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Top 20 Most Read Articles
October 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Carrier mobility, structural order, and solar cell efficiency of organic heterojunction devices Appl. Phys. Lett. 93, 133308 (2008); http://dx.doi.org/10.1063/1.2996029 (3 pages) Online Publication Date: 2 October 2008
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The loss mechanisms limiting solar cell efficiency in organic heterojunction photodiodes are discussed. We show that the energy loss when the exciton is split, and absence of Langevin recombination at the interface, are related to the magnitude of the electron-phonon interaction and the wavefunction extent of the electron and hole. In contrast to several recent publications, it is argued that well-ordered, high mobility organic materials will provide higher efficiency cells.
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Electron trapping in pentacene based p- and n-type organic field-effect transistors Appl. Phys. Lett. 93, 133303 (2008); http://dx.doi.org/10.1063/1.2992031 (3 pages) Online Publication Date: 30 September 2008
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We report on electron trapping in the channel of pentacene field-effect transistors due to electron injection over large energy barriers. Using scanning Kelvin-probe microscopy, one observes temporary and permanent electron trapping in the transistor channel for both n- and p-type organic field-effect transistors driven in ambipolar mode, proving the ability of electrons to be injected from Au contacts into pentacene despite a large energy barrier. The trapped negative charge carriers are determined to be the origin of hysteresis in n-type devices and appear to be the source of threshold voltage shifts for p-type transistors.
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Low noise superconducting single photon detectors on silicon Appl. Phys. Lett. 93, 131101 (2008); http://dx.doi.org/10.1063/1.2990646 (3 pages) Online Publication Date: 29 September 2008
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We have fabricated superconducting nanowire single photon detectors made of NbTiN on a silicon substrate. This type of material reduces the dark count rate by a factor of 10 compared to identical NbN detectors, enabling single photon detection with unprecedented signal to noise ratio: we report a noise equivalent power of 10−19 W Hz−1/2 at 4.2 K. The compatibility of our superconducting device with silicon enables its integration with complex structures.
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Plasmonic nanoparticle enhanced light absorption in GaAs solar cells Appl. Phys. Lett. 93, 121904 (2008); http://dx.doi.org/10.1063/1.2988288 (3 pages) Online Publication Date: 22 September 2008
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We demonstrate an improvement in efficiency of optically thin GaAs solar cells decorated with size-controlled Ag nanoparticles fabricated by masked deposition through anodic aluminum oxide templates. The strong scattering by the interacting surface plasmons in densely formed high aspect-ratio nanoparticles effectively increases the optical path of the incident light in the absorber layers resulting in an 8% increase in the short circuit current density of the cell. The nanoparticle array sheet conductivity also reduces the cell surface sheet resistance evidenced by an improved fill factor. This dual function of plasmonic nanoparticles has potential to enable thinner photovoltaic layers in solar cells.
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Enhanced spontaneous emission in a photonic-crystal light-emitting diode Appl. Phys. Lett. 93, 143102 (2008); http://dx.doi.org/10.1063/1.2964186 (3 pages) Online Publication Date: 6 October 2008
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We report direct evidence of enhanced spontaneous emission in a photonic-crystal (PhC) light-emitting diode. The device consists of p-i-n heterojunction embedded in a suspended membrane, comprising a layer of self-assembled quantum dots. Current is injected laterally from the periphery to the center of the PhC. A well-isolated emission peak at 1.3 μm from the PhC cavity mode is observed, and the enhancement of the spontaneous emission rate is clearly evidenced by time-resolved electroluminescence measurements, showing that our diode switches off in a time shorter than the bulk radiative and nonradiative lifetimes.
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Fluoropolymer indium-tin-oxide buffer layers for improved power conversion in organic photovoltaics Appl. Phys. Lett. 93, 133302 (2008); http://dx.doi.org/10.1063/1.2983742 (3 pages) Online Publication Date: 29 September 2008
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We investigate the effects of polytetrafluoroethylene (PTFE) on poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based organic photovoltaic (OPV) devices by inserting thermally evaporated PTFE films between indium-tin-oxide (ITO) and P3HT:PCBM layers. Significant improvement in terms of open-circuit voltage, short-circuit current, and thereby in its commensurate power conversion efficiency is achieved compared to devices with poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) layers. The OPVs performance enhancement is attributed to the formation of an artificial dipole layer resulting from the rich, negatively charged fluorine that facilitates the hole extraction process. This result shows the high potential of PTFE as a low cost and stable ITO buffer layer for OPV devices.
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Ferromagnetism induced by defect complex in Co-doped ZnO Appl. Phys. Lett. 93, 132506 (2008); http://dx.doi.org/10.1063/1.2995997 (3 pages) Online Publication Date: 2 October 2008
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The effect of Al donor and O vacancy (VO) on the magnetic properties of Co-doped ZnO has been studied by first-principles calculations. It is found that only Al donor cannot induce ferromagnetism (FM) in Co-doped ZnO but can provide additional electrons, which results in the enhancement of Fermi level. The presence of VO makes the Co empty 3d-t2g minority state broadened, and a t2g-VO hybrid level at the conduction band minimum forms. The combination of Al donor and VO results in a charge transfer to the Co empty 3d-t2g minority states, which induces a strong carrier-mediated FM interaction. Our results give an explanation for the controversial magnetic properties of (Co, Al)-codoped ZnO reported in the literature.
