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Top 20 Most Read Articles
February 2009
The 20 articles with the most full-text downloads during the month, in descending order.
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Self-assembled GaN hexagonal micropyramid and microdisk Appl. Phys. Lett. 94, 062105 (2009); http://dx.doi.org/10.1063/1.3079078 (3 pages) Online Publication Date: 11 February 2009
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The self-assembled GaN hexagonal micropyramid and microdisk were grown on LiAlO2 by plasma-assisted molecular-beam epitaxy. It was found that the (000
 ) disk was established with the capture of N atoms by most-outside Ga atoms as the (1×1) surface was constructing, while the pyramid was obtained due to the missing of most-outside N atoms. The intensity of cathode luminescence excited from the microdisk was one order of amplitude greater than that from M-plane GaN. |
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Room temperature midinfrared electroluminescence from InAs quantum dots Appl. Phys. Lett. 94, 061101 (2009); http://dx.doi.org/10.1063/1.3080688 (3 pages) Online Publication Date: 9 February 2009
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We demonstrate room temperature midinfrared electroluminescence from intersublevel transitions in self-assembled InAs quantum dots. The dots are grown in GaAs/AlGaAs heterostructures designed to maximize current injection into dot excited states while preferentially removing electrons from the ground states. As such, these devices resemble quantum cascade lasers. However, rigorous modeling of carrier transport through the devices indicates that the current transport mechanism for quantum dot active regions differs from that of quantum-well-based midinfrared lasers. We present the calculated energy states and transport mechanism for an intersublevel quantum dot emitter, as well as experimental electroluminescence data for these structures.
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Light scattering and Fano resonances in high-Q photonic crystal nanocavities Appl. Phys. Lett. 94, 071101 (2009); http://dx.doi.org/10.1063/1.3080683 (3 pages) Online Publication Date: 17 February 2009
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The authors show that light scattering from high-Q planar photonic crystal nanocavities can display Fano-like resonances corresponding to the excitation of localized cavity modes. By changing the scattering conditions, we are able to tune the observed lineshapes from strongly asymmetric and dispersivelike resonances to symmetric Lorentzians. Results are interpreted according to the Fano model of quantum interference between two coupled scattering channels. Combined measurements and line shape analysis on a series of silicon L3 nanocavities as a function of nearby hole displacement demonstrate that Q factors as high as 1.1×105 can be directly measured in these structures. Furthermore, a comparison with theoretically calculated Q factors allows to extract the rms deviation of hole radii due to weak disorder of the photonic lattice.
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Nonpolar m-plane InGaN multiple quantum well laser diodes with a lasing wavelength of 499.8 nm Appl. Phys. Lett. 94, 071105 (2009); http://dx.doi.org/10.1063/1.3078818 (3 pages) Online Publication Date: 18 February 2009
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We demonstrated nonpolar m-plane InGaN multiple quantum well laser diodes (LDs) under continuous-wave (cw) operation with a lasing wavelength of 499.8 nm, which is the longest reported for GaN-based LDs. A maximum optical output power of 15 mW was achieved, with the threshold current and the corresponding threshold current density (Jth) of 46 mA and 3.1 kA/cm2, respectively. The correlation between lasing wavelength shift and electrical input power (Pin) under cw operation was investigated using LDs of which reflectivity of front facet were varied from 70% to 97%. The lasing wavelength increased with increasing Pin with a slope of 4.56 and 4.34 nm/W for 70% and 97% mirror, respectively. The result suggested that the redshift due to self-heating is more predominant than the blueshift due to band filling above Jth even at near green region for nonpolar GaN-based LDs and reduction in Pin is indispensable to improve wavelength stabilization.
