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26 Dec 2011

Volume 99, Issue 26, Articles (26xxxx)

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Appl. Phys. Lett. 99, 261101 (2011); http://dx.doi.org/10.1063/1.3660243 (3 pages)

T. Schwarzbäck, H. Kahle, M. Eichfelder, R. Roßbach, M. Jetter, and P. Michler
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Determination of miscibility in MgO-ZnO nanocrystal alloys by x-ray absorption spectroscopy

Sukit Limpijumnong, Jaru Jutimoosik, Nirawith Palakawong, Wantana Klysubun, Jiti Nukeaw, Mao-Hua Du, and Saroj Rujirawat

Appl. Phys. Lett. 99, 261901 (2011); http://dx.doi.org/10.1063/1.3671987 (4 pages) | Cited 3 times

Online Publication Date: 27 December 2011

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The local structure of MgxZn1−xO nanocrystals is studied using synchrotron x-ray absorption near edge structures (XANES) over the full range of composition, from x = 0 to 1. Mg and Zn K-edges XANES measurements allow us to selectively study the local environments around Mg and Zn atoms in these nanocrystalline samples. Our results indicate that, for MgO-ZnO alloys, Zn is highly miscible in the rocksalt domain (i.e., up to ∼50 at. %) while the miscibility of Mg in the wurtzite domain is much less but is still substantial (i.e., up to ∼20 at. %). The simulated XANES spectra, based on first principles methods, are consistent with the observed spectra, confirming our finding. Because it is short-ranged and element-specific, the technique is useful for local structure and crystal phase determination of nanostructures, quantum dots, and mixed-phase alloys in general.
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
61.46.Hk Nanocrystals
64.75.Bc Solubility
78.70.Dm X-ray absorption spectra
61.05.cj X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc.

Revealing low-temperature atomic ordering in bulk Co-Pt with the high-throughput ab-initio method

Roman V. Chepulskii and Stefano Curtarolo

Appl. Phys. Lett. 99, 261902 (2011); http://dx.doi.org/10.1063/1.3671992 (3 pages) | Cited 5 times

Online Publication Date: 27 December 2011

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The low-temperature phase diagram of bulk Co-Pt is studied with a high-throughput ab-initio method. Global, hcp-, and fcc-restricted convex hulls are constructed to evaluate stable and metastable phases. It is found that fcc-L10 is energetically degenerate with hcp-B19. Both structures are unstable with respect to phase decomposition into hcp-D019↔fcc-β2 at low temperature. Furthermore, L10 is an adaptive structure on the fcc-restricted convex hull which relates to the low energies of antiphase boundaries. Fcc-L12 is energetically degenerate with fcc-D023 for both Co3Pt and CoPt3. L12-Co3Pt and L10-CoPt belong to the fcc-restricted convex hull. They might stabilize above the Co hcp/fcc transition and remain kinetically frozen below. L12-CoPt3 is energetically well above the convex hull. Its experimental observation may result from yet unexplained finite-temperature effects.
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81.30.Bx Phase diagrams of metals, alloys, and oxides
71.15.-m Methods of electronic structure calculations
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
64.70.kd Metals and alloys
71.15.Dx Computational methodology (Brillouin zone sampling, iterative diagonalization, pseudopotential construction)
71.20.Be Transition metals and alloys

Microsecond linear optical response in the unusual nematic phase of achiral bimesogens

V. P. Panov, R. Balachandran, M. Nagaraj, J. K. Vij, M. G. Tamba, A. Kohlmeier, and G. H. Mehl

Appl. Phys. Lett. 99, 261903 (2011); http://dx.doi.org/10.1063/1.3671996 (3 pages) | Cited 7 times

Online Publication Date: 28 December 2011

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Some hydrocarbon linked mesogenic dimers are known to exhibit an additional nematic phase (Nx) below a conventional uniaxial nematic (Nu) phase. Although composed of non-chiral molecules, the Nx phase is found to exhibit linear (polar) switching under applied electric field. This switching has remarkably low response time of the order of a few microseconds. Two chiral domains with opposite handedness and consequently opposite responses are found in planar cells. Uniformly lying helix, electroclinic, and flexoelectric effects are given as possible causes for this intriguing phenomenon.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
77.65.-j Piezoelectricity and electromechanical effects

