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Top 20 Most Read Articles
January 2009
The 20 articles with the most full-text downloads during the month, in descending order.
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Three dimensional silicon photonic crystals fabricated by two photon phase mask lithography Appl. Phys. Lett. 94, 011101 (2009); http://dx.doi.org/10.1063/1.3036955 (3 pages) Online Publication Date: 5 January 2009
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We describe the fabrication of silicon three dimensional photonic crystals using polymer templates defined by a single step, two-photon exposure through a layer of photopolymer with relief molded on its surface. The resulting crystals exhibit high structural quality over large areas, displaying geometries consistent with calculation. Spectroscopic measurements of transmission and reflection through the silicon and polymer structures reveal excellent optical properties, approaching properties predicted by simulations that assume ideal layouts.
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Appl. Phys. Lett. 94, 013303 (2009); http://dx.doi.org/10.1063/1.3059552 (3 pages) Online Publication Date: 7 January 2009
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We present zinc phthalocyanine (ZnPc):C60 bulk-heterojunction top-illuminated organic solar cells using ultrathin metal layers as transparent top contacts. We show that solar cell performance sensitively depends on the interface and morphology of the cathode, which can be influenced by varying the composition and layer structure of the metal contact. We investigate various metal combinations, such as 3 nm Al/8 nm Ag and 7 nm Al/14 nm Ag, to illustrate the necessity to find a suitable combination of morphology and electrical and optical properties. Solar cells using standard materials and a 1 nm Al/14 nm Ag cathode exhibit promising efficiencies of over 2.2%.
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Electronic structure and band-gap modulation of graphene via substrate surface chemistry Appl. Phys. Lett. 94, 032101 (2009); http://dx.doi.org/10.1063/1.3070238 (3 pages) Online Publication Date: 20 January 2009
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We have studied the electronic structure of graphene deposited on a SiO2 surface using density functional methods. The band structure of the graphene monolayer strongly depends on surface characteristics of the underlying SiO2 surface; for an oxygen-terminated surface, the monolayer exhibits a finite energy band gap while the band gap is closed when the oxygen atoms on the substrate are passivated with hydrogen atoms. We find that at least a graphene bilayer is required for a near zero energy gap when deposited on a substrate without H-passivation. Our results are discussed in the light of recent experiments.
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High efficiency blue phosphorescent organic light emitting diodes using a simple device structure Appl. Phys. Lett. 94, 013301 (2009); http://dx.doi.org/10.1063/1.3046288 (3 pages) Online Publication Date: 5 January 2009
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High efficiency blue phosphorescent organic light emitting diodes have been developed by using a simple device structure. A derivative of spirobifluorene based phosphine oxide was used both as a host and an electron transport layer with an exciton blocking function. A maximum quantum efficiency of 19.2% and a current efficiency of 37.2 cd/A were obtained by using a simple device structure without a hole blocking layer.
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Appl. Phys. Lett. 94, 022101 (2009); http://dx.doi.org/10.1063/1.3065067 (3 pages) Online Publication Date: 12 January 2009
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A single-walled carbon nanotube field-effect transistor has been fabricated with two single-walled carbon nanotube bundles as its top gates and a heavily doped p-type silicon substrate as its global back gate. The channel conductance is found to oscillate significantly as a function of the top- and back-gate voltages when the device is measured at 100 K or below. “Diamond”-shaped current forbidden regions can be clearly observed under both positive and negative top-gate voltages. A single-electron transistor model is proposed to qualitatively explain the observations.
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Real-time imaging of acoustic waves on a bulk acoustic resonator Appl. Phys. Lett. 93, 261101 (2008); http://dx.doi.org/10.1063/1.3053074 (3 pages) Online Publication Date: 29 December 2008
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Time resolved images of acoustic waves in the 100 MHz–2.2 GHz range are obtained for an electrically excited thin-film bulk acoustic wave resonator by means of an ultrafast optical technique. Electrical pulses, synchronized to ultrashort laser pulses, piezoelectrically excite the device, and synchronous near-infrared laser pulses interferometrically detect surface motion. The frequency dispersion is extracted using spatiotemporal Fourier transforms, revealing both longitudinal and surface acoustic modes.
