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Top 20 Most Read Articles
November 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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Enhanced light extraction from GaN-based green light-emitting diode with photonic crystal Appl. Phys. Lett. 91, 181109 (2007); http://dx.doi.org/10.1063/1.2804005 (3 pages) Online Publication Date: 31 October 2007
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This letter reports the properties of GaN-based green light-emitting diodes (LEDs) having a p-GaN photonic crystal layer with a photonic bandgap (PCWG) and without a photonic bandgap (PCOG). With decreasing the photoluminescence (PL) detection angle from 140° to 60°, the enhancement of PL intensity of LED with PCWG was largely increased from 9 to 25 times, compared to that of LEDs without a patterned structure, while the PL intensity of LED with PCOG was increased from 4.6 to 5.6 times. The electroluminescence output power of green LEDs with a PCWG was enhanced about two times compared to LEDs with a PCOG. These results suggest that the light extraction of green LEDs can be greatly increased by using PCWG instead of PCOG.
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Ordered, uniform-sized ZnO nanolaser arrays Appl. Phys. Lett. 91, 181112 (2007); http://dx.doi.org/10.1063/1.2805073 (3 pages) Online Publication Date: 31 October 2007
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Ordered ZnO nanorod arrays with almost uniform rod size have been grown perpendicularly on GaN/Al2O3 substrates by a controlled vapor phase transport growth method. The ZnO nanorods are [0001] oriented single crystals with diameter of 200 nm and length of 4.7 μm, with a rod-to-rod spacing of 500 nm. Photoluminescence spectra of the rod arrays indicate that the rods are of high crystal quality: very strong, well-separated bound and free exciton emission in the ultraviolet (UV) region are resolved at low temperature. Time resolved microphotoluminescence measurements are performed on single nanorods standing on the substrate which demonstrates lasing behavior with multiple UV lasing modes. Under quasistationary excitation lasing is observed up to room temperature. The lasing emission peaks are sharp, with a linewidth about 0.1 nm, and have a fast decay time of ∼ 8 ps. These high crystal quality nanorod arrays may be promising candidates for UV nanolaser devices.
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Hexagonal structures on metal-coated two-dimensional microlens arrays Appl. Phys. Lett. 91, 191106 (2007); http://dx.doi.org/10.1063/1.2804016 (3 pages) Online Publication Date: 6 November 2007
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Hexagonally shaped apertures on metal-coated colloidal lattices of microspheres have been observed within certain parameter regimes of femtosecond Ti:sapphire-laser irradiation. The occurrence of such structures is explained by electromagnetic field interferences caused by the array of microspheres. The calculations are based on the splitting of the incident laser field into narrow paraxial Gaussian beams and their subsequent tracing and summation.
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Soldering to a single atomic layer Appl. Phys. Lett. 91, 193512 (2007); http://dx.doi.org/10.1063/1.2812571 (3 pages) Online Publication Date: 9 November 2007
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The standard technique to make electrical contact to nanostructures is electron beam lithography. This method has several drawbacks including complexity, cost, and sample contamination. We present a simple technique to cleanly solder submicron sized, Ohmic contacts to nanostructures. To demonstrate, we contact graphene, a single atomic layer of carbon, and investigate low- and high-bias electronic transport. We set lower bounds on the current carrying capacity of graphene. A simple model allows us to obtain device characteristics such as mobility, minimum conductance, and contact resistance.
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Photoinduced charge transfer between poly(3-hexylthiophene) and germanium nanowires Appl. Phys. Lett. 91, 183501 (2007); http://dx.doi.org/10.1063/1.2801554 (3 pages) Online Publication Date: 29 October 2007
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Germanium nanowires (GeNWs) were used to enhance the properties of organic photovoltaic devices. GeNWs were grown to a length of 1–5 μm on SiO2 by the vapor-liquid-solid method catalyzed by 20 nm Au seeds. Once grown, the GeNWs were dispersed in solution with poly(3-hexylthiophene) and spin cast into films. The photoluminescence and external quantum efficiency of the films indicated a significant increase in exciton dissociation and photocurrent generation. The results imply that the GeNWs may act as an electron acceptor for bulk heterojunction hybrid-inorganic/organic photovoltaic devices. The impacts of GeNW on device characteristics are discussed.
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Enhanced forward emission from ZnO via surface plasmons Appl. Phys. Lett. 91, 211107 (2007); http://dx.doi.org/10.1063/1.2805022 (3 pages) Online Publication Date: 21 November 2007
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By using metal-insulator-metal (MIM) as the cap layer, we have enhanced the photoluminescent emission from ZnO in the forward direction via coupling of surface plasmons. The basic principle behind the enhancement involves two steps. First, the overall light emission efficiency of ZnO is increased significantly due to the coupling and scattering of nonradiative surface plasmons at the metal/ZnO interface. The forward emission is then increased by enhanced transmission due to the excitation of radiative surface plasmons in the MIM structure. As a result, the forward emission from MIM/ZnO is found to be 7 and 3.5 times stronger than that from uncapped ZnO and M/ZnO. We expect that the MIM structure could evolve as an effective means in fabricating high brightness light emitting diodes from semiconductors.
