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24 Sep 2007

Volume 91, Issue 13, Articles (13xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 132501 (2007); http://dx.doi.org/10.1063/1.2786856 (3 pages)

V. Rose, X. M. Cheng, D. J. Keavney, J. W. Freeland, K. S. Buchanan, B. Ilic, and V. Metlushko
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Narrow-band detection of propagating coherent acoustic phonons in piezoelectric InGaN/GaN multiple-quantum wells

Cheng-Ying Chen, Yu-Chieh Wen, Hung-Ping Chen, Tzu-Ming Liu, Chang-Chi Pan, Jen-Inn Chyi, and Chi-Kuang Sun

Appl. Phys. Lett. 91, 133101 (2007); http://dx.doi.org/10.1063/1.2785126 (3 pages) | Cited 4 times

Online Publication Date: 24 September 2007

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The authors demonstrated that the piezoelectric superlattice, can serve as narrow-band detectors for propagating coherent longitudinal acoustic phonons at multiple frequencies corresponding to the spatial frequency of the superlattice and its higher harmonics, and its detection bandwidth is determined by the total structure width. By optically exciting a broadband propagating longitudinal acoustic pulse from a thin Ni film, the authors studied the acoustic spectral sensitivity function of a ten-period In0.12Ga0.88N/GaN multiple quantum well. Because the barriers (19 nm) and wells (3.6 nm) are of different widths, the second detection band, corresponding to the second harmonic of the fundamental frequency, can be resolved.
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63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
78.20.hb Piezo-optical, elasto-optical, acousto-optical, and photoelastic effects

In situ health monitoring and repair in composites using carbon nanotube additives

W. Zhang, V. Sakalkar, and N. Koratkar

Appl. Phys. Lett. 91, 133102 (2007); http://dx.doi.org/10.1063/1.2783970 (3 pages) | Cited 7 times

Online Publication Date: 24 September 2007

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The authors demonstrate a simple, effective and real-time diagnostic, and repair technique featuring electrically conductive carbon nanotube additives that are infiltrated into the host structure. They show that by monitoring volume and through-thickness resistance, one can determine the extent and propagation of fatigue-induced damage such as crack and delamination growth in the vicinity of stress concentrations. The conductive nanotube network also provides opportunities to repair damage by enabling fast heating of the crack interfaces; the authors show up to 70% recovery of the strength of the undamaged composite. These advances could result in enhanced safety, reliability, and service life for polymer composites that are used in engineering applications.
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61.46.Fg Nanotubes
46.50.+a Fracture mechanics, fatigue and cracks

Bright white upconversion luminescence in rare-earth-ion-doped Y2O3 nanocrystals

G. Y. Chen, Y. Liu, Y. G. Zhang, G. Somesfalean, Z. G. Zhang, Q. Sun, and F. P. Wang

Appl. Phys. Lett. 91, 133103 (2007); http://dx.doi.org/10.1063/1.2787893 (3 pages) | Cited 66 times

Online Publication Date: 25 September 2007

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Room temperature bright white upconversion (UC) luminescence in Yb3+–Tm3+–Er3+ ions doped Y2O3 nanocrystals was obtained under single-wavelength diode laser excitation of 976 nm. The white light consists of the blue, green, and red UC radiations which correspond to the transitions 1G4→3H6 of Tm3+, 2H11/2/4S3/2→4I15/2, and 4F9/2→4I15/2 of Er3+ ions, respectively. The UC mechanisms were proposed based on spectral, kinetic, and pump power dependence analyses. The calculated color coordinates display that white light can be achieved in a wide range of pumping powers, which promises their potential applications in the field of displays, lasers, photonics, and biomedicine.
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78.55.Hx Other solid inorganic materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters

Single dot spectroscopy of site-controlled InAs quantum dots nucleated on GaAs nanopyramids

T. Tran, A. Muller, C. K. Shih, P. S. Wong, G. Balakrishnan, N. Nuntawong, J. Tatebayashi, and D. L. Huffaker

Appl. Phys. Lett. 91, 133104 (2007); http://dx.doi.org/10.1063/1.2790498 (3 pages) | Cited 15 times