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Open circuit voltage enhancement due to reduced dark current in small molecule photovoltaic cells Appl. Phys. Lett. 94, 023307 (2009); http://dx.doi.org/10.1063/1.3072807 (3 pages) Online Publication Date: 16 January 2009
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We demonstrate high open circuit voltage photovoltaic cells achieved by reducing the electron leakage current through the introduction of both organic and inorganic electron blocking layers between the donor layer and the anode contact. As an example, the blocking layers reduce the dark current in tin (II) phthalocyanine (SnPc)/C60 solar cells with response across the visible and near infrared spectral region up to a wavelength of 1000 nm, is decreased by two orders of magnitude compared to cells lacking the layers, resulting in a doubling of the open circuit voltage. The structure: indium tin oxide/electron blocker/SnPc (100 Å)/C60 (400 Å)/bathocuproine (100 Å)/Al, has a power conversion efficiency of (2.1±0.1)% at 1 sun, standard AM1.5G solar illumination. This work demonstrates the importance of reducing dark current to achieve high organic thin film photovoltaic cell efficiencies.
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Thermoelectric power measurements of wide band gap semiconducting nanowires Appl. Phys. Lett. 94, 022106 (2009); http://dx.doi.org/10.1063/1.3067868 (3 pages) Online Publication Date: 13 January 2009
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We investigated the temperature-dependent thermoelectric power (TEP) of individual wide band gap ZnO and GaN semiconducting nanowires by fabricating the devices with good Ohmic contacts. In the temperature range of 10–300 K, the measured TEP of both nanowires was linearly dependent on temperature, indicating the degenerate doping nature of these nanowires. The room temperature TEP value of ZnO nanowires was as high as −400 μV/K while an order of magnitude smaller TEP value was observed in GaN. The negative sign of TEP values shows that electrons are the majority carriers in these wide band gap nanowires. More importantly, in comparison with gate-dependent transport measurements of the nanowire field effect transistors, analysis of temperature-dependent TEP measurements provides a reliable way of estimating the majority carrier concentration of nanowires, where conventional Hall effect measurements cannot be used.
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Modification of graphene properties due to electron-beam irradiation Appl. Phys. Lett. 94, 013101 (2009); http://dx.doi.org/10.1063/1.3062851 (3 pages) Online Publication Date: 6 January 2009
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The authors report micro-Raman investigation of changes in the single and bilayer graphene crystal lattice induced by the low and medium energy electron-beam irradiation (5–20 keV). It was found that the radiation exposures result in the appearance of the strong disorder D band around 1345 cm−1, indicating damage to the lattice. The D and G peak evolution with increasing radiation dose follows the amorphization trajectory, which suggests graphene’s transformation to the nanocrystalline and then to amorphous form. The results have important implications for graphene characterization and device fabrication, which rely on the electron microscopy and focused ion beam processing.
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Appl. Phys. Lett. 94, 023101 (2009); http://dx.doi.org/10.1063/1.3067837 (3 pages) Online Publication Date: 12 January 2009
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A noninvasive fabrication process involving soft nanoimprint lithography is used to pattern a photonic crystal (PhC) in titania film for enhanced light extraction from a GaN light emitting diode (LED). This technique avoids damaging the LED structure by the etching process, while photoluminescence measurements show extracted modes emitted from the quantum wells which agree well with modeling. A light extraction improvement of 1.8 times is measured using this noninvasive PhC.
