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3 Dec 2007

Volume 91, Issue 23, Articles (23xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 232101 (2007); http://dx.doi.org/10.1063/1.2818712 (3 pages)

T. Kita, D. Chiba, Y. Ohno, and H. Ohno
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Trace gas detection using nanostructured graphite layers

Muhammad Qazi, Thomas Vogt, and Goutam Koley

Appl. Phys. Lett. 91, 233101 (2007); http://dx.doi.org/10.1063/1.2820387 (3 pages) | Cited 18 times

Online Publication Date: 3 December 2007

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Nanostructured graphite (NG) has been investigated as a sensing material using a highly sensitive potentiometric detection technique. NO2 concentration down to 60 ppb was detected in ambient conditions using NG functionalization layer. Simultaneous current and surface work function (SWF) change transients measured using NG functionalization layer reveal much shorter response time for the later, which is attributed to its dependence solely on surface molecular adsorption. The gradient of SWF with respect to the current transient was found to be independent of gaseous concentration and fraction of preoccupied surface states.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
68.43.-h Chemisorption/physisorption: adsorbates on surfaces

Epitaxial aluminum nitride tunnel barriers grown by nitridation with a plasma source

T. Zijlstra, C. F. J. Lodewijk, N. Vercruyssen, F. D. Tichelaar, D. N. Loudkov, and T. M. Klapwijk

Appl. Phys. Lett. 91, 233102 (2007); http://dx.doi.org/10.1063/1.2819532 (3 pages) | Cited 11 times

Online Publication Date: 4 December 2007

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High critical current-density (10 to 420 kA/cm2) superconductor-insulator-superconductor tunnel junctions with aluminum nitride barriers have been realized using a remote nitrogen plasma from an inductively coupled plasma source operated in a pressure range of 10−3–10−1 mbar. We find a much better reproducibility and control compared to previous work. From the current-voltage characteristics and cross-sectional transmission electron microscopy images it is inferred that, compared to the commonly used AlOx barriers, the polycrystalline AlN barriers are much more uniform in transmissivity, leading to a better quality at high critical current densities.
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74.50.+r Tunneling phenomena; Josephson effects
74.25.Sv Critical currents

Rheological properties of nanofluids flowing through microchannels

J. Chevalier, O. Tillement, and F. Ayela

Appl. Phys. Lett. 91, 233103 (2007); http://dx.doi.org/10.1063/1.2821117 (3 pages) | Cited 30 times

Online Publication Date: 4 December 2007

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Results on the viscosity of SiO2 nanofluids submitted to very strong shear rates are reported. Experiments were conducted with micromachined capillary viscometers equipped with local pressure probes. For particle sizes of 35, 94, and 190 nm, and solid volume fractions from 1.4% to 7%, a Newtonian behavior has been observed up to 5×104 s−1. The increase in the nanofluid viscosity obeys a classical model but with a crowding factor which is a function of the diameter of the particles. This size dependence is explained by the influence of the shearing motion on the aggregates aspect ratio.
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07.10.Cm Micromechanical devices and systems
47.61.Jd Multiphase flows
47.57.Qk Rheological aspects
47.55.Kf Particle-laden flows

Emission enhancement behaviors in the coupling between surface plasmon polariton on a one-dimensional metallic grating and a light emitter

Jyh-Yang Wang, Yean-Woei Kiang, and C. C. Yang

Appl. Phys. Lett. 91, 233104 (2007); http://dx.doi.org/10.1063/1.2821829 (3 pages) | Cited 11 times

Online Publication Date: 5 December 2007

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The authors study numerically the enhancements of dipole radiation rate and emission efficiency in a structure of a sinusoidal interface between a half space of metal and a half space of dielectric. The surface plasmon polaritons (SPPs) generated at the interface couple with the dipole to enhance its radiation rate. The energy dependence of radiation rate relies on the factors of SPP density of state, dipole position, and the emission efficiency. Below the light line, the emission efficiency is controlled by the localized surface plasmon (SP) field distribution in coupling two counterpropagating SPP modes near the Brillouin zone boundary. A localized SP field distribution of higher intensity and higher emission efficiency leads to a higher dipole radiation rate and a higher emission fraction.
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78.66.Bz Metals and metallic alloys
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.68.+m Optical properties of surfaces

