AIP Advances

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Vibrational stability of graphene

Yangfan Hu and Biao Wang

AIP Advances 3, 052101 (2013); http://dx.doi.org/10.1063/1.4804244 (10 pages)

Online Publication Date: 2 May 2013

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The mechanical stability of graphene as temperature rises is analyzed based on three different self-consistent phonon (SCP) models. Compared with three-dimensional (3-D) materials, the critical temperature T_i at which instability occurs for graphene is much closer to its melting temperature T_m obtained from Monte Carlo simulation (T_i ≃ 2T_m, K. V. Zakharchenko, A. Fasolino, J. H. Los, and M. I. Katsnelson, J. Phys. Condens. Matter 23, 202202). This suggests that thermal vibration plays a significant role in melting of graphene while melting for 3-D materials is often dominated by topologic defects. This peculiar property of graphene derives from its high structural anisotropy, which is characterized by the vibrational anisotropic coefficient (VAC), defined upon its Lindermann ratios in different directions. For any carbon based material with a graphene-like structure, the VAC value must be smaller than 5.4 to maintain its stability. It is also found that the high VAC value of graphene is responsible for its negative thermal expansion coefficient at low temperature range. We believe that the VAC can be regarded as a new criterion concerning the vibrational stability of any low-dimensional (low-D) materials.

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63.22.Rc	Phonons in graphene
61.48.Gh	Structure of graphene
81.05.ue	Graphene
02.70.Uu	Applications of Monte Carlo methods
65.80.Ck	Thermal properties of graphene

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Strain induced phase transitions in silicene bilayers: a first principles and tight-binding study

Chao Lian and Jun Ni

AIP Advances 3, 052102 (2013); http://dx.doi.org/10.1063/1.4804246 (10 pages)

Online Publication Date: 2 May 2013

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Using first principles and tight-binding calculations, we have investigated the structures of silicene bilayers under the isotropic tensile strain. We find that (i) the strain induce several barrierless phase transitions. (ii) After the phase transitions, the bilayer structures become planar, similar with the AA-stacking graphene bilayers, but combined with the strong covalent interlayer bonds. The tight-binding results demonstrate that this silicene bilayer is characterized by intralayer sp² hybridization and the interlayer sp¹ hybridization. (iii) The electronic properties of the silicene bilayers change from semiconducting to metallic with the increase of strain.

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71.15.Ap	Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
64.70.Nd	Structural transitions in nanoscale materials

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Control of two-dimensional growth of AlN and high Al-content AlGaN-based MQWs for deep-UV LEDs

Weihuang Yang, Jinchai Li, Wei Lin, Shuping Li, Hangyang Chen, Dayi Liu, Xu Yang, and Junyong Kang

AIP Advances 3, 052103 (2013); http://dx.doi.org/10.1063/1.4804247 (6 pages)

Online Publication Date: 2 May 2013

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Dense and atomically flat AlN film with root-mean-square roughness value of 0.32 nm was grown on sapphire substrate at a relatively lower temperature by using a three-step epitaxy technique. On the basis of this AlN template, AlGaN-based multiple quantum wells (MQWs) with atomically flat hetero-interfaces were epitaxially grown to suppress nonradiative recombination by introducing In as a surfactant during simultaneous source supply. As a result, single intense- and narrow-peaked photoluminescence was obtained from the MQWs. Finally, the deep ultraviolet light emitting diodes with well-behaved I-V characteristic and strong electroluminescence in the range of 256–312 nm were fabricated successfully.

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85.60.Jb	Light-emitting devices
85.35.Be	Quantum well devices (quantum dots, quantum wires, etc.)
81.07.St	Quantum wells
78.55.Cr	III-V semiconductors

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Extended generalized Riccati equation mapping method for the fifth-order Sawada-Kotera equation

Hasibun Naher, Farah Aini Abdullah, and Syed Tauseef Mohyud-Din

AIP Advances 3, 052104 (2013); http://dx.doi.org/10.1063/1.4804433 (14 pages)

Online Publication Date: 8 May 2013

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In this article, the generalized Riccati equation mapping together with the basic (G′/G)-expansion method is implemented which is advance mathematical tool to investigate nonlinear partial differential equations. Moreover, the auxiliary equation G′(ϕ) = h + f G(ϕ) + g G²(ϕ) is used with arbitrary constant coefficients and called the generalized Riccati equation. By applying this method, we have constructed abundant traveling wave solutions in a uniform way for the Sawada-Kotera equation. The obtained solutions of this equation have vital and noteworthy explanations for some practical physical phenomena.

