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5 Sep 2011

Volume 99, Issue 10, Articles (10xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 99, 103701 (2011); http://dx.doi.org/10.1063/1.3633066 (3 pages)

Yinan Zhang, David J. S. Birch, and Yu Chen
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Enhanced spin signal in nonlocal devices based on a ferromagnetic CoFeAl alloy

G. Bridoux, M. V. Costache, J. Van de Vondel, I. Neumann, and S. O. Valenzuela

Appl. Phys. Lett. 99, 102107 (2011); http://dx.doi.org/10.1063/1.3635391 (3 pages) | Cited 7 times

Online Publication Date: 9 September 2011

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We systematically study the nonlocal spin signal in lateral spin valves based on CoFeAl injectors and detectors and compare the results with identically fabricated devices based on CoFe. The devices are fabricated by electron beam evaporation at room temperature. We observe a > 10-fold enhancement of the spin signal in the CoFeAl devices. We explain this increase as due to the formation of a highly spin-polarized Co2FeAl Heusler compound with large resistivity. These results suggest that Heusler compounds are promising candidates as spin polarized electrodes in lateral spin devices for future spintronic applications.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
72.25.-b Spin polarized transport
75.50.Bb Fe and its alloys

Dielectric dipole mitigated Schottky barrier height tuning using atomic layer deposited aluminum oxide for contact resistance reduction

Brian E. Coss, Wei-Yip Loh, Herman Carlo Floresca, Moon J. Kim, Prashant Majhi, Robert M. Wallace, Jiyoung Kim, and Raj Jammy

Appl. Phys. Lett. 99, 102108 (2011); http://dx.doi.org/10.1063/1.3633117 (3 pages) | Cited 1 time

Online Publication Date: 9 September 2011

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Schottky barrier height tuning is reported from the insertion of thin layers of AlOx and SiO2 at the interface between tantalum nitride and p-type silicon. The magnitude of the change in the barrier height is found to be dependent on the conditions of AlOx and SiO2 formation. The largest change in barrier height is over 350 meV and correlates well with the intrinsic dipole found at this interface. These findings are then interpreted using a model of the dipole formation at the high-κ and SiO2 interface. The application of these findings for low resistance contacts as well as options to achieve greater performance are discussed.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.40.Cg Contact resistance, contact potential

Low-temperature photocarrier dynamics in monolayer MoS2

T. Korn, S. Heydrich, M. Hirmer, J. Schmutzler, and C. Schüller

Appl. Phys. Lett. 99, 102109 (2011); http://dx.doi.org/10.1063/1.3636402 (3 pages) | Cited 38 times

Online Publication Date: 9 September 2011

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The band structure of MoS2 strongly depends on the number of layers, and a transition from indirect to direct-gap semiconductor has been observed recently for a single layer of MoS2. Single-layer MoS2 therefore becomes an efficient emitter of photoluminescence even at room temperature. Here, we report on scanning Raman and on temperature-dependent, as well as time-resolved photoluminescence measurements on single-layer MoS2 flakes prepared by exfoliation. We observe the emergence of two distinct photoluminescence peaks at low temperatures. The photocarrier recombination at low temperatures occurs on the few-picosecond timescale, but with increasing temperatures, a biexponential photoluminescence decay with a longer-lived component is observed.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.61.Le Other inorganic semiconductors
78.30.Hv Other nonmetallic inorganics
78.55.Hx Other solid inorganic materials
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces

Effect of cationic substitution on the thermoelectric properties of In4−xMxSe2.95 compounds (M = Na, Ca, Zn, Ga, Sn, Pb; x = 0.1)

Kyunghan Ahn, Eunseog Cho, Jong-Soo Rhyee, Sang Il Kim, Sang Mock Lee, and Kyu Hyoung Lee

Appl. Phys. Lett. 99, 102110 (2011); http://dx.doi.org/10.1063/1.3637053 (3 pages) | Cited 4 times

