Thermodynamics of the Au-Si-O System: Application to the Synthesis and Growth of Silicon-Silicon dioxide Nanowires

Abstract

The Solid-Vapor-Liquid-Solid (SVLS) process is a fundamental mechanism for the growth of nanowires. In this article, experimental observations and assessment of thermodynamic data have been used to explain the Solid-Vapor-Liquid-Solid (SVLS) mechanism for the growth of silicon-based nanowires. The binary phase diagram of nanoparticle (Au-Si) systems has been evaluated from information on Gibbs energy of the bulk and surface tension of the liquid phase. At 1100°C, temperature commonly used for the growth of nanowires by the SVLS mechanism, it has been shown that the nanometric decreases the melting point of pure Au and Si and more generally the liquidus temperatures. Moreover, the liquid phase region in the binary Au-Si phase diagram is enlarged as the particle size becomes smaller. The presence of SiO in the gaseous phase is a necessary but not sufficient condition for the formation of nanowires. The nanowiresÕgrowth cannot be explained by the modification of the Au-Si phase diagram with the size of the particles, neither by the presence of SiO, but by the existence of a metastable equilibrium involving the silicon of the wafer, the deposit of vitreous silica and supersaturated SiO in the gaseous phase.

Synthesis of Amorphous SiOx Nanowires and Nanofibers by Thermal Evaporation with Gold as Catalyst

“…As the duration time increased, more and more silicon and oxygen diffused and dissolved into the droplet until supersaturated. The substrate provides the droplet with silicon until it get saturated and hence, no more silicon atoms were attracted [5]. Therefore, the SLS mechanism is not responsible of SiOxNWs complete growth processes.…”

“…If the SiO gas not enough to dissolve into the droplet the growth will stop and the final shape will look like tree roots or comet shape, but if the deposition time is more enough, these Au balls will aggregates and form bigger ball as in stage (4), which then let the wires continue to rise and form SiO x nanowires with Au cap at the end of the wire, see stage (5). The percentage of gold is changing in the same wire or a fiber depending on the location.…”

“…One-dimensional SiO x nanostructures have recently attracted considerable research attention due to their unique properties and promising applications [1][2][3][4][5][6][7]. The stable and bright blue emission of SiO x nanowires makes them potential sources of highintensity light, near-field optical microscope probes and waveguides.…”

“…The later argument cannot be accepted as there is no thermodynamic (energetic) reason for the silicon of the wafer to supersaturate from the binary liquid phase. Then a Solid-VaporLiquid-Solid (SVLS) mechanism was suggested by Bahloul [5], which is similar to VLS mechanism, but differs in the starting phase of silicon whether it is solid or vapour.…”

Direct Growth and Photoluminescence of SiOx Nanowires and Aligned Nanocakes by Simple Carbothermal Evaporation

“…This argument cannot be accepted as there is no thermodynamic (energetic) reason for the silicon of the wafer to be continually attracted after the droplet become saturated. Therefore, the availability of silicon monoxide gas at a temperature >900°C can merge the previous two mechanisms to be Solid-VLS mechanism [21]. So both solid and gas phases can play an important role as the two main sources of silicon atoms.…”