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28 Nov 2011

Volume 99, Issue 22, Articles (22xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 99, 221902 (2011); http://dx.doi.org/10.1063/1.3663578 (3 pages)

Aparna Deshpande, Kai Felix Braun, and Saw-Wai Hla
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Solar blind metal-semiconductor-metal ultraviolet photodetectors using quasi-alloy of BGaN/GaN superlattices

H. Srour, J. P. Salvestrini, A. Ahaitouf, S. Gautier, T. Moudakir, B. Assouar, M. Abarkan, S. Hamady, and A. Ougazzaden

Appl. Phys. Lett. 99, 221101 (2011); http://dx.doi.org/10.1063/1.3662974 (3 pages)

Online Publication Date: 28 November 2011

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Large internal gains that can be obtained in wide band gap semiconductors-based (GaN and ZnO types) Schottky and/or metal-semiconductor-metal photodetectors are generally accompanied by large dark current and time response. We show that, using quasi-alloy of BGaN/GaN superlattices as the active layer, the dark current can be lowered while maintaining high internal gain (up to 3 × 104) for optical power in the nW range and low time response (few tens of ns) for optical power in the W range. Furthermore, the boron incorporation allows the tuning of the cutoff wavelength.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
68.65.Cd Superlattices
73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Sx Metal-semiconductor-metal structures
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Polarization dependent state to polarization independent state change in THz metamaterials

W. M. Zhu, A. Q. Liu, W. Zhang, J. F. Tao, T. Bourouina, J. H. Teng, X. H. Zhang, Q. Y. Wu, H. Tanoto, H. C. Guo, G. Q. Lo, and D. L. Kwong

Appl. Phys. Lett. 99, 221102 (2011); http://dx.doi.org/10.1063/1.3664131 (3 pages) | Cited 2 times

Online Publication Date: 28 November 2011

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We experimentally demonstrated a polarization dependent state to polarization independent state change in terahertz (THz) metamaterials. This is accomplished by reconfiguring the lattice structure of metamaterials from 2-fold to 4-fold rotational symmetry by using micromachined actuators. In experiment, it measures resonance frequency shift of 25.8% and 12.1% for TE and TM polarized incidence, respectively. Furthermore, single-band to dual-band switching is also demonstrated. Compared with the previous reported tunable metamaterials, lattice reconfiguration promises not only large tuning range but also changing of polarization dependent states, which can be used in photonic devices such as sensors, optical switches, and filters.
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42.70.-a Optical materials
42.79.Ci Filters, zone plates, and polarizers
42.65.Pc Optical bistability, multistability, and switching, including local field effects
81.20.Wk Machining, milling
07.10.Cm Micromechanical devices and systems
81.05.Xj Metamaterials for chiral, bianisotropic and other complex media

Performance enhancement of blue light-emitting diodes with a special designed AlGaN/GaN superlattice electron-blocking layer

Yun Yan Zhang and Yi An Yin

Appl. Phys. Lett. 99, 221103 (2011); http://dx.doi.org/10.1063/1.3653390 (3 pages) | Cited 6 times

Online Publication Date: 28 November 2011

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The characteristics of the nitride-based blue light-emitting diode (LED) with an AlGaN/GaN superlattice (SL) electron-blocking layer (EBL) of gradual Al mole fraction are analyzed numerically and experimentally. The emission spectra, carrier concentrations in the quantum wells, energy band diagrams, electrostatic fields, and internal quantum efficiency are investigated. The results indicate that the LED with an AlGaN/GaN SL EBL of gradual Al mole fraction has a better hole injection efficiency, lower electron leakage, and smaller electrostatic fields in its active region over the LED with a conventional rectangular AlGaN EBL or with a normal AlGaN/GaN SL EBL. The results also show that the efficiency droop is markedly improved when the SL EBL of gradual Al mole fraction is used.
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85.60.Jb Light-emitting devices

At-wavelength characterization of refractive x-ray lenses using a two-dimensional grating interferometer

Simon Rutishauser, Irene Zanette, Timm Weitkamp, Tilman Donath, and Christian David

Appl. Phys. Lett. 99, 221104 (2011); http://dx.doi.org/10.1063/1.3665063 (3 pages) | Cited 9 times

