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15 Aug 2006

Volume 100, Issue 4, Articles (04xxxx)

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Thermally induced self-hardening of nanocrystalline Ti–B–N thin films

P. H. Mayrhofer, C. Mitterer, J. G. Wen, I. Petrov, and J. E. Greene

J. Appl. Phys. 100, 044301 (2006); http://dx.doi.org/10.1063/1.2222406 (7 pages) | Cited 13 times

Online Publication Date: 16 August 2006

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Nanocrystalline films with high hardness have attracted increasing interest for wear resistant applications. Specifically, nanocrystalline Ti–B–N layers have been demonstrated to exhibit enhanced hardness and thermal stability. Here, we show that Ti–B–N films grown at 300 °C and consisting of a high volume fraction, ∼ 50%, of a fully percolated disordered phase encapsulating 2–3 nm wide TiN and TiB2 grains, have a hardness of 37 GPa and an elastic modulus of 332 GPa which increase with annealing to 43 and 362 GPa, respectively, at Ta = 800 °C. The structural rearrangement which occurs during annealing results in the formation of compact interface boundaries which lead, in turn, to the observed hardness increase. Annealing at Ta>900 °C decreases the hardness, although the elastic modulus continues to increase, due to the combination of grain growth and B loss via the formation of volatile boron oxides and hydroxides. These conclusions, obtained based upon a combination of x-ray diffraction, nanoindentation, electron probe microanalysis, and transmission electron microscopy, are corroborated by calorimetric investigations. The overall results provide insight toward developing “design rules” for high-temperature superhard nanoscale based coatings.
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68.60.Bs Mechanical and acoustical properties
68.60.Dv Thermal stability; thermal effects
68.55.-a Thin film structure and morphology
61.46.Hk Nanocrystals
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Jj Elasticity and anelasticity, stress-strain relations

Magnetic and transport properties of nanostructured ferric oxide produced by mechanical attrition

P. Brahma, S. Dutta, M. Pal, and D. Chakravorty

J. Appl. Phys. 100, 044302 (2006); http://dx.doi.org/10.1063/1.2227669 (6 pages) | Cited 2 times

Online Publication Date: 16 August 2006

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Nanosized particles of α-Fe2O3 in the range of 7–11 nm were produced by subjecting micron-sized α-Fe2O3 powder to ball milling for durations extending to 10 h. The magnetization of the powder was found to decrease as the particle size was reduced. Detailed electrical measurements of the compacted powder of different ball milled samples showed a drastic change in dc resistivity variation as a function of temperature. Two activated processes were observed with activation energies of ∼ 0.06 and ∼ 0.7 eV, respectively. These are ascribed to a polaron hopping conduction between Fe2+ and Fe3+ sites of neighboring nanoparticles. Large and small particle sizes are responsible for the two activation energies.
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81.07.Wx Nanopowders
81.07.Bc Nanocrystalline materials
75.50.Tt Fine-particle systems; nanocrystalline materials
73.63.Bd Nanocrystalline materials
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Spin relaxation in a germanium nanowire

S. Patibandla, S. Pramanik, S. Bandyopadhyay, and G. C. Tepper

J. Appl. Phys. 100, 044303 (2006); http://dx.doi.org/10.1063/1.2230012 (5 pages) | Cited 9 times

Online Publication Date: 16 August 2006

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We report experimental study of spin transport in nanowire spin valve structures consisting of three layers—cobalt, germanium, and nickel. The spin diffusion length in the Ge is estimated to be about 400 nm at 1.9 K and the corresponding spin relaxation time is about 4 ns. At 100 K, the spin diffusion length drops to 180 nm and the relaxation time is about 0.81 ns. These short relaxation times, which depend weakly on temperature, are caused by strong surface roughness scattering that causes rapid spin relaxation via the Elliott-Yafet mode [ Elliott, Phys. Rev. 96, 266 (1954) ].
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72.20.My Galvanomagnetic and other magnetotransport effects
73.63.Nm Quantum wires
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect)
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
73.40.Ns Metal-nonmetal contacts

Effect of tip geometry on contrast and spatial resolution of the near-field microwave microscope

