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Top 20 Most Read Articles
September 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Structural transition in LaVO3/SrVO3 superlattices and its influence on transport properties Appl. Phys. Lett. 98, 212106 (2011); http://dx.doi.org/10.1063/1.3593489 (3 pages) Online Publication Date: 24 May 2011
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Measurements of the resistive properties and the lattice parameters of a (LaVO3)[6 unit cells]/(SrVO3)[1 unit cell] superlattice between 10 K and room temperature are presented. A low temperature metallic phase compatible with a Fermi liquid behavior is evidenced. It disappears in the vicinity of a structural transition from a monoclinic to tetragonal phase, in which disorder seems to strongly influence the transport. Our results will enrich the understanding of the electronic properties of complex heterostructures.
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Unique prospects for graphene-based terahertz modulators Appl. Phys. Lett. 99, 113104 (2011); http://dx.doi.org/10.1063/1.3636435 (3 pages) Online Publication Date: 12 September 2011
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The modulation depth of two-dimensional electron-gas (2DEG) based terahertz (THz) modulators using AlGaAs/GaAs hetero-structures with metal gates is inherently limited to <30%. The metal gate not only attenuates the THz signal but also severely degrades modulation depth. Metal losses can be significantly reduced employing an alternative material with tunable conductivity. Graphene presents a unique solution to this problem due to its symmetric band structure and extraordinarily high hole mobility. In this work, we show that it is possible to achieve a modulation depth of >90% while simultaneously minimizing signal attenuation to <5% by tuning the Fermi level at its Dirac point.
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Spin transfer torque switching for multi-bit per cell magnetic memory with perpendicular anisotropy Appl. Phys. Lett. 99, 092506 (2011); http://dx.doi.org/10.1063/1.3632075 (3 pages) Online Publication Date: 30 August 2011
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A novel multi-bit dual pseudo spin valve with perpendicular magnetic anisotropy is investigated for spin transfer torque (STT) switching. The structure consists of two free layers and one reference layer, and all are based on Co/Pd multilayer. STT switching of the multi-bit device shows distinct four resistance levels. The selection of intrinsic properties of each ferromagnetic layer can be controlled for distinct separation of the resistance levels as well as the respective STT switching current. Reversible transitions between different states can be achieved by a pulsed current, in which its critical value is found to be linearly dependent on pulse duration.
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Experimental evidence of ferroelectric negative capacitance in nanoscale heterostructures Appl. Phys. Lett. 99, 113501 (2011); http://dx.doi.org/10.1063/1.3634072 (3 pages) Online Publication Date: 12 September 2011
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We report a proof-of-concept demonstration of negative capacitance effect in a nanoscale ferroelectric-dielectric heterostructure. In a bilayer of ferroelectric Pb(Zr0.2Ti0.8)O3 and dielectric SrTiO3, the composite capacitance was observed to be larger than the constituent SrTiO3 capacitance, indicating an effective negative capacitance of the constituent Pb(Zr0.2Ti0.8)O3 layer. Temperature is shown to be an effective tuning parameter for the ferroelectric negative capacitance and the degree of capacitance enhancement in the heterostructure. Landau’s mean field theory based calculations show qualitative agreement with observed effects. This work underpins the possibility that by replacing gate oxides by ferroelectrics in nanoscale transistors, the sub threshold slope can be lowered below the classical limit (60 mV/decade).
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Localized surface plasmon resonance of nanoporous gold Appl. Phys. Lett. 98, 093701 (2011); http://dx.doi.org/10.1063/1.3560482 (3 pages) Online Publication Date: 28 February 2011
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We report the plasmonic properties of free-standing nanoporous gold (NPG) films with an intricate bicontinuous nanostructure. Two characteristic plasmon bands of NPG have been detected in absorption spectra. One at ∼ 490 nm, resulting from the resonant absorption of gold films, is independent of nanopore sizes and dielectric surroundings. The other at ∼ 550–650 nm, arising from the excitation of localized surface plasmon resonance, shows obvious band shift with the nanopore sizes and dielectric indices of surrounding media, suggesting that NPG is a promising candidate as plasmonic sensors for organic and biologic molecule detection. This study also shines light on the underlying mechanisms of surface enhanced spectroscopy of NPG.
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Ionic polarization of liquid-liquid interfaces; dynamic control of the rate of electro-coalescence Appl. Phys. Lett. 99, 094101 (2011); http://dx.doi.org/10.1063/1.3629783 (3 pages) Online Publication Date: 29 August 2011
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Electrostatic forces are the strongest interactions in soft matter physics, yet they are usually screened by ions present in the solutions. Here we demonstrate that the extent of ionic polarization of liquid-liquid interfaces can be dynamically controlled via appropriate choice of the strength and frequency of the external electric field. Results of microfluidic experiments on electro-coalescence of droplets of aqueous solutions of salt provide guidelines for optimization of the process.