Online Publication Date: 29 November 2011

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We report on the application of a two-dimensional hard x-ray grating interferometer to x-ray optics metrology. The interferometer is sensitive to refraction angles in two perpendicular directions with a precision of 10 nrad. It is used to observe the wavefront changes induced by a single parabolic beryllium focusing lens of large radius of curvature. The lens shape is reconstructed and its residual aberrations are analyzed. Its profile differs from an ideal parabolic shape by less than 2 μm or λ/50 at λ = 0.54 Å wavelength.
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42.79.Bh Lenses, prisms and mirrors
07.85.-m X- and γ-ray instruments

GaN-based photonic crystal surface emitting lasers with central defects

Tzeng-Tsong Wu, Peng-Hsiang Weng, Yen-Ju Hou, and Tien-Chang Lu

Appl. Phys. Lett. 99, 221105 (2011); http://dx.doi.org/10.1063/1.3665251 (3 pages) | Cited 1 time

Online Publication Date: 29 November 2011

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The GaN-based photonic crystal surface emitting lasers (PCSELs) with different central defects were fabricated and investigated. The threshold energy densities increased from 3.23 to 3.51 mJ/cm2 when the central defect size increased. In addition, lasing wavelengths decreased from 400 nm to 390 nm due to the guided mode shifting phenomenon for the PCSEL cavities with larger central defects. The tendency of threshold gain and resonance wavelength for PCSELs with different central defects were calculated by the multiple scattering method and well matched to the experimental results.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.70.Qs Photonic bandgap materials

Experimental realization and modeling of a subwavelength frequency-selective plasmonic metasurface

Ping-Chun Li, Yang Zhao, Andrea Alù, and Edward T. Yu

Appl. Phys. Lett. 99, 221106 (2011); http://dx.doi.org/10.1063/1.3664634 (3 pages) | Cited 5 times

Online Publication Date: 29 November 2011

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We have modeled, fabricated, and characterized a plasmonic metasurface with subwavelength features, whose dominant resonance is the independent of incident angle and polarization, and sensitive only to the material composition and geometry of a single element. Higher-order resonances, associated with surface plasmon polariton (SPP) coupling and higher diffraction orders, are sensitive to the incident angle and the array periodicity and less pronounced compared with the metasurface resonance. Numerical simulations and theoretical analysis highlight a clear physical difference between the SPP resonances and the dominant metasurface collective resonance, whose properties may be of great interest for plasmonic solar cells and subwavelength color filters.
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42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
42.86.+b Optical workshop techniques
42.79.-e Optical elements, devices, and systems

Gain assisted nanocomposite multilayers with near zero permittivity modulus at visible frequencies

Carlo Rizza, Andrea Di Falco, and Alessandro Ciattoni

Appl. Phys. Lett. 99, 221107 (2011); http://dx.doi.org/10.1063/1.3665414 (3 pages) | Cited 4 times

Online Publication Date: 30 November 2011

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We have fabricated a nano-laminate by alternating metal and gain medium layers, the gain dielectric consisting of a polymer incorporating optically pumped dye molecules. From standard reflection-transmission experiments, we show that, at a visible wavelength, both the real and the imaginary parts of the permittivity ε∥ attain very small values and we measure, at λ = 604 nm, |ε∥| = 0.04 which is 21.5% smaller than its value in the absence of optical pumping. Our investigation thus proves that a medium with a permittivity with very small modulus, a key condition promising efficient subwavelength optical steering, can be actually synthesized.
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81.16.-c Methods of micro- and nanofabrication and processing
77.22.Ch Permittivity (dielectric function)

Homeotropically aligning phase separated columnar structures for fabrication of flexible electrooptical devices

Bharat R. Acharya, Hyunchul Choi, Mohan Srinivasarao, and Satyendra Kumar

Appl. Phys. Lett. 99, 221108 (2011); http://dx.doi.org/10.1063/1.3663966 (3 pages)

Online Publication Date: 30 November 2011

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A method of achieving homeotropic alignment of liquid crystals (LCs) by ultraviolet light induced phase separation of LC from its mixture with photo-curable pre-polymer is reported. Vertical polymer columns and micro-fibrils developed during the phase separation promote homeotropic alignment of the LC director (i.e., alignment perpendicular to the LC-substrate interface), suitable for devices based on LCs possessing negative dielectric anisotropy. These vertical structures extend between two substrates and permit the fabrication of highly flexible electro-optical devices with high contrast coupled with fast response times. This simple single-step technique eliminates the need for the traditional polymer alignment layer pre-deposited on substrates.
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85.60.Pg Display systems

Up to 3 μm light emission on InP substrate using GaInAs/GaAsSb type-II quantum wells

