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21 May 2007

Volume 90, Issue 21, Articles (21xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 90, 213501 (2007); http://dx.doi.org/10.1063/1.2742294 (3 pages)

Jong-Hyun Ahn, Hoon-Sik Kim, Etienne Menard, Keon Jae Lee, Zhengtao Zhu, Dae-Hyeong Kim, Ralph G. Nuzzo, John A. Rogers, Islamshah Amlani, Vadim Kushner, Shawn G. Thomas, and Terrisa Duenas
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Enhanced switching current density due to resonant precession in current-induced magnetization switching

Woojin Kim, Taek-Dong Lee, Jang-Eun Lee, Se-Chung Oh, Kyung-Ho Shin, Hong-Ju Suh, and Kyung-Jin Lee

Appl. Phys. Lett. 90, 212504 (2007); http://dx.doi.org/10.1063/1.2742282 (3 pages) | Cited 3 times

Online Publication Date: 22 May 2007

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Micromagnetic investigation was performed on the influence of neighboring cells in a conventional rectangular array to the switching current density (Jc) in the current-induced magnetization switching. We found that the Jc was increased regardless of the relative switching direction to the overall stray field. It is because the precession of the magnetization in the switching cell induces a resonant precession in neighboring cells through magnetostatic interaction. The resonant precession disturbs a fast reversal and results in the enhanced Jc for a fixed pulse width. The enhancement increases as the distance between the cells decreases. Here we proposed a zigzag array which significantly suppresses the enhancement of Jc due to the resonant precession.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Gw Magnetic anisotropy
84.30.Sk Pulse and digital circuits
85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)

Self-homodyne rf demodulator using a ferromagnetic nanowire

A. Yamaguchi, H. Miyajima, S. Kasai, and T. Ono

Appl. Phys. Lett. 90, 212505 (2007); http://dx.doi.org/10.1063/1.2742588 (3 pages) | Cited 7 times

Online Publication Date: 22 May 2007

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The radio frequency (rf) demodulator using a spin-torque diode effect in a single ferromagnetic nanowire is proposed. This is the first self-homodyne rf demodulator with ferromagnetic nanowire, and it can demodulate the amplitude modulation rf signal whose baseband frequency corresponds to the ferromagnetic resonance frequency of the wire.
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84.30.Qi Modulators and demodulators; discriminators, comparators, mixers, limiters, and compressors
84.40.Ua Telecommunications: signal transmission and processing; communication satellites

Controlling the magnetic anisotropy of CoPt/AlN multilayer films

Y. Hodumi, J. Shi, and Y. Nakamura

Appl. Phys. Lett. 90, 212506 (2007); http://dx.doi.org/10.1063/1.2742793 (3 pages) | Cited 6 times

Online Publication Date: 23 May 2007

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The magnetic anisotropy of sputter-deposited CoPt/AlN multilayer films has been studied in contrast with the multilayer films consisting of magnetic and noble metals. It has been found that the as-deposited multilayer film shows enhanced in-plane magnetic anisotropy compared with CoPt alloy film of equivalent thickness. However, upon thermal annealing in a vacuum, the film undergoes a smooth transition of magnetic anisotropy from in-plane to perpendicular direction; moreover, the anisotropy field and coercivity can be controlled by controlling the annealing temperature. Structural characterization has revealed that the perpendicular magnetic anisotropy of the annealed CoPt/AlN multilayer films is mainly of magnetoelastic origin.
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75.30.Gw Magnetic anisotropy
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Effect of electrode composition on the tunnel magnetoresistance of pseudo-spin-valve magnetic tunnel junction with a MgO tunnel barrier

Y. M. Lee, J. Hayakawa, S. Ikeda, F. Matsukura, and H. Ohno

Appl. Phys. Lett. 90, 212507 (2007); http://dx.doi.org/10.1063/1.2742576 (3 pages) | Cited 102 times