The study of electrical characteristics of heterojunction based on ZnO nanowires using ultrahigh-vacuum conducting atomic force microscopy

J. H. He and C. H. Ho

Appl. Phys. Lett. 91, 233105 (2007); http://dx.doi.org/10.1063/1.2821831 (3 pages) | Cited 19 times

Online Publication Date: 5 December 2007

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The electrical performances of the heterojunction of n-ZnO nanowires with p-Si substrate at the nanometer scale have been characterized using an ultrahigh-vacuum conducting atomic force microscopy. Compared with the expected values of 1.0–2.0 reported in p-n junction in the previous studies, the abnormally high diode ideality factor (⪢2) was obtained. It elucidates that a ZnO–Si p-n junction can be modeled by a series of diodes, the actual ZnO–Si junction diode and two Schottky diodes at the metal/ZnO and metal/Si junctions. The tunneling across p-n junction would also play a role in the externally measured high ideality factor.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
81.07.Vb Quantum wires
85.30.Kk Junction diodes

Stranski-Krastanow growth of tensile strained Si islands on Ge (001)

D. Pachinger, H. Groiss, H. Lichtenberger, J. Stangl, G. Hesser, and F. Schäffler

Appl. Phys. Lett. 91, 233106 (2007); http://dx.doi.org/10.1063/1.2820605 (3 pages) | Cited 7 times

Online Publication Date: 5 December 2007

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Stranski-Krastanow island growth is demonstrated for tensile strained silicon epilayers on Ge (001) substrates over a wide range of growth temperatures. Small, Si-rich islands show sidewall faces near {1,1,10}, whereas larger islands are {113}-terminated truncated pyramids with an aspect ratio near 0.1. In contrast to compressively strained Ge on Si, we find for Si on Ge a significantly thicker wetting layer of >8 ML and coexistence of islands and dislocations.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.60.Bs Mechanical and acoustical properties

FeCo–Au core-shell nanocrystals

Yun-Hao Xu and Jian-Ping Wang

Appl. Phys. Lett. 91, 233107 (2007); http://dx.doi.org/10.1063/1.2823574 (3 pages) | Cited 6 times

Online Publication Date: 5 December 2007

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A one-step synthesis of heterostructured high magnetic moment FeCo–Au nanocrystals was demonstrated. The nanocrystals possess a cubic shape and have an average size of 30 nm. Compositional analysis revealed the detailed distribution of each element within the nanocrystal. The crystalline structure of the components and their epitaxial relationship were determined by structural analysis. The heterostructured FeCo–Au nanocrystals are promising in biomedical applications as well as in fundamental studies of interactions in the nanoscale.
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73.22.-f Electronic structure of nanoscale materials and related systems
75.30.Cr Saturation moments and magnetic susceptibilities

Raman fingerprint of charged impurities in graphene

C. Casiraghi, S. Pisana, K. S. Novoselov, A. K. Geim, and A. C. Ferrari

Appl. Phys. Lett. 91, 233108 (2007); http://dx.doi.org/10.1063/1.2818692 (3 pages) | Cited 56 times

Online Publication Date: 5 December 2007

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We report strong variations in the Raman spectra for different single-layer graphene samples obtained by micromechanical cleavage. This reveals the presence of excess charges, even in the absence of intentional doping. Doping concentrations up to ∼ 1013 cm−2 are estimated from the G peak shift and width and the variation of both position and relative intensity of the second order 2D peak. Asymmetric G peaks indicate charge inhomogeneity on a scale of less than 1 μm.
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78.30.Na Fullerenes and related materials

Tailoring the intrinsic metallic states of double-walled nanotube films by self-soldered laser welding

Yong Zhang, Tao Gong, Yi Jia, Wenjin Liu, Jinquan Wei, Mingxing Ma, Kunlin Wang, Minlin Zhong, Dehai Wu, and Anyuan Cao

Appl. Phys. Lett. 91, 233109 (2007); http://dx.doi.org/10.1063/1.2821830 (3 pages) | Cited 2 times

