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17 Oct 2011

Volume 99, Issue 16, Articles (16xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 99, 163301 (2011); http://dx.doi.org/10.1063/1.3651509 (3 pages)

Jonathan E. Allen, Kevin G. Yager, Htay Hlaing, Chang-Yong Nam, Benjamin M. Ocko, and Charles T. Black
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Interface effects on perpendicular magnetic anisotropy for molecular-capped cobalt ultrathin films

Xianmin Zhang, Shigemi Mizukami, Takahide Kubota, Mikihiko Oogane, Hiroshi Naganuma, Yasuo Ando, and Terunobu Miyazaki

Appl. Phys. Lett. 99, 162509 (2011); http://dx.doi.org/10.1063/1.3651766 (3 pages) | Cited 2 times

Online Publication Date: 19 October 2011

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The perpendicular magnetic anisotropy (PMA) of cobalt (0.5-1.8 nm) films capped separately by pentacene (Pc), fullerene (C60), and 8-hydroxyquinoline-aluminum (Alq3) are investigated. For all three series, the thickness of Co is around 0.7 nm for maximum out-of-plane coercivity. It is found that the coercivity of C60-capped films is nearly equal to that for Alq3-capped samples, although both are smaller than for Pc-capped films. The different interface effects of Co/molecules are discussed to explain this observation. This work highlights the PMA of ferromagnetic metal, which can be markedly infected depending on the nature of organic molecule.
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75.30.Gw Magnetic anisotropy
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Ak Magnetic properties of monolayers and thin films
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
81.15.Cd Deposition by sputtering
68.55.at Other materials

Structure and magnetic properties of ternary Tb-Fe-B nanoparticles and nanoflakes

R. M. Liu, M. Yue, W. Q. Liu, D. T. Zhang, J. X. Zhang, Z. H. Guo, and W. Li

Appl. Phys. Lett. 99, 162510 (2011); http://dx.doi.org/10.1063/1.3653256 (3 pages) | Cited 2 times

Online Publication Date: 19 October 2011

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The structure and magnetic properties were studied for the ternary Tb-Fe-B nanoparticles and nanoflakes prepared by surfactant-aid high energy ball milling and subsequent size-selection. For the nanoparticles, significant room-temperature coercivity up to 10.6 kOe was achieved with the average particle size of 31.4 nm, but it decreases substantially to 400 Oe as the size is down to 8.2 nm. For the nanoflakes, the thickness is in the range of 50-80 nm while the length is about 1.1 μm, and high coercivity of 22.1 kOe and strong magnetic anisotropy were obtained simultaneously, indicating that the nanoflakes possess excellent permanent magnetic properties.
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
75.50.Bb Fe and its alloys
75.75.-c Magnetic properties of nanostructures
75.50.Tt Fine-particle systems; nanocrystalline materials
81.16.Rf Micro- and nanoscale pattern formation
81.07.Wx Nanopowders
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Electric control of magnetization relaxation in thin film magnetic insulators

Zihui Wang, Yiyan Sun, Young-Yeal Song, Mingzhong Wu, Helmut Schultheiß, John E. Pearson, and Axel Hoffmann

Appl. Phys. Lett. 99, 162511 (2011); http://dx.doi.org/10.1063/1.3654148 (3 pages) | Cited 5 times

Online Publication Date: 20 October 2011

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Control of magnetization relaxation in magnetic insulators via interfacial spin scattering is demonstrated. The experiments use nanometer-thick yttrium iron garnet (YIG)/Pt layered structures, with the Pt layer biased by an electric voltage. The bias voltage produces a spin current across the Pt thickness. As this current scatters off the YIG surface, it exerts a torque on the YIG surface spins. This torque can reduce or enhance the damping and thereby decrease or increase the ferromagnetic resonance linewidth of the YIG film, depending on the field/current configuration.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Experimental evidence of Ga-vacancy induced room temperature ferromagnetic behavior in GaN films

