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15 Aug 2007

Volume 102, Issue 4, Articles (04xxxx)

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Self-organization of 6H‐SiC (0001) surface under keV ion irradiation

Y. S. Katharria, Sandeep Kumar, P. S. Lakshmy, D. Kanjilal, and A. T. Sharma

J. Appl. Phys. 102, 044301 (2007); http://dx.doi.org/10.1063/1.2769804 (6 pages) | Cited 6 times

Online Publication Date: 16 August 2007

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In the present study, we have investigated the temporal evolution of 6H‐SiC (0001) surface under 100 keV Ar+ ion irradiation at oblique incidence (θ = 60°). The topographical changes introduced by ion beam were examined using scanning force microscopy, and it is demonstrated that while at short time scales, surface morphology is dominated by dots with average diameter of 30 nm, periodic height modulations or ripples emerge at the later time scales. Existing theories of ripple formation have been invoked to explain various features of the observed ripples. Ripple structures developed on a physically stable material such as SiC are expected to show very small time degradation and therefore, would be more advantageous for various technological applications as compared to those grown on conventional semiconductors such as Si, GaAs, InP, etc.
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81.65.-b Surface treatments
61.80.Jh Ion radiation effects
68.37.Ps Atomic force microscopy (AFM)

Synthesis and analysis of abnormal wurtzite ZnSe nanowheels

Lei Jin, Wallace C. H. Choy, Yee P. Leung, Tong I. Yuk, Hock C. Ong, and Jian-bo Wang

J. Appl. Phys. 102, 044302 (2007); http://dx.doi.org/10.1063/1.2769329 (6 pages) | Cited 11 times

Online Publication Date: 17 August 2007

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An abnormal structure of the ZnSe nanowheels composed of teethlike extended patterns on nanoring bases has been successfully synthesized by thermal evaporation method. It is interesting to note that the as-synthesized ZnSe nanowheels are metastable wurtzite phase with the dominant exposed surfaces of ±(2mathmath0) while the stable ZnSe is typically zinc blende phase. A full picture of the growth mechanism of the metastable wurtzite phase ZnSe nanostructures will be proposed from the thermodynamic point of view. Meanwhile, the formation of the nanowheels is also explained by a two-stage mechanism. In the first stage, the base of the nanowheel begins to form by vapor-solid mechanism, while in the second stage, the teethlike extended structures grow through the self-catalyzed growth process. The cathodoluminescence spectrum of ZnSe nanowheel exhibited a band edge transition at about 460 nm and a strong self-activated luminescence at 610 nm. It is important to note that the discussions of the nanostructure thermodynamics and stability can be applied to understand the growth mechanism of other nanostructures which are critical for optimization of the nanostructures.
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81.07.Bc Nanocrystalline materials
64.70.Nd Structural transitions in nanoscale materials
78.60.Hk Cathodoluminescence, ionoluminescence

Nucleation and growth of carbon nanotubes in catalytic chemical vapor deposition

Stanislav A. Moshkalev and Carla Verissimo

J. Appl. Phys. 102, 044303 (2007); http://dx.doi.org/10.1063/1.2769354 (6 pages) | Cited 8 times

Online Publication Date: 17 August 2007

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The process of nucleation of multiwall carbon nanotubes in chemical vapor deposition process with nickel as catalyst and methane as a carbon precursor is analyzed. The nucleation is considered as a specific instability developed on the surface of a metal catalyst particle supersaturated with carbon. The energy released in graphitization of carbon from the metal-carbon solution is shown to be crucial for the nanotube nucleation. The energy released may be high enough for substantial metal heating resulting in partial liquefaction of the catalyst particle. The proposed mechanism can be called vapor-solid-liquid-solid (VSLS) as the catalyst particle may be in a mixed solid-liquid (or liquidlike) state during nucleation and unstable phases of nanotube growth.
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81.05.U- Carbon/carbon-based materials
64.60.Q- Nucleation
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Three-dimensional structure of CdX (X = Se,Te) nanocrystals by total x-ray diffraction

