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15 Feb 2010

Volume 107, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 107, 041101 (2010); http://dx.doi.org/10.1063/1.3318287 (13 pages)

Y. C. Tao and J. G. Hu
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back to top Nanoscale Science and Design

van der Waals adhesion of graphene membranes

Zhixing Lu and Martin L. Dunn

J. Appl. Phys. 107, 044301 (2010); http://dx.doi.org/10.1063/1.3270425 (5 pages) | Cited 6 times

Online Publication Date: 16 February 2010

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We study the nanomechanics of graphene and other ultrathin membranes adhered to substrates with open cavities by van der Waals forces, and subjected to mechanical loads (concentrated or pressure) that cause stretching, decohesion, and/or sliding. We develop analytical models to describe equilibrium configurations similar to those observed in recent nanomechanical experiments, as well as various deformation regimes that occur upon mechanical loading. We apply the analytical models to single layers of graphene and carry out atomistic simulations of graphene adhered to a substrate to validate the models and illustrate the operative phenomena. We obtain excellent agreement between theory and atomistic simulations and identify the influence of van der Waals adhesion energy, membrane elasticity, geometry, and loading on membrane decohesion from and/or sliding along a substrate.
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62.25.-g Mechanical properties of nanoscale systems
62.20.D- Elasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.F- Deformation and plasticity

Electron spin resonance study of proton-irradiation-induced defects in graphite

Kyu Won Lee, H. Kweon, J. J. Kweon, and Cheol Eui Lee

J. Appl. Phys. 107, 044302 (2010); http://dx.doi.org/10.1063/1.3305334 (3 pages) | Cited 5 times

Online Publication Date: 18 February 2010

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Electron spin resonance measurements of proton-irradiated graphite have revealed detailed nature of proton-irradiation-induced defects. Our results indicate that proton-irradiation creates confined defect regions of a metallic island surrounded by an insulating magnetic region which “isolates” the metallic island inside from the metallic graphite background outside. We have thus come up with a picture of phase separation in proton-irradiated graphite comprising three regions of distinct electrical and magnetic properties.
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76.30.Mi Color centers and other defects
75.50.Cc Other ferromagnetic metals and alloys
61.72.-y Defects and impurities in crystals; microstructure
61.82.-d Radiation effects on specific materials
61.80.Jh Ion radiation effects

Shape effects in tapered metal rods during adiabatic nanofocusing of plasmons

Michael W. Vogel and Dmitri K. Gramotnev

J. Appl. Phys. 107, 044303 (2010); http://dx.doi.org/10.1063/1.3309409 (8 pages) | Cited 8 times

Online Publication Date: 18 February 2010

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We studied adiabatic nanofocusing of strongly localized surface plasmons in tapered metal rods with parabolic perturbations of the conical shape of the taper. Typical plasmon propagation parameters and local field enhancements are determined along the tapered rod, depending on the shape and plasmon frequency (wavelength). In particular, it is demonstrated that metal rods with tapered convex shape are more efficient in achieving larger local field enhancements at the tip, as compared to rods with the concave shape of the taper. This is explained by weaker dissipative losses in tapered rods with convex shape. In addition, plasmon nanofocusing in tapered rods with convex shape is less sensitive to further variations of the shape. As a result, it is demonstrated that tapered rods with the convex shape could be advantageous for practical applications where strong local field enhancement near the tip is required, and/or where metal rods with relatively large dissipation are used. The obtained results will be important for optimization and practical fabrication of efficient nanofocusing structures using tapered metal rods.
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78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
73.22.Lp Collective excitations

Tunneling between carbon nanofiber and gold electrodes

Toshishige Yamada, Tsutomu Saito, Makoto Suzuki, Patrick Wilhite, Xuhui Sun, Navid Akhavantafti, Drazen Fabris, and Cary Y. Yang