Basanta Roul, Mohana K. Rajpalke, Thirumaleshwara N. Bhat, Mahesh Kumar, A. T. Kalghatgi, S. B. Krupanidhi, Nitesh Kumar, and A. Sundaresan

Appl. Phys. Lett. 99, 162512 (2011); http://dx.doi.org/10.1063/1.3654151 (3 pages) | Cited 5 times

Online Publication Date: 20 October 2011

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We have grown Ga deficient GaN epitaxial films on (0001) sapphire substrate by plasma-assisted molecular beam epitaxy and report the experimental evidence of room temperature ferromagnetic behavior. The observed yellow emission peak in room temperature photoluminescence spectra and the peak positioning at 300 cm−1 in Raman spectra confirms the existence of Ga vacancies. The x-ray photoelectron spectroscopic measurements further confirmed the formation of Ga vacancies; since the N/Ga is found to be >1. The ferromagnetism is believed to originate from the polarization of the unpaired 2p electrons of N surrounding the Ga vacancy.
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75.70.Ak Magnetic properties of monolayers and thin films
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.ag Semiconductors
75.50.Dd Nonmetallic ferromagnetic materials

Table-like magnetocaloric effect and enhanced refrigerant capacity in Eu8Ga16Ge30-EuO composite materials

Anurag Chaturvedi, Stevce Stefanoski, Manh-Huong Phan, George S. Nolas, and Hariharan Srikanth

Appl. Phys. Lett. 99, 162513 (2011); http://dx.doi.org/10.1063/1.3654157 (3 pages) | Cited 17 times

Online Publication Date: 21 October 2011

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A large reversible magnetocaloric effect (MCE) and enhanced refrigerant capacity (RC) were observed in multiphase composite materials composed of type-I clathrate Eu8Ga16Ge30 and EuO. Eu8Ga16Ge30 undergoes two successive ferromagnetic transitions at 10 K and 35 K, and EuO exhibits a ferromagnetic transition at 75 K. A large RC of 794 J/kg for a field change of 5 T over a temperature interval of 70 K was achieved in the Eu8Ga16Ge30–EuO composite with a 40%-60% weight ratio. This is the largest value ever achieved among existing magnetocaloric materials for magnetic refrigeration in the temperature range 10 K-100 K. Adjusting the Eu8Ga16Ge30 to EuO ratio is shown to produce composites with table-like MCE, desirable for ideal Ericsson-cycle magnetic refrigeration. The excellent magnetocaloric properties of these Eu8Ga16Ge30–EuO composites make them attractive for active magnetic refrigeration in the liquid nitrogen temperature range.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Magnetic force microscopy in the presence of a strong probe field

Inhee Lee, Jongjoo Kim, Yuri Obukhov, Palash Banerjee, Gang Xiang, Denis V. Pelekhov, Adam Hauser, Fengyuan Yang, and P. Chris Hammel

Appl. Phys. Lett. 99, 162514 (2011); http://dx.doi.org/10.1063/1.3653281 (3 pages) | Cited 3 times

Online Publication Date: 21 October 2011

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We describe a magnetic force microscopy (MFM) imaging approach in which we take advantage of the strong, localized magnetic field of the MFM probe to deterministically modify the magnetization of the sample. This technique enables quantitative mapping of sample magnetic properties including saturation magnetization and anisotropy, a capability not generally available using conventional MFM methods. This approach yields a fruitful theoretical analysis that accurately describes representative experimental data we obtain from an isolated permalloy disk.
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07.79.Pk Magnetic force microscopes
75.30.Gw Magnetic anisotropy
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Electromechanical properties and anisotropy of single- and multi-domain 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 single crystals

Gang Liu, Wenhua Jiang, Jiaqi Zhu, and Wenwu Cao

Appl. Phys. Lett. 99, 162901 (2011); http://dx.doi.org/10.1063/1.3652703 (3 pages) | Cited 3 times