S. K. Pradhan, Z. T. Deng, F. Tang, C. Wang, Y. Ren, P. Moeck, and V. Petkov

J. Appl. Phys. 102, 044304 (2007); http://dx.doi.org/10.1063/1.2767615 (6 pages) | Cited 4 times

Online Publication Date: 20 August 2007

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The three-dimensional structure of oleic acid-capped CdSe and thiol-capped CdTe nanocrystals used as quantum dots has been determined by total synchrotron radiation x-ray diffraction and atomic pair distribution function analysis. Both CdSe and CdTe are found to exhibit the zinc-blende-type atomic ordering. It is only slightly distorted in CdSe implying the presence of nanosize domains and very heavily distorted in CdTe due to the presence of distinct core-shell regions. The results well demonstrate the great potential of the experimental approach and thus encourage its wider application in quantum dot research.
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61.46.Hk Nanocrystals
68.65.Hb Quantum dots (patterned in quantum wells)

Formation and characterization of carbon nanowires

Amit Kumar, D. K. Avasthi, A. Tripathi, L. D. Filip, J. D. Carey, and J. C. Pivin

J. Appl. Phys. 102, 044305 (2007); http://dx.doi.org/10.1063/1.2767227 (4 pages) | Cited 11 times

Online Publication Date: 21 August 2007

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This article reports on the formation and electronic characteristics of conducting carbon nanowires produced by swift heavy ion irradiation of a fullerene thin film. This study shows that it is possible to create arrays of carbon nanowires, which are perfectly parallel to each other and perpendicular to the substrate. As-deposited fullerene films exhibit poor field emission characteristics with breakdown fields as high as 51 V/μm, whereas low dose ion irradiated fullerene film produces a threshold field as low as 9 V/μm. The present approach of making conducting carbon nanowires by ion irradiation for potential field emitters and large area applications is also discussed.
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79.70.+q Field emission, ionization, evaporation, and desorption
61.80.Jh Ion radiation effects

Size effect in polymer nanofibers under tension

Xiang-Fa Wu and Yuris A. Dzenis

J. Appl. Phys. 102, 044306 (2007); http://dx.doi.org/10.1063/1.2769266 (6 pages) | Cited 19 times

Online Publication Date: 22 August 2007

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This article studies the size effect on the elastic behavior of solid and hollow polymer nanofibers (e.g., electrospun nanofibers) subjected to uniaxial tension. A one-dimensional nonlinear elastic tension model is proposed that takes into account the coupling effect of fiber elastic deformation and surface tension. The fiber axial force-displacement and stress-strain relations are obtained in explicit forms. It is shown that, at nanoscale, fiber radius has appreciable effect on the elastic response of polymer nanofibers. With consideration of the fiber radial effect, it is shown that the actual contribution of surface energy of the solid polymer fibers to the axial tensile force is πr0γ rather than 2πr0γ (where r0 is the fiber radius after deformation and γ is the surface tension), as commonly used in literature. Compared to solid polymer fibers, the tensile behavior of hollow polymer nanofibers appears more complex with greater axial stiffening effect depending upon the combination effect of the fiber exterior and interior radii and the material properties. The results presented in this study can be utilized for data reduction of the nanoscale tension tests of polymer nanofibers and the analysis and design of nanofiber devices.
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61.46.-w Structure of nanoscale materials
61.41.+e Polymers, elastomers, and plastics
62.20.F- Deformation and plasticity
62.25.-g Mechanical properties of nanoscale systems

Flow-induced instability of double-walled carbon nanotubes based on an elastic shell model

