'a' is suitable for all three, because all three are concepts and multiples, rather than a single specific object.
The simplest explanation is to use "the" for things which are singular. A diameter is a property of a nanotube...Since there are many nanotubes with many diameters (even if they are all the same size, each has it's own diameter), there are many diameters.
The only exception to this would be when we are talking about "The diameter" of a single nanotube... because now there is only one, and it is the subject of our sentence. Note that this only applies where we know there is only a single diameter.
The diameter of the nanotube is 200 nanometers
We may still use "a" in the following sentence:
The nanotube has a diameter of 200 nanometers
This is because in theory a nanotube may have multiple diameters, and (unlikely in the previous stencence) we have not specified that there is only one.
If we did know that it only had a single diameter, we would likely use the earlier sentence, or could construct it as:
The nanotube's diameter is 200 nanometers
The nanotube has the diameter 200 nanometers
The first of these is good, the second sounds a bit "wrong" while being technically correct.