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5 Mar 2012

Volume 100, Issue 10, Articles (10xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 100, 101903 (2012); http://dx.doi.org/10.1063/1.3673327 (3 pages)

Michael Ian Lapsley, Anaram Shahravan, Qingzhen Hao, Bala Krishna Juluri, Stephen Giardinelli, Mengqian Lu, Yanhui Zhao, I-Kao Chiang, Themis Matsoukas, and Tony Jun Huang
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Multiferroic PbZrxTi1−xO3/Fe3O4 epitaxial sub-micron sized structures

Ionela Vrejoiu, Daniele Preziosi, Alessio Morelli, and Eckhard Pippel

Appl. Phys. Lett. 100, 102903 (2012); http://dx.doi.org/10.1063/1.3692583 (4 pages) | Cited 1 time

Online Publication Date: 7 March 2012

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Large range well ordered epitaxial ferrimagnetic nominally Fe3O4 structures were fabricated by pulsed-laser deposition and embedded in ferroelectric PbZrxTi1−xO3 (x = 0.2, 0.52) epitaxial films. Magnetite dots were investigated by magnetic force microscopy and exhibited magnetic domain contrast at room temperature (RT). Embedding ferroelectric PbZrxTi1−xO3 layers exhibit remnant polarization values close to the values of single epitaxial layers. Transmission electron microscopy demonstrated the epitaxial growth of the composites and the formation of the ferrimagnetic and ferroelectric phases. Physical and structural properties of these composites recommend them for investigations of stress mediated magneto-electric coupling at room temperature.
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77.55.Nv Multiferroic/magnetoelectric films
75.85.+t Magnetoelectric effects, multiferroics
75.70.Ak Magnetic properties of monolayers and thin films
75.50.Gg Ferrimagnetics
75.60.Ch Domain walls and domain structure
77.22.Ej Polarization and depolarization

Correlation between growth dynamics and dielectric properties of epitaxial BaTiO3 films

G. Radaelli, S. Brivio, I. Fina, and R. Bertacco

Appl. Phys. Lett. 100, 102904 (2012); http://dx.doi.org/10.1063/1.3692732 (4 pages) | Cited 1 time

Online Publication Date: 7 March 2012

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We report on the effect of the substrate temperature on the growth dynamics and dielectric properties of BaTiO3 epitaxial films on Nb:SrTiO3 (001) substrates. By real-time reflection high energy electron diffraction analysis, we quantitatively estimated the adatoms surface diffusivity as a function of temperature. The increase of the substrate temperature promotes surface diffusion, which reflects in the improvement of BaTiO3 ferroelectric properties up to 680 °C. Above this temperature, Ba re-evaporation occurs, leading to a rapid deterioration of the dielectric and ferroelectric properties. This work shed light on thermally activated physical mechanisms which determine the ferroelectric properties of BaTiO3 films.
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77.55.fe BaTiO3-based films
77.55.Px Epitaxial and superlattice films
68.35.Fx Diffusion; interface formation
68.55.aj Insulators

Multiferroicity in 0.7Pb(Zr0.52Ti0.48)O3-0.3Pb(Ni1/3Nb2/3)O3 ceramics

Jie Su, Xiaomei Lu, Yaoyang Liu, Junting Zhang, Guorong Li, Xuezheng Ruan, Fengzhen Huang, Jun Du, and Jinsong Zhu

Appl. Phys. Lett. 100, 102905 (2012); http://dx.doi.org/10.1063/1.3693145 (4 pages)

Online Publication Date: 9 March 2012

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0.7Pb(Zr0.52Ti0.48)O3-0.3Pb(Ni1/3Nb2/3)O3 (abbreviated as PZT-PNN) ceramics were prepared using the solid-state reaction method. The results reveal that ferroelectric and anti-ferromagnetic orderings coexist (multiferroicity) in PZT-PNN ceramics. Furthermore, the abnormal behavior of dielectric constant and loss observed at Néel temperature gives an evidence of the coupling between the ferroelectric and anti-ferromagnetic orderings (magnetoelectric coupling). The ferroelectric loops become slimmer with increasing temperature and the remnant polarization persists up to above the temperature of dielectric maximum (Tm), indicates a lead-induced ferroelectric behavior.
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75.85.+t Magnetoelectric effects, multiferroics
75.50.Ee Antiferromagnetics
77.22.Ch Permittivity (dielectric function)
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
77.22.Ej Polarization and depolarization
77.80.Jk Relaxor ferroelectrics

