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Top 20 Most Read Articles
February 2012
The 20 articles with the most full-text downloads during the month, in descending order.
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Drawing graphene nanoribbons on SiC by ion implantation Appl. Phys. Lett. 100, 073501 (2012); http://dx.doi.org/10.1063/1.3682479 (3 pages) Online Publication Date: 13 February 2012
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We describe a straightforward technique for selective graphene growth and nanoribbon production onto 4H- and 6H-SiC. The technique presented is as easy as ion implanting regions where graphene layers are desired followed by annealing to 100 °C below the graphitization temperature (TG) of SiC. We find that ion implantation of SiC lowers the TG, allowing selective graphene growth at temperatures below the TG of pristine SiC and above TG of implanted SiC. This results in an approach for patterning device structures ranging from a couple tens of nanometers to microns in size without using conventional lithography and chemical processing.
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Appl. Phys. Lett. 100, 043512 (2012); http://dx.doi.org/10.1063/1.3679132 (4 pages) Online Publication Date: 27 January 2012
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The effect of a GaAs1−xSbx strain reducing layer on the performance of InAs/GaAs quantum-dot infrared photodetectors was investigated. The results suggest that increasing Sb composition x from 0 to 0.2 leads to an enhanced peak response and a pronounced narrowing of the band width of the spectral response from 3.3 to 1.5 μm. For a photodetector with GaAs0.8Sb0.2 strain reducing layer, the best responsivity obtained is 533 mA/W, which is 380 times higher than that without strain reducing layer. In addition, the operating temperature increases from 50 to 90 K when increasing Sb composition from 0 to 0.2.
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Appl. Phys. Lett. 100, 041119 (2012); http://dx.doi.org/10.1063/1.3681162 (4 pages) Online Publication Date: 27 January 2012
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We report the effect of an electron blocking layer (EBL) on the piezoelectric field in InGaN/GaN multiple quantum well light-emitting diodes (LEDs). Electric-field-dependent ER measurements showed an enhanced piezoelectric field in LEDs with a p-AlGaN EBL compared with LEDs without EBL. In contrast, LEDs with a p-AlGaN EBL exhibited reduced blueshift and a sublinear increase of full width at half maximum in EL spectra at low current densities. These behaviors can be explained by the strong localization of injected carriers in dominant InGaN regions due to an increase of the piezoelectric field by subsequent growing EBL.
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Position dependent photodetector from large area reduced graphene oxide thin films Appl. Phys. Lett. 96, 163109 (2010); http://dx.doi.org/10.1063/1.3415499 (3 pages) Online Publication Date: 22 April 2010
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We fabricated large area infrared photodetector devices from thin film of chemically reduced graphene oxide (RGO) sheets and studied their photoresponse as a function of laser position. We found that the photocurrent either increases, decreases, or remain almost zero depending upon the position of the laser spot with respect to the electrodes. The position sensitive photoresponse is explained by Schottky barrier modulation at the RGO film-electrode interface. The time response of the photocurrent is dramatically slower than single sheet of graphene possibly due to disorder from the chemical synthesis and interconnecting sheets.
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Appl. Phys. Lett. 100, 053504 (2012); http://dx.doi.org/10.1063/1.3681797 (3 pages) Online Publication Date: 1 February 2012
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We report on the effect of a graded AlGaN electron blocking layer (GEBL) on the emission properties of InGaN/GaN multiple quantum wells light-emitting diode (LED). The adoption of GEBL in the LED enhances the electroluminescence intensity and reduces the wavelength blue-shift with increasing injection current. The light output power of the GEBL LED is enhanced by 163% and 415% at 20 and 350 mA, respectively. Moreover, the forward voltage of the GEBL LED is reduced by 0.38 V at the forward current of 20 mA.