Ultra-coherent single photon source

H. S. Nguyen, G. Sallen, C. Voisin, Ph. Roussignol, C. Diederichs, and G. Cassabois

Appl. Phys. Lett. 99, 261904 (2011); http://dx.doi.org/10.1063/1.3672034 (3 pages) | Cited 11 times

Online Publication Date: 28 December 2011

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We present an original type of single photon source in solid state, based on the coherent laser light scattering by a single InAs quantum dot. We demonstrate that the coherence of the emitted single photons is tailored by the resonant excitation with a spectral linewidth below the radiative limit. Our ultra-coherent source opens the way for integrated quantum devices dedicated to the generation of single photons with high degrees of indistinguishability.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Facile creation of bio-inspired superhydrophobic Ce-based metallic glass surfaces

Kesong Liu, Zhou Li, Weihua Wang, and Lei Jiang

Appl. Phys. Lett. 99, 261905 (2011); http://dx.doi.org/10.1063/1.3672036 (4 pages) | Cited 7 times

Online Publication Date: 29 December 2011

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A bio-inspired synthesis strategy was conducted to fabricate superhydrophobic Ce-based bulk metallic glass (BMG) surfaces with self-cleaning properties. Micro-nanoscale hierarchical structures were first constructed on BMG surfaces and then modified with the low surface energy coating. Surface structures, surface chemical compositions, and wettability were characterized by combining scanning electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy, and contact angle measurements. Research indicated that both surface multiscale structures and the low surface free energy coating result in the final formation of superhydrophobicity.
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61.43.Dq Amorphous semiconductors, metals, and alloys
81.05.Kf Glasses (including metallic glasses)
68.35.bj Amorphous semiconductors, glasses
68.08.Bc Wetting
68.35.Md Surface thermodynamics, surface energies
79.60.Bm Clean metal, semiconductor, and insulator surfaces

High temperature thermal properties of thin tantalum nitride films

Elah Bozorg-Grayeli, Zijian Li, Mehdi Asheghi, Gil Delgado, Alexander Pokrovsky, Matthew Panzer, Daniel Wack, and Kenneth E. Goodson

Appl. Phys. Lett. 99, 261906 (2011); http://dx.doi.org/10.1063/1.3672098 (3 pages) | Cited 2 times

Online Publication Date: 29 December 2011

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Tantalum Nitride (TaN) films carry high heat fluxes in a variety of applications including diffusion barriers in magnetoresistive random access memory and buffer/absorbers in extreme ultraviolet masks. The thicknesses of these films are usually of the same order as the thermal energy carrier mean free path, which complicates the study of heat conduction. This paper presents thermal (cross-plane) and electrical (in-plane) conductivity measurements on TaN films with thicknesses of 50, 75, and 100 nm. Picosecond thermoreflectance is used to extract the thermal boundary resistance between TaN and Al and the intrinsic thermal conductivity of TaN for temperatures of 300–700 K. The data and the relative importance of boundary resistances, electron-boundary scattering, and electron-defect scattering are interpreted using the electrical and thermal transport data. These data facilitate comparison of the phonon and electron contributions to thermal conduction in TaN.
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66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
73.61.Ng Insulators
78.20.N- Thermo-optic effects
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Structural variability in La0.5Sr0.5TiO3±δ thin films

Meng Gu, Craig R. Dearden, Chengyu Song, Nigel D. Browning, and Yayoi Takamura

Appl. Phys. Lett. 99, 261907 (2011); http://dx.doi.org/10.1063/1.3672217 (3 pages) | Cited 3 times

