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15 Sep 2009

Volume 106, Issue 6, Articles (06xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 106, 061101 (2009); http://dx.doi.org/10.1063/1.3216463 (14 pages)

Carolyn A. Koh, Amadeu K. Sum, and E. Dendy Sloan
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A study on stability and thermophysical properties (density and viscosity) of Al2O3 in water nanofluid

M. J. Pastoriza-Gallego, C. Casanova, R. Páramo, B. Barbés, J. L. Legido, and M. M. Piñeiro

J. Appl. Phys. 106, 064301 (2009); http://dx.doi.org/10.1063/1.3187732 (8 pages) | Cited 21 times

Online Publication Date: 16 September 2009

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The dispersion and stability of nanofluids obtained by dispersing Al2O3 nanoparticles (obtained from different sources) in water have been analyzed. The differences arising from different dispersion techniques, the resulting particle size distribution, and time stability among the different samples are evaluated. Then the volumetric behavior up to high pressures (25 MPa) and atmospheric pressure viscosity were experimentally determined. It has been found that the influence of particle size in density is subtle but not negligible, but the differences in viscosity are very large and must be taken into account for any practical application. These viscosity differences can be rationalized by considering a theory describing the aggregation state of the nanofluid.
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66.20.-d Viscosity of liquids; diffusive momentum transport
62.50.-p High-pressure effects in solids and liquids

Synthesis and microwave electromagnetic properties of CoFe alloy nanoflakes prepared with hydrogen-thermal reduction method

Y. X. Gong, L. Zhen, J. T. Jiang, C. Y. Xu, and W. Z. Shao

J. Appl. Phys. 106, 064302 (2009); http://dx.doi.org/10.1063/1.3211987 (5 pages) | Cited 8 times

Online Publication Date: 18 September 2009

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CoFe alloy nanoflakes (NFs) with diameter and thickness on nanoscale were prepared by hydrogen-thermal reduction in CoFe2O4 flakes at 400 °C for 60 min. The effective complex permittivity and permeability of CoFe alloy NFs/paraffin composites were measured and compared with that of CoFe alloy nanoparticles (NPs)/paraffin composites. Due to the two-dimensional shape character, the real part of permittivity and permeability of CoFe alloy NFs was rather higher than that of CoFe alloy NPs. Electromagnetic wave absorbing (EMA) performance of both CoFe alloy NFs and NPs was evaluated by using transmission line theory. The effective EMA band position of the coating with CoFe alloy NFs as fillers was found to locate in the range of 2–4 GHz, while the effective EMA band position of the coating containing CoFe alloy NPs as fillers was located in the range 8–18 GHz. A maximum reflection loss (RLmax) of −57.8 dB was achieved in a coating containing CoFe alloy NFs as fillers, which is much higher than the −16.6 dB of coatings with CoFe alloy NPs.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
77.22.Ch Permittivity (dielectric function)
41.20.Jb Electromagnetic wave propagation; radiowave propagation

ZnO nanowire arrays grown on Al:ZnO buffer layers and their enhanced electron field emission

Z. H. Chen, Y. B. Tang, Y. Liu, G. D. Yuan, W. F. Zhang, J. A. Zapien, I. Bello, W. J. Zhang, C. S. Lee, and S. T. Lee

J. Appl. Phys. 106, 064303 (2009); http://dx.doi.org/10.1063/1.3213091 (6 pages) | Cited 6 times

Online Publication Date: 18 September 2009

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Arrays of highly ordered ZnO nanowires have been synthesized on polycrystalline Al-doped ZnO (AZO) buffer layers prepared on p-Si substrates (7–13 Ω cm) with assistance of a thermal deposition method. The diameter and interspacing of the nanowires have been controlled by the growth conditions and properties of AZO films. The optimized array of ZnO nanowires shows low turn-on and threshold fields ( ∼ 1.1 and ∼ 3.0 V/μm, respectively) and displays exceptional time stability of electron field emission. The time-fluctuation instability was found to be less than 0.6% at a current density of 10 mA/cm2, as measured for 500 min. The low turn-on and threshold fields as well as the stable electron emission current suggest that the arrays of ZnO nanowires could be considered in some electron field emission applications.
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79.70.+q Field emission, ionization, evaporation, and desorption
81.16.Be Chemical synthesis methods
61.46.Np Structure of nanotubes (hollow 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)