Y. Yan, X. Q. He, L. X. Zhang, and Q. Wang

J. Appl. Phys. 102, 044307 (2007); http://dx.doi.org/10.1063/1.2763955 (8 pages) | Cited 13 times

Online Publication Date: 23 August 2007

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Double-walled carbon nanotubes (DWCNTs) are modeled based on Donnell’s shell theory, and flow-induced instability that is induced when pressure-driven fluid goes through the inner tube at a steady flow velocity is studied. The van der Waals (vdW) interaction between the inner and outer walls is taken into account in the modeling. The numerical simulations show that the vdW interaction has significant effects on the flow-induced instability of DWCNTs. The critical flow velocities and loss of stability are closely related to the ratio of the length to the outer radius. Donnell’s shell model for carbon nanotubes (CNTs) is preferred in simulations because it takes into account the shear effects in the walls. A comparison between the CNTs that are based on a Eulerian beam model and those that are based on Donnell’s shell model shows that when the 50-nm-radius tube length is shorter than 10 μm, the comparative errors between the Eulerian beam and Donnell’s shell models are greatly increased.
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61.48.-c Structure of fullerenes and related hollow and planar molecular structures
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations
62.20.D- Elasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations

Focused ion beam induced synthesis of a porous antimony nanowire network

Christoph Schoendorfer, Alois Lugstein, Youn-Joo Hyun, Emmerich Bertagnolli, Lothar Bischoff, Philipp M. Nellen, Victor Callegari, and Peter Pongratz

J. Appl. Phys. 102, 044308 (2007); http://dx.doi.org/10.1063/1.2771044 (5 pages) | Cited 5 times

Online Publication Date: 24 August 2007

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We present a focused ion beam-based approach for the synthesis of an antimony nanofiber network. The nanofibers, with a homogeneous distribution of diameters of about 25 nm and lengths up to several microns, are synthesized in a self-assembling process without any additional material source at room temperature. It is possible to recrystallize the as-grown amorphous nanofibers by moderate rapid thermal annealing at 473 K. These results have been verified by means of scanning electron microscopy, Auger electron spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, and energy dispersive x-ray analysis. As this approach is not limited solely to the material discussed here, other substrates (e.g., GaSb and Ge) and ion sources should extend this method to other materials, which offers a great potential for future nanoscale devices and applications.
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81.07.Bc Nanocrystalline materials
81.05.Rm Porous materials; granular materials
81.16.Dn Self-assembly
81.20.-n Methods of materials synthesis and materials processing
61.80.Jh Ion radiation effects
79.20.Fv Electron impact: Auger emission

Paramagnetic defects and amorphous network reconstruction of magnetron sputtered a-SiO2:Ge films

R. N. Pereira, J. Skov Jensen, J. Chevallier, B. Bech Nielsen, and A. Nylandsted Larsen

J. Appl. Phys. 102, 044309 (2007); http://dx.doi.org/10.1063/1.2769780 (7 pages) | Cited 2 times

Online Publication Date: 24 August 2007

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We have investigated the paramagnetic defects and the structure of magnetron sputtered amorphous SiO2 films containing 3.8 at. % Ge (a-SiO2:Ge) over the 500–1000 °C annealing temperature range using electron paramagnetic resonance (EPR), Fourier-transform infrared (FTIR) absorption, and transmission electron microscopy (TEM). The EPR spectra of as-grown a-SiO2:Ge films reveal three different defects: Si-E′ centers with g‖ = 2.0019 and g⊥ = 2.0004, •Ge ≡ Si3 dangling bonds with g‖ = 2.001 and g⊥ = 2.024, and •Si ≡ Si2O or •Si ≡ SiO2 defects with g = 2.004. While the Si-E′ and g = 2.004 lines are removed by heat treatments at 500 °C, the signal from •Ge ≡ Si3 dangling bonds persists up to annealing temperatures of 700 °C. The structural changes induced upon annealing on the a-SiO2:Ge films have been studied by monitoring the frequency and linewidth of the asymmetric stretching vibration of the Si–O–Si linkage using FTIR. We find that the rearrangement of the amorphous oxide network occurs primarily within the 500–700 °C temperature range and no further significant recovery happens upon annealing at temperatures above 700 °C, in line with the EPR results. TEM images reveal the formation of Ge nanocrystals (Ge ncs) with diameters of 2–4 nm already upon heat treatments at 500 °C. Moreover, it is shown that the mean size of the Ge ncs increases quite significantly as the temperature of the heat treatments increases. The mean diameter of Ge ncs observed after annealing at temperatures above 600 °C is above that expected for Ge ncs with efficient photoluminescence properties. The implications of our experimental results for the understanding of the quenching of the photoluminescence from quantum-confined excitons within Ge ncs are briefly discussed.
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81.15.Cd Deposition by sputtering
68.55.-a Thin film structure and morphology
81.40.Gh Other heat and thermomechanical treatments
81.05.Gc Amorphous semiconductors

