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7 May 2012

Volume 100, Issue 19, Articles (19xxxx)

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Appl. Phys. Lett. 100, 191901 (2012); http://dx.doi.org/10.1063/1.4709436 (4 pages)

Muamer Kadic, Tiemo Bückmann, Nicolas Stenger, Michael Thiel, and Martin Wegener
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Reduction of spectral phonon relaxation times from suspended to supported graphene

Bo Qiu and Xiulin Ruan

Appl. Phys. Lett. 100, 193101 (2012); http://dx.doi.org/10.1063/1.4712041 (4 pages) | Cited 4 times

Online Publication Date: 7 May 2012

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We have performed molecular dynamics simulations with phonon spectral analysis to predict the mode-wise phonon relaxation times (RT) of suspended and supported graphene at room temperature, and the findings are consistent with recent optical measurements. For acoustic phonons, RTs reduce from up to 50 ps to less than 5 ps when graphene is put on silicon dioxide substrate. Similarly, optical phonon RTs reduce by half when supported. Stronger interfacial bonding is found to result in more RT reduction. Our results provide a fundamental understanding at the spectral phonon property level for the observed thermal conductivity reduction in supported graphene.
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63.22.Rc Phonons in graphene
82.70.Kj Emulsions and suspensions
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.

Nanoscale integration of SiC/SiO2 core-shell nanocables in diamond through a simultaneous hybrid structure fabrication

Hao Zhuang, Lei Zhang, Thorsten Staedler, and Xin Jiang

Appl. Phys. Lett. 100, 193102 (2012); http://dx.doi.org/10.1063/1.4712044 (4 pages)

Online Publication Date: 7 May 2012

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The nanoscale integration of SiC nanocables in a diamond thin film is achieved through a novel synthetic pathway, which combines Fe catalyst and detonation nanodiamond seeding technique in a microwave plasma chemical vapor deposition process. The obtained hybrid structures show controllable SiC nanocable fraction depending on the relative fraction of iron catalyst and diamond seeds. The SiC nanocable has a conical structure with 10 nm diameter SiC core, surrounded by SiO2 shell. The diamond crystals show high quality/crystallinity even for hybrid structures featuring an increasing SiC nanocable fraction. In the end, the growth behavior of the hybrid structure is discussed.
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81.16.-c Methods of micro- and nanofabrication and processing
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
52.77.-j Plasma applications
61.46.-w Structure of nanoscale materials
68.55.-a Thin film structure and morphology
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Schottky solar cells based on graphene nanoribbon/multiple silicon nanowires junctions

Chao Xie, Jiansheng Jie, Biao Nie, Tianxin Yan, Qiang Li, Peng Lv, Fangze Li, Mingzheng Wang, Chunyan Wu, Li Wang, and Linbao Luo

Appl. Phys. Lett. 100, 193103 (2012); http://dx.doi.org/10.1063/1.4711205 (4 pages) | Cited 3 times

Online Publication Date: 7 May 2012

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We report the construction of Schottky solar cells based on graphene nanoribbon/multiple silicon nanowires (SiNWs) junctions. Only a few (∼10) SiNWs were involved to miniaturize the solar cell for nanoscale power source applications. It was found that doping level of the SiNWs played an important role in determining the device performance. By increasing the doping level, solar cell with open circuit voltage of 0.59 V and energy conversion efficiency of 1.47% were achieved under AM 1.5G illumination. The large effective junction area of the radial Schottky junction was responsible for the high device performance.
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88.40.jp Multijunction solar cells
85.30.Hi Surface barrier, boundary, and point contact devices
85.30.Kk Junction diodes
88.40.hj Efficiency and performance of solar cells

Optimizing fiber cross-sectional shape for improving stability of air–water interface over superhydrophobic fibrous coatings

B. Emami and H. Vahedi Tafreshi

Appl. Phys. Lett. 100, 193104 (2012); http://dx.doi.org/10.1063/1.4711800 (4 pages)

