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17 Sep 2012

Volume 101, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 101, 123901 (2012); http://dx.doi.org/10.1063/1.4751469 (4 pages)

Chin-An Lin, K. P. Huang, S. T. Ho, Mei-Wen Huang, and Jr-Hau He
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High sensitivity SQUID-detection and feedback-cooling of an ultrasoft microcantilever

A. Vinante, A. Kirste, A. den Haan, O. Usenko, G. Wijts, E. Jeffrey, P. Sonin, D. Bouwmeester, and T. H. Oosterkamp

Appl. Phys. Lett. 101, 123101 (2012); http://dx.doi.org/10.1063/1.4752766 (4 pages)

Online Publication Date: 17 September 2012

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We measure the motion of an ultrasoft cantilever, carrying a ferromagnetic particle, by means of a superconducting quantum interference device (SQUID). In our scheme, the cantilever motion modulates the magnetic flux in the SQUID due to the coupling with the magnetic particle. For the cantilever fundamental mode, cooled to temperatures below 100 mK, we achieve a dimensionless coupling factor as large as 0.07, displacement sensitivity of 200 fm/math, and subattonewton force sensitivity. We demonstrate the outstanding combination of very low displacement and force noise by feedback-cooling the cantilever mode to an effective mode temperature of 160 μK.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.10.Cm Micromechanical devices and systems
85.25.Dq Superconducting quantum interference devices (SQUIDs)

Depolarization effect in optical absorption measurements of one- and two-dimensional nanostructures

Lihong H. Herman, Cheol-Joo Kim, Zenghui Wang, Moon-Ho Jo, and Jiwoong Park

Appl. Phys. Lett. 101, 123102 (2012); http://dx.doi.org/10.1063/1.4752889 (4 pages)

Online Publication Date: 17 September 2012

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We measure absolute optical absorption cross-sections of one- (1D) and two-dimensional (2D) nanostructures using a focused laser beam while varying the numerical aperture (NA) of the focusing lens. We find the optical absorption deviates at higher NA. In the high NA regime, absorption by graphene decreases from 2.2% to below 1.8%; for Ge nanowires, it decreases from an expected value by a factor of 1.2. We explain this using the depolarization effect at the focal spot and conclude that these corrections allow for accurate quantitative measurements of optical and optoelectronic processes in 1D or 2D nanostructures.
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78.67.Uh Nanowires
78.67.Wj Optical properties of graphene
81.07.Gf Nanowires
78.40.Fy Semiconductors
78.40.Ri Fullerenes and related materials
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)

Growth processes of lithium titanate thin films deposited by using pulsed laser deposition

Akichika Kumatani, Takeo Ohsawa, Ryota Shimizu, Yoshitaka Takagi, Susumu Shiraki, and Taro Hitosugi

Appl. Phys. Lett. 101, 123103 (2012); http://dx.doi.org/10.1063/1.4752466 (4 pages)

Online Publication Date: 17 September 2012

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We have investigated the pulsed laser deposition (PLD) growth processes of spinel lithium titanates based on the preparation of Li4Ti5O12 and LiTi2O4 from a Li4Ti5O12 target. The Li/Ti atomic ratio of the species arriving at substrate during the deposition was only ∼0.5. The LiTi2O4 epitaxial thin films fabricated on MgAl2O4 (111) substrate exhibited high conductivity at room temperature (∼3.0 × 103 Ω−1 cm−1) and a superconducting transition temperature of ∼12 K. These values are the highest reported for epitaxial thin films. Our results demonstrate the importance of the target composition, providing further insights into the Li-containing metal oxide deposition processes using PLD.
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81.15.Fg Pulsed laser ablation deposition
74.62.Yb Other effects
74.70.-b Superconducting materials other than cuprates
74.78.-w Superconducting films and low-dimensional structures
68.55.-a Thin film structure and morphology

Nanoscale frictional characteristics of graphene nanoribbons

Jiangnan Zhang, Wei Lu, James M. Tour, and Jun Lou

Appl. Phys. Lett. 101, 123104 (2012); http://dx.doi.org/10.1063/1.4752724 (5 pages)

Online Publication Date: 18 September 2012

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We compare nanoscale frictional characteristics of three types of chemically treated graphene nanoribbons (GNRs) via atomic force microscope. Measurements in both ambient and low humidity conditions demonstrate that covalently bonded functional groups have a significant impact on the frictional response of GNRs. The measured tip-sample adhesion forces seem to support the observed friction force dependence on functionalizations, and the friction and adhesion differences could also be attributed to the surface reactivity and hydrophilicity differences induced by different chemical functionalization schemes.
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62.25.-g Mechanical properties of nanoscale systems
81.07.Bc Nanocrystalline materials
62.20.Qp Friction, tribology, and hardness
81.40.Pq Friction, lubrication, and wear
61.48.Gh Structure of graphene
81.05.ue Graphene

