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Top 20 Most Read Articles
November 2010
The 20 articles with the most full-text downloads during the month, in descending order.
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Passivated TiN nanocrystals/SiN trapping layer for enhanced erasing in nonvolatile memory Appl. Phys. Lett. 97, 152112 (2010); http://dx.doi.org/10.1063/1.3501129 (3 pages) Online Publication Date: 14 October 2010
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We present chemical vapor deposition of titanium nitride nanocrystals (ncs) on silicon nitride (SiN). Ncs are passivated in situ by a silicon shell and encapsulated in SiN. High density (3×1012 cm−2), crystalline and isolated ncs are observed by transmission electron microscopy and characterized by x-ray photoelectron spectroscopy. TiN ncs/SiN are integrated as charge trapping layer in a nonvolatile memory. Devices show large memory window (10 V) and fast erasing compared to devices using pure SiN trapping layer, explained by enhanced electrical field in SiN. Acceptable reliability in terms of cycling and data retention is also demonstrated.
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Carbon impurities and the yellow luminescence in GaN Appl. Phys. Lett. 97, 152108 (2010); http://dx.doi.org/10.1063/1.3492841 (3 pages) Online Publication Date: 13 October 2010
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Using hybrid functional calculations we investigate the effects of carbon on the electrical and optical properties of GaN. In contrast to the currently accepted view that C substituting for N (CN) is a shallow acceptor, we find that CN has an ionization energy of 0.90 eV. Our calculated absorption and emission lines also indicate that CN is a likely source for the yellow luminescence that is frequently observed in GaN, solving the longstanding puzzle of the nature of the C-related defect involved in yellow emission. Our results suggest that previous experimental data, analyzed under the assumption that CN acts as a shallow acceptor, should be re-examined.
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Appl. Phys. Lett. 97, 181101 (2010); http://dx.doi.org/10.1063/1.3507891 (3 pages) Online Publication Date: 1 November 2010
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InGaN/GaN light-emitting diodes (LEDs) with graded-thickness multiple quantum wells (GQW) was designed and grown by metal-organic chemical vapor deposition. The GQW structure, in which the well-thickness increases along [0001] direction, was found to have superior hole distribution as well as radiative recombination distribution by performing simulation modeling. Accordingly, the experimental investigation of electroluminescence spectrum reveals additional emission from the narrower wells within GQWs. Consequently, the efficiency droop can be alleviated to be about 16% from maximum at current density of 30 to 200 A/cm2, which is much smaller than that for conventional LED (32%). Moreover, the light output power was enhanced from 18.0 to 24.3 mW at 20 A/cm2.
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Graphene synthesis by ion implantation Appl. Phys. Lett. 97, 183103 (2010); http://dx.doi.org/10.1063/1.3507287 (3 pages) Online Publication Date: 2 November 2010
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We demonstrate an ion implantation method for large-scale synthesis of high quality graphene films with controllable thickness. Thermally annealing polycrystalline nickel substrates that have been ion implanted with carbon atoms results in the surface growth of graphene films whose average thickness is controlled by implantation dose. The graphene film quality, as probed with Raman and electrical measurements, is comparable to previously reported synthesis methods. The implantation synthesis method can be generalized to a variety of metallic substrates and growth temperatures, since it does not require a decomposition of chemical precursors or a solvation of carbon into the substrate.
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Appl. Phys. Lett. 91, 063124 (2007); http://dx.doi.org/10.1063/1.2768624 (3 pages) Online Publication Date: 10 August 2007
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Microfabrication of graphene devices used in many experimental studies currently relies on the fact that graphene crystallites can be visualized using optical microscopy if prepared on top of Si wafers with a certain thickness of SiO2. The authors study graphene’s visibility and show that it depends strongly on both thickness of SiO2 and light wavelength. They have found that by using monochromatic illumination, graphene can be isolated for any SiO2 thickness, albeit 300 nm (the current standard) and, especially, ≈ 100 nm are most suitable for its visual detection. By using a Fresnel-law-based model, they quantitatively describe the experimental data.
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Appl. Phys. Lett. 97, 183109 (2010); http://dx.doi.org/10.1063/1.3512865 (3 pages) Online Publication Date: 4 November 2010
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We developed an ambient pressure chemical vapor deposition (CVD) for rapid growth of high-quality graphene films on Cu foils. The quality and growth rate of graphene films are dramatically increased with decreasing H2 concentration. Without the presence of H2, continuous graphene films are obtained with a mean sheet resistance of <350 Ω/sq and light transmittance of 96.3% at 550 nm. Because of the ambient pressure, rapid growth rate, absence of H2 and readily available Cu foils, this CVD process enables inexpensive and high-throughput growth of high-quality graphene films.
