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15 Jan 2012

Volume 111, Issue 2, Articles (02xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 111, 023508 (2012); http://dx.doi.org/10.1063/1.3677988 (7 pages)

Li-Yong Jiang, Hong Wu, Wei Jia, and Xiang-Yin Li

Polarization-independent negative refraction effect in SiO2-GaAs annular photonic crystals.

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back to top Nanoscale Science and Design

Control of the optical and crystalline properties of TiO2 in visible-light active TiO2/TiN bi-layer thin-film stacks

Wilson Smith, Houssam Fakhouri, Jérôme Pulpytel, and Farzaneh Arefi-Khonsari

J. Appl. Phys. 111, 024301 (2012); http://dx.doi.org/10.1063/1.3671428 (10 pages) | Cited 2 times

Online Publication Date: 18 January 2012

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Multi-layered thin films of TiO2 and TiN were created by rf reactive magnetron sputtering, and their crystalline, optical, and photoelectrochemical properties were measured. The overall composition of the films (TiO2-to-TiN ratio) was kept constant with the height of each film. The number of layers and thickness of each layer was controlled to create bi-layer thin films that were composed of: 9 bi-layers, 18 bi-layers, 27 bi-layers, 36 bi-layers, and 45 bi-layers. XRD patterns were observed for each film after annealing to measure the grain size and composition of anatase and rutile as a function of temperature. It was found that the phase-transition temperature is able to be substantially controlled (between 550 °C and 850 °C) for the anatase to rutile transition by varying the number of layers/thickness of each layer. In addition, bi-layer stacking significantly affected the film’s optical properties by lowering the bandgap into the visible-light region, and also showed up to three times the improvement in photoelectrochemical performance under uv and visible irradiation. Overall, bi-layer stacking of TiO2/TiN films has shown a unique and highly desirable control over several important physical characteristics that can be beneficial for many applications, such as high-temperature sensors and optoelectronic devices.
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81.05.Hd Other semiconductors
81.15.Cd Deposition by sputtering
82.45.-h Electrochemistry and electrophoresis
82.50.-m Photochemistry
73.61.Le Other inorganic semiconductors
78.66.Li Other semiconductors

Stability and topological transformations of liquid droplets on vapor-liquid-solid nanowires

Edwin J. Schwalbach, Stephen H. Davis, Peter W. Voorhees, James A. Warren, and Daniel Wheeler

J. Appl. Phys. 111, 024302 (2012); http://dx.doi.org/10.1063/1.3676451 (10 pages) | Cited 1 time

Online Publication Date: 19 January 2012

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The Vapor-Liquid-Solid method is one of the most popular techniques for growing semiconducting nanowires, and the stability of the liquid droplet is an important factor controlling wire morphology and, ultimately, functionality. Earlier theoretical work on axisymmetric systems indicates that the lowest-energy liquid configuration varies with surface energies, wire radius, and fluid volume. We test these predictions with a fully dynamic phase-field model that incorporates viscous fluid flow. Under conditions predicted by this earlier theoretical work, we observe the pinning of the liquid to the top face of a nanowire, a condition necessary for wire growth. To study the stability of the droplet, we apply perturbations to the liquid shape and find that the system can transition to a metastable configuration, a local minimum in the energy landscape. Furthermore, the transition pathway to this local minimum depends on the magnitude of the perturbations. Under conditions that favor a liquid on the sidewalls of the wire, we observe a spontaneous transition of the liquid from a droplet to an annular configuration through an intermediate state that is not predicted by theory. The time scales and contact-line speeds for these transitions are determined through simulation and are consistent with approximations based on simple dimensional analysis.
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81.16.-c Methods of micro- and nanofabrication and processing
64.70.D- Solid-liquid transitions
66.20.-d Viscosity of liquids; diffusive momentum transport
81.07.Gf Nanowires
47.55.D- Drops and bubbles
68.03.Cd Surface tension and related phenomena

