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18 Jul 2005

Volume 87, Issue 3, Articles (03xxxx)

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Appl. Phys. Lett. 87, 033501 (2005); http://dx.doi.org/10.1063/1.1992665 (3 pages)

Yoshihito Miyoshi, Fumito Nakajima, Junichi Motohisa, and Takashi Fukui
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Near-field measurement of spectral anisotropy and optical absorption of isolated ZnO nanorod single-quantum-well structures

Takashi Yatsui, Motoichi Ohtsu, Jinkyoung Yoo, Sung Jin An, and Gyu-Chul Yi

Appl. Phys. Lett. 87, 033101 (2005); http://dx.doi.org/10.1063/1.1990247 (3 pages) | Cited 10 times

Online Publication Date: 11 July 2005

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We report low-temperature near-field spectroscopy of isolated ZnO/ZnMgO single-quantum-well structures (SQWs) on the end of ZnO nanorod to define their potential for nanophotonics. First, absorption spectra of isolated ZnO/ZnMgO nanorod SQWs with the Stokes shift as small as 3 meV and very sharp photoluminescent peaks indicate that the nanorod SQWs are of very high optical quality. Furthermore, we performed polarization spectroscopy of isolated ZnO SQWs, and observed valence-band anisotropy of ZnO SQWs in photoluminescence spectra directly. Since the exciton in a quantum structure is an ideal two-level system with long coherence times, our results provide criteria for designing nanophotonic devices.
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78.67.De Quantum wells
78.55.Et II-VI semiconductors
78.40.Fy Semiconductors
73.21.Fg Quantum wells

Study of the current stressing in nanomanipulated three-dimensional carbon nanotube structures

D. C. Cox, R. D. Forrest, P. R. Smith, V. Stolojan, and S. R. P. Silva

Appl. Phys. Lett. 87, 033102 (2005); http://dx.doi.org/10.1063/1.1990268 (3 pages) | Cited 5 times

Online Publication Date: 11 July 2005

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We report the fabrication of free-standing carbon nanotube structures. The welding of individual carbon nanotubes to other nanotubes and metal substrates has been performed, on a selective basis, to produce joints of both good electrical conductivity and mechanical integrity, without the need for a joining material. As a result of this unique process, we study the damage to the microstructure of the nanotube as a function of current. When the current densities are in excess of 2×106 A/cm2, particular care must be taken with regard to the quality of the nanotube and the heat dissipation. This is crucial for the use and application of nanotubes in any future device structure for it gives the upper limits to the “average” current density calculations. This process now allows for the fabrication of bespoke carbon nanotube devices for the prototyping of device performance.
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81.07.De Nanotubes
73.63.Fg Nanotubes
81.16.-c Methods of micro- and nanofabrication and processing
61.46.-w Structure of nanoscale materials

Patterned growth of carbon nanotubes on Si substrates without predeposition of metal catalysts

Y. Chen and J. Yu

Appl. Phys. Lett. 87, 033103 (2005); http://dx.doi.org/10.1063/1.1995961 (3 pages) | Cited 6 times

Online Publication Date: 11 July 2005

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Aligned carbon nanotubes (CNTs) can be readily synthesized on quartz or silicon-oxide-coated Si substrates using a chemical vapor deposition method, but it is difficult to grow them on pure Si substrates without predeposition of metal catalysts. We report that aligned CNTs were grown by pyrolysis of iron phthalocyanine at 1000 °C on the templates created on Si substrates with simple mechanical scratching. Scanning electron microscopy and x-ray energy spectroscopy analysis revealed that the trenches and patterns created on the surface of Si substrates were preferred nucleation sites for nanotube growth due to a high surface energy, metastable surface structure, and possible capillarity effect. A two-step pyrolysis process maintained Fe as an active catalyst.
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81.07.De Nanotubes
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.16.Hc Catalytic methods
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
68.35.Md Surface thermodynamics, surface energies

Tuning of electronic coupling between self-assembled quantum dots

S. I. Rybchenko, I. E. Itskevich, M. S. Skolnick, J. Cahill, A. I. Tartakovskii, G. Hill, and M. Hopkinson

Appl. Phys. Lett. 87, 033104 (2005); http://dx.doi.org/10.1063/1.1995953 (3 pages) | Cited 1 time

