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17 Aug 2009

Volume 95, Issue 7, Articles (07xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 95, 073703 (2009); http://dx.doi.org/10.1063/1.3152768 (3 pages)

Mohammud R. Noor, Swati Goyal, Shawn M. Christensen, and Samir M. Iqbal
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Morphology of graphene on SiC(000math) surfaces

Luxmi, P. J. Fisher, N. Srivastava, R. M. Feenstra, Yugang Sun, J. Kedzierski, P. Healey, and Gong Gu

Appl. Phys. Lett. 95, 073101 (2009); http://dx.doi.org/10.1063/1.3207757 (3 pages) | Cited 12 times

Online Publication Date: 17 August 2009

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Graphene is formed on SiC(000math) surfaces (the so-called C-face of the crystal) by annealing in vacuum, with the resulting films characterized by atomic force microscopy, Auger electron spectroscopy, scanning Auger microscopy, and Raman spectroscopy. Morphology of these films is compared with the graphene films grown on SiC(0001) surfaces (the Si-face). Graphene forms a terraced morphology on the C-face, whereas it forms with a flatter morphology on the Si-face. It is argued that this difference occurs because of differing interface structures in the two cases. For certain SiC wafers, nanocrystalline graphite is found to form on top of the graphene.
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68.35.Ct Interface structure and roughness
68.37.Ps Atomic force microscopy (AFM)
79.20.Fv Electron impact: Auger emission
81.40.Gh Other heat and thermomechanical treatments
78.66.Tr Fullerenes and related materials
68.55.ap Fullerenes
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
78.30.Na Fullerenes and related materials

Dephasing of Si spin qubits due to charge noise

Dimitrie Culcer, Xuedong Hu, and S. Das Sarma

Appl. Phys. Lett. 95, 073102 (2009); http://dx.doi.org/10.1063/1.3194778 (3 pages) | Cited 8 times

Online Publication Date: 17 August 2009

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Spin qubits in silicon quantum dots can have long coherence times, yet their manipulation relies on the exchange interaction, through which charge noise can induce decoherence. Charge traps near the interface of a Si heterostructure lead to fluctuations in the quantum-dot confinement and barrier potentials, which cause gating errors and two-spin dephasing. We quantify these effects in Si double quantum dots using a realistic model of noise. Specifically, we consider both random telegraph noise from a few traps good for dots grown on submicron wafers and 1/f noise from many traps good for larger wafers appropriate for quantum dot arrays. We give estimates of gate errors for single-spin qubit architectures and dephasing in singlet-triplet qubits.
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68.65.Hb Quantum dots (patterned in quantum wells)
81.05.Cy Elemental semiconductors
81.07.Ta Quantum dots
71.70.Gm Exchange interactions
75.30.Ds Spin waves

Strain mapping of tensiley strained silicon transistors with embedded Si1−yCy source and drain by dark-field holography

Florian Hüe, Martin Hÿtch, Florent Houdellier, Hugo Bender, and Alain Claverie

Appl. Phys. Lett. 95, 073103 (2009); http://dx.doi.org/10.1063/1.3192356 (3 pages) | Cited 18 times

Online Publication Date: 18 August 2009

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Dark-field holography, a new transmission electron microscopy technique for mapping strain distributions at the nanoscale, is used to characterize strained-silicon n-type transistors with a channel width of 65 nm. The strain in the channel region, which enhances electron mobilities, is engineered by recessed Si0.99C0.01 source and drain stressors. The strain distribution is measured across an array of five transistors over a total area of 1.6 μm wide. The longitudinal tensile strain reaches a maximum of 0.58%±0.02% under the gate oxide. Theoretical strain maps obtained by finite element method agree well with the experimental results.
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85.30.Tv Field effect devices
42.40.-i Holography

Characterization of field emission from carbon nanofibers on a metal tip

Y. Sakai, D. Tone, S. Nagatsu, T. Endo, S. Kita, and F. Okuyama

Appl. Phys. Lett. 95, 073104 (2009); http://dx.doi.org/10.1063/1.3205475 (3 pages) | Cited 7 times

Online Publication Date: 18 August 2009

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Field electron emission from carbon nanofibers (CNFs) grown on a tungsten tip has been characterized by measuring emission current-voltage (I-V) curves and observing emission patterns on a phosphor screen. CNFs were vertically grown on the tip by plasma-enhanced chemical vapor deposition. Field emission from the CNFs over 100 μA was strongly dependent on emitter-anode distance, and the dominant field electrons were emitted within an angular spread of Δθ ∼ 25°, indicating the electron emission took place mainly from the emitter’s apex area. By analyzing the I-V curves with the aid of the Fowler–Nordheim theory, the maximum current density was estimated to be about J = 2×109 A/m2.
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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.)
52.77.Dq Plasma-based ion implantation and deposition
81.07.-b Nanoscale materials and structures: fabrication and characterization
81.16.-c Methods of micro- and nanofabrication and processing

