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12 Nov 2007

Volume 91, Issue 20, Articles (20xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 203501 (2007); http://dx.doi.org/10.1063/1.2806922 (3 pages)

Michael N. Feiginov and Dibakar Roy Chowdhury
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Elegant SiOX heliotropes composed of assembled flexural SiOX nanowires

Renbing Wu, Baosheng Li, Mingxia Gao, Qimiao Zhu, Yi Pan, Guangyi Yang, and Jianjun Chen

Appl. Phys. Lett. 91, 203101 (2007); http://dx.doi.org/10.1063/1.2807270 (3 pages) | Cited 1 time

Online Publication Date: 12 November 2007

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Silicon oxide nanowires assembled with elegant heliotrope-shape have been synthesized by the modified evaporation of Fe and Si mixture sources. Structures and morphologies of the obtained microheliotropes were thoroughly studied by field emission scanning electron microscopy and high resolution transmission electron microscopy. It is suggested that the multinucleation sites around the perimeter of Fe droplet are responsible for the growth of SiOX nanowires and then via self-assembly process, which results in the formation of microheliotropes. These interesting results and discussion may be beneficial to the understanding of complex nanostructures formation and hopefully enrich the conventional knowledge of vapor-liquid-solid growth phenomena.
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81.16.Dn Self-assembly
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
81.07.Bc Nanocrystalline materials

Very small-size and high-density β-FeSi2 nanocrystal assemblies grown on a Si(100) substrate using an embedded solid-phase epitaxy and bionanoprocess with protein ferritin

Yuji Nakama, Kyousuke Minakawa, Jun Ohta, and Masahiro Nunoshita

Appl. Phys. Lett. 91, 203102 (2007); http://dx.doi.org/10.1063/1.2813642 (3 pages) | Cited 1 time

Online Publication Date: 12 November 2007

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An embedded solid-phase epitaxy technique has been studied to produce very small-size and high-density β-FeSi2 nanocrystal (NC) assemblies on a Si (100) substrate. In this procedure, a bionanoprocess is utilized, with protein “ferritin” containing Fe2O3 cores, for solid-phase epitaxial β-FeSi2-NC growth (at 500–800 °C) embedded in an amorphous Si thin film. X-ray diffraction and transmission electron microscopy revealed β-FeSi2 NCs of uniform diameter (6.3±0.3 nm) and high density (6.2×1011 dots/cm2). By measuring the photoluminescence spectra at 7 K, a photoluminescence peak of 0.91 eV is obtained as an excitonic emission from the β-FeSi2 NCs.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
81.16.Dn Self-assembly
68.37.Lp Transmission electron microscopy (TEM)

Hot electron relaxation and phonon dynamics in graphene

S. Butscher, F. Milde, M. Hirtschulz, E. Malić, and A. Knorr

Appl. Phys. Lett. 91, 203103 (2007); http://dx.doi.org/10.1063/1.2809413 (3 pages) | Cited 40 times

Online Publication Date: 12 November 2007

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Using density-matrix theory, we microscopically calculate the relaxation dynamics of photoexcited electrons in graphene. Electron-phonon coupling leads to an initially ultrafast energy dissipation and to a nonthermal phonon occupation of the highest optical phonon modes. We also calculate the temporal evolution of the electronic temperature and find good agreement with recent experimental work.
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72.20.Ht High-field and nonlinear effects
71.38.-k Polarons and electron-phonon interactions
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Large scale, highly ordered assembly of nanowire parallel arrays by differential roll printing

Roie Yerushalmi, Zachery A. Jacobson, Johnny C. Ho, Zhiyong Fan, and Ali Javey

Appl. Phys. Lett. 91, 203104 (2007); http://dx.doi.org/10.1063/1.2813618 (3 pages) | Cited 26 times

Online Publication Date: 13 November 2007

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A differential roll printing strategy is developed to enable large-scale and uniform assembly of highly aligned and ordered nanowire arrays on various rigid and flexible substrate materials. The dynamics of the process are explored by tuning the linear sliding motion of the roller with respect to the rolling motion, clearly demonstrating the importance of the differential rolling process in the controlled assembly of nanowires. The potency and versatility of the method is further demonstrated by fabrication of nanowire transistor arrays on flexible substrates.
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81.16.-c Methods of micro- and nanofabrication and processing
85.30.Tv Field effect devices

