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Top 20 Most Read Articles
July 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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Enhanced outcoupling from organic light-emitting diodes using aperiodic dielectric mirrors Appl. Phys. Lett. 90, 241112 (2007); http://dx.doi.org/10.1063/1.2748859 (3 pages) Online Publication Date: 14 June 2007
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Aperiodic dielectric stacks between the substrate and transparent anode in organic light-emitting diodes are used to improve the optical outcoupling efficiency. The authors demonstrate that a nine-layer SiO2/SiNx aperiodic dielectric stack improves the brightness by 80% within a 60° viewing cone for a red-emitting organic light-emitting diode, while maintaining a Lambertian emission pattern. As the refractive index contrast between the two materials used in a two-component multilayer dielectric stack is increased, a brightness improvement of 170% in a 60° viewing cone is achievable while maintaining a Lambertian emission profile.
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Directed assembly of nanowire contacts using electrodeposition Appl. Phys. Lett. 91, 033106 (2007); http://dx.doi.org/10.1063/1.2757609 (3 pages) Online Publication Date: 17 July 2007
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A maskless process for the directed assembly of Ni contacts to Si nanowires on prepatterned electrodes is reported. Microarrays of thin Au/Cr electrodes were lithographically formed on oxidized Si substrates followed by electric-field assisted alignment of Si nanowires between the electrodes. The nanowire ends were then embedded in Ni by selective electrodeposition over the prepatterned electrodes. Annealing to 300 °C provided good electrical contacts for transport through the doped nanowires. This approach provides a parallel, maskless method to establish metal contacts to the nanowires without the need of high resolution electron beam lithography for electrical and mechanical applications.
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Electrophotoluminescence of ZnO film Appl. Phys. Lett. 91, 021105 (2007); http://dx.doi.org/10.1063/1.2753760 (3 pages) Online Publication Date: 10 July 2007
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The electric-field-controlled photoluminescence (PL), i.e., electrophotoluminescence (EPL) of ZnO film has been investigated via a ZnO-based metal-insulator-semiconductor (MIS) structure on a silicon substrate applied with different biases. Compared with the PL of ZnO film in the case where there is no bias on the MIS structure, the positive bias with negative voltage applied on silicon substrate significantly enhances the near-band-edge ultraviolet emission while suppressing the deep-level-related visible emissions, whereas the negative bias hardly changes the PL of ZnO film. The mechanism for EPL of ZnO film is proposed in terms of the electric-field effect on the bending of energy bands of ZnO.
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40% efficient metamorphic GaInP/GaInAs/Ge multijunction solar cells Appl. Phys. Lett. 90, 183516 (2007); http://dx.doi.org/10.1063/1.2734507 (3 pages) Online Publication Date: 4 May 2007
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An efficiency of 40.7% was measured and independently confirmed for a metamorphic three-junction GaInP/GaInAs/Ge cell under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0 W/cm2, AM1.5D, low aerosol optical depth, 25 °C). This is the initial demonstration of a solar cell with over 40% efficiency, and is the highest solar conversion efficiency yet achieved for any type of photovoltaic device. Lattice-matched concentrator cells have now reached 40.1% efficiency. Electron-hole recombination mechanisms are analyzed in metamorphic GaxIn1−xAs and GaxIn1−xP materials, and fundamental power losses are quantified to identify paths to still higher efficiencies.
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Lasing characteristics of a GaN photonic crystal nanocavity light source Appl. Phys. Lett. 91, 041101 (2007); http://dx.doi.org/10.1063/1.2759467 (3 pages) Online Publication Date: 23 July 2007
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Lasing characteristics from photonic crystal defects fabricated on bulk GaN are investigated. The device demonstrates multimode lasing with linewidths as narrow as 2–3 Å, and an enhanced spontaneous emission factor β ∼ 0.045. The emission spectra indicate that the laser emission is initiated horizontally in the defect nanocavity and then coupled to the vertical radiation, possibly via photonic crystal Bloch modes or by scattering.
