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15 Feb 2012

Volume 111, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 111, 043501 (2012); http://dx.doi.org/10.1063/1.3680881 (8 pages)

Gregory J. McGraw and Stephen R. Forrest
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back to top Nanoscale Science and Design

Faster radial strain relaxation in InAs–GaAs core–shell heterowires

Karen L. Kavanagh, Igor Saveliev, Marina Blumin, Greg Swadener, and Harry E. Ruda

J. Appl. Phys. 111, 044301 (2012); http://dx.doi.org/10.1063/1.3684964 (9 pages) | Cited 9 times

Online Publication Date: 17 February 2012

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The structure of wurtzite and zinc blende InAs–GaAs (001) core–shell nanowires grown by molecular beam epitaxy on GaAs (001) substrates has been investigated by transmission electron microscopy. Heterowires with InAs core radii exceeding 11 nm, strain relax through the generation of misfit dislocations, given a GaAs shell thickness greater than 2.5 nm. Strain relaxation is larger in radial directions than axial, particularly for shell thicknesses greater than 5.0 nm, consistent with molecular statics calculations that predict a large shear stress concentration at each interface corner.
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81.07.Gf Nanowires
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Fabrication of single-walled carbon nanohorns containing iodine and cesium

J. H. Cho, S. T. Lim, S. R. Huh, and G. H. Kim

J. Appl. Phys. 111, 044302 (2012); http://dx.doi.org/10.1063/1.3685751 (8 pages)

Online Publication Date: 17 February 2012

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Iodine and cesium atoms were encapsulated in single-walled carbon nanohorns (SWCNHs). Atom encapsulation was carried out with sequential plasma aided procedures which consisted of opening SWCNH tips with an oxygen plasma, atom insertion in an iodine-mixed or cesium-mixed argon plasma, and closing the open tip in an argon plasma. Results reveal that oxidation plays a role in the tip opening procedure, and capillary forces are the driving force for the permeation of the atoms through the open tip of the SWCNHs. The open tip of the atom inserted SWCNH can be closed under the ion irradiation. It demonstrated the fabrication process of encapsulating atoms in SWCNH by using the sequential plasma assisted processes.
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81.07.Bc Nanocrystalline materials
81.16.-c Methods of micro- and nanofabrication and processing
52.77.Dq Plasma-based ion implantation and deposition
82.30.Nr Association, addition, insertion, cluster formation
61.80.Jh Ion radiation effects

Stable vortex magnetite nanorings colloid: Micromagnetic simulation and experimental demonstration

Yong Yang, Xiao-Li Liu, Jia-bao Yi, Yang Yang, Hai-Ming Fan, and Jun Ding

J. Appl. Phys. 111, 044303 (2012); http://dx.doi.org/10.1063/1.3684963 (9 pages) | Cited 1 time

Online Publication Date: 21 February 2012

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Magnetite nanoring with vortex domain structure may form stable magnetic colloid for biomedical applications due to its weak magnetic interaction without superparamagnetic (SPM) limitation. In the present study, we perform three-dimensional (3 D) Landau-Liftshitz-Gilbert (LLG) micromagnetics simulation for magnetite nanorings. The ground state phase diagram and stable vortex area (SVA) as a function of outer diameter (Dout), thickness (T), and inner to outer diameter ratios (β) within 100 nm are obtained. The influence of notch, eccentricity, and crystallographic orientation are taken carefully into consideration. In the SVA, the vortex state is not only the ground state but also the remanence state after in-plane is fully magnetized. In particular, the results suggest that a 20 nm inter-rings distance for a typical magnetite nanoring (Dout = 70 nm, T = 50 nm, and β = 0.6) can achieve the stable colloid based on vortex domain structure. Furthermore, these simulation results have been confirmed experimentally and demonstrated by using phosphorylated-mPEG modified magnetite nanorings. The optimization of magnetite nanorings from both simulation and experiments in this work pave the way to achieve such novel and stable vortex domain based magnetic suspension for various biomedical applications.
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75.75.-c Magnetic properties of nanostructures
75.50.Dd Nonmetallic ferromagnetic materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.60.Ch Domain walls and domain structure
75.78.Cd Micromagnetic simulations

Phonon thermal conductivity in silicon nanowires: The effects of surface roughness at low temperatures

