this optical storage medium.
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Two-state competition in 1.3 μm multilayer InAs/InGaAs quantum dot lasers Appl. Phys. Lett. 95, 191101 (2009); http://dx.doi.org/10.1063/1.3259659 (3 pages) Online Publication Date: 9 November 2009
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The competition of ground state (GS) and excited state (ES) is investigated from the as-grown and thermally annealed 1.3 μm ten-layer p-doped InAs/GaAs quantum dot (QD) lasers. The modal gain competition between GS and ES are measured and analyzed around the ES threshold characteristics. Our results show that two-state competition is more significant in devices with short cavity length operating at high temperature. By comparing the as-grown and annealed devices, we demonstrate enhanced GS and suppressed ES lasing from the QD laser annealed at 600 °C for 15 s.
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ZnO/TiO2 nanonetwork as efficient photoanode in excitonic solar cells Appl. Phys. Lett. 95, 193104 (2009); http://dx.doi.org/10.1063/1.3257370 (3 pages) Online Publication Date: 11 November 2009
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An innovative nanonetwork composed of TiO2 nanoparticles and single-crystalline ZnO nanowires is demonstrated as efficient photoanode in excitonic solar cells. Such architecture benefits of the capability of high sensitizer loading offered by the nanoparticles and of the direct conduction path for electrons guaranteed by the nanowires. The combination of these features leads to improved light absorption, electron photogeneration, and charge collection. The nanonetwork was implemented in a dye-sensitized solar-cell architecture demonstrating threefold enhancement of the efficiency with respect to a nanowire photoanode of the same thickness. Cell efficiency of 1.6% was obtained in 1.5 μm thick nanonetwork.
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Appl. Phys. Lett. 95, 173304 (2009); http://dx.doi.org/10.1063/1.3257383 (3 pages) Online Publication Date: 30 October 2009
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We demonstrate photocurrent enhancement of up to 20% in polymer:fullerene bulk heterojunction photovoltaic cells via the incorporation of a phosphorescent dopant, without degradation in the open-circuit voltage or fill factor of the device. The enhancement is shown to originate from multiple sources. First, the phosphor is able to populate the long-lived triplet state of the polymer, leading to longer diffusion length and a larger polymer contribution. Also, there is direct absorption on the dopant leading to enhanced spectral coverage. Finally, the dopant acts as a donor site and therefore increases the fullerene signal.
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Vertically aligned ZnO nanostructures grown on graphene layers Appl. Phys. Lett. 95, 213101 (2009); http://dx.doi.org/10.1063/1.3266836 (3 pages) Online Publication Date: 24 November 2009
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We report the vertical growth of ZnO nanostructures on graphene layers and their photoluminescence (PL) characteristics. ZnO nanostructures were grown vertically on the graphene layers using catalyst-free metal-organic vapor-phase epitaxy. The surface morphology of the ZnO nanostructures on the graphene layers depended strongly on the growth temperature. Further, interesting growth behavior leading to the formation of aligned ZnO nanoneedles in a row and vertically aligned nanowalls was also observed and explained in terms of enhanced nucleation on graphene step edges and kinks. Additionally, the optical characteristics and carbon incorporation into ZnO were investigated using variable-temperature PL spectroscopy.
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Appl. Phys. Lett. 95, 182502 (2009); http://dx.doi.org/10.1063/1.3258069 (3 pages) Online Publication Date: 3 November 2009
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Magnetoresistance ratio up to 330% at room temperature (700% at 10 K) has been obtained in a spin-valve-type magnetic tunnel junction (MTJ) consisting of a full-Heusler alloy Co2FeAl electrode and a MgO tunnel barrier fabricated on a single crystal MgO (001) substrate by sputtering method. The output voltage of the MTJ at one-half of the zero-bias value was found to be as high as 425 mV, which is the largest reported to date in MTJs using Heusler alloy electrodes. The present finding suggests that Co2FeAl may be one of the most promising candidates for future spintronics devices applications.
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