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24 May 2010

Volume 96, Issue 21, Articles (21xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 96, 213701 (2010); http://dx.doi.org/10.1063/1.3431628 (3 pages)

Satish Rao, Saurabh Raj, Stefan Balint, Carlota Bardina Fons, Susana Campoy, Montserrat Llagostera, and Dmitri Petrov
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Uniaxial strain modulated band gap of ZnO nanostructures

S. Li, Q. Jiang, and G. W. Yang

Appl. Phys. Lett. 96, 213101 (2010); http://dx.doi.org/10.1063/1.3435479 (3 pages) | Cited 8 times

Online Publication Date: 24 May 2010

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Influence of the uniaxial strain on the electronic structures of ZnO nanowires (NWs) and nanotubes (NTs) has been studied using the density functional theory. It was found that the uniaxial compress strain can lead to a band gap transition from direct to indirect band for both ZnO NWs and NTs. Under the same strain, the phase transition of the thick NW is easier than that of the thin one, and the phase transition of NWs seems easier than that of NTs. These results suggested the uniaxial strain can be used to tune the band structures of ZnO nanostructures, which may be help for design of ZnO-based nanodevices.
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71.20.Nr Semiconductor compounds
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)
64.70.K- Solid-solid transitions
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Enhanced photoluminescence efficiency of mid-infrared InAsSb nanostructures using a carrier blocking layer

W. Lei, H. H. Tan, and C. Jagadish

Appl. Phys. Lett. 96, 213102 (2010); http://dx.doi.org/10.1063/1.3436562 (3 pages) | Cited 1 time

Online Publication Date: 24 May 2010

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This paper presents a study on the emission efficiency enhancement of InAsSb nanostructures using a carrier blocking layer. InP is proposed to serve as the carrier blocking layer to suppress the thermal escape of carriers in InAsSb nanostructures and significantly enhance their emission efficiency at high temperature (good photoluminescence signal even at 330 K). However, this leads to a blueshift in their emission wavelength due to the significantly increased quantum confinement of the nanostructures. By inserting a thin InGaAs layer between InP blocking layer and InAsSb nanostructures, longer emission wavelength can be maintained. This provides an approach to achieve InAsSb nanostructures with both good high-temperature optical characteristics and long emission wavelength, which is very useful for fabricating mid-infrared emitters operating at room temperature.
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78.67.Hc Quantum dots
81.07.Ta Quantum dots
81.16.-c Methods of micro- and nanofabrication and processing
78.55.Cr III-V semiconductors

A symmetry-breaking electromechanical detector

I. Mahboob, C. Froitier, and H. Yamaguchi

Appl. Phys. Lett. 96, 213103 (2010); http://dx.doi.org/10.1063/1.3429589 (3 pages) | Cited 1 time

Online Publication Date: 25 May 2010

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The dynamical double well potential underpinning the stable oscillation phases in an electromechanical parametric resonator is manipulated via a secondary field excitation applied at the natural frequency of the oscillator. This enables symmetry to be lifted in the dynamical potential well and results in the parametric resonator oscillating with a preferred phase. The ability to break symmetry in the dynamical double well potential permits the realization of a symmetry-breaking detector which can resolve resonance frequency (f0) shifts of δf0/f0 ∼ 10−7 in a single-shot measurement.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
84.30.Ng Oscillators, pulse generators, and function generators
77.65.-j Piezoelectricity and electromechanical effects

Surface transfer hole doping of epitaxial graphene using MoO3 thin film

Zhenyu Chen, Iman Santoso, Rui Wang, Lan Fei Xie, Hong Ying Mao, Han Huang, Yu Zhan Wang, Xing Yu Gao, Zhi Kuan Chen, Dongge Ma, Andrew Thye Shen Wee, and Wei Chen

Appl. Phys. Lett. 96, 213104 (2010); http://dx.doi.org/10.1063/1.3441263 (3 pages) | Cited 17 times

