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20 Aug 2012

Volume 101, Issue 8, Articles (08xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 101, 081102 (2012); http://dx.doi.org/10.1063/1.4745791 (3 pages)

M. K. Wu, M. Feng, and N. Holonyak, Jr.
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Conductance fluctuations as a function of sliding motion in bilayer graphene nanoribbon junction: A first-principles investigation

Jiming Zheng, Ping Guo, Zhaoyu Ren, Zhenyi Jiang, Jintao Bai, and Zhiyong Zhang

Appl. Phys. Lett. 101, 083101 (2012); http://dx.doi.org/10.1063/1.4739838 (4 pages) | Cited 1 time

Online Publication Date: 20 August 2012

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A bilayer graphene nanoribbon based strain sensor is designed and investigated via first principles simulations. The interlayer sliding induces dramatically oscillation of current in order of several magnitudes. Electronic structure analysis attributes this oscillation of current to periodic variation of HOMO-LUMO gap which comes from the periodic change of interlayer stacking configuration. This kind of behaviors could potentially be implemented in future strain sensor and other nanoelectronic devices.
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85.65.+h Molecular electronic devices
85.35.-p Nanoelectronic devices

Electrical and lattice vibrational behaviors of graphene devices on flexible substrate under small mechanical strain

Yun-Hi Lee and Yoon-Joong Kim

Appl. Phys. Lett. 101, 083102 (2012); http://dx.doi.org/10.1063/1.4746285 (4 pages)

Online Publication Date: 20 August 2012

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We present systematic experimental study on the electrical response and two-phonon Raman scattering mode of graphene under small uniaxial strain. The graphene, which was initially grown by chemical vapor deposition, was transferred to a transparent-flexible-polyethylene-terephthalate substrate. It was found that the electrical resistance increases as the stain is increased after a slight decrease in very small strain regimes of <0.20%. This is due to a relaxation of intrinsic ripples created during the transfer of the graphene to the polyethylene-terephthalate substrate. The gauge factor in the linear response regime was found to be about 22. Also, the 2D Raman bands of the strained graphene showed a distinct red-shift of −37 cm−1 per 1% strain for the 2D+ mode and −46 cm−1 per 1% strain for the 2D− mode. Finally, we determined the Gruneisen parameters of γ2Δ+ ∼ 2.05 and γ2Δ−∼ 2.55 for the phonons in free-standing graphene without a substrate. Our results provide electro-mechanical parameters for graphene-based flexible devices and show the potential of graphene for measuring strain in future flexible electronics.
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85.65.+h Molecular electronic devices

Role of ion density in growth, transport, and morphology of nanoparticles generated in plasmas

Kil Byoung Chai and Wonho Choe

Appl. Phys. Lett. 101, 083103 (2012); http://dx.doi.org/10.1063/1.4746396 (4 pages)

Online Publication Date: 20 August 2012

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Spatial distribution, growth, and morphology of the nanoparticle were investigated in the plasmas with relatively low and high ion densities. Our experimental results reveal that cauliflower-shaped amorphous nanoparticles are dominantly distributed throughout the entire plasma in the low ion density plasma while spherical crystalline particles are spread near the plasma edge in the high ion density plasma. Only agglomeration growth step of the nanoparticles was observed without molecular accretion growth step in the high density plasma. Based on the experimental and numerical results, the role of ion density in the growth mechanism and transport of the nanoparticles is discussed.
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81.16.Rf Micro- and nanoscale pattern formation
52.77.-j Plasma applications
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Raman spectroscopy as a probe for the coupling of light into ensembles of sub-wavelength-sized nanowires

C. Pfüller, M. Ramsteiner, O. Brandt, F. Grosse, A. Rathsfeld, G. Schmidt, L. Geelhaar, and H. Riechert

Appl. Phys. Lett. 101, 083104 (2012); http://dx.doi.org/10.1063/1.4747208 (4 pages)

Online Publication Date: 21 August 2012

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We use Raman scattering to investigate how light penetrates into and escapes out of GaN nanowires with diameters and interspacings smaller than the effective wavelength of the light. Quantitatively, the efficiencies for light coupling into and out of the nanowires can be explained by considering the nanowire ensemble as a homogeneous effective medium. The observed phonon modes in the Raman spectra, however, suggest that light enters the nanowires to a considerable extent through their side facets even for normal incidence. This finding is supported by rigorous calculations of the spatially distributed light intensity within a periodic nanowire structure.
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68.65.La Quantum wires (patterned in quantum wells)
78.30.Fs III-V and II-VI semiconductors
81.05.Ea III-V semiconductors
63.22.Gh Nanotubes and nanowires
81.07.Gf Nanowires