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Plasmonic nanocavity arrays for enhanced efficiency in organic photovoltaic cells Appl. Phys. Lett. 93, 123308 (2008); http://dx.doi.org/10.1063/1.2988287 (3 pages) Online Publication Date: 23 September 2008
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We demonstrate enhanced power conversion efficiency in organic photovoltaic (OPV) cells incorporated into a plasmonic nanocavity array. The nanocavity array is formed between a patterned Ag anode and an unpatterned Al cathode. This structure leads to the confinement of optical energy and enhanced absorption in the OPV. Devices characterized under simulated solar illumination show a 3.2-fold increase in power conversion efficiency compared to OPVs with unpatterned Ag anodes. The observed enhancement is also reflected in the external quantum efficiency, and the spectral response is consistent with optical finite-difference time-domain simulations of the structure.
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Measurement of the optical absorption spectra of epitaxial graphene from terahertz to visible Appl. Phys. Lett. 93, 131905 (2008); http://dx.doi.org/10.1063/1.2990753 (3 pages) Online Publication Date: 30 September 2008
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We present experimental results on the optical absorption spectra of epitaxial graphene from the visible to the terahertz frequency range. In the terahertz range, the absorption is dominated by intraband processes with a frequency dependence similar to the Drude model. In the near-IR range, the absorption is due to interband processes and the measured optical conductivity is close to the theoretical value of e2/4ℏ. We extract values for the carrier densities, the number of carbon atom layers, and the intraband scattering times from the measurements.
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High quality GaN-based Schottky barrier diodes Appl. Phys. Lett. 93, 132110 (2008); http://dx.doi.org/10.1063/1.2993353 (3 pages) Online Publication Date: 30 September 2008
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We report the fabrication of GaN-based Schottky barrier diodes with multi-MgxNy/GaN buffer. Compared to conventional devices with a low-temperature GaN buffer, we achieved a six orders of magnitude smaller leakage current. It was also found that effective Schottky barrier height is larger for the proposed device due to the reduction in surface defect density by using the multi-MgxNy/GaN buffer.
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Appl. Phys. Lett. 93, 122106 (2008); http://dx.doi.org/10.1063/1.2988825 (3 pages) Online Publication Date: 24 September 2008
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Ballistic transport calculations, which are treated exactly within the nearest-neighbor tight-binding theory, on graphene strips connected to a pair of contacts are presented. In strips with high width-to-length ratio, the calculations reveal the presence of groups of resonances from a large number of nonequivalent conductance channels. While the energies of the collective resonances are, to first order, uniformly spaced and only depend on the contact separation, their contrast also depends on the strip width and contact interfaces.
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n-InGaN/p-GaN single heterostructure light emitting diode with p-side down Appl. Phys. Lett. 93, 133505 (2008); http://dx.doi.org/10.1063/1.2992582 (3 pages) Online Publication Date: 30 September 2008
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The effects of negative polarization charge at the n-InGaN/p-GaN interface on the performance of hydride vapor phase-epitaxy deposited single heterostructure n-InGaN/p-GaN LEDs with p-side down are investigated. The strong peak emission wavelength blueshift and concomitant superlinear increase in light output as the injection current increases below 25 A/cm2 are characteristic of radiative tunneling. We show that the combination of two-dimensional hole gas formation on the n-InGaN side of the heterointerface and enhancement of the electron barrier to transport across this interface results in only ∼ 10% efficiency droop up to 500 A/cm2 without implementation of an AlGaN electron-blocking layer or a second heterointerface.
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InAs–GaSb laser: Prospects for efficient terahertz emission Appl. Phys. Lett. 93, 131104 (2008); http://dx.doi.org/10.1063/1.2993232 (3 pages) Online Publication Date: 30 September 2008
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We suggest to use InAs/GaSb coupled quantum wells for terahertz lasing. In these heterostructures terahertz lasing is based not on intersubband but on interband transitions. Crucial advantages of this design in comparison with intersubband lasers are (i) a large value of the interband dipole matrix element and (ii) easier maintenance of population inversion. These advantages lead to a gain of two orders of magnitude higher than that for intersubband lasing. Even higher gain can be obtained in special design InAs/GaSb W-structures where a hybridization gap of 1–3 THz is formed and optical density of states is singular.
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Dark carrier recombination in organic solar cell Appl. Phys. Lett. 93, 133501 (2008); http://dx.doi.org/10.1063/1.2972115 (3 pages) Online Publication Date: 29 September 2008
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The carrier recombination in organic solar cells is investigated by numerical modeling to understand the weak dependence of the open-circuit voltage on the workfunction of the electrodes. In Ohmic contact structures, photocarriers recombine predominantly with dark carriers diffused from the electrode into the semiconductor. Such dark carrier recombination becomes the main limit of power conversion efficiency and open-circuit voltage. For a given semiconductor decreasing the workfunction difference of the electrodes reduces simultaneously the dark carrier recombination and the flat band voltage. The balance between these two opposite factors gives a nearly constant open-circuit voltage. In an ideal bilayer structure there is no dark carrier recombination and the efficiency is demonstrated to be 60% higher than single layer blend.
