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Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2009"


Journal ArticleDOI
TL;DR: In this article, the elastic properties and frictional properties of graphene samples of varying thickness using an atomic force microscope were investigated for tensile testing, and it was shown that the frictional force between an AFM tip and graphene decreases with thickness for samples from 1 to 4 layers, and does not depend on the presence of a substrate.
Abstract: We descnbe studies of the elastic properties and frictional characteristics of graphene samples of varying thickness using an atomic force microscope. For tensile testing, graphene is suspended over micron-sized circular holes and indented by atomic force microscope (AFM) tips. Fitting of the force-displacement curves yields the prestress and elastic stiffness, while comparison of the breaking force to simulation gives the ultimate strength, which is the highest measured for any material. Experiments on samples with 1-3 atomic layers yield similar values for the intrinsic stiffness and strength of a single sheet, but also reveal differences in mechanical behavior with thickness. The frictional force between an AFM tip and graphene decreases with thickness for samples from 1 to 4 layers, and does not depend on the presence of a substrate. High-resolution friction force imaging in stick-slip mode shows the same trend, and allows direct imaging of the crystal lattice.

360 citations


Journal ArticleDOI
TL;DR: For more than 25 years, the Physikalisch-Technische Bundesanstalt has been strongly engaged in the field of metrology using synchrotron radiation as mentioned in this paper.
Abstract: For more than 25 years, the Physikalisch-Technische Bundesanstalt has been strongly engaged in the field of metrology using synchrotron radiation. In Berlin, this research programme started together with the user operation of the electron storage ring BESSY I in the early 1980s. At the beginning, the work was focused on fundamental radiometry, i.e. using the storage ring as a primary radiation source standard and operating beamlines for source and detector calibration in the vacuum ultraviolet spectral range. Meanwhile, at the electron storage rings BESSY II and Metrology Light Source in Berlin-Adlershof, the activities have been extended to a broad range of fundamental and applied photon metrology in the range from the far infrared to hard X-rays, including methods like cryogenic radiometry, reflectometry and X-ray fluorescence spectroscopy. In the present review, we give a short historical introduction to this work, describe our laboratories and the basic radiometric principles, and present examples of recent applications, largely performed within the framework of scientific cooperations with external partners from industry and research. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

143 citations


Journal ArticleDOI
TL;DR: In this article, the authors describe the progress in the science and technology of stabilized a-Se from its early use in xerography and xeroradiography to its present use in commercial modern flat panel X-ray imagers and ultrasensitive video tubes which utilize impact ionization of drifting holes.
Abstract: We describe the progress in the science and technology of stabilized a-Se from its early use in xerography and xeroradiography to its present use in commercial modern flat panel X-ray imagers and ultrasensitive video tubes which utilize impact ionization of drifting holes. Both electrons and holes can drift in stabilized a-Se, which is a distinct advantage since X-ray photogeneration of charge carriers occurs throughout the bulk of the photoconductive layer. An a-Se photoconductor has to be operated at high fields to ensure that the photogeneration efficiency is sufficiently large to provide reasonable X-ray sensitivity. However, at high fields, the dark current is unacceptably large in simple metal/a-Se/metal devices, and special multilayer device structures need to be designed. The dark current decays with time and increases with the nominal applied field. The reduction of the dark current to a tolerable level was one of the key factors that lead to the commercialization of a-Se X-ray detectors. We discuss the origin of the dark current, and highlight some of the current challenges in the design of next generation detectors. We also discuss the origin of impact ionization in a-Se, and its fruitful utilization in ultrasensitive imaging devices, including the Harpicon, which are likely to lead to new high detective quantum efficiency detectors.

