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23 Aug 2010

Volume 97, Issue 8, Articles (08xxxx)

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Appl. Phys. Lett. 97, 081901 (2010); http://dx.doi.org/10.1063/1.3457448 (3 pages)

Zhaofeng Li, Rongkuo Zhao, Thomas Koschny, Maria Kafesaki, Kamil Boratay Alici, Evrim Colak, Humeyra Caglayan, Ekmel Ozbay, and C. M. Soukoulis
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Chiral metamaterials with negative refractive index based on four “U” split ring resonators

Zhaofeng Li, Rongkuo Zhao, Thomas Koschny, Maria Kafesaki, Kamil Boratay Alici, Evrim Colak, Humeyra Caglayan, Ekmel Ozbay, and C. M. Soukoulis

Appl. Phys. Lett. 97, 081901 (2010); http://dx.doi.org/10.1063/1.3457448 (3 pages) | Cited 19 times

Online Publication Date: 23 August 2010

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A uniaxial chiral metamaterial is constructed by double-layered four “U” split ring resonators mutually twisted by 90°. It shows a giant optical activity and circular dichroism. The retrieval results reveal that a negative refractive index is realized for circularly polarized waves due to the large chirality. The experimental results are in good agreement with the numerical results.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.20.Fm Birefringence
78.20.Ek Optical activity

Polarization sensitive lateral photoconductivity in GaAs/AlGaAs quantum well based structures on low-temperature grown GaAs(001)

Ashish Arora, Sandip Ghosh, B. M. Arora, Stefan Malzer, and Gottfried Döhler

Appl. Phys. Lett. 97, 081902 (2010); http://dx.doi.org/10.1063/1.3479501 (3 pages)

Online Publication Date: 23 August 2010

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Polarization-resolved lateral-photoconductivity measurements are reported on device structures made of GaAs/Al0.3Ga0.7As quantum wells sandwiched between low-temperature grown GaAs(001) layers. The mesa device structures have long length (3 mm∥y) and narrow width (10 and 20 μm∥x) in the (001) plane. For light incident along [001], the ground state light-hole exciton transition is much stronger for light polarization E∥x, compared to E∥y. The heavy-hole exciton transition shows a weaker polarization anisotropy of opposite sign, being stronger for E∥y. Through calculations based on the Bir–Pikus Hamiltonian, the observed in-plane optical polarization anisotropy is shown to arise from valence band mixing induced by anisotropic strain in the plane of quantum wells.
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85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Benefits of homoepitaxy on the properties of nonpolar (Zn,Mg)O/ZnO quantum wells on a-plane ZnO substrates

J.-M. Chauveau, M. Teisseire, H. Kim-Chauveau, C. Deparis, C. Morhain, and B. Vinter

Appl. Phys. Lett. 97, 081903 (2010); http://dx.doi.org/10.1063/1.3481078 (3 pages) | Cited 18 times

Online Publication Date: 24 August 2010

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We report on the properties of nonpolar (Zn,Mg)O/ZnO quantum wells (QW) homoepitaxially grown by molecular beam epitaxy on a-plane ZnO substrates. We demonstrate a drastic improvement of the structural properties. We compare the photoluminescence properties of nonpolar homoepitaxial QWs and nonpolar heteroepitaxial QWs grown on sapphire and show that the reduction in structural defects and the improvement of surface morphology are correlated with a strong enhancement of the photoluminescence properties: reduction in full width at half maximum, strong increase in the luminescence intensities and their thermal stability. The comparison convincingly demonstrates the interest of homoepitaxial nonpolar QWs for bright UV emission applications.
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81.07.St Quantum wells
78.55.Et II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.35.B- Structure of clean surfaces (and surface reconstruction)

Passive self-tuning energy harvester for extracting energy from rotational motion

Lei Gu and Carol Livermore

Appl. Phys. Lett. 97, 081904 (2010); http://dx.doi.org/10.1063/1.3481689 (3 pages) | Cited 7 times