Online Publication Date: 9 September 2011

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We report an electrical and thermal transport study for polycrystalline samples of In4Se3−x substituted with various metals. Our experimental and theoretical investigation revealed that the Na, Ca, and Pb substitutions at the In4 site in the 4:3 indium selenide lattice are more effective for increase in electron concentration than the Zn, Ga, and Sn substitutions at other sites, and the Peierls distortion in cationic substituted compounds may not be as strong as In4Se3−x based on higher lattice thermal conductivities of the former than the latter. It is found that the cationic substitutions may weaken the effect of Peierls distortion on thermal properties for In4Se3−x while they maintain the same trend for electrical properties of In4Se3−x with different electron concentrations.
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72.20.Pa Thermoelectric and thermomagnetic effects
72.80.Jc Other crystalline inorganic semiconductors
72.20.My Galvanomagnetic and other magnetotransport effects
66.70.Df Metals, alloys, and semiconductors
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Ferromagnetic resonance in Y3Fe5O12 nanofibers

A. Jalalian, M. S. Kavrik, S. I. Khartsev, and A. M. Grishin

Appl. Phys. Lett. 99, 102501 (2011); http://dx.doi.org/10.1063/1.3633351 (3 pages) | Cited 2 times

Online Publication Date: 6 September 2011

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Continuous bead-free polycrystalline yttrium iron garnet (YIG) nanofibers 100 μm long and 50-100 nm in diameter were sintered by sol-gel calcination assisted electrospinning. Room temperature ferromagnetism in YIG fibers is indicated by a well formed M-H hysteresis loop with about 70% of remnant magnetization to the saturated value reached at 150 Oe. Broad band microwave spectroscopy revealed strong asymmetry of ferromagnetic resonance lines that conforms to a three orders of magnitude shape aspect ratio of YIG nanofibers.
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76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Gg Ferrimagnetics
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.75.Cd Fabrication of magnetic nanostructures
75.50.Tt Fine-particle systems; nanocrystalline materials

Rapid thermal annealing study of magnetoresistance and perpendicular anisotropy in magnetic tunnel junctions based on MgO and CoFeB

Wei-Gang Wang, Stephen Hageman, Mingen Li, Sunxiang Huang, Xiaoming Kou, Xin Fan, John Q. Xiao, and C. L. Chien

Appl. Phys. Lett. 99, 102502 (2011); http://dx.doi.org/10.1063/1.3634026 (3 pages) | Cited 7 times

Online Publication Date: 6 September 2011

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The tunneling magnetoresistance and perpendicular magnetic anisotropy in CoFeB(1.1-1.2 nm)/MgO/CoFeB(1.2-1.7 nm) junctions were found to be very sensitively dependent on annealing time. During annealing at a given temperature, decay of magnetoresistance occurs much earlier compared to junctions with in-plane magnetic anisotropy. Through a rapid thermal annealing study, the decrease of magnetoresistance is found to be associated with the degradation of perpendicular anisotropy, instead of impurity diffusion as observed in common in-plane junctions. The origin of the evolution of perpendicular anisotropy as well as possible means to further enhance tunneling magnetoresistance is discussed.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.-m Magnetotransport phenomena; materials for magnetotransport
75.30.Gw Magnetic anisotropy
61.72.Cc Kinetics of defect formation and annealing

Non-collinear magnetization configuration in interlayer exchange coupled magnetic thin films

J. Choi, B.-C. Min, J.-Y. Kim, B.-G. Park, J. H. Park, Y. S. Lee, and K.-H. Shin

Appl. Phys. Lett. 99, 102503 (2011); http://dx.doi.org/10.1063/1.3634027 (3 pages) | Cited 2 times

Online Publication Date: 6 September 2011

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Element specific magnetic hysteresis loops of the interlayer exchange coupled CoFeB/Ru/[Co/Ni]4 structure were measured utilizing x-ray magnetic circular dichroism. It was found that the Co/Ni multilayer and the CoFeB layer have Ru thickness dependent oscillatory interlayer coupling. Due to its interlayer coupling with the perpendicularly magnetized Co/Ni multilayer, the CoFeB magnetization direction is slightly tilted out-of-plane from its in-plane magnetic easy axis. Quantitative measurements show that the tilting angle is small (<12°) and that a small in-plane magnetic field (∼50 Oe) applied to this structure will result in a completely in-plane CoFeB magnetization.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
78.20.Ls Magneto-optical effects

