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Top 20 Most Read Articles
May 2012
The 20 articles with the most full-text downloads during the month, in descending order.
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Luminescence decay in disordered low‐dimensional semiconductors Appl. Phys. Lett. 60, 2672 (1992); http://dx.doi.org/10.1063/1.106891 (3 pages)
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The luminescence decay of excitons in disordered low‐dimensional semiconductors with quantum confinement is shown experimentally to be characterized by a nonexponential profile and an absence of spectral diffusion. We are able to describe this luminescence as a hopping‐assisted recombination using the correlation function approach to nondispersive transport developed by H. Scher, M. F. Shlesinger, and J. T. Bendler [Phys. Today 41, 26 (1991)]. We suggest a simple derivation of analytical functions which accurately describe the anomalous luminescence decays of disordered II‐VI superlattices and of porous silicon, and show that this model includes exponential and Kohlrausch [Pogg. Ann. Phys. 119, 352 (1863)] (stretched‐exponential) relaxations as special cases. |
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Controlling light propagation with nanowires Appl. Phys. Lett. 100, 171903 (2012); http://dx.doi.org/10.1063/1.4704193 (3 pages) Online Publication Date: 23 April 2012
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We study the interaction of electromagnetic waves (EM) with metallic nanowire systems for frequencies not far from the surface plasma frequency (polaritonic range). We employ calculation and simulation to show that when excited at one end with axially polarized EM waves, nanowires can function as efficient waveguides of surface plasmon polaritons (SPPs). From the Fabry-Perot resonances of standing SPP waves, we study their dispersion relation and show that for a vanishing SPP wavelength it is identical to that for a planar metallic surface. Nanowire systems can be employed in various nanophotonic applications, and we assess this potential by studying propagation characteristics of these nano-waveguides and their interactions.
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Photo-origami—Bending and folding polymers with light Appl. Phys. Lett. 100, 161908 (2012); http://dx.doi.org/10.1063/1.3700719 (5 pages) Online Publication Date: 20 April 2012
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Photo-origami uses the dynamic control of the molecular architecture of a polymer by a combination of mechanical and non-contact optical stimuli to design and program spatially and temporally variable mechanical and optical fields into a material. The fields are essentially actuators, embedded in the material at molecular resolution, designed to enable controllable, sequenced, macroscopic bending and folding to create three-dimensional material structures. Here, we demonstrate, through a combination of theory, simulation-based design, synthesis, and experiment, the operative phenomena and capabilities of photo-origami that highlight its potential as a powerful, and potentially manufacturable, approach to create three-dimensional material structures.
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Appl. Phys. Lett. 100, 143501 (2012); http://dx.doi.org/10.1063/1.3700729 (4 pages) Online Publication Date: 2 April 2012
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The ultrafast metal-gate silicon quantum-dot (Si-QD) nonvolatile memory (NVM) with program/erase speed of 1 μs under low operating voltages of ± 7 V is achieved by thin tunneling oxide, in situ Si-QD-embedded dielectrics, and metal gate. Selective source/drain activation by green nanosecond laser spike annealing, due to metal-gate as light-blocking layer, responds to low thermal damage on gate structures and, therefore, suppresses re-crystallization/deformation/diffusion of embedded Si-QDs. Accordingly, it greatly sustains efficient charge trapping/de-trapping in numerous deep charge-trapping sites in discrete Si-QDs. Such a gate nanostructure also ensures excellent endurance and retention in the microsecond-operation Si-QD NVM.
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Physics of ultra-high bioproductivity in algal photobioreactors Appl. Phys. Lett. 100, 143703 (2012); http://dx.doi.org/10.1063/1.3701168 (4 pages) Online Publication Date: 4 April 2012
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Cultivating algae at high densities in thin photobioreactors engenders time scales for random cell motion that approach photosynthetic rate-limiting time scales. This synchronization allows bioproductivity above that achieved with conventional strategies. We show that a diffusion model for cell motion (1) accounts for high bioproductivity at irradiance values previously deemed restricted by photoinhibition, (2) predicts the existence of optimal culture densities and their dependence on irradiance, consistent with available data, (3) accounts for the observed degree to which mixing improves bioproductivity, and (4) provides an estimate of effective cell diffusion coefficients, in accord with independent hydrodynamic estimates.