Atif Imtiaz and Steven M. Anlage

J. Appl. Phys. 100, 044304 (2006); http://dx.doi.org/10.1063/1.2234801 (8 pages) | Cited 14 times

Online Publication Date: 16 August 2006

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The near-field scanning microwave microscope (NSMM) can quantitatively image materials properties at length scales far shorter than the free space wavelength (λ). Here we report a study of the effect of tip geometry on the NSMM signals. This particular NSMM utilizes scanning tunneling microscopy (STM) for distance-following control. We systematically examined many commercially available STM tips and found them to have a conical structure on the macroscopic scale, with an embedded sphere (of radius rsphere) at the apex of the tip. The rsphere values used in the study ranged from 0.1 to 12.6 μm. Tips with larger rsphere show good signal contrast [as measured by the frequency shift (Δf) signal between tunneling height and 2 μm away from the sample] with NSMM. For example, the tips with rsphere = 8 μm give signal contrast of 1000 kHz compared to 85 kHz with a tip of rsphere = 0.55 μm. However, large rsphere tips distort the topographic features acquired through STM. A theoretical model is used to understand the tip-to-sample interaction. The model quantitatively explains the measured change in quality factor (Q) as a function of height over bulk copper and silicon samples.
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07.79.Fc Near-field scanning optical microscopes
07.79.Cz Scanning tunneling microscopes

Coupled optoelectronic modeling and simulation of nanowire lasers

L. Chen and E. Towe

J. Appl. Phys. 100, 044305 (2006); http://dx.doi.org/10.1063/1.2221514 (9 pages) | Cited 6 times

Online Publication Date: 18 August 2006

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A self-consistent, coupled optoelectronic simulation approach for studying microcavity nanowire lasers is presented. To focus the discussion, we use GaN nanowires to illustrate the methodology. The comprehensive model solves, simultaneously and self-consistently, the carrier transport equations and the photon rate equations. The basic physical model takes into account both bulk and surface dark carrier recombination processes. It includes stimulated emission, the anisotropic optical gain typical of the wurtzite GaN structure, the modified spontaneous emission, and its coupling into the lasing modes as a consequence of microcavity effects. The model further incorporates band-gap shrinkage effects due to band renormalization; it includes the complex dispersion and reflectivity relations of the guided modes, and the effects of multiple lateral and longitudinal lasing and nonlasing optical modes. We provide a detailed discussion of the influence of other important effects on the operation of optically pumped GaN nanowire lasers. Finally, we want to point out that the method is sufficiently general that it can be used to study any class of nanowire lasers.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.55.Sa Microcavity and microdisk lasers
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Multidentate functionalized lubricant for ultralow head/disk spacing in a disk drive

X.-C. Guo, B. Knigge, B. Marchon, R. J. Waltman, M. Carter, and J. Burns

J. Appl. Phys. 100, 044306 (2006); http://dx.doi.org/10.1063/1.2221510 (8 pages) | Cited 21 times

Online Publication Date: 21 August 2006

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As areal density in disk drive technology is marching towards the Tbit/in.2 mark, the slider-to-disk spacing will be required to be within only a few nanometers. Coupled with very high slider velocities (>40 m/s), this leads to extremely high shear stresses on the nanometer-thin lubricant film at the disk surface. As a result, the lubricant film tends to exhibit local redistribution on a micrometer lateral level, decreasing the overall clearance and impacting the integrity of the interface. This paper describes a different approach to functionalized lubricant, where in addition to functional groups placed at the end of the chain, additional attachment moieties are introduced within the main polymer chain. The benefits of this approach is twofold: it increases adhesive interaction with the disk surface, therefore increasing the effective surface viscosity, while at the same time “tying” down the free backbone length for increased clearance. Fabrication, characterization, and performance data obtained on actual magnetic disks will be described.
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82.35.Gh Polymers on surfaces; adhesion
83.60.Fg Shear rate dependent viscosity
83.50.-v Deformation and flow
85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)

