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Top 20 Most Read Articles
September 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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Electron transport through single carbon nanotubes Appl. Phys. Lett. 91, 103101 (2007); http://dx.doi.org/10.1063/1.2778551 (3 pages) Online Publication Date: 4 September 2007
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The authors report on the transport of energetic electrons through single, well aligned multiwall carbon nanotubes (CNTs). Embedding of CNTs in a protective carbon fiber coating enables the application of focused ion beam based sample preparation techniques for the nondestructive isolation and alignment of individual tubes. Aligned tubes with lengths of 0.7–3 μm allow transport of 300 keV electrons in a transmission electron microscope through their hollow cores at zero degree incident angles and for a misalignment of up to 1°.
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Highly efficient simplified organic light emitting diodes Appl. Phys. Lett. 91, 113506 (2007); http://dx.doi.org/10.1063/1.2784176 (3 pages) Online Publication Date: 11 September 2007
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The authors report on highly efficient organic light emitting diodes (OLEDs) consisting of only two organic layers. The key to the simplification is the direct injection of holes into the wide band gap hole transport material 4,4′,4″-tris(N-carbazolyl)-triphenyl amine (highest occupied molecular orbital is 5.9 eV) through an indium tin oxide/tungsten oxide (WO3) anode. Kelvin probe analysis has revealed an extremely high work function of 6.4 eV for WO3. The efficiencies of the simplified OLEDs exceed 40 lm/W and 45 cd/A at a brightness of 100 cd/m2, unsurpassed by other comparably simple OLED devices. Therefore, our OLED architecture demonstrates highly efficient, yet easy to fabricate devices.
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ZnO light-emitting diodes fabricated on Si substrates with homobuffer layers Appl. Phys. Lett. 91, 113503 (2007); http://dx.doi.org/10.1063/1.2783262 (3 pages) Online Publication Date: 10 September 2007
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ZnO homojunction light-emitting diodes (LEDs), comprised of N–Al codoped p-type ZnO and Al-doped n-type ZnO layers, were fabricated on Si substrates with homobuffer layers. The current-voltage measurements showed typical diode characteristic with a threshold voltage of about 3.3 V. The electroluminescence (EL) bands at 110 K consisted of a near-band-edge emission at 3.18 eV and a deep level emission at 2.58 eV. The EL emissions were assigned as radiative recombinations, presumably of donor-acceptor pairs, in the p-type layer of the LED. The quenching of EL with temperature was attributed to the degradation of p-type conducting of the ZnO:(N,Al) layer.
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Appl. Phys. Lett. 91, 093117 (2007); http://dx.doi.org/10.1063/1.2778454 (3 pages) Online Publication Date: 29 August 2007
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A significant improvement of the efficiency of the ZnO nanowire (NW) dye-sensitized solar cell (DSSC) has been achieved by the chemical bath deposition of the dense nanoparticles (NPs) within the interstices of the vertical ZnO-NW anode. Impedance analyses of the electron transports in DSSCs reveal that the effective diffusion coefficient of an electron in the ZnO-NW array/NP composite anode falls between those in the ZnO-NW and TiO2-NP anodes. The superior performance of the ZnO-NW array/NP composite DSSC to the ZnO-NW cell is mainly ascribed to the enrichment of the light harvesting without significantly sacrificing the electron transport efficiency.
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Two laterally arranged quantum dot systems with strong capacitive interdot coupling Appl. Phys. Lett. 91, 102101 (2007); http://dx.doi.org/10.1063/1.2778542 (3 pages) Online Publication Date: 4 September 2007
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A method has been developed to form two quantum dot systems in lateral arrangement in a two-dimensional electron system of a GaAs–AlGaAs heterostructure with strong capacitive interdot coupling. In the authors’ design, the interdot capacitance can reach more than one-third of the single-dot capacitance while tunneling between the dots is excluded. This has been achieved by a floating metallic electrode covering both quantum dots, a method already used in split-gate designs before. Here, however, they have reduced the capacitive coupling of this floating gate to other electrodes in the surroundings by an etching technique to obtain a large interdot coupling.