Online Publication Date: 7 May 2012

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In this letter, a mathematical force-balance formulation is developed that can be used to predict the critical pressure, the hydrostatic pressure above which the surface starts to depart from the non-wetting state, for superhydrophobic surfaces comprised of highly aligned fibers (e.g., biased AC-electrospun coatings) with arbitrary cross-sectional shapes. We have also developed a methodology for optimizing the fiber cross-sections to maximize the critical pressure of the surface, using the Euler–Lagrange equation. A case study is presented to better demonstrate the application of our method.
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81.65.-b Surface treatments

Low-temperature, site selective graphitization of SiC via ion implantation and pulsed laser annealing

Maxime G. Lemaitre, Sefaattin Tongay, Xiaotie Wang, Dinesh K. Venkatachalam, Joel Fridmann, Brent P. Gila, Arthur F. Hebard, Fan Ren, Robert G. Elliman, and Bill R. Appleton

Appl. Phys. Lett. 100, 193105 (2012); http://dx.doi.org/10.1063/1.4707383 (4 pages) | Cited 1 time

Online Publication Date: 8 May 2012

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A technique is presented to selectively graphitize regions of SiC by ion implantation and pulsed laser annealing (PLA). Nanoscale features are patterned over large areas by multi-ion beam lithography and subsequently converted to few-layer graphene via PLA in air. Graphitization occurs only where ions have been implanted and without elevating the temperature of the surrounding substrate. Samples were characterized using Raman spectroscopy, ion scattering/channeling, SEM, and AFM, from which the degree of graphitization was determined to vary with implantation species, damage and dose, laser fluence, and pulsing. Contrasting growth regimes and graphitization mechanisms during PLA are discussed.
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61.72.up Other materials
61.85.+p Channeling phenomena (blocking, energy loss, etc.)
78.30.Hv Other nonmetallic inorganics
61.72.Cc Kinetics of defect formation and annealing

Stochastic processes in light-assisted nanoparticle formation

Makoto Naruse, Yang Liu, Wataru Nomura, Takashi Yatsui, Masaki Aida, Laszlo B. Kish, and Motoichi Ohtsu

Appl. Phys. Lett. 100, 193106 (2012); http://dx.doi.org/10.1063/1.4711808 (5 pages) | Cited 2 times

Online Publication Date: 8 May 2012

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Recently, light-assisted nanofabrication have been introduced, such as the synthesis of quantum dots using photo-induced desorption that yields reduced size fluctuations or metal sputtering under light illumination resulting in self-organized, nanoparticle chains. The physical mechanisms have originally been attributed to material desorption or plasmon resonance effects. However, significant stochastic phenomena are also present that have not been explained yet. We introduce stochastic models taking account of the light-assisted processes that reproduce phenomenological characteristics consistent with the experimental observations.
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81.07.Ta Quantum dots
81.16.Dn Self-assembly
68.43.Nr Desorption kinetics
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.21.La Quantum dots
73.22.Lp Collective excitations

Direct assembly of three-dimensional mesh plasmonic rolls

Fu Min Huang, Jatin K. Sinha, Nicholas Gibbons, Philip N. Bartlett, and Jeremy J. Baumberg

Appl. Phys. Lett. 100, 193107 (2012); http://dx.doi.org/10.1063/1.4711923 (4 pages) | Cited 2 times

Online Publication Date: 10 May 2012

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A direct-assembly method to construct three-dimensional (3D) plasmonic nanostructures yields porous plasmonic rolls through the strain-induced self-rolling up of two-dimensional metallic nanopore films. This route is scalable to different hole sizes and film thicknesses, and applicable to a variety of materials, providing general routes towards a diverse family of 3D metamaterials with nano-engineerable optical properties. These plasmonic rolls can be dynamically driven by light irradiation, rolling or unrolling with increasing or decreasing light intensity. Such dynamically controllable 3D plasmonic nanostructures offer opportunities both for sensing and feedback in active nano-actuators.
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81.16.-c Methods of micro- and nanofabrication and processing
68.55.jd Thickness
78.67.Rb Nanoporous materials
61.43.Gt Powders, porous materials
73.22.Lp Collective excitations
78.66.Bz Metals and metallic alloys

Engineering direct-indirect band gap transition in wurtzite GaAs nanowires through size and uniaxial strain

