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17 Sep 2007

Volume 91, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 123101 (2007); http://dx.doi.org/10.1063/1.2784389 (3 pages)

Thierry Laroche, Alexandre Vial, and Matthieu Roussey
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Confinement of enhanced field investigated by tip-sample gap regulation in tapping-mode tip-enhanced Raman microscopy

Taka-aki Yano, Taro Ichimura, Atsushi Taguchi, Norihiko Hayazawa, Prabhat Verma, Yasushi Inouye, and Satoshi Kawata

Appl. Phys. Lett. 91, 121101 (2007); http://dx.doi.org/10.1063/1.2785115 (3 pages) | Cited 15 times

Online Publication Date: 17 September 2007

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The authors developed a tip-enhanced near field Raman microscope that can precisely regulate longitudinal distance between a metallic tip and sample molecules. This was done by employing a time-gated photoncounting scheme that enabled us to observe exponentially decaying near field Raman intensity with the tip-sample distance. The exponential decay shows a characteristic of the enhanced field generated by the localization of the surface plasmon polaritons near the tip apex. This microscope was applied to evaluate metal-coated tips and also to investigate confinement of the field generated at a gap between two metal nanostructures from the decay curves.
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07.79.Lh Atomic force microscopes

Studies of polymer microring lasers subject to uniaxial stress

A. Tulek and Z. V. Vardeny

Appl. Phys. Lett. 91, 121102 (2007); http://dx.doi.org/10.1063/1.2785955 (3 pages) | Cited 2 times

Online Publication Date: 17 September 2007

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The emission spectra of microring lasers fabricated from π-conjugated polymer films casted on nylon microfibers with diameters in the range of 35–90 μm were studied upon application of uniaxial stress with strain up to ∼ 12%. The laser emission spectra substantially change with the applied stress, showing enhanced sensitivity to stress over changes induced in the fiber diameter alone. This is explained as due to the induced change in the polymer refractive index spectrum upon stress, causing an unexpected increase in the refractive index dispersion and, consequently, also in the effective refraction index for lasing at emission wavelengths.
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78.66.Qn Polymers; organic compounds
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.55.Wd Fiber lasers
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Microfluidic photonic crystal double heterostructures

Cameron L. C. Smith, Darran K. C. Wu, Michael W. Lee, Christelle Monat, Snjezana Tomljenovic-Hanic, Christian Grillet, Benjamin J. Eggleton, Darren Freeman, Yinlan Ruan, Steve Madden, Barry Luther-Davies, Harald Giessen, and Yong-Hee Lee

Appl. Phys. Lett. 91, 121103 (2007); http://dx.doi.org/10.1063/1.2785988 (3 pages) | Cited 32 times

Online Publication Date: 17 September 2007

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We demonstrate postprocessed and reconfigurable photonic crystal double-heterostructure cavities via selective fluid infiltration. We experimentally investigate the microfluidic cavities via evanescent probing from a tapered fiber at telecommunication wavelengths. Fabry-Pérot fringes associated with modes of the induced cavity are in good agreement with the theory. We also demonstrate a cavity with quality factor Q = 4300. Our defect-writing technique does not require nanometer-scale alterations in lattice geometry and may be undertaken at any time after photonic crystal waveguide fabrication.
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61.72.Qq Microscopic defects (voids, inclusions, etc.)
42.70.Qs Photonic bandgap materials
47.85.Np Fluidics
81.20.-n Methods of materials synthesis and materials processing

Tunable terahertz quantum cascade lasers with an external cavity

Jihua Xu, Joel M. Hensley, D. B. Fenner, Richard P. Green, Lukas Mahler, Alessandro Tredicucci, Mark G. Allen, Fabio Beltram, Harvey E. Beere, and David A. Ritchie

Appl. Phys. Lett. 91, 121104 (2007); http://dx.doi.org/10.1063/1.2786587 (3 pages) | Cited 22 times

Online Publication Date: 17 September 2007

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Tunable quantum cascade lasers operating in the terahertz frequency range are demonstrated. By using an external cavity based on the reflection from a movable mirror, both broad and fine tuning of the frequency are achieved by varying the cavity length. Coarse tuning up to 3 cm−1 is obtained near the center frequency of 4.8 THz ( ∼ 160 cm−1), and continuous mode-hop-free tuning is observed over ∼ 0.4 cm−1, nearly corresponding to the cavity free spectral range.
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42.55.Px Semiconductor lasers; laser diodes