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Gas sensing properties of defect-controlled ZnO-nanowire gas sensor Appl. Phys. Lett. 93, 263103 (2008); http://dx.doi.org/10.1063/1.3046726 (3 pages) Online Publication Date: 29 December 2008
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The effect of oxygen-vacancy-related defects on gas-sensing properties of ZnO-nanowire gas sensors was investigated. Gas sensors were fabricated by growing ZnO nanowires bridging the gap between two prepatterned Au catalysts. The sensor displayed fast response and recovery behavior with a maximum sensitivity to NO2 gas at 225 °C. Gas sensitivity was found to be linearly proportional to the photoluminescence intensity of oxygen-vacancy-related defects in both as-fabricated and defect-controlled gas sensors by postannealing in Ar and H2 atmosphere. This result agrees well with previous theoretical prediction that oxygen vacancies play a role of preferential adsorption sites for NO2 molecules.
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Luminescent properties in the strain adjusted phosphor-free GaN based white light-emitting diode Appl. Phys. Lett. 93, 261117 (2008); http://dx.doi.org/10.1063/1.3063044 (3 pages) Online Publication Date: 31 December 2008
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A kind of phosphor-free GaN based white light-emitting diode was fabricated with a strain adjusting InGaN interlayer. The origin of the strain adjusted white luminescent properties was studied with cathodoluminescence, asymmetrically reciprocal space mapping with high resolution x-ray diffraction, and scanning electron microscopy. The yellow and blue components of the electroluminescence spectrum were attributed to the high indium core and the adjacent indium depleted region in the inverted pyramidal pits on the device surface, respectively. These pits existed at the end of the dislocations induced by the strain relaxation process of the InGaN interlayer.
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Observation of excited states in a graphene quantum dot Appl. Phys. Lett. 94, 012107 (2009); http://dx.doi.org/10.1063/1.3064128 (3 pages) Online Publication Date: 7 January 2009
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We demonstrate that excited states in single-layer graphene quantum dots can be detected via direct transport experiments. Coulomb diamond measurements show distinct features of sequential tunneling through an excited state. Moreover, the onset of inelastic cotunneling in the diamond region could be detected. For low magnetic fields, the positions of the single-particle energy levels fluctuate on the scale of a flux quantum penetrating the dot area. For higher magnetic fields, the transition to the formation of Landau levels is observed. Estimates based on the linear energy-momentum relation of graphene give carrier numbers of the order of 10 for our device.
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High-quality InGaN/GaN heterojunctions and their photovoltaic effects Appl. Phys. Lett. 93, 261108 (2008); http://dx.doi.org/10.1063/1.3056628 (3 pages) Online Publication Date: 29 December 2008
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High-quality p-GaN/i-In0.1Ga0.9N/n-GaN heterojunctional epilayers are grown on (0001)-oriented sapphire substrates by metal organic chemical vapor deposition. The Pendellösung fringes around the InGaN peak in high-resolution x-ray diffraction (HRXRD) confirm a sharp interface between InGaN and GaN films. The corresponding HRXRD and photoluminescence measurements demonstrate that there is no observable phase separation. The improvement in crystal quality yields high-performance photovoltaic cells with open-circuit voltage of around 2.1 eV and fill factor up to 81% under standard AM 1.5 condition. The dark current-voltage measurements show very large shunt resistance, implying an insignificant leakage current in the devices and therefore achieving the high fill factor in the illuminated case.
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Reducing the critical current for spin-transfer switching of perpendicularly magnetized nanomagnets Appl. Phys. Lett. 94, 012502 (2009); http://dx.doi.org/10.1063/1.3058680 (3 pages) Online Publication Date: 6 January 2009
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We describe nanopillar spin valves with perpendicular anisotropy designed to reduce the critical current needed for spin transfer magnetization reversal while maintaining thermal stability. By adjusting the perpendicular anisotropy and volume of the free element consisting of a [Co/Ni] multilayer, we observe that the critical current scales with the height of the anisotropy energy barrier and we achieve critical currents as low as 120 μA in quasistatic room-temperature measurements of a 45 nm diameter device. The field-current phase diagram of such a device is presented.