Online Publication Date: 25 May 2010

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Synchrotron-based in situ photoelectron spectroscopy investigations demonstrate effective surface transfer p-type doping of epitaxial graphene (EG) thermally grown on 4H–SiC(0001) via the deposition of MoO3 thin film on top. The large work function difference between EG and MoO3 facilitates electron transfer from EG to the MoO3 thin film. This leads to hole accumulation in the EG layer with an areal hole density of about 1.0×1013 cm−2, and places the Fermi level 0.38 eV below the graphene Dirac point.
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72.80.Vp Electronic transport in graphene
61.72.up Other materials
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
68.55.A- Nucleation and growth
79.60.Bm Clean metal, semiconductor, and insulator surfaces
73.40.Cg Contact resistance, contact potential

Flexible room-temperature NO2 gas sensors based on carbon nanotubes/reduced graphene hybrid films

Hu Young Jeong, Dae-Sik Lee, Hong Kyw Choi, Duck Hyun Lee, Ji-Eun Kim, Jeong Yong Lee, Won Jong Lee, Sang Ouk Kim, and Sung-Yool Choi

Appl. Phys. Lett. 96, 213105 (2010); http://dx.doi.org/10.1063/1.3432446 (3 pages) | Cited 34 times

Online Publication Date: 25 May 2010

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We present a flexible room temperature NO2 gas sensor consisting of vertical carbon nanotubes (CNTs)/reduced graphene hybrid film supported by a polyimide substrate. The reduced graphene film alone showed a negligible sensor response, exhibiting abnormal N–P transitions during the initial NO2 injection. A hybrid film, formed by the growth of a vertically aligned CNT array (with CNTs 20 μm in length) on the reduced graphene film surface, exhibited remarkably enhanced sensitivities with weak N–P transitions. The increase in sensitivity was mainly attributed to the high sensitivity of the CNT arrays. The outstanding flexibility of the reduced graphene films ensured stable sensing performances in devices submitted to extreme bending stress.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Nanoscale imaging and control of resistance switching in VO2 at room temperature

Jeehoon Kim, Changhyun Ko, Alex Frenzel, Shriram Ramanathan, and Jennifer E. Hoffman

Appl. Phys. Lett. 96, 213106 (2010); http://dx.doi.org/10.1063/1.3435466 (3 pages) | Cited 16 times

Online Publication Date: 25 May 2010

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We demonstrate controlled local phase switching of a VO2 film using a biased conducting atomic force microscope tip. After application of an initial, higher “training” voltage, the resistance transition is hysteretic with IV loops converging upon repeated voltage sweep. The threshold Vset to initiate the insulator-to-metal transition is on order ∼ 5 V at room temperature, and increases at low temperature. We image large variations in Vset from grain to grain. Our imaging technique opens up the possibility for an understanding of the microscopic mechanism of phase transition in VO2 as well as its potential relevance to solid state devices.
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73.61.Ng Insulators
71.30.+h Metal-insulator transitions and other electronic transitions
72.60.+g Mixed conductivity and conductivity transitions

High-performance organic charge trap flash memory devices based on ink-jet printed 6,13-bis(triisopropylsilylethynyl) pentacene transistors

Young-Su Park, Seungjun Chung, Soo-Jin Kim, Si-Hoon Lyu, Jae-Wan Jang, Soon-Ki Kwon, Yongtaek Hong, and Jang-Sik Lee

Appl. Phys. Lett. 96, 213107 (2010); http://dx.doi.org/10.1063/1.3435470 (3 pages) | Cited 6 times

Online Publication Date: 25 May 2010

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Organic nanofloating gate memory devices were developed based on ink-jet printed 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene thin-film transistors (TFTs) embedding gold nanoparticles. The programming/erasing operations showed that the organic memory devices exhibited good programmable memory characteristics that resulted in a gate-voltage controlled reliable threshold voltage shift of the programmed/erased states. The data retention and endurance measurements also showed the reliable nonvolatile memory properties. Solution processes were used for synthesis of the charge trapping elements and TIPS-pentacene TFTs were made by the ink-jet printing technique at low temperatures. Therefore, these processes can readily be adopted in all-printed organic memory devices on flexible substrates.
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84.30.Sk Pulse and digital circuits

