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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.
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.
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
The VLS process takes place as follows:
- A thin (~1-10 nm) Au film is deposited onto a silicon (Si) wafer substrate by sputter deposition or thermal evaporation.
- 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 Â°C) when the ratio of its constituents is 4:1 Au:Si, also known as the Au:Si eutectic point.
- 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).
- 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 Â°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 Â°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).
- Si has a much higher melting point (~1414 Â°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, â€œroughâ€� 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.
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 (Î²0, 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:
where r0is the radius of the contact area and Î²0 is defined by a modified You
liquid one of the three commonly recognized states in which matter occurs, i.e., that state, as distinguished from solid and gas, in which a substance has a definite volume but no definite shape. Properties of Liquids In general, liquids show expansion on heating, contraction on cooling; water, however, does not follow the rule exactly. A liquid changes at its boiling point to a gas and at its freezing point, or melting point , to a solid. The boiling point is especially important because, since liquids change their states at different temperatures, those in a mixture can be separated from one another by raising the temperature of the mixture gradually so that each component in turn undergoes vaporization at its boiling point. This process is known as fractional distillation. Liquids, like gases, exhibit the property of diffusion. When two miscible liquids (i.e., they mix without separation) are poured carefully into a container so that the denser one forms a separate layer on the bottom, each will diffuse slowly into the other until they are thoroughly mixed. Liquids, like gases, differ from solids in that they are fluids, that is, they flow into the shape of a containing vessel. Liquids exert pressure on the sides of a containing vessel and on any body immersed in them, and pressure is transmitted through a liquid undiminished and in all directions. Liquids exert a buoyant force on an immersed body equal to the weight of the liquid displaced by the body (see Archimedes' principle and specific gravity ). Unlike gases, liquids are very nearly incompressible, and for that reason are useful in such devices as the hydraulic press. Liquids are useful as solvents. No one liquid can dissolve all substances; each takes into solution only certain specific substances. Molecular Structure of Liquids The molecules (or atoms or ions) of a liquid, like those of a solid (and unlike those of a gas), are quite close together; however, while molecules in a solid are held in fixed positions by intermolecular forces, molecules in a liquid have too much thermal energy to be bound by these forces and move about freely within the liquid, although they cannot escape the liquid easily. Although the molecules of a liquid have greater cohesion than those of a gas, it is not sufficient to prevent some of those at the free surface of the liquid from bounding off (see evaporation ). On the other hand, the cohesive forces between the molecules at the surface of a mass of liquid and those within cause the free surface to act somewhat like a stretched elastic membrane; it tends to draw inward toward the center of the liquid mass, to draw the liquid into the shape of a sphere, thus exhibiting the phenomenon known as surface tension . A liquid is said to "wet" a solid substance when the attractive force between the molecules of the liquid and those of the solid is great enough to hold the liquid's molecules at the solid surface. For example, water "wets" glass since its molecules cling to glass surfaces, whereas mercury does not since the adhesive force between its molecules and those of glass is not strong enough to hold them together. Capillarity is an example of surface tension and adhesion acting at the same time.
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Answers:First, you must define homogenous. According to http://www.chemicool.com/definition/homogeneous.html , the definition of homogeneous is: "A substance or material that contains only one kind of compound or one element. Homogeneous is Latin for "the same kind". An example of a homogeneous substance would be pure water, which only contains the compound H2O or pure table salt that only contains the compound NaCl." Now that you know the definition, some examples are: Homogenous liquid: water, paint after you've opened the can and stirred it (before you stirred it, it was not homogenous, see?), and any fruits you put in the blender long enough, ha ha ha Homogenous gases: Any gas given enough time will become homogenous because of gas diffusion and natural mixture caused by kinetic energy and the nature of entropy. So air, any hydrocarbon of less than six carbons at room temperature (methane, ethane, propane...), and any mixture of gases given enough time and the generally-same density and boyancy and weight and stuff. Of course a huge difference would make a heterogenous mixture of gasses, like if one gas was rock heavy and sank to the bottom of the container. Solid solution or alloy: Anything that looks the same. Like how a steel bar is steel all the way through and all the way along it, that steel bar is homogenous all the way. Any refined metal made in a factory with modern production standards will be homogenous, like aluminum foil or stainless steel sheeting. Things that are not homogenous: 1. Dipping dots ice cream. See picture if you've never eaten it and don't know what it is: http://www.acatinthekitchen.com/photo/joypolisglass2.jpg See how there are pink dots and green dots, and it's not the same all the way through? That is Not Homogenous, or "Heterogenous". 2. Sand in water. The sand settles down to the bottom, and if you look at the bottom it's sand and in the middle is sandy water and in the top is clean water. That's heterogenous, not homogenous. 3. trail mix. It's not the same all the way through. There are little bits of peanuts and crackers, and you're never sure what you're going to get when you reach in and pull out a handfull. If you're still confused, there's a website with more information and pictures here: http://elaineanne.learnerblogs.org/ Just scroll down or use 'control f ' to find the word "homogenous". It'll explain more.
Answers:a) a) c)
Answers:I'm a bit confused here: there are two cups, one of which had a solid in it and caught fire, and the other of which the teacher drank from? Is the liquid in the two cups the same? Calcium carbide reacts with water to produce acetylene gas, which is flammable. However, it won't ignite spontaneously. If the liquid in the cup was an acid, the solid could have been a silicide - magnesium silicide can be made in low yield by heating sand with magnesium metal. It reacts with acid (HCl for example) to produce silane gas (SiH4), which is pyrophoric and ignites spontaneously on contact with air. There's not too many pure liquids you can drink in decent quantities without harm. Water, ethanol, fats, and maybe glycerin and propylene glycol. If this liquid was the same as the one used for the catching fire experiment, maybe it was a solution of some salt in water?