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15 Jun 2012

Volume 111, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 111, 123510 (2012); http://dx.doi.org/10.1063/1.4729803 (4 pages)

Sz-Chin Steven Lin, Bernhard R. Tittmann, and Tony Jun Huang
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back to top Nanoscale Science and Design

The effect of particle vertical positioning on the absorption enhancement in plasmonic organic solar cells

Shu-Yi Wang, Diana-Andra Borca-Tasciuc, and Deborah A. Kaminski

J. Appl. Phys. 111, 124301 (2012); http://dx.doi.org/10.1063/1.4729293 (5 pages) | Cited 1 time

Online Publication Date: 18 June 2012

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The light absorption enhancement of an organic solar cell with plasmonic nanoparticles (NP) embedded in the active layer is studied employing 3D finite element simulation. The effect of the vertical positioning of the particle monolayer inside the active layer is elucidated. The results indicate that the highest enhancement is obtained when the particles lay at the bottom of the active layer, an organization less difficult to control accurately in practice. The paper also discusses the difference in the absorption enhancement obtained for two existing definitions currently used in the literature. The results show that models assessing absorption by taking both host and nanoparticles into consideration may overpredict the enhancement even when integration is carried out only over the wavelength interval where the host absorption dominates.
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88.40.jr Organic photovoltaics

Surface fingerprints of individual silicon nanocrystals in laser-annealed Si/SiO2 superlattice: Evidence of nanoeruptions of laser-pressurized silicon

Timur Nikitin, Marianna Kemell, Esa Puukilainen, Simona Boninelli, Fabio Iacona, Markku Räsänen, Mikko Ritala, Sergei Novikov, and Leonid Khriachtchev

J. Appl. Phys. 111, 124302 (2012); http://dx.doi.org/10.1063/1.4729303 (7 pages)

Online Publication Date: 18 June 2012

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Silicon nanocrystals prepared by continuous-wave laser annealing of a free-standing Si/SiO2 superlattice are studied for the first time by using methods of surface analysis (scanning electron microscopy and atomic force microscopy). The surface topology and composition are compared with transmission electron microscopy images that show a projection through the whole film, allowing us to discriminate silicon nanocrystals located near the film surface. These nanocrystals have an unusual pear-like shape with the thinner part sticking out of the laser-illuminated surface. The non-spherical shape of these nanocrystals is explained by eruption of silicon pressurized at the stage of crystallization from the melt phase. This hypothesis is supported by the micro-Raman spectra which show low stress near the surface features, in contrast to the neighbouring regions having high compressive stress.
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61.46.Hk Nanocrystals
61.72.Cc Kinetics of defect formation and annealing
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
78.30.Am Elemental semiconductors and insulators
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
68.35.bg Semiconductors

A fundamental numerical and theoretical study for the vibrational properties of nanowires

H. F. Zhan and Y. T. Gu

J. Appl. Phys. 111, 124303 (2012); http://dx.doi.org/10.1063/1.4729485 (9 pages) | Cited 3 times

Online Publication Date: 18 June 2012

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Based on the molecular dynamics (MD) simulation and the classical Euler-Bernoulli beam theory, a fundamental study of the vibrational performance of the Ag nanowire (NW) is carried out. A comprehensive analysis of the quality (Q)-factor, natural frequency, beat vibration, as well as high vibration mode is presented. Two excitation approaches, i.e., velocity excitation and displacement excitation, have been successfully implemented to achieve the vibration of NWs. Upon these two kinds of excitations, consistent results are obtained, i.e., the increase of the initial excitation amplitude will lead to a decrease to the Q-factor, and moderate plastic deformation could increase the first natural frequency. Meanwhile, the beat vibration driven by a single relatively large excitation or two uniform excitations in both two lateral directions is observed. It is concluded that the nonlinear changing trend of external energy magnitude does not necessarily mean a non-constant Q-factor. In particular, the first order natural frequency of the Ag NW is observed to decrease with the increase of temperature. Furthermore, comparing with the predictions by Euler-Bernoulli beam theory, the MD simulation provides a larger and smaller first vibration frequencies for the clamped-clamped and clamped-free thin Ag NWs, respectively. Additionally, for thin NWs, the first order natural frequency exhibits a parabolic relationship with the excitation magnitudes. The frequencies of the higher vibration modes tend to be low in comparison to Euler-Bernoulli beam theory predictions. A combined initial excitation is proposed which is capable to drive the NW under a multi-mode vibration and arrows the coexistence of all the following low vibration modes. This work sheds lights on the better understanding of the mechanical properties of NWs and benefits the increasing utilities of NWs in diverse nano-electronic devices.
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63.22.Gh Nanotubes and nanowires
62.23.Hj Nanowires
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)
81.40.Lm Deformation, plasticity, and creep
62.20.fq Plasticity and superplasticity