Online Publication Date: 6 December 2007

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The metallic state and resistivity of interconnected double-walled carbon nanotube bundles have been tuned by modifying two major components, including (1) structure of the bundles and (2) contacts at the heterojunctions between the bundles. Infrared laser illumination at controlled intensity on the nanotubes produces an amorphous carbon coating that subsequently serves as the solder material to weld the bundles at their heterojunction sites. The results show that without selective etching of metallic or semiconductor species, the crossover temperature at which metallic behavior changes to semiconducting can be tuned within a wide range (25–150 K).
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73.63.Fg Nanotubes
72.60.+g Mixed conductivity and conductivity transitions
81.07.De Nanotubes
81.20.Vj Joining; welding
42.62.Cf Industrial applications
61.46.Fg Nanotubes

Magnetic nanocheckerboards with tunable sizes in the Mn-doped CoFe2O4 spinel

C. L. Zhang, C. M. Tseng, C. H. Chen, S. Yeo, Y. J. Choi, and S.-W. Cheong

Appl. Phys. Lett. 91, 233110 (2007); http://dx.doi.org/10.1063/1.2821838 (3 pages) | Cited 8 times

Online Publication Date: 6 December 2007

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Formation of magnetically ordered array of two types of rectangular nanorods, ∼ 300 nm in length and a few nanometers in size, is achieved in the Mn-doped CoFe2O4 spinel through chemical phase separation mediated by cooperative Jahn-Teller distortions. At room temperature, the magnetic nanorods, with composition close to CoFe2O4, interlace with the paramagnetic counterparts and form a highly organized checkerboard pattern in the cross section. The checkerboard size in the range of ∼ 13.8×7.9– ∼ 17.3×14.0 nm2 is tunable with composition and, particularly, with the isothermal annealing time. These three-dimensional nanocheckerboards exhibit a nearly ideal configuration for the patterned perpendicular recording medium.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.20.-g Diamagnetism, paramagnetism, and superparamagnetism
75.50.Dd Nonmetallic ferromagnetic materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.16.-c Methods of micro- and nanofabrication and processing

Optical characterization of silicon on insulator photonic crystal nanocavities infiltrated with colloidal PbS quantum dots

D. F. Dorfner, T. Hürlimann, G. Abstreiter, and J. J. Finley

Appl. Phys. Lett. 91, 233111 (2007); http://dx.doi.org/10.1063/1.2822441 (3 pages) | Cited 4 times

Online Publication Date: 6 December 2007

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The authors present the fabrication of silicon on insulator (SOI) membrane nanocavities for use as sensitive optical biosensors and investigations of their optical properties. Simulations illustrate the evolution of the cavity mode frequency as a function of structural parameters. The SOI nanocavities were characterized using μ-photoluminescence spectroscopy by doping the photonic crystals with colloidal PbS quantum dots embedded in a polymer matrix. Experiments lead to cavity mode quality factors of Q = 800±35 and the dependence on geometric parameters is in excellent agreement with our calculations.
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82.70.Dd Colloids
81.07.Ta Quantum dots
78.67.Hc Quantum dots
78.55.-m Photoluminescence, properties and materials
42.70.Qs Photonic bandgap materials

Selective growth of catalyst-free ZnO nanowire arrays on Al:ZnO for device application

T. F. Chung, L. B. Luo, Z. B. He, Y. H. Leung, I. Shafiq, Z. Q. Yao, and S. T. Lee

Appl. Phys. Lett. 91, 233112 (2007); http://dx.doi.org/10.1063/1.2811717 (3 pages) | Cited 15 times

Online Publication Date: 6 December 2007

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Vertically aligned ZnO nanowire (NW) arrays have been synthesized selectively on patterned aluminum-doped zinc oxide (AZO) layer deposited on silicon substrates without using any metal catalysts. The growth region was defined by conventional photolithography with an insulating template. Careful control of the types of template materials and growth conditions allows good alignment and growth selectivity for ZnO NW arrays. Sharp ultraviolet band-edge peak observed in the photoluminescence spectra of the patterned ZnO NW arrays reveals good optical qualities. The current-voltage characteristics of ZnO NWs/AZO/p-Si device suggest that patterned and aligned ZnO NW arrays on AZO may be used in optoelectronic devices.
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81.16.Nd Micro- and nanolithography
81.15.Cd Deposition by sputtering
68.65.La Quantum wires (patterned in quantum wells)
78.67.Lt Quantum wires
78.55.Et II-VI semiconductors

Near-surface modification of polystyrene by Ar+: Molecular dynamics simulations and experimental validation