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Open circuit voltage enhancement due to reduced dark current in small molecule photovoltaic cells Appl. Phys. Lett. 94, 023307 (2009); http://dx.doi.org/10.1063/1.3072807 (3 pages) Online Publication Date: 16 January 2009
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We demonstrate high open circuit voltage photovoltaic cells achieved by reducing the electron leakage current through the introduction of both organic and inorganic electron blocking layers between the donor layer and the anode contact. As an example, the blocking layers reduce the dark current in tin (II) phthalocyanine (SnPc)/C60 solar cells with response across the visible and near infrared spectral region up to a wavelength of 1000 nm, is decreased by two orders of magnitude compared to cells lacking the layers, resulting in a doubling of the open circuit voltage. The structure: indium tin oxide/electron blocker/SnPc (100 Å)/C60 (400 Å)/bathocuproine (100 Å)/Al, has a power conversion efficiency of (2.1±0.1)% at 1 sun, standard AM1.5G solar illumination. This work demonstrates the importance of reducing dark current to achieve high organic thin film photovoltaic cell efficiencies.
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Fabry–Pérot-type enhancement in plasmonic visible nanosource Appl. Phys. Lett. 94, 051105 (2009); http://dx.doi.org/10.1063/1.3039075 (3 pages) Online Publication Date: 3 February 2009
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Starting from thin film planar technology, we designed and fabricated a visible optical source that produces a localized bright spot with nanometric dimensions. The structure consists of exciting surface plasmons through the illumination of a subwavelength hole in a silver film and in confining them at the vicinity of the aperture by surrounding the hole of Bragg mirrors resonant with the plasmons generated. Both finite element method computations and experimental results evidence the performances of this device that could find applications, for example, in nanolithography or optical data storage.
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Top-down approach to align single-walled carbon nanotubes on silicon substrate Appl. Phys. Lett. 94, 053113 (2009); http://dx.doi.org/10.1063/1.3078280 (3 pages) Online Publication Date: 3 February 2009
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We report controlled horizontal alignment of single-walled carbon nanotubes (SWNTs) directly grown on trenched SiO2/Si substrate. The nanotubes were found to align along the trenches, which were created via electron beam lithography followed by reactive ion etching. From the experimental observations, the alignment mechanism was proposed. Furthermore, field-effect transistors fabricated from these substrates showed acceptable mobility and on/off ratio as high as 104. The method offers the possibility of large-scale integrated SWNT electronics for mass production.
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Electronic structure and band-gap modulation of graphene via substrate surface chemistry Appl. Phys. Lett. 94, 032101 (2009); http://dx.doi.org/10.1063/1.3070238 (3 pages) Online Publication Date: 20 January 2009
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We have studied the electronic structure of graphene deposited on a SiO2 surface using density functional methods. The band structure of the graphene monolayer strongly depends on surface characteristics of the underlying SiO2 surface; for an oxygen-terminated surface, the monolayer exhibits a finite energy band gap while the band gap is closed when the oxygen atoms on the substrate are passivated with hydrogen atoms. We find that at least a graphene bilayer is required for a near zero energy gap when deposited on a substrate without H-passivation. Our results are discussed in the light of recent experiments.
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High-yield dielectrophoretic assembly of two-dimensional graphene nanostructures Appl. Phys. Lett. 94, 053110 (2009); http://dx.doi.org/10.1063/1.3077197 (3 pages) Online Publication Date: 3 February 2009
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Graphene handling is still dominated by serial mechanical exfoliation, which may well facilitate measurements in a laboratory environment but does not allow reliable larger-scale integration. Herein we demonstrate the controlled, high-yield (>90%), site-selective deposition of ultrathin few-layer (three to ten) graphene oxide by dielectrophoresis between prefabricated electrodes. Individual layers are found near the edges. Initially insulating, thermal reduction at 450 °C thins out the two-dimensional few-atom thick films and dramatically reduces electrical resistances down to 40 kΩ. Conductivities between 15 and 36 S/cm are obtained. The introduced method permits the nonintrusive, parallel, large-scale assembly of soluble two-dimensional nanostructures and sheets.
