What are the roles of gas and solid in nanowire growth using the VLS method?

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Discussion Overview

The discussion focuses on the roles of gas and solid phases in the growth of nanowires using the vapor-liquid-solid (VLS) method. Participants explore the mechanisms of material transfer and deposition during the growth process, as well as the implications of different phases involved in this method.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant describes the VLS method, noting that it involves a solid, liquid, and gas phase, and seeks to clarify the specific roles of each phase in nanowire growth.
  • Another participant asserts that the gas supplies the material for growth, while the liquid acts as an intermediary that absorbs material from the gas and deposits it onto the solid surface.
  • A question is raised about the solid's role beyond serving as a surface for the catalyst deposition.
  • It is mentioned that the liquid catalyst accelerates the growth process by facilitating the deposition of material from the gas onto the solid surface.
  • A participant expresses interest in the thermodynamical theory behind (III/V)-nanowire growth and seeks clarification on the concept of chemical potential and its reference in the context of solid materials.

Areas of Agreement / Disagreement

Participants generally agree on the roles of the gas and liquid phases in the growth process, but there is uncertainty regarding the specific function of the solid substrate and how it interacts with the other phases. The discussion remains unresolved on these points.

Contextual Notes

Participants express confusion about the interpretation of chemical potential and its equilibrium reference, indicating a need for further clarification on these concepts. There are also assumptions about the behavior of the solid phase that are not fully explored.

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If you don't know anything about this please just read the first 20 lines on the below link and help me understand (I don't know anything either).
The vapour liquid solid method:
http://en.wikipedia.org/wiki/Vapor–liquid–solid_method
uses a solid, liquid and gas phase to grow nanowires. While I understand the overall ideas I am unsure about some things. As far as I can see the idea is:
Place a thin layer of an alloy on a substrate. Heat up until the alloy liquifies. At this point it will absorb some of the gaseous phase to form supersaturated droplets from which the nanowires are grown.
But what exactly are the roles of gas/solid in this proces? Which of these phases supplies the atoms that form the nanowires. On the link both the solid and the gas contain Si, but it seems that it is the gaseous phase which the liquid catalyst absorbs and uses to grow the nanowire. So if this is understood correctly what is the role of the solid substrate?
 
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The gas supplies the material from which the growth occurs; the growth can be seen in the solid whiskers.

The liquid is an intermediary: it absorbs the material from the gas, and as it becomes super-saturated it deposits it onto the solid surface.

By choosing different liquids (referred to as catalysts because they are no used up) one can obtain different growth characteristics. Thus an additional degree of control is provided when attempting to engineer specific materials.
 
Okay but what is then the effect of the solid other than being a surface where to deposit the catalyst?
 
The material being drawn from the gas is deposited on the solid surface.

The liquid catalyst accelerates the process.
 
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Okay thank you. You have been very helpful. I am now starting to read about the basic thermodynamical theory behind (III/V)-nanowire growth which I think refers to growth of nanowires of atoms from group (III) and (V), e.g. GaAs.
But the general ideas are giving me some trouble. I have attached a few lines, which I hope you might be able to help me understand.
Can you explain to me what is the basic idea of this section? I have some trouble understanding it, maybe because I am not really sure how to interpret chemical potential. The most fruitful definition of the chemical potential for me is the energy required to add another particle to the system.
So what is it that is used as an equilibrium reference (and what does this mean exactly?) - is it the solid. I should expect so since the solid is practically infinite and thus its chemical potential doesn't change as the growth proceeds (apart from if the temperature changes).
But how do they arrive at the condition of the last equality?
 

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