Online Publication Date: 17 October 2011

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Complete sets of elastic, piezoelectric, and dielectric constants of 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 single crystal poled along [111]c (single domain) as well as non-polar axes [001]c and [011]c (multidomain) have been measured under natural conditions. These data allowed us to evaluate accurately the extrinsic contributions to the superior piezoelectric properties. Very large extrinsic contributions to the unusual anisotropies in multidomain crystals are confirmed. We found that the instability of domain structures is the origin of the low mechanical quality factor Q for the multidomain relaxor-based ferroelectric single crystals. Our results can provide useful guidance in future design of domain engineered materials.
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77.80.Dj Domain structure; hysteresis
77.22.Ch Permittivity (dielectric function)
77.80.Jk Relaxor ferroelectrics
77.22.Ej Polarization and depolarization
62.20.D- Elasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations

Mapping and statistics of ferroelectric domain boundary angles and types

Joseph Desmarais, Jon F. Ihlefeld, Tassilo Heeg, Jürgen Schubert, Darrell G. Schlom, and Bryan D. Huey

Appl. Phys. Lett. 99, 162902 (2011); http://dx.doi.org/10.1063/1.3643155 (3 pages) | Cited 6 times

Online Publication Date: 17 October 2011

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Ferroelectric domain orientations have been mapped using piezo-force microscopy, allowing the calculation and statistical analysis of interfacial polarization angles, the head-to-tail or head-to-head configuration, and any cross-coupling terms. Within 1 µm2 of an epitaxial (001)p-oriented BiFeO3 film, there are >40 µm of linear domain boundary based on over 500 interfaces. 71° domain walls dominate the interfacial polarization angles, with a 2:1 preference for uncharged head-to-tail versus charged head-to-head boundary types. This mapping technique offers a unique perspective on domain boundary distributions, important for ferroelectric and multiferroic applications where domain wall parameters are critical.
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77.55.fp Other ferroelectric films
77.22.Ej Polarization and depolarization
77.80.Dj Domain structure; hysteresis
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.55.Nv Multiferroic/magnetoelectric films
77.55.Px Epitaxial and superlattice films

The effects of polarization dynamics and domain switching energies on field induced phase transformations of perovskite ferroelectrics

Daniel J. Franzbach, Bai-Xiang Xu, Ralf Mueller, and Kyle G. Webber

Appl. Phys. Lett. 99, 162903 (2011); http://dx.doi.org/10.1063/1.3644957 (3 pages) | Cited 3 times

Online Publication Date: 19 October 2011

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A time-dependent phase field model was utilized to investigate the electric field induced tetragonal-to-orthorhombic (T→O) planar phase transformations of perovskite materials, indicated by the simulated semi-discontinuous polarization-electric field hysteresis. Simulations show that transformations are sensitive to the crystallographic orientation of the applied electric field, the energy threshold for 90° domain switching and the rotation path between tetragonal variants. This indicates that the field induced phase transformation behavior has a significant influence on the evaluation and validation of the Landau coefficients.
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77.22.Ej Polarization and depolarization
77.80.Jk Relaxor ferroelectrics
77.80.B- Phase transitions and Curie point
77.80.Fm Switching phenomena
77.80.Dj Domain structure; hysteresis
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Method for measuring energy generation and efficiency of dielectric elastomer generators

Rainer Kaltseis, Christoph Keplinger, Richard Baumgartner, Martin Kaltenbrunner, Tiefeng Li, Philipp Mächler, Reinhard Schwödiauer, Zhigang Suo, and Siegfried Bauer

Appl. Phys. Lett. 99, 162904 (2011); http://dx.doi.org/10.1063/1.3653239 (3 pages) | Cited 11 times