Superior thermal conductivity and extremely high mechanical strength in polyethylene chains from ab initio calculation

Jin-Wu Jiang, Junhua Zhao, Kun Zhou, and Timon Rabczuk

J. Appl. Phys. 111, 124304 (2012); http://dx.doi.org/10.1063/1.4729489 (4 pages) | Cited 4 times

Online Publication Date: 18 June 2012

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The upper limit of the thermal conductivity and the mechanical strength are predicted for the polyethylene chain, by performing the ab initio calculation and applying the quantum mechanical non-equilibrium Green’s function approach. Specially, there are two main findings from our calculation: (1) the thermal conductivity can reach a high value of 310 Wm−1 K−1 in a 100 nm polyethylene chain at room temperature and the thermal conductivity increases with the length of the chain; (2) the Young’s modulus in the polyethylene chain is as high as 374.5 GPa, and the polyethylene chain can sustain 32.85%±0.05% (ultimate) strain before undergoing structural phase transition into gaseous ethylene.
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66.70.Hk Glasses and polymers
61.41.+e Polymers, elastomers, and plastics
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli
81.40.Lm Deformation, plasticity, and creep
63.50.-x Vibrational states in disordered systems

Characterization of surface and nonlinear elasticity in wurtzite ZnO nanowires

J. Yvonnet, A. Mitrushchenkov, G. Chambaud, Q.-C. He, and S.-T. Gu

J. Appl. Phys. 111, 124305 (2012); http://dx.doi.org/10.1063/1.4729545 (6 pages) | Cited 1 time

Online Publication Date: 19 June 2012

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Surface elasticity and nonlinear effects are reported in ZnO nanowires and characterized by ab initio calculations. Fully anisotropic elastic and stress coefficients related to (10math0) surfaces are provided and used to construct a continuum model of nanowires based on the Gurtin-Murdoch surface elasticity theory, able to capture mechanical size effects. Nonlinear elasticity is observed through non-zero third order energy derivative terms with respect to axial strain in the direction of the nanowire. The associated material parameters are found to be themselves size-dependent.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli
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)
81.05.Dz II-VI semiconductors
68.35.Gy Mechanical properties; surface strains

Surface effect on electronic and optical properties of Bi2Ti2O7 nanowires for visible light photocatalysis

Q. Fu, T. He, J. L. Li, and G. W. Yang

J. Appl. Phys. 111, 124306 (2012); http://dx.doi.org/10.1063/1.4729553 (6 pages) | Cited 1 time

Online Publication Date: 19 June 2012

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We theoretically performed the structural, electronic, and optical properties of the pyrochlore Bi2Ti2O7 nanowires for photocatalytic applications using pseudopotential density-functional theory calculations. The groundwork of electronic structure calculations showed a possible band gap modification of the Bi2Ti2O7 nanowires compared to that of the bulk. The midband states induced by the oxygen atoms and bismuth atoms on the surface of nanowires leads to a shift in the valence band toward the conduction band, which enables reduction of the band gap. The calculated optical results indicated that the absorption edges shift of nanowires towards the red-light region. These theoretical results suggested that the pyrochlore Bi2Ti2O7 nanowires can be expected to be a promising candidate for photocatalytic applications such as solar-assisted water splitting reactions.
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73.22.-f Electronic structure of nanoscale materials and related systems
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.15.Dx Computational methodology (Brillouin zone sampling, iterative diagonalization, pseudopotential construction)
81.07.Gf Nanowires

High-temperature gold metallization for ZnO nanowire device on a SiC substrate

Ron Gurwitz, Guy Tuboul, Boaz Shikler, and Ilan Shalish

J. Appl. Phys. 111, 124307 (2012); http://dx.doi.org/10.1063/1.4729802 (4 pages)

Online Publication Date: 19 June 2012

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Gold is commonly used nowadays in metal contacts to nanowire devices. Due to their small size, nanowire devices often get heat up enough to cause a reaction of the contact and substrate, whether during operation or as a result of a spontaneous pulse of an electrostatic discharge. In most cases, the point of failure is the metallization, as is the case studied here. Gold is useful not only for its good electrical conductance but also because it is a good heat conductor and inert to the ambient. To improve the survivability of a gold metallization for nanowire devices incorporating ZnO nanowire atop a SiC substrate, we used a sputter-deposited Ti-Si-N ternary diffusion barrier layer and a Ti adhesion layer between the top gold layer and a 4H-SiC substrate that survives 30 min of vacuum annealing at 850 °C and 5 days of annealing at 500 °C in Ar. Rutherford backscattering spectrometry and x-ray photoelectron spectroscopy were used to test the integrity of the layers before and after annealing both with and without the diffusion barrier. Current-voltage characteristics were measured up to 75 V in air to test the metallization.
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85.40.Ls Metallization, contacts, interconnects; device isolation
85.30.-z Semiconductor devices

