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Top 20 Most Read Articles
July 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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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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Preparation of nanochain and nanosphere by self-assembly of gold nanoparticles Appl. Phys. Lett. 92, 263108 (2008); http://dx.doi.org/10.1063/1.2955504 (3 pages) Online Publication Date: 2 July 2008
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A self-assembly method is demonstrated to link nanoparticles into nanostructure of nanochain or nanosphere. Gold nanoparticles were covered with capping molecules by forming Au–S bonds with thiol group at one terminate. Another terminating group, carboxylic acid, showed strong complex interaction with inorganic linker Zr4+ to form covalent complex bond. The different nanostructures were obtained by moving a balance between two opposite interactions, the linking interaction of Zr4+ and the electrostatic repulsive interaction of net surface charge. When the capping molecule with different chain length was used, the linked nanochain feature exhibited a tunable interdistance between the neighboring nanoparticles.
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Appl. Phys. Lett. 93, 013303 (2008); http://dx.doi.org/10.1063/1.2954015 (3 pages) Online Publication Date: 8 July 2008
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Reduction in the operating voltage of organic transistors is of high importance for successful implementation in low-power electronic applications. Here we report on low-voltage n-channel transistors fabricated employing a combination of soluble organic semiconductors and a self-assembled gate dielectric. The high geometric capacitance of the nanodielectric allows transistor operation below 2 V. Solution processing is enabled by analysis of the surface energy compatibility of the dielectric and semiconductor solutions. Electron mobilities in the range of 0.01–0.04 cm2/V s and threshold voltages ⩽ 0.35 V are demonstrated. The present work paves the way toward solution processable low-voltage/power, organic complementary circuits.
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On the importance of radiative and Auger losses in GaN-based quantum wells Appl. Phys. Lett. 92, 261103 (2008); http://dx.doi.org/10.1063/1.2953543 (3 pages) Online Publication Date: 1 July 2008
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Fully microscopic many-body models are used to study the importance of radiative and Auger carrier losses in InGaN/GaN quantum wells. Auger losses are found to be negligible in contrast to recent speculations on their importance for the experimentally observed efficiency droop. Good agreement with experimentally measured threshold losses is demonstrated. The results show no significant dependence on details of the well alloy profile.
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High-mobility organic single crystal transistors with submicrometer channels Appl. Phys. Lett. 93, 023303 (2008); http://dx.doi.org/10.1063/1.2953179 (3 pages) Online Publication Date: 15 July 2008
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We demonstrate high-performance electric-field effects in submicrometer-channel organic transistors with rubrene single crystals. Platinum source and drain electrodes are embedded in silicon dioxide gate insulators to reduce thickness of the dielectrics and to minimize the short-channel effect. The miniaturized devices exhibit typical output characteristics with Ohmic linear region, well-defined current saturation, and on-off ratio of 106. Mobility values are in the range of 0.1–0.3 cm2/V s, which is comparable to those of the best submicrometer organic transistors. Anisotropy in the mobility is detected, indicating that bandlike transport is responsible for the high transistor performance of the short-channel devices.
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Appl. Phys. Lett. 93, 013301 (2008); http://dx.doi.org/10.1063/1.2955528 (3 pages) Online Publication Date: 7 July 2008
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Transparent organic light emitting diodes were developed by using a thermally evaporable WO3/Ag/WO3 (WAW) as a transparent cathode. A thin Ag layer was introduced as an interlayer between the Li doped electron transport layer and the WAW electrode. A high transparency over 80% was obtained and electron injection was greatly improved by using the thin Ag interlayer between the Li doped layer and the WAW electrode. The driving voltage at 1000 cd/m2 was only 4.5 V and the sheet resistance of the WAW electrode was as low as 12 Ω/◻.
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Appl. Phys. Lett. 93, 021102 (2008); http://dx.doi.org/10.1063/1.2957667 (3 pages) Online Publication Date: 14 July 2008
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Carrier distributions governed by hole transport in InGaN/GaN multiple quantum well (MQW) visible light-emitting diodes (LEDs) were investigated using conventional blue LEDs and dual-wavelength blue-green LEDs. It was found that holes were dominantly distributed in the QW close to the p-GaN layer in LEDs with conventional MQW active regions at a current of 20 mA. A decrease in the thickness or the height of the quantum-well potential barrier enhanced hole injection into the MQWs located near the n-GaN layer. Reducing the thickness of a GaN quantum-well barrier between the blue QW and green QW did not degrade the electroluminescence (EL) intensity of the LED. In contrast, reducing the potential height of the barrier with material of possibly compromised quality resulted in a degradation of the EL intensity of the LED.
