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Top 20 Most Read Articles
August 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 93, 051101 (2008); http://dx.doi.org/10.1063/1.2965797 (3 pages) Online Publication Date: 4 August 2008
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Direct evidence of the transition from amplified spontaneous emission to laser action in optically pumped zinc oxide (ZnO) nanowires, at room temperature, is presented. The optical power evolves from a superlinear to a linear regime as the pump power exceeds threshold, concomitant with a transition to directional emission along the nanowire and the emergence of well defined cavity Fabry–Pérot modes around a wavelength of ≈ 385 nm, the intensity of which exceeds the spontaneous emission background by orders of magnitude. The laser oscillation threshold is found to be strongly dependent on nanowire diameter, with no laser oscillation observed for diameters smaller than ∼ 150 nm. Finally, we use an alternative “head on” detection geometry to measure the output power of a single nanowire laser.
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Efficient multilayer organic solar cells using the optical interference peak Appl. Phys. Lett. 93, 043307 (2008); http://dx.doi.org/10.1063/1.2962986 (3 pages) Online Publication Date: 1 August 2008
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A multilayer structure of copper phthalocyanine/poly(3-hexylthiophene-2,5-diyl): [6,6]-phenyl-C61-butyric acid methyl ester (CuPc/P3HT:PCBM) is used to extend the light absorption spectrum covering almost the entire visible spectrum. To maximize the light absorption, the total number of excitons created in the multilayer structure as a function of layer thickness of both CuPc and P3HT:PCBM is simulated by using the optical transfer matrix formalism. The solar cells with a device structure of ITO/PEDOT:PSS/CuPc/P3HT:PCBM/Al are fabricated with different layers thicknesses. The optimized solar cell with a high short circuit current density of 12.54 mA/cm2 and power conversion efficiency as high as 4.13% is achieved, owing to the utilization of the second optical interference peak in the multilayer structure for the enhanced light absorption.
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Room temperature polariton lasing in a GaN/AlGaN multiple quantum well microcavity Appl. Phys. Lett. 93, 051102 (2008); http://dx.doi.org/10.1063/1.2966369 (3 pages) Online Publication Date: 4 August 2008
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The authors report room temperature polariton lasing at λ ∼ 345 nm in a hybrid AlInN/AlGaN multiple quantum well microcavity (MQW-MC) containing a GaN/AlGaN MQW active region, i.e., the achievement under nonresonant optical excitation of coherent light emission of a macroscopic population of polaritons occupying the lowest energy state of the lower polariton branch. This was made possible by taking advantage of the efficient relaxation of polaritons in a MQW-MC exhibiting a large vacuum Rabi splitting ΩVRS = 56 meV.
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High-mobility spin-cast organic thin film transistors Appl. Phys. Lett. 93, 043301 (2008); http://dx.doi.org/10.1063/1.2952769 (3 pages) Online Publication Date: 28 July 2008
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We report high-mobility organic thin film transistors (OTFTs) with good uniformity using the small molecule organic semiconductor 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT). diF-TESADT was spin cast to form OTFTs with a carrier mobility of more than 1.5 cm2/V s and shows a useful differential microstructure on or near pentaflorobenzenethiol (PFBT) treated Au source/drain electrodes compared to untreated Au or gate dielectric areas with improved molecular order observed on PFBT treated Au electrodes. For short channel length devices diF-TESADT crystal grains extend between the source and drain electrodes, resulting in increasing OTFT field effect mobility for decreasing gate length.
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Appl. Phys. Lett. 93, 052901 (2008); http://dx.doi.org/10.1063/1.2965799 (3 pages) Online Publication Date: 4 August 2008
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Cr, Mn, Co, Ni, and Cu were added to polycrystalline BiFeO3 films, and their influence on the ferroelectric, electrical, and magnetic properties was investigated. All the additives except Ni reduced the leakage current density in the high electric field region. The addition of Cu and Co decreased the coercive field without reducing remanent polarization. The addition of Co caused spontaneous magnetization at room temperature, which exhibited a large coercive field of 16 kOe at 10 K. It was revealed that Co addition suppressed the leakage current density, decreased the electric coercive field, and induced spontaneous magnetization and large magnetic coercivity.
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Infrared emission from the substrate of GaAs-based semiconductor lasers Appl. Phys. Lett. 93, 041101 (2008); http://dx.doi.org/10.1063/1.2959854 (3 pages) Online Publication Date: 28 July 2008
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We report on the origin of three additional low-energy spontaneously emitted bands in GaAs-based broad-area laser diodes. Spectrally and spatially resolved scanning optical microscopy and Fourier-transform infrared spectroscopy assign the different contributions to bandtail-related luminescence from the gain region as well as interband and deep-level-related luminescences from the GaAs substrate. The latter processes are photoexcited due to spontaneous emission from the active region followed by a cascaded photon-recycling process within the substrate.