Online Publication Date: 11 July 2005

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Semiconductor self-assembled quantum dots (SAQDs) normally have zero-dimensional properties, but become coupled and acquire higher-dimensional character if the distance between the dots is small. Using photoluminescence spectroscopy under high hydrostatic pressure, we have obtained clear evidence for electronic coupling due to quantum-mechanical tunneling in stacks of InGaAs/GaAs SAQDs. We demonstrate that application of pressure allows controllable tuning and suppression of the electronic coupling. The effect originates from a pressure-induced increase in the effective mass of Γ-electrons and a related increase in the interdot-barrier height.
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81.07.Ta Quantum dots
81.05.Ea III-V semiconductors
73.21.La Quantum dots
73.63.Kv Quantum dots
73.40.Gk Tunneling
78.67.Hc Quantum dots
78.55.Cr III-V semiconductors
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor

Nanoscopic friction as a probe of local phase transitions

Robert Szoszkiewicz and Elisa Riedo

Appl. Phys. Lett. 87, 033105 (2005); http://dx.doi.org/10.1063/1.1995954 (3 pages) | Cited 9 times

Online Publication Date: 11 July 2005

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We study nanoscopic friction forces between an atomic force microscope tip and a glass sample. We show how and why it is possible to tune friction forces in a predictable way by changing either the sample temperature, or the humidity in the experimental chamber. We relate the friction behavior to confined water phase transitions. We find that the water gas-liquid phase diagram is the same at the macroscopic scale as at the nanoscopic tip-sample contact.
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68.35.Af Atomic scale friction
62.20.Qp Friction, tribology, and hardness
64.70.F- Liquid-vapor transitions

Reversible blue light emission from self-assembled silica nanocords

Yufeng Hao, Guowen Meng, Changhui Ye, and Lide Zhang

Appl. Phys. Lett. 87, 033106 (2005); http://dx.doi.org/10.1063/1.1996846 (3 pages) | Cited 13 times

Online Publication Date: 12 July 2005

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Self-assembled silica nanocords were synthesized through a vapor-solid process without the assistance of catalyst. The rope-like nanostructures were formed by splitting and connecting of lots of amorphous silica nanowires. Appropriate water vapor partial pressure and reaction temperature play critical roles in determining the morphologies. Blue light emission of these nanocords can dramatically be strengthened and weakened in the process of alternative dry and humid treatments. We propose that hydroxyl groups and adsorbed–desorbed ambient water on the nanocords surface are responsible for the reversible blue light emission behavior through analyzing photoluminescence and infrared spectra. Silica nanocords with such blue light emission may be used as effective optoelectronic devices and optical signal humid sensors.
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81.07.Bc Nanocrystalline materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Hx Other solid inorganic materials
78.30.Hv Other nonmetallic inorganics
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
81.16.Dn Self-assembly
61.46.-w Structure of nanoscale materials

Strong polarization-dependent photoluminescence from silicon nanowire fibers

D. D. D. Ma, S. T. Lee, and J. Shinar

Appl. Phys. Lett. 87, 033107 (2005); http://dx.doi.org/10.1063/1.1996838 (3 pages) | Cited 19 times

Online Publication Date: 13 July 2005

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Fibers of highly oriented Si nanowires (SiNWs) were formed by drawing from a condensed SiNW suspension. The SiNW fiber, excited at 514.5 nm, produces a strong photoluminescence (PL) at room temperature. The PL spectrum shows three bands at 565–580, 605–640, and 680–690 nm, respectively, which are consistent with the PL of porous silicon. The relative intensity of these bands and the integrated intensity of the PL vary with the angle θ between the electric field of the polarized laser excitation and the fiber axis. The dependence on θ is attributed to the combined effects of the one-dimensional shape of the SiNW and the large dielectric contrast between the SiNW and the ambient.
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78.55.Ap Elemental semiconductors
78.67.Lt Quantum wires
81.05.Cy Elemental semiconductors
81.07.Vb Quantum wires
68.65.La Quantum wires (patterned in quantum wells)
77.22.Ch Permittivity (dielectric function)

ZnxCd1−xSe alloy nanowires covering the entire compositional range grown by metalorganic chemical vapor deposition

C. X. Shan, Z. Liu, C. M. Ng, and S. K. Hark

Appl. Phys. Lett. 87, 033108 (2005); http://dx.doi.org/10.1063/1.1997271 (3 pages) | Cited 10 times

