Gas-Phase Growth of Nanowires on Transition-Metal Catalysts

DoctorOne-dimensional (1-D) nanocrystal synthesis is an interesting picture from fundamental crystal growth view, because single crystalline growth and hetero-epitaxial growth via 1-D nanocrystal synthesis are relatively easier than bulk crystal synthesis. The most popular way to grow 1-D nanocrystal is transition-metal catalytic nanowire growth. This dissertation describes an experimental study on transition-metal catalytic nanowire growth. The first chapter presents changes in nanowire growth patterns depending on the type of metal catalyst. There are three growth mode concerning with metal catalystsi) liquid-catalytic nanowire growth, ii) solid-catalytic nanowire growth, iii) non catalytic spontaneous nanowire growth. The first case, liquid-catalytic nanowire growth, is the earliest and prevailing examples of such one-dimensional growth, so-called vapor-liquid-solid (VLS) syntheses of semiconductor nanowires. I describe this growth mode using a case of liquid Au-catalytic nanowire growth, which is already well studied process. Other two growth mode, solid catalytic nanowire growth and non-catalytic spontaneous nanowire growth, are so-called unconventional growth mode, and also my main experimental work in this chapter. I illustrated solid Cu-catalytic growth of Ge nanowires and non-catalytic spontaneous NiSix nanowire growth from continuous Ni bulks, in direct comparisons to the growth from the eutectic liquid catalyst of Au. The second chapter describes the comparative study between a bulk crystal growth versus a catalytic nanowire crystal growth. This comparative study treats disadvantages cause by transition-metal catalysts in crystal growth rather than size effect. As above mentioned, single crystalline and hetero-epitaxial structure are relatively easily obtained via catalytic nanowire growth. However wurtzite/zincblende mixtures containing stacking fault defect are often generated during catalytic single crystalline nanowire growth. Also nanowire kinks at the heterojunction are observed during catalytic nanowire heterostructure growth. In this chapter, I discuss the role of surface energy relation around the catalyst concerned with these phenomena, and how to solve them. Previous chapters mainly focused on fundamental study about nanowire crystal growth mediated on metal catalysts, rather than applications. In this final chapter I will discuss the practical applications of catalytic nanowire growth. Over the past decade, many researchers have spent effort to adopt nanowires for electronic devices. However nanowires are not still component in commercial electronic devices due to difficulty to grow position and direction controlled nanowire array with smaller diameter than lithographical technique. In this vein, I tried to demonstrate the nanowires as a Li-ion battery anode or field emission tip, which do not need nanowire array