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Reproducible resistive switching in nonvolatile organic memories Appl. Phys. Lett. 91, 192103 (2007); http://dx.doi.org/10.1063/1.2806275 (3 pages) Online Publication Date: 7 November 2007
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Resistive switching in nonvolatile, two terminal organic memories can be due to the presence of a native oxide layer at an aluminum electrode. Reproducible solid state memories can be realized by deliberately adding a thin sputtered Al2O3 layer to nominal electron-only, hole-only, and bipolar organic diodes. Before memory operation, the devices have to be formed at an electric field of 109 V/m, corresponding to soft breakdown of Al2O3. After forming, the structures show pronounced negative differential resistance and the local maximum in the current scales with the thickness of the oxide layer. The polymer acts as a current limiting series resistance.
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Large scale, highly ordered assembly of nanowire parallel arrays by differential roll printing Appl. Phys. Lett. 91, 203104 (2007); http://dx.doi.org/10.1063/1.2813618 (3 pages) Online Publication Date: 13 November 2007
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A differential roll printing strategy is developed to enable large-scale and uniform assembly of highly aligned and ordered nanowire arrays on various rigid and flexible substrate materials. The dynamics of the process are explored by tuning the linear sliding motion of the roller with respect to the rolling motion, clearly demonstrating the importance of the differential rolling process in the controlled assembly of nanowires. The potency and versatility of the method is further demonstrated by fabrication of nanowire transistor arrays on flexible substrates.
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Improving efficiency of organic photovoltaic cells with pentacene-doped CuPc layer Appl. Phys. Lett. 91, 191109 (2007); http://dx.doi.org/10.1063/1.2806195 (3 pages) Online Publication Date: 6 November 2007
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We have fabricated efficient heterojunction organic photovoltaic (OPV) cells based on pentacene-doped copper(II) phthalocyanine (CuPc) layer as donor and fullerene (C60) layer as acceptor. The power conversion efficiency of 4% pentacene-doped CuPc/C60 OPV cell (3.06%) is increased by 77% compared with that of the standard CuPc/C60 OPV cell (1.73%). The efficiency improvement can be attributed to the higher carrier mobility instead of the stronger photon absorption of the pentacene-doped CuPc layer.
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Appl. Phys. Lett. 91, 211104 (2007); http://dx.doi.org/10.1063/1.2816892 (3 pages) Online Publication Date: 21 November 2007
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We fabricated air-bridge-type silicon photonic crystal (PC) nanocavities and observed significant enhancement of photoluminescence (PL) from crystalline silicon at room temperature. Cavity-resonant peaks shifted toward longer wavelengths when the period of PC was increased and their polarization dependences agree with the calculation. At a cavity mode wavelength, a 310-fold enhancement of PL intensity, compared with that of unpatterned silicon-on-insulator substrate, was demonstrated.
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Appl. Phys. Lett. 91, 196101 (2007); http://dx.doi.org/10.1063/1.2806183 (2 pages) Online Publication Date: 6 November 2007
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Enhancement of ferromagnetism upon thermal annealing in pure ZnO Appl. Phys. Lett. 91, 182501 (2007); http://dx.doi.org/10.1063/1.2804081 (3 pages) Online Publication Date: 29 October 2007
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We report here the enhancement of ferromagnetism in pure ZnO upon thermal annealing with the ferromagnetic transition temperature Tc above room temperature. We observe a finite coercive field up to 300 K and a finite thermoremanent magnetization up to 340 K for the annealed sample. We propose that magnetic moments can be formed at anionic vacancy clusters. Ferromagnetism can occur due to either superexchange between vacancy clusters via isolated F+ centers or through a limited electron delocalization between vacancy clusters. Isolated vacancy clusters or isolated F+ centers give rise to a strong paramagneticlike behavior below 10 K.
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Gallium-nitride-based microcavity light-emitting diodes with air-gap distributed Bragg reflectors Appl. Phys. Lett. 91, 211108 (2007); http://dx.doi.org/10.1063/1.2805028 (3 pages) Online Publication Date: 21 November 2007
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We report on the realization of highly efficient InGaN microcavity light-emitting diodes incorporating a high index contrast air-gap distributed Bragg reflector (DBR). Detailed analysis deduces an effective cavity length of ∼ 500 nm and cavity mode orders of 5 and 6 for measured Fabry-Pérot fringes. A value reflectivity of ∼ 70% was determined for the 4.5 period air∕Al0.08Ga0.92N DBR through the analysis of cavity finesse based on the angle-resolved photoluminescence (PL) data. A fivefold improvement in light extraction efficiency was verified by electrical probing as well as angle-resolved PL measurements.