Highly directional emission from photonic crystals with a wide bandwidth

Zhaofeng Li, Koray Aydin, and Ekmel Ozbay

Appl. Phys. Lett. 91, 121105 (2007); http://dx.doi.org/10.1063/1.2786590 (3 pages) | Cited 14 times

Online Publication Date: 17 September 2007

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The authors numerically and experimentally demonstrated highly directional emission from photonic crystals. This was achieved by first splitting the incident electromagnetic wave into multiple beams using photonic crystal waveguide structures. The beams were then emitted out of the surface of a photonic crystal with the same phase, which resulted in a highly directional radiation pattern. The measured half power beam width was 4.8°, which was in good agreement with the calculated value of 4.1°. In contrast to the traditional beaming structures, their design did not involve gratinglike structures, which resulted in a wider operation bandwidth.
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42.70.Qs Photonic bandgap materials
78.45.+h Stimulated emission
42.55.Tv Photonic crystal lasers and coherent effects

Self-sustained pulsation in the oxide-confined vertical-cavity surface-emitting lasers based on submonolayer InGaAs quantum dots

A. G. Kuzmenkov, V. M. Ustinov, G. S. Sokolovskii, N. A. Maleev, S. A. Blokhin, A. G. Deryagin, S. V. Chumak, A. S. Shulenkov, S. S. Mikhrin, A. R. Kovsh, A. D. McRobbie, W. Sibbett, M. A. Cataluna, and E. U. Rafailov

Appl. Phys. Lett. 91, 121106 (2007); http://dx.doi.org/10.1063/1.2784937 (3 pages) | Cited 3 times

Online Publication Date: 18 September 2007

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The authors report the observation of strong self-pulsations in molecular-beam epitaxy–grown oxide-confined vertical-cavity surface-emitting lasers based on submonolayer InGaAs quantum dots. At continuous-wave operation, self-pulsations with pulse durations of 100–300 ps and repetition rates of 0.2–0.6 GHz were measured. The average optical power of the pulsations was 0.5–1.0 mW at the laser continuous-wave current values of 1.5–2.5 mA.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Enhancement of optical absorption and photocurrent of 6H-SiC by laser surface nanostructuring

Q. Z. Zhao, F. Ciobanu, S. Malzer, and L. J. Wang

Appl. Phys. Lett. 91, 121107 (2007); http://dx.doi.org/10.1063/1.2786863 (3 pages) | Cited 12 times

Online Publication Date: 18 September 2007

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Nanoscale ripple structures with spatial periods of up to 155 nm are generated by a single beam ultrashort pulsed laser structuring of 6H-SiC surface. The period of nanoripples does not depend on the laser fluence and the number of pulses. Optical absorption and photocurrent measurements on SiC samples are investigated before and after laser nanostructuring. Nearly 40% enhancement of optical absorption and a factor of 3 increase of photocurrent are achieved after laser nanostructuring.
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81.16.Mk Laser-assisted deposition
81.16.Rf Micro- and nanoscale pattern formation
81.05.Hd Other semiconductors
72.40.+w Photoconduction and photovoltaic effects
78.47.-p Spectroscopy of solid state dynamics
78.66.Li Other semiconductors

The impact of thermal effects on emission characteristics of asymmetrical AlGAO-waveguide microdisks based on quantum dots

S. A. Blokhin, A. V. Sakharov, A. M. Nadtochy, M. M. Kulagina, Yu. M. Zadiranov, N. Yu. Gordeev, M. V. Maximov, V. M. Ustinov, N. N. Ledentsov, E. Stock, T. Warming, and D. Bimberg

Appl. Phys. Lett. 91, 121108 (2007); http://dx.doi.org/10.1063/1.2785127 (3 pages)

Online Publication Date: 18 September 2007

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Microphotoluminescence studies of AlGaO-based asymmetric waveguide microdisk structures incorporating a single sheet of self-organized InAs quantum dots (QD) under photoexcitation are performed. The authors have observed a strong redshift and broadening of the QD optical transition energy at high excitation densities due to excitation-density-induced overheating and many-particle effects and monitored the impact of these effects on the whispering gallery modes (WGMs) in spontaneous and stimulated emission regimes. They have observed strongly nonmonotonic dependencies of the linewidth and intensity of the WGMs with excitation density increase and underline possible reasons for the observed behavior.
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42.82.Et Waveguides, couplers, and arrays
42.79.Gn Optical waveguides and couplers

