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Top 20 Most Read Articles
July 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 98, 262102 (2011); http://dx.doi.org/10.1063/1.3600787 (3 pages) Online Publication Date: 27 June 2011
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We report the electrical spin accumulation with enhanced bias voltage dependence in n-type Si, employing a crystalline CoFe/MgO tunnel contact. A sizable spin signal of ∼4.8 kΩμm2, a spin lifetime of ∼155 ps, and a spin diffusion length of ∼220 nm were obtained at 300 K. The spin signal and lifetime obtained in this system show consistent behavior with the temperature variation irrespective of the bias voltage. Notably, the spin signal exhibits nearly symmetric dependence with respect to the bias polarity, which is ascribed to the improved bias dependence of tunnel spin polarization.
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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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Improved endurance of resistive switching TiO2 thin film by hourglass shaped Magnéli filaments Appl. Phys. Lett. 98, 262901 (2011); http://dx.doi.org/10.1063/1.3600784 (3 pages) Online Publication Date: 27 June 2011
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A modified biasing scheme was adopted to improve the electrical endurance characteristics of conducting filamentary resistive switching (RS) in a Pt/TiO2/Pt RS cell. The modified bias scheme included the application of bias voltages with alternating polarity, even though RS proceeds in non-polar mode, which results in the stable distribution of each resistance states as well as improved endurance. This was attributed to the minimized consumption of oxygen ions in the TiO2 film, which can be induced by the formation of hourglass-shaped conducting filament (HSCF). The presence of a HSCF was confirmed by high-resolution transmission electron microscopy.
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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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Transparent conductive oxides: Plasmonic materials for telecom wavelengths Appl. Phys. Lett. 99, 021101 (2011); http://dx.doi.org/10.1063/1.3604792 (3 pages) Online Publication Date: 11 July 2011
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We show that despite of low loss, silver and gold are not suitable for a variety of nanoplasmonic applications in the infrared range, which require compact modes in single-interface plasmonic waveguides. At the same time, degenerate wide-band-gap semiconductors can serve as high-quality plasmonic materials at telecom wavelengths, combining fairly high compactness and relatively low loss. Their plasmonic properties in the near-infrared can be compared to those of gold in the visible range. The same materials can be used in a variety of non-plasmonic metamaterials applications, including transformation optics and invisibility cloaking.
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Homogeneous nano-patterning using plasmon-assisted photolithography Appl. Phys. Lett. 99, 011107 (2011); http://dx.doi.org/10.1063/1.3606505 (3 pages) Online Publication Date: 6 July 2011
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We report an innovative lithography system appropriate for fabricating sharp-edged nanodot patterns with nanoscale accuracy using plasmon-assisted photolithography. The key technology is two-photon photochemical reactions of a photoresist induced by plasmonic near-field light and the scattering component of the light in a photoresist film. The scattering component of the light is a radiation mode from higher order localized surface plasmon resonances scattered by metallic nanostructures.
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Advanced core/multishell germanium/silicon nanowire heterostructures: The Au-diffusion bottleneck Appl. Phys. Lett. 99, 023102 (2011); http://dx.doi.org/10.1063/1.3567932 (3 pages) Online Publication Date: 11 July 2011
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Synthesis of germanium/silicon (Ge/Si) core/shell nanowire heterostructures is typically accompanied by unwanted gold (Au) diffusion on the Ge nanowire sidewalls, resulting in rough surface morphology, undesired whisker growth, and detrimental performance of electronic devices. Here, we advance understanding of this Au diffusion on nanowires, its diameter dependence and its kinetic origin. We devise a growth procedure to form a blocking layer between the Au seed and Ge nanowire sidewalls leading to elimination the Au diffusion for in situ synthesis of high quality Ge/Si core/shell heterostructures.
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Influence of an Au capping layer on the magnetic properties of CoPt nanowires Appl. Phys. Lett. 98, 252507 (2011); http://dx.doi.org/10.1063/1.3601748 (3 pages) Online Publication Date: 24 June 2011
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Equiatomic CoPt nanowires with a gold cap layer have been fabricated by electrochemical template synthesis. Au improves the structural and magnetic properties of the L10 ordered CoPt, leading to coercivities of up to 1.43 T. The anisotropy constant K1 was estimated as 3.9(9) MJ m−3 from the approach to saturation, and four effective anisotropy constants were determined from analysis of torque curves in a 14 T field. Multisegmented magnetic nanowires are produced using gold interlayers.