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Direct printing of aligned carbon nanotube patterns for high-performance thin film devices Appl. Phys. Lett. 94, 053109 (2009); http://dx.doi.org/10.1063/1.3073748 (3 pages) Online Publication Date: 3 February 2009
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The aligned assembly of carbon nanotubes (CNTs) on substrate presents a significant bottleneck in the fabrication of high-performance thin film devices. Here, we report a direct printing method to prepare laterally aligned thick CNT patterns over large surface regions. In this method, CNT forests were grown selectively on specific regions of one substrate, and the forest patterns were transferred on another SiO2 substrate in a laterally aligned formation while keeping their original shapes. The degree of alignment was characterized via electrical measurement and polarized Raman spectroscopy. Furthermore, we demonstrated high-performance field-effect transistors and gas sensors using our method.
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Appl. Phys. Lett. 94, 063308 (2009); http://dx.doi.org/10.1063/1.3080203 (3 pages) Online Publication Date: 13 February 2009
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We have demonstrated an improvement of photovoltaic performance based on the nanostructured ZnO/poly(3-hexylthiophene) (P3HT) hybrid through interface molecular modification on ZnO nanorod surface. By probing the carrier dynamics at ZnO/P3HT interfaces, we have found that the interfacial molecules can play the role of assisting charge separation and suppression of back recombination at interfaces, which accounts for the observed enhanced short circuit current (Jsc) and open circuit voltage (Voc) in photovoltaic performance.
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Harvesting triplet excitons for application in polymer solar cells Appl. Phys. Lett. 94, 063307 (2009); http://dx.doi.org/10.1063/1.3082081 (3 pages) Online Publication Date: 13 February 2009
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Triplet enhanced ladder-type poly (para-phenylene) polymer (PhLPPP) with covalently bound trace amounts of palladium blended with a fullerene derivative [[6,6]-phenyl C61-butyric acid methyl ester (PCBM)] shows power conversion efficiencies (PCE) almost ten times greater than with pristine ladder-type polymer (with no palladium atom) blended with PCBM. The steady state optical properties of the triplet and nontriplet-enhanced polymers are comparable; the enhanced PCE and external quantum efficiency in PhLPPP photovoltaics are attributed to the presence of long-lived mobile triplet excitons. Furthermore, the luminescence from PhLPPP blends measured in a delayed setup correlates very well with the efficiency of the solar cells.
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An optical spacer is no panacea for light collection in organic solar cells Appl. Phys. Lett. 94, 043302 (2009); http://dx.doi.org/10.1063/1.3073710 (3 pages) Online Publication Date: 26 January 2009
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The role of an optical spacer layer has been examined by optical simulations of organic solar cells with various bandgaps. The simulations have been performed with the transfer matrix method and the finite element method. The results show that no beneficial effect can be expected by adding an optical spacer to a solar cell with an already optimized active layer thickness.
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Inkjet printing of light emitting quantum dots Appl. Phys. Lett. 94, 073108 (2009); http://dx.doi.org/10.1063/1.3085771 (3 pages) Online Publication Date: 18 February 2009
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We demonstrate the fabrication of diodes having inkjet printed light emitting quantum dots layer. Close packing of printed layer is shown to be influenced by surface morphology of the underlying polymer layer and size variance of quantum dots used. We extend our approach to printing quantum dots onto a quarter video graphics array substrate (76 800 monochrome pixels). The purity of emitted electroluminescent spectra of resulting devices is related to coverage integrity of printed layer, which in turn is shown to be affected by the number of printed drops per pixel.
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Metallic conductivity and the role of copper in ZnO/Cu/ZnO thin films for flexible electronics Appl. Phys. Lett. 94, 052104 (2009); http://dx.doi.org/10.1063/1.3077184 (3 pages) Online Publication Date: 3 February 2009
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ZnO/Cu/ZnO multilayer structures are obtained with the highest conductivity of dielectric-metal-dielectric films reported in literature with a carrier concentration of 1.2×1022 cm−3 and resistivity of 6.9×10−5 Ω-cm at the optimum copper layer thickness. The peak transmittance, photopic averaged transmittance, and Haacke figure of merit are 88%, 75%, and 8.7×10−3 Ω−1, respectively. The conduction mechanism involves metal to oxide carrier injection prior to the formation of a continuous metal conduction pathway. Optical transmission is elucidated in terms of copper’s absorption due to d-band to Fermi surface transitions at short wavelengths and reflectance combined with scattering losses at long wavelengths.