Proton nuclear magnetic resonance studies of hydrogen diffusion and electron tunneling in Ni-Nb-Zr-H glassy alloys

Haruo Niki, Hiroyuki Okuda, Morihito Oshiro, Mamoru Yogi, Ichiro Seki, and Mikio Fukuhara

J. Appl. Phys. 111, 124308 (2012); http://dx.doi.org/10.1063/1.4729544 (5 pages) | Cited 1 time

Online Publication Date: 20 June 2012

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Using the Fourier transform of the echo envelope, the proton line shapes, spin-lattice relaxation time, and spin-spin relaxation time have been measured in a (Ni0.36Nb0.24Zr0.40)90H10 glassy alloy at 1.83 T (∼78 MHz) and at temperatures between 1.8 and 300 K. First, the spectral line width decreases abruptly between 1.8 and 2.1 K. Next, it remains almost constant at 13 kHz up to ∼150 K. Finally, the line width decreases as the temperature increases from ∼150 to 300 K. The initial decrease in the spectral line width is ascribed to the distribution of the external field, which is caused by the penetration of vortices in the superconducting state. The subsequent leveling off in the spectral line width is ascribed to the dipole-dipole interaction between protons when hydrogen atoms are trapped into vacancies among the Zr-centered icosahedral Zr5Ni5Nb3 clusters. The final decrease in the spectral line width is ascribed to the motional narrowing of the width that is caused by the movement of hydrogen atoms. The temperature dependences of the spin-lattice and spin-spin relaxation time showed that at temperature above 150 K and the activation energy of 8.7 kJ/mol allowed the hydrogen atoms to migrate among the clusters. The distance between the hydrogen atoms is estimated to be 2.75 Å. Hydrogen occupancies among clusters in the (Ni0.36Nb0.24Zr0.40)90H10 glassy alloy play an important role in the diffusion behavior and in the electronic properties of this alloy.
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81.05.Kf Glasses (including metallic glasses)
81.05.Bx Metals, semimetals, and alloys
76.60.Lz Spin echoes
76.60.Es Relaxation effects
73.40.Gk Tunneling
66.30.Fq Self-diffusion in metals, semimetals, and alloys

Direct observation and mechanism of increased emission sites in Fe-coated microcrystalline diamond films

Kalpataru Panda, B. Sundaravel, B. K. Panigrahi, Pin-Chang Huang, Wen-Ching Shih, Huang-Chin Chen, and I-Nan Lin

J. Appl. Phys. 111, 124309 (2012); http://dx.doi.org/10.1063/1.4729836 (9 pages) | Cited 2 times

Online Publication Date: 20 June 2012

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The electron field emission (EFE) properties of microcrystalline diamond (MCD) films are significantly enhanced due to the Fe coating and post-annealing processes. The 900 °C post-annealed Fe coated diamond films exhibit the best EFE properties, with a turn on field (E0) of 3.42 V/μm and attain EFE current density (Je) of 170 μA/cm2 at 7.5 V/μm. Scanning tunnelling spectroscopy (STS) in current imaging tunnelling spectroscopy mode clearly shows the increased number density of emission sites in Fe-coated and post-annealed MCD films than the as-prepared ones. Emission is seen from the boundaries of the Fe (or Fe3C) nanoparticles formed during the annealing process. In STS measurement, the normalized conductance math versus V curves indicate nearly metallic band gap, at the boundaries of Fe (or Fe3C) nanoparticles. Microstructural analysis indicates that the mechanism for improved EFE properties is due to the formation of nanographite that surrounds the Fe (or Fe3C) nanoparticles.
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79.70.+q Field emission, ionization, evaporation, and desorption
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
81.40.Gh Other heat and thermomechanical treatments
52.77.Dq Plasma-based ion implantation and deposition
73.61.Wp Fullerenes and related materials

Tuning of terahertz intrinsic oscillations in asymmetric triple-barrier resonant tunneling diodes