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Appl. Phys. Lett. 98, 263101 (2011); http://dx.doi.org/10.1063/1.3587576 (3 pages) Online Publication Date: 27 June 2011
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The authors report the fabrication of heterojunction light emitting diodes (LEDs) based on two-dimensional (2D) hexagonal ordered n-type ZnO nanomesh and p-type GaN. The 2D ZnO nanomesh array was prepared by employing polystyrene spheres as a template. When a forward bias was applied to the LED, a strong ultraviolet (UV) electroluminescence peaked at 385 nm can be observed. The peak deconvolution revealed three emission peaks at 370, 388, and 420 nm. The origin of these emission peaks will be discussed. In addition, the LED was capable of exciting a red phosphor to convert UV light into red light.
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A circular dielectric grating for vertical extraction of single quantum dot emission Appl. Phys. Lett. 99, 041102 (2011); http://dx.doi.org/10.1063/1.3615051 (3 pages) Online Publication Date: 25 July 2011
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We demonstrate a nanostructure composed of partially etched annular trenches in a suspended GaAs membrane, designed for efficient and moderately broadband (≈5 nm) emission extraction from single InAs quantum dots. Simulations indicate that a dipole embedded in the nanostructure center radiates upward into free space with a nearly Gaussian far field, allowing a collection efficiency >80% with a high numerical aperture (NA = 0.7) optic and with ≈12× Purcell radiative rate enhancement. Fabricated devices exhibit a ≈10% photon collection efficiency with a NA = 0.42 objective, a 20× improvement over quantum dots in unpatterned GaAs. A fourfold exciton lifetime reduction indicates moderate Purcell enhancement.
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Appl. Phys. Lett. 99, 013304 (2011); http://dx.doi.org/10.1063/1.3607478 (3 pages) Online Publication Date: 7 July 2011
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We report increased cell performance for boron subphthalocyanine chloride (SubPc)/fullerene (C60) organic photovoltaic (OPV) cells when thermally evaporated vanadium oxide (V2OX) thin films are incorporated as a hole-extracting layer at the indium-tin oxide (ITO)/SubPc interface. Ultra-violet photoemission spectroscopy (UPS) studies of the V2OX films reveal highly n-type character, with a large work function of 6.8 eV. This correlates well with recently reported data for other metal oxide hole-extracting layers, such as molybdenum oxide and tungsten oxide, in contrast to the p-type character previously reported for V2OX films. There is significant improvement in energy level alignment for hole-extraction when cells utilise the V2OX layer at the ITO/SubPc interface, resulting in substantial increases in open circuit voltage (VOC) and power conversion efficiency (ηp).
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Capacitance-voltage characteristics of organic Schottky diode with and without deep traps Appl. Phys. Lett. 99, 023301 (2011); http://dx.doi.org/10.1063/1.3607955 (3 pages) Online Publication Date: 11 July 2011
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Capacitance based spectroscopic techniques have been used to characterize defects in organic Schottky diode based on copper phthalocyanine. Deep traps in organic thin films introduced by varying growth conditions have been identified and characterized by voltage and temperature dependence of capacitance. These results are interpreted using a consistent modelling of capacitance of organic Schottky diode with and without deep traps.
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Revealing the grain structure of graphene grown by chemical vapor deposition Appl. Phys. Lett. 99, 023104 (2011); http://dx.doi.org/10.1063/1.3610941 (3 pages) Online Publication Date: 13 July 2011
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The physical processes occurring in the presence of disorder: point defects, grain boundaries, etc. may have detrimental effects on the electronic properties of graphene. Here we present an approach to reveal the grain structure of graphene by the selective oxidation of defects and subsequent atomic force microscopy analysis. This technique offers a quick and easy alternative to different electron microscopy and diffraction methods and may be used to give quick feedback on the quality of graphene samples grown by chemical vapor deposition.
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Omnidirectional absorption enhancement in hybrid waveguide-plasmon system Appl. Phys. Lett. 98, 261101 (2011); http://dx.doi.org/10.1063/1.3603934 (3 pages) Online Publication Date: 27 June 2011
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We investigate the omnidirectional absorption enhancement induced by the excitation of the localized surface plasmon in the hybrid system consisting of a gold nanowire array embedded in a slab waveguide. Assisted by the waveguide layer, the hybrid system can support the localized waveguide-plasmon resonances for a wide range of incident angles. Theoretical and experimental results are both presented to demonstrate the omnidirectional absorption enhancement which could find important applications on plasmonic-assisted photovoltaic devices or photodetectors.
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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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Investigation of structural and electronic properties of graphene oxide Appl. Phys. Lett. 99, 013104 (2011); http://dx.doi.org/10.1063/1.3607305 (3 pages) Online Publication Date: 5 July 2011
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The local atomic structure of graphene oxide has been probed using synchrotron radiations. Detailed investigations of recently proposed simplistic model of graphene oxide using x-ray absorption near edge spectroscopy have been performed. X-ray diffraction measurements and calculations indicate loss of coherence between graphene-like layers. However, larger in-plane structural coherence is understood to be present. Selected area electron diffraction measurements indicate the presence of graphitic regions in graphene oxide which is expected to produce interesting confinement effects in graphene oxide which could be important for the development of tunable electronic and photonic devices.
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Appl. Phys. Lett. 99, 013302 (2011); http://dx.doi.org/10.1063/1.3607481 (3 pages) Online Publication Date: 6 July 2011
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It is a matter of controversy why excitons can efficiently dissociate into free carriers at an intrinsic polymer/fullerene interface, despite the strong Coulomb interaction between the charges provided by the very low dielectric constant in organic materials. The effect has been ascribed to the presence of intrinsic dipoles on the polymer/fullerene interface, though assuming an unrealistically small carrier effective mass necessary for exciton dissociation. We improve the model showing that it allows realistic carrier effective masses. The dissociation probability is calculated as a function of electric field acting on the dissociating electron-hole pairs.
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Highly efficient light emission from stacking faults intersecting nonpolar GaInN quantum wells Appl. Phys. Lett. 99, 011901 (2011); http://dx.doi.org/10.1063/1.3607301 (3 pages) Online Publication Date: 5 July 2011
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We report on the optical properties of m-plane GaInN/GaN quantum wells (QWs). We found that the emission energy of GaInN QWs grown on m-plane SiC is significantly lower than on non-polar bulk GaN, which we attribute to the high density of stacking faults. Temperature and power dependent photoluminescence reveals that the GaInN QWs on SiC have almost as large internal quantum efficiencies as on bulk GaN despite the much higher defect density. Our results indicate that quantum-wire-like features formed by stacking faults intersecting the quantum wells provide a highly efficient light emission completely dominating the optical properties of the structures.
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Domain wall induced magnetoresistance in a superconductor/ferromagnet nanowire Appl. Phys. Lett. 99, 032501 (2011); http://dx.doi.org/10.1063/1.3610947 (3 pages) Online Publication Date: 18 July 2011
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In a nanowire consisting of a ferromagnet/insulator/superconductor multilayer structure, the superconductivity is shown to depend strongly on the configuration of the magnetic domain walls in the neighboring ferromagnetic layer, yielding a high magnetoresistance within a temperature range near the superconducting transition temperature TC. Micromagnetic simulations confirmed that out-of-plane stray magnetic fields from uncompensated magnetic poles play a dominant role in inducing magnetoresistance in this particular system.
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Appl. Phys. Lett. 99, 012502 (2011); http://dx.doi.org/10.1063/1.3605564 (3 pages) Online Publication Date: 5 July 2011
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Magnetic anisotropy of Co40Fe40B20 thin films sandwiched between Ta and MgO layers was investigated. Magnetic properties of CoFeB layers deposited on top and bottom of MgO layer are different. The thickness of the CoFeB layer and annealing temperature are the critical parameters to achieve and keep the perpendicular magnetic anisotropy. The phase diagram of perpendicular anisotropic strength of CoFeB layers on annealing temperatures and thicknesses of CoFeB layers is observed. According to phase diagrams, perpendicular CoFeB/MgO/CoFeB tunnel junctions were fabricated, and tunneling magnetoresistance (TMR) ratio was higher than 30% at low temperatures.
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