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Top 20 Most Read Articles
January 2010
The 20 articles with the most full-text downloads during the month, in descending order.
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Why nitrogen cannot lead to p-type conductivity in ZnO Appl. Phys. Lett. 95, 252105 (2009); http://dx.doi.org/10.1063/1.3274043 (3 pages) Online Publication Date: 22 December 2009
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Based on electronic structure and atomic size considerations, nitrogen has been regarded as the most suitable impurity for p-type doping in ZnO. However, numerous experimental efforts by many different groups have not resulted in stable and reproducible p-type material, casting doubt on the efficacy of nitrogen as a shallow acceptor. Based on advanced first-principles calculations we find that nitrogen is actually a deep acceptor, with an exceedingly high ionization energy of 1.3 eV, and hence cannot lead to hole conductivity in ZnO. In light of this result, we reexamine prior experiments on nitrogen doping of ZnO.
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Integrated nanoplasmonic-nanofluidic biosensors with targeted delivery of analytes Appl. Phys. Lett. 96, 021101 (2010); http://dx.doi.org/10.1063/1.3290633 (3 pages) Online Publication Date: 11 January 2010
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Performances of the biosensors are often limited by the depletion zones created around the sensing area which impede the effective analyte transport. To overcome this limitation, we propose and demonstrate a nanoplasmonic-nanofluidic sensor enabling targeted delivery of analytes to the sensor surface with dramatic improvements in mass transport efficiency. Our sensing platform is based on extraordinary light transmission effect in suspended plasmonic nanoholes. This scheme allows three-dimensional control of the fluidic flow by connecting separate layers of microfluidic channels through plasmonic/nanofluidic holes. To implement the proposed sensor platform, we also introduce a lift-off free nanofabrication method.
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Mechanics of hemispherical electronics Appl. Phys. Lett. 95, 181912 (2009); http://dx.doi.org/10.1063/1.3256185 (3 pages) Online Publication Date: 5 November 2009
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A simple analytical model is established for the development of hemisphere electronics, which has many important applications in electronic-eye cameras and related curvilinear systems. The photodetector arrays, made in planar mesh layouts with conventional techniques, are deformed and transferred onto a hemisphere. The model gives accurately the positions of photodetectors on the hemisphere, and has been validated by experiments and finite element analysis. The results also indicate very small residual strains in the photodetectors. The model provides a tool to define a pattern of photodetectors in the planar, as-fabricated layout to yield any desired spatial configuration on the hemisphere.
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Super-resolution imaging using a three-dimensional metamaterials nanolens Appl. Phys. Lett. 96, 023114 (2010); http://dx.doi.org/10.1063/1.3291677 (3 pages) Online Publication Date: 14 January 2010
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Super-resolution imaging beyond Abbe’s diffraction limit can be achieved by utilizing an optical medium or “metamaterial” that can either amplify or transport the decaying near-field evanescent waves that carry subwavelength features of objects. Earlier approaches at optical frequencies mostly utilized the amplification of evanescent waves in thin metallic films or metal-dielectric multilayers, but were restricted to very small thicknesses (⪡λ, wavelength) and accordingly short object-image distances, due to losses in the material. Here, we present an experimental demonstration of super-resolution imaging by a low-loss three-dimensional metamaterial nanolens consisting of aligned gold nanowires embedded in a porous alumina matrix. This composite medium possesses strongly anisotropic optical properties with negative permittivity in the nanowire axis direction, which enables the transport of both far-field and near-field components with low-loss over significant distances (>6λ), and over a broad spectral range. We demonstrate the imaging of large objects, having subwavelength features, with a resolution of at least λ/4 at near-infrared wavelengths. The results are in good agreement with a theoretical model of wave propagation in anisotropic media.
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Room temperature-dipolelike single photon source with a colloidal dot-in-rod Appl. Phys. Lett. 96, 033101 (2010); http://dx.doi.org/10.1063/1.3291849 (3 pages) Online Publication Date: 19 January 2010
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We propose colloidal CdSe/CdS dots in rods as nonclassical sources for quantum information technology. Such nanoemitters show specific properties such as strongly polarized emission of on-demand single photons at room temperature, dipolelike behavior and mono-exponential recombination rates, making us envision their suitability as sources of single photons with well defined quantum states in quantum cryptography based devices.
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Dual band terahertz waveguiding on a planar metal surface patterned with annular holes Appl. Phys. Lett. 96, 011101 (2010); http://dx.doi.org/10.1063/1.3276545 (3 pages) Online Publication Date: 4 January 2010
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We report studies of the guiding of terahertz radiation on a copper surface textured with an array of blind annular holes. The structure supports two tightly bound surface plasmon-polaritonlike electromagnetic waves associated with TE11 and TEM coaxial waveguide modes. The TE11-like surface mode has a cutoff determined by the array period while the TEM mode can be arranged to have a lower frequency cut-off by adjusting the hole depth. The ability to guide two modes in independently variable bands could be useful in quantitative chemical sensing.
