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2 Apr 2012

Volume 100, Issue 14, Articles (14xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 100, 144101 (2012); http://dx.doi.org/10.1063/1.3697983 (4 pages)

H. Xu (徐涵), Wei Yu (余玮), M. Y. Yu (郁明阳), A. Y. Wong (黄燿煇), Z. M. Sheng (盛政明), M. Murakami (村上匡且), and J. Zhang (张杰)
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Tunable clover-shaped GaN photonic bandgap structures patterned by dual-step nanosphere lithography

K. H. Li, Zetao Ma, and H. W. Choi

Appl. Phys. Lett. 100, 141101 (2012); http://dx.doi.org/10.1063/1.3698392 (5 pages) | Cited 2 times

Online Publication Date: 2 April 2012

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The fabrication of close-packed clover-shaped photonic crystal structure on GaN by dual-step nanosphere lithography is demonstrated. By shrinkage of spheres prior to pattern transfer, a non-closed-packed clover-shaped photonic bandgap (PBG) structure, as designed by modified 3D finite-difference time-domain simulation, is also realized. The PBG of the close-packed and non-close-packed clover-shaped structures is verified through optical transmission spectroscopy, found to agree well with simulated results. A threefold enhancement in photoluminescence (PL) intensity is observed from the optimized structure, when the PBG is tuned to overlap with the emission band of the InGaN/GaN multi-quantum wells. From time-resolved PL measurements, shortened decay lifetimes are observed.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
81.16.Nd Micro- and nanolithography
81.16.Rf Micro- and nanoscale pattern formation
78.47.je Time resolved light scattering spectroscopy
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

Optical generation of polarized photoluminescence from GaAs(100)

Zhan Hu, Sima Singha, Daniel H. Rich, and Robert J. Gordon

Appl. Phys. Lett. 100, 141102 (2012); http://dx.doi.org/10.1063/1.3698469 (4 pages) | Cited 2 times

Online Publication Date: 2 April 2012

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Polarized photoluminescence from GaAs(100) was generated using shaped ultrashort laser pulses. A train of three pulses separated by an integer multiple of the longitudinal optical phonon period produced p-polarized continuum emission, whereas trains with half-integer multiples of the phonon period as well as single Gaussian pulses produced s-polarized emission. The p-polarized emission is attributed to recombination of carriers in the L-valley, resulting from plasma generation and coherent phonon-excitation by the pulse train, whereas the s-polarized emission is caused by reflection by the melted surface of unpolarized plasma emission.
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78.55.Cr III-V semiconductors
63.20.dd Measurements
78.47.jd Time resolved luminescence

Phase sensitive monitoring of electron bunch form and arrival time in superconducting linear accelerators

C. Kaya, C. Schneider, A. Al-Shemmary, W. Seidel, M. Kuntzsch, J. Bhattacharyya, M. Mittendorff, P. Evtushenko, S. Winnerl, G. Staats, M. Helm, N. Stojanovic, P. Michel, and M. Gensch

Appl. Phys. Lett. 100, 141103 (2012); http://dx.doi.org/10.1063/1.3699025 (3 pages) | Cited 1 time

Online Publication Date: 2 April 2012

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In this Letter, we present a simple approach for monitoring electron bunch form and arrival time combining electro-optic sampling and phase and frequency sensitive signal detection. The sensitivity of the technique has the potential to allow online diagnostics to be performed down to bunch charges in the femto coulomb regime. The concept has high impact for the developments of the next generation of 4th generation x-ray light sources working with long pulse trains or continuous wave mode of operation.
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29.20.Ej Linear accelerators

Transmission and temperature sensing characteristics of a selectively liquid-filled photonic-bandgap-fiber-based Sagnac interferometer

Xibao Zheng, Yan-ge Liu, Zhi Wang, Tingting Han, Chengli Wei, and Jinjie Chen

Appl. Phys. Lett. 100, 141104 (2012); http://dx.doi.org/10.1063/1.3699026 (4 pages) | Cited 2 times

