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.
Studying 2D materials brings obvious challenges. It is especially challenging for those on rough substrates. This is the case for graphene grown on copper foils by CVD. Detecting graphene on copper is quite difficult and there are many different microscopes that have certain advantages. Here I want to introduce friction force microscopy in an atomic force microscope.
The key parts of an atomic force microscope (AFM) are shown in the schematic below. A laser light is reflected off a beam onto a photodiode, which collects the light in each quadrant and measures how the beam bends. A sharp tip on the end of the beam pushes it up when it moves over a lump on the surface; we measure the lump with the movement of the laser.
We can measure the graphene on copper surface like this, but this height image is governed by the rough, wavy copper surface, and seeing a single atomic step of graphene becomes impossible.
However, AFM can probe many surface forces, and we can exploit a different property of graphene to image it: graphene has a very low friction coefficient, much smaller than copper. We can map the friction at each point on a surface and use this to identify graphene.
To measure the friction we have to see how the beam twists instead of how it bends. We measure a lateral signal when the beam twists as the spot moves left or right on the photodiode.
The schematic below shows what we record as a tip slides over two surfaces with different friction values. The copper colour represents high friction copper and the grey represents the low friction graphene.
What we are most interested in is the difference between the trace and retrace lateral signals; the larger the difference at that point on the surface, the greater the friction. When we map the same area as above in this way we are left with the image below (part e). The graphene is now clearly visible from the copper in the gaps.
More information can be found in the paper:
EPSRC run a photo competition every year, for science related photos. Above is the photo I submitted for last years competition. It shows Jon Peters from our group using our very latest transmission electron microscope (TEM). Even with the introduction of digital cameras for TEMs, they are still sometimes operated by looking directly at the microscope screen. This needs to be done in the dark, and the back-lit user panels make for attractive photos. The winners of last years competition can be found here.