Anderson localization in strongly coupled gold-nanoparticle assemblies near the metal–insulator transition

Cheng-Wei Jiang, I-Chi Ni, Shien-Der Tzeng, and Watson Kuo

Appl. Phys. Lett. 101, 083105 (2012); http://dx.doi.org/10.1063/1.4747322 (4 pages)

Online Publication Date: 21 August 2012

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An experimental study was made to explore the charge transport properties of a two-dimensional network of self-assembled gold nanoparticles, which were prepared near the metal-insulator transition. The insulating devices demonstrated single-charge tunneling and resonant tunneling at mK temperatures. A magnetic field perpendicular to the substrate could suppress the Coulomb oscillations, suggesting that the charge interactions were due to dynamical charge inhomogeneities, rather than single-nanoparticle charging effects.
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71.30.+h Metal-insulator transitions and other electronic transitions
73.20.Fz Weak or Anderson localization
73.40.Gk Tunneling
73.61.At Metal and metallic alloys
81.16.Dn Self-assembly

Metamaterial filter for the near-visible spectrum

Haider Butt, Qing Dai, Niraj N. Lal, Timothy D. Wilkinson, Jeremy J. Baumberg, and Gehan A. J. Amaratunga

Appl. Phys. Lett. 101, 083106 (2012); http://dx.doi.org/10.1063/1.4747323 (4 pages)

Online Publication Date: 21 August 2012

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We demonstrate metamaterials operating in the near-visible regime based on two-dimensional arrays of gold-coated silicon nanopillars. The nanopillar arrays demonstrate a cutoff response at the metamaterial plasma frequency in accordance with theory and can be utilized for filtering applications. A plasma frequency in the near visible region of λ = 1 μm is calculated numerically for an array with a lattice constant of 300 nm and wire radius of 50 nm, with reflection measurements in agreement with numerical calculations. Such structures can be utilized for achieving negative-index based metamaterials for the visible spectrum.
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42.79.Ci Filters, zone plates, and polarizers
42.70.-a Optical materials

Conductivity mapping of nanoparticles by torsional resonance tunneling atomic force microscopy

C. Prastani, A. Vetushka, A. Fejfar, M. Nanu, D. Nanu, J. K. Rath, and R. E. I. Schropp

Appl. Phys. Lett. 101, 083107 (2012); http://dx.doi.org/10.1063/1.4744601 (4 pages) | Cited 1 time

Online Publication Date: 21 August 2012

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In this paper, torsional resonance tunneling mode atomic force microscopy is used to study the conductivity of nanoparticles. SnS nanoparticles capped with trioctylphosphine oxide (TOPO) and with In2S3 shell are analyzed. This contactless technique allows carrying out measurements on nanoparticles without destroying them and to obtain simultaneously topography and conductivity maps. This made it possible to achieve complete characterization of individual particles in a single measurement. The results demonstrate that the particles have conductive properties. The results have also showed that the TOPO capping layer may hinder tunneling currents, therefore should be avoided when performing these measurements.
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72.80.Jc Other crystalline inorganic semiconductors
73.40.Gk Tunneling
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
68.37.Ps Atomic force microscopy (AFM)

Role of phonon anharmonicity in time-domain thermoreflectance measurements

Carolina Abs da Cruz, Wu Li, Nebil A. Katcho, and Natalio Mingo

Appl. Phys. Lett. 101, 083108 (2012); http://dx.doi.org/10.1063/1.4746275 (4 pages)

Online Publication Date: 21 August 2012

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Using ab-initio calculations, we argue that the apparent frequency dependence of the thermal conductivity measured by time-domain thermoreflectance (TDTR) is majorly determined by the balance between harmonic (elastic) and anharmonic (inelastic) phonon channels, and not by ballistic phonons. The match between this theory and experiment is closer than that obtained with previous models. Concrete trends in frequency and temperature dependence are predicted. Reinterpreting frequency modulated time-domain thermoreflectance in terms of elastic vs. inelastic channels explains the markedly different behavior between alloys and non-alloys, and the approach can in principle be applied to other nanostructured material systems.
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78.20.N- Thermo-optic effects
05.70.Ce Thermodynamic functions and equations of state
71.15.-m Methods of electronic structure calculations