Reversible atomic modification of nanostructures on surfaces using direction-depended tip-surface interaction with a trimer-apex tip

Yiqun Xie, Liuxue Ma, Peng Zhang, Xiulong Cai, Wenxian Zhang, Fuxi Gan, Xi-Jing Ning, and Jun Zhuang

Appl. Phys. Lett. 95, 073105 (2009); http://dx.doi.org/10.1063/1.3180814 (3 pages) | Cited 4 times

Online Publication Date: 18 August 2009

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Using first-principles simulations, we propose a simple mechanism and an easy-controlled method for reversible modification of supported nanoclusters on surfaces with atomic precision. As illustrated, individual atoms at edges of a Al nanocluster on a Al(111) surface can be extracted vertically and repositioned with a Al trimer-apex tip, which allows to rearrange a ten-atom hexagonal nanocluster to a triangular one in a reversible way. The governing mechanism is the atomic tip-surface interaction whose distinct dependence on directions plays a key role in manipulations.
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79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)

Universal scaling of semiconductor nanowires bandgap

S. Li and G. W. Yang

Appl. Phys. Lett. 95, 073106 (2009); http://dx.doi.org/10.1063/1.3211128 (3 pages) | Cited 4 times

Online Publication Date: 19 August 2009

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We have established an analytical model to investigate the bandgap energy of III-V and II-V semiconductor nanowires (NWs) by taking size and surface effect into account and found that there is a universal scaling of bandgap between bandgap energy Eg and structural factor K of NWs, Eg ∼ K−1, which meaning that the bandgap energy of semiconductor NWs usually increases with the structural factor decreasing. This scaling rule has a general insight into the basic physics involved in size effect of semiconductor NWs bandgap.
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73.22.-f Electronic structure of nanoscale materials and related systems
73.21.Hb Quantum wires
71.15.-m Methods of electronic structure calculations
73.50.Mx High-frequency effects; plasma effects
73.63.Nm Quantum wires

Microwave propagation in graphene

G. Deligeorgis, M. Dragoman, D. Neculoiu, D. Dragoman, G. Konstantinidis, A. Cismaru, and R. Plana

Appl. Phys. Lett. 95, 073107 (2009); http://dx.doi.org/10.1063/1.3202413 (3 pages) | Cited 12 times

Online Publication Date: 19 August 2009

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We report on measurements and modeling of microwave propagation in graphene. In deep contrast with carbon nanotubes, which display very high impedances in the microwave range, planar waveguides patterned directly on graphene display a 50 Ω impedance, which is tuned slightly by an applied dc. The high values of kinetic impedance in carbon nanotubes were not observed in graphene.
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81.05.ub Fullerenes and related materials
84.40.-x Radiowave and microwave (including millimeter wave) technology

Self-supporting nanowire arrays templated in sacrificial branched porous anodic alumina for thermoelectric devices

Kalapi G. Biswas, Hatem El Matbouly, Vijay Rawat, Jeremy L. Schroeder, and Timothy D. Sands

Appl. Phys. Lett. 95, 073108 (2009); http://dx.doi.org/10.1063/1.3207756 (3 pages) | Cited 2 times

Online Publication Date: 19 August 2009

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Templated synthesis of thermoelectric nanowires in porous anodic alumina (PAA) have potential for enhanced performance relative to bulk materials. A significant challenge is the template material, which can serve as a thermal shunt. In this work, an approach for creating a branched PAA template is described. The process utilizes localized self-heating to destabilize the planar anodization front, yielding branched and interconnected pores growing at a rate of 300 μm/h. The template is selectively etched after electrodeposition of desired materials, yielding self-supporting nanowire arrays with thicknesses up to about 300 μm, thereby eliminating the thermal shunt through the template.
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81.07.Vb Quantum wires
61.43.Gt Powders, porous materials
85.80.Fi Thermoelectric devices

Strong UV absorption and emission from L-cysteine capped monodispersed gold nanoparticles

S. N. Sarangi, A. M. P. Hussain, and S. N. Sahu

Appl. Phys. Lett. 95, 073109 (2009); http://dx.doi.org/10.1063/1.3210788 (3 pages) | Cited 8 times