Vortex pinning in Nb thin films modulated by nanospheres

Wim Vinckx, Johan Vanacken, and Victor V. Moshchalkov

J. Appl. Phys. 100, 044307 (2006); http://dx.doi.org/10.1063/1.2227711 (7 pages) | Cited 5 times

Online Publication Date: 22 August 2006

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Superconducting Nb thin films deposited on top of self-assembled spherical nanoparticles have been investigated by transport measurements. Colloid polystyrene nanospheres have been organized by convective self-assembly into colloid crystals. These crystals serve as modulation templates for the Nb thin films which are deposited on top. These periodic arrays provide a natural topographic pattern to modulate laterally the superconducting order parameter in deposited Nb films. The nanoparticle diameter effectively controls the matching fields and flux pinning properties of these films. The superconducting phase boundary and matching effects have been studied for Nb films grown on the templates formed by polystyrene particles with diameter d = 202 and d = 357 nm. The triangular colloid lattice geometry induces strong modulation in thin Nb layers. In combination with the short range crystalline order of the templates, this leads to geometrical vortex ordering effects and vortex density dependent matching phenomena.
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74.78.-w Superconducting films and low-dimensional structures
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.25.F- Transport properties
81.16.Dn Self-assembly

Structure of latent tracks in rutile single crystal of titanium dioxide induced by swift heavy ions

Koichi Awazu, Xiaomin Wang, Makoto Fujimaki, Tetsuo Komatsubara, Takahiro Ikeda, and Yoshimichi Ohki

J. Appl. Phys. 100, 044308 (2006); http://dx.doi.org/10.1063/1.2229432 (5 pages) | Cited 8 times

Online Publication Date: 22 August 2006

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The structurally damaged zone in titanium dioxide rutile single crystal induced by MeV-order heavy ions was observed using high resolution electronic microscopy (HREM). Stressed regions as well as amorphous regions were identified in the damaged areas. Both stressed and amorphous regions were etched with hydrofluoric acid. The thermal spike model was used to calculate the track radii variation versus electron stopping power. When the calculated lattice temperature did not exceed the melting point of rutile titanium dioxide (2130 K), no structural change introduced by ions, such as 90 MeV Cl, was observed by HREM. It was found that the radius of the lattice temperature over the melting point corresponded closely to the radius of the stressed region. It was concluded that both stressed and amorphous regions are the result of quenching by molten titanium dioxide.
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61.80.Jh Ion radiation effects
61.82.Ms Insulators
64.70.D- Solid-liquid transitions
81.40.Gh Other heat and thermomechanical treatments

Growth modes of carbon nanotubes on metal substrates

Kristopher D. Matthews, Maxime G. Lemaitre, Taekyung Kim, Hao Chen, Moonsub Shim, and Jian-Min Zuo

J. Appl. Phys. 100, 044309 (2006); http://dx.doi.org/10.1063/1.2219000 (10 pages) | Cited 8 times

Online Publication Date: 23 August 2006

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Growth temperature induced changes in Al/Fe thin film catalysts are examined for chemical vapor deposition of carbon nanotubes directly on metal substrates. The film thickness, growth temperature, and supporting substrate affect the size and the density of Fe catalyst nanoparticles which in turn control the diameter, length, and single versus multiwalled nature of carbon nanotubes. Growths on two metal substrates, Au and Mo, using sputter deposited Al/Fe thin films are compared by transmission and scanning electron microscopy, and Raman analyses. Striking differences in the growth modes are observed with Au substrate enhancing multiwalled nanotube growth with metal catalyst particles at the tip away from the substrate and Mo substrate promoting single-walled nanotube growth with the catalyst nanoparticles remaining on the substrate. Oxidative treatment of Mo underlayer (i.e., relatively thick layer Mo sputtered on Au prior to Al/Fe catalyst deposition) can also induce nanotube growth with catalyst particles at the tips but with single-walled structure. These results suggest the importance of the support catalyst (Al/AlxOy) adhesion strength on substrates in determining nanotube growth modes.
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81.07.De Nanotubes
61.46.Fg Nanotubes
81.16.Hc Catalytic methods
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.35.Np Adhesion
78.30.Na Fullerenes and related materials

Thermal transport in nanocrystalline materials

Zhanrong Zhong and Xinwei Wang

J. Appl. Phys. 100, 044310 (2006); http://dx.doi.org/10.1063/1.2266206 (8 pages) | Cited 11 times

Online Publication Date: 23 August 2006

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In this work, thermal transport in nanocrystalline materials is studied using large-scale equilibrium molecular dynamics simulation. Nanocrystalline materials with different grain sizes are studied to explore how and to what extent the size of nanograins affects the thermal conductivity and specific heat. Substantial thermal conductivity reduction is observed and the reduction is stronger for nanocrystalline materials with smaller grains. On the other hand, the specific heat of nanocrystalline materials shows little change with the grain size. Based on the calculated thermal conductivity, the thermal resistance at grain boundaries is calculated and found to be in the order of 10−9 m2 K/W. The simulation results are compared with the thermal transport in freestanding nanograins based on molecular dynamics simulation. Further discussions are provided to explain the fundamental physics behind the observed thermal phenomena in this work.
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65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
61.46.Hk Nanocrystals
65.40.Ba Heat capacity
61.72.Mm Grain and twin boundaries

