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Top 20 Most Read Articles
June 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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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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Coupling between quantum Hall state and electromechanics in suspended graphene resonator Appl. Phys. Lett. 100, 233103 (2012); http://dx.doi.org/10.1063/1.3703763 (4 pages) Online Publication Date: 4 June 2012
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Using graphene resonator, we perform electromechanical measurements in quantum Hall regime to probe the coupling between a quantum Hall (QH) system and its mechanical motion. Mechanically perturbing the QH state through resonance modifies the DC resistance of the system and results in a Fano-lineshape due to electronic interference. Magnetization of the system modifies the resonator’s equilibrium position and effective stiffness leading to changes in resonant frequency. Our experiments show that there is an intimate coupling between the quantum Hall state and mechanics—electron transport is affected by physical motion and in turn the magnetization modifies the electromechanical response.
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Experimental isolation of degradation mechanisms in capacitive microelectromechanical switches Appl. Phys. Lett. 100, 233505 (2012); http://dx.doi.org/10.1063/1.4726116 (4 pages) Online Publication Date: 6 June 2012
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DC and bipolar voltage stresses are used to isolate mechanical degradation of the movable electrode from charging mechanism in microelectromechanical capacitive switches. Switches with different metals as the movable electrode were investigated. In titanium switches, a shift in the pull-in voltages is observed after dc stressing whereas no shift occurs after the bipolar stressing, which is to be expected from charging theory. On switches with similar dielectric but made of aluminium, the narrowing effect occurs regardless if dc or bipolar stressing is used, which indicates the mechanical degradation as the mechanism responsible.
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Continuous-wave terahertz system with a 60 dB dynamic range Appl. Phys. Lett. 86, 204104 (2005); http://dx.doi.org/10.1063/1.1935032 (3 pages) Online Publication Date: 13 May 2005
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We have developed a high-performance continuous-wave terahertz imaging system based on photomixing. The emitter and detector are driven by compact, unstabilized, single-mode diode lasers. The all-optoelectronic, homodyne detection scheme yields both amplitude and phase information, and with careful optimization and matching of both emitter and receiver, a 60 dB dynamic range, at 0.53 THz, can be routinely achieved. This replicates the performance of established pulsed THz imagers at this frequency.
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Novel millimeter‐wave near‐field resistivity microscope Appl. Phys. Lett. 68, 1579 (1996); http://dx.doi.org/10.1063/1.116685 (3 pages)
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We demonstrate a technique for contactless mapping of resistivity or dielectric constant of surfaces and films with a spatial resolution better than 100 μm. This technique may be used for the nondestructive testing of semiconducting wafers, conducting polymers, oxide superconductors, and printed circuits. The principle of operation consists of the scanning of a tiny millimeter‐wave antenna at a very small height above an inhomogeneous conducting surface and measuring the intensity and phase of the reflected (transmitted) wave. We use a specially designed resonant slit antenna and achieve subwavelength spatial resolution of λ/50. © 1996 American Institute of Physics. |
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Appl. Phys. Lett. 100, 231110 (2012); http://dx.doi.org/10.1063/1.4726106 (4 pages) Online Publication Date: 6 June 2012
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We investigate the influence of polarity on carrier transport in single-quantum-well and multiple-quantum-well (MQW) light-emitting diodes (LEDs) grown on the semipolar (20
 1) and (20 ) orientations of free-standing GaN. For semipolar MQW LEDs with the opposite polarity to conventional Ga-polar c-plane LEDs, the polarization-related electric field in the QWs results in an additional energy barrier for carriers to escape the QWs. We show that semipolar (20) MQW LEDs with the same polarity to Ga-polar c-plane LEDs have a more uniform carrier distribution and lower forward voltage than (201) MQW LEDs. |
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Appl. Phys. Lett. 100, 241101 (2012); http://dx.doi.org/10.1063/1.4724309 (3 pages) Online Publication Date: 11 June 2012
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We report on the development of nanowire-based field-effect transistors operating as high sensitivity terahertz (THz) detectors. By feeding the 1.5 THz radiation field of a quantum cascade laser (QCL) at the gate-source electrodes with a wide band dipole antenna, we record a photovoltage signal corresponding to responsivity values >10 V/W, with impressive noise equivalent power levels <6 × 10−11 W/√Hz at room temperature and a wide modulation bandwidth. The potential scalability to even higher frequencies and the technological feasibility of realizing multi-pixel arrays coupled with QCL sources make the proposed technology highly competitive for a future generation of THz detection systems.