J. J. Végh, D. Nest, D. B. Graves, R. Bruce, S. Engelmann, T. Kwon, R. J. Phaneuf, G. S. Oehrlein, B. K. Long, and C. G. Willson

Appl. Phys. Lett. 91, 233113 (2007); http://dx.doi.org/10.1063/1.2821226 (3 pages) | Cited 23 times

Online Publication Date: 6 December 2007

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Results are presented from molecular dynamics (MD) simulations of 100 eV Ar+ bombardment of a model polystyrene (PS) surface. The simulations show that the system transitions from an initially high sputter yield (SY) for the virgin polymer to a drastically lower SY as steady state is approached. This is consistent with corresponding ion beam experiments. The MD indicates that this drop in SY is due to the formation of a heavily cross-linked, dehydrogenated damaged layer. The thickness and structure of this layer are also consistent with ellipsometry and x-ray photoelectron spectroscopy measurements of Ar plasma-exposed PS samples.
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79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
52.77.-j Plasma applications
79.60.Fr Polymers; organic compounds

Structural templating as a route to improved photovoltaic performance in copper phthalocyanine/fullerene (C60) heterojunctions

P. Sullivan, T. S. Jones, A. J. Ferguson, and S. Heutz

Appl. Phys. Lett. 91, 233114 (2007); http://dx.doi.org/10.1063/1.2821229 (3 pages) | Cited 38 times

Online Publication Date: 6 December 2007

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We have developed a method to improve the short circuit current density in copper phthalocyanine (CuPc)/fullerene (C60) organic solar cells by ∼ 60% by modifying the CuPc crystal orientation through use of a molecular interlayer to maximize charge transport in the direction between the two electrodes. Powder x-ray diffraction and electronic absorption spectroscopy show that a thin 3,4,9,10-perylenetetracarboxylic dianhydride interlayer before CuPc growth templates the CuPc film structure, forcing the molecules to lie flat with respect to the substrate surface, although the intrastack orientation is unaffected. This modified stacking configuration facilitates charge transport and improves charge collection.
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84.60.Jt Photoelectric conversion
73.61.Ph Polymers; organic compounds

Nanopatterning by dual-ion-beam sputtering

M. Joe, C. Choi, B. Kahng, and J.-S. Kim

Appl. Phys. Lett. 91, 233115 (2007); http://dx.doi.org/10.1063/1.2816236 (3 pages) | Cited 4 times

Online Publication Date: 7 December 2007

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We studied the development of ordered nanopatterns during dual-ion-beam sputtering (DIBS) of Au(001) in which two ion beams that cross perpendicular to each other at their azimuth are incident on the surface at a grazing angle. In the erosion (diffusion) regime, a square-symmetric two-dimensional (2D) pattern of nanodots (holes) is formed. The 2D pattern is achieved only when the two beams are balanced in the erosion regime. In the diffusion regime, no such condition is required. The observations cannot be explained by the Kuramoto-Sivashinsky (KS) equation derived from Sigmund theory with two ion beams.
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81.16.Rf Micro- and nanoscale pattern formation
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Photoelectron spectroscopy of individual nanowires of Si and Ge

Richard Haight, George Sirinakis, and Mark Reuter

Appl. Phys. Lett. 91, 233116 (2007); http://dx.doi.org/10.1063/1.2822415 (3 pages) | Cited 1 time

Online Publication Date: 7 December 2007

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We describe an experiment designed to carry out photoelectron spectroscopy on individual nanowires of Si and Ge. Laser generated, 150 fs pulses of 200 nm light (6.2 eV) were focused onto a single Si or Ge nanowire; the ensuing photoemitted electrons were measured with 20 meV resolution. Fermi level locations within the individual Si and Ge nanowire band gaps and work functions of hydrogen terminated nanowires were measured. Polarization dependent electron emission was observed and compared with Mie theory.
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79.60.Jv Interfaces; heterostructures; nanostructures
79.60.Bm Clean metal, semiconductor, and insulator surfaces
73.21.Hb Quantum wires
71.20.Mq Elemental semiconductors

Silicon nanowire solar cells

L. Tsakalakos, J. Balch, J. Fronheiser, B. A. Korevaar, O. Sulima, and J. Rand

Appl. Phys. Lett. 91, 233117 (2007); http://dx.doi.org/10.1063/1.2821113 (3 pages) | Cited 248 times