Band alignment of TiN/HfO2 interface of TiN/HfO2/SiO2/Si stack

Xiaolei Wang, Kai Han, Wenwu Wang, Hong Yang, Jing Zhang, Xueli Ma, Jinjuan Xiang, Dapeng Chen, and Tianchun Ye

Appl. Phys. Lett. 100, 102906 (2012); http://dx.doi.org/10.1063/1.3693188 (5 pages)

Online Publication Date: 9 March 2012

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Band alignment of TiN/HfO2 interface of TiN/HfO2/SiO2/Si stack is investigated by x-ray photoelectron spectroscopy (XPS). The p-type Schottky barrier height (p-SBH) is found to increase with thicker HfO2 thickness. Since considering only the metal/dielectric interface cannot explain this phenomenon, band alignment of TiN/HfO2 interface of TiN/HfO2/SiO2/Si stack is demonstrated based on band alignment of entire gate stack. Dependence of p-SBH on HfO2 thickness is interpreted and contributed to fixed charges in gate stack, interfacial gap state charges at HfO2/SiO2 interface, and space charges in Si substrate. Electrical measurements of capacitor structures further support XPS results and corresponding explanation.
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85.30.De Semiconductor-device characterization, design, and modeling
85.30.Tv Field effect devices
84.32.Tt Capacitors

Equivalence of direct and converse magnetoelectric coefficients in strain-coupled two-phase systems

Jing Lou, Gerry N. Pellegrini, Ming Liu, Neil D. Mathur, and Nian X. Sun

Appl. Phys. Lett. 100, 102907 (2012); http://dx.doi.org/10.1063/1.3693375 (2 pages) | Cited 1 time

Online Publication Date: 9 March 2012

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We demonstrate an equivalence between direct and converse magnetoelectric effects in two-phase systems comprising piezoelectric and magnetostrictive materials. This was achieved by recasting the Maxwell relation in terms of the effective electrical and magnetic dipole moments for the system and comparing coupling strengths at the same electrical and magnetic d.c. biases. Our findings therefore apply to magnetoelectric systems comprising more than two phases and correct the two widely held but opposing views that are compromised by incorrect parameterization or inconsistent bias conditions.
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75.85.+t Magnetoelectric effects, multiferroics
77.65.-j Piezoelectricity and electromechanical effects
75.30.Cr Saturation moments and magnetic susceptibilities

Origin of the dielectric response in Ba0.767Ca0.233TiO3

Takao Shimizu, Desheng Fu, Hiroki Taniguchi, Tomoyasu Taniyama, and Mitsuru Itoh

Appl. Phys. Lett. 100, 102908 (2012); http://dx.doi.org/10.1063/1.3693524 (4 pages)

Online Publication Date: 9 March 2012

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We perform a Raman scattering study to elucidate the mechanism of temperature independent dielectric response of Ba0.767Ca0.233TiO3 (BCTO-0.233) from a viewpoint of phonon dynamics. BCTO-0.233 remains tetragonal below Tc down to 3.5 K. Over-damped soft phonon observed in a pure BaTiO3 is found to become underdamped in BCTO-0.233, suggesting that local dipoles induced by Ca2+ off-centering suppress the hopping of Ti ions to reduce damping of the soft-mode. The temperature dependence of the soft-mode frequency agrees qualitatively with the dielectric permittivity through Lyddane-Sachs-Teller relationship, indicating that the dielectric response of BCTO-0.233 is governed by the soft-mode.
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77.22.Ch Permittivity (dielectric function)
77.22.Ej Polarization and depolarization
78.30.Hv Other nonmetallic inorganics
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
77.80.B- Phase transitions and Curie point
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The enhanced optical conductivity for zigzag-edge graphene nanoribbons with applied gate voltage

Wen-Huan Zhu, Guo-Hui Ding, and Bing Dong

Appl. Phys. Lett. 100, 103101 (2012); http://dx.doi.org/10.1063/1.3692171 (3 pages) | Cited 1 time

