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9 Jan 2012

Volume 100, Issue 2, Articles (02xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 100, 023701 (2012); http://dx.doi.org/10.1063/1.3673335 (3 pages)

Biswarup Pathak, Henrik Löfås, Jariyanee Prasongkit, Anton Grigoriev, Rajeev Ahuja, and Ralph H. Scheicher
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Ultrahigh dielectric constant of thin films obtained by electrostatic force microscopy and artificial neural networks

E. Castellano-Hernández and G. M. Sacha

Appl. Phys. Lett. 100, 023101 (2012); http://dx.doi.org/10.1063/1.3675446 (3 pages) | Cited 2 times

Online Publication Date: 9 January 2012

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A detailed analysis of the electrostatic interaction between an electrostatic force microscope tip and a thin film is presented. By using artificial neural networks, an equivalent semiinfinite sample has been described as an excellent approximation to characterize the whole thin film sample. A useful analytical expression has been also developed. In the case of very small thin film thicknesses (around 1 nm), the electric response of the material differs even for very high dielectric constants. This effect can be very important for thin materials where the finite size effect can be described by an ultrahigh thin film dielectric constant.
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77.55.F- High-permittivity capacitive films
77.22.Ch Permittivity (dielectric function)

The roles of ruthenium nanoparticles decorated on thin multi-walled carbon nanotubes in the enhancement of field emission properties

Seung Youb Lee, Cheolho Jeon, Yooseok Kim, Won Chel Choi, Kyuwook Ihm, Tai-Hee Kang, Young-Hwan Kim, Chun Keun Kim, and Chong-Yun Park

Appl. Phys. Lett. 100, 023102 (2012); http://dx.doi.org/10.1063/1.3675460 (3 pages)

Online Publication Date: 9 January 2012

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The roles of metal nanoparticles (NPs) decorated on carbon nanotubes (CNTs) in the enhancement of field emission properties were investigated by measuring the work functions (WF), densities of state (DOS), turn-on fields, current densities, and sizes of NPs. RuO2 was deposited on CNTs and reduced to Ru by annealing at elevated temperatures. The WF decreased during reduction, and integrated DOS increased after the formation of Ru NPs. Consequently, the turn-on field showed strong correlation with the WF, and the shape of the Ru NPs had a more significant impact on current density than the WF and DOS.
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73.20.Hb Impurity and defect levels; energy states of adsorbed species
73.22.-f Electronic structure of nanoscale materials and related systems
79.70.+q Field emission, ionization, evaporation, and desorption

Nanocomposite lanthanum strontium manganite thin films formed by using a chemical solution deposition

C. Moreno, P. Abellán, F. Sandiumenge, M.-J. Casanove, and X. Obradors

Appl. Phys. Lett. 100, 023103 (2012); http://dx.doi.org/10.1063/1.3675461 (3 pages) | Cited 1 time

Online Publication Date: 9 January 2012

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A single-step process to prepare epitaxial nanocomposite La0.7Sr0.3MnO3/La2O3 thin films formed by spontaneous phase separation using a chemical solution deposition is presented. High magnetic field and temperature dependences of magnetic, transport, and magneto-transport properties of the films were investigated. The relationship between the enhanced magnetoresistance and microstructure and chemical inhomogeneities of the films are discussed.
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75.70.Ak Magnetic properties of monolayers and thin films
75.47.Pq Other materials
75.30.Sg Magnetocaloric effect, magnetic cooling
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
68.65.Ac Multilayers
73.50.Lw Thermoelectric effects

Magnetoresistance oscillations arising from edge-localized electrons in low-defect graphene antidot-lattices

T. Shimizu, J. Nakamura, K. Tada, Y. Yagi, and J. Haruyama

Appl. Phys. Lett. 100, 023104 (2012); http://dx.doi.org/10.1063/1.3675547 (4 pages) | Cited 4 times

Online Publication Date: 9 January 2012

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The so-called zigzag edge of graphenes has localized and strongly spin-polarized electrons. However, magnetoresistance (MR) behavior associated with the edge electrons has not been reported in graphenes. Here, we measure MR of graphene antidot-lattices, honeycomb-like arrays of hexagonal antidots with a large ensemble of hydrogen-terminated and low-defect antidot edges, prepared by a nonlithographic method using nanoporous alumina templates. We find anomalous MR oscillations arising from localized electron spins existing at the antidot edges. These are promising for realization of spintronic devices.
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72.20.My Galvanomagnetic and other magnetotransport effects
75.47.Pq Other materials
72.80.Vp Electronic transport in graphene
73.63.Kv Quantum dots
81.05.ue Graphene
81.07.Ta Quantum dots

