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19 Oct 2009

Volume 95, Issue 16, Articles (16xxxx)

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Appl. Phys. Lett. 95, 162501 (2009); http://dx.doi.org/10.1063/1.3248257 (3 pages)

W. W. Lei, D. Liu, P. W. Zhu, X. H. Chen, Q. Zhao, G. H. Wen, Q. L. Cui, and G. T. Zou
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Influence of bin time and excitation intensity on fluorescence lifetime distribution and blinking statistics of single quantum dots

Der-Hau Lee, Chi-Tsu Yuan, M. Tachiya, and Jau Tang

Appl. Phys. Lett. 95, 163101 (2009); http://dx.doi.org/10.1063/1.3236772 (3 pages) | Cited 3 times

Online Publication Date: 19 October 2009

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In this study of single CdSe/ZnS quantum dots in a polymethyl methacrylate (PMMA) matrix, we report narrowing of fluorescence lifetime distribution with increased bin time or excitation intensity mostly due to statistical variance of photon counts per bin and Auger relaxation. We also observed saturation for the excitation intensity dependence of the exponential bending rate for the blinking statistics for quantum dots in PMMA but not on glass.
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78.67.Hc Quantum dots
78.55.Et II-VI semiconductors
68.65.Hb Quantum dots (patterned in quantum wells)
81.07.Ta Quantum dots

Suspended metal wire array as a thermoacoustic sound source

A. O. Niskanen, J. Hassel, M. Tikander, P. Maijala, L. Grönberg, and P. Helistö

Appl. Phys. Lett. 95, 163102 (2009); http://dx.doi.org/10.1063/1.3249770 (3 pages) | Cited 10 times

Online Publication Date: 19 October 2009

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We demonstrate that a suspended metal wire array can be used to produce high-pressure sound waves over a wide spectrum using the thermoacoustic effect. We fabricated air-bridge arrays containing up to 2×105 wires covering an area of a few square centimeters. The supporting silicon wafer was isotropically plasma etched to release the wires thereby avoiding heat contact with the substrate. Sound pressure levels reaching 110 dB at a distance of 8 cm were demonstrated near 40 kHz in free field. The devices are also able to reproduce music and speech. They have potential for applications especially in the ultrasound range.
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07.64.+z Acoustic instruments and equipment
43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect

Thermal conductivity of graphene nanoribbons

Zhixin Guo, Dier Zhang, and Xin-Gao Gong

Appl. Phys. Lett. 95, 163103 (2009); http://dx.doi.org/10.1063/1.3246155 (3 pages) | Cited 57 times

Online Publication Date: 20 October 2009

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We have investigated the thermal conductivity of graphene nanoribbons (GNRs) with different edge shapes as a function of length, width, and strain using nonequilibrium molecular dynamics method. The initial GNR for the functional variations has dimensions of 2×11 nm2. Strong length dependence of thermal conductivity is obtained, indicating high thermal conductivities of GNRs, which is consistent with the experimental results for graphene. A tensile/compressive uniaxial strain can remarkably decrease the thermal conductivity of GNR.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep

Stretching induced Hall current and conductance anisotropy in graphene

A. R. Wright and C. Zhang

Appl. Phys. Lett. 95, 163104 (2009); http://dx.doi.org/10.1063/1.3251074 (3 pages) | Cited 3 times

Online Publication Date: 20 October 2009

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We evaluate the effect of stretching on the optical conductance of graphene. It is found that the low energy (Dirac regime) isotropy that leads to the “universal conductance” is lost. More significantly, due to the loss of C3 symmetry, a nonzero Hall conductance emerges for stretching along chiral directions, reaching a maximum at a stretching angle of 45°, and being as high as σ0 = e2/4ℏ at van Hove singular point for bond angle changes of about 2°. Our results indicate that the optical properties of graphene can be tuned by a weak mechanical deformation.
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78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
73.63.-b Electronic transport in nanoscale materials and structures
73.20.At Surface states, band structure, electron density of states
81.05.U- Carbon/carbon-based materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.40.Lm Deformation, plasticity, and creep
72.20.My Galvanomagnetic and other magnetotransport effects