Photoreduction and oxidation behavior of In2O3 nanoparticles by metal organic chemical vapor deposition

Ch. Y. Wang, V. Cimalla, Th. Kups, C.-C. Röhlig, H. Romanus, V Lebedev, J. Pezoldt, Th. Stauden, and O. Ambacher

J. Appl. Phys. 102, 044310 (2007); http://dx.doi.org/10.1063/1.2770831 (6 pages) | Cited 6 times

Online Publication Date: 27 August 2007

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In2O3 nanoparticles were synthesized at low substrate temperatures by the metal organic chemical vapor deposition technique. Nanoparticles with a mean diameter from 3 to 33 nm can be obtained by varying the growth temperature. Photoreduction and oxidation studies were carried out for particle-containing layers exhibiting a resistance change of more than five orders of magnitude after ultraviolet irradiation and oxidation by ozone. A grain boundary model was proposed to understand the photoreduction and oxidation mechanism for the nanoparticle layers. It was suggested that by photoreduction the nanoparticles are reactivated throughout the layer. The Schottky barrier between the nanoparticles decreases inducing a reduction of the space-charge-limited region. After oxidation, a completely depleted space-charge region covering the whole volume of In2O3 nanoparticles is formed. Furthermore, the bulk diffusion process dominates the response of thick layers during the oxidation process. By decreasing the layer thickness down to 10 nm, surface effects dominate, resulting in an ultrafast response to changes in ozone concentration. The typical response time of very thin In2O3 nanoparticle layers was determined to be less than 1 s.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.16.Pr Micro- and nano-oxidation
81.65.Mq Oxidation
82.30.-b Specific chemical reactions; reaction mechanisms
73.30.+y Surface double layers, Schottky barriers, and work functions

Using pn junction depletion regions to position epitaxial nanowires

Nathaniel J. Quitoriano and Theodore I. Kamins

J. Appl. Phys. 102, 044311 (2007); http://dx.doi.org/10.1063/1.2770820 (5 pages) | Cited 7 times

Online Publication Date: 28 August 2007

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Si nanowires were grown horizontally using the vapor-liquid-solid method from vertical {111} surfaces etched into a (110) Si substrate. The nanowires were catalyzed by negatively charged, citrate-stabilized, Au nanoparticles. The negative charge on the nanoparticles was used to position them along a positively charged depletion region formed by a pn junction. By positioning the nanoparticle catalysts, the epitaxial Si nanowires catalyzed by the nanoparticles were also positioned along this junction. The structure that best positioned the nanowires was highly doped n-type material on a lightly doped p-type substrate. Enhanced positioning of the nanowires was accomplished using a reverse bias across the pn junction.
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81.05.Cy Elemental semiconductors
81.07.Bc Nanocrystalline materials
73.63.Nm Quantum wires
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
61.46.Hk Nanocrystals

Arsenic-induced etched nanovoids on GaSb (100)

