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3 Nov 2003

Volume 83, Issue 18, pp. 3647-3835

Issue Cover Spotlight Figure

Appl. Phys. Lett. 83, 3737 (2003); http://dx.doi.org/10.1063/1.1623315 (3 pages)

Ian Appelbaum, D. J. Monsma, K. J. Russell, V. Narayanamurti, and C. M. Marcus
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Growth mechanism and characterization of zinc oxide hexagonal columns

Jason B. Baxter, Feng Wu, and Eray S. Aydil

Appl. Phys. Lett. 83, 3797 (2003); http://dx.doi.org/10.1063/1.1624467 (3 pages) | Cited 43 times

Online Publication Date: 28 October 2003

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We report on the growth mechanism, structure, and luminescence properties of ZnO hexagonal columns grown from Zn vapor and air plasma. Single-crystal ZnO columns grow in the [0001] direction through repeated nucleation and growth of epitaxial hexagonal pyramids on the c-planes. Homoepitaxial nucleation of three-dimensional ZnO pyramids is most likely due to the Ehrlich–Schwoebel effect. This mechanism produces columns that are a few hundred nanometers in diameter and up to 2 μm in length. Convergent beam electron diffraction shows that the columns grow with Zn polarity in the [0001] direction. Cathodoluminescence and photoluminescence measurements show near-bandedge emission (3.29 eV) with no emission associated with oxygen vacancies at 2.5 eV. © 2003 American Institute of Physics.
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81.07.Bc Nanocrystalline materials
61.46.-w Structure of nanoscale materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.55.Et II-VI semiconductors
68.55.A- Nucleation and growth
78.60.Hk Cathodoluminescence, ionoluminescence

Fabrication and electrical characterization of polyaniline-based nanofibers with diameter below 30 nm

Yangxin Zhou, Marcus Freitag, James Hone, Cristian Staii, A. T. Johnson, Nicholas J. Pinto, and A. G. MacDiarmid

Appl. Phys. Lett. 83, 3800 (2003); http://dx.doi.org/10.1063/1.1622108 (3 pages) | Cited 72 times

Online Publication Date: 28 October 2003

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We fabricate and electrically characterize electrospun nanofibers of doped polyaniline/polyethylene oxide (PAn/PEO) blend with sub-30 nm diameter. Fiber diameters near 5 nm are obtained for optimized process parameters. Scanning conductance microscopy (SCM) shows that fibers with diameter below 15 nm are electrically insulating; the small diameter may allow complete dedoping in air or be smaller than phase-separated grains of PAn and PEO. Electrical contacts to nanofibers are made by shadow mask evaporation with no chemical or thermal damage to the fibers. Single fiber I–V characteristics show that thin fibers conduct more poorly than thick ones, in agreement with SCM data. I–Vs of asymmetric fibers are rectifying, consistent with formation of Schottky barriers at the nanofiber-metal contacts. © 2003 American Institute of Physics.
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73.63.Rt Nanoscale contacts
81.07.Lk Nanocontacts
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
72.80.Le Polymers; organic compounds (including organic semiconductors)
72.15.Nj Collective modes (e.g., in one-dimensional conductors)
73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ei Rectification

Effect of the large current flow on the low-temperature transport properties in a bundle of single-walled carbon nanotubes

T. Fuse, S. Moriyama, M. Suzuki, Y. Aoyagi, and K. Ishibashi

Appl. Phys. Lett. 83, 3803 (2003); http://dx.doi.org/10.1063/1.1624641 (3 pages) | Cited 2 times

Online Publication Date: 28 October 2003

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A large current flowing process has been used at 1.5 K to modify transport properties of quantum dots in a bundle of single-walled carbon nanotubes. After the processes were applied to the samples a few times, Coulomb oscillations started to be observed that did not appear before the process. The number of Coulomb peaks decreased as the process was repeated. The experimental observations could be explained by the simple model that the nanotube bundle was composed of many single and multiquantum dots in parallel, and some of them were broken by the current flowing process. © 2003 American Institute of Physics.
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73.63.Fg Nanotubes
73.63.Kv Quantum dots
73.23.Hk Coulomb blockade; single-electron tunneling

Field emission from zinc oxide nanopins

C. X. Xu and X. W. Sun

Appl. Phys. Lett. 83, 3806 (2003); http://dx.doi.org/10.1063/1.1625774 (3 pages) | Cited 152 times