Initial phases in sputter deposited HfO2–Al2O3 nanolaminate films

E. E. Hoppe, C. R. Aita, and M. Gajdardziska-Josifovska

Appl. Phys. Lett. 91, 203105 (2007); http://dx.doi.org/10.1063/1.2813624 (3 pages) | Cited 8 times

Online Publication Date: 13 November 2007

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Nanolaminate films of crystalline HfO2 and amorphous Al2O3 were grown by reactive sputter deposition on unheated fused SiO2 and the surface oxide of ⟨111⟩ Si. X-ray diffraction showed the amount of monoclinic (m) HfO2 decreased with decreasing HfO2 layer thickness, consistent with a finite crystal size effect. High resolution transmission electron microscopy of individual crystallites detected tetragonal (t) and orthorhombic (o) HfO2 as the initial phases formed. Whereas the t→m transition is accomplished by a shear mechanism, we demonstrate the important role of polysynthetic twinning for the o→m transition.
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64.70.Nd Structural transitions in nanoscale materials
81.07.Bc Nanocrystalline materials
61.46.Hk Nanocrystals
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
81.15.Cd Deposition by sputtering
81.16.-c Methods of micro- and nanofabrication and processing
61.72.Mm Grain and twin boundaries

Direct measurement of effective diffusion coefficients in nanochannels using steady-state dispersion effects

Nicolas F. Y. Durand, Arnaud Bertsch, Mina Todorova, and Philippe Renaud

Appl. Phys. Lett. 91, 203106 (2007); http://dx.doi.org/10.1063/1.2801625 (3 pages) | Cited 13 times

Online Publication Date: 13 November 2007

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We present a method to measure effective diffusion coefficients of fluorescently labeled molecules inside a nanofluidic system. Molecules with small diffusion coefficients show a larger lateral dispersion than highly diffusive species, which is counterintuitive. We performed measurements with wheat germ agglutinin proteins and obtained an effective diffusion coefficient which is four orders of magnitude lower than its free diffusion coefficient. Our technique which is a direct and relatively simple measurement of the effective diffusion coefficients inside nanochannels of well controlled dimensions could help fundamental studies in membranes and separation sciences.
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87.15.Vv Diffusion
87.14.E- Proteins

Simultaneous fabrication of nanogap gold electrodes by electroless gold plating using a common medical liquid

Yuhsuke Yasutake, Keijiro Kono, Masayuki Kanehara, Toshiharu Teranishi, Mark R. Buitelaar, Charles G. Smith, and Yutaka Majima

Appl. Phys. Lett. 91, 203107 (2007); http://dx.doi.org/10.1063/1.2805035 (3 pages) | Cited 20 times

Online Publication Date: 13 November 2007

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We report a simple and high yield method for fabricating multiple nanogaps simultaneously by an electroless gold plating technique using electroless gold plating solution which consists of common medical liquid of iodine tincture and L(+)-ascorbic acid (vitamin C). The distance between the gold electrodes (33 nm in average) on the SiO2/Si substrate was decreased by selective deposition of gold onto the surface of the gold electrodes. By electroless gold plating, we fabricated nanogaps below 5 nm in width with a 41% process yield. We also demonstrated the Coulomb blockade effect in octanethiol(C8)-protected Au nanoparticles by using such a fabricated nanogap.
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81.16.-c Methods of micro- and nanofabrication and processing
81.07.Bc Nanocrystalline materials
73.63.Bd Nanocrystalline materials
73.22.-f Electronic structure of nanoscale materials and related systems

Enhancement of thermal conductivity in magnetite based nanofluid due to chainlike structures

John Philip, P. D. Shima, and Baldev Raj

Appl. Phys. Lett. 91, 203108 (2007); http://dx.doi.org/10.1063/1.2812699 (3 pages) | Cited 48 times