J. Appl. Phys. 107, 044304 (2010); http://dx.doi.org/10.1063/1.3295901 (5 pages) | Cited 3 times

Online Publication Date: 18 February 2010

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In a carbon nanofiber (CNF)-metal system such as a bridge between two gold electrodes, passing high current (current stressing) reduces the total resistance of the system (CNF resistance RCNF plus contact resistance Rc) by orders of magnitude. The role of current stressing is modeled as a reduction in the interfacial tunneling gap with transport characteristics attributed to tunneling between Au and CNF. The model predicts a reduction in Rc and gradual disappearance of the nonlinearity in the current-voltage (I-V) characteristics as Rc decreases. These results are consistent with measured I-V behavior.
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
73.40.Gk Tunneling
73.40.Cg Contact resistance, contact potential
82.45.Fk Electrodes

Quantitative assessment of sample stiffness and sliding friction from force curves in atomic force microscopy

Jon R. Pratt, Gordon A. Shaw, Lee Kumanchik, and Nancy A. Burnham

J. Appl. Phys. 107, 044305 (2010); http://dx.doi.org/10.1063/1.3284957 (6 pages) | Cited 6 times

Online Publication Date: 18 February 2010

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It has long been recognized that the angular deflection of an atomic force microscope (AFM) cantilever under “normal” loading conditions can be profoundly influenced by the friction between the tip and the surface. It is shown here that a remarkably quantifiable hysteresis occurs in the slope of loading curves whenever the normal flexural stiffness of the AFM cantilever is greater than that of the sample. This situation arises naturally in cantilever-on-cantilever calibration, but also when trying to measure the stiffness of nanomechanical devices or test structures, or when probing any type of surface or structure that is much more compliant along the surface normal than in transverse directions. Expressions and techniques for evaluating the coefficient of sliding friction between the cantilever tip and sample from normal force curves, as well as relations for determining the stiffness of a mechanically compliant specimen are presented. The model is experimentally supported by the results of cantilever-on-cantilever spring constant calibrations. The cantilever spring constants determined here agree with the values determined using the NIST electrostatic force balance within the limits of the largest uncertainty component, which had a relative value of less than 2.5%. This points the way for quantitative testing of micromechanical and nanomechanical components, more accurate calibration of AFM force, and provides nanotribologists access to information about contact friction from normal force curves.
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07.79.Lh Atomic force microscopes
06.20.fb Standards and calibration
46.55.+d Tribology and mechanical contacts
46.25.-y Static elasticity

On the measured current in electrospinning

P. K. Bhattacharjee, T. M. Schneider, M. P. Brenner, G. H. McKinley, and G. C. Rutledge

J. Appl. Phys. 107, 044306 (2010); http://dx.doi.org/10.1063/1.3277018 (7 pages) | Cited 5 times

Online Publication Date: 19 February 2010

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The origin and scaling of the current measured during steady electrospinning of polymer solutions in organic solvents are considered. For a specified electric field strength E, flow rate Q, and conductivity K, the total measured current is shown empirically to scale as Itotal ∼ EQ0.5K0.4, for a wide variety of polymer solutions with different electrical conductivities. It is also shown that Itotal is composed of two distinct components: one that varies linearly with E, and another that is independent of E, but varies with the conductivity K of the fluid and the flow rate Q. The experimental evidence suggests that the latter component arises due to a secondary electrospray emanating from the surface of the jet. The consequence of this secondary electrospray mechanism on the final fiber size achieved during the electrospinning process is also discussed.
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61.25.he Polymer solutions
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
81.20.-n Methods of materials synthesis and materials processing
72.80.Le Polymers; organic compounds (including organic semiconductors)

Enhancement of silicon nanostructures generation using dual wavelength double pulse femtosecond laser under ambient condition