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Acoustic cloak for airborne sound by inverse design Appl. Phys. Lett. 99, 074102 (2011); http://dx.doi.org/10.1063/1.3623761 (3 pages) Online Publication Date: 16 August 2011
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This Letter presents practical realization of a two-dimensional low loss acoustic cloak for airborne sound obtained by inverse design. The cloak consists of 120 aluminum cylinders of 15 mm diameter surrounding the cloaked object—a cylinder of diameter 22.5 cm. The position of each cylinder in the cloak is optimized using the data from two different techniques: genetic algorithm and simulated annealing. The operation frequency of this cloak is 3061 Hz with the bandwidth of about 100 Hz. Being a multi-step approach to the desired cloaking, the inverse design is also valid, in principle, for non-symmetric cylinders and even for three-dimensional objects.
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Appl. Phys. Lett. 99, 121101 (2011); http://dx.doi.org/10.1063/1.3617472 (3 pages) Online Publication Date: 19 September 2011
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We demonstrate a spectrally broadband and efficient technique for collecting emission from a single InAs quantum dot directly into a standard single mode optical fiber. In this approach, an optical fiber taper waveguide is placed in contact with a suspended GaAs nanophotonic waveguide with embedded quantum dots, forming a broadband directional coupler with standard optical fiber input and output. Efficient photoluminescence collection over a wavelength range of tens of nanometers is demonstrated, and a maximum collection efficiency of 6% (corresponding single photon rate of 3.0 MHz) into a single mode optical fiber is estimated for a single quantum dot exciton.
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The design of efficient surface-plasmon-enhanced ultra-thin polymer-based solar cells Appl. Phys. Lett. 99, 093307 (2011); http://dx.doi.org/10.1063/1.3633349 (3 pages) Online Publication Date: 2 September 2011
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Polymer based solar cells are particularly attractive because of their mechanical flexibility and potential for low-cost fabrication. Although significant progress has been made, their efficiency is reduced strongly due to recombination processes that scale with the thickness of the active layer. A theoretical study of periodic plasmonic solar cell enhancement is presented, including a design for demonstrating high efficiency while using a significantly reduced active layer thicknesses. This is achieved through the superposition of toothgrating structures of multiple periodicities along a silver reflecting layer. Through finite-difference time-domain calculations, it was possible to optimize the overall spectral response of the cell yielding surface plasmon resonances at predetermined wavelengths. The improved solar cell design results in a system with increased absorption, allowing for the desired reduction in active layer thickness while also enhancing the performance of the cell over a wide wavelength range.
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Origin of photocurrent generation and collection losses in large area organic solar cells Appl. Phys. Lett. 99, 093309 (2011); http://dx.doi.org/10.1063/1.3637041 (3 pages) Online Publication Date: 2 September 2011
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Loss of performance in large area devices is a major concern for organic solar cells. We present studies highlighting up-scaling loss mechanisms caused by interfacial resistance at organic-metal interface in pentacene/C60 heterojunction solar cells. Devices with different active areas and electrodes are investigated, revealing the origin of higher series resistance in large area devices. We also simulate influence of spatial non-uniformity and anisotropic charge collection in active layer through selective coverage using customized electrode geometries and identify grain boundaries as the dominating link behind loss in photocurrent generation and collection.
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Radiation patterns from coupled photonic crystal nanocavities Appl. Phys. Lett. 99, 111101 (2011); http://dx.doi.org/10.1063/1.3633106 (3 pages) Online Publication Date: 12 September 2011
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We report on far field measurements on two coupled photonic crystal nanocavities. The distinct features of the antisymmetric modes (minima of intensity at zero-emission angles) allow us to demonstrate a π-phase difference between the cavity fields, a clear signature of evanescent coupling. Good agreement between experimental results and simulated radiation patterns has been found.
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Advantages of InGaN light-emitting diodes with GaN-InGaN-GaN barriers Appl. Phys. Lett. 99, 091107 (2011); http://dx.doi.org/10.1063/1.3633268 (3 pages) Online Publication Date: 31 August 2011
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The advantages of InGaN light-emitting diodes with GaN-InGaN-GaN barriers are studied. The energy band diagrams, carrier concentrations in the quantum wells, radiative recombination rate in the active region, light-current performance curves, and internal quantum efficiency are investigated. The simulation results show that the InGaN/GaN-InGaN-GaN light-emitting diode has better performance over its conventional InGaN/GaN and InGaN/InGaN counterparts due to the appropriately modified energy band diagrams which are favorable for the injection of electrons and holes and uniform distribution of these carriers in the quantum wells.
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Appl. Phys. Lett. 99, 091104 (2011); http://dx.doi.org/10.1063/1.3605572 (3 pages) Online Publication Date: 29 August 2011
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In this work, the dislocation-related efficiency droop in InGaN/GaN blue light-emitting diodes (LEDs) was investigated by comparing the external quantum efficiency (EQE) of GaN grown on c-plane sapphire and free-standing GaN substrate over a wide range of operation conditions. The values of A, B, and C coefficients had been iteratively obtained by fitting quantum efficiency in the rate equation model. Analysis revealed that threading dislocation density was strongly related to the decrease in EQE of InGaN LEDs at elevated currents by introducing a number of acceptor-like levels with the energy EA lying within the band gap.