Growth direction determination of a single RuO2 nanowire by polarized Raman spectroscopy

Myung Hwa Kim, Jeong Min Baik, Seung Joon Lee, Hae-Young Shin, Jaeyeon Lee, Seokhyun Yoon, Galen D. Stucky, Martin Moskovits, and Alec M. Wodtke

Appl. Phys. Lett. 96, 213108 (2010); http://dx.doi.org/10.1063/1.3435475 (3 pages) | Cited 2 times

Online Publication Date: 25 May 2010

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The dependence of band intensities in the Raman spectrum of individual single-crystal ruthenium dioxide (RuO2) nanowires on the angle between the plane of polarization of the exciting (and collected) light and the long axis of the nanowire, is shown to be a simple, complementary technique to high resolution transmission electron microscopy (HRTEM) for determining nanowire growth direction. We show that excellent agreement exists between what is observed and what is predicted for the polarization angle dependence of the intensities of the nanowires’ Eg (525 cm−1) and the B2g (714 cm−1) Raman bands, only by assuming that the nanowires grow along the (001) crystallographic direction, as confirmed by HRTEM.
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81.07.Gf Nanowires
68.65.La Quantum wires (patterned in quantum wells)
78.30.Hv Other nonmetallic inorganics
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)

Thermophysical property-related comparison criteria for nanofluid heat transfer enhancement in turbulent flow

W. Yu, D. M. France, E. V. Timofeeva, D. Singh, and J. L. Routbort

Appl. Phys. Lett. 96, 213109 (2010); http://dx.doi.org/10.1063/1.3435487 (3 pages) | Cited 8 times

Online Publication Date: 25 May 2010

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Heat transfer enhancement criteria for nanofluids over their base fluids are presented based on three separate considerations: Reynolds number, flow velocity, and pumping power. Analyses presented show that, among the three comparisons, the constant pumping power comparison is the most unambiguous; the constant flow velocity comparison can be quite reasonable under certain conditions but the constant Reynolds number comparison (the most commonly used in the engineering literature for nanofluids) distorts the physical situation, and therefore, should not be used.
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47.85.Np Fluidics
47.27.te Turbulent convective heat transfer
47.61.Fg Flows in micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS)

Imaging dielectric relaxation in nanostructured polymers by frequency modulation electrostatic force microscopy

C. Riedel, R. Sweeney, N. E. Israeloff, R. Arinero, G. A. Schwartz, A. Alegria, Ph. Tordjeman, and J. Colmenero

Appl. Phys. Lett. 96, 213110 (2010); http://dx.doi.org/10.1063/1.3431288 (3 pages) | Cited 12 times

Online Publication Date: 25 May 2010

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We have developed a method for imaging the temperature-frequency dependence of the dynamics of nanostructured polymer films with spatial resolution. This method provides images with dielectric compositional contrast well decoupled from topography. Using frequency-modulation electrostatic-force-microscopy, we probe the local frequency-dependent (0.1–100 Hz) dielectric response through measurement of the amplitude and phase of the force gradient in response to an oscillating applied electric field. When the phase is imaged at fixed frequency, it reveals the spatial variation in dielectric losses, i.e., the spatial variation in molecular/dipolar dynamics, with 40 nm lateral resolution. This is demonstrated by using as a model system; a phase separated polystyrene/polyvinyl-acetate (PVAc) blend. We show that nanoscale dynamic domains of PVAc are clearly identifiable in phase images as those which light-up in a band of temperature, reflecting the variations in the molecular/dipolar dynamics approaching the glass transition temperature of PVAc.
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77.22.Gm Dielectric loss and relaxation
61.41.+e Polymers, elastomers, and plastics
61.43.Bn Structural modeling: serial-addition models, computer simulation
64.70.Nd Structural transitions in nanoscale materials

