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16 Jan 2006

Volume 88, Issue 3, Articles (03xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 88, 034101 (2006); http://dx.doi.org/10.1063/1.2164910 (3 pages)

W. K. Hensinger, S. Olmschenk, D. Stick, D. Hucul, M. Yeo, M. Acton, L. Deslauriers, C. Monroe, and J. Rabchuk
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NiO films consisting of vertically aligned cone-shaped NiO rods

Zhengjun Zhang, Ye Zhao, and Minmin Zhu

Appl. Phys. Lett. 88, 033101 (2006); http://dx.doi.org/10.1063/1.2166479 (3 pages) | Cited 26 times

Online Publication Date: 18 January 2006

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By thermally heating a nickel foil in a vacuum of ∼ 5×10−2 Torr, films consisting of vertically aligned cone-shaped NiO microrods were deposited on Si (001) substrates at a temperature of <300 °C. The NiO rods were grown along 〈001〉 directions by stacking the NiO (001) nanoslices, and are ∼ 10 μm long with a sharp nanosized tip. Due to this morphology, the NiO film exhibited a threshold field of ∼ 6.5 V/μm in field emission and a field enhancement factor of 2130 that is sufficiently high for field emission applications. The optical band gap of the NiO film was estimated to be ∼ 3.68 eV from the optical absorption measurement and was almost a constant upon heating. In addition, the NiO film exhibited a strong photoluminescence at ∼ 674 nm when excited by a 514 nm Ar+ laser, which might be attributed to the oxygen vacancies.
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68.55.-a Thin film structure and morphology
79.70.+q Field emission, ionization, evaporation, and desorption
68.55.A- Nucleation and growth
81.40.Gh Other heat and thermomechanical treatments

Enhanced field emission from ZnO nanorods via thermal annealing in oxygen

Q. Zhao, X. Y. Xu, X. F. Song, X. Z. Zhang, D. P. Yu, C. P. Li, and L. Guo

Appl. Phys. Lett. 88, 033102 (2006); http://dx.doi.org/10.1063/1.2166483 (3 pages) | Cited 71 times

Online Publication Date: 18 January 2006

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To optimize the field emission behavior of the ZnO nanorods, postthermal annealing in different ambience was conducted. The field emission properties of the ZnO nanorods are considerably improved after annealing in oxygen and getting worse when annealing in air or ammonia. Photoluminescence and Raman spectroscopy were employed to elucidate the reason for such a significant improvement of the field emission when annealing in oxygen. Those detailed analyses suggested that oxygen annealing can reduce the oxygen vacancy concentration, improve the crystal quality, lower the work function, and increase the conductivity of the ZnO nanorods. Our work is important for applications of ZnO nanorods as a promising candidate in flat panel displays and high brightness electron sources.
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79.70.+q Field emission, ionization, evaporation, and desorption
73.63.Bd Nanocrystalline materials
78.30.Fs III-V and II-VI semiconductors
78.55.Et II-VI semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
61.72.Cc Kinetics of defect formation and annealing

Growth of aligned carbon nanotubes on carbon microfibers by dc plasma-enhanced chemical vapor deposition

L.-H. Chen, J. F. AuBuchon, I.-C. Chen, C. Daraio, X.-R. Ye, A. Gapin, S. Jin, and C. M. Wang

Appl. Phys. Lett. 88, 033103 (2006); http://dx.doi.org/10.1063/1.2166472 (3 pages) | Cited 10 times