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02.60.Lj

Ordinary and partial differential equations; boundary value problems

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First-principles study of native point defects in Bi₂Se₃

L. Xue, P. Zhou, C. X. Zhang, C. Y. He, G. L. Hao, L. Z. Sun, and J. X. Zhong

AIP Advances 3, 052105 (2013); http://dx.doi.org/10.1063/1.4804439 (10 pages)

Online Publication Date: 8 May 2013

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Using first-principles method within the framework of the density functional theory, we study the influence of native point defect on the structural and electronic properties of Bi₂Se₃. Se vacancy in Bi₂Se₃ is a double donor, and Bi vacancy is a triple acceptor. Se antisite (Se_Bi) is always an active donor in the system because its donor level (ɛ(+1/0)) enters into the conduction band. Interestingly, Bi antisite (Bi_Se1) in Bi₂Se₃ is an amphoteric dopant, acting as a donor when μ_e < 0.119 eV (the material is typical p-type) and as an acceptor when μ_e > 0.251 eV (the material is typical n-type). The formation energies under different growth environments (such as Bi-rich or Se-rich) indicate that under Se-rich condition, Se_Bi is the most stable native defect independent of electron chemical potential μ_e. Under Bi-rich condition, Se vacancy is the most stable native defect except for under the growth window as μ_e > 0.262 eV (the material is typical n-type) and Δμ_Se < −0.459 eV (Bi-rich), under such growth window Bi_Se1 carrying one negative charge is the most stable one.

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61.72.jd	Vacancies
71.55.Ht	Other nonmetals
71.15.Mb	Density functional theory, local density approximation, gradient and other corrections
71.20.Nr	Semiconductor compounds
61.66.Fn	Inorganic compounds

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Annealing assisted substrate coherency and high-temperature antiferromagnetic insulating transition in epitaxial La_0.67Ca_0.33MnO₃/NdGaO₃(001) films

L. F. Wang, X. L. Tan, P. F. Chen, B. W. Zhi, B. B. Chen, Z. Huang, G. Y. Gao, and W. B. Wu

AIP Advances 3, 052106 (2013); http://dx.doi.org/10.1063/1.4804541 (14 pages)

Online Publication Date: 8 May 2013

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Bulk La_0.67Ca_0.33MnO₃ (LCMO) and NdGaO₃ (NGO) have the same Pbnm symmetry but different orthorhombic lattice distortions, yielding an anisotropic strain state in the LCMO epitaxial film grown on the NGO(001) substrate. The films are optimally doped in a ferromagnetic-metal ground state, after being ex-situ annealed in oxygen atmosphere, however, they show strikingly an antiferromagnetic-insulating (AFI) transition near 250 K, leading to a phase separation state with tunable phase instability at the temperatures below. To explain this drastic strain effect, the films with various thicknesses were ex-situ annealed under various annealing parameters. We demonstrate that the ex-situ annealing can surprisingly improve the epitaxial quality, resulting in the films with true substrate coherency and the AFI ground state. And the close linkage between the film morphology and electronic phase evolution implies that the strain-mediated octahedral deformation and rotation could be assisted by ex-situ annealing, and moreover, play a key role in controlling the properties of oxide heterostructures.

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75.70.Ak	Magnetic properties of monolayers and thin films
75.50.Ee	Antiferromagnetics
75.50.Dd	Nonmetallic ferromagnetic materials
62.20.F-	Deformation and plasticity
81.40.Lm	Deformation, plasticity, and creep
75.30.Kz	Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

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Au-free InAs nanowires grown in In-particle-assisted vapor-liquid-solid mode: growth, structure, and electrical property

Guoqiang Zhang, Satoshi Sasaki, Kouta Tateno, Hideki Gotoh, and Tetsuomi Sogawa

AIP Advances 3, 052107 (2013); http://dx.doi.org/10.1063/1.4804542 (12 pages)