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On the practicability of pentamode mechanical metamaterials Appl. Phys. Lett. 100, 191901 (2012); http://dx.doi.org/10.1063/1.4709436 (4 pages) Online Publication Date: 7 May 2012
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Conceptually, all conceivable three-dimensional mechanical materials can be built from pentamode materials. Pentamodes also enable to implement three-dimensional transformation elastodynamics—the analogue of transformation optics. However, pentamodes have not been realized experimentally. Here, we investigate inasmuch the pentamode theoretical ideal suggested by Milton and Cherkaev in 1995 can be approximated by a metamaterial with current state-of-the-art lithography. Using numerical calculations calibrated by our fabricated three-dimensional microstructures, we find that the figure of merit, i.e., the ratio of bulk modulus to shear modulus, can realistically be made as large as about 1000.
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Vacuum nanoelectronics: Back to the future?—Gate insulated nanoscale vacuum channel transistor Appl. Phys. Lett. 100, 213505 (2012); http://dx.doi.org/10.1063/1.4717751 (4 pages) Online Publication Date: 23 May 2012
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A gate-insulated vacuum channel transistor was fabricated using standard silicon semiconductor processing. Advantages of the vacuum tube and transistor are combined here by nanofabrication. A photoresist ashing technique enabled the nanogap separation of the emitter and the collector, thus allowing operation at less than 10 V. A cut-off frequency fT of 0.46 THz has been obtained. The nanoscale vacuum tubes can provide high frequency/power output while satisfying the metrics of lightness, cost, lifetime, and stability at harsh conditions, and the operation voltage can be decreased comparable to the modern semiconductor devices.
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Appl. Phys. Lett. 100, 122405 (2012); http://dx.doi.org/10.1063/1.3695168 (3 pages) Online Publication Date: 20 March 2012
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We have investigated the electric field effects in low resistance perpendicular magnetic tunnel junction (MTJ) devices and found that the electric field can effectively reduce the coercivity (Hc) of free layer (FL) by 30% for a bias voltage Vb = −0.2 V. In addition, the bias field (Hb) on free layer is almost linearly dependent on Vb yet independent on the device size. The demonstrated Vb dependences of Hc and Hb in low resistance MTJ devices present the potential to extend the scalability of the electric field assisted spin transfer torque magnetic random access memory and improve its access speed.
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Nonlinear graphene metamaterial Appl. Phys. Lett. 100, 181109 (2012); http://dx.doi.org/10.1063/1.4711044 (3 pages) Online Publication Date: 3 May 2012
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We demonstrate that the broadband nonlinear optical response of graphene can be resonantly enhanced by more than an order of magnitude through hybridization with a plasmonic metamaterial, while retaining an ultrafast nonlinear response time of ∼ 1 ps. Transmission modulation close to ∼ 1% is seen at a pump fluence of ∼ 30 μJ/cm2 at the wavelength of ∼ 1.6 μm. This approach allows to engineer and enhance graphene’s nonlinearity within a broad wavelength range enabling applications in optical switching, mode-locking, and pulse shaping.
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Organic electroluminescent diodes Appl. Phys. Lett. 51, 913 (1987); http://dx.doi.org/10.1063/1.98799 (3 pages)
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A novel electroluminescent device is constructed using organic materials as the emitting elements. The diode has a double‐layer structure of organic thin films, prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium‐tin‐oxide anode and an alloyed Mg:Ag cathode. Electron‐hole recombination and green electroluminescent emission are confined near the organic interface region. High external quantum efficiency (1% photon/electron), luminous efficiency (1.5 lm/W), and brightness (>1000 cd/m2) are achievable at a driving voltage below 10 V. |
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Plasmons in electrostatically doped graphene Appl. Phys. Lett. 100, 201105 (2012); http://dx.doi.org/10.1063/1.4714688 (4 pages) Online Publication Date: 15 May 2012
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Graphene has raised high expectations as a low-loss plasmonic material in which the plasmon properties can be controlled via electrostatic doping. Here, we analyze realistic configurations, which produce inhomogeneous doping, in contrast to what has been so far assumed in the study of plasmons in nanostructured graphene. Specifically, we investigate backgated ribbons, co-planar ribbon pairs placed at opposite potentials, and individual ribbons subject to a uniform electric field. Plasmons in backgated ribbons and ribbon pairs are similar to those of uniformly doped ribbons, provided the Fermi energy is appropriately scaled to compensate for finite-size effects such as the divergence of the carrier density at the edges. In contrast, the plasmons of a ribbon exposed to a uniform field exhibit distinct dispersion and spatial profiles that considerably differ from uniformly doped ribbons. Our results provide a road map to understand graphene plasmons under realistic electrostatic doping conditions.