Enhancement of the light output performance for GaN-based light-emitting diodes by bottom pillar structure

Chien-Chun Wang, Han Ku, Chien-Chih Liu, Kwok-Keung Chong, Chen-I Hung, Yeong-Her Wang, and Mau-Phon Houng

Appl. Phys. Lett. 91, 121109 (2007); http://dx.doi.org/10.1063/1.2786015 (3 pages) | Cited 9 times

Online Publication Date: 18 September 2007

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A three-dimensional model with finite difference and time domain was established to investigate the enhancement of the light output intensity of GaN light-emitting diodes (LEDs) with bottom pillar (BP) structure. Through comparing the normalized light extraction intensity of GaN LEDs with or without BP in different dimensions, the theoretical results show that the light output intensity in the LED with BP structure involved could be enhanced by about 30%. The influence of BP structure on the light output intensity of a LED could be explained by the physical model of light interaction. In addition, the experimental results also show the same trend to the theoretical calculations.
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85.60.Jb Light-emitting devices
02.70.Bf Finite-difference methods

Parasitic sub-band-gap emission originating from compensating native defects in Si doped AlGaN

K. X. Chen, Q. Dai, W. Lee, J. K. Kim, E. F. Schubert, W. Liu, S. Wu, X. Li, and J. A. Smart

Appl. Phys. Lett. 91, 121110 (2007); http://dx.doi.org/10.1063/1.2786838 (3 pages) | Cited 2 times

Online Publication Date: 19 September 2007

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The authors investigate the dependence of the photoluminescence intensity ratio between the near-band-edge and the parasitic blue emission as a function of silane flow in n-type Al0.3Ga0.7N. It is found that when the silane flow rate is higher than 1.13×10−9 mol/min, the UV-to-blue ratio is a constant. When the silane flow rate is lower than 1.13×10−9 mol/min, the UV-to-blue ratio increases rapidly as the silane flow rate decreases. A theoretical model is proposed, which assumes that the parasitic blue emission is caused by an acceptorlike compensating native defect. The model is fully consistent with the experimental results.
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78.55.Cr III-V semiconductors
71.55.Eq III-V semiconductors

Vibrational resonance enhanced broadband multiphoton absorption in a triphenylamine derivative

Changgui Lu, Yiping Cui, Wei Huang, Binfeng Yun, Zhuyuan Wang, Guohua Hu, Jing Cui, Zhifeng Lu, and Ying Qian

Appl. Phys. Lett. 91, 121111 (2007); http://dx.doi.org/10.1063/1.2783270 (3 pages) | Cited 2 times

Online Publication Date: 19 September 2007

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Multiphoton absorption of 2,5-bis[4-(2-N,N-diphenylaminostyryl)phenyl]-1,3,4-oxadiazole was experimentally studied by using femtosecond laser pulses. This material demonstrates a very broad multiphoton absorption band of around 300 nm width with two peaks of 1250 and 1475 nm. The first peak results from the three-photon absorption process while the second is attributed to the vibrational resonance enhanced four-photon absorption process. Combination of these two processes provides a much broader multiphoton absorption band. In this letter, the analytical solution to nonlinear transmission of a three-photon absorption process is also given when the incident beam has a Gaussian transverse spatial profile.
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42.65.Re Ultrafast processes; optical pulse generation and pulse compression
78.47.-p Spectroscopy of solid state dynamics
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Pv Polymers, organic compounds
42.70.Jk Polymers and organics

Growth of plasmonic gold nanostructures by electron beam induced deposition

S. Graells, R. Alcubilla, G. Badenes, and R. Quidant

Appl. Phys. Lett. 91, 121112 (2007); http://dx.doi.org/10.1063/1.2786600 (3 pages) | Cited 15 times

Online Publication Date: 19 September 2007

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The authors report on the growth of organometallic dots for optical applications using electron beam induced gold deposition on a transparent substrate. The effects of the substrate material and the deposition parameters, such as beam current and water vapor pressure, on both the deposition rate and gold purity are investigated. Ex situ annealing of the sample is used as a purity improvement method. Scattering optical measurements on the purified dots reveal that they support localized surface plasmon resonances. This technique opens new perspectives in the fabrication of substantial height-base aspect ratio plasmonic nanostructures and may become particularly relevant for nonflat substrates.
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81.07.Bc Nanocrystalline materials
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.22.Lp Collective excitations
78.66.Bz Metals and metallic alloys
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.40.Gh Other heat and thermomechanical treatments