Magnetic properties of MoS2: Existence of ferromagnetism

Sefaattin Tongay, Sima S. Varnoosfaderani, Bill R. Appleton, Junqiao Wu, and Arthur F. Hebard

Appl. Phys. Lett. 101, 123105 (2012); http://dx.doi.org/10.1063/1.4753797 (4 pages) | Cited 5 times

Online Publication Date: 18 September 2012

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We report on the magnetic properties of MoS2 measured from room temperature down to 10 K and magnetic fields up to 5 T. We find that single crystals of MoS2 display ferromagnetism superimposed onto large temperature-dependent diamagnetism and have observed that ferromagnetism persists from 10 K up to room temperature. We attribute the existence of ferromagnetism partly to the presence of zigzag edges in the magnetic ground state at the grain boundaries. Since the magnetic measurements are relatively insensitive to the interlayer coupling, these results are expected to be valid in the single layer limit.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.72.Mm Grain and twin boundaries
75.20.Ck Nonmetals
75.50.Dd Nonmetallic ferromagnetic materials

Influence of surface plasmon propagation on leakage radiation microscopy imaging

S. Aberra Guebrou, J. Laverdant, C. Symonds, S. Vignoli, F. Bessueille, and J. Bellessa

Appl. Phys. Lett. 101, 123106 (2012); http://dx.doi.org/10.1063/1.4753809 (4 pages)

Online Publication Date: 18 September 2012

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We study in this Letter, the effect of the surface plasmon (SP) propagation and coherence on the images obtained by leakage radiation microscopy. The studied system is a set of nanocrystals deposited on a thin silver film supporting surface plasmon modes. More than 70% of the emission in this typical system comes from non-local emission. The diameter of the influence circle around the detection point is of the order of magnitude of the plasmon propagation length. We also present an original method to measure the propagation length (Lspp) of surface plasmons in complex systems by a two Young's slits experiment. This method can be useful for complex systems with a very short propagation length.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.21.La Quantum dots
73.22.Lp Collective excitations
81.07.Ta Quantum dots
81.16.-c Methods of micro- and nanofabrication and processing
68.65.Hb Quantum dots (patterned in quantum wells)

Catalyst-free synthesis of reduced graphene oxide–carbon nanotube hybrid materials by acetylene-assisted annealing graphene oxide

Fuchi Liu, Ming Li, Qian Feng, Nujiang Tang, Wei Zhong, Wei Huang, and Youwei Du

Appl. Phys. Lett. 101, 123107 (2012); http://dx.doi.org/10.1063/1.4753928 (3 pages)

Online Publication Date: 18 September 2012

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A facile catalyst-free approach for synthesis of reduced graphene oxide-carbon nanotube (RGO-CNT) hybrid materials was presented by acetylene-assisted annealing graphene oxide. The weight ratio of CNTs to RGO can be adjusted by regulating the annealing temperature, and it can be tuned in a relatively wide range of 0.33 to 7.039. The catalyst-free method opens up the possibility for the synthesis of RGO-CNTs for various applications.
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81.05.uj Diamond/nanocarbon composites
81.07.De Nanotubes
81.40.Gh Other heat and thermomechanical treatments
81.05.ue Graphene

Spectroscopic investigation of nitrogen doped graphene

R. Podila, J. Chacón-Torres, J. T. Spear, T. Pichler, P. Ayala, and A. M. Rao

Appl. Phys. Lett. 101, 123108 (2012); http://dx.doi.org/10.1063/1.4752736 (4 pages) | Cited 1 time

Online Publication Date: 19 September 2012

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Current research efforts are aimed at controlling the electronic properties via doping graphene. Previously, dopant-induced changes in the Fermi velocity were observed to result in an effectively downshifted Raman peak below the G′-band for n-doped carbon nanotubes. However, in the case of N-doped graphene, we find that several Raman features vary depending upon both dopant concentration and its bonding environment. For instance, only pyridinic/pyrrolic dopants were observed to result in intense D/D′-bands with a concomitant downshift in the G′-band. Here, we correlate x-ray photoelectron measurements with Raman spectra to elucidate effects of dopant bonding configuration on vibrational properties of graphene.
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78.67.Wj Optical properties of graphene
61.72.up Other materials
63.22.Rc Phonons in graphene
73.22.Pr Electronic structure of graphene
78.30.Na Fullerenes and related materials
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)

Intrinsic effect of porosity on mechanical and fracture properties of nanoporous ultralow-k dielectrics

K. Vanstreels, C. Wu, P. Verdonck, and M. R. Baklanov

Appl. Phys. Lett. 101, 123109 (2012); http://dx.doi.org/10.1063/1.4753972 (3 pages)