Online Publication Date: 24 May 2007

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The authors investigate the effect of electrode composition on the tunnel magnetoresistance (TMR) ratio of (CoxFe100−x)80B20/MgO/(CoxFe100−x)80B20 pseudo-spin-valve magnetic tunnel junctions (MTJs). TMR ratio is found to strongly depend on the composition and thicknesses of CoFeB. High resolution transmission electron microscopy shows that the crystallization process of CoFeB during annealing depends on the composition and the thicknesses of the CoFeB film, resulting in different TMR ratios. A TMR ratio of 500% at room temperature and of 1010% at 5 K are observed in a MTJ having 4.3 nm and 4-nm-thick (Co25Fe75)80B20 electrodes with a 2.1-nm-thick MgO barrier annealed at 475 °C.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.-m Magnetotransport phenomena; materials for magnetotransport
81.40.Gh Other heat and thermomechanical treatments
64.70.K- Solid-solid transitions

Nanocrystalline soft magnetic ribbons with high relative strain at fracture

T. M. Heil, K. J. Wahl, A. C. Lewis, J. D. Mattison, and M. A. Willard

Appl. Phys. Lett. 90, 212508 (2007); http://dx.doi.org/10.1063/1.2742598 (3 pages) | Cited 6 times

Online Publication Date: 24 May 2007

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Melt-spun nanocrystalline/amorphous composite ribbons (e.g., Finemet and Nanoperm) have excellent soft magnetic properties but are notoriously brittle after crystallization. In this letter, the authors present their findings of increased relative strain at fracture in Co-rich nanocomposite soft magnetic ribbons compared to conventional nanocomposite ribbons. Bending tests of the Co-rich ribbons show relative strain at fracture values more than twice as high as a Nanoperm ribbon. These Co-rich alloys have excellent soft magnetic properties with potential for high temperature applications. A (Co0.90Fe0.08Ni0.02)88Zr7B4Cu1 ribbon annealed at 823 K for 3600 s has a high saturation magnetization of 131 A m2/kg and a low coercivity of 8 A/m.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Lm Deformation, plasticity, and creep
68.35.Gy Mechanical properties; surface strains
62.20.M- Structural failure of materials
62.20.F- Deformation and plasticity
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

All MgB2 tunnel junctions with Al2O3 or MgO tunnel barriers

Heejae Shim, K. S. Yoon, J. S. Moodera, and J. P. Hong

Appl. Phys. Lett. 90, 212509 (2007); http://dx.doi.org/10.1063/1.2742909 (3 pages) | Cited 5 times

Online Publication Date: 24 May 2007

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All MgB2 thin film tunnel junctions with Al2O3 or MgO tunnel barriers were fabricated in situ on Si substrates in a molecular beam epitaxy system and their tunneling characteristics were investigated. In the quasiparticle tunneling spectra of the junction with Al2O3 tunnel barrier, we observed both superconducting gaps of MgB2, while only a small gap was seen with MgO tunnel barrier. Using a microscopic structural analysis, we found that the difference in the spectra is due to the crystal orientation difference of the MgB2 films: the film grown on Al2O3 was polycrystalline whereas the film grown on MgO was c-axis oriented.
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74.50.+r Tunneling phenomena; Josephson effects
74.78.-w Superconducting films and low-dimensional structures
74.25.Jb Electronic structure (photoemission, etc.)

Ab initio study of magnetic interaction of Fe doped ZnO with intrinsic vacancies

A. Debernardi and M. Fanciulli

Appl. Phys. Lett. 90, 212510 (2007); http://dx.doi.org/10.1063/1.2742597 (3 pages) | Cited 11 times

Online Publication Date: 25 May 2007

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The authors report on the calculation, by density functional theory, of the total energy and of the magnetic properties of wurtzite ZnO doped with diluted Fe impurities in the presence of vacancies. They found that the complex FeZn–VZn—where the Zn vacancy (VZn) occupies the next nearest neighbor position to Fe atom—is energetically favored compared with the isolated FeZn and VZn. An opposite situation was obtained for FeZn and VO. For Zn-poor growth condition, the room temperature population of the FeZn–VZn complex provides a key to understand the magnetism of this system.
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71.15.Nc Total energy and cohesive energy calculations
61.72.J- Point defects and defect clusters
75.30.Et Exchange and superexchange interactions
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
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Ferroelectric and paraelectric Ba0.5Sr0.5TiO3 film structure distortions at room temperature and their effects on tunable microwave properties