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The role of mobility in bulk heterojunction solar cells Appl. Phys. Lett. 99, 113303 (2011); http://dx.doi.org/10.1063/1.3639273 (3 pages) Online Publication Date: 14 September 2011
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In this letter, we employ a three-dimensional master equation calculation to investigate the mobility dependence of bulk heterojunction (BHJ) solar cell performance. By taking energetic disorder and morphology into consideration, we show mobility-enhanced device efficiency with a remarkable charge transport loss induced by molecular disorder and an open circuit voltage loss in high mobility region due to morphological defect-assisted bimolecular recombination. The result suggests that the description of interfacial processes is crucial in the modeling of BHJ photovoltaic devices.
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Efficient bulk heterojunction photovoltaic cells with a pre-organized poly(3-hexylthiophene) phase Appl. Phys. Lett. 99, 093303 (2011); http://dx.doi.org/10.1063/1.3628658 (3 pages) Online Publication Date: 31 August 2011
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This paper presents the controlled generation of nanostructured poly(3-hexylthiophene) (P3HT) from solution to fully fibrillate solid-state films. The fibrillate morphology of P3HT is further maintained when the deposition is carried out from a mixed solution with the acceptor material ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM). This enables the fabrication of active layers for bulk heterojunction organic photovoltaic devices with a controlled morphology. The contribution of these nanostructures to the photovoltaic performances is evidenced. High-resolution electrical characterization with conductive atomic force microscopy (C-AFM) confirms an improved charge transport throughout the fibrillate P3HT matrix. A local hole mobility value of 0.9 × 10−4 cm2/V s is derived from the C-AFM measurements.
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Graphene/GaN Schottky diodes: Stability at elevated temperatures Appl. Phys. Lett. 99, 102102 (2011); http://dx.doi.org/10.1063/1.3628315 (3 pages) Online Publication Date: 7 September 2011
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Rectification and thermal stability of diodes formed at graphene/GaN interfaces have been investigated using Raman Spectroscopy and temperature-dependent current-voltage measurements. The Schottky barriers formed between GaN and mechanically transferred graphene display rectification that is preserved up to 550 K with the diodes eventually becoming non-rectifying above 650 K. Upon cooling, the diodes show excellent recovery with improved rectification. We attribute these effects to the thermal stability of graphene, which acts like an impenetrable barrier to the diffusion of contaminants across the interface, and to changes in the interface band alignment associated with thermally induced dedoping of graphene.
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Appl. Phys. Lett. 99, 113305 (2011); http://dx.doi.org/10.1063/1.3634015 (3 pages) Online Publication Date: 15 September 2011
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We report on efficient and stable ITO-free small molecule organic solar cells with conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electrodes using a post-treatment process, causing selective removal of PSS. The solar cells with post-treated PEDOT:PSS electrodes show significantly improved short circuit current densities and efficiencies compared to untreated devices. Moreover, the removal of PSS by the post-treatment significantly improves the lifetime of devices, which are more resistant to loss of fill factor compared to untreated devices.
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Graphene bubbles with controllable curvature Appl. Phys. Lett. 99, 093103 (2011); http://dx.doi.org/10.1063/1.3631632 (3 pages) Online Publication Date: 30 August 2011
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Raised above the substrate and elastically deformed areas of graphene in the form of bubbles are found on different substrates. They come in a variety of shapes, including those which allow strong modification of the electronic properties of graphene. We show that the shape of the bubble can be controlled by an external electric field. This effect can be used to make graphene-based adaptive focus lenses.
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Appl. Phys. Lett. 99, 102104 (2011); http://dx.doi.org/10.1063/1.3634018 (3 pages) Online Publication Date: 8 September 2011
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This paper will show that, while ZT is an appropriate performance index when optimizing the heat conversion rate, it may mislead research in view of applications aiming at large electric power production. This is of special relevance when related to the surge of research in the area of low-dimensionality semiconductors where ZT is increased by lowering the thermal conductivity κ. It will be shown that, when operating between sources at fixed temperature, the highest power output can be obtained by increasing κ, not decreasing it, the larger electric power output economically enabling thermoelectric generators for massive electric power production.
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Ultra-wide bandwidth piezoelectric energy harvesting Appl. Phys. Lett. 99, 083105 (2011); http://dx.doi.org/10.1063/1.3629551 (3 pages) Online Publication Date: 23 August 2011
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Here, we present an ultra wide-bandwidth energy harvester by exploiting the nonlinear stiffness of a doubly clamped microelectromechanical systems (MEMSs) resonator. The stretching strain in a doubly clamped beam shows a nonlinear stiffness, which provides a passive feedback and results in amplitude-stiffened Duffing mode resonance. This design has been fabricated into a compact MEMS device, which is about the size of a US quarter coin. Based on the open circuit voltage measurement, it is expected to have more than one order of magnitude improvement in both bandwidth (more than 20% of the peak frequency) and power density (up to 2 W/cm3) in comparison to the devices previously reported.
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