Online Publication Date: 8 May 2013

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We investigated the growth, structure, and electrical properties of InAs nanowires grown in the Au-free vapor-liquid-solid mode. We demonstrated the self-assisted vapor-liquid-solid growth of InAs nanowire with self-assembled In particles on InP substrates. We found that the III/V source mole ratio has a significant effect on the growth behavior and tapering shape. With a high III/V mole ratio (>0.11), a pure In particle at the tip of a nanowire makes it possible to grow the InAs nanowire in the self-assisted vapor-liquid-solid mode. We also found that the growth temperature range of the self-assisted vapor-liquid-solid growth was quite narrow compared with the conventional Au-assisted vapor-liquid-solid mode. A single InAs nanowire grown with a high III/V mole ratio exhibits distinct TO phonon peak in a Raman spectroscopy observation. We further verified that an undoped InAs nanowire grown in the self-assisted vapor-liquid-solid mode could function as a channel in a field-effect transistor device and the undoped nanowire exhibits n-type conduction behavior.

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81.16.Dn	Self-assembly
78.30.Fs	III-V and II-VI semiconductors
85.30.Tv	Field effect devices
63.22.Gh	Nanotubes and nanowires
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

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Growth and characterization of cooperative quantum dot chains in quaternary InAsSbP material system

K. M. Gambaryan and V. M. Aroutiounian

AIP Advances 3, 052108 (2013); http://dx.doi.org/10.1063/1.4804547 (5 pages)

Online Publication Date: 8 May 2013

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The cooperative quantum dot chains (CQDCs) are grown from In-As-Sb-P quaternary liquid phase on InAs(100) substrate with a deviation of surface orientation from (100) of about 0.3° along [010] direction. The wet chemical etching is utilized to create an additional artificial disorientation of the substrate. AFM investigations show that CQDCs mainly consist of central coupled InAsSb quantum dot (QD) sub-chains surrounded by InAsP-leaf chains. Cooperative chains have a ∼120 nm total width, over 5 μm length and directed along [010]. The separation between QDs within sub-chains is about 40 nm. The red shift of CQDCs’ absorption edge is detected.

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81.07.Ta	Quantum dots
61.46.Df	Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
81.65.Cf	Surface cleaning, etching, patterning
68.37.Ps	Atomic force microscopy (AFM)
81.05.Ea	III-V semiconductors

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Synthesis, characterizations and antimicrobial activities of well dispersed ultra-long CdO nanowires

Sumeet Kumar and Animesh K. Ojha

AIP Advances 3, 052109 (2013); http://dx.doi.org/10.1063/1.4804930 (6 pages)

Online Publication Date: 8 May 2013

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We present a simple, efficient, low cost and template free method for preparation of well dispersed ultra-long (1 μm) CdO nanowires. The CdO nanowires were characterized by x-ray diffraction (XRD), Transmission electron microscopy (TEM), UV-visible spectroscopy and Raman measurements. The direct and indirect band gaps were calculated to be 3.5 eV and 2.6 eV, respectively. In the Raman spectra only second order features were observed. The CdO nanowires were used to study antimicrobial activities against B.subtilis and E.coli microbes. It shows antimicrobial activity against B.subtilis and E.coli. However, the antimicrobial activities are better against B.subtilis than that of E.coli.

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81.16.Be	Chemical synthesis methods
78.30.Hv	Other nonmetallic inorganics
78.40.Fy	Semiconductors
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

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Energy in a finite two-dimensional spinless electron gas

Orion Ciftja, Bradley Sutton, and Ashley Way

AIP Advances 3, 052110 (2013); http://dx.doi.org/10.1063/1.4804933 (9 pages)

Online Publication Date: 8 May 2013

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We study the properties of a finite two-dimensional electron gas system in the Hartree-Fock approximation. We obtain exact analytical expressions for the energy in a finite two-dimensional fully spin-polarized (spinless) system of electrons interacting with a Coulomb potential immersed in a finite square region uniformly filled with a neutralizing positive charge. The difficult two-electron integrals over the finite square domain are reduced to simple compact expressions involving analytic auxiliary functions. We provide results for the potential energy of systems with a finite number of electrons and show how the energy slowly converges towards its thermodynamic limit bulk value.

