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18 Oct 2010

Volume 97, Issue 16, Articles (16xxxx)

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Appl. Phys. Lett. 97, 162901 (2010); http://dx.doi.org/10.1063/1.3501139 (3 pages)

Wei-Feng Rao, Ke-Wei Xiao, Tian-Le Cheng, Jie E. Zhou, and Yu U. Wang
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Modified, semiconducting graphene in contact with a metal: Characterization of the Schottky diode

Amirhasan Nourbakhsh, Mirco Cantoro, Afshin Hadipour, Tom Vosch, Marleen H. van der Veen, Marc M. Heyns, Bert F. Sels, and Stefan De Gendt

Appl. Phys. Lett. 97, 163101 (2010); http://dx.doi.org/10.1063/1.3495777 (3 pages) | Cited 3 times

Online Publication Date: 18 October 2010

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In this paper, we report the fabrication and characterization of Schottky rectifying junctions between semiconducting, modified single-layer graphene and a metal. The pristine, semimetallic behavior of graphene is altered by controlled exposure to an oxygen plasma, resulting in the opening of an optical band gap as shown by photoluminescence spectroscopy. The occurrence of a Schottky barrier between semiconducting graphene and metals with different work functions (Al, Cr, Pd, and Yb) is investigated by electrically characterizing the as-fabricated junctions. The rectifying properties of our Schottky diodes show the potential of semiconducting, modified graphene as building block of elementary logic circuits.
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85.30.Kk Junction diodes
81.05.ue Graphene
81.05.Cy Elemental semiconductors
73.40.Ns Metal-nonmetal contacts
73.40.Ei Rectification
73.30.+y Surface double layers, Schottky barriers, and work functions

Metamaterial high pass filter based on periodic wire arrays of multiwalled carbon nanotubes

Haider Butt, Qing Dai, Petros Farah, Tim Butler, Timothy D. Wilkinson, Jeremy J. Baumberg, and Gehan A. J. Amaratunga

Appl. Phys. Lett. 97, 163102 (2010); http://dx.doi.org/10.1063/1.3491840 (3 pages) | Cited 3 times

Online Publication Date: 18 October 2010

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In this manuscript, we demonstrate metamaterials based on two-dimensional high density arrays of metallic multiwalled carbon nanotubes. They demonstrate a cutoff response toward electromagnetic waves and can be utilized for filtering applications. The plasma frequency, where the metamaterial displayed a sharp change in the reflection and transmission, depends on the geometry of their two-dimensional cubic lattice. A plasma frequency in the near infrared region of 1.5 μm was calculated numerically, for an array consisting of multiwalled nanotubes, having radius of 50 nm and lattice constant of 400 nm. Reflection experiments conducted on the nanoscale structures were in excellent agreement with numerical calculations.
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42.79.Ci Filters, zone plates, and polarizers
42.70.-a Optical materials

Femtosecond UV-pump/visible-probe measurements of carrier dynamics in stacked graphene films

Jingzhi Shang, Zhiqiang Luo, Chunxiao Cong, Jianyi Lin, Ting Yu, and Gagik G. Gurzadyan

Appl. Phys. Lett. 97, 163103 (2010); http://dx.doi.org/10.1063/1.3504704 (3 pages) | Cited 2 times

Online Publication Date: 18 October 2010

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The transient differential transmission (ΔT/T) spectra of graphene were obtained from 380 (3.3 eV) to 670 nm (1.9 eV). The intraband carrier equilibration by carrier–carrier scattering occurred within 60 fs. The subsequent carrier relaxation process was governed by carrier-optical phonon scattering and had linear dependence on the probe photon energy (Epr); lifetimes ranged from 180 to 90 fs for Epr from 2.1 to 2.8 eV. Negative ΔT/T signals in kinetic curves were discussed and assigned to thermal diffusion and shrinkage of band separation caused by lattice heating.
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78.66.Tr Fullerenes and related materials
78.40.Ri Fullerenes and related materials
66.30.Xj Thermal diffusivity
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Single-material multilayer with enhanced photoactivity