Online Publication Date: 19 October 2011

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Dielectric elastomer generators convert mechanical into electrical energy at high energy density, showing promise for large and small scale energy harvesting. We present an experiment to monitor electrical and mechanical energy flows separately and show the cycle of energy conversion in work-conjugate planes. A specific electrical energy generated per cycle of 102mJ/g, at a specific average power of 17mW/g, is demonstrated with an acrylic elastomer in a showcase generation cycle. The measured mechanical to electrical energy conversion efficiency is 7.5%. The experiment may be used to assess the aptitude of specifically designed elastomers for energy harvesting.
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84.60.-h Direct energy conversion and storage
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
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Dramatic thermal conductivity reduction by nanostructures for large increase in thermoelectric figure-of-merit of FeSb2

Huaizhou Zhao, Mani Pokharel, Gaohua Zhu, Shuo Chen, Kevin Lukas, Qing Jie, Cyril Opeil, Gang Chen, and Zhifeng Ren

Appl. Phys. Lett. 99, 163101 (2011); http://dx.doi.org/10.1063/1.3651757 (3 pages) | Cited 7 times

Online Publication Date: 17 October 2011

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In this report, thermal conductivity reduction by more than three orders of magnitude over its single crystal counterpart for the strongly correlated system FeSb2 through a nanostructure approach was presented, leading to a significant increase of thermoelectric figure-of-merit (ZT). For the samples processed with the optimal parameters, the thermal conductivity reached 0.34 Wm−1 K−1 at 50 K, leading to a ZT peak of about 0.013, compared to 0.005 for single crystal FeSb2, an increase of about 160%. This work suggests that nanostructure method is effective and can be possibly extended to other strongly correlated low temperature thermoelectric materials, paving the way for future cryogenic temperature cooling applications.
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66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
61.46.Hk Nanocrystals
72.20.Pa Thermoelectric and thermomagnetic effects

Electrostatic studies of π–π interaction for benzene stacking on a graphene layer

Yen Hsun Su, Yang Kai Wu, Sheng‐Lung Tu, and Shoou-Jinn Chang

Appl. Phys. Lett. 99, 163102 (2011); http://dx.doi.org/10.1063/1.3653284 (3 pages) | Cited 2 times

Online Publication Date: 17 October 2011

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Recently, aromatic molecules have been stacked on graphene for applications in biosensors and chemical sensors, although the interaction between them is not well understood. In this paper, we use electrostatic model, double charge rings, and its image charges model to simulate the π–π interaction between benzene and a graphene layer. Furthermore, the results of our model are confirmed by the numerical results from density functional theory and experimental reviews. This model has potential for use in predicting the interactions between aromatic molecules and graphene.
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71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Temperature dependence of the field emission from the few-layer graphene film

Jun Li (李军), Jiangtao Chen (陈江涛), Baoshou Shen (申保收), Xingbin Yan (阎兴斌), and Qunji Xue (薛群基)

Appl. Phys. Lett. 99, 163103 (2011); http://dx.doi.org/10.1063/1.3651323 (3 pages) | Cited 7 times

Online Publication Date: 17 October 2011

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Temperature dependence of field-emission (FE) characteristics was investigated for the spray-coated few-layer graphene (FLG) film. The results show that the turn-on field and work function both decrease with increasing temperature from room temperature to 623 K. The possible physical mechanism was proposed based on that the FLG sheets with different stacking orders are nonzero or zero band gap semiconductors.
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79.70.+q Field emission, ionization, evaporation, and desorption
73.30.+y Surface double layers, Schottky barriers, and work functions

Sc-phthalocyanine sheet: Promising material for hydrogen storage

Kun Lü, Jian Zhou, Le Zhou, Qian Wang, Qiang Sun, and Puru Jena

Appl. Phys. Lett. 99, 163104 (2011); http://dx.doi.org/10.1063/1.3653465 (3 pages) | Cited 6 times