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73.20.-r	Electron states at surfaces and interfaces
73.21.-b	Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
71.15.Ap	Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
72.25.-b	Spin polarized transport

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A generic tight-binding model for monolayer, bilayer and bulk MoS₂

Ferdows Zahid, Lei Liu, Yu Zhu, Jian Wang, and Hong Guo

AIP Advances 3, 052111 (2013); http://dx.doi.org/10.1063/1.4804936 (6 pages)

Online Publication Date: 8 May 2013

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Molybdenum disulfide (MoS₂) is a layered semiconductor which has become very important recently as an emerging electronic device material. Being an intrinsic semiconductor the two-dimensional MoS₂ has major advantages as the channel material in field-effect transistors. In this work we determine the electronic structures of MoS₂ with the highly accurate screened hybrid functional within the density functional theory (DFT) including the spin-orbit coupling. Using the DFT electronic structures as target, we have developed a single generic tight-binding (TB) model that accurately produces the electronic structures for three different forms of MoS₂ - bulk, bilayer and monolayer. Our TB model is based on the Slater-Koster method with non-orthogonal sp³d⁵ orbitals, nearest-neighbor interactions and spin-orbit coupling. The TB model is useful for atomistic modeling of quantum transport in MoS₂ based electronic devices.

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71.23.Cq	Amorphous semiconductors, metallic glasses, glasses
71.15.Mb	Density functional theory, local density approximation, gradient and other corrections
71.15.Ap	Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
71.70.Ej	Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect

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High-temperature ferromagnetism of helical carbon nanotubes

Ye Zhuang, Jianfeng Wen, Nujiang Tang, Ming Li, Liya Lv, and Youwei Du

AIP Advances 3, 052112 (2013); http://dx.doi.org/10.1063/1.4804947 (5 pages)

Online Publication Date: 8 May 2013

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We report the experimental results on the magnetism of curvature-induced helical carbon nanotubes (HCNTs). It is demonstrated that without any magnetic impurities in the sample, the as-prepared HCNTs show clear ferromagnetism with a Curie point as high as 970 K.

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71.20.Tx	Fullerenes and related materials; intercalation compounds
75.50.Dd	Nonmetallic ferromagnetic materials
75.30.Kz	Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.40.-s	Critical-point effects, specific heats, short-range order
77.80.B-	Phase transitions and Curie point

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Organic nonvolatile resistive memory devices based on thermally deposited Au nanoparticle

Zhiwen Jin, Guo Liu, and Jizheng Wang

AIP Advances 3, 052113 (2013); http://dx.doi.org/10.1063/1.4804948 (7 pages)

Online Publication Date: 8 May 2013

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Uniform Au nanoparticles (NPs) are formed by thermally depositing nominal 2-nm thick Au film on a 10-nm thick polyimide film formed on a Al electrode, and then covered by a thin polymer semiconductor film, which acts as an energy barrier for electrons to be injected from the other Al electrode (on top of polymer film) into the Au NPs, which are energetically electron traps in such a resistive random access memory (RRAM) device. The Au NPs based RRAM device exhibits estimated retention time of 10⁴ s, cycle times of more than 100, and ON-OFF ratio of 10² to 10³. The carrier transport properties are also analyzed by fitting the measured I-V curves with several conduction models.

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84.30.Sk

Pulse and digital circuits

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Horizontal cloaking and vertical reflection by transformation acoustics

Min Kyung Lee and Yoon Young Kim

AIP Advances 3, 052114 (2013); http://dx.doi.org/10.1063/1.4805353 (7 pages)

Online Publication Date: 9 May 2013

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This investigation shows that if an acoustic metamaterial bounded by an external rectangle and an internal circular cavity is properly engineered by a set of transformation equations that satisfy certain requirements, it can virtually cloak an object against incoming acoustic waves in one direction and make an incoming wave along the orthogonal direction reflected by an object located inside its inner cavity. The specific transformation equations realizing the metamaterial are suggested and an analysis is carried out to investigate the wave phenomena taking place along the cavity boundary.