Online Publication Date: 13 November 2007

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We observe a dramatic enhancement of thermal conductivity in a nanofluid containing magnetite particles of average diameter of 6.7 nm under the influence of an applied magnetic field. The maximum enhancement in the thermal conductivity observed is 300% (k/kf = 4.0) at a particle loading of 6.3 vol %. The increase in thermal conductivity is attributed to the effective conduction of heat through the chainlike structures formed in the nanofluid. This finding is consistent with the theoretical prediction of enhanced thermal conductivity in nanofluid containing fractal aggregates [ R. Prasher et al., Appl. Phys. Lett.89, 143119 (2006) ].
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Tailoring surface plasmon polariton propagation via specific symmetry properties of nanostructures

B. Ashall, M. Berndt, and D. Zerulla

Appl. Phys. Lett. 91, 203109 (2007); http://dx.doi.org/10.1063/1.2806224 (3 pages) | Cited 7 times

Online Publication Date: 13 November 2007

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We report on an experimental investigation on surface plasmon polariton (SPP) propagation and interaction on two-dimensional arrays of differing symmetry properties. Providing the required symmetry variations and forming the basis of the arrays are tailor designed nanostructures. We demonstrate that as a result of a 120° symmetry presence, our triquetra-rotor nanostructures can be used for SPP guiding and propagation direction control. As a result, the polarization angle at which the far field SPP related minimum reflectivity occurs can be predetermined by design characteristics and orientation of the nanostructures.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.22.Lp Collective excitations
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
61.46.-w Structure of nanoscale materials

Microwave absorption properties and mechanism of cagelike ZnO/SiO2 nanocomposites

Mao-Sheng Cao, Xiao-Ling Shi, Xiao-Yong Fang, Hai-Bo Jin, Zhi-Ling Hou, Wei Zhou, and Yu-Jin Chen

Appl. Phys. Lett. 91, 203110 (2007); http://dx.doi.org/10.1063/1.2803764 (3 pages) | Cited 72 times

Online Publication Date: 13 November 2007

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In this paper, cagelike ZnO/SiO2 nanocomposites were prepared and their microwave absorption properties were investigated in detail. Dielectric constants and losses of the pure cagelike ZnO nanostructures were measured in a frequency range of 8.2–12.4 GHz. The measured results indicate that the cagelike ZnO nanostructures are low-loss material for microwave absorption in X band. However, the cagelike ZnO/SiO2 nanocomposites exhibit a relatively strong attenuation to microwave in X band. Such strong absorption is related to the unique geometrical morphology of the cagelike ZnO nanostructures in the composites. The microcurrent network can be produced in the cagelike ZnO nanostructures, which contributes to the conductive loss.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
77.22.Ch Permittivity (dielectric function)
77.22.Gm Dielectric loss and relaxation
84.40.-x Radiowave and microwave (including millimeter wave) technology

Use of nanocomposites to increase electrical “gain” in chemical sensors

Sara M. C. Vieira, Paul Beecher, Ibraheem Haneef, Florin Udrea, William I. Milne, Manoj A. G. Namboothiry, David L. Carroll, Jonghyurk Park, and Sunglyul Maeng

Appl. Phys. Lett. 91, 203111 (2007); http://dx.doi.org/10.1063/1.2811716 (3 pages) | Cited 5 times

Online Publication Date: 14 November 2007

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We have investigated chemical sensors by combining silicon-on-insulator complementary-metal-oxide-semiconducting microtechnology with nanotechnology. The sensing materials were single-walled carbon nanotubes and poly(3,3‴-dialkyl-quarterthiophone). The devices containing only nanotubes or pure polymer provided minimal response, whereas the nanocomposite material (1 wt. % of nanotubes in the polymer) provided excellent sensitivity/selectivity to the particular analyte monitored (hydrogen, ammonia, and acetone). We observed that even small amounts of gas doping (10 ppb) resulted in exponential changes in the overall conductivity profile of the nanocomposite sensor, thus anticipating an element of “gain” within the chemical sensor.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
82.80.-d Chemical analysis and related physical methods of analysis
85.35.Kt Nanotube devices