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Appl. Phys. Lett. 58, 2921 (1991); http://dx.doi.org/10.1063/1.105227 (3 pages)
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Measurements of the contact potential difference between different materials have been performed for the first time using scanning force microscopy. The instrument has a high resolution for both the contact potential difference (better than 0.1 mV) and the lateral dimension (<50 nm) and allows the simultaneous imaging of topography and contact potential difference. Images of gold, platinum, and palladium surfaces, taken in air, show a large contrast in the contact potential difference and demonstrate the basic concept. |
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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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Appl. Phys. Lett. 100, 241112 (2012); http://dx.doi.org/10.1063/1.4729414 (4 pages) Online Publication Date: 14 June 2012
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Crack-free InGaN multiple quantum wells (MQWs) light-emitting diodes with embedded electrode structures (EE-LEDs) were transferred from Si (111) substrate onto the electroplating copper submount. Crystalline quality was investigated by the high resolution x-ray diffraction (HR-XRD) measurement, in which no obvious deteriorations were found in the MQWs structure after the LEDs transferred from silicon substrate onto copper except for a partial residual strain relaxation in the film. The strain relaxation after silicon removal leads to a reduction in quantum confined stark effect (QCSE), which results in the enhancement of internal quantum efficiency (IQE). In comparison to the conventional LEDs on silicon substrate, the light output of the EE-LEDs on copper was enhanced by 122% at an injection current of 350 mA. Besides the enhancement of IQE, the improvement is also attributed to the following factors: the removal of the absorptive substrate, the inserting of the metal reflector between the EE-LEDs structure and the copper submount, the elimination of the electrode-shading, and the rough surface of the exposed AlN buffer layer.
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Very high open-circuit voltage of 5.89 V in organic solar cells with 10-fold-tandem structure Appl. Phys. Lett. 100, 243302 (2012); http://dx.doi.org/10.1063/1.4729009 (4 pages) Online Publication Date: 12 June 2012
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Organic solar cells (OSCs) based on chloroaluminum phthalocyanine (ClAlPc) as donor and fullerene C60 as acceptor with a multi-tandem structure were fabricated. We demonstrated very high open-circuit voltage (VOC) and enhanced power conversion efficiency (PCE) for the multi-tandem OSCs. Using a fivefold structure, we obtained PCE of 2.49% with a VOC of 3.50 V, in comparison with PCE of ∼2% and VOC of 0.72–0.81 V for the single device. We also fabricated a tenfold-stacked OSC showing an extremely high VOC of 5.89 V. The multi-tandem OSCs with very high VOC have great potential for applications in limited-area low-power electronics.
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Mechanism of non-metal catalytic growth of graphene on silicon Appl. Phys. Lett. 100, 231604 (2012); http://dx.doi.org/10.1063/1.4726114 (5 pages) Online Publication Date: 6 June 2012
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Compared to preparation on metal substrates, graphene synthesis on non-metal surfaces is highly desirable to avoid the deleterious metallic effects in fabrication of electronic devices. However, study of graphene growth mechanism on non-metal surfaces is rare and little understood. Here, we report that few-layers graphene films can be grown directly on silicon-on-insulator surface. Furthermore, the graphene growth mechanism on non-metal surfaces is proposed as a surface reaction, adsorption, decomposition, and accumulation process.