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43.20.El

Reflection, refraction, diffraction of acoustic waves

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Dielectric response and magnetoelectric coupling in single crystal gallium ferrite

Somdutta Mukherjee, Rajeev Gupta, and Ashish Garg

AIP Advances 3, 052115 (2013); http://dx.doi.org/10.1063/1.4806762 (8 pages)

Online Publication Date: 9 May 2013

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Here we report the dielectric response and electric conduction behavior of magnetoelectric gallium ferrite single crystals studied using impedance analysis in time and temperature domain. The material exhibits two distinct relaxation processes: a high frequency bulk response and a low frequency interfacial boundary layer response. Calculated bulk capacitance as a function of temperature showed an anomaly at ferri- to paramagnetic transition temperature (∼ 300 K), suggestive of magneto-dielectric coupling in the material. Interestingly, we also witness an abrupt change in the activation energy at ∼ 220 K, in the vicinity of spin-glass transition temperature in GaFeO₃.

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75.50.Gg	Ferrimagnetics
75.85.+t	Magnetoelectric effects, multiferroics
77.55.Nv	Multiferroic/magnetoelectric films
75.30.Kz	Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.40.-s	Critical-point effects, specific heats, short-range order

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Low temperature solid state processing of pure P3HT fibers

Alexandre Rodrigues, Deb Nabankur, Loic Hilliou, Julio Viana, David G. Bucknall, and Gabriel Bernardo

AIP Advances 3, 052116 (2013); http://dx.doi.org/10.1063/1.4806764 (8 pages)

Online Publication Date: 9 May 2013

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Fibers of pure poly(3-hexylthiophene) (P3HT) of 1 and 2 mm diameter, were produced from solid state processing at temperatures of 200, 150 and 100ºC, i.e. up to more than 100 ºC below the melting point of P3HT (∼240 ºC), using a small device reminiscent with the early stage polymer extrusion machines. The fibers produced by this method are continuous and mechanically robust. WAXS results show that the macromolecular chains of P3HT are preferably oriented along the fiber axis. This study clearly shows, for the first time, that P3HT fibers can be processed without solvents well below the polymer's melting temperature.

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81.05.Lg	Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
61.41.+e	Polymers, elastomers, and plastics
36.20.Fz	Constitution (chains and sequences)

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Resonant mode behavior of lumped-resistor-loaded electric-inductive-capacitive resonator and its absorber application

Hong-Min Lee and Hyung-Sup Lee

AIP Advances 3, 052117 (2013); http://dx.doi.org/10.1063/1.4806995 (7 pages)

Online Publication Date: 14 May 2013

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This paper presents investigations into the resonant mode behavior of a lumped-resistor-loaded electric-inductive-capacitive (ELC) resonator, which is illuminated with a parallel polarization external electromagnetic wave. An ELC resonator exhibits a negative effective permittivity for both parallel and perpendicular polarizations. In contrast to a common ELC resonator, the lumped-resistor-loaded ELC resonator exhibits a switchable resonant mode behavior, thereby revealing a negative effective permeability. In addition, this resonator exhibits a low quality factor owing to the loaded lumped resistors. A metamaterial absorber, which consists of a lumped-resistor-loaded ELC resonator and a cut-wire strip, is designed to confirm the effectiveness of the resonator.

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42.79.-e	Optical elements, devices, and systems
42.70.-a	Optical materials

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Comparative study of magnetic and magnetotransport properties of Sm_0.55Sr_0.45MnO₃ thin films grown on different substrates

Manoj K. Srivastava, Sandeep Singh, P. K. Siwach, Amarjeet Kaur, V. P. S. Awana, K. K. Maurya, and H. K. Singh

AIP Advances 3, 052118 (2013); http://dx.doi.org/10.1063/1.4805077 (13 pages)