143 citations


Journal ArticleDOI
TL;DR: In this paper, extended defects in 4H-SiC epilayers and reports recent results concerning fast epitaxial growth were surveyed and reported. But the authors focused on the development of the 4HSiC fast epilayer in a newly developed vertical hot-wall-type reactor under low system pressure using a H2 + SiH4 + C3H8 system.
Abstract: This paper surveys extended defects in 4H-SiC epilayers and reports recent results concerning fast epitaxial growth. Synchrotron X-ray topography, transmission electron microscopy, Nomarski optical microscopy and defect selective etching analysis are applied to investigate the nucleation and propagation of carrot defects, basal plane Frank-type defects, polytype inclusions and basal plane dislocations (BPDs) in 4H-SiC epitaxial growth. In the development of the 4H-SiC fast epitaxial growth technique, a very high growth rate of up to 250 μm/h is obtained in a newly developed vertical hot-wall-type reactor under low system pressure using a H2 + SiH4 + C3H8 system. Good thickness and impurity doping uniformity are also obtained simultaneously over a large area, with the retention of a high growth rate. A 4H-SiC epilayer virtually free from BPDs is obtained on a 4° off Si-face substrate. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

127 citations


Journal ArticleDOI
TL;DR: A review of recent theoretical and experimental advances toward understanding the effects of nuclear spins in confined nanostructures can be found in this article, where a sequence of spectacular new results have provided understanding of spin-bath decoherence, nuclear spin diffusion, and preparation of the nuclear state through dynamic polarization and more general manipulation of nuclear-spin density matrix through "state narrowing".
Abstract: We review recent theoretical and experimental advances toward understanding the effects of nuclear spins in confined nanostructures. These systems, which include quantum dots, defect centers, and molecular magnets, are particularly interesting for their importance in quantum information processing devices, which aim to coherently manipulate single electron spins with high precision. On one hand, interactions between confined electron spins and a nuclear-spin environment provide a decoherence source for the electron, and on the other, a strong effective magnetic field that can be used to execute local coherent rotations. A great deal of effort has been directed toward understanding the details of the relevant decoherence processes and to find new methods to manipulate the coupled electron-nuclear system. A sequence of spectacular new results have provided understanding of spin-bath decoherence, nuclear spin diffusion, and preparation of the nuclear state through dynamic polarization and more general manipulation of the nuclear-spin density matrix through "state narrowing." These results demonstrate the richness of this physical system and promise many new mysteries for the future.

108 citations


Journal ArticleDOI
TL;DR: In this paper, the electronic states of semiconductors and insulators without taking into account experimental parameters are calculated based on the solution of the quasiparticle equation starting with a reasonable zeroth order approximation for the states and the exchange-correlation self-energy.
Abstract: We present most recent developments to calculate the electronic states of semiconductors and insulators without taking into account experimental parameters. They are based on the solution of the quasiparticle equation starting with a reasonable zeroth order approximation for the electronic states and the GW approximation for the exchange–correlation self-energy. Due to inclusion of screened exchange effects from the very beginning, self-consistency can be easily reached. The advantages with respect to a starting point based on single-particle eigenfunctions and eigenvalues of the ground-state density functional theory (DFT) are clearly shown for band gaps, positions of semicore d-bands, and densities of states. The progress is demonstrated for compounds containing first-row elements such as metal oxides and nitrides whose gaps are much too small or even negative within the conventional DFT. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

100 citations


Journal ArticleDOI
TL;DR: A general expression for the directionally anisotropic Poisson's ratio (PR) of cubic materials under external pressure is discussed in this article, expressed in terms of the elastic moduli ratios, where K is the bulk modulus and G, Ware shear moduli, and where the mechanical stability criteria are taken into account.
Abstract: A general expression for the directionally anisotropic Poisson's ratio (PR) of cubic materials under external pressure is discussed. It is expressed in terms of the elastic moduli ratios, X = G/K, Y= G/W, where K is the bulk modulus and G, Ware shear moduli, and where the mechanical stability criteria are taken into account. The global maximum and global minimum PR surfaces are shown. In the X, Y-plane, regions of different auxetic behavior are identified, and a straightforward way of classifying any cubic material as auxetic, nonauxetic, and completely auxetic is given. The domains of the different extreme directions, for which the PR shows either a global maximum or a global minimum for given X, Y, are identified and discussed. There are three extreme directions: [100], [110], and a novel, noncrystallographic one denoted as the V3-direction. The main features of the V3 extreme direction and corresponding XY-regions are described. It is found that the most extreme PR values can be achieved exclusively in the very limited domain of the X, Y-plane, corresponding to the V3-direction, and for X, Y → 0, the absolute value of the PR becomes unbounded. Analysis of literature data on real materials demonstrates that the great majority of cubic materials are nonauxetics or auxetics (with the [110] extreme direction). The materials possessing very large (i.e., PR > 2) and very small (i.e., PR < -1) PR values are identified.