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Powering pacemakers from heartbeat vibrations using linear and nonlinear energy harvesters Appl. Phys. Lett. 100, 042901 (2012); http://dx.doi.org/10.1063/1.3679102 (4 pages) Online Publication Date: 23 January 2012
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Linear and nonlinear piezoelectric devices are introduced to continuously recharge the batteries of the pacemakers by converting the vibrations from the heartbeats to electrical energy. The power requirement of a pacemaker is very low. However, after few years, patients require another surgical operation just to replace their pacemaker battery. Linear low frequency and nonlinear mono-stable and bi-stable energy harvesters are designed according to the especial signature of heart vibrations. The proposed energy harvesters are robust to variation of heart rate and can meet the power requirement of pacemakers.
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Giant electro-mechanical energy conversion in [011] cut ferroelectric single crystals Appl. Phys. Lett. 100, 042903 (2012); http://dx.doi.org/10.1063/1.3679644 (3 pages) Online Publication Date: 26 January 2012
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Giant electro-mechanical energy conversion is demonstrated under a ferroelectric/ferroelectric phase transformation in [011] cut and poled lead titanate-based relaxor perovskite morphotropic single crystals. It is found that under mechanical pre-stress, a relatively small oscillatory stress drives the material reversibly between rhombohedral and orthorhombic phases with a remarkably high polarization and strain jump induced at zero bias electric field and room temperature. The measured electrical output per cycle is more than an order of magnitude larger than that reported for linear piezoelectric materials. Ideal thermodynamic cycles are presented for this electro-mechanical energy conversion followed by a presentation and discussion of the experimental data.
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Appl. Phys. Lett. 100, 053901 (2012); http://dx.doi.org/10.1063/1.3681397 (3 pages) Online Publication Date: 2 February 2012
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In this letter, we demonstrate the effectiveness of the controlled spalling technology for producing high-efficiency (28.7%) thin-film InGaP/(In)GaAs/Ge tandem solar cells. The controlled spalling technique was employed to separate the as-grown solar cell structure from the host Ge wafer followed by its transfer to an arbitrary Si support substrate. The structural and electrical properties of the thin-film tandem cells were examined and compared against those on the original bulk Ge substrate. The comparison of the electrical data suggests the equivalency in cell parameters for both the thin-film (spalled) and bulk (non-spalled) cells, confirming that the controlled spalling technology does maintain the integrity of all layers in such an elaborate solar cell structure.
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Dye-sensitized solar cell with a titanium-oxide-modified carbon nanotube transparent electrode Appl. Phys. Lett. 99, 021107 (2011); http://dx.doi.org/10.1063/1.3610488 (3 pages) Online Publication Date: 15 July 2011
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Transparent and conductive carbon-based materials are promising for window electrodes in solid-state optoelectronic devices. However, the catalytic activity to redox reaction limits their application as a working electrode in a liquid-type dye-sensitized solar cell (DSSC). In this letter, we propose and demonstrate a transparent carbon nanotubes (CNTs) film as the working electrode in a DSSC containing iodide/triiodide redox couples. This implementation is realized by inhibiting the charge-transfer kinetics at CNT/redox solution interface with an aid of thin titanium oxide film that facilitates the unidirectional flow of electrons in the cell without sacrificing the electrical and optical properties of CNT.
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Elastomeric silicone substrates for terahertz fishnet metamaterials Appl. Phys. Lett. 100, 061101 (2012); http://dx.doi.org/10.1063/1.3665180 (3 pages) Online Publication Date: 6 February 2012
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In this work, we characterize the electromagnetic properties of polydimethylsiloxane (PDMS) and use this as a free-standing substrate for the realization of flexible fishnet metamaterials at terahertz frequencies. Across the 0.2–2.5 THz band, the refractive index and absorption coefficient of PDMS are estimated as 1.55 and 0–22 cm−1, respectively. Electromagnetic modeling, multi-layer flexible electronics microfabrication, and terahertz time-domain spectroscopy are used in the design, fabrication, and characterization of the metamaterials, respectively. The properties of PDMS add a degree of freedom to terahertz metamaterials, with the potential for tuning by elastic deformation or integrated microfluidics.