Online Publication Date: 18 January 2006

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It is shown that unidirectionally aligned carbon nanotubes can be grown on electrically conductive network of carbon microfibers via control of buffer layer material and applied electric field during dc plasma chemical vapor deposition growth. Ni catalyst deposition on carbon microfiber produces relatively poorly aligned nanotubes with significantly varying diameters and lengths obtained. The insertion of Ti 5 nm thick underlayer between Ni catalyst layer and C microfiber substrate significantly alters the morphology of nanotubes, resulting in much better aligned, finer diameter, and longer array of nanotubes. This beneficial effect is attributed to the reduced reaction between Ni and carbon paper, as well as prevention of plasma etching of carbon paper by inserting a Ti buffer layer. Such a unidirectionally aligned nanotube structure on an open-pore conductive substrate structure may conveniently be utilized as a high-surface-area base electrodes for fuel cells, batteries, and other electrochemical and catalytic reactions.
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81.07.De Nanotubes
81.05.U- Carbon/carbon-based materials
82.45.Yz Nanostructured materials in electrochemistry
52.77.Dq Plasma-based ion implantation and deposition
61.46.Fg Nanotubes
81.16.Hc Catalytic methods
82.45.Fk Electrodes

Direct deposition of continuous metal nanostructures by thermal dip-pen nanolithography

B. A. Nelson, W. P. King, A. R. Laracuente, P. E. Sheehan, and L. J. Whitman

Appl. Phys. Lett. 88, 033104 (2006); http://dx.doi.org/10.1063/1.2164394 (3 pages) | Cited 32 times

Online Publication Date: 18 January 2006

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We describe the deposition of continuous metal nanostructures onto glass and silicon using a heated atomic force microscope cantilever. Like a miniature soldering iron, the cantilever tip is coated with indium metal, which can be deposited onto a surface forming lines of a width less than 80 nm. Deposition is controlled using a heater integrated into the cantilever. When the cantilever is unheated, no metal is deposited from the tip, allowing the writing to be registered to existing features on the surface. We demonstrate direct-write circuit repair by writing an electrical connection between two metal electrodes separated by a submicron gap.
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81.07.Bc Nanocrystalline materials
81.16.Nd Micro- and nanolithography
81.16.Ta Atom manipulation
85.40.Hp Lithography, masks and pattern transfer
68.37.Ps Atomic force microscopy (AFM)

Fabrication and microwave absorption of carbon nanotubes/CoFe2O4 spinel nanocomposite

R. C. Che, C. Y. Zhi, C. Y. Liang, and X. G. Zhou

Appl. Phys. Lett. 88, 033105 (2006); http://dx.doi.org/10.1063/1.2165276 (3 pages) | Cited 90 times

Online Publication Date: 18 January 2006

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A large-scale carbon nanotube/CoFe2O4 (CNTs/CoFe2O4) spinel nanocomposite has been fabricated by a chemical vapor deposition method using CoFe2O4 nanoparticles as catalysts. A uniform mixture of CNTs and CoFe2O4 nanoparticles was obtained simultaneously. The structure and chemical composition of the product were investigated using various techniques, such as x-ray diffraction, high-resolution transmission electron microscopy, and electron energy loss spectroscopy. It was found that the particles functionalized on CNTs were cubic phase CoFe2O4. Microwave absorption of the CNT/CoFe2O4 nanocomposites at 2–18 GHz is evidently enhanced, as compared with that of both pure CNTs and CoFe2O4 nanoparticles. The enhancement mechanism is discussed based on magnetization hysteresis loop measurement and electromagnetic theory.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
81.16.Hc Catalytic methods
82.80.-d Chemical analysis and related physical methods of analysis
68.37.Lp Transmission electron microscopy (TEM)

Electrospinning of silica nanochannels for single molecule detection

Miao Wang, Nan Jing, Chin B. Su, Jun Kameoka, Chao-Kai Chou, Mien-Chie Hung, and Kuang-An Chang

Appl. Phys. Lett. 88, 033106 (2006); http://dx.doi.org/10.1063/1.2165277 (3 pages) | Cited 23 times

Online Publication Date: 18 January 2006

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We have fabricated silica nanochannels with inner diameter as small as 20 nm using a scanned coaxial electrospinning and demonstrated their application for single molecule detection. A coaxial jet, with the use of motor oil as the core and silica sol-gel solution as the shell, is extruded through a coaxial source and deposited on the rotating collector as oriented nanofibers. They are then annealed to cross-link silica and eliminate motor oil, thereby forming nanochannels. Subsequently, a fluorescent dye was injected into the individual nanochannels via a capillary force and single molecule detection was performed by monitoring the photon signals from 5-Iodoacetamidofluorescein.
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81.07.Bc Nanocrystalline materials
81.16.-c Methods of micro- and nanofabrication and processing