Online Publication Date: 26 September 2007

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Single InAs quantum dots, site-selectively grown by a patterning and regrowth technique, were probed using high-resolution low-temperature microphotoluminescence spectroscopy. Systematic measurements on many individual dots show a statistical distribution of homogeneous linewidths with a peak value of ∼ 120 μeV, exceeding that of unpatterned dots but comparing well with previously reported patterning approaches. The linewidths do not appear to depend upon the specific facet on which the dots grow and often can reach the spectrometer resolution limit (<100 μeV). These measurements show that the site-selective growth approach can controllably position the dots with good optical quality, suitable for constrained structures such as microcavities.
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78.67.Hc Quantum dots
78.55.Cr III-V semiconductors
81.07.Ta Quantum dots
81.16.Rf Micro- and nanoscale pattern formation

Green light-emitting diodes with self-assembled In-rich InGaN quantum dots

Il-Kyu Park, Min-Ki Kwon, Jeom-Oh Kim, Seong-Bum Seo, Ja-Yeon Kim, Jae-Hong Lim, Seong-Ju Park, and Yoon-Seok Kim

Appl. Phys. Lett. 91, 133105 (2007); http://dx.doi.org/10.1063/1.2790783 (3 pages) | Cited 20 times

Online Publication Date: 26 September 2007

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A green light-emitting diode (LED) was fabricated using self-assembled In-rich InGaN quantum dots (QDs). The photoluminescence studies showed that the QDs provide thermally stable deeply localized recombination sites for carriers with negligibly small piezoelectric field. The electroluminescence spectra of the LED showed a peak in the green spectral range and the dominant peak was blueshifted with increasing injection current due to the distribution of depth of the potential wells of QDs. The output power of the LED increased with increasing injection current, indicating that the potential wells are thermally stable and deeply localized in the QDs.
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85.60.Jb Light-emitting devices

Controlled production of emulsion drops using an electric field in a flow-focusing microfluidic device

Haejune Kim, Dawei Luo, Darren Link, David A. Weitz, Manuel Marquez, and Zhengdong Cheng

Appl. Phys. Lett. 91, 133106 (2007); http://dx.doi.org/10.1063/1.2790785 (3 pages) | Cited 30 times

Online Publication Date: 26 September 2007

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We describe a flexible emulsification method using an electric field to generate droplets in a hydrodynamic-flow-focusing geometry in microchannels. The droplet size is controlled by the ratio of inner and outer flow rates as well as by the electric field. As the voltage increases, the droplet size decreases. A Taylor cone is formed and generates very fine droplets, less than 1 μm in diameter. Small inner flow rates and high electric fields are required to form a stable Taylor cone in a dc electric field. An ac electric field produces tiny droplets periodically.
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47.55.db Drop and bubble formation
47.57.Bc Foams and emulsions
47.65.-d Magnetohydrodynamics and electrohydrodynamics
47.61.Fg Flows in micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS)
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.85.Dh Hydrodynamics, hydraulics, hydrostatics

Highly confined energy propagation in a gap waveguide composed of two coupled nanorod chains

F. M. Wang, H. Liu, T. Li, S. M. Wang, S. N. Zhu, Jie Zhu, and Wenwu Cao

Appl. Phys. Lett. 91, 133107 (2007); http://dx.doi.org/10.1063/1.2790786 (3 pages) | Cited 2 times

Online Publication Date: 26 September 2007

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We propose a subwavelength waveguide composed of two parallel nanorod chains. Based on the finite-difference time-domain analysis, we find that the electromagnetic energy can be highly confined in the gaps of nanorod pairs and transported in the gap waveguide through strong magnetic coupling interaction between neighboring nanorod pairs. In a structure with the rod length of 500 nm and the gap size of 100 nm, the energy flow cross section of the propagation mode can be restricted to the size of λ/33×λ/16 at the frequency of 130.0 THz. The corresponding attenuation length of energy propagation reaches 7.2λ. Moreover, these propagation modes exhibit a broad continuous frequency band from zero up to a cutoff frequency ωc ∼ 162.6 THz.
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42.79.Gn Optical waveguides and couplers

Doped gallium oxide nanowires with waveguiding behavior

Emilio Nogales, José Ángel García, Bianchi Méndez, and Javier Piqueras

Appl. Phys. Lett. 91, 133108 (2007); http://dx.doi.org/10.1063/1.2790809 (3 pages) | Cited 22 times