97 citations


Journal ArticleDOI
TL;DR: In this article, two sets of carbon fiber laminate configurations have been designed, manufactured and examined, and they are shown to be less notch sensitive than their conventional counterpart for the negative in-plane Poisson's ratio.
Abstract: Two sets of carbon fibre laminate configurations have been designed, manufactured and examined. The first consisted of one in-plane auxetic (i.e. having a negative Poisson's ratio) and one with matched elastic moduli, but a positive in-plane Poisson's ratio. The second consisted of three laminates, again with matched elastic moduli, but with negative, near zero and positive thru-thickness Poisson's ratio. Fracture toughness is ; predicted to be enhanced if a negative thru-thickness Poisson's ratio is present. This study also shows that more energy is required to propagate a crack in the auxetic laminate and that it is less notch sensitive than its conventional counterpart for the negative in-plane Poisson's ratio.

94 citations


Journal ArticleDOI
TL;DR: In this article, the unit cell of the Bi1-xLax FeO3 system is described with the R3c space group in the concentration interval 0.05 and the mixed-phase state was observed in the range 1.5 020, whereas no evidence for Imma phase was found.
Abstract: Crystal structures of Bi1–xLnx FeO3 systems (Ln = La, Nd, Eu) are studied using X-ray and neutron diffraction, magnetization measurements as well as electron microscopy It was shown that the unit cell of the Bi1–xLax FeO3 system is described with the R3c space group in the concentration interval 0 05 The mixed-phase state was observed in the range 015 020, whereas no evidence for Imma phase was found The magnetization study has revealed the spontaneous magnetization to be associated with Pnma and Imma phases that do not allow conventional ferroelectricity The ferroelectric R3c phase exhibits metamagnetic behavior due to transition from an incommensurately modulated antiferromagnetic state to a weak ferromagnetic state Substitution of Bi3+ with Ln3+ leads to a strong decrease of external magnetic field inducing a metamagnetic transition (© 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)

78 citations


Journal ArticleDOI
TL;DR: By reviewing the experimental and theoretical literature on γ'-Fe 4 N, and by a systematic survey of predictions by the LDA, PBE, WC, LDA + U (2x), PBE + U(2x) and B3PW91 exchange-correlation functionals, the structural, magnetic and hyperfine properties of this material as well as their pressure dependencies are interpreted as discussed by the authors.
Abstract: By reviewing the experimental and theoretical literature on γ'-Fe 4 N, and by a systematic survey of predictions by the LDA, PBE, WC, LDA + U (2x), PBE + U (2x) and B3PW91 exchange-correlation functionals, the structural, magnetic and hyperfine properties of this material as well as their pressure dependencies are interpreted The hypothesis is put forward that γ'-Fe 4 N as found in Nature is exactly at a steep transition between low-spin and high-spin behaviour PBE + U (U = 04 eV) is identified as the most accurate exchange-correlation functional for this material, although it is needed to fix the magnetization at the experimental value to obtain a satisfying description Remaining disagreement between theory and experiment is pointed out A recent experimental claim for a giant magnetic moment in γ'-Fe 4 N is discussed, and is not reproduced by our calculations We expect that the new insight obtained in the present work can lead to a consistent ab initio modeling of other materials in the iron-nitrogen binary system In an accompanying didactic section, the physics behind some common exchange-correlation functionals is outlined

