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Top 20 Most Read Articles
November 2006
The 20 articles with the most full-text downloads during the month, in descending order.
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Lasing in GaN microdisks pivoted on Si Appl. Phys. Lett. 89, 211101 (2006); http://dx.doi.org/10.1063/1.2392673 (3 pages) Online Publication Date: 20 November 2006
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Arrays of pivoted GaN microdisks have been fabricated on a GaN/Si material by a combination of dry and wet etching. The Si material beneath the GaN microdisks is removed by wet etching, leaving behind a fine pillar to support the disks. Raman spectroscopy reveals substantial strain relaxation in these structures. Resonant modes, corresponding to whispering gallery modes, are observed in the photoluminescence spectra. Stimulated emission is achieved at higher optical pumping intensities.
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Large scale directed assembly of nanoparticles using nanotrench templates Appl. Phys. Lett. 89, 193108 (2006); http://dx.doi.org/10.1063/1.2385067 (3 pages) Online Publication Date: 7 November 2006
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The authors describe a general high throughput directed assembly technique to address some of the challenges to enable high rate∕high volume nanomanufacturing. The directed assembly of colloidal particles using an applied electric field shows the ability of precise control of nanoparticles by controlling assembly voltage, time, and geometric design of templates. The results show that single nanoparticle lines as small as 10 nm wide and 100 000 nm long over a 2.25 cm2 area as well as other nanoparticle structures can be fabricated using electrophoresis. This approach offers a simple, robust, and fast means of directed assembly of nanoelements for many applications.
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InGaAs quantum dot molecules around self-assembled GaAs nanomound templates Appl. Phys. Lett. 89, 202101 (2006); http://dx.doi.org/10.1063/1.2388049 (3 pages) Online Publication Date: 13 November 2006
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Several distinctive self-assembled InGaAs quantum dot molecules (QDMs) are studied. The QDMs self-assemble around nanoscale-sized GaAs moundlike templates fabricated by droplet homoepitaxy. Depending on the specific InAs monolayer coverage, the number of QDs per GaAs mound ranges from two to six (bi-QDMs to hexa-QDMs). The Ga contribution from the mounds is analyzed in determining the morphologies of the QDMs, with respect to the InAs coverages ranging between 0.8 and 2.4 ML. Optical characterization shows that the resulting nanostructures are high-quality nanocrystals.
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Appl. Phys. Lett. 89, 181101 (2006); http://dx.doi.org/10.1063/1.2372737 (3 pages) Online Publication Date: 30 October 2006
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A compact, pulsed, red, green, and blue laser source based on cascaded quasi-phase-matched wavelength conversion in two periodic superlattices set in tandem in a single stoichiometric LiTaO3 crystal and pumped by a laser source at 532 nm is reported. The white light equivalent flux obtained was 80 lm per 1 W input with a green to white light power conversion efficiency of >30%. Unity power conversion is feasible in this monolithic approach.
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Investigation on the p-type formation mechanism of arsenic doped p-type ZnO thin film Appl. Phys. Lett. 89, 181103 (2006); http://dx.doi.org/10.1063/1.2364865 (3 pages) Online Publication Date: 30 October 2006
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The photoluminescence spectra of As doped p-type ZnO thin films reveal neutral acceptor bound exciton of 3.3437 eV and a transition between free electrons and acceptor levels of 3.2924 eV. Calculated acceptor binding energy is about 0.1455 eV. Thermal activation and doping mechanism of this film have been suggested by the analysis of x-ray photoelectron spectroscopy. p-type formation mechanism of As doped ZnO thin film is related to the AsZn–2VZn complex model. ZnO-based p-n junction was fabricated by the deposition of an undoped n-type ZnO layer on an As doped p-type ZnO layer.
