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Top 20 Most Read Articles
March 2008
The 20 articles with the most full-text downloads during the month, in descending order.
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Coherent control of molecular modulation Appl. Phys. Lett. 91, 241101 (2007); http://dx.doi.org/10.1063/1.2821382 (3 pages) Online Publication Date: 10 December 2007
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We demonstrate coherent control of a molecular modulation process using an incident set of seven optical sidebands spanning two octaves of bandwidth. We utilize a genetic algorithm to optimize the relative phases of the incident sidebands to generate additional UV sidebands with nearly 1% efficiency, change the ratio of energy between sidebands by more than a factor of 50, and efficiently alter individual sideband energies by millijoules.
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Single-electron-based flexible multivalued logic gates Appl. Phys. Lett. 92, 093101 (2008); http://dx.doi.org/10.1063/1.2888164 (3 pages) Online Publication Date: 3 March 2008
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Single-electron transistor (SET)-based multivalued (MV) not-AND (NAND) and not-OR (NOR) logic cells were implemented on a silicon-on-insulator chip. Depending on the ways of connecting two SETs with a field-effect transistor, the voltage transfer characteristics show typical NAND or NOR gate functions for various input voltages, which are binary, MV, and binary-MV mixed. Moreover, the switching functionality of our NAND (NOR) can convert to OR (AND) operation by simply adjusting their initial input voltages. These flexible two-input logic gates are expected to provide four basic arithmetic cells for the SET MV logic gate family.
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Appl. Phys. Lett. 91, 233117 (2007); http://dx.doi.org/10.1063/1.2821113 (3 pages) Online Publication Date: 7 December 2007
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Silicon nanowire-based solar cells on metal foil are described. The key benefits of such devices are discussed, followed by optical reflectance, current-voltage, and external quantum efficiency data for a cell design employing a thin amorphous silicon layer deposited on the nanowire array to form the p-n junction. A promising current density of ∼ 1.6 mA/cm2 for 1.8 cm2 cells was obtained, and a broad external quantum efficiency was measured with a maximum value of ∼ 12% at 690 nm. The optical reflectance of the silicon nanowire solar cells is reduced by one to two orders of magnitude compared to planar cells.
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Appl. Phys. Lett. 92, 101101 (2008); http://dx.doi.org/10.1063/1.2891067 (3 pages) Online Publication Date: 10 March 2008
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A high-performance, GaN-based light emitting diode (LED) was prepared using a metal organic chemical vapor deposition method on a silica-sphere, monolayer-coated sapphire substrate. Various surface coverage ratios of the silica submicron spheres with diameters ranging from 300 to 550 nm were deposited on the sapphire substrate using a spin-coating method. The LED output power was increased 2.5-fold compared with the LED constructed without silica spheres and uniform light distribution was achieved. In addition, LED output power was dependent on silica-sphere size and surface coverage of the substrates.
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Interface modification for highly efficient organic photovoltaics Appl. Phys. Lett. 92, 093303 (2008); http://dx.doi.org/10.1063/1.2885724 (3 pages) Online Publication Date: 4 March 2008
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We present highly efficient inverted polymer:fullerene bulk-heterojunction solar cells by incorporation of a nanoscale organic interfacial layer between the indium tin oxide (ITO) and the metal oxide electron-conducting layer. We demonstrate that stacking of solution-processed organic and metal oxide interfacial layers gives highly charged selective low ohmic cathodes. The incorporation of a polyoxyethylene tridecyl ether interfacial layer between ITO and solution-processed titanium oxide (TiOx) raised the power conversion efficiency of inverted organic photovoltaics to 3.6%, an improvement of around 15% in their performance over comparable devices without the organic interfacial layer.