Online Publication Date: 2 April 2012

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A selectively liquid-filled photonic-bandgap-fiber-based Sagnac interferometer is proposed and demonstrated. Extraordinary transmission and sensing characteristics of the interferometer are observed and investigated theoretically and experimentally. Due to the unique modal and group birefringence characteristics of the material-infiltrated photonic bandgap fiber, the temperature sensitivity of the interference peaks strongly depends on the wavelength. Furthermore, the interference peaks at the two edges of the bandgap appear to shift in the opposite direction alongside changes in the temperature, providing a method of achieving two- or multi-parameter simultaneous sensing measurement with a compact structure.
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42.81.Pa Sensors, gyros
07.20.Dt Thermometers
07.60.Ly Interferometers
07.60.Vg Fiber-optic instruments
42.70.Qs Photonic bandgap materials
42.81.Dp Propagation, scattering, and losses; solitons

Lens-less edge-pumped high power microchip laser

Weipeng Kong and Takunori Taira

Appl. Phys. Lett. 100, 141105 (2012); http://dx.doi.org/10.1063/1.3699225 (4 pages) | Cited 1 time

Online Publication Date: 2 April 2012

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We report the demonstration of compact lens-less microchip laser, which is directly pumped by single-emitter diode chips from multiple directions. The lens-less design makes the microchip laser more compact; the multi-direction designable pump schemes can realize a Gaussian shape. As a preliminary result, continuous wave output power of 32.5 W and slope efficiency of 45% was obtained by 9-direction pumped Yb:YAG ceramic microchip. Further, 27.2 W single peak Gaussian beam was obtained by a small adjustment of the output mirror. Power scalability is easy by increasing the number of diodes to 27 to achieve 100-watt-level microchip laser.
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42.55.Sa Microcavity and microdisk lasers

Advantages of GaN based light-emitting diodes with a p-InGaN hole reservoir layer

Taiping Lu, Shuti Li, Chao Liu, Kang Zhang, Yiqin Xu, Jinhui Tong, Lejuan Wu, Hailong Wang, Xiaodong Yang, Yian Yin, Guowei Xiao, and Yugang Zhou

Appl. Phys. Lett. 100, 141106 (2012); http://dx.doi.org/10.1063/1.3700722 (3 pages) | Cited 7 times

Online Publication Date: 3 April 2012

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A p-type InGaN hole reservoir layer (HRL) was designed and incorporated in GaN based light-emitting diodes (LEDs) to enhance hole injection efficiency and alleviate efficiency droop. The fabricated LEDs with p-type HRL exhibited higher light output power, smaller emission energy shift and broadening as compared to its counterpart. Based on electrical and optical characteristics analysis and numerical simulation, these improvements are mainly attributed to the alleviated band bending in the last couple of quantum well and electron blocking layer, and thus better hole injection efficiency. Meanwhile, the efficiency droop can be effectively mitigated when the p-InGaN HRL was used.
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85.60.Jb Light-emitting devices
02.60.-x Numerical approximation and analysis

Bistability in silicon microring resonator based on strain induced by a piezoelectric lead zirconate titanate thin film

Y. Sebbag, I. Goykhman, B. Desiatov, T. Nachmias, O. Yoshaei, M. Kabla, S. E. Meltzer, and U. Levy

Appl. Phys. Lett. 100, 141107 (2012); http://dx.doi.org/10.1063/1.3701587 (4 pages)

Online Publication Date: 3 April 2012

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We demonstrate bistability in a submicron silicon optical phase shifter based on the photoelastic effect. The strain magnitude is electrically controlled by a piezoelectric thin film placed on top of the device. The hysteresis behavior of the piezoelectric response shows potential application as bistable device independent of the optical intensity.
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42.65.Pc Optical bistability, multistability, and switching, including local field effects
42.79.-e Optical elements, devices, and systems
77.65.-j Piezoelectricity and electromechanical effects
77.55.H- Piezoelectric and electrostrictive films
78.20.hb Piezo-optical, elasto-optical, acousto-optical, and photoelastic effects

The effect of viewing angle on the spectral behavior of a Gd plasma source near 6.7 nm

Colm O’Gorman, Takamitsu Otsuka, Noboru Yugami, Weihua Jiang, Akira Endo, Bowen Li, Thomas Cummins, Padraig Dunne, Emma Sokell, Gerry O’Sullivan, and Takeshi Higashiguchi