Online Publication Date: 18 October 2011

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It has been a long-standing dream to have high surface area materials with isolated and exposed transition-metal ions for hydrogen storage. The flexible synthesis procedure proposed recently by M. Abel, et al. [J. Am. Chem. Soc. 133, 1203 (2011)] and A. Sperl et al. [J. Am. Chem. Soc. 133, 11007 (2011)] provides a different pathway to achieve this goal. Using first-principles theory and grand canonical Monte Carlo simulation, we carry out a systematic study of 3d transition metals (Sc to Zn)-phthalocyanine porous sheets and find that Sc-phthalocyanine can store 4.6 wt. % hydrogen at 298 K and 100 bar.
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88.30.rf Organics
61.43.Gt Powders, porous materials

High quality factor graphene varactors for wireless sensing applications

Steven J. Koester

Appl. Phys. Lett. 99, 163105 (2011); http://dx.doi.org/10.1063/1.3651334 (3 pages) | Cited 3 times

Online Publication Date: 18 October 2011

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A graphene wireless sensor concept is described. By utilizing thin gate dielectrics, the capacitance in a metal-insulator-graphene structure varies with charge concentration through the quantum capacitance effect. Simulations using realistic structural and transport parameters predict quality factors, Q, >60 at 1 GHz. When placed in series with an ideal inductor, a resonant frequency tuning ratio of 25% (54%) is predicted for sense charge densities ranging from 0.32 to 1.6 μC/cm2 at an equivalent oxide thickness of 2.0 nm (0.5 nm). The resonant frequency has a temperature sensitivity, df/dT, less than 0.025%/K for sense charge densities >0.32 μC/cm2.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
84.32.Tt Capacitors
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
84.32.Hh Inductors and coils; wiring

Control of spontaneous emission in InGaAs/GaAs quantum structure lattices

K. Y. Cheng, Chien-Chia Cheng, and K. C. Hsieh

Appl. Phys. Lett. 99, 163106 (2011); http://dx.doi.org/10.1063/1.3654152 (3 pages) | Cited 1 time

Online Publication Date: 19 October 2011

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We report the control of spontaneous emission wavelength of quantum wells (QWs) using an artificial design of semiconductor lattice, i.e., quantum structure lattice (QSL). Two-dimension square quantum box arrays fabricated in strained InGaAs/GaAs QW samples are used as active lattice units in QSL for this study. The photoluminescence peak wavelength of the QSL can be adjusted to blue-shift, red-shift, or unchanged from that of the as-grown QW by fitting the in-plane Bragg diffraction condition of the QSL. The wavelength-shift has values of multiple phonon energies indicating that the phonon-assisted recombination process may play a significant role in the observed spontaneous emission.
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78.67.De Quantum wells
63.22.Np Layered systems
78.55.Cr III-V semiconductors

Intercalation of metal islands and films at the interface of epitaxially grown graphene and Ru(0001) surfaces

Li Huang, Yi Pan, Lida Pan, Min Gao, Wenyan Xu, Yande Que, Haitao Zhou, Yeliang Wang, Shixuan Du, and H.-J. Gao

Appl. Phys. Lett. 99, 163107 (2011); http://dx.doi.org/10.1063/1.3653241 (3 pages) | Cited 12 times

Online Publication Date: 19 October 2011

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We report on intercalation of seven kinds of metals—Pt, Pd, Ni, Co, Au, In, and Ce—at the interface between an epitaxially grown graphene layer and a Ru(0001) substrate. Atomic resolution scanning tunneling microscopy images of perfect graphene lattice are obtained on top of these intercalated metals, showing that the high quality of the original graphene is, in the end, undisturbed by the intercalation. A model based theoretical calculation is proposed for the intercalation mechanism: metal atom-aided defect formation and self-healing of C–C bonds at high temperature. These intercalated materials include noble metals, magnetic metals, a IIIA group metal, and a rare earth metal, which indicates that intercalation through epitaxial graphene on Ru(0001) is a universal approach for metals.
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68.55.-a Thin film structure and morphology
75.70.Ak Magnetic properties of monolayers and thin films
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.55.A- Nucleation and growth

Doping of graphene adsorbed on the a-SiO2 surface

R. H. Miwa, Tome M. Schmidt, W. L. Scopel, and A. Fazzio

Appl. Phys. Lett. 99, 163108 (2011); http://dx.doi.org/10.1063/1.3653261 (3 pages) | Cited 6 times