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Room temperature ferromagnetism in ZnO films due to defects Appl. Phys. Lett. 92, 082508 (2008); http://dx.doi.org/10.1063/1.2885730 (3 pages) Online Publication Date: 29 February 2008
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ZnO films were prepared by pulsed laser deposition on a-plane sapphire substrates under N2 atmosphere. Ferromagnetic loops were obtained with the superconducting quantum interference device at room temperature, which indicate a Curie temperature much above room temperature. No clear ferromagnetism was observed in intentionally Cu-doped ZnO films. This excludes that Cu doping into ZnO plays a key role in tuning the ferromagnetism in ZnO. 8.8% negative magnetoresistance probed at 5 K at 60 kOe on ferromagnetic ZnO proves the lack of s-d exchange interaction. Anomalous Hall effect (AHE) was observed in ferromagnetic ZnO as well as in nonferromagnetic Cu-doped ZnO films, indicating that AHE does not uniquely prove ferromagnetism. The observed ferromagnetism in ZnO is attributed to intrinsic defects.
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In-plane microelectromechanical resonator with integrated Fabry–Pérot cavity Appl. Phys. Lett. 92, 081101 (2008); http://dx.doi.org/10.1063/1.2883874 (3 pages) Online Publication Date: 25 February 2008
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A silicon-on-insulator in-plane microelectromechanical resonator coupled to a high-Q (Q ≈ 4,200), high finesse (FMax = 265) optical Fabry–Pérot microcavity is presented. The cavity utilizes high reflectance dry-etched silicon/air distributed Bragg reflectors. By suspending one of the Bragg mirrors to a microbridge resonator, the mirror can be displaced and the cavity is tuned. Using electrostatic actuation, bidirectional cavity tuning from −12.1 to +17.0 nm (29.1 nm total range) is demonstrated near 1601 nm wavelength. The device also enables measurement of thermal-mechanical noise with sensitivity better than 10 fm/Hz1/2 and may find application in high resolution sensors.
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Branched SnO2 nanowires on metallic nanowire backbones for ethanol sensors application Appl. Phys. Lett. 92, 102101 (2008); http://dx.doi.org/10.1063/1.2890735 (3 pages) Online Publication Date: 10 March 2008
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We report the synthesis of hierarchically branched semiconducting SnO2 nanowire on metallic Sb-doped SnO2 nanowires by the sequential seeding of multiple nanowire generations with Au nanoparticles as catalysts. Such semiconducting nanowire/metallic backbone complex structures increase the potential functionality of SnO2 nanowires. Branched SnO2 nanowire films are used as sensing materials for high-performance ethanol sensor fabrication. The nanowire sensors show sub-ppm sensitivity and fast response and recovery times at 300 °C. A linear equation relationship between sensitivity and the ethanol vapor concentration was observed.
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Appl. Phys. Lett. 92, 091112 (2008); http://dx.doi.org/10.1063/1.2890730 (3 pages) Online Publication Date: 4 March 2008
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The authors demonstrate the implementation of a white-light device by spin-coating CdSe/ZnS nanocrystals (NCs) on the top of a blue/green two-color InGaN/GaN quantum-well light-emitting diode for converting blue and green emissions into red light through the absorption/reemission process. Meanwhile, Au nanoparticles are mixed with CdSe/ZnS NCs for generating localized surface plasmon (LSP) modes to couple with the CdSe/ZnS NCs. The LSP modes can absorb green emission and effectively transfer the energy into the CdSe/ZnS NCs through the coupling process for enhancing red emission. With the LSP coupling process, the conversion efficiency from the blue/green range into red light can be increased by around 30%. The conversion quantum efficiency can reach 52.8%.
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Correlating reduced fill factor in polymer solar cells to contact effects Appl. Phys. Lett. 92, 093301 (2008); http://dx.doi.org/10.1063/1.2841062 (3 pages) Online Publication Date: 3 March 2008
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A probable limiting factor for efficiency and fill factors of organic solar cells originates from the cathode-polymer interface. We utilize various forms of cathode layer such as Al, Ca, oxidized Ca, and low melting point alloys in model systems to emphasize this aspect in our studies. The current-voltage (JV) response in the fourth quadrant indicates a general trend of convex shaped JV characteristics (d2J/dV2>0) for illuminated devices with good cathode-polymer interfaces and linear or concave JV responses (d2J/dV2<0) for inefficient cathode-polymer interfaces.
