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Top 20 Most Read Articles
February 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Imaging ellipsometry of graphene Appl. Phys. Lett. 97, 231901 (2010); http://dx.doi.org/10.1063/1.3524226 (3 pages) Online Publication Date: 6 December 2010
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Imaging ellipsometry studies of graphene on SiO2/Si and crystalline GaAs are presented. We demonstrate that imaging ellipsometry is a powerful tool to detect and characterize graphene on any flat substrate. Variable angle spectroscopic ellipsometry is used to explore the dispersion of the optical constants of graphene in the visible range with high lateral resolution. In this way, the influence of the substrate on graphene’s optical properties can be investigated.
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Appl. Phys. Lett. 98, 053103 (2011); http://dx.doi.org/10.1063/1.3549183 (3 pages) Online Publication Date: 1 February 2011
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We present a robust method for forming high quality ohmic contacts to graphene, which improves the contact resistance by nearly 6000 times compared to untreated metal/graphene interfaces. The optimal specific contact resistance for treated Ti/Au contacts is found to average <10−7 Ω cm2. Additionally, we examine Al/Au, Ti/Au, Ni/Au, Cu/Au, Pt/Au, and Pd/Au contact metallizations and find that most metallizations result in similar specific contact resistances in this work regardless of the work function difference between graphene and the metal overlayer. The results presented in this work serve as a foundation for achieving ultralow resistance ohmic contacts to graphene for high speed electronic and optoelectronic applications.
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Ultrathin Co/Pt and Co/Pd superlattice films for MgO-based perpendicular magnetic tunnel junctions Appl. Phys. Lett. 97, 232508 (2010); http://dx.doi.org/10.1063/1.3524230 (3 pages) Online Publication Date: 9 December 2010
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Ultrathin [Co/Pt]n and [Co/Pd]n superlattice films consisting of 0.14–0.20-nm-thick Co and Pt(Pd) layers were deposited by sputtering. A large perpendicular magnetic anisotropy [(3–9)×106 ergs/cm3] and an ideal square out-of-plane hysteresis loop were attained even for ultrathin superlattice films with a total thickness of 1.2–2.4 nm. The films were stable against annealing up to 370 °C. MgO-based perpendicular magnetic tunnel junctions with this superlattice layer as the free layer showed a relatively high magnetoresistance ratio (62%) and an ultralow resistance-area product (3.9 Ω μm2) at room temperature. The use of these films will enable the development of gigabit-scale nonvolatile memory.
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Kerr detection of acoustic spin transport in GaAs (110) quantum wells Appl. Phys. Lett. 97, 242110 (2010); http://dx.doi.org/10.1063/1.3524218 (3 pages) Online Publication Date: 16 December 2010
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Time-resolved Kerr reflectometry (TRKR) is used to investigate the long-range transport of spins by surface acoustic waves in undoped GaAs (110) quantum wells. TRKR measurements under an applied magnetic field demonstrate the coherent precession of the optically generated electron spin during acoustic transport over several micrometers and yield information about the relaxation processes for moving spins.
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Dynamic ripples in single layer graphene Appl. Phys. Lett. 98, 063101 (2011); http://dx.doi.org/10.1063/1.3551574 (3 pages) Online Publication Date: 7 February 2011
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Similar to the water wave on a pond caused by dropping a pebble, the formation of ripples is shown in a single layer graphene (SLG) when it is stroked by a C60 molecule, no matter whether the graphene is plane or corrugated. The controllable ripple in SLG is investigated by molecular dynamics simulations. Noticeable diffraction and interference of the ripples are observed. This study indicates that the ripple propagation in graphene can be used to detect defects.