76 citations


Journal ArticleDOI
TL;DR: In this article, the authors review recent ultrafast optical studies of carrier dynamics in semiconductor nanowires and provide a tantalizing hint of the rich physics yet to be discovered in these quasi-1D systems, which offers great promise for applications in areas including nanoelectronics, thermoelectrics, sensing, and nanophotonics.
Abstract: Semiconductor nanowires (NWs) are nanostructures with a number of novel optical and electronic properties that offer great promise for applications in areas including nanoelectronics, thermoelectrics, sensing, and nanophotonics. To realize the full potential of these unique nanosystems, however, a deep understanding of their response to optical excitation on a sub-picosecond time scale is required. Here, we review recent ultrafast optical studies of carrier dynamics in semiconductor NWs. These experiments have been performed on different materials as a function of both intrinsic NW parameters such as diameter and doping as well as experimental parameters including photoexcited carrier density and wavelength. A variety of phenomena, including one-dimensional (1D) exciton dynamics, rapid carrier trapping at surface and bulk defects, and lasing from an electron-hole plasma (EHP) have been observed. These first measurements of ultrafast carrier dynamics are a tantalizing hint of the rich physics yet to be discovered in these quasi-1D systems. Ultrafast optical spectroscopy can track the temporal evolution ; of carrier populations in semiconductor NWs with femtosecond time resolution.

Journal ArticleDOI
TL;DR: In this paper, an approach to searching for the structures of crystals is described, where an ensemble of "random" starting structures and relax them, often constraining the starting structures to bias the search in some manner.
Abstract: An approach to searching for the structures of crystals is described. We choose an ensemble of "random" starting structures and relax them, often constraining the starting structures and/or relaxation to bias the search in some manner. The methodology is illustrated by applications to high-pressure phases of group IV hydrides, especially silane (SiH 4 ), and to solid hydrogen, using first-principles density-functional-theory methods. We discuss the issue of possible decomposition of group IV hydrides into their constituent elements under pressure. A new molecular-hydrogen structure is described whose enthalpy is calculated to be slightly lower than all other known structures in the pressure range 75-108 GPa, and we mention its possible relevance to phase II of hydrogen.

Journal ArticleDOI
TL;DR: The photoviscous effect, that is, the athermal decrease of viscosity of a non-crystalline chalcogenide upon illumination, is the key for a plethora of photoinduced effects reported so far in the literature under different names.
Abstract: A synopsis of various photoinduced changes of rheological, mechanical and elastic properties is presented in the first part of the article. After a critical appraisal of a large body of experimental data, it is suggested that the photoviscous effect, that is, the athermal decrease of viscosity of a non-crystalline chalcogenide upon illumination, is the key for a plethora of photoinduced effects reported so far in the literature under different names. Morphic effects (shape or surface morphology) may appear either in the presence or absence of external mechanical stimuli leading to the fabrication of a variety of technologically important photoprocessed structures. A few representative examples of photoplastic effects are described in some detail in the second part of the paper, based on information provided by in situ Raman scattering and nanoindentation experiments.

Journal ArticleDOI
TL;DR: In this article, X-ray diffraction (XRD) measurements and photoluminescence (PL) spectra show that Mn ions are doped into the lattice positions of ZnO.
Abstract: Mn-doped ZnO was synthesized using a co-precipitation technique. X-ray diffraction (XRD) measurements and photoluminescence (PL) spectra show that Mn ions are doped into the lattice positions of ZnO. The modes at 202, 330, and 437 cm -1 in the Raman spectrum are assigned as 2E 2 (low), E 2 (high)-E 2 (low), and E 2 (high) modes of ZnO base, respectively. The mode at 528cm -1 is ascribed to a local vibrational mode related to Mn. The mode at 580cm -1 should be an intrinsic mode of ZnO and assigned to E 1 longitudinal optical (LO). Its reinforcement should result from a combination of resonance at the excitation wavelength and impurity-induced scattering.

