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Top 20 Most Read Articles
January 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 91, 233117 (2007); http://dx.doi.org/10.1063/1.2821113 (3 pages) Online Publication Date: 7 December 2007
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Silicon nanowire-based solar cells on metal foil are described. The key benefits of such devices are discussed, followed by optical reflectance, current-voltage, and external quantum efficiency data for a cell design employing a thin amorphous silicon layer deposited on the nanowire array to form the p-n junction. A promising current density of ∼ 1.6 mA/cm2 for 1.8 cm2 cells was obtained, and a broad external quantum efficiency was measured with a maximum value of ∼ 12% at 690 nm. The optical reflectance of the silicon nanowire solar cells is reduced by one to two orders of magnitude compared to planar cells.
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Atomic-layer-deposited nanostructures for graphene-based nanoelectronics Appl. Phys. Lett. 92, 013101 (2008); http://dx.doi.org/10.1063/1.2828338 (3 pages) Online Publication Date: 2 January 2008
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Graphene is a hexagonally bonded sheet of carbon atoms that exhibits superior transport properties with a velocity of 108 cm/s and a room-temperature mobility of >15 000 cm2/V s. How to grow gate dielectrics on graphene with low defect states is a challenge for graphene-based nanoelectronics. Here, we present the growth behavior of Al2O3 and HfO2 films on highly ordered pyrolytic graphite (HOPG) by atomic layer deposition (ALD). To our surprise, large numbers of Al2O3 and HfO2 nanoribbons, with dimensions of 5–200 nm in width and >50 μm in length, are observed on HOPG surfaces at growth temperature between 200 and 250 °C. This is due to the large numbers of step edges of graphene on HOPG surfaces, which serve as nucleation sites for the ALD process. These Al2O3 and HfO2 nanoribbons can be used as hard masks to generate graphene nanoribbons or as top-gate dielectrics for graphene devices. This methodology could be extended to synthesize insulating, semiconducting, and metallic nanostructures and their combinations.
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All-optical delay line using semiconductor cavity solitons Appl. Phys. Lett. 92, 011101 (2008); http://dx.doi.org/10.1063/1.2828458 (3 pages) Online Publication Date: 2 January 2008
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An all-optical delay line based on the lateral drift of cavity solitons in semiconductor microresonators is proposed and experimentally demonstrated. The functionalities of the device proposed as well as its performance is analyzed and compared with recent alternative methods based on the decrease of group velocity in the vicinity of resonances. We show that the current limitations can be overcome using broader devices with tailored material responses.
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Influence of polymer coating on the low-temperature photoluminescence properties of ZnO nanowires Appl. Phys. Lett. 92, 011103 (2008); http://dx.doi.org/10.1063/1.2829598 (3 pages) Online Publication Date: 2 January 2008
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We report on low temperature photoluminescence studies of ZnO nanowires embedded in different polymers. Comparing the spectra of as-grown and embedded ZnO nanowires, we find a decrease of the deep-level emission and an increase of the near band-edge emission after the embedding process. The near band-edge emission of the embedded ZnO nanowires is dominated by a surface exciton band. The observed effects are independent of the selected polymer. The decrease of the deep-level emission scales with the balling abilities of the different polymers. We propose a model to explain the spectral changes.
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Plasmon-enhanced solar energy conversion in organic bulk heterojunction photovoltaics Appl. Phys. Lett. 92, 013504 (2008); http://dx.doi.org/10.1063/1.2823578 (3 pages) Online Publication Date: 3 January 2008
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Plasmon-active silver nanoparticle layers were included in solution-processed bulk-heterojunction solar cells. Nanoparticle layers were fabricated using vapor-phase deposition on indium tin oxide electrodes. Owing to the increase in optical electrical field inside the photoactive layer, the inclusion of such particle films lead to increased optical absorption and consequently increased photoconversion at solar-conversion relevant wavelengths. The resulting solar energy conversion efficiency for a bulk heterojunction photovoltaic device of poly(3-hexylthiophene)/[6,6]-phenyl C61 butyric acid methyl ester was found to increase from 1.3%±0.2% to 2.2%±0.1% for devices employing thin plasmon-active layers. Based on six measurements, the improvement factor of 1.7 was demonstrated to be statistically significant.