Negative permeability with arrays of aperiodic silver nanoclusters

Anurag Agrawal, Wounjhang Park, and Rafael Piestun

Appl. Phys. Lett. 101, 083109 (2012); http://dx.doi.org/10.1063/1.4746753 (4 pages)

Online Publication Date: 21 August 2012

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Materials that exhibit magnetic resonance have recently attracted interest for synthesizing exotic optical properties of metamaterials. However, none of the known naturally occurring materials are magnetic in the optical regime. Here, we present a metamaterial architecture that exhibits strong magnetic resonance at optical frequencies. The building blocks of the structure are aperiodic clusters of silver nanowires that reveal stronger magnetic resonance than their periodic counterparts. A particular realization exhibits more than three times stronger peak magnetic response than an equivalent periodic array. These results suggest that a larger design space is available for the generation of metamaterials.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
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)
78.67.Uh Nanowires
81.07.Gf Nanowires
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
73.21.Hb Quantum wires

Ultra high density three dimensional capacitors based on Si nanowires array grown on a metal layer

P. H. Morel, G. Haberfehlner, D. Lafond, G. Audoit, V. Jousseaume, C. Leroux, M. Fayolle-Lecocq, T. Baron, and T. Ernst

Appl. Phys. Lett. 101, 083110 (2012); http://dx.doi.org/10.1063/1.4746762 (4 pages)

Online Publication Date: 21 August 2012

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We report the fabrication and the characterization of chemical vapor deposition (CVD) grown silicon nanowires capacitors using a complementary-metal-oxide-semiconductor (CMOS) circuit interconnect level compatible process. Silicon nanowires have been grown by CVD on metallic interconnect lines used in today’s CMOS circuits at low temperature (<425 °C) and using copper as catalyst. The nanowire assembly develops a huge surface leading to very high measured capacitance densities reaching 18 μF/cm2, and featuring a ×23 gain when compared to the same structure without nanowires. This opens the path toward embedded capacitances technologies by using bottom-up nanowires.
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84.32.Tt Capacitors
85.40.Ls Metallization, contacts, interconnects; device isolation

Opposite ReD-dependencies of nanofluid (Al2O3) thermal conductivities between heating and cooling modes

Young Su Na, Kenneth David Kihm, and Joon Sik Lee

Appl. Phys. Lett. 101, 083111 (2012); http://dx.doi.org/10.1063/1.4747329 (5 pages) | Cited 1 time

Online Publication Date: 22 August 2012

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The dynamic thermal conductivities of nanofluids (Al2O3) in heating or cooling under fully developed laminar flow conditions show opposite dependence on Reynolds numbers, i.e., the dynamic conductivities under the heating conditions increase with increasing ReD but under the cooling conditions, the dynamic conductivities decrease with increasing ReD. Furthermore, the dynamic conductivities for cooling are higher than those for heating, and the thermal conductivities of stationary nanofluids with uniform distributions fall between these two values, for the entire tested ReD range from 300 to 800. We believe that the main reason for this distinction is because of the drastically different cross-sectional nanoparticle concentration distributions that are in turn attributed to the opposite thermophoretic behavior near the tube wall between heating and cooling. The near-wall nanoparticle concentrations for cooling are substantially higher than those for heating; however, the stationary nanofluid with no thermophoresis maintains its uniform concentration in the middle between the two concentrations.
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66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
81.40.Gh Other heat and thermomechanical treatments

Reversal mechanisms of coupled bi-component magnetic nanostructures

G. Shimon, A. O. Adeyeye, and C. A. Ross

Appl. Phys. Lett. 101, 083112 (2012); http://dx.doi.org/10.1063/1.4747446 (5 pages) | Cited 1 time

Online Publication Date: 22 August 2012

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Magnetostatically coupled bi-component (NiFe/Fe) thin film structures were fabricated using angle deposition and liftoff processes, including bi-component dots consisting of a crescent of Fe adjacent to a lens of NiFe and rectangular rings with two or four NiFe sides plus two Fe sides. The NiFe and Fe regions are self-aligned and are separated by narrow gaps, and the reversal processes of the bi-component structures are governed by magnetostatic coupling between the NiFe and Fe. In the ring structures, the formation of vortex states of specific chirality could be controlled.
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75.75.Cd Fabrication of magnetic nanostructures
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Jk Magnetization reversal mechanisms
75.70.Ak Magnetic properties of monolayers and thin films