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Terahertz intersubband absorption in GaN/AlGaN step quantum wells Appl. Phys. Lett. 97, 191101 (2010); http://dx.doi.org/10.1063/1.3515423 (3 pages) Online Publication Date: 8 November 2010
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We demonstrate terahertz intersubband absorptions at frequencies of 2.1 THz (λ ≈ 143 μm) and 4.2 THz (λ ≈ 70 μm) in nitride-based semiconductor quantum wells. The structures consist of a 3 nm thick GaN well, an Al0.05Ga0.95N step barrier, and a 3 nm thick Al0.1Ga0.9N barrier. The absorption is detected at 4.7 K. The structure design has been optimized to approach a flat-band potential in the wells to allow for an intersubband absorption in the terahertz frequency range and to maximize the optical dipole moments.
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Appl. Phys. Lett. 97, 193101 (2010); http://dx.doi.org/10.1063/1.3504664 (3 pages) Online Publication Date: 8 November 2010
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An efficient technique is developed to fabricate optically thin metallic films with subwavelength patterns and their complements simultaneously. By comparing the spectra of the complementary films, we show that Babinet’s principle nearly holds for these structures in the optical domain. Rigorous full-wave simulations are employed to verify the experimental observations. It is further demonstrated that a discrete-dipole approximation can qualitatively describe the spectral dependence of the metallic membranes on the geometry of the constituent particles as well as the illuminating polarization.
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Hemispherical resonators with embedded nanocrystal quantum rod emitters Appl. Phys. Lett. 97, 211101 (2010); http://dx.doi.org/10.1063/1.3517566 (3 pages) Online Publication Date: 22 November 2010
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We report a technique to prepare hemispherical resonators on a distributed Bragg reflector (DBR). This so-called hydrophobic spreading allows the creation of hemispherical structures with diameters ranging from 5 to 50 μm. By embedding semiconductor nanocrystal quantum rods (NQRs) into these structures, we achieve a coupling of their emission into whispering-gallery modes. Although the NQR-emission is confined in three dimensions, the DBR is transparent for the excitation, allowing selective excitation of different regions of the hemisphere. Employing a two-dimensional model to approximately describe relevant modes, we are able to estimate the refractive index and diameters of the hemispheres from spectral data.
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Appl. Phys. Lett. 97, 053110 (2010); http://dx.doi.org/10.1063/1.3467468 (3 pages) Online Publication Date: 4 August 2010
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In this letter, we show how high-resolution scanning tunneling microscopy (STM) imaging can be used to reveal that certain edges of micromechanically exfoliated single layer graphene crystals on silicon oxide follow either zigzag or armchair orientation. Using the cleavage technique, graphene flakes are obtained that very often show terminating edges seemingly following the crystallographic directions of the underlying honeycomb lattice. Performing atomic resolution STM-imaging on such flakes, we were able to directly prove this assumption. Raman imaging carried out on the same flakes further validated our findings.
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Gate dependent photocurrents at a graphene p-n junction Appl. Phys. Lett. 97, 193102 (2010); http://dx.doi.org/10.1063/1.3505926 (3 pages) Online Publication Date: 9 November 2010
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We have used scanning photocurrent microscopy to explore the electronic characteristics of a graphene p-n junction fabricated by local chemical doping of a graphene sheet. The photocurrent signal at the junction was found to be most prominent for gate voltages between the two Dirac points of the oppositely doped graphene regions. The gate dependence of this signal agrees well with simulations based upon the Fermi level difference between the two differently doped sections. It is concluded that the photocurrent maps are dominated by the built-in electric field, with only a minor photothermoelectric contribution.
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The origin of oxygen in oxide thin films: Role of the substrate Appl. Phys. Lett. 97, 192107 (2010); http://dx.doi.org/10.1063/1.3515849 (3 pages) Online Publication Date: 11 November 2010
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During the growth of oxide thin films by pulsed laser deposition, a strong oxygen substrate-to-film transfer has been experimentally observed for SrTiO3 and LaAlO3 thin films epitaxially grown on 18O exchanged SrTiO3 and LaAlO3 substrates by secondary ion mass spectrometry depth profiling. This oxygen transfer effect can seriously change the respective thin film properties. Taking the oxygen substrate contribution to the overall oxygen balance into account, original ways to design material properties of oxide thin films can be envisioned like a controlled charge carrier doping of SrTiO3 thin films.
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Appl. Phys. Lett. 97, 201101 (2010); http://dx.doi.org/10.1063/1.3518065 (3 pages) Online Publication Date: 15 November 2010
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We propose a convenient approach for generating compact radially polarized laser beam by integrating concentric gold rings on the surface of a vertical-cavity surface-emitting laser. Due to the polarization selection of the surface plasmon polariton mode excited by the gold rings, a radially polarized beam at 850 nm is experimentally demonstrated. The physical mechanisms of the generation are numerically and analytically analyzed by employing finite-difference time-domain method and metal-insulator-metal plasmonic waveguide theory.