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Appl. Phys. Lett. 96, 033302 (2010); http://dx.doi.org/10.1063/1.3297878 (3 pages) Online Publication Date: 22 January 2010
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Controlled gold nanoparticle (AuNP)-based nonvolatile memory devices were developed based on pentacene organic transistors and polymethylmethacrylate (PMMA) insulator layers. The memory device had the following configuration: n+Si gate/SiO2 blocking oxide/polyelectrolytes/AuNP/PMMA tunneling dielectric layer/Au source-drain. According to the programming/erasing operations, the memory device showed good programmable memory characteristics with a large memory window. In addition, good reliability was confirmed by the data retention characteristics. The fabrication procedures for the charge trapping and tunneling layers were based on simple solution processes (by dipping and spin-coating) and the maximum processing temperature was <100 °C, so this method has potential applications in plastic/flexible electronics.
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Contact resistance in few and multilayer graphene devices Appl. Phys. Lett. 96, 013512 (2010); http://dx.doi.org/10.1063/1.3290248 (3 pages) Online Publication Date: 8 January 2010
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The contact resistance of metals on backgated graphene field-effect transistors is studied. The residual resistance obtained at high backgate voltage is found to be in excellent agreement with the extracted values of contact resistance from transfer length measurements on graphene flakes. The contact resistance is found to be a significant contributor to the total resistance of graphene-based devices. The specific contact resistance is shown to be independent of the applied backgate voltage and the number of graphene layers.
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Appl. Phys. Lett. 96, 013304 (2010); http://dx.doi.org/10.1063/1.3280370 (3 pages) Online Publication Date: 7 January 2010
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We report on bulk-heterojunction hybrid solar cells based on blends of non-ligand-exchanged CdSe quantum dots (QDs) and the conjugated polymer poly(3-hexylthiophene) with improved power conversion efficiencies of about 2% under AM1.5G illumination after spectral mismatch correction. This is the highest reported value for a spherical CdSe QD based photovoltaic device. After synthesis, the CdSe QDs are treated by a simple and fast acid-assisted washing procedure, which has been identified as a crucial factor in enhancing the device performance. A simple model of a reduced ligand sphere is proposed explaining the power conversion efficiency improvement.
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Optical resonances created by photonic transitions Appl. Phys. Lett. 96, 011108 (2010); http://dx.doi.org/10.1063/1.3279130 (3 pages) Online Publication Date: 7 January 2010
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We show that a high-Q optical resonance can be created dynamically, by inducing a photonic transition between a localized state and a one-dimensional continuum through refractive index modulation. In this mechanism, both the frequency and the external linewidth of a single resonance are specified by the dynamics, allowing complete control of the resonance properties. An example using photonic crystal heterostructure cavity is demonstrated with numerical simulation. We also show that the reported effect can be accomplished with realistic index modulation strength and frequencies.
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Voltage-induced perpendicular magnetic anisotropy change in magnetic tunnel junctions Appl. Phys. Lett. 96, 022506 (2010); http://dx.doi.org/10.1063/1.3279157 (3 pages) Online Publication Date: 11 January 2010
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A voltage-induced perpendicular magnetic anisotropy change in an ultrathin FeCo layer was observed in an epitaxial magnetic tunnel junction (MTJ) structure. A spin-transfer induced ferromagnetic resonance measurement technique was used under various bias voltage applications to evaluate the anisotropy change. From the peak frequency shifts, we could estimate that a surface magnetic anisotropy change of 15 μJ/m2 was induced by an electric field application of 400 mV/nm in the MTJ with a 0.5 nm thick FeCo layer. The realization of voltage-induced anisotropy changes in an MTJ structure should have a large impact on the development of electric-field driven spintronic devices.
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Polarization-charge tunnel junctions for ultraviolet light-emitters without p-type contact Appl. Phys. Lett. 96, 031102 (2010); http://dx.doi.org/10.1063/1.3280866 (3 pages) Online Publication Date: 19 January 2010
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Polarization-charge tunnel junctions for ultraviolet light-emitters (λ<360 nm) are modeled using the k⋅p multiband quantum transmitting boundary method. It is shown that polarization-charge tunnel junctions can carry sufficient current to be viable for ultraviolet emitters. Sources of inefficiency in existing ultraviolet emitters are discussed, including poor hole-injection efficiency and optical absorption in the p-type GaN contact layers. It is demonstrated that polarization-charge tunnel junctions can deliver improvements in both areas, by eliminating the need for p-type contacts, and by enabling the use of a p-side-down structure, which is shown to have favorable characteristics for carrier transport compared to conventional p-side-up structures.
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Appl. Phys. Lett. 96, 023111 (2010); http://dx.doi.org/10.1063/1.3291106 (3 pages) Online Publication Date: 14 January 2010
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We investigated the photoluminescence (PL) properties of ZnO nanowires with and without covering with polymethyl methacrylate (PMMA). Low temperature PL spectra of as-grown ZnO nanowires are dominated by near band edge (NBE) emission due to donor bound excitons and free-to-bound recombination (FB). FB emission persists till 300 K and together with free exciton emission governs the lineshape of the PL spectra. After covering with PMMA, the integral intensity of NBE emission increases about three times, indicating significantly improved excitonic emission efficiency. A model based on surface states and energy bands theory was proposed to interpret this emission enhancement.