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Photoinduced write-once read-many-times memory device based on DNA biopolymer nanocomposite Appl. Phys. Lett. 99, 253301 (2011); http://dx.doi.org/10.1063/1.3671153 (3 pages) Online Publication Date: 19 December 2011
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We demonstrate a photoinduced write-once read-many-times (WORM) organic memory device based on DNA biopolymer nanocomposite. The device consists of a single biopolymer layer sandwiched between electrodes, in which electrical bistability is activated by in situ formation of silver nanoparticles embedded in biopolymer upon light irradiation. The device exhibits a switching effect to high conductivity above a threshold of 2.6 V and a good retention property. This facile technique, taking advantage of DNA’s affinity for metals and solution processing, can optically manipulate the properties of DNA nanocomposite thin films, which holds promise for optical storage and plasmonic applications.
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Appl. Phys. Lett. 100, 061107 (2012); http://dx.doi.org/10.1063/1.3683484 (5 pages) Online Publication Date: 7 February 2012
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One-dimensional and two-dimensional hybrid structures, composed of vertical ZnO nanorods grown on large-area graphene, are successfully integrated onto the GaN/InGaN light emitting diodes (LEDs). Compared with GaN LED without transparent conducting electrode, current injection and light emission increased almost 2–3 times, respectively, by the introduction of graphene based conducting electrode. Additional ∼66% increase in light emission was achieved by growing the ZnO nanorods on the graphene, which is consistent with the finite difference time domain modeling result. Furthermore, electroluminescence intensity profiles confirm the uniform light emission with high brightness in GaN LED with the ZnO nanorods-graphene hybrid electrode.
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Understanding metal doping for organic electron transport layers Appl. Phys. Lett. 100, 053305 (2012); http://dx.doi.org/10.1063/1.3681383 (3 pages) Online Publication Date: 1 February 2012
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This work concerns the physical mechanisms of metal n-doping in charge transport layers for optoelectronic devices, for which the doping level is constrained by transparency requirements so as to avoid parasitic absorption. Comparing various metal dopants, we claim that enhanced conductivity at low doping is initiated by the electrical doping effect, namely, metal-semiconductor charge donation. Electrical measurements show that doping effects at low concentration strongly depend on the work function of the introduced metal, and not every metal works as an efficient dopant. Practical applicability is demonstrated by introducing doped transport layers in prototypical bilayer solar cells in conventional and inverted architectures.
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Advantages of blue InGaN light-emitting diodes with InGaN-AlGaN-InGaN barriers Appl. Phys. Lett. 100, 031112 (2012); http://dx.doi.org/10.1063/1.3678341 (3 pages) Online Publication Date: 20 January 2012
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Efficiency enhancement of the blue InGaN light-emitting diodes (LEDs) with InGaN-AlGaN-InGaN barriers is studied numerically. The energy band diagrams, carrier concentrations in quantum wells, radiative recombination rate in active region, light-current performance curves, and internal quantum efficiency are investigated. The simulation results suggest that the blue InGaN/InGaN-AlGaN-InGaN LED has better performance over its conventional InGaN/GaN and InGaN/InGaN counterparts due to the appropriately modified energy band diagrams, which are caused mainly by the reduced polarization charges at the interface between the well and barrier.
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Room-temperature single molecular memory Appl. Phys. Lett. 100, 053101 (2012); http://dx.doi.org/10.1063/1.3679127 (3 pages) Online Publication Date: 30 January 2012
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Single molecular memory operation was observed on a porphyrin derivative by scanning tunneling microscopy at room temperature. A porphyrin derivative with four disulfide groups was chemically synthesized and chemisorbed on a Au(111) surface. Coulomb blockade behaviors and switching behaviors in current-voltage (I-V) characteristics were observed on a single porphyrin derivative by scanning tunneling spectroscopy. Based on the switching behaviors, the memory operation of electrical conductance in the porphyrin derivative was demonstrated by applying a programmed pulse sequence with an on/off ratio of 2.9 at room temperature.
