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Top 20 Most Read Articles
October 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Electric-field control of spin accumulation signals in silicon at room temperature Appl. Phys. Lett. 99, 132511 (2011); http://dx.doi.org/10.1063/1.3643141 (3 pages) Online Publication Date: 28 September 2011
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We demonstrate spin accumulation signals controlled by the gate voltage in a metal-oxide-semiconductor field effect transistor structure with a Si channel and a CoFe/n+-Si contact at room temperature. Under the application of a back-gate voltage, we clearly observe the three-terminal Hanle-effect curves, i.e., spin accumulation signals. The magnitude of spin accumulation signals can be reduced with increasing the gate voltage. We consider that the gate controlled spin signals are attributed to the change in the carrier density in the Si channel beneath the CoFe/n+-Si contact. This study is not only a technological jump for Si-based spintronic applications with gate structures but also reliable evidence for the spin injection into the semiconducting Si channel at room temperature.
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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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Enhanced charge collection in confined bulk heterojunction organic solar cells Appl. Phys. Lett. 99, 163301 (2011); http://dx.doi.org/10.1063/1.3651509 (3 pages) Online Publication Date: 17 October 2011
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Confining blended poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester organic solar cell active layers within nanometer-scale cylindrical pores nearly double the supported short-circuit photocurrent density compared to equivalent unconfined volumes of the same blend and increases the poly(3-hexylthiophene) hole mobility in the blend by nearly 500 times. Grazing incidence x-ray diffraction measurements show that the confinement changes the polymer orientation distribution, suppressing low charge conductivity orientations while simultaneously disrupting polymer ordering.
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Appl. Phys. Lett. 99, 121101 (2011); http://dx.doi.org/10.1063/1.3617472 (3 pages) Online Publication Date: 19 September 2011
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We demonstrate a spectrally broadband and efficient technique for collecting emission from a single InAs quantum dot directly into a standard single mode optical fiber. In this approach, an optical fiber taper waveguide is placed in contact with a suspended GaAs nanophotonic waveguide with embedded quantum dots, forming a broadband directional coupler with standard optical fiber input and output. Efficient photoluminescence collection over a wavelength range of tens of nanometers is demonstrated, and a maximum collection efficiency of 6% (corresponding single photon rate of 3.0 MHz) into a single mode optical fiber is estimated for a single quantum dot exciton.
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Experimental evidence of ferroelectric negative capacitance in nanoscale heterostructures Appl. Phys. Lett. 99, 113501 (2011); http://dx.doi.org/10.1063/1.3634072 (3 pages) Online Publication Date: 12 September 2011
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We report a proof-of-concept demonstration of negative capacitance effect in a nanoscale ferroelectric-dielectric heterostructure. In a bilayer of ferroelectric Pb(Zr0.2Ti0.8)O3 and dielectric SrTiO3, the composite capacitance was observed to be larger than the constituent SrTiO3 capacitance, indicating an effective negative capacitance of the constituent Pb(Zr0.2Ti0.8)O3 layer. Temperature is shown to be an effective tuning parameter for the ferroelectric negative capacitance and the degree of capacitance enhancement in the heterostructure. Landau’s mean field theory based calculations show qualitative agreement with observed effects. This work underpins the possibility that by replacing gate oxides by ferroelectrics in nanoscale transistors, the sub threshold slope can be lowered below the classical limit (60 mV/decade).
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Analytical modeling of organic solar cells and photodiodes Appl. Phys. Lett. 99, 143301 (2011); http://dx.doi.org/10.1063/1.3643126 (3 pages) Online Publication Date: 3 October 2011
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An analytical and physically based expression of organic solar cell I-V characteristic under dark and illuminated conditions has been derived. This model has been found in very good agreement with both experimental data and drift-diffusion numerical simulations accounting for the coupling with Poisson equation and optical propagation.
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Appl. Phys. Lett. 99, 153101 (2011); http://dx.doi.org/10.1063/1.3646406 (3 pages) Online Publication Date: 10 October 2011
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Using epitaxially grown graphene on Ru(0001) as a buffer layer, the intrinsic molecular orbitals of perylene-3,4,9,10-tetracarboxylic dianhydride, pentacene, and C60 molecules were imaged by means of scanning tunneling microscope (STM). Combined with density functional theory calculations, our high resolution STM images of the molecules reveal that the graphene layer decouples the individual molecules electronically from the metallic substrate. Our results show that graphene-based moiré pattern can be used as a unique way to probe the intrinsic electronic structures of molecular adsorbates and their interactions.