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Top-gated graphene field-effect-transistors formed by decomposition of SiC Appl. Phys. Lett. 92, 092102 (2008); http://dx.doi.org/10.1063/1.2889959 (3 pages) Online Publication Date: 3 March 2008
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Top-gated, few-layer graphene field-effect transistors (FETs) fabricated on thermally decomposed semi-insulating 4H-SiC substrates are demonstrated. Physical vapor deposited SiO2 is used as the gate dielectric. A two-dimensional hexagonal arrangement of carbon atoms with the correct lattice vectors, observed by high-resolution scanning tunneling microscopy, confirms the formation of multiple graphene layers on top of the SiC substrates. The observation of n-type and p-type transition further verifies Dirac Fermions’ unique transport properties in graphene layers. The measured electron and hole mobilities on these fabricated graphene FETs are as high as 5400 and 4400 cm2/V s, respectively, which are much larger than the corresponding values from conventional SiC or silicon.
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Appl. Phys. Lett. 92, 083304 (2008); http://dx.doi.org/10.1063/1.2888176 (3 pages) Online Publication Date: 27 February 2008
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The authors have achieved an efficient organic solar cell based on copper phthalocyanine (CuPc) layer as donor and fullerene (C60) as acceptor. The aluminum doped zinc oxide (ZnO:Al) instead of indium tin oxide (ITO) is used as the anode. An ultrathin gold film is introduced among ZnO:Al, transparent conductor oxide, and the CuPc donor layer. We show that the power conversion efficiency of this cell is enhanced by one order of magnitude compared to that achieved with a ZnO anode without ultrathin gold film. Therefore, the power conversion efficiency of this cell is comparable to that with an ITO anode.
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Dielectric passivation effects on ZnO light emitting diodes Appl. Phys. Lett. 92, 112101 (2008); http://dx.doi.org/10.1063/1.2898709 (3 pages) Online Publication Date: 17 March 2008
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Plasma-enhanced chemical vapor-deposited SiO2 and SiNx were used to passivate ZnO heterojunction light emitting diodes (LEDs). Postdielectric deposition annealing was critical in obtaining good LED electrical and optical characteristics. No diode characteristics or light emission was observed unless the structures were annealed at 350 °C after fabrication. Annealed diodes showed a band-edge electroluminescence (EL) (385 nm) and a broad defect band with a peak at 930 nm at room temperature. The SiO2 and SiNx had very different passivation effects in terms of the electrical and EL characteristics of the LEDs. After annealing, the SiO2 passivated ZnO LEDs showed diode I-V characteristics and emitted light. However, the annealed SiNx-passivated ZnO LEDs showed leaky diode characteristics and no light emission. We attribute these differences to the role of hydrogen on the LEDs.
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A write time of 6 ns for quantum dot–based memory structures Appl. Phys. Lett. 92, 092108 (2008); http://dx.doi.org/10.1063/1.2890731 (3 pages) Online Publication Date: 4 March 2008
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The concept of a memory device based on self-organized quantum dots (QDs) is presented, enabling extremely fast write times, limited only by the charge carrier relaxation time being in the picosecond range. For a first device structure with embedded InAs/GaAs QDs, a write time of 6 ns is demonstrated. A similar structure containing GaSb/GaAs QDs shows a write time of 14 ns. These write times are independent of the localization energy (e.g., storage time) of the charge carriers and at the moment are limited only by the experimental setup and the parasitic cutoff frequency of the RC low pass of the device.
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Characteristics of long wavelength InGaN quantum well laser diodes Appl. Phys. Lett. 92, 101103 (2008); http://dx.doi.org/10.1063/1.2892634 (3 pages) Online Publication Date: 10 March 2008
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We demonstrated the long wavelength (485 nm) lasing of InGaN laser diodes under continuous wave condition at room temperature over 10 mW. Two InGaN laser structures were adapted with different indium composition for InGaN optical confinement layers (OCLs) below quantum wells. The blue shift of electroluminescence (EL) was reduced in InGaN laser diodes grown on 3% In concentration in InGaN OCL compared with 1.5% In concentration in InGaN OCL. The EL peak for laser diode with 3% In concentration in InGaN OCL occurs at longer wavelength for all current levels compared to the laser with 1.5% In concentration in InGaN OCL. In addition, the laterally nonuniform InGaN wells grown on 1.5% In concentration in InGaN OCL was verified by the cross-sectional view of InGaN active layer using high-resolution transmission electron microscopy.