Andrew Copple, Nathaniel Ralston, and Xihong Peng

Appl. Phys. Lett. 100, 193108 (2012); http://dx.doi.org/10.1063/1.4718026 (4 pages) | Cited 1 time

Online Publication Date: 10 May 2012

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Electronic structures of wurtzite GaAs nanowires in the [0001] direction were studied using first-principles calculations. It was found that the band gap of GaAs nanowires experiences a direct-to-indirect transition when the diameter of the nanowires is smaller than ∼28 Å. For those thin GaAs nanowires with an indirect band gap, it was found that the gap can be tuned to be direct if a moderate external uniaxial strain is applied. Both tensile and compressive strain can trigger the indirect-to-direct gap transition. The critical strains for the gap-transition are determined by the energy crossover of two states in conduction bands.
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71.20.Nr Semiconductor compounds
73.22.Dj Single particle states
73.61.Ey III-V semiconductors
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Study of charge distribution and charge loss in dual-layer metal-nanocrystal-embedded high-κ/SiO2 gate stack

Z. Z. Lwin, K. L. Pey, Q. Zhang, M. Bosman, Q. Liu, C. L. Gan, P. K. Singh, and S. Mahapatra

Appl. Phys. Lett. 100, 193109 (2012); http://dx.doi.org/10.1063/1.4712565 (4 pages)

Online Publication Date: 10 May 2012

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In this work, we present a comprehensive experimental study of charge loss mechanisms in a dual-layer metal nanocrystal (DL-MNC) embedded high-κ/SiO2 gate stack. Kelvin force microscopy characterization reveals that the internal-electric-field assisted tunneling could be a dominant charge loss mechanism in DL devices that mainly depends on the charge distribution in two MNC-layers and inter-layer dielectric (ILD) thickness between the two layers of nanocrystals. Our findings suggest that an optimized DL-MNCs embedded memory cell could be achieved by defining the ILD thickness larger than the average MNC-spacing for enhancement of retention ability in MNC embedded gate stacks. It implies the possibility of reducing MNC spacing in DL structure of scaled memory devices by controlling the thickness of ILD.
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84.30.Sk Pulse and digital circuits

Anomalous enhancement of the thermoelectric figure of merit by V co-doping of Nb-SrTiO3

K. Ozdogan, M. Upadhyay Kahaly, H. N. Alshareef, and U. Schwingenschlögl

Appl. Phys. Lett. 100, 193110 (2012); http://dx.doi.org/10.1063/1.4714541 (4 pages)

Online Publication Date: 10 May 2012

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The effect of V co-doping of Nb-SrTiO3 is studied by full-potential density functional theory. We obtain a stronger increase of the carrier density for V than for Nb dopants. While in Nb-SrTiO3 a high carrier density counteracts a high thermoelectric figure of merit, the trend is inverted by V co-doping. The mechanism leading to this behavior is explained in terms of a local spin-polarization introduced by the V ions. Our results indicate that magnetic co-doping can be a prominent tool for improving the thermoelectric figure of merit.
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72.20.Pa Thermoelectric and thermomagnetic effects
72.25.-b Spin polarized transport
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.20.Ps Other inorganic compounds
72.20.Fr Low-field transport and mobility; piezoresistance

Fullerene as electrical hinge

Neng Wan, Pascal Perriat, Li-Tao Sun, Qing-An Huang, Jun Sun, and Tao Xu

Appl. Phys. Lett. 100, 193111 (2012); http://dx.doi.org/10.1063/1.4714682 (5 pages) | Cited 2 times

Online Publication Date: 10 May 2012

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The current study demonstrates that fullerenes with sizes between 1.9 nm and 3.3 nm serve as an efficient electrical hinges for interconnecting sub-5 nm carbon nanotubes (CNTs). Three modes of such “soft connections” were validated via transmission electron microscopy employing in-situ nano-manipulation and electrical transportation measurement. A high stability of the electrical connectivity caused by the forces acting between the fullerene and nanotubes was detected. Current densities as high as ∼108 A/cm2, which almost reached the current carrier capability of CNTs, were observed within the nanocontact and a low contact resistivity (∼10−8 Ω·cm2) was observed between the fullerene and nanotubes.
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61.48.-c Structure of fullerenes and related hollow and planar molecular structures
68.37.Lp Transmission electron microscopy (TEM)
73.61.Wp Fullerenes and related materials

