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18 Feb 2013

Volume 102, Issue 7, Articles (07xxxx)

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Appl. Phys. Lett. 102, 073101 (2013); http://dx.doi.org/10.1063/1.4790646 (4 pages)

V. Reboud, J. Romero-Vivas, P. Lovera, N. Kehagias, T. Kehoe, G. Redmond, and C. M. Sotomayor Torres
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Lasing in nanoimprinted two-dimensional photonic crystal band-edge lasers

V. Reboud, J. Romero-Vivas, P. Lovera, N. Kehagias, T. Kehoe, G. Redmond, and C. M. Sotomayor Torres

Appl. Phys. Lett. 102, 073101 (2013); http://dx.doi.org/10.1063/1.4790646 (4 pages) | Cited 1 time

Online Publication Date: 19 February 2013

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We demonstrate optically pumped polymer band-edge lasers based on a two-dimensional photonic crystal slab fabricated by nanoimprint lithography (NIL). Lasing was obtained at the photonic band-edge, where the light exhibits a low group velocity at the Γ point of the triangular lattice photonic crystal band structure. The active medium was composed of a dye chromophore-loaded polymer matrix directly patterned in a single step by nanoimprint lithography. Plane-wave and finite difference time domain algorithms were used to predict experimental lasing frequencies and the lasing thresholds obtained at different Γ points. A low laser threshold of 3 μJ/mm2 was achieved in a defect-free photonic crystal thus showing the suitability of nanoimprint lithography to produce cost-efficient optically pumped lasers.
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42.55.Tv Photonic crystal lasers and coherent effects
42.60.By Design of specific laser systems
42.70.Qs Photonic bandgap materials
42.55.Mv Dye lasers

Probing the electrical transport properties of intrinsic InN nanowires

S. Zhao, O. Salehzadeh, S. Alagha, K. L. Kavanagh, S. P. Watkins, and Z. Mi

Appl. Phys. Lett. 102, 073102 (2013); http://dx.doi.org/10.1063/1.4792699 (5 pages)

Online Publication Date: 19 February 2013

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We have studied the electrical transport properties of intrinsic InN nanowires using an electrical nanoprobing technique in a scanning electron microscope environment. It is found that such intrinsic InN nanowires exhibit an ohmic conduction at low bias and a space charge limited conduction at high bias. It is further derived that such InN nanowires can exhibit a free carrier concentration as low as ∼ 1013 cm−3 and possess a very large electron mobility in the range of 8000–12 000 cm2/V s, approaching the theoretically predicted maximum electron mobility at room temperature. In addition, charge traps are found to distribute exponentially just below the conduction band edge, with a characteristic energy ∼ 65 meV.
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73.63.Nm Quantum wires
72.20.Ht High-field and nonlinear effects
72.20.Fr Low-field transport and mobility; piezoresistance

Long wavelength (>1.55 μm) room temperature emission and anomalous structural properties of InAs/GaAs quantum dots obtained by conversion of In nanocrystals

A. Urbańczyk, J. G. Keizer, P. M. Koenraad, and R. Nötzel

Appl. Phys. Lett. 102, 073103 (2013); http://dx.doi.org/10.1063/1.4792700 (3 pages)

Online Publication Date: 19 February 2013

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We demonstrate that molecular beam epitaxy-grown InAs quantum dots (QDs) on (100) GaAs obtained by conversion of In nanocrystals enable long wavelength emission in the InAs/GaAs material system. At room temperature they exhibit a broad photoluminescence band that extends well beyond 1.55 μm. We correlate this finding with cross-sectional scanning tunneling microscopy measurements. They reveal that the QDs are composed of pure InAs which is in agreement with their long-wavelength emission. Additionally, the measurements reveal that the QDs have an anomalously undulated top surface which is very different to that observed for Stranski-Krastanow grown QDs.
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78.67.Hc Quantum dots
81.07.Ta Quantum dots
81.16.Ta Atom manipulation
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Diffusion enhancement in on/off ratchets

W. Chr. Germs, E. M. Roeling, L. J. van IJzendoorn, R. A. J. Janssen, and M. Kemerink

Appl. Phys. Lett. 102, 073104 (2013); http://dx.doi.org/10.1063/1.4793198 (5 pages)

Online Publication Date: 20 February 2013

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We show a diffusion enhancement of suspended polystyrene particles in an electrical on/off ratchet. The enhancement can be described by a simple master equation model. Furthermore, we find that the diffusion enhancement can be described by a general curve whose shape is only determined by the asymmetry of the ratchet repeat unit. The scaling of this curve can be explained from an analytical expression valid for small off-times. Finally, we demonstrate how the master equation model can be used to find the driving parameters for optimal particle separation.
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81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
66.30.Dn Theory of diffusion and ionic conduction in solids

Investigate the role of the active layers' structures and morphology in the performance of the organic solar cell devices

Ronak Rahimi, Alex Roberts, V. Narang, and D. Korakakis

Appl. Phys. Lett. 102, 073105 (2013); http://dx.doi.org/10.1063/1.4793201 (3 pages)

