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Top 20 Most Read Articles
August 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 99, 032105 (2011); http://dx.doi.org/10.1063/1.3614553 (3 pages) Online Publication Date: 22 July 2011
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The phase change memory with monolayer chalcogenide film (Si18Sb52Te30) is investigated for the feasibility of multilevel data storage. During the annealing of the film, a relatively stable intermediate resistance can be obtained at an appropriate heating rate. The transmission electron microscopy in situ analysis reveals a conversion of crystallization mechanism from nucleation to crystal growth, which leads a continuous reduction in the degree of disorder. It is indicated from the electrical properties of the devices that the fall edge of the voltage pulse is the critical factor that determines a reliable triple-level resistance state of the phase change memory cell.
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Appl. Phys. Lett. 98, 262102 (2011); http://dx.doi.org/10.1063/1.3600787 (3 pages) Online Publication Date: 27 June 2011
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We report the electrical spin accumulation with enhanced bias voltage dependence in n-type Si, employing a crystalline CoFe/MgO tunnel contact. A sizable spin signal of ∼4.8 kΩμm2, a spin lifetime of ∼155 ps, and a spin diffusion length of ∼220 nm were obtained at 300 K. The spin signal and lifetime obtained in this system show consistent behavior with the temperature variation irrespective of the bias voltage. Notably, the spin signal exhibits nearly symmetric dependence with respect to the bias polarity, which is ascribed to the improved bias dependence of tunnel spin polarization.
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Improved endurance of resistive switching TiO2 thin film by hourglass shaped Magnéli filaments Appl. Phys. Lett. 98, 262901 (2011); http://dx.doi.org/10.1063/1.3600784 (3 pages) Online Publication Date: 27 June 2011
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A modified biasing scheme was adopted to improve the electrical endurance characteristics of conducting filamentary resistive switching (RS) in a Pt/TiO2/Pt RS cell. The modified bias scheme included the application of bias voltages with alternating polarity, even though RS proceeds in non-polar mode, which results in the stable distribution of each resistance states as well as improved endurance. This was attributed to the minimized consumption of oxygen ions in the TiO2 film, which can be induced by the formation of hourglass-shaped conducting filament (HSCF). The presence of a HSCF was confirmed by high-resolution transmission electron microscopy.
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Appl. Phys. Lett. 99, 042901 (2011); http://dx.doi.org/10.1063/1.3615675 (3 pages) Online Publication Date: 25 July 2011
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Phase stability of (1 − x) (Bi1/2Na1/2)TiO3-x BaTiO3 (0 ≤ x ≤ 0.15) under electric field was investigated by measuring volume changes during a bipolar poling cycle. The unique nature of field-dependent phase stability with three distinctive regions is revealed by comparative studies using commercial soft PZT and relaxor PLZT. For x ≤ 0.06 and x ≥ 0.13, similarly with PZT and PLZT, the axial strain expands with the contracting radial strain, but the former results in a remanent volume demonstrating an electric-field-induced phase transition. For 0.08 ≤ x ≤ 0.12, this field-induced phase transition is distinguished by negligible radial contractions implying polarization rotation. A “poling-induced” morphotropic phase boundary forms at x = 0.07.
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Nanoparticle-based protein detection by optical shift of a resonant microcavity Appl. Phys. Lett. 99, 073701 (2011); http://dx.doi.org/10.1063/1.3599706 (3 pages) Online Publication Date: 15 August 2011
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We demonstrated a biosensing approach which, for the first time, combines the high sensitivity of whispering gallery modes (WGMs) with a metallic nanoparticle-based assay. We provided a computational model based on generalized Mie theory to explain the higher sensitivity of protein detection. We quantitatively analyzed the binding of a model protein (i.e., Bovine Serum Albumin) to gold nanoparticles from high-Q WGM resonance frequency shifts, and fit the results to an adsorption isotherm, which agrees with the theoretical predictions of a two-component adsorption model.