Structural properties and magic structures in hydrogenated finite and infinite silicon nanowires

A. D. Zdetsis, E. N. Koukaras, and C. S. Garoufalis

Appl. Phys. Lett. 91, 203112 (2007); http://dx.doi.org/10.1063/1.2813019 (3 pages) | Cited 11 times

Online Publication Date: 14 November 2007

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Unusual effects such as bending and “canting,” related with the stability, have been identified by ab initio real-space calculations for hydrogenated silicon nanowires. We have examined in detail the electronic and structural properties of finite and infinite nanowires as a function of length (and width) and have developed stability and bending rules, demonstrating that “magic” wires do not bend. Reconstructed 2×1 nanowires are practically as stable as the magic ones. Our calculations are in good agreement with the experimental data of Ma et al. [Science 299, 1874 (2003). ].
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68.65.La Quantum wires (patterned in quantum wells)
71.15.-m Methods of electronic structure calculations

Ultralow thermal conductivity of nanoparticle packed bed

X. Jack Hu, Ravi Prasher, and Kelly Lofgreen

Appl. Phys. Lett. 91, 203113 (2007); http://dx.doi.org/10.1063/1.2814959 (3 pages) | Cited 4 times

Online Publication Date: 15 November 2007

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We show that thermal conductivity of packed bed of alumina nanoparticles can be as low as 0.035 W/m K which is only 35% higher than the thermal conductivity of air and is smaller than the recently reported lowest thermal conductivity of solids using disordered layered WeS2. These findings show a promising approach for making low-cost and ultralow thermal conductivity thermal insulation materials with high density and good sustainability at high pressures.
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Ductility at the nanoscale: Deformation and fracture of adhesive contacts using atomic force microscopy

N. Pradeep, D.-I. Kim, J. Grobelny, T. Hawa, B. Henz, and M. R. Zachariah

Appl. Phys. Lett. 91, 203114 (2007); http://dx.doi.org/10.1063/1.2815648 (3 pages) | Cited 6 times

Online Publication Date: 15 November 2007

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Fracture of nanosize contacts formed between spherical probes and flat surfaces is studied using an atomic force microscope in an ultrahigh vacuum environment. Analysis of the observed deformation during the fracture process indicates significant material extensions for both gold and silica contacts. The separation process begins with an elastic deformation followed by plastic flow of material with atomic rearrangements close to the separation. Classical molecular dynamics studies show similarity between gold and silicon, materials that exhibit entirely different fracture behavior at macroscopic scale. This direct experimental evidence suggests that fracture at nanoscale occurs through a ductile process.
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81.40.Lm Deformation, plasticity, and creep
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
68.35.Gy Mechanical properties; surface strains

The mechanism for polarity inversion of GaN via a thin AlN layer: Direct experimental evidence

Fude Liu, Ramon Collazo, Seiji Mita, Zlatko Sitar, Gerd Duscher, and Stephen J. Pennycook

Appl. Phys. Lett. 91, 203115 (2007); http://dx.doi.org/10.1063/1.2815748 (3 pages) | Cited 17 times

Online Publication Date: 15 November 2007

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Lateral-polarity heterostructures of GaN on c sapphire were prepared by deposition and patterning of a thin low-temperature AlN nucleation layer. Adjacent macroscopic domains were found to have opposite polarity; domains grown on the AlN nucleation layer were Ga polar while those grown on the nitrided sapphire were N polar, as confirmed by convergent-beam electron diffraction and Z-contrast images. We directly determined the atomic interface structure between the AlN and c sapphire with an aberration-corrected scanning transmission electron microscope at ∼ 1.0 Å resolution. This is the direct experimental evidence for the origin of the polarity control in III nitrides. This understanding is an important step toward manipulating polarity in these semiconductors.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)

Giant thermoelectric effect in graphene

D. Dragoman and M. Dragoman

Appl. Phys. Lett. 91, 203116 (2007); http://dx.doi.org/10.1063/1.2814080 (3 pages) | Cited 23 times