Stability of magnetic configurations in nanorings

P. Landeros, J. Escrig, D. Altbir, M. Bahiana, and J. d’Albuquerque e Castro

J. Appl. Phys. 100, 044311 (2006); http://dx.doi.org/10.1063/1.2218997 (6 pages) | Cited 18 times

Online Publication Date: 24 August 2006

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The relative stability of the vortex, onion, and ferromagnetic phases in nanorings is examined as a function of the ring geometry. Total energy calculations are carried out analytically, based on simple models for each configuration. Results are summarized by phase diagrams, which might be used as a guide to the production of rings with specific magnetic properties.
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75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.50.Tt Fine-particle systems; nanocrystalline materials

Rabi oscillations in a strongly driven semiconductor quantum well

Emmanuel Paspalakis, Margarita Tsaousidou, and Andreas F. Terzis

J. Appl. Phys. 100, 044312 (2006); http://dx.doi.org/10.1063/1.2227647 (8 pages) | Cited 24 times

Online Publication Date: 24 August 2006

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We study the interaction of an ac electric field with a semiconductor quantum well by using the effective nonlinear Bloch equations. Only the first two electron subbands in the well are considered. We apply the rotating wave approximation and derive analytical solutions for the Bloch equations for two different values of the detuning. At exact resonance we find a critical value of the Rabi frequency around which the dynamics of the system changes abruptly. Above this critical value one obtains electron oscillations with complete inversion in the two-subband system, while below this value we obtain electron oscillations without complete inversion and with the majority of the electron population on average in the lower subband. We also present numerical calculations for a specific quantum well structure and assess the limits of validity of the analytical results.
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73.21.Fg Quantum wells
78.67.De Quantum wells
73.50.Fq High-field and nonlinear effects

Formation of ultrahigh density and ultrasmall coherent β‐FeSi2 nanodots on Si (111) substrates using Si and Fe codeposition method

Yoshiaki Nakamura, Yasushi Nagadomi, Sung-Pyo Cho, Nobuo Tanaka, and Masakazu Ichikawa

J. Appl. Phys. 100, 044313 (2006); http://dx.doi.org/10.1063/1.2266322 (5 pages) | Cited 15 times

Online Publication Date: 24 August 2006

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We studied the formation of β‐FeSi2 nanodots by codeposition at disilicide stoichiometric deposition rates of Fe and Si on Si (111) substrates covered with ultrathin SiO2 films. Hemispherical β‐FeSi2 nanodots with an ultrahigh density (>1012 cm−2) and with a narrow size distribution at the average size of ∼ 5 nm diameter were epitaxially grown by the codeposition at 500 °C on ultrathin SiO2 films with predeposited Si. High-resolution transmission electron microscope observations showed that the formed nanodots were strained with a main configuration of β‐FeSi2 (110)∕Si (111). We propose the formation mechanism that a reaction between the SiO2 films and deposited Si atoms formed voids in the SiO2 films to work as nucleation sites, followed by direct β‐FeSi2 formation from Fe and Si without passing through other iron silicide phases.
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81.07.Bc Nanocrystalline materials
81.05.Hd Other semiconductors
81.16.-c Methods of micro- and nanofabrication and processing
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Enhanced low field magnetoresistance of Fe3O4 nanosphere compact

