The phase of iron catalyst nanoparticles during carbon nanotube growth

We study the Fe-catalyzed chemical vapor deposition of carbon nanotubes by complementary in situ grazing-incidence X-ray diffraction, in situ X-ray reflectivity, and environmental transmission electron microscopy. We find that typical oxide supported Fe catalyst films form widely varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction, which we ascribe to variations in minor commonly present carbon contamination levels. Depending on the as-formed phase composition, different growth modes occur upon hydrocarbon exposure: For γ-rich Fe nanoparticle distributions, metallic Fe is the active catalyst phase, implying that carbide formation is not a prerequisite for nanotube growth. For α-rich catalyst mixtures, Fe3C formation more readily occurs and constitutes part of the nanotube growth process. We propose that this behavior can be rationalized in terms of kinetically accessible pathways, which we discuss in the context of the bulk iron-carbon phase diagram with the inclusion of phase equilibrium lines for metastable Fe3C. Our results indicate that kinetic effects dominate the complex catalyst phase evolution during realistic CNT growth recipes. © 2012 American Chemical Society.

The effect of Ln(III)/An(III) separation on TRU multi-recycling in CONFU assembly

This work examines the basic feasibility of the net-zero-balance TRU multi-recycling concept in which trivalent lanthanide fission products (Ln(III) ) are not separated from trivalent actinides (An(III)). The TRU together with Eu and Gd isotopes are recycled in a standard PWR using Combined Non-Fertile and UO2 (CONFU) assembly design. The assembly assumes a heterogeneous structure where about 20% of U02 fuel pins on the assembly periphery are replaced with Inert Matrix Fuel (IMF) pins hosting TRU, Gd, and Eu generated in the previous cycles. The 2-D neutronic analysis show potential feasibility of Ln / An recycling in PWR using CONFU assembly. Recycling of Ln reduces the fuel cycle length by about 30 effective full power days (EFPD) and TRU destruction efficiency by about 5%. Power peaking factors and reactivity feedback coefficients are close to those of CONFU assembly without Ln recycling.

III-V semiconductor nanowires for optoelectronic device applications

Semiconductor nanowires have recently emerged as a new class of materials with significant potential to reveal new fundamental physics and to propel new applications in quantum electronic and optoelectronic devices. Semiconductor nanowires show exceptional promise as nanostructured materials for exploring physics in reduced dimensions and in complex geometries, as well as in one-dimensional nanowire devices. They are compatible with existing semiconductor technologies and can be tailored into unique axial and radial heterostructures. In this contribution we review the recent efforts of our international collaboration which have resulted in significant advances in the growth of exceptionally high quality IIIV nanowires and nanowire heterostructures, and major developments in understanding the electronic energy landscapes of these nanowires and the dynamics of carriers in these nanowires using photoluminescence, time-resolved photoluminescence and terahertz conductivity spectroscopy. © 2011 Elsevier Ltd. All rights reserved.

鳜鱼视觉特性及其对捕食习性适应的研究III．视觉对猎物运动和形状的反应

作者采用行为学方法测定了伏击型凶猛鱼类鳜鱼视觉对猎物运动和形状特征的反应特性。鳜鱼对３种不同体形饵料鱼有最强的跟踪反应和攻击反应，对虾则有较强的跟踪反应而几乎没有攻击反应，对蜻蜒幼虫仅有不强的跟踪反应而完全没有攻击反应。它对低速（ｖ≤５ｃｍ／ｓ）一连续和等间歇不连续运动的饵料鱼有较强的跟踪反应和攻击反应，对中速和高速（ｖ≥１０ｃｍ／ｓ）连续运动的饵料鱼有最强的跟踪反应而几乎没有或完全没有攻击反应，对中速和高速等间歇不连续运动的饵料鱼则有最强的跟踪反应和最强的攻击反应。它对不连续运动的ａ、ｂ、ｃ、ｄ、ｅ、ｆ

Phase perfection in zinc Blende and Wurtzite III-V nanowires using basic growth parameters.