Gisia Beydaghyan, Mathieu Boudreau, Jason Riordon, Alain Haché, and P. V. Ashrit

Appl. Phys. Lett. 97, 163104 (2010); http://dx.doi.org/10.1063/1.3502480 (3 pages)

Online Publication Date: 18 October 2010

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Molybdenum trioxide (MoO3) patterned at the nanometer scale is combined with the same material in its bulk form to produce Bragg mirrors with enhanced photoactive properties. MoO3 undergoes coloration with exposure to UV light but a multilayer structure which alternates between nanostructured and bulk MoO3 is 2.5 times more effective. Measurements with various multilayer arrangements suggest the proximity of bulk and nanostructured MoO3 favors the photoreaction with structural water. A possible minor contribution from electronic band shifting is also discussed.
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81.16.Rf Micro- and nanoscale pattern formation
82.50.-m Photochemistry
78.40.Ha Other nonmetallic inorganics
68.65.Ac Multilayers

Separation of junction and bundle resistance in single wall carbon nanotube percolation networks by impedance spectroscopy

Matthew P. Garrett, Ilia N. Ivanov, Rosario A. Gerhardt, Alex A. Puretzky, and David B. Geohegan

Appl. Phys. Lett. 97, 163105 (2010); http://dx.doi.org/10.1063/1.3490650 (3 pages) | Cited 1 time

Online Publication Date: 18 October 2010

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Single wall carbon nanotube (SWNT) networks of different loadings were measured by impedance spectroscopy. The resistances of the junctions and bundles have been separated by modeling ac impedance spectroscopy data to an equivalent circuit of two parallel resistance-capacitance elements in series. The junction resistance was found to be 3–3.5 times higher than the bundle resistance. The dc and ac properties of the SWNT networks were found to obey a percolation scaling law, with parameters determined by dispersant type and SWNT purity. The values of the critical exponent in all cases were higher than the expected value of 1.3, which is related to widely distributed bundle and junction conductivities.
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73.63.Fg Nanotubes
81.07.De Nanotubes
82.80.Fk Electrochemical methods
81.16.-c Methods of micro- and nanofabrication and processing

Tuning plasmonic interaction between gold nanorings and a gold film for surface enhanced Raman scattering

Jian Ye (叶坚), Masahiko Shioi, Kristof Lodewijks, Liesbet Lagae, Tatsuro Kawamura, and Pol Van Dorpe

Appl. Phys. Lett. 97, 163106 (2010); http://dx.doi.org/10.1063/1.3504187 (3 pages) | Cited 4 times

Online Publication Date: 18 October 2010

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We investigate the plasmonic properties of gold nanorings in close proximity to a gold film. The rings have been fabricated using nanosphere lithography and are optimized to boost their near-infrared surface enhanced Raman scattering (SERS) effects. A SERS enhancement factor as large as 1.4×107 has been achieved by tuning the separation between the gold nanorings and the gold film. In addition, we have numerically and experimentally demonstrated an enhanced tunability of the plasmon resonance wavelength and a narrowing of the plasmon linewidth for increasing ring-film interaction.
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81.05.Bx Metals, semimetals, and alloys
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
78.66.Bz Metals and metallic alloys
78.30.Er Solid metals and alloys
81.16.Nd Micro- and nanolithography

Selenium adsorbed single wall carbon nanotubes as a potential candidate for nanoscale interconnects

Sridevi Krishnan, Hulusi Yilmaz, Rajasekarakumar Vadapoo, and Carlos Marin

Appl. Phys. Lett. 97, 163107 (2010); http://dx.doi.org/10.1063/1.3499750 (3 pages) | Cited 2 times