Irradiation enhanced paramagnetism on graphene nanoflakes

Andreas Ney, Pagona Papakonstantinou, Ajay Kumar, Nai-Gui Shang, and Nianhua Peng

Appl. Phys. Lett. 99, 102504 (2011); http://dx.doi.org/10.1063/1.3628245 (3 pages) | Cited 7 times

Online Publication Date: 6 September 2011

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We have studied the magnetization of vertically aligned graphene nanoflakes irradiated with nitrogen ions of 100 KeV energy and doses in the range 1011–1017 ions/cm2. The non-irradiated graphene nanoflakes show a paramagnetic contribution, which is increased progressively by ion irradiation at low doses up to 1015/cm2. However, further increase on implantation dose reduces the magnetic moment which coincides with the onset of amorphization as verified by both Raman and x-ray photoelectron spectroscopic data. Overall, our results demonstrate the absence of ferromagnetism on either implanted or unimplanted samples from room temperature down to a temperature of 5 K.
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75.20.-g Diamagnetism, paramagnetism, and superparamagnetism
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
78.30.Na Fullerenes and related materials
75.30.Cr Saturation moments and magnetic susceptibilities
61.48.Gh Structure of graphene

Unidirectional anisotropy in the spin pumping voltage in yttrium iron garnet/platinum bilayers

L. H. Vilela-Leão, C. Salvador, A. Azevedo, and S. M. Rezende

Appl. Phys. Lett. 99, 102505 (2011); http://dx.doi.org/10.1063/1.3631683 (3 pages) | Cited 7 times

Online Publication Date: 6 September 2011

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Detailed measurements of the dc voltage generated in a thin Pt layer deposited on films of yttrium iron garnet (YIG) have been carried out to study the spin pumping effect produced by magnetostatic (MS) modes excited by a microwave field. In relatively thick YIG films the modes are far apart so that one can identify clearly the spin pumping voltage in VSP produced by each MS mode. We have discovered that when the sputter deposition of the thin Pt layer is made on the YIG film magnetized by a static magnetic field, VSP exhibits a strong unidirectional anisotropy.
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75.30.Gw Magnetic anisotropy
75.50.Gg Ferrimagnetics
75.70.Ak Magnetic properties of monolayers and thin films
81.15.Cd Deposition by sputtering
75.30.Ds Spin waves

Manipulation of magnetic coercivity of Fe film in Fe/BaTiO3 heterostructure by electric field

G. Venkataiah, Y. Shirahata, M. Itoh, and T. Taniyama

Appl. Phys. Lett. 99, 102506 (2011); http://dx.doi.org/10.1063/1.3628464 (3 pages) | Cited 11 times

Online Publication Date: 6 September 2011

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The manipulation of magnetism at Fe/BaTiO3 interfaces is demonstrated via lattice distortion induced by thermal and electrical means. We find that the magnetic coercivity shows similar electric field dependence for positive and negative electric fields in the tetragonal phase of BaTiO3, whereas those in the orthorhombic and rhombohedral phases vary asymmetrically with respect to the polarity. The temperature dependent magnetization also reveals that the effect has its origin in the strong magnetoelastic coupling at the interface. The underlying mechanisms of the electric field dependence of the coercivity are discussed, associated with the different ferroelectric poling processes of BaTiO3.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Gw Magnetic anisotropy
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Bb Fe and its alloys
77.80.-e Ferroelectricity and antiferroelectricity
75.80.+q Magnetomechanical effects, magnetostriction

Spin-torque-driven ballistic precessional switching with 50 ps impulses

O. J. Lee, D. C. Ralph, and R. A. Buhrman

Appl. Phys. Lett. 99, 102507 (2011); http://dx.doi.org/10.1063/1.3635782 (3 pages) | Cited 8 times

Online Publication Date: 7 September 2011

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We demonstrate reliable spin-torque-driven ballistic precessional switching using 50 ps current impulses in a spin-valve device that includes both in-plane and out-of-plane spin polarizers. Different threshold currents as the function of switching direction and current polarity enable the final orientation of the magnetic free layer to be steered, in accord with a macrospin analysis, by the sign of the pulse, eliminating the need for read-before-write toggle operation. The pulse amplitude windows for this deterministic operation are wider and more symmetric as a function of current polarity for shorter impulses, while inhomogeneous fringe fields from the polarizers lead to asymmetries as a function of current direction.
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85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.

