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Top 20 Most Read Articles
December 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Two-dimensional electron gas related emissions in ZnMgO/ZnO heterostructures Appl. Phys. Lett. 99, 211906 (2011); http://dx.doi.org/10.1063/1.3662964 (3 pages) Online Publication Date: 22 November 2011
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Radiative recombination of two-dimensional electron gas (2DEG), induced by polarization and validated by Hall effect measurements, is investigated in ZnMgO/ZnO heterostructures grown by metal-organic chemical vapor deposition. The Mg composition, the depth profile distribution of Mg, the residual strain in ZnMgO caplayer, and the thickness of caplayer all significantly influence the 2DEG-related transitions in ZnMgO/ZnO heterostructures. Below or above ZnO donor bound exciton, three additional broad emissions persisting up to 100 K are assigned to the spatially indirect transitions from 2DEG electrons to the photoexcited holes towards the ZnO flat-band region or remaining at the heterointerface.
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Spin injection from two-dimensional electron and hole gases in resonant tunneling diodes Appl. Phys. Lett. 99, 233507 (2011); http://dx.doi.org/10.1063/1.3668087 (4 pages) Online Publication Date: 8 December 2011
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We have investigated the polarized-resolved photoluminescence from the contact layers and the quantum-well in an n-type GaAs/GaAlAs resonant tunneling diode for magnetic fields up to 19 T. The optical emission from the GaAs contact layers comprises the recombination from highly spin-polarized two-dimensional electron and hole gases with free tunneling carriers. Both the energy position and intensity of this indirect recombination are voltage-dependent and show remarkably abrupt variations near scattering-assisted tunneling resonances. Our results show that these two dimensional gases act as spin-polarized sources for carriers tunneling through the well in resonant tunneling diodes.
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Photo-induced spin filtering in a double quantum dot Appl. Phys. Lett. 99, 192101 (2011); http://dx.doi.org/10.1063/1.3660227 (3 pages) Online Publication Date: 7 November 2011
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We investigate the spin-dependent electron dynamics in a double quantum dot driven by sub-picosecond asymmetric electromagnetic pulses. We show analytically that applying the appropriate pulses, specified here, allows a spin separation on a femtosecond time scale in the sense that states with a desired spin projection are localized mainly on one of the dots. It is shown how to maintain in time this photo-induced spin-dependent filtering.
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Flexible distributed-feedback colloidal quantum dot laser Appl. Phys. Lett. 99, 241103 (2011); http://dx.doi.org/10.1063/1.3659305 (3 pages) Online Publication Date: 12 December 2011
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By fabricating a submicron-scale grating structure on a bendable polymer substrate, we demonstrate a flexible distributed-feedback colloidal quantum dot laser. This laser uses cadmium selenide/zinc sulfide core-shell nanostructures, operating in transverse electric polarized multiple-modes, and has a typical threshold pump fluence of ∼4 mJ/cm2.
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Sensory and short-term memory formations observed in a Ag2S gap-type atomic switch Appl. Phys. Lett. 99, 203108 (2011); http://dx.doi.org/10.1063/1.3662390 (3 pages) Online Publication Date: 16 November 2011
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Memorization caused by the change in conductance in a Ag2S gap-type atomic switch was investigated as a function of the amplitude and width of input voltage pulses (Vin). The conductance changed little for the first few Vin, but the information of the input was stored as a redistribution of Ag-ions in the Ag2S, indicating the formation of sensory memory. After a certain number of Vin, the conductance increased abruptly followed by a gradual decrease, indicating the formation of short-term memory (STM). We found that the probability of STM formation depends strongly on the amplitude and width of Vin, which resembles the learning behavior of the human brain.