Online Publication Date: 28 October 2003

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Nanostructural zinc oxide pins have been fabricated by vapor transport on copper-coated silicon wafer. The nanopins are composed of hexagonal wurtzite-phase zinc oxide with single crystal quality. The growth process includes two steps: (1) growth of a micron-sized zinc oxide dot on the substrate and (2) growth of a sharp tip from the zinc oxide dot. The field emission of the nanopins shows a low field emission threshold (1.92 V/μm at a current density of 0.1 μA/cm−2) and high current density with a field enhancement factor of 657. The emission current density and the electric field follow Fowler–Nordheim relationship. The good performance for field emission is attributed to the single-crystalline structure and the nanopin geometry. © 2003 American Institute of Physics.
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79.70.+q Field emission, ionization, evaporation, and desorption
81.07.Bc Nanocrystalline materials
61.46.-w Structure of nanoscale materials
81.05.Dz II-VI semiconductors
81.10.Bk Growth from vapor
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Anode material based on Zr-doped ZnO thin films for organic light-emitting diodes

H. Kim, J. S. Horwitz, W. H. Kim, S. B. Qadri, and Z. H. Kafafi

Appl. Phys. Lett. 83, 3809 (2003); http://dx.doi.org/10.1063/1.1623933 (3 pages) | Cited 31 times

Online Publication Date: 28 October 2003

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Zr-doped ZnO (ZZO) thin films have been investigated as an anode material, a potential alternative to indium tin oxide (ITO), for organic light emitting diode (OLED) devices. ZZO films have been deposited on glass substrates by pulsed laser deposition. The electrical and optical properties of these films were studied as a function of substrate temperature and oxygen pressure during deposition. For a 200-nm-thick ZZO film grown at 250 °C in 1 mTorr of oxygen, a resistivity of 5.6×10−4 Ω cm was measured and an average optical transmittance of 84% was measured in the visible range (400–700 nm). The ZZO films, grown at different oxygen pressures, were used as an anode contact for OLED devices. External electroluminescence quantum efficiencies (0.8%–0.9%) comparable to those (0.9%–1.0%) measured for control devices fabricated on commercial ITO anodes were obtained at high current densities (1000 A/m2). These results demonstrate that ZZO is a good anode material. In addition, it is an attractive alternative to ITO due to its low price and lack of toxicity. © 2003 American Institute of Physics.
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68.55.-a Thin film structure and morphology
85.60.Jb Light-emitting devices
73.61.-r Electrical properties of specific thin films
73.50.-h Electronic transport phenomena in thin films
78.66.-w Optical properties of specific thin films
81.15.Fg Pulsed laser ablation deposition
78.60.Fi Electroluminescence
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Investigation of transport properties in polymer/fullerene blends using time-of-flight photocurrent measurements

S. A. Choulis, J. Nelson, Y. Kim, D. Poplavskyy, T. Kreouzis, J. R. Durrant, and D. D. C. Bradley

Appl. Phys. Lett. 83, 3812 (2003); http://dx.doi.org/10.1063/1.1624636 (3 pages) | Cited 69 times

Online Publication Date: 28 October 2003

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The electron and hole transport properties of blends of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene], (MDMO-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) are investigated at room temperature using time-of-flight photocurrent measurements as a function of blend composition and laser excitation intensity. The experimental results are consistent with the notion of a mobility edge for holes in this material that delineates long-lived trapping and mobile carrier states. Electron transport in PCBM appears to be less dispersive than that of holes in MDMO-PPV in the blend, with electron mobilities in the former exceeding hole mobilities in the latter by up to two orders of magnitude. Photocurrent generation in corresponding solar cell devices may therefore be limited by the low hole mobility of MDMO-PPV. © 2003 American Institute of Physics.
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72.40.+w Photoconduction and photovoltaic effects
72.80.Rj Fullerenes and related materials
72.80.Le Polymers; organic compounds (including organic semiconductors)

Shell-type micromechanical actuator and resonator

Maxim Zalalutdinov, Keith L. Aubin, Robert B. Reichenbach, Alan T. Zehnder, Brian Houston, Jeevak M. Parpia, and Harold G. Craighead

Appl. Phys. Lett. 83, 3815 (2003); http://dx.doi.org/10.1063/1.1622792 (3 pages) | Cited 14 times