Unusual high-temperature ferromagnetism of PbPd0.81Co0.19O2 nanograin film

H. L. Su, S. Y. Huang, Y. F. Chiang, J. C. A. Huang, C. C. Kuo, Y. W. Du, Y. C. Wu, and R. Z. Zuo

Appl. Phys. Lett. 99, 102508 (2011); http://dx.doi.org/10.1063/1.3634070 (3 pages) | Cited 2 times

Online Publication Date: 8 September 2011

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Single-phase PbPd0.81Co0.19O2 film with a body-centered orthorhombic structure was prepared using the sol-gel spin-coating technique and an oxidation treatment. Film resistivity has a power dependence on temperature. The insulator-metal transition temperature was 358 K, markedly higher than the reported values of similar material systems. Ferromagnetism and superparamagnetism coexisted in the film and the ferromagnetism persisted up to 380 K. As temperature increased, the notable increasing tendencies were found for the film’s saturation magnetization and for the magnetic field where saturation magnetization decreases abruptly. The special spin gapless band structure and the film’s nanograin microstructure are likely responsible for these interesting properties.
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75.70.Ak Magnetic properties of monolayers and thin films
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.75.Cd Fabrication of magnetic nanostructures
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Kj Amorphous and quasicrystalline magnetic materials
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)

Ho substitution suppresses collinear Dy spin order and enhances polarization in DyMnO3

N. Zhang, Y. Y. Guo, L. Lin, S. Dong, Z. B. Yan, X. G. Li, and J.-M. Liu

Appl. Phys. Lett. 99, 102509 (2011); http://dx.doi.org/10.1063/1.3636399 (3 pages) | Cited 9 times

Online Publication Date: 9 September 2011

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The multiferroic behaviors of Dy1−xHoxMnO3 are investigated in order to reveal the effects of Ho-substitution on the independent collinear Dy spin order and ferroelectric polarization at low temperature. It is demonstrated that a partial Ho-substitution of Dy ions significantly suppresses the independent Dy spin order at low temperature on one hand, and maintains the R-Mn spin coupling on the other hand, thus leading to a remarkable enhancement of the polarization at low temperature.
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77.22.Ej Polarization and depolarization
75.85.+t Magnetoelectric effects, multiferroics
75.80.+q Magnetomechanical effects, magnetostriction
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Electromagnetic properties of sapphire, ruby, and irradiated ruby at frequencies of 30–40 GHz

Jerzy Krupka, Takeshi Shimada, Itaru Ueda, and Waldemar Karcz

Appl. Phys. Lett. 99, 102901 (2011); http://dx.doi.org/10.1063/1.3635787 (3 pages)

Online Publication Date: 6 September 2011

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Permittivity and electromagnetic losses of sapphire, ruby, and irradiated ruby samples were examined at frequencies of 30–40 GHz and temperatures of 4–300 K employing the whispering gallery mode technique. At room temperature, the dielectric properties of the sapphire, ruby, and irradiated ruby samples were similar. Significant differences in electromagnetic losses took place at cryogenic temperatures. Losses were related to the imaginary part of the susceptibility associated with the presence of paramagnetic Cr3+ ions as well as those introduced by structural defects caused by the irradiation of ruby samples. The presence of impurities and the structural defects result in the dielectric losses being larger than those of pure sapphire.
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77.22.Ch Permittivity (dielectric function)
77.22.Gm Dielectric loss and relaxation
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
61.72.-y Defects and impurities in crystals; microstructure
75.20.Ck Nonmetals
75.30.Cr Saturation moments and magnetic susceptibilities