Online Publication Date: 13 July 2005

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We show that preferentially oriented, single-crystalline ZnxCd1−xSe alloy nanowires can be grown on GaAs (100) surface using Au as a catalyst over the entire compositional range in a metalorganic chemical vapor deposition system. The composition of the alloy nanowires can be simply adjusted through the ratio of the flow rates of group-II precursors. Electron microscopy shows that the nanowires are smooth and uniform in shape; their diameters range from 20 to 80 nm and lengths exceed a few micrometers. Nanowires containing more than 13% Zn are zinc blende structured and grow along the ⟨110⟩ direction. Those containing less Zn are wurtzite structured and grow along the ⟨210⟩ direction. Compared with the bulk alloy, the change from zinc blende to wurtzite structure in nanowires occurs at far smaller x. The preferred orientation and the persistence of the zinc blende structure both reflect the influence of the substrate on the growth of the nanowires. Photoluminescence measurements identify a strong near-band-edge emission for all samples and show that its peak energy tracks the band gap of ZnxCd1−xSe epilayer for x>0.13. The growth of alloy nanowires at many compositions opens up the possibility of realizing quasi-one-dimensional heterojunctions.
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68.65.La Quantum wires (patterned in quantum wells)
78.67.Lt Quantum wires
78.55.Et II-VI semiconductors
81.05.Dz II-VI semiconductors
61.46.-w Structure of nanoscale materials
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
81.07.Vb Quantum wires

Thermal processes in metal-coated fiber probes for near-field experiments

A. Ambrosio, M. Allegrini, G. Latini, and F. Cacialli

Appl. Phys. Lett. 87, 033109 (2005); http://dx.doi.org/10.1063/1.1999019 (3 pages) | Cited 5 times

Online Publication Date: 13 July 2005

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We have used a ray optics model to calculate the optical power absorbed in the metal coating of apertured probes for scanning near-field optical microscopy. We have then introduced the absorbed power profile into the heat balance equation to calculate the temperature of the probe as a function of the distance from the apex. By comparing our results with available experimental data, we demonstrate accurate prediction of both the temperature profile along the probe, and the temperature increase per mW of power launched into the fiber (60.7 versus 60 K/mW at 25 μm from the apex).
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07.79.Fc Near-field scanning optical microscopes

Nucleation of pentacene on silicon dioxide at hyperthermal energies

Aravind S. Killampalli, Todd W. Schroeder, and James R. Engstrom

Appl. Phys. Lett. 87, 033110 (2005); http://dx.doi.org/10.1063/1.1990254 (3 pages) | Cited 12 times

Online Publication Date: 13 July 2005

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The nucleation of pentacene on silicon dioxide, incident at hyperthermal energies, has been investigated with atomic force microscopy. The incident kinetic energy of the pentacene molecules strongly influences the process of adsorption—the adsorption probability decreases with increasing incident energy, indicative of trapping-mediated adsorption. In addition, the trapping probability of pentacene decreases with more glancing angles of incidence, a result inconsistent with so-called normal energy scaling. Analysis of the dependence of the island density on the growth rate in the submonolayer regime indicates that growth at all energies is consistent with a critical cluster containing four molecules.
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81.05.Hd Other semiconductors
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
68.43.Mn Adsorption kinetics
68.37.Ps Atomic force microscopy (AFM)

Enhanced carrier confinement in quantum dots by raising wetting layer state energy

Sebastian Moehl, Laurent Maingault, Kuntheak Kheng, and Henri Mariette

Appl. Phys. Lett. 87, 033111 (2005); http://dx.doi.org/10.1063/1.2000335 (3 pages) | Cited 3 times

Online Publication Date: 13 July 2005

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A quantum dot design is proposed where the wetting layer states are shifted to higher energies. It is realized by including CdTe quantum dots between two thin MgTe layers. As both materials have nearly the same lattice parameter, the first MgTe layer forms a wetting layer with high carrier state energy. Consequently, the radiative regime of the dots is significantly extended to higher temperatures. The unusual temperature-dependence of the decay time is discussed using a model for localized and delocalized states.
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73.21.La Quantum dots
73.20.Jc Delocalization processes

Memory effect of Al-rich AlN films synthesized with rf magnetron sputtering

Y. Liu, T. P. Chen, P. Zhao, S. Zhang, S. Fung, and Y. Q. Fu

Appl. Phys. Lett. 87, 033112 (2005); http://dx.doi.org/10.1063/1.2000337 (3 pages) | Cited 11 times