Online Publication Date: 24 August 2010

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This paper presents experiments and models of a passive self-tuning energy harvester for rotational vibration applications. Tensile stress due to centrifugal force in a radially oriented piezoelectric cantilever beam passively tunes the resonant frequency so that the harvester remains at or near its resonant frequency. Because centrifugal force is proportional to the square of driving frequency, the resonant frequency of an optimized harvester can track and match the driving frequency over a wide frequency range. An analytical model is presented to explain the harvester’s operation, advantages, and design parameter selection. A prototype demonstrated significantly improved performance compared with an untuned harvester.
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84.60.-h Direct energy conversion and storage
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White light emission from amorphous silicon oxycarbide (a-SiCxOy) thin films: Role of composition and postdeposition annealing

Spyros Gallis, Vasileios Nikas, Himani Suhag, Mengbing Huang, and Alain E. Kaloyeros

Appl. Phys. Lett. 97, 081905 (2010); http://dx.doi.org/10.1063/1.3482938 (3 pages) | Cited 3 times

Online Publication Date: 24 August 2010

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The effects of carbon and postdeposition annealing on white luminescence are studied in amorphous silicon oxycarbide (a-SiCxOy) films grown by chemical vapor deposition. The films showed strong room-temperature luminescence in a broad spectral range from blue-violet to near infrared, depending on excitation energy. Photoluminescence (PL) intensity exhibited good correlation with SiOC bond concentration. At low C (<5%), matrix PL was completely quenched after annealing in O2 even at 500 °C. PL was unaffected by O2 annealing at higher C, and could be enhanced when excited by an ultraviolet laser. These findings are correlated to C- and Si-related O defect centers as luminescence sources in a-SiCxOy.
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78.55.Qr Amorphous materials; glasses and other disordered solids
61.43.Dq Amorphous semiconductors, metals, and alloys
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
78.66.Jg Amorphous semiconductors; glasses
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Long-range spontaneous structural ordering in barium stannate thin films

R. Takahashi, K. Valset, E. Folven, E. Eberg, J. K. Grepstad, and T. Tybell

Appl. Phys. Lett. 97, 081906 (2010); http://dx.doi.org/10.1063/1.3481364 (3 pages) | Cited 1 time

Online Publication Date: 25 August 2010

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We have studied spontaneous structural ordering in epitaxial barium stannate thin films, grown on SrTiO3(001) substrates by pulsed laser deposition. X-ray diffraction analysis revealed satellite reflections around the Bragg peaks, indicative of nanoscale periodic ordering in the out-of-plane direction. We find that mass transport strongly affects this spontaneous ordering, and that the thin film growth rate can be used to tune the periodicity of the superlattices, here between 4.3 and 57.7 nm.
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77.55.Px Epitaxial and superlattice films
81.15.Fg Pulsed laser ablation deposition
68.55.A- Nucleation and growth

Effects of release holes on microscale solid–solid phononic crystals

Y. M. Soliman, M. F. Su, Z. C. Leseman, C. M. Reinke, I. El-Kady, and R. H. Olsson, III

Appl. Phys. Lett. 97, 081907 (2010); http://dx.doi.org/10.1063/1.3476354 (3 pages) | Cited 2 times

Online Publication Date: 25 August 2010

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Solid–solid phononic crystals exhibit wider band gaps than those observed with air–solid phononic crystals. For micromachined phononic crystal devices it is advantageous to release the phononic crystal to avoid propagation losses. In a solid–solid phononic crystal operating in the low megahertz range, due to the large lattice constant, it is necessary to place release holes in the center of the inclusions to release devices from the substrate while minimizing the effect the release hole has on the band gap. In this report, we investigate the effect of release holes on phononic band gaps and highlight the need for careful design. It was determined that release holes of radius rair holes/rinclusion = 0.26 can reliably release a phononic crystal membrane composed of W inclusions in SiO2 without significantly compromising the phononic band gap.
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62.65.+k Acoustical properties of solids
07.10.Cm Micromechanical devices and systems
61.72.Qq Microscopic defects (voids, inclusions, etc.)
61.66.Fn Inorganic compounds

Optical properties of surface modified polypropylene by plasma immersion ion implantation technique

Sk. Faruque Ahmed, Myoung-Woon Moon, Chansoo Kim, Yong-Jun Jang, Seonghee Han, Jin-Young Choi, Won-Woong Park, and Kwang-Ryeol Lee