Journal ArticleDOI
TL;DR: In this paper, a review of recent developments in magnetic plasmonics arising from the coupling effect in metamaterials is given, where it is shown that the coupling between these units produces multiple discrete resonance modes due to hybridization.
Abstract: Magnetic metamaterials consist of magnetic resonators smaller in size than their excitation wavelengths. Their unique electromagnetic properties were characterized by the effective media theory at the early stage. However, the effective media model does not take into account the interactions between magnetic elements; thus, the effective properties of bulk metamaterials are the result of the “averaged effect” of many uncoupled resonators. In recent years, it has been shown that the interaction between magnetic resonators could lead to some novel phenomena and interesting applications that do not exist in conventional uncoupled metamaterials. In this paper, we will give a review of recent developments in magnetic plasmonics arising from the coupling effect in metamaterials. For the system composed of several identical magnetic resonators, the coupling between these units produces multiple discrete resonance modes due to hybridization. In the case of a system comprising an infinite number of magnetic elements, these multiple discrete resonances can be extended to form a continuous frequency band by strong coupling. This kind of broadband and tunable magnetic metamaterial may have interesting applications. Many novel metamaterials and nanophotonic devices could be developed from coupled resonator systems in the future. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal ArticleDOI
TL;DR: In this paper, the molecular orientation of phthalocyanines as representatives of flat, π-conjugated molecules on different polycrystalline substrates is reviewed.
Abstract: We review the molecular orientation of phthalocyanines as representatives of flat, π-conjugated molecules on different polycrystalline substrates. Thin films with a thickness of ∼10–50 nm are often highly ordered on several substrates. The molecular orientation, however, can be radically different for single crystalline model substrates and for relatively ill defined polycrystalline substrates. Moreover, it is important to distinguish between the growth of the first organic layer(s) at the substrate surface and the growth in films of a thickness of several nanometers: Differently oriented, buried interfacial layers of few monolayers were observed in particular for polycrystalline substrates. The growth mode is discussed in terms of different molecule–molecule and molecule–substrate interactions. Consequences for our understanding of the behavior of such films in devices are considered in context with electronic interface properties. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal ArticleDOI
TL;DR: In this paper, the authors give a brief description of the experimental and theoretical methods employed in the study of solid matter at high pressures and summarize the high-pressure phases of the most relevant elements and inorganic compounds of the AX, A2X, AX2, ABX2, A 2X3 ABX3, AB X4 and AB2X4 families giving an overview of the state of the art in high pressure research.
Abstract: Pressure is an important thermodynamic parameter since it allows an increase of matter density by reducing volume. The reduction of volume by applying high pressures leads to an overall decrease of interatomic and intermolecular distances that allows exploring in detail atomic and molecular interactions. Therefore, high-pressure research has improved our fundamental understanding of these interactions in solids, liquids and gasses. The study of the structure of matter under compression is a rapid developing field that is receiving increasing attention especially due to continuous experimental and theoretical developments. In this article, we give a brief description of the experimental and theoretical methods employed in the study of solid matter at high pressures and summarize the high-pressure phases of the most relevant elements and inorganic compounds of the AX, A2X, AX2, ABX2, A2X3, ABX3, ABX4 and AB2X4 families giving an overview of the state of the art in high-pressure research by highlighting recent discoveries, hot spots, controversial questions, and future directions of research. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal ArticleDOI
TL;DR: In this paper, the magnetic phase stability was determined from the total energy calculations for both the nonmagnetic (NM) and magnetic (M) phases, and the elastic constants at equilibrium were also determined.
Abstract: We have performed ab-initio density-functional theory self-consistent calculations using the full-potential linear muffin-tin orbital method within local spin-density approximation to study the electronic and magnetic properties of Ni2MnZ (Z = Al, Ga and In) in L21 structure. The magnetic phase stability is determined from the total energy calculations for both the nonmagnetic (NM) and magnetic (M) phases. The theoretical calculations clearly indicate that at both ambient and high pressures, the magnetic phase is more stable than the nonmagnetic phase. The elastic constants at equilibrium are also determined. We derived the bulk and shear moduli, Young's modulus, and Poisson's ratio. The Debye temperature of Ni2MnZ was estimated from the average sound velocity. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal ArticleDOI
TL;DR: A theoretical study was carried out by first-principles calculations using plane-wave pseudopotential method to investigate the crystal structures, relative stabilities, and elastic properties of seven known titanium dioxide (TiO 2 ) polymorphs as discussed by the authors.
Abstract: A theoretical study was carried out by first-principles calculations using plane-wave pseudopotential method to investigate the crystal structures, relative stabilities, and elastic properties of seven known titanium dioxide (TiO 2 ) polymorphs. Our calculated equilibrium structural parameters of TiO 2 polymorphs are in good agreement with the experimental results. Firstly, it is worth to note that from our energy-minimized generalized gradient approximation (GGA) calculations, the four phases of TiO 2 - anatase, rutile, columbite, and baddeleyite - are seen to lie within a very close energy range (0.02eV/atom) of each other, which implies easy phase transition between them. Secondly, on the basis of enthalpy versus pressure data obtained from our GGA calculations for high-pressure forms, we demonstrate the phase transition pressure and the expected sequence of the phase transition of TiO 2 , which are in reasonable agreement with experimental observations. Finally, we calculate the elastic constants of all the TiO 2 polymorphs, and based on them, the bulk modulus, shear modulus, Young's modulus, and Poisson's ratio are estimated. The results show that cotunnite-type TiO 2 is a potential low-compressible material.

