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Top 20 Most Read Articles
December 2009
The 20 articles with the most full-text downloads during the month, in descending order.
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Stimulated emission of near-infrared radiation by current injection into silicon (100) quantum well Appl. Phys. Lett. 95, 241101 (2009); http://dx.doi.org/10.1063/1.3273367 (3 pages) Online Publication Date: 14 December 2009
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We describe the observation of stimulated emissions by current injections into a silicon quantum well. The device consists of a free standing membrane with a distributed feedback resonant cavity fabricated by state-of-the-art silicon processes. The emission spectra have multimode structures peaked in the near-infrared region above the submilliampere threshold currents at room temperatures. Consequently, electronics and photonics should be able to be converged on chips by using silicon quantum well laser diodes.
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Appl. Phys. Lett. 95, 231102 (2009); http://dx.doi.org/10.1063/1.3269593 (3 pages) Online Publication Date: 7 December 2009
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Photonic crystal back-reflectors offer enhanced optical absorption in thin-film solar cells, without undesirable losses. We fabricated metallic photonic crystal back-reflectors using photolithography and reactive-ion etching and deposited a-Si:H solar cells. The photonic crystal has triangular lattice symmetry, a pitch of 760 nm, and was designed with rigorous simulations. Scanning electron microscopy demonstrates excellent long range periodicity and conformal a-Si:H growth. The average light absorption increases by 7%, relative to a flat reference device, with an enhancement factor approaching 6 at near-infrared wavelengths. The photonic crystal back reflector strongly diffracts light and increases optical path lengths of solar photons.
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Nonvolatile resistive switching in graphene oxide thin films Appl. Phys. Lett. 95, 232101 (2009); http://dx.doi.org/10.1063/1.3271177 (3 pages) Online Publication Date: 7 December 2009
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Reliable and reproducible resistive switching behaviors were observed in graphene oxide (GO) thin films prepared by the vacuum filtration method. The Cu/GO/Pt structure showed an on/off ratio of about 20, a retention time of more than 104 s, and switching threshold voltages of less than 1 V. The switching effect could be understood by considering the desorption/absorption of oxygen-related groups on the GO sheets as well as the diffusion of the top electrodes. Our experiments indicate that GO is potentially useful for future nonvolatile memory applications.
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Mechanics of hemispherical electronics Appl. Phys. Lett. 95, 181912 (2009); http://dx.doi.org/10.1063/1.3256185 (3 pages) Online Publication Date: 5 November 2009
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A simple analytical model is established for the development of hemisphere electronics, which has many important applications in electronic-eye cameras and related curvilinear systems. The photodetector arrays, made in planar mesh layouts with conventional techniques, are deformed and transferred onto a hemisphere. The model gives accurately the positions of photodetectors on the hemisphere, and has been validated by experiments and finite element analysis. The results also indicate very small residual strains in the photodetectors. The model provides a tool to define a pattern of photodetectors in the planar, as-fabricated layout to yield any desired spatial configuration on the hemisphere.
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Why nitrogen cannot lead to p-type conductivity in ZnO Appl. Phys. Lett. 95, 252105 (2009); http://dx.doi.org/10.1063/1.3274043 (3 pages) Online Publication Date: 22 December 2009
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Based on electronic structure and atomic size considerations, nitrogen has been regarded as the most suitable impurity for p-type doping in ZnO. However, numerous experimental efforts by many different groups have not resulted in stable and reproducible p-type material, casting doubt on the efficacy of nitrogen as a shallow acceptor. Based on advanced first-principles calculations we find that nitrogen is actually a deep acceptor, with an exceedingly high ionization energy of 1.3 eV, and hence cannot lead to hole conductivity in ZnO. In light of this result, we reexamine prior experiments on nitrogen doping of ZnO.
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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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All-optical conditional logic with a nonlinear photonic crystal nanocavity Appl. Phys. Lett. 95, 221102 (2009); http://dx.doi.org/10.1063/1.3265736 (3 pages) Online Publication Date: 30 November 2009
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We demonstrate tunable frequency-converted light mediated by a χ(2) nonlinear photonic crystal nanocavity. The InP-based cavity supports two closely spaced localized modes near 1550 nm, which are resonantly excited by a 130 fs laser pulse. The cavity is simultaneously irradiated with a nonresonant probe beam, giving rise to rich second-order scattering spectra showing nonlinear mixing of the different resonant and nonresonant components. We highlight the radiation at the sum frequencies of the probe beam and the respective cavity modes. This would be a useful, minimally invasive monitor of the joint occupancy state of multiple cavities in an integrated optical circuit.