Appl. Phys. Lett. 100, 141108 (2012); http://dx.doi.org/10.1063/1.3701593 (4 pages) | Cited 1 time

Online Publication Date: 5 April 2012

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We have demonstrated the effect of viewing angle on the extreme ultraviolet (EUV) emission spectra of gadolinium (Gd) near 6.7 nm. The spectra are shown to have a strong dependence on viewing angle when produced with a laser pulse duration of 10 ns, which may be attributed to absorption by low ion stages of Gd and an angular variation in the ion distribution. Absorption effects are less pronounced at a 150-ps pulse duration due to reduced opacity resulting from plasma expansion. Thus for evaluating source intensity, it is necessary to allow for variation with both viewing angle and target orientation.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.50.Dg Plasma sources
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.25.Fi Transport properties

Ultrafast excited state dynamics and dispersion studies of nonlinear optical properties in dinaphthoporphycenes

Debasis Swain, P. T. Anusha, T. Shuvan Prashant, Surya P. Tewari, Tridib Sarma, Pradeepta K. Panda, and S. Venugopal Rao

Appl. Phys. Lett. 100, 141109 (2012); http://dx.doi.org/10.1063/1.3701274 (5 pages) | Cited 5 times

Online Publication Date: 6 April 2012

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Ultrafast excited state dynamics of dinaphthoporphycenes were investigated using femtosecond and picosecond degenerate pump-probe techniques at 600 nm and 800 nm, respectively. Femtosecond pump-probe data indicated photo-induced absorption at 600 nm resulting from two-photon/single photon excitation, whereas picosecond pump-probe data demonstrated photo-bleaching which was a consequence of three-photon absorption. The fastest lifetimes (100–120 fs) observed are attributed to the intramolecular vibrational relaxation, the slower ones (1–3 ps) to internal conversion, and the slowest components (7–10 ps) to non-radiative decay back to ground state. Z-scan studies in the 560–600 nm range were also carried out.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.50.-j Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion)
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The critical growth velocity for planar-to-faceted interfaces transformation in SiGe crystals

Xinbo Yang, K. Fujiwara, N. V. Abrosimov, R. Gotoh, J. Nozawa, H. Koizumi, A. Kwasniewski, and S. Uda

Appl. Phys. Lett. 100, 141601 (2012); http://dx.doi.org/10.1063/1.3698336 (4 pages) | Cited 1 time

Online Publication Date: 2 April 2012

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Crystal-melt interface morphological transformation of differently oriented SiGe crystals with different Ge concentrations was observed, and the effect of Ge concentration on critical growth velocity (Vc) for the interface morphological transformation was investigated. A planar-to-faceted morphological transformation for the 〈110〉, 〈112〉, and 〈100〉 interfaces was observed. Vc for planar-to-faceted transformation of the 〈110〉, 〈112〉, and 〈100〉 interfaces decreases nonlinearly with increasing Ge concentration. SiGe faceted interfaces can be attributed to the fact that the perturbation induced in a planar interface was amplified when the constitutional undercooled zone was formed at high growth velocities.
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81.05.Hd Other semiconductors
81.10.Fq Growth from melts; zone melting and refining
68.35.Rh Phase transitions and critical phenomena

Interface barriers at the interfaces of polar GaAs(111) faces with Al2O3

H. Y. Chou, E. O’Connor, P. K. Hurley, V. V. Afanas’ev, M. Houssa, A. Stesmans, P. D. Ye, and S. B. Newcomb

Appl. Phys. Lett. 100, 141602 (2012); http://dx.doi.org/10.1063/1.3698461 (4 pages)

Online Publication Date: 2 April 2012

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Internal photoemission measurements of barriers for electrons at interfaces between GaAs(111) and atomic-layer deposited Al2O3 indicate that changing the GaAs polar crystal face orientation from the Ga-terminated (111)A to the As-terminated (111)B has no effect on the barrier height and remains the same as at the non-polar GaAs(100)/Al2O3 interface. Moreover, the presence of native oxide on GaAs(111) or passivation of this surface with sulphur also have no measurable influence on the GaAs(111)/Al2O3 barrier. These results suggest that the orientation and composition-sensitive surface dipoles conventionally observed at GaAs surfaces are effectively compensated at GaAs/oxide interfaces.
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79.60.Bm Clean metal, semiconductor, and insulator surfaces
81.65.Rv Passivation