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Tunable Coulomb blockade in nanostructured graphene Appl. Phys. Lett. 92, 012102 (2008); http://dx.doi.org/10.1063/1.2827188 (3 pages) Online Publication Date: 2 January 2008
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We report on Coulomb blockade and Coulomb diamond measurements on an etched, tunable single-layer graphene quantum dot. The device consisting of a graphene island connected via two narrow graphene constrictions is fully tunable by three lateral graphene gates. Coulomb blockade resonances are observed and from Coulomb diamond measurements, a charging energy of ≈ 3.5 meV is extracted. For increasing temperatures, we detect a peak broadening and a transmission increase of the nanostructured graphene barriers.
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Physical properties and efficiency of GaNP light emitting diodes Appl. Phys. Lett. 92, 021101 (2008); http://dx.doi.org/10.1063/1.2830696 (3 pages) Online Publication Date: 14 January 2008
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GaNP/GaP is promising for yellow-amber-red light emitting diodes (LEDs). In this study, pressure and temperature dependent electroluminescence and photocurrent measurements on bulk GaP/GaN0.006P0.994/GaP LED structures are presented. Below ∼ 110 K, emission is observed from several localized nitrogen states. At room temperature, the band-edge energy increases weakly with pressure at a rate of +1.6 meV/kbar, substantially lower than the Γ band gap of GaP (+9.5 meV/kbar). Thus, despite the multiplicity of nitrogen levels, the band anticrossing model reasonably describes this system based on an average of the nitrogen states. Furthermore, carrier leakage into the X minima of GaP reduces the efficiency in GaNP-LEDs with increasing pressure.
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Enhanced charge carrier generation in dye sensitized solar cells by nanoparticle plasmons Appl. Phys. Lett. 92, 013113 (2008); http://dx.doi.org/10.1063/1.2830817 (3 pages) Online Publication Date: 4 January 2008
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An interesting possibility to improve the conversion and cost efficiencies of photovoltaic solar cells is to exploit the large optical cross sections of localized (nanoparticle) surface plasmon resonances (LSPRs). We have investigated this prospect for dye sensitized solar cells. Photoconductivity measurements were performed on flat TiO2 films, sensitized by a combination of dye molecules and arrays of nanofabricated elliptical gold disks. An enhanced dye charge carrier generation rate was found and shown to derive from the LSPR contribution by means of the polarization dependent resonance frequency in the anisotropic, aligned gold disks.
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Effect of traps on the performance of bulk heterojunction organic solar cells Appl. Phys. Lett. 91, 263505 (2007); http://dx.doi.org/10.1063/1.2821368 (3 pages) Online Publication Date: 27 December 2007
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The effect of electron traps on the performance of bulk heterojunction organic solar cells made of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is investigated. By deliberately introducing 7,7,8,8-tetracyanoquinodimethane, which is a stronger electron accepting molecule than PCBM, the electron transport in the solar cells becomes trap limited. We demonstrate that the open circuit voltage and its light-intensity dependence are strongly affected by recombination of holes with trapped electrons. Depending on the amount of traps, their recombination strength, and the light intensity, the losses due to trap-assisted recombination can even dominate over the intrinsic bimolecular recombination.
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An effective light trapping configuration for thin-film solar cells Appl. Phys. Lett. 91, 243501 (2007); http://dx.doi.org/10.1063/1.2789677 (3 pages) Online Publication Date: 10 December 2007
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Many thin-film solar cells make a compromise between achieving complete optical absorption using films that are thicker than the optical absorption length and achieving efficient conversion of the absorbed photons into photocurrent which is favored in thinner structures. We evaluate the performance of a V-shaped light trapping configuration that substantially increases the photocurrent generation efficiency for all angles of incidence and that is applicable to a broad class of low-cost thin-film solar cells. We experimentally demonstrate its effectiveness for small molecular weight and polymer organic solar cells. A 52% efficiency enhancement is obtained for a 170-nm-thick polymer cell.