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Low voltage blue-phase liquid crystal displays Appl. Phys. Lett. 95, 231101 (2009); http://dx.doi.org/10.1063/1.3271771 (3 pages) Online Publication Date: 7 December 2009
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A protrusion electrode structure is proposed to dramatically lower the operation voltage of the emerging blue-phase liquid crystal displays (BP-LCDs). Simulation results indicate that the generated horizontal electric field is not only strong but also penetrates deeply into the bulk LC layer. As a result, a low voltage ( ∼ 10 Vrms) and reasonably high transmittance ( ∼ 70%) BP-LCD can be achieved. This approach enables the BP-LCDs to be addressed by amorphous silicon thin-film transistors (TFTs). Widespread application of TFT BP-LCDs is foreseeable.
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Appl. Phys. Lett. 95, 233302 (2009); http://dx.doi.org/10.1063/1.3270105 (3 pages) Online Publication Date: 7 December 2009
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Energy distributions [density-of-states (DOS)] of defects in the effective band gap of organic bulk heterojunctions are determined by means of capacitance methods. The technique consists of calculating the junction capacitance derivative with respect to the angular frequency of the small voltage perturbation applied to thin film poly(3-hexylthiophene) (P3HT) [6,6]-phenyl C61-butyric acid methyl ester (PCBM) solar cells. The analysis, which was performed on blends of different composition, reveals the presence of defect bands exhibiting Gaussian shape located at E ≈ 0.38 eV above the highest occupied molecular orbital level of the P3HT. The disorder parameter σ, which accounts for the broadening of the Gaussian DOS, lies within the range of 49–66 meV. The total density of defects results of order 1016 cm−3.
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Graphene field effect transistors with parylene gate dielectric Appl. Phys. Lett. 95, 242104 (2009); http://dx.doi.org/10.1063/1.3273396 (3 pages) Online Publication Date: 14 December 2009
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We report the fabrication and characterization of graphene field effect transistors with parylene back gate and exposed graphene top surface. A back gate stack of 168 nm parylene on 94 nm thermal silicon oxide permitted optical reflection microscopy to be used for identifying exfoliated graphene flakes. Room temperature mobilities of 10 000 cm2/Vs at 1012/cm2 electron/hole densities were observed in electrically contacted graphene. Parylene gated devices exhibited stable neutrality point gate voltage under ambient conditions and less hysteresis than that observed in graphene flakes directly exfoliated on silicon oxide.
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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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Electroluminescence from n-ZnO nanowires/p-GaN heterostructure light-emitting diodes Appl. Phys. Lett. 95, 223101 (2009); http://dx.doi.org/10.1063/1.3263720 (3 pages) Online Publication Date: 2 December 2009
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The investigation explores the fabrication and characteristics of ZnO nanowire (NW)/p-GaN/ZnO NW heterojunction light-emitting diodes (LEDs). Vertically aligned ZnO NWs arrays were grown on the p-GaN substrate. The n-p-n heterojunction LED was fabricated by combining indium tin oxide/glass substrate with the prepared ZnO NWs/p-GaN substrate. The symmetrical rectifying behavior demonstrates that the heterostructure herein was formed with two p-n junction diodes and connected back to back. The room-temperature electroluminescent emission peak at 415 nm was attributed to the band offset at the interface between n-ZnO and p-GaN and defect-related emission from ZnO and GaN. Finally, the photograph indicated the LED clearly emitted blue light.
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Multi-beam multi-wavelength semiconductor lasers Appl. Phys. Lett. 95, 161108 (2009); http://dx.doi.org/10.1063/1.3253713 (3 pages) Online Publication Date: 23 October 2009
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Multibeam emission and spatial wavelength demultiplexing in semiconductor lasers by patterning their facets with plasmonic structures is reported. Specifically, a single-wavelength laser was made to emit beams in two directions by defining on its facet two metallic gratings with different periods. The output of a dual-color laser was spatially separated according to wavelength by using a single metallic grating. The designs can be integrated with a broad range of active or passive optical components for applications such as interferometry and demultiplexing.
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Observation of the single-electron regime in a highly tunable silicon quantum dot Appl. Phys. Lett. 95, 242102 (2009); http://dx.doi.org/10.1063/1.3272858 (3 pages) Online Publication Date: 14 December 2009
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We report on low-temperature electronic transport measurements of a silicon metal-oxidesemiconductor quantum dot, with independent gate control of electron densities in the leads and the quantum dot island. This architecture allows the dot energy levels to be probed without affecting the electron density in the leads and vice versa. Appropriate gate biasing enables the dot occupancy to be reduced to the single-electron level, as evidenced by magnetospectroscopy measurements of the ground state of the first two charge transitions. Independent gate control of the electron reservoirs also enables discrimination between excited states of the dot and density of states modulations in the leads.