Journal ArticleDOI
TL;DR: In this article, the spin-orbit interaction produces splittings of bands in both compounds, especially in the conduction band, which are in very good agreement with measured values of the core spectra.
Abstract: While the orthorhombic IV–VI compounds show the typical layered behavior of that crystallography, we show that the presence of sulphur induces important changes to the band gap behaviour: both in its location, and in its character as well. Our modelling of tin sulfide (SnS) and tin selenide (SnSe), performed within an ab initio density-functional theory (DFT) with a FP-LAPW method, shows that the spin–orbit interaction produces slight splittings of bands in both compounds, especially in the conduction band, which are in very good agreement with measured values of the core spectra. In addition to these splittings, for SnS we found the novel feature that the spin–orbit relocates the band gap in the BZ, while it does not affect that of SnSe. Furthermore, several aspects of the crystals anisotropy may be explained upon our results on the hybridized band structure, the density of states (DOS) and the optical spectra (complex dielectric function and absorption coefficient). We satisfactorily compare our results with those available in the literature. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal ArticleDOI
TL;DR: In this paper, the photoreduction properties of pristine CeO 2 specimens (bulk crystal, film, and nanocrystals) have been studied at different temperatures between 7 K and room temperature.
Abstract: The photoluminescence properties of different pristine CeO 2 specimens (bulk crystal, film, and nanocrystals) have been studied at different temperatures between 7 K and room temperature. The photoinduced luminescence spectral change has been observed at room temperature for the first time. The phenomenon observed in a vacuum is explained as a photoinduced associative detachment of 0 2 on the specimen surface, while that observed in 0 2 gas is explained as a photoinduced dissociative adsorption of O 2 on the specimen surface. The photoreduction is accompanied both by a valence number change of cerium ions (Ce 4+ → Ce 3+ ) and by oxygen-defect formation. Such reversible photoinduced phenomena of CeO 2 can be applied to light control of the oxygen-storage and oxygen-release processes at metal-oxide surfaces. A tentative result obtained at room temperature for an oxygen-storage material (0.3)CeO 2 -(0.7)ZrO 2 solid solution is also described.

Journal ArticleDOI
TL;DR: In this paper, the authors used Hedin's GW framework to compute quasiparticle bands for group-II monoxides in wurtzite (wz) structure and derived band parameters and effective masses for the three uppermost valence bands as well as the lowest conduction band including spinorbit coupling.
Abstract: We use Hedin's GW framework to compute quasiparticle bands for group-II monoxides in wurtzite (wz) structure. Modern hybrid density functional theory is applied to compute the starting electronic structure for the solution of the quasiparticle equation. We derive band parameters and effective masses for the three uppermost valence bands as well as the lowest conduction band including spin–orbit coupling. Optical transition matrix elements and exciton binding energies are also calculated. In addition to the prediction of parameters for wz-MgO and wz-CdO we discuss the chemical trend.