Structure and magnetic properties of nanophase-LiFe1.5P2O7

C. V. Ramana, M. Kopec, A. Mauger, F. Gendron, and C. M. Julien

J. Appl. Phys. 106, 064304 (2009); http://dx.doi.org/10.1063/1.3213093 (6 pages) | Cited 1 time

Online Publication Date: 18 September 2009

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The structure and magnetic properties of lithium iron pyrophosphate, i.e., Li2Fe3(P2O7)2 or LiFe1.5P2O7, synthesized using a facile metal acetate approach for application in lithium-ion batteries, are investigated in detail. The high-resolution transmission electron microscopy, selected area electron diffraction, and x-ray diffraction measurements indicate that Li2Fe3(P2O7)2 is crystallized in the monoclinic structure, without any indication of crystallographic defects such as dislocations or misfits, and exhibit smooth surface morphology. The evaluated lattice parameters are a = 0.698 76 nm, b = 0.812 36 nm, c = 0.964 22 nm, and β = 111.83° (P21/c space group). Infrared spectroscopic measurements indicate the presence of P2O7 groups, which are formed by the two PO4 tetrahedral groups connected together. The magnetic measurements indicate that Li2Fe3(P2O7)2 is a weak antiferromagnetic material with TN = 20 K exhibiting a Curie constant Cp = 3.38 emu K/mol per Fe ion and a negative value of the Weiss temperature (Θp = −15 K). The absence of higher valence state Fe impurities and antiferromagnetic interactions due to the greater distance between two equivalent magnetic ions, which vanishes the Fe–O–Fe superexchange interactions, is confirmed.
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61.46.-w Structure of nanoscale materials
75.50.Tt Fine-particle systems; nanocrystalline materials
75.30.Et Exchange and superexchange interactions
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
82.47.Aa Lithium-ion batteries
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
81.07.Bc Nanocrystalline materials
75.50.Ee Antiferromagnetics
61.66.Fn Inorganic compounds
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.30.Hv Other nonmetallic inorganics

Longitudinal thermal conductivity of radial nanowire heterostructures

Xiang Lü

J. Appl. Phys. 106, 064305 (2009); http://dx.doi.org/10.1063/1.3223329 (7 pages) | Cited 4 times

Online Publication Date: 21 September 2009

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Thermal conductivity of tubular nanowires and radial nanowire heterostructures is analytically modeled along the longitudinal direction by using Boltzmann transport equation. This work is on the basis of Dingle [Proc. R. Soc. London, Ser. A 201, 545 (1950)] and Lucas [J. Appl. Phys. 36, 1632 (1965)] formalisms on thin wires and films, respectively. To investigate the thermal conductivity dependence on the interface conditions, we have generalized Prasher’s analytical solution [ Appl. Phys. Lett. 89, 063121 (2006) ] to cover the case where the scattering events at the interfaces are not totally diffuse scattering. The calculation of the size-dependent thermal conductivity includes the partly diffuse and partly specular scatterings at both internal and external interfaces of the tubular nanowires. It is found that the calculated thermal conductivities are in good agreement with the numerical solution of Yang et al. [Nano Lett. 5, 1111 (2005)] . Comparison is also made with the thermal conductivity of thin films and solid nanowires with the same dimensions. Results show that the thermal conductivity of the structures can be modulated by changing the radius ratio between the shell layer and the core layer of the radial nanowire heterostructures. The obtained results may serve as a possible way for tuning the thermal conductivity in nanostructures.
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65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
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)
72.10.Bg General formulation of transport theory
68.60.Dv Thermal stability; thermal effects
73.63.-b Electronic transport in nanoscale materials and structures

Charge spectroscopy of Si nanocrystallites embedded in a SiO2 matrix

Irina V. Antonova, Vladimir A. Volodin, Efim P. Neustroev, Svetlana A. Smagulova, Jedrzej Jedrzejewsi, and Isaac Balberg