L. M. B. Alldredge, Wontae Chang, Syed B. Qadri, Steven W. Kirchoefer, and Jeffrey M. Pond

Appl. Phys. Lett. 90, 212901 (2007); http://dx.doi.org/10.1063/1.2741610 (3 pages) | Cited 11 times

Online Publication Date: 21 May 2007

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Sputter-deposited Ba0.5Sr0.5TiO3 films on (001) MgO were characterized for their dielectric properties with different lattice structures. With varying Ar:O2 ratios during deposition, the films showed either in-plane (c<a) or out-of-plane (c>a) tetragonal distortions, significantly affecting the dielectric constant and tunability. The dielectric constant exhibited clear hysteresis with dc bias at room temperature, indicating that the films were ferroelectric. The relationship between the dielectric properties and the distortions was the reverse of that observed in films deposited by pulsed laser deposition. The anisotropic in-plane dielectric behavior can be understood by relating polarization to film distortions and to the presence of permanent dipoles.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
77.80.Dj Domain structure; hysteresis
77.22.Ch Permittivity (dielectric function)
77.22.Ej Polarization and depolarization
68.55.-a Thin film structure and morphology

Improved ferroelectric and leakage properties in symmetric BiFeO3/SrTiO3 superlattice

Somenath Bose and S. B. Krupanidhi

Appl. Phys. Lett. 90, 212902 (2007); http://dx.doi.org/10.1063/1.2742322 (3 pages) | Cited 13 times

Online Publication Date: 22 May 2007

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Symmetric BiFeO3/SrTiO3 superlattices were fabricated by pulsed laser deposition on SrTiO3 (100) substrates. Frequency independent and near saturated P-E hysteresis was observed in the case of a superlattice (periodicity of ∼ 11 nm) as compared to leaky hysteresis observed in epitaxial BiFeO3. Room temperature leakage current density of the superlattice was almost two orders of magnitude lower than that of BiFeO3. Observed leakage current behavior in case of both BiFeO3 and superlattice indicates the dominance of space charge limited conduction. Improvement in ferroelectric property was discussed in connection to enhanced epitaxial strain, reduced leakage current, and electrostatic interaction between BiFeO3 and SrTiO3.
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77.55.-g Dielectric thin films
77.80.Dj Domain structure; hysteresis
77.22.Jp Dielectric breakdown and space-charge effects
68.65.Cd Superlattices
73.63.-b Electronic transport in nanoscale materials and structures

Complete set of elastic, dielectric, and piezoelectric constants of orthorhombic 0.71Pb(Mg1/3Nb2/3)O3–0.29PbTiO3 single crystal

Feifei Wang, Laihui Luo, Dan Zhou, Xiangyong Zhao, and Haosu Luo

Appl. Phys. Lett. 90, 212903 (2007); http://dx.doi.org/10.1063/1.2743393 (3 pages) | Cited 26 times

Online Publication Date: 23 May 2007

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In order to better utilize the superior transverse piezoelectric properties of [011] poled (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 single crystals with the composition near the morphotropic phase boundary, a complete set of elastic, dielectric, and piezoelectric constants of [011] poled 0.71Pb(Mg1/3Nb2/3)O3–0.29PbTiO3 single crystal was measured by ultrasonic and resonance methods at room temperature. The electromechanical coupling coefficient k32 and transverse piezoelectric constant d32 can reach 0.94 and −1883 pC/N, respectively. This complete set of material properties will provide convenience for device designs and fundamental theoretical studies.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.65.Bn Piezoelectric and electrostrictive constants
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
77.22.Ch Permittivity (dielectric function)

Partial decoupling between strain and polarization in mono-oriented Pb(Zr0.2Ti0.8)O3 thin film

Pierre-Eymeric Janolin, Bernard Fraisse, Françoise Le Marrec, and Brahim Dkhil

Appl. Phys. Lett. 90, 212904 (2007); http://dx.doi.org/10.1063/1.2742313 (3 pages) | Cited 8 times