Online Publication Date: 20 October 2011

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We have performed an ab initio theoretical investigation of a graphene sheet adsorbed on amorphous SiO2 surface (G/a-SiO2). We find that graphene adsorbs on the a-SiO2 surface through van der Waals interactions. The inhomogeneous topology of the a-SiO2 clean surface promotes a total charge density displacement on the adsorbed graphene sheet, giving rise to electron-rich as well as hole-rich regions on the graphene. Furthermore, the adsorbed graphene sheet exhibits a net total charge density gain. In this case, the graphene sheet becomes n-type doped, however, no chemical bonds form at the graphene–SiO2 interface. The electronic charge transfer from a-SiO2 to the graphene sheet occurs upon the formation of a partially occupied level lying above the Dirac point. We find that this partially occupied level comes from the three-fold coordinated oxygen atoms in the a-SiO2 substrate.
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68.43.Mn Adsorption kinetics
68.43.Bc Ab initio calculations of adsorbate structure and reactions
68.35.bt Other materials
61.72.up Other materials
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
73.20.At Surface states, band structure, electron density of states

Number-of-layer discriminated graphene phonon softening and stiffening

Y. Wang, X. X. Yang, J. W. Li, Z. F. Zhou, W. T. Zheng, and Chang Q. Sun

Appl. Phys. Lett. 99, 163109 (2011); http://dx.doi.org/10.1063/1.3656704 (3 pages) | Cited 3 times

Online Publication Date: 21 October 2011

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From the perspective of bond order-length-strength correlation and the local bond averaging approach, we have formulated the number-of-layer resolved Raman shifts of graphene, with quantification of the referential origins from which the Raman shifts proceed and clarification of their origins. It is found that the primary D mode and the secondary 2D mode are dominated by the interaction between a specific atom and its nearest neighbors while the G mode by the dimer interaction, and therefore red shift happens to the D/2D phonons and blue shift to the G mode upon the number-of-layer is reduced.
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63.70.+h Statistical mechanics of lattice vibrations and displacive phase transitions
78.67.Wj Optical properties of graphene
78.30.Am Elemental semiconductors and insulators

Optical metasurfaces with robust angular response on flexible substrates

Andrea Di Falco, Yang Zhao, and Andrea Alú

Appl. Phys. Lett. 99, 163110 (2011); http://dx.doi.org/10.1063/1.3655332 (3 pages) | Cited 1 time

Online Publication Date: 21 October 2011

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We have fabricated optical metasurfaces on flexible substrates that show an optical response independent on the polarization and angles of incidence of light. The realized devices operate as ultrathin selective filters at visible frequencies, with a bandwidth of ∼200 nm, and their response is robust to membrane bending for possible integration in conformal optical devices.
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42.70.-a Optical materials
42.79.Ci Filters, zone plates, and polarizers
42.82.Cr Fabrication techniques; lithography, pattern transfer
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Enhanced charge collection in confined bulk heterojunction organic solar cells

Jonathan E. Allen, Kevin G. Yager, Htay Hlaing, Chang-Yong Nam, Benjamin M. Ocko, and Charles T. Black

Appl. Phys. Lett. 99, 163301 (2011); http://dx.doi.org/10.1063/1.3651509 (3 pages) | Cited 9 times

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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88.40.jr Organic photovoltaics
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Evidence for non-isotropic emitter orientation in a red phosphorescent organic light-emitting diode and its implications for determining the emitter’s radiative quantum efficiency

Tobias D. Schmidt, Daniel S. Setz, Michael Flämmich, Jörg Frischeisen, Dirk Michaelis, Benjamin C. Krummacher, Norbert Danz, and Wolfgang Brütting

Appl. Phys. Lett. 99, 163302 (2011); http://dx.doi.org/10.1063/1.3653475 (3 pages) | Cited 7 times

