The Experts below are selected from a list of 261 Experts worldwide ranked by ideXlab platform
Duncan T. L. Alexander - One of the best experts on this subject based on the ideXlab platform.
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zinc blende wurtzite Polytypism in nanocrystalline zno films
Acta Materialia, 2017Co-Authors: Brian A Aebersold, Lorenzo Fanni, Sylvain Nicolay, Christophe Ballif, Cécile Hébert, Aicha Hesslerwyser, Duncan T. L. AlexanderAbstract:Abstract ZnO is commonly observed and assumed to crystallize fully in a wurtzite structure. Indeed, while Polytypism between a wurtzite and a zinc blende phase is often seen in some II-VI compounds, it has only rarely been observed in ZnO. Here, by means of automated crystal orientation mapping and high-resolution imaging in transmission electron microscopy, we identify nanometer-sized regions of zinc blende forming during the growth of compact nanocrystalline metal–organic chemical vapor deposited ZnO films. We analyze a c-fiber-textured wurtzite ZnO film, where the columnar grain growth, typical for polycrystalline films, is interrupted by the formation of these zinc blende regions. These regions in turn provide surfaces for the heteroepitaxial renucleation of newly oriented wurtzite grains. In contrast to the columnar grains, which grow fastest along their c-axis, these new grains are identified to grow fastest along their basal plane. By means of convergent beam electron diffraction, we show that this difference in fast growth direction relates to the polarity of exposed facets. A growth model is proposed, showing how this Polytypism affects the microstructure evolution of the film in ways that were not predicted by existing film growth models.
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Zinc blende–wurtzite Polytypism in nanocrystalline ZnO films
Acta Materialia, 2017Co-Authors: A. Brian Aebersold, Lorenzo Fanni, Aïcha Hessler-wyser, Sylvain Nicolay, Christophe Ballif, Cécile Hébert, Duncan T. L. AlexanderAbstract:Abstract ZnO is commonly observed and assumed to crystallize fully in a wurtzite structure. Indeed, while Polytypism between a wurtzite and a zinc blende phase is often seen in some II-VI compounds, it has only rarely been observed in ZnO. Here, by means of automated crystal orientation mapping and high-resolution imaging in transmission electron microscopy, we identify nanometer-sized regions of zinc blende forming during the growth of compact nanocrystalline metal–organic chemical vapor deposited ZnO films. We analyze a c-fiber-textured wurtzite ZnO film, where the columnar grain growth, typical for polycrystalline films, is interrupted by the formation of these zinc blende regions. These regions in turn provide surfaces for the heteroepitaxial renucleation of newly oriented wurtzite grains. In contrast to the columnar grains, which grow fastest along their c-axis, these new grains are identified to grow fastest along their basal plane. By means of convergent beam electron diffraction, we show that this difference in fast growth direction relates to the polarity of exposed facets. A growth model is proposed, showing how this Polytypism affects the microstructure evolution of the film in ways that were not predicted by existing film growth models.
Robert Joseph Cava - One of the best experts on this subject based on the ideXlab platform.
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Polytypism polymorphism and superconductivity in tase2 xtex
Proceedings of the National Academy of Sciences of the United States of America, 2015Co-Authors: Huixia Luo, Weiwei Xie, Jing Tao, Hiroyuki Inoue, Andras Gyenis, Jason W. Krizan, Ali Yazdani, Yimei Zhu, Robert Joseph CavaAbstract:Polymorphism in materials often leads to significantly different physical properties—the rutile and anatase polymorphs of TiO2 are a prime example. Polytypism is a special type of polymorphism, occurring in layered materials when the geometry of a repeating structural layer is maintained but the layer-stacking sequence of the overall crystal structure can be varied; SiC is an example of a material with many polytypes. Although polymorphs can have radically different physical properties, it is much rarer for Polytypism to impact physical properties in a dramatic fashion. Here we study the effects of Polytypism and polymorphism on the superconductivity of TaSe2, one of the archetypal members of the large family of layered dichalcogenides. We show that it is possible to access two stable polytypes and two stable polymorphs in the TaSe2−xTex solid solution and find that the 3R polytype shows a superconducting transition temperature that is between 6 and 17 times higher than that of the much more commonly found 2H polytype. The reason for this dramatic change is not apparent, but we propose that it arises either from a remarkable dependence of Tc on subtle differences in the characteristics of the single layers present or from a surprising effect of the layer-stacking sequence on electronic properties that are typically expected to be dominated by the properties of a single layer in materials of this kind.