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A photonic-plasmonic structure for enhancing light absorption in thin film solar cells Appl. Phys. Lett. 99, 131114 (2011); http://dx.doi.org/10.1063/1.3641469 (3 pages) Online Publication Date: 30 September 2011
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We describe a photonic-plasmonic nanostructure, for significantly enhancing the absorption of long-wavelength photons in thin-film silicon solar cells, with the promise of exceeding the classical 4n2 limit for enhancement. We compare identical solar cells deposited on the photonic-plasmonic structure, randomly textured back reflectors and silver-coated flat reflectors. The state-of-the-art back reflectors, using annealed Ag or etched ZnO, had high diffuse and total reflectance. For nano-crystalline Si absorbers with comparable thickness, the highest absorption and photo-current of 21.5 mA/cm2 was obtained for photonic-plasmonic back-reflectors. The periodic photonic plasmonic structures scatter and reradiate light more effectively than a randomly roughened surface.
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Appl. Phys. Lett. 99, 143306 (2011); http://dx.doi.org/10.1063/1.3646547 (3 pages) Online Publication Date: 4 October 2011
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A method has been developed to fabricate organic solar cell based on poly(3-hexylthiopene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) film with a thickness as thin as ∼60 nm. The P3HT chain ordering can be dramatically enhanced by slow solvent evaporation process, which is verified by obvious vibronic features and enhanced absorption capability. Due to the improvement of P3HT chain ordering degree, the solar cell based on ultrathin film achieves the power conversion efficiency of 3.7%. Furthermore, the fabrication process does not require any post-treatments such as solvent-vapor or thermal annealing process, which is more compatible with future plastic substrates.
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Appl. Phys. Lett. 99, 153304 (2011); http://dx.doi.org/10.1063/1.3650707 (3 pages) Online Publication Date: 11 October 2011
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We demonstrate efficiency improvement in polymer solar cells (PSCs) by ∼22% through incorporating Au nanoparticles (NPs) into all polymer layers. Au NPs are found to have distinct mechanisms in improving device performance when incorporated in different polymer layers. Au NPs in poly‐(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) mainly contribute to better hole collection while Au NPs in active layer contributes to the enhanced optical absorption and more balanced charge‐transport. Our theoretical result shows that the absorption enhancement at the active layer is attributed to plasmon resonances with strong near‐field distributions penetrated into absorption polymers. These findings can be applied to design high‐efficiency metallic NPs‐incorporated PSCs.
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Facile fabrication of lateral nanowire wrap-gate devices with improved performance Appl. Phys. Lett. 99, 173101 (2011); http://dx.doi.org/10.1063/1.3634010 (3 pages) Online Publication Date: 24 October 2011
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We present a simple fabrication technique for lateral nanowire wrap-gate devices with high capacitive coupling and field-effect mobility. Our process uses e-beam lithography with a single resist-spinning step and does not require chemical etching. We measure, in the temperature range 1.5–250 K, a subthreshold slope of 5–54 mV/decade and mobility of 2800–2500 cm2/Vs—significantly larger than previously reported lateral wrap-gate devices. At depletion, the barrier height due to the gated region is proportional to applied wrap-gate voltage.
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Appl. Phys. Lett. 99, 153302 (2011); http://dx.doi.org/10.1063/1.3650472 (3 pages) Online Publication Date: 11 October 2011
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We demonstrate enhanced power conversion efficiency (ηPCE) for small molecular-based organic photovoltaic cells with an exciton blocking layer (ExBL) at the anode/donor interface. Although poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS) films are widely used as anodic buffer layers, they also act as exciton quenchers. To prevent exciton quenching, we introduced a tris[4-(5-phenyl thiophen-2-yl)phenyl]amine layer between the donor and the PEDOT:PSS layer and clarified its effect. By a combination of dual ExBLs at both the anode and cathode sides, we achieved significantly enhanced short circuit current and ηPCE values; the highest ηPCE = 5.24% was obtained by optimizing the device parameters.
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Appl. Phys. Lett. 99, 141901 (2011); http://dx.doi.org/10.1063/1.3644948 (3 pages) Online Publication Date: 3 October 2011
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We report on the structural characterization of Ge clusters selectively grown by chemical vapor deposition on free-standing 50 nm wide Si(001) nanopillars. Synchrotron based x-ray diffraction studies and transmission electron microscopy were performed to experimentally verify the nanoheteroepitaxy theory as a technique to grow high quality Ge on Si(001). Although the structure dimensions are comparable to the theoretical values required for the strain partitioning phenomenon, the compliant character of Si is not unambiguously proven. In consequence, the strain is relieved by nucleation of misfit dislocations at the Ge/Si interface. By gliding out of threading arms, high quality Ge nanostructures are achieved.