Paweł Wójcik, Bartłomiej J. Spisak, Maciej Wołoszyn, and Janusz Adamowski

J. Appl. Phys. 111, 124310 (2012); http://dx.doi.org/10.1063/1.4729895 (6 pages)

Online Publication Date: 21 June 2012

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Intrinsic terahertz oscillations of the electronic current in the asymmetric triple-barrier resonant tunneling diode are investigated by means of the time-dependent Wigner-Poisson method. The current-voltage characteristics calculated for the nanodevice exhibits four separate bias voltage windows with the current oscillations, which are caused by two different mechanisms. One of these bias voltage windows, for which the electronic current oscillations stem from the negative feedback between the electronic current and the coupled quasi-bound states in the quantum wells embedded in the active region of the nanodevice, is considered in detail. It is demonstrated that the amplitude and frequency of the current oscillations in this bias voltage window depend on the coupling between the quasi-bound states formed in the quantum wells. Strength of this coupling is controlled by the thickness of the central barrier separating the quantum wells, which allows to tune the amplitude and frequency of the terahertz oscillations. Additionally, it was shown that the amplitude of the current oscillations can be tuned by shifting up or down the energy of the bottom of the wider quantum well. These properties suggest that the considered nanodevice may be of interest because of its possible application as tunable terahertz generator.
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85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)

Electrical conductivity of cationized ferritin decorated gold nanoshells

Rebecca Cortez, Joseph M. Slocik, Joseph E. Van Nostrand, Naomi J. Halas, and Rajesh R. Naik

J. Appl. Phys. 111, 124311 (2012); http://dx.doi.org/10.1063/1.4729800 (5 pages)

Online Publication Date: 21 June 2012

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We report on a novel method of controlling the resistance of nanodimensional, gold-coated SiO2 nanoparticles by utilizing biomolecules chemisorbed to the nanoshell surface. Local electronic transport properties of gold-coated nanoshells were measured using scanning conductance microscopy. These results were compared to transport properties of identical gold nanoshells biofunctionalized with cationized ferritin protein both with and without an iron oxide core (apoferritin). Measured resistances were on the order of mega-ohms. White light irradiation effects on transport properties were also explored. The results suggest that the light energy influences the nanoshells’ conductivity. A mechanism for assembly of gold nanoshells with cationized ferritin or cationized apoferritin is proposed to explain the resistivity dependence on irradiation.
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87.15.K- Molecular interactions; membrane-protein interactions
87.15.Pc Electronic and electrical properties
87.15.R- Reactions and kinetics
73.63.Bd Nanocrystalline materials
87.14.E- Proteins

The impact of quantum dots magnetization on spin separation and spin current in a multiple quantum-dot ring in the presence of Rashba spin-orbit coupling

Edris Faizabadi and Leila Eslami

J. Appl. Phys. 111, 124312 (2012); http://dx.doi.org/10.1063/1.4729912 (6 pages) | Cited 1 time

Online Publication Date: 22 June 2012

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The influence of quantum dot magnetization on electronic spin-dependent transport is investigated through a triple-quantum-dot ring structure in which one of the quantum dots is non-magnetic subjected to the Rashba spin-orbit interaction and the two other ones possess magnetic structure. Evaluated results, based on single particle Green’s function formalism, indicate that the presence of magnetic moment on the quantum dots leads to additional spin-dependent phase factor which affects electronic transport through the system. For both antiferromagnetic and ferromagnetic quantum dots, the system can operate as a spin-splitter but differently; by tuning Rashba spin-orbit strength and in the presence of magnetic flux, respectively. Besides, in the absence of one of the outgoing leads, spin current in the output is calculated and demonstrated that magnetization of quantum dots leads to spin current even in the absence of Rashba spin-orbit effect. Moreover, it is shown that in the presence of Rashba spin orbit interaction, magnetic quantum dots, and magnetic flux, the two terminal system produces a completely tunable spin current.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.07.Ta Quantum dots
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
72.25.-b Spin polarized transport
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Tt Fine-particle systems; nanocrystalline materials

Layer thickness dependent tensile deformation mechanisms in sub-10 nm multilayer nanowires

Fuping Yuan and Xiaolei Wu

J. Appl. Phys. 111, 124313 (2012); http://dx.doi.org/10.1063/1.4730337 (7 pages)

Online Publication Date: 22 June 2012

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Using molecular dynamics simulations, the tensile deformation behavior for two types of sub-10 nm multilayer nanowires (NWs) have been investigated. For the structure with interfaces perpendicular to the wire axis, the deformation mechanism is changed from interface crossing by dislocations to interface rotation as the layer thickness is decreasing, causing a significant reduction in yield strength. However, the deformation mechanisms are all accommodated through interface crossing by dislocations regardless of layer thickness for the structure with interfaces parallel to the wire axis. Moreover, the yield strengths in the second structure are found to be controlled by two competing mechanisms: the interface strengthening by increased repulsive force and interface softening by increased dislocation source sites. The sudden stress drop after yielding point in NWs could be explained by the dislocation source-limited hardening mechanism: the more atomic fraction of newly formed stacking faults (SF) after stress drop, the larger normalized stress drop and the larger uniform tensile elongation. For the second structure, the larger total tensile elongation for larger layer thickness could be related to the twinning induced plasticity at the necking position. These findings should have implications for designing functionalized structures and devices in nanoelectromechanical systems.
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81.40.Lm Deformation, plasticity, and creep
62.23.Hj Nanowires
62.20.fg Shape-memory effect; yield stress; superelasticity
61.72.Lk Linear defects: dislocations, disclinations
61.72.Nn Stacking faults and other planar or extended defects
68.65.Ac Multilayers