Defect-induced activation of symmetry forbidden infrared resonances in individual metallic nanorods

F. Neubrech, A. Garcia-Etxarri, D. Weber, J. Bochterle, H. Shen, M. Lamy de la Chapelle, G. W. Bryant, J. Aizpurua, and A. Pucci

Appl. Phys. Lett. 96, 213111 (2010); http://dx.doi.org/10.1063/1.3437093 (3 pages) | Cited 8 times

Online Publication Date: 26 May 2010

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We report on the observation of second-order infrared (IR) plasmon resonances in lithographically prepared gold nanorods investigated by means of far-field microscopic IR spectroscopy. In addition to the fundamental antennalike mode, even and odd higher order resonances are observed under normal incidence of light. The activation of even-order modes under normal incidence is surprising since even orders are dipole-forbidden because of their centrosymmetric charge density oscillation. Performing atomic force microscopy and calculations with the boundary element method, we determine that excitation of even modes is enabled by symmetry breaking by structural deviations of the rods from an ideal, straight shape.
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78.67.Qa Nanorods
73.22.Lp Collective excitations
81.16.Nd Micro- and nanolithography
02.60.Lj Ordinary and partial differential equations; boundary value problems
78.30.Er Solid metals and alloys

Electronic structure study of Li+/OH− modified single-walled carbon nanotubes by soft-x-ray absorption and resonant emission spectroscopy

Jun Zhong, Li Song, Jauwern Chiou, Chungli Dong, Xianqing Liang, Dongliang Chen, Sishen Xie, Way-Faung Pong, Chinglin Chang, Jinghua Guo, and Ziyu Wu

Appl. Phys. Lett. 96, 213112 (2010); http://dx.doi.org/10.1063/1.3441027 (3 pages) | Cited 1 time

Online Publication Date: 27 May 2010

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Li+ and OH− modified single-walled carbon nanotubes (SWNTs) treated with the LiOH water solution have been investigated by soft-x-ray absorption and resonant emission spectroscopy. A reconstruction of the band structure after hydroxyl modification and intensity changes between π and σ states in the resonant emission spectra are presented and discussed. A charge transfer induced valence state near Fermi level has been detected at the resonant excitation energy of 285.5 eV, which indicates the tuning of electronic properties of SWNTs by Li+ adsorption.
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73.22.-f Electronic structure of nanoscale materials and related systems
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
78.55.Ap Elemental semiconductors
73.20.At Surface states, band structure, electron density of states
71.70.-d Level splitting and interactions
68.43.Mn Adsorption kinetics

Ultrahigh secondary electron emission of carbon nanotubes

Jun Luo, Jamie H. Warner, Chaoqun Feng, Yagang Yao, Zhong Jin, Huiliang Wang, Caofeng Pan, Sheng Wang, Leijing Yang, Yan Li, Jin Zhang, Andrew A. R. Watt, Lian-mao Peng, Jing Zhu, and G. Andrew D. Briggs

Appl. Phys. Lett. 96, 213113 (2010); http://dx.doi.org/10.1063/1.3442491 (3 pages) | Cited 9 times

Online Publication Date: 27 May 2010

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The secondary electron emission of the tube bodies of single-walled carbon nanotubes is found to be ultrahigh and comparable with that of diamond, when the nanotubes are connected with electron reservoir. Both of semiconducting and metallic nanotubes possess this property.
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79.20.Hx Electron impact: secondary emission
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems

Comparison of thermal conductivity in nanodot nanocomposites and nanograined nanocomposites

Chanyoung Kang, Hyoungjoon Kim, Sung-Geun Park, and Woochul Kim

Appl. Phys. Lett. 96, 213114 (2010); http://dx.doi.org/10.1063/1.3436568 (3 pages) | Cited 3 times

