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Top 20 Most Read Articles
April 2012
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 100, 122405 (2012); http://dx.doi.org/10.1063/1.3695168 (3 pages) Online Publication Date: 20 March 2012
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We have investigated the electric field effects in low resistance perpendicular magnetic tunnel junction (MTJ) devices and found that the electric field can effectively reduce the coercivity (Hc) of free layer (FL) by 30% for a bias voltage Vb = −0.2 V. In addition, the bias field (Hb) on free layer is almost linearly dependent on Vb yet independent on the device size. The demonstrated Vb dependences of Hc and Hb in low resistance MTJ devices present the potential to extend the scalability of the electric field assisted spin transfer torque magnetic random access memory and improve its access speed.
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Flexible solid-state paper based carbon nanotube supercapacitor Appl. Phys. Lett. 100, 104103 (2012); http://dx.doi.org/10.1063/1.3691948 (4 pages) Online Publication Date: 7 March 2012
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This paper presents a flexible solid-state supercapacitor of high energy density. The electrodes of the supercapacitor are made of porous and absorbent cotton paper coated with single-wall carbon nanotubes. To ensure all solid-state configuration, a solid-state polymer-based electrolyte (poly (vinyl alcohol)/phosphoric acid) is used. The as-fabricated supercapacitor can be charged to over 3 V. It has high specific capacitance and high energy density of 115.8301 F/g carbon and 48.8587 Wh/kg carbon. Its performance is comparable to that of commercial supercapacitors, which need to utilize liquid electrolytes. Flexible solid-state supercapacitors offer several significant advantages for use in hybrid electric vehicles.
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Appl. Phys. Lett. 100, 121108 (2012); http://dx.doi.org/10.1063/1.3695164 (4 pages) Online Publication Date: 20 March 2012
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We measured the relative arrival time between an optical pulse and a soft x-ray pulse from a free-electron laser. This femtosecond cross-correlation measurement was achieved by observing the change in optical reflectivity induced through the absorption of a fraction of the x-ray pulse. The main x-ray pulse energy remained available for an independent pump-probe experiment where the sample may be opaque to soft x-rays. The method was employed to correct the two-pulse delay data from a canonical pump-probe experiment and demonstrate 130 ± 20 fs (FWHM) temporal resolution. We further analyze possible timing jitter sources and point to future improvements.
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Low cost and high performance Al nanoparticles for broadband light trapping in Si wafer solar cells Appl. Phys. Lett. 100, 151101 (2012); http://dx.doi.org/10.1063/1.3703121 (4 pages) Online Publication Date: 9 April 2012
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In this paper low cost and earth abundant Al nanoparticles are simulated and compared with noble metal nanoparticles Ag and Au for plasmonic light trapping in Si wafer solar cells. It has been found tailored Al nanoparticles enable broadband light trapping leading to a 28.7% photon absorption enhancement in Si wafers, which is much larger than that induced by Ag or Au. Once combined with the SiNx anti-reflection coating, Al nanoparticles can produce a 42.5% enhancement, which is 4.3% higher than the standard SiNx due to the increased absorption in both the blue and near-infrared regions.
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Controlling light propagation with nanowires Appl. Phys. Lett. 100, 171903 (2012); http://dx.doi.org/10.1063/1.4704193 (3 pages) Online Publication Date: 23 April 2012
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We study the interaction of electromagnetic waves (EM) with metallic nanowire systems for frequencies not far from the surface plasma frequency (polaritonic range). We employ calculation and simulation to show that when excited at one end with axially polarized EM waves, nanowires can function as efficient waveguides of surface plasmon polaritons (SPPs). From the Fabry-Perot resonances of standing SPP waves, we study their dispersion relation and show that for a vanishing SPP wavelength it is identical to that for a planar metallic surface. Nanowire systems can be employed in various nanophotonic applications, and we assess this potential by studying propagation characteristics of these nano-waveguides and their interactions.
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Metal-oxide-semiconductor field effect transistor humidity sensor using surface conductance Appl. Phys. Lett. 100, 101603 (2012); http://dx.doi.org/10.1063/1.3691936 (3 pages) Online Publication Date: 7 March 2012
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This letter presents a metal-oxide-semiconductor field effect transistor based humidity sensor which does not use any specific materials to sense the relative humidity. We simply make use of the low pressure chemical vapor deposited (LPCVD) silicon dioxide’s surface conductance change. When the gate is biased and then floated, the electrical charge in the gate is dissipated through the LPCVD silicon dioxide’s surface to the surrounding ground with a time constant depending on the surface conductance which, in turn, varies with humidity. With this method, extremely high sensitivity was achieved—the charge dissipation speed increased thousand times as the relative humidity increased.
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Appl. Phys. Lett. 100, 161101 (2012); http://dx.doi.org/10.1063/1.4704189 (4 pages) Online Publication Date: 16 April 2012
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A two-step GaN barrier growth methodology was developed for InxGa1−xN/GaN multiple quantum well solar cells in which a lower temperature GaN cap layer was grown on top of the quantum wells (QWs) and then followed by a higher temperature GaN barrier layer. The performance of the solar cells improved markedly by increasing the low temperature GaN cap layer thickness from 1.5 to 3.0 nm. High-angle annular dark field scanning transmission electron microscopy and atom probe tomography measurements showed that increasing the GaN cap layer thickness improved the uniformity and increased the average indium content of the QWs.