Room-temperature chemical vapor deposition and mass detection on a heated atomic force microscope cantilever

Erik O. Sunden, Tanya L. Wright, Jungchul Lee, William P. King, and Samuel Graham

Appl. Phys. Lett. 88, 033107 (2006); http://dx.doi.org/10.1063/1.2164916 (3 pages) | Cited 23 times

Online Publication Date: 19 January 2006

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This letter reports the localized room-temperature chemical vapor deposition of carbon nanotubes (CNTs) onto an atomic force microscope cantilever having an integrated heater, using the cantilever self-heating to provide temperatures required for CNT growth. Precise temperature calibration of the cantilever was possible and the CNTs were synthesized at a cantilever heater temperature of 800 °C in reactive gases at room temperature. Scanning electron microscopy confirmed the CNTs were vertically aligned and highly localized to only the heater area of the cantilever. The cantilever mechanical resonance decreased from 119.10 kHz to 118.23 kHz upon CNT growth, and then returned to 119.09 kHz following cantilever cleaning, indicating a CNT mass of 1.4×10−14 kg. This technique for highly local growth and measurement of deposited CNTs creates new opportunities for interfacing nanomaterials with microstructures.
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81.07.De Nanotubes
61.46.Fg Nanotubes
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Temperature-dependent resistance changes in invar alloy nanocontact

P. Xu, K. Xia, H. F. Yang, J. J. Li, and C. Z. Gu

Appl. Phys. Lett. 88, 033108 (2006); http://dx.doi.org/10.1063/1.2166694 (3 pages)

Online Publication Date: 19 January 2006

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A nanocontact structure of invar alloy is fabricated by using electron beam lithography and lift-off technique. The current-voltage (I-V) characteristic is measured under various temperatures from 10 to 300 K. We find that the I-V curves are nonlinear and asymmetric, and the resistance change increases when the temperature decreases down to 50 K. We attribute this effect to spin electron scattering by a domain wall trapped in the nanocontact. We also show that the anomaly is not observed in Cu nanocontacts. There is almost no resistance change in a Cu nanocontact with a change in the bias voltage.
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73.63.Rt Nanoscale contacts
75.70.Kw Domain structure (including magnetic bubbles and vortices)

Quadratic electro-optic effect in a nano-optical material based on the nonconjugated conductive polymer, poly(β-pinene)

H. Rajagopalan, P. Vippa, and M. Thakur

Appl. Phys. Lett. 88, 033109 (2006); http://dx.doi.org/10.1063/1.2166197 (3 pages) | Cited 10 times

Online Publication Date: 19 January 2006

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Large quadratic electro-optic effect has been observed in a nano-optical material based on a nonconjugated conductive polymer, poly(β-pinene). Doping of poly(β-pinene) leads to subnanometer size charge-transfer sites with positive charges (holes). The observed large electro-optic effect has been attributed to these confined systems with special structures. The magnitude of the Kerr constant increases with the concentration of the dopant or charge-transfer sites. The maximum Kerr coefficient at 633 nm which is close to resonance is about 50 times that of nitrobenzene.
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78.20.Jq Electro-optical effects
42.65.-k Nonlinear optics
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
42.70.Jk Polymers and organics

Formation of nanoporous noble metal thin films by electrochemical dealloying of PtxSi1−x

J. C. Thorp, K. Sieradzki, Lei Tang, P. A. Crozier, Amit Misra, Michael Nastasi, David Mitlin, and S. T. Picraux

Appl. Phys. Lett. 88, 033110 (2006); http://dx.doi.org/10.1063/1.2161939 (3 pages) | Cited 45 times