J. Appl. Phys. 106, 064306 (2009); http://dx.doi.org/10.1063/1.3224865 (6 pages) | Cited 5 times

Online Publication Date: 21 September 2009

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In the present work we have determined the electronic levels in systems of Si nanocrystallites (NCs) embedded in the insulating matrix of silicon dioxide, SiO2, by employing the charge deep-level transient spectroscopy (Q-DLTS) technique. We have clearly shown that these levels are associated with the NCs. Correspondingly, we suggest that the levels that we found are associated mainly with two quantum confinement energies, 0.14 and 0.19 eV. These energies are shown to be consistent with the corresponding theoretical estimates for the presently studied Si–NCs/SiO2 systems. The fact that these levels are almost fixed for the various samples studied suggests the importance of the bulk-surface coupling under quantum confinement conditions.
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81.07.Bc Nanocrystalline materials
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
71.55.-i Impurity and defect levels
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems

A comparative study of thermal behavior of iron and copper nanofluids

Kaustav Sinha, Barkan Kavlicoglu, Yanming Liu, Faramarz Gordaninejad, and Olivia A. Graeve

J. Appl. Phys. 106, 064307 (2009); http://dx.doi.org/10.1063/1.3225574 (7 pages) | Cited 9 times

Online Publication Date: 21 September 2009

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Nanofluids consist of nanoparticles dispersed in heat transfer carrier fluid and are typically used for enhancing thermal conductivity in devices and systems. This study investigated the synthesis of iron and copper nanoparticle-based thermal fluids prepared using a two-step process. Chemical precipitation was used for the synthesis of the powders, and ultrasonic irradiation was used to disperse the nanoparticles in the carrier fluid (ethylene glycol). The size distributions of the nanopowders in the carrier fluid were determined using dynamic light scattering resulting in average particle sizes of around 500 nm. The crystallite sizes of the powders were below 20 nm. Thus, both types of nanofluids are comparable with regard to crystallite size, particle size, and morphology resulting in a direct comparison of material properties and their effect on thermal conductivity of the nanofluids. A guarded hot parallel-plate method and dynamic tests were used to compare the thermal conductivities of the nanofluids. It was shown that thermal conductivity can be enhanced by up to 70% for copper nanofluids. It was also demonstrated that for a given particle concentration, copper nanofluids are superior in thermal conductivity compared to iron nanofluids.
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73.63.Bd Nanocrystalline materials
81.16.-c Methods of micro- and nanofabrication and processing
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
62.60.+v Acoustical properties of liquids
72.15.Cz Electrical and thermal conduction in amorphous and liquid metals and alloys
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters

Modulated reflectance study of InAs quantum dot stacks embedded in GaAs/AlAs superlattice

R. Nedzinskas, B. Čechavičius, J. Kavaliauskas, V. Karpus, D. Seliuta, V. Tamošiūnas, G. Valušis, G. Fasching, K. Unterrainer, and G. Strasser

J. Appl. Phys. 106, 064308 (2009); http://dx.doi.org/10.1063/1.3212980 (5 pages) | Cited 1 time

Online Publication Date: 23 September 2009

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Optical transitions in vertically stacked InAs quantum dot (QD) superlattice (SL) with and without AlAs barriers were examined by photo- and electroreflectance techniques. The interband transitions corresponding to the QD, wetting layer (WL), and InAs/GaAs/AlAs SL have been identified. Experimental data and numerical calculations show that blueshifts and enhancement in the intensity of WL-related optical transitions in an InAs/GaAs/AlAs SL originate mainly due to off-center position of the QD layers in the quantum wells. The appearance of multiple WL-related features in the modulated reflectance spectra was revealed and discussed.
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78.67.Hc Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
78.20.Jq Electro-optical effects
68.65.Cd Superlattices
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials

Laser-induced self-assembly of iron oxide nanostructures with controllable dimensionality