Thermal boundary resistance at the graphene-oil interface

Deepthi Konatham and Alberto Striolo

Appl. Phys. Lett. 95, 163105 (2009); http://dx.doi.org/10.1063/1.3251794 (3 pages) | Cited 19 times

Online Publication Date: 20 October 2009

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In this work, using molecular dynamics simulations, we demonstrate that it is possible to significantly reduce the Kapitza resistance [ P. L. Kapitza, J. Phys. (USSR) 4, 181 (1941) ] at the graphene sheet-liquid octane interface by appropriately functionalizing the graphene sheets. The key concept is that the functional groups, to be effective, must show vibrational modes compatible with those of the organic matrix. Because functionalizing graphene sheets at their edges should not compromise their exceptional intrinsic thermal-transport properties, our results suggest a practical recipe for manufacturing high-thermal-transport polymeric nanocomposites.
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68.35.Ja Surface and interface dynamics and vibrations

Number sensitive detection and direct imaging of dipolar coupled magnetic nanoparticles by tunnel magnetoresistive sensors

C. Albon, A. Weddemann, A. Auge, D. Meißner, K. Rott, P. Jutzi, and A. Hütten

Appl. Phys. Lett. 95, 163106 (2009); http://dx.doi.org/10.1063/1.3253410 (3 pages) | Cited 5 times

Online Publication Date: 20 October 2009

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The suitability of magnetic tunnel junctions for the detection of magnetic nanoparticles is related to their scalability onto the nanoscale size regime without a significant loss of sensitivity. Elliptically shaped MgO based tunnel magnetoresistance sensors are used to provide a sharp detection of 14 nm Co nanoparticles. The measured signal is related to the degree of coverage of the sensor area by a nanoparticle layer. Moreover, the nanoparticles magnetostatic interaction on the sensor surface is clearly distinguished by the presence of a coercitive field in the detected signal. Experimentally obtained results are compared to theoretical models.
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85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.55.-w Magnetic instruments and components
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Tt Fine-particle systems; nanocrystalline materials
75.47.-m Magnetotransport phenomena; materials for magnetotransport

Controllable local modification of fractured Nb-doped SrTiO3 surfaces

TeYu Chien, Tiffany S. Santos, Matthias Bode, Nathan P. Guisinger, and John W. Freeland

Appl. Phys. Lett. 95, 163107 (2009); http://dx.doi.org/10.1063/1.3254184 (3 pages) | Cited 9 times

Online Publication Date: 21 October 2009

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Nanoscale surface modification of a fractured Nb-doped SrTiO3 surface is demonstrated in a controlled way by scanning tunneling microscopy. By applying positive voltage pulses, holes can be created and the width and depth of the hole can be controlled by selecting the appropriate bias and pulse duration. The process shows a threshold condition for creation of the holes and change in the local electronic density of state consistent with exposure of the underlying TiO2 layer by removal of SrO. By applying negative bias, the hole can be partially refilled from the transfer of adsorbates on the tip.
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68.35.Gy Mechanical properties; surface strains
62.20.mm Fracture
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
73.20.At Surface states, band structure, electron density of states
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)

Dual nonlinear dielectric resonance and nesting microwave absorption peaks of hollow cobalt nanochains composites with negative permeability

Xiao-Ling Shi, Mao-Sheng Cao, Jie Yuan, and Xiao-Yong Fang

Appl. Phys. Lett. 95, 163108 (2009); http://dx.doi.org/10.1063/1.3250170 (3 pages) | Cited 33 times

Online Publication Date: 21 October 2009

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The permittivity and permeability behaviors of the hollow cobalt nanochains composites have been investigated in 2–18 GHz. The permittivity presents two dielectric resonance peaks at about 12.3 and 14.5 GHz, respectively, which mainly results from the cooperative consequence of the hollow structure and the one-dimensional structure of the as-synthesized Co nanochains. The negative permeability behavior within 12.3–18 GHz is attributed to radiation of the magnetic energy according to the as-established equivalent circuit model. Two strong absorption peaks of the composites nest at the resonance frequencies due to the effect of the dual nonlinear dielectric resonance and the negative permeability behavior.
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77.22.Ch Permittivity (dielectric function)
81.16.-c Methods of micro- and nanofabrication and processing
61.46.-w Structure of nanoscale materials