Online Publication Date: 28 October 2003

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Dome-shaped radio-frequency micromechanical resonators were fabricated by utilizing the buckling of a prestressed thin polysilicon film. The enhanced rigidity of the dome structure leads to a significant increase of its resonant frequency compared to a flat plate resonator. The shell-type geometry of the structure also provides an imbedded actuation mechanism. Significant out-of plane deflections are actuated by mechanical stress introduced within the plane of the shell. We demonstrate that thermomechanical stress generated by a focused laser beam, or microfabricated resistive heater, provides an effective and fast mechanism to operate the dome as an acoustic resonator in the radio-frequency range. All-optical operation of the shell resonator and an integrated approach are discussed. © 2003 American Institute of Physics.
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07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
81.05.Cy Elemental semiconductors
43.58.Kr Spectrum and frequency analyzers and filters; acoustical and electrical oscillographs; photoacoustic spectrometers; acoustical delay lines and resonators
42.79.Jq Acousto-optical devices
43.38.Zp Acoustooptic and photoacoustic transducers

Efficient, deep-blue organic electrophosphorescence by guest charge trapping

R. J. Holmes, B. W. D’Andrade, S. R. Forrest, X. Ren, J. Li, and M. E. Thompson

Appl. Phys. Lett. 83, 3818 (2003); http://dx.doi.org/10.1063/1.1624639 (3 pages) | Cited 324 times

Online Publication Date: 28 October 2003

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We demonstrate efficient, deep-blue organic electrophosphorescence using a charge-trapping phosphorescent guest, iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) doped in the wide-energy-gap hosts, diphenyldi(o-tolyl)silane (UGH1) and p-bis(triphenylsilyly)benzene (UGH2), where exciton formation occurs directly on the guest molecules. Charge trapping on the guest is confirmed by the dependence of the drive voltage and electroluminescence spectrum on guest concentration. Ultraviolet photoemission spectroscopy measurements establish the relative highest occupied molecular orbital positions of FIr6 in UGH1 and UGH2. Peak quantum and power efficiencies of (8.8±0.9)% and (11.0±1.1) lm/W in UGH1 and (11.6±1.2)% and (13.9±1.4) lm/W in UGH2 are obtained, while the emission in both cases is from FIr6 and is characterized by Commission Internationale de l’Eclairage coordinates of (x = 0.16, y = 0.26) in UGH2. © 2003 American Institute of Physics.
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78.60.Fi Electroluminescence
79.60.Fr Polymers; organic compounds
71.35.Cc Intrinsic properties of excitons; optical absorption spectra

Short-channel effects in contact-passivated nanotube chemical sensors

Keith Bradley, Jean-Christophe P. Gabriel, Alexander Star, and George Grüner

Appl. Phys. Lett. 83, 3821 (2003); http://dx.doi.org/10.1063/1.1619222 (3 pages) | Cited 42 times

Online Publication Date: 28 October 2003

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We report a design for carbon nanotube field-effect transistors which tests the nanotube depletion length. In this design, the metal contacts and adjacent nanotubes were coated with impermeable silicon oxide, while the central region of nanotubes was exposed. We tested the devices by measuring sensitivity to NH3 and poly(ethylene imine). NH3 caused similar responses in passivated devices and in normal, nonpassivated devices. Thus, the device design passivates the metal-nanotube contacts while preserving chemical sensor characteristics. Poly(ethylene imine) produced negative threshold shifts of tens of volts, despite being in contact with only the center region of devices. Based on the observed device characteristics, we conclude that the length scale of the covered nanotubes in our structure is comparable to the decay length of the depletion charge in nanotube transistors. © 2003 American Institute of Physics.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
85.35.Kt Nanotube devices
85.30.Tv Field effect devices
81.65.Rv Passivation
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Towards dynamical force microscopy using optical probing of thermomechanical noise

P. Vairac, B. Cretin, and A. J. Kulik

Appl. Phys. Lett. 83, 3824 (2003); http://dx.doi.org/10.1063/1.1624634 (3 pages) | Cited 2 times