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Organic solar cells with solution-processed graphene transparent electrodes Appl. Phys. Lett. 92, 263302 (2008); http://dx.doi.org/10.1063/1.2924771 (3 pages) Online Publication Date: 1 July 2008
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We demonstrate that solution-processed graphene thin films can serve as transparent conductive anodes for organic photovoltaic cells. The graphene electrodes were deposited on quartz substrates by spin coating of an aqueous dispersion of functionalized graphene, followed by a reduction process to reduce the sheet resistance. Small molecular weight organic solar cells can be directly deposited on such graphene anodes. The short-circuit current and fill factor of these devices on graphene are lower than those of control device on indium tin oxide due to the higher sheet resistance of the graphene films. We anticipate that further optimization of the reduction conditions will improve the performance of these graphene anodes.
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Indium tin oxide-free and metal-free semitransparent organic solar cells Appl. Phys. Lett. 97, 153304 (2010); http://dx.doi.org/10.1063/1.3499299 (3 pages) Online Publication Date: 12 October 2010
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We report on indium tin oxide (ITO)-free and metal-free semitransparent organic solar cells with a high-conductivity poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (PH1000) as both the bottom and the top electrodes. The PH1000 film showed a conductivity of 680±50 S/cm. A ZnO layer was used as an interlayer to produce an electron-selective electrode. The semitransparent devices with a structure of glass/PH1000/ZnO/poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester/PEDOT:PSS (CPP 105 D)/PH1000 exhibited an average power conversion efficiency of 1.8% estimated for 100 mW/cm2 air mass 1.5 global illumination. This geometry alleviates the need of vacuum deposition of a top electrode.
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Bimetallic nanopetals for thousand-fold fluorescence enhancements Appl. Phys. Lett. 97, 203101 (2010); http://dx.doi.org/10.1063/1.3495773 (3 pages) Online Publication Date: 15 November 2010
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We present a simple, ultra-rapid and robust method to create sharp nanostructures—nanopetals—in a shape memory polymer substrate demonstrating unprecedented enhancements for surface enhanced sensing over large surface areas. These bimetallic nanostructures demonstrate extremely strong surface plasmon resonance effects due to the high density multifaceted petal structures that increase the probability of forming nanogaps. We demonstrate that our nanopetals exhibit extremely strong surface plasmons, confining the emission and enhancing the fluorescence intensity of the nearby high-quantum yield fluorescein by >4000×. The enhancements are confined to the extremely small volumes at the nanopetal borders. This enables us to achieve single molecule detection at relatively high and physiological concentrations.
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Increased open-circuit voltage in bulk-heterojunction solar cells using a C60 derivative Appl. Phys. Lett. 97, 193309 (2010); http://dx.doi.org/10.1063/1.3518066 (3 pages) Online Publication Date: 12 November 2010
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The fullerene derivative C60-fused N-methyl-2-(3-hexylthiophen-2-yl)pyrrolidine (C60-TH-Hx) is used as the acceptor material in bulk-heterojunction (BHJ) solar cells fabricated with the low band-gap polymer poly[(4,4′-bis(2-ethylhexyl)dithiene[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl]. Direct comparison with BHJ solar cells based on [6,6]-phenyl-C61-butyric methyl ester and Si-PCPDTBT indicates that the C60-TH-Hx acceptor yields a larger open-circuit voltage because of higher lowest unoccupied molecular orbital energy level of C60-TH-Hx.
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Appl. Phys. Lett. 97, 213301 (2010); http://dx.doi.org/10.1063/1.3514551 (3 pages) Online Publication Date: 23 November 2010
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We demonstrate the fabrication of inverted-structure organic solar cells (OSCs) with graphene cathodes. The graphene film used in this work was work-function-engineered with an interfacial dipole layer to reduce the work function of graphene, which resulted in an increase in the built-in potential and enhancement of the charge extraction, thereby enhancing the overall device performance. Our demonstration of inverted-structure OSCs with work-function-engineering of graphene electrodes will foster the fabrication of more advanced structure OSCs with higher efficiency.
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Plasmon modes of silver nanowire on a silica substrate Appl. Phys. Lett. 97, 183102 (2010); http://dx.doi.org/10.1063/1.3509415 (3 pages) Online Publication Date: 1 November 2010
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Plasmon mode in a silver nanowire is theoretically studied when the nanowire is placed on or near a silica substrate. It is found that the substrate has much influence on the plasmon mode. For the nanowire on the substrate, the plasmon (hybrid) mode possesses not only a long propagation length but also an ultrasmall mode area. From the experimental point of view, this cavity-free structure holds a great potential to study a strong coherent interaction between the plasmon mode and single quantum system (for example, quantum dots) embedded in the substrate.
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Appl. Phys. Lett. 97, 193307 (2010); http://dx.doi.org/10.1063/1.3516469 (3 pages) Online Publication Date: 10 November 2010
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The current-voltage characteristics of benzoporphine-fullerene solar cells were measured subsequent to the deposition of Al as a cathode material. Even in vacuum, a shift in the open circuit voltage was observed at 20 min after Al deposition. Moreover, the displacement of inert gases (N2 or Ar) in the evaporation chamber enhanced the photovoltaic parameters. The power conversion efficiency was increased by 24% over the initial characteristics (from 1.04% to 1.29%), which indicates that the structure of the organic-metal interface changed rapidly after Al deposition, even if the process was performed in an air-free glovebox.
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