M. Sivakumar, Bo Tan, and Krishnan Venkatakrishnan

J. Appl. Phys. 107, 044307 (2010); http://dx.doi.org/10.1063/1.3309422 (3 pages) | Cited 1 time

Online Publication Date: 22 February 2010

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In this study we propose a unique method to increase the weblike silicon nanofibrous structures formation using dual wavelength double pulse femtosecond laser radiation under ambient condition. The augmentation of nanostructures is evidenced from the difference in nanofibrous structure layer thickness. Enhancement in generation is explained through the increase in excited state electrons with double pulse as compared to single pulse. Moreover the absorption characteristic of irradiated surface undergoes significant changes after the first pulse (515 nm) which enhances absorption for the second pulse (1030 nm) and consequently results in an increase in nanostructures.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
81.05.Cy Elemental semiconductors
42.62.-b Laser applications
61.46.-w Structure of nanoscale materials
68.35.bg Semiconductors
81.16.-c Methods of micro- and nanofabrication and processing

In situ optical emission study on the role of C2 in the synthesis of single-walled carbon nanotubes

David Edmond Motaung, Mathew Kisten Moodley, E. Manikandan, and Neil J. Coville

J. Appl. Phys. 107, 044308 (2010); http://dx.doi.org/10.1063/1.3311563 (15 pages) | Cited 7 times

Online Publication Date: 22 February 2010

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In situ optical emission spectroscopy was used to study the temporal and spatial behavior of laser induced plasmas in the laser-furnace synthesis of single-walled carbon nanotubes (SWCNTs). A graphite composite target located within a sealed quartz tube with a chemical stoichiometric composition of 95:4:1 at. wt % of carbon, yttrium, and nickel, respectively, was ablated by a Q-switched Nd:YAG laser delivering colinear, focused laser pulses of 1064 and 532 nm temporarily separated by 20 ns. The ablation process was done at a furnace temperature of 1273 K in a flow of argon gas at either 150 or 200 SCCM (SCCM denotes cubic centimeter per minute at STP). The pressure was varied (100, 400, and 600 Torr) for each gas flow setting. The temporal and spatial behavior of the emission intensity associated with C2 Swan bands (d mathg−a mathu) was investigated and found to be influenced by the pressure and flow rate of the argon gas. At conditions optimal to SWCNT production, a sharp drop in C2 intensity followed by a rise in C2 intensity was observed. The temporal and spatial behavior of the electron density was determined by the Stark broadening profile of the CII emission peak at 283.7 nm and was found to decrease with the adiabatic expansion of the plume. We propose that the sharp drop in C2 intensity and the rise in electron density and electron temperature observed in this study are due to the accompanying rapid nucleation and growth of SWCNTs.
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52.77.Dq Plasma-based ion implantation and deposition
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
81.07.De Nanotubes
81.16.Mk Laser-assisted deposition

Local conductivity and electric field analysis of Ag-based conductive adhesive by transmission electron microscopy

N. Kawamoto, Y. Murakami, and D. Shindo

J. Appl. Phys. 107, 044309 (2010); http://dx.doi.org/10.1063/1.3305637 (6 pages) | Cited 2 times

Online Publication Date: 23 February 2010

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The conduction mechanism in Ag-based conductive adhesive, which has attracted the attention of researchers, has been intensively studied by the in situ observations of transmission electron microscopy (TEM) and by using special methods that employ the use of microprobes; Ag-based conductive adhesive is considered as an alternative to Pb-free solders. A current versus voltage (I-V) curve measured by using a microscope exhibited peculiar fluctuations, which implied an irreversible change in the internal structure of a cured adhesive. TEM observations showed a local shape change in Ag agglomerations, such as the formation of small horns, in a sample that was subjected to a large electric current of 1 μA. Electron holography data, which revealed the inhomogeneous distribution of equipotential lines in the sample, also implied an essential role of morphological change in the conduction properties of Ag-based conductive adhesive.
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85.40.-e Microelectronics: LSI, VLSI, ULSI; integrated circuit fabrication technology

Large-scale graphitic thin films synthesized on Ni and transferred to insulators: Structural and electronic properties

Helin Cao, Qingkai Yu, Robert Colby, Deepak Pandey, C. S. Park, Jie Lian, Dmitry Zemlyanov, Isaac Childres, Vladimir Drachev, Eric A. Stach, Muhammad Hussain, Hao Li, Steven S. Pei, and Yong P. Chen