Online Publication Date: 16 May 2013

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Highly oriented polycrystalline Sm_0.55Sr_0.45MnO₃ thin films (thickness ∼100 nm) deposited on LaAlO₃ (LAO, (001)), SrTiO₃ (STO, (001)) and (La_0.18Sr_0.82) (Al_0.59Ta_0.41)O₃ (LSAT, (001)) single crystal substrates by ultrasonic nebulized spray pyrolysis have been studied. The out of plane lattice parameter (OPLP) of the film on LAO is slightly larger than that of the corresponding bulk. In contrast, the OPLP of the films on STO and LSAT are slightly smaller than the corresponding bulk value. This suggests that the film on LAO is under compressive strain while LSAT and STO are under tensile strain. The films on LAO and LSAT show simultaneous paramagnetic-ferromagnetic (PM–FM) and insulator-metal transition (IMT) temperature at T_C/T_IM ∼ 165 K and 130 K, respectively. The PM–FM and IM transition occur at T_C ∼ 120 K and T_IM ∼ 105 K, respectively in the film on STO substrate. At T < T_C, the zero field cooled–field cooled (ZFC–FC) magnetization of all the films shows strong bifurcation. This suggests the presence of a metamagnetic state akin to cluster glass formed due to coexisting FM and antiferromagnetic–charge order (AFM–CO) clusters. All the films show colossal magnetoresistance but its temperature and magnetic field dependence are drastically different. The films on LAO and STO show peak CMR around T_C/T_IM, while the film on LSAT shows MR > 99 % over a very wide temperature range of ∼40 K centred on T_C/T_IM. In the lower temperature region the magnetic field dependent isothermal resistivity also shows signature of metamagnetic transitions. The observed results have been explained in terms of the variation of the relative fractions of the coexisting FM and AFM–CO phases as a function of the substrate induced strain and oxygen vacancy induced quenched disorder.

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75.70.Ak	Magnetic properties of monolayers and thin films
75.20.Ck	Nonmetals
75.30.Kz	Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.47.Gk	Colossal magnetoresistance
75.50.Dd	Nonmetallic ferromagnetic materials
75.50.Ee	Antiferromagnetics

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Effect of germanium fraction on the effective minority carrier lifetime in thin film amorphous-Si/crystalline-Si_1xGe_x/crystalline-Si heterojunction solar cells

Sabina Abdul Hadi, Pouya Hashemi, Nicole DiLello, Evelina Polyzoeva, Ammar Nayfeh, and Judy L. Hoyt

AIP Advances 3, 052119 (2013); http://dx.doi.org/10.1063/1.4805078 (6 pages)

Online Publication Date: 16 May 2013

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The effect of germanium fraction on the effective minority carrier lifetime (τ_eff) for epitaxial Si_1-xGe_x layers is extracted using measurements on amorphous(a) Si(n+)/crystalline(c)-Si_1-xGe_x(p)/crystalline(c)-Si(p+) heterojunction solar cells with x = 0.25, 0.41 and 0.56. The τ_eff extracted for Si_0.75Ge_0.25 is ∼1 μs, decreasing to ∼ 40 ns for Si_0.44Ge_0.56. In addition, the band-gap voltage offset (W_oc) increases from 0.5 eV for Si to 0.65 eV for 56% Ge indicating an increase in non-radiative recombination consistent with the reduction in effective lifetime.

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88.40.jp	Multijunction solar cells
88.40.hj	Efficiency and performance of solar cells
88.40.jj	Silicon solar cells

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Photothermal formation of vortex flows by 1.55 μm light

Rui Xu, Hongbao Xin, and Baojun Li

AIP Advances 3, 052120 (2013); http://dx.doi.org/10.1063/1.4805080 (7 pages)

Online Publication Date: 17 May 2013

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Fluid flows with four vortices were simultaneously formed on water surface based on photothermal effect induced by a laser beam of 1.55 μm wavelength. With the illumination of an optical power of 60 mW, the peak velocity of each vortex flow was reached to 1200 μm/s. Influence of optical power and attack angle on the vortex flows has been investigated. The mechanism of photothermal formation was analyzed with the assistance of a finite-element simulation.

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47.32.C-	Vortex dynamics
47.80.Jk	Flow visualization and imaging
02.70.Dh	Finite-element and Galerkin methods
47.11.Fg	Finite element methods

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High-visibility ghost imaging from artificially generated non-Gaussian intensity fluctuations

Xue-Feng Liu, Ming-Fei Li, Xu-Ri Yao, Wen-Kai Yu, Guang-Jie Zhai, and Ling-An Wu

AIP Advances 3, 052121 (2013); http://dx.doi.org/10.1063/1.4807655 (7 pages)

Online Publication Date: 17 May 2013

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The image quality in ghost imaging is vital in practical applications. Through theoretical analysis, we find that for thermal light the average intensity as well as the fluctuations of an arbitrary incident field can greatly influence the image quality. Based on this, we suggest an easily realizable scheme to improve the visibility by generating speckles of non-Gaussian intensity distributions with a spatial light modulator. Numerical simulation demonstrates that this method can significantly improve the visibility, and the effect on the imaging resolution is also discussed. This method may thus be helpful in promoting the implementation of ghost imaging in real applications.