Structural phase transitions of FeCo and FeNi nanoparticles: A molecular dynamics study

L. J. Meng, X. Y. Peng, K. W. Zhang, C. Tang, and J. X. Zhong

J. Appl. Phys. 111, 024303 (2012); http://dx.doi.org/10.1063/1.3675275 (5 pages)

Online Publication Date: 19 January 2012

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We have investigated the structural phase transition of FeCo and FeNi nanoparticles by molecular dynamics (MD) simulation using the generalized embedded atom potential (GEAM). It is found that the phase varies with the atomic compositions and annealing processes. By using the Honeycutt and Andersen index (HA index), bond order parameters (BOP) and pair correlation function (PCF), we found that a BCC to defective icosahedra phase transition occurs in the FeCo nanoparticle when Co composition is increased to about 60 at %. In the FeNi nanoparticle, three phases have been identified, namely, the BCC phase, the mixed BCC/FCC phase, and the multilayer defective icosahedral phase, which correspond to the Ni compositions of 0–20 at %, 20–70 at %, and 70–100 at %, respectively. Our simulations have well reproduced the phase transition points and most of the phases observed in recent experiments.
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64.70.Nd Structural transitions in nanoscale materials
64.70.kd Metals and alloys
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Propagating waves in quasi-one dimensional MoO3 ribbons investigated by Raman spectroscopy and scanning near-field optical microscope

Zhipeng Li, Ning Liu, Zhilin Yang, and Bin Dong

J. Appl. Phys. 111, 024304 (2012); http://dx.doi.org/10.1063/1.3677972 (4 pages)

Online Publication Date: 23 January 2012

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MoO3 ribbons have proven to be a good quasi-one dimensional waveguide. Here, we show that propagating waves can be launched in very thin MoO3 ribbons with the thickness of only 150 nm. By using Raman image method, it was found that the 1/e light damping length at 632.8 nm is 5 ∼ 6 μm. Scanning near-field optical measurement was carried to provide distribution of propagating waves near the surface of the waveguide, where Fabry-Pérot interference with the period ∼ 0.33 μm was observed. Furthermore, numerical calculations reveal the guiding waves in thin ribbons are in leak mode with a large portion of energy guided over the MoO3 surface. Applications of these kinds of thin MoO3 ribbons can be found in combined nano-photonic devices and remote surface enhanced Raman sensing.
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78.30.Hv Other nonmetallic inorganics
78.66.Nk Insulators

Anti-Stokes bright yellowish emission of NdAlO3 nanocrystals

Wieslaw Strek, Lukasz Marciniak, Dariusz Hreniak, and Anna Lukowiak

J. Appl. Phys. 111, 024305 (2012); http://dx.doi.org/10.1063/1.3674272 (6 pages) | Cited 2 times

Online Publication Date: 23 January 2012

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Infrared laser diode–induced anti-Stokes bright yellowish emission of NdAlO3 nanocrystalline powder was observed at room temperature in ambient atmosphere. The emission intensity was found to be unaltered with lowering temperature and to increase by two orders of magnitude in vacuum. The temperature of bright emission under ambient atmosphere was determined to be 350 °C. It was found that the yellowish emission was accompanied by a giant photocurrent of 0.5 microamperes at relatively low applied voltage. The power dependence of the photocurrent was governed by an avalanche-like mechanism. The origin of the bright emission is discussed in terms of charge transfer luminescence of Nd3+.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Hx Other solid inorganic materials
72.40.+w Photoconduction and photovoltaic effects

Nucleation of lateral compositional modulation in InGaP epitaxial films grown on (001) GaAs