Quantum and thermo-mechanical noise squeezing in nanoresonators: A comparative study

Yang Xu, Kaicheng Zhu, Sheping Yan, Zhonghe Jin, Yuelin Wang, Hongsheng Chen, Jikui Luo, and Bin Yu

Appl. Phys. Lett. 100, 023105 (2012); http://dx.doi.org/10.1063/1.3675548 (4 pages)

Online Publication Date: 9 January 2012

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Based on theoretical modeling, we performed comprehensive investigation on the quantum and thermomechanical noise squeezing effect in nanoresonators made by two platform material systems: silicon and graphene. We discovered that quantum noise squeezing plays an important role in the effort to improve signal-to-noise ratio in graphene-based nanoresonators. The research suggests a viable strategy for implementing graphene-based nanoscale transducers with ultra-high sensitivity that can be potentially utilized to detect a variety of sensing targets, including atomic force, single biological molecule, and gravitational wave.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.07.Mp Transducers
07.10.Cm Micromechanical devices and systems

Huge enhancement of optical nonlinearities in coupled Au and Ag nanoparticles induced by conjugated polymers

Lakshminarayana Polavarapu, Venkatesh Mamidala, Zhenping Guan, Wei Ji, and Qing-Hua Xu

Appl. Phys. Lett. 100, 023106 (2012); http://dx.doi.org/10.1063/1.3675911 (3 pages) | Cited 1 time

Online Publication Date: 10 January 2012

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Exceptional optical limiting properties were observed in coupled Au and Ag nanoparticles that are induced by conjugated polymers. Fluence-dependent transmission measurements using 7-ns laser pulses of 532-nm wavelength showed that the optical limiting properties of Au and Ag nanoparticles were significantly enhanced upon assembly induced by addition of cationic conjugated polymers. The optical limiting performances of coupled Au and Ag nanoparticles (with optical limiting threshold as low as 2.8 J/cm2 and 2.6 J/cm2, respectively) are even better than that of the benchmark optical limiter-carbon nanotube suspensions (with threshold of 3.6 J/cm2). In addition, these coupled Au and Ag nanoparticle solutions are very stable and suitable for practical applications. Input fluence and angle-dependent scattering experiments suggested that nonlinear scattering should play an important role in the observed optical limiting effects.
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42.50.Md Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
78.30.Er Solid metals and alloys
78.40.Kc Metals, semimetals, and alloys
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.47.J- Ultrafast spectroscopy (<1 psec)

Surface enhanced Raman spectroscopy by interfered femtosecond laser created nanostructures

Chao Wang, Yun-Ching Chang, Jimmy Yao, Claire Luo, Stuart (Shizhuo) Yin, Paul Ruffin, Christina Brantley, and Eugene Edwards

Appl. Phys. Lett. 100, 023107 (2012); http://dx.doi.org/10.1063/1.3676040 (4 pages) | Cited 4 times

Online Publication Date: 10 January 2012

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A type of surface enhanced Raman spectroscopy (SERS) by interfered femtosecond laser created nanostructures on Cu metal is presented. It is found out that finer and more uniform nanostructures (with an average feature size 100 nm or smaller) can be created on Cu metal by interfered femtosecond illumination with a phase mask. Significantly enhanced Raman signal (with an enhancement factor around 863) can be realized by using the nanostructured Cu substrate created by the interfered femtosecond laser illumination. The experimentally measured enhancement factor agrees relatively well with the theoretical analyses. Since the nanostructures can be inscribed in real time and at remote locations by the femtosecond laser inscription, the proposed SERS can be particularly useful for the standoff detection of chemicals.
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81.07.Bc Nanocrystalline materials
78.47.J- Ultrafast spectroscopy (<1 psec)
61.82.Rx Nanocrystalline materials
78.30.Er Solid metals and alloys

The influence of anisotropic gate potentials on the phonon induced spin-flip rate in GaAs quantum dots

Sanjay Prabhakar, Roderick V. N. Melnik, and Luis L. Bonilla

Appl. Phys. Lett. 100, 023108 (2012); http://dx.doi.org/10.1063/1.3675620 (4 pages) | Cited 1 time

Online Publication Date: 10 January 2012

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We study the anisotropic orbital effect in the electric field tunability of the phonon induced spin-flip rate in quantum dots (QDs). Our study shows that anisotropic gate potential enhances the spin-flip rate and reduces the level crossing point to a lower QDs radius due to the suppression of the Landé g-factor towards bulk crystal. In the range of 104−106 V/cm, the electric field tunability of the phonon induced spin-flip rate can be manipulated through strong Dresselhaus spin-orbit coupling. These results might assist the development of a spin based solid state quantum computer by manipulating spin-flip rate through spin-orbit coupling in a regime where the g-factor changes its sign.
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73.21.La Quantum dots
81.05.Ea III-V semiconductors
81.07.Ta Quantum dots
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect

Impact of gate work-function on memory characteristics in Al2O3/HfOx/Al2O3/graphene charge-trap memory devices

Sejoon Lee, Emil B. Song, Sungmin Kim, David H. Seo, Sunae Seo, Tae Won Kang, and Kang L. Wang

Appl. Phys. Lett. 100, 023109 (2012); http://dx.doi.org/10.1063/1.3675633 (4 pages) | Cited 4 times

Online Publication Date: 10 January 2012

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Graphene-based non-volatile memory devices composed of a single-layer graphene channel and an Al2O3/HfOx/Al2O3 charge-storage layer exhibit memory functionality. The impact of the gate material’s work-function (Φ) on the memory characteristics is investigated using different types of metals [Ti (ΦTi = 4.3 eV) and Ni (ΦNi = 5.2 eV)]. The ambipolar carrier conduction of graphene results in an enlargement of memory window (ΔVM), which is ∼4.5 V for the Ti-gate device and ∼9.1 V for the Ni-gate device. The increase in ΔVM is attributed to the change in the flat-band condition and the suppression of electron back-injection within the gate stack.
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84.30.Sk Pulse and digital circuits

Laser direct growth of graphene on silicon substrate

Dapeng Wei and Xianfan Xu

Appl. Phys. Lett. 100, 023110 (2012); http://dx.doi.org/10.1063/1.3675636 (3 pages) | Cited 5 times

Online Publication Date: 10 January 2012

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We demonstrate laser direct growth of few layer graphene on a silicon substrate. In our study, a continuous wave laser beam was focused on a poly(methyl methacrylate) (PMMA)-coated silicon wafer to evaporate PMMA and melt the silicon wafer. Carbon atoms, decomposed from PMMA, were absorbed by the molten silicon surface, and then separated from silicon in the cooling process to form few-layer graphene. This Si-catalyzed method will provide a new approach and platform for applications of graphene.
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42.62.-b Laser applications
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
64.70.dj Melting of specific substances
81.05.ue Graphene

Piezoresistive silicon nanowire based nanoelectromechanical system cantilever air flow sensor

Songsong Zhang, Liang Lou, and Chengkuo Lee

Appl. Phys. Lett. 100, 023111 (2012); http://dx.doi.org/10.1063/1.3675878 (3 pages) | Cited 2 times

Online Publication Date: 10 January 2012

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We present nanoelectromechanical system based cantilever air flow sensor using silicon nanowires (SiNWs). The cantilever is fabricated in the complementary metal-oxide-semiconductor compatible process with dimension of 90 μm × 20 μm × 3 μm. SiNWs with the size of 2 μm × 90 nm × 90 nm (length × width × height) are embedded at the edge of the cantilever fixed end to experience the maximum air flow induced strain. Compared with recently reported air flow sensors, our device shows a better sensitivity of 198 Ω/m/s and a flow sensing range up to 65 m/s. In addition, improvements in terms of linearity, hysteresis, and lower power consumption are reported as well.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Structure dependence in hybrid Si nanowire/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) solar cells: Understanding photovoltaic conversion in nanowire radial junctions

Wenhui Lu (吕文辉), Qi Chen (陈琪), Bing Wang (王兵), and Liwei Chen (陈立桅)

Appl. Phys. Lett. 100, 023112 (2012); http://dx.doi.org/10.1063/1.3676041 (4 pages) | Cited 3 times

Online Publication Date: 11 January 2012

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Hybrid Si/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solar cells based on nanowire array structures with radial and axial junctions are prepared and investigated. Compared to axial junction cells, radial junction cells exhibit much higher external quantum efficiency for long wavelength light due to improved separation of photogenerated charge carriers, resulting in more than 10 times higher power conversion efficiency (PCE). By tuning the height of nanowire array for the hybrid radial junction cells and investigating the external quantum efficiency and open circuit voltage decay, we identify that inefficient carrier collection due to interfacial recombination is the major limiting factor for the hybrid radial junction cell performance.
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88.40.J- Types of solar cells
88.40.H- Solar cells (photovoltaics)

Graphene covered SiC powder as advanced photocatalytic material

Kaixing Zhu, Liwei Guo, Jingjing Lin, Weichang Hao, Jun Shang, Yuping Jia, Lianlian Chen, Shifeng Jin, Wenjun Wang, and Xiaolong Chen