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Influence of annealing on stimulated emission in ZnO nanorods Appl. Phys. Lett. 89, 183112 (2006); http://dx.doi.org/10.1063/1.2378560 (3 pages) Online Publication Date: 2 November 2006
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Vertically aligned ZnO nanorod arrays with rod lengths in the range of 200–1500 nm were fabricated by a hydrothermal method. No stimulated emission was observed in as grown nanorods. Annealing of the rods in forming gas and oxygen significantly affected their optical properties and enabled the achievement of stimulated emission. The lowest lasing threshold and defect emission as well as the longest spontaneous emission decay times were obtained for nanorods annealed in oxygen flow. This indicates that interstitial oxygen, which is commonly assumed to be the cause of yellow-green defect emission, is not the dominant defect in hydrothermally grown nanorods.
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Appl. Phys. Lett. 89, 201102 (2006); http://dx.doi.org/10.1063/1.2386915 (3 pages) Online Publication Date: 13 November 2006
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Freestanding hexagonal two-dimensional photonic crystal (PhC) microcavities with Ge self-assembled quantum dots were fabricated on silicon-on-insulator substrates. Strong photoluminescence associated with Ge quantum dots was observed in the wavelength region of 1.2–1.6 μm at room temperature. Sharp peaks dominated the spectrum, showing strong optical resonance inside the cavity. A resonant peak with a quality factor of 560 was observed at 1.58 μm along with a significant enhancement of the luminescence in the microphotoluminescence spectrum. The peaks were also observed to reasonably shift when the structural parameter of PhC was changed.
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Doping of ZnO by group-IB elements Appl. Phys. Lett. 89, 181912 (2006); http://dx.doi.org/10.1063/1.2378404 (3 pages) Online Publication Date: 31 October 2006
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The authors present their first-principles calculations of doping effects in ZnO with group-IB elements such as Cu, Ag, and Au. The calculated transition energies ε(0/−) for substitutional Cu, Ag, and Au are 0.7, 0.4, and 0.5 eV, respectively. The calculated formation energies are very low for these group-IB elements on the substitutional sites, but rather high at the interstitial sites under oxygen-rich growth conditions. Under the conditions, the formation of major hole-killer defects, such as oxygen vacancies and Zn interstitial, are suppressed. Thus, Ag may be a good candidate for producing p-type ZnO.
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On-chip detection of ferromagnetic resonance of a single submicron Permalloy strip Appl. Phys. Lett. 89, 192506 (2006); http://dx.doi.org/10.1063/1.2385405 (3 pages) Online Publication Date: 8 November 2006
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The authors measured ferromagnetic resonance of a single submicron ferromagnetic strip, embedded in an on-chip microwave transmission line device. The method used is based on detection of the oscillating magnetic flux due to the magnetization dynamics, with an inductive pickup loop. The dependence of the resonance frequency on applied static magnetic field agrees very well with the Kittel formula, demonstrating that the uniform magnetization precession mode is being driven.
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Comparative study of the (0001) and (000 Appl. Phys. Lett. 89, 182111 (2006); http://dx.doi.org/10.1063/1.2378589 (3 pages) Online Publication Date: 1 November 2006
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The authors compare the surface and optical properties of the Zn-polar (0001) and O-polar (000
 ) surfaces of bulk ZnO samples. For optical characterization, steady-state photoluminescence using a He–Cd laser was measured at 15 and 300 K. At room temperature, the (000) surface demonstrates nearly double the near-band-edge emission intensity seen for the (0001) surface. Using scanning Kelvin probe microscopy, the authors have measured surface contact potentials of 0.39±0.05 and 0.50±0.05 V for the (0001) and (000) surfaces, respectively. The resulting small difference in band bending for these two surfaces indicates that charge transfer between the surfaces is not a dominant stabilizing mechanism. Conductive atomic force microscopy studies show enhanced reverse-bias conduction in localized regions on the (000) vs (0001) surface. The differences in surface conduction and band bending between the two polar surfaces can be attributed to their chemical interactions with hydrogen and water in the ambient. |
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Appl. Phys. Lett. 89, 211117 (2006); http://dx.doi.org/10.1063/1.2392720 (3 pages) Online Publication Date: 22 November 2006
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The authors present a technique that allows to modify the local characteristics of two-dimensional photonic crystals by controlled microinfiltration of liquids. They demonstrate experimentally that by addressing and infiltrating each pore with a simple liquid, e.g., water, it is possible to write pixel by pixel optical devices of any geometry and shape. Calculations confirm that the obtained structures indeed constitute the desired resonators and waveguide structures.