Jung Hyun Oh, Mincheol Shin, and Moon-Gyu Jang

J. Appl. Phys. 111, 044304 (2012); http://dx.doi.org/10.1063/1.3684973 (7 pages) | Cited 5 times

Online Publication Date: 21 February 2012

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Using a Green’s function method based on an elastic wave equation, the effects of surface roughness and the nanowire-contact interface scattering on phonon thermal conductivity are studied at low temperatures. It is found that the interface geometry between a nanowire and its contacts affects the transmission function at small energies related to the gapless modes and it gives rise to deviated results from the universal conductance. It is also shown that the surface roughness is crucial in the suppression of phonon thermal conductivity with reducing the nanowire size by averaging the transmission function over the rough-surface configurations. Furthermore, the phonon mean free path is proportional to the ratio of the correlation length and roughness heights quadratically as well as the cross-section area of the nanowire.
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63.22.Gh Nanotubes and nanowires
66.70.Df Metals, alloys, and semiconductors
68.47.Fg Semiconductor surfaces
62.30.+d Mechanical and elastic waves; vibrations
68.35.Ct Interface structure and roughness
62.25.Jk Mechanical modes of vibration

Magneto-optical spectrum of ZnO nanorods

Wen Xiong

J. Appl. Phys. 111, 044305 (2012); http://dx.doi.org/10.1063/1.3686139 (6 pages) | Cited 1 time

Online Publication Date: 21 February 2012

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Based on the framework of the effective-mass theory, the Hamiltonians of the electron and hole states of the wurtzite structure in the magnetic field are derived. The obtained Hamiltonians can then be solved by the Bessel function expanding method in which the electron and the hole are assumed to be confined in the infinitely high potential barrier. The conduction and valence subbands are denoted after solving the Schrödinger equation. Several possible low interband transitions are calculated, and the magnetic field is found to split the eight transitions to 18 individual transitions. The optical circularly polarized properties of each transition are also labeled when the magnetic field is applied along the z axis.
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78.20.Ls Magneto-optical effects
71.20.Nr Semiconductor compounds
73.22.-f Electronic structure of nanoscale materials and related systems
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
78.67.Qa Nanorods
73.20.-r Electron states at surfaces and interfaces

SiGe superlattice nanocrystal infrared and Raman spectra: A density functional theory study

Mudar A. Abdulsattar

J. Appl. Phys. 111, 044306 (2012); http://dx.doi.org/10.1063/1.3686610 (4 pages)

Online Publication Date: 21 February 2012

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Infrared and Raman vibrational spectrum are calculated using ab initio density functional theory for SiGe superlattice nanocrystal of approximately 1.6 nm length. After obtaining the optimum positions of atoms via geometrical optimization using density functional theory, coupled perturbed Hartree-Fock equations are solved iteratively to obtain vibrational spectrum. Frequencies of vibrations are analyzed against intensities, reduced masses, and vibrational force constants. A scale factor of 0.81 is suggested to correct the frequencies of the present calculations that are obtained using STO-3 G basis functions. Results show that SiGe nanocrystals have complex and rich vibrational spectrum that can be generally divided into three regions. The highest reduced masses are in the first region where Si and Ge atoms are the main contributors to vibrations with a smaller number of vibrations attributed to hydrogen atoms. The highest intensity lines in SiGe superlattice nanocrystals are in the middle region where most of the modes of vibration can be excited. The third region is characterized by high force constants. The first region shows a redshift of the original Ge-Si bond vibration from the calculated bulk 418 cm−1 to the present nanocrystal 395 cm−1. Hydrogen vibrations interferences are found in the same redshift region that might induce uncertainties in the experimentally measured redshift. Si-H and Ge-H vibrations are observed mainly in the second and third region and less frequently in the first region. These vibrations include modes of vibration such as symmetric, asymmetric, wagging, scissor, rocking, and twisting modes.
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78.30.Hv Other nonmetallic inorganics
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
63.20.-e Phonons in crystal lattices
81.07.Bc Nanocrystalline materials

High emission currents and low threshold fields in multi-wall carbon nanotube-polymer composites in the vertical configuration