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Electroluminescence from monolayer ZnO nanoparticles using dry coating technique Appl. Phys. Lett. 92, 261107 (2008); http://dx.doi.org/10.1063/1.2952283 (3 pages) Online Publication Date: 1 July 2008
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We report ultraviolet electroluminescence from ZnO nanoparticle-based devices prepared by the dry-coating technique. With dry-coating process, the structure of the ZnO nanoparticle monolayer (90 nm) in the device can be easily achieved. The method reduces the density of pinhole defects in the ZnO nanoparticles. The confirmation for dry coating is investigated using field-emission scanning electron microscopy. The devices show the ZnO band-gap emission peak at 380 nm and the background emission from the interface between the host matrix and Aluminum tris-8-hydroxyquinoline. The origins of the ZnO band-gap emission and background emission are also discussed.
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Surface electronic properties of ZnO nanoparticles Appl. Phys. Lett. 92, 261916 (2008); http://dx.doi.org/10.1063/1.2952955 (3 pages) Online Publication Date: 3 July 2008
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The surface electronic structure of ZnO nanoparticles has been studied with photoemission and x-ray absorption spectroscopies. Contrary to expectation, ZnO:Zn phosphor nanoparticles were found to contain a lower oxygen vacancy density on the surface than undoped ZnO counterparts, but oxygen vacancies are in different chemical environments. Cathodoluminescence shows intense green luminescence from the ZnO:Zn surface, while the undoped nanoparticles exhibit only the near-band-edge emission. The results indicate the roles of surface oxygen vacancies and their environment in the previously unexplained green luminescence from the ZnO:Zn material.
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Grating-coupled surface emitting quantum cascade ring lasers Appl. Phys. Lett. 93, 011108 (2008); http://dx.doi.org/10.1063/1.2958910 (3 pages) Online Publication Date: 10 July 2008
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We report on the fabrication and operation of quantum cascade ring lasers providing grating-coupled surface emission. The devices exhibit tunable far fields, ranging from spot- to ring-shaped symmetric beam cross sections, depending on the grating period. This—along with threshold current densities as low as for comparable Fabry–Pérot lasers—demonstrates the compatibility of reduced beam divergence and two-dimensional integrability, resulting in an attractive light source for applications in midinfrared spectroscopy and imaging.
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Appl. Phys. Lett. 92, 253301 (2008); http://dx.doi.org/10.1063/1.2945281 (3 pages) Online Publication Date: 23 June 2008
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The performance and stability of unencapsulated inverted bulk-heterojunction solar cells with zinc oxide (ZnO) made by different processes as the electron selective contact are compared to conventional bulk-heterojunction solar cells. The low temperature processed inverted devices using ZnO nanoparticles on indium tin oxide plastic substrates showed high power conversion efficiency of ∼ 3.3%. This inverted device structure possessed much better stability under ambient conditions retaining over 80% of its original conversion efficiency after 40 days while the conventional one showed negligible photovoltaic activity after 4 days. This is due to the improved stability at the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/Ag interface.
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Photonic crystal cavity based gas sensor Appl. Phys. Lett. 92, 261112 (2008); http://dx.doi.org/10.1063/1.2955523 (3 pages) Online Publication Date: 2 July 2008
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We have studied the response of a photonic crystal cavity to changes of the ambient refractive index. Transmission measurements of the cavity under different gaseous environments and pressures showed a linear dependence of the resonance wavelength on the refractive index of the ambient gas. A change of the refractive index by 10−4 leads to a shift of the resonance by 8 pm, which is readily detectable due to the high quality factor of the cavity. The observed wavelength shifts agree well with finite-difference time domain simulations of the cavity.
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Appl. Phys. Lett. 92, 263109 (2008); http://dx.doi.org/10.1063/1.2955512 (3 pages) Online Publication Date: 2 July 2008
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We investigated the passivation effects on the electrical characteristics of ZnO nanowire field effect transistors (FETs) under the various oxygen environments of ambient air, dry O2, and vacuum. When the ZnO nanowire FET was exposed to more oxygen, the current decreased and the threshold voltage shifted to the positive gate bias direction, due to electrons trapping to the oxygen molecules at the nanowire surface. On the contrary, the electrical properties of the nanowire FET remained unchanged under different environments with passivation by a polymethyl methacrylate layer, which demonstrates the importance of surface passivation for ZnO nanowire-based electronic device applications.
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Appl. Phys. Lett. 93, 031101 (2008); http://dx.doi.org/10.1063/1.2959092 (3 pages) Online Publication Date: 21 July 2008
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This letter demonstrates holographic fabrication of three-dimensional diamondlike photonic crystal templates in SU8 photoresist using a single diffractive optical element. Five coherent laser beams produced by a two-dimensional phase mask were used to construct face-centered-cubic or tetragonal interference patterns. The superposition of two interference patterns through double exposures yields diamondlike photonic crystal templates in SU8. Photonic bandgap calculation reveals a full bandgap in inverse structures based on the template. The utilization of the two-dimensional phase mask simplifies the fabrication configuration in multiple beam holographic lithography for three-dimensional photonic fabrication.