ZnO-based light-emitting metal-insulator-semiconductor diodes

Dae-Kue Hwang, Min-Suk Oh, Jae-Hong Lim, Yong-Seok Choi, and Seong-Ju Park

Appl. Phys. Lett. 91, 121113 (2007); http://dx.doi.org/10.1063/1.2786603 (3 pages) | Cited 15 times

Online Publication Date: 19 September 2007

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The ZnO-based metal-insulator-semiconductor (MIS) diode was fabricated by using an insulator ZnO layer and an n-ZnO layer grown by radio frequency magnetron sputtering. The current-voltage of the ZnO MIS diodes showed a good diode characteristic with a threshold voltage of 8.9 V and a band-edge emission at 380 nm at room temperature. The electroluminescence emission of ZnO MIS was attributed to the generation of holes in the insulating ZnO layer at the high threshold voltage of 8.9 V via an impact ionization process.
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85.60.Jb Light-emitting devices
85.40.Sz Deposition technology

Study of the structural damage in the (0001) GaN epilayer processed by laser lift-off techniques

W. H. Chen, X. N. Kang, X. D. Hu, R. Lee, Y. J. Wang, T. J. Yu, Z. J. Yang, G. Y. Zhang, L. Shan, K. X. Liu, X. D. Shan, L. P. You, and D. P. Yu

Appl. Phys. Lett. 91, 121114 (2007); http://dx.doi.org/10.1063/1.2783134 (3 pages) | Cited 9 times

Online Publication Date: 20 September 2007

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The structural influences of the laser lift-off (LLO) techniques on the created (0001) GaN surface region are characterized by cross-sectional high-resolution transmission electron microscopy and fitted using the model of stress waves caused by a longitudinal impact at the end of a cylindrical bar extending to infinity. The authors study reveals that, in addition to the superficial damage caused by laser absorption, the stress saltation in GaN crystal where the shock waves come into being induces deformation of the lattices and generates a cluster of half loops above the LLO interface. After that, the lattice deformation will be induced every time the partial dissipation of the steady-state shock waves takes place until the shock wave is dissipated to elastic mode.
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81.05.Ea III-V semiconductors
68.60.Bs Mechanical and acoustical properties
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
42.62.-b Laser applications
62.50.-p High-pressure effects in solids and liquids

Noninvasive optical amplification and detection of Faraday rotation

J. M. LaForge and G. M. Steeves

Appl. Phys. Lett. 91, 121115 (2007); http://dx.doi.org/10.1063/1.2785111 (3 pages) | Cited 1 time

Online Publication Date: 20 September 2007

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The authors present a Mach-Zehnder-type interferometer with orthogonally polarized arms that passively, noninvasely, and optically amplifies Faraday rotation. At ambient temperatures, the interferometer, with an optical gain factor of ∼ 4.5, outperforms the optical bridge (OB) technique with a detection limit of approximately 50 mrad, compared to 250 mrad for the OB, when both techniques are used for time resolved faraday rotation under similar experimental conditions. Isolation and active stabilization of the interferometer have reduced sources of noise so that the interferometer’s signal is also quieter than the OB.
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07.60.Ly Interferometers
78.20.Ls Magneto-optical effects

Tunable magneto-optical wavelength filter of long-period fiber grating with magnetic fluids

Ting Liu, Xianfeng Chen, Ziyun Di, Junfeng Zhang, Xinwan Li, and Jianping Chen

Appl. Phys. Lett. 91, 121116 (2007); http://dx.doi.org/10.1063/1.2787970 (3 pages) | Cited 14 times

Online Publication Date: 20 September 2007

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In this paper, the authors propose a magneto-optical tunable filter based on a long-period fiber grating (LPG) coated with magnetic fluids (MFs) as the ambient media. By applying a tunable magnetic field, the center wavelength shift of the attenuation band of LPG is found as large as 7.4 nm. The refractive index dependence of MF on the external magnetic intensity is measured and the simulation results show that it is well agreeable with the experimental observations.
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85.70.Sq Magnetooptical devices
42.79.Ci Filters, zone plates, and polarizers
75.50.Mm Magnetic liquids

Correlation between biaxial stress and free exciton transition in AlN epilayers

B. N. Pantha, N. Nepal, T. M. Al Tahtamouni, M. L. Nakarmi, J. Li, J. Y. Lin, and H. X. Jiang

Appl. Phys. Lett. 91, 121117 (2007); http://dx.doi.org/10.1063/1.2789182 (3 pages) | Cited 15 times