Origin of 90° domain wall pinning in Pb(Zr0.2Ti0.8)O3 heteroepitaxial thin films

Dong Su, Qingping Meng, C. A. F. Vaz, Myung-Geun Han, Yaron Segal, Fred J. Walker, Monica Sawicki, Christine Broadbridge, and Charles H. Ahn

Appl. Phys. Lett. 99, 102902 (2011); http://dx.doi.org/10.1063/1.3634028 (3 pages) | Cited 4 times

Online Publication Date: 8 September 2011

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We describe a transmission-electron-microscopy study of the ferroelectric domains in an epitaxial Pb(Zr0.2Ti0.8)O3 (PZT) film grown on La0.7Sr0.3MnO3/SrTiO3(001). We directly observe the pinning of 90° domain walls by pairs of misfit dislocations, respectively, with Burgers vectors a [100] and a [001]. Model calculations based on the elastic theory confirm our finding that, in addition to the depolarization field surrounding the dislocation, the strain field of misfit dislocation-pairs plays the primary role in the formation and pinning of a domains.
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77.80.Dj Domain structure; hysteresis
77.55.fg Pb(Zr,Ti)O3-based films
61.72.Lk Linear defects: dislocations, disclinations
77.22.Ej Polarization and depolarization
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity

Ferroelectricity in hafnium oxide thin films

T. S. Böscke, J. Müller, D. Bräuhaus, U. Schröder, and U. Böttger

Appl. Phys. Lett. 99, 102903 (2011); http://dx.doi.org/10.1063/1.3634052 (3 pages) | Cited 12 times

Online Publication Date: 8 September 2011

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We report that crystalline phases with ferroelectric behavior can be formed in thin films of SiO2 doped hafnium oxide. Films with a thickness of 10 nm and with less than 4 mol. % of SiO2 crystallize in a monoclinic/tetragonal phase mixture. We observed that the formation of the monoclinic phase is inhibited if crystallization occurs under mechanical encapsulation and an orthorhombic phase is obtained. This phase shows a distinct piezoelectric response, while polarization measurements exhibit a remanent polarization above 10 μC/cm2 at a coercive field of 1 MV/cm, suggesting that this phase is ferroelectric. Ferroelectric hafnium oxide is ideally suited for ferroelectric field effect transistors and capacitors due to its excellent compatibility to silicon technology.
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77.80.bg Compositional effects
61.66.Fn Inorganic compounds
77.55.hn Other piezoelectric or electrostrictive films
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.65.-j Piezoelectricity and electromechanical effects
77.22.Ej Polarization and depolarization
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Threshold conductivity switching in sulfurized antimony selenide nanowires

C. Karthik, Rutvik J. Mehta, Wei Jiang, Eduardo Castillo, Theodorian Borca-Tasciuc, and Ganpati Ramanath

Appl. Phys. Lett. 99, 103101 (2011); http://dx.doi.org/10.1063/1.3624465 (3 pages) | Cited 1 time

Online Publication Date: 6 September 2011

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We report reversible switching between Ohmic and negative differential resistance states at a threshold voltage in sub-100-nm diameter sulfurized antimony selenide nanowires. We show that threshold switching in our nanowires arises due to high non-equilibrium free carrier concentrations resulting from impact ionization of carriers from defect states traceable to sulfurization and surface dangling bonds. Threshold switching is suppressed because of inhibited carrier generation at air-passivated defect states or at high temperatures due to thermally induced carrier depletion from deep states which preempts impact ionization. Such non-linear phenomena would be important for designing phase-change memories, thermoelectric devices, and sensors using pnictogen chalcogenide nanowires.
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73.21.Hb Quantum wires
73.50.-h Electronic transport phenomena in thin films
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Switching magnetoresistance in vertically interfaced Pr0.5Ca0.5MnO3 grown on ZnO nanowires

R. V. K. Mangalam, Z. Zhang, T. Wu, and W. Prellier

Appl. Phys. Lett. 99, 103102 (2011); http://dx.doi.org/10.1063/1.3627182 (3 pages) | Cited 1 time