M. Schmidbauer, A. Ugur, C. Wollstein, F. Hatami, F. Katmis, O. Caha, and W. T. Masselink

J. Appl. Phys. 111, 024306 (2012); http://dx.doi.org/10.1063/1.3677995 (5 pages) | Cited 1 time

Online Publication Date: 23 January 2012

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The nucleation of the one dimensional periodic surface corrugations that form during epitaxy along the [-110] direction on the In0.48Ga0.52 P lattice matched to (001) GaAs is investigated using x-ray diffuse scattering in the grazing incidence geometry. Comparing samples with differing In0.48Ga0.52 P thickness shows that the corrugations are not detectable for a 10-nm film, but develop gradually for thicker films. Very pronounced corrugations are well developed for 200-nm thick films and are associated with a lateral strain field and enhanced In composition. The data support a lateral compositional modulation as the cause of the corrugations.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.ag Semiconductors
78.66.Fd III-V semiconductors
78.70.Ck X-ray scattering
81.05.Ea III-V semiconductors

Nanoscale magnetic structure and properties of solution-derived self-assembled La0.7Sr0.3MnO3 islands

J. Zabaleta, M. Jaafar, P. Abellán, C. Montón, O. Iglesias-Freire, F. Sandiumenge, C. A. Ramos, R. D. Zysler, T. Puig, A. Asenjo, N. Mestres, and X. Obradors

J. Appl. Phys. 111, 024307 (2012); http://dx.doi.org/10.1063/1.3677985 (8 pages) | Cited 3 times

Online Publication Date: 24 January 2012

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Strain-induced self-assembled La0.7Sr0.3MnO3 nanoislands of lateral size 50−150 nm and height 10−40 nm have been grown on yttria-stabilized zirconia (001)-substrates from ultradiluted chemical solutions based on metal propionates. The nanoislands grow highly relaxed withstanding the epitaxial relation (001)LSMO[110]//(001)YSZ[010] and show bulk-like average magnetic properties in terms of Curie temperature and saturation magnetization. The interplay of the magnetocrystalline and shape anisotropy within the nanoisland ensemble results in an in-plane magnetic anisotropy with a magnetocrystalline constant K1(150  K) = -(5±1)  kJ/m3 and in-plane easy axis along the [110]-La0.7Sr0.3MnO3 direction as measured, for the first time, through ferromagnetic resonance experiments. Magnetic force microscopy studies reveal the correlation between nanoisland size and its magnetic domain structure in agreement with micromagnetic simulations. In particular, we have established the required geometric conditions for single domain, multidomain, and vortex configurations.
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75.75.Cd Fabrication of magnetic nanostructures
61.46.-w Structure of nanoscale materials
75.30.Gw Magnetic anisotropy
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.16.Dn Self-assembly

Ge redistribution in SiO2/SiGe structures under thermal oxidation: Dynamics and predictions

Ethan Long, Alexander Azarov, Frode Kløw, Augustinas Galeckas, Andrej Yu Kuznetsov, and Spyridon Diplas

J. Appl. Phys. 111, 024308 (2012); http://dx.doi.org/10.1063/1.3677987 (10 pages) | Cited 3 times

Online Publication Date: 24 January 2012

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Several fundamental aspects of the oxidation-induced redistribution of Ge in thin films of SiGe are studied. This includes the incorporation of Ge into the oxide and the formation of what is alternatively referred to as pile-up, snow-plow, or a germanium-rich layer. Experimental data from the present work shows longer oxidation times leading to an increase of Ge content in the pile-up region and eventually creating a single high Ge content pile-up layer by entirely consuming the initial SiGe layer. The pile-up effect was shown to occur at the oxidation interface, with the highest Ge content occurring at the same interface. For a given oxide thickness, the redistribution of Ge and the formation of a pile-up region was shown experimentally to be independent of temperature in the range between 800 °C and 1000 °C. Simulations using common models for the oxidation of Si and diffusion of Si in SiGe indicate that temperature does have an influence on the composition of the pile-up layer, though the range of achievable compositions is limited. The flux of Si due to diffusion of Si in SiGe relative to the oxidation-induced flux of Si out of the SiGe is integral to the formation and dimensions of a pile-up region. Two predictive relations were derived for describing the dynamics of oxidation of SiGe. The first relation is given for determining the pile-up layer thickness as a function of oxide thickness and the composition of the pile-up layer. The second relation assumes a limited supply of Si and is for determination of the minimum initial thickness of a SiGe layer to avoid oxidation of Ge. The validity of these equations was confirmed experimentally by RBS and XPS data from the present work. The proposed models may be used in nanostructuring of thin films of SiGe by oxidation and in the design of core-shell structures and transistors. This is all done with a focus on oxidation of epitaxial thin films (< 100 nm) of Si1-XGeX in dry O2 at 1 atm between 800 °C and 1000 °C.
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81.65.Mq Oxidation
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
82.80.Yc Rutherford backscattering (RBS), and other methods of chemical analysis
68.55.ag Semiconductors
68.35.Fx Diffusion; interface formation
79.60.Jv Interfaces; heterostructures; nanostructures