Online Publication Date: 26 September 2007

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Er and Cr doped β-Ga2O3 nano- and microwires have been grown by thermal treatments of Ga2O3 powder containing a fraction of Er2O3 powder or in the presence of Cr2O3 powder, respectively. Doping gives rise to the characteristic red photoluminescence (PL) of Cr3+ ions at about 700 nm and to green emission of Er3+ at about 555 nm. Waveguiding of the ion related PL light excited in the wires has been studied for wires with different sizes. Waveguide behavior for incident visible light of different wavelengths was demonstrated.
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81.16.-c Methods of micro- and nanofabrication and processing
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
78.55.Hx Other solid inorganic materials
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
81.40.Gh Other heat and thermomechanical treatments
61.72.up Other materials

Role of oxygen migration in the kinetics of the phase separation of nonstoichiometric silicon oxide films during high-temperature annealing

A. Sarikov, V. Litovchenko, I. Lisovskyy, I. Maidanchuk, and S. Zlobin

Appl. Phys. Lett. 91, 133109 (2007); http://dx.doi.org/10.1063/1.2790814 (3 pages) | Cited 5 times

Online Publication Date: 26 September 2007

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The kinetics of phase separation and growth of Si precipitates during high-temperature annealing of nonstoichiometric SiOx films is theoretically studied. The mechanisms of silicon diffusion and capture by Si precipitates as well as oxygen emission and out diffusion from the Si precipitate interface toward the silicon oxide bulk are compared. The experimental kinetics of phase separation within 1 s annealing SiOx films at 1000 °C can be explained by the latter mechanism involving either neutral or negatively charged oxygen atoms. The maximum values of the activation energy of oxygen emission are estimated to match the simulation results to the experimental data.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.A- Nucleation and growth
64.75.-g Phase equilibria
66.30.J- Diffusion of impurities
81.40.Gh Other heat and thermomechanical treatments

Room temperature gas sensing properties of SnO2/multiwall-carbon-nanotube composite nanofibers

An Yang, Xiaoming Tao, Rongxing Wang, Shuncheng Lee, and Charles Surya

Appl. Phys. Lett. 91, 133110 (2007); http://dx.doi.org/10.1063/1.2783479 (3 pages) | Cited 19 times

Online Publication Date: 26 September 2007

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Pure SnO2 and SnO2 polycrystalline nanofibers doped with multiwall carbon nanotubes (MWCNTs) are synthesized by electrospinning followed by calcination in air at 500 °C. The measurement results by sensors fabricated from these fiber mats at steady state show that the n-type SnO2/MWCNT nanofibers are able to detect carbon monoxide at 50 ppm at room temperature, while the pure SnO2 nanofibers are insensitive up to 500 ppm. The MWCNT doped SnO2 nanofibers have demonstrated their potentials for wearable room temperature gas sensors with low cost and power consumption.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
82.80.-d Chemical analysis and related physical methods of analysis
85.35.Kt Nanotube devices

Magnetic circular dichroism in Co1−xPtx nanowire bundles at the Co L2,3 edges

Wuxia Li, Jun Zhang, Jamie Thompson, Tiehan H. Shen, Elke Arenholz, Simon A. Morton, and Mark Ellerby

Appl. Phys. Lett. 91, 133111 (2007); http://dx.doi.org/10.1063/1.2784186 (3 pages) | Cited 3 times

Online Publication Date: 26 September 2007

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Co1−xPtx nanowire arrays embedded in anodic aluminum oxide templates were fabricated by an electrochemical route. X-ray magnetic circular dichroism (XMCD) measurements were conducted at the Co L2,3 edges on samples with an array of nanowire bundles at the substrate surfaces. The ratios between the orbital and the spin moments R were estimated to be about 0.19 (±0.03) and 0.07 (±0.03) for 20 nm Co and 14 nm Co91Pt9 nanowires, respectively. A strong temperature dependence of the R value was observed. The x-ray absorption spectra also indicated the presence of CoO, which did not contribute to the XMCD signals.
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78.67.Lt Quantum wires
78.20.Ls Magneto-optical effects
78.70.Dm X-ray absorption spectra
75.75.-c Magnetic properties of nanostructures
75.30.Cr Saturation moments and magnetic susceptibilities
68.65.La Quantum wires (patterned in quantum wells)

