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16 Mar 2009

Volume 94, Issue 11, Articles (11xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 94, 111101 (2009); http://dx.doi.org/10.1063/1.3097278 (3 pages)

Ling Lu, Adam Mock, Tian Yang, Min Hsiung Shih, Eui Hyun Hwang, Mahmood Bagheri, Andrew Stapleton, Stephen Farrell, John O’Brien, and P. Daniel Dapkus
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Stranski–Krastanow growth of (11math2)-oriented GaN/AlN quantum dots

L. Lahourcade, S. Valdueza-Felip, T. Kehagias, G. P. Dimitrakopulos, P. Komninou, and E. Monroy

Appl. Phys. Lett. 94, 111901 (2009); http://dx.doi.org/10.1063/1.3095499 (3 pages) | Cited 7 times

Online Publication Date: 16 March 2009

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Semipolar GaN(11math2) deposited on AlN(11math2) by plasma-assisted molecular-beam epitaxy can follow the Frank–Van der Merwe or the Stranski–Krastanow growth mode as a function of the Ga/N ratio. N-rich grown GaN relaxes elastically at a critical thickness but the resulting GaN islands present multiple crystallographic orientations. In contrast, after deposition of a few two-dimensional GaN monolayers under Ga-rich conditions, a growth interruption in vacuum induces (11math2)-oriented islanding. Applying this latter procedure, we have synthesized GaN/AlN quantum dot superlattices with reduced internal electric field.
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68.65.Cd Superlattices
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.07.Ta Quantum dots

Effect of atomic ordering on hydrogen dissociation on Ni3Fe surfaces

X. Y. Cheng, Yip-Wah Chung, Bo Zhao, B. Hong, and C. T. Liu

Appl. Phys. Lett. 94, 111902 (2009); http://dx.doi.org/10.1063/1.3089575 (3 pages) | Cited 2 times

Online Publication Date: 16 March 2009

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The dissociation of molecular to atomic hydrogen on clean surfaces of Ni3Fe with either an ordered or disordered structure has been directly investigated by thermal desorption studies. Results show that when exposed to an ordered Ni3Fe surface, molecular hydrogen dissociates to produce atomic hydrogen, which desorbs from the surface around 350 K. On the other hand, hydrogen does not dissociate on a disordered Ni3Fe surface. These results indicate that atomic ordering has a marked effect on surface chemical reactivity. Furthermore, our study provides a mechanistic basis to explain the hydrogen-induced embrittlement in ordered Ni3Fe but not in disordered Ni3Fe.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
68.43.Nr Desorption kinetics
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Localized modes in capped single-walled carbon nanotubes

Alexander V. Savin and Yuri S. Kivshar

Appl. Phys. Lett. 94, 111903 (2009); http://dx.doi.org/10.1063/1.3100199 (3 pages) | Cited 4 times

Online Publication Date: 16 March 2009

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We study numerically small-amplitude oscillations in capped single-walled carbon nanotubes and predict the existence of surface modes localized at capped tips of the nanotubes. These modes are similar to the surface Tamm states found in solids and optics at the edges of truncated lattices.
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63.22.Gh Nanotubes and nanowires
68.35.Ja Surface and interface dynamics and vibrations
81.07.De Nanotubes

Photoactive periodic media

Tahar Ben-Messaoud, Jason Riordon, Alexandre Melanson, P. V. Ashrit, and Alain Haché

Appl. Phys. Lett. 94, 111904 (2009); http://dx.doi.org/10.1063/1.3095478 (3 pages) | Cited 3 times

Online Publication Date: 17 March 2009

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Photoactive (photochromic) multilayers of MoO3/SiO2 are studied optically before and after UV excitation. Enhancement in photochromic activity is observed over a wide spectrum and in particular near the photonic band edges where optical changes are up to 25 times greater than in similarly prepared bulk, nonperiodic samples. Applications to light-sensitive devices are discussed.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
68.65.Ac Multilayers