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The interlayer screening effect of graphene sheets investigated by Kelvin probe force microscopy Appl. Phys. Lett. 95, 222107 (2009); http://dx.doi.org/10.1063/1.3269597 (3 pages) Online Publication Date: 2 December 2009
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We report on the interlayer screening effect of graphene using Kelvin probe force microscopy (KPFM). By using a gate device configuration that enables the supply of electronic carriers in graphene sheets, the vertical screening properties were studied from measuring the surface potential gradient. The results show layer-dependence of graphene sheets, as the number of graphene layers increases, the surface potential decreases exponentially. In addition, we calculate the work function-related information of the graphene layers using KPFM.
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Appl. Phys. Lett. 95, 231105 (2009); http://dx.doi.org/10.1063/1.3271520 (3 pages) Online Publication Date: 8 December 2009
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We have observed optical birefringence in liquids induced by single-cycle terahertz pulses with field strengths exceeding 100 kV/cm. The induced change in polarization is proportional to the square of the terahertz electric field. The time-dependent terahertz Kerr signal is composed of a fast electronic response that follows the individual cycles of the electric field and a slow exponential response associated with molecular orientation.
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Coupling of light from an optical fiber taper into silver nanowires Appl. Phys. Lett. 95, 221109 (2009); http://dx.doi.org/10.1063/1.3270530 (3 pages) Online Publication Date: 3 December 2009
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We report the coupling of photons from an optical fiber taper to surface plasmon modes of silver nanowires. The coupling efficiency can be modulated by adjusting the cross angle and the polarization of the input light. The launch of propagating plasmons can be realized not only at ends of the nanowires but also at the midsection. In addition, we present the coupling of light into multiple nanowires from a single optical fiber taper simultaneously. Our demonstration offers an efficient method for optimizing plasmon coupling into nanoscale metallic waveguides and promotes the realization of highly integrated plasmonic devices.
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Organic vapor phase deposition for the growth of large area organic electronic devices Appl. Phys. Lett. 95, 233305 (2009); http://dx.doi.org/10.1063/1.3271797 (3 pages) Online Publication Date: 11 December 2009
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We demonstrate that material utilization efficiencies of 50% and deposition nonuniformities ≤ 2.5% are achievable over substrate diameters of 200 mm using a simplified, organic vapor phase deposition (OVPD) system. The OVPD system is used to demonstrate doped electrophosphorescent organic light emitting diodes whose performance is comparable to those grown by vacuum thermal evaporation. Through continuum modeling, we demonstrate that analogous systems whose chamber dimensions are comparable to the substrate width are scalable to substrate sizes of at least 1500×1800 mm2 with deposition nonuniformities between 1.5% and 2.5%. These results indicate that OVPD is useful in the large area deposition of displays, lighting, and other organic electronic devices.
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Fabrication of high efficiency III-V quantum nanostructures at low thermal budget on Si Appl. Phys. Lett. 95, 241102 (2009); http://dx.doi.org/10.1063/1.3273860 (3 pages) Online Publication Date: 14 December 2009
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We fabricate high efficiency GaAs/AlGaAs quantum nanostructure active layer for intersubband detectors and light emitting devices on a silicon substrate. The whole process of formation of the GaAs/AlGaAs active layer was realized via droplet epitaxy and migration enhanced epitaxy maintaining the growth temperature ⩽ 350 °C, thus resulting in a low thermal budget procedure compatible with back-end integration of the fabricated materials on integrated circuits. The realized quantum nanostructures show optical efficiencies comparable to those achievable with state of the art quantum dot materials grown on GaAs substrates.
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Low temperature growth of epitaxial graphene on SiC induced by carbon evaporation Appl. Phys. Lett. 95, 231907 (2009); http://dx.doi.org/10.1063/1.3265916 (3 pages) Online Publication Date: 8 December 2009
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Low temperature growth of epitaxial graphene on SiC is facilitated by carbon evaporation under ultrahigh vacuum (UHV) conditions. By counteracting the need for complete Si depletion as in the conventional sublimation method, monolayer graphene evolves at significantly lower temperatures by depositing additional carbon, so that a degradation of the initial SiC surface quality can be avoided. The original, well ordered terrace structure of SiC(0001) is preserved, the graphene layers grow on top and show the typical linear π-band dispersion. On SiC(000
 ) the graphene lattice is rotated by 30° in comparison to the conventional UHV preparation method. |
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