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Device scaling in sub-100 nm pentacene field-effect transistors Appl. Phys. Lett. 89, 183101 (2006); http://dx.doi.org/10.1063/1.2364154 (3 pages) Online Publication Date: 30 October 2006
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Reported here is the fabrication of 20–100 nm channel length pentacene field-effect transistors (FETs) with well-behaved current-voltage characteristics. Using a solution deposition method, pentacene grains span entire devices, providing superior contacts. Varying the gate oxide thickness, the effects of scaling on transistor performance is studied. When the channel length to oxide thickness exceeds 5:1, electrostatically well-scaled nanometer FETs are prepared. The results show that the device characteristics are dominated by the contacts. Decreasing the oxide thickness lowers the device turn-on voltage beyond simple field scaling, as sharper bending of the gate potential lines around the contacts more effectively reduces the molecule/source interfacial resistance.
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Appl. Phys. Lett. 89, 182107 (2006); http://dx.doi.org/10.1063/1.2364146 (3 pages) Online Publication Date: 31 October 2006
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Room-temperature near-band-edge photoluminescence of ZnO is composed of contributions from free-exciton recombination and its longitudinal-optical phonon replica. By tracking the photoluminescence of ZnO nanowires from 4 K up to room temperature, the authors show that the relative contributions of these emission lines show a strong variation for samples grown under different conditions. The varying coupling strengths of the excitons and phonons thus lead to a significant shift of the energy position of the room-temperature photoluminescence. They verify that this is not caused by laser heating or stress/strain but is most probably related to crystalline imperfections in the surface region.
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Appl. Phys. Lett. 89, 193506 (2006); http://dx.doi.org/10.1063/1.2378445 (3 pages) Online Publication Date: 9 November 2006
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The effects of bias stress (gate stress or drain stress) on nanowire field-effect transistor (NW-FET) stability were investigated as a function of stress bias and stress time. The n-channel NW-FETs used a nanoscopic self-assembled organic gate insulator, and each device contained a single ZnO nanowire. Before stress, the off current is limited by a leakage current in the 1 nA range, which increases as the gate to source bias becomes increasingly negative. The devices also exhibited significant changes in threshold voltage (Vth) and off current over 500 repeated measurement sweeps. The leakage current was significantly reduced after gate stress, but not after drain stress. Vth variations observed upon successive bias sweeps for devices following gate stress or drain stress were smaller than the Vth variation of unstressed devices. These observations suggest that gate stress and drain stress modify the ZnO nanowire-gate insulator interface, which can reduce electron trapping at the surface and therefore reduce the off current levels and variations in Vth. These results confirm that gate and drain stresses are effective means to stabilize device operation and provide high performance transistors with impressive reliabilities.
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Inserting one single Mn ion into a quantum dot Appl. Phys. Lett. 89, 193109 (2006); http://dx.doi.org/10.1063/1.2387116 (3 pages) Online Publication Date: 8 November 2006
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A method of growth to get one single Mn in self-assembled semiconductor quantum dot is presented. With a simple quantitative model, the appropriate low Mn density needed prior to the quantum dot nucleation is estimated. Such a low Mn concentration was reached by inserting a thin ZnTe spacer between a Zn1−xMnxTe buffer and the CdTe quantum dot layer. The control of Mn density is made by changing the thickness of the ZnTe spacer, with good reproducibility. Qualitative and quantitative comparisons of optical spectra for different samples assess the relevance of this growth method.
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Electrical and photoconductive properties of vertical ZnO nanowires in high density arrays Appl. Phys. Lett. 89, 213110 (2006); http://dx.doi.org/10.1063/1.2387868 (3 pages) Online Publication Date: 22 November 2006
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High density vertical zinc oxide nanowire arrays were fabricated using highly ordered channels in anodic alumina membranes via chemical vapor deposition assisted by electrochemical deposition methods. Using conductive atomic force microscopy, the electrical transport and photoconduction of individual vertical nanowires were investigated. A negative photoconductivity was observed as a result of electron trapping in the alumina membrane. In contrast, positive photoconductivity was observed using a thermally annealed anodic alumina membrane as the nanowire growth template. These studies provide a pathway for constructing highly integrated nanoscale electronic and optoelectronic circuits, such as logic circuits, light emitting diodes, solar cells, and ultrahigh resolution imaging sensors.