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Appl. Phys. Lett. 92, 261105 (2008); http://dx.doi.org/10.1063/1.2953700 (3 pages) Online Publication Date: 1 July 2008
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We have studied the characteristics of blue InGaN/GaN multiquantum-well light-emitting diodes (LEDs) after reducing the length of the lateral current path through the transparent layer through formation of a peripheral high-resistance current-blocking region in the Mg-doped GaN layer. To study the mechanism of selective activation in the Mg-doped GaN layer, we deposited titanium (Ti), gold (Au), Ti/Au, silver, and copper individually onto the Mg-doped GaN layer and investigated their effects on the hole concentration in the p-GaN layer. The Mg-doped GaN layer capped with Ti effectively depressed the hole concentration in the p-GaN layer by over one order of magnitude relative to that of the as-grown layer. This may suggest that high resistive regions are formed by diffusion of Ti and depth of high resistive region from the p-GaN surface depends on the capped Ti film thickness. Selective activation of the Mg-doped GaN layer could be used to modulate the length of the lateral current path. Furthermore, the external quantum efficiency of the LEDs was improved significantly after reducing the lateral current spreading length. In our best result, the external quantum efficiency was 52.3% higher (at 100 mA) than that of the as-grown blue LEDs.
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Appl. Phys. Lett. 92, 251110 (2008); http://dx.doi.org/10.1063/1.2948851 (3 pages) Online Publication Date: 26 June 2008
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A photonic crystal (PC) structure of periodic SiO2 pillar cubic array is embedded in n-GaN layer of InGaN/GaN multiple quantum well (MQW) blue (480 nm) light-emitting diode (LED). The diameter, period, and depth of SiO2 pillar are 124±6, 230±10, and 130±10 nm, respectively. The increments of 70% for external quantum efficiency, 17% for internal quantum efficiency, and 45% for light extraction efficiency from photoluminescence measurement, and 33% for optical output power at 20 mA are observed for LEDs with an embedded PC layer. This improvement can be attributed to the increased extraction efficiency by PC effect as well as increased internal quantum efficiency due to the decrease of dislocation density in n-GaN layer because of an epitaxial lateral over-growth process.
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Photovoltaic effect and charge storage in single ZnO nanowires Appl. Phys. Lett. 93, 023111 (2008); http://dx.doi.org/10.1063/1.2957470 (3 pages) Online Publication Date: 16 July 2008
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Asymmetric Schottky barriers between ZnO nanowire and metal electrode have been fabricated at the two ends of the nanowire. An obvious photocurrent generated from the device at zero voltage bias can be switched on/off with quick response by controlling the light irradiation. Moreover, the device can still afford a current at zero bias after switching off light illumination, which is ascribed to the charge storage effect in single ZnO nanowires. The underlying mechanisms related to the photovoltaic effect and charge storage were discussed.
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Appl. Phys. Lett. 92, 263303 (2008); http://dx.doi.org/10.1063/1.2955515 (3 pages) Online Publication Date: 2 July 2008
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The mobility and contact resistance of transistors based on regioregular poly(3-hexylthiophene) (P3HT) with Ti/Pt electrodes were investigated as a function of the molecular weight (MW) of P3HT. For an increase in MW from 5.5 to 11 kDa, the mobility increased from 0.04 to 0.16 cm2 V−1 s−1, whereas the contact resistance decreased from 1.7 to 0.6 M Ω. Further increases in MW yielded an apparent saturation in both the mobility and the contact resistance. A nanofibrilar morphology was observed where the width of the nanofibrils increases with MW. A qualitative model based on polymer chain folding is proposed to explain the electrical results.
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Surface-plasmon enhanced transparent electrodes in organic photovoltaics Appl. Phys. Lett. 92, 243304 (2008); http://dx.doi.org/10.1063/1.2938089 (3 pages) Online Publication Date: 17 June 2008
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Random silver nanohole films were created through colloidal lithography techniques and metal vapor deposition. The transparent electrodes were characterized by uv-visible spectroscopy and incorporated into an organic solar cell. The test cells were evaluated for solar power-conversion efficiency and incident photon-to-current conversion efficiency. The incident photon-to-current conversion efficiency spectra displayed evidence that a nanohole film with 92 nm diameter holes induces surface-plasmon-enhanced photoconversion. The nanohole silver films demonstrate a promising route to removing the indium tin oxide transparent electrode that is ubiquitous in organic optoelectronics.
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The underlying terahertz vibrational spectrum of explosives solids Appl. Phys. Lett. 93, 011102 (2008); http://dx.doi.org/10.1063/1.2949068 (3 pages) Online Publication Date: 8 July 2008
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Using waveguide terahertz time-domain spectroscopy, we demonstrate the measurement of the underlying terahertz vibrational spectrum of the explosive solids hexahydro-1,3,5-trinitro-1,3,5-triazine, and 2,4,6-trinitrotoluene. Each explosive is cast as a polycrystalline thin film with planar ordering on the inner surface of metal parallel plate waveguide. For measurements near 10 K, each explosive reveals a complex spectrum of approximately 20 vibrational modes between 0.5 and 3.5 THz. The explosive films are of sufficient quality to produce vibrational linewidths as much as an order of magnitude sharper compared to conventional terahertz measurements of corresponding pellet samples of explosives.
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