Online Publication Date: 18 October 2011

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The efficiency of organic light-emitting diodes is limited as only a fraction of the consumed electrical power is converted into light that is finally extracted to air. Especially, the radiative quantum efficiency of the guest-host system is of interest and should be close to unity to achieve highly efficient devices. We show that the red phosphorescent emitter Ir(MDQ)2(acac) doped in an α-NPD matrix exhibits a profound non-isotropic dipole orientation. Ignoring this feature leads to a significant overestimation of the emitter efficiency. Furthermore, we demonstrate the huge potential for efficiency enhancement of mainly parallel emitter orientation in phosphorescent organic light-emitting diodes.
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85.60.Jb Light-emitting devices
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Realization of high efficiency orange and white organic light emitting diodes by introducing an ultra-thin undoped orange emitting layer

Yongbiao Zhao, Jiangshan Chen, and Dongge Ma

Appl. Phys. Lett. 99, 163303 (2011); http://dx.doi.org/10.1063/1.3654150 (3 pages) | Cited 9 times

Online Publication Date: 19 October 2011

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In this paper, we report on high efficiency orange and white organic light emitting diodes (OLEDs) by introducing an ultra-thin undoped orange emitting layer (less than 1 nm) adjacent to the electron-transporting layer. The orange OLEDs showed a high external quantum efficiency (EQE) of 16.4% at 1000 cd/m2, and the resulting four-color white OLEDs emitted an EQE of 15.5% and a maximum color render index (CRI) of 87 at luminance of 1000 cd/m2. Further analysis on the EQE of the white OLEDs makes us confident that this design has the possibility to realize 100% internal quantum efficiency (IQE) by using much more highly efficient phosphorescent emitters.
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85.60.Jb Light-emitting devices
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High speed and high density organic electrochemical transistor arrays

Dion Khodagholy, Moshe Gurfinkel, Eleni Stavrinidou, Pierre Leleux, Thierry Herve, Sébastien Sanaur, and George G. Malliaras

Appl. Phys. Lett. 99, 163304 (2011); http://dx.doi.org/10.1063/1.3652912 (3 pages) | Cited 4 times

Online Publication Date: 19 October 2011

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A generic lithographic process is presented that allows the fabrication of high density organic electrochemical transistor arrays meant to interface with aqueous electrolytes. The channels of the transistors, which were 6 μm long, were made of the conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) and were in direct contact with phosphate buffered saline. Source and drain electrodes and interconnects were insulated by parylene C, a biocompatible material. The transistors operated at low voltages and showed a response time of the order of 100 μs.
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85.30.Tv Field effect devices
85.40.Hp Lithography, masks and pattern transfer
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High performance polymer light-emitting diodes with N-type metal oxide/conjugated polyelectrolyte hybrid charge transport layers

Ji Sun Park, Bo Ram Lee, Eunjae Jeong, Hyun-Jung Lee, Ju Min Lee, Ji-Seon Kim, Jin Young Kim, Han Young Woo, Sang Ouk Kim, and Myoung Hoon Song

Appl. Phys. Lett. 99, 163305 (2011); http://dx.doi.org/10.1063/1.3653962 (3 pages) | Cited 4 times

Online Publication Date: 20 October 2011

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We present an interfacial engineering strategy employing n-type-metal-oxide/conjugated-polyelectrolyte (CPE) hybrid charge-transport layers for highly efficient polymer light-emitting diodes (PLEDs). The hybrid metal-oxide/CPE layer facilitates electron-injection, while blocking hole-transport, and thereby maximizes electron-hole recombination within the emitting layer. A series of metal-oxide/CPE combinations were tested in inverted PLEDs (FTO/metal-oxide/CPE/F8BT/MoO3/Au). Specifically, HfO2/CPE double layer achieved an electroluminescence (EL) efficiency of up to 25.8 cd/A (@ 6.4 V, one of the highest values reported for fluorescent PLEDs).
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85.60.Jb Light-emitting devices
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