Electrode conformation-induced negative differential resistance and rectifying performance in a molecular device

X. Q. Deng, J. C. Zhou, Z. H. Zhang, H. Zhang, M. Qiu, and G. P. Tang

Appl. Phys. Lett. 95, 163109 (2009); http://dx.doi.org/10.1063/1.3246163 (3 pages) | Cited 12 times

Online Publication Date: 21 October 2009

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Carrying out theoretical calculations using the nonequilibrium Green’s function method combined with the density functional theory, the transport properties of a carbon wire connected to two Au electrodes are investigated. The results show that the negative differential resistance and rectifying performance can be observed apparently when a pure carbon chain is connected to two asymmetric Au electrodes. The main origin of the negative differential resistance behavior is a suppression of the highest occupied molecular orbital resonance at certain bias voltage. Also shown is that it is possible to make the negative differential resistance disappear and rectifying performance be weakened only by adding side groups to a wire.
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71.15.Mb Density functional theory, local density approximation, gradient and other corrections
02.30.-f Function theory, analysis

Manipulation of a one dimensional molecular assembly of helical superstructures by dielectrophoresis

Wakana Kubo and Shigenori Fujikawa

Appl. Phys. Lett. 95, 163110 (2009); http://dx.doi.org/10.1063/1.3253708 (3 pages) | Cited 1 time

Online Publication Date: 22 October 2009

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We demonstrate that it is possible to manipulate helical superstructures composed of self-assembled chiral lipids by dielectrophoresis while preserving the shape of the superstructures. Alignment and migration of the helical fibers were only observed for an applied ac field of 1 kHz–1 MHz. The structural and physical properties of the helical fibers were preserved even under ac electric fields. This is the first report on the manipulation of a helical molecular assembly wherein the original properties of the assembly remained unchanged even after manipulation.
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81.16.Dn Self-assembly
64.75.Yz Self-assembly
82.45.-h Electrochemistry and electrophoresis

On the hydrogen storage capacity of carbon nanotube bundles

Andre R. Muniz, M. Meyyappan, and Dimitrios Maroudas

Appl. Phys. Lett. 95, 163111 (2009); http://dx.doi.org/10.1063/1.3253711 (3 pages) | Cited 6 times

Online Publication Date: 22 October 2009

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An analytical model is presented to describe the effect of carbon nanotube (CNT) swelling upon hydrogenation on the hydrogen storage capacity of single-walled CNT bundles; the model is properly parameterized using atomistic calculations for the relationship between CNT swelling and the degree of hydrogenation as measured by the coverage of the CNTs by chemisorbed atomic H. The model generates experimentally testable hypotheses, which can be used to explain the lower H storage capacities reported for CNT bundles and the experimentally observed nonuniformity of hydrogenation of CNT bundles.
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68.43.Mn Adsorption kinetics
84.60.Ve Energy storage systems, including capacitor banks
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Large excess volume in grain boundaries of stressed, nanocrystalline metallic thin films: Its effect on grain-growth kinetics

Y. Kuru, M. Wohlschlögel, U. Welzel, and E. J. Mittemeijer

Appl. Phys. Lett. 95, 163112 (2009); http://dx.doi.org/10.1063/1.3248070 (3 pages) | Cited 7 times

Online Publication Date: 22 October 2009

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The excess volumes per unit grain-boundary area of nanocrystalline Pd and Ni thin films were measured by an efficacious method based on real time in situ x-ray diffraction measurements. The obtained large values for the grain-boundary excess volume reveal the background of surprising, yet unexplained observations of grain growth in nanocrystalline materials.
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61.72.Mm Grain and twin boundaries
68.55.jd Thickness

Gigahertz breathing oscillators based on carbon nanoscrolls

Xinghua Shi, Nicola M. Pugno, Yuan Cheng, and Huajian Gao

Appl. Phys. Lett. 95, 163113 (2009); http://dx.doi.org/10.1063/1.3253423 (3 pages) | Cited 12 times

