Interlopers at the interface –
Ultrathin nanowires have a diameter of less than 50 nanometers (1 nm is one thousandth of a millionth of a meter) and promise to make electronic devices, such as computers and mobile phones, even more compact in the future. However, to realize this goal, precise control of the growth of nanowires will be necessary, and this requires a detailed understanding of the underlying mechanisms. Most nanowires produced at present grow by a process termed ‘Vapour-Liquid-Solid Growth’ (VLS).
VLS involves the uptake of atoms from the gas phase into the liquid phase, followed by their incorporation into the growing wire. “To make silicon nanowires, for example, silicon atoms are first dissolved in a droplet of liquid gold, and then adsorbed onto the interface of the nanowire,“ explains Professor Christina Scheu of the Department of Chemistry and the Center for NanoScience (CeNS) at LMU Munich.
But what if the atoms required for nanowire growth are poorly soluble in the liquid phase? “This is the case in our system,“ says Scheu, who coordinated the collaborative efforts of teams based in Korea, Israel, USA and Germany to answer this question. “The droplets of liquid aluminium that are involved in the growth of the aluminium oxide nanowires cannot take up oxygen at high temperatures.“ To study the influence of this effect, aluminium oxide crystals were observed at 750 degrees Celsius in an electron microscope at the Max Planck Institute for Metals Research in Stuttgart. At this temperature the aluminium is liquid, and the growth of nanowires from the droplets can be observed in real time.
The experimental design permitted monitoring of the growth of aluminium oxide wires with atomic precision. The team observed that the self-catalytic growth process occurs in cycles of two steps. At the triple junction where gas, liquid and solid phases meet, tiny crystalline facets repeatedly form and dissolve as the junction shifts back and forth across the surface abutting the rim of the growing wire. “Longitudinal growth of these crystalline facets is accompanied by the liberation of oxygen, which can be adsorbed at the interface between liquid phase and wire,“ says Scheu, “enabling a new atomic layer of aluminium oxide to be added to the wire.“ Thus, growth actually occurs discontinuously.
The whole reaction process was captured on video. ”Without such high-resolution investigations, it would not have been possible to dissect the fundamental steps involved in the two-stage growth of aluminium oxide nanowires,“ says Scheu. “In addition, we would have overlooked the dynamic nature of events at the triple point.“ A detailed understanding of the growth of nanowires is a prerequisite for the targeted manipulation of their structure and properties. The next goal for Scheu and her team is to elucidate the basic processes that govern the growth of nanowires with other chemical compositions.
"Oscillatory Mass Transport in Vapor-Liquid-Solid Growth of Sapphire Nanowires"
Sang Ho Oh, Matthew F. Chisholm, Yaron Kauffmann, Wayne D. Kaplan,
Weidong Luo, Manfred Rühle, Christina Scheu
Science, vol. 330, no. 6003, pp. 489-493, 21 October 2010