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Appl. Phys. Lett. 94, 063304 (2009); http://dx.doi.org/10.1063/1.3079399 (3 pages) Online Publication Date: 11 February 2009
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pn-Heterojunction organic photovoltaic devices (OPVs) using amorphous molecular materials with high hole drift mobilities of 10−2 cm2 V−1 s−1, tris[4-(2-thienyl)phenyl]amine and tris[4-(5-phenylthiophen-2-yl)phenyl]amine, as electron donors and fullerene as an electron acceptor were fabricated. In spite of the use of amorphous materials instead of crystalline materials, the devices exhibited high performance with fill factors of 0.62–0.71 and power conversion efficiencies of 1.5%–1.7% under air-mass 1.5 G illumination at an intensity of 100 mW cm−2.
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Practical limits of absorption enhancement near metal nanoparticles Appl. Phys. Lett. 94, 071103 (2009); http://dx.doi.org/10.1063/1.3081631 (3 pages) Online Publication Date: 17 February 2009
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We consider the enhanced absorption of optical radiation by molecules placed in the vicinity of spherical metal nanoparticles in the realistic situation that includes perturbation of the optical field by the absorbing molecules. We show that there is an optimal nanosphere radius that gives the strongest enhancement for each combination of the number of absorbing molecules, their absorption strength, and their distance from the nanosphere surface and that the enhancement is strong only for relatively weak and diluted absorbers.
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Appl. Phys. Lett. 94, 071118 (2009); http://dx.doi.org/10.1063/1.3077606 (3 pages) Online Publication Date: 20 February 2009
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We report a dramatic increase in the light extraction efficiency of GaN-based blue light-emitting diodes (LEDs) by ZnO nanorod arrays on a planar indium tin oxide (ITO) transparent electrode. ZnO nanorods were grown into aqueous solution at the low temperature of 90 °C. With 20 mA current injection, the light output efficiency of the LED with ZnO nanorod arrays on ITO was increased by about 57% with no increase in a forward voltage over the conventional LEDs with planar ITO. The increased light extraction by the ZnO nanorod arrays is due to the formation of sidewalls and a rough surface, resulting in a multiple photon scattering at the LED surface.
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Logic gates with a single graphene transistor Appl. Phys. Lett. 94, 073305 (2009); http://dx.doi.org/10.1063/1.3079663 (3 pages) Online Publication Date: 18 February 2009
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The operation of four basic two-input logic gates fabricated with a single graphene transistor is demonstrated. Single-transistor operation is obtained in a circuit designed to exploit the charge neutrality point of graphene to perform Boolean logic. The type of logic function is selected by offset of the input digital signals. The merits and limitations of the fabricated gates are assessed by comparing their performance with that of conventional logic gates.
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Two-photon excited whispering-gallery mode ultraviolet laser from an individual ZnO microneedle Appl. Phys. Lett. 94, 051106 (2009); http://dx.doi.org/10.1063/1.3077011 (3 pages) Online Publication Date: 3 February 2009
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Wurtzite structural ZnO microneedles with hexagonal cross section were fabricated by vapor-phase transport method and an individual microneedle was employed as a lasing microcavity. Under excitation of a femtosecond pulse laser with 800 nm wavelength, the ultraviolet (UV) laser emission was obtained, which presented narrow linewidth and high Q value. The UV emission, resonant mechanism, and laser mode characteristics were discussed in detail. The results demonstrated that the UV laser originated from the whispering-gallery mode induced by two-photon absorption assisted by Rabi oscillation.
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