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Plasmonic very-small-aperture lasers Appl. Phys. Lett. 91, 021103 (2007); http://dx.doi.org/10.1063/1.2755784 (3 pages) Online Publication Date: 9 July 2007
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The fabrication of plasmonic very-small-aperture lasers is demonstrated in this letter. It is an integration of the surface plasmon structures and very-small-aperture lasers (VSAL). The experimental and numerical results demonstrate that the transmission field can be confined to a spot with subwavelength width in the far field, and the power output can be enhanced 140% of the normal VSAL. Such a device can be useful in the application of a high resolution far-field scanning optical microscope.
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Secondary growth of small ZnO tripodlike arms on the end of nanowires Appl. Phys. Lett. 91, 013106 (2007); http://dx.doi.org/10.1063/1.2752605 (3 pages) Online Publication Date: 3 July 2007
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Small zinc oxide tripodlike arms were assembled at the end of nanowires through thermal evaporation of zinc powder in a horizontal tube furnace. These arms are hundreds of nanometers in length, and they are tens of nanometers in diameter, similar to that of the based nanowires. High-resolution transmission electron microscopy investigation exhibits that they are wurtzite structure and have clear twin planes at the junction of the arms and the based nanowires. The growth mechanism, Raman, and photoluminescence properties were discussed.
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Where does the spin reside in ferromagnetic Cu-doped ZnO? Appl. Phys. Lett. 91, 012501 (2007); http://dx.doi.org/10.1063/1.2751587 (3 pages) Online Publication Date: 2 July 2007
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The authors report a soft x-ray absorption spectroscopy and magnetic circular dichroism study of the ferromagnetism in pulsed-laser-deposited Cu-doped ZnO thin films, which display robust room-temperature ferromagnetic signatures using bulk magnetization probes. In this work, the authors probe the spin asymmetries in the Cu 3d, O 2p, and Zn 3d states individually. They find a paramagnetic component originating from the Cu 3d, and no magnetic signal in the O or Zn. They find no dichroic signal consistent with ferromagnetism originating from any of these states.
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Scanning photovoltage microscopy of potential modulations in carbon nanotubes Appl. Phys. Lett. 91, 031101 (2007); http://dx.doi.org/10.1063/1.2757100 (3 pages) Online Publication Date: 16 July 2007
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We use the locally generated photovoltage in carbon nanotubes to image potential modulations produced by defects. The correlation with spatially resolved spectroscopy of the disorder-enhanced Raman band allows us to locate the defects and to determine their effect on band bending. The defects produce local potential maxima, which are consistent with trapped electrons near the tube. An offset photovoltage is generated when the laser populates metastable traps in the oxide. Previously uncharged defects can thus be imaged. To demonstrate the versatility of the technique, we show that tube-tube junction at the base of nanotube loops can be electronically characterized.
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Appl. Phys. Lett. 91, 013502 (2007); http://dx.doi.org/10.1063/1.2753724 (3 pages) Online Publication Date: 2 July 2007
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Oxide-semiconductor-based thin-film transistors (TFTs), particularly the amorphous ones, are becoming an important emerging technology. Since oxide semiconductors easily form polycrystalline phases, usually more complicated oxide mixtures are needed for growing amorphous phases. In this letter, we report that by simply reducing the thickness, ZnO can be intentionally grown into the amorphous phase. Furthermore, both top-gate and bottom-gate amorphous ZnO TFTs of micrometer scales were effectively implemented using fully lithographic and etching processes. Rather high field-effect mobilities of 25 and 4 cm2/V s and on∕off current ratios of >107 and >106 were achieved for top-gate and bottom-gate configurations, respectively.