Online Publication Date: 19 October 2010

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Using ab initio calculations, we show that atomic selenium (Se) strongly adsorbs to the outer surface of single wall carbon nanotubes (SWCNTs), converting the semiconductor nanotubes to metallic and enhancing the conductance of the metallic and semimetallic ones. Considering the ease of vaporizing and depositing selenium, due to its volatile nature and low melting point, this system could be potentially viable. Thus, selenium adsorption offers a single process for enhancing the conductance of SWCNTs regardless of their chirality making them a promising candidate for nanoscale interconnects applications.
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68.43.Bc Ab initio calculations of adsorbate structure and reactions
68.43.Mn Adsorption kinetics
73.63.Fg Nanotubes
85.40.Ls Metallization, contacts, interconnects; device isolation

Sculpting nanoscale precipitation patterns in nanocomposite thin films via hyperthermal ion deposition

Gintautas Abrasonis, György J. Kovács, Mark D. Tucker, René Heller, Matthias Krause, Mathew C. Guenette, Frans Munnik, Jan Lehmann, Anton Tadich, Bruce C. C. Cowie, Lars Thomsen, Marcela M. M. Bilek, and Wolfhard Möller

Appl. Phys. Lett. 97, 163108 (2010); http://dx.doi.org/10.1063/1.3503967 (3 pages) | Cited 2 times

Online Publication Date: 19 October 2010

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Control of the morphology of self-organized nanostructures is the key issue in bottom-up approaches. Here, morphological transitions of precipitation patterns in C:Cu nanocomposite films are studied. The films have been grown by oblique incidence ionized physical vapor deposition. We show that the ion energy and directionality are transferred into the C–Cu phase separation process resulting in nanopattern formation and tilt. Increasing metal content induces the “tilted”-“lying” transition accompanied with Cu nanoparticle prolate-spherical-oblate shape transformations. The results allow the identification of metal subplantation as the key atomistic mechanism, and demonstrate the possibility to achieve nanoscale sculpting via energetic ion deposition.
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81.16.Rf Micro- and nanoscale pattern formation
68.55.A- Nucleation and growth
81.15.Kk Vapor phase epitaxy; growth from vapor phase

Hierarchical multilevel arrays of self-assembled gold nanoparticles: Control of resistivity-temperature dependence

M. M. A. Yajadda, I. Levchenko, Z. J. Han, and K. Ostrikov

Appl. Phys. Lett. 97, 163109 (2010); http://dx.doi.org/10.1063/1.3504260 (3 pages) | Cited 1 time

Online Publication Date: 19 October 2010

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The possibility to control the electric resistivity-temperature dependence of the nanosized resistive components made using hierarchical multilevel arrays of self-assembled gold nanoparticles prepared by multiple deposition/annealing is demonstrated. It is experimentally shown that the hierarchical three-level patterns, where the nanoparticles of sizes ranging from several nanometers to several tens of nanometer play a competitive roles in the electric conductivity, demonstrate sharp changes in the activation energy. These patterns can be used for the precise tuning of the resistivity-temperature behavior of nanoelectronic components.
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81.07.Bc Nanocrystalline materials
81.16.Dn Self-assembly
81.40.Gh Other heat and thermomechanical treatments
73.63.Bd Nanocrystalline materials
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Gate voltage dependence of weak localization in bilayer graphene

Zhi-Min Liao, Bing-Hong Han, Han-Chun Wu, and Da-Peng Yu

Appl. Phys. Lett. 97, 163110 (2010); http://dx.doi.org/10.1063/1.3505310 (3 pages) | Cited 1 time

Online Publication Date: 20 October 2010

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Weak localization modulated by gate voltage in bilayer graphene was studied experimentally. A transition from weak localization [near the carrier charge neutrality point (CNP)] to weak antilocalization (away from the CNP) was found. The suppressed intervalley scattering due to screening of atomically sharp defects and spin-orbit coupling regulated by gate voltage can explain the experimental results well. Our experimental results confirm the theoretical prediction that the weak localization in bilayer graphene is strongly suppressed by the trigonal warping and it is only present in systems with pronounced intervalley scattering.
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81.05.ue Graphene
73.20.Fz Weak or Anderson localization
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect)
73.23.-b Electronic transport in mesoscopic systems
73.22.Pr Electronic structure of graphene