Ultra-thin titanium oxide

M. Bareiß, D. Kälblein, C. Jirauschek, A. Exner, I. Pavlichenko, B. Lotsch, U. Zschieschang, H. Klauk, G. Scarpa, B. Fabel, W. Porod, and P. Lugli

Appl. Phys. Lett. 101, 083113 (2012); http://dx.doi.org/10.1063/1.4745651 (4 pages) | Cited 1 time

Online Publication Date: 23 August 2012

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We demonstrate the fabrication of ultra-thin titanium oxide films by plasma-induced surface oxidation. Ellipsometry measurements indicate an oxide thickness of about 2 nm. Electrical characterization was performed on microscale and nanoscale metal-insulator-metal tunneling diodes. Electrical fields up to 22 MV/cm were applied without destroying the titanium oxide films. The current-voltage-characteristic of the diodes are found to be asymmetric with respect to zero bias when employing electrodes with different work functions. The permittivity of the ultra-thin titanium oxide was determined to be less than 6, which is the smallest permittivity that has been reported for titanium oxide.
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85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)
68.55.aj Insulators
68.55.jd Thickness
52.77.Dq Plasma-based ion implantation and deposition
73.40.Rw Metal-insulator-metal structures
77.22.Ch Permittivity (dielectric function)
81.65.Mq Oxidation

Luminescence-based magnetic imaging with scanning x-ray transmission microscopy

C. A. F. Vaz, C. Moutafis, C. Quitmann, and J. Raabe

Appl. Phys. Lett. 101, 083114 (2012); http://dx.doi.org/10.1063/1.4747697 (3 pages) | Cited 1 time

Online Publication Date: 23 August 2012

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We demonstrate the imaging of the magnetic domain configuration of cobalt structures fabricated on MgO(001) using x-ray induced optical luminescence in a scanning transmission microscope. The technique relies on the measurement of the magnetization-dependent x-ray absorption probed by the optical luminescence radiated from the MgO substrate and induced by the x-rays transmitted through the magnetic layer. This method enables the measurement of the electronic and magnetic spectroscopic properties of single crystalline layers and buried heterostructures with nanometer lateral resolution and elemental sensitivity and opens scanning transmission x-ray microscopy to materials which cannot be grown on membranes or as freestanding thin films.
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75.70.Kw Domain structure (including magnetic bubbles and vortices)
07.85.Tt X-ray microscopes
75.60.Ch Domain walls and domain structure
78.70.Dm X-ray absorption spectra
78.70.En X-ray emission spectra and fluorescence
75.70.Ak Magnetic properties of monolayers and thin films

Characterization of thiol-functionalized oligo(phenylene-ethynylene)-protected Au nanoparticles by scanning tunneling microscopy and spectroscopy

Hyunmo Koo, Shinya Kano, Daisuke Tanaka, Masanori Sakamoto, Toshiharu Teranishi, Gyoujin Cho, and Yutaka Majima

Appl. Phys. Lett. 101, 083115 (2012); http://dx.doi.org/10.1063/1.4747720 (5 pages) | Cited 2 times

Online Publication Date: 23 August 2012

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The electrical properties of Au nanoparticles protected by thiol-functionalized oligo(phenylene-ethynylene) (OPE), 1,4-bis-(3-mercapto-phenylethynyl)benzene have been investigated by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The STM and scanning electron microscopy images of chemisorbed OPE-protected Au nanoparticles on Au(111) surface were similar, and the densities were almost identical. OPE-protected Au nanoparticles exhibited stochastic conductance switching behaviors, and current-voltage (I–V) and log I–V characteristics by STS at 100 K showed Coulomb blockade behaviors. The charging energy of Au nanoparticles was as high as 0.57 eV when the core diameter was 2.1 nm. Our results are significant for single-electron transistor memory applications.
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81.16.Rf Micro- and nanoscale pattern formation
72.80.Tm Composite materials
73.23.Hk Coulomb blockade; single-electron tunneling
68.35.bt Other materials
68.43.Mn Adsorption kinetics
68.43.-h Chemisorption/physisorption: adsorbates on surfaces

Optimal electromagnetic-wave absorption by enhanced dipole polarization in Ni/C nanocapsules

Han Wang, Huaihong Guo, Yingying Dai, Dianyu Geng, Zheng Han, Da Li, Teng Yang, Song Ma, Wei Liu, and Zhidong Zhang