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One-dimensional photonic crystal nanobeam lasers on a flexible substrate Appl. Phys. Lett. 99, 071101 (2011); http://dx.doi.org/10.1063/1.3626592 (3 pages) Online Publication Date: 15 August 2011
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We demonstrate a one-dimensional (1D) photonic crystal (PhC) nanocavity laser composed of hybrid PhC mirrors on a suspended nanobeam (NB) with very small device footprint of 8.5 × 0.57 μm2. The 0th-order mode lasing action with low threshold of 280 μW is observed. Via the optical glue stamping process, the devices are directly transferred onto a flexible polypropylene substrate. Single mode lasing action with effective threshold of 17 μW is achieved. The robust lasing properties of the device with different bending radii R from ∞ to 2.5 mm are obtained. Via finite-element method, we also theoretically address that the lasing wavelength is almost invariant when R > 1.0 mm. This flexible 1D PhC NB laser will be a good candidate for efficient nanolaser in future flexible photonic integrated circuits with ultrahigh component density.
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Appl. Phys. Lett. 99, 063108 (2011); http://dx.doi.org/10.1063/1.3624900 (3 pages) Online Publication Date: 9 August 2011
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The authors show that the magnetization of a 2-phase magnetostrictive/piezoelectric multiferroic single-domain shape-anisotropic nanomagnet can be switched with very small voltages that generate strain in the magnetostrictive layer. This can be the basis of ultralow power computing and signal processing. With appropriate material choice, the energy dissipated per switching event can be reduced to ∼45 kT at room temperature for a switching delay of ∼100 ns and ∼70 kT for a switching delay of ∼10 ns, if the energy barrier separating the two stable magnetization directions is ∼32 kT. Such devices can be powered by harvesting energy exclusively from the environment without the need for a battery.
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Theory of plasmonic near-field enhanced absorption in solar cells Appl. Phys. Lett. 99, 063304 (2011); http://dx.doi.org/10.1063/1.3623759 (3 pages) Online Publication Date: 11 August 2011
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We derive analytical expressions for the absorption enhancement expected when dilute suspensions of small metallic nanoparticles are inserted inside an organic semiconductor. A comparison with accurate numerical simulations shows excellent agreement for a wide range of volume filling ratios and even in the case of mixing different types of metals. These results are invaluable tools in optimizing the absorption performance of plasmonic thin-film organic solar cells.
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Metal/graphene sheets as p-type transparent conducting electrodes in GaN light emitting diodes Appl. Phys. Lett. 99, 041115 (2011); http://dx.doi.org/10.1063/1.3595941 (3 pages) Online Publication Date: 29 July 2011
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We demonstrate the use of graphene based transparent sheets as a p-type current spreading layer in GaN light emitting diodes (LEDs). Very thin Ni/Au was inserted between graphene and p-type GaN to reduce contact resistance, which reduced contact resistance from ∼ 5.5 to ∼ 0.6Ω/cm2, with no critical optical loss. As a result, LEDs with metal-graphene provided current spreading and injection into the p-type GaN layer, enabling three times enhanced electroluminescent intensity compared with those with graphene alone. We confirmed very strong blue light emission in a large area of the metal-graphene layer by analyzing image brightness.
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Nanobeam photonic crystal cavity light-emitting diodes Appl. Phys. Lett. 99, 071105 (2011); http://dx.doi.org/10.1063/1.3625432 (3 pages) Online Publication Date: 16 August 2011
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We present results on electrically driven nanobeam photonic crystal cavities formed out of a lateral p-i-n junction in gallium arsenide. Despite their small conducting dimensions, nanobeams have robust electrical properties with high current densities possible at low drive powers. Much like their two-dimensional counterparts, the nanobeam cavities exhibit bright electroluminescence at room temperature from embedded 1250 nm InAs quantum dots. A small room temperature differential gain is observed in the cavities with minor beam self-heating suggesting that lasing is possible. These results open the door for efficient electrical control of active nanobeam cavities for diverse nanophotonic applications.