Online Publication Date: 5 March 2012

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We study the optical absorption properties of zigzag-edge graphene nanoribbons (ZGNRs) taking into account the Coulomb interaction effect in the Hartree-Fock approximation. The optical selection rules for the incident light polarized along the longitudinal and transverse directions are investigated. We demonstrate that the excitations from the edge states are essential for the optical properties of ZGNRs in the neutral case. With the chemical potential shifting away from the Dirac point, the optical conductivity is drastically enhanced in the low frequency region for the transverse polarized incident light.
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81.16.-c Methods of micro- and nanofabrication and processing
61.46.-w Structure of nanoscale materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.48.Gh Structure of graphene
78.67.Wj Optical properties of graphene
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Site-selective photoluminescence in thiol-capped gold nanoclusters

T. N. Lin, C. H. Liu, G. W. Shu, J. L. Shen, C. A. J. Lin, W. H. Chang, H. H. Wang, H. I. Yeh, and W. H. Chan

Appl. Phys. Lett. 100, 103102 (2012); http://dx.doi.org/10.1063/1.3692575 (4 pages)

Online Publication Date: 5 March 2012

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Photoluminescence (PL) from the thiol-capped Au nonoclusters (NCs) has been investigated under site-selective excitation. Upon scanning the excitation light with energy below 2.1 eV down to 1.6 eV, the PL narrows and begins shifting linearly with excitation energy. The time-resolved PL was studied and the PL decay traces of Au NCs were found to depend on the excitation and emission energies. The slow carrier relaxation in the localized states is suggested to be responsible for the line narrowing and peak-shift in the site-selective PL.
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78.55.-m Photoluminescence, properties and materials
78.47.jd Time resolved luminescence

Contacting nanowires and nanotubes with atomic precision for electronic transport

Shengyong Qin, Sondra Hellstrom, Zhenan Bao, Boyan Boyanov, and An-Ping Li

Appl. Phys. Lett. 100, 103103 (2012); http://dx.doi.org/10.1063/1.3692585 (3 pages) | Cited 1 time

Online Publication Date: 5 March 2012

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Making contacts to nanostructures with atomic precision is an important process in the bottom-up fabrication and characterization of electronic nanodevices. Existing contacting techniques use top-down lithography and chemical etching, but lack atomic precision and introduce the possibility of contamination. Here, we report that a field-induced emission process can be used to make local contacts onto individual nanowires and nanotubes with atomic spatial precision. The gold nano-islands are deposited onto nanostructures precisely by using a scanning tunneling microscope tip, which provides a clean and controllable method to ensure both electrically conductive and mechanically reliable contacts. To demonstrate the wide applicability of the technique, nano-contacts are fabricated on silicide atomic wires, carbon nanotubes, and copper nanowires. The electrical transport measurements are performed in situ by utilizing the nanocontacts to bridge the nanostructures to the transport probes.
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73.63.Rt Nanoscale contacts
81.07.Lk Nanocontacts
73.40.Ns Metal-nonmetal contacts
73.40.Jn Metal-to-metal contacts
81.16.-c Methods of micro- and nanofabrication and processing

Effects of nanowire texturing on the performance of Si/organic hybrid solar cells fabricated with a 2.2 μm thin-film Si absorber

Lining He, Changyun Jiang, Hao Wang, Donny Lai, Yew Heng Tan, Chuan Seng Tan, and Rusli

Appl. Phys. Lett. 100, 103104 (2012); http://dx.doi.org/10.1063/1.3692590 (4 pages) | Cited 6 times

Online Publication Date: 6 March 2012

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Hybrid solar cells are fabricated by spin coating poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) on planar Si and Si-nanowires (SiNWs) arrays prepared by electroless chemical etching. With only a 2.2 μm thick Si absorber thin film, the short-circuit current density and power conversion efficiency (PCE) of SiNWs/PEDOT cell increase from 12.5 to 13.6 mA/cm2 and from 5.4% to 5.6%, respectively, as compared to planar Si/PEDOT cell. A maximum external quantum efficiency of 56.6% is obtained for the SiNWs/PEDOT cell. The promising PCE obtained demonstrates the potential of realizing Si/PEDOT and SiNWs/PEDOT hybrid cells using low-cost Si thin films instead of bulk Si substrate.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells

Interface engineering of epitaxial graphene on SiC(000math) via fluorine intercalation: A first principles study