Online Publication Date: 29 December 2011

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La0.5Sr0.5TiO3±δ films were grown by pulsed laser deposition and characterized using aberration-corrected scanning transmission electron microscopy. Single-phase films with low resistivity and a near Ti3+ valence state were stabilized by a large concentration of oxygen vacancies under reducing conductions. In contrast, single-phase films transformed into “superlattices” upon exposure to an oxygen rich environment through the formation of stacking faults and cation vacancies in order to reach the stable Ti4+ valence state. The resistivity of the superlattices exceeded that of single phase films by two orders of magnitude, indicating that the observed structural order had a significant effect on the overall film properties.
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68.55.aj Insulators
81.15.Fg Pulsed laser ablation deposition
61.72.Nn Stacking faults and other planar or extended defects
61.72.jd Vacancies
68.65.Cd Superlattices
73.61.Ng Insulators

The indirect to direct band gap transition in multilayered MoS2 as predicted by screened hybrid density functional theory

Jason K. Ellis, Melissa J. Lucero, and Gustavo E. Scuseria

Appl. Phys. Lett. 99, 261908 (2011); http://dx.doi.org/10.1063/1.3672219 (3 pages) | Cited 12 times

Online Publication Date: 29 December 2011

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Molybdenite (MoS2) undergoes a transition from an indirect to direct gap semiconductor exhibiting strong photoluminescence when confined in a 2D monolayer. We investigate the effect of interlayer interactions on the band structure and density of states using the screened hybrid functional of Heyd, Scuseria, and Ernzerhof. We show that for the bulk and monolayer systems, our short-range screened hybrid functional produces band gaps in good agreement with experiment. Our functional includes only interlayer interactions of non-van der Waals origin, predicts properties consistent with recent experiments, and provides predictions for few-layered systems.
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64.70.K- Solid-solid transitions
78.55.Hx Other solid inorganic materials
71.20.Nr Semiconductor compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
73.21.Ac Multilayers
73.20.At Surface states, band structure, electron density of states

Formation of metallic colloids in CaF2 by intense ultraviolet light

Stephan Rix, Ute Natura, Felix Loske, Martin Letz, Claudia Felser, and Michael Reichling

Appl. Phys. Lett. 99, 261909 (2011); http://dx.doi.org/10.1063/1.3673301 (3 pages)

Online Publication Date: 29 December 2011

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Highest purity CaF2 single crystals are irreversibly modified when irradiated with millions of pulses of 193 nm light at fluences of 120 mJ/cm2. Mie theory explains the observed haze by attributing the wavelength dependent extinction and the ratio between absorption and scattering to metallic colloids with radii in the range of 20 to 30 nm and a fractional volume of up to 2.8·10-7. Non-contact scanning force microscopy (NC-AFM) measurements performed on a surface produced by in-vacuo cleavage reveals that laser irradiation additionally produces a 104 times higher volume density of colloids with a radius of 1 to 2 nm.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
79.20.Ds Laser-beam impact phenomena
82.70.Dd Colloids
68.35.bt Other materials
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Optical properties of a-plane (Al, Ga)N/GaN multiple quantum wells grown on strain engineered Zn1−xMgxO layers by molecular beam epitaxy

Y. Xia, J. Brault, M. Nemoz, M. Teisseire, B. Vinter, M. Leroux, and J.-M. Chauveau

Appl. Phys. Lett. 99, 261910 (2011); http://dx.doi.org/10.1063/1.3673325 (3 pages)

Online Publication Date: 29 December 2011

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Nonpolar (11math0) Al0.2Ga0.8N/GaN multiple quantum wells (MQWs) have been grown by molecular beam epitaxy on (11math0) Zn0.74Mg0.26O templates on r-plane sapphire substrates. The quantum wells exhibit well-resolved photoluminescence peaks in the ultra-violet region, and no sign of quantum confined Stark effect is observed in the complete multiple quantum well series. The results agree well with flat band quantum well calculations. Furthermore, we show that the MQW structures are strongly polarized along the [0001] direction. The origin of the polarization is discussed in terms of the strain anisotropy dependence of the exciton optical oscillator strengths.
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78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
71.35.-y Excitons and related phenomena
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
78.55.Cr III-V semiconductors

Quantitative strain mapping of InAs/InP quantum dots with 1 nm spatial resolution using dark field electron holography

David Cooper, Jean-Luc Rouviere, Armand Béché, Shima Kadkhodazadeh, Elizaveta S. Semenova, Kresten Yvind, and Rafal Dunin-Borkowski