Time-resolved photoluminescence properties of CuInS2/ZnS nanocrystals: Influence of intrinsic defects and external impurities

Vamsi K. Komarala, Chuang Xie, Yongqiang Wang, Jian Xu, and Min Xiao

J. Appl. Phys. 111, 124314 (2012); http://dx.doi.org/10.1063/1.4730345 (4 pages)

Online Publication Date: 22 June 2012

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Photoluminescence (PL) lifetime studies of CuInS2 nanocrystals (NCs) are carried out after synthesizing core-shell and compositionally variant structures using time-resolved PL spectroscopy. Long-lived excited state decay times are observed for the NCs, and decay times are very much dependent on the size of the CuInS2 NCs. The emission bands are attributed to the surface (shorter PL lifetime) and defect (longer PL lifetime) related trap states, respectively. The decay dynamics of the CuInS2 NC’s excited-state carriers is very sensitive to the surface, intrinsic defects, and extrinsic impurities. The observed large Stokes shifts and broad PL spectra also reveal the involvement of the defect-related trapping sites in the emission process.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.47.jd Time resolved luminescence
78.55.Hx Other solid inorganic materials
78.55.Et II-VI semiconductors
72.20.Fr Low-field transport and mobility; piezoresistance
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
71.55.Gs II-VI semiconductors
71.55.Ht Other nonmetals

Surface plasmon polariton model of high-spatial frequency laser-induced periodic surface structure generation in silicon

M. Straub, M. Afshar, D. Feili, H. Seidel, and K. König

J. Appl. Phys. 111, 124315 (2012); http://dx.doi.org/10.1063/1.4730381 (6 pages) | Cited 4 times

Online Publication Date: 22 June 2012

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In recent years, high-spatial frequency laser-induced surfaces structures have been generated in a large variety of dielectrics. In silicon subwavelength ripples, some of which featured periodicities below 100 nm, were formed using ultrafast lasers. We demonstrate for Si(100) surfaces that generation of a dense electron-hole plasma in the focal spot of ultrashort-pulsed laser light followed by massive excitation of plasma waves provides an explanation for the formation of such high-spatial frequency surface structures. The applied Drude-like model includes carrier-carrier collisions and is in excellent agreement with the experimentally observed ripple period.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
42.62.Cf Industrial applications
68.35.bg Semiconductors

Correlation between quantum conductance and atomic arrangement of atomic-size silver nanowires

M. J. Lagos, P. A. S. Autreto, D. S. Galvao, and D. Ugarte

J. Appl. Phys. 111, 124316 (2012); http://dx.doi.org/10.1063/1.4729805 (7 pages)

Online Publication Date: 22 June 2012

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We have studied the effect of thermal effects on the structural and transport response of Ag atomic-size nanowires (NWs) generated by mechanical elongation. Our study involves both time-resolved atomic resolution transmission electron microscopy imaging and quantum conductance measurement using an ultra-high-vacuum mechanically controllable break junction. We have observed drastic changes in conductance and structural properties of Ag nanowires generated at different temperatures (150 and 300 K). By combining electron microscopy images, electronic transport measurements, and quantum transport calculations, we have been able to obtain a consistent correlation between the conductance and structural properties of Ag NWs. In particular, our study has revealed the formation of metastable rectangular rod-like Ag wire (3/3) along the [001] crystallographic direction, whose formation is enhanced. These results illustrate the high complexity of analyzing structural and quantum conductance behaviour of metal atomic-size wires; also, they reveal that it is extremely difficult to compare NW conductance experiments performed at different temperatures due to the fundamental modifications of the mechanical behavior.
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73.63.Nm Quantum wires
81.40.Lm Deformation, plasticity, and creep
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)
78.47.jg Time resolved reflection spectroscopy
81.07.Gf Nanowires
72.20.Fr Low-field transport and mobility; piezoresistance

The influence of the substrate thermal conductivity on scanning thermochemical lithography