Chen Si, Gang Zhou, Yuanchang Li, Jian Wu, and Wenhui Duan

Appl. Phys. Lett. 100, 103105 (2012); http://dx.doi.org/10.1063/1.3692586 (4 pages) | Cited 3 times

Online Publication Date: 7 March 2012

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Our first-principles calculations show that a change of carrier type from electron to hole can be achieved in monolayer epitaxial graphene on SiC(000math) by fluorine (F) intercalation. The p-doping level in graphene, however, is not monotonously enhanced as the F coverage increases, and an interesting interface magnetism is observed at the partially passivated interface. Because intercalated F atoms prefer to bond to the substrate than to the graphene, F-intercalation provides a promising way of “interface modulation doping” to tailor the electronic properties of epitaxial graphene on SiC(000math) without appreciably degrading its intrinsic high mobility.
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71.20.Tx Fullerenes and related materials; intercalation compounds
75.70.Ak Magnetic properties of monolayers and thin films
72.20.Fr Low-field transport and mobility; piezoresistance
71.15.Dx Computational methodology (Brillouin zone sampling, iterative diagonalization, pseudopotential construction)
61.72.sm Impurity gradients

Patterning graphene nanoribbons using copper oxide nanowires

Alexander Sinitskii and James M. Tour

Appl. Phys. Lett. 100, 103106 (2012); http://dx.doi.org/10.1063/1.3692744 (3 pages) | Cited 1 time

Online Publication Date: 7 March 2012

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We present a fabrication technique for graphene nanoribbons (GNRs) that employs copper oxide nanowires as the etch masks. We demonstrate that these etch masks have numerous advantages: they can be synthesized simply by heating a copper foil in air, deposited on graphene from a solution, they are inert to oxygen plasma, and can be removed from the substrate by dissolution in mild acids. We fabricated GNRs in the device configuration and tested their electrical properties. Depending on the duration of the plasma etching, GNR devices exhibiting either standard ambipolar electric field effects or p-type transistor behaviors with ON-OFF ratios > 50 can be fabricated. The resulting devices based on narrow GNRs are demonstrated to exhibit promising electronic properties, which can be exploited in studies where GNR devices are required.
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81.16.Rf Micro- and nanoscale pattern formation
72.80.Vp Electronic transport in graphene
73.63.Bd Nanocrystalline materials
52.77.Bn Etching and cleaning
81.65.Cf Surface cleaning, etching, patterning

Intrinsic half-metallicity in hydrogenated boron-nitride nanoribbons

Duminda K. Samarakoon and Xiao-Qian Wang

Appl. Phys. Lett. 100, 103107 (2012); http://dx.doi.org/10.1063/1.3693174 (4 pages) | Cited 4 times

Online Publication Date: 7 March 2012

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We have investigated the structural and electronic characteristics of fully hydrogenated boron-nitride (BN) layer and zigzag-edged nanoribbons using dispersion-corrected density-functional calculations. In the fully hydrogenated BN structure, the hydrogen atoms adsorb on top of the B and N sites, alternating on both sides of the hexagonal BN-plane in a specific periodic manner. Among various low-energy hydrogenated membranes referred to as chair, boat, twist-boat, and stirrup, the stirrup conformation is the most energetically favorable one. The zigzag-edged BN nanoribbon, prominently fabricated in experiments, possesses intrinsic half-metallicity with full hydrogenation. The half-metallicity can be tuned by applying a transverse electric bias, thereby providing a promising route for spintronics device applications.
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61.46.-w Structure of nanoscale materials
73.22.-f Electronic structure of nanoscale materials and related systems
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.15.Nc Total energy and cohesive energy calculations

Detection of electromagnetic waves using charged cantilevers

P. G. Datskos, N. V. Lavrik, J. D. Tobin, and L. T. Bowland

Appl. Phys. Lett. 100, 103108 (2012); http://dx.doi.org/10.1063/1.3689017 (4 pages)