Online Publication Date: 27 May 2010

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Most recent increases in thermoelectric performance have come by reducing thermal conductivity through nanostructuring. Therefore, current research efforts focus mainly on bulk nanocomposites. We simulated the thermal conductivities of two types of nanocomposites. We nanostructured Tl0.02Pb0.98Te by (i) embedding InSb nanodots in it, creating a nanodot nanocomposite, and (ii) polycrystallizing it, creating a nanograined nanocomposite. The nanograined nanocomposite achieved lower thermal conductivity than did the nanodot nanocomposite due to the ability of the nanosized grains in nanograined nanocomposites to effectively scatter phonons over a wide range of frequencies, as long as the nanograined nanocomposite has sufficiently small grain size.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
68.65.Hb Quantum dots (patterned in quantum wells)

A synthesis route of gold nanoparticles without using a reducing agent

C. Vargas-Hernandez, M. M. Mariscal, R. Esparza, and M. J. Yacaman

Appl. Phys. Lett. 96, 213115 (2010); http://dx.doi.org/10.1063/1.3442479 (3 pages) | Cited 3 times

Online Publication Date: 28 May 2010

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In the present work we show that synthesis of gold nanoparticles (NPs) could be performed by microwave-assisted technique without the need of adding any reducing agent. Only water and the gold salt precursor are necessary to generate the NPs under the influence of microwaves. The produced NPs have been characterized by state-of-art microscopy techniques, like high resolution transmission electron microscopy, scanning electron microscopy, and energy-dispersive x-ray. Theoretical calculations have been performed to support the experimental findings. It is expected that the present work opens routes for synthesis of NPs using green, fast, and safe methods.
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81.16.-c Methods of micro- and nanofabrication and processing
81.07.Bc Nanocrystalline materials

Optical identification of atomically thin dichalcogenide crystals

A Castellanos-Gomez, N Agraït, and G Rubio-Bollinger

Appl. Phys. Lett. 96, 213116 (2010); http://dx.doi.org/10.1063/1.3442495 (3 pages) | Cited 16 times

Online Publication Date: 28 May 2010

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We present a systematic study of the optical contrast of niobium diselenide and molybdenum disulfide flakes deposited onto silicon wafers with a thermally grown silicon oxide layer. We measure the optical contrast of flakes whose thickness, which is obtained by atomic force microscopy, ranges from 200 layers down to a monolayer using different illumination wavelengths in the visible spectrum. The refractive index of these thin crystals has been obtained from the optical contrast using Fresnel law. In this way the optical microscopy data can be quantitatively analyzed to determine the thickness of the flakes in a fast and nondestructive way.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Electronic structures of fully fluorinated and semifluorinated zinc oxide sheets

Yanli Wang, Yi Ding, Jun Ni, Siqi Shi, Chaorong Li, and Jianjun Shi

Appl. Phys. Lett. 96, 213117 (2010); http://dx.doi.org/10.1063/1.3442506 (3 pages) | Cited 9 times

Online Publication Date: 28 May 2010

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Using first-principles calculations, we investigate the structural, electronic, and magnetic properties of fully fluorinated and semifluorinated ZnO sheets. We find the fully fluorinated ZnO sheet prefers a twist-chair conformation, which is a nonmagnetic semiconductor. While for the semifluorinated sheet, the F-center conformation is the most stable and exhibits half-metallic behavior. The half-metallicity is attributed to the decrease in the charge transfer from Zn to O atoms after fluorination, which leads to the partial occupancy of the O 2p orbitals and spin-polarization in the semifluorinated sheet. Our studies demonstrate that fluorination is an efficient route to tune the electronic structures of ZnO nanostructures.
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71.20.Nr Semiconductor compounds
61.46.-w Structure of nanoscale materials
71.15.-m Methods of electronic structure calculations
75.75.-c Magnetic properties of nanostructures
73.22.-f Electronic structure of nanoscale materials and related systems
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