Online Publication Date: 24 May 2007

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The structural evolution of epitaxial mono-oriented (i.e., with the c axis perpendicular to the interface) ferroelectric Pb(Zr0.2Ti0.8)O3 thin film has been investigated, using high-resolution, temperature dependent, x-ray diffraction. The full set of lattice parameters was obtained; it allowed to estimate the variation of the polarization as a function of temperature, underlying the difference between the polarization-induced tetragonality and the elastic one. The temperature evolution of the misfit strain has been calculated and found to be in good agreement with the theoretical temperature-misfit strain phase diagram.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
77.80.-e Ferroelectricity and antiferroelectricity
77.22.Ej Polarization and depolarization
81.40.Jj Elasticity and anelasticity, stress-strain relations
68.60.Bs Mechanical and acoustical properties

Quantitative determination of tip parameters in piezoresponse force microscopy

Sergei V. Kalinin, Stephen Jesse, Brian J. Rodriguez, Eugene A. Eliseev, Venkatraman Gopalan, and Anna N. Morozovska

Appl. Phys. Lett. 90, 212905 (2007); http://dx.doi.org/10.1063/1.2742900 (3 pages) | Cited 16 times

Online Publication Date: 24 May 2007

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One of the key limiting factors in the quantitative interpretation of piezoresponse force microscopy (PFM) is the lack of knowledge on the effective tip geometry. Here the authors derive analytical expressions for a 180° domain wall profile in PFM for the point charge, sphere plane, and disk electrode models of the tip. An approach for the determination of the effective tip parameters from the wall profile is suggested and illustrated for several ferroelectric materials. The calculated tip parameters can be used self-consistently for the interpretation of PFM resolution and spectroscopy data, i.e., linear imaging processes.
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77.80.Dj Domain structure; hysteresis

Strong charge trapping and bistable electrowetting on nanocomposite fluoropolymer:BaTiO3 dielectrics

M. K. Kilaru, J. Heikenfeld, G. Lin, and J. E. Mark

Appl. Phys. Lett. 90, 212906 (2007); http://dx.doi.org/10.1063/1.2743388 (3 pages) | Cited 3 times

Online Publication Date: 24 May 2007

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Strong charge trapping and bistable electrowetting on nanocomposite fluoropolymer:BaTiO3 dielectrics are reported. Thin nanocomposite dielectrics were spin casted from BaTiO3 nanopowder dispersed in a fluoropolymer/fluorosolvent solution. Electrowetting contact angle versus voltage and capacitance measurements confirm a severalfold increase in film dielectric constant with increasing BaTiO3 content. Bistable electrowetting was observed as droplets retained a decreased contact angle at 0 V but would dewet the surface by briefly applying a reverse polarity voltage. Strong charge trapping in the nanocomposite was confirmed by charge-voltage hysteresis. These results could prove important for low-power applications such as bistable displays and electronic paper.
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77.84.Lf Composite materials
68.08.Bc Wetting
68.03.Cd Surface tension and related phenomena
77.22.Ch Permittivity (dielectric function)

Optical measurement of the converse piezoelectric d33 coefficients of bulk and microtubular zinc oxide crystals

John Y. Fu, Paris Y. Liu, Jiping Cheng, Amar S. Bhalla, and Ruyan Guo

Appl. Phys. Lett. 90, 212907 (2007); http://dx.doi.org/10.1063/1.2742587 (3 pages) | Cited 3 times

Online Publication Date: 25 May 2007

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Zinc oxide (ZnO) crystal is a semiconductor material; the direct measurement and the interpretation of its piezoelectric coefficients are challenging. In this letter, the authors investigate the piezoelectric properties (d33 coefficient) of a bulk ZnO sample by measuring its converse piezoelectric effect using an optical method. The experimental data agree with recent theoretical calculations. The piezoelectric properties of a ZnO hexagonal microtube, which has a highly symmetrical hexagonal tubular structure, are also studied. The results demonstrate that the microtubular ZnO has significantly different piezoelectric properties from that of ZnO bulk crystal, especially under the applied electric fields of low frequencies.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.65.Bn Piezoelectric and electrostrictive constants

Effect of annealing on dielectric behavior and electrical conduction of W6+ doped Bi3TiNbO9 ceramics

