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Top 20 Most Read Articles
January 2012
The 20 articles with the most full-text downloads during the month, in descending order.
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Photoinduced write-once read-many-times memory device based on DNA biopolymer nanocomposite Appl. Phys. Lett. 99, 253301 (2011); http://dx.doi.org/10.1063/1.3671153 (3 pages) Online Publication Date: 19 December 2011
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We demonstrate a photoinduced write-once read-many-times (WORM) organic memory device based on DNA biopolymer nanocomposite. The device consists of a single biopolymer layer sandwiched between electrodes, in which electrical bistability is activated by in situ formation of silver nanoparticles embedded in biopolymer upon light irradiation. The device exhibits a switching effect to high conductivity above a threshold of 2.6 V and a good retention property. This facile technique, taking advantage of DNA’s affinity for metals and solution processing, can optically manipulate the properties of DNA nanocomposite thin films, which holds promise for optical storage and plasmonic applications.
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Resonant-tunnelling-diode oscillators operating at frequencies above 1.1 THz Appl. Phys. Lett. 99, 233506 (2011); http://dx.doi.org/10.1063/1.3667191 (3 pages) Online Publication Date: 8 December 2011
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We present resonant-tunnelling-diode (RTD) oscillators operating at the fundamental frequency of 1111 GHz. We show that our RTDs and RTD oscillators have much room for further improvement of their parameters and for further increase of their operating frequencies. The operating frequencies of several THz should be achievable with RTD oscillators. Our study also shows that operation of RTDs beyond the relaxation-time limit at THz frequencies should be possible. RTD oscillators under study are extremely compact (less than a square millimeter) room-temperature sources of coherent cw THz radiation. Such sources should enable plenty of real-world THz applications.
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A high-reflectivity, ambient-stable graphene mirror for neutral atomic and molecular beams Appl. Phys. Lett. 99, 211907 (2011); http://dx.doi.org/10.1063/1.3663866 (3 pages) Online Publication Date: 22 November 2011
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We report a He and H2 diffraction study of graphene-terminated Ru(0001) thin films grown epitaxially on c-axis sapphire. Even for samples exposed for several weeks to ambient conditions, brief annealing in ultrahigh vacuum restored extraordinarily high specular reflectivities for He and H2 beams (23% and 7% of the incident beam, respectively). The quality of the angular distributions recorded with both probes exceeds the one obtained from in-situ prepared graphene on Ru(0001) single crystals. Our results for graphene-terminated Ru thin films represent a significant step toward ambient tolerant, high-reflectivity curved surface mirrors for He-atom microscopy.
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Appl. Phys. Lett. 99, 252905 (2011); http://dx.doi.org/10.1063/1.3671392 (4 pages) Online Publication Date: 22 December 2011
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Ferroelectric BiFeO3 (BFO) nanoparticles deposited on epitaxial substrates of SrRuO3 (SRO) and La1−xSrxMnO3 (LSMO) were studied using band excitation piezoresponse spectroscopy (BEPS), piezoresponse force microscopy (PFM), and ferromagnetic resonance (FMR). BEPS confirms that the nanoparticles are ferroelectric in nature. Switching behavior of nanoparticle clusters were studied and showed evidence for inhomogeneous switching. The dimensionality of domains within nanoparticles was found to be fractal in nature, with a dimensionality constant of ∼1.4, on par with ferroelectric BFO thin-films under 100 nm in thickness. Ferromagnetic resonance studies indicate BFO nanoparticles only weakly affect the magnetic response of LSMO.
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Spin injection from two-dimensional electron and hole gases in resonant tunneling diodes Appl. Phys. Lett. 99, 233507 (2011); http://dx.doi.org/10.1063/1.3668087 (4 pages) Online Publication Date: 8 December 2011
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We have investigated the polarized-resolved photoluminescence from the contact layers and the quantum-well in an n-type GaAs/GaAlAs resonant tunneling diode for magnetic fields up to 19 T. The optical emission from the GaAs contact layers comprises the recombination from highly spin-polarized two-dimensional electron and hole gases with free tunneling carriers. Both the energy position and intensity of this indirect recombination are voltage-dependent and show remarkably abrupt variations near scattering-assisted tunneling resonances. Our results show that these two dimensional gases act as spin-polarized sources for carriers tunneling through the well in resonant tunneling diodes.