I. Sameera, Ravi Bhatia, V. Prasad, and Reghu Menon

J. Appl. Phys. 111, 044307 (2012); http://dx.doi.org/10.1063/1.3685754 (5 pages) | Cited 3 times

Online Publication Date: 22 February 2012

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In this work, we present field emission characteristics of multi-wall carbon nanotube (MWCNT)-polystyrene composites at various weight fractions along the cross-section of sample. Scanning electron microscope images in cross-sectional view reveal that MWCNTs are homogeneously distributed across the thickness and the density of protruding tubes can be scaled with weight fraction of the composite film. Field emission from composites has been observed to vary considerably with density of MWCNTs in the polymer matrix. High current density of 100 mA/cm2 was achieved at a field of 2.2 V/μm for 0.15 weight fraction. The field emission is observed to follow the Fowler–Nordheim tunneling mechanism, however, electrostatic screening is observed to play a role in limiting the current density at higher weight fractions.
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79.70.+q Field emission, ionization, evaporation, and desorption
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
68.55.-a Thin film structure and morphology
61.41.+e Polymers, elastomers, and plastics

Field-emission properties of individual GaN nanowires grown by chemical vapor deposition

Yongho Choi, Mario Michan, Jason L. Johnson, Ali Kashefian Naieni, Ant Ural, and Alireza Nojeh

J. Appl. Phys. 111, 044308 (2012); http://dx.doi.org/10.1063/1.3685903 (6 pages) | Cited 2 times

Online Publication Date: 22 February 2012

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Single crystalline GaN nanowires were synthesized using chemical vapor deposition. Devices containing individual GaN nanowires were fabricated using contact printing. The local turn-on electric field at the tip of the GaN nanowires was compared to that of other nanomaterials. The quality of contact between GaN nanowires and metal electrodes was found to affect the field-emission behavior significantly. It was also observed that the field-emission behavior of individual GaN nanowires follows the conventional Fowler-Nordheim model in the range of applied electric fields.
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79.70.+q Field emission, ionization, evaporation, and desorption
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.07.Gf Nanowires
85.35.-p Nanoelectronic devices

Interface charge transfer in polypyrrole coated perovskite manganite magnetic nanoparticles

O. Pana, M. L. Soran, C. Leostean, S. Macavei, E. Gautron, C. M. Teodorescu, N. Gheorghe, and O. Chauvet

J. Appl. Phys. 111, 044309 (2012); http://dx.doi.org/10.1063/1.3686662 (8 pages)

Online Publication Date: 22 February 2012

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Different hybrid structures were obtained by coating magnetic nanoparticles of perovskite type manganite at optimal doping (La0.67Sr0.33MnO3,LSMO) with different quantities of polypyrrole (PPy). The amorphous layer of polypyrrole surrounding the crystalline magnetic core was observed by high resolution transmission electron microscopy (HRTEM) and analyzed by using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements in near edge structure (XANES) techniques. By analyzing the magnetic behavior of the samples one can observe that the surface modification of magnetic nanoparticles by PPy results in an increase in the saturation magnetization of the composites. The process is ascribed to paired electrons transferred from the delocalized π states of the PPy into the outer disordered layers of the manganite. The analysis of pre-edge peak of the Mn K-edge XANES spectra in the case of PPy coated LSMO nanoparticles indicates that the charge transfer between polymer and nanoparticles is (directed) going to missing or distorted oxygen positions, hence increasing the 3d electrons’ mobility and orbital hybridization between the neighboring manganese ion. As a consequence, within the surface layers of LSMO nanoparticles, both energy bands disrupted the structure, and the double exchange process between Mn ions was reestablished determining the saturation magnetizations and pre-edge features increase, respectively.
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75.75.Cd Fabrication of magnetic nanostructures
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
81.07.Wx Nanopowders
75.50.Kj Amorphous and quasicrystalline magnetic materials
68.37.Og High-resolution transmission electron microscopy (HRTEM)
75.50.Tt Fine-particle systems; nanocrystalline materials

Vibration analysis of circular double-layered graphene sheets

Toshiaki Natsuki, Jin-Xing Shi, and Qing-Qing Ni

J. Appl. Phys. 111, 044310 (2012); http://dx.doi.org/10.1063/1.3686689 (6 pages) | Cited 1 time

Online Publication Date: 22 February 2012

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This paper presents a study on the natural vibration of circular double-layer graphene sheets (DLGSs) using circular plate theory. The circular DLGSs are assumed to be coupled together through the carbon–carbon van der Waals (vdW) force between their two layers. An analytical solution of coupled governing equations is proposed to predict the natural frequencies of circular DLGSs. The vibration behaviors in circular DLGSs are found to have an in-phase mode (IPM) associated with the classical natural frequency, and an anti-phase mode (APM) owing to the influence of the vdW interaction. The natural frequencies of the IPM are independent of vdW interactions between adjacent layers, while those of the APM depend on the vdW interaction, and are larger. Based on the exact solution, the influences of wavenumbers (m, n) on the natural frequencies of simply supported circular DLGSs are also investigated in detail.
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62.30.+d Mechanical and elastic waves; vibrations
62.20.mq Buckling
81.40.Lm Deformation, plasticity, and creep