Zhiyong Zhou, Yuchen Li, Lihui Yang, Xianlin Dong, and Haixue Yan

Appl. Phys. Lett. 90, 212908 (2007); http://dx.doi.org/10.1063/1.2740189 (3 pages) | Cited 1 time

Online Publication Date: 25 May 2007

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Bi3Ti1−xWxNbO9+δ (x = 0.15) (WBTN-15) ceramics were prepared using the solid state reaction method, and the behaviors of oxygen vacancies were confirmed and investigated by annealing the ceramic samples in O2 and Ar gas atmospheres. Dielectric measurements showed that oxygen vacancies had great contributions to the dielectric loss and constant of Ar-annealed sample, compared with those of as-sintered and O2-annealed samples. dc conductivity measurements indicate that oxygen vacancies played an important role mainly at lower temperature. The WBTN-15 ceramics exhibit intrinsic conduction mechanism at high temperature range and electronic and oxygen-vacancy ionic mixed conduction mechanism at low temperature range.
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77.22.Gm Dielectric loss and relaxation
81.40.Gh Other heat and thermomechanical treatments
77.22.Ch Permittivity (dielectric function)
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
72.60.+g Mixed conductivity and conductivity transitions
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Transmission through randomly arranged microcells of subwavelength holes on an aluminum film

Yi-Tsung Chang, Tzu-Hung Chuang, Chieh-Hung Yang, Ming-Wei Tsai, and Si-Chen Lee

Appl. Phys. Lett. 90, 213101 (2007); http://dx.doi.org/10.1063/1.2740175 (3 pages) | Cited 3 times

Online Publication Date: 21 May 2007

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This investigation presents an observation of enhanced optical transmission through an Al film that is perforated with microcells that are arranged in random structures. The dispersion relations of the Al/p-Si surface plasmon polariton in these structures with individual microcells with 3×3, 6×6, 9×9, 12×12, and 16×16 hole arrays of hexagonal were deduced. The transmission peak wavelength is determined from the spatial period of the microcell arrays. The random structure provides multicolor light transmission, which can be exploited in infrared wavelength-selective devices.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.66.Bz Metals and metallic alloys
61.66.Bi Elemental solids
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)

Magnetism in dodecanethiol-capped gold nanoparticles: Role of size and capping agent

P. Dutta, S. Pal, M. S. Seehra, M. Anand, and C. B. Roberts

Appl. Phys. Lett. 90, 213102 (2007); http://dx.doi.org/10.1063/1.2740577 (3 pages) | Cited 24 times

Online Publication Date: 21 May 2007

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In gold nanoparticles (Au NPs) capped with dodecanethiol (DT), the authors report the observation of superparamagnetic blocking temperature TB ≃ 50 K in D ≃ 5 nm NPs but only diamagnetism in 12 nm NPs. For T<TB = 50 K, the strong temperature dependence of coercivity Hc, saturation magnetization Ms, and exchange bias He (in the field-cooled sample) confirm the blocked state resembling ferromagnetism with Hc ≃ 250 Oe, He ≃ −40 Oe, and Ms ≃ 10−2 emu/g at 5 K. The observed electron magnetic resonance line shows expected shift, broadening, and reduced intensity below TB. A magnetic moment μ ≃ 0.006μB per Au atom attached to DT is determined using a model which yields Ms varying as 1/D, with its source being holes in the 5d band of Au produced by charge transfer from Au to S atoms in DT.
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75.50.Tt Fine-particle systems; nanocrystalline materials
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
75.20.En Metals and alloys
75.50.Cc Other ferromagnetic metals and alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions

Antiadhesion considerations for UV nanoimprint lithography

F. A. Houle, C. T. Rettner, D. C. Miller, and R. Sooriyakumaran

Appl. Phys. Lett. 90, 213103 (2007); http://dx.doi.org/10.1063/1.2740578 (3 pages) | Cited 18 times

Online Publication Date: 21 May 2007

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Low surface energy fluorosilane layers are widely used as release coatings for quartz templates in UV nanoimprint lithography, yet they are generally found to degrade with use. It is found that these layers are chemically attacked when used with UV cured methacrylate and vinyl ether resists, as found previously for acrylate resists, leading to the conclusion that low reactivity and not low surface energy is of importance for effective release layers. It is shown that an ion-beam deposited diamondlike carbon release coating is a useful alternative, having both stability in a reactive environment and lower adhesion despite its higher surface energy.
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81.16.Nd Micro- and nanolithography
81.15.Jj Ion and electron beam-assisted deposition; ion plating
68.35.Md Surface thermodynamics, surface energies