Stephan Sprengel, Christian Grasse, Kristijonas Vizbaras, Tobias Gruendl, and Markus-Christian Amann

Appl. Phys. Lett. 99, 221109 (2011); http://dx.doi.org/10.1063/1.3665256 (3 pages) | Cited 6 times

Online Publication Date: 1 December 2011

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We present 3 μm photoluminescence at room temperature, which is achieved with GaInAs/GaAsSb type-II quantum wells on InP substrate. This long-wavelength emission became feasible by using highly compressive strained Ga0.25In0.75As and GaAs0.4Sb0.6 layers. Furthermore, a comparison between standard superlattice and so called “W” shaped quantum wells revealed that the emission linewidth can be drastically reduced by using the latter design, which is necessary for low threshold laser operation.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors

Room temperature mid-infrared surface-emitting photonic crystal laser on silicon

Binbin Weng, Jiangang Ma, Lai Wei, Lin Li, Jijun Qiu, Jian Xu, and Zhisheng Shi

Appl. Phys. Lett. 99, 221110 (2011); http://dx.doi.org/10.1063/1.3665402 (3 pages) | Cited 5 times

Online Publication Date: 1 December 2011

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We demonstrate a mid-infrared surface-emitting photonic crystal laser on silicon substrate operating at room temperature. The active region consisting of PbSe/PbSrSe multiple quantum wells was grown by molecular beam epitaxy on Si(111) substrate patterned with a photonic crystal (PC) array. The PC array forms a transverse magnetic polarized photonic bandgap at around 2840 cm−1. Under pulsed optical pumping, room temperature multimode lasing emissions were observed at wavelength ∼3.5 μm with estimated threshold peak pumping intensity of 24 kW/cm2. Angular-dependent measurement indicates the lasing is of a Gaussian-like profile with full width at half maximum of 4.66°.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.55.Tv Photonic crystal lasers and coherent effects

Continuous-wave cascaded-harmonic generation and multi-photon Raman lasing in lithium niobate whispering-gallery resonators

Jeremy Moore, Matthew Tomes, Tal Carmon, and Mona Jarrahi

Appl. Phys. Lett. 99, 221111 (2011); http://dx.doi.org/10.1063/1.3665947 (3 pages)

Online Publication Date: 1 December 2011

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We report experimental demonstration of continuous-wave cascaded-harmonic generation and Raman lasing in a millimeter-scale lithium niobate whispering-gallery resonator pumped at a telecommunication-compatible infrared wavelength. Intensity enhancement through multiple recirculations in the whispering-gallery resonator and quasi phase-matching through a nonuniform crystal poling enable simultaneous cascaded-harmonic generation up to the fourth-harmonic accompanied by stimulated Raman, two-photon, three-photon, and four-photon Raman scattering corresponding the molecular vibrational wavenumbers 632 and 255 cm−1 in z-cut lithium niobate at pump power levels as low as 200 mW. We demonstrate simultaneous cascaded-harmonic generation and Raman lasing by observing the spectrum of the scattered light from the resonator and by capturing the image of the decoupled light from the resonator on a camera.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.55.Rz Doped-insulator lasers and other solid state lasers
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.65.Dr Stimulated Raman scattering; CARS
42.65.Es Stimulated Brillouin and Rayleigh scattering

Induced terahertz emission as a probe for semiconductor devices

Patrick Hoyer, Gabor Matthäus, Ulrike Blumröder, Kevin Füchsel, and Stefan Nolte

Appl. Phys. Lett. 99, 221112 (2011); http://dx.doi.org/10.1063/1.3664769 (3 pages)

Online Publication Date: 2 December 2011

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A nondestructive and contact free method for the characterization of semiconductor devices is presented using stimulated terahertz (THz) emission. For demonstration purposes, nanostructured semiconductor-insulator-semiconductor solar cells are investigated. These solar cells are based on indium tin oxide (ITO) upon black silicon (BS). During illumination with fs laser pulses, free charge carriers are generated at the junction between ITO and BS yielding the emission of broadband THz radiation. Since the THz field strength depends on the acceleration characteristics of the photoinduced charge carriers, phase sensitive detection of the emitted THz signal reflects the existing electric field distribution at the boundary zone. In contrast to existing methods where the sample is illuminated by an additional THz generator, here, the THz emission itself characterizes the sample. Moreover, only the region of THz generation is probed yielding a depth-resolved measurement setup that can be applied for the investigation of semiconductor multilayer systems in general as far as THz generation is supported.
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88.40.hj Efficiency and performance of solar cells
88.40.J- Types of solar cells
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Kinetic solutions for electrons in multi-cusp ion source