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Photonic crystal cavities in silicon dioxide Appl. Phys. Lett. 96, 031107 (2010); http://dx.doi.org/10.1063/1.3297877 (3 pages) Online Publication Date: 21 January 2010
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One dimensional nanobeam photonic crystal cavities fabricated in silicon dioxide are considered in both simulation and experiment. Quality factors of over 104 are found via simulation, while quality factors of over 5×103 are found in experiment, for cavities with mode volumes of 2.0(λ/n)3 and in the visible wavelength range 600–716 nm. The dependences of the cavity quality factor and mode volume for different design parameters are also considered.
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Effect of dielectric thin films on reflection properties of metal hole arrays Appl. Phys. Lett. 96, 021106 (2010); http://dx.doi.org/10.1063/1.3292024 (3 pages) Online Publication Date: 12 January 2010
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We study the effect of a dielectric film attached to the surface of a metal hole array (MHA) on the reflection spectrum in the terahertz (THz) region. The frequency of the reflection dip, attributed to the excitation of surface waves in the vicinity of the MHA surface, shifts to lower frequencies with increasing dielectric film thickness. This resonant characteristic of MHAs can be applied to highly sensitive THz sensing for samples attached to the MHA surface. We also investigate the dependence of the reflection spectrum on the MHA’s thickness and the side to which the dielectric film is attached.
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Multiple levels in intermediate band solar cells Appl. Phys. Lett. 96, 013501 (2010); http://dx.doi.org/10.1063/1.3280387 (3 pages) Online Publication Date: 4 January 2010
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The presence of multiple energy levels in the intermediate band solar cell (IBSC) is studied by detailed balance calculations under ideal conditions. Multiple levels are found experimentally in IBSCs made with quantum dots (QDs) which act to reduce the limiting efficiency determined from detailed balance calculations. JL-VOC measurements up to 1000 suns on IBSCs are presented together with their fitting to modified detailed balance calculations. It is found that the introduction of the QDs degrades the performance of the host cell but the sub-bandgap cell operates close to ideality.
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Band bowing and band alignment in InGaN alloys Appl. Phys. Lett. 96, 021908 (2010); http://dx.doi.org/10.1063/1.3291055 (3 pages) Online Publication Date: 15 January 2010
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We use density functional theory calculations with the HSE06 hybrid exchange-correlation functional to investigate InGaN alloys and accurately determine band gaps and band alignments. We find a strong band-gap bowing at low In content. Band positions on an absolute energy scale are determined from surface calculations. The resulting GaN/InN valence-band offset is 0.62 eV. The dependence of InGaN valence-band alignment on In content is found to be almost linear. Based on the values of band gaps and band alignments, we conclude that InGaN fulfills the requirements for a photoelectrochemical electrode for In contents up to 50%.
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High extraction efficiency light-emitting diodes based on embedded air-gap photonic-crystals Appl. Phys. Lett. 96, 031108 (2010); http://dx.doi.org/10.1063/1.3293442 (3 pages) Online Publication Date: 22 January 2010
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This letter reports on high extraction efficiency light-emitting diodes (LEDs) based on embedded two-dimensional air-gap photonic crystals (PhCs). High refractive index contrast provided by the air gaps along with high interaction of the embedded PhCs with the guided light resulted in an efficient extraction of all guided modes in the LED, in contrast to the common surface PhC configuration. Embedded PhC LEDs presented an enhanced directional light emission compared to non-PhC LEDs. High extraction efficiency, close to unity, provided by the encapsulated embedded PhC LEDs demonstrates the capability of this approach to achieve high efficiency devices with directional light emission.
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Self-organized growth of high density magnetic Co nanodot arrays on a Moiré template Appl. Phys. Lett. 96, 013107 (2010); http://dx.doi.org/10.1063/1.3280900 (3 pages) Online Publication Date: 6 January 2010
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We report the self-organized growth of cobalt nanodot arrays using a Gd-Au Moiré superlattice as a template. After analyzing the influence of the Co flux and the substrate temperature, we obtain the suitable parameters to maximize nanodot density, homogeneity, and individual size. Depending on the growth conditions an areal density of up to 54 Teradots/inch2 can be achieved. Below the limit of lateral coalescence, independent nanodots made of ∼ 1000 Co atoms exhibit room temperature remanent magnetization.
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Electrically driven quantum dot-micropillar single photon source with 34% overall efficiency Appl. Phys. Lett. 96, 011107 (2010); http://dx.doi.org/10.1063/1.3284514 (3 pages) Online Publication Date: 7 January 2010
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We report on triggered single photon emission from low mode volume electrically driven quantum dot-micropillar cavities at repetition rates of up to 220 MHz. Due to an optimized layout of the doped planar microcavity and an advanced lateral current injection scheme, highly efficient single photon sources are realized. While g(2)(0)-values as low as 0.13±0.05 and a Purcell factor of 4 are observed for a 2.0 μm diameter micropillar, single photon emission at a rate of (35±7) MHz and an overall efficiency of (34±7)% are demonstrated for a 3.0 μm device.
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