ZnO coated nanospring-based chemiresistors

Vladimir Dobrokhotov, Landon Oakes, Dewayne Sowell, Alexander Larin, Jessica Hall, Alex Kengne, Pavel Bakharev, Giancarlo Corti, Timothy Cantrell, Tej Prakash, Joseph Williams, and D. N. McIlroy

J. Appl. Phys. 111, 044311 (2012); http://dx.doi.org/10.1063/1.3686212 (8 pages) | Cited 1 time

Online Publication Date: 23 February 2012

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Chemiresistors were constructed using 3-D silica nanospring mats coated with a contiguous film of ZnO nanocrystals. Chemiresistors with an average ZnO nanocrystal radius <3 nm, or >20 nm, were found to exhibit a relative change in conductance of a factor of 50 upon exposure to a gas flow of 20% O2 and 80% N2 with ∼500 ppm of toluene and an operational temperature of 400 °C. Samples with an average ZnO nanocrystal radius of 15 nm were found to be the most responsive with a relative conductance change of a factor of 1000. The addition of metal nanoparticles (average radius equal to 2.4 nm) onto the surface of the ZnO nanocrystals (average radius equal to 15 nm) produced a relative change in conductance of a factor of 1500. For the optimum conditions (T = 400 °C, grain size ∼15 nm) well-defined spikes in conductance to explosive vapors (TNT, TATP) were obtained for 0.1 ms exposure time at ppb levels.
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82.80.-d Chemical analysis and related physical methods of analysis
68.55.ag Semiconductors
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
81.05.Dz II-VI semiconductors
81.07.Bc Nanocrystalline materials
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Magnetic properties of few nanometers ɛ-Fe2O3 nanoparticles supported on the silica

S. S. Yakushkin, A. A. Dubrovskiy, D. A. Balaev, K. A. Shaykhutdinov, G. A. Bukhtiyarova, and O. N. Martyanov

J. Appl. Phys. 111, 044312 (2012); http://dx.doi.org/10.1063/1.3686647 (5 pages)

Online Publication Date: 23 February 2012

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Magnetic properties of ɛ-Fe2O3 nanoparticles supported on silica with the average size of few nanometers, narrow size distribution and no admixture of any other iron oxide polymorphs are investigated. The investigation of the temperature behavior of magnetization within the temperature range from 4.2 to 1000 K revealed the presence of several magnetic subsystems in the species under study. The temperatures’ behavior of the magnetic moment value indicates ferrimagnetic ordering in the ɛ-Fe2O3 nanoparticles with a Curie temperature of about 800 K and points to the existence of a significant paramagnetic contribution becoming apparent at low temperatures. According to the electron spin resonance data, the particles possess superparamagnetic behavior at temperature higher ∼120 K. The model of the magnetic structure of monophase system of few nanometers ɛ-Fe2O3 nanoparticles supported on silica is proposed.
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75.75.-c Magnetic properties of nanostructures
75.78.-n Magnetization dynamics
75.30.Cr Saturation moments and magnetic susceptibilities
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Gg Ferrimagnetics

Charge transport in two different conductive polymer and semiconducting quantum dot nanocomposite systems

Sushmita Biswas, Yang Li, Michael A. Stroscio, and Mitra Dutta

J. Appl. Phys. 111, 044313 (2012); http://dx.doi.org/10.1063/1.3682106 (6 pages)

Online Publication Date: 24 February 2012

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Charge transport mechanisms have been investigated in two different nanocomposite structures made of conductive polymers embedded with II-VI semiconducting quantum dots. Photoluminescence data indicated charge transport in the two systems. Higher photocurrents observed in the poly(3-hexylthiophene-2,5-diyl) polymer-based heterostructure in comparison with the poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) polymer-based heterostructure have been analyzed with hole and electron conduction. For larger concentrations of quantum dots, both electron transport through nanocrystals and hole transport through polymer become relevant. Based on the electron tunneling mechanism, current voltage characteristics are modeled for a double barrier quantum well device formed by semiconducting quantum dots and polymer molecules.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
78.55.-m Photoluminescence, properties and materials
72.40.+w Photoconduction and photovoltaic effects
73.40.Gk Tunneling
72.20.Fr Low-field transport and mobility; piezoresistance
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters

Tip geometry effects in dopant profiling by scanning microwave microscopy

I. Humer, C. Eckhardt, H. P. Huber, F. Kienberger, and J. Smoliner

J. Appl. Phys. 111, 044314 (2012); http://dx.doi.org/10.1063/1.3686748 (5 pages) | Cited 3 times

Online Publication Date: 24 February 2012

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In this paper, the impact of the tip radius on dopant profiling by scanning microwave microscopy is investigated. The cantilevers are very likely to erode in such measurements, and thus, a two-dimensional Poisson solver was used to calculate the lateral spatial resolution as a function of tip radius and doping. Moreover, a strong correlation between the slope of the calibration curves and the tip diameter was found. The slope of the calibration curves increases toward −0.5 and saturates as the radius approaches values of 150 nm, which is in agreement with experimental data obtained from scanning microwave microscopy.
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61.72.U- Doping and impurity implantation
07.79.-v Scanning probe microscopes and components
06.20.fb Standards and calibration

Thermal-magnetic-electric oscillator based on spin-valve effect

A. M. Kadigrobov, S. Andersson, Hee Chul Park, D. Radić, R. I. Shekhter, M. Jonson, and V. Korenivski

J. Appl. Phys. 111, 044315 (2012); http://dx.doi.org/10.1063/1.3686735 (8 pages) | Cited 1 time

Online Publication Date: 24 February 2012

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A thermal-magnetic-electric valve with the free layer of exchange-spring type and inverse magnetoresistance is investigated. The structure has S-shaped current-voltage characteristics and can exhibit spontaneous oscillations when integrated with a conventional capacitor within a resonator circuit. The frequency of the oscillations can be controlled from essentially dc to the GHz range by the circuit capacitance.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.-m Magnetotransport phenomena; materials for magnetotransport
72.15.Eb Electrical and thermal conduction in crystalline metals and alloys
71.70.Gm Exchange interactions
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons

Space charge effects in ultrafast electron diffraction and imaging

Zhensheng Tao, He Zhang, P. M. Duxbury, Martin Berz, and Chong-Yu Ruan

J. Appl. Phys. 111, 044316 (2012); http://dx.doi.org/10.1063/1.3685747 (10 pages)

Online Publication Date: 24 February 2012

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Understanding space charge effects is central for the development of high-brightness ultrafast electron diffraction and microscopy techniques for imaging material transformation with atomic scale detail at the fs to ps timescales. We present methods and results for direct ultrafast photoelectron beam characterization employing a shadow projection imaging technique to investigate the generation of ultrafast, non-uniform, intense photoelectron pulses in a dc photo-gun geometry. Combined with N-particle simulations and an analytical Gaussian model, we elucidate three essential space-charge-led features: the pulse lengthening following a power-law scaling, the broadening of the initial energy distribution, and the virtual cathode threshold. The impacts of these space charge effects on the performance of the next generation high-brightness ultrafast electron diffraction and imaging systems are evaluated.
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52.80.-s Electric discharges
61.72.Dd Experimental determination of defects by diffraction and scattering