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Green laser emission from monolithic II-VI-based pillar microcavities near room temperature Appl. Phys. Lett. 92, 031101 (2008); http://dx.doi.org/10.1063/1.2832644 (3 pages) Online Publication Date: 22 January 2008
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We report on single-mode stimulated emission at a wavelength of 510 nm in monolithic high-Q ZnSe-based vertical-cavity surface-emitting laser (VCSEL) micropillars under optical excitation at 280 K. Calculations reveal the influence of polariton coupling and the dynamic detuning of the gain maximum to the cavity modes on the spectral characteristics of the VCSEL emission. In accordance with theory, we find a systematic decrease in threshold excitation energy for decreasing pillar diameter to a minimum value of 0.09 pJ for a pillar diameter of 1.2 μm as a combined effect of reduced active area and reduced mode volume in the microcavity lasers.
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Extraction enhancement in organic light emitting devices by using metallic nanowire arrays Appl. Phys. Lett. 92, 013303 (2008); http://dx.doi.org/10.1063/1.2828712 (3 pages) Online Publication Date: 4 January 2008
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The extraction efficiency of organic light emitting devices is enhanced by depositing metallic nanowires on the glass surface and indium tin oxide (ITO) anode. For the aluminum tris-(8-hydroxyquinoline) (Alq3) based devices, a 100 nm-width and 450 nm-period gold nanowire array increases light extraction up to 46% from glass substrate and 80% from the organic layer. Such metallic nanowire arrays double the brightness with small absorption, only 10% lower than ITO glass. In addition, colors of the devices can be selected by the period of nanowire array. We demonstrated blue to red light emission by using single Alq3-based device.
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Effect of epitaxial strain on ferroelectric polarization in multiferroic BiFeO3 films Appl. Phys. Lett. 92, 012911 (2008); http://dx.doi.org/10.1063/1.2830799 (3 pages) Online Publication Date: 4 January 2008
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Multiferroic BiFeO3 epitaxial films with thicknesses ranging from 40 to 960 nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 bottom electrodes. X-ray characterization shows that the structure evolves from angularly distorted tetragonal with c/a ≈ 1.04 to more bulklike distorted rhombohedral (c/a ≈ 1.01) as the strain relaxes with increasing thickness. Despite this significant structural evolution, the ferroelectric polarization along the body diagonal of the distorted pseudocubic unit cells, as calculated from measurements along the normal direction, barely changes.
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Appl. Phys. Lett. 91, 263503 (2007); http://dx.doi.org/10.1063/1.2827178 (3 pages) Online Publication Date: 27 December 2007
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We demonstrate high-efficiency white organic light emitting devices (WOLEDs) employing three adjacent phosphorescent emission layers (3-EMLs). The metal-organic dopants for red, green, and blue emissions are each doped in separate hosts, allowing for separate optimization of the three dopant-host material combinations. This structure distributes the exciton generation region across the three hosts to form a stepped progression of highest occupied and lowest unoccupied molecular orbitals. The 3-EML WOLED has a color rendering index of 81 and peak forward-viewing external quantum (EQE) and power efficiencies (PE) of (16.6±0.8)% and 32±1 lm/W, respectively, corresponding to a total EQE = (28±1)% and a total PE = 54±3 lm/W. When an n-doped electron transporting layer is used, the total PE peaks at 64±3 lm/W, and rolls off to 34±2 lm/W at 1000 cd/m2.
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Selective formation of Ohmic junctions and Schottky barriers with electrodeposited ZnO Appl. Phys. Lett. 92, 012103 (2008); http://dx.doi.org/10.1063/1.2828702 (3 pages) Online Publication Date: 2 January 2008
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Constant-potential electrochemical synthesis of ZnO on metal substrates enables selective formation of either Ohmic or Schottky-barrier contacts. Using a mildly acidic nitrate-based aqueous electrolyte, there is a substrate-dependent deposition potential below which electrodeposited ZnO heterojunctions display Schottky response with high contact resistances ( ∼ 105 Ω) and above which Ohmic behavior and low contact resistances ( ∼ 1 Ω) occur. Voltammetric evidence for Zn metal deposition, in conjunction with Schottky-barrier heights that are consistent with values expected for a ZnO–Zn junction, suggests that more negative deposition potentials create a Zn-based interface between the substrate and ZnO that leads to rectifying behavior.