Nanosecond in situ transmission electron microscope studies of the reversible Ge2Sb2Te5 crystalline ⇔ amorphous phase transformation

M. K. Santala, B. W. Reed, T. Topuria, S. Raoux, S. Meister, Y. Cui, T. LaGrange, G. H. Campbell, and N. D. Browning

J. Appl. Phys. 111, 024309 (2012); http://dx.doi.org/10.1063/1.3678447 (6 pages) | Cited 3 times

Online Publication Date: 25 January 2012

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Chalcogenide-based phase-change materials have wide use in optical recording media and are growing in importance for use in non-volatile electronic memory. For both applications, rapid switching between the amorphous and crystalline phases is necessary, and understanding the changes during rapidly driven phase transitions is of scientific and technological significance. Laser-induced crystallization and amorphization occur rapidly and changes in atomic structure, microstructure, and temperature are difficult to observe experimentally and determine computationally. We have used nanosecond-scale time-resolved diffraction with intense electron pulses to study Ge2Sb2Te5 during laser crystallization. Using a unique and unconventional specimen geometry, cycling between the amorphous and crystalline phases was achieved, enabling in situ transmission electron microscope (TEM) study of both microstructural and crystallographic changes caused by repeated switching. Finite element analysis was used to simulate interactions of the laser with the nano-structured specimens and to model the rapidly changing specimen temperature. Such time-resolved experimental methods combined with simulation of experimentally inaccessible physical characteristics will be fundamental to advancing the understanding of rapidly driven phase transformations.
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64.70.dg Crystallization of specific substances
78.47.D- Time resolved spectroscopy (>1 psec)
61.43.Fs Glasses
61.46.Hk Nanocrystals
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Rx Nanocrystalline materials

Optical anisotropy in self-assembled InAs nanostructures grown on GaAs high index substrate

M. Bennour, F. Saidi, L. Bouzaïene, L. Sfaxi, and H. Maaref

J. Appl. Phys. 111, 024310 (2012); http://dx.doi.org/10.1063/1.3677952 (7 pages)

Online Publication Date: 25 January 2012

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We present a study of the optical properties of InAs self-assembled nanostructures grown by molecular beam epitaxy on GaAs(11N)A substrates (N = 3–5). Photoluminescence (PL) measurements revealed good optical properties of InAs quantum dots (QDs) grown on GaAs(115)A compared to those grown on GaAs(113)A and (114)A orientations substrate. An additional peak localized at 1.39 eV has been shown on PL spectra of both GaAs(114)A and (113)A samples. This peak persists even at lower power density. Supporting on the polarized photoluminescence characterization, we have attributed this additional peak to the quantum strings (QSTs) emission. A theoretical study based on the resolution of the three dimensional Schrödinger equation, using the finite element method, including strain and piezoelectric-field effect was adopted to distinguish the observed photoluminescence emission peaks. The mechanism of QDs and QSTs formation on such a high index GaAs substrates was explained in terms of piezoelectric driven atoms and the equilibrium surfaces at edges.
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78.67.Hc Quantum dots
81.16.Dn Self-assembly
68.65.Hb Quantum dots (patterned in quantum wells)
78.55.Cr III-V semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters

Thermal conductivity of semiconductor nanowires from micro to nano length scales

Martin Maldovan

J. Appl. Phys. 111, 024311 (2012); http://dx.doi.org/10.1063/1.3677973 (6 pages) | Cited 3 times

Online Publication Date: 25 January 2012

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We study the influence of phonon boundary scattering on the transport of thermal energy in semiconductor nanowires from micro to nano length scales. We use a kinetic theory model based on the Boltzmann transport equation that accurately calculates the reduction of the phonon mean free paths by considering their directional dependence and the fundamental statistical definition of the mean free path. As a result, our model does not use phenomenological formulas to account for the reduction of the phonon mean free paths due to boundary scattering. The transport of thermal energy is also fully divided into that carried by different polarizations by separating phonon group velocities and relaxation times for transverse and longitudinal phonons. We study the correctness of using frequency independent versus frequency dependent models for describing the specularity of the nanowire boundary. We also examine the validity of the assumption that phonons in the semiconductor nanowire maintain their bulk phonon dispersion relations and that modifications to the dispersion relations due to phonon confinement effects can be neglected. The thermal conductivities of silicon nanowires are calculated for different length scales and temperatures and good agreement is obtained with experiments. The theoretical results in this paper can be used to understand and quantitatively predict heat transport in nanowires, which is critical for increasing the efficiency of thermoelectric and electronic devices.
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66.70.Df Metals, alloys, and semiconductors
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Thermosize effects and thermodynamic analysis of a macro/nano scaled refrigerator cycle

Hao Wang, Guoxing Wu, and Xinbin Chen

J. Appl. Phys. 111, 024312 (2012); http://dx.doi.org/10.1063/1.3678597 (4 pages) | Cited 4 times

Online Publication Date: 26 January 2012

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We have established a macro/nano refrigerator model. A possibly new thermodynamic refrigerator based on thermosize effects is analyzed. Expressions for some important parameters, such as the refrigeration load, coefficient of performance, and work input, are derived. By means of numerical calculation and illustration, the influence of the thermosize effects and the additional regenerative loss on the performance of the cycle are discussed and evaluated in detail. Some fundamental optimal relations and general performance characteristic curves of the cycle are obtained. The results obtained here will provide theoretical guidance for the design of some new macro/nano scaled devices.
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07.20.Mc Cryogenics; refrigerators, low-temperature detectors, and other low-temperature equipment

Catalyst solubility and self-doping in ZnS nanostructures

M. Hafeez, U. Manzoor, A. S. Bhatti, M. Burak Kaynar, and S. Ismat Shah

J. Appl. Phys. 111, 024313 (2012); http://dx.doi.org/10.1063/1.3677950 (7 pages) | Cited 2 times

Online Publication Date: 26 January 2012

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We demonstrate that the variation in the solubility of the catalyst during nanostructure growth via vapor-liquid-solid technique is dependent on the catalyst dimensions. This property can be exploited in a simple way for controlled doping of the nanostructures. Specifically, we investigate the role of Au droplet size on its solubility in ZnS nanostructures. The size-dependent variations in the surface tension of the catalyst droplet can change its solubility in the nanostructures. The solubility of the catalyst (Au) has been observed to decrease drastically with the increase in its droplet radius. X ray diffraction and scanning electron microscopy were used to determine the phase, shape, size, and density of nanostructures. XPS results revealed that concentration of Au in the ZnS nanostructures was high for a small Au droplet and dropped considerably with increase in the droplet size. The experimental findings were in good agreement with the theoretical model, based on the thermodynamical equations for the doping in nanowires. The critical size of the catalyst for the surface and core doping is determined for the zinc blende and Wurtzite phases of ZnS. In the end, we show that, by using a suitable catalyst with high miscibility, nanostructures can be doped during growth, thus eliminating a step in the device fabrication.
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64.75.Bc Solubility
81.07.Gf Nanowires
79.60.Bm Clean metal, semiconductor, and insulator surfaces
81.05.Dz II-VI semiconductors
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
61.72.uj III-V and II-VI semiconductors