Synthesis of high purity Au nanobelts via the one-dimensional self-assembly of triangular Au nanoplates

Jianhui Zhang, Huaiyong Liu, Zhenlin Wang, and Naiben Ming

Appl. Phys. Lett. 91, 133112 (2007); http://dx.doi.org/10.1063/1.2790578 (3 pages) | Cited 12 times

Online Publication Date: 27 September 2007

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High purity ( ∼ 88%) gold nanobelts have been synthesized in the water/polyvinylpyrrolidone (PVP)/n-pentanol system created to realize the selective-adsorption of PVP on Au which directs Au to grow in belt form via the one-dimensional (1D) self-assembly of triangular Au nanoplates. These nanobelts present uniform thickness and lateral dimension, large aspect ratio up to 160, unique 60° angle end structure, and double-peak plasmon resonance property. The PVP directed 1D self-assembly method demonstrated here may provide a general route for the shape-selective synthesis of the other 1D functional nanostructures.
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61.46.-w Structure of nanoscale materials
81.16.Dn Self-assembly
68.43.Mn Adsorption kinetics
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Edge effects in buckled thin films on elastomeric substrates

C. T. Koh, Z. J. Liu, D.-Y. Khang, J. Song, C. Lu, Y. Huang, J. A. Rogers, and C. G. Koh

Appl. Phys. Lett. 91, 133113 (2007); http://dx.doi.org/10.1063/1.2791004 (3 pages) | Cited 8 times

Online Publication Date: 27 September 2007

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Buckled thin films on elastomeric substrates have many applications. Films of this type exhibit periodic, sinusoidal “wavy” relief profiles, except near edges that lie perpendicular to the wavevector associated with waves. In these locations, the amplitudes of the waves steadily decrease until the films become completely flat, in a manner that can be used to advantage in applications. This paper quantitatively describes the mechanics of this phenomenon. The finite element analysis shows that the edge effect results from the traction-free boundary condition. The edge-effect length is proportional to the thin-film thickness, and decreases with the increasing prestrain and substrate modulus.
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68.60.Bs Mechanical and acoustical properties
62.30.+d Mechanical and elastic waves; vibrations
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

InAs nanocrystals on SiO2/Si by molecular beam epitaxy for memory applications

Moïra Hocevar, Philippe Regreny, Armel Descamps, David Albertini, Guillaume Saint-Girons, Abdelkader Souifi, Michel Gendry, and Gilles Patriarche

Appl. Phys. Lett. 91, 133114 (2007); http://dx.doi.org/10.1063/1.2793694 (3 pages) | Cited 7 times

Online Publication Date: 28 September 2007

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We studied a memory structure based on InAs nanocrystals grown by molecular beam epitaxy directly on thermal SiO2 on silicon. Both nanocrystal diameter and density can be controlled by growth parameters. Transmission electron microscopy analysis shows high crystallinity and low size dispersion. In an electrical test structure with a 3.5 nm tunnel oxide, we observed that 80% of the initial injected electrons remain stored in the InAs nanocrystals after 3 months and that the retention time for electrons in InAs nanocrystals is four orders of magnitude higher than in silicon nanocrystals.
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81.05.Ea III-V semiconductors
81.07.Bc Nanocrystalline materials
68.55.A- Nucleation and growth
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
73.61.Ey III-V semiconductors
73.40.Ty Semiconductor-insulator-semiconductor structures

Evolution of InAs branches in InAs/GaAs nanowire heterostructures

M. Paladugu, J. Zou, G. J. Auchterlonie, Y. N. Guo, Y. Kim, H. J. Joyce, Q. Gao, H. H. Tan, and C. Jagadish

Appl. Phys. Lett. 91, 133115 (2007); http://dx.doi.org/10.1063/1.2790486 (3 pages) | Cited 18 times