Spatially resolved electron energy loss spectroscopy on n-type ultrananocrystalline diamond films

R. Arenal, O. Stephan, P. Bruno, and D. M. Gruen

Appl. Phys. Lett. 94, 111905 (2009); http://dx.doi.org/10.1063/1.3095663 (3 pages) | Cited 2 times

Online Publication Date: 17 March 2009

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The addition of nitrogen to the synthesis gas during synthesis of ultrananocrystalline-diamond (UNCD) films results in films uniquely exhibiting very high n-type electrical conductivity even at ambient temperatures. This result is due to the formation of nanowires having elongated diamond core nanostructures and a sp2-bonded C sheath surrounding the core. The work presented here provides detailed confirmation of this important result through spatially resolved-electron energy loss spectroscopy. The direct observation of nitrogen incorporated in the sheath has been enabled. The incorporation of this nitrogen provides strong support to a plausible mechanism for the n-type conduction characteristic of the UNCD films.
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79.20.Uv Electron energy loss spectroscopy
73.61.-r Electrical properties of specific thin films

UV photovoltaic cells based on conjugated ZnO quantum dot/multiwalled carbon nanotube heterostructures

Fushan Li, Sung Hwan Cho, Dong Ick Son, Tae Whan Kim, Sun-Kyun Lee, Yong-Hoon Cho, and Sungho Jin

Appl. Phys. Lett. 94, 111906 (2009); http://dx.doi.org/10.1063/1.3098400 (3 pages) | Cited 9 times

Online Publication Date: 17 March 2009

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In situ growth of ZnO quantum dots (QDs) on the surface of multiwalled carbon nanotubes (MWCNTs) was realized via a mild solution-process method, which resulted in an improvement in photoinduced charge separation and transport of carriers to the collecting electrode. The charge transfer efficiency was significantly increased by more than 90% due to the conjugation of ZnO QDs with MWCNTs, as confirmed by photoluminescence measurements. Ultraviolet photovoltaic cells based on the charge transfer at the ZnO QD-MWCNT heterostructures were fabricated, and their power conversion efficiency was measured to be above 1%.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.63.Fg Nanotubes
78.67.Ch Nanotubes
84.60.Jt Photoelectric conversion
78.67.Hc Quantum dots
73.63.Kv Quantum dots

Hydrogen diffusion in NaH as derived from isotope exchange experiments

A. Borgschulte, F. Pendolino, R. Gremaud, and A. Züttel

Appl. Phys. Lett. 94, 111907 (2009); http://dx.doi.org/10.1063/1.3103277 (3 pages) | Cited 3 times

Online Publication Date: 17 March 2009

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We report on hydrogen-deuterium-exchange experiments on NaH probed by thermogravimetry. From the measurements a diffusion parameter of deuterium in NaH of D = 1.1×10−17 m2/s at 523 K is derived. The activation energy of tracer diffusion of D in NaH is found to be 1.0 eV. The results are hints for a diffusion process mediated by neutral hydrogen vacancies.
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66.30.H- Self-diffusion and ionic conduction in nonmetals
61.72.jd Vacancies

Time reversal of continuous-wave, steady-state signals in elastic media

Brian E. Anderson, Robert A. Guyer, Timothy J. Ulrich, and Paul A. Johnson

Appl. Phys. Lett. 94, 111908 (2009); http://dx.doi.org/10.1063/1.3097811 (3 pages) | Cited 3 times

Online Publication Date: 19 March 2009

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Experimental observations of spatial focusing of continuous-wave, steady-state elastic waves in a reverberant elastic cavity using time reversal are reported here. Spatially localized focusing is achieved when multiple channels are employed, while a single channel does not yield such focusing. The amplitude of the energy at the focal location increases as the square of the number of channels used, while the amplitude elsewhere in the medium increases proportionally with the number of channels used. The observation is important in the context of imaging in solid laboratory samples as well as problems involving continuous-wave signals in Earth.
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62.30.+d Mechanical and elastic waves; vibrations