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Role of Ru nano-dots embedded in TiO2 thin films for improving the resistive switching behavior Appl. Phys. Lett. 97, 232904 (2010); http://dx.doi.org/10.1063/1.3525801 (3 pages) Online Publication Date: 9 December 2010
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Ru nano-dots were embedded in a Pt/TiO2/Pt resistive switching cell to improve the uniformity of the switching parameters. The TiO2 film grown on the Ru nano-dots had a rutile structure locally whereas other parts of the TiO2 film had an anatase structure. The rutile-structured TiO2 formed conducting filaments easily and their rupture was much more uniform than the randomized ones in anatase TiO2. This largely improved the resistance uniformity at the reading voltage during the repeated resistance switching events. The improvement was also attributed to the high leakage current of the pristine sample at the reading voltage.
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Ultimate performance of polymer:fullerene bulk heterojunction tandem solar cells Appl. Phys. Lett. 98, 053301 (2011); http://dx.doi.org/10.1063/1.3549693 (3 pages) Online Publication Date: 1 February 2011
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We present the model calculations to explore the potential of polymer:fullerene tandem solar cells. As an approach we use a combined optical and electrical device model, where the absorption profiles are used as starting point for the numerical current-voltage calculations. With this model a maximum power efficiency of 11.7% for single cells has been achieved as a reference. For tandem structures with a ZnO/poly(3,4-ethylenedioxythiophene)/poly(styrenesulphonic acid) middle electrode an ultimate efficiency of 14.1% has been calculated. In the optimum configuration the subcell with the narrowest band gap is placed closest to the incoming light. Consequently, tandem structures are expected to enhance the performance of optimized single cells by about 20%.
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Appl. Phys. Lett. 98, 051106 (2011); http://dx.doi.org/10.1063/1.3552968 (3 pages) Online Publication Date: 3 February 2011
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We demonstrate the surface plasmon (SP) enhanced green light-emitting diodes (LEDs). The Au nanoparticles were embedded in the p-GaN of LEDs. The photoluminescence and electroluminescence measurements showed improved optical properties of LEDs with Au nanoparticles due to an increase in the spontaneous emission rate by resonance coupling between the excitons in multiple quantum wells and localized surface plasmons in Au nanoparticles. The optical output power of SP-enhanced green LEDs with Au nanoparticles was increased by 86% without showing degradation of the electrical characteristics of LEDs compared to LEDs without Au nanoparticles.
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Evaporative self-assembly of nanowires on superhydrophobic surfaces of nanotip latching structures Appl. Phys. Lett. 98, 073101 (2011); http://dx.doi.org/10.1063/1.3554360 (3 pages) Online Publication Date: 14 February 2011
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In this letter, we report a site-specific self-assembly of nanowires during the evaporation of a colloid droplet of nanowires on nanoengineered superhydrophobic surfaces. The self-assembly of nanowires is achieved by the interactions between nanowires and the superhydrophobic surface engineered with sharp-tip latching nanostructures of micropillars, provided by the convective hydrodynamic flow and the receding three-phase contact line of the evaporating droplet. The experimental results show that the spatial density of surface structures, the relative dimension of surface patterns to nanowires, and the morphology of tip surface influence the self-assembly and alignment of nanowires on the evaporative superhydrophobic surface.
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Appl. Phys. Lett. 98, 071102 (2011); http://dx.doi.org/10.1063/1.3554426 (3 pages) Online Publication Date: 14 February 2011
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We report on a self-organized nanoscale patterning method by using oblique angle deposition to enhance the light extraction in a GaInN light-emitting diode (LED). The method offers one-step processing with good controllability of the feature size and density of the nanopatterns by varying the deposition angle during oblique angle deposition, eliminating the need for photolithography and annealing. A 5-nm-thick silver (Ag) film, when deposited by using oblique angle deposition, spontaneously forms a nanoscale island-like morphology on the substrate. This method is used to texture p-type GaN with nanoscale features, which results in increased light extraction from a GaInN LED. At 100 mA, the nanotextured LED shows a 46% higher light output than a standard LED with unpatterned (planar) p-type GaN.