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Light-harvesting antennae for polymer solar cells Appl. Phys. Lett. 89, 213505 (2006); http://dx.doi.org/10.1063/1.2396909 (3 pages) Online Publication Date: 20 November 2006
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The authors demonstrate the incorporation of an artificial, extrinsic antenna into organic, conjugated polymer layers. This can lead to the independent optimization of spectral and spatial cross section for light absorption in organic solar devices. They report studies of three antenna layers, anthracene, 9,10-dichloroanthracene, and 9,10-diphenylanthracene, that are spin coated onto a poly[2-methoxy,5-(2methyl hexoxy)-1,4-phenylenevinylene] thin film.
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Search for fully spin-polarized semiconductor heterostructures: The candidate (Zn,Co)O Appl. Phys. Lett. 89, 192101 (2006); http://dx.doi.org/10.1063/1.2370751 (3 pages) Online Publication Date: 6 November 2006
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The authors suggest geometrical arrangements in (Zn,Co)O multilayered structures which maximize the spin polarization densities. The theoretical argument is supported by electronic structure calculations based on a two-band effective mass model adapted to describe these systems and solved self-consistently with the aid of Poisson’s equation. The exchange correlation, the strain terms, and the magnetic interactions are all included in the Hamiltonian. From their results a set of parameters can be determined to maximize the presence of spin-polarized charge distributions inside or outside the nonmagnetic layers, which enhances the transition temperature to ferromagnetism or the carrier mobility in the system.
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High open circuit voltage organic photovoltaic cells based on oligothiophene derivatives Appl. Phys. Lett. 89, 213501 (2006); http://dx.doi.org/10.1063/1.2392823 (3 pages) Online Publication Date: 20 November 2006
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For the purpose of developing organic photovoltaic devices with good performance characteristics, The authors have fabricated two cells using 5-vinyl-2,2′:5′,2″:5″,2‴-quarterthiophene (V4T), 5-vinyl-2′:5′,2″:5″,2‴:5‴,2″″-quinquethiophene (V5T) and 3,4,9,10-perylenetertracarboxylic dianhydride (PTCDA). The indium tin oxide (ITO)/V4T/PTCDA/Al cell has an open circuit voltage (Voc) of 2.07 V and a photoelectric conversion efficiency of 2.32%. The ITO/V5T/PTCDA/Al cell has an Voc of 1.87 V and a photoelectric conversion efficiency of 2.67%. The two cells have higher Voc (1.87–2.07 V). It is possible that intermolecular hydrogen bonding between –CHCH2 group of VnT with carboxylic dianhydride of PTCDA contributes to the enhancement by promoting forward interfacial electron transfer and eliminating the subconducting band trap sites.
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p-type Zn1−xMgxO films with Sb doping by radio-frequency magnetron sputtering Appl. Phys. Lett. 89, 202102 (2006); http://dx.doi.org/10.1063/1.2388254 (3 pages) Online Publication Date: 13 November 2006
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Sb-doped Zn1−xMgxO films were grown on c-plane sapphire substrates by radio-frequency magnetron sputtering. The p-type conduction of the films (0.05 ⩽ x ⩽ 0.13) was confirmed by Hall measurements, revealing a hole concentration of 1015–1016 cm−3 and a mobility of 0.6–4.5 cm2/V s. A p-n homojunction comprising an undoped ZnO layer and an Sb-doped Zn0.95Mg0.05O layer shows a typical rectifying characteristic. Sb-doped p-type Zn1−xMgxO films also exhibit a changeable wider band gap as a function of x, implying that they can probably be used for fabrication of ZnO-based quantum wells and ultraviolet optoelectronic devices.
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