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Graphene/ZnO nanowire/graphene vertical structure based fast-response ultraviolet photodetector Appl. Phys. Lett. 100, 223114 (2012); http://dx.doi.org/10.1063/1.4724208 (4 pages) Online Publication Date: 1 June 2012
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We report the high performance vertical ultraviolet (UV) photodetectors based on single ZnO nanowire (NW) sandwiched between two graphene sheets. The photocurrent on-off ratio, rising time, and recovery speed of our UV detectors are 8 × 102, 0.7 s, and 0.5 s, respectively, which are significantly improved compared to the conventional ZnO NWs photodetectors. The improved performance is attributed to the existence of Schottky barriers between ZnO NW and graphene electrodes. The graphene/ZnO NW/graphene vertical sandwiched structures may be promising candidates for integrated optoelectronic sensor devices.
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Electrically pumped wavelength-tunable blue random lasing from CdZnO films on silicon Appl. Phys. Lett. 100, 231101 (2012); http://dx.doi.org/10.1063/1.4725486 (4 pages) Online Publication Date: 4 June 2012
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Electrically pumped lasing actions from the metastable CdZnO films have hardly been achieved as yet. Herein, we have demonstrated electrically pumped wavelength-tunable blue random lasing from the hexagonal CdZnO films with different Cd contents, with central wavelength changing from ∼490 to 425 nm. The devices based on the metal-insulator-semiconductor structures of Au/SiO2/CdZnO on silicon substrates are constructed for electrical pumping of the CdZnO films. The insulator layers of SiO2 onto the CdZnO films in the devices should be annealed at sufficiently low temperature such as 400 °C so that the CdZnO films can be kept their integrity in terms of near-band-edge emissions.
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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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Controlled addressing of quantum dots by nanowire plasmons Appl. Phys. Lett. 100, 231102 (2012); http://dx.doi.org/10.1063/1.4725490 (3 pages) Online Publication Date: 4 June 2012
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We demonstrate optical near field coupling of small quantum dot (QD) ensembles and surface plasmons propagating along a silver nanowire. The nanowire fabrication and the aligned QD deposition close to one nanowire end rely on a two-step electron beam lithography procedure. We observe both the addressing of QDs by plasmons and the excitation of plasmonic nanowire modes by QDs. We use the fluorescence signals to quantify the QD/plasmon coupling and show that part of the plasmon-induced QD fluorescence couples back to plasmonic modes.
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Efficient single-photon extraction from quantum-dots embedded in GaAs micro-pyramids Appl. Phys. Lett. 100, 251101 (2012); http://dx.doi.org/10.1063/1.4729482 (4 pages) Online Publication Date: 18 June 2012
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We demonstrate an easy method to fabricate efficient single-photon sources based on In(Ga)As quantum-dots embedded in reversed GaAs micro-pyramids. It relies on a single wet-chemical etching step utilizing an AlAs sacrificial layer. Due to the pyramidal shape of the cavities, we have been able to separate a small number of quantum-dots from the self-assembled ensemble and improve the extraction efficiency for single photons. The latter is predicted by finite difference time domain and finite elements method simulations to be about 80%–90% over a broad spectral range of 40 nm. Single-photon emission has been proven experimentally by means of auto-correlation measurements.
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Phase separation of co-evaporated ZnPc:C60 blend film for highly efficient organic photovoltaics Appl. Phys. Lett. 100, 233302 (2012); http://dx.doi.org/10.1063/1.4726118 (5 pages) Online Publication Date: 5 June 2012
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We demonstrate phase separation of co-evaporated zinc phthalocyanine (ZnPc) and fullerene (C60) for efficient organic photovoltaic cells. With introducing a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) film and a crystalline copper iodide film on indium tin oxide, 20-nm-thick ZnPc film adopts a lying-down crystalline geometry with grain sizes of about 50 nm. This surface distributed with strong interaction areas and weak interaction areas enables the selective growth of ZnPc and C60 molecules during following co-evaporation, which not only results in a phase separation but also improve the crystalline growth of C60. This blend film greatly enhances the efficiencies in photocurrent generation and carrier transport, resulting in a high power conversion efficiency of 4.56% under 1 sun.
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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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