Dark field TEM – From December 2012

When graphene was first produced in 2004, it was using the famous “sticky-tape” method. This simple method produces high-quality, single sheets of graphene, but it doesn’t produce very much of it; you end up with flakes that are (at the larger end of the scale) hundredths of a millimetre across. In 2009, researchers developed a technique that could produce large areas of graphene, and this was quickly expanded up to 30 inch pieces. The technique is called chemical vapour deposition, and involves breaking down carbon-containing molecules on a metal surface.

Here we are investigating the most common CVD route: using methane and copper. Although it can produce huge sheets of graphene, the quality of the material is much lower. CVD graphene did not quite have the same amazing properties that had been measured before.

It didn’t take too long to work out why. CVD-graphene is actually formed of lots of smaller graphene sheets stitched together, like a patchwork quilt, with all the different patches (or grains, as they are actually called) rotated a bit compared to those nearby.

Understanding why graphene grows with different orientations is key to improving the quality of CVD-graphene.

It is possible to visualise the graphene grains using a technique called “dark-field” imaging in a transmission electron microscope (TEM). In a normal TEM image, you cannot see very much of graphene because it is so thin: it often looks like the grey image below (the darker strands hold the graphene in place). But in the dark field image – seen by hovering your mouse over the image – we have managed to separate the electrons from each orientation, and used these to make another image. Each colour corresponds to a different orientation.

The image has only two colours, and we interpret this to mean that this small area of graphene is formed of grains that have only two orientations. This is an interesting result for growing graphene on copper, and is discussed more in the paper we recently published here.

 

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