Journal ArticleDOI
TL;DR: In this article, the amplitude and the period of the Aharonov-Bohm effect over a magnetic field range of ±5 T were investigated for a single-layer graphene ring.
Abstract: We experimentally investigate the conductance of a single-layer graphene ring. The Aharonov-Bohm oscillation amplitude of the four-terminal resistance is very high with a visibility up to 10%. Additionally, we investigate the amplitude and the period of the Aharonov-Bohm effect over a magnetic field range of ±5 T. We find that, while the period remains constant, the amplitude rises by a factor of 2.

Journal ArticleDOI
TL;DR: In this paper, a small steady shear applied for 1 h to a nonconductive composite with initially well-dispersed nanotubes was found to induce the insulator-to-conductor transition resulting in a conductivity increase by about six orders of magnitude.
Abstract: We report on shear-induced nanotube network formation in multi-wall nanotubes/polycarbonate (MWNT/PC) composite melts which was directly monitored by the time-resolved DC-conductivity measurements during steady shear. A small steady shear applied for 1 h to a non-conductive composite with initially well-dispersed nanotubes was found to induce the insulator-to-conductor transition resulting in a conductivity increase by about six orders of magnitude. Similar composite melt annealed without steady shear demonstrates much slower process of the network formation what can be attributed to an agglomeration of attractively interacting nanotubes in polymer melts. The rheologic properties were also measured for shear-stimulated agglomeration. Unexpected difference between the electrical and mechanical networks in MWNT/PC composites was found. For the modeling of the network formation a shear-dependent kinetic equation for the nanotube agglomeration was coupled with empirical formula for insulator-to-conductor transition. Electric DC-conductivity and shear modulus (G' and G") of MWNT/PC melt (230 °C) measured simultaneously during shear-stimulated network formation.

Journal ArticleDOI
TL;DR: In this paper, the mechanical properties of rotating rigid parallelograms have been analyzed and compared with the properties of the Type IIα, Iβ and IIβ rotating parallelogram.
Abstract: Auxetic systems have the anomalous property of becoming wider when uniaxially stretched, i. e. exhibit a negative Poisson's ratio. One of the mechanisms which can give rise to this property is based on rotating rigid units, in particular 2D rigid polygons which are connected together at their corners through hinges and rotate relative to each other when uniaxially stretched. This work extends earlier preliminary work on connected rigid parallelograms and presents expressions for the mechanical properties for all the types of planar systems that can be constructed from rigid parallelograms of equal size connected at their vertices through flexible hinges. In particular, we derive and discuss the mechanical properties for the Type Iα, Iβ and IIβ rotating parallelograms which were not previously analysed and compare them with the properties of the Type IIα systems. We show that despite being rather similar to each other, the different types of 'rotating parallelograms' have very different mechanical properties and different abilities to exhibit auxetic behaviour.