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42.30.Va	Image forming and processing
42.79.Hp	Optical processors, correlators, and modulators
42.30.Ms	Speckle and moiré patterns

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Hysteresis mechanism and control in pentacene organic field-effect transistors with polymer dielectric

Wei Huang, Wei Shi, Shijiao Han, and Junsheng Yu

AIP Advances 3, 052122 (2013); http://dx.doi.org/10.1063/1.4807660 (6 pages)

Online Publication Date: 17 May 2013

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Hysteresis mechanism of pentacene organic field-effect transistors (OFETs) with polyvinyl alcohol (PVA) and/or polymethyl methacrylate (PMMA) dielectrics is studied. Through analyzing the electrical characteristics of OFETs with various PVA/PMMA arrangements, it shows that charge, which is trapped in PVA bulk and at the interface of pentacene/PVA, is one of the origins of hysteresis. The results also show that memory window is proportional to both trap amount in PVA and charge density at the gate/PVA or PVA/pentacene interfaces. Hence, the controllable memory window of around 0 ∼ 10 V can be realized by controlling the thickness and combination of triple-layer polymer dielectrics.

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85.30.Tv

Field effect devices

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Flexible field-effect transistor arrays with patterned solution-processed organic crystals

Yun Li, Chuan Liu, Yu Wang, Yang Yang, Xinran Wang, Yi Shi, and Kazuhito Tsukagoshi

AIP Advances 3, 052123 (2013); http://dx.doi.org/10.1063/1.4807669 (6 pages)

Online Publication Date: 17 May 2013

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We demonstrate a fabrication process to develop field-effect transistor arrays based on patterned organic crystals as active semiconductor materials on flexible plastic substrates. Large plate-like organic crystals are produced by a direct spin-coating process on a substrate with patterned wettability. Resulting transistor arrays exhibit high device performance, indicating that the proposed method has great potential in flexible electronics.

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85.30.Tv

Field effect devices

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Determination of electric field at and near the focus of a cylindrical lens for applications in fluorescence microscopy

Subhajit B. Purnapatra and Partha P. Mondal

AIP Advances 3, 052124 (2013); http://dx.doi.org/10.1063/1.4807670 (10 pages)

Online Publication Date: 17 May 2013

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We present an explicit computable integral solution of the electric field generated at the focal region of a cylindrical lens. This representation is based on vectorial diffraction theory and further enables the computation of the system point spread function of a cylindrical lens. It is assumed that there is no back-scattering and the contribution from the evanescent field is negligible. Stationary phase approximation along with the Fresnel transmission coefficients are employed for evaluating the polarization dependent electric field components. Studies were carried out to determine the polarization effects and to calculate the system resolution. The effect of s −, p − and randomly polarized light is studied on the fixed sample (electric dipole is fixed in space). Proposed approach allows better understanding of electric field effects at the focus of a cylindrical aplanatic system. This opens up future developments in the field of fluorescence microscopy and optical imaging.

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42.79.Bh	Lenses, prisms and mirrors
42.30.Lr	Modulation and optical transfer functions

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White-light-emitting diode based on a single-layer polymer

B. Z. Wang, X. P. Zhang, and H. M. Liu

AIP Advances 3, 052125 (2013); http://dx.doi.org/10.1063/1.4807735 (7 pages)

Online Publication Date: 20 May 2013

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A broad-band light-emitting diode was achieved in a single-layer device based on pure poly(9,9^′-dioctylfluorene-co-bis-N,N^′-(4-butylphenyl)-bis-N,N^′-phenyl-1,4-phenylenediamine) (PFB). Electromer emission was observed in the red with a center wavelength of about 620 nm in electroluminescence (EL) spectrum. This kind of emission exhibits strong dependence on the thickness of the PFB layer, so that the shape of the EL spectrum may be adjusted through changing the thickness of the active polymer layer to balance between the intrinsic PFB emission in the blue and the electromer emission in the red. Thus, white light emission may be achieved from such a single-layer single-material diode.

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85.60.Jb	Light-emitting devices
78.60.Fi	Electroluminescence

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May 2013

Volume 3, Issue 5 (partial)

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