Enhanced electron field emission from plasma-nitrogenated carbon nanotips

B. B. Wang, Q. J. Cheng, X. X. Zhong, Y. Q. Wang, Y. A. Chen, and K. Ostrikov

J. Appl. Phys. 111, 044317 (2012); http://dx.doi.org/10.1063/1.3688252 (8 pages) | Cited 6 times

Online Publication Date: 24 February 2012

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Nitrogenated carbon nanotips (NCNTPs) are synthesized by plasma-enhanced hot filament chemical vapor deposition from the hydrogen, methane, and nitrogen gas mixtures with different flow rate ratios of hydrogen to nitrogen. The morphological, structural, compositional, and electron field emission (EFE) properties of the NCNTPs were investigated by field emission scanning electron microscopy, Raman spectroscopy, x ray photoelectron spectroscopy, and EFE high-vacuum system. It is shown that the NCNTPs deposited at an intermediate flow rate ratio of hydrogen to nitrogen feature the best size/shape and pattern uniformity, the highest nanotip density, the highest nitrogen concentration, as well as the best electron field emission performance. Several factors that come into play along with the nitrogen incorporation, such as the combined effect of the plasma sputtering and etching, the transition of sp3 carbon clusters to sp2 carbon clusters, the increase of the size of the sp2 clusters, as well as the reduction of the work function, have been examined to interpret these experimental findings. Our results are highly relevant to the development of the next generation electron field emitters, flat panel displays, atomic force microscope probes, and several other advanced applications.
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79.70.+q Field emission, ionization, evaporation, and desorption
52.77.Dq Plasma-based ion implantation and deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.46.-w Structure of nanoscale materials
81.07.Bc Nanocrystalline materials
78.30.Hv Other nonmetallic inorganics

Simulation on initial growth stages of graphene on Pt (111) surface

J. Y. Guo, C. X. Xu, F. Y. Sheng, Z. L. Shi, J. Dai, Z. H. Li, and X. Hu

J. Appl. Phys. 111, 044318 (2012); http://dx.doi.org/10.1063/1.3686609 (6 pages) | Cited 1 time

Online Publication Date: 27 February 2012

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Epitaxial growth process and morphology evolution in the initial growth stages of graphene on Pt (111) surface have been studied by means of canonical Monte Carlo simulation. It is found that the nucleation, carbon chains, carbon rings, and then graphene domain are formed orderly in the early growth stages during the annealing process. The dynamic processes of the morphology evolution are visualized through the simulation. The formed structures of graphene are investigated quantitatively by pair distribution function, atomic intervals, and the bend angles among the three contiguous carbon atoms. The lattice mismatch between graphene and the Pt (111) surface is accommodated by the main structure of hcp-fcc. These simulation results are consistent with experimental observations and may give further insights to the epitaxial growth of graphene in the atomic scale.
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81.05.ue Graphene
81.40.Gh Other heat and thermomechanical treatments
68.35.bp Fullerenes
68.55.aj Insulators
61.48.Gh Structure of graphene
68.65.Pq Graphene films
FREE

A model for the effective thermal conductivity of metal-nonmetal particulate composites

J. Ordonez-Miranda, Ronggui Yang, and J. J. Alvarado-Gil

J. Appl. Phys. 111, 044319 (2012); http://dx.doi.org/10.1063/1.3688044 (12 pages) | Cited 2 times

Online Publication Date: 27 February 2012

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The effective thermal conductivity of particulate composites with oriented spheroidal metallic particles embedded in a dielectric matrix is analyzed under the framework of the two-temperature model of heat conduction. The obtained analytical results show that the effective thermal conductivity depends strongly on (1) the relative size of the particle inclusions with respect to the electron-phonon coupling length and (2) the ratio between the electron and phonon thermal conductivities. The effect of the electron-phonon coupling inside metallic particles is expressed by the reduction of the composite thermal conductivity with respect to its corresponding values obtained for an infinite electron-phonon coupling factor, where the analysis could be established based on the Fourier law of heat conduction. It is shown that the composite thermal conductivity has upper and lower bounds, which are determined by the particle size in comparison with the electron-phonon coupling length. The generalized model for spheroidal particles is then used to analyze the thermal conductivity for limiting cases on the particle shape including spheres, cylinders, and flat plates. For perfect electron-phonon coupling, the proposed model reduces to various previously-reported results. This study shows that the particle size dependence of the thermal conductivity of metal-nonmetal composites appears not only through the interfacial thermal resistance but also by means of the electron-phonon coupling. The results of this work could be useful for guiding the design of particulate composites with spheroidal metallic inclusions from macro/micro- to nanoscales.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
61.72.Qq Microscopic defects (voids, inclusions, etc.)
63.20.kd Phonon-electron interactions
77.84.Lf Composite materials

ZnO/anthracene based inorganic/organic nanowire heterostructure: Photoresponse and photoluminescence studies