Gold on graphene as a substrate for surface enhanced Raman scattering study

Yingying Wang, Zhenhua Ni, Hailong Hu, Yufeng Hao, Choun Pei Wong, Ting Yu, John T. L. Thong, and Ze Xiang Shen

Appl. Phys. Lett. 97, 163111 (2010); http://dx.doi.org/10.1063/1.3505335 (3 pages) | Cited 1 time

Online Publication Date: 20 October 2010

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In this paper, we report our study on gold (Au) films with different thicknesses deposited on single layer graphene (SLG) as surface enhanced Raman scattering (SERS) substrates for the characterization of rhodamine (R6G) molecules. We find that an Au film with a thickness of ∼ 7 nm deposited on SLG is an ideal substrate for SERS, giving the strongest Raman signals for the molecules and the weakest photoluminescence (PL) background. While Au films effectively enhance both the Raman and PL signals of molecules, SLG effectively quenches the PL signals from the Au film and molecules. The former is due to the electromagnetic mechanism involved while the latter is due to the strong resonance energy transfer from Au to SLG. Hence, the combination of Au films and SLG can be widely used in the characterization of low concentration molecules with relatively weak Raman signals.
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78.66.Bz Metals and metallic alloys
78.30.Er Solid metals and alloys
78.55.Hx Other solid inorganic materials
42.79.Wc Optical coatings

Narrow bandwidth and highly polarized ratio infrared thermal emitter

Hung-Hsin Chen, Yu-Wei Jiang, Yi-Ting Wu, Pei-En Chang, Yi-Tsung Chang, Hao-Fu Huang, and Si-Chen Lee

Appl. Phys. Lett. 97, 163112 (2010); http://dx.doi.org/10.1063/1.3503631 (3 pages) | Cited 1 time

Online Publication Date: 20 October 2010

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Polarized infrared thermal emitters consisting of a waveguide thermal emitter combined with silver grating structure were studied. For a device containing a perforated silver film under the grating, polarized infrared light was emitted only when the wavelength satisfied the standing wave condition, therefore the broad bandwidth SiO2 phonon vibration modes were suppressed in the thermal radiation spectrum. The polarized ratio with different grating thickness was investigated and the highly polarized ratio up to 0.875 was achieved. This study demonstrated that the integrated structure can be used as a narrow bandwidth and highly polarized ratio infrared light source.
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42.72.Ai Infrared sources
44.40.+a Thermal radiation
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials

Memory effect in cadmium telluride quantum dots doped ferroelectric liquid crystals

A. Kumar, J. Prakash, Mohd Taukeer Khan, S. K. Dhawan, and A. M. Biradar

Appl. Phys. Lett. 97, 163113 (2010); http://dx.doi.org/10.1063/1.3495780 (3 pages) | Cited 6 times

Online Publication Date: 21 October 2010

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A pronounced memory effect has been observed in cadmium telluride quantum dots (CdTe-QDs) doped ferroelectric liquid crystals (FLCs) by using dielectric and electro-optical methods. The memory effect has been attributed to the charge storage on the CdTe-QDs upon the application of dc bias across the sample cell. The FLC molecules remain in the switched state in vicinity of the charge stored on QDs even after removal of bias. It has been observed that the memory effect depends on doping concentrations of CdTe-QDs and the FLC material used.
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77.84.Nh Liquids, emulsions, and suspensions; liquid crystals
78.20.Jq Electro-optical effects
78.15.+e Optical properties of fluid materials, supercritical fluids and liquid crystals
61.72.up Other materials
42.70.Df Liquid crystals
61.30.-v Liquid crystals

Hydrogen compounds of group-IV nanosheets

L. C. Lew Yan Voon, E. Sandberg, R. S. Aga, and A. A. Farajian

Appl. Phys. Lett. 97, 163114 (2010); http://dx.doi.org/10.1063/1.3495786 (3 pages) | Cited 6 times