Online Publication Date: 19 January 2006

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We demonstrate the synthesis of nanoporous Pt thin films on Si by electrochemical dealloying. Amorphous PtxSi1−x films ( ∼ 100–250 nm thick) are formed by electron beam codeposition and dealloyed in aqueous HF solutions at an electrochemical potential sufficient to selectively remove Si while allowing self-assembly of Pt into a nanoporous structure. The Pt nanoporous layers have a pore size of 5–20 nm, ligament thickness ∼ 5 nm, a surface area enhancements >20 times, and an ultrafine grain polycrystalline microstructure.
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81.05.Rm Porous materials; granular materials
81.07.-b Nanoscale materials and structures: fabrication and characterization
61.43.Gt Powders, porous materials
61.43.Dq Amorphous semiconductors, metals, and alloys
82.45.Aa Electrochemical synthesis
81.16.Dn Self-assembly

Electrodeposition of iron core-shell nanoparticles on a H-terminated Si(100) surface

L. Y. Zhao, K. R. Eldridge, K. Sukhija, H. Jalili, N. F. Heinig, and K. T. Leung

Appl. Phys. Lett. 88, 033111 (2006); http://dx.doi.org/10.1063/1.2165180 (3 pages) | Cited 13 times

Online Publication Date: 19 January 2006

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Iron core-shell nanoparticles with different morphologies were obtained on a H-terminated Si(100) surface for the first time by electrodeposition at different FeCl3 concentrations. At 10 mM, near-monosized and uniformly distributed oval-shaped nanorod Fe particles (130 nm length×25 nm diameter), with a mixed FeOOH and FeO shell, have been obtained. At 0.1 mM, spherical Fe nanoparticles (6–40 nm diameter) with a broader size distribution and a mixed Fe2O3 and FeO shell were observed. The morphologies and compositions of these two types of Fe core-shell nanoparticles could be easily manipulated to provide magnetic properties of potential industrial interest.
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81.07.Bc Nanocrystalline materials
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)
82.45.Qr Electrodeposition and electrodissolution

Filling of single silicon nanocrystals within multiwalled carbon nanotubes

V. Švrček, C. Pham-Huu, M.-J. Ledoux, F. Le Normand, O. Ersen, and S. Joulie

Appl. Phys. Lett. 88, 033112 (2006); http://dx.doi.org/10.1063/1.2166482 (3 pages) | Cited 7 times

Online Publication Date: 20 January 2006

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We demonstrate here a simple approach to introduce the ex situ synthesized silicon nanocrystals (Si-ncs) embedded in SiO2 based spin on glass (SOG) within multiwalled carbon nanotubes (MWNTs) at room temperature and atmospheric pressure. After opening the ends of MWNTs and declustering the Si-ncs dispersed in the SOG solution, it is possible to introduce them inside MWNTs channel through capillary effect. In this letter, we present the initial explorations of carrying the stabilized Si-ncs in SOG inside nanotubes far from their ends by capillary forces. The results here described have important implications to further expand the useability of Si-ncs/MWNTs and might find very important applications in optoelectronics.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
81.05.Cy Elemental semiconductors
81.16.-c Methods of micro- and nanofabrication and processing
61.46.-w Structure of nanoscale materials
61.46.Fg Nanotubes
68.03.Kn Dynamics (capillary waves)

Electron microscopy study on structure of rolled-up semiconductor nanotubes

N. Y. Jin-Phillipp, J. Thomas, M. Kelsch, Ch. Deneke, R. Songmuang, and O. G. Schmidt

Appl. Phys. Lett. 88, 033113 (2006); http://dx.doi.org/10.1063/1.2164913 (3 pages) | Cited 17 times