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Microfluidic photonic crystal double heterostructures Appl. Phys. Lett. 91, 121103 (2007); http://dx.doi.org/10.1063/1.2785988 (3 pages) Online Publication Date: 17 September 2007
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We demonstrate postprocessed and reconfigurable photonic crystal double-heterostructure cavities via selective fluid infiltration. We experimentally investigate the microfluidic cavities via evanescent probing from a tapered fiber at telecommunication wavelengths. Fabry-Pérot fringes associated with modes of the induced cavity are in good agreement with the theory. We also demonstrate a cavity with quality factor Q = 4300. Our defect-writing technique does not require nanometer-scale alterations in lattice geometry and may be undertaken at any time after photonic crystal waveguide fabrication.
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Appl. Phys. Lett. 91, 091106 (2007); http://dx.doi.org/10.1063/1.2776980 (3 pages) Online Publication Date: 27 August 2007
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The fabrication process of photonic crystals in a p‐GaN layer was established to improve the light extraction efficiency of light-emitting diodes (LEDs) by using nanoimprint lithography and inductively coupled plasma (ICP) etching process. Due to low etch selectivity of imprinted pattern, Cr mask patterns were lifted-off from the p‐GaN surface and ICP etch process was followed using SiCl4-based plasma. As a result, two-dimensional pillar array patterns were uniformly fabricated on the p‐GaN layer and the photoluminescence intensity of the photonic crystal patterned LED was increased by 2.6 fold compared to that of the same LED sample without photonic crystal patterns.
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Crystalline structure’s influence on the near-field optical properties of single plasmonic nanowires Appl. Phys. Lett. 91, 123101 (2007); http://dx.doi.org/10.1063/1.2784389 (3 pages) Online Publication Date: 17 September 2007
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The finite difference time domain method is employed to study the crystalline structure’s influence on the propagation of a local excitation along metallic nanowires of subwavelength cross section. The metallic nanowires are elongated cylinders deposited on a transparent substrate. A tightly focused gaussian beam illuminates one end of the nanowires. According to recent experimental studies, the authors show that the propagation length of the localized surface plasmon excitations depends on the crystalline structure of the nanowire. Thus, they are able to determine the effective permittivity of metals in such a nanostructure versus its crystalline properties. The authors also demonstrate that the field of optical information transport could greatly benefit from the care of the subwavelength optical waveguide’s crystallinity.
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Via-hole-based vertical GaN light emitting diodes Appl. Phys. Lett. 91, 111106 (2007); http://dx.doi.org/10.1063/1.2785110 (3 pages) Online Publication Date: 11 September 2007
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A vertical GaN-light emitting diode (LED) has been fabricated on a sapphire substrate with periodic via holes formed by a laser drilling technique. n-contact metal which was deposited on the backside of sapphire substrate was directly connected with an Ohmic metal of n-GaN layer through the via holes. The via-hole-based vertical GaN-LED demonstrated an optical power improvement of up to 12.5% with lower forward operating voltage compared with a conventional GaN-LED. In addition, this vertical LED showed just 0.8% and 1.5% variations of optical power and operation voltage at the 500 h reliability test.
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Current-induced magnetic vortex core switching in a Permalloy nanodisk Appl. Phys. Lett. 91, 112501 (2007); http://dx.doi.org/10.1063/1.2780107 (3 pages) Online Publication Date: 10 September 2007
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The authors report on the switching of a magnetic vortex core in a submicron Permalloy disk, induced by a short current pulse applied in the film plane. Micromagnetic simulations including the adiabatic and nonadiabatic spin-torque terms are used to investigate the current-driven magnetization dynamics. They predict that a core reversal can be triggered by current bursts a tenth of a nanosecond long. The vortex core reversal process is found to be the same as when an external field pulse is applied. The control of a vortex core’s orientation using current pulses introduces the technologically relevant possibility to address individual nanomagnets within dense arrays.