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Appl. Phys. Lett. 93, 043308 (2008); http://dx.doi.org/10.1063/1.2965120 (3 pages) Online Publication Date: 1 August 2008
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We investigated the interfacial electronic structures of indium tin oxide (ITO)/molybdenum trioxide (MoO3)/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) using in situ ultraviolet and x-ray photoemission spectroscopy to understand the origin of hole injection improvements in organic light-emitting devices (OLEDs). Inserting a MoO3 layer between ITO and NPB, the hole injection barrier was remarkably reduced. Moreover, a gap state in the band gap of NPB was found which assisted the Ohmic hole injection at the interface. The hole injection barrier lowering and Ohmic injection explain why the OLED in combination with MoO3 showed improved performance.
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Appl. Phys. Lett. 93, 051106 (2008); http://dx.doi.org/10.1063/1.2968309 (3 pages) Online Publication Date: 5 August 2008
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We report enhanced top emission from organic light-emitting devices by surface plasmon-mediated radiative energy transfer. A dye-doped dielectric acceptor layer was deposited onto the surface of a one-dimensionally corrugated silver cathode and was excited by the electroluminescence of a donor layer, which is located at the other side of the cathode. Ten times enhancement in emission intensity from the acceptor was observed compared to flat devices; this is due to the enhanced radiative energy transfer from the donor to the acceptor by the coupled surface plasmons on the opposite interfaces of the silver cathode.
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High-efficiency monochrome organic light emitting diodes employing enhanced microcavities Appl. Phys. Lett. 93, 043310 (2008); http://dx.doi.org/10.1063/1.2966784 (3 pages) Online Publication Date: 1 August 2008
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We demonstrate enhanced light outcoupling from bottom emitting monochrome high-efficiency red, green, and blue organic light emitting diodes by adding silver layers on the indium tin oxide (ITO) anode. The devices contain the phosphorescent emitting dyes tris(2-phenylpyridine)iridium and iridium(III)bis[2-methyldibenzo-(f, h)quinoxaline](acetylacetonate), and the blue singlet emitter 2,2′,7,7′-tetrakis 2,2-(diphenylvinyl)spiro-9,9′-bifluorene. We follow the p-i-n doping concept to increase the power efficiencies. We reach 81 lm/W for red, 101 lm/W for green, and 4.0 lm/W for blue color. These efficiencies are improved by up to a factor of 2.3 compared to standard ITO devices, which we attribute to microcavity amplification between the cathode and the Ag layer.
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Self-aligned top-gate amorphous gallium indium zinc oxide thin film transistors Appl. Phys. Lett. 93, 053501 (2008); http://dx.doi.org/10.1063/1.2966145 (3 pages) Online Publication Date: 4 August 2008
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We have demonstrated a self-aligned top-gate amorphous gallium indium zinc oxide thin film transistor (a-GIZO TFT). It had a field effect mobility of 5 cm2/V s, a threshold voltage of 0.2 V, and a subthreshold swing of 0.2 V/decade. Ar plasma was treated on the source/drain region of the a-GIZO active layer to reduce the series resistance. After Ar plasma treatment, the surface of the source/drain region was divided into In-rich and In-deficient regions. The a-GIZO TFT also had a constant sheet resistance of 1 kΩ/◻ for a film thickness of over 40 nm. The interface between the source/drain Mo metal and the Ar plasma-treated a-GIZO indicated a good Ohmic contact and a contact resistivity of 50 μΩ cm2.
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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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Appl. Phys. Lett. 93, 043303 (2008); http://dx.doi.org/10.1063/1.2959728 (3 pages) Online Publication Date: 28 July 2008
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We have fabricated high-performance organic thin-film transistors with top contact geometry using silver source and drain electrodes that are directly patterned on pentacene film by inkjet printing. Ink droplets containing Ag nanoparticles were ejected from inkjet nozzles. By controlling wave forms applied to piezoelectric actuators in these nozzles, the volume of the droplets can be changed from 17 to 1.4 pl. When the volume of the droplets is reduced, dc characteristics of manufactured transistors are improved significantly. The transistors manufactured with droplets of 1.4 pl exhibit a mobility of 0.3 cm2/Vs and an on-off current ratio that exceeding 106.
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Inducing ferromagnetism in ZnO through doping of nonmagnetic elements Appl. Phys. Lett. 93, 042514 (2008); http://dx.doi.org/10.1063/1.2966360 (3 pages) Online Publication Date: 1 August 2008
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In this work, Zn, Al, Pt, Ag, and Au nonmagnetic metallic films were deposited on the surface of ZnO film, followed by high vacuum annealing at different temperatures. Results showed that (Zn,Al,Pt)/ZnO films possessed room temperature ferromagnetism (RTF) after the vacuum annealing, while (Ag,Au)/ZnO films did not. Our detailed structural investigations (transmission electron microscopy and x-ray photoelectron spectroscopy) revealed that this RTF was associated with the presence of metal clusters. The RTF disappeared in Al/ZnO after a subsequent annealing in air, as metal clusters were oxidized. Pt/ZnO remained ferromagnetic, as the metal cluster structure was stable subjected to the air annealing.
