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Top 20 Most Read Articles
March 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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Formation of single crystalline ZnO nanotubes without catalysts and templates Appl. Phys. Lett. 90, 113108 (2007); http://dx.doi.org/10.1063/1.2714186 (3 pages) Online Publication Date: 13 March 2007
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Oxide and nitride nanotubes have gained attention for their large surface areas, wide energy band gaps, and hydrophilic natures for various innovative applications. These nanotubes were either grown by templates or multistep processes with uncontrollable crystallinity. Here the authors show that single crystal ZnO nanotubes can be directly grown on planar substrates without using catalysts and templates. These results are guided by the theory of nucleation and the vapor-solid crystal growth mechanism, which is applicable for transforming other nanowires or nanorods into nanotubular structures.
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Finite size effect in ZnO nanowires Appl. Phys. Lett. 90, 113101 (2007); http://dx.doi.org/10.1063/1.2712507 (3 pages) Online Publication Date: 12 March 2007
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To clarify the size effect in semiconductor nanowires with decreasing diameters but not yet reaching the quantum confinement region, single crystalline zinc oxide nanowires with diameters around 10 nm are synthesized. Electrical transport measurements of these thin nanowires show significant increase in conductivity accompanied by diminished gate modulation and reduced mobility. This phenomenon is a result of the enrichment of surface states owing to the increased surface-to-volume ratio. The enhanced surface effect is confirmed by the temperature dependent photoluminescence measurements and contributes to the “anomalous” blueshift. This study shows that surface states play a dominant role in the electrical and optical properties of quasi-one-dimensional materials.
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Focusing of light by a nanohole array Appl. Phys. Lett. 90, 091119 (2007); http://dx.doi.org/10.1063/1.2710775 (3 pages) Online Publication Date: 2 March 2007
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The authors demonstrate a mechanism for focusing at optical frequencies based on the use of nanohole quasiperiodic arrays in metal screens. Using coherent illumination at 660 nm and scanning aperture optical microscopy, ∼ 290 nm “hot spots” were observed at a distance of ∼ 12.5 μm from the array. Even smaller hot spots of about ∼ 200 nm in waist were observed closer to the plane of the array.
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Photoluminescence properties of a single GaN nanorod with GaN/AlGaN multilayer quantum disks Appl. Phys. Lett. 90, 101901 (2007); http://dx.doi.org/10.1063/1.2712772 (3 pages) Online Publication Date: 5 March 2007
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Photoluminescence (PL) properties of a single nanorod containing multiple GaN quantum disks separated by AlGaN potential barriers are investigated using micro-PL spectroscopy. Previous studies reported ensemble spectra from many nanorods. The PL spectra show different features depending on the region of the nanorod excited by the laser, including a sharp feature originating from the quantum disk region. The distinct differences between the PL from the different regions are discussed. The results imply that excitons are strongly confined in the quantum disks, and the authors suggest that small quantum disks can be regarded as quantum dots having a discrete density of states.
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Heteroepitaxial growth and optical properties of ZnS nanowire arrays on CdS nanoribbons Appl. Phys. Lett. 90, 093127 (2007); http://dx.doi.org/10.1063/1.2710743 (3 pages) Online Publication Date: 2 March 2007
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The authors present the results of heteroepitaxial growth of single-crystalline ZnS nanowire arrays on CdS nanoribbon substrates by the metal-catalyzed vapor-liquid-solid growth method. ZnS nanowire arrays were vertically or crosswise aligned to the surface of CdS nanoribbon substrates. Room-temperature lasing from ZnS nanowire arrays was demonstrated. The present synthesis provides a new approach to the rational design of building blocks for nanodevices.
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High-symmetry ZnS hepta- and tetrapods composed of assembled ZnS nanowire arrays Appl. Phys. Lett. 90, 123101 (2007); http://dx.doi.org/10.1063/1.2716242 (3 pages) Online Publication Date: 19 March 2007
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Sixfold symmetry heptapodlike and threefold symmetry tetrapodlike ZnS structures have been fabricated by thermal evaporation of a ZnS and SiO mixture source in N2 at 1300 °C. Both the heptapods and tetrapods have been formed by the self-assembly of ZnS nanowires with the preferred (0001) orientations. Structures and morphologies of the obtained heptapods and tetrapods are thoroughly studied using x-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. Room-temperature photoluminescence spectra taken from the nanostructures show a strong green emission centered at 550 nm.
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Self-assembled magnetic nanowire arrays Appl. Phys. Lett. 90, 103105 (2007); http://dx.doi.org/10.1063/1.2711522 (3 pages) Online Publication Date: 6 March 2007
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Different self-assembled magnetic nanowire arrays were achieved with ∼ 100% trapping rate on templates with nanomagnet arrays under a low external magnetic field ( ∼ 10 Oe). The principles of magnetic charge matching and dimension matching between the magnetic nanowires to be assembled and the gaps between two nanomagnets were proposed and demonstrated to be crucial for achieving low magnetostatic energy and high trapping rate. This templated self-assembly technique and the proposed template design principles have great potential for nanomanufacturing of regular arrays of magnetic nanostructures.