Structure of Si-capped Ge/SiC/Si (001) epitaxial nanodots: Implications for quantum dot patterning

C. W. Petz, D. Yang, J. Levy, and J. A. Floro

Appl. Phys. Lett. 100, 141603 (2012); http://dx.doi.org/10.1063/1.3699223 (4 pages) | Cited 1 time

Online Publication Date: 2 April 2012

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Artificially ordered quantum dot (QD) arrays, where confined carriers can interact via direct exchange coupling, may create unique functionalities such as cluster qubits and spintronic bandgap systems. Development of such arrays for quantum computing requires fine control over QD size and spatial arrangement on the sub-35 nm length scale. We employ electron-beam irradiation to locally decompose ambient hydrocarbons onto a bare Si (001) surface. These carbonaceous patterns are annealed in ultra-high vacuum (UHV), forming ordered arrays of nanoscale SiC precipitates that have been suggested to template subsequent epitaxial Ge growth to form ordered QD arrays. We show that 3C-SiC nanodots form, in cube-on-cube epitaxial registry with the Si substrate. The SiC nanodots are fully relaxed by misfit dislocations and exhibit small lattice rotations with respect to the substrate. Ge overgrowth at elevated deposition temperatures, followed by Si capping, results in expulsion of the Ge from SiC template sites due to the large chemical and lattice mismatch between Ge and C. Maintaining an epitaxial, low-defectivity Si matrix around the quantum dots is important for creating reproducible electronic and spintronic coupling of states localized at the QDs.
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
61.80.Fe Electron and positron radiation effects
71.20.Nr Semiconductor compounds
81.16.-c Methods of micro- and nanofabrication and processing
81.30.Mh Solid-phase precipitation
68.55.A- Nucleation and growth

Highly ionized physical vapor deposition plasma source working at very low pressure

V. Stranak, A.-P. Herrendorf, S. Drache, M. Cada, Z. Hubicka, M. Tichy, and R. Hippler

Appl. Phys. Lett. 100, 141604 (2012); http://dx.doi.org/10.1063/1.3699229 (3 pages) | Cited 2 times

Online Publication Date: 2 April 2012

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Highly ionized discharge for physical vapor deposition at very low pressure is presented in the paper. The discharge is generated by electron cyclotron wave resonance (ECWR) which assists with ignition of high power impulse magnetron sputtering (HiPIMS) discharge. The magnetron gun (with Ti target) was built into the single-turn coil RF electrode of the ECWR facility. ECWR assistance provides pre-ionization effect which allows significant reduction of pressure during HiPIMS operation down to p = 0.05 Pa; this is nearly more than an order of magnitude lower than at typical pressure ranges of HiPIMS discharges. We can confirm that nearly all sputtered particles are ionized (only Ti+ and Ti++ peaks are observed in the mass scan spectra). This corresponds well with high plasma density ne ∼ 1018 m−3, measured during the HiPIMS pulse.
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52.50.Dg Plasma sources
52.77.Dq Plasma-based ion implantation and deposition
52.80.Pi High-frequency and RF discharges
81.15.Cd Deposition by sputtering
52.25.Jm Ionization of plasmas
52.25.Os Emission, absorption, and scattering of electromagnetic radiation

Smallest separation of nanorods from physical vapor deposition

L. G. Zhou and Hanchen Huang

Appl. Phys. Lett. 100, 141605 (2012); http://dx.doi.org/10.1063/1.3700740 (3 pages) | Cited 1 time

Online Publication Date: 3 April 2012

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In catalysis applications, it is desirable to have nanorods of smaller diameter. However, if the separation of nanorods is smaller than their diameter, nanorods will merge to become dense films. The separation of nanorods derives from the separation of nucleating clusters on the substrate. This letter reports a theory of the smallest separation of nanorods. This theory is in closed form and is verified by accompanying lattice kinetic Monte Carlo simulations. The theoretical prediction of transition from nanorods to dense films—when the separation of nanorods is smaller than their diameter—is in good agreement with in-house experimental observation.
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81.05.Bx Metals, semimetals, and alloys
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
81.16.-c Methods of micro- and nanofabrication and processing