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Appl. Phys. Lett. 94, 011113 (2009); http://dx.doi.org/10.1063/1.3058687 (3 pages) Online Publication Date: 9 January 2009
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Blue light-emitting diodes (LEDs) with polarization-matched GaInN/GaInN multi-quantum-well (MQW) active regions are grown by metal-organic vapor-phase epitaxy. The GaInN/GaInN MQW structure reduces the magnitude of polarization sheet charges at heterointerfaces in the active region. The GaInN/GaInN MQW LEDs are shown to have enhanced light-output power, reduced efficiency droop, a lower forward voltage, a smaller diode ideality factor, and decreased wavelength shift, compared with conventional GaInN/GaN MQW LEDs.
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Appl. Phys. Lett. 94, 023302 (2009); http://dx.doi.org/10.1063/1.3070574 (3 pages) Online Publication Date: 13 January 2009
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This paper reports a study of a high band gap conjugated polymer poly(9,9-dihexylfluorene-alt-bithiophene) (F6T2) in an organic photovoltaic device. F6T2 blended with [6,6]-phenyl C61 butyric acid methyl ester (1:4 weight ratio) produced films exhibiting phase segregated domains ∼ 5 nm in size, and possessing a hole mobility of 3×10−8 m2/V s. Open circuit voltages of photovoltaic devices were greater than 1 V, and power conversion efficiencies of 2.7% based on AM 1.5 G illumination were measured. External quantum efficiencies of up to 48 %, and internal quantum efficiencies of up to 87 % were calculated.
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Ferromagnetism in Ni-doped ZnO films: Extrinsic or intrinsic? Appl. Phys. Lett. 94, 012510 (2009); http://dx.doi.org/10.1063/1.3067998 (3 pages) Online Publication Date: 9 January 2009
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Here we report a detailed study aimed on understanding the origin of ferromagnetism in Ni-doped ZnO films. A pulsed laser deposition technique was used to deposit Ni-doped (5 at. %) ZnO films on sapphire (0001) substrates under different oxygen pressures ranging from 10−6 to 0.1 Torr. Films were characterized using numerous characterization techniques including x-ray diffraction, x-ray photospectroscopy, energy dispersive x-ray spectroscopy, optical absorption spectroscopy, and electrical transport, magnetotransport, and magnetization measurements. A detailed structure-property correlation and analysis of our results revealed that the ferromagnetism in ZnO:Ni films is not an inherent property of the material but results due to a strong tendency of Ni to precipitate out in the ZnO matrix.
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Optical constants of graphene layers in the visible range Appl. Phys. Lett. 94, 031901 (2009); http://dx.doi.org/10.1063/1.3073717 (3 pages) Online Publication Date: 21 January 2009
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We show that the optical constants of graphene in the visible range can be estimated by means of a very simple procedure involving their consistence with universal optical conductivity and experimentally measured optical spectra, within the framework of Fresnel coefficients calculation. The obtained complex refractive index allows for accurate prediction of the optical behavior of graphene in the visible range, from the two-dimensional limit (single atomically thick graphene layer) to the bulk limit (graphite). Therefore, it may result very useful for quantitative optical analysis of graphene layers and graphitic structures in general.
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Appl. Phys. Lett. 94, 013102 (2009); http://dx.doi.org/10.1063/1.3062938 (3 pages) Online Publication Date: 6 January 2009
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This letter presents studies of multiwavelength flexible photodetectors on a plastic substrate by use of printing transferred single-crystal germanium (Ge) membranes. Ge membranes of 250 nm thickness with selectively ion-implantation doped regions were released from a germanium-on-insulator substrate and integrated with a 175-μm-thick polyethylene terephthalate substrate via a dry printing technique. Photodiodes configured in lateral p-i-n configuration using the flexible Ge membranes with an intrinsic region width of 10 μm exhibit an external quantum efficiency that varies from 5% at 411 nm to 42% at 633 nm under −1 V bias condition. These results demonstrate the potential of utilizing single-crystal Ge-membrane photodiodes for imaging applications and as solar cells on objects with arbitrary curvatures and shapes.
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