Online Publication Date: 7 December 2007

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Silicon nanowire-based solar cells on metal foil are described. The key benefits of such devices are discussed, followed by optical reflectance, current-voltage, and external quantum efficiency data for a cell design employing a thin amorphous silicon layer deposited on the nanowire array to form the p-n junction. A promising current density of ∼ 1.6 mA/cm2 for 1.8 cm2 cells was obtained, and a broad external quantum efficiency was measured with a maximum value of ∼ 12% at 690 nm. The optical reflectance of the silicon nanowire solar cells is reduced by one to two orders of magnitude compared to planar cells.
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84.60.Jt Photoelectric conversion

Improved charge injection characteristics of Ge nanocrystals embedded in hafnium oxide for floating gate devices

S. Das, K. Das, R. K. Singha, A. Dhar, and S. K. Ray

Appl. Phys. Lett. 91, 233118 (2007); http://dx.doi.org/10.1063/1.2821114 (3 pages) | Cited 19 times

Online Publication Date: 7 December 2007

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Metal-oxide-semiconductor capacitors with a trilayer structure consisting of Ge+HfO2 layers sandwiched between HfO2 tunnel and cap oxides were fabricated on p-Si substrates. Ge nanocrystals embedded in SiO2 were also studied for comparison. Cross-sectional transmission electron micrographs revealed the formation of spherical shaped Ge nanocrystals. The optical and charge storage characteristics of trilayer structures were studied through photoluminescence spectroscopy and capacitance-voltage measurements, respectively. An enhancement of the charge injection capability into nanocrystals was observed for the device with HfO2 as tunnel and cap oxide. The optical emission characteristics support the carrier confinement in Ge nanocrystals embedded in oxide matrices.
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84.32.Tt Capacitors
81.16.-c Methods of micro- and nanofabrication and processing
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)

The effect of contact length on adhesion between carbon nanotubes on silicon dioxide

Kaylee McElroy, Robert C. Davis, and Aaron Hawkins

Appl. Phys. Lett. 91, 233119 (2007); http://dx.doi.org/10.1063/1.2821228 (3 pages) | Cited 5 times

Online Publication Date: 7 December 2007

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The force of adhesion was measured for single walled carbon nanotubes grown over lithographically defined silicon dioxide trenches. We varied contact lengths between the nanotubes and silicon dioxide from 230 to 850 nm. Suspended nanotubes were pushed vertically into the trenches with an atomic force microscope tip, causing them to slip along the surface. Previous work done at shorter contact lengths found that tension was constant with contact length [ J. D. Whittaker et al., Nano Lett. 6, 953 (2006) ]. This study finds that when the nanotube contact length approaches 1 μm, the tension at which nanotubes slip begins to increase with contact length. This indicates that contact length independent adhesion is a uniquely nanoscale behavior.
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68.35.Np Adhesion
68.37.Ps Atomic force microscopy (AFM)
81.16.Ta Atom manipulation
61.46.Fg Nanotubes

Effect of anodic TiO2 powder as additive on electron transport properties in nanocrystalline TiO2 dye-sensitized solar cells

Chin-Jung Lin, Wen-Yueh Yu, and Shu-Hua Chien

Appl. Phys. Lett. 91, 233120 (2007); http://dx.doi.org/10.1063/1.2823604 (3 pages) | Cited 17 times

Online Publication Date: 7 December 2007

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Highly crystalline anodic TiO2 (AO-TiO2) powder was obtained by grinding of annealed TiO2 nanotube arrays grown using anodization of Ti foil. The influence of AO-TiO2 as additive in nanocrystalline TiO2 film on electron transport properties affecting the performance of these dye-sensitized solar cell was investigated using electrochemical impedance spectroscopy and the open-circuit voltage decay technique under AM 1.5 illumination. As a result of the enhanced charge-collection and light harvesting efficiencies, an increase of above 20% in photocurrent density was observed after the addition of AO-TiO2.
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84.60.Jt Photoelectric conversion
82.45.Rr Electroanalytical chemistry
82.45.Cc Anodic films
82.45.Yz Nanostructured materials in electrochemistry
82.80.Fk Electrochemical methods
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