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Monolayer graphene growth on Ni(111) by low temperature chemical vapor deposition Appl. Phys. Lett. 100, 021601 (2012); http://dx.doi.org/10.1063/1.3675481 (3 pages) Online Publication Date: 9 January 2012
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In contrast to the commonly employed high temperature chemical vapor deposition growth that leads to multilayer graphene formation by carbon segregation from the bulk, we demonstrate that below 600 °C graphene can be grown in a self-limiting monolayer growth process. Optimum growth is achieved at ∼550 °C. Above this temperature, carbon diffusion into the bulk is limiting the surface growth rate, while at temperatures below ∼500 °C a competing surface carbide phase impedes graphene formation.
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Appl. Phys. Lett. 100, 011914 (2012); http://dx.doi.org/10.1063/1.3671162 (3 pages) Online Publication Date: 6 January 2012
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Transparent conducting oxides combine high electrical conductivity with transparency to visible light. However, the large concentration of free electrons introduces a source of absorption that limits the transparency. Here, we evaluate the importance of phonon-assisted free-carrier absorption in SnO2 completely from first principles. Our results show that absorption is modest in the visible and much stronger in the UV and infrared. We also provide insight into the mechanisms that govern absorption in different wavelength regimes.
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Impact of unbalanced charge transport on the efficiency of normal and inverted solar cells Appl. Phys. Lett. 100, 013306 (2012); http://dx.doi.org/10.1063/1.3663860 (3 pages) Online Publication Date: 5 January 2012
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In a normal solar cell, most charge carriers are generated close to the anode, such that electrons have to travel a longer distance as compared to the holes. In an inverted solar cell, holes have to travel a longer distance. We use a combined optical and electronic model to simulate the effect of unbalanced transport on the efficiency of normal and inverted single and tandem solar cells. When the electrons are ten times more mobile than the holes, the efficiency for a single cell with a thickness of 250 nm drops from 7.5% to 4.5% when changing from a normal to an inverted structure. For opposite mobility ratio, the inverted structure clearly outperforms the normal structure.
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Appl. Phys. Lett. 100, 021101 (2012); http://dx.doi.org/10.1063/1.3675451 (4 pages) Online Publication Date: 9 January 2012
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For AlGaN-based multi-quantum-well light emitters grown on c-plane substrates there is a tendency for the polarization of the emitted light to switch from transverse electric (TE) polarization to transverse magnetic (TM) polarization as the wavelength decreases. This transition depends on various factors that include the strain in the quantum well. Experimental results are presented that illustrate the phenomenon for nitride light emitting diodes (LEDs) grown on sapphire and on bulk AlN. Model calculations are presented which quantify the dependence of the TE/TM switch on the quantum well strain and the Al composition in the barriers surrounding the well.
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Appl. Phys. Lett. 100, 013507 (2012); http://dx.doi.org/10.1063/1.3675453 (3 pages) Online Publication Date: 6 January 2012
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High quality, nearly lattice-matched InAlN/GaN/InAlN/GaN double-channel heterostructures were grown on sapphire by pulsed-metal-organic-chemical-vapor-deposition (PMOCVD). High electron mobility of 1414 cm2/Vs was achieved along with a two-dimensional-electron-gas density of 2.55 × 1013 cm−2. We attribute it to the high quality PMOCVD-grown InAlN barriers and, additionally, to the novel GaN layer growth between two InAlN barriers, which consists of a thin GaN spacer to prevent indium-redistribution and indium-cluster formation during the subsequent growth and a relatively thick GaN channel to enhance electron mobility. High-electron-mobility-transistors fabricated on these heterostructures with 0.8-μm-length gate exhibit a maximum drain current of 906 mA/mm and a transconductance of 186 mS/mm.