P. Y. Song, J. F. Wang, C. P. Chen, H. Deng, and Y. D. Li

J. Appl. Phys. 100, 044314 (2006); http://dx.doi.org/10.1063/1.2335386 (4 pages) | Cited 5 times

Online Publication Date: 24 August 2006

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Unusually large low field magnetoresistance (LFMR), ∼ 10%, at 300 K has been observed with the sample of monodispersed Fe3O4 magnetite nanospheres, ∼ 200 nm, compactly cold pressed and sintered at 800 °C. A detailed analysis on the transport and magnetic measurements indicates that the electron conduction is dominated by the spin-dependent scattering or tunneling at the grain boundaries. At low temperatures, 140 and 100 K near the Verwey transition, ∼ 115 K, the LFMR (below 2 kOe) does not show any sign of dependence on the transition and does not follow the variation of magnetization to reach the saturation region either. On the other hand, at 300 K, the MR saturates fast with the magnetization below 2 kOe. This temperature dependent property in LFMR is very likely attributed to the scattering or tunneling of the conduction electron passing through the grain boundary layer with spin disordered state.
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81.07.Wx Nanopowders
75.50.Tt Fine-particle systems; nanocrystalline materials
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
72.20.My Galvanomagnetic and other magnetotransport effects
75.47.Pq Other materials
61.72.Mm Grain and twin boundaries

Vapor-liquid-solid mechanisms: Challenges for nanosized quantum cluster/dot/wire materials

P. Cheyssac, M. Sacilotti, and G. Patriarche

J. Appl. Phys. 100, 044315 (2006); http://dx.doi.org/10.1063/1.2236163 (12 pages) | Cited 19 times

Online Publication Date: 25 August 2006

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The growth mechanism model of a nanoscaled material is a critical step that has to be refined for a better understanding of a nanostructure’s dot/wire fabrication. To do so, the growth mechanism will be discussed in this paper and the influence of the size of the metallic nanocluster starting point, referred to later as “size effect,” will be studied. Among many of the so-called size effects, a tremendous decrease of the melting point of the metallic nanocluster changes the physical properties as well as the physical/mechanical interactions inside the growing structure composed of a metallic dot on top of a column. The thermodynamic size effect is related to the bending or curvature of chains of atoms, giving rise to the weakening of bonds between them; this size or curvature effect is described and approached to crystal nanodot/wire growth. We will describe this effect as that of a “cooking machine” when the number of atoms decreases from ∼ 1023 at./cm3 for a bulk material to a few tens of them in a 1–2 nm diameter sphere. The decrease of the number of atoms in a metallic cluster from such an enormous quantity is accompanied by a lowering of the melting temperature that extends from 200 up to 1000 K, depending on the metallic material and its size under study. In this respect, the vapor-liquid-solid (VLS) model, which is the most utilized growth mechanism for quantum nanowires and nanodots, is critically exposed to size or curvature effects (CEs). More precisely, interactions in the vicinity of the growth regions should be reexamined. Some results illustrating the growth of micrometer-/nanometer-sized materials are presented in order to corroborate the CE/VLS models utilized by many research groups in today’s nanosciences world. Examples of metallic clusters and semiconducting wires will be presented. The results and comments presented in this paper can be seen as a challenge to be overcome. From them, we expect that in a near future an improved model can be exposed to the scientific community.
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68.65.La Quantum wires (patterned in quantum wells)
68.65.Hb Quantum dots (patterned in quantum wells)
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
64.70.D- Solid-liquid transitions

Sensitivity enhancement of AlGaAs/InGaAs/GaAs quantum well-based Hall device

H. Sghaier, L. Bouzaiene, L. Sfaxi, and H. Maaref

J. Appl. Phys. 100, 044316 (2006); http://dx.doi.org/10.1063/1.2335398 (5 pages) | Cited 1 time

Online Publication Date: 25 August 2006

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We have investigated the consequences of a modified design of modulation doping pseudomorphic AlGaAs/InGaAs/GaAs and we propose a quantum well structure for a Hall device with the goal of improving its performances. From self-consistent calculations we find that the electron concentration ns in the interface region is increased. This implies that one can have a wider spacer layer and still have the same ns with the result that the mobility is improved. This result should be valuable for many types of devices. We specifically consider Hall sensors, where it is desirable to have a low electron concentration and a high mobility.
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85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
85.30.Fg Bulk semiconductor and conductivity oscillation devices (including Hall effect devices, space-charge-limited devices, and Gunn effect devices)
85.30.De Semiconductor-device characterization, design, and modeling