Interfacial oxide re-growth in thin film metal oxide III-V semiconductor systems

S. McDonnell, H. Dong, J. M. Hawkins, B. Brennan, M. Milojevic, F. S. Aguirre-Tostado, D. M. Zhernokletov, C. L. Hinkle, J. Kim, and R. M. Wallace

Appl. Phys. Lett. 100, 141606 (2012); http://dx.doi.org/10.1063/1.3700863 (4 pages) | Cited 3 times

Online Publication Date: 4 April 2012

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The Al2O3/GaAs and HfO2/GaAs interfaces after atomic layer deposition are studied using in situ monochromatic x-ray photoelectron spectroscopy. Samples are deliberately exposed to atmospheric conditions and interfacial oxide re-growth is observed. The extent of this re-growth is found to depend on the dielectric material and the exposure temperature. Comparisons with previous studies show that ex situ characterization can result in misleading conclusions about the interface reactions occurring during the metal oxide deposition process.
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77.55.-g Dielectric thin films
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)

Surface area enhancement of microcantilevers by femto-second laser irradiation

A. Kumar, S. Rajauria, H. Huo, O. Ozsun, K. Rykaczewski, J. Kumar, and K. L. Ekinci

Appl. Phys. Lett. 100, 141607 (2012); http://dx.doi.org/10.1063/1.3701163 (3 pages)

Online Publication Date: 4 April 2012

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A dry single-step process for enhancing the surface area of a silicon microcantilever is described. In this process, a flat microcantilever is irradiated with ∼ 100-femto-second-long laser pulses. The silicon surface melts and rapidly cools, resulting in the formation of nanoscale pillars. The shape and size of these nanostructures can be tuned by changing the energy of the pulses. Resonance measurements on surface-enhanced microcantilevers show that the irradiation process reduces the stiffness and the resonance frequency of the cantilevers. Fluidic dissipation measurements provide an estimate for the surface area increase. Both the enhanced surfaces and the fluidic characteristics of these microcantilevers may be useful in bio-chemical sensing applications.
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07.10.Cm Micromechanical devices and systems

Effects of polarization azimuth in dynamics of electrically assisted light-induced gliding of nematic liquid-crystal easy axis

A. V. Dubtsov, D. V. Shmeliova, S. V. Pasechnik, Alexei D. Kiselev, and V. G. Chigrinov

Appl. Phys. Lett. 100, 141608 (2012); http://dx.doi.org/10.1063/1.3701781 (4 pages) | Cited 2 times

Online Publication Date: 5 April 2012

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We experimentally study the reorientation dynamics of the nematic liquid crystal easy axis at photoaligned azo-dye films under the combined action of in-plane electric field and linearly polarized reorienting UV light at varying polarization azimuth, ϕp. At non-zero values of the azimuth, ϕp ≠ 0, as opposed to the case where the polarization vector of the light is parallel to the initial easy axis (ϕp = 0), the easy axis reorientation was observed to be most pronounced outside the interelectrode gaps. In the regions between electrodes with non-vanishing electric field, it is found that the dynamics of reorientation slows down with ϕp and the sense of easy axis rotation is independent of the sign of ϕp. A generalized version of the phenomenological model that was previously developed to describe the electrically assisted light-induced gliding is applied to interpret the experimental data.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
68.15.+e Liquid thin films

Modification of the surface-state occupancy on noble metal films with stacking fault arrays

Puneet Mishra, Takashi Uchihashi, and Tomonobu Nakayama

Appl. Phys. Lett. 100, 141609 (2012); http://dx.doi.org/10.1063/1.3701777 (4 pages)