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Air stable hybrid inverted tandem solar cell design Appl. Phys. Lett. 99, 063301 (2011); http://dx.doi.org/10.1063/1.3622119 (3 pages) Online Publication Date: 8 August 2011
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In order to get an air stable solar cell with high open circuit voltage (Voc), we fabricated an inverted tandem solar cell based on hybrid wet chemistry and vacuum thermal deposition techniques. A thin metallic interfacial layer was applied to improve charge recombination and maximize both the fill factor and Voc of the tandem solar cell. A cationic dye doped electron transport layer was used to minimize space charge induced Voc loss. The tandem cell Voc reached 1.02 V, which equals the sum of the two subcells’ Voc. Increase of the metal nanoparticles’ layer thickness reduces the short circuit current density of the tandem owing to increasing light extinction. Our tandem cell design offers superior air stability due to additional encapsulation effect from top metal oxide layers. It retains about 80% of its original efficiency after storage in air for three months.
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Appl. Phys. Lett. 86, 252902 (2005); http://dx.doi.org/10.1063/1.1952567 (3 pages) Online Publication Date: 14 June 2005
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A method for thin-film fabrication employing high oxygen-pressure processing (HOPP) was developed. With this method, the highly (100) oriented Pb(ZrxTi1−x)O3 (PZT) thin film was fabricated at temperature as low as 400 °C. HOPP is compatible to the ferroelectric PZT film integration with a readout integrated circuit. The sol-gel-derived PZT 50/50 thin film showed a well-saturated hysteresis loop at an applied electric field of 367 kV/cm with Pr and Ec of 45 μC/cm2 and 121 kV/cm, respectively. Large electric leakage was attributed to remnant organic components, which was demonstrated by sputtered organic-free PZT films. The optimized Pr and Ec are of 26 μC/cm2 and 93 kV/cm under an applied electric field of 400 kV/cm.
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Appl. Phys. Lett. 98, 113305 (2011); http://dx.doi.org/10.1063/1.3567516 (3 pages) Online Publication Date: 17 March 2011
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We fabricate near-infrared absorbing organic photovoltaics that are highly transparent to visible light. By optimizing near-infrared optical-interference, we demonstrate power efficiencies of 1.3±0.1% with simultaneous average visible transmission of >65%. Subsequent incorporation of near-infrared distributed-Bragg-reflector mirrors leads to an increase in the efficiency to 1.7±0.1%, approaching the 2.4±0.2% efficiency of the opaque cell, while maintaining high visible-transparency of >55%. Finally, we demonstrate that a series-integrated array of these transparent cells is capable of powering electronic devices under near-ambient lighting. This architecture suggests strategies for high-efficiency power-generating windows and highlights an application uniquely benefiting from excitonic electronics.
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Strain control spin reorientation transition in DyFeO3/SrTiO3 epitaxial film Appl. Phys. Lett. 99, 072501 (2011); http://dx.doi.org/10.1063/1.3623756 (3 pages) Online Publication Date: 15 August 2011
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We grew single phase DyFeO3 (110) epitaxial films on SrTiO3(001) substrates in the layer-by-layer mode using the pulsed laser deposition. The Fe L2,3-edge x-ray magnetic linear dichroism and magnetic hysteresis confirm that the film shows the spin-canted antiferromagnetism at room temperature and undergoes the spin reorientation transition (SRT) upon cooling as the bulk. The SRT temperature is significantly reduced in the thin films due to the compressive strain and becomes below 5 K for the thickness ≲ 50 Å.
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Role of oxygen-bonds in the degradation process of phosphorescent organic light emitting diodes Appl. Phys. Lett. 99, 053302 (2011); http://dx.doi.org/10.1063/1.3617459 (3 pages) Online Publication Date: 1 August 2011
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For improving the lifetime of organic light emitting diodes (OLEDs), it is essential to understand the chemical reaction pathways involved in the degradation process of these devices. Using the laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) technique, an OLED based on the phosphorescent emitter tris(phenylpyridine)iridium (Ir(ppy)3) was investigated. We show that the chemical dissociation reaction of the Ir(ppy)3 molecule and an additional complexation of its fragments with the used hole blocking material 4,7-diphenyl-1,10-phenanthroline is the main intrinsic degradation mechanism. This finding underlines the exciton induced dissociation mechanism of phosphorescent emitters, even at emitters without any metal-oxygen-bonds in the ligand system.