J. Appl. Phys. 107, 044310 (2010); http://dx.doi.org/10.1063/1.3309018 (7 pages) | Cited 24 times

Online Publication Date: 23 February 2010

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We present a comprehensive study of the structural and electronic properties of ultrathin films containing graphene layers synthesized by chemical vapor deposition based surface segregation on polycrystalline Ni foils then transferred onto insulating SiO2/Si substrates. Films of size up to several mm’s have been synthesized. Structural characterizations by atomic force microscopy, scanning tunneling microscopy, cross-sectional transmission electron microscopy (XTEM), and Raman spectroscopy confirm that such large-scale graphitic thin films (GTF) contain both thick graphite regions and thin regions of few-layer graphene. The films also contain many wrinkles, with sharply-bent tips and dislocations revealed by XTEM, yielding insights on the growth and buckling processes of the GTF. Measurements on mm-scale back-gated transistor devices fabricated from the transferred GTF show ambipolar field effect with resistance modulation ∼ 50% and carrier mobilities reaching ∼ 2000 cm2/V s. We also demonstrate quantum transport of carriers with phase coherence length over 0.2 μm from the observation of two-dimensional weak localization in low temperature magnetotransport measurements. Our results show that despite the nonuniformity and surface roughness, such large-scale, flexible thin films can have electronic properties promising for device applications.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.48.Gh Structure of graphene
78.67.Wj Optical properties of graphene
78.30.Na Fullerenes and related materials
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
73.22.Pr Electronic structure of graphene

Spin glass behavior of mechanically alloyed fcc-(Fe79Mn21)1−xCux (0.00<x<0.30)

M. Mizrahi, A. F. Cabrera, and J. Desimoni

J. Appl. Phys. 107, 044311 (2010); http://dx.doi.org/10.1063/1.3309223 (10 pages)

Online Publication Date: 23 February 2010

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The structural, hyperfine, and magnetic properties of mechanically milled alloys of (Fe79Mn21)1−xCux (x from 0.00 to 0.30) have been characterized by x-ray diffraction (XRD), extended x-ray absorption fine structure spectroscopy (EXAFS), Mössbauer spectroscopy, and ac susceptibility measurements. XRD and EXAFS results indicated that Cu addition favored the formation of a Fe–Mn–Cu random solid solution with fcc structure. The lattice parameter exhibited linear dependence on the Cu content, which is compatible with the larger atomic radii of the Cu atoms when compared with the Mn or the Fe ones. The dependence of Mössbauer hyperfine parameters on the Cu content at room temperature is coherent with the formation of a ternary phase. The ac susceptibility curves jointly with the Mössbauer results are representative of a spin-glass-like system.
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61.66.Dk Alloys
71.70.Jp Nuclear states and interactions
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Lk Spin glasses and other random magnets
75.50.Bb Fe and its alloys
78.70.Dm X-ray absorption spectra

Tunable electrical superlattices in periodically gated bilayer graphene

D. Dragoman, M. Dragoman, and R. Plana

J. Appl. Phys. 107, 044312 (2010); http://dx.doi.org/10.1063/1.3309408 (5 pages) | Cited 1 time

Online Publication Date: 23 February 2010

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The paper demonstrates that a single flake of bilayer graphene patterned with a periodic array of metallic gate electrodes behaves like a programmable superlattice formed by heterostructures of type I, II, or III, depending on the dc gate voltage values. The engineering of the width and position of the band gap in each region is performed only by tuning the dc voltages applied on the gate electrodes. Such a single programmable superlattice on bilayer graphene could replace the existing superlattices that require different semiconductor materials for each heterostructure type.
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73.21.Cd Superlattices
81.07.-b Nanoscale materials and structures: fabrication and characterization
73.61.Wp Fullerenes and related materials

Insights into the reactive ion etching mechanism of nanocrystalline diamond films as a function of film microstructure and the presence of fluorine gas