Online Publication Date: 22 October 2009

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Theoretical study and molecular dynamics simulations are performed to investigate the gigahertz “breathing” oscillatory motion of a carbon nanoscroll (CNS). It is shown that the oscillation frequency depends on surface energy, bending stiffness, interlayer spacing, and length of the basal graphene sheet of the CNS, and that energy dissipation in the system can be controlled by adjusting temperature, graphene length, and surface energy. The analysis indicates potential applications of CNS in nanomechanical devices such as nanooscillators, nanoactuators, as well as drug and gene delivery systems.
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87.85.Qr Nanotechnologies-design
81.05.ub Fullerenes and related materials
81.05.U- Carbon/carbon-based materials
61.46.-w Structure of nanoscale materials
68.35.Md Surface thermodynamics, surface energies
84.30.Ng Oscillators, pulse generators, and function generators

Influence of alloy buffer and capping layers on InAs/GaAs quantum dot formation

V. D. Dasika, J. D. Song, W. J. Choi, N. K. Cho, J. I. Lee, and R. S. Goldman

Appl. Phys. Lett. 95, 163114 (2009); http://dx.doi.org/10.1063/1.3243688 (3 pages) | Cited 9 times

Online Publication Date: 22 October 2009

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We have investigated the influence of alloy buffer and capping layers on the shape, size, and density of self-assembled InAs/GaAs quantum dots. Cross-sectional scanning tunneling microscopy (XSTM) images reveal ellipse-shaped dots with highest (lowest) diameter, height, and density, for dots with (without) surrounding alloy layers. Furthermore, the wetting layer is thicker in the presence of the alloy layers. We propose a strain-based mechanism for dot formation and collapse in the absence and presence of alloy buffer and capping layers. This mechanism is likely to be applicable to a wide range of lattice-mismatched thin-film systems.
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68.65.Hb Quantum dots (patterned in quantum wells)
81.05.Ea III-V semiconductors
81.07.Ta Quantum dots

Enhanced three-photon absorption and nonlinear refraction in ZnS and Mn2+ doped ZnS quantum dots

M. Chattopadhyay, P. Kumbhakar, R. Sarkar, and A. K. Mitra

Appl. Phys. Lett. 95, 163115 (2009); http://dx.doi.org/10.1063/1.3254186 (3 pages) | Cited 12 times

Online Publication Date: 23 October 2009

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Enhanced three-photon absorption (3PA) and self-focusing nonlinear refraction are reported for the first time in ZnS and (1%–2.5%) Mn2+ doped ZnS quantum dots (QDs) of average size of 1.5 nm by using z-scan technique at 532 nm laser radiations. At this wavelength, the obtained maximum value of the 3PA coefficient both in the ZnS and in the doped ZnS QDs is ∼ 107 times that of bulk ZnS. Also intensity dependent saturation of 3PA has been observed and the characteristic saturation intensity is estimated to be 0.85±0.09 GW/cm2 for ZnS QDs.
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78.67.Hc Quantum dots
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
61.72.uj III-V and II-VI semiconductors
81.07.Ta Quantum dots

10 μm minority-carrier diffusion lengths in Si wires synthesized by Cu-catalyzed vapor-liquid-solid growth

Morgan C. Putnam, Daniel B. Turner-Evans, Michael D. Kelzenberg, Shannon W. Boettcher, Nathan S. Lewis, and Harry A. Atwater

Appl. Phys. Lett. 95, 163116 (2009); http://dx.doi.org/10.1063/1.3247969 (3 pages) | Cited 28 times

Online Publication Date: 23 October 2009

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The effective electron minority-carrier diffusion length, Ln,eff, for 2.0 μm diameter Si wires that were synthesized by Cu-catalyzed vapor-liquid-solid growth was measured by scanning photocurrent microscopy. In dark, ambient conditions, Ln,eff was limited by surface recombination to a value of ≤ 0.7 μm. However, a value of Ln,eff = 10.5±1 μm was measured under broad-area illumination in low-level injection. The relatively long minority-carrier diffusion length observed under illumination is consistent with an increased surface passivation resulting from filling of the surface states of the Si wires by photogenerated carriers. These relatively large Ln,eff values have important implications for the design of high-efficiency, radial-junction photovoltaic cells from arrays of Si wires synthesized by metal-catalyzed growth processes.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.20.At Surface states, band structure, electron density of states
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