Online Publication Date: 7 March 2012

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We describe micromechanical structures that are capable of sensing both electrostatic fields and electromagnetic fields over a wide frequency range. Typically, sensing of electromagnetic waves is achieved with electrically conducting antennas, which despite the many advantages do not exhibit high sensitivity over a broad frequency range. An important aspect of our present work is that, in contrast to traditional antennas, the dimensions of micromechanical oscillators sensitive to electromagnetic waves can be much smaller than the wavelength. We characterized the micromechanical oscillators and measured responses to electric fields and estimated the performance limits by evaluating the signal-to-noise ratio theoretically and experimentally.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
84.30.Ng Oscillators, pulse generators, and function generators
84.40.Ba Antennas: theory, components and accessories
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Transport properties of hybrid graphene/graphane nanoribbons

Wei Zou, Zhizhou Yu, C. X. Zhang, J. X. Zhong, and L. Z. Sun

Appl. Phys. Lett. 100, 103109 (2012); http://dx.doi.org/10.1063/1.3692725 (4 pages)

Online Publication Date: 9 March 2012

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The transport properties of hybrid nanoribbons constructed by substituting zigzag graphane nanoribbons into zigzag graphene nanoribbons are investigated using the first-principles calculations and non-equilibrium Green’s function. The transport properties of the hybrid systems are improved due to the appearance of transport platform with nontrivial conductance around the Fermi level. This enhancement attributes to the extra bands induced by the substituted graphane nanoribbons. Moreover, this enhancement is sensitive to the position and concentration of the substituted graphane nanoribbons in the system. Our results indicate that such hybrid system is an effective approach to modulate the transport properties of zigzag graphene nanoribbons.
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73.22.Pr Electronic structure of graphene
72.80.Vp Electronic transport in graphene
73.63.Bd Nanocrystalline materials
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Bias-dependent oscillatory electron transport of monatomic sulfur chains

Jing-Xin Yu, Yan Cheng, Stefano Sanvito, and Xiang-Rong Chen

Appl. Phys. Lett. 100, 103110 (2012); http://dx.doi.org/10.1063/1.3693380 (4 pages)

Online Publication Date: 9 March 2012

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The bias-dependent oscillatory electron transport of monatomic sulfur chains sandwiched between gold electrodes is investigated with density functional theory and non-equilibrium Green’s function method. At zero bias, in contrast to the typical odd-even oscillations observed in most metallic chains, we find that the conductance oscillates with a period of four atoms. However, as the bias voltage is increased the current displays a two-atom periodicity. This emerges gradually, first for the longer chains and then, at voltages larger than 0.7 V, for lengths. The oscillatory behaviors are analyzed by the density of states and the energy-dependent and bias-dependent transmission coefficients.
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73.20.At Surface states, band structure, electron density of states
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
61.46.-w Structure of nanoscale materials
73.40.-c Electronic transport in interface structures

Ultra-long, free-standing, single-crystalline vanadium dioxide micro/nanowires grown by simple thermal evaporation

Chun Cheng, Kai Liu, Bin Xiang, Joonki Suh, and Junqiao Wu

Appl. Phys. Lett. 100, 103111 (2012); http://dx.doi.org/10.1063/1.3693381 (4 pages) | Cited 3 times

Online Publication Date: 9 March 2012

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Recently, it was discovered that single-crystalline VO2 nanostructures exhibit unique, single-domain metal-insulator phase transition. They enable a wide range of device applications as well as discoveries of oxide physics beyond those can be achieved with VO2 bulk or thin films. Previous syntheses of these nanostructures are limited in density, aspect ratio, single-crystallinity, or by substrate clamping. Here we break these limitations and synthesize ultra-long, ultra-dense, and free-standing VO2 micro/nanowires using a simple vapor transport method. These are achieved by enhancing the VO2 nucleation and growth rates using rough-surface quartz as the substrate and V2O5 powder as the evaporation source.
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81.16.-c Methods of micro- and nanofabrication and processing
71.30.+h Metal-insulator transitions and other electronic transitions
64.60.Q- Nucleation
68.55.-a Thin film structure and morphology
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)
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy

Influence of Si Co-doping on electrical transport properties of magnesium-doped boron nanoswords

Yuan Tian, Hongliang Lu, Jifa Tian, Chen Li, Chao Hui, Xuezhao Shi, Yuan Huang, Chengmin Shen, and Hong-jun Gao

Appl. Phys. Lett. 100, 103112 (2012); http://dx.doi.org/10.1063/1.3693383 (5 pages)

