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Nanotubes and Nanowires
Nanotubes and Nanowires
Though not as well known as their little brother, the quantum dot, nanowires and nanotubes have their own unique uses that will get any nano-scientist excited.
Like the quantum dot, a nanowire exhibits quantum confinement. Instead of confinement in three directions, a nanowire has confinement in two directions (or free electron movement in 1-D). Instead of looking like a sphere, it naturally looks like a wire or cylinder. Nanowires can be metal, semiconductor, or some compound between the two.
Synthesis techniques are mainly focused on a CVD process with a gold monolayer on the substrate. When the substrate is heated up, the gold monolayer dewets (unbinds) from the substrate in liquid form. From there, the liquid gold coalesces into nanoclusters and will naturally self-assemble into a grid of gold nanoclusters. The gold acts as the catalyst in the VLS model of nanowire growth. VLS stands for vapor-liquid-solid. As researchers understand it, the source material enters the CVD chamber as a gas (vapor!). From there, it enters the gold nanoclusters as liquid form. Once the percentage of source material has supersaturated the nanoclusters, it begins to solidify and grow outward in a perfect single crystal nanowire! The diameter of the nanowire can be roughly controlled by controlling the diameter of the nanoclusters. The length of a nanowire can be controlled by leaving the source on until the desired length is reached.
Semiconductor nanowires are supremely important for the future of nanoelectronics. They can be doped and physically crossed to form p-n junctions that are the basis of all of our modern electronics. We'll discuss their applications in the applications section.
Nanotubes are a bit different. They are an entirely new class of material in their own right. Carbon nanotubes have generated a lot of interest in the media because of their unique properties.
The structure of a nanotube is essentially a sheet of graphite (carbon) that has folded over to form a tube. Carbon nanotubes, in particular, have demonstrated unheard of electronic conduction properties. On the physical side, a carbon nanotube is stronger, more ductile, and more elastic than any other manmade material. Unfortunately, they're so small that no bulk material can be made right now that can take advantage of their strengths. They have, however, found their way into many proof of concept demonstrations of nanoelectronics devices.