Using Carbon Nanotubes for Electron Guns

[WarrickF]

Hi All,

I was reading about carbon nanotubes on wikipedia and they seem to be a pretty interesting development. Wikipedia indicates that they are using in producting better electron guns, and I was wondering if anyone knew how they are being used to do so.

My unserstanding of current electron guns is that there is a charge placed ocross a + and - plate ... then the - plate is heated electrons released from the negative plate are attracted towards the positive plate. The + plate has a hold in it, so some of the electrons fly through.

How are nanotubes used in this exercise?

Thanks
Warrick

 

[Kane O'Donnell]

The process you have described is thermionic emission of electrons from some filament (usually tungsten or something involving tungsten because of it's high melting point).

With nanotubes, we are interested in a phenomena called field emission, where electrons, instead of just stepping over the metal-vacuum interface to become free electrons, actually tunnel through the barrier. Do a google search on electron field emission, there are *lots* of papers about it.

I'm setting up an experiment on CNT field emission right now, actually, so I better get back to work.

Incidently, field emission from nanotubes has another interesting application, that of bright electron sources to light up pixels in a flat screen display - hence why so much research money is going into it. The reason we use nanotubes is because firstly they're tiny (you need massive local electric fields to make the tunnelling barrier narrow enough for reasonable currents) and secondly they're mechanically very strong so the devices survive longer under the high field conditions.

There are also thermionic nanotube devices, I believe, and even nanotube lightbulbs!

Kane O'Donnell

 

[WarrickF]

Wow, thanks I took a look into field emission and it's quite an interesting topic.

How does one work with a device this small, from what I've read they are a fraction of the diameter of the cross section of a hair. I can't really imagie myself pickig one of these guys up :) . Surely you must do this under some serious microscope, but even then how on Earth do you workwith it?

Thanks
Warrick

 

[Kane O'Donnell]

Well typically the 'working end' of a FE tip is of the order of 10s of nanometres in diameter, so yes, they're very small! On the other hand, the tip itself is often sharpened or etched from a larger piece of wire, for example the Tungsten wires in my experimental apparatus start off 0.38mm in diameter and then I electrochemically etch one end to a sharp point.

It's quite a bit of work to make sure tips are clean, protected, well-formed and so on, but once you get them in vacuum conditions it's ok - then you just have to make sure you don't melt them from too much field emission current!

Kane

 

[Kane O'Donnell]

I should point out though, with nanotubes, typically what you do is form them on some larger surface area (via chemical vapour deposition or whatever) and then deal with lots of nanotubes emitting instead of just trying to deal with one individual one.

I work with single-walled CNTs so I never get to see them, but you can grow aligned arrays of multi-wall nanotubes and image them with an SEM to see what's going on at the surface.

Kane