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Polytypism, polymorphism, and superconductivity in TaSe2−xTex
Proceedings of the National Academy of Sciences of the United States of America, 2015Co-Authors: Huixia Luo, Weiwei Xie, Jing Tao, Hiroyuki Inoue, Andras Gyenis, Jason W. Krizan, Ali Yazdani, Yimei Zhu, Robert Joseph CavaAbstract:Polymorphism in materials often leads to significantly different physical properties—the rutile and anatase polymorphs of TiO2 are a prime example. Polytypism is a special type of polymorphism, occurring in layered materials when the geometry of a repeating structural layer is maintained but the layer-stacking sequence of the overall crystal structure can be varied; SiC is an example of a material with many polytypes. Although polymorphs can have radically different physical properties, it is much rarer for Polytypism to impact physical properties in a dramatic fashion. Here we study the effects of Polytypism and polymorphism on the superconductivity of TaSe2, one of the archetypal members of the large family of layered dichalcogenides. We show that it is possible to access two stable polytypes and two stable polymorphs in the TaSe2−xTex solid solution and find that the 3R polytype shows a superconducting transition temperature that is between 6 and 17 times higher than that of the much more commonly found 2H polytype. The reason for this dramatic change is not apparent, but we propose that it arises either from a remarkable dependence of Tc on subtle differences in the characteristics of the single layers present or from a surprising effect of the layer-stacking sequence on electronic properties that are typically expected to be dominated by the properties of a single layer in materials of this kind.
Friedhelm Bechstedt - One of the best experts on this subject based on the ideXlab platform.
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Polytypism and surface structure of sic
Diamond and Related Materials, 1997Co-Authors: P. Käckell, J. Furthmüller, Friedhelm BechstedtAbstract:Abstract Within the framework of density functional theory (DFT) in the local density approximation (LDA) using ultrasoft Vanderbilt pseudopotentials in a plane wave basis we investigate the atomic and electronic structure of 3C SiC(111) and n H SiC(0001) surfaces (1 × 1 and √3 × √3 reconstructions) and of 3C SiC(001) surfaces (2 × 2 reconstructions). Since (111)/(0001) is the natural growth direction, the 3C SiC(111)/ n H SiC(0001) surfaces are of special interest. Atomic geometries of various reconstructions, their relative stability and their electronic structure (surface band structure) are discussed. The theoretical results are compared with available experiments.
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Polytypism and properties of silicon carbide
Physica Status Solidi (B) Basic Research, 1997Co-Authors: Friedhelm Bechstedt, P. Käckell, A. Zywietz, K. Tenelsen, Brigitte Adolph, K Karch, J. FurthmüllerAbstract:The relationship between crystal structure and related material properties is discussed for the common 3C, 6H, 4H, and 2H polytypes of SiC. The theoretical results are derived in the framework of well converged density-functional calculations within the local-density approximation and the pseudopotential-plane-wave approach. In the case of electronic excitations additionally quasiparticle corrections are included. The lattice-dynamical properties of the noncubic polytypes are described within a bond-charge model. We focus our attention on the actual atomic structures, the accompanying lattice vibrations, thermodynamical properties, properties of layered combinations of polytypes, optical spectra, and surface equilibrium structures. On the one hand, the influence of the polytype on the material properties is considered. On the other hand, indications for driving forces of the Polytypism are extracted.
