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Top 20 Most Read Articles
October 2010
The 20 articles with the most full-text downloads during the month, in descending order.
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Appl. Phys. Lett. 97, 053110 (2010); http://dx.doi.org/10.1063/1.3467468 (3 pages) Online Publication Date: 4 August 2010
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In this letter, we show how high-resolution scanning tunneling microscopy (STM) imaging can be used to reveal that certain edges of micromechanically exfoliated single layer graphene crystals on silicon oxide follow either zigzag or armchair orientation. Using the cleavage technique, graphene flakes are obtained that very often show terminating edges seemingly following the crystallographic directions of the underlying honeycomb lattice. Performing atomic resolution STM-imaging on such flakes, we were able to directly prove this assumption. Raman imaging carried out on the same flakes further validated our findings.
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Appl. Phys. Lett. 91, 063124 (2007); http://dx.doi.org/10.1063/1.2768624 (3 pages) Online Publication Date: 10 August 2007
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Microfabrication of graphene devices used in many experimental studies currently relies on the fact that graphene crystallites can be visualized using optical microscopy if prepared on top of Si wafers with a certain thickness of SiO2. The authors study graphene’s visibility and show that it depends strongly on both thickness of SiO2 and light wavelength. They have found that by using monochromatic illumination, graphene can be isolated for any SiO2 thickness, albeit 300 nm (the current standard) and, especially, ≈ 100 nm are most suitable for its visual detection. By using a Fresnel-law-based model, they quantitatively describe the experimental data.
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Raman fingerprint of charged impurities in graphene Appl. Phys. Lett. 91, 233108 (2007); http://dx.doi.org/10.1063/1.2818692 (3 pages) Online Publication Date: 5 December 2007
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We report strong variations in the Raman spectra for different single-layer graphene samples obtained by micromechanical cleavage. This reveals the presence of excess charges, even in the absence of intentional doping. Doping concentrations up to ∼ 1013 cm−2 are estimated from the G peak shift and width and the variation of both position and relative intensity of the second order 2D peak. Asymmetric G peaks indicate charge inhomogeneity on a scale of less than 1 μm.
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Quantum resistance metrology in graphene Appl. Phys. Lett. 93, 222109 (2008); http://dx.doi.org/10.1063/1.3043426 (3 pages) Online Publication Date: 5 December 2008
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We performed a metrological characterization of the quantum Hall resistance in a 1 μm wide graphene Hall bar. The longitudinal resistivity in the center of the ν = ±2 quantum Hall plateaus vanishes within the measurement noise of 20 mΩ up to 2 μA. Our results show that the quantization of these plateaus is within the experimental uncertainty (15 ppm for 1.5 μA current) equal to that in conventional semiconductors. The principal limitation of the present experiments is the relatively high contact resistances in the quantum Hall regime, leading to a significantly increased noise across the voltage contacts and a heating of the sample when a high current is applied.
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Submicron sensors of local electric field with single-electron resolution at room temperature Appl. Phys. Lett. 88, 013901 (2006); http://dx.doi.org/10.1063/1.2159564 (3 pages) Online Publication Date: 3 January 2006
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We describe probes of a local electric field, which are capable of detecting an electric charge as small as the charge of one electron e, operational under ambient conditions and having a spatial resolution down to 100 nm. The submicron-sized probes were made from a high-density high-mobility two-dimensional electron gas, which is sensitive to the presence of electric charges near its surface. We demonstrate the possibility of using such microprobes for life-science applications by measuring an electric response of individual yeast cells to abrupt changes in their environment.
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Spin-polarized electron tunneling across magnetic dielectric Appl. Phys. Lett. 86, 212501 (2005); http://dx.doi.org/10.1063/1.1925785 (3 pages) Online Publication Date: 16 May 2005
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This letter deals with a magnetic tunnel junction having spin filtering by a magnetic barrier. We performed experiments in which a relatively strong external field rotates magnetizations of both ferromagnetic electrodes in the tunnel junction with the magnetic barrier simultaneously so that the two are always parallel to each other. The tunnel magnetoresistance induced in this way was over 16% at 300 K. The angular dependency of the tunnel current on the layer magnetizations indicates that the barrier contains antiferromagnetic oxide. To achieve the described effect the magnetic electrode of the junction was oxidized prior to forming the Al2O3 layer.
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Early development of cutaneous cancer revealed by intravital nonlinear optical microscopy Appl. Phys. Lett. 97, 113702 (2010); http://dx.doi.org/10.1063/1.3490644 (3 pages) Online Publication Date: 16 September 2010
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We performed intravital multiphoton microscopy to image and analyze normal and carcinogen treated skin tissues of nude mice in vivo. Using intravital images and the quantitative pixel to pixel ratiometric processing of multiphoton autofluorescence to second harmonic generation index (MAFSI), we can visualize the interaction between epithelial cells and extracellular matrix. We found that as the imaging depth increases, MAFSI has different distribution in normal and treated cutaneous specimens. Since the treated skin eventually became squamous cell carcinoma, our results show that the physiological changes to mouse skin en route to become cancer can be effectively tracked by multiphoton microscopy.