Online Publication Date: 16 November 2007

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The paper predicts a giant thermoelectric coefficient in a nanostructure consisting of metallic electrodes periodically patterned over graphene, which is deposited on a silicon dioxide substrate. The Seebeck coefficient in this device attains 30 mV/K, this value being among the largest reported ever. The calculations are based on a transfer matrix approach that takes a particular form for graphene-based devices. The results are important for future nanogenerators with applications in the area of sensors, energy harvesting, and scavenging.
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72.20.Pa Thermoelectric and thermomagnetic effects
72.80.Sk Insulators
85.35.Kt Nanotube devices

High-Q GaN nanowire resonators and oscillators

S. M. Tanner, J. M. Gray, C. T. Rogers, K. A. Bertness, and N. A. Sanford

Appl. Phys. Lett. 91, 203117 (2007); http://dx.doi.org/10.1063/1.2815747 (3 pages) | Cited 24 times

Online Publication Date: 16 November 2007

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We report high mechanical quality factors Q for GaN nanowire cantilevers grown by molecular beam epitaxy. Nanowires with 30–500 nm diameters and 5–20 μm lengths having resonance frequencies from 400 kHz to 2.8 MHz were measured. Q near room temperature and 10−4 Pa ranged from 2700 to above 60 000 with most above 10 000. Positive feedback to a piezoelectric stack caused spontaneous nanowire oscillations with Q exceeding 106. Spontaneous oscillations also occurred with direct e-beam excitation of unintentionally doped nanowires. Doped nanowires showed no oscillations, consistent with oscillation arising via direct actuation of piezoelectric GaN.
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85.50.-n Dielectric, ferroelectric, and piezoelectric devices
77.65.Fs Electromechanical resonance; quartz resonators
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
85.35.-p Nanoelectronic devices
81.16.-c Methods of micro- and nanofabrication and processing

Temperature-dependent Raman study of Ce0.75Nd0.25O2−δ nanocrystals

Z. D. Dohčević-Mitrović, M. Radović, M. Šćepanović, M. Grujić-Brojčin, Z. V. Popović, B. Matović, and S. Bošković

Appl. Phys. Lett. 91, 203118 (2007); http://dx.doi.org/10.1063/1.2815928 (3 pages) | Cited 6 times

Online Publication Date: 16 November 2007

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Raman spectra of Nd doped ceria nanocrystals were measured by gradual heating and cooling over the temperature range of 293–1073 K and analyzed using the phonon confinement model that incorporates inhomogeneous strain and anharmonic effects. We have demonstrated that in nanograins, four-phonon anharmonic processes are more dominant at higher temperatures than size effects. After the heat treatment, Nd doped ceria nanocrystals remain of nanometric range ( ∼ 20 nm) while the concentration of oxygen vacancies is still high in ceria lattice, making this material convenient for solid oxide fuel cells application.
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81.40.Gh Other heat and thermomechanical treatments
61.46.Hk Nanocrystals
78.30.Hv Other nonmetallic inorganics
82.47.Ed Solid-oxide fuel cells (SOFC)

Nanoscale thick layer transfer of hydrogen-implanted wafer by using polycrystalline silicon sacrificial layer

T.-H. Lee, C.-H. Huang, Y. Y. Yang, T. Suryasindhu, and P. W. Li

Appl. Phys. Lett. 91, 203119 (2007); http://dx.doi.org/10.1063/1.2806913 (3 pages) | Cited 1 time

Online Publication Date: 16 November 2007

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This article presents a sacrificial layer method of forming a nanoscale thick silicon-on-insulator thin film, avoiding the channeling effect of implantation and eliminating a subsequent thinning process. However, because of the light mass of hydrogen, it is difficult with the implantation technique to have a shallow implant depth for splitting a layer at a thickness less than 100 nm by a traditional Smart-Cut® process. This study proves that using a polycrystalline-Si layer as a sacrificial layer in the initial implantation step can easily define a silicon transfer layer down to a thickness of tens of nanometers.
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81.07.Bc Nanocrystalline materials
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
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