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Appl. Phys. Lett. 93, 142502 (2008); http://dx.doi.org/10.1063/1.2993354 (3 pages) Online Publication Date: 6 October 2008
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Monochromatic cathodoluminescence image of Mn-doped ZnO nanorods shows that Mn elements are mainly distributed on the surface of nanorods. The intensity of ultraviolet luminescence decreases with the increment in Mn-doped concentration. Mn-doped ZnO nanorods exhibit a room temperature ferromagnetic characteristic with a saturation magnetization (MS) of 0.39 μB/Mn and a coercive field of 50 Oe.
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Size effects on thin film ferroelectrics: Experiments on isolated single crystal sheets Appl. Phys. Lett. 93, 132904 (2008); http://dx.doi.org/10.1063/1.2990760 (3 pages) Online Publication Date: 1 October 2008
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Thin lamellae were cut from bulk single crystal BaTiO3 using a focused ion beam microscope. They were then removed and transferred onto single crystal MgO substrates, so that their functional properties could be measured independent of the original host bulk ferroelectric. The temperature dependence of the capacitance of these isolated single crystal films was found to be strongly bulklike, demonstrating a sharp Curie anomaly, as well as Curie–Weiss behavior. In addition, the sudden change in the remanent polarization as a function of temperature at TC was characteristic of a first order phase change. The work represents a dramatic improvement on that previously published by
Saad et al. [J. Phys.: Condens. Matter 16, L451 (2004)
], as critical shortcomings in the original specimen geometry, involving potential signal contributions from bulk BaTiO3, have now been obviated. That the functional properties of single crystal thin film lamellae are comparable to bulk, and not like those of conventionally deposited heterogeneous thin film systems, has therefore been confirmed.
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Appl. Phys. Lett. 93, 132901 (2008); http://dx.doi.org/10.1063/1.2985815 (3 pages) Online Publication Date: 29 September 2008
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We have directly grown single-walled carbon nanotubes on epitaxial BaTiO3 thin films, fabricating prototype carbon nanotube-ferroelectric devices. We demonstrate polarization switching using the nanotube as a local electric field source and compare the results to switching with an atomic force microscopy tip. The observed variation of domain growth rates in the two cases agrees with the changes in electric field intensity at the ferroelectric surface.
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Low-voltage C60 organic field-effect transistors with high mobility and low contact resistance Appl. Phys. Lett. 93, 133305 (2008); http://dx.doi.org/10.1063/1.2993349 (3 pages) Online Publication Date: 2 October 2008
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State-of-the-art C60 organic transistors are reported here by engineering the essential electrode/semiconductor and dielectric/semiconductor interfaces. By using calcium (Ca) as the source and drain electrodes, the width-normalized contact resistance (RCW) at the electrode/semiconductor interface could be reduced to a constant value of 2 kΩ cm at a gate-source voltage (VGS) of 2.6 V, for devices with channel lengths ranging from 25 to 200 μm. Channel transconductance is observed to follow channel length scaling, and charge mobility average value of 2.5 cm2/Vs at VGS<5 V is found independent of channel length within the studied range.
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Low voltage and very high efficiency deep-blue phosphorescent organic light-emitting devices Appl. Phys. Lett. 93, 133309 (2008); http://dx.doi.org/10.1063/1.2996274 (3 pages) Online Publication Date: 2 October 2008
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We report on very high efficiency deep-blue phosphorescent organic light-emitting devices (PHOLEDs) based on iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6). Dual emissive layers consisting of an N,N′-dicarbazolyl-3,5-benzene layer doped with 4 wt % FIr6 and a p-bis(triphenylsilyly)benzene layer doped with 25 wt % FIr6 were employed to maximize exciton generation on FIr6 molecules. Combined with the p-i-n device structure, we achieved a low turn-on voltage of 3.2 V and very high power efficiencies of 25±2 lm/W at 100 cd/m2 and 20±2 lm/W at 1000 cd/m2 for such deep-blue PHOLEDs with peak emission at a wavelength of 458 nm.
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Appl. Phys. Lett. 93, 133301 (2008); http://dx.doi.org/10.1063/1.2960349 (3 pages) Online Publication Date: 29 September 2008
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A facile way to fabricate highly efficient organic light emitting devices with insulator MnO as an electron injecting and transporting material was devised, which eliminates the problem of the oxidation of reactive dopants. The power efficiency of 1.1 lm/W by inserting 3-nm-thick MnO as the electron injecting layer was obtained, higher than the 0.8 lm/W efficiency for the reference device with 0.5-nm-thick LiF. A thermal coevaporation layer containing 10% weight of MnO and tris(8-hydroxyquinolato)aluminum (Alq3) as the electron transporting layer showed more efficient electron transport ability, with turn-on voltage of 3.8 V, lower than 7.4 V for the intrinsic Alq3.
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