Online Publication Date: 20 January 2006

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By releasing strained epitaxial layers from their substrates, the layers roll up to form nanotubes. The structures of InAs/GaAs and SiGe/Si rolled-up nanotubes (RUNTs) are characterized by using transmission electron microscopy (TEM), electron diffraction, high-resolution TEM, and spatially resolved electron energy-loss spectroscopy. Freestanding RUNTs as well as their cross sections are investigated. It is found that the walls of the nanotubes are composed of alternating crystalline and noncrystalline oxide-containing layers. Defects may form in some nanotubes, where the rolling involves a misorientation.
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81.07.De Nanotubes
61.46.Fg Nanotubes
79.20.Uv Electron energy loss spectroscopy

Optical emission spectroscopy study for optimization of carbon nanotubes growth by a triode plasma chemical vapor deposition

Sung Hoon Lim, Hyun Sik Yoon, Jong Hyun Moon, Kyu Chang Park, and Jin Jang

Appl. Phys. Lett. 88, 033114 (2006); http://dx.doi.org/10.1063/1.2166690 (3 pages) | Cited 15 times

Online Publication Date: 20 January 2006

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We carried out the in situ analysis of chemical species for the growth of carbon nanotubes (CNTs), deposited by a triode plasma enhanced chemical vapor deposition with a C2H2 and NH3 mixture, using optical emission spectroscopy (OES). A positive mesh bias enhances the radical density, thus increasing the growth rate. The vertically aligned CNTs were grown at a 50% C2H2 flow rate ratio to NH3 and mesh bias voltage of +300 V, resulting from the increased CH radical density and the decreased H and CN radical density through the OES analysis.
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78.67.Ch Nanotubes
78.60.-b Other luminescence and radiative recombination
81.07.De Nanotubes
52.77.Dq Plasma-based ion implantation and deposition

Quantum dot formation and dynamic scaling behavior of SnO2 nanocrystals induced by pulsed delivery

Z. W. Chen, J. K. L. Lai, and C. H. Shek

Appl. Phys. Lett. 88, 033115 (2006); http://dx.doi.org/10.1063/1.2162689 (3 pages) | Cited 6 times

Online Publication Date: 20 January 2006

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Quantum dot formation and dynamic scaling behavior of SnO2 nanocrystals in coalescence regime for growth by pulsed-laser deposition is explored experimentally and theoretically, and the same is compared with that for continuous vapor deposition such as molecular-beam epitaxy. Using high-resolution transmission electron microscopy, unusual quantum dots of SnO2 nanocrystals are studied. We present kinetic Monte-Carlo simulations for pulsed-laser deposition in the submonolayer regime and give a description of the island distance versus pulse intensity. We found that the scaling exponent for pulsed-laser deposition is 1.28±0.03, which is significantly lower as compared to that for molecular-beam epitaxy (1.62±0.03). Theoretical simulations reveal that this attractive difference can be pursued to the large fraction of multiple droplet coalescence under pulsed vapor delivery.
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81.07.Ta Quantum dots
81.15.Fg Pulsed laser ablation deposition
68.65.Hb Quantum dots (patterned in quantum wells)

Enhancement of field emission characteristics of tungsten emitters by single-walled carbon nanotube modification

D. Ferrer, T. Tanii, I. Matsuya, G. Zhong, S. Okamoto, H. Kawarada, T. Shinada, and I. Ohdomari

Appl. Phys. Lett. 88, 033116 (2006); http://dx.doi.org/10.1063/1.2165205 (3 pages) | Cited 15 times

Online Publication Date: 20 January 2006

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We present a simple method for improving the field emission performance of tungsten-tip electron sources based on single-walled carbon nanotube (SWCNT) modification. By coating a sandwich-like thin film of Al–Fe–Al (with Fe as a catalyst) on a tungsten tip, SWCNTs were synthesized at 600 °C in a chemical vapor deposition (CVD) reactor. The influence of CNT modification on the electron emission characteristics of the emitters was investigated by means of a triode structure. We have found that CNT-modified tungsten tips exhibit low threshold-voltage for electron emission, and improved emission-current stability, compared with nonmodified and Al–Fe–Al-coated needles.
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79.70.+q Field emission, ionization, evaporation, and desorption
85.35.Kt Nanotube devices
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