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5 Dec 2011

Volume 99, Issue 23, Articles (23xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 99, 233701 (2011); http://dx.doi.org/10.1063/1.3651756 (3 pages)

Melis Hazar, Robert L. Steward, Jr., Chia-Jung Chang, Cynthia J. Orndoff, Yukai Zeng, Mon-Shu Ho, Philip R. LeDuc, and Chao-Min Cheng
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Thermal conductance modulator based on folded graphene nanoribbons

Tao Ouyang, Yuanping Chen, Yuee Xie, G. M. Stocks, and Jianxin Zhong

Appl. Phys. Lett. 99, 233101 (2011); http://dx.doi.org/10.1063/1.3665184 (4 pages) | Cited 3 times

Online Publication Date: 5 December 2011

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Based on folded graphene nanoribbons, we report a thermal conductance modulator which performs analogous operations as the rheostat in electronic circuits. This fundamental device can controllably and reversibly modulate the thermal conductance by varying the geometric structures and its tuning range can be up to 40% of the conductance of unfolded nanoribbons (∼1 nm wide and 7–15 nm long). Under this modulation, the conductance shows a linearly dependence on the folded angle, while undergoes a transition with the variation of the inter-layer distance. This primary thermal device may have great potential applications for phononic circuits and nanoscale thermal management.
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65.80.Ck Thermal properties of graphene
66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
81.05.ue Graphene

Local contact potential difference of molecular self-assemblies investigated by Kelvin probe force microscopy

Evan J. Spadafora, Mathieu Linares, Wan Zaireen Nisa Yahya, Frédéric Lincker, Renaud Demadrille, and Benjamin Grevin

Appl. Phys. Lett. 99, 233102 (2011); http://dx.doi.org/10.1063/1.3662850 (3 pages) | Cited 1 time

Online Publication Date: 5 December 2011

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Self-assembled pi-conjugated oligomer nanowires have been investigated by frequency modulation atomic force microscopy and amplitude modulation Kelvin probe force microscopy under ultra high vacuum. The distance dependence of the contact potential difference (CPD) has been analyzed by combining high resolution imaging with distance-spectroscopy measurements. It is shown that the apparition of a damping contrast characterizes the onset of short range electrostatic (SRE) forces, which are responsible for the occurrence of local CPD (LCPD) modulations correlated with the molecular lattice. By working at the onset of the damping contrast, the tip-surface separation can be adjusted to minimize the contribution of SRE forces to the measured CPD.
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81.16.Dn Self-assembly
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)
73.40.Cg Contact resistance, contact potential

Surface enhanced Raman scattering of aged graphene: Effects of annealing in vacuum

Yingying Wang, Zhenhua Ni, Aizhi Li, Zainab Zafar, Yan Zhang, Zhonghua Ni, Shiliang Qu, Teng Qiu, Ting Yu, and Ze Xiang Shen

Appl. Phys. Lett. 99, 233103 (2011); http://dx.doi.org/10.1063/1.3665624 (3 pages) | Cited 2 times

Online Publication Date: 5 December 2011

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In this paper, we report a simple method to recover the surface enhanced Raman scattering activity of aged graphene. The Raman signals of Rhodamine molecules absorbed on aged graphene are dramatically increased after vacuum annealing and comparable to those on fresh graphene. Atomic force microscopy measurements indicate that residues on aged graphene surface can efficiently be removed by vacuum annealing, which makes target molecule closely contact with graphene. We also find that the hole doping in graphene will facilitate charge transfer between graphene and molecule. These results confirm the strong Raman enhancement of target molecule absorbed on graphene is due to the charge transfer mechanism.
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78.30.Na Fullerenes and related materials
78.68.+m Optical properties of surfaces
81.40.Gh Other heat and thermomechanical treatments
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
61.48.Gh Structure of graphene
78.67.Wj Optical properties of graphene

Optically switchable molecular device using microsphere based junctions

V. Faramarzi, C. Raimondo, F. Reinders, M. Mayor, P. Samorì, and B. Doudin

Appl. Phys. Lett. 99, 233104 (2011); http://dx.doi.org/10.1063/1.3665940 (3 pages) | Cited 1 time