Hualin Zhan (占华琳) and Chundong Hu (胡纯栋)

Appl. Phys. Lett. 99, 221501 (2011); http://dx.doi.org/10.1063/1.3664347 (3 pages)

Online Publication Date: 29 November 2011

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Some analytical kinetic solutions are obtained for the primary and plasma electrons in multi-cusp ion source. The characteristics based on these solutions are discussed, such as frictional coefficient, collision frequency, electron current, electron diffusion, electron density, and electrical conductivity. A code based on one of these solutions is developed to discuss the plasma electrons diffusion; the simulation results are qualitatively consistent with our current experiences.
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52.25.Fi Transport properties
52.40.Mj Particle beam interactions in plasmas
52.65.-y Plasma simulation
52.20.Fs Electron collisions

Effects of ionization distribution on plasma beam focusing characteristics in Hall thrusters

Zhongxi Ning, Hui Liu, Daren Yu, and Zhongxiang Zhou

Appl. Phys. Lett. 99, 221502 (2011); http://dx.doi.org/10.1063/1.3665632 (3 pages) | Cited 3 times

Online Publication Date: 30 November 2011

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The relationship between ionization distribution and divergence of plasma beam in a Hall thruster is investigated using spectrum and probe methods. Experimental results indicate that the shift of ionization region towards the exit of channel causes the reduction of accelerating field and the enhancement of electron thermal pressure effect, which result in further deviation of equipotential lines to magnetic field lines and further increase in divergence of plasma beam. It is, therefore, suggested that to put the ionization region deep inside the channel and separate it from the acceleration region at the design, and development stage is helpful to improve the plasma beam focusing characteristics of a Hall thruster.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.70.Ds Electric and magnetic measurements
52.75.Di Ion and plasma propulsion
52.80.-s Electric discharges
52.25.Fi Transport properties
52.25.Jm Ionization of plasmas

Electric field step in air gap streamer discharges

Rong Zeng, Chijie Zhuang, Zhanqing Yu, Zhizhao Li, and Yinan Geng

Appl. Phys. Lett. 99, 221503 (2011); http://dx.doi.org/10.1063/1.3665633 (3 pages)

Online Publication Date: 30 November 2011

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Electric field (E-field) in air gap streamer discharges under positive lightning impulse was measured by specifically developed integrated electro-optic sensors. An E-field step phenomenon was observed. The E-field firstly agreed with the Laplace field, then suddenly increased with a rise time of μs. The occurrence probability of this phenomenon increased as the applied voltage increased. The discharge current waveforms and photos taken by a fast camera prove the E-field step was caused by the space net charge. From the E-step rise time and the corona area range, the average electron drift speed under the experiment situation was estimated about 0.2 × 106 – 0.6× 106 m/s.
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52.80.Hc Glow; corona
52.25.Fi Transport properties
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
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X-ray interference effects on the determination of structural data in ultrathin La2/3Sr1/3MnO3 epitaxial thin films

D. Pesquera, X. Marti, V. Holy, R. Bachelet, G. Herranz, and J. Fontcuberta

Appl. Phys. Lett. 99, 221901 (2011); http://dx.doi.org/10.1063/1.3663574 (3 pages) | Cited 2 times

Online Publication Date: 28 November 2011

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We analyze x-ray diffraction data to extract cell parameters of ultrathin films on closely matching substrates. We focus on epitaxial La2/3Sr1/3MnO3 films grown on (001) SrTiO3 single crystalline substrates. It will be shown that, due to extremely high structural similarity of film and substrate, the data analysis must explicitly consider the distinct phase of the diffracted waves by substrate and films to extract reliable unit cell parameters. The implications of this finding for the understanding of strain effects in ultrathin films and interfaces will be underlined.
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68.55.-a Thin film structure and morphology
62.20.D- Elasticity
62.20.F- Deformation and plasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations

Determination of chemical specific atomic interaction with scanning tunneling microscope

Aparna Deshpande, Kai Felix Braun, and Saw-Wai Hla

Appl. Phys. Lett. 99, 221902 (2011); http://dx.doi.org/10.1063/1.3663578 (3 pages) | Cited 1 time