Photoluminescence and band gap modulation in graphene oxide

Zhengtang Luo, Patrick M. Vora, Eugene J. Mele, A. T. Charlie Johnson, and James M. Kikkawa

Appl. Phys. Lett. 94, 111909 (2009); http://dx.doi.org/10.1063/1.3098358 (3 pages) | Cited 75 times

Online Publication Date: 19 March 2009

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We report broadband visible photoluminescence from solid graphene oxide, and modifications of the emission spectrum by progressive chemical reduction. The data suggest a gapping of the two-dimensional electronic system by removal of π-electrons. We discuss possible gapping mechanisms, and propose that a Kekule pattern of bond distortions may account for the observed behavior.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.05.ub Fullerenes and related materials
82.30.-b Specific chemical reactions; reaction mechanisms
71.20.Tx Fullerenes and related materials; intercalation compounds
78.55.Hx Other solid inorganic materials

Semiconductor point defect concentration profiles measured using coherent acoustic phonon waves

A. Steigerwald, Y. Xu, J. Qi, J. Gregory, X. Liu, J. K. Furdyna, K. Varga, A. B. Hmelo, G. Lüpke, L. C. Feldman, and N. Tolk

Appl. Phys. Lett. 94, 111910 (2009); http://dx.doi.org/10.1063/1.3099341 (3 pages) | Cited 5 times

Online Publication Date: 19 March 2009

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Coherent acoustic phonon interferometry is used to quantitatively measure depth-dependent point defect concentrations in semiconductor systems with a depth range of the order of tens of microns. Using time-resolved pump-probe techniques, the optical response of ion-beam irradiated GaAs crystals is analyzed as a function of defect concentration ranging over four orders of magnitude. Varying the ion dose quantitatively relates changes in the optical response to local defect concentrations. Thermal annealing is shown to reduce the effect on the optical response, indicating recovery of the crystal lattice through self-interstitial-vacancy recombination.
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61.72.jd Vacancies
61.72.jj Interstitials
61.80.Jh Ion radiation effects
61.72.Cc Kinetics of defect formation and annealing
78.47.D- Time resolved spectroscopy (>1 psec)
62.65.+k Acoustical properties of solids

Conformational characteristics of β-glucan in laminarin probed by terahertz spectroscopy

Hee Jun Shin, Seung Jae Oh, Sung In Kim, Ha Won Kim, and Joo-Hiuk Son

Appl. Phys. Lett. 94, 111911 (2009); http://dx.doi.org/10.1063/1.3100778 (3 pages) | Cited 1 time

Online Publication Date: 19 March 2009

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We measured the binding-state-dependent power absorptions, refractive indices, and dielectric constants of triple-stranded helices (TSHs) and single-stranded helices (SSHs) β-glucans in laminarin using terahertz time-domain spectroscopy (TDS). The SSH β-glucan was obtained from a TSH β-glucan laminarin by a chemical treatment with NaOH solution. The power absorption of TSH β-glucan increased more rapidly than that of the SSH β-glucan with the frequency increment. The refractive index and dielectric constants of TSH β-glucan were also larger than those of the SSH β-glucan. This result implies that terahertz-TDS is a very effective method in classifying the conformational state of β-glucans.
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87.64.K- Spectroscopy
87.15.Pc Electronic and electrical properties
87.15.hp Conformational changes
87.14.Df Carbohydrates
87.15.Fh Bonding; mechanisms of bond breakage

Multilayered optical bit memory with a high signal-to-noise ratio in fluorescent polymethylmethacrylate

Zhaogang Nie, Heungyeol Lee, Hyeonggeun Yoo, Youlee Lee, Younshil Kim, Ki-Soo Lim, and Myeongkyu Lee