Online Publication Date: 20 August 2009

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We report a synthesis of L-cysteine capped monodispersed gold (Au) nanoparticles (NPs) with size ∼ 2.0 nm exhibiting a strong surface plasmon resonance optical absorption at 3.13 eV, which is blueshifted by 1.01 eV compared to the uncapped Au NPs of size 20.0 nm. A strong fluorescence (FL) of the capped Au NPs appears at 3.25 eV, whereas the uncapped Au NPs do not show any FL in this range. The L-cysteine concentration has been optimized to achieve one of the strongest ultraviolet absorption and luminescence. The capping of Au NPs has been confirmed by Fourier transform infra red measurement.
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81.16.-c Methods of micro- and nanofabrication and processing
87.14.E- Proteins
87.15.mq Luminescence
87.85.Qr Nanotechnologies-design
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.40.-q Absorption and reflection spectra: visible and ultraviolet

Control morphology of nanostructures with electric field

Jonghyun Park and Wei Lu

Appl. Phys. Lett. 95, 073110 (2009); http://dx.doi.org/10.1063/1.3211415 (3 pages)

Online Publication Date: 20 August 2009

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We showed that the morphology of nanostructures on a dielectric substrate may be tuned by electric field. The collective action of surface energy, interface energy between nanostructures and the substrate, and electrostatic energy defines a thermodynamic force that drives surface diffusion. The evolution is characterized by a quick adjustment of the dihedral angle at the triple junction, followed by an extrusion of a thin layer from the edges, and subsequent significant overall morphology change through long range mass transport. A high voltage can induce instability of the extrusion, causing the formation of a pattern of tiny islands.
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
77.22.Ch Permittivity (dielectric function)
68.55.J- Morphology of films
65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
68.35.Fx Diffusion; interface formation
81.10.Fq Growth from melts; zone melting and refining
68.35.Md Surface thermodynamics, surface energies

Electrical anisotropy in multiscale nanotube/fiber hybrid composites

Erik T. Thostenson, John J. Gangloff, Jr., Chunyu Li, and Joon-Hyung Byun

Appl. Phys. Lett. 95, 073111 (2009); http://dx.doi.org/10.1063/1.3202788 (3 pages) | Cited 12 times

Online Publication Date: 20 August 2009

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This letter reports an experimental and theoretical study on the electrical properties of carbon nanotube/glass fiber composites. Experimental measurements on unidirectional glass fiber composites with nanotubes dispersed in the polymer matrix show a high degree of anisotropy. The composites, manufactured with a vacuum infusion technique, do not show any significant process-induced anisotropy. Theoretical modeling reveals that the microstructure of the fiber composite plays a dominant role in the electrical behavior due to alteration of percolating paths in the carbon nanotube network.
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73.63.Fg Nanotubes
81.16.-c Methods of micro- and nanofabrication and processing
81.05.Ni Dispersion-, fiber-, and platelet-reinforced metal-based composites

Pull-in/out analysis of nano/microelectromechanical switches with defective oxide layers

Yang Xu and N. R. Aluru

Appl. Phys. Lett. 95, 073112 (2009); http://dx.doi.org/10.1063/1.3211111 (3 pages) | Cited 6 times

Online Publication Date: 21 August 2009

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We investigate the effect of surface and interior defects such as vacancies and broken bonds on the performance of nano/microelectromechanical (N/MEMS) switches. By combining multiscale electrostatic analysis with mechanical analysis, we compute the capacitance-voltage and pull-in/out voltages of N/MEMS switches in the presence of defects in the dielectric oxide layer. Our results indicate that both surface and interior defects can change the pull-in/out voltages leading to significant voltage offsets. These voltage offsets can lead to an eventual failure of the N/MEMS switch.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
84.32.Dd Connectors, relays, and switches
07.10.Cm Micromechanical devices and systems
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
61.72.jd Vacancies

Enhanced performance of silicon quantum dot light-emitting diodes grown on nanoroughened silicon substrate

Baek Hyun Kim, Robert F. Davis, Chang-Hee Cho, and Seong-Ju Park

Appl. Phys. Lett. 95, 073113 (2009); http://dx.doi.org/10.1063/1.3211113 (3 pages) | Cited 4 times

Online Publication Date: 21 August 2009

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We report the effect of a nanoroughened Si substrate on silicon quantum dot (Si QD) light-emitting diodes (LEDs). The electroluminescence of Si QD LEDs grown on the nanoroughened Si substrate was remarkably improved by 493% at an injection current of 90 mA compared to those of Si QD LEDs grown on the flat Si substrate. The electrical and optical enhancements were attributed to the enhanced inhomogeneous local electric field on the nanoroughened Si surface and the angular randomization of photons emitted from Si QDs at the nanoroughened surface of silicon nitride layer containing Si QDs.
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85.60.Jb Light-emitting devices

Trimming of aqueous chemically grown ZnO nanorods into ZnO nanotubes and their comparative optical properties