Journal ArticleDOI
TL;DR: In this paper, the authors review pitfalls in recent efforts to make a conventional semiconductor, namely ZnO, ferromagnetic by means of doping with transition metal ions, which can lead to unwanted magnetic effects.
Abstract: We review pitfalls in recent efforts to make a conventional semiconductor, namely ZnO, ferromagnetic by means of doping with transition metal ions. Since the solubility of those elements is rather low, formation of secondary phases and the creation of defects upon low temperature processing can lead to unwanted magnetic effects. Among others, ion implantation is a method of doping, which is highly suited for the investigation of those effects. By focussing mainly on Fe, Co or Ni implanted ZnO single crystals we show that there are manifold sources for ferromagnetism in this material which can easily be confused with the formation of a ferromagnetic diluted magnetic semiconductor (DMS). We will focus on metallic as well as oxide precipitates and the difficulties of their identification. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal ArticleDOI
TL;DR: In this paper, the full set of Raman tensor elements was experimentally determined for the corundum structure of a sapphire single-crystal, and a tensorial algorithm was proposed for the practical determination of unknown crystallographic textures in polycrystalline alumina materials.
Abstract: Raman selection rules were theoretically put forward and, according to the obtained relationships, the full set of Raman tensor elements was experimentally determined for the corundum structure of a sapphire single-crystal. The knowledge of the Raman selection rules allowed us to represent the change in polarizability of the aluminium oxide molecule with respect to each of the available vibrational modes. Restrictions related to the symmetry of the scattering system are also discussed together with directional properties, thus leading to unambiguous assignment of the fundamental modes available in sapphire single-crystal. From the experimental side, scattering intensities of Raman bands located at 378 cm–1 and 417 cm–1, which correspond to the Eg and the A1g phonon modes of the corundum structure, respectively, were collected in selected polarization configurations as a function of in-plane rotation angle on different crystallographic planes. Fitting to theoretical equations (i.e., worked out for different crystallographic orientations) enabled us to unequivocally define the elements of the Raman tensor. Finally, a tensorial algorithm was proposed for the practical determination of unknown crystallographic textures in polycrystalline alumina materials, an issue of significant importance in materials science. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal ArticleDOI
TL;DR: In this paper, the authors have attempted to prepare graphene samples by mechanical exfoliation of HOPG (highly oriented pyrolytic graphite) using scotch tape.
Abstract: We have attempted to prepare graphene samples by mechanical exfoliation of HOPG (highly oriented pyrolytic graphite) using scotch tape. Random testing of the flakes by AFM has shown in majority single layer graphene. Nevertheless, the presence of ultrathine graphite cannot be excluded in the large assembly of flakes needed for electron spin resonance (ESR) measurements. Graphene flakes sitting on ESR-silent scotch tapes were stacked parallel to form a multilayer sandwich. The ESR measurements performed in the 4-300 K range yielded narrow Lorentzian line. The spin susceptibility was decreasing linearly with decreasing temperature as expected for the conical band dispersion of graphene. Below 70 K the spin susceptibility started to deviate from the linear temperature dependence and a Curie-like behavior was observed. This contribution to the susceptibility is due to the existence of defects or impurities, which are in strong exchange coupling limit with conduction electrons. The temperature dependence of the linewidth suggests Elliott's mechanism for spin relaxation in graphene flakes. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal ArticleDOI
TL;DR: In this paper, optical absorption and cathodoluminescence properties of AlN epilayers were investigated using optical transition analysis and the endpoints of the observed optical transitions were assigned to ionization levels of oxygen.
Abstract: Different zones of nominally undoped AlN bulk single crystals are investigated using optical absorption and cathodoluminescence. Incorporation of impurities and formation of intrinsic defects during growth strongly depend on the facet which forms the zone. Following first principles calculations and earlier observations on AlN epilayers, we assign the endpoints of the observed optical transitions to ionization levels of oxygen, VAl2–/3–, and (VAl–ON)–/2–. According to the strength of the respective optical transitions in different zones, we find that oxygen contamination increases in the order Al-polar (0001), rhombohedral {102} and prismatic {100}, N-polar (000). We conclude that the low UV transmittance typically observed in bulk AlN is caused by the formation of VAl and (VAl–ON). These deep levels form in the presence of oxygen, which is the major contaminant in bulk AlN. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal ArticleDOI
TL;DR: In this article, a gas evaporation method was used to extract ZnO nanoparticles and they were processed and surface treated to be compatible for bio-conjugation.
Abstract: ZnO nanoparticles were prepared by using a gas evaporation method. As ZnO is a luminescent and less toxic material, it can be useful in imaging applications. The nanoparticles prepared in our laboratory showed intense exciton emission in the UV-blue region. They were processed and surface treated to be compatible for bio-conjugation. We were able to prepare good dispersions and water-soluble ZnO nanoparticles from our experiments. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)