Online Publication Date: 13 July 2005

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Al-rich AlN thin film, which is deposited onto n-type Si substrate by radio frequency sputtering of Al target in an argon and N2 gas mixture, can exhibit a large memory effect as a result of charge trapping in the Al nanoparticles/nanoclusters embedded in the AlN matrix. For the metal-insulator-semiconductor structure with a 60 nm Al-rich AlN thin film, a voltage of −15 V applied to the metal electrode for 10−6 s causes a flatband voltage shift of ∼ 1.5 V. Both electron trapping and hole trapping are possible, depending on the polarity of the applied voltage. In addition, whether the electron trapping or the hole trapping is the dominant process also depends on the charging time and the magnitude of the voltage. The Al-rich AlN thin films provide the possibility of memory applications with low cost.
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81.05.Ea III-V semiconductors
73.61.Ey III-V semiconductors
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
68.55.A- Nucleation and growth
81.15.Cd Deposition by sputtering

Highly ordered self-organized dot patterns on Si surfaces by low-energy ion-beam erosion

B. Ziberi, F. Frost, B. Rauschenbach, and Th. Höche

Appl. Phys. Lett. 87, 033113 (2005); http://dx.doi.org/10.1063/1.2000342 (3 pages) | Cited 10 times

Online Publication Date: 13 July 2005

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Scanning force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM) have been used to investigate the complex topography evolution of Si surfaces during low-energy ion beam erosion. Depending on ion-beam parameters, a variety of different topographies can develop on the surface. At oblique ion-incidence angles, nanodots are formed for ion energies ≥ 300 eV upon sample rotation. Properly chosen parameters of the broad-beam ion source result in dots possessing a very high degree of lateral ordering with a mean dot size λ ∼ 30 nm. Both, degree of ordering and size homogeneity of these nanostructures increases with erosion time leading to the most ordered self-organized patterns on Si surfaces reported thus far.
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81.07.Ta Quantum dots
81.16.Dn Self-assembly
81.65.Cf Surface cleaning, etching, patterning
61.82.Fk Semiconductors

Anomalous electrorheological behavior of ZnO nanowires

P. Feng, Q. Wan, X. Q. Fu, T. H. Wang, and Y. Tian

Appl. Phys. Lett. 87, 033114 (2005); http://dx.doi.org/10.1063/1.1999848 (3 pages) | Cited 10 times

Online Publication Date: 14 July 2005

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We observe an anomalous electrorheological (ER) behavior of suspensions composed of ZnO nanowires and silicone oil. In contrast to the usual ER behavior, a decrease in viscosity of the suspensions is observed. Such an anomalous behavior results from the migration of ZnO nanowires to the electrodes under a dc electric field. The migration leads to a relatively pure silicone oil zone between the electrodes, as confirmed by optical microscope observations. The occurrence of the electrophoresis is proposed as the origin of the decrease in shear stress.
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83.80.Gv Electro- and magnetorheological fluids
82.70.Kj Emulsions and suspensions
83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
82.45.-h Electrochemistry and electrophoresis

Self-alignment of Fe nanoparticles on a tunnel barrier

F. Ernult, S. Mitani, K. Takanashi, Y. K. Takahashi, K. Hono, Y. Takahashi, and E. Matsubara

Appl. Phys. Lett. 87, 033115 (2005); http://dx.doi.org/10.1063/1.2000339 (3 pages) | Cited 5 times

Online Publication Date: 14 July 2005

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Nanometric metallic particles were prepared on top of a thin epitaxial oxide layer. Samples with the following structure: Fe electrode/MgO/Fe particles were fabricated and the arrangement of the Fe particles could be tuned from random to self-aligned by simply varying the thickness of the Fe electrode. Under appropriate deposition conditions, the particles were found to be self-aligned along the ⟨110⟩ directions of the underlying Fe electrode. Scanning tunneling microscope (STM) showed that their mean diameter and size distribution were then significantly reduced compared to randomly organized particles. Transmission electron microscope (TEM) images indicated that the self-alignment process originates from the strain relaxation of the Fe electrode which favors faceting of its surface and the formation of pyramidal structures. These self-aligned particles may be straightly used for applications based on a thin oxide tunnel barrier such as single-electron tunneling devices.
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81.07.Bc Nanocrystalline materials
81.16.Dn Self-assembly
68.35.B- Structure of clean surfaces (and surface reconstruction)
61.46.-w Structure of nanoscale materials
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Lp Transmission electron microscopy (TEM)
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