Online Publication Date: 6 April 2012

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Low-temperature scanning tunneling spectroscopy reveals that striped Au films grown on Ag templates containing periodic arrays of stacking faults exhibit partially populated one-dimensional (1D) surface states, in contrast to the depopulated 1D surface states on the Ag template. Detailed spectroscopic analysis on Au stripes shows the presence of 1D states with onset slightly below the Fermi level, while two-dimensional surface states exist at lower energies. These findings indicate that the Au stripes, owing to their occupied 1D surface states, may provide an effective means for 1D transport of charge and spin information between magnetic adsorbates.
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73.20.At Surface states, band structure, electron density of states
68.55.A- Nucleation and growth
61.72.Nn Stacking faults and other planar or extended defects
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Design of ductile bulk metallic glasses by adding “soft” atoms

N. Zheng, R. T. Qu, S. Pauly, M. Calin, T. Gemming, Z. F. Zhang, and J. Eckert

Appl. Phys. Lett. 100, 141901 (2012); http://dx.doi.org/10.1063/1.3700721 (4 pages) | Cited 6 times

Online Publication Date: 2 April 2012

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We propose a strategy for the design of ductile bulk metallic glasses (BMGs) through minor substitution using relatively large atoms, which make the bonding nature become more metallic and with it less shear resistant. Such a locally modified structure results in topological heterogeneity, which appears to be crucial for achieving enhanced plasticity. This strategy is verified for Ti-Zr-Cu-Pd glassy alloys, in which Cu was replaced by In, and seems to be extendable to other BMG systems. The atomic-scale heterogeneity in BMGs is somewhat analog to defects in crystalline alloys and helps to improve the overall plasticity of BMGs.
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81.05.Kf Glasses (including metallic glasses)
61.72.-y Defects and impurities in crystals; microstructure
81.20.-n Methods of materials synthesis and materials processing
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.dj Poisson's ratio
81.40.Lm Deformation, plasticity, and creep
62.20.fq Plasticity and superplasticity

Surface morphological stabilization of stressed crystalline solids by simultaneous action of applied electric and thermal fields

Dwaipayan Dasgupta, Georgios I. Sfyris, M. Rauf Gungor, and Dimitrios Maroudas

Appl. Phys. Lett. 100, 141902 (2012); http://dx.doi.org/10.1063/1.3698360 (4 pages)

Online Publication Date: 2 April 2012

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We examine the surface morphological stability of electrically and thermally conducting crystalline elastic solids in uniaxial tension under the simultaneous action of an electric field and a temperature gradient. We use linear stability analysis of a surface mass transport model that accounts for surface electromigration and thermomigration induced by the applied fields and for surface diffusional anisotropy. We find that a properly oriented applied thermal gradient can reduce the critical electric-field strength requirement for stabilization of the planar surface morphology. The temperature dependence of the solid material’s properties does not affect the critical strength requirement of the applied fields.
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68.35.B- Structure of clean surfaces (and surface reconstruction)
66.30.Qa Electromigration
68.35.Fx Diffusion; interface formation
73.25.+i Surface conductivity and carrier phenomena
62.20.D- Elasticity
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves

Origin of size-dependent photoluminescence decay dynamics in colloidal γ-Ga2O3 nanocrystals

Manu Hegde, Ting Wang, Zoran L. Miskovic, and Pavle V. Radovanovic

Appl. Phys. Lett. 100, 141903 (2012); http://dx.doi.org/10.1063/1.3698390 (5 pages) | Cited 2 times

Online Publication Date: 2 April 2012

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We studied size-dependent dynamics of defect-based photoluminescence of colloidal γ-Ga2O3 nanocrystals in the framework of the donor-acceptor pair model. Two theoretical models were developed based on relative positioning of donor and acceptor sites: (1) for random distribution of defects throughout the nanocrystal volume and (2) for surface segregation of defects. The results of the modeling indicate that defect sites are predominantly located in the vicinity of nanocrystal surfaces and that the density of defects increases with decreasing nanocrystal size. The donor Bohr radius obtained as a fitting parameter suggests an increase in the donor binding energy with decreasing nanocrystal size.
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78.55.Hx Other solid inorganic materials
82.70.Dd Colloids
61.72.jd Vacancies
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
68.35.Dv Composition, segregation; defects and impurities
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Notable internal thermal effect on the yielding of metallic glasses

W. D. Liu and K. X. Liu

Appl. Phys. Lett. 100, 141904 (2012); http://dx.doi.org/10.1063/1.3700923 (4 pages) | Cited 1 time