Appl. Phys. Lett. 101, 083116 (2012); http://dx.doi.org/10.1063/1.4747811 (4 pages) | Cited 3 times

Online Publication Date: 23 August 2012

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Electromagnetic-wave (EMW) absorption by Ni/C nanocapsules with similar permeability but different permittivity mainly due to differences in the graphite-shell thickness has been investigated. The optimal working frequency could appear at S-band and C-band and considerable strong EMW absorption was achieved. For the optimal Ni/C nanocapsules, a reflection loss exceeding −20 dB was reached from 2.6 to 8.2 GHz with a maximum value of −40 dB at 3 GHz. The improved absorption can be attributed to an optimal electromagnetic match and an enhanced dipole polarization upon increasing of shell thickness.
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61.46.-w Structure of nanoscale materials
41.20.Jb Electromagnetic wave propagation; radiowave propagation
77.22.Ch Permittivity (dielectric function)

Bipolar-unipolar transition in thermospin transport through a graphene-based transistor

Zhiyun Zhao, Xuechao Zhai, and Guojun Jin

Appl. Phys. Lett. 101, 083117 (2012); http://dx.doi.org/10.1063/1.4748110 (5 pages) | Cited 1 time

Online Publication Date: 24 August 2012

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Based on the mean-field Hubbard model, we study the thermally driven spin-polarized transport through a local-gated magnetic zigzag graphene nanoribbon by using the nonequilibrium Green’s function method. The spin currents are tuned by the source temperature, the temperature bias, and the gate voltage. We find this transistor exhibits a transition from the bipolar to unipolar spin transport under associated modulations of thermal bias and gate voltage. It is argued that the result originates from the band selective rule related to parity conservation of wave functions in quantum tunneling. We also find the thermal magnetoresistance of the ribbon between the ferromagnetic excited state and antiferromagnetic ground state could reach up to 105% under a small local gate voltage. This proposed device provides possibility for bettering control of the spin freedom of electrons in graphene materials.
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85.30.Tv Field effect devices
72.25.Dc Spin polarized transport in semiconductors
66.70.Df Metals, alloys, and semiconductors
72.80.Vp Electronic transport in graphene
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Ee Antiferromagnetics

A method to measure specific absorption rate of nanoparticles in colloidal suspension using different configurations of radio-frequency fields

Dhivya Ketharnath, Rohit Pande, Leiming Xie, Srimeenakshi Srinivasan, Biana Godin, and Jarek Wosik

Appl. Phys. Lett. 101, 083118 (2012); http://dx.doi.org/10.1063/1.4748285 (4 pages)

Online Publication Date: 24 August 2012

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We report a method for characterization of the efficiency of radio-frequency (rf) heating of nanoparticles (NPs) suspended in an aqueous medium. Measurements were carried out for water suspended 5 nm superparamagnetic iron-oxide NPs with 30 nm dextran matrix for three different configurations of rf electric and magnetic fields. A 30 MHz high-Q resonator was designed to measure samples placed inside a parallel plate capacitor and solenoid coil with or without an rf electric field shield. All components of rf losses were analyzed and rf electric and magnetic field induced heating of NPs and the dispersion medium was determined and discussed.
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82.70.Kj Emulsions and suspensions
82.70.Dd Colloids
81.07.Bc Nanocrystalline materials
75.50.Tt Fine-particle systems; nanocrystalline materials
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Effect of tip polarity on Kelvin probe force microscopy images of thin insulator CaF2 films on Si(111)

Ayhan Yurtsever, Yoshiaki Sugimoto, Masaki Fukumoto, Masayuki Abe, and Seizo Morita

Appl. Phys. Lett. 101, 083119 (2012); http://dx.doi.org/10.1063/1.4748291 (4 pages)

Online Publication Date: 24 August 2012

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We investigate thin insulating CaF2 films on a Si (111) surface using a combination of noncontact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM). Atomic-scale NC-AFM and KPFM images are obtained in different imaging modes by employing two different tip polarities. The KPFM image contrast and the distance-dependent variation of the local contact potential difference (LCPD) give rise to a tip-polarity-dependent contrast inversion. Ca2+ cations had a higher LCPD contrast than F− anions for a positively terminated tip, while the LCPD provided by a negatively charged tip gave a higher contrast for F− anions. Thus, this result implies that it is essential to determine the tip apex polarity to correctly interpret LCPD signals acquired by KPFM.
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73.40.Cg Contact resistance, contact potential
68.55.at Other materials
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