Online Publication Date: 28 October 2003

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Dynamic force microscopy is often used to investigate local tip–sample interaction in order to obtain the material elastic properties. In this letter, we demonstrate the feasibility of the detection of thermal noise of microcantilever in contact with hard samples. Optical sensing is based on a high sensitivity heterodyne interferometer. The detected power spectral density clearly shows the first flexural vibration mode of the microcantilever. The variation of the resonant frequency of this vibration mode is related to the local elastic properties of the sample. © 2003 American Institute of Physics.
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07.79.Lh Atomic force microscopes
07.60.Ly Interferometers
46.80.+j Measurement methods and techniques in continuum mechanics of solids
81.70.Bt Mechanical testing, impact tests, static and dynamic loads
81.70.Fy Nondestructive testing: optical methods

Low temperature Si layer transfer by direct bonding and mechanical ion cut

Y. Cho and N. W. Cheung

Appl. Phys. Lett. 83, 3827 (2003); http://dx.doi.org/10.1063/1.1623336 (3 pages) | Cited 3 times

Online Publication Date: 28 October 2003

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Edge initiated crack propagation was employed to transfer a thin Si layer from a hydrogen implanted donor wafer to Si and oxidized Si receptor wafers. Referred to as mechanical ion cut, successful Si layer transfer was achieved after directly bonding a Si donor wafer implanted with a hydrogen dose of 8×1016/cm2 at H+ energy of 28 keV with Si and SiO2 receptor wafers and annealing above 105 and 170 °C, respectively. The mechanical cut enables Si layer transfer without annealing at 400–600 °C as in thermal ion cut. Our data verify the condition for successful layer transfer to be that mechanical strength of the bonding interface is greater than that of the cutting interface. © 2003 American Institute of Physics.
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85.40.-e Microelectronics: LSI, VLSI, ULSI; integrated circuit fabrication technology
81.05.Cy Elemental semiconductors
61.72.Cc Kinetics of defect formation and annealing
68.35.Np Adhesion
62.20.M- Structural failure of materials
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.20.Wk Machining, milling
68.60.Bs Mechanical and acoustical properties
68.55.-a Thin film structure and morphology

Chemical imaging of interfaces by sum-frequency generation microscopy: Application to patterned self-assembled monolayers

K. Kuhnke, D. M. P. Hoffmann, X. C. Wu, A. M. Bittner, and K. Kern

Appl. Phys. Lett. 83, 3830 (2003); http://dx.doi.org/10.1063/1.1624465 (3 pages) | Cited 14 times

Online Publication Date: 28 October 2003

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We demonstrate molecule-specific imaging of a chemically patterned self-assembled monolayer by IR-visible sum-frequency microscopy. The pattern on an Au substrate consists of microcontact printed 10 μm wide alkanethiolate stripes embedded in ω-carboxyalkanethiolate adsorbed from solution. We use both electronic and vibrational contrast mechanisms for a quantitative analysis of thiolate density and the coverage of the two molecular species. The evaluation of images taken at three different IR wavelengths suggests a substantial intermixing of the two thiolates occuring in the preparation procedure. © 2003 American Institute of Physics.
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68.35.Dv Composition, segregation; defects and impurities
68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
82.80.Gk Analytical methods involving vibrational spectroscopy
68.35.Fx Diffusion; interface formation
66.30.Ny Chemical interdiffusion; diffusion barriers

Dielectrophoresis and electrohydrodynamics-mediated fluidic assembly of silicon resistors

S. W. Lee and R. Bashir

Appl. Phys. Lett. 83, 3833 (2003); http://dx.doi.org/10.1063/1.1624642 (3 pages) | Cited 14 times

Online Publication Date: 28 October 2003

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In this letter, we present techniques, utilizing dielectrophoresis and electrohydrodynamics, which can be used for assembling single-crystal silicon devices suspended in a solution onto a binding site on a heterogeneous substrate. Silicon resistors with gold/chromium layers located at the end of the resistors and silicon blocks without metal were fabricated on bonded and etched-backed silicon-on-insulator wafers and successfully released into deionized water. The devices were subsequently assembled on a different substrate at specific binding sites by dielectrophoretic and electrohydrodynamic forces with submicron precision. Current–voltage measurements of the assembled resistors exhibited low contact resistance after the solution was completely evaporated and the contacts were annealed. © 2003 American Institute of Physics.
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85.30.-z Semiconductor devices
84.32.Ff Conductors, resistors (including thermistors, varistors, and photoresistors)
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
47.85.Np Fluidics
47.65.-d Magnetohydrodynamics and electrohydrodynamics
73.40.Cg Contact resistance, contact potential
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