Online Publication Date: 9 March 2012

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Magnesium-doped boron nanoswords were synthesized via a thermoreduction method. The as-prepared nanoswords are single crystalline and β-rhombohedral (β-rh) phase. Electrical transport measurements show that variable range hopping conductivity increases with temperature, and carrier mobility has a greater influence than carrier concentration. These results are consistent with the three dimensional Mott’s model (M. Cutler and N. F. Mott, Phys. Rev. 181, 1336 (1969)) besides a high density of localized states at the Fermi level compared with bulk β-rh boron. Conductivity of Mg-doped boron nanoswords is significantly lower than that of “pure” (free of magnesium) boron nanoswords. Electron energy loss spectroscopy studies confirm that the poorer conductivity arises from silicon against magnesium doping.
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81.05.Bx Metals, semimetals, and alloys
79.20.Uv Electron energy loss spectroscopy
73.22.-f Electronic structure of nanoscale materials and related systems
73.63.Bd Nanocrystalline materials
81.16.-c Methods of micro- and nanofabrication and processing
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
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Direct probing of selective electron and hole accumulation processes along the channel of an ambipolar double-layer field-effect transistor by optical modulation spectroscopy

Le Zhang, Dai Taguchi, Takaaki Manaka, and Mitsumasa Iwamoto

Appl. Phys. Lett. 100, 103301 (2012); http://dx.doi.org/10.1063/1.3692581 (4 pages) | Cited 1 time

Online Publication Date: 6 March 2012

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By using optical modulation spectroscopy, the electronic state and selective carrier accumulation processes in the channel region of unipolar and ambipolar organic field effect transistors (OFETs) were directly probed. The modulated spectra of unipolar single-layer pentacene and C60 OFETs corresponding well with their I–V characteristics were understood in terms of the injected carrier induced effect. On the other hand, for the double-layer C60/pentacene OFETs, it is indicated that the modulated spectra under both hole and electron accumulation cases generated mainly from the pentacene layer but with different characteristics, implying that the injected carriers accumulated at spatially separated locations.
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85.30.Tv Field effect devices
FREE

Electron depletion and accumulation regions in n-type copper-hexadecafluoro-phthalocyanine and their effects on electronic properties

Haibo Wang, Zengtao Liu, Ming Fai Lo, Tsz Wai Ng, Donghang Yan, and Chun-Sing Lee

Appl. Phys. Lett. 100, 103302 (2012); http://dx.doi.org/10.1063/1.3691257 (3 pages)

Online Publication Date: 6 March 2012

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We investigated interfacial electronic structures of organic/inorganic heterojunctions formed between copper-hexadecafluoro-phthalocyanine (F16CuPc) and molybdenum trioxide or lead monoxide by ultraviolet and x-ray photoemission spectroscopies. Obvious energy level bending in F16CuPc shows the formation of electron depletion and accumulation regions in its contact with molybdenum and lead oxides, respectively. Effects of these depletion/accumulation zones were studied via electronic characteristics of field-effect transistors constructed with these heterojunctions as active layers. Finally, a summary about the heterojunction including organic semiconductor (n-type and p-type) and metal oxide was given, which demonstrated that organic semiconductors can form various space charge regions like conventional inorganic semiconductors.
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73.21.Ac Multilayers
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
77.22.Jp Dielectric breakdown and space-charge effects
78.40.-q Absorption and reflection spectra: visible and ultraviolet
79.60.Bm Clean metal, semiconductor, and insulator surfaces
FREE

Efficient polymer solar cells based on light-trapping transparent electrodes

Ziyang Hu, Jianjun Zhang, and Ying Zhao

Appl. Phys. Lett. 100, 103303 (2012); http://dx.doi.org/10.1063/1.3691906 (4 pages) | Cited 2 times

Online Publication Date: 6 March 2012

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Highly efficient and cost-effective polymer solar cells (PSCs) fabricated on textured fluorine doped transparent conductive (FTO) electrodes are achieved. Such electrodes with rough surface in combination with reflective back contacts provide efficient light-trapping by light scattering and multiple reflections to increase the path length of the light. Due to the increased light absorption in active layer, the resultant PSCs base on polymer:fullerene system show 10% and 8% improvement in short current density and efficiency, respectively, compared with the reference cell based on the flat electrodes.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
FREE