Appl. Phys. Lett. 99, 261911 (2011); http://dx.doi.org/10.1063/1.3672194 (3 pages) | Cited 5 times

Online Publication Date: 30 December 2011

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The optical properties of semiconductor quantum dots are greatly influenced by their strain state. Dark field electron holography has been used to measure the strain in InAs quantum dots grown in InP with a spatial resolution of 1 nm. A strain value of 5.4% ± 0.1% has been determined which is consistent with both measurements made by geometrical phase analysis of high angle annular dark field scanning transmission electron microscopy images and with simulations.
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78.67.Hc Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
61.05.jp Electron holography

Thermal expansion coefficients of Bi2Se3 and Sb2Te3 crystals from 10 K to 270 K

X. Chen, H. D. Zhou, A. Kiswandhi, I. Miotkowski, Y. P. Chen, P. A. Sharma, A. L. Lima Sharma, M. A. Hekmaty, D. Smirnov, and Z. Jiang

Appl. Phys. Lett. 99, 261912 (2011); http://dx.doi.org/10.1063/1.3672198 (3 pages) | Cited 6 times

Online Publication Date: 30 December 2011

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Lattice constant of Bi2Se3 and Sb2Te3 crystals is determined by x-ray powder diffraction measurement in a wide temperature range. Linear thermal expansion coefficients (α) of the crystals are extracted, and considerable anisotropy between α|| and α⊥ is observed. The low temperature values of α can be fit well by the Debye model, while an anomalous behavior at above 150 K is evidenced and explained. Grüneisen parameters of the materials are also estimated at room temperature.
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65.40.De Thermal expansion; thermomechanical effects
61.66.Fn Inorganic compounds

Interplay of index contrast with periodicity in polymer photonic crystals

Chris E. Finlayson, Andrew I. Haines, David R. E. Snoswell, Andreas Kontogeorgos, Silvia Vignolini, Jeremy J. Baumberg, Peter Spahn, and G. Peter Hellmann

Appl. Phys. Lett. 99, 261913 (2011); http://dx.doi.org/10.1063/1.3672215 (4 pages) | Cited 3 times

Online Publication Date: 30 December 2011

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We report how the strength of resonant Bragg reflection from polymeric photonic crystals (polymer opals) varies linearly with the refractive-index contrast, Δn, in contrast to the quadratic buildup of Fresnel reflections scaling as (Δn)2. This occurs due to the interplay of disorder and periodicity, in agreement with a simple 1-dimensional periodic model. Goniometry experiments show that opal films exhibit “cones” of resonantly scattered light, which extend to ±20° angular deviation from the specular direction. The intensity of the scattering cones varies super-linearly with Δn. Such medium contrast photonic crystals are of significant interest for understanding structural colors exhibited in nature, by structures with inherent disorder.
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42.70.Qs Photonic bandgap materials
42.70.Jk Polymers and organics

Influence of silicon on the thermally-induced crystallization process of Si-Sb4Te phase change materials

Yan Cheng, Zhitang Song, Yifeng Gu, Sannian Song, Feng Rao, Liangcai Wu, Bo Liu, and Songlin Feng

Appl. Phys. Lett. 99, 261914 (2011); http://dx.doi.org/10.1063/1.3673281 (4 pages)

Online Publication Date: 30 December 2011

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Si-Sb4Te phase change thin films with different silicon contents have been investigated by in situ heating technique in transmission electron microscopy (TEM). The studies show that Si-doping can significantly improve the thermal stability of Si-Sb4Te thin films, refine their grain size, and change the nucleation characters with the increase of silicon content. By in situ annealing in TEM, the crystalline phase of Si-Sb4Te thin films can be indexed as hexagonal Sb structure, and Si is still holding amorphous state which is considered as the reason for the change, by destroying the long-range order lattice of crystal grains.
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61.72.up Other materials
68.55.at Other materials
64.70.dg Crystallization of specific substances
81.40.Gh Other heat and thermomechanical treatments
64.60.qj Studies of nucleation in specific substances
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