M. Q. Israr, J. R. Sadaf, L. L. Yang, O. Nur, M. Willander, J. Palisaitis, and P. O. Å. Persson

Appl. Phys. Lett. 95, 073114 (2009); http://dx.doi.org/10.1063/1.3211124 (3 pages) | Cited 14 times

Online Publication Date: 21 August 2009

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Highly oriented ZnO nanotubes were fabricated on a silicon substrate by aqueous chemical growth at low temperature (<100 °C) by trimming of ZnO nanorods. The yield of nanotubes in the sample was 100%. Photoluminescence spectroscopy of the nanotubes reveals an enhanced and broadened ultraviolet (UV) emission peak, compared with the initial nanorods. This effect is attributed to whispering gallery mode resonance. In addition, a redshift of the UV emission peak is also observed. Enhancement in the deep defect band emission in the nanotubes compared to nanorods was also manifested as a result of the increased surface area.
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81.07.De Nanotubes
78.67.Ch Nanotubes
81.16.-c Methods of micro- and nanofabrication and processing
81.10.Dn Growth from solutions
61.46.Fg Nanotubes
78.55.Et II-VI semiconductors
81.05.Dz II-VI semiconductors

Photoluminescence and electron paramagnetic resonance studies of springlike carbon nanofibers

Bipin Kumar Gupta, V. Shanker, Manju Arora, and D. Haranath

Appl. Phys. Lett. 95, 073115 (2009); http://dx.doi.org/10.1063/1.3206663 (3 pages) | Cited 4 times

Online Publication Date: 21 August 2009

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Show Abstract
Carbon nanofiber (CNF) with springlike and double-helix structures has been synthesized by catalytic thermal pyrolysis of an acetylene precursor at 850–950 °C using iron nanopowder and thiophene as catalyst and promoter, respectively. High resolution electron microscopy revealed a higher d-spacing ( ∼ 3.46 Å) of (002) crystal plane than customary multiwalled carbon nanotube (MWCNT) (3.37 Å) that helps in sustaining mechanical shocks better than MWCNTs. The large surface to volume ratio of springlike CNF does provide many delocalized free electrons to enhance the photoluminescence activity. Electron paramagnetic resonance signal showed a single narrow line having g-value 2.0024±0.0002 and spin contribution 3.4956×10−16 spins/g.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
81.16.Be Chemical synthesis methods
81.16.Hc Catalytic methods
76.30.-v Electron paramagnetic resonance and relaxation
78.55.Hx Other solid inorganic materials
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Controlled nanoporous Pt morphologies by varying deposition parameters

Antonia Antoniou, Dhriti Bhattacharrya, J. Kevin Baldwin, Peter Goodwin, Michael Nastasi, S. T. Picraux, and Amit Misra

Appl. Phys. Lett. 95, 073116 (2009); http://dx.doi.org/10.1063/1.3194109 (3 pages) | Cited 6 times

Online Publication Date: 21 August 2009

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We synthesize nanoporous platinum (np-Pt) through electrochemical dealloying in aqueous HF from cosputtered PtxSi1−x amorphous films for different initial composition and sputter bias conditions. We demonstrate that, in addition to the expected isotropic open cell np-Pt foam, anisotropic (columnar and Voronoi) np-Pt is obtained. There are two levels of anisotropy: on the micron scale, 100 nm columns or 1 μm Voronoi polygons form. Inside the columnar and Voronoi hyperstructures, the ligaments and pores are anisotropic ranging from 5 to 25 nm. The ligament diameter and grain size is 5 nm for all reported structures. A processing-structure map is developed that correlate np-Pt structures to processing conditions.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
61.43.Gt Powders, porous materials
81.16.-c Methods of micro- and nanofabrication and processing
82.70.Rr Aerosols and foams
81.15.Cd Deposition by sputtering
81.05.Rm Porous materials; granular materials

Tunable thermal conductivity of Si1−xGex nanowires

Jie Chen, Gang Zhang, and Baowen Li

Appl. Phys. Lett. 95, 073117 (2009); http://dx.doi.org/10.1063/1.3212737 (3 pages) | Cited 22 times

Online Publication Date: 21 August 2009

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By using molecular dynamics simulation, we demonstrate that the thermal conductivity of silicon-germanium nanowires (Si1−xGex NWs) depends on the composition remarkably. The thermal conductivity reaches the minimum, which is about 18% of that of pure Si NW, when Ge content is 50%. More interesting, with only 5% Ge atoms (Si0.95Ge0.05 NW), SiNW’s thermal conductivity is reduced to 50%. The reduction of thermal conductivity mainly comes from the localization of phonon modes due to random scattering. Our results demonstrate that Si1−xGex NW might have promising application in thermoelectrics.
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66.70.Df Metals, alloys, and semiconductors
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)
63.22.Gh Nanotubes and nanowires
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
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