Soumen Dhara and P. K. Giri

J. Appl. Phys. 111, 044320 (2012); http://dx.doi.org/10.1063/1.3687936 (7 pages) | Cited 1 time

Online Publication Date: 27 February 2012

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The effects of surface modification of vertically aligned ZnO nanowires (NWs) with anthracene for the improved photocurrent, photoresponse, and UV photoluminescence have been investigated in this work. The formation of ZnO/anthracene based inorganic/organic NWs heterostructure by surface capping with anthracene solution was confirmed from the high resolution transmission electron microscopy and Fourier transport infrared spectroscopy analyses. After the anthracene capping of ZnO NWs, despite an increase in dark current, we obtained a significant improvement in the photocurrent and photoresponsivity. A sixfold improvement in the UV photocurrent-to-dark current ratio is obtained with capping. Compared to the uncapped NWs, the photoresponse is significantly faster for the ZnO/anthracene system with response and reset times of 1.5 and 1.6 s, respectively. The room temperature photoluminescence spectra show threefold enhanced UV emission with large enhancement in the ratio of UV to green emission intensities. The faster photoresponse and enhanced photocurrent from the ZnO/anthracene heterostructure are explained on the basis of modification of surface defects and interfacial charge transfer process.
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78.55.-m Photoluminescence, properties and materials
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
79.60.Jv Interfaces; heterostructures; nanostructures
78.30.-j Infrared and Raman spectra
82.80.Nj Fourier transform mass spectrometry

Mechanisms of Stranski-Krastanov growth

Arvind Baskaran and Peter Smereka

J. Appl. Phys. 111, 044321 (2012); http://dx.doi.org/10.1063/1.3679068 (6 pages) | Cited 4 times

Online Publication Date: 27 February 2012

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Stranski-Krastanov (SK) growth is reported experimentally as the growth mode that is responsible for the transition to three dimensional islands in heteroepitaxial growth. A kinetic Monte Carlo (KMC) model is proposed that can replicate many of the experimentally observed features of this growth mode. Simulations reveal that this model effectively captures the SK transition and subsequent growth. Annealing simulations demonstrate that the wetting layer formed during SK growth is stable, with entropy playing a key role in its stability. It is shown that this model also captures the apparent critical thickness that tends to occur at higher deposition rates and for alloy films (where intermixing is significant). This work shows that the wetting layer thickness increases with increasing temperature, whereas the apparent critical thickness decreases with increasing temperature. Both of which are in agreement with experiments.
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81.05.Cy Elemental semiconductors
65.40.gd Entropy
68.55.ag Semiconductors
61.72.Cc Kinetics of defect formation and annealing
68.08.Bc Wetting

Atomic simulations of effect of grain size on deformation behavior of nano-polycrystal magnesium

H. Y. Song and Y. L. Li

J. Appl. Phys. 111, 044322 (2012); http://dx.doi.org/10.1063/1.3687908 (5 pages) | Cited 1 time

Online Publication Date: 27 February 2012

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The effect of grain size and temperature on the mechanical behavior of nano-polycrystal magnesium under tensile load is investigated using molecular dynamics simulation. The results indicate that the average flow stress of nano-polycrystal magnesium decreases with decreasing grain size, exhibiting a breakdown in the Hall-Petch relation when grain size is smaller than a critical size. The results also show that crack formation during tension is a cause of reduced flow stress of nano-polycrystal magnesium with a large grain size and that the Young’s modulus of nano-polycrystal magnesium increases with increasing grain size. Furthermore, we find that the deformation behavior of nano-polycrystal magnesium obviously depends on temperatures.
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81.40.Lm Deformation, plasticity, and creep
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.de Elastic moduli
62.20.fq Plasticity and superplasticity
62.20.mt Cracks
81.40.Jj Elasticity and anelasticity, stress-strain relations

Hybrid polymer:colloidal nanoparticle photovoltaic cells incorporating a solution-processed, multi-functioned ZnO nanocrystal layer

Jihua Yang, Lei Qian, Renjia Zhou, Ying Zheng, Aiwei Tang, Paul H. Holloway, and Jiangeng Xue

J. Appl. Phys. 111, 044323 (2012); http://dx.doi.org/10.1063/1.3689154 (8 pages) | Cited 1 time

Online Publication Date: 28 February 2012

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We report significant improvement in both the power conversion efficiency and the environmental stability of solution-processed hybrid organic-inorganic solar cells by including a solution-processed ZnO nanocrystal layer between the photoactive layer and the cathode. For devices based on blends of poly(3-hexylthiophene) (P3HT) and mostly-spherical CdSe nanocrystals, incorporation of the ZnO layer leads to an up to 70% increase in the power conversion efficiency. Compared to only a few hours of shelf lifetime for unencapsulated devices with the metal cathode directly deposited on the hybrid active layer, devices with the ZnO layer can retain approximately 70% of the original efficiency when they are exposed to the laboratory ambient without encapsulation for more than two months. We attribute the function of this ZnO nanocrystal layer to a combination of optical, electronic, morphological, and chemical effects, including blocking leakage of photogenerated holes to the cathode, optimizing the optical intensity profile in the hybrid active layer, minimizing recombination or quenching of photogenerated excitons and charge carriers, significantly reducing the transport rate of oxygen and water molecules to the active layer and reducing degradation/oxidation of any low work function layer at the cathode interface.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells

Study of focused-ion-beam–induced structural and compositional modifications in nanoscale bilayer systems by combined grazing incidence x ray reflectivity and fluorescence

Erhan Arac, David M. Burn, David S. Eastwood, Thomas P. A. Hase, and Del Atkinson

J. Appl. Phys. 111, 044324 (2012); http://dx.doi.org/10.1063/1.3689016 (9 pages) | Cited 2 times

Online Publication Date: 28 February 2012

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A detailed analysis of the structural and compositional changes in NiFe/Au bilayers induced by a focused ion beam (FIB) is presented. NiFe/Au bilayers with different thickness were irradiated with a focused 30 keV Ga+ ion beam, and the evaluation of the individual layers and interfaces were investigated systematically as a function of a broad range of irradiation fluence using grazing incidence x ray reflectivity (GIXRR) and angular dependent x ray fluorescence (ADXRF) techniques carried out at synchrotron radiation sources. Experimental data were collected from 1.3 mm × 4.5 mm structures, and irradiation of such a broad areas with a 100-nm-wide focused ion beam is a challenging task. Two irradiation regimes were identified: For Ga+ fluences < 15.6 × 1014 ion/cm2 (low dose regime), the main influence of the focused ion beam is on the interface and, beyond this dose (high dose regime), sputtering effects and ion implantation becomes significant, eventually causing amorphization of the bilayer system. The broadening of the NiFe/Au interface occurs even at the lowest dose, and above a critical fluence (Φ = 1.56 × 1014 ion/cm2) can be represented by an interfacial-intermixed layer (NixFeyAu(1-x-y); x = 0.5-0.6, y = 0.1-0.15) formed between the NiFe and Au layers. The thickness of this layer increases with irradiation fluence in the low dose regime. A linear relationship is found between the squared intermixing length and irradiation fluence, indicating that FIB-induced mixing is diffusion controlled. The ballistic model fails to describe FIB-induced intermixing, indicating that thermodynamical factors, which might be originated from FIB specific features, should be taken into account. Despite the complexity of the chemical and structural formation, good agreement between the experiment and theory highlights the functionality of the combined GIXRR and ADXRF techniques for studying intermixing in high resolution.
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61.80.Jh Ion radiation effects
61.82.Bg Metals and alloys
78.55.Hx Other solid inorganic materials
78.70.En X-ray emission spectra and fluorescence
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods

Over 100 ns intrinsic radiative recombination lifetime in type II InAs/GaAs1−xSbx quantum dots

Kazutaka Nishikawa, Yasuhiko Takeda, Ken-ichi Yamanaka, Tomoyoshi Motohiro, Daisuke Sato, Junya Ota, Naoya Miyashita, and Yoshitaka Okada

J. Appl. Phys. 111, 044325 (2012); http://dx.doi.org/10.1063/1.3688864 (6 pages) | Cited 2 times

Online Publication Date: 28 February 2012

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We report very long intrinsic radiative recombination lifetime τrad in type II InAs quantum dots embedded in GaAs1−xSbx. The dependence of photoluminescence (PL) decay time τPL on both the Sb composition (x = 0–0.18) and excitation intensity (38–460 mW/cm2) was systematically investigated by time-resolved PL measurements with a time-correlated single-photon counting method. All PL decay curves exhibited non-exponential profiles, and τPL was strongly dependent on the excitation intensity. These properties were well explained by solving rate equations of carrier density with neglecting nonradiative process, in which τrad is inversely proportional to carrier density. The 18% Sb sample exhibited a τPL of over 100 ns under weak excitation, which is longer than twice the previously reported values. We evaluated the value of τrad in InAs/GaAs1−xSbx QDs relative to that in type I InAs/GaAs QDs based on an effective mass approximation and found that the observed extremely long τPL corresponds to τrad.
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78.67.Hc Quantum dots
78.47.jd Time resolved luminescence
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
78.55.Cr III-V semiconductors
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