Online Publication Date: 3 April 2012

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We revealed a notable internal thermal effect on the yielding of metallic glasses with the aid of a newly developed microscopic model focusing on the interaction of incipient localized shear events. It is shown that at the high strain rate, localized heating in metallic glasses instead of localized structure change plays the dominant role in shear softening. Using the modified cooperative shear model with the ingredient of the thermal effect, we captured the sudden decrease of the strength of metallic glasses after a critical strain rate, which was found in experiments.
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81.40.Lm Deformation, plasticity, and creep
61.43.Fs Glasses
63.20.Pw Localized modes
62.20.F- Deformation and plasticity

Above-room-temperature photoluminescence from a strain-compensated Ge/Si0.15Ge0.85 multiple-quantum-well structure

P. H. Wu, D. Dumcenco, Y. S. Huang, H. P. Hsu, C. H. Lai, T. Y. Lin, D. Chrastina, G. Isella, E. Gatti, and K. K. Tiong

Appl. Phys. Lett. 100, 141905 (2012); http://dx.doi.org/10.1063/1.3700804 (3 pages) | Cited 7 times

Online Publication Date: 4 April 2012

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Photoluminescence (PL) of a strain-compensated Ge/Si0.15Ge0.85 multiple-quantum-well (MQW) structure was studied above room temperature, in the range of 300–440 K. Both direct and indirect radiative recombination PL features were observed. The relative intensity of direct to indirect recombination markedly increases with the increase of temperature. The enhancement of PL from direct recombination above RT has been attributed to the thermal excitation of carriers from L-type to Γ-type confined states. This extends the potential applicability of Ge/SiGe MQW as light emitters on a Si-based platform and is favorable for applications in metal-oxide-semiconductor integrated circuits which normally operate above RT.
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78.55.Ap Elemental semiconductors
78.67.De Quantum wells

Nano-precipitates made of atomic pillars revealed by single atom detection in a Mg-Nd alloy

W. Lefebvre, V. Kopp, and C. Pareige

Appl. Phys. Lett. 100, 141906 (2012); http://dx.doi.org/10.1063/1.3701272 (5 pages)

Online Publication Date: 5 April 2012

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We report on the atomic architecture of metastable precipitates in a model Mg-0.5 at. %Nd system. High resolution Z-contrast observations in scanning transmission electron microscopy demonstrate that a common elementary structure, made of Nd atomic pillars, is shared by precipitates involved in the early decomposition of the system. A combination of Z-contrast projections has been used to establish the accurate location of Nd atoms in the atomic pillars, therefore allowing the determination of structure and composition of precipitates. Other precipitates compositions are extrapolated by means of a quantitative analysis.
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81.16.-c Methods of micro- and nanofabrication and processing
61.46.-w Structure of nanoscale materials
81.30.Mh Solid-phase precipitation

Probing the electrode-polymer interface in conjugated polymer devices with surface-enhanced Raman scattering

Dongbo Li, Nicholas J. Borys, and John M. Lupton

Appl. Phys. Lett. 100, 141907 (2012); http://dx.doi.org/10.1063/1.3701278 (4 pages) | Cited 1 time

Online Publication Date: 6 April 2012

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A crucial consideration in organic devices is the role of the interface between a metal electrode and the active polymer material. Here, we use the conjugated polymer poly[2-methoxy-5-(2-ethylhexyl-oxy)-1,4-phenylene-vinylene] (MEH-PPV) with model gold and silver electrodes to perform surface-enhanced Raman scattering (SERS) on the metal-MEH-PPV interface. We observe significant differences between the SERS spectra on the two metals, which we assign to conformational changes of the phenyl rings within the polymer. The difference between gold and silver interfaces can be removed upon thermal annealing, suggesting structural relaxation. Resonance Raman spectra of the two interfaces are identical, implying that the overall conformation of the polymer backbone which supports the pi-electron transition remains unaffected by the different metals. SERS is uniquely sensitive to the interfacial metal/organic layer and provides an important in situ tool to optimizing organic device structures.
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85.60.Jb Light-emitting devices
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