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Top 20 Most Read Articles
January 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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Subwavelength waveguide for visible light Appl. Phys. Lett. 90, 021104 (2007); http://dx.doi.org/10.1063/1.2430400 (3 pages) Online Publication Date: 8 January 2007
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The authors demonstrate transmission of visible light through metallic coaxial nanostructures many wavelengths in length, with coaxial electrode spacing much less than a wavelength. Since the light frequency is well below the plasma resonance in the metal of the electrodes, the propagating mode reduces to the well-known transverse electromagnetic mode of a coaxial waveguide. They have thus achieved a faithful analog of the conventional coaxial cable for visible light.
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Temperature-dependent photoluminescence of quasialigned Al-doped ZnO nanorods Appl. Phys. Lett. 90, 023104 (2007); http://dx.doi.org/10.1063/1.2429906 (3 pages) Online Publication Date: 8 January 2007
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Temperature-dependent photoluminescence (PL) properties of quasialigned Al-doped ZnO nanorods grown by thermal evaporation method were investigated. The ionization energy of the Al donor was determined to be ∼ 90 meV. A PL peak at 3.315 eV was observed at low temperature and was tentatively related to excitons bound to surface defects. The emission, along with its first longitudinal optical phonon replica, persists up to room temperature and dominates the near band edge (NBE) emission of the nanorods. The doping of Al results in a redshift of ∼ 0.04 eV of the room-temperature NBE emission of the ZnO nanorods.
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Electrical properties of nominally undoped silicon nanowires grown by molecular-beam epitaxy Appl. Phys. Lett. 90, 012105 (2007); http://dx.doi.org/10.1063/1.2428402 (3 pages) Online Publication Date: 3 January 2007
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Single undoped Si nanowires were electrically characterized. The nanowires were grown by molecular-beam epitaxy on n+ silicon substrates and were contacted by platinum/iridium tips. I-V curves were measured and electron beam induced current investigations were performed on single nanowires. It was found that the nanowires have an apparent resistivity of 0.85 Ω cm, which is much smaller than expected for undoped Si nanowires. The conductance is explained by hopping conductivity at the Si–SiO2 interface of the nanowire surface.
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Structural transformation of ZnO nanostructures Appl. Phys. Lett. 90, 023115 (2007); http://dx.doi.org/10.1063/1.2431073 (3 pages) Online Publication Date: 11 January 2007
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Many nanostructures experience structural transformation, either from one type of crystalline structure to another or from one orientation to another orientation within the same crystalline structure. Using a combination of ab initio calculations and analytical formulation, the authors show that such structural transformation is a direct result of surface reconstruction. In particular, the ab initio results show that infinite large ZnO nanoplates and infinite long ZnO nanowires transform from wurtzite to graphitic structure below a critical thickness or diameter. The analytical formulation further shows that nanowires of finite length can go through the same structural transformation at larger critical diameters.
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Effect of annealing temperature and ambient gas on phosphorus doped p-type ZnO Appl. Phys. Lett. 90, 021106 (2007); http://dx.doi.org/10.1063/1.2430937 (3 pages) Online Publication Date: 10 January 2007
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The authors report on the thermal activation of phosphorus doped p-type ZnO thin films grown by radio frequency magnetron sputtering. The p-type ZnO was produced by activating phosphorus doped ZnO thin films in N2, Ar, or O2 ambients. The hole concentration of the p-type ZnO, prepared in an O2 ambient, showed a lower hole concentration compared to samples annealed in N2 and Ar ambients. The activation energies of the phosphorus dopant in the p-type ZnO under different ambient gases indicate that phosphorus atoms replace oxygen atoms in the ZnO to form PO which acts as an acceptor.