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Organic light-emitting diodes with structured cathode Appl. Phys. Lett. 91, 093514 (2007); http://dx.doi.org/10.1063/1.2776018 (3 pages) Online Publication Date: 29 August 2007
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Organic light-emitting diode (OLED) devices were fabricated with a structured polymer-cathode interface. The devices have a layered structure indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/poly(2-methoxy-5-[(2′-ethylhexyl)oxy]-p-phenylenevinylene)/Al. The light-emitting polymer layers were patterned via a stamp imprinting process, followed by vapor deposition of the cathode. Devices employing the structured cathode showed improved electron injection and increased brightness when compared to conventional flat cathode devices. Electrostatic modeling of the structured cathode geometry revealed localized increase in electric field leading to enhanced electron tunneling through the polymer-cathode interface. Thus, improved OLED performance was demonstrated from cathode microstructuring.
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ZnO-based light-emitting metal-insulator-semiconductor diodes Appl. Phys. Lett. 91, 121113 (2007); http://dx.doi.org/10.1063/1.2786603 (3 pages) Online Publication Date: 19 September 2007
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The ZnO-based metal-insulator-semiconductor (MIS) diode was fabricated by using an insulator ZnO layer and an n-ZnO layer grown by radio frequency magnetron sputtering. The current-voltage of the ZnO MIS diodes showed a good diode characteristic with a threshold voltage of 8.9 V and a band-edge emission at 380 nm at room temperature. The electroluminescence emission of ZnO MIS was attributed to the generation of holes in the insulating ZnO layer at the high threshold voltage of 8.9 V via an impact ionization process.
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Appl. Phys. Lett. 91, 103507 (2007); http://dx.doi.org/10.1063/1.2775323 (3 pages) Online Publication Date: 4 September 2007
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The authors report on highly efficient phosphorescent organic light-emitting diodes (OLEDs) based on a low-molecular weight electron-conducting, bis-spirobifluorene host doped with a soluble derivative of the green emitter fac-tris(2-phenylpyridine) iridium (III) [Ir(ppy)3]. All organic layers were spin coated and a strong improvement of performance was achieved by introduction of a hole-transporting double layer based on cross-linkable low-molecular weight molecules. The devices combine the easy fabrication procedure known from polymer-based OLEDs with the higher efficiency of small molecules. Maximum luminous and power efficiencies of 59 cd/A and 58 lm/W, respectively, are obtained, combined with a low driving voltage and high efficiencies even at high brightnesses. At 1000 cd/m2 the efficiencies are as high as 55 cd/A and 49 lm/W.
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Resonant Raman scattering in hydrogen and nitrogen doped ZnO Appl. Phys. Lett. 91, 111903 (2007); http://dx.doi.org/10.1063/1.2783222 (3 pages) Online Publication Date: 11 September 2007
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Raman backscattering measurements were performed on single crystal ZnO for different excitation wavelengths before and after ion implantation with hydrogen and nitrogen. In addition to the formation of H- and N-related defects due to implantation, anomalous Raman modes were observed. Recently, the anomalous Raman modes have been attributed to the disorder-induced activation of silent modes. However, we will show that part of the observed modes are due to the resonantly enhanced longitudinal optical phonons.
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Highly directional emission from photonic crystals with a wide bandwidth Appl. Phys. Lett. 91, 121105 (2007); http://dx.doi.org/10.1063/1.2786590 (3 pages) Online Publication Date: 17 September 2007
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The authors numerically and experimentally demonstrated highly directional emission from photonic crystals. This was achieved by first splitting the incident electromagnetic wave into multiple beams using photonic crystal waveguide structures. The beams were then emitted out of the surface of a photonic crystal with the same phase, which resulted in a highly directional radiation pattern. The measured half power beam width was 4.8°, which was in good agreement with the calculated value of 4.1°. In contrast to the traditional beaming structures, their design did not involve gratinglike structures, which resulted in a wider operation bandwidth.
