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Critical Diameter (critical + diameter)
Selected AbstractsCritical diameters and temperature domains for MBE growth of III,V nanowires on lattice mismatched substratesPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 4 2009G. E. Cirlin Abstract We report on the growth properties of InAs, InP and GaAs nanowires (NWs) on different lattice mismatched substrates, in particular, on Si(111), during Au-assisted molecular beam epitaxy (MBE). We show that the critical diameter for the epitaxial growth of dislocation-free III,V NWs decreases as the lattice mismatch increases and equals 24 nm for InAs NWs on Si(111), 39 nm for InP NWs on Si(111), 44 nm for InAs NWs on GaAs(111)B, and 110 nm for GaAs NWs on Si(111). When the diameters exceed these critical values, the NWs are dislocated or do not grow at all. The corresponding temperature domains for NW growth extend from 320 °C to 340 °C for InAs NWs on Si(111), 330 °C to 360 °C for InP NWs on Si(111), 370 °C to 420 °C for InAs NWs on GaAs(111)B and 380 °C to 540 °C for GaAs NWs on Si(111). Experimental values for critical diameters are compared to the previous findings and are discussed within the frame of a theoretical model. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Nanostructure Fracturing: Brittle-to-Ductile Transition in Uniaxial Compression of Silicon Pillars at Room Temperature (Adv. Funct.ADVANCED FUNCTIONAL MATERIALS, Issue 15 2009Mater. On page 2439, F. Östlund et al. report on an interesting effect observed during the compression of sub-micrometer silicon pillars; a critical diameter separates pillars that are observed to crack from pillars that exhibit metal-like ductility. This observation allows for the development of a quantitative method for measuring the fracture toughness of such structures, which can be used to predict and explain small-volume fracture behavior. [source] Bed Stability and Sedimentation Associated With Human Disturbances in Pacific Northwest Streams,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 2 2009Philip R. Kaufmann Abstract:, To evaluate anthropogenic sedimentation in United States (U.S.) Pacific Northwest coastal streams, we applied an index of relative bed stability (LRBS*) to summer low flow survey data collected using the U.S. Environmental Protection Agency's Environmental Monitoring and Assessment Program field methods in a probability sample of 101 wadeable stream reaches. LRBS* is the log of the ratio of bed surface geometric mean particle diameter (Dgm) to critical diameter (D*cbf) at bankfull flow, based on a modified Shield's criterion for incipient motion. We used a formulation of LRBS* that explicitly accounts for reductions in bed shear stress that result from channel form roughness due to pools and wood. LRBS* ranged from ,1.9 to +0.5 in streams within the lower quartile of human riparian and basin disturbance, and was substantially lower (,4.2 to ,1.1) in streams within the upper quartile of human disturbance. Modeling results suggest that the expected range of LRBS* in streams without human disturbances in this region might be generally between ,0.7 and +0.5 in either sedimentary or volcanic lithology. However, streams draining relatively soft, erodible sedimentary lithology showed greater reductions in LRBS* associated with disturbance than did those having harder, more resistant volcanic (basalt) lithology with similar levels of basin and riparian disturbance. At any given level of disturbance, smaller streams had lower LRBS* than those with larger drainages. In sedimentary lithology (sandstone and siltstone), high-gradient streams had higher LRBS* than did low-gradient streams of the same size and level of human disturbance. High gradient streams in volcanic lithology, in contrast, had lower LRBS* than low-gradient streams of similar size and disturbance. Correlations between Dgm and land disturbance were stronger than those observed between D*cbf and land disturbance. This pattern suggests that land use has augmented sediment supplies and increased streambed fine sediments in the most disturbed streams. However, we also show evidence that some of the apparent reductions in LRBS*, particularly in steep streams draining small volcanic drainages, may have resulted in part from anthropogenic increases in bed shear stress. The synoptic survey methods and designs we use appear adequate to evaluate regional patterns in bed stability and sedimentation and their general relationship to human disturbances. More precise field measurements of channel slope, cross-section geometry, and bed surface particle size would be required to use LRBS* in applications requiring a higher degree of accuracy and precision, such as site-specific assessments at individual streams. [source] Critical diameters and temperature domains for MBE growth of III,V nanowires on lattice mismatched substratesPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 4 2009G. E. Cirlin Abstract We report on the growth properties of InAs, InP and GaAs nanowires (NWs) on different lattice mismatched substrates, in particular, on Si(111), during Au-assisted molecular beam epitaxy (MBE). We show that the critical diameter for the epitaxial growth of dislocation-free III,V NWs decreases as the lattice mismatch increases and equals 24 nm for InAs NWs on Si(111), 39 nm for InP NWs on Si(111), 44 nm for InAs NWs on GaAs(111)B, and 110 nm for GaAs NWs on Si(111). When the diameters exceed these critical values, the NWs are dislocated or do not grow at all. The corresponding temperature domains for NW growth extend from 320 °C to 340 °C for InAs NWs on Si(111), 330 °C to 360 °C for InP NWs on Si(111), 370 °C to 420 °C for InAs NWs on GaAs(111)B and 380 °C to 540 °C for GaAs NWs on Si(111). Experimental values for critical diameters are compared to the previous findings and are discussed within the frame of a theoretical model. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Structural stability of clean GaAs nanowires grown along the [111] directionPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2010Rita Magri Abstract Using a first-principles approach we have calculated the formation energies of small diameter GaAs nanowires (NWs) with both zinc-blende and wurtzite structure grown along the [111] direction. The section of the wires is hexagonal and the side facets are oriented either {11-20} and {10-10} in the case of the wurtzite structure, and {110} and {112} for the zinc-blende structure. The formation energy of the nanowires as a function of their radius is then interpreted in terms of a model in which the energy contributions from the bulks, the flat surfaces and the ridges are taken explicitly into account. We find that the nanowire stability is mainly explained by the competition between the bulk energy, favoring the zincblende structure and the surface energies favoring the wurtzite structure. We find also that the directly calculated formation energies of some small diameter wurtzite NWs can be reproduced by our model taking into account only the bulk and flat surface contributions. That is, the ridges do not contribute substantially to the nanowire formation energy. Inspection of the ridge structure and band structure reveals that this good agreement occurs when the NWs are semiconducting and the ridges do not add more dangling bonds to the surface with respect to those provided by the sidewalls. Within our model we find the critical diameter for the wurtzite-zinc-blende transition at 6.3 nm. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Critical diameters and temperature domains for MBE growth of III,V nanowires on lattice mismatched substratesPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 4 2009G. E. Cirlin Abstract We report on the growth properties of InAs, InP and GaAs nanowires (NWs) on different lattice mismatched substrates, in particular, on Si(111), during Au-assisted molecular beam epitaxy (MBE). We show that the critical diameter for the epitaxial growth of dislocation-free III,V NWs decreases as the lattice mismatch increases and equals 24 nm for InAs NWs on Si(111), 39 nm for InP NWs on Si(111), 44 nm for InAs NWs on GaAs(111)B, and 110 nm for GaAs NWs on Si(111). When the diameters exceed these critical values, the NWs are dislocated or do not grow at all. The corresponding temperature domains for NW growth extend from 320 °C to 340 °C for InAs NWs on Si(111), 330 °C to 360 °C for InP NWs on Si(111), 370 °C to 420 °C for InAs NWs on GaAs(111)B and 380 °C to 540 °C for GaAs NWs on Si(111). Experimental values for critical diameters are compared to the previous findings and are discussed within the frame of a theoretical model. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||