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Terahertz composite right-left handed transmission-line metamaterial waveguides Appl. Phys. Lett. 100, 071101 (2012); http://dx.doi.org/10.1063/1.3684250 (4 pages) Online Publication Date: 13 February 2012
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We report terahertz metamaterial waveguides based on the concept of composite right/left-handed transmission-lines. The waveguides are implemented in a metal-insulator-metal geometry fabricated with spin-coated Benzocyclobutene and contact photolithography. Angle-resolved reflection spectroscopy shows strong resonant absorption features corresponding to both right-handed and left-handed (backward wave) propagating modes within the leaky-wave bandwidth. Tuning of the waveguide dispersion is achieved by varying the effective lumped element series capacitance. The experimental results are in good agreement with full-wave finite element method simulations as well as an intuitive transmission-line circuit model.
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Organic electroluminescent diodes Appl. Phys. Lett. 51, 913 (1987); http://dx.doi.org/10.1063/1.98799 (3 pages)
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A novel electroluminescent device is constructed using organic materials as the emitting elements. The diode has a double‐layer structure of organic thin films, prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium‐tin‐oxide anode and an alloyed Mg:Ag cathode. Electron‐hole recombination and green electroluminescent emission are confined near the organic interface region. High external quantum efficiency (1% photon/electron), luminous efficiency (1.5 lm/W), and brightness (>1000 cd/m2) are achievable at a driving voltage below 10 V. |
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Metal contact to graphene nanoribbon Appl. Phys. Lett. 100, 063108 (2012); http://dx.doi.org/10.1063/1.3682319 (4 pages) Online Publication Date: 7 February 2012
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Metal to graphene nanoribbon (GNR) contact is explored by self‐consistent numerical simulations. The Schottky barrier height of the GNR contact can be modulated by either the gate voltage or the workfunction of the metal contact. An Ohmic contact with multiple‐subband injection can be achieved with increase of the GNR width. The contact resistance normalized by the GNR width is smaller than that of the two‐dimensional graphene, and the effect of the quasi‐one‐dimensional subbands for GNRs is manifested when the GNR is narrower than about 5 nm.
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Mapping the electronic properties of individual graphene grain boundaries Appl. Phys. Lett. 100, 053114 (2012); http://dx.doi.org/10.1063/1.3681375 (4 pages) Online Publication Date: 1 February 2012
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Grain boundaries, the characteristic topological defects of chemical vapor deposition grown graphene samples, are expected to substantially alter the electronic properties of the unperturbed graphene lattice. However, there is very little experimental insight into the underlying mechanisms. Here, we systematically map the electronic properties of individual graphene grain boundaries by scanning tunneling microscopy and spatially resolved tunneling spectroscopy measurements. The tunneling spectroscopy data reveal that the conductivity inside the boundaries is markedly suppressed for both electron and hole-type charge carriers. Furthermore, graphene grain boundaries can give rise to n-type inversion channels within the p-doped graphene sheets, forming p-n junctions with sharp interfaces on the nanometer scale. These properties persist for grain boundaries of various configurations and are robust against structural disorder.
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Graphene induced tunability of the surface plasmon resonance Appl. Phys. Lett. 100, 061116 (2012); http://dx.doi.org/10.1063/1.3683534 (4 pages) Online Publication Date: 9 February 2012
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Tunability of the surface plasmon resonance wavelength is demonstrated by varying the thickness of Al2O3 spacer layer inserted between the graphene and nanoparticles. By varying the spacer layer thickness from 0.3 to 1.8 nm, the resonance wavelength is shifted from 583 to 566 nm. The shift is due to a change in the electromagnetic field coupling strength between the localized surface plasmons excited in the gold nanoparticles and a single layer graphene film. In contrast, when the graphene film is absent from the system, no noticeable shift in the resonance wavelength is observed upon varying the spacer thickness.
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