Multiphonon Raman scattering in GaN nanowires

S. Dhara, Sharat Chandra, G. Mangamma, S. Kalavathi, P. Shankar, K. G. M. Nair, A. K. Tyagi, C. W. Hsu, C. C. Kuo, L. C. Chen, K. H. Chen, and K. K. Sriram

Appl. Phys. Lett. 90, 213104 (2007); http://dx.doi.org/10.1063/1.2741410 (3 pages) | Cited 12 times

Online Publication Date: 21 May 2007

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UV Raman scattering studies show longitudinal optical (LO) mode up to fourth order in wurtzite GaN nanowire system. Fröhlich interaction of electron with the long range electrostatic field of ionic bonded GaN gives rise to enhancement in LO phonon modes. Good crystalline quality, as indicated by the crystallographic as well as luminescence studies, is thought to be responsible for this significant observation. Calculated size dependence, incorporating size corrected dielectric constants, of electron-phonon interaction energy agrees well with measured values and also predict stronger interaction energy than that of the bulk for diameter below ∼ 3 nm.
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78.67.Lt Quantum wires
78.30.Fs III-V and II-VI semiconductors
78.55.Cr III-V semiconductors
68.65.La Quantum wires (patterned in quantum wells)
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
77.22.Ch Permittivity (dielectric function)

Suppression of InAs/GaAs quantum dot decomposition by the incorporation of a GaAsSb capping layer

J. M. Ulloa, I. W. D. Drouzas, P. M. Koenraad, D. J. Mowbray, M. J. Steer, H. Y. Liu, and M. Hopkinson

Appl. Phys. Lett. 90, 213105 (2007); http://dx.doi.org/10.1063/1.2741608 (3 pages) | Cited 29 times

Online Publication Date: 23 May 2007

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The influence of a GaAsSb capping layer on the structural properties of self-assembled InAs/GaAs quantum dots (QDs) is studied on the atomic scale by cross-sectional scanning tunneling microscopy. QDs capped with GaAs0.75Sb0.25 exhibit a full pyramidal shape and a height more than twice that of the typical GaAs-capped QDs, indicating that capping with GaAsSb suppresses dot decomposition. This behavior is most likely related to the reduced lattice mismatch between the dot and the capping layer.
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68.65.Hb Quantum dots (patterned in quantum wells)
64.75.-g Phase equilibria

Fabrication and enhanced magnetoresistance of SiO2-coated Fe3O4 nanosphere compact

Junfeng Wang, Jing Shi, Decheng Tian, Hong Deng, Yadong Li, Pengyun Song, and Chinping Chen

Appl. Phys. Lett. 90, 213106 (2007); http://dx.doi.org/10.1063/1.2741612 (3 pages) | Cited 14 times

Online Publication Date: 23 May 2007

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Magnetoresistance (MR) of a nanostructured material, monodisperse Fe3O4 nanospheres of about 200 nm coated with thin SiO2 and compactly cold pressed and sintered, has been investigated. A high MR, up to 10.8% at 1 T and 17% at 8 T, has been observed at 100 K. This enhanced MR is likely arising from the spin-polarized tunneling of conducting electrons through the insulating SiO2 boundaries. The decrease of the MR with the temperature increasing was attributed to the existence of the local spins in the grain boundaries. In addition, the MR ratio decreases with the SiO2 thickness increasing and sintering temperature decreasing. It suggests that the magnetic property of the insulating barrier is crucial for enhancing the low-field MR at a high temperature.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.47.Gk Colossal magnetoresistance
81.07.Bc Nanocrystalline materials
81.10.Fq Growth from melts; zone melting and refining
72.25.-b Spin polarized transport
73.40.Gk Tunneling

Electrodeposition of Pd nanoparticles on single-walled carbon nanotubes for flexible hydrogen sensors