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Chip cooling with integrated carbon nanotube microfin architectures Appl. Phys. Lett. 90, 123105 (2007); http://dx.doi.org/10.1063/1.2714281 (3 pages) Online Publication Date: 20 March 2007
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Efficient cooling of silicon chips using microfin structures made of aligned multiwalled carbon nanotube arrays is achieved. The tiny cooling elements mounted on the back side of the chips enable power dissipation from the heated chips on the level of modern electronics demands. The nanotube fins are mechanically superior compared to other materials being ten times lighter, flexible, and stiff at the same time. These properties accompanied with the relative simplicity of the fabrication makes the nanotube structures strong candidates for future on-chip thermal management applications.
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Plasmonic excitation of organic double heterostructure solar cells Appl. Phys. Lett. 90, 121102 (2007); http://dx.doi.org/10.1063/1.2714193 (3 pages) Online Publication Date: 19 March 2007
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The authors demonstrate that thin film organic photovoltaic cells are efficient detectors of surface plasmon polaritons (SPPs). For λ = 532 nm radiation in a Kretschmann configuration, the external quantum efficiency in fullerene–copper phthalocyanine photovoltaic cells is doubled at resonance to 12%. In thin heterojunction organic photovoltaics, SPP detection relies on a substantial increase in absorption when the incoming radiation is coupled to guided SPPs rather than unguided photons. SPP scattering and nonradiative losses are negligible; however, optical modeling shows that cathode metal penetration into the neighboring organic semiconductor is a major source of loss for SPP or photonic excitation.
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Appl. Phys. Lett. 90, 111114 (2007); http://dx.doi.org/10.1063/1.2713237 (3 pages) Online Publication Date: 15 March 2007
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The authors have fabricated organic light-emitting diodes (OLEDs) having two-dimensional photonic crystals (PC) as light extraction elements by employing nanoimprint lithography technique. PCs were imprinted on the glass substrate and OLED layers were formed on the imprinted side of the glass substrate. The device having PC showed the improvement of luminance by a factor of 1.5 compared to normal devices. The authors conclude that the nanoimprint lithography is very useful for the fabrication of the OLEDs with PC.
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Room temperature fast synthesis of zinc oxide nanowires by inductive heating Appl. Phys. Lett. 90, 093101 (2007); http://dx.doi.org/10.1063/1.2709618 (3 pages) Online Publication Date: 26 February 2007
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ZnO nanowires have been rapidly synthesized using inductive heating in a room temperature environment. Nanowires with random and aligned orientations were grown on silicon and 4H-SiC (0001) substrates in less than 5 min, respectively, using ZnO/graphite as the solid source powder. Scanning electron microscopy showed nanowire diameters of 20–120 nm and lengths up to 5 μm, and transmission electron microscopy verified the single-crystalline lattice of the nanowires. Electrical properties were studied by connecting a single ZnO nanowire in the field-effect transistor configuration. This demonstration further illustrates the feasibility of a simple and fast nanoscale synthesis using inductive heating for nanomaterial synthesis.
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Appl. Phys. Lett. 90, 101504 (2007); http://dx.doi.org/10.1063/1.2711753 (3 pages) Online Publication Date: 5 March 2007
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A complex three-dimensional, nanowire based nanoarchitecture is presented, which can be processed by high-throughput bottom-up procedures without any high-resolution lithography. It combines the benefits of three self-organization mechanisms to produce nanostructures, i.e., the formation of nanoneedles, the droplet formation out of a thin metal film, and the vapor-liquid-solid growth of nanowires. The principle is demonstrated for a silicon based suspended nanowire web. Cell adherence on this assembly was found to be superior to other nanostructures. The possibility of fluid transport beneath the nanowire web enables improved microcatalyst principles and the realization of novel interfaces for biosensing or bioelectronics.
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Enhanced light outcoupling in a thin film by texturing meshed surfaces Appl. Phys. Lett. 90, 091102 (2007); http://dx.doi.org/10.1063/1.2709920 (3 pages) Online Publication Date: 27 February 2007
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The authors demonstrate a method of texturing a meshed surface on a poly(dimethyl siloxane) (PDMS) film for improving light extraction. This meshed surface is fabricated through a casting process by using a self-organized porous film as a template. Experimental results show that the light outcoupling efficiency increases on the meshed surface of a freestanding PDMS film with large incident angles. The external quantum efficiency of an organic light-emitting diode with the textured PDMS film was also demonstrated to have an enhancement of 46%.