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Surface-depletion controlled gas sensing of ZnO nanorods grown at room temperature Appl. Phys. Lett. 91, 032101 (2007); http://dx.doi.org/10.1063/1.2752541 (3 pages) Online Publication Date: 16 July 2007
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The authors report on surface-depletion controlled gas sensing ZnO nanorods. These nanorods were synthesized through a simple wet chemical route at room temperature. The diameter of nanorods is about 15 nm, which is close to two times of the Debye length of ZnO. In contrast to the previous report, the sensing is surface-depletion controlled rather than contact controlled and the sensitivity is up to 29.7 against 100 ppm ethanol. Such high sensitivity is due to an almost complete depletion of the rods in air, which is confirmed from photoluminescence spectrum and x-ray photoelectron spectroscopy. The gas sensing mechanism controlled by surface depletion provides another approach to realize high-performance gas sensors.
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Silicon-based light emitters fabricated by embedding Ge self-assembled quantum dots in microdisks Appl. Phys. Lett. 91, 011104 (2007); http://dx.doi.org/10.1063/1.2754356 (3 pages) Online Publication Date: 2 July 2007
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Silicon-based light emitters have been fabricated on silicon-on-insulator substrates by embedding Ge self-assembled quantum dots in microdisk resonators. Strong room-temperature resonant luminescence is observed from the microdisks under optical pumping. Photoluminescence from Ge quantum dots is significantly enhanced by the microdisk resonators. Different kinds of cavity modes, whispering-gallery modes, and Fabry-Pérot modes are observed in the wavelength range from 1.2 to 1.6 μm for microdisks with different diameters.
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Transit-time spin field-effect transistor Appl. Phys. Lett. 90, 262501 (2007); http://dx.doi.org/10.1063/1.2752015 (3 pages) Online Publication Date: 25 June 2007
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The authors propose and analyze a four-terminal metal-semiconductor device that uses hot-electron transport through thin ferromagnetic films to inject and detect a charge-coupled spin current transported through the conduction band of an arbitrary semiconductor. This provides the possibility of realizing a spin field-effect transistor in Si using electrostatic transit-time control of coherent spin precession in a perpendicular magnetic field.
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On the lattice parameters of GaN Appl. Phys. Lett. 91, 031911 (2007); http://dx.doi.org/10.1063/1.2753122 (3 pages) Online Publication Date: 19 July 2007
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The lattice parameters of low-defect density, undoped bulk GaN fabricated by hydride vapor phase epitaxy (HVPE) on (0001) sapphire and subsequent substrate removal, are precisely determined using high-resolution x-ray diffraction. The obtained values, c = 5.18523 Å and a = 3.18926 Å, are compared with the lattice parameters of freestanding HVPE GaN from different sources and found to be representative for state-of-the-art undoped HVPE bulk GaN material. A comparison with bulk GaN fabricated by the high-pressure technique and homoepitaxial GaN is made, and significant differences in the lattice parameters are found. The observed differences are discussed and a possible explanation is suggested.
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Optical quenching of NiO/Ni coated ZnO nanowires Appl. Phys. Lett. 91, 012102 (2007); http://dx.doi.org/10.1063/1.2753717 (3 pages) Online Publication Date: 2 July 2007
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Low temperature photoluminescence (PL) measurements were carried out to investigate optical transition characteristics of ZnO nanowires, Ni nanodot, and NiO coated ZnO nanowires (NWs). PL emission spectra show emission peaks of distinctive bound exciton to neutral donor (D0X) and to acceptor (A0X) transition. The authors found that the optical quenching was drastic with increasing temperature and the activation energies are unusually small. The major PL emission peak for ZnO NW was switched from D0X to A0X for Ni–ZnO nanodot NWs. This is due to the reason that diffused hydrogen atoms into ZnO NWs during thermal annealing played as acceptors.