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Efficient optical trapping using small arrays of plasmonic nanoblock pairs Appl. Phys. Lett. 100, 021102 (2012); http://dx.doi.org/10.1063/1.3675550 (3 pages) Online Publication Date: 9 January 2012
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We report that a small two-dimensional array of gold nanoblock pairs separated by a nanometric gap significantly improves the performance of optical trapping compared to a single nanoblock pair. The array of 4 × 4 pairs suppresses the Brownian motion of a trapped 1 μm diameter particle by a factor of six compared to the single pair. In addition, the array enables particle trapping for a longer period of time. These results are essential for biological applications where intense optical irradiation is a concern.
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Can morphology tailoring improve the open circuit voltage of organic solar cells? Appl. Phys. Lett. 100, 013307 (2012); http://dx.doi.org/10.1063/1.3672221 (3 pages) Online Publication Date: 5 January 2012
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While the effect of interfacial morphology on the short circuit current (ISC) of organic photovoltaic devices (OPVs) is well known, its impact on open circuit voltage (VOC) and fill-factor (FF) are less clear. Since the output power of a solar cell Pout = ISCVOCFF, such understanding is critical for designing high-performance, morphology-engineered OPVs. In this letter, we provide an explicit analytical proof that any effort to radically improve VOC by tailoring bulk heterojunction morphology is futile, because any increase in ISC due to larger interface area is counterbalanced by corresponding increase in recombination current, so that the upper limit of VOCBHJ cannot exceed that of the corresponding planar heterojunction devices, i.e., VOCBHJ ≤ VOCPHJ. We discuss the implication of this VOC-constraint on the efficiency optimization of organic solar cells.
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Laser direct growth of graphene on silicon substrate Appl. Phys. Lett. 100, 023110 (2012); http://dx.doi.org/10.1063/1.3675636 (3 pages) Online Publication Date: 10 January 2012
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We demonstrate laser direct growth of few layer graphene on a silicon substrate. In our study, a continuous wave laser beam was focused on a poly(methyl methacrylate) (PMMA)-coated silicon wafer to evaporate PMMA and melt the silicon wafer. Carbon atoms, decomposed from PMMA, were absorbed by the molten silicon surface, and then separated from silicon in the cooling process to form few-layer graphene. This Si-catalyzed method will provide a new approach and platform for applications of graphene.
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Ultra-thin plasmonic optical vortex plate based on phase discontinuities Appl. Phys. Lett. 100, 013101 (2012); http://dx.doi.org/10.1063/1.3673334 (3 pages) Online Publication Date: 3 January 2012
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A flat optical device that generates optical vortices with a variety of topological charges is demonstrated. This device spatially modulates light beams over a distance much smaller than the wavelength in the direction of propagation by means of an array of V-shaped plasmonic antennas with sub-wavelength separation. Optical vortices are shown to develop after a sub-wavelength propagation distance from the array, a feature that has major potential implications for integrated optics.
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Appl. Phys. Lett. 100, 022102 (2012); http://dx.doi.org/10.1063/1.3675632 (3 pages) Online Publication Date: 9 January 2012
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A noncatalytic chemical vapor deposition mechanism is proposed, where high precursor concentration, long deposition time, high temperature, and flat substrate are needed to grow large-area nanocrystalline graphene using hydrocarbon pyrolysis. The graphene is scalable, uniform, and with controlled thickness. It can be deposited on virtually any nonmetallic substrate that withstands ∼1000 °C. For typical examples, graphene grown directly on quartz and sapphire shows transmittance and conductivity similar to exfoliated or metal-catalyzed graphene, as evidenced by transmission spectroscopy and transport measurements. Raman spectroscopy confirms the sp2-C structure. The model and results demonstrate a promising transfer-free technique for transparent electrode production.