Online Publication Date: 5 December 2011

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Metallic planar electrodes are bridged using microspheres coated with chemisorbed azobenzene self-assembled monolayers. The circuit exhibits light-induced switching, with reproducibility over 90%, as statistically determined and compared to junctions incorporating photo-insensitive alkanethiol layers. Microsphere interconnects provide direct access to molecular transport properties, with reliability and stability, making multifunctional molecular electronics devices possible.
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85.65.+h Molecular electronic devices

Ballistic thermal conductance in graphene nanoribbon with double-cavity structure

Xiao-Fang Peng, Xin-Jun Wang, Zhi-Qiang Gong, and Ke-Qiu Chen

Appl. Phys. Lett. 99, 233105 (2011); http://dx.doi.org/10.1063/1.3666221 (3 pages) | Cited 4 times

Online Publication Date: 6 December 2011

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We investigate phonon transport and thermal conductance in a Graphene Nanoribbon modulated with a double-cavity quantum structure at low temperatures. Two methods are compared: the force-constant and elastic wave continuum models. Calculations show that both the models show the similar thermal conductance property at low temperatures despite the excited theory of the discrete phonon modes in quantum structure being not the same. However, in the higher temperature region, the thermal conductance in the force-constant model is bigger than that in the elastic wave continuum model. The difference originates from the inequable cutoff frequencies of the phonon modes. A brief analysis of these results is given.
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66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
63.22.Rc Phonons in graphene
62.30.+d Mechanical and elastic waves; vibrations

Surface effects in a semiconductor photonic nanowire and spectral stability of an embedded single quantum dot

Inah Yeo, Nitin S. Malik, Mathieu Munsch, Emmanuel Dupuy, Joël Bleuse, Yann-Michel Niquet, Jean-Michel Gérard, Julien Claudon, Édouard Wagner, Signe Seidelin, Alexia Auffèves, Jean-Philippe Poizat, and Gilles Nogues

Appl. Phys. Lett. 99, 233106 (2011); http://dx.doi.org/10.1063/1.3665629 (3 pages) | Cited 4 times

Online Publication Date: 6 December 2011

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We evidence the influence of surface effects for InAs quantum dots embedded into GaAs photonic nanowires used as efficient single photon sources. We observe a continuous temporal drift of the emission energy that is an obstacle to resonant quantum optics experiments at the single photon level. We attribute the drift to the sticking of oxygen molecules onto the wire, which modifies the surface charge and hence the electric field seen by the quantum dot. The influence of temperature and excitation laser power on this phenomenon is studied. Most importantly, we demonstrate a proper treatment of the nanowire surface to suppress the drift.
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78.67.Hc Quantum dots
78.67.Uh Nanowires
73.40.-c Electronic transport in interface structures

Audio mixing in a tri-port nano-electro-mechanical device

M. Defoort, K. Lulla, J.-S. Heron, O. Bourgeois, E. Collin, and F. Pistolesi

Appl. Phys. Lett. 99, 233107 (2011); http://dx.doi.org/10.1063/1.3665958 (3 pages) | Cited 2 times

Online Publication Date: 7 December 2011

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We report on experiments performed on a cantilever-based tri-port nano-electro-mechanical (NEMS) device. Two ports are used for actuation and detection through the magnetomotive scheme, while the third port is a capacitively coupled gate electrode. By applying a low frequency voltage signal on the gate, we demonstrate mixing in the mechanical response of the device, even for low magnetomotive drives without resorting to conduction measurements through the NEMS. The technique can thus be used, in particular, in the linear regime as an alternative to nonlinear mixing for normal conducting devices. An analytic theory is presented reproducing the data without free parameters.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
84.30.Qi Modulators and demodulators; discriminators, comparators, mixers, limiters, and compressors

Tunable singlet-triplet splitting in a few-electron Si/SiGe quantum dot

Zhan Shi, C. B. Simmons, J. R. Prance, John King Gamble, Mark Friesen, D. E. Savage, M. G. Lagally, S. N. Coppersmith, and M. A. Eriksson

Appl. Phys. Lett. 99, 233108 (2011); http://dx.doi.org/10.1063/1.3666232 (3 pages) | Cited 8 times