Top-to-bottom optimization of the optical performance of the tandem organic solar cells with thin metal film as interlayer

Yongbing Long (龙拥兵), Liang Shen (沈亮), Shenping Ruan (阮圣平), Wenjuan Yu (于文娟), Yi Wang (王忆), Qingguang Zeng (曾庆光), and Jianyi Luo (罗坚义)

Appl. Phys. Lett. 100, 103304 (2012); http://dx.doi.org/10.1063/1.3692597 (4 pages)

Online Publication Date: 7 March 2012

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Top-to-bottom optimization is developed to maximize the absorption for tandem organic solar cells with thin Ag interlayer connecting two subcells. By redshifting the cavity modes of the microcavity between the Ag interlayer and the top electrode, the absorption spectrum of the top cell can be shifted to the near-infrared wavelength range where the bottom cell has weak absorption. Correspondingly, subcells with highly complementary absorption spectrum are achieved, and there is an improvement of 17.8% in the total absorbed photons for the tandem device. Additionally, it is revealed that high transparency is not an essential property for the interlayer in tandem devices.
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88.40.jr Organic photovoltaics
FREE

Simultaneous enhancement of charge transport and exciton diffusion in single-crystal-like organic semiconductors

Jianbing Yang, Feng Zhu, Bo Yu, Haibo Wang, and Donghang Yan

Appl. Phys. Lett. 100, 103305 (2012); http://dx.doi.org/10.1063/1.3692106 (4 pages) | Cited 2 times

Online Publication Date: 7 March 2012

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The charge transport and exciton diffusion have been simultaneously enhanced in the highly ordered and oriented zinc phthalocyanine (ZnPc) films which are fabricated by weak epitaxy growth method compared to the normal ZnPc films (charge mobility from 4 × 10−5 cm2/Vs to 1.1 × 10−4 cm2/Vs and exciton diffusion length from 9 nm to 16 nm). The temperature dependent current-voltage (I-V) characteristics are utilized to investigate the charge transport mechanism, which reveal the clear mechanism and the role of traps in different films. These results reveal the direct relationship between traps properties and transport behavior of both exciton and charge.
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71.35.-y Excitons and related phenomena
66.30.-h Diffusion in solids
68.55.-a Thin film structure and morphology
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Mode discretization in an organic microcavity including a perforated silver layer

C. Reinhardt, R. Brückner, J. Haase, M. Sudzius, S. I. Hintschich, H. Fröb, V. G. Lyssenko, and K. Leo

Appl. Phys. Lett. 100, 103306 (2012); http://dx.doi.org/10.1063/1.3690053 (4 pages)

Online Publication Date: 8 March 2012

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Two optical Tamm plasmons and a discretized microcavity state are observed simultaneously in an organic microcavity by angle-resolved photoluminescence spectroscopy. The Tamm plasmons form as a result of a 40 nm silver layer placed between the bottom distributed Bragg reflector and the λ/2 cavity layer. This silver layer is perforated by round holes of a few microns size, generating optical mesas from which discretized microcavity states are observed concurrently. The discretization and the intensity of the different states are studied as a function of angle and hole diameter and compared to analytical calculations.
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42.79.Bh Lenses, prisms and mirrors
42.82.-m Integrated optics
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
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High brightness alternating current electroluminescence with organic light emitting material

Ajay Perumal, Björn Lüssem, and Karl Leo

Appl. Phys. Lett. 100, 103307 (2012); http://dx.doi.org/10.1063/1.3692776 (4 pages) | Cited 3 times

Online Publication Date: 8 March 2012

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We report on an alternating current (AC) organic electroluminance (EL) device with high brightness of over 1500 cdm−2 and very low threshold voltage for the onset of AC luminance (10 V). The device consists of organic layers enclosed in a pair of either silicon dioxide (SiO2) or hafnium dioxide (HfO2) insulating dielectric layers. We observe internal charge carrier generation and luminance due to charge carrier recombination within the device under the AC drive without charge injection. We compare AC-EL demonstrated for the two devices and show that the threshold voltage for the onset of luminance can be significantly lowered using high dielectric constant (κ) HfO2 insulating layers. Further, we discuss the limitations of internal charge carrier generation and the device performance supported with the results from time resolved measurements.
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85.60.Jb Light-emitting devices
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