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Optical response of large scale single layer graphene Appl. Phys. Lett. 98, 071905 (2011); http://dx.doi.org/10.1063/1.3555425 (3 pages) Online Publication Date: 14 February 2011
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We have measured optical transmission and reflection spectra of large scale graphene grown by chemical vapor deposition technique over the extensive frequency range from far-IR to uv (4 meV–6.2 eV). Large scale graphene exhibits an excitonic absorption peak in the uv-region (ℏω = 4.6 eV) and the constant interband absorption with σ1(ω) = e2/4ℏ in the IR-visible region, respectively. In the far-IR range, Drude peak is observed, and its strength ωp,2d2 indicates the induced carrier density N2d = 1.95×1012 cm−2. These results are highly consistent with the theoretical prediction/experimental results of the single layer graphene. It proves that, contrary to the doubts about its quality due to the chemical growth process, the sample has single layer optical response over the entire photon energy; therefore, it can be applied to large scale devices such as terahertz-IR detector, solar cell material, and visible uv-transparent conductor.
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Addressable subwavelength grids of confined light in a multislotted nanoresonator Appl. Phys. Lett. 98, 081101 (2011); http://dx.doi.org/10.1063/1.3555489 (3 pages) Online Publication Date: 22 February 2011
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In this letter, we fabricate a multislotted optical nanoresonator with several spatial field distributions which are all addressable by the wavelength. The reported structure consists in an array of evanescently coupled single mode photonic crystal nanocavities. By using a scanning near-field optical microscope, we quantify the morphology of the different optical mode volumes and show that they consist in grids of light confined at the subwavelength scale.
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Charge and spin transport in graphene-based heterostructure Appl. Phys. Lett. 98, 053101 (2011); http://dx.doi.org/10.1063/1.3549154 (3 pages) Online Publication Date: 31 January 2011
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We investigate electron transport properties of a heterostructure based on zigzag graphene nanoribbon (ZGNR) by first-principles calculations. This heterostructure consists of hydrogen-terminated ZGNR (ZGNR-H) and oxygen-terminated ZGNR (ZGNR-O). We find that both charge and spin transport can be well controlled with the ZGNR-H/ZGNR-O heterostructure. A large charge transmission gap appears near the Fermi energy, and rectification behavior is observed. Moreover, the ZGNR-H/ZGNR-O heterostructure can act as a perfect bipolar spin filter or magnetoresistance device. Our results show that the ZGNR-H/ZGNR-O heterostructure hold promise for combining magnetoelectronics and conventional charge-based electronics.
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Spin current generation by adiabatic pumping in monolayer graphene Appl. Phys. Lett. 98, 032106 (2011); http://dx.doi.org/10.1063/1.3544581 (3 pages) Online Publication Date: 20 January 2011
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We propose a method of generating spin currents in monolayer graphene through adiabatic quantum pumping by two oscillating potentials. Spin splitting is induced in the graphene layer by ferromagnetic proximity. The pumped charge and spin currents are sensitive functions of the Fermi energy, which can thus be used to control the degree of polarization. The predicted pumped currents are measurable using the current technology. The proposed method is useful in the development of graphene spintronics.
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Imbalanced mobilities causing S-shaped IV curves in planar heterojunction organic solar cells Appl. Phys. Lett. 98, 063301 (2011); http://dx.doi.org/10.1063/1.3553764 (3 pages) Online Publication Date: 8 February 2011
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We show that S-kinks in the current voltage characteristics, which decrease the fill factor significantly, can be caused by a strong imbalance of charge carrier mobilities (hole mobility in donor and electron mobility in acceptor) in planar/flat heterojunction organic solar cells. Electrical simulations according to a drift-diffusion model predict the occurrence of an S-kink for a mobility mismatch factor larger than 100. By combining a low-mobility donor material, (1,2,3,4,9,10,11,12-octaphenyl-diindeno[1,2,3-cd:1′,2′,3′-lm]perylene), with the acceptors C60 and N,N′-dimethylperylene-3,4:9,10-dicarboximide, which show different electron mobilities, we experimentally verify the predictions. Our results demonstrate that not only interface effects but also the photoactive material itself can cause S-kinks.