Online Publication Date: 8 December 2011

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We measure the excited-state spectrum of a Si/SiGe quantum dot as a function of in-plane magnetic field and identify the spin of the lowest three eigenstates in an effective two-electron regime. We extract the singlet-triplet splitting, an essential parameter for spin qubits, from the data. We find it to be tunable by lateral displacement of the dot, which is realized by changing two gate voltages on opposite sides of the device. We present calculations showing the data are consistent with a spectrum in which the first excited state of the dot is a valley-orbit state.
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73.21.La Quantum dots
81.07.Ta Quantum dots
71.20.Mq Elemental semiconductors
71.35.-y Excitons and related phenomena

Ab-initio multiplet calculation of oxygen vacancy effect on Ti-L2,3 electron energy loss near edge structures of BaTiO3

S. Ootsuki, H. Ikeno, Y. Umeda, H. Moriwake, A. Kuwabara, O. Kido, S. Ueda, I. Tanaka, Y. Fujikawa, and T. Mizoguchi

Appl. Phys. Lett. 99, 233109 (2011); http://dx.doi.org/10.1063/1.3663543 (3 pages)

Online Publication Date: 8 December 2011

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The effect of oxygen vacancy on Ti-L2,3 electron energy-loss near-edge structures (ELNES) of BaTiO3 was theoretically investigated through ab initio multiplet calculation. The presence of an oxygen vacancy influences spectral features not only at the nearest neighbor Ti site but also at Ti sites further from the oxygen vacancy. The effects of different oxygen vacancy concentrations were also investigated. Based on this study, it was concluded that the detection limit for oxygen vacancy with Ti-L2,3 ELNES is approximately 1%.
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79.20.Uv Electron energy loss spectroscopy
61.72.jd Vacancies

Unintentional doping induced splitting of G peak in bilayer graphene

S. S. Lin, B. G. Chen, C. T. Pan, S. Hu, P. Tian, and L. M. Tong

Appl. Phys. Lett. 99, 233110 (2011); http://dx.doi.org/10.1063/1.3666821 (3 pages) | Cited 1 time

Online Publication Date: 8 December 2011

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Raman characterizations show the G peak of an unintentional doped single crystal bilayer graphene (BLG) splits into two peaks: S and AS peaks. From the relative shift between S and AS peaks, the doping concentration is estimated to be from 8.8 × 1012 cm−2 to 2 × 1013 cm−2, as in the same order of that in monolayer graphene prepared under the same condition. The dopants distribute relatively homogeneously in a 0.7 mm× 0.3 mm large BLG judged through the G peak splitting. The 2D peak of heavy doped BLG can only be deconvoluted into three peaks, corresponding to 2D1B, 2D1A, and 2D2A peaks.
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61.72.up Other materials
78.30.Na Fullerenes and related materials
78.67.Wj Optical properties of graphene
63.20.kd Phonon-electron interactions
63.22.Rc Phonons in graphene

Mismatched alloy nanowires for electronic structure tuning

Joanne W. L. Yim, Costas P. Grigoropoulos, and Junqiao Wu

Appl. Phys. Lett. 99, 233111 (2011); http://dx.doi.org/10.1063/1.3666223 (3 pages)

Online Publication Date: 9 December 2011

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Electronic structure engineering is essential for producing materials suited for efficient solid-state devices. Mismatched semiconductors offer wide tunability of electronic structure with only a small change in composition. Here, we report a combined compound-elemental source vapor transport method for synthesis of mismatched alloy nanowires (NWs) of ZnSe1−xTex across the composition range. The alloy composition can be continuously tuned by varying the growth temperature from ZnSe (x = 0) at higher temperature to ZnTe (x = 1) at lower temperature. The nanowires have structure and bandgaps consistent with their compositions, with lattice parameters varying with Vegard’s law and emissions following predicted extreme bandgap bowing.
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81.07.Gf Nanowires
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)
81.16.-c Methods of micro- and nanofabrication and processing
81.05.Dz II-VI semiconductors
73.22.-f Electronic structure of nanoscale materials and related systems

Band-gap-dependent emissions from conjugated polymers coupled silver nanocap array

Fan Kong, Xueqin Zhang, Xianzhong Lang, Baoping Lin, Yimin Yang, and Teng Qiu

Appl. Phys. Lett. 99, 233112 (2011); http://dx.doi.org/10.1063/1.3667195 (3 pages) | Cited 1 time