Periodic lines and holes produced in thin Au films by pulsed laser irradiation

Yu. Kaganovskii, H. Vladomirsky, and M. Rosenbluh

J. Appl. Phys. 100, 044317 (2006); http://dx.doi.org/10.1063/1.2234548 (7 pages) | Cited 11 times

Online Publication Date: 25 August 2006

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A technique for the fabrication of periodic lines and holes in thin gold films (5–50 nm thick) on a glass substrate by irradiation with two and four intersecting beams of a nanosecond pulsed laser is demonstrated. The two- or four-beam interference creates a one-dimensional or two-dimensional periodically alternating intensity distribution, respectively. It is shown that the threshold intensity necessary for the fabrication of periodic structures is not a monotonous function of increasing film thickness and also depends on the period of the structure. In films thicker than 17 nm a unique redistribution of the film material was observed at the high intensity lines or spots. At these “hot” locations we observed the formation of tall and narrow ridges and rims caused by the hydrodynamic flow of the molten metallic film in the optically induced temperature gradient, with subsequent crystallization in the “cold” regions. A model was developed which allows us to calculate the temperature distribution around irradiated regions of the film as a function of time and position for various film thicknesses, periodicities, and beam intensities, taking into account the film reflectivity and the beam intensity profile. From the temperature distribution we calculate the expected film morphology subsequent to laser irradiation and find good agreement with the measured distances between the ridges and the rim diameters.
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68.55.-a Thin film structure and morphology
78.66.Bz Metals and metallic alloys
61.82.Bg Metals and alloys
78.47.-p Spectroscopy of solid state dynamics

Poling-assisted bleaching of soda-lime float glasses containing silver nanoparticles with a decreasing filling factor across the depth

Olivier Deparis, Peter G. Kazansky, Alexander Podlipensky, Amin Abdolvand, Gerhard Seifert, and Heinrich Graener

J. Appl. Phys. 100, 044318 (2006); http://dx.doi.org/10.1063/1.2234813 (9 pages) | Cited 11 times

Online Publication Date: 29 August 2006

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The recently discovered poling-assisted bleaching of glass with embedded silver nanoparticles has renewed the interest in thermal poling as a simple, reliable, and low-cost technique for controlling locally the surface-plasmon-resonant optical properties of metal-doped nanocomposite glasses. In the present study, the emphasis is put on the influence of the volume filling factor of metallic clusters on poling-assisted bleaching. Soda-lime silicate glass samples containing spherical silver nanoparticles with a decreasing filling factor across the depth were subject to thermal poling experiments with various poling temperatures, voltages, and times. Optical extinction spectra were measured from ultraviolet to near-infrared ranges and the surface-plasmon-resonant extinction due to silver nanoparticles (around 410 nm) was modeled by the Maxwell Garnett [Philos. Trans. R. Soc. London, Ser. A 203, 385 (1904); 205, 237 (1906)] effective medium theory which was adapted in order to take into account the filling factor depth profile. A method was proposed for the retrieval of the filling factor depth profile from optical extinction spectra recorded in fresh and chemically etched samples. A stretched exponential depth profile turned out to be necessary in order to model samples having a high filling factor near the surface. Based on the fact that the electric-field-assisted dissolution of embedded metallic nanoparticles proceeded progressively from the top surface, a bleaching front was defined that moved forward in depth as time elapsed. The position of the bleaching front was determined after each poling experiment by fitting the measured extinction spectrum to the theoretical one. In samples with higher peak value and steeper gradient of the filling factor, the bleaching front reached more rapidly a steady-state depth as poling time increased. Also it increased less strongly with increasing poling voltage. These results were in agreement with the physics of the dissolution process. Finally, clear evidence of injection of hydrogenated ionic species from the atmosphere into the sample during poling was obtained from the growth of the infrared extinction peak associated with OH radicals.
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42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption
78.40.Pg Disordered solids
78.35.+c Brillouin and Rayleigh scattering; other light scattering
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

A first-principles model of birefringent porous silicon

Yuri Bonder and Chumin Wang

J. Appl. Phys. 100, 044319 (2006); http://dx.doi.org/10.1063/1.2335669 (5 pages) | Cited 6 times

Online Publication Date: 29 August 2006

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Optical properties of birefringent porous silicon (b‐PSi) layers are studied by means of the density functional theory (DFT) within the local density approximation (LDA). A systematic study of crystalline silicon (c‐Si) is performed in order to validate this DFT-LDA calculation of optical properties of semiconductors. In order to simulate b‐PSi, elliptical columns of 1–4 atoms are removed from a c‐Si supercell of 16 atoms in the [100] and [010] directions. The dangling bonds are saturated with hydrogen atoms. A geometry optimization is carried out to get the minimum energy configuration. The results of the refractive index (n) show an enhanced anisotropy and the difference Δn = math−n[001] agrees well with experimental data. In particular, measurements in p+ and p++ doped b‐PSi samples are consistent with the results obtained in the limit cases of pore branches along the [001] direction and the perfectly straight pores, respectively.
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78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
78.20.Fm Birefringence
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.43.Gt Powders, porous materials
71.55.Cn Elemental semiconductors