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Appl. Phys. Lett. 93, 053109 (2008); http://dx.doi.org/10.1063/1.2967871 (3 pages) Online Publication Date: 6 August 2008
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We developed step edge decoration method for the fabrication of semiconductor ZnO nanodots and nanowires using pulsed laser deposition. We synthesized high quality ZnO nanowires with the small diameter of about 20 nm and the uniform interval of about 80 nm between each nanowire, which has a simple structure for the formation of contact electrodes. The ZnO nanowire-based sensor was prepared only with the simple process of a gold electrode formation. The ZnO nanowire-based sensor exhibited the high surface-to-volume ratio of 58.6 μm−1 and the significantly high sensitivity of about 10 even for the low ethanol concentration of 0.2 ppm.
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Appl. Phys. Lett. 93, 041105 (2008); http://dx.doi.org/10.1063/1.2963030 (3 pages) Online Publication Date: 29 July 2008
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We investigate the use of photonic crystals for light extraction from high-brightness thin-film AlGaInP light-emitting diodes with different etch depths, lattice constants, and two types of lattices (hexagonal and Archimedean). Both simulations and experimental results show that the extraction of high order modes with a low effective index neff is most efficient. The highest external quantum efficiency without encapsulation is 19% with an Archimedean A7 lattice with reciprocal lattice constant G = 1.5 k0, which is 47% better than an unstructured reference device.
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Low voltage organic light-emitting devices with triphenylphosphine oxide layer Appl. Phys. Lett. 93, 043306 (2008); http://dx.doi.org/10.1063/1.2960348 (3 pages) Online Publication Date: 1 August 2008
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We have developed low voltage driving organic light-emitting devices using triphenylphosphine oxide (Ph3PO) layers. The devices with a Ph3PO layer show high current density at a low voltage. For example, the current density of 20 mA/cm2 is achieved at a low voltage of 2.9 V for the device consisted of 4,4′,4″-tris[N-(2-naphthyl)-N-phenyl-amino]-triphenylamine (2-TNATA), 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi), and Ph3PO layers. Due to the good electron conduction property of Ph3PO, a luminance of 1017 cd/m2 is achieved at a low voltage of 3.0 V in a device with a structure of ITO/2-TNATA/DPVBi:rubrene (1%,10 nm)/DPVBi (30 nm)/Ph3PO (60 nm)/LiF/Al.
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Absence of substrate roughness effects on an all-printed organic transistor operating at one volt Appl. Phys. Lett. 93, 053302 (2008); http://dx.doi.org/10.1063/1.2958225 (3 pages) Online Publication Date: 4 August 2008
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A hygroscopic insulator transistor (HIFET) operating at 1 V was manufactured using roll-to-roll techniques on a rough, low-cost plastic substrate. The effects of the substrate roughness on the active channel were studied by using two different plastic substrates and comparing HIFETs and organic field-effect transistors (OFETs). We found that the HIFET, as opposed to OFETs, is rather insensitive to changes in the roughness of plastic substrates. Hence, a robust feature of ion modulated transistors is shown.
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X-ray diffraction study of polycrystalline BiFeO3 thin films under electric field Appl. Phys. Lett. 93, 042907 (2008); http://dx.doi.org/10.1063/1.2967736 (3 pages) Online Publication Date: 1 August 2008
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Diffraction measurements using 12.4 keV x-ray of synchrotron radiation have been performed in (001)pc- and (110)pc-oriented polycrystalline 350-nm-thick BiFeO3 thin films on a Pt/TiO2/SiO2/Si substrate under electric field in air at RT. Unipolar rectangular pulse voltages having a 150 ns width and a 804.09 ns period have been applied to BiFeO3 with a Pt top electrode. A diffraction peak of the (001)pc [(110)pc] plane shifts from 14.602° (20.520°) to 14.588 (20.505°) due to piezoelectric response when a 12 V (11 V) pulse is applied. Piezoelectric constants (d33) of (001)pc-oriented and (110)pc-oriented domains estimated from these peak shifts are 27.8 and 26.4 pm/V, respectively.
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Appl. Phys. Lett. 93, 041102 (2008); http://dx.doi.org/10.1063/1.2963029 (3 pages) Online Publication Date: 28 July 2008
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Blue multi-quantum-well light-emitting diodes (LEDs) with GaInN quantum wells and polarization-matched AlGaInN barriers are grown by metal-organic chemical vapor deposition. The use of quaternary alloys enables an independent control over interface polarization charges and bandgap and has been suggested as a method to reduce electron leakage from the active region, a carrier loss mechanism that can reduce efficiency at high injection currents—an effect known as the efficiency droop. The GaInN/AlGaInN LEDs show reduced forward voltage, reduced efficiency droop, and improved light-output power at large currents compared to conventional GaInN/GaN LEDs.
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Terahertz conductivity of thin metal films Appl. Phys. Lett. 93, 051105 (2008); http://dx.doi.org/10.1063/1.2968308 (3 pages) Online Publication Date: 5 August 2008
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The conductivities of thin Al, Au, and Ag films were measured via their transmission at terahertz frequencies. The conductivities of all the films, particularly the thinner films and Al films, were much smaller than their bulk dc values. This reduced conductivity can be quantitatively understood in terms of an increased scattering rate from defects. The transmission is consistent with a frequency independent conductivity, implying a very fast electron scattering time.
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