Simultaneous visualization of oxygen vacancies and the accompanying cation shifts in a perovskite oxide by combining annular imaging techniques

Shunsuke Kobayashi, Scott D. Findlay, Naoya Shibata, Teruyasu Mizoguchi, Yukio Sato, Eiji Okunishi, Yuichi Ikuhara, and Takahisa Yamamoto

Appl. Phys. Lett. 100, 193112 (2012); http://dx.doi.org/10.1063/1.4714920 (5 pages) | Cited 1 time

Online Publication Date: 10 May 2012

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The defect structures around oxygen vacancies in perovskite-type oxides play very important roles in determining material properties through the change of the electronic state. Therefore, from the viewpoint of developing condensed matter physics and technological applications, it is crucial to obtain direct images of these structures. We report the simultaneous visualization of oxygen vacancies and the cation shifts around them in perovskite-type manganite by scanning transmission electron microscopy. The defective structure of the material makes it amenable to using Fourier analysis to obtain important structural information like the oxygen vacancy locations and the shift of nearby cation columns.
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61.72.jd Vacancies

Lattice thermal conductivity diminution and high thermoelectric power factor retention in nanoporous macroassemblies of sulfur-doped bismuth telluride nanocrystals

Yanliang Zhang, Rutvik J. Mehta, Matthew Belley, Liang Han, Ganpati Ramanath, and Theodorian Borca-Tasciuc

Appl. Phys. Lett. 100, 193113 (2012); http://dx.doi.org/10.1063/1.4711774 (4 pages) | Cited 1 time

Online Publication Date: 10 May 2012

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We report ultralow lattice thermal conductivity in the 0.3 ≤ κL ≤ 0.6 W m−1 K−1 range in nanoporous bulk bismuth telluride pellets obtained by sintering chemically synthesized nanostructures, together with single-crystal-like electron mobilities and Seebeck coefficients at comparable charge carrier concentrations. The observed κL is up to 35% lower than classical effective medium predictions, and can be quantitatively explained by increased phonon scattering at nanopores and nanograins. Our findings are germane to tailoring nanoporous thermoelectric materials for efficient solid-state refrigeration, thermal energy harvesting, and thermal management applications.
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72.20.Pa Thermoelectric and thermomagnetic effects
63.22.Kn Clusters and nanocrystals
66.70.Df Metals, alloys, and semiconductors
61.43.Gt Powders, porous materials
61.46.Hk Nanocrystals
72.20.Fr Low-field transport and mobility; piezoresistance

Investigation of CuSb4Te2 alloy for high-speed phase change random access memory applications

Yegang Lu, Sannian Song, Zhitang Song, Feng Rao, Liangcai Wu, Min Zhu, Bo Liu, and Dongning Yao

Appl. Phys. Lett. 100, 193114 (2012); http://dx.doi.org/10.1063/1.4711811 (4 pages) | Cited 3 times

Online Publication Date: 10 May 2012

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The thermal stability of amorphous Sb2Te film can be significantly improved by the addition of Cu. CuSb4Te2 alloy is considered to be a potential candidate for phase change random access memory (PCRAM), as evidenced by a higher crystallization temperature, a better data retention ability, and a faster switching speed in comparison with those of Ge2Sb2Te5. A reversible switching between set and reset states can be realized by an electric pulse as short as 7 ns for CuSb4Te2-based PCRAM cell. In addition, CuSb4Te2 shows endurance up to 1.5 × 105 cycles with a resistance ratio of about two orders of magnitude.
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73.61.Jc Amorphous semiconductors; glasses
84.30.Sk Pulse and digital circuits
68.55.ag Semiconductors

Electrical contact resistance in filaments

Xiang-Fa Wu, Zhengping Zhou, and Wang-Min Zhou

Appl. Phys. Lett. 100, 193115 (2012); http://dx.doi.org/10.1063/1.4718019 (4 pages) | Cited 1 time