Online Publication Date: 19 September 2012

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The intrinsic effect of porosity on the mechanical and fracture properties of ultralow-k dielectrics is demonstrated using a special curing process that allows a separate control of porosity and matrix properties. The stiffness and fracture energy highly depend on the precise pore structure of the film. A change in relative Young's modulus behavior with open porosity and average pore diameter was found and attributed to the percolation of mesopores.
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77.55.Bh Low-permittivity dielectric films
81.05.Rm Porous materials; granular materials
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.de Elastic moduli
62.20.mm Fracture

Tunable electroluminescence from polymer-passivated 3C-SiC quantum dot thin films

B. Xiao, X. L. Wu, W. Xu, and Paul K. Chu

Appl. Phys. Lett. 101, 123110 (2012); http://dx.doi.org/10.1063/1.4753995 (4 pages)

Online Publication Date: 19 September 2012

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Strong room-temperature photoluminescence in the wavelength range of 400-540 nm is achieved from a thin film containing polymer encapsulated 3C-SiC quantum dots (QDs). The QD surface is completely passivated by the polymer so that the QD film possesses voltage-tunable electroluminescence. The electroluminescence spectrum blue-shifts from 490 to 460 nm when the applied voltage is increased from 5 to 10 V. The tunable electroluminescence is attributed to carrier recombination in the core quantum confinement states as a result of Pool-Frenkel emission. Owing to the environmental friendliness, nontoxicity, easy fabrication, and stability, the polymer passivated 3C-SiC QD thin films have promising applications.
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78.67.Hc Quantum dots
81.65.Rv Passivation
68.55.ag Semiconductors
78.55.Hx Other solid inorganic materials
78.60.Fi Electroluminescence
78.66.Li Other semiconductors

Highly tunable electron transport in epitaxial topological insulator (Bi1-xSbx)2Te3 thin films

Xiaoyue He, Tong Guan, Xiuxia Wang, Baojie Feng, Peng Cheng, Lan Chen, Yongqing Li, and Kehui Wu

Appl. Phys. Lett. 101, 123111 (2012); http://dx.doi.org/10.1063/1.4754108 (5 pages) | Cited 2 times

Online Publication Date: 19 September 2012

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Atomically smooth, single crystalline (Bi1−xSbx)2Te3 films have been grown on SrTiO3(111) substrates by molecular beam epitaxy. A full range of Sb-Bi compositions have been studied in order to obtain the lowest possible bulk conductivity. For the samples with optimized Sb compositions (x = 0.5±0.1), the carrier type can be tuned from n-type to p-type across the whole thickness with the help of a back-gate. Linear magnetoresistance has been observed at gate voltages close to the maximum in the longitudinal resistance of a (Bi0.5Sb0.5)2Te3 sample. These highly tunable (Bi1−xSbx)2Te3 thin films provide an excellent platform to explore the intrinsic transport properties of the three-dimensional topological insulators.
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73.61.Ng Insulators
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.aj Insulators
72.20.My Galvanomagnetic and other magnetotransport effects
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)

Direct actuation of cantilever in aqueous solutions by electrostatic force using high-frequency electric fields

Ken-ichi Umeda, Kei Kobayashi, Kazumi Matsushige, and Hirofumi Yamada

Appl. Phys. Lett. 101, 123112 (2012); http://dx.doi.org/10.1063/1.4754289 (4 pages) | Cited 1 time

Online Publication Date: 19 September 2012

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We recently developed a method to directly actuate a cantilever in aqueous solutions by electrostatic force [K.-I. Umeda et al., Appl. Phys. Express 3, 065205 (2010)]. However, the cantilever was actuated by surface stress in a low frequency regime. We solved this problem by applying amplitude-modulated high-frequency electric fields, which actuates the cantilever solely by electrostatic force. The time variations of the self-oscillation frequency of a cantilever and the Au(111) surface images by frequency-modulation atomic force microscopy using acoustic, photothermal, and the improved electrostatic actuation methods were compared, which demonstrates the advantages of the last method such as stability and simplicity in instrumentation.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.10.Cm Micromechanical devices and systems

Imaging current paths in complex conductors by scanning fluorescence microscopy

L. Aigouy, J. Lesueur, N. Bergeal, and M. Mortier

Appl. Phys. Lett. 101, 123113 (2012); http://dx.doi.org/10.1063/1.4754553 (3 pages)

Online Publication Date: 20 September 2012

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Using a fluorescence-based scanning thermal microscope, we observed the Joule heating of a complex circuit made of submicron-wide rectangular stripes interconnected between them. The thermal images reveal the presence of localized hot spots which allow to visualize the main current paths in the networks. By suppressing a single elementary conductive element in the device, we observed a different hot spot repartition showing the new electrical current paths. The experimental results are in good agreement with simulations and open the way to directly characterize the electrical and thermal behaviour of complex devices and metal-dielectric composites.
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68.37.-d Microscopy of surfaces, interfaces, and thin films
07.79.-v Scanning probe microscopes and components