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Appl. Phys. Lett. 99, 172103 (2011); http://dx.doi.org/10.1063/1.3656703 (3 pages) Online Publication Date: 26 October 2011
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Two anomalous bias dependent resistive peaks induced by the SrTiO3 structural phase transitions at 55 and 110 K were observed in a LaAlO3/SrTiO3 and Nb:SrTiO3 rectifying junction when the LaAlO3/SrTiO3 was depleted under reverse bias. At these transition temperatures, the barrier between LaAlO3/SrTiO3 and Nb:SrTiO3 showed abrupt changes in the tunneling energy under forward bias. The peak at 110 K was an insulator-metal phase transition while the peak at 55 K was a metal-insulator one. We propose that the phase transitions of the SrTiO3 substrate influence the charge transfer to the LaAlO3/SrTiO3 layer, giving rise to these anomalous resistive peaks.
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Comprehensive control of optical polarization anisotropy in semiconducting nanowires Appl. Phys. Lett. 99, 141101 (2011); http://dx.doi.org/10.1063/1.3631630 (3 pages) Online Publication Date: 3 October 2011
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The demonstration of strong photoluminescence polarization anisotropy in semiconducting nanowires embodies both technological promise and scientific challenge. Here, we present progress on both fronts through the study of the photoluminescence polarization anisotropy of randomly oriented nanowire ensembles in materials without/with crystalline anisotropy, small/wide bandgap, and both III-V/II-VI chemistry (InP/ZnO nanowires, respectively). Comprehensive control of the polarization anisotropy is realized by dielectric matching with conformally deposited Ta2O5 (dielectric ratios of 9.6:4.41 and 4.0:4.41 for InP and ZnO, respectively). After dielectric matching, the polarization anisotropy of the nanowire ensembles is reduced by 86% for InP:Ta2O5 and 84% for ZnO:Ta2O5.
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Appl. Phys. Lett. 99, 143101 (2011); http://dx.doi.org/10.1063/1.3644496 (3 pages) Online Publication Date: 4 October 2011
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We report on the development of a large-area few-layer graphene (FLG)—based transparent conductive electrode as a current spreading layer for GaN-based ultraviolet (UV) light-emitting diodes (LEDs). Large-area FLG was deposited on Cu using the chemical vapor deposition (CVD) method and subsequently transferred to the surface of the UV LED. UV light at a peak of 372 nm was emitted through the FLG-based transparent conductive electrode. The current spreading effects of FLG were clearly evident in both the optical images of electroluminescence (EL) and current-voltage (I-V) characteristics. Degradation of the FLG-based transparent conductive electrode could be induced by high power operation. Our results indicate that a large-area FLG-based electrode on GaN offers excellent current spreading and ultra-violet transparency properties when compared to the standard optoelectronic indium tin oxide (ITO) contact layer.
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Graphene bubbles with controllable curvature Appl. Phys. Lett. 99, 093103 (2011); http://dx.doi.org/10.1063/1.3631632 (3 pages) Online Publication Date: 30 August 2011
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Raised above the substrate and elastically deformed areas of graphene in the form of bubbles are found on different substrates. They come in a variety of shapes, including those which allow strong modification of the electronic properties of graphene. We show that the shape of the bubble can be controlled by an external electric field. This effect can be used to make graphene-based adaptive focus lenses.
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High-reflectivity II-VI-based distributed Bragg reflectors for the blue-violet spectral range Appl. Phys. Lett. 99, 151101 (2011); http://dx.doi.org/10.1063/1.3644955 (3 pages) Online Publication Date: 10 October 2011
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We report on the realization of a high quality distributed Bragg reflector for the blue-violet spectral range, with both high and low refractive index layers lattice matched to the GaAs substrate. Our structure is grown by molecular beam epitaxy (MBE). The high refractive index layer is made of ZnMgSSe, while the low index material consists of a short period superlattice containing MgS and ZnCdSe. The refractive index step of Δn = 0.43 results in a stop band width of 40 nm and the normalized reflectivity exceeds 99% for 21 Bragg pairs.
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Monolayer graphene film/silicon nanowire array Schottky junction solar cells Appl. Phys. Lett. 99, 133113 (2011); http://dx.doi.org/10.1063/1.3643473 (3 pages) Online Publication Date: 28 September 2011
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Schottky junction solar cells were constructed by combining the monolayer graphene (MLG) films and the Si nanowire (SiNW) arrays. Pronounced photovoltaic characteristics were investigated for devices with both p-MLG/n-SiNWs and n-MLG/p-SiNWs structures. Due to the balance between light absorption and surface carrier recombination, devices made of SiNW arrays with a medium length showed better performance and could be further improved by enhancing the MLG conductivity via appropriate surface treatment or doping. Eventually, a photoconversion efficiency up to 2.15% is obtained by the means of filling the interspace of SiNW array with graphene suspension.
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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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