Online Publication Date: 21 October 2010

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The structural and electronic properties of the hydrides of silicene and germanene have been studied using ab initio calculations. The trend for the M–H (M = C, Si, and Ge) bond lengths, and corresponding bond energies, is consistent with the atomic size trend, and comparable to those of MH4 hydrides. Band structures were also obtained for the buckled configuration, which is the stable form for both silicene and germanene. Upon hydrogenation, both silicane (indirect gap) and germanane (direct gap) are semiconducting.
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61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
73.22.-f Electronic structure of nanoscale materials and related systems
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Gap-mode plasmonic nanocavity

Kasey J. Russell and Evelyn L. Hu

Appl. Phys. Lett. 97, 163115 (2010); http://dx.doi.org/10.1063/1.3505154 (3 pages) | Cited 1 time

Online Publication Date: 21 October 2010

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Here we describe the fabrication and characterization of a plasmonic nanocavity formed in the narrow gap between a Ag nanowire and a flat Ag substrate. The fluorescence spectrum of nanocrystals within the gap was strongly modified by the cavity modes, showing peaks of position and width (Q ∼ 30–60) in quantitative agreement with numerical calculations. At gap spacings of ∼ 15 nm, the noncavity background fluorescence is largely quenched by the Ag substrate, while the modal fluorescence remains strong, indicating that gap-type structures are more robust to fluorescence quenching.
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81.07.Gf Nanowires
78.67.Uh Nanowires
78.55.Hx Other solid inorganic materials
73.22.Lp Collective excitations

Highly specular carbon nanotube absorbers

X. J. Wang, L. P. Wang, O. S. Adewuyi, B. A. Cola, and Z. M. Zhang

Appl. Phys. Lett. 97, 163116 (2010); http://dx.doi.org/10.1063/1.3502597 (3 pages) | Cited 2 times

Online Publication Date: 21 October 2010

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Specular black materials have important applications, such as in absolute cryogenic radiometers, space-borne spectroradiometers, and some energy conversion devices. While vertically aligned carbon nanotubes (VACNT) can have close-to-unity absorptance, so far the reported reflection has been essentially diffuse. This letter describes a highly specular black absorber made of VACNT. Both the bidirectional reflectance distribution function and specular reflectance were measured at the wavelength λ = 635 nm using a laser scatterometer. The ordinary and extraordinary optical constants were obtained by fitting the specular reflectance, calculated from modified reflectance formulae for light incident from air to a uniaxial medium, considering surface roughness. Furthermore, the absorptance at λ = 635 nm was shown to be 0.994±0.002, based on the measured directional-hemispherical reflectance.
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78.67.Ch Nanotubes
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
68.37.Ps Atomic force microscopy (AFM)
78.40.Ri Fullerenes and related materials
61.46.Fg Nanotubes
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Structural and charge trapping properties of two bilayer (Ge+SiO2)/SiO2 films deposited on rippled substrate

M. Buljan, J. Grenzer, V. Holý, N. Radić, T. Mišić-Radić, S. Levichev, S. Bernstorff, B. Pivac, and I. Capan

Appl. Phys. Lett. 97, 163117 (2010); http://dx.doi.org/10.1063/1.3504249 (3 pages)

Online Publication Date: 21 October 2010

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We report on structural properties and charge trapping in [(Ge+SiO2)/SiO2]×2 films deposited by magnetron sputtering on a periodically corrugated-rippled substrate and annealed in vacuum and forming gas. The rippled substrate caused a self-ordered growth of Ge quantum dots, while annealing in different environments enabled us to separate charge trapping in quantum dots from the trapping at the dot-matrix and matrix-substrate interfaces. We show that the charge trapping occurs mainly in Ge quantum dots in the films annealed in the forming gas, while Si–SiO2 interface trapping is dominant for the vacuum annealed films.
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68.65.Hb Quantum dots (patterned in quantum wells)
73.63.Kv Quantum dots
73.40.-c Electronic transport in interface structures
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
61.72.Cc Kinetics of defect formation and annealing
81.15.Cd Deposition by sputtering
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