Study of ferromagnetic transition in Pd nanometer-scale constrictions using a mechanically controllable break junction technique

K. Ienaga, N. Nakashima, Y. Inagaki, H. Tsujii, T. Kimura, and T. Kawae

Appl. Phys. Lett. 101, 123114 (2012); http://dx.doi.org/10.1063/1.4754565 (4 pages) | Cited 2 times

Online Publication Date: 20 September 2012

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We measured the magnetoresistance (MR) of Pd nanometer-scale constrictions while changing the constriction size by stretching the Pd wire using a mechanically controllable break junction technique. In large constrictions, the resistance is constant for |H| ≤ 400 Oe, reflecting bulk Pd paramagnetic properties. As size decreases to ∼ 10 nm, the MR effect with hysteresis is observed with a sweeping field, indicating the ferromagnetic transition in Pd constrictions. We proposed this technique to detect ferromagnetic transitions caused by downsizing in paramagnetic metals and to study magnetic properties after the transitions.
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72.15.Gd Galvanomagnetic and other magnetotransport effects
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Efficient broadband energy transfer via momentum matching at hybrid junctions of guided-waves

Charles Lin, Herman M. K. Wong, Benedict Lau, Mohamed A. Swillam, and Amr S. Helmy

Appl. Phys. Lett. 101, 123115 (2012); http://dx.doi.org/10.1063/1.4753985 (4 pages)

Online Publication Date: 20 September 2012

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Momentum matching at hybrid junctions is examined for efficient broadband energy transfer between internal reflection guided waves and evanescence-based plasmonic-gap guided waves. We demonstrate a nanoscale orthogonal junction coupler between 50 nm air-filled plasmonic slot waveguides (PSWs) and 450 nm silicon rib waveguides. Non-resonant junction coupling efficiency of 50 ± 2 % between 1450 nm and 1650 nm is achieved experimentally and PSW propagation loss is directly measured to be only 2.5 dB/μm. This taperless hybrid junction reduces PSW-based device footprint and enhances device tolerance to temperature and fabrication process variations, serving as a potential platform for hybrid silicon-plasmonic interconnects.
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42.79.Gn Optical waveguides and couplers
42.82.Ds Interconnects, including holographic interconnects

Improving the field emission of carbon nanotubes by lanthanum-hexaboride nano-particles decoration

Monika Kumari, S. Gautam, P. V. Shah, S. Pal, U. S. Ojha, A. Kumar, A. A. Naik, J. S. Rawat, P. K. Chaudhury, Harsh, and R. P. Tandon

Appl. Phys. Lett. 101, 123116 (2012); http://dx.doi.org/10.1063/1.4754110 (5 pages) | Cited 1 time

Online Publication Date: 20 September 2012

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Field emission of carbon nanotubes (CNTs) was remarkably improved by lanthanum-hexaboride (LaB6) nano-particles (NPs) decoration. CNTs were grown on the silicon substrate by chemical vapor deposition. Field emission of bare CNTs and LaB6-NPs decorated CNTs was carried out under same conditions. Physical, morphological, elemental, and graphitic nature changes were ascertained by scanning electron microscope, transmission electron microscope, energy dispersive x-ray spectroscopy, and Raman analysis. LaB6-NPs decorated CNTs show significant reduction of turn-on electric-field from 3.0 to 2.1 V/μm. A remarkable improvement of more than six-fold from 2.05 to 13.19 mA/cm2 at a field of 4.6 V/μm was also observed in the current-density.
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79.70.+q Field emission, ionization, evaporation, and desorption
81.07.De Nanotubes
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.46.Fg Nanotubes
78.30.Hv Other nonmetallic inorganics

Negative and positive photoconductivity modulated by light wavelengths in carbon nanotube film

Jia-Lin Zhu, Guowei Zhang, Jinquan Wei, and Jia-Lin Sun

Appl. Phys. Lett. 101, 123117 (2012); http://dx.doi.org/10.1063/1.4754624 (4 pages)

Online Publication Date: 21 September 2012

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We explore the photoconductive properties of double-walled carbon nanotube film under laser illumination at different wavelengths in both ambient air and vacuum. In our experimental measurements, the photoconductivity of the film shows a strong dependence on the wavelength of light and can be changed from negative to positive by fully removing oxygen. We propose that competition among photoexcitation, photodesorption, and surface plasmon polaritons is responsible for these interesting phenomena. Our results are expected to provide potential applications in the field of nano-optoelectronic sensors based on carbon nanotubes.
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72.40.+w Photoconduction and photovoltaic effects
73.22.Lp Collective excitations
73.50.Pz Photoconduction and photovoltaic effects
73.61.Wp Fullerenes and related materials
73.63.Fg Nanotubes
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
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