Online Publication Date: 8 September 2011

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The synthesis, morphology, and magneto-transport properties of nanostructure-engineered charge-ordered Pr0.5Ca0.5MnO3 grown on ZnO nanowires are reported. The stability of the charge-ordering can be tuned, but more interestingly, the sign of the magnetoresistance is inverted at low temperatures. Coexistence of ferromagnetic clusters on the surface and antiferromagnetic phase in the core of the grains were considered in order to understand these features. This work suggests that such a process of growing on nanowires (NW) network can be readily extended to other transition metal oxides and open doors towards tailoring their functionalities.
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81.07.Gf Nanowires
75.75.-c Magnetic properties of nanostructures
72.20.My Galvanomagnetic and other magnetotransport effects
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Ee Antiferromagnetics
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)

Luminescence blue-shift of CdSe nanowires beyond the quantum confinement regime

Yuan Yan, Zhi-Min Liao, Ya-Qing Bie, Han-Chun Wu, Yang-Bo Zhou, Xue-Wen Fu, and Da-Peng Yu

Appl. Phys. Lett. 99, 103103 (2011); http://dx.doi.org/10.1063/1.3634047 (3 pages) | Cited 3 times

Online Publication Date: 8 September 2011

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Photoluminescence (PL) properties of individual CdSe nanowires with diameters beyond the quantum confinement regime have been studied. A blue-shift in the PL spectra was observed with decreasing nanowire diameter. We attribute the blue-shift to band-filling effect. Carrier density induced by surface vacancy doping and laser excitation is found to be high enough to meet the criterion of the band-filling effect and increases with decreasing nanowire diameter. Temperature dependent PL analysis and characterizations of a single CdSe nanowire based field-effect transistor were also performed.
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85.30.Tv Field effect devices
78.55.Et II-VI semiconductors
81.07.Gf Nanowires
81.05.Dz II-VI semiconductors
78.67.Uh Nanowires
61.72.uj III-V and II-VI semiconductors
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Influence of carrier density on the electronic cooling channels of bilayer graphene

T. Limmer, A. J. Houtepen, A. Niggebaum, R. Tautz, and E. Da Como

Appl. Phys. Lett. 99, 103104 (2011); http://dx.doi.org/10.1063/1.3633099 (3 pages) | Cited 2 times

Online Publication Date: 9 September 2011

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We study the electronic cooling dynamics in a single flake of bilayer graphene by femtosecond transient absorption probing the photon-energy range 0.25–1.3 eV. From the transients, we extract the carrier cooling curves for different initial temperatures and densities of the photoexcited electrons and holes. Two regimes of carrier cooling, dominated by optical and acoustic phonons emission, are clearly identified. For increasing carrier density, the crossover between the two regimes occurs at larger carrier temperatures, since cooling via optical phonons experiences a bottleneck. Acoustic phonons, which are less sensitive to saturation, show an increasing contribution at high density.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
62.65.+k Acoustical properties of solids
78.20.Pa Photoacoustic effects

Remote actuation of a mechanical resonator

D. Hatanaka, I. Mahboob, and H. Yamaguchi

Appl. Phys. Lett. 99, 103105 (2011); http://dx.doi.org/10.1063/1.3635781 (3 pages)

Online Publication Date: 9 September 2011

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A micron-sized mechanical resonator is placed inside a coil operating at radio frequencies (RFs). The mechanical response is highly sensitive to continuous RF irradiation with the observation of both resonance frequency and quality factor shifts. If the RF amplitude is modulated at the natural mechanical frequency, it can drive the mechanical oscillator to resonance with an actuation efficiency of 4.0 μm/V. This remote actuation method is a promising technique to access performance and functionality beyond conventional actuation schemes.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
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Realization of size controllable graphene micro/nanogap with a micro/nanowire mask method for organic field-effect transistors

Zhiyu Liao, Qing Wan, Huixuan Liu, and Qingxin Tang

Appl. Phys. Lett. 99, 103301 (2011); http://dx.doi.org/10.1063/1.3634065 (3 pages) | Cited 1 time