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Room-temperature photoluminescence from ZnO/ZnMgO multiple quantum wells grown on Si(111) substrates Appl. Phys. Lett. 91, 022103 (2007); http://dx.doi.org/10.1063/1.2755922 (3 pages) Online Publication Date: 9 July 2007
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A set of ten-period ZnO/Zn0.85Mg0.15O multiple quantum wells with well thickness varying from 2.5 to 5 nm has been grown on Si(111) substrates by pulsed laser deposition. A periodic structure with sharp interfaces was observed by cross-sectional transmission electron microscopy. The room-temperature photoluminescence resulting from the well regions exhibits a significant blueshift with respect to the ZnO single layer. The well layer thickness dependence of the emission energy from the well regions was investigated and compared with a simple theoretical model. The results suggest that the quantum confinement effects in the quantum wells can be observed up to room temperature.
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Dielectric properties of single-walled carbon nanotubes in the terahertz frequency range Appl. Phys. Lett. 91, 011108 (2007); http://dx.doi.org/10.1063/1.2753747 (3 pages) Online Publication Date: 3 July 2007
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The authors report on the terahertz time-domain spectroscopy of single-walled carbon nanotubes (SWCNTs) dispersed in a polymer film in a broad frequency range from 0.2 to 20 THz. The complex dielectric function of the composite film is obtained without using Kramers-Kronig analysis, from which the dielectric function of SWCNTs is extracted by using the effective medium theory. The real part of the dielectric function exhibits very large values at low frequency below 2 THz. The conductivity spectrum shows a peak around 3 THz. The spectral behavior is consistent with overdamped Lorentz oscillator model, indicating the response of curvature-induced small-gap SWCNTs.
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High-efficiency nondoped white organic light-emitting devices Appl. Phys. Lett. 91, 023503 (2007); http://dx.doi.org/10.1063/1.2756137 (3 pages) Online Publication Date: 10 July 2007
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High-efficiency nondoped white organic light-emitting devices (WOLEDs) were demonstrated by using both the intrinsic and exciplex emissions from a single electroluminescent material, 4,4′,4″-trispyrenylphenylamine (TPyPA). The simple device structure of indium tin oxide/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine/TPyPA/4,7-diphenyl-1,10-phenanthroline/LiF/Al exhibited a luminance of 10 000 cd/m2 at a low driving voltage of 4.5 V, and high current and power efficiencies of 9.4 cd/A and 9.0 lm/W, respectively. Such WOLED showed excellent color stability and purity with the Commission Internationale de L’Eclairage coordinates of (0.31, 0.35), which remained unchanged over a wide range of luminance from 100 to 20 000 cd/m2.
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Organic molecular nanowires: N,N′-dimethylperylene-3,4,9,10-bis(dicarboximide) on KBr(001) Appl. Phys. Lett. 91, 023101 (2007); http://dx.doi.org/10.1063/1.2755879 (3 pages) Online Publication Date: 9 July 2007
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The growth of N,N′-dimethylperylene-3,4,9,10-bis(dicarboximide) (DiMe-PTCDI) on KBr(001) has been studied by frequency modulation atomic force microscopy. At low coverages, DiMe-PTCDI forms molecular wires with a length of up to 600 nm; at higher coverages, wire networks were found. The height of the wires is two or more molecular layers; if the sample is cooled to 250 K during evaporation, also wires of one layer are found. All wires grow along the [110] and [
 10] directions of the KBr(001) substrate. There is no wetting layer of molecules. Atomic resolution of the substrate could be achieved between the wires. Due to their size and shape, the molecular nanowires might act as a model system for organic electronics research on insulating substrates. |
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Broadband efficient second harmonic generation in media with a short-range order Appl. Phys. Lett. 91, 011101 (2007); http://dx.doi.org/10.1063/1.2754365 (3 pages) Online Publication Date: 2 July 2007
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The authors demonstrate broadband efficient quasiphase matched second harmonic generation (SHG) in nonlinear LiNbO3 crystals. The distribution of ferroelectric domains in a short-range order provides the possibility for broadband second harmonic generation in the visible range. This process is similar to the broadband SHG in random media, but it results in much higher conversion efficiency. The unique distribution of reciprocal vectors provided by the structural short-range order plays an important role in the enhancement. The achieved broadband SHG with high conversion efficiency will aid in the development of optics integration and multiple-channel devices.
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