Controlling the crystallographic phase purity of III-V nanowires is notoriously difficult, yet this is essential for future nanowire devices. Reported methods for controlling nanowire phase require dopant addition, or a restricted choice of nanowire diameter, and only rarely yield a pure phase. Here we demonstrate that phase-perfect nanowires, of arbitrary diameter, can be achieved simply by tailoring basic growth parameters: temperature and V/III ratio. Phase purity is achieved without sacrificing important specifications of diameter and dopant levels. Pure zinc blende nanowires, free of twin defects, were achieved using a low growth temperature coupled with a high V/III ratio. Conversely, a high growth temperature coupled with a low V/III ratio produced pure wurtzite nanowires free of stacking faults. We present a comprehensive nucleation model to explain the formation of these markedly different crystal phases under these growth conditions. Critical to achieving phase purity are changes in surface energy of the nanowire side facets, which in turn are controlled by the basic growth parameters of temperature and V/III ratio. This ability to tune crystal structure between twin-free zinc blende and stacking-fault-free wurtzite not only will enhance the performance of nanowire devices but also opens new possibilities for engineering nanowire devices, without restrictions on nanowire diameters or doping.

Unexpected benefits of rapid growth rate for III-V nanowires.

In conventional planar growth of bulk III-V materials, a slow growth rate favors high crystallographic quality, optical quality, and purity of the resulting material. Surprisingly, we observe exactly the opposite effect for Au-assisted GaAs nanowire growth. By employing a rapid growth rate, the resulting nanowires are markedly less tapered, are free of planar crystallographic defects, and have very high purity with minimal intrinsic dopant incorporation. Importantly, carrier lifetimes are not adversely affected. These results reveal intriguing behavior in the growth of nanoscale materials, and represent a significant advance toward the rational growth of nanowires for device applications.

III-V compound semiconductor nanowires for optoelectronic device applications

GaAs and InP based III-V compound semiconductor nanowires were grown epitaxially on GaAs (or Si) (111)B and InP (111)B substrates, respectively, by metalorganic chemical vapor deposition using Au nanoparticles as catalyst. In this paper, we will give an overview of nanowire research activities in our group. In particular, the effects of growth parameters on the crystal structure and optical properties of various nanowires were studied in detail. We have successfully obtained defect-free GaAs nanowires with nearly intrinsic exciton lifetime and vertical straight nanowires on Si (111)B substrates. The crystal structure of InP nanowires, i.e., WZ or ZB, can also be engineered by carefully controlling the V/III ratio and catalyst size. © 2011 World Scientific Publishing Company.

Au-catalyzed InP nanowires: The influence of growth temperature and V/III ratio

Growth of Au-catalyzed InP nanowires (NWs) by metalorganic chemical vapor deposition (MOCVD) has been studied in the temperature range of 400-510 °C and V/III ratio of 44-700. We demonstrate that minimal tapering of InP NWs can be achieved at 400 °C and V/III ratio of 350. Zinc-blende (ZB) or wurtzite (WZ) NWs is obtained depending on the growth conditions. 4K microphotoluminescence (μ-PL) studies show that emission energy is blue-shifted as growth temperature increases. By changing these growth parameters, one can tune the emission wavelength of InP NWs which is attractive for applications in developing novel optoelectronic devices. © 2010 IEEE.

Growth temperature and V/III ratio effects on the morphology and crystal structure of InP nanowires

The effects of growth temperature and V/III ratio on the morphology and crystallographic phases of InP nanowires that are grown by metal organic chemical vapour deposition have been studied. We show that higher growth temperatures or higher V/III ratios promote the formation of wurtzite nanowires while zinc-blende nanowires are favourableat lower growth temperatures and lower V/III ratios. A schematic map of distribution of zinc-blende and wurtzite structures has been developed in the range of growth temperatures (400-510 °C) and V/III ratios (44 to 700) investigated in this study. © 2010 IOP Publishing Ltd.

III-V compound semiconductor nanowires

InP and GaAs based nanowires were grown epitaxially on InP or GaAs (111)B substrates by metalorganic chemical vapor deposition via vapor-liquid-solid (VLS) mechanism. In this report, I will give an overview of nanowire research activities in our group. In particular, the effects of growth parameters for InP and GaAs nanowires on the crystal quality have been studied in detail. We demonstrated the ability to obtain defect-free GaAs nanowires and control the crystal structure of InP nanowires, ie, WZ or ZB, by choosing a combination of growth parameters, such as temperature, V/III ratio and nanowire diameter. © 2009 IEEE NANO Organizers.

III-V compound semiconductor nanowires

GaAs and InP based nanowires were grown epitaxially on GaAs or InP (111)B substrates by metalorganic chemical vapor deposition using Au nanoparticles as catalyst. In this talk, I will give an overview of nanowire research activities in our group. Especially, the effects of growth parameters for GaAs and InP nanowires on the crystal quality have been studied in detail. We demonstrated the ability to obtain defect-free GaAs nanowires and control the crystal structure of InP nanowires, ie, WZ or ZB, by choosing a combination of growth parameters, such as temperature, V/III ratio and nanowire diameter. © 2009 IEEE.