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Appl. Phys. Lett. 90, 011101 (2007); http://dx.doi.org/10.1063/1.2424642 (3 pages) Online Publication Date: 2 January 2007
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The authors present the design, growth, fabrication, experimental characterization, and theoretical analysis of blue quantum electroabsorption modulators that incorporate ∼ 5 nm thick In0.35Ga0.65N/GaN quantum structures for operation between 420 and 430 nm. Growing on polar c plane on sapphire, they obtain quantum structures with zigzag potential profile due to alternating polarization fields and demonstrate that their optical absorption blueshifts with applied electric field, unlike the redshift of conventional quantum confined Stark effect. In InGaN/GaN quantum structures, they report the largest absorption change of 6000 cm−1 for 6 V bias swing around 424 nm, holding promise for blue optical clock generation and injection directly into silicon chips.
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Efficient, single-layer molecular organic light-emitting diodes Appl. Phys. Lett. 90, 023511 (2007); http://dx.doi.org/10.1063/1.2426882 (3 pages) Online Publication Date: 11 January 2007
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The authors demonstrate efficient molecular organic light-emitting diodes that use direct hole injection from poly(3,4-ethylene-dioxythio-phene):poly(styrene-sulfonate) into a single layer of tris(8-hydroxyquinoline) aluminum (III) for carrier transport and electroluminescence. Single-layer devices have a lower operating bias and higher luminous power efficiency than conventional bilayer devices with a 4,4-bis[N-1-napthyl-N-phenyl-amino]biphenyl hole transport layer. The current density-voltage characteristics of single-layer devices follow Schottky-Richardson behavior and are consistent with an Ohmic contact at the anode.
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Appl. Phys. Lett. 90, 021101 (2007); http://dx.doi.org/10.1063/1.2431445 (3 pages) Online Publication Date: 8 January 2007
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The authors demonstrate a nonblocked all-fiber mode locker operated by the interaction of carbon nanotubes with the evanescent field of propagating light in a tapered fiber. Symmetric cross section of the device with the randomly oriented nanotubes guarantees the polarization insensitive operation of the pulse formation. In order to minimize the scattering, the carbon nanotubes are deposited within a designed area around the tapered waist. The demonstrated passively pulsed laser has the repetition rate of 7.3 MHz and the pulse width of 829 fs.
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Carbon nanotubes grown on In2O3:Sn glass as large area electrodes for organic photovoltaics Appl. Phys. Lett. 90, 023105 (2007); http://dx.doi.org/10.1063/1.2431437 (3 pages) Online Publication Date: 9 January 2007
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The authors report the growth of multiwall carbon nanotubes directly onto indium tin oxide glass via chemical vapor deposition as large area semitransparent electrodes for organic solar cell applications. The rate of nanotube growth on this ternary oxide is greatly reduced as compared to that of silicon dioxide and glass substrates enabling a high degree of control over nanotube height. The strong potential of this nanostructured semitransparent substrate as an interpenetrating hole-extracting electrode in bulk-heterojunction organic solar cells is also demonstrated.
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Impact of disorder on high quality factor III-V nitride microcavities Appl. Phys. Lett. 89, 261101 (2006); http://dx.doi.org/10.1063/1.2420788 (3 pages) Online Publication Date: 26 December 2006
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The authors report on the micron scale characterization of a monolithic GaN microcavity (MC) with lattice matched AlInN/GaN distributed Bragg reflectors by means of a microtransmission setup. This technique allows extracting very high quality factors (Q up to 2800), in accordance with theoretical predictions, contrary to what was previously reported for nitride based MCs. Furthermore, two-dimensional mappings of the MC transmission spectrum allow probing the disorder in this MC. The direct relationship between an increased disorder and a reduction in the Q factor is clearly observed.