A cross-sectional scanning tunneling microscopy study of a quantum dot infrared photodetector structure

L. Ouattara, A. Mikkelsen, E. Lundgren, L. Höglund, C. Asplund, and J. Y. Andersson

J. Appl. Phys. 100, 044320 (2006); http://dx.doi.org/10.1063/1.2245195 (5 pages) | Cited 12 times

Online Publication Date: 30 August 2006

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We report on cross-sectional scanning tunneling microscopy studies of a quantum dot infrared photodetector structure consisting of multiple InGaAs quantum wells containing InAs quantum dots and separated by GaAs. We have investigated the composition and size distribution of the InAs quantum dots in this structure. Using cross-sectional scanning tunneling microscopy images displaying atomic resolution, we reveal that the InAs quantum dots are strongly intermixed with the InGaAs quantum well layer. We estimate the size distribution of the dots and find this to be nonuniform. Further, defects related to dopants introduced in the structure are identified and their concentration is estimated.
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68.65.Hb Quantum dots (patterned in quantum wells)
68.65.Fg Quantum wells
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
81.07.Ta Quantum dots
85.60.Gz Photodetectors (including infrared and CCD detectors)

Evolution of catalyst particle size during carbon single walled nanotube growth and its effect on the tube characteristics

Avetik R. Harutyunyan, Toshio Tokune, Elena Mora, Jung-Woo Yoo, and Arthur J. Epstein

J. Appl. Phys. 100, 044321 (2006); http://dx.doi.org/10.1063/1.2335396 (8 pages) | Cited 9 times

Online Publication Date: 30 August 2006

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A series of Fe catalysts, with different mean diameters, supported on alumina with different molar ratios, was studied before and after carbon single walled nanotubes growth using magnetic measurements and Raman scattering techniques (laser excitation wavelengths from 1.17 to 2.54 eV) to follow changes on catalyst particle size and composition, as well as the relationship between particle size and diameter of nanotubes grown. In all cases, an increase and redistribution of the particle size after the growth was concluded based on the blocking temperature values and Langevin function analysis. This is explained in terms of agglomeration of particles due to carbon-induced liquefaction accompanied with an increase in the catalyst mobility. For large particles no direct correlation between the catalyst size and the nanotube diameters was observed.
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75.50.Bb Fe and its alloys
75.50.Tt Fine-particle systems; nanocrystalline materials
81.05.Bx Metals, semimetals, and alloys
81.07.De Nanotubes
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Narrow bandpass optical filters fabricated with one-dimensionally periodic inhomogeneous thin films

Matthew M. Hawkeye and Michael J. Brett

J. Appl. Phys. 100, 044322 (2006); http://dx.doi.org/10.1063/1.2335397 (7 pages) | Cited 26 times

Online Publication Date: 30 August 2006

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Thin films with sinusoidally varying refractive index profiles display photonic band gap effects. Intentional deviations from the periodic index profile can be used to tailor the optical properties of the resulting thin film. We present experimental characterization of TiO2 films with periodic index profiles fabricated using a deposition technique known as glancing angle deposition (GLAD). The resulting porous thin films have a microstructure consisting of vertically aligned columns. Sinusoidal porosity gradients, and therefore sinusoidal index profiles, can be introduced in the direction of the substrate normal by fabricating columns with a periodically varying diameter. Local modifications of the index profile are achieved by inserting thin layers of constant porosity into the center of the film, or by discontinuously changing the phase of the sinusoidal gradient. The introduction of these structural defects creates a narrow optical passband inside the larger band gap, and we demonstrate how the properties of this passband can be controlled through modification of the defect parameters. The magnitude of the phase shift constituting the defect is shown to control the location of the resulting passband. By inserting a layer with in-plane birefringence, we show that it is possible to create two separate passbands, one for each polarization, within a single stop band. The results illustrate how nanoscale porosity engineering using GLAD is a precise technique for fabricating one-dimensionally periodic films with a variety of optical characteristics.
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42.79.Ci Filters, zone plates, and polarizers
42.79.Wc Optical coatings
42.79.Ry Gradient-index (GRIN) devices
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.20.Fm Birefringence