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Appl. Phys. Lett. 97, 153102 (2010); http://dx.doi.org/10.1063/1.3492845 (3 pages) Online Publication Date: 11 October 2010
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We demonstrate the application of graphene as a support for imaging individual biological molecules in transmission electron microscope (TEM). A simple procedure to produce free-standing graphene membranes has been designed. Such membranes are extremely robust and can support practically any submicrometer object. Tobacco mosaic virus has been deposited on graphene samples and observed in a TEM. High contrast has been achieved even though no staining has been applied.
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Carbon impurities and the yellow luminescence in GaN Appl. Phys. Lett. 97, 152108 (2010); http://dx.doi.org/10.1063/1.3492841 (3 pages) Online Publication Date: 13 October 2010
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Using hybrid functional calculations we investigate the effects of carbon on the electrical and optical properties of GaN. In contrast to the currently accepted view that C substituting for N (CN) is a shallow acceptor, we find that CN has an ionization energy of 0.90 eV. Our calculated absorption and emission lines also indicate that CN is a likely source for the yellow luminescence that is frequently observed in GaN, solving the longstanding puzzle of the nature of the C-related defect involved in yellow emission. Our results suggest that previous experimental data, analyzed under the assumption that CN acts as a shallow acceptor, should be re-examined.
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Appl. Phys. Lett. 84, 3459 (2004); http://dx.doi.org/10.1063/1.1737072 (3 pages) Online Publication Date: 20 April 2004
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Electroluminescent devices combining two families of PbS colloidal quantum dots to achieve spectrally tailored two-color emission are reported. Depending on device structure selected—the use of two separated layers versus a mixture of nanocrystals—the structures demonstrated light emission either in two infrared frequency peaks corresponding to the spectral region 1.1–1.6 μm or in a wide band spanning this same spectral region. Separated-layer devices exhibit wide tunability in the relative intensity of the two peaks by varying excitation conditions. Replacing oleate with octodecylamine ligands increases the internal electroluminescence efficiency to 3.1%. © 2004 American Institute of Physics. |
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Optical suppression of ionized impurity scattering in vertical hot‐electron transport Appl. Phys. Lett. 61, 3157 (1992); http://dx.doi.org/10.1063/1.107962 (3 pages)
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A striking effect of illumination on the vertical nonequilibrium electron transport has been observed in the GaAs‐based tunneling hot electron transfer amplifier (THETA). Weak illumination can considerably increase the transparency of a THETA structure for quasiballistic electrons if the photon energy exceeds the GaAs band gap. The temperature and illumination intensity dependencies indicate that the effect is caused by photoneutralization of ionized impurities which are a major source of hot electron scattering. |
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Zero‐dimensional states in macroscopic resonant tunneling devices Appl. Phys. Lett. 64, 2563 (1994); http://dx.doi.org/10.1063/1.111574 (3 pages)
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We demonstrate that it is possible to observe transport through individual tunneling channels due to zero‐dimensional states in large area resonant tunneling devices (RTD). These localized states are found to be related to the presence of donor impurities in the vicinity of the quantum well but their binding energies are larger than that due to a single isolated hydrogenic donor. The states give rise to additional peaks in current voltage below the threshold for the main resonant peak. These peaks are visible in RTD with essentially any lateral dimension provided the current is measured with sufficient sensitivity. |
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Ballistic Hall micromagnetometry Appl. Phys. Lett. 71, 2379 (1997); http://dx.doi.org/10.1063/1.120034 (3 pages)
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We report a magnetization measurement technique which allows quantitative studies of thermodynamic properties of individual submicron superconducting and ferromagnetic particles. © 1997 American Institute of Physics. |
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Appl. Phys. Lett. 97, 133303 (2010); http://dx.doi.org/10.1063/1.3491815 (3 pages) Online Publication Date: 27 September 2010
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Air-stable n-type organic thin-film transistor (TFT) arrays and a complementary inverter circuit were fabricated on a flexible substrate. A benzobis(thiadiazole) (BBT) derivative-based TFT showed excellent air- stability and performances such as an electron mobility of over 0.1 cm2/V s, a large ON/OFF ratio over 108 when combined with a cross-linkable olefin-type polymer gate dielectric. In addition, an organic complementary inverter that combined the BBT derivative and a pentacene TFT demonstrated a sharp switching behavior and a high gain of over 150. We attribute these excellent characteristics to a combination of the low-lying lowest unoccupied molecular orbital level of n-type semiconductor material and the low interface trap of the gate dielectric.