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VAPOR‐LIQUID‐SOLID MECHANISM OF SINGLE CRYSTAL GROWTH Appl. Phys. Lett. 4, 89 (1964); http://dx.doi.org/10.1063/1.1753975 (2 pages) Online Publication Date: 23 December 2004
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Appl. Phys. Lett. 100, 181108 (2012); http://dx.doi.org/10.1063/1.4711033 (5 pages) Online Publication Date: 2 May 2012
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We report an enhancement in light emission efficiency form Si nanocrystal (NC) light-emitting diodes (LEDs) via surface plasmons (SPs) by employing Au nanoparticles (NPs). Photoluminescence intensity of Si NCs with Au NPs was enhanced by 2 factors of magnitude due to the strong coupling of Si NCs and SP resonance modes of Au NPs. The electrical characteristics of Si NC LED were significantly improved, which was attributed to an increase in an electron injection into the Si NCs due to the formation of inhomogeneous Schottky barrier at the SiC-indium tin oxide interface. Moreover, light output power from the Si NC LED was enhanced by 50% due to both SP coupling and improved electrical properties. The results presented here can provide a very promising way to significantly enhance the performance of Si NC LED.
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Appl. Phys. Lett. 100, 193302 (2012); http://dx.doi.org/10.1063/1.4711849 (4 pages) Online Publication Date: 8 May 2012
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We investigated photogeneration yield and recombination dynamics in blends of poly(3-hexyl thiophene) (P3HT) and poly[2-methoxy-5 -(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) with [6,6]-phenyl-C61butyric acid methyl ester (PC61BM) by means of temperature dependent time delayed collection field measurements. In MDMO-PPV:PC61BM, we find a strongly field dependent polaron pair dissociation which can be attributed to geminate recombination in the device. Our findings are in good agreement with field dependent photoluminescence measurements published before, supporting a scenario of polaron pair dissociation via an intermediate charge transfer state. In contrast, polaron pair dissociation in P3HT:PC61BM shows only a very weak field dependence, indicating an almost field independent polaron pair dissociation or a direct photogeneration. Furthermore, we found Langevin recombination for MDMO-PPV:PC61BM and strongly reduced Langevin recombination for P3HT:PC61BM.
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Appl. Phys. Lett. 100, 183304 (2012); http://dx.doi.org/10.1063/1.4709426 (4 pages) Online Publication Date: 1 May 2012
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The characteristics of green phosphorescent organic light-emitting diodes (OLEDs) fabricated on ITO/glass substrates pretreated with low-energy O2 and Cl2 plasma were compared. At 20 mA/cm2, the OLEDs with O2 and Cl2 plasma-treated indium tin oxide (ITO) had voltages of 9.6 and 7.6 eV, and brightness of 9580 and 12380 cd/m2, respectively. At ∼104 cd/m2, the latter had a 30% higher external quantum efficiency and a 74% higher power efficiency. Photoelectron spectroscopies revealed that Cl2 plasma treatment created stable In-Cl bonds and raised the work function of ITO by up to 0.9 eV. These results suggest that the better energy level alignment at the chlorinated ITO/organic interface enhances hole injection, leading to more efficient and more reliable operation of the OLEDs. The developed plasma chlorination process is very effective for surface modification of ITO and compatible with the fabrication of various organic electronics.