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Influence of Polytypism on thermal properties of silicon carbide.
Physical review. B Condensed matter, 1996Co-Authors: A. Zywietz, K Karch, Friedhelm BechstedtAbstract:We present calculations of thermal properties of the 3C, 6H, 4H, and 2H polytypes of silicon carbide (SiC). The underlying lattice-dynamical properties are calculated within a generalized bond-charge model which gives also correct phonon eigenvectors. In the case of the zinc-blende structure the results are checked by comparison with those of ab initio density-functional calculations. Explicitly, we determine the free energy, the specific heat, the Debye temperature, and the Debye-Waller factors. The influence of the Polytypism, in particular of the anisotropy in the hexagonal cases, is studied in detail. The theoretical results are in good agreement with available experimental data. A temperature-dependent axial next-nearest-neighbor Ising model is derived. Consequences are discussed for the Polytypism and the thermodynamics of the different SiC phases. \textcopyright{} 1996 The American Physical Society.
Kevin M. Ryan - One of the best experts on this subject based on the ideXlab platform.
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Precursor-mediated linear- and branched- Polytypism control in CuαZnβSnySeδ colloidal nanocrystals using a dual-injection method
Chemistry of Materials, 2020Co-Authors: Huan Ren, Yuanwei Sun, Peng Gao, Conor T. Mccarthy, Ning Liu, Kevin M. RyanAbstract:Control of Polytypism in colloidal nanocrystals allows both for a shape evolution from 0D to 3D but also provides an opportunity to tailor physical properties that are crystal phase-dependent. Init...
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Occurrence of Polytypism in Compound Colloidal Metal Chalcogenide Nanocrystals, Opportunities, and Challenges
The Journal of Physical Chemistry Letters, 2015Co-Authors: Shalini Singh, Kevin M. RyanAbstract:This Perspective will look at the occurrence of Polytypism in colloidal nanocrystals of metal chalcogenides. The low energy barrier between growth in wurtzite and zincblende crystal phases allows the occurrence of both in a single particle which has a dramatic influence on shape control. To date, various strategies have been employed to understand and control this phenomenon leading to a range of particle shapes across a wide range of semiconductor compositions from the archetypal cadmium selenide to the more recently studied compound copper chalcogenide nanocrystals such as dicopper–zinc–tin tetrasulfide and copper indium gallium selenide systems. The Perspective will also look at the control factors for Polytypism in colloidal nanocrystals and the impact of Polytypism on end-use applications.
Brian A. Korgel - One of the best experts on this subject based on the ideXlab platform.
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Wurtzite−Chalcopyrite Polytypism in CuInS2 Nanodisks
Chemistry of Materials, 2009Co-Authors: Bonil Koo, Reken N. Patel, Brian A. KorgelAbstract:Monodisperse CuInS2 nanodisks were synthesized by heating metal chlorides and thiourea in oleylamine. X-ray diffraction showed the predominant phase to be wurtzite (hexagonal) CuInS2 instead of chalcopyrite (tetragonal) or compositionally disordered sphalerite (cubic). High-resolution transmission electron microscopy, however, revealed Polytypism in the nanodisks, with the wurtzite phase interfaced with significant chalcopyrite domains.
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wurtzite chalcopyrite Polytypism in cuins2 nanodisks
Chemistry of Materials, 2009Co-Authors: Bonil Koo, Reken N. Patel, Brian A. KorgelAbstract:Monodisperse CuInS2 nanodisks were synthesized by heating metal chlorides and thiourea in oleylamine. X-ray diffraction showed the predominant phase to be wurtzite (hexagonal) CuInS2 instead of chalcopyrite (tetragonal) or compositionally disordered sphalerite (cubic). High-resolution transmission electron microscopy, however, revealed Polytypism in the nanodisks, with the wurtzite phase interfaced with significant chalcopyrite domains.