Simon J. Henley, Shafikuddin Mollah, Christina E. Giusca, and S. Ravi P. Silva

J. Appl. Phys. 106, 064309 (2009); http://dx.doi.org/10.1063/1.3224854 (8 pages) | Cited 3 times

Online Publication Date: 23 September 2009

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The nanosecond pulsed laser ablation of fine iron powder submerged under different liquid media (water, methanol, ethanol, and isopropanol) is used to rapidly produce a variety of iron oxide nanostructures from nanoparticles to nanowires and nanosheets. The dimensionality of the nanostructures is shown to be a consequence of two controllable mechanisms. The rapid oxidation, collisional quenching, and coalescence of the ablation products are suggested as the dominant mechanisms for the formation of zero-dimensional nanostructures such as hematite (α-Fe2O3) nanoparticles in water, or iron oxyhydroxide nanoparticles under alcohols. By employing different laser wavelengths (248 and 532 nm) it is demonstrated that the growth of extended iron oxyhydroxide nanostructures (one-dimensional nanowires and two-dimensional nanosheets) under methanol is possible and is a consequence of a second self-assembly mechanism driven by interaction between the UV laser pulses and the ablation products.
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81.16.Dn Self-assembly
81.16.Mk Laser-assisted deposition
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)
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
81.40.Gh Other heat and thermomechanical treatments
81.16.Pr Micro- and nano-oxidation
81.65.Mq Oxidation
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Design optimization of piezoresistive cantilevers for force sensing in air and water

Joseph C. Doll, Sung-Jin Park, and Beth L. Pruitt

J. Appl. Phys. 106, 064310 (2009); http://dx.doi.org/10.1063/1.3224965 (12 pages) | Cited 13 times

Online Publication Date: 23 September 2009

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Piezoresistive cantilevers fabricated from doped silicon or metal films are commonly used for force, topography, and chemical sensing at the micro- and macroscales. Proper design is required to optimize the achievable resolution by maximizing sensitivity while simultaneously minimizing the integrated noise over the bandwidth of interest. Existing analytical design methods are insufficient for modeling complex dopant profiles, design constraints, and nonlinear phenomena such as damping in fluid. Here we present an optimization method based on an analytical piezoresistive cantilever model. We use an existing iterative optimizer to minimimize a performance goal, such as minimum detectable force. The design tool is available as open source software. Optimal cantilever design and performance are found to strongly depend on the measurement bandwidth and the constraints applied. We discuss results for silicon piezoresistors fabricated by epitaxy and diffusion, but the method can be applied to any dopant profile or material which can be modeled in a similar fashion or extended to other microelectromechanical systems.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
84.32.Ff Conductors, resistors (including thermistors, varistors, and photoresistors)
85.30.-z Semiconductor devices
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
85.40.Ry Impurity doping, diffusion and ion implantation technology

An energy-based model to predict wear in nanocrystalline diamond atomic force microscopy tips

R. Agrawal, N. Moldovan, and H. D. Espinosa

J. Appl. Phys. 106, 064311 (2009); http://dx.doi.org/10.1063/1.3223316 (6 pages) | Cited 10 times

Online Publication Date: 24 September 2009

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Atomic force microscopy (AFM) is one of the most powerful techniques to probe surfaces and material properties at the nanoscale, and pattern organic and inorganic molecules. In all cases, knowledge of the tip geometry and its evolution with continued use is essential. In this work, a broadly applicable energy model for the evolution of scanning probe tip radii during use is presented based on quantitative wear experiments. Experiments were conducted using AFM probes made of both undoped and nitrogen-doped diamond. Undoped diamond probes were found to be nearly ten times more wear resistant than commercially available silicon nitride probes. For a constant applied force, a linear relationship between wear volume and total dissipation energy is identified. The change in tip radius was also found to be proportional to the square root of scan distance, x0.5.
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07.79.Lh Atomic force microscopes
81.16.Ta Atom manipulation
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
62.20.Qp Friction, tribology, and hardness
81.40.Pq Friction, lubrication, and wear

Magnetoelectric effect in BaTiO3/Ni particulate nanocomposites at microwave frequencies