Marten Tolk, Oliver Fenwick, Sadi Ahmad, and Franco Cacialli

J. Appl. Phys. 111, 124317 (2012); http://dx.doi.org/10.1063/1.4729809 (8 pages) | Cited 1 time

Online Publication Date: 22 June 2012

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We present a joint experimental and computational study of the role of the substrate thermal conductivity on scanning thermochemical lithography (SThL) of thin organic films. We aim this study at lithography of the luminescent conjugated polymer poly(p-phenylene vinylene) (PPV) from its soluble precursor poly(p-xylene tetrahydrothiophenium chloride) (PXT), but our results provide relevant insights into the SThL of thermosensitive polymers in general, and into a wide range of nanoscale thermal and thermochemical processes in thin films. As high thermal conductivity substrates we used gold films on silicon, and indium-tin oxide (ITO) films on glass, successfully patterning PPV on both substrates. We find that a higher probe temperature (>300 °C instead of ≈250 °C) is necessary for lithography of PXT films on ITO compared to those on fused silica (for the same scanning speed and comparable precursor thickness). Surprisingly, however, our experiments show that minimum feature sizes are nearly independent of the underlying substrate. While a lateral resolution (full width at half maximum, FWHM) of 37 nm was achieved previously on fused silica for a 40 nm thick PXT film, we obtain here a FWHM of 36 nm for a 35 nm thick PXT layer on ITO. We compare our experiments with finite element simulations and gain further insight into the possibilities of thermochemical lithography, the necessary minimum probe temperature and the highest attainable resolutions. The model shows that for high thermal conductivity substrates there should be a region of unconverted polymer near the polymer-substrate interface. Our experiments demonstrate that patterned features are able to adhere to the substrate despite this unconverted layer, thus allowing SThL to work on very high thermal conductivity substrates such as gold. Our model builds on this experimental finding and accounts for the experimental lack of dependence of lateral size with substrate conductivity, i.e. it predicts that the minimum feature size increases only slightly for increasing thermal conductivities of the substrates.
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81.16.Nd Micro- and nanolithography
82.35.Cd Conducting polymers
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
02.70.Dh Finite-element and Galerkin methods
78.60.Kn Thermoluminescence

Scanning transmission X-ray microscopy and X-ray absorption near-edge structure studies of N-doped carbon nanotubes sealed with N2 gas

Tian Xie, Yu Zhao, Jun Zhong, Zheng Hu, and Xuhui Sun

J. Appl. Phys. 111, 124318 (2012); http://dx.doi.org/10.1063/1.4729903 (4 pages) | Cited 2 times

Online Publication Date: 25 June 2012

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N-doped carbon nanotubes (NCNTs) were synthesized and their electronic structures have been explored by X-ray absorption near-edge structure (XANES) spectroscopy. With a surface sensitive mode, XANES confirms the N-doping in NCNTs. However, with a more bulk sensitive detection mode of XANES, large amount of gaseous N2 have been found to be sealed in NCNTs, even in a high vacuum environment. The encapsulation of the ferrocene residues in carbon nanotubes had been revealed by scanning transmission X-ray microscopy (STXM), which may help for the N2 sealing. The results suggest that the easily sealed gas should be taken into consideration for CNT-based applications.
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78.70.Dm X-ray absorption spectra
71.20.Tx Fullerenes and related materials; intercalation compounds
61.72.up Other materials
81.16.-c Methods of micro- and nanofabrication and processing
81.07.De Nanotubes

Modeling and theoretical efficiency of a silicon nanowire based thermoelectric junction with area enhancement

M. Seong, J. S. Sadhu, J. Ma, M. G. Ghossoub, and S. Sinha

J. Appl. Phys. 111, 124319 (2012); http://dx.doi.org/10.1063/1.4728189 (10 pages) | Cited 1 time

Online Publication Date: 25 June 2012

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Recent experimental work suggests that individual silicon nanowires with rough surfaces possess a thermoelectric figure of merit as high as 0.6 near room temperature. This paper addresses the possibility of using an array of such nanowires in a thermoelectric junction for generation. Employing a model of frequency dependent phonon boundary scattering, we estimate the effective thermal conductivity of the array and investigate heat flow through the junction. We show that charge transport is largely unaffected by the roughness scales considered. Enhancing the area for heat exchange at an individual 200 μm × 200 μm p-n junction yields significant temperature differences across the junction leading to power >0.6 mW and efficiency >1.5% for a junction with effective thermal conductivity <5 W/mK, when the source and sink are at 450 K and 300 K, respectively. We show that relatively short nanowires of ∼50 μm length are sufficient for obtaining peak power and reasonable efficiency. This substantially reduces the challenge of engineering low resistivity electrical contacts that critically affect power and efficiency. This paper provides insight into how fundamental transport in relation to bulk heat transfer and charge transport, affects the performance of thermoelectric junctions based on nanostructured materials.
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85.80.Fi Thermoelectric devices
85.30.De Semiconductor-device characterization, design, and modeling
84.32.Dd Connectors, relays, and switches