Perovskite phase transformation in 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 nanoparticles derived by sol-gel

Satyendra Singh and S. B. Krupanidhi

J. Appl. Phys. 111, 024314 (2012); http://dx.doi.org/10.1063/1.3677974 (5 pages) | Cited 2 times

Online Publication Date: 26 January 2012

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Fabrication of 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 (PMN-PT) nanoparticles with an average size of about 40 nm and their phase transformation behavior from pyrochlore to perovskite phase is investigated. A novel sol-gel method was used for the synthesis of air-stable and precipitate-free diol-based sol of PMN-PT which was dried and partially calcined at 450 °C for 1 h to decompose organics and bring down the free energy barrier for perovskite crystallization and then finally annealed in the temperature range 600 to 700 °C. Annealed at around 700 °C for 1 h, PMN-PT gel powder exhibited nanocrystalline morphology with perovskite phase as confirmed by the transmission electron microscopy and X-ray diffraction techniques.
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81.07.Wx Nanopowders
81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
77.80.Jk Relaxor ferroelectrics
77.84.Cg PZT ceramics and other titanates
64.70.Nd Structural transitions in nanoscale materials
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)

A nano-cheese-cutter to directly measure interfacial adhesion of freestanding nano-fibers

Xin Wang, Johnny F. Najem, Shing-Chung Wong, and Kai-tak Wan

J. Appl. Phys. 111, 024315 (2012); http://dx.doi.org/10.1063/1.3677947 (6 pages) | Cited 2 times

Online Publication Date: 27 January 2012

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A nano-cheese-cutter is fabricated to directly measure the adhesion between two freestanding nano-fibers. A single electrospun fiber is attached to the free end of an atomic force microscope cantilever, while a similar fiber is similarly prepared on a mica substrate in an orthogonal direction. External load is applied to deform the two fibers into complementary V-shapes, and the force measurement allows the elastic modulus to be determined. At a critical tensile load, “pull-off” occurs when the adhering fibers spontaneously detach from each other, yielding the interfacial adhesion energy. Loading-unloading cycles are performed to investigate repeated adhesion-detachment and surface degradation.
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81.16.-c Methods of micro- and nanofabrication and processing

A fractal permeability model for gas flow through dual-porosity media

Qian Zheng and Boming Yu

J. Appl. Phys. 111, 024316 (2012); http://dx.doi.org/10.1063/1.3679070 (7 pages) | Cited 2 times

Online Publication Date: 27 January 2012

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The dual-porosity medium, i.e., a matrix porous medium coupled with fractured networks, extensively exists in fissured rocks, natural gas reservoirs, and other natural underground reservoirs or in resolving subsurface contamination problems. This work investigates gas permeability through matrix porous media embedded with randomly distributed fractal-like tree networks. The analytical expression for gas permeability in dual-porosity media is derived based on both the pore size of matrix and the mother channel diameter of embedded fractal-like tree networks having fractal distribution. It is found that gas permeability is a function of structural parameters (the fractal dimensions for pore area and tortuous capillaries, porosity and the maximum diameter of matrix, the length ratio, the diameter ratio, the branching levels and angle of the embedded networks) for dual-porosity media. The proposed model does not contain any empirical constant. The model predictions are compared with the available experimental data and simulating results, a fair agreement among them is found. The influences of geometrical parameters on the gas permeability in the media are also analyzed.
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47.53.+n Fractals in fluid dynamics
47.56.+r Flows through porous media
02.50.-r Probability theory, stochastic processes, and statistics
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
47.11.-j Computational methods in fluid dynamics
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