Metallic contamination was key to the discovery of semiconductor nanowires,
but today it stands in the way of their adoption by the semiconductor industry.
This is because many of the metallic catalysts required for nanowire growth are
not compatible with standard CMOS (complementary metal oxide semiconductor)
fabrication processes. Nanowire synthesis with those metals which are CMOS
compatible, such as aluminium and copper, necessitate temperatures higher than
450 C, which is the maximum temperature allowed in CMOS processing. Here, we
demonstrate that the synthesis temperature of silicon nanowires using copper
based catalysts is limited by catalyst preparation. We show that the
appropriate catalyst can be produced by chemical means at temperatures as low
as 400 C. This is achieved by oxidizing the catalyst precursor, contradicting
the accepted wisdom that oxygen prevents metal-catalyzed nanowire growth. By
simultaneously solving material compatibility and temperature issues, this
catalyst synthesis could represent an important step towards real-world
applications of semiconductor nanowires.Comment: Supplementary video can be downloaded on Nature Nanotechnology
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Self-assembled catalyst-free GaN micropillars grown on (0001) sapphire substrates by metal organic vapor phase epitaxy are investigated. Transmission electron microscopy, as well as KOH etching, shows the systematic presence of two domains of opposite polarity within each single micropillar. The analysis of the initial growth stages indicates that such double polarity originates at the micropillar/substrate interface, i.e., during the micropillar nucleation, and it propagates along the micropillar. Furthermore, dislocations are also generated at the wire/substrate interface, but bend after several hundreds of nanometers. This leads to micropillars several tens of micrometers in length that are dislocation-free. Spatially resolved cathodoluminescence and microphotoluminescence show large differences in the optical properties of each polarity domain, suggesting unequal impurity/dopant/vacancy incorporation depending on the polarity.
Oxidation behavior of nano-Fe(0) particles in an anoxic environment was determined using different state-of-the-art analytical approaches, including high resolution transmission electron microscopy (HR-TEM) combined with energy filtered transmission electron microscopy (EFTEM), X-ray absorption spectroscopy (XAS), and magnetic measurements. Oxidation in controlled experiments was compared in standard double distilled (DD) water, DD water spiked with trichloroethene (TCE), and TCE contaminated site water. Using HR-TEM and EFTEM, we observed a surface oxide layer (∼3 nm) formed immediately after the particles were exposed to water. XAS analysis followed the dynamic change in total metallic iron concentration and iron oxide concentration for the experimental duration of 35 days. The metallic iron concentration in nano-Fe(0) particles exposed to water, was ∼40% after 35 days; in contrast, the samples containing TCE were reduced to ∼15% and even to nil in the case of TCE contaminated site water, suggesting that the contaminants enhance the oxidation of nano-Fe(0). Frequency dependence measurements confirmed the formation of superparamagnetic particles in the system. Overall, our results suggest that nano-Fe(0) oxidized via the Fe(0) - Fe(OH)2 - Fe3O4 - (γ-Fe2O3) route and the formation of superparamagnetic maghemite nanoparticles due to disruption of the surface oxide layer.
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