Ju-Heon Yoon, Wook-Seong Lee, Jong-Keuk Park, Gyu Weon Hwang, Young-Joon Baik, Tae-Yeon Seong, and Jeung-hyun Jeong

J. Appl. Phys. 107, 044313 (2010); http://dx.doi.org/10.1063/1.3309420 (6 pages) | Cited 1 time

Online Publication Date: 23 February 2010

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Inhomogeneous etching of nanocrystalline diamond (NCD) films, which produces nanopillars during reactive ion etching process, is problematic to the microfabrication of NCD films for the sensor and actuator applications. Thus, its origin was investigated for various initial microstructures of the NCD films, SF6/O2 gas ratios during etching, and plasma powers. The etched NCD film surface roughness became more pronounced (leading to larger pillar diameters and heights) for larger initial microstructural features (larger grain and cluster sizes), particularly at low plasma powers. The surface roughening was significantly reduced with the addition of SF6, almost disappearing at SF6/O2 of 5% to 10%. These results indicate that the etch rate was locally enhanced at the interfaces between grains or clusters, and the etch rate disparity between intragranular and intergranular (or cluster) carbons increased with decreasing ion energy, implying a chemical reaction rate-limited etching mechanism. The role of SF6 could be explained to reduce the energy barrier for the chemical reaction of intragranular carbons. Here we suggest that the etching rate is limited by an energy barrier that could be reduced by defect generation during ion bombardment or by catalytic radicals.
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81.65.Cf Surface cleaning, etching, patterning
52.77.Bn Etching and cleaning
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.55.-a Thin film structure and morphology
61.80.Jh Ion radiation effects
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Thermal characterization of the SiO2-Ge2Sb2Te5 interface from room temperature up to 400 °C

J.-L. Battaglia, A. Kusiak, V. Schick, A. Cappella, C. Wiemer, M. Longo, and E. Varesi

J. Appl. Phys. 107, 044314 (2010); http://dx.doi.org/10.1063/1.3284084 (6 pages) | Cited 6 times

Online Publication Date: 24 February 2010

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The thermal conductivity of Ge2Sb2Te5 (GST) layers, as well as the thermal boundary resistance at the interface between the GST and amorphous SiO2, was measured using a photothermal radiometry experiment. The two phase changes in the Ge2Sb2Te5 were retrieved, starting from the amorphous and sweeping to the face centered cubic (fcc) crystalline state at 130 °C and then to the hexagonal crystalline phase (hcp) at 310 °C. The thermal conductivity resulted to be constant in the amorphous phase, whereas it evolved between the two crystalline states. The thermal boundary resistance at the GST-SiO2 interface was estimated to be higher for the hcp phase than for the amorphous and fcc ones.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
64.70.K- Solid-solid transitions
61.66.-f Structure of specific crystalline solids

Ag dendritic nanostructures for rapid detection of polychlorinated biphenyls based on surface-enhanced Raman scattering effect

Yajun Yang and Guowen Meng

J. Appl. Phys. 107, 044315 (2010); http://dx.doi.org/10.1063/1.3298473 (5 pages) | Cited 20 times

Online Publication Date: 24 February 2010

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We report a facile and efficient synthetic route for Ag dendritic nanostructures on Si wafer via an electroless deposition process. The formation of the Ag dendritic nanostructures is based on a self-assembled localized microscopic electrochemical cell model. These Ag dendritic nanostructures have exhibited very strong surface-enhanced Raman scattering (SERS) effect using rhodamine 6G as probe molecules, and have been used as SERS substrate for detection of low concentration polychlorinated biphenyl-77 with fast time response. The Ag dendritic nanostructures reported here have potentials as SERS substrates for fast detecting other polychlorobiphenyls.
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61.46.-w Structure of nanoscale materials
81.15.Pq Electrodeposition, electroplating
81.30.-t Phase diagrams and microstructures developed by solidification and solid-solid phase transformations
78.40.Kc Metals, semimetals, and alloys