Online Publication Date: 20 September 2007

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Photoluminescence (PL) spectroscopy and x-ray diffraction measurements were employed to study biaxial strain in AlN epilayers grown on different substrates. X-ray diffraction revealed that AlN epilayers grown on AlN bulk substrates (or homoepilayers) have the same lattice parameters as AlN bulk crystals and are almost strain-free. Compared to the free exciton (FX) transition in an AlN homoepilayer, the FX line was 31 meV higher in AlN/sapphire due to a compressive strain and 55 (69) meV lower in AlN/SiC (AlN/Si) due to a tensile strain. A linear relationship between the FX transition energy peak position and in-plane stress was obtained, and a value of 45 meV/GPa for the linear coefficient of the stress-induced bandgap shift in AlN epilayers was deduced. The work here establishes PL as another simple and effective method for monitoring the biaxial stress in AlN epilayers.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
68.60.Bs Mechanical and acoustical properties
71.35.Cc Intrinsic properties of excitons; optical absorption spectra

Reversible color switching from blue to red in a polymer stabilized chiral nematic liquid crystals

Jing Chen, Stephen M. Morris, Timothy D. Wilkinson, and Harry J. Coles

Appl. Phys. Lett. 91, 121118 (2007); http://dx.doi.org/10.1063/1.2789290 (3 pages) | Cited 16 times

Online Publication Date: 20 September 2007

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In this letter, the authors demonstrate reversible color switching from blue to red in polymer stabilized chiral nematic liquid crystals when viewed at oblique angles. A series of unusual colored scattering states, different from those noted in polymer-free samples, was observed in cells that had been prepared using a technique of cooling the sample from the isotropic phase in the presence of a low frequency (80 Hz) applied electric field and then photopolymerizing the sample on both sides of the cell. This process “freezes-in” a texture whereby the helix axis lies preferentially in the plane of the cell.
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61.30.Vx Polymer liquid crystals

All semiconductor swept laser source utilizing quantum dots

B. J. Stevens, D. T. D. Childs, K. M. Groom, M. Hopkinson, and R. A. Hogg

Appl. Phys. Lett. 91, 121119 (2007); http://dx.doi.org/10.1063/1.2789179 (3 pages) | Cited 4 times

Online Publication Date: 21 September 2007

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An all semiconductor swept laser source demonstrating continuous sweeping over a >11 nm range with a linewidth suitable for optical coherence tomography is proposed and demonstrated. The operation of this device relies upon state filling in a multiple contact laser utilizing quantum dots with strongly overlapping ground and excited states.
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42.55.Px Semiconductor lasers; laser diodes

Optical gain measurements in porous silicon planar waveguides codoped by erbium and ytterbium ions at 1.53 μm

Adel Najar, Joël Charrier, Nathalie Lorrain, Lazhar Haji, and Mehrezi Oueslati

Appl. Phys. Lett. 91, 121120 (2007); http://dx.doi.org/10.1063/1.2789185 (3 pages) | Cited 6 times

Online Publication Date: 21 September 2007

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The on-off optical gain measurements as a function of the pump power were performed on porous silicon planar waveguides codoped by erbium and ytterbium ions. These measurements were obtained for different ratios of Yb concentration to Er concentration. The highest value of the gain was reached when the Yb concentration is three times higher than that of Er at a moderate 980 nm pump power value equal to 70 mW. Optical losses measurements have been performed on these waveguides and were equal to 2.1 dB/cm and an internal gain of about 6.4 dB/cm was obtained.
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42.79.Gn Optical waveguides and couplers
42.82.-m Integrated optics

A direct first principles study on the structure and electronic properties of BexZn1−xO

X. F. Fan, Zexuan Zhu, Yew-Soon Ong, Y. M. Lu, Z. X. Shen, and Jer-Lai Kuo

Appl. Phys. Lett. 91, 121121 (2007); http://dx.doi.org/10.1063/1.2789692 (3 pages) | Cited 11 times