Appl. Phys. Lett. 97, 081908 (2010); http://dx.doi.org/10.1063/1.3481417 (3 pages) | Cited 3 times

Online Publication Date: 25 August 2010

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The optical band gap and activation energy of polypropylene (PP) induced by an Ar plasma immersion ion implantation technique were studied in detail. It was revealed that the structural alternation with an increase in polymer chain cross-linking in the ion beam affected layer enhanced the optical properties of PP. The optical band gap, calculated from the transmittance spectra, decreased from 3.44 to 2.85 eV with the Ar plasma ion energy from 10 to 50 keV. The activation energy, determined from the band tail of the transmittance spectra, decreased while the electrical conductivity increased with the Ar plasma ion energy.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
61.41.+e Polymers, elastomers, and plastics
72.80.Le Polymers; organic compounds (including organic semiconductors)
78.40.Me Organic compounds and polymers
78.30.Jw Organic compounds, polymers
52.77.Dq Plasma-based ion implantation and deposition

Carbon nanotube initiated formation of carbon nanoscrolls

Zhao Zhang and Teng Li

Appl. Phys. Lett. 97, 081909 (2010); http://dx.doi.org/10.1063/1.3479050 (3 pages) | Cited 4 times

Online Publication Date: 26 August 2010

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The unique topology and exceptional properties of carbon nanoscrolls (CNSs) have inspired unconventional nanodevice concepts, yet the fabrication of CNSs remains rather challenging. Using molecular dynamics simulations, we demonstrate the spontaneous formation of a CNS from graphene on a substrate, initiated by a carbon nanotube (CNT). The rolling of graphene into a CNS is modulated by the CNT size, the carbon–carbon interlayer adhesion, and the graphene–substrate interaction. A phase diagram emerging from the simulations can offer quantitative guideline toward a feasible and robust physical approach to fabricating CNSs.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
81.05.U- Carbon/carbon-based materials
81.30.Dz Phase diagrams of other materials
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems

Optical spectroscopy of cubic GaN in nanowires

J. Renard, G. Tourbot, D. Sam-Giao, C. Bougerol, B. Daudin, and B. Gayral

Appl. Phys. Lett. 97, 081910 (2010); http://dx.doi.org/10.1063/1.3478004 (3 pages) | Cited 4 times

Online Publication Date: 26 August 2010

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We show that highly homogeneous cubic GaN can be grown by plasma-assisted molecular beam epitaxy on wurtzite GaN nanowires. The line width of the donor bound exciton is below 3 meV and can reach 1.6 meV in the best parts of the studied sample. This allows to perform a detailed spectroscopy of cubic GaN, and, in particular, to determine the precise spectral positions of the donor bound exciton, the fundamental free exciton and the split-off exciton in a photoluminescence experiment.
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78.67.Uh Nanowires
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.ag Semiconductors
71.35.-y Excitons and related phenomena
78.55.Cr III-V semiconductors
52.77.Dq Plasma-based ion implantation and deposition

Thermodynamic model for heterogeneous bubble nucleation in a temperature gradient

Di Wu, Yuan-Yuan Duan, and Zhen Yang

Appl. Phys. Lett. 97, 081911 (2010); http://dx.doi.org/10.1063/1.3483191 (2 pages) | Cited 3 times

Online Publication Date: 26 August 2010

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A thermodynamic model for heterogeneous bubble nucleation was developed to include the effect of the fluid temperature gradient next to a heated wall. An increase of the wall temperature gradient (heat flux) is shown to suppress bubble nucleation or increase the superheat needed for nucleation. Bubble nucleation is completely suppressed above a critical temperature gradient. The onset of nucleate boiling is shown to be related to the intersection of the qw″−Tw curve of the boiling system and the qw,c″−Tw curve of the fluid. The theoretical predictions agree with experimental results.
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65.20.De General theory of thermodynamic properties of liquids, including computer simulation
47.55.dp Cavitation and boiling
64.60.qe General theory and computer simulations of nucleation
64.70.fh Boiling and bubble dynamics
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