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Surface plasmon increase absorption in polymer photovoltaic cells Appl. Phys. Lett. 91, 113514 (2007); http://dx.doi.org/10.1063/1.2782910 (3 pages) Online Publication Date: 13 September 2007
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The authors demonstrate the triggering of surface plasmons at the interface of a metal grating and a photovoltaic bulk heterojunction blend of alternating polyfluorenes and a fullerene derivative. An increased absorption originating from surface plasmon resonances is confirmed by experimental reflection studies and theoretical modeling. Plasmonic resonances are further confirmed to influence the extracted photocurrent from devices. More current is generated at the wavelength position of the plasmon resonance peak. High conductivity polymer electrodes are used to build inverted sandwich structures with top anode and bottom metal grating, facilitating for triggering and characterization of the surface plasmon effects.
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Optical loss and interface morphology in AlGaAs/GaAs distributed Bragg reflectors Appl. Phys. Lett. 91, 101101 (2007); http://dx.doi.org/10.1063/1.2779242 (3 pages) Online Publication Date: 4 September 2007
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It is shown that n-type doping of AlGaAs/GaAs distributed Bragg reflectors (DBRs) grown by metal-organic vapor-phase epitaxy has a profound negative impact on the performance of vertical-cavity surface-emitting lasers (VCSELs) based on such mirrors. Using an intracavity contact scheme, 1.3-μm-range InGaAs VCSELs with and without doping in the bottom DBR are directly compared. Doped mirrors lead to lower slope efficiency, lower output power, and higher threshold current. From x-ray diffraction, high-accuracy reflectance measurements, and atomic force microscopy studies, it is suggested that this performance degradation is due to the doping-enhanced Al–Ga interdiffusion, leading to interface roughening and increased scattering loss.
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Donor/acceptor doping and electrical tailoring in ZnO quantum dots Appl. Phys. Lett. 91, 112110 (2007); http://dx.doi.org/10.1063/1.2784198 (3 pages) Online Publication Date: 12 September 2007
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The authors report on donor/acceptor doping in ZnO quantum dots (QDs) grown by a metal-organic chemical vapor deposition method. The Ga donor and N acceptor, as identified by x-ray photoelectron spectroscopy (XPS), are introduced into ZnO QDs. They demonstrate, with a combination of valence band XPS and scanning tunneling microscopy, that the electrical properties as well as Fermi level of the ZnO QDs can be well tuned by the donor/acceptor doping. In addition, photoluminescence from the ZnO QDs with quantum confinement effect is observed.
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Self-assembled biomimetic antireflection coatings Appl. Phys. Lett. 91, 101108 (2007); http://dx.doi.org/10.1063/1.2783475 (3 pages) Online Publication Date: 6 September 2007
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The authors report a simple self-assembly technique for fabricating antireflection coatings that mimic antireflective moth eyes. Wafer-scale, nonclose-packed colloidal crystals with remarkable large hexagonal domains are created by a spin-coating technology. The resulting polymer-embedded colloidal crystals exhibit highly ordered surface modulation and can be used directly as templates to cast poly(dimethylsiloxane) (PDMS) molds. Moth-eye antireflection coatings with adjustable reflectivity can then be molded against the PDMS master. The specular reflection of replicated nipple arrays matches the theoretical prediction using a thin-film multilayer model. These biomimetic films may find important technological application in optical coatings and solar cells.
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Organic electroluminescent diodes Appl. Phys. Lett. 51, 913 (1987); http://dx.doi.org/10.1063/1.98799 (3 pages)
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A novel electroluminescent device is constructed using organic materials as the emitting elements. The diode has a double‐layer structure of organic thin films, prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium‐tin‐oxide anode and an alloyed Mg:Ag cathode. Electron‐hole recombination and green electroluminescent emission are confined near the organic interface region. High external quantum efficiency (1% photon/electron), luminous efficiency (1.5 lm/W), and brightness (>1000 cd/m2) are achievable at a driving voltage below 10 V. |
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