Online Publication Date: 28 November 2011

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We present an experimental technique to comparatively determine the shape of tip-atom interaction potential and interaction strength of individual atoms on a surface using a scanning tunnelling microscope. From the atom manipulation signals, the tip-atom force vector angle is measured as a function of distance. Remarkably, the force angle is found to increase linearly with reducing tip-height for a small distance variation, which enables to deduce the shape of tip-atom interaction potential, and the lateral force required to move individual atoms on a surface. This technique will be useful to comparatively determine interaction strengths of molecules and biomolecular systems on surfaces.
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79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Ultrafast decay of high frequency optical phonon mode in KTiOPO4

Shan Yang (杨山), Sanjay Adhikari, Manoj Dobbala, and Feruz Ganikhanov

Appl. Phys. Lett. 99, 221903 (2011); http://dx.doi.org/10.1063/1.3664218 (3 pages)

Online Publication Date: 28 November 2011

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Decay of the strongest optical phonon mode in KTiOPO4 was directly traced using femtosecond coherent anti-Stokes Raman scattering spectroscopy. Dephasing of the Raman active mode at ∼700 cm−1 proceeds with the nonlinear polarization dephasing time of 495 ± 10 fs. The dephasing is solely due to the phonon energy decay with corresponding homogeneous linewidth of 21.4 ± 0.5 cm−1. Low temperature linewidth of 14.7 cm−1 is estimated from our data assuming that down-conversion phonon relaxation process is dominant. Our results can help to understand stimulated Raman generation and oscillation of Stokes wave in laser systems where Raman gain is critically dependent on dephasing time.
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63.20.dd Measurements
78.30.Hv Other nonmetallic inorganics
78.47.jh Coherent nonlinear optical spectroscopy

Role of correlation effects in the superconducting material: InV6S8

M. Ramzan and R. Ahuja

Appl. Phys. Lett. 99, 221904 (2011); http://dx.doi.org/10.1063/1.3664219 (3 pages)

Online Publication Date: 28 November 2011

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In this letter, we present the high pressure studies to compute the structural parameters and electronic structure of InV6S8, using the first principles density functional theory (DFT) and DFT + U methods. While LDA and GGA fail to give the correct electronic and structural properties, the DFT + U method performs well to reproduce the experimental observed quantities of InV6S8. We analyze the electronic structures and nesting of InV6S8 at different pressure values and show that the nesting of energy bands reduces with compression. Finally, we conclude that the Hubbard like correction is necessary to take into account the correlation effects, which are very important in the correct description of InV6S8, by choosing the moderate value of U.
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74.20.Pq Electronic structure calculations
74.25.Jb Electronic structure (photoemission, etc.)
71.10.Fd Lattice fermion models (Hubbard model, etc.)
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Energy-filtered phase retrieval using the transport of intensity equation

V. J. Keast, M. J. Gladys, T. C. Petersen, C. Dwyer, C. T. Koch, T. Haber, and G. Kothleitner

Appl. Phys. Lett. 99, 221905 (2011); http://dx.doi.org/10.1063/1.3663572 (3 pages)

Online Publication Date: 29 November 2011

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Energy-filtered imaging was combined with phase retrieval to study plasmon scattering contributions to electron phase shifts surrounding gold nanoparticles. Spectral windows with sufficient energy resolution to exclude plasmon scattering were used to measure through-focal intensity derivatives and retrieve phase maps via the transport of intensity equation. Phase excursions in the phase maps that included plasmon scattering were found to disagree with theory by at least an order of magnitude. This discrepancy was attributed to subtle particle instabilities under the electron beam, which lead to systematic errors in the retrieved phase maps.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.22.Lp Collective excitations
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters

Enhanced and reduced transmission of acoustic waves with bubble meta-screens

Alice Bretagne, Arnaud Tourin, and Valentin Leroy

Appl. Phys. Lett. 99, 221906 (2011); http://dx.doi.org/10.1063/1.3663623 (3 pages) | Cited 2 times

Online Publication Date: 29 November 2011

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We present a class of sonic meta-screens for manipulating air-borne acoustic waves at ultrasonic or audible frequencies. Our screens consist of periodic arrangements of air bubbles in water or possibly embedded in a soft elastic matrix. They can be used for soundproofing but also for exalting transmission at an air/water interface or even to achieve enhanced absorption.
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62.60.+v Acoustical properties of liquids
47.55.D- Drops and bubbles
81.40.Jj Elasticity and anelasticity, stress-strain relations

Broadband polarization transformation via enhanced asymmetric transmission through arrays of twisted complementary split-ring resonators