Carbon segregation as a strain relaxation mechanism in thin germanium-carbon layers deposited directly on silicon

D. I. Garcia-Gutierrez, M. José-Yacamán, Shifeng Lu, D. Q. Kelly, and S. K. Banerjee

J. Appl. Phys. 100, 044323 (2006); http://dx.doi.org/10.1063/1.2336305 (6 pages) | Cited 3 times

Online Publication Date: 30 August 2006

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We report experimental evidence for the segregation and preferential localization of C atoms at the surface and substrate interfaces in thin Ge1−xCx films deposited directly on Si (100). The results are interpreted in the context of C segregation providing a mechanism for strain relaxation. Four different experimental techniques, including energy-dispersive spectroscopy, electron energy loss spectroscopy (EELS), energy-filtering transmission electron microscopy, and secondary ion mass spectrometry, support our claims. The EELS analyses showed that the C bonding near the Ge1−xCx/Si substrate interface presented a higher sp3 character than in the central region or at the surface. Two interpretations are given for this observation; one is that structural relaxation occurs when C atoms occupy substitutional sites in the Ge crystal closer to the Ge1−xCx/Si substrate interface; the other is that the higher sp3 character of the C atoms might be an indication that C-containing tetrahedral interstitial complexes form at the interface (chemical relaxation).
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81.05.Cy Elemental semiconductors
68.35.Dv Composition, segregation; defects and impurities
68.60.Bs Mechanical and acoustical properties
79.20.Uv Electron energy loss spectroscopy
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
81.40.Jj Elasticity and anelasticity, stress-strain relations

Determination of the aspect ratio statistical distribution of gold nanorods in solution from a theoretical fit of the observed inhomogeneously broadened longitudinal plasmon resonance absorption spectrum

Susie Eustis and Mostafa A. El-Sayed

J. Appl. Phys. 100, 044324 (2006); http://dx.doi.org/10.1063/1.2244520 (7 pages) | Cited 21 times

Online Publication Date: 31 August 2006

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The determination of the statistical distribution of aspect ratios of a nanorod solution is desirable for experimentally synthesized solutions. The traditional method of using transmission electron microscopy (TEM) images for size determination gives statistically incorrect values due to distortions introduced by TEM sample preparation and by difficulties in counting a sufficiently large number of rods. In the present work, we propose a method to obtain the aspect ratio distribution using the observed longitudinal surface plasmon resonance absorption spectrum of gold nanorods in solution. The observed inhomogeneously broadened spectrum is fitted with a collection of homogeneously broadened spectra of nanorods each with a specific aspect ratio and population contribution using Gans extension [ Ann. Phys. 47, 270 (1915) ] of Mie theory. The fit generates an aspect ratio distribution for the rods in solution from which the median value and the statistical distribution are determined. This method is statistically more accurate, more convenient, and less expensive than the traditional method of TEM analysis on a solid substrate.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.68.+m Optical properties of surfaces
61.46.-w Structure of nanoscale materials

Effective thermal conductivity and thermal diffusivity of nanofluids containing spherical and cylindrical nanoparticles

Xing Zhang, Hua Gu, and Motoo Fujii

J. Appl. Phys. 100, 044325 (2006); http://dx.doi.org/10.1063/1.2259789 (5 pages) | Cited 41 times

Online Publication Date: 31 August 2006

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This paper reports on measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al2O3 thin film and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Au/toluene, Al2O3/water, and carbon nanofiber (CNF)/water nanofluids are measured and the effects of the volume fraction and thermal conductivity of the nanoparticles and temperature are clarified. The average diameters of Au and Al2O3 spherical particles are 1.65 and 20 nm, respectively. The average length and diameter of CNFs are 10 μm and 150 nm, respectively. The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity. The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancements and can be predicted accurately by the model equation of Hamilton and Crosser [Ind. Eng. Chem. Fundam. 1, 187 (1962)] for the spherical nanoparticles and by the unit-cell model equation of Yamada and Ota [Waerme-Stoffuebertrag. 13, 27 (1980)] for carbon nanofibers.
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66.25.+g Thermal conduction in nonmetallic liquids
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
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