Appl. Phys. Lett. 94, 111912 (2009); http://dx.doi.org/10.1063/1.3103365 (3 pages) | Cited 9 times

Online Publication Date: 20 March 2009

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We report on the three-dimensional optical memory utilizing a photoluminescence (PL) change in polymethylmethacrylate. Irradiation with a femtosecond pulsed laser (800 nm, 1 kHz, 100 fs) induced a strong PL spectrum in the visible range, which may result from the photogeneration of emissive radicals. Multilayered patterns were recorded inside the bulk sample by tightly focusing a pulsed laser beam. The pattern images were read out by a reflection-type fluorescent confocal microscope which detected the blue-green emission at 410–510 nm. The stored bits were retrieved with a high signal-to-noise ratio in the absence of any cross-talk.
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42.79.Vb Optical storage systems, optical disks
78.55.Hx Other solid inorganic materials
78.55.Kz Solid organic materials
42.70.-a Optical materials
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Control performance of a single-chip white light emitting diode by adjusting strain in InGaN underlying layer

X. H. Wang, L. W. Guo, H. Q. Jia, Z. G. Xing, Y. Wang, X. J. Pei, J. M. Zhou, and H. Chen

Appl. Phys. Lett. 94, 111913 (2009); http://dx.doi.org/10.1063/1.3103559 (3 pages) | Cited 5 times

Online Publication Date: 20 March 2009

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Light emission from green to white in a single-chip light emitting diode is modulated by adjusting the strain in InGaN underlying layer (UL) embedded below an active layer of InGaN/GaN multiple quantum wells. Transmission electron microscopy combined with x-ray reciprocal space mapping reveals that indium phase separation in InGaN quantum well active layer is enhanced by using a partly relaxed InGaN UL and In-rich quantum dots with different size and indium composition are formed. They emit multicolor lights whose mixing produces white light. Quality of the white light could be controlled by modulation on relaxation degree of the InGaN UL.
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85.60.Jb Light-emitting devices
85.60.Bt Optoelectronic device characterization, design, and modeling
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Valence-differential spectroscopy of Co–Fe cyanide films

Y. Moritomo, F. Nakada, and Y. Kurihara

Appl. Phys. Lett. 94, 111914 (2009); http://dx.doi.org/10.1063/1.3103561 (3 pages) | Cited 3 times

Online Publication Date: 20 March 2009

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Electrochromism is extensively investigated for practical application of display and memory devices. To develop the material, reliable information on the optical and electronic properties of the solid film is indispensable. Here, we propose valence-differential spectroscopy that can selectively extract the spectral components related to the oxidized/reduced metal site. We applied the spectroscopy to Co2+–Fe2+δ and Co2+δ–Fe2+ cyanide films with finely control of averaged valence (δ) of the transition metal by external electric pulses. The spectroscopy revealed transition energy E, width Γ, and oscillator strength f of the spectral components related to the transition metal.
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78.66.-w Optical properties of specific thin films
78.20.Jq Electro-optical effects

Electric field controlled formation and dissociation of multiwalled carbon nanotube conductive pathways in a polymer melt

Cheng Zhang, Jun Zhu, Mi Ouyang, and Chun-an Ma

Appl. Phys. Lett. 94, 111915 (2009); http://dx.doi.org/10.1063/1.3103549 (3 pages) | Cited 10 times

Online Publication Date: 20 March 2009

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Electric field controlled formation and dissociation of multiwalled carbon nanotube (MWCNT) conductive pathways in a polycarbonate (PC) melt are investigated by the dynamic percolation measurement. The results show that field-induced MWCNT alignment causes the decrease in the activation energy of conductive pathway formation. The directional or disordered alignment of MWCNTs in the PC melt results in the transition from a conductor to an insulator as the electric field changes from 500 to 1 V/cm. This electric-controllable directional or disordered alignment technology is promising for the fabrication of low-dimensional conductive materials and applications of voltage-switch devices.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
72.80.Tm Composite materials
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
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