  Semiconductor nanowires offer the opportunity to incorporate novel structures
and functionality into electronic and optoelectronic devices. A clear
understanding of the nanowire growth mechanism is essential for well-controlled
growth of structures with desired properties, but the understanding is
currently limited by a lack of empirical measurements of important parameters
during growth, such as catalyst particle composition. However, this is
difficult to accurately determine by investigating post-growth. We report
direct measurement of the catalyst composition of individual gold seeded GaAs
nanowires inside an electron microscope as they grow. The Ga content in the
catalyst during growth increased with both temperature and Ga precursor flux. A
direct comparison of the calculated phase diagrams of the Au-Ga-As ternary
system to the measured catalyst composition not only lets us estimate the As
content in the catalyst but also indicates the relevance of phase diagrams to
understanding nanowire growth.

Applied Physics;Mesoscale and Nanoscale Physics