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Blue light emitting diodes grown on freestanding (11-20) a-plane GaN substrates Appl. Phys. Lett. 92, 011123 (2008); http://dx.doi.org/10.1063/1.2832645 (3 pages) Online Publication Date: 8 January 2008
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Visible blue light emitting diodes have been produced on freestanding nonpolar GaN (11-20) a-plane substrates by metal-organic chemical vapor deposition. The growth conditions have been optimized for smooth growth morphology of GaN nonpolar homoepitaxial layers without surface features, leading to light emitting diode epitaxial structures that are free of crystalline defects such as threading dislocations and stacking faults. Electroluminescence of light emitting diodes exhibit peak wavelengths of ∼ 450 nm and are independent of current level at low current densities before the heating effects are evidenced.
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Micromachined optical well structure for thermo-optic switching Appl. Phys. Lett. 91, 261106 (2007); http://dx.doi.org/10.1063/1.2825578 (3 pages) Online Publication Date: 28 December 2007
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This letter demonstrates the thermo-optic switching function using an adjustable optical well structure, which is constructed by a thin air gap sandwiched between two micromachined hemicylindrical prisms. The device is etched on a silicon-on-insulator wafer within a footprint of 400×400 μm2. In experiment, it measures an extinction ratio of 30.2 dB and a switching time of 2.2 μs. Compared with the other demonstrated switches that have optical barrier structures, this device is unique in the working principle and optical design, and shows various merits such as high extinction ratio, fast speed, low power consumption, and small size.
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Size dependence of the electronic structures and electron-phonon coupling in ZnO quantum dots Appl. Phys. Lett. 91, 262101 (2007); http://dx.doi.org/10.1063/1.2824396 (3 pages) Online Publication Date: 26 December 2007
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The electronic structures and optical properties of various sizes of ZnO quantum dots (QDs) were studied using x-ray absorption, photoluminescence, and Raman spectroscopy. The increase in the intensity ratio of the second-order Raman spectra of longitudinal optical mode and its fundamental mode, which is related to the strength of the electron-phonon coupling (EPC), is found to increase with the size of QD. The trend of EPC also correlates with the increase of the intensity ratio of the O 2pπ (Iπ) and 2pσ (Iσ) orbital features in the O K-edge x-ray absorption near-edge structure (XANES) as the size of QD increases. The EPC and XANES results suggest that the crystal orientations of ZnO QDs are approximately aligned with the c axis parallel with the polarization of x-ray photons.
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Appl. Phys. Lett. 92, 023504 (2008); http://dx.doi.org/10.1063/1.2834697 (3 pages) Online Publication Date: 14 January 2008
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An anode interfacial layer is reported for bulk-heterojunction (BHJ) polymer solar cells to replace the commonly used poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). A poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB)+4,4′-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (TPDSi2) blend is crosslinked, forming robust ∼ 10 nm thick films covalently bound to indium tin oxide, which transport holes while blocking misdirected electrons. The thermal stability and photovoltaic performance metrics of TFB:TPDSi2-modified BHJ cells are significantly greater than those of cells fabricated in parallel with PEDOT:PSS or with no interfacial layer. For a poly[2-methoxy-5-(3′,7′-dimethyloctyloxyl]-1,4-phenylene vinylene: methanofullerene[6 6]-phenyl C61-butyric acid methyl ester cell, Voc = 0.89 V, Jsc = 4.62 mA/cm2, FF = 54.4%, and ηp = 2.23%.
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Concept of a nanowire array magnetoresistance device Appl. Phys. Lett. 92, 023107 (2008); http://dx.doi.org/10.1063/1.2834371 (3 pages) Online Publication Date: 15 January 2008
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We describe a concept of the magnetoresistive medium comprising of a planar array of intermingled nanowires with pinned and unpinned magnetic moments. We propose a bottom up method of forming the medium and demonstrate the feasibility of the proposed approach. We present the results of the simulations of the magnetic moments in the nanowires under an external magnetic field to illustrate the concept.
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