Epitaxial two dimensional aluminum films on silicon (111) by ultra-fast thermal deposition

Igal Levine, Alexander Yoffe, Adi Salomon, Wenjie Li, Yishay Feldman, and Ayelet Vilan

J. Appl. Phys. 111, 124320 (2012); http://dx.doi.org/10.1063/1.4730411 (9 pages) | Cited 1 time

Online Publication Date: 25 June 2012

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Aluminum thin films are known for their extremely rough surface, which is detrimental for applications such as molecular electronics and photonics, where protrusions cause electrical shorts or strong scattering. We achieved atomically flat Al films using a highly non-equilibrium approach. Ultra-fast thermal deposition (UFTD), at rates >10 nm/s, yields RMS roughness of 0.4 to 0.8 nm for 30–50 nm thick Al films on variety of substrates. For UFTD on Si(111) substrates, the top surface follows closely the substrate topography (etch pits), indicating a 2D, layer-by-layer growth. The Al film is a mixture of (100) and (111) grains, where the latter are commensurate with the in-plane orientation of the underlying Si (epitaxy). We show the use of these ultra-smooth Al films for highly reproducible charge-transport measurements across a monolayer of alkyl phosphonic acid as well as for plasmonics applications by directly patterning them by focused ion beam to form a long-range ordered array of holes. UFTD is a one-step process, with no need for annealing, peeling, or primer layers. It is conceptually opposite to high quality deposition methods, such as MBE or ALD, which are slow and near-equilibrium processes. For Al, though, we find that limited diffusion length (and good wetting) is critical for achieving ultra-smooth thin films.
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81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.05.Bx Metals, semimetals, and alloys
68.55.at Other materials
68.35.bd Metals and alloys
68.35.Fx Diffusion; interface formation
68.08.Bc Wetting
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Effect of size-dependent grain structures on the dynamics of nanoparticle coalescence

Yiyang Zhang, Shuiqing Li, Wen Yan, and Stephen D. Tse

J. Appl. Phys. 111, 124321 (2012); http://dx.doi.org/10.1063/1.4730773 (12 pages) | Cited 1 time

Online Publication Date: 26 June 2012

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The effect of grain structure on the coalescence dynamics of anatase TiO2 nanoparticles at different temperatures is investigated using classical molecular dynamics (MD) simulation. Examination of local-lattice-orientation distributions reveals that the grain morphology of particles is highly dependent on size. For a single anatase nanoparticle below the melting temperature, an amorphous-to-crystalline transition occurs for diameters ranging from 2 to 2.5 nm as temperature increases. Below the transition diameter (for a given temperature), the entire nanoparticle is amorphous. Above the transition diameter, the nanoparticle consists of a crystalline core and an amorphous shell (4–6 Å). Considering that such grain-structure characteristics may lead to different dynamic behaviors, the coalescence between pairs of 2 nm–2 nm, 3 nm–3 nm, and 2 nm–3 nm nanoparticles is investigated. For 2 nm–2 nm nanoparticle coalescence, the process is independent of initial temperature and is seemingly viscosity-controlled with a dynamic temperature rise due to energy transfer from surface to internal kinetic (thermal). For 3 nm–3 nm nanoparticle coalescence, the process is sensitive to initial temperature. Above the melting temperature, the dynamics are similar to the 2 nm–2 nm amorphous case. Just below the melting point, coalescence consists of melting of the crystalline cores with subsequent large increase in temperature due to recrystallization. For 2 nm–3 nm nanoparticle coalescence, recrystallization of the 2 nm particle significantly increases the total temperature compared to the 2 nm–2 nm case.
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
61.72.-y Defects and impurities in crystals; microstructure
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
64.70.dj Melting of specific substances

Effect of nanoparticles on charge transport in nanofluid-impregnated pressboard

Yuefan Du, Yuzhen Lv, Chengrong Li, Mutian Chen, Yuxiang Zhong, Shengnan Zhang, and You Zhou

J. Appl. Phys. 111, 124322 (2012); http://dx.doi.org/10.1063/1.4730778 (4 pages) | Cited 1 time

Online Publication Date: 26 June 2012

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Transformer pressboard impregnated in mineral oil modified by nanoparticles (nanofluid) exhibits substantially higher AC and DC breakdown voltage than that of the pure oil-impregnated pressboard (OP). Charge transport and decay characteristics of both pressboards were measured by pulse electroacoustic technique. It reveals that nanofluid-impregnated pressboard (NP) has a more uniform internal electric field and a higher charge decay rate compared to OP, which is caused by an increase of shallow trap density in NP related to the difference of internal structures, based on the test results of thermally stimulated current and Fourier transformed infra-red spectroscopy.
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84.70.+p High-current and high-voltage technology: power systems; power transmission lines and cables