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Highly efficient semitransparent tandem organic solar cells with complementary absorber materials Appl. Phys. Lett. 99, 043301 (2011); http://dx.doi.org/10.1063/1.3610551 (3 pages) Online Publication Date: 25 July 2011
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We present highly efficient, semitransparent small molecule organic solar cells. The devices employ an indium tin oxide-free top contact, consisting of thin metal films. An additional organic layer is used to enhance light outcoupling. The solar cell incorporates two stacked subcells, each containing a donor:acceptor bulk heterojunction. The two subcells have complementary absorbers, with separate blue (C60), red (fluorinated zinc phthalocyanine), and green (dicyanovinyl oligothiophene derivative) absorbing molecules. A power conversion efficiency of 4.9 ± 0.2% is obtained for the device having an average transmission of 24% in the visible range.
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On-a-chip surface plasmon tweezers Appl. Phys. Lett. 99, 061107 (2011); http://dx.doi.org/10.1063/1.3625936 (3 pages) Online Publication Date: 11 August 2011
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We report on an integrated optical trapping platform operated by simple fiber coupling. The system consists of a dielectric channel optical waveguide decorated with an array of gold micro-pads. Through a suitable engineering of the waveguide mode, we achieve light coupling to the surface plasmon resonance of the gold pads that act as individual plasmonic traps. We demonstrate parallel trapping of both micrometer size polystyrene beads and yeast cells at predetermined locations on the chip with only 20 mW total incident laser power.
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On the optical polarization properties of semipolar InGaN quantum wells Appl. Phys. Lett. 99, 051103 (2011); http://dx.doi.org/10.1063/1.3618676 (3 pages) Online Publication Date: 2 August 2011
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Polarized photoluminescence of strained quantum wells grown on c–plane, semipolar (10
 2), (11 2), (101), (201) planes, and nonpolar GaN substrates was studied experimentally and in theory. The observed optical polarization switching between the substrate orientations (102) and (112) is in accordance with our general model of polarization switching, based on a  · model of arbitrary substrate orientation. Spectrally resolved measurements of the polarization degree stemming from (102) samples show that the maximum of the polarization degree is red–shifted with respect to the maximum of the photoluminescence intensity. We ascribe this effect to an increased polarization of the transitions for higher indium content. |
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Appl. Phys. Lett. 99, 053305 (2011); http://dx.doi.org/10.1063/1.3623477 (3 pages) Online Publication Date: 4 August 2011
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Ultra-low frequency AC field was employed to align p/n polymers during organic photovoltaic device fabrication. The resulting devices show 15% increase in power conversion efficiency and four-fold increase in parallel resistance. Supported by the transmission electron microscopy and atomic force microscopy images, the performance enhancement is attributed to the optimized morphology and enlarged p/n interface by AC field, which is more effective than DC, possibly explained by the argument of better mixing via back-and-forth shaking than a single swing.
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Appl. Phys. Lett. 99, 053301 (2011); http://dx.doi.org/10.1063/1.3617427 (3 pages) Online Publication Date: 1 August 2011
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Organic p-i-n homojunctions that function both as ultra violet light emitting diode (peak wavelengths around 375 nm and 415 nm) and visible-blind photodiode are reported. They are considered as the organic counterpart to what has exclusively been known from inorganic semiconductors, as yet. The diodes are based on the ambipolar material 4,4′-Bis(carbazol-9-yl)-biphenyl (CBP) which is p- and n-type doped adjacent to the electrodes. We study the emission characteristics of the p-i-n homojunction for varied doping concentrations and subsequently focus on its characteristics as photodiode. A superlinear relation of photocurrent and incident light intensity is found and attributed to an intensity-dependent photoconductivity.
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