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Resonant-tunnelling-diode oscillators operating at frequencies above 1.1 THz Appl. Phys. Lett. 99, 233506 (2011); http://dx.doi.org/10.1063/1.3667191 (3 pages) Online Publication Date: 8 December 2011
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We present resonant-tunnelling-diode (RTD) oscillators operating at the fundamental frequency of 1111 GHz. We show that our RTDs and RTD oscillators have much room for further improvement of their parameters and for further increase of their operating frequencies. The operating frequencies of several THz should be achievable with RTD oscillators. Our study also shows that operation of RTDs beyond the relaxation-time limit at THz frequencies should be possible. RTD oscillators under study are extremely compact (less than a square millimeter) room-temperature sources of coherent cw THz radiation. Such sources should enable plenty of real-world THz applications.
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Appl. Phys. Lett. 99, 232104 (2011); http://dx.doi.org/10.1063/1.3665405 (3 pages) Online Publication Date: 6 December 2011
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We present electronic transport measurements of single and bilayer graphene on commercially available hexagonal boron nitride. We extract mobilities as high as 125 000 cm2 V−1 s−1 at room temperature and 275 000 cm2 V−1 s−1 at 4.2 K. The excellent quality is supported by the early development of the ν = 1 quantum Hall plateau at a magnetic field of 5 T and temperature of 4.2 K. We also present a fast, simple, and accurate transfer technique of graphene to hexagonal boron nitride crystals. This technique yields atomically flat graphene on boron nitride which is almost completely free of bubbles or wrinkles. The potential of commercially available boron nitride combined with our transfer technique makes high mobility graphene devices more accessible.
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Direct observation of internal potential distributions in a bulk heterojunction solar cell Appl. Phys. Lett. 99, 243301 (2011); http://dx.doi.org/10.1063/1.3669533 (4 pages) Online Publication Date: 12 December 2011
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Cross-sectional potential distributions of a polymer bulk heterojunction (BHJ) solar cell, consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester blends, were directly observed by using Kelvin probe force microscopy. It was found that the bulk electric field of BHJ is nearly field-free compared with that of classical metal-insulator-metal model, whereas the electric field of BHJ device is confined to the cathode interface. The cathode-interfaced rectification junction is studied to originate from the formation of bilayered p-n junction in the BHJ.
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Appl. Phys. Lett. 99, 223305 (2011); http://dx.doi.org/10.1063/1.3664127 (3 pages) Online Publication Date: 1 December 2011
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We investigate the effect of a high hole mobility triarylamine-based conjugated polymer on a bulk hetero-junction organic solar cell. We employed a polymer blend consisting of poly(3-hexylthiophene) (P3HT), [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and poly(N-(4 -(9,9-dioctyl-fluoren-2-yl)phenyl)-N,N′,N′-triphenyl-l,4-phenylenediamine) (PFLAM) as active materials. The hole mobility of PFLAM is ∼10−3 cm2 V−1 s−1, which is similar to the electron mobility of PCBM. Addition of PFLAM improves the hole mobility of the photovoltaic cell augmenting the charge balance of the system. The overall efficiency gain for such a device is 34%.
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Appl. Phys. Lett. 99, 221103 (2011); http://dx.doi.org/10.1063/1.3653390 (3 pages) Online Publication Date: 28 November 2011
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The characteristics of the nitride-based blue light-emitting diode (LED) with an AlGaN/GaN superlattice (SL) electron-blocking layer (EBL) of gradual Al mole fraction are analyzed numerically and experimentally. The emission spectra, carrier concentrations in the quantum wells, energy band diagrams, electrostatic fields, and internal quantum efficiency are investigated. The results indicate that the LED with an AlGaN/GaN SL EBL of gradual Al mole fraction has a better hole injection efficiency, lower electron leakage, and smaller electrostatic fields in its active region over the LED with a conventional rectangular AlGaN EBL or with a normal AlGaN/GaN SL EBL. The results also show that the efficiency droop is markedly improved when the SL EBL of gradual Al mole fraction is used.
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Temperature-dependence of the internal efficiency droop in GaN-based diodes Appl. Phys. Lett. 99, 181127 (2011); http://dx.doi.org/10.1063/1.3658031 (3 pages) Online Publication Date: 4 November 2011
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The temperature dependence of the measured internal efficiencies of green and blue emitting InGaN-based diodes is analyzed. With increasing temperature, a strongly decreasing strength of the loss mechanism responsible for droop is found which is in contrast to the usually assumed behavior of Auger losses. However, the experimental observations can be well reproduced assuming density activated defect recombination with a temperature independent recombination time.