S. H. Huang, G. Balakrishnan, M. Mehta, L. R. Dawson, D. L. Huffaker, and P. Li

J. Appl. Phys. 102, 044312 (2007); http://dx.doi.org/10.1063/1.2772532 (4 pages) | Cited 1 time

Online Publication Date: 28 August 2007

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We describe in situ nanoscale etch-pit formation on GaSb (100) surfaces as a result of exposure to an As2 flux in molecular beam epitaxy. The pits form as a result of an Sb-displacement reaction that occurs between the GaSb substrate and the impinging As adatoms. The nanoscale surface features are highly crystallographic with a strong preference for {111} planes, similar to other etching techniques. Nanopit dimensions and density increase with As exposure time. For the 60 s exposure analyzed in this article, the pits vary in both size and shape with average dimensions ∼ 25 nm wide and 50–80 nm long and 10–70 nm deep, with density of ∼ 1×109/cm2. Subsequent GaAs overgrowth proceeds by a coalescence mechanism leaving interfacial nanovoids and finally highly planar bulk layers.
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61.72.Qq Microscopic defects (voids, inclusions, etc.)
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

Effects of nanotube alignment and measurement direction on percolation resistivity in single-walled carbon nanotube films

Ashkan Behnam, Jing Guo, and Ant Ural

J. Appl. Phys. 102, 044313 (2007); http://dx.doi.org/10.1063/1.2769953 (7 pages) | Cited 21 times

Online Publication Date: 28 August 2007

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We have used Monte Carlo simulations to study the effects of nanotube alignment and measurement direction on the resistivity in single-walled carbon nanotube films. These films consist of multiple layers of conductive nanotube networks with percolative transport as the dominant conduction mechanism. We find that minimum resistivity occurs for a partially aligned rather than a perfectly aligned nanotube film. When nanotubes are strongly aligned, the film resistivity becomes highly dependent on the measurement direction. We also find that aligning the nanotubes too strongly or measuring the resistivity in a direction which is very different from the alignment direction causes the film to approach the percolation threshold, as evidenced by the inverse power law increase in resistivity. Furthermore, the location of the resistivity minimum and the values of the inverse power law critical exponents are not universal, but depend strongly on other nanotube and device parameters. To illustrate this explicitly, we have studied the effect of three parameters, namely, nanotube length, nanotube density per layer, and device length on the scaling of nanotube film resistivity with nanotube alignment and measurement direction. We find that longer nanotubes, denser films, and shorter device lengths decrease the alignment critical exponent and the alignment angle at which minimum resistivity occurs, but increase the measurement direction critical exponent. However, the amount of increase or decrease in the critical exponents or the minima locations is different for each parameter. We explain these results by simple physical and geometrical arguments. Characterizing and understanding the effects of alignment and measurement direction on the percolation resistivity in films and composites made up of one-dimensional conductors, such as nanotubes, give valuable insights into the optimal way to arrange these nanomaterials for potential applications in optoelectronics, sensors, and flexible microelectronics.
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73.63.Fg Nanotubes
73.61.Wp Fullerenes and related materials
85.35.Kt Nanotube devices

Comparison of thermodynamic properties of cubic Cr1−xAlxN and Ti1−xAlxN from first-principles calculations

B. Alling, T. Marten, I. A. Abrikosov, and A. Karimi

J. Appl. Phys. 102, 044314 (2007); http://dx.doi.org/10.1063/1.2773625 (8 pages) | Cited 19 times

Online Publication Date: 31 August 2007

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In order to investigate the stability of the cubic phase of Cr1−xAlxN at high AlN content, first principles calculations of magnetic properties, lattice parameters, electronic structure, and mixing enthalpies of the system were performed. The mixing enthalpy was calculated on a fine concentration mesh to make possible the accurate determination of its second concentration derivative. The results are compared to calculations performed for the related compound Ti1−xAlxN and with experiments. The mixing enthalpy is discussed in the context of isostructural spinodal decomposition. It is shown that the magnetism is the key to understand the difference between the Cr- and Ti-containing systems. Cr1−xAlxN turns out to be more stable against spinodal decomposition than Ti1−xAlxN, especially for AlN-rich samples which are of interest in cutting tools applications.
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65.40.G- Other thermodynamical quantities
71.20.Ps Other inorganic compounds
61.66.Fn Inorganic compounds
71.15.-m Methods of electronic structure calculations
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