Spatially correlated erbium and Si nanocrystals in coimplanted SiO2 after a single high temperature anneal

Iain F. Crowe, Reza J. Kashtiban, Ben Sherliker, Ursel Bangert, Matthew P. Halsall, Andrew P. Knights, and Russell M. Gwilliam

J. Appl. Phys. 107, 044316 (2010); http://dx.doi.org/10.1063/1.3294645 (6 pages) | Cited 3 times

Online Publication Date: 25 February 2010

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We present a study of silicon (Si) and erbium (Er) coimplanted silica (SiO2) in which we observe, by combining high resolution scanning transmission electron microscopy and selective electron energy loss spectroscopy (EELS), a high spatial correlation between silicon nanocrystals (Si-NCs), Er, and oxygen (O) after a single high temperature (1100 °C) anneal. The observation of a spatial overlap of the EELS chemical maps of dark field (DF) images at the Er N4,5, Si L2,3, and O K edges is concomitant with an intense room temperature infrared luminescence around 1534 nm. We suggest that these observations correspond to Er–O complexes within an amorphous silicon (a-Si) shell at the Si-NC/SiO2 interface. The presence of a crystalline phase at the Si-NC center, verified by high resolution electron micrographs and DF diffraction contrast images and the low solubility of Er in crystalline Si (c-Si) would tend to suggest a preferential Er agglomeration toward the Si-NC/SiO2 interface during formation, particularly when high concentrations of both Si and Er are obtained in a narrow region of the SiO2 after coimplantation. The absence of narrow Stark related features in the Er emission spectrum at low temperature and an inhomogeneous broadening with increasing temperature, which are characteristic of Er confined by an amorphous, rather than a crystalline host further support these hypotheses. After comparing the luminescence to that from a SiO2:Er control sample prepared in exactly the same manner but without Si-NCs, we find that, despite the observed spatial correlation, only a small fraction ( ∼ 7%) of the Er are sensitized by the Si-NCs. We ascribe this low fraction to a combination of low sensitizer (Si-NC) density and Auger-type losses arising principally from Er ion-ion interactions.
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78.55.Qr Amorphous materials; glasses and other disordered solids
81.40.Gh Other heat and thermomechanical treatments
79.20.Uv Electron energy loss spectroscopy

Structural investigations and magnetic properties of sol-gel Ni0.5Zn0.5Fe2O4 thin films for microwave heating

Pengzhao Gao, Evgeny V. Rebrov, Tiny M. W. G. M. Verhoeven, Jaap C. Schouten, Richard Kleismit, Gregory Kozlowski, John Cetnar, Zafer Turgut, and Guru Subramanyam

J. Appl. Phys. 107, 044317 (2010); http://dx.doi.org/10.1063/1.3309767 (8 pages) | Cited 5 times

Online Publication Date: 26 February 2010

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Nanocrystalline Ni0.5Zn0.5Fe2O4 thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology, magnetic, and microwave absorption properties of the films calcined in the 673–1073 K range were studied with x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, vibrating sample magnetometry, and evanescent microwave microscopy. All films were uniform without microcracks. Increasing the calcination temperature from 873 to 1073 K and time from 1 to 3 h resulted in an increase of the grain size from 12 to 27 nm. The saturation and remnant magnetization increased with increasing the grain size, while the coercivity demonstrated a maximum near a critical grain size of 21 nm due to the transition from monodomain to multidomain behavior. The complex permittivity of the Ni–Zn ferrite films was measured in the frequency range of 2–15 GHz. The heating behavior was studied in a multimode microwave cavity at 2.4 GHz. The highest microwave heating rate in the temperature range of 315–355 K was observed in the film close to the critical grain size.
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75.70.-i Magnetic properties of thin films, surfaces, and interfaces
81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Vv High coercivity materials
77.22.Ch Permittivity (dielectric function)
77.55.-g Dielectric thin films
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
81.40.Gh Other heat and thermomechanical treatments
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