Appl. Phys. Lett. 100, 023113 (2012); http://dx.doi.org/10.1063/1.3676042 (4 pages) | Cited 4 times

Online Publication Date: 12 January 2012

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Graphene covered SiC powder (GCSP) has been fabricated by well established method of high temperature thermal decomposition of SiC. The structural and photocatalystic characteristics of the prepared GCSP were investigated and compared with that of the pristine SiC powder. Under UV illumination, more than 100% enhancement in photocatalystic activity is achieved in degradation of Rhodamine B (Rh B) by GCSP catalyst than by pristine SiC powder. The possible mechanisms underlining the observed results are discussed. The results suggested that GCSP as a composite of graphene based material has great potential for use as a high performance photocatalyst.
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82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
61.43.Gt Powders, porous materials
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.50.Hp Processes caused by visible and UV light

Coaxing graphene to be piezoelectric

Swapnil Chandratre and Pradeep Sharma

Appl. Phys. Lett. 100, 023114 (2012); http://dx.doi.org/10.1063/1.3676084 (3 pages) | Cited 5 times

Online Publication Date: 13 January 2012

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Several exotic characteristics and properties have made graphene a well-studied material from both a basic science viewpoint as well as tantalizing applications ranging from nanoelectronics, gas separation membranes to ultracapacitors. Graphene, however, is non-piezoelectric. This is obvious when graphene is in its metallic or semi-metallic state. However, even when graphene is in dielectric form—which can be engineered through porosity or by using nanoribbons—graphene is non-piezoelectric due to its centrosymmetric crystal structure. Using quantum mechanical calculations, we show that merely by creating holes of the right symmetry, graphene can be coaxed to act as a piezoelectric. We find that certain specifically tailored porous graphene sheets can “acquire” piezoelectricity coefficient that is nearly 72% of the well-known piezoelectric (quartz) or 36% of boron nitride nanotubes.
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77.65.-j Piezoelectricity and electromechanical effects
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
61.43.Gt Powders, porous materials

Electrospun carbon nanofibers surface-grafted with vapor-grown carbon nanotubes as hierarchical electrodes for supercapacitors

Zhengping Zhou, Xiang-Fa Wu, and Hao Fong

Appl. Phys. Lett. 100, 023115 (2012); http://dx.doi.org/10.1063/1.3676193 (4 pages) | Cited 3 times

Online Publication Date: 13 January 2012

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This letter reports the fabrication and electrochemical properties of electrospun carbon nanofibers surface-grafted with vapor-grown carbon nanotubes (CNTs) as hierarchical electrodes for supercapacitors. The specific capacitance of the fabricated electrodes was measured up to 185 F/g at the low discharge current density of 625 mA/g; a decrease of 38% was detected at the high discharge current density of 2.5 A/g. The morphology and microstructure of the electrodes were examined by electron microscopy, and the unique connectivity of the hybrid nanomaterials was responsible for the high specific capacitance and low intrinsic contact electric resistance of the hierarchical electrodes.
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81.16.-c Methods of micro- and nanofabrication and processing
82.80.Fk Electrochemical methods
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
73.40.Cg Contact resistance, contact potential
73.63.Rt Nanoscale contacts
81.07.Lk Nanocontacts

Single-photon emitters based on epitaxial isolated InP/InGaP quantum dots

Asli Ugur, Stefan Kremling, Fariba Hatami, Sven Höfling, Lukas Worschech, Alfred Forchel, and W. Ted Masselink

Appl. Phys. Lett. 100, 023116 (2012); http://dx.doi.org/10.1063/1.3676273 (3 pages) | Cited 2 times

Online Publication Date: 13 January 2012

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Quantum dots as single-photon sources have several advantages, such as emitting light over a broad spectral range and being photostable. Quantum dots with densities as low as 1 dot/μm2 have been achieved using ultra-low-rate epitaxy and single-dot emission measured without apertures or post-growth processing. Both excitionic and biexcitonic emissions are observed from single dots created in this way, appearing as doublets with a fine-structure splitting of 320 μeV. The polarization of the split states is also investigated. Hanbury Brown-Twiss correlation measurements for the excitonic emission under cw excitation show anti-bunching behavior with an autocorrelation value of g(2)(0) = 0.2.
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78.55.Cr III-V semiconductors
78.67.Hc Quantum dots
79.60.Bm Clean metal, semiconductor, and insulator surfaces
81.05.Ea III-V semiconductors
81.07.Ta Quantum dots
71.35.-y Excitons and related phenomena
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