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Organic solar cells with solution-processed graphene transparent electrodes Appl. Phys. Lett. 92, 263302 (2008); http://dx.doi.org/10.1063/1.2924771 (3 pages) Online Publication Date: 1 July 2008
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We demonstrate that solution-processed graphene thin films can serve as transparent conductive anodes for organic photovoltaic cells. The graphene electrodes were deposited on quartz substrates by spin coating of an aqueous dispersion of functionalized graphene, followed by a reduction process to reduce the sheet resistance. Small molecular weight organic solar cells can be directly deposited on such graphene anodes. The short-circuit current and fill factor of these devices on graphene are lower than those of control device on indium tin oxide due to the higher sheet resistance of the graphene films. We anticipate that further optimization of the reduction conditions will improve the performance of these graphene anodes.
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Surface patterning on periodicity of femtosecond laser-induced ripples Appl. Phys. Lett. 97, 141101 (2010); http://dx.doi.org/10.1063/1.3495785 (3 pages) Online Publication Date: 4 October 2010
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Properties of femtosecond laser-induced ripples on patterned metal surfaces are investigated through using a crossed two-step line-scribing method. It is found that the ripple periodicity tends to decrease with larger surface roughness but increase with higher laser fluence. For the increased roughness, the change in ripple periodicity becomes more sensitive to the incident laser fluence. A cut-off surface roughness that prevents from altering the ripple periodicity is also revealed to increase with the lower laser fluence. These phenomena are discussed in the view of surface plasmon polaritons in the laser-induced plasma and the modified dielectric constant of the roughened surfaces.
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Concentration dependence of the transport energy level for charge carriers in organic semiconductors Appl. Phys. Lett. 97, 143302 (2010); http://dx.doi.org/10.1063/1.3496045 (3 pages) Online Publication Date: 4 October 2010
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The concept of the transport energy (TE) has proven to be one of the most powerful theoretical approaches to describe charge transport in organic semiconductors. In the recent paper
L. Li, G. Meller, and H. Kosina [Appl. Phys. Lett. 92, 013307 (2008)]
have studied the effect of the partially filled localized states on the position of the TE level. We show that the position of the TE is essentially different to the one suggested by
L. Li, G. Meller, and H. Kosina [Appl. Phys. Lett. 92, 013307 (2008)]
We further modify the standard TE approach taking into account the percolation nature of the transport path. Our calculations show that the TE becomes dependent on the concentration of charge carriers n at much higher n values than those, at which the carrier mobility already strongly depends on n. Hence the calculations of the concentration-dependent carrier mobility cannot be performed within the approach, in which only the concentration dependence of the TE is taken into account.
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Tailoring GaAs terahertz radiative properties with surface phonons polaritons Appl. Phys. Lett. 97, 161101 (2010); http://dx.doi.org/10.1063/1.3497645 (3 pages) Online Publication Date: 18 October 2010
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Absorption of visible and infrared radiation by gratings due to the resonant excitation of surface waves is a well-known phenomenon. In this paper, we study the resonant absorption of terahertz (THz) radiation due to the excitation of surface phonon polaritons on a grating ruled on a GaAs substrate. We report the design and fabrication of such a grating. Reflectivity measurements clearly show the excitation of surface phonon polariton. Numerical simulations indicate that the radiative properties depend critically on the surface profile. We finally discuss potential applications to THz thermal emission.
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Indium tin oxide-free and metal-free semitransparent organic solar cells Appl. Phys. Lett. 97, 153304 (2010); http://dx.doi.org/10.1063/1.3499299 (3 pages) Online Publication Date: 12 October 2010
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We report on indium tin oxide (ITO)-free and metal-free semitransparent organic solar cells with a high-conductivity poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (PH1000) as both the bottom and the top electrodes. The PH1000 film showed a conductivity of 680±50 S/cm. A ZnO layer was used as an interlayer to produce an electron-selective electrode. The semitransparent devices with a structure of glass/PH1000/ZnO/poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester/PEDOT:PSS (CPP 105 D)/PH1000 exhibited an average power conversion efficiency of 1.8% estimated for 100 mW/cm2 air mass 1.5 global illumination. This geometry alleviates the need of vacuum deposition of a top electrode.
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High reflectivity electrofluidic pixels with zero-power grayscale operation Appl. Phys. Lett. 97, 143501 (2010); http://dx.doi.org/10.1063/1.3494552 (3 pages) Online Publication Date: 4 October 2010
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Electrofluidic display pixels are demonstrated with zero-power grayscale operation for 3 months and with >70% reflectance. The color of the pixel is changed as electrowetting moves the pigment dispersion between a top and bottom channel. When voltage is removed, a near zero Laplace pressure and a hysteresis pressure of 0.11 kN/m2 stabilizes the position. For 450 μm pixels, an electromechanical pressure of 1.4 kN/m2 moves the pigment dispersion at a speed of ∼ 2650 μm/s. The predicted switching speed for ∼ 150 μm pixels is consistent with video rate operation (20 ms). The geometrically sophisticated pixel structure is fabricated with only simple photolithography and wet chemical processing.
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