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.