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Simple tandem organic photovoltaic cells for improved energy conversion efficiency Appl. Phys. Lett. 92, 083310 (2008); http://dx.doi.org/10.1063/1.2885721 (3 pages) Online Publication Date: 28 February 2008
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We proposed and demonstrated a simple tandem structure of organic photovoltaic (PV) cell for efficient light harvesting. In this device structure, a soluble fullerene derivative of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is employed simultaneously to form a bilayer heterojunction PV subcell with the underlying copper phthalocyanine (CuPc) and a bulk heterojunction PV subcell with blended poly(3-hexylthiophene-2,5-diyl) (P3HT). In comparison with the conventional tandem structure, the omission of the semitransparent intercellular connection layer reduces the complexity of the device and the light loss. The enhanced short circuit current density (JSC = 8.63 mA/cm2) and power conversion efficiency (PCE) (2.79%) of the tandem structure are nearly the sum of those of the stand-alone cells of CuPc/PCBM (JSC = 2.09 mA/cm2, PCE = 0.43%) and P3HT:PCBM (JSC = 6.87 mA/cm2, PCE = 2.50%).
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Electrically driven high-Q quantum dot-micropillar cavities Appl. Phys. Lett. 92, 091107 (2008); http://dx.doi.org/10.1063/1.2890166 (3 pages) Online Publication Date: 4 March 2008
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We report on electrically pumped high-Q quantum dot-micropillar cavities with quality factors of up to 16.000. A special current injection scheme using a ring-shaped upper contact is presented which ensures an efficient light out-coupling through the uncapped upper surface of the micropillar. The devices feature excellent single-quantum dot cavity quantum electrodynamic effects with a Purcell enhancement of about 10 for a micropillar with a diameter of 2.5 μm.
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The role of dislocations as nonradiative recombination centers in InGaN quantum wells Appl. Phys. Lett. 92, 091901 (2008); http://dx.doi.org/10.1063/1.2889444 (3 pages) Online Publication Date: 3 March 2008
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InGaN multiple quantum wells (MQWs) were grown on atomically smooth c-GaN templates and identical c-GaN templates etched to reveal hexagonal pits associated with screw dislocations. We found that the room temperature internal quantum efficiency of the MQWs grown on the etched c-GaN templates is a factor of 2 higher than that of the smooth QWs. This finding is accounted for by the fact that the QWs on the nonplanar surfaces are thinner than the c-plane QWs, and thus the carriers are prevented from reaching the dislocations due to the energy barrier around each defect.
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Appl. Phys. Lett. 92, 091105 (2008); http://dx.doi.org/10.1063/1.2890050 (3 pages) Online Publication Date: 4 March 2008
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Optical polarization characteristics of InGaN/GaN light-emitting diodes (LEDs) were studied. Light-emitting diode samples were fabricated on four types of GaN substrates near (10
 0) orientation with intentional off-axis cuts of 0°, 5°, 10°, and 27° towards [000]. A confocal microscope was used to characterize the optical polarization of electroluminescence at various currents. The highest polarization ratio of 0.91 was measured on samples fabricated on a 5° off-cut substrate. First moments were calculated on emission spectra to assess emission peak shifts of two polarization components. We drew a conclusion that substrate off-axis cut is a technique to improve optical polarization characteristics of nonpolar-oriented InGaN/GaN LEDs. |
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Surface plasmon coupled electroluminescent emission Appl. Phys. Lett. 92, 103304 (2008); http://dx.doi.org/10.1063/1.2884268 (3 pages) Online Publication Date: 10 March 2008
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Besides directly emitting light, electroluminescence in organic light emitting diodes (OLEDs) can excite surface plasmons (SPs) on the metal electrodes of the device. By designing a microcavity OLED with thin film metal electrodes, we can directly probe a SP mode by leaky wave extraction with a high refractive index glass prism coupler. Additional angle and polarization resolved reflection measurements on the OLED multilayer structure together with transfer matrix calculations allow us to unequivocally characterize the electroluminescent-driven SP mode.
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