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Practicable enhancement of spontaneous emission using surface plasmons Appl. Phys. Lett. 90, 111107 (2007); http://dx.doi.org/10.1063/1.2539745 (3 pages) Online Publication Date: 14 March 2007
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The authors develop a rigorous theory of the enhancement of spontaneous emission from a light emitting device via coupling the radiant energy in and out of surface plasmon polaritons (SPPs) on the metal-dielectric interface. Using the GaN/Ag system as an example, the authors show that using SPP pays off only for emitters that have a low luminescence efficiency.
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Appl. Phys. Lett. 90, 111103 (2007); http://dx.doi.org/10.1063/1.2713145 (3 pages) Online Publication Date: 12 March 2007
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The authors report the experimental observation of the splitting of the degenerate (0,±1) surface plasmon polariton modes excited in a metallic nanohole array integrated with microfluidic channel for delivery and precise control of the index of refraction of overlaying layer. They use high-resolution wavelength and angle scans for excitation of the surface modes and a polarizer-analyzer pair to suppress the nonresonant transmission. The two resultant modes include an intense, spectrally broad low-energy mode and a weaker, much narrower bandwidth high-energy mode.
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Synthesis and optical properties of InN nanowires and nanotubes Appl. Phys. Lett. 90, 113105 (2007); http://dx.doi.org/10.1063/1.2712801 (3 pages) Online Publication Date: 13 March 2007
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InN nanowires and faceted hexagonal InN nanotubes are synthesized by catalyst-free chemical vapor deposition at different temperatures. Both have the single crystalline wurtzite structure and grow along the c axis. Different growth dynamics are suggested for the difference in morphology. Observations of phonon-plasmon coupled modes in their Raman scattering suggest of high electron concentrations. Absorption edges in their optical spectra have energies slightly higher than 1 eV, showing blueshifts from the fundamental band gap of ∼ 0.7 eV, recently observed in epitaxial films. The shifts are argued to be the result of the Burstein-Moss effect.
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Appl. Phys. Lett. 90, 103107 (2007); http://dx.doi.org/10.1063/1.2711419 (3 pages) Online Publication Date: 7 March 2007
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Vertically aligned ZnO nanowire arrays are grown homoepitaxially on the ZnO seeded indium tin oxide substrate by electrochemical deposition from aqueous solution at low temperature (70 °C) without using any template. ZnO nanowires exhibit single crystalline, wurtzite crystal structure determined by transmission electron microscopy and powder x-ray diffraction. The ZnO nanowire arrays show high transmittance in the visible wavelengths. Interestingly, the optical band gap of the ZnO nanowire arrays has been tuned by simply changing zinc salts in the electrodeposition from aqueous solution.
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Electrochemical route to the synthesis of ultrathin ZnO nanorod/nanobelt arrays on zinc substrate Appl. Phys. Lett. 90, 103109 (2007); http://dx.doi.org/10.1063/1.2711532 (3 pages) Online Publication Date: 7 March 2007
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An electrochemical route has been developed for the synthesis of highly ordered ZnO ultrathin nanorod and hierarchical nanobelt arrays on zinc substrate. A H2 gas sensor based on the ZnO nanobelt array exhibits excellent sensitivity, rapid response, and good reproducibility even at room temperature.
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Origins of shallow level and hole mobility in codoped p-type ZnO thin films Appl. Phys. Lett. 90, 102115 (2007); http://dx.doi.org/10.1063/1.2711538 (3 pages) Online Publication Date: 8 March 2007
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A combination study of structural, optical, and electrical properties has been carried out on N–In codoped p-type ZnO thin films for the origins of shallow level and hole mobility. The observed small activation energy of ∼ 20 meV for the hole concentration corresponds well to the results from photoluminescence and conductivity data, revealing the grain boundary trapping nature of the shallow level. The achieved hole mobility is mainly due to the lack of grain boundary barrier effect, and the codoping yielded weak ionized impurity scattering. The authors have also revealed the scattering and conduction mechanisms in these p-ZnO films.
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Selectively coated photonic crystal fiber for highly sensitive fluorescence detection Appl. Phys. Lett. 90, 111101 (2007); http://dx.doi.org/10.1063/1.2713132 (3 pages) Online Publication Date: 12 March 2007
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The authors investigate the potential of hollow core photonic crystal fibers (HCPCFs) for highly sensitive fluorescence detection by selectively coating the central hole of a HCPCF with fluorophores. After studying the influence of reabsorption and intermolecular effects on the emission, they show that dye concentrations down to 10−9M can be detected using only nanoliter sample volumes. The results indicate that HCPCF are well suited for sensing applications involving internal emission or scattering processes, such as fluorescence based gas sensing or surface-enhanced Raman scattering, and may be integrated into compact optical devices.
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