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Stress-relaxed growth of n-GaN epilayers Appl. Phys. Lett. 100, 181904 (2012); http://dx.doi.org/10.1063/1.4710561 (4 pages) Online Publication Date: 2 May 2012
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A significant stress-relaxation was observed in GaN epilayers by integrating a heavily Si-doped GaN (n+-GaN) sacrificial layer in the undoped GaN templates grown on sapphire substrates by metal-organic chemical vapor deposition. Selective GaN growth and electrochemical etching were exploited to achieve embedded air-gaps. Stress-relaxation and its local variations were probed by Raman mapping of high-frequency transverse-optical E2 (high) phonon mode of GaN. Enhanced In incorporation and improved light emission were observed in InGaN/GaN multi-quantum well visible light emitting diode structures fabricated on stress-relaxed GaN-epilayers with embedded air-gaps. Relevant sources for stress reduction and improved optical emission have been discussed.
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Ultra-low resistance ohmic contacts in graphene field effect transistors Appl. Phys. Lett. 100, 203512 (2012); http://dx.doi.org/10.1063/1.4719579 (3 pages) Online Publication Date: 18 May 2012
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We report on an experimental demonstration of graphene-metal ohmic contacts with contact resistance below 100 Ω µm. These have been fabricated on graphene wafers, both with and without hydrogen intercalation, and measured using the transmission line method. Specific contact resistivities of 3 × 10−7 to 1.2 × 10−8 Ω cm2 have been obtained. The ultra-low contact resistance yielded short-channel (source-drain distance of 0.45 µm) HfO2/graphene field effect transistors (FETs) with a low on-resistance (Ron) of 550 Ω µm and a high current density of 1.7 A/mm at a source-drain voltage of 1 V. These values represent state-of-the-art (SOA) performance in graphene-metal contacts and graphene FETs. This ohmic contact resistance is comparable to that of SOA high-speed III–V high electron mobility transistors.
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Appl. Phys. Lett. 100, 183306 (2012); http://dx.doi.org/10.1063/1.4711129 (4 pages) Online Publication Date: 3 May 2012
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We exposed regions of green phosphorescent organic light emitting devices (PHOLEDTMs) consisting of a fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) as the phosphorescent emitter to a partial pressure of water of 3 × 10−4 Pa during device fabrication to induce degradation in a specific region of the multi-layered devices. We identified the interface between the hole transport layer and the emissive layer as the most susceptive region to degradation. We discuss the luminance loss mechanism and estimate an operational lifetime of 10 000 h, after 20% loss of the initial luminance from 1000 cd/m2, is attainable from an Ir(ppy)3 PHOLED fabricated under ultra-high vacuum conditions.
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Appl. Phys. Lett. 100, 203104 (2012); http://dx.doi.org/10.1063/1.3701731 (4 pages) Online Publication Date: 14 May 2012
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Silicon-based active and reproducible surface-enhanced Raman scattering (SERS) substrate, i.e., silver nanoparticles decorated-silicon wafers (AgNPs@Si), is employed for constructing high-performance sensors. Significantly, the AgNPs@Si, facilely prepared via in situ AgNPs growth on silicon wafers, features excellent SERS reproducibility and high enhancement factor. Our experiment further demonstrates such resultant silicon-based SERS substrate is efficacious for multiplex, sensitive, and specific DNA detection. In particular, single-base mismatched DNA with low concentrations is readily discriminated by using the AgNPs@Si. Moreover, the silicon-based sensor exhibits adequate multiplexing capacity, enabling unambiguous identification of the dual-target DNA detection.
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Modeling charge transfer at organic donor-acceptor semiconductor interfaces Appl. Phys. Lett. 100, 203302 (2012); http://dx.doi.org/10.1063/1.4717985 (4 pages) Online Publication Date: 15 May 2012
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We develop an integer charge transfer model for the potential steps observed at interfaces between donor and acceptor molecular semiconductors. The potential step can be expressed as the difference between the Fermi energy pinning levels of electrons on the acceptor material and holes on the donor material, as determined from metal-organic semiconductor contacts. These pinning levels can be obtained from simple density functional theory calculations.
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