Online Publication Date: 6 September 2011

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A size controllable graphene micro/nanogap fabrication method using micro/nanowire as mask is presented. The gap dimension can be adjusted by the diameter of the mask wire. As a typical application, copper phthalocyanine (CuPc) film organic field-effect transistors (OFETs) were fabricated with the graphene micro/nanogap bottom electrodes. The ultrathin thickness of the graphene, combined with its good compatibility with organic semiconductors, and high electrical conductivity produced high-performance CuPc film device with mobility at 0.053 cm2/Vs and on/off ratio at 105, showing promising potential in low-cost and light-weight electronics.
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85.30.Tv Field effect devices
81.07.Gf Nanowires
81.05.ue Graphene
72.80.Vp Electronic transport in graphene
81.16.-c Methods of micro- and nanofabrication and processing
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Influence of the semiconductor oxidation potential on the operational stability of organic field-effect transistors

A. Sharma, S. G. J. Mathijssen, P. A. Bobbert, and D. M. de Leeuw

Appl. Phys. Lett. 99, 103302 (2011); http://dx.doi.org/10.1063/1.3634066 (3 pages) | Cited 5 times

Online Publication Date: 7 September 2011

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During prolonged application of a gate bias, organic field-effect transistors show a gradual shift of the threshold voltage towards the applied gate bias voltage. The shift follows a stretched-exponential time dependence governed by a relaxation time. Here, we show that a thermodynamic analysis reproduces the observed exponential dependence of the relaxation time on the oxidation potential of the semiconductor. The good fit with the experimental data validates the underlying assumptions. It demonstrates that this operational instability is a straightforward thermodynamically driven process that can only be eliminated by eliminating water from the transistor.
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85.30.Tv Field effect devices
FREE

Metallic Bragg-gratings for light management in organic light-emitting devices

Julian Hauss, Tobias Bocksrocker, Boris Riedel, Ulf Geyer, Uli Lemmer, and Martina Gerken

Appl. Phys. Lett. 99, 103303 (2011); http://dx.doi.org/10.1063/1.3631728 (3 pages) | Cited 4 times

Online Publication Date: 7 September 2011

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We used laser interference lithography to fabricate organic light-emitting diodes (OLEDs) with gold Bragg gratings on top of the indium-tin-oxide layer for efficient light management. We built polymer OLEDs with 15 and 30 nm gold grating thickness as well as reference devices with continuous gold layers and without gold layer. Electrical and optical device characterization was performed and compared to optical simulations. The gratings extract waveguide and substrate modes and change the angular and spectral emission characteristics by cavity effects. The combination of light extraction and cavity effects leads to 25–30% enhancement in power efficiency compared to reference devices.
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85.60.Jb Light-emitting devices
42.62.-b Laser applications
42.79.Dj Gratings
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Tandem white phosphorescent organic light-emitting diodes based on interface-modified C60/pentacene organic heterojunction as charge generation layer

Yonghua Chen, Jiangshan Chen, Dongge Ma, Donghang Yan, and Lixiang Wang

Appl. Phys. Lett. 99, 103304 (2011); http://dx.doi.org/10.1063/1.3628317 (3 pages) | Cited 9 times

Online Publication Date: 8 September 2011

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The significant improvement of power efficiency and efficiency roll-off in tandem white organic light-emitting diodes (WOLEDs) was achieved using interface-modified C60/pentacene organic heterojunction as charge generation layer (CGL). The resulting tandem WOLED exhibits a maximum power efficiency of 53.8 lm W−1 and a maximum current efficiency of 101.5 cd A−1 without any out-coupling techniques. More importantly, the efficiency roll-off is greatly improved, yet remaining power efficiency of 53 lm W−1 at 100 cd m−2 and 45 lm W−1 at 1000 cd m−2, which outperforms that of the conventional CGL-based tandem WOLED. This clearly demonstrates that the organic heterojunction is a promising CGL for high-performance tandem WOLEDs.
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85.60.Jb Light-emitting devices
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