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Highly efficient simplified organic light emitting diodes Appl. Phys. Lett. 91, 113506 (2007); http://dx.doi.org/10.1063/1.2784176 (3 pages) Online Publication Date: 11 September 2007
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The authors report on highly efficient organic light emitting diodes (OLEDs) consisting of only two organic layers. The key to the simplification is the direct injection of holes into the wide band gap hole transport material 4,4′,4″-tris(N-carbazolyl)-triphenyl amine (highest occupied molecular orbital is 5.9 eV) through an indium tin oxide/tungsten oxide (WO3) anode. Kelvin probe analysis has revealed an extremely high work function of 6.4 eV for WO3. The efficiencies of the simplified OLEDs exceed 40 lm/W and 45 cd/A at a brightness of 100 cd/m2, unsurpassed by other comparably simple OLED devices. Therefore, our OLED architecture demonstrates highly efficient, yet easy to fabricate devices.
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Active metamaterials: Sign of refractive index and gain-assisted dispersion management Appl. Phys. Lett. 91, 191103 (2007); http://dx.doi.org/10.1063/1.2800309 (3 pages) Online Publication Date: 5 November 2007
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We derive an approach to determine the causal direction of wavevectors of modes in optical metamaterials, which, in turn, determines signs of refractive index and impedance as a function of real and imaginary parts of dielectric permittivity and magnetic permeability. We use the developed technique to demonstrate that the interplay between resonant response of constituents of metamaterials can be used to achieve efficient dispersion management. Finally, we demonstrate broadband dispersionless index and impedance matching in active nanowire-based negative index materials. Our work has a potential to open new practical applications of negative index composites for broadband lensing, imaging, and pulse routing.
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Appl. Phys. Lett. 91, 183517 (2007); http://dx.doi.org/10.1063/1.2803219 (3 pages) Online Publication Date: 2 November 2007
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High performance thin film transistor (TFT) with atomic layer deposition (ALD) nitrogen doped ZnO (ZnO:N) as an active layer is demonstrated. The electrical properties of ZnO thin films were effectively controlled by in situ nitrogen doping using NH4OH as a source for reactants. Especially, the electron concentration in ZnO was lowered to below 1015 cm−3. Good device characteristics were obtained from the inverted staggered type TFTs with ZnO:N channel and ALD Al2O3 gate insulator; μsat = 6.7 cm2/V s, Ioff = 2.03×10−12 A, Ion/off = 9.46×107, and subthreshold swing = 0.67 V/decade. The entire TFT fabrication processes were carried out at below 150 °C, which is a favorable process for plastic based flexible display.
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Electronic transport in locally gated graphene nanoconstrictions Appl. Phys. Lett. 91, 192107 (2007); http://dx.doi.org/10.1063/1.2803074 (3 pages) Online Publication Date: 8 November 2007
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We have developed the combination of an etching and deposition techniques that enables the fabrication of locally gated graphene nanostructures of arbitrary design. Employing this method, we have fabricated graphene nanoconstrictions with local tunable transmission and characterized their electronic properties. An order of magnitude enhanced gate efficiency is achieved adopting the local gate geometry with thin dielectric gate oxide. A complete turn off of the device is demonstrated as a function of the local gate voltage. Such strong suppression of device conductance was found to be due to both quantum confinement and Coulomb blockade effects in the constricted graphene nanostructures.
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Plasmonic quantum cascade laser antenna Appl. Phys. Lett. 91, 173113 (2007); http://dx.doi.org/10.1063/1.2801551 (3 pages) Online Publication Date: 23 October 2007
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We report a plasmonic quantum cascade laser antenna that confines coherent midinfrared radiation well below the diffraction limit. The antenna was fabricated on the facet of a midinfrared quantum cascade laser and consists of a pair of gold nanorods separated by a gap. The antenna near field was characterized by an apertureless near-field scanning optical microscope; field confinement of about 100 and 70 nm, limited by the gap size, was demonstrated at wavelengths of 7.0 and 5.3 μm, respectively. This device may find important applications in midinfrared subwavelength chemical and biological imaging and spectroscopy.
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Appl. Phys. Lett. 91, 193501 (2007); http://dx.doi.org/10.1063/1.2807843 (3 pages) Online Publication Date: 5 November 2007
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We fabricated high mobility, low voltage n-channel transistors on plastic substrates by combining an amorphous phase C60 film and a high dielectric constant gate insulator titanium silicon oxide (TiSiO2). The transistors exhibited high performance with a threshold voltage of 1.13 V, an inverse subthreshold swing of 252 mV/decade, and a field-effect mobility up to 1 cm2/V s at an operating voltage as low as 5 V. The amorphous phase C60 films can be formed at room temperature, implying that this transistor is suitable for corresponding n-channel transistors in flexible organic logic devices.
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Appl. Phys. Lett. 91, 196102 (2007); http://dx.doi.org/10.1063/1.2806185 (1 page) Online Publication Date: 6 November 2007
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