Online Publication Date: 28 September 2007

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Branched nanowire heterostructures of InAs/GaAs were observed during Au-assisted growth of InAs on GaAs nanowires. The evolution of these branches has been determined through detailed electron microscopy characterization with the following sequence: (1) in the initial stage of InAs growth, the Au droplet is observed to slide down the side of the GaAs nanowire, (2) the downward movement of Au nanoparticle later terminates when the nanoparticle encounters InAs growing radially on the GaAs nanowire sidewalls, and (3) with further supply of In and As vapor reactants, the Au nanoparticles assist the formation of InAs branches with a well-defined orientation relationship with GaAs/InAs core/shell stems. We anticipate that these observations advance the understanding of the kink formation in axial nanowire heterostructures.
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81.07.Vb Quantum wires
81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.65.La Quantum wires (patterned in quantum wells)
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Lp Transmission electron microscopy (TEM)

Coupling in concentric double quantum rings

A. Mühle, W. Wegscheider, and R. J. Haug

Appl. Phys. Lett. 91, 133116 (2007); http://dx.doi.org/10.1063/1.2790828 (3 pages) | Cited 14 times

Online Publication Date: 28 September 2007

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We designed a device consisting of two concentric quantum rings in a GaAs/AlGaAs heterostructure. The outer ring is connected to leads while the inner ring is only capacitively coupled to the rest of the system. Measuring electronic transport in a regime with strong coupling to the leads we found that the conductivity not only shows an Aharonov-Bohm effect with a periodicity fitting to the diameter of the outer ring but also another component matching the inner ring. This can be explained by magnetic-field dependent charge redistributions in the inner ring which affect the outer ring via Coulomb interaction.
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73.21.La Quantum dots
72.80.Ey III-V and II-VI semiconductors
73.23.Ad Ballistic transport

Electron tunneling from quantum dots characterized by deep level transient spectroscopy

O. Engström, M. Kaniewska, M. Kaczmarczyk, and W. Jung

Appl. Phys. Lett. 91, 133117 (2007); http://dx.doi.org/10.1063/1.2790846 (3 pages) | Cited 10 times

Online Publication Date: 28 September 2007

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Electron tunneling from InAs/GaAs quantum dots has been studied by deep level transient spectroscopy (DLTS). Comparing DLTS data with theory, we demonstrate how the results can be interpreted for situations where the emission mechanism is pure tunneling. An illusory anomalous tunneling dependence on electric field is resolved by taking into account the energy level distribution originating from size fluctuations in the quantum dot ensemble.
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73.40.Gk Tunneling
73.63.Kv Quantum dots
73.61.Ey III-V semiconductors
73.21.La Quantum dots

A dynamic approach to liquid crystal simulations

Jones T. K. Wan

Appl. Phys. Lett. 91, 133118 (2007); http://dx.doi.org/10.1063/1.2789695 (3 pages)

Online Publication Date: 28 September 2007

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An efficient simulation scheme is proposed to determine liquid crystal configurations in complex physical environments. In this simulation scheme, liquid crystal elastic energy, electrostatic interaction, and the surface anchoring effect are calculated via a dynamic approach that is analogous to molecular dynamics simulation. As a result, various techniques that have been established for molecular dynamics are readily adopted in the proposed scheme. The method is demonstrated by calculating the director field of a liquid crystal interacting with a patchy pattern.
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61.30.Cz Molecular and microscopic models and theories of liquid crystal structure
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order

Indium selenide nanowire phase-change memory

Bin Yu, Sanghyun Ju, Xuhui Sun, Garrick Ng, Thuc Dinh Nguyen, M. Meyyappan, and David B. Janes

Appl. Phys. Lett. 91, 133119 (2007); http://dx.doi.org/10.1063/1.2793505 (3 pages) | Cited 17 times

Online Publication Date: 28 September 2007

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Nonvolatile memory device using indium selenide nanowire as programmable resistive element was fabricated and its resistive switching property was studied as functions of electrical pulse width and voltage magnitude. The nanowire memory can be repeatedly switched between high-resistance ( ∼ 1011 Ω) and low-resistance ( ∼ 6×105 Ω) states which are attributed to amorphous and crystalline states, respectively. Once set to a specific state, the nanowire resistance is stable as measured at voltages up to 2 V. This observation suggests that the nanowire can be programed into two distinct states with a large on-off resistance ratio of ∼ 105 with significant potential for nonvolatile information storage.
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85.35.-p Nanoelectronic devices
85.30.-z Semiconductor devices
84.30.Sk Pulse and digital circuits
73.63.Nm Quantum wires
72.80.Ey III-V and II-VI semiconductors
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