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From Wikipedia

Vapor-liquid-solid method

The vapor-liquid-solid method (VLS) is a mechanism for the growth of one-dimensional structures, such as nanowires, from chemical vapor deposition. Growth of a crystal through direct adsorption of a gas phase on to a solid surface is generally very slow. The VLS mechanism circumvents this by introducing a catalytic liquid alloy phase which can rapidly adsorb a vapor to supersaturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid-solid interface. The physical characteristics of nanowires grown in this manner depend, in a controllable way, upon the size and physical properties of the liquid alloy.

Historical background

The VLS mechanism was proposed in 1964 as an explanation for silicon whisker growth from the gas phase in the presence of a liquid gold droplet placed upon a silicon substrate. The explanation was motivated by the absence of axial screw dislocations in the whiskers (which in themselves are a growth mechanism), the requirement of the gold droplet for growth, and the presence of the droplet at the tip of the whisker during the entire growth process.

Introduction

The VLS mechanism is typically described in three stages:

  • Preparation of a liquid alloy droplet upon the substrate from which a wire is to be grown
  • Introduction of the substance to be grown as a vapor, which adsorbs on to the liquid surface, and diffuses in to the droplet
  • Supersaturation and nucleation at the liquid/solid interface leading to axial crystal growth

Experimental technique

The VLS process takes place as follows:

  1. A thin (~1-10 nm) Au film is deposited onto a silicon (Si) wafer substrate by sputter deposition or thermal evaporation.
  2. The wafer is annealed at temperatures higher than the Au-Si eutectic point, creating Au-Si alloy droplets on the wafer surface (the thicker the Au film, the larger the droplets). Mixing Au with Si greatly reduces the melting temperature of the alloy as compared to the alloy constituents. The melting temperature of the Au:Si alloy reaches a minimum (~363 °C) when the ratio of its constituents is 4:1 Au:Si, also known as the Au:Si eutectic point.
  3. Lithography techniques can also be used to controllably manipulate the diameter and position of the droplets (and as you will see below, the resultant nanowires).
  4. One-dimensional crystalline nanowires are then grown by a liquid metal-alloy droplet-catalyzed chemical or physical vapor deposition process, which takes place in a vacuum deposition system. Au-Si droplets on the surface of the substrate act to lower the activation energy of normal vapor-solid growth. For example, Si can be deposited by means of a SiCl4:H2 gaseous mixture reaction (chemical vapor deposition), only at temperatures above 800 °C, in normal vapor-solid growth. Moreover, below this temperature almost no Si is deposited on the growth surface. However, Au particles can form Au-Si eutectic droplets at temperatures above 363 °C and adsorb Si from the vapor state (due to the fact that Au can form a solid-solution with all Si concentrations up to 100%) until reaching a supersaturated state of Si in Au. Furthermore, nanosized Au-Si droplets have much lower melting points (ref) due to the fact that the surface area-to-volume ratio is increasing, becoming energetically unfavorable, and nanometer-sized particles act to minimize their surface energy by forming droplets (spheres or half-spheres).
  5. Si has a much higher melting point (~1414 °C) than that of the eutectic alloy, therefore Si atoms precipitate out of the supersaturated liquid-alloy droplet at the liquid-alloy/solid-Si interface, and the droplet rises from the surface. This process is illustrated in figure 1.

Typical features of the VLS method

  • Greatly lowered reaction energy compared to normal vapor-solid growth.
  • Wires grow only in the areas activated by the metal catalysts and the size and position of the wires are determined by that of the metal catalysts.
  • This growth mechanism can also produce highly anisotropic nanowire arrays from a variety of materials.

Requirements for catalyst particles

The requirements for catalyst are:

  • It must form a liquid solution with the crystalline material to be grown at the nanowire growth temperature.
  • The solid solubility of the catalyzing agent is low in the solid and liquid phases of the substrate material.
  • The equilibrium vapor pressure of the catalyst over the liquid alloy be small so that the droplet does not vaporize, shrink in volume (and therefore radius), and decrease the radius of the growing wire until, ultimately, growth is terminated.
  • The catalyst must be inert (non-reacting) to the reaction products (during CVD nanowire growth).
  • The vapor-solid, vapor-liquid, and liquid-solid interfacial energies play a key role in the shape of the droplets and therefore must be examined before choosing a suitable catalyst; small contact angle between the droplet and solid are more suitable for large area growth, while large contacts angles result in smaller (decreased radius) whisker formations.
  • The solid-liquid interface must be well-defined crystallographically in order to produce highly directional growth of nanowires. It is also important to point out that the solid-liquid interface cannot, however, be completely smooth. Furthermore, if the solid liquid interface was atomically smooth, atoms near the interface trying to attach to the solid would have no place to attach to until a new island nucleates (atoms attach at step ledges), leading to an extremely long growth rate. Therefore, “roughâ€� solid surfaces, or surfaces containing a large number of surface atomic steps (ideally 1 atom wide, for large growth rates) are needed for depositing atoms to attach and nanowire growth to proceed.

Growth mechanism

Catalyst droplet formation

The materials system used, as well as the cleanliness of the vacuum system and therefore the amount of contamination and/or the presence of oxide layers at the droplet and wafer surface during the experiment, both greatly influence the absolute magnitude of the forces present at the droplet/surface interface and, in turn, determine the shape of the droplets. The shape of the droplet, i.e. the contact angle (β0, see Figure 4) can, be modeled mathematically, however, the actual forces present during growth are extremely difficult to measure experimentally. Nevertheless, the shape of a catalyst particle at the surface of a crystalline substrate is determined by a balance of the forces of surface tension and the liquid-solid interface tension. The radius of the droplet varies with the contact angle as:

R=\frac{r_\mathrm{o}}{\sin(\beta_\mathrm{o})},

where r0is the radius of the contact area and β0 is defined by a modified You


From Yahoo Answers

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