Online Publication Date: 9 December 2011

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The optical properties of poly(2,5-dioctyloxy-p-phenylene) (PPP) and poly[3-(2,5,8-trioxanonyl) thiophene] (P3TT) coupled silver nanocap array have been investigated. The absorption spectrum of P3TT and the emission spectrum of PPP match the absorption of the silver nanocap array. Plasmon energy transfer occurs from the silver nanocap array to P3TT or from PPP to the silver nanocap array, resulting in the largely increased or decreased photoluminescence (PL), respectively. The PL enhancement of the P3TT coupled silver nanocap array is more than 9-fold; whereas the PL intensity of the PPP coupled silver nanocap array is less than one-tenth of the PPP film.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
78.55.-m Photoluminescence, properties and materials
78.40.-q Absorption and reflection spectra: visible and ultraviolet
78.30.-j Infrared and Raman spectra
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Doping profile of InP nanowires directly imaged by photoemission electron microscopy

M. Hjort, J. Wallentin, R. Timm, A. A. Zakharov, J. N. Andersen, L. Samuelson, M. T. Borgström, and A. Mikkelsen

Appl. Phys. Lett. 99, 233113 (2011); http://dx.doi.org/10.1063/1.3662933 (3 pages) | Cited 3 times

Online Publication Date: 9 December 2011

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InP nanowires (NWs) with differently doped segments were studied with nanoscale resolution using synchrotron based photoemission electron microscopy. We clearly resolved axially stacked n-type and undoped segments of the NWs without the need of additional processing or contacting. The lengths and relative doping levels of different NW segments as well as space charge regions were determined indicating memory effects of sulfur during growth. The surface chemistry of the nanowires was monitored simultaneously, showing that in the present case, the doping contrast was independent of the presence or absence of a native oxide.
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61.72.uj III-V and II-VI semiconductors
73.61.Ey III-V semiconductors
79.60.Bm Clean metal, semiconductor, and insulator surfaces
79.60.Jv Interfaces; heterostructures; nanostructures

Strong substrate effects of Joule heating in graphene electronics

X. Li, B. D. Kong, J. M. Zavada, and K. W. Kim

Appl. Phys. Lett. 99, 233114 (2011); http://dx.doi.org/10.1063/1.3668113 (4 pages) | Cited 2 times

Online Publication Date: 9 December 2011

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The effect of Joule heating on graphene electronic properties is investigated by self-consistent use of full-band Monte Carlo electron dynamics and three-dimensional heat transfer simulations. Several technologically important substrate materials are examined: SiO2, SiC, hexagonal BN, and diamond. Results illustrate that the choice of substrate has a major impact via heat conduction and surface polar phonon scattering. Particularly, the poor thermal conductivity of SiO2 leads to significant Joule heating and saturation velocity degradation in graphene characterized by the 1/math decay. Considering the overall characteristics, BN appears to compare favorably against the other substrate choices for graphene electronic applications.
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66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
63.22.Rc Phonons in graphene
72.80.Vp Electronic transport in graphene
85.35.-p Nanoelectronic devices

Broadband energy-harvesting hybrid solar cells employing nanocomposites of polythiophene:ternary PbSSe nanocrystals

Minwoo Nam, Sungwoo Kim, Taehoon Kim, Sang-Wook Kim, and Kee-Keun Lee

Appl. Phys. Lett. 99, 233115 (2011); http://dx.doi.org/10.1063/1.3669515 (3 pages)

Online Publication Date: 9 December 2011

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Hybrid solar cells (HSCs) incorporating narrow band gap PbSSe nanocrystals (NCs) in a conjugated polymer were developed for the extensive use of solar radiation up to the near infrared (NIR) range. The developed cell, composed of poly(3-hexylthiophene) (P3HT):PbSSe NC composites with 1:1 weight ratio, exhibited a large light-harvesting efficiency over a broad range, in which the external quantum efficiency reached 0.3% at 1100 nm. These promising results were attributed to the well-assembled percolation networks with large interfacial junctions and the efficient use of broad solar spectrum due to the use of narrow band gap ternary NCs.
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88.40.hj Efficiency and performance of solar cells
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