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Temperature-dependence of the internal efficiency droop in GaN-based diodes Appl. Phys. Lett. 99, 181127 (2011); http://dx.doi.org/10.1063/1.3658031 (3 pages) Online Publication Date: 4 November 2011
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The temperature dependence of the measured internal efficiencies of green and blue emitting InGaN-based diodes is analyzed. With increasing temperature, a strongly decreasing strength of the loss mechanism responsible for droop is found which is in contrast to the usually assumed behavior of Auger losses. However, the experimental observations can be well reproduced assuming density activated defect recombination with a temperature independent recombination time.
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Appl. Phys. Lett. 100, 013308 (2012); http://dx.doi.org/10.1063/1.3675970 (4 pages) Online Publication Date: 6 January 2012
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Surface plasmon-enhanced electroluminescence (EL) in an organic light-emitting diode is demonstrated by incorporating the synthesized Au nanoparticles (NPs) in the hole injection layer of poly(3,4-ethylene dioxythiophene):polystyrene sulfonic acid. An increase of ∼25% in the EL intensity and efficiency are achieved for devices with Au NPs, whereas the spectral and electrical properties remain almost identical to the control device. Time-resolved photoluminescence spectroscopy reveals that the EL enhancement is ascribed to the increase in spontaneous emission rate due to the plasmonic near-field effect induced by Au NPs.
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Diffusion, aggregation, and the thermal conductivity of nanofluids Appl. Phys. Lett. 93, 103110 (2008); http://dx.doi.org/10.1063/1.2977868 (3 pages) Online Publication Date: 11 September 2008
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The effects of nanoparticle aggregation and diffusion are difficult to separate using most nanofluid thermal conductivity data, for which the temperature dependence is collected sequentially. The present work captures the instantaneous temperature-dependent thermal conductivity using cross-sectional infrared microscopy and tracks the effects of aggregation and diffusion over time. The resulting data are strongly influenced by spatial and temperature variations in particle size and concentration and are interpreted using a Monte Carlo simulation and rate equations for particle and heat transport. These experiments improve our understanding of nanofluid behavior in practical systems including microscale heat exchangers.
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Advantages of blue InGaN light-emitting diodes with InGaN-AlGaN-InGaN barriers Appl. Phys. Lett. 100, 031112 (2012); http://dx.doi.org/10.1063/1.3678341 (3 pages) Online Publication Date: 20 January 2012
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Efficiency enhancement of the blue InGaN light-emitting diodes (LEDs) with InGaN-AlGaN-InGaN barriers is studied numerically. The energy band diagrams, carrier concentrations in quantum wells, radiative recombination rate in active region, light-current performance curves, and internal quantum efficiency are investigated. The simulation results suggest that the blue InGaN/InGaN-AlGaN-InGaN LED has better performance over its conventional InGaN/GaN and InGaN/InGaN counterparts due to the appropriately modified energy band diagrams, which are caused mainly by the reduced polarization charges at the interface between the well and barrier.
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Appl. Phys. Lett. 100, 023307 (2012); http://dx.doi.org/10.1063/1.3671181 (3 pages) Online Publication Date: 12 January 2012
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We demonstrate organic discrete heterojunction photovoltaic cells based on fullerene (C60) and copper hexadecafluorophthalocyanine (F16CuPc), in which the C60 and F16CuPc act as the electron donor and the electron acceptor, respectively. The C60/F16CuPc cells fabricated with conventional and inverted architectures both exhibit comparable power conversion efficiencies. Furthermore, we show that the photocurrent in both cells is generated by a conventional exciton dissociation mechanism rather than the exciton recombination mechanism recently proposed for a similar C60/F16ZnPc system [Song et al., J. Am. Chem. Soc. 132, 4554 (2010)]. These results demonstrate that new unconventional material systems are a potential way to fabricate organic photovoltaic cells with inverted as well as conventional architectures.
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