Online Publication Date: 20 February 2013

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Several CuPc/PTCDI-C8 films with different structures (co-deposited, layered, and bilayer) were prepared and their structural properties were studied using X-ray diffraction. In order to study the effects of the active layers' structures on the performance of the device, organic solar cells based on these films were fabricated and their electrical characteristics have been explored. Absorbed prominent diffraction peaks for CuPc/PTCDI-C8 bilayer films indicate the formation of higher degree of crystallinity for this structure compared to the films with layered or co-deposited structures. This crystallinity results in higher device performance due to reduced recombination losses and microscopic shorts within the devices.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells

Quantum simulation of topological insulator based spin transfer torque device

Yang Lu and Jing Guo

Appl. Phys. Lett. 102, 073106 (2013); http://dx.doi.org/10.1063/1.4793202 (5 pages)

Online Publication Date: 20 February 2013

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We developed a quantum transport model to simulate transport properties of topological insulator (TI)-based spintronic memory devices. The model captures the effects of spin-momentum locking, Klein tunneling, and coupled spin dynamics. Based on the model, we present a design of spin-transfer torque (STT) device, which consists of a thin layer TI coupled to a top ferromagnetic film. The device removes the requirement of spin-polarized contacts and magnetic tunnel junctions in conventional STT memory cells by exploiting intrinsic spin-momentum locking of the TI surface states. The analysis shows that by introducing partial perpendicular magnetic anisotropy, both fast switching and low switching current can be achieved.
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85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.

Highly localized heat generation by femtosecond laser induced plasmon excitation in Ag nanowires

Lei Liu, Peng Peng, Anming Hu, Guisheng Zou, W. W. Duley, and Y. Norman Zhou

Appl. Phys. Lett. 102, 073107 (2013); http://dx.doi.org/10.1063/1.4790189 (4 pages)

Online Publication Date: 21 February 2013

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Photo-excitation of plasmons in nano-systems initially results in highly localized heating, but the final temperature distribution in irradiated nanostructures is almost uniform because heat diffusion equilibrates the overall temperature within ∼10−12 s. Here, we show that irradiation with femtosecond pulses enables visualization of the location of plasmonic heating because thermal effects such as plasmon-induced melting are frozen in at the initial location of energy deposition. Simulations show plasmonic heating is related to the orientation of the laser polarization and to the geometry of partially melted nanowires. This may provide a useful tool in joining, cutting, and reshaping nano-objects.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.22.Lp Collective excitations
81.40.Gh Other heat and thermomechanical treatments
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)
78.67.Uh Nanowires
81.07.Gf Nanowires

Sensitivity enhancement of carbon nanotube based ammonium ion sensors through surface modification by using oxygen plasma treatment

Sanghak Yeo, Changrok Choi, Chi Woong Jang, Seok Lee, and Young Min Jhon

Appl. Phys. Lett. 102, 073108 (2013); http://dx.doi.org/10.1063/1.4793482 (4 pages)

Online Publication Date: 21 February 2013

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We have shown that the sensitivity of carbon nanotube (CNT) based sensors can be enhanced as high as 74 times through surface modification by using the inductively coupled plasma chemical vapor deposition method with oxygen. The plasma treatment power was maintained as low as 10 W within 20 s, and the oxygen plasma was generated far away from the sensors to minimize the plasma damage. From X-ray photoelectron spectroscopy analysis, we found that the concentration of oxygen increased with the plasma treatment time, which implies that oxygen functional groups or defect sites were generated on the CNT surface.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
52.77.Dq Plasma-based ion implantation and deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
82.80.-d Chemical analysis and related physical methods of analysis
85.35.Kt Nanotube devices

Different tips for high-resolution atomic force microscopy and scanning tunneling microscopy of single molecules

Fabian Mohn, Bruno Schuler, Leo Gross, and Gerhard Meyer

Appl. Phys. Lett. 102, 073109 (2013); http://dx.doi.org/10.1063/1.4793200 (4 pages)

Online Publication Date: 21 February 2013

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We explore different tip functionalizations for atomic force microscopy (AFM), scanning tunneling microscopy (STM), and Kelvin probe force microscopy (KPFM) of organic molecules on thin insulating films. We describe in detail how tips terminated with single Br and Xe atoms can be created. The performance of these tips in AFM, STM, and KPFM imaging of single molecules is compared to other tip terminations, and the advantages and disadvantages of the different tips are discussed. The Br tip was found to be particularly useful for AFM and lateral manipulation, whereas the Xe tip excelled in STM and KPFM.
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81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
07.79.Cz Scanning tunneling microscopes
07.79.Lh Atomic force microscopes
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)

Bias controlled capacitive driven cantilever oscillation for high resolution dynamic force microscopy

Jinjin Zhang, Daniel M. Czajkowsky, Yi Shen, Jielin Sun, Chunhai Fan, Jun Hu, and Zhifeng Shao

Appl. Phys. Lett. 102, 073110 (2013); http://dx.doi.org/10.1063/1.4793205 (4 pages)