Yugang Sun and H. Hau Wang

Appl. Phys. Lett. 90, 213107 (2007); http://dx.doi.org/10.1063/1.2742596 (3 pages) | Cited 35 times

Online Publication Date: 23 May 2007

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Single-walled carbon nanotubes on thin plastic substrates have been decorated with Pd nanoparticles through a simple electrochemical deposition process and have been used to fabricate high-performance hydrogen sensors with excellent mechanical flexibility. The typical flexible sensors exhibit sensitivity (i.e., the change of resistance) of ∼ 75% for 0.05% hydrogen in air and response time of ∼ 3 s for 1% hydrogen at room temperature. The performance of the as-fabricated devices keeps essentially unchanged even when they are bent to curving profile with bending radius of 7.5 mm and after they are operated for 2000 cycles of bending/relaxing.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
82.80.-d Chemical analysis and related physical methods of analysis
81.15.Pq Electrodeposition, electroplating
82.45.Qr Electrodeposition and electrodissolution

Field emission patterns from multiwall carbon nanotubes with a cone-shaped tip

Yahachi Saito, Yuhki Tsujimoto, Akira Koshio, and Fumio Kokai

Appl. Phys. Lett. 90, 213108 (2007); http://dx.doi.org/10.1063/1.2742637 (3 pages) | Cited 11 times

Online Publication Date: 23 May 2007

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Electron emission from multiwall carbon nanotubes (CNTs) with a cone-shaped tip, on the apex of which five pentagons are present, has been studied by field emission microscopy (FEM). Two types of FEM patterns were observed: one is a well-defined “pentagon” pattern that is typically observed for ordinal multiwall CNTs though the number of pentagons is five in the case of cone-shaped CNTs, and the other is a “dim” pattern that is usually observed for single-wall CNTs. Appearance voltages of the respective patterns and transmission electron microscopy study of the cone-shaped CNTs suggest that the pentagon patterns originate from CNTs with apex radii larger than approximately 2 nm while the dim patterns originate from those with the smaller apex radii.
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79.70.+q Field emission, ionization, evaporation, and desorption
61.46.Fg Nanotubes

Effects of dye adsorption on the electron transport properties in ZnO-nanowire dye-sensitized solar cells

Jih-Jen Wu, Guan-Ren Chen, Hung-Hsien Yang, Chen-Hao Ku, and Jr-Yuan Lai

Appl. Phys. Lett. 90, 213109 (2007); http://dx.doi.org/10.1063/1.2742639 (3 pages) | Cited 73 times

Online Publication Date: 23 May 2007

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Mercurochrome and N3 dyes are employed to be the sensitizers in the ZnO-nanowire (NW) dye-sensitized solar cells (DSSCs). A lower fill factor is obtained in the N3-sensitized cell which results in comparable efficiencies in both ZnO-NW DSSCs although the N3 molecules possess a wider absorptive range for light harvesting. Electrochemical impedance spectroscopy and open-circuit photovoltage decay measurements are employed to investigate the electron transport properties in both ZnO-NW DSSCs. The results indicate that more abundant electron interfacial recombination occurs in the N3-sensitized ZnO-NW DSSC due to the higher surface trap density in the ZnO-NW photoanode after N3 dye adsorption.
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73.63.Nm Quantum wires
68.43.Mn Adsorption kinetics
84.60.Jt Photoelectric conversion
82.80.Fk Electrochemical methods
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Local quantum dot tuning on photonic crystal chips

Andrei Faraon, Dirk Englund, Ilya Fushman, Jelena Vučković, Nick Stoltz, and Pierre Petroff

Appl. Phys. Lett. 90, 213110 (2007); http://dx.doi.org/10.1063/1.2742789 (3 pages) | Cited 36 times

Online Publication Date: 23 May 2007

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Quantum networks based on InAs quantum dots embedded in photonic crystal devices rely on quantum dots being in resonance with each other and with the cavities they are embedded in. The authors developed a technique based on temperature tuning to spectrally align different quantum dots located on the same chip. The technique allows for up to 1.8 nm reversible on-chip quantum dot tuning.
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81.07.Ta Quantum dots
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
42.70.Qs Photonic bandgap materials
78.67.Hc Quantum dots
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