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Catalyst-free growth of uniform ZnO nanowire arrays on prepatterned substrate Appl. Phys. Lett. 89, 263116 (2006); http://dx.doi.org/10.1063/1.2425036 (3 pages) Online Publication Date: 29 December 2006
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Uniform and ordered ZnO nanowire arrays have been fabricated on the nanopatterned SiO2/GaN substrate without metal catalysts using hydrothermal synthesis. The nanopatterns on SiO2/GaN substrate with an average diameter of 65 nm are produced by inductively coupled plasma etching using anodic alumina template as a mask, which provides nucleation sites for the vertical ZnO nanowires growth. High quality of the aligned uniform ZnO nanowire arrays grown on GaN substrate was confirmed by x-ray diffraction, transmission electron microscopy, and photoluminescence. This growth technique provides a cost-effective approach to fabricate ordered nanowire arrays with controlled size, which may benefit the nanowire device applications.
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Spontaneous emission control of colloidal nanocrystals using nanoimprinted photonic crystals Appl. Phys. Lett. 90, 011115 (2007); http://dx.doi.org/10.1063/1.2430625 (3 pages) Online Publication Date: 4 January 2007
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The authors report on the fabrication and optical characterizations of two-dimensional photonic crystals fabricated by nanoimprint lithography in a nanocomposite polymer incorporating highly luminescent and red emitting (CdSe)ZnS core-shell colloidal nanocrystals. Photonic crystal structures enhance the light emitted from the quantum sized nanoparticles in the composite layer by slowing the propagation speed of the photons, thus increasing the coupling to the out-of-plane radiative modes. A 200% enhancement of the light collection is achieved compared to an unpatterned sample.
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High performance ZnO nanowire field effect transistor using self-aligned nanogap gate electrodes Appl. Phys. Lett. 89, 263102 (2006); http://dx.doi.org/10.1063/1.2416249 (3 pages) Online Publication Date: 26 December 2006
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A field effect transistor (FET) using a zinc oxide nanowire with significantly enhanced performance is demonstrated. The device consists of single nanowire and self-aligned gate electrodes with well defined nanosize gaps separating them from the suspended nanowire. The fabricated FET exhibits excellent performance with a transconductance of 3.06 μS, a field effect mobility of 928 cm2/V s, and an on/off current ratio of 106. The electrical characteristics are the best obtained to date for a ZnO transistor. The FET has a n-type channel and operates in enhancement mode. The results are close to those reported previously for p-type carbon nanotube (CNT) FETs. This raises the possibility of using ZnO as the n-type FET with a CNT as the p-type FET in nanoscale complementary logic circuits.
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Appl. Phys. Lett. 90, 012113 (2007); http://dx.doi.org/10.1063/1.2404590 (3 pages) Online Publication Date: 4 January 2007
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Top-gate thin film transistors with n-type ZnO active channel were performed under 230 °C. Especially, ZnO film was deposited by a combined method of sol-gel and chemical bath deposition without any preactivation for film growth. Silicon nitride and indium tin oxide were used as the gate insulator and the conducting electrodes (source, drain, and gate). These transistors were highly transparent in the visible spectrum, with transmittance as high as 75% to approximately 85% at wavelength from 500 to 700 nm. The optimum device has field-effect mobility of 0.67 cm2/V s and an on-off ratio more than 107.
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Sound beyond the speed of light: Measurement of negative group velocity in an acoustic loop filter Appl. Phys. Lett. 90, 014102 (2007); http://dx.doi.org/10.1063/1.2423240 (3 pages) Online Publication Date: 2 January 2007
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The authors describe the experimental observation of negative group velocity propagation of sound waves through an asymmetric loop filter. The characteristics of the filter are established using impulse response and direct tunneling of narrow bandwidth Gaussian pulses. The results confirm recent theoretical predictions that faster-than-light group velocity propagation of sound is possible. Further, the results show that the spectral rephasing achieved in a loop filter is sufficient to produce negative group velocities independent of the phase velocity of the spectral components themselves. Thus, superluminal propagation is realized despite almost six orders of magnitude difference between the speeds of sound and light.