Online Publication Date: 21 February 2013

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Well controlled cantilever oscillations are essential for precise measurements in dynamic force microscopy and spectroscopy, especially in demanding conditions such as under solution or at high frequencies. Here, we show that, with a capacitive driving system, an externally introduced DC-bias significantly increases the driving efficiency in solution and in air, as well as at high resonant frequencies, in agreement with theoretical predictions. This DC-voltage dependence not only improves the robustness of the method but also offers the possibility for precise amplitude control. The simplicity of this design further reinforces its potential for widespread implementation in dynamic force microscopy.
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07.79.Lh Atomic force microscopes

A neutron spin echo resolved grazing incidence scattering study of crystallites in organic photovoltaic thin films

A. J. Parnell, R. M. Dalgliesh, R. A. L. Jones, and A. D. F. Dunbar

Appl. Phys. Lett. 102, 073111 (2013); http://dx.doi.org/10.1063/1.4793513 (5 pages)

Online Publication Date: 22 February 2013

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Neutron spin echo resolved grazing incidence scattering (SERGIS) was used to probe crystallites of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) produced by extensive thermal annealing of a poly(3-hexylthiophene-2,5-diyl)(P3HT):PCBM organic photovoltaic layer. After annealing a thin film of P3HT:PCBM, PCBM crystallites appear on the sample surface, and a strong SERGIS signal is observed superimposed on the specular reflection. Features in the data can be readily correlated with length scales of the crystallites determined using atomic force microscopy and indicate that in such cases the SERGIS signal may be interpreted as a form of small angle neutron scattering.
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61.41.+e Polymers, elastomers, and plastics
61.72.Cc Kinetics of defect formation and annealing
68.47.Mn Polymer surfaces
81.40.Gh Other heat and thermomechanical treatments
68.35.bm Polymers, organics
68.55.am Polymers and organics

Gate voltage induced phase transition in magnetite nanowires

Johannes Gooth, Robert Zierold, Jan G. Gluschke, Tim Boehnert, Stefan Edinger, Sven Barth, and Kornelius Nielsch

Appl. Phys. Lett. 102, 073112 (2013); http://dx.doi.org/10.1063/1.4793529 (4 pages)

Online Publication Date: 22 February 2013

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Since its discovery in 1939 the origin of the phase transition in magnetite (Fe3O4) has been an object of intensive research and great controversy. Here, electrical resistance measurements as a function of gate voltage have been performed on single-crystalline Fe3O4 nanowires, showing that high electric fields trigger the breakdown of the insulating phase into a highly conductive state. Furthermore, the Verwey transition itself is suppressed by the gate voltage.
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71.30.+h Metal-insulator transitions and other electronic transitions
72.60.+g Mixed conductivity and conductivity transitions
73.61.Ng Insulators

Dimensionality and heat transport in Si-Ge superlattices

Ivana Savić, Davide Donadio, François Gygi, and Giulia Galli

Appl. Phys. Lett. 102, 073113 (2013); http://dx.doi.org/10.1063/1.4792748 (4 pages)

Online Publication Date: 22 February 2013

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We investigated how dimensionality affects heat transport in Si-Ge superlattices (SLs) by computing the thermal conductivity of planar superlattices and arrays of Ge nanowires (NWs) and nanodots embedded in Si. We studied superlattices with ∼10 nm periods using a fully atomistic Monte Carlo solution of the Boltzmann transport equation in the relaxation time approximation. We found that for periods larger than 4 nm, the room temperature cross-plane conductivity of planar superlattices with equally thick Si and Ge layers is larger than that of their nanowire and dot counterparts of similar sizes (up to 100%), while the trend is reversed below 4 nm.
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73.63.-b Electronic transport in nanoscale materials and structures
66.70.Df Metals, alloys, and semiconductors
72.20.Fr Low-field transport and mobility; piezoresistance
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.50.Dn Low-field transport and mobility; piezoresistance
73.61.Le Other inorganic semiconductors

Structure, magnetic, and electronic properties of hydrogenated two-dimensional diamond films

Jia Li, Hongdong Li, Zhigang Wang, and Guangtian Zou

Appl. Phys. Lett. 102, 073114 (2013); http://dx.doi.org/10.1063/1.4793204 (5 pages)

Online Publication Date: 22 February 2013

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In this paper, we study the effects of semi-hydrogenation (SH) and full-hydrogenation (FH) on the structural evolution and properties of two-dimensional (2D) diamond nanofilms by first-principles calculations. Both the hydrogenation processes play an important role in stabilizing the 2D diamond structures. For the FH cases, the direct bandgaps are localized in the region of 2.54–3.55 eV and decreased following an inverse law with layer number (n). For the SH cases, a ferrimagnetism characteristic is presented determining by the unpaired electrons on the un-hydrogenated side, and the spin-related bandgaps are in an infrared region of 0.74–1.17 eV, which are strongly dependent on n. As a result, the hydrogenation is favorable for tuning the electronic and magnetic properties of 2D diamond nanofilms to achieve high performance diamond-based nanodevices.
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75.70.Ak Magnetic properties of monolayers and thin films
75.75.-c Magnetic properties of nanostructures
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
71.20.Ps Other inorganic compounds
75.30.Ds Spin waves
75.50.Gg Ferrimagnetics
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