Online Publication Date: 21 September 2007

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We present a systematic study on the structural and electronic properties of all alloy configurations of BexZn1−xO in a unit cell with 16 cations using density functional theory (DFT) methods. The 216 complexity is reduced by considering the symmetry of the parent structures. The experimental structures and electronic properties of the bulk material are reasonably reproduced by the DFT methods. The lattice constants of the alloy are found to follow Vegard’s law [Z. Phys. 5, 17 (1921)] and are comparable with the experimental values. Examining the formation enthalpy of all alloy configurations suggests the possible existence of three metastable order states. The calculated band gap of the BexZn1−xO is also compared with the experimental measurements and the authors found that some alloy configurations with the same concentration can have band gaps differed by ∼ 1.5 eV.
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71.20.Nr Semiconductor compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
61.66.Fn Inorganic compounds
65.40.G- Other thermodynamical quantities
82.60.Cx Enthalpies of combustion, reaction, and formation
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Multibubble plasma production and solvent decomposition in water by slot-excited microwave discharge

T. Ishijima, H. Hotta, H. Sugai, and M. Sato

Appl. Phys. Lett. 91, 121501 (2007); http://dx.doi.org/10.1063/1.2783209 (3 pages) | Cited 20 times

Online Publication Date: 17 September 2007

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Intense microwaves are injected from a slot antenna into water partly filling a metal vessel. When the vessel is evacuated to saturated vapor pressure ( ∼ 5×103 Pa) of water, microwave breakdown gives rise to plasmas in many bubbles in the boiling water. Gas bubbling technique enables production of multibubble plasmas in water even at atmospheric pressure. Optical emissions from the exited species are investigated to identify radical species in water. In order to demonstrate application to purification of polluted water, methylene blue and trichlorethylene solution in 8 l water were observed to rapidly decrease with multibubble plasma treatment.
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52.50.-b Plasma production and heating
52.80.Pi High-frequency and RF discharges

Direct measurements of neutral density depletion by two-photon absorption laser-induced fluorescence spectroscopy

A. Aanesland, L. Liard, G. Leray, J. Jolly, and P. Chabert

Appl. Phys. Lett. 91, 121502 (2007); http://dx.doi.org/10.1063/1.2786601 (3 pages) | Cited 18 times

Online Publication Date: 19 September 2007

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The ground state density of xenon atoms has been measured by spatially resolved laser-induced fluorescence spectroscopy with two-photon excitation in the diffusion chamber of a magnetized Helicon plasma. This technique allows the authors to directly measure the relative variations of the xenon atom density without any assumptions. A significant neutral gas density depletion was measured in the core of the magnetized plasma, in agreement with previous theoretical and experimental works. It was also found that the neutral gas density was depleted near the radial walls.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.25.Xz Magnetized plasmas
52.25.Ya Neutrals in plasmas
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.40.Hf Plasma-material interactions; boundary layer effects

Microplasma synthesis of metal nanoparticles for gas-phase studies of catalyzed carbon nanotube growth

Wei-Hung Chiang and R. Mohan Sankaran

Appl. Phys. Lett. 91, 121503 (2007); http://dx.doi.org/10.1063/1.2786835 (3 pages) | Cited 23 times

Online Publication Date: 20 September 2007

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Catalytic properties of metal nanoparticles toward gas-phase carbon nanotube (CNT) growth are presented. Narrow dispersions of iron (Fe) and nickel (Ni) nanoparticles are prepared in a direct current microplasma reactor and subsequently introduced with acetylene (C2H2) and hydrogen (H2) into a heated flow furnace to catalyze CNT growth. Aerosol size classification and high-resolution transmission electron microscopy show that CNT growth occurs on Ni particles at lower temperatures than that for similarly produced Fe nanoparticles. Activation energies of 117 and 73 kJ/mol are found for Fe and Ni catalyst particles, respectively, suggesting that CNT growth occurs by carbon surface diffusion.
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81.16.Hc Catalytic methods
61.46.Fg Nanotubes
81.07.De Nanotubes
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Displacement current and surface flashover

J. R. Harris, G. J. Caporaso, D. Blackfield, and Y.-J. Chen

Appl. Phys. Lett. 91, 121504 (2007); http://dx.doi.org/10.1063/1.2785116 (3 pages) | Cited 3 times

Online Publication Date: 21 September 2007

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High-voltage vacuum insulator failure is generally due to surface flashover rather than insulator bulk breakdown. Vacuum surface flashover is widely believed to be initiated by a secondary electron emission avalanche along the vacuum-insulator interface. This process requires a physical mechanism to cause secondary electrons emitted from the insulator surface to return to that surface. Here, it is shown that when an insulator is subjected to a fast high-voltage pulse, the magnetic field due to displacement current through the insulator can provide this mechanism. This indicates the importance of the voltage pulse shape, especially the rise time, in the flashover initiation process.
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52.80.Vp Discharge in vacuum
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