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Identification of oxygen and zinc vacancy optical signals in ZnO Appl. Phys. Lett. 89, 262112 (2006); http://dx.doi.org/10.1063/1.2424641 (3 pages) Online Publication Date: 28 December 2006
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Photoluminescence spectroscopy has been used to study single crystalline ZnO samples systematically annealed in inert, Zn-rich and O-rich atmospheres. A striking correlation is observed between the choice of annealing ambient and the position of the deep band emission (DBE) often detected in ZnO. In particular, annealing in O2 results in a DBE at 2.35±0.05 eV, whereas annealing in the presence of metallic Zn results in DBE at 2.53±0.05 eV. The authors attribute the former band to zinc vacancy (VZn) related defects and the latter to oxygen vacancy (VO) related defects. Additional confirmation for the VO and VZn peak identification comes from the observation that the effect is reversible when O- and Zn-rich annealing conditions are switched. After annealing in the presence of ZnO powder, there is no indication for the VZn- or VO-related bands, but the authors observe a low intensity yellow luminescence band peaking at 2.17 eV, probably related to Li, a common impurity in hydrothermally grown ZnO.
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Multisegmented one-dimensional hybrid structures of carbon nanotubes and metal nanowires Appl. Phys. Lett. 89, 243122 (2006); http://dx.doi.org/10.1063/1.2405390 (3 pages) Online Publication Date: 15 December 2006
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Multisegmented one-dimensional hybrid structures of carbon nanotubes and metal nanowires were fabricated using the alumina templates. Metal nanowires are first grown inside part of the nanochannel using electrodeposition technique, which is followed by the growth of carbon nanotubes using chemical vapor deposition. Well-adhered interfaces formed between the carbon nanotubes and the metal nanowires. The hybrid structure reported here results in nanoscale metal contact with carbon nanotube and will provide a solution to problem of using carbon nanotubes in interconnects. Nanotube-nanowire interfaces for different metals have also been examined and are characterized using scanning electron microscopy and transmission electron microscopy.
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Manipulating exciton fine structure in quantum dots with a lateral electric field Appl. Phys. Lett. 90, 041101 (2007); http://dx.doi.org/10.1063/1.2431758 (3 pages) Online Publication Date: 22 January 2007
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The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5 meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. The authors show that the lateral electric field strongly affects the exciton fine-structure splitting due to active manipulation of the single particle wave functions. Remarkably, the splitting can be tuned over large values and through zero.
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Liquid-state field-effect transistors using electrowetting Appl. Phys. Lett. 90, 043507 (2007); http://dx.doi.org/10.1063/1.2435508 (3 pages) Online Publication Date: 23 January 2007
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The authors report the demonstration of transistor action in the liquid state. The control of current flow in a liquid field-effect transistor (LiquiFET) was achieved by electrowetting between competitive insulating/conducting fluids. The LiquiFET structure included dielectric/hydrophobic layers, source/drain regions, a gate electrode, and hydrophilic/hydrophobic grids to contain the liquids. For a 400 μm long channel, turn-on occurs at 2.5–3 V drain voltage. On/off current ratios >10 000:1 were measured. Linear gate voltage control over drain current was obtained with a transconductance up to 40 nS. A calculated channel mobility of ∼ 1 cm2/V s indicates that electronic charge transport dominates transistor operation.
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Doping-induced efficiency enhancement in organic photovoltaic devices Appl. Phys. Lett. 90, 023504 (2007); http://dx.doi.org/10.1063/1.2430783 (3 pages) Online Publication Date: 11 January 2007
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Performance of organic photovoltaic (OPV) devices is dramatically enhanced by doping suitable fluorescent dyes into the donor and/or acceptor layers. By doping rubrene into standard CuPc/C60 OPV cell, a high JSC of 30.1 mA/cm2, VOC of 0.58 V, and an exceptionally high power conversion efficiency of 5.58% are achieved. The performance improvement is mainly attributed to efficient light absorption by rubrene in the range of 460–530 nm where two hosts have low absorbance, leading to more effective exciton formation. Their findings motivate the use of fluorescent dyes for maximizing absorption spectral coverage as well as increasing photon harvesting.
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