Online Publication Date: 11 May 2012

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Electrical contact resistance (ECR) influences the electrochemical performance of porous electrodes made of stacked discrete materials (e.g., carbon nanotubes, nanofibers, etc.) for use in supercapacitors and rechargeable batteries. This study establishes a simple elasticity-conductivity model for the ECR of filaments in adhesive contact. The elastic deformation and size of electrical contact zone of the filaments are determined by using an adhesive contact model of filaments, and the ECR of adhesive filaments is obtained in explicit form. Dependencies of the ECR upon the filament geometries, surface energy, and elasticity are examined.
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73.40.Cg Contact resistance, contact potential
84.32.Dd Connectors, relays, and switches
84.32.Tt Capacitors

Room temperature nonlinear operation of a graphene-based three-branch nanojunction device with chemical doping

Shaharin Fadzli Abd Rahman, Seiya Kasai, and Abdul Manaf Hashim

Appl. Phys. Lett. 100, 193116 (2012); http://dx.doi.org/10.1063/1.4711035 (3 pages)

Online Publication Date: 11 May 2012

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A chemically doped graphene-based three-branch nanojunction device is fabricated on a SiO2/p-Si substrate, and its nonlinear operation is characterized at room temperature (RT). By polyethyleneimine doping, the fabricated device shows improved field effect mobility of 14 800 and 16 100 cm2/Vs for electron and holes, respectively. The device clearly exhibits nonlinearity in voltage transfer curves at RT. The curvature of the transfer curve can be controlled by using the back gate voltage, and its polarity abruptly switches near the Dirac point because of the carrier type change. The observed behaviour can be quantitatively explained in terms of the difference in the amounts of gate-induced carriers in the two input branches.
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85.30.Tv Field effect devices

Low-κ organic layer as a top gate dielectric for graphene field effect transistors

G. Mordi, S. Jandhyala, C. Floresca, S. McDonnell, M. J. Kim, R. M. Wallace, L. Colombo, and J. Kim

Appl. Phys. Lett. 100, 193117 (2012); http://dx.doi.org/10.1063/1.4711776 (3 pages) | Cited 1 time

Online Publication Date: 11 May 2012

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We demonstrate the characteristics of dual gated graphene field effect transistors using a thin layer (∼7 nm) of parylene-C as a top-gate dielectric. Our devices exhibit good dielectric properties with minimal doping, low leakage current (∼10−6 A/cm2 at ±2 V), and a dielectric constant of ∼2.1. Additionally, Raman spectroscopy did not reveal any process induced defects after dielectric deposition. Electrical characterization performed in air showed a carrier mobility of ∼5050 cm2/Vs with hysteresis less than 30 mV during top gate operation (−2.5 V to 2.5 V) which indicates that parylene and its interface with graphene does not have a significant amount of trapped charges.
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85.30.Tv Field effect devices
77.55.Bh Low-permittivity dielectric films
72.20.Fr Low-field transport and mobility; piezoresistance
77.22.Ch Permittivity (dielectric function)
77.80.Dj Domain structure; hysteresis

Highly efficient thermal glue for carbon nanotubes based on azide polymers

Yuxiang Ni, Hung Le Khanh, Yann Chalopin, Jinbo Bai, Pierre Lebarny, Laurent Divay, and Sebastian Volz

Appl. Phys. Lett. 100, 193118 (2012); http://dx.doi.org/10.1063/1.4711809 (4 pages) | Cited 1 time

Online Publication Date: 11 May 2012

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Equilibrium molecular dynamics (EMD) simulations and experimental data show that the thermal contact resistance (TCR) between carbon nanotube (CNT) and azide-functionalized polymer with C-N bond is significantly decreased compared to that with Van der Waals force interaction. EMD simulations indicate that C-N covalent bond between CNT and polymer is the most efficient way to reduce TCR, and we measured the lowest thermal interface resistance of Si/CNT/Polymer/Cu thermal interface material as 1.40 mm2 KW−1 with CNTs of 10 μm length. These results provide useful information for future designs of thermal glue for carbon-based materials with better thermal conduction.
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72.80.Cw Elemental semiconductors
72.80.Rj Fullerenes and related materials
73.40.Cg Contact resistance, contact potential
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations
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