Zeyong Wei, Yang Cao, Yuancheng Fan, Xing Yu, and Hongqiang Li

Appl. Phys. Lett. 99, 221907 (2011); http://dx.doi.org/10.1063/1.3664774 (3 pages) | Cited 8 times

Online Publication Date: 29 November 2011

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This study proposes an ultrathin chiral metamaterial slab stacked with twisted complementary split-ring resonators (CSRRs) for highly efficient broadband polarization transformation. The polarization of linearly polarized electromagnetic waves can be rotated in a specific direction by passing it through such a slab having a thickness of about one-tenth the operational wavelength. Microwave experiments verified the theoretically predicted conversion efficiency of up to 96% covering a bandwidth of 24% of the central wavelength. CSRRs with circular symmetry provide increased interlayer coupling strength, which produces a high-efficiency broadband response and strong isolation of the original polarization.
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41.20.Jb Electromagnetic wave propagation; radiowave propagation
81.05.Xj Metamaterials for chiral, bianisotropic and other complex media

Electronic structure and symmetry of valence states of epitaxial NiTiSn and NiZr0.5Hf0.5Sn thin films by hard x-ray photoelectron spectroscopy

Xeniya Kozina, Tino Jaeger, Siham Ouardi, Andrei Gloskowskij, Gregory Stryganyuk, Gerhard Jakob, Takeharu Sugiyama, Eiji Ikenaga, Gerhard H. Fecher, and Claudia Felser

Appl. Phys. Lett. 99, 221908 (2011); http://dx.doi.org/10.1063/1.3665621 (3 pages) | Cited 4 times

Online Publication Date: 30 November 2011

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The electronic band structure of thin films and superlattices made of Heusler compounds with NiTiSn and NiZr0.5Hf0.5Sn composition was studied by means of polarization dependent hard x-ray photoelectron spectroscopy. The linear dichroism allowed to distinguish the symmetry of the valence states of the different types of layered structures. The films exhibit a larger amount of "in-gap" states compared to bulk samples. It is shown that the films and superlattices grown with NiTiSn as starting layer exhibit an electronic structure close to bulk materials.
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71.20.Be Transition metals and alloys
73.21.Cd Superlattices
73.50.Lw Thermoelectric effects
73.61.At Metal and metallic alloys
79.60.Bm Clean metal, semiconductor, and insulator surfaces
79.60.Dp Adsorbed layers and thin films
79.60.Jv Interfaces; heterostructures; nanostructures

Energy-level and optical properties of nitrogen doped TiO2: An experimental and theoretical study

P. P. González-Borrero, H. S. Bernabé, N. G. C. Astrath, A. C. Bento, M. L. Baesso, M. V. Castro Meira, J. S. de Almeida, and A. Ferreira da Silva

Appl. Phys. Lett. 99, 221909 (2011); http://dx.doi.org/10.1063/1.3664104 (3 pages) | Cited 5 times

Online Publication Date: 30 November 2011

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Photoacoustic spectroscopy was used to study nitrogen-doped titanium dioxide film. The energy positions of defect and impurity centre levels are reported. The energy levels were obtained using the excitation method and the mechanisms of the photoacoustic signal generation are discussed. The visible light absorption of the yellow film was explained considering electronic transitions between localized states within the band gap and the transitions from these states into the conduction band. Moreover, first principles calculations revealed that nitrogen doping and oxygen vacancies in titanium dioxide induce defect levels within the gap which account for the absorption in the visible light.
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78.66.Nk Insulators
61.72.jd Vacancies
78.20.Pa Photoacoustic effects
62.65.+k Acoustical properties of solids
71.20.Ps Other inorganic compounds
71.55.Ht Other nonmetals

Measuring the ordering of closely packed particles

Nizan Meitav and Erez N. Ribak

Appl. Phys. Lett. 99, 221910 (2011); http://dx.doi.org/10.1063/1.3665187 (3 pages) | Cited 2 times

Online Publication Date: 1 December 2011

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We look at irregularity and phase transitions in two-dimensional patterns. Two independent methods are provided for identifying and measuring hexagonal close packing of particles. We implemented these methods on images of melting colloidal films at different melting processes. We could easily spot various defects in the hexagonal crystal film. The feasibility of the methods is also demonstrated on human retinal images, for measuring the unit cell size and fraction of close-packed cone photoreceptors.
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64.70.dj Melting of specific substances
82.70.Dd Colloids
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