Anisotropic surface properties of micro/nanostructured a-C:H:F thin films with self-assembly applications

V.-M. Freire, C. Corbella, E. Bertran, S. Portal-Marco, M. Rubio-Roy, and J.-L. Andújar

J. Appl. Phys. 111, 124323 (2012); http://dx.doi.org/10.1063/1.4730783 (8 pages)

Online Publication Date: 26 June 2012

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The singular properties of hydrogenated amorphous carbon (a-C:H) thin films deposited by pulsed DC plasma enhanced chemical vapor deposition (PECVD), such as hardness and wear resistance, make it suitable as protective coating with low surface energy for self-assembly applications. In this paper, we designed fluorine-containing a-C:H (a-C:H:F) nanostructured surfaces and we characterized them for self-assembly applications. Sub-micron patterns were generated on silicon through laser lithography while contact angle measurements, nanotribometer, atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to characterize the surface. a-C:H:F properties on lithographied surfaces such as hydrophobicity and friction were improved with the proper relative quantity of CH4 and CHF3 during deposition, resulting in ultrahydrophobic samples and low friction coefficients. Furthermore, these properties were enhanced along the direction of the lithography patterns (in-plane anisotropy). Finally, self-assembly properties were tested with silica nanoparticles, which were successfully assembled in linear arrays following the generated patterns. Among the main applications, these surfaces could be suitable as particle filter selector and cell colony substrate.
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68.35.bt Other materials
68.55.ap Fullerenes
62.20.Qp Friction, tribology, and hardness
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.16.Dn Self-assembly
81.16.Nd Micro- and nanolithography
81.40.Pq Friction, lubrication, and wear

Adhesion of E. coli to silver- or copper-coated porous clay ceramic surfaces

I. Yakub and W. O. Soboyejo

J. Appl. Phys. 111, 124324 (2012); http://dx.doi.org/10.1063/1.4722326 (9 pages) | Cited 1 time

Online Publication Date: 26 June 2012

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Porous ceramic water filters (CWFs), produced by sintering a mixture of clay and a combustible material (such as woodchips), are often used in point-of-use water filtration systems that occlude microbes by size exclusion. They are also coated with colloidal silver, which serves as a microbial disinfectant. However, the adhesion of microbes to porous clay surfaces and colloidal silver coated clay surfaces has not been studied. This paper presents the results of atomic force microscopy (AFM) measurements of the adhesion force between Escherichia coli bacteria, colloidal silver, and porous clay-based ceramic surfaces. The adhesion of silver and copper nanoparticles is also studied in control experiments on these alternative disinfectant materials. The adhesive force between the wide range of possible bi-materials was measured using pull-off measurements during force microscopy. These were combined with measurements of AFM tip radii/substrate roughness that were incorporated into adhesion models to obtain the adhesion energies for the pair wise interaction. Of the three antimicrobial metals studied, the colloidal silver had the highest affinity for porous ceramic surface (125 ± 32 nN and ∼0.29 J/m2) while the silver nanoparticles had the highest affinity for E. coli bacteria (133 ± 21 nN and ∼0.39 J/m2). The implications of the results are then discussed for the design of ceramic water filter that can purify water by adsorption and size exclusion.
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81.65.-b Surface treatments
82.70.Dd Colloids
87.85.J- Biomaterials
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
68.37.Ps Atomic force microscopy (AFM)
68.43.Mn Adsorption kinetics

Understanding the effect of the layer-to-layer distance on Li-intercalated graphite

B. Xu, M. S. Wu, G. Liu, and C. Y. Ouyang

J. Appl. Phys. 111, 124325 (2012); http://dx.doi.org/10.1063/1.4730969 (5 pages) | Cited 1 time

Online Publication Date: 26 June 2012

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The lithium ion dynamics in graphite, an important anode material for lithium ion batteries, is strongly related with the layer-to-layer distance (d-spacing) and the stacking modes of graphite. We studied these relationships by first-principles calculations. It is found that a larger d-spacing results in the easier transformation of the stacking mode from A-B stacking to A-A stacking when lithium atoms are inserted. This transformation is unfavorable to the lithium diffusion because of the larger diffusion energy barrier for lithium in A-A stacking graphite compared to that in A-B one. On the other hand, as the d-spacing increases, the diffusion energy barrier for lithium in A-A stacking graphite decreases substantially, thus being favorable to the lithium diffusion. These results give a better understanding of the lithium ion dynamics in graphite, and show that it is possible to optimize the lithium ion dynamics in graphite by properly adjusting the d-spacing.
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68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
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