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Polymer solar cells with gold nanoclusters decorated multi-layer graphene as transparent electrode Appl. Phys. Lett. 99, 223302 (2011); http://dx.doi.org/10.1063/1.3664120 (3 pages) Online Publication Date: 30 November 2011
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A thin layer of ultraviolet-ozone (UVO) treated gold (Au) is introduced on multi-layer graphene (MLG) to enable the MLG as an effective anode for polymer solar cells (PSCs). By optimizing the Au thickness and the durations of the UVO treatments at different stages, MLG PSCs with enhanced fill factor and power conversion efficiency are obtained, exhibiting better performance compared with MLG devices directly modified with UVO and poly(3,4-ethylenedioythiophene):poly(styrenesulfonate). Further analysis shows that UVO treated Au provides favorable band alignment at the MLG/polymer interface. Moreover, the improved interfacial contact and shortened UVO durations reduce the series resistance of PSCs significantly.
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Dynamically reconfigurable terahertz metamaterial through photo-doped semiconductor Appl. Phys. Lett. 99, 231101 (2011); http://dx.doi.org/10.1063/1.3667197 (3 pages) Online Publication Date: 5 December 2011
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We demonstrate reconfigurable terahertz metamaterial (MM) in which constituent resonators can be switched from split-ring resonators (SRRs) to closed-ring resonators via optical excitation of silicon islands strategically placed in the split gap. Both the fundamental and the third-order resonance modes experience monotonic damping due to increasing conductive losses in the photo-doped silicon region. More importantly, increasing the optical fluence (>200 μJ/cm2) results in the excitation of the second-order resonance mode, which is otherwise forbidden in a split-ring resonator for the incidence polarization in our experiments. Such dynamical control of metamaterial resonances could be implemented in active terahertz devices to achieve additional functionalities.
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Appl. Phys. Lett. 99, 223303 (2011); http://dx.doi.org/10.1063/1.3664771 (3 pages) Online Publication Date: 30 November 2011
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We demonstrate suppressed singlet-triplet (S-T) quenching, and hence increased quantum efficiency, in high-brightness fluorescent organic light emitting diodes (OLEDs) by reducing the guest triplet population through the introduction of a triplet manager molecule into the emission layer (EML). As an example, an OLED whose EML consists of the red fluorophore, 4-(dicyanomethylene)-2-methyl-6-julolidyl-9-enyl-4H-pyran doped into the host, tris(8-hydroxyquinoline) Al (Alq3) is blended with the triplet manager, 9,10-di(naphtha-2-yl)anthracene. The manager triplet energy is less than that of the host or dopant, leading to efficient triplet removal from the dopant without affecting the radiative singlet population. Measurements suggest the complete suppression of S-T quenching using the triplet management strategy, leading to >100% increase in the steady-state OLED external quantum efficiency.
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Demonstration of interfacial charge transfer in an organic charge injection device Appl. Phys. Lett. 99, 223304 (2011); http://dx.doi.org/10.1063/1.3665190 (3 pages) Online Publication Date: 1 December 2011
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We report the fabrication and testing of an organic charge injection device composed of two adjacent metal-insulator-semiconductor capacitors in which the active semiconductor is poly(3-hexylthiophene). One capacitor, C1, is biased into partial depletion and illuminated with 550 nm light. By applying higher depletion voltages to the second capacitor, C2, photogenerated electrons are induced to flow along the semiconductor-insulator interface from C1 to C2 where they are detected either by simultaneously measuring the C-V characteristic of C2 or the current flow into it. We estimate that the transfer efficiency is in excess of 40% but is capable of significant improvement.