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Enhancing the electron mobility via delta-doping in SrTiO3 Appl. Phys. Lett. 97, 222115 (2010); http://dx.doi.org/10.1063/1.3524198 (3 pages) Online Publication Date: 2 December 2010
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We fabricated high-mobility δ-doped structures in SrTiO3 thin films in order to investigate the low temperature electronic transport properties of confined carriers in this system. An enhancement of the electron mobility above the bulk value was observed as the doped layer thickness decreased. High-field Hall measurements revealed that this mobility enhancement originates from higher-mobility electrons in the undoped clean regions, which have quantum-mechanically broadened from the doped layer. Because of the absence of apparent lattice misfit between the layers, this structure is highly suitable for investigating two-dimensional electron gases in SrTiO3.
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Generation of correlated photon pairs in a chalcogenide As2S3 waveguide Appl. Phys. Lett. 98, 051101 (2011); http://dx.doi.org/10.1063/1.3549744 (3 pages) Online Publication Date: 31 January 2011
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We demonstrate a 1550 nm correlated photon-pair source in an integrated glass platform—a chalcogenide As2S3 waveguide. A measured pair coincidence rate of 80 s−1 was achieved using 57 mW of continuous-wave pump. The coincidence to accidental ratio was shown to be limited by spontaneous Raman scattering effects that are expected to be mitigated by using a pulsed pump source.
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Appl. Phys. Lett. 97, 232106 (2010); http://dx.doi.org/10.1063/1.3525713 (3 pages) Online Publication Date: 9 December 2010
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A correlation between microstructures and high gate leakage current density of Schottky contacts on lattice-matched InxAl1−xN/GaN heterostructures has been investigated by means of current-voltage measurements, conductive atom force microscopy (C-AFM), and transmission electron microscopy (TEM) investigations. It is shown that the reverse-bias gate leakage current density of Ni/Au Schottky contacts on InxAl1−xN/GaN heterostructures is more than two orders of magnitude larger than that on AlxGa1−xN/GaN ones. C-AFM and TEM observations indicate that screw- and mixed-type threading dislocations (S/M-TDs) are efficient leakage current channels in InxAl1−xN barrier and In segregation is formed around S/M-TDs. It is believed that In segregation around S/M-TDs reduces local Schottky barrier height to form conductive channels and leads to high leakage current density of Schottky contacts on InxAl1−xN/GaN heterostructures.
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The p-type ZnO film realized by a hydrothermal treatment method Appl. Phys. Lett. 98, 062102 (2011); http://dx.doi.org/10.1063/1.3549304 (3 pages) Online Publication Date: 7 February 2011
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Reproducible and high quality N-doped ZnO (ZnO:N) films were achieved by a hydrothermal treatment method. The ZnO:N films exhibited p-type characteristics by means of the Hall-effect and the photoluminescence measurements. At room temperature, the electrical properties of ZnO:N film showed a hole concentration of 1×1016 cm−3 and hole mobility of 8.6 cm2 V−1 s−1. At 83 K two acceptor related emission peaks could be observed located at 3.353 and 3.237 eV, which were assigned to the acceptor-bound exciton and the donor-acceptor pair emissions. This result gave a direct evidence for the generation of the acceptor energy level after the hydrothermal treatment process. Also, a ZnO homojunction diode was fabricated by this method, which displayed a good rectification characteristic at room temperature. This study revealed that the hydrothermal treatment method was effective and practicable in producing p-type ZnO.
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Magnetoresistance of Fe3O4-graphene-Fe3O4 junctions Appl. Phys. Lett. 98, 052511 (2011); http://dx.doi.org/10.1063/1.3552679 (3 pages) Online Publication Date: 3 February 2011
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The magnetoresistance (MR) of Fe3O4-graphene-Fe3O4 junctions has been experimentally studied at different temperatures. It is found that a barrier exists at the Fe3O4/graphene interface. The existence of the interfacial barrier was further confirmed by the nonlinear I-V characteristics and nonmetallic temperature dependence of the interfacial resistance. Moreover, spin dependent transport at the interfaces contributes −1.6% MR to the whole device at room temperature and can be regulated by an external electric field.
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