V. Castel, C. Brosseau, and J. Ben Youssef

J. Appl. Phys. 106, 064312 (2009); http://dx.doi.org/10.1063/1.3225567 (15 pages) | Cited 10 times

Online Publication Date: 25 September 2009

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We present a comprehensive study of the magnetic and microwave properties of piezoelectric BaTiO3/magnetostrictive Ni nanocomposites (NCs), fabricated under uniaxial compression, at room temperature. In the current work, we investigated samples in the compositional range between 0 ≤ fNi ≤ 33.5 vol % and from 0.1 to 6 GHz using broadband microwave spectroscopy in combination with atomic and magnetic force microscopy (MFM), x-ray diffraction (XRD), electron transport, and broadband (6–28 GHz) ferromagnetic resonance (FMR) experiments in the microwave regime to correlate magnetization dynamics, electromagnetic materials parameters, and microstructural information. The static magnetic response is consistent with a model of a composite medium with an unmodified Ni phase in a nonmagnetic matrix. We provide the experimental evidence for a magnetoelectric (ME) effect, i.e., the effective permittivity at microwave frequencies can be controlled by an external magnetic field, which makes these nanostructures ready for microwave tunable devices, sensors, and transducers. We show in the analysis that this magnetic field dependence is inconsistent with expectations from magnetoresistance and magnetocapacitance effects, and propose as an alternative an explanation based on the striction across the interfaces between the magnetic and piezoelectric phases. By varying the Ni content and frequency, room temperature broadband FMR was performed in order to investigate the different contributions, e.g., inhomogeneous broadening, to the effective linewidth and microwave damping. The line broadening and asymmetry of the FMR features are not intrinsic properties of the metallic nanophase but reflects the local nonmagnetic environment in which they are embedded. The increase in the effective Gilbert damping coefficient as function of the Ni content is related to the strong increase in the damping experienced by the precessing magnetization in the Ni phase. One of the characteristic features of the present results is the significant correlation between the internal field probed by FMR and the ME coupling coefficient evaluated by microwave spectroscopy which was not observed in our previous study of ZnO/Ni NCs. The present results highlight the strong influence of interfaces of the composite constituent play a crucial role in the analysis of the ME coupling. In addition MFM has been successfully used to detect the strong magnetic contrast between the phases of these nanostructures which indicates local changes in composition and structure.
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75.80.+q Magnetomechanical effects, magnetostriction
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
68.37.Ps Atomic force microscopy (AFM)
68.37.Rt Magnetic force microscopy (MFM)
75.50.Cc Other ferromagnetic metals and alloys
77.22.Ch Permittivity (dielectric function)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
78.70.Gq Microwave and radio-frequency interactions
75.50.Tt Fine-particle systems; nanocrystalline materials

Surface chemistry dependence of native oxidation formation on silicon nanocrystals

R. W. Liptak, U. Kortshagen, and S. A. Campbell

J. Appl. Phys. 106, 064313 (2009); http://dx.doi.org/10.1063/1.3225570 (5 pages) | Cited 8 times

Online Publication Date: 25 September 2009

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The growth of silicon oxide on bare and SF6-etched silicon nanocrystals (Si-NCs), which were synthesized by an all gas phase approach, was investigated by examining the surface chemistry and optical properties of the NCs over time. Consistent with previous work in the low temperature oxidation of silicon, the oxidation follows the Cabrera–Mott mechanism, and the measured data are well fitted to the Elovich equation. The use of the SF6 plasma is found to reduce the surface Si–H bond density and dramatically increase the monolayer growth rate. This is believed to be due to the much larger volatility of Si–F bonds compared to Si–H bonds on the surface of the NC.
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81.65.Mq Oxidation
81.07.Bc Nanocrystalline materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
81.16.Pr Micro- and nano-oxidation
78.30.Hv Other nonmetallic inorganics
81.65.Cf Surface cleaning, etching, patterning
52.77.Bn Etching and cleaning

Surface plasmon resonance and super-resolution imaging by anisotropic superlens

Changtao Wang, Chunlei Du, and Xiangang Luo

J. Appl. Phys. 106, 064314 (2009); http://dx.doi.org/10.1063/1.3224959 (4 pages) | Cited 1 time

Online Publication Date: 28 September 2009

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Surface plasmon resonance with large transversal wave vector occurs at the interface of effective anisotropic metallic media comprising layered metallodielectric films, provided that appropriate permittivity and geometrical parameters are selected. This results in, as demonstrated with analytical investigation and numerical simulation, evanescent waves amplification and super-resolution imaging with extra working distance as a generalization of the superlens [ J. B. Pendry, Phys. Rev. Lett. 85, 3966 (2000) ].
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
77.22.Ch Permittivity (dielectric function)