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Measurement of exciton diffusion lengths in optically thin organic films Appl. Phys. Lett. 99, 243303 (2011); http://dx.doi.org/10.1063/1.3668106 (3 pages) Online Publication Date: 13 December 2011
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Spectrally resolved photoluminescence quenching (SR-PLQ) is a convenient and accurate method for measuring the exciton diffusion length of organic materials; however, the requirement of optically thick films demanded by this technique poses practical limitations to its implementation. Through simulations of the optical field and exciton dynamics, we extend SR-PLQ to the case of optically thin organic films; i.e., films whose thickness is comparable to or less than that of the optical absorption length across the entire optical absorption spectrum. This allows for the characterization of films whose thickness is comparable to that used in practical organic optoelectronic devices. Using this method, we measure the diffusion lengths of several squaraine donors, the acceptor 3,4,9,10 perylenetetracarboxylic dianhydride, and the relationship between the donor, boron subphthalocyanine thickness and diffusion length.
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A high-reflectivity, ambient-stable graphene mirror for neutral atomic and molecular beams Appl. Phys. Lett. 99, 211907 (2011); http://dx.doi.org/10.1063/1.3663866 (3 pages) Online Publication Date: 22 November 2011
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We report a He and H2 diffraction study of graphene-terminated Ru(0001) thin films grown epitaxially on c-axis sapphire. Even for samples exposed for several weeks to ambient conditions, brief annealing in ultrahigh vacuum restored extraordinarily high specular reflectivities for He and H2 beams (23% and 7% of the incident beam, respectively). The quality of the angular distributions recorded with both probes exceeds the one obtained from in-situ prepared graphene on Ru(0001) single crystals. Our results for graphene-terminated Ru thin films represent a significant step toward ambient tolerant, high-reflectivity curved surface mirrors for He-atom microscopy.
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Oxygen sensors made by monolayer graphene under room temperature Appl. Phys. Lett. 99, 243502 (2011); http://dx.doi.org/10.1063/1.3668105 (3 pages) Online Publication Date: 13 December 2011
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The electrical resistivity of monolayer graphene exhibit significant changes upon expose to different concentration of oxygen (O2) at room temperature. The monolayer graphene, grown by chemical vapor deposition with perfect uniformity within 1 cm × 1 cm will attach O2 molecules and enhance the hole conductivity, which will lead to a change of resistivity of graphene thin film. We quantified the change of resistivity of graphene versus different O2 concentration and the detection limit of the simple O2 sensor was 1.25% in volume ratio.
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Unique prospects for graphene-based terahertz modulators Appl. Phys. Lett. 99, 113104 (2011); http://dx.doi.org/10.1063/1.3636435 (3 pages) Online Publication Date: 12 September 2011
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The modulation depth of two-dimensional electron-gas (2DEG) based terahertz (THz) modulators using AlGaAs/GaAs hetero-structures with metal gates is inherently limited to <30%. The metal gate not only attenuates the THz signal but also severely degrades modulation depth. Metal losses can be significantly reduced employing an alternative material with tunable conductivity. Graphene presents a unique solution to this problem due to its symmetric band structure and extraordinarily high hole mobility. In this work, we show that it is possible to achieve a modulation depth of >90% while simultaneously minimizing signal attenuation to <5% by tuning the Fermi level at its Dirac point.
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Ag-nanowire films coated with ZnO nanoparticles as a transparent electrode for solar cells Appl. Phys. Lett. 99, 183307 (2011); http://dx.doi.org/10.1063/1.3656973 (3 pages) Online Publication Date: 2 November 2011
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We demonstrate that solution-processible silver-nanowire films coated with zinc-oxide-nanoparticles (ZnO-NPs) can be used as transparent electrodes in organic photovoltaic devices. The ZnO-NP coating acts as electron extraction layer and as encapsulating agent, protecting the wires from oxidation and improving their mechanical stability. Scanning photocurrent microscopy showed photocurrent generation to be more efficient at the active material surrounding the wires. Ultra-violet illumination as present in the solar spectrum was found to enhance photocurrent by improving the ZnO in-layer conductivity through the photoconductive effect. Inverted polythiophene:fullerene devices using ZnO-NP coated silver-nanowires or indium-tin-oxide as transparent electrode reached power conversion efficiencies of 2.4%.
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