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Low temperature processing solid-state dye sensitized solar cells Appl. Phys. Lett. 100, 113901 (2012); http://dx.doi.org/10.1063/1.3693399 (4 pages) Online Publication Date: 12 March 2012
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A study on low temperature processed solid state dye sensitized solar cell (LT-SDSC) is reported. The LT-SDSC uses a photoelectrode with a mesoporous TiO2 (mp-TiO2) film fabricated from a binder-free nanoparticle-TiO2 paste at room temperature, and a blocking layer of an amorphous TiO2 thin film deposited by atomic layer deposition (ALD) at 150 °C. A power conversion efficiency of 1.30% is obtained from the LT-SDSC with 0.9 μm mp-TiO2 layer and 20 nm ALD-TiO2 blocking layer, in cooperating with organic indoline dyes and a hole conductor, 2,2′,7,7′-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD). The lower electron conductivity of the low-temperature-processed mp-TiO2 film and the amorphous blocking layer is equilibrated by using smaller thicknesses of the films. Ways to further boost the LT-SDSC performance are proposed. These LT-SDSC are potentially compatible with low cost plastic substrates and show promising manufacturing potential for low cost flexible SDSCs.
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The critical growth velocity for planar-to-faceted interfaces transformation in SiGe crystals Appl. Phys. Lett. 100, 141601 (2012); http://dx.doi.org/10.1063/1.3698336 (4 pages) Online Publication Date: 2 April 2012
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Crystal-melt interface morphological transformation of differently oriented SiGe crystals with different Ge concentrations was observed, and the effect of Ge concentration on critical growth velocity (Vc) for the interface morphological transformation was investigated. A planar-to-faceted morphological transformation for the 〈110〉, 〈112〉, and 〈100〉 interfaces was observed. Vc for planar-to-faceted transformation of the 〈110〉, 〈112〉, and 〈100〉 interfaces decreases nonlinearly with increasing Ge concentration. SiGe faceted interfaces can be attributed to the fact that the perturbation induced in a planar interface was amplified when the constitutional undercooled zone was formed at high growth velocities.
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Tunable clover-shaped GaN photonic bandgap structures patterned by dual-step nanosphere lithography Appl. Phys. Lett. 100, 141101 (2012); http://dx.doi.org/10.1063/1.3698392 (5 pages) Online Publication Date: 2 April 2012
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The fabrication of close-packed clover-shaped photonic crystal structure on GaN by dual-step nanosphere lithography is demonstrated. By shrinkage of spheres prior to pattern transfer, a non-closed-packed clover-shaped photonic bandgap (PBG) structure, as designed by modified 3D finite-difference time-domain simulation, is also realized. The PBG of the close-packed and non-close-packed clover-shaped structures is verified through optical transmission spectroscopy, found to agree well with simulated results. A threefold enhancement in photoluminescence (PL) intensity is observed from the optimized structure, when the PBG is tuned to overlap with the emission band of the InGaN/GaN multi-quantum wells. From time-resolved PL measurements, shortened decay lifetimes are observed.
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Appl. Phys. Lett. 100, 133701 (2012); http://dx.doi.org/10.1063/1.3696019 (3 pages) Online Publication Date: 26 March 2012
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Microlens arrays on curvilinear surfaces are highly desirable for wide field-of-view imaging and sensing systems. However, it is technically challenging to fabricate these structures. This letter reports a simple method to machine close-packed microlenses on curvilinear surfaces as inspired by the insect eyes, which involves a femtosecond-laser-based microfabrication and a thermomechanical bending process. Over 7600 hexagonal-shaped microlenses with a diameter of 50 μm were fabricated on a hemispherical poly (methyl methacrylate) shell, which is similar to the compound eyes of insects. The optical performances of the microlens array were demonstrated by the abilities of high-resolution imaging and large view-angle focusing.
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Appl. Phys. Lett. 100, 153106 (2012); http://dx.doi.org/10.1063/1.3701594 (3 pages) Online Publication Date: 10 April 2012
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An epitaxial layer of graphene was grown on a pre patterned 6H-SiC(0001) crystal. The graphene smoothly covers the hexagonal nano-holes in the substrate without the introduction of small angle grain boundaries or dislocations. This is achieved by an elastic deformation of the graphene by ≈0.3% in accordance to its large elastic strain limit. This elastic stretching of the graphene leads to a modification of the band structure and to a local lowering of the electron group velocity of the graphene. We propose to use this effect to focus two-dimensional electrons in analogy to simple optical lenses.