Local etching of nanoholes and quantum rings with InxGa1−x droplets

A. Stemmann, T. Köppen, M. Grave, S. Wildfang, S. Mendach, W. Hansen, and Ch. Heyn

J. Appl. Phys. 106, 064315 (2009); http://dx.doi.org/10.1063/1.3225759 (4 pages) | Cited 8 times

Online Publication Date: 28 September 2009

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We study the formation of nanoholes and quantum rings in GaAs and AlGaAs surfaces by local droplet etching with InxGa1−x. The rings are crystallized from droplet material and surround the nanohole openings. In particular, the influence of the In content x on density, diameter, and depth of the nanoholes is investigated. Our data establish an exponential dependence of these quantities on x, which is quantitatively reproduced by a model that considers different surface diffusion energy barriers for Ga and In. By etching with pure In, hole densities as low as 5×106 cm−2 have been achieved. In addition, for low In content incompletely removed initial droplets are visible on the surface. These droplets are not visible on samples with x>0.5 which indicates a higher desorption rate of In compared to Ga. As a consequence, even in the case of etching with InGa the quantum rings consist of nearly pure GaAs. This is confirmed by photoluminescence experiments of quantum rings overgrown with AlGaAs barrier material.
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81.05.Ea III-V semiconductors
81.07.Ta Quantum dots
81.65.Cf Surface cleaning, etching, patterning
81.16.-c Methods of micro- and nanofabrication and processing
68.65.Hb Quantum dots (patterned in quantum wells)
68.35.Fx Diffusion; interface formation

Stepped light-induced transient measurements of photocurrent and voltage in dye-sensitized solar cells based on ZnO and ZnO:Ga

Agnaldo de Souza Gonçalves, Marian R. Davolos, Naruhiko Masaki, Shozo Yanagida, Shogo Mori, and Ana F. Nogueira

J. Appl. Phys. 106, 064316 (2009); http://dx.doi.org/10.1063/1.3226073 (4 pages) | Cited 2 times

Online Publication Date: 29 September 2009

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In order to explain the higher short-circuit current (Jsc) with comparable open-circuit voltage (Voc) from dye-sensitized solar cells (DSCs) based on gallium-modified ZnO (ZnO:Ga) porous electrodes, the diffusion coefficient (D) and electron lifetime (τ) in DSCs with and without Ga-modified ZnO were studied by stepped light-induced transient measurements of photocurrent and voltage. In comparison to DSCs based on ZnO electrodes, the ZnO:Ga-based solar cells provided lower D and higher τ values. The results were interpreted according to the transport-limited recombination model, where the Ga modification induced a higher density of intraband charge traps. At matched electron densities, a decrease in Voc from DSCs based on ZnO:Ga was observed, suggesting a positive shift of the ZnO:Ga conduction band edge. The higher Jsc can be explained by the positive shift of the ZnO:Ga conduction band edge in addition to the increased roughness factor of the electrode due to the Ga modification.
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84.60.Jt Photoelectric conversion
71.20.Nr Semiconductor compounds
72.40.+w Photoconduction and photovoltaic effects
66.30.H- Self-diffusion and ionic conduction in nonmetals
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.80.Ey III-V and II-VI semiconductors

Dwell time in graphene-based magnetic barrier nanostructures

Yiyang Gong and Yong Guo

J. Appl. Phys. 106, 064317 (2009); http://dx.doi.org/10.1063/1.3225914 (6 pages) | Cited 4 times

Online Publication Date: 30 September 2009

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The authors have investigated the dwell time of electrons tunneling through nonuniform magnetically modulated graphene monolayer. Two types of models, i.e., the square magnetic barrier and the δ-function magnetic barrier, are introduced to simulate the magnetic modulation realized by depositing nanoscale ferromagnetic stripes on top of the graphene monolayer. It is found that both the dwell time and the transmission probability show remarkable anisotropy that varies in different magnetically modulated configurations. Particularly, when the electrons tunnel through the graphene monolayer modulated by two antiparallelly aligned ferromagnetic stripes, the corresponding transmission probability exhibits angularly symmetric property, whereas the dwell time does not. Moreover, there exists great discrepancy of the dwell time between in the Klein tunneling region and in the resonant tunneling region, where each region corresponds to the perfect transmission peaks.
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75.70.Ak Magnetic properties of monolayers and thin films
75.50.Tt Fine-particle systems; nanocrystalline materials
75.75.-c Magnetic properties of nanostructures
75.30.Gw Magnetic anisotropy
73.63.-b Electronic transport in nanoscale materials and structures
75.50.Dd Nonmetallic ferromagnetic materials
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