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Advantages of GaN based light-emitting diodes with a p-InGaN hole reservoir layer Appl. Phys. Lett. 100, 141106 (2012); http://dx.doi.org/10.1063/1.3700722 (3 pages) Online Publication Date: 3 April 2012
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A p-type InGaN hole reservoir layer (HRL) was designed and incorporated in GaN based light-emitting diodes (LEDs) to enhance hole injection efficiency and alleviate efficiency droop. The fabricated LEDs with p-type HRL exhibited higher light output power, smaller emission energy shift and broadening as compared to its counterpart. Based on electrical and optical characteristics analysis and numerical simulation, these improvements are mainly attributed to the alleviated band bending in the last couple of quantum well and electron blocking layer, and thus better hole injection efficiency. Meanwhile, the efficiency droop can be effectively mitigated when the p-InGaN HRL was used.
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Shifts in plasmon resonance due to charging of a nanodisk array in argon plasma Appl. Phys. Lett. 100, 101903 (2012); http://dx.doi.org/10.1063/1.3673327 (3 pages) Online Publication Date: 5 March 2012
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A method for generating charge-induced plasmonic shifts, using argon plasma to charge nanoparticle arrays, is presented. Particles develop a negative charge, due to enhanced collisions with high-temperature electrons, in low-temperature plasmas. The negative charge generated causes a blue shift in the localized surface plasmon resonance. The dynamics of the shift were recorded and discussed. This effect could be used as a real-time method for studying the dynamics for charging in plasma.
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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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Optical coupling of surface plasmons between graphene sheets Appl. Phys. Lett. 100, 131111 (2012); http://dx.doi.org/10.1063/1.3698133 (4 pages) Online Publication Date: 27 March 2012
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In this letter, we theoretically investigate the coupling of far-infrared surface plasmon polaritons (SPPs) between spatially separated graphene sheets. By using the finite-difference frequency-domain method, we numerically illustrate the SPP propagation and modulation in the graphene sheets. The coupling of SPPs is employed to design zero insertion loss optical splitters, 1 × 2 digital optical spatial switches, and ultra-compact Mach-Zehnder interferometers with the arm length far below the diffraction limit. The study provides an effective way in designing graphene based high-speed and ultra-compact optoelectronic devices.
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Tunable terahertz optical antennas based on graphene ring structures Appl. Phys. Lett. 100, 153111 (2012); http://dx.doi.org/10.1063/1.3702819 (5 pages) Online Publication Date: 11 April 2012
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Highly tunable optical antennas in teraherz range based on graphene ring structures are proposed, which employ graphene plasmons instead of traditional metallic plasmons. The plasmon resonances of the perfect graphene ring (PGR) can be understood with the edge plasmons in graphene ribbons. While in the nonconcentric graphene ring, the multipolar plasmon modes appear and anti-symmetric mode splits due to symmetry breaking. Furthermore, the symmetric plasmon mode in a graphene ring can concentrate electromagnetic field with an enhancement factor as large as 103 in terahertz waveband, which is almost 20 times larger than a gold ring with the same size.
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Role of nitrogen vacancies in the luminescence of Mg-doped GaN Appl. Phys. Lett. 100, 142110 (2012); http://dx.doi.org/10.1063/1.3699009 (4 pages) Online Publication Date: 3 April 2012
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Defects may cause compensation or act as recombination centers in Mg-doped GaN. Using hybrid functional calculations, we investigate the effects of nitrogen vacancies (VN) and Mg-vacancy complexes (MgGa-VN) on the electrical and optical properties of GaN. We find that MgGa-VN are compensating centers in p-type but electrically inactive in n-type GaN. They give rise to a broad emission at 1.8 eV, explaining the red luminescence that is frequently observed in Mg-doped GaN, regardless of the Fermi level. Nitrogen vacancies are also compensating centers in p-type GaN and likely contribute to the yellow luminescence that has been observed in Mg-doped GaN.
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Multilayered graphene anode for blue phosphorescent organic light emitting diodes Appl. Phys. Lett. 100, 133304 (2012); http://dx.doi.org/10.1063/1.3697639 (4 pages) Online Publication Date: 27 March 2012
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In this work, we report on blue organic light emitting devices (OLEDs), which have multilayered graphene as its anode. Our graphene films have been grown catalytically and transferred to the support. The fabricated blue OLEDs with graphene anode showed outstanding external quantum efficiency of 15.6% and power efficiency of 24.1 lm/W at 1000 cd/m2. Weak oxygen plasma treatments on graphene film surfaces improved the injection property between the anode and hole injection layer.
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Electron drift-mobility measurements in polycrystalline CuIn1−xGaxSe2 solar cells Appl. Phys. Lett. 100, 103901 (2012); http://dx.doi.org/10.1063/1.3692165 (3 pages) Online Publication Date: 7 March 2012
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We report photocarrier time-of-flight measurements of electron drift mobilities for the p-type CuIn1−xGaxSe2 films incorporated in solar cells. The electron mobilities range from 0.02 to 0.05 cm2/Vs and are weakly temperature-dependent from 100–300 K. These values are lower than the range of electron Hall mobilities (2-1100 cm2/Vs) reported for n-